POSS Polymers: Physical Properties and Biomaterials Applications

R Journal of Macromolecular Science , Part C: Polymer Reviews, 49:25–63, 2009 Copyright © Taylor & Francis Group, LLC ISSN: 1558-3724 print / 1558-3716 online DOI: 10.1080/15583720802656237

POSS Polymers: Physical Properties and Biomaterials Applications JIAN WU AND PATRICK T. MATHER

Downloaded By: [Mather, Patrick] At: 22:18 28 January 2009

Syracuse Biomaterials Institute and Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, NY 13244 Research into polymers incorporating polyhedral oligomeric silsesquioxane (POSS) has intensified during the past several years, revealing new fundamental polymer physics, new synthetic routes, and unexpected applications. The present review article critically examines the recent scientific literature on POSS polymers with an emphasis on structure-property relationships. We conclude that it is an exciting time to work on such materials and we expect the field to continue to grow in the foreseeable future. Keywords

POSS, hybrid materials, rheology, microstructure, biomaterials

1. Introduction 1.1. Polymeric Nanocomposites To reinforce polymers, it is common to physically disperse in the polymeric host inorganic fillers chosen from a variety of different shapes, such as fibers or whiskers, platelets, or spheres. This ubiquitous approach attempts to combine acceptable processibility typical of thermoplastic polymers with desired characteristics from the inorganic filler, such as high modulus, high oxidation resistance, or high use temperature. Ideally, the resultant properties will represent not only the volumetric averaging of contributions from individual components, but also the synergic effects of the components included. In such cases, the improvement of physical properties can often be found at relatively lower filler inclusion.1−4 As the filler is decreased to smaller than 100 nm, the resulting composites, termed nanocomposites, may achieve dramatic improvements in such physical properties as gas barrier, thermal stability, elastic modulus, and ultimate mechanical properties.5−9 Such substantial enhancement can be attributed mainly to the filler particle surface properties and interfacial interactions that become increasingly important with decreasing particle size. Nanofillers exist in various shapes, such as spherical (metallic particles10−12 and semi-conductive particles10−12 ), layered (layered silicate13−15 ), and fibrous (nanofibers16,17 and carbon nanotubes15,18,19 ). Applications ranging from automotive components to food packaging and to biomaterials have been pursued.

Received November 29, 2008; Accepted November 30, 2008. Address correspondence to Patrick T. Mather, Syracuse Biomaterials Institute, Biomedical and Chemical Engineering, 121 Link Hall, Syracuse University, Syracuse, NY 13244, E-mail: [email protected]

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The physical properties exhibited by polymeric nanocomposites are determined by the quality and the nature of dispersion of the nano-fillers in the polymer matrix. However, it is known that the surface energy substantially increases with the decrease of particle size. Consequently, nanoparticles tend to aggregate in order to reduce the total surface energy, creating a pervasive manufacturing challenge. To ameliorate this problem, nanoparticles are often grafted or otherwise modified with organic groups (commonly alkyl ammonium surfactants) similar or compatible with the polymer matrix, followed by melt-mixing or in-situ polymerization. The resulting materials feature microstructure and properties that are quite sensitive to processing conditions. The complexities mentioned concerning “topdown” nanocomposites processed by dispersion, coupled with the fact that the size scale of fillers approaches the molecular scale, present the need for a synthetic approach utilizing nanoscale monomers that would naturally disperse and feature covalent incorporation. Polyhedral oligosilsesquioxane, so-called “POSS,” is just such a nanoscale monomer and is the focus on the remainder of this review.

1.2. What is POSS? Polyhedral oligosilsesquioxane (POSS) is one of many kinds of silsesquioxane molecules. The term silsesquioxane refers to the molecules, whose chemical structure follows the basic composition of Rn Sin O1.5n , for example Me8 Si8 O12 . Here, the R-group, also called the vertex group for polyhedral molecules, may be hydrogen, alkyl, alkylene, aryl arylene, among others. Such silsesquioxanes can form oligomeric organosilsesquioxanes (CH3 SiO1.5 )n through chemical reactions and the chemical structures of the derivative silsesquioxanes are quite versatile and the interested readers are referred to the review by Barney et al.,20 which focuses on the preparation, structures, and applications of such silsesquioxanes. The molecular architecture of silsesquioxanes can be classified into two categories: (a) noncaged structure and (b) caged structure, each shown in Scheme 1(a) and Scheme 1(b). As shown in Scheme 1(a), the non-caged silsesquioxane molecules can be further classified into: (a) random structure; (b) ladder structure, and (c) partial-cage structure.

Scheme 1. Chemical structures of silsesquioxanes. (a) non-caged silsesquioxanes: (i) random, (ii) ladder; (iii) partial caged structures, and (b) caged silsesquioxanes: (i) T8 , (ii) T10 , (iii) T12 structures.

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Cage-like silsesquioxanes are usually called polyhedral oligosilsesquioxanes or Polyhedral Oligomeric Silsesquioxanes, abbreviated as POSS. This class of well-defined, highly symmetric molecules usually features a nanoscopic size, approximately 1.5 nm in diameter when the vertex (R) groups are included. They can be loosely regarded as the smallest possible silica particles.21 POSS molecules with a T8 cubic inorganic core composed of silicon-oxygen (R8 Si8 O12 or R 1 R7 Si8 O12 ) are the most prevalent system studied, although the Q8 structure (R8 Si8 O20 ) has also been given significant attention. Here, “T” and “Q” refer to conventional nomenclature from silicon nuclear magnetic resonance (NMR) literature, with T and Q referring to silicon atoms bonded to three (3) or four (4) oxygen atoms, respectively.22 The hybrid organic-inorganic framework renders POSS thermally and chemically robust, so much so that one of the promising applications of POSS-based polymers is for use in the highly oxidizing environment of orbiting space vehicles.23,24 This aspect will be discussed later, in Section 3.3 on Surface and Interfacial Phenomena. In a similar fashion to atomic oxygen resistance, POSS can improve the oxidative stability and flame retardance of polymers in terrestrial applications.25−27

1.3. POSS-Based Polymeric Nanocomposites From the microscopic viewpoint, the characteristic nanoscopic size of the POSS molecule (1.5 nm) is comparable to the dimensions of polymeric segments or “blobs”28 in the condensed phase (molten or solid), yet nearly double typical intermolecular spacing. Undoubtedly, the incorporation of POSS moieties into linear polymer chains and/or polymer networks will modify the local molecular interactions, local molecular topology, and the resulting polymer chain and segment mobility. These microscopic modifications are manifested in the macroscopic physical properties and performance, such as modulus, strength, glass transition temperature, thermal stability, and dimensional stability. Considering dimensionality, nano-fillers can be classified as one-, two-, or three-dimensional (1D, 2D, or 3D), depending on their geometrical symmetry. Layered nano-clays29−31 are aluminosilicate sheets that extend in two dimensions and featuring one dimension, the layer normal, nˆ for interaction with a polymer host. In this sense such fillers are considered one-dimensional. In contrast, single and multi-walled nanotubes, carbon nanofibers, and molecular rigid rods32 extend in one dimension and interact with a polymer host in two dimensions of the surrounding space (ˆr and θˆ of cylindrical coordinates), and thus are considered 2D. Finally, POSS is like other highly symmetric molecules, including dendrimers, by being roughly spherical and interacting with the polymer host in the three ˆ and ϕˆ of spherical coordinates). As we will dimensions of the surrounding space (ˆr , θ, show, POSS moieties can aggregate or crystallize into supramolecular objects of lower symmetry and then interact with the polymer host in a geometrically distinct way. While non-reactive POSS is often studied as a filler capable of molecular level dispersion due to the small size, we feel that the more powerful implementation of POSS in nanocomposites is through copolymerization with the POSS monomer. In this bottom-up approach, good dispersion is assured through covalent attachment to the host polymer, while nanocomposite reinforcement occurs by self-assembly—aggregation or crystallization—that can be tuned architecturally and compositionally. This self-assembly impacts the resulting macroscopic physical properties in a rational manner. Unlike silica or silicones, each POSS molecule is tethered to eight organic groups surrounding its cage and bonded to the silicon vertices, these groups varying in composition to include methyl, isobutyl, cyclopentyl, cyclohexyl, phenyl, aniline, among others. Positioned


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at the cage vertex, all but the smallest of tethers collectively form a voluminous shell—as much as 80% of the POSS volume33 —around the Si8 O12 core that mediates the interactions between POSS moieties and polymer matrix. Thus, POSS molecules embody a truly hybrid inorganic core/organic shell architecture that is naturally compatible with organic hosts, such as polymers and natural biomaterials. Furthermore, one or more corner groups can be substituted by a functional group through conventional organic conversions.34−37 These versatile functional groups, such as methacrylate, acrylate, styrene, norbornene, amine, epoxy, alcohol, and phenol, provide the possibility to incorporate POSS into a polymer chain or network through polymerization or grafting. In this manner, a large diversity of POSS-polymer architectures is possible. Early POSS research focused on random tethering of POSS along the polymer chain through free radical chain-growth polymerization38−40 and step-growth polyadditions of copolymers.41 However, as shown in Scheme 2, polymerizations and reactions with POSS can be designed that architecturally locate the POSS moiety at a single end of a polymer chain to form a hemi-telechelic POSS polymer, at both ends to yield POSS telechelic molecules, and tethered to a single block of block copolymers or multiblock polyurethanes. Examples and their references will be given later in this review.

Scheme 2. Molecular architectures of polymer chain with POSS incorporation. (a) random; (b) random block; (c) tri-block; (d) di-block; (e) end-capped telechelics; (f) end-capped hemi-telechelics and (g) centered telechelics. The straight line and solid circles represent polymer chain and POSS molecules, respectively.

Regardless of the preparation approaches, the key factor in determining physical properties is the dispersion and self-assembly of POSS moieties in polymer host. This behavior depends on the thermodynamic interaction between POSS and the polymer matrix, particularly the constituent polymer segments. If the interaction is favorable or mutually


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unfavorable relative to POSS-POSS interactions, POSS moieties will disperse well; otherwise POSS will aggregate. Unlike a filled system or blend, however, POSS aggregation will be limited due to covalent attachment to the polymer backbone that prevents aggregation beyond a scale of about one radius of gyration. Naturally, the resulting physical properties will vary with the POSS dispersion (or the aggregation) level. It is critical, therefore, to understand the nanostructure-property-processing relationship for given systems if one is to successfully target properties per intended application. The significant influence of incorporating POSS moieties on the resulting properties of POSS-related polymeric materials stimulates scientists to also find a rational way to explain the correlation between nanoscopic microstructures and macroscopic properties in these polymeric nanocomposites employing modeling and simulation.33,42−46 The first successful synthesis of a well-defined POSS structure was reported by Scott47 in 1946, which was primarily used for electrical insulation at high temperature. After a couple decades of inactivity, research on POSS was reinvigorated in the 1990s, spawned by the discovery of a method to prepare polymerizable POSS. At the Air Force Research Lab at Edwards Air Force Base, CA, (then the US Air Force Phillips Laboratory) Lichtenhan and Haddad et al. successfully synthesized a series of linear random copolymers incorporated by POSS molecules. The systems include styryl-POSS,38 methacrylate-POSS,40 norbornyl-POSS,48 siloxane-POSS copolymers41 (Scheme 3). While research on POSS-based polymers has continued in earnest at the Air Force Research Lab, the establishment of Hybrid Plastics, Inc. now headquartered in Hattiesburg, MS, has led to commercial availability of POSS monomers and polymers, enabling much fascinating POSS-related materials research49 across academic, industrial, and government laboratories. A number of research groups devote themselves to the studies of POSS-related polymeric nanocomposites, varying from synthesis to materials characterization, attempting to understand the relationships among processing, structure, and property. The field of POSS polymer research is expanding rapidly; indeed, the number of publications pertaining to POSS research has increased 10-fold during the past decade, following an exponential growth in time. In response to this high level of research activity, review articles in the area have appeared every two or three years. These review articles have introduced the latest developments with varying emphasis ranging from the synthesis of POSS polymer materials to the formation of POSS nanocomposites and associated enhanced properties.21,50−52 As indicated previously, the prevailing methodology to incorporate POSS groups into a polymeric system is to copolymerize the POSS macromer, bearing one or two polymerizable groups and the remaining inert vertex groups, with a suitable host comonomer to obtain the desired organic-inorganic hybrid polymeric nanocomposites. It has been observed that polymerization reactivity for POSS monomers is high and that even homopolymers of the POSS monomer are possible, so long as a spacer molecule between the POSS moiety and the polymerizable group of sufficient length is utilized. Thus far, copolymers achieved and studied by this methodology include poly(styrene),53−56 poly(methacrylate),57−60 epoxies,61−65 polyurethanes,66−68 polyimide,69−71 polyolefin,72−74 poly(siloxane)75−78 and polycarbonate.79,80 When compared to the top-down approach of filler-based composites requiring dispersion, such as silica, POSS copolymers have been observed to yield nanocomposites with excellent properties, such as higher glass transition and usage temperature, increased thermal stability and oxygen permeability, enhanced mechanical properties and lowered dielectric constant, while largely preserving processability. The rest of our review is divided into two parts. First, we will discuss the latest advances in the studies of morphology and rheological behavior of polymeric nanocomposites


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Scheme 3. Schematic drawing of chemical structures for POSS-based polymeric nanocomposites: (a) norbornyl-POSS copolymers; (b) siloxane-POSS copolymers; (c) methacrylate-POSS copolymers and (d) styryl-POSS copolymers.

incorporating POSS, along with studies on the morphology and self-assembly behavior of amphiphilic POSS nanocomposites at the surface and the interfaces. Following that, we turn to a focused examination of applications of POSS nanocomposites. Indeed, many application areas have emerged for POSS-based materials, including biomaterials,81−83 dielectric materials,69,84−87 organic light emitting diode devices,88−91 lithography resists,92−95 catalyst,96−98 and fuel cell and battery membranes.99−101 For our review we will turn our attention to biomaterials.

2. Microstructure and Rheological Behavior of POSS-Based Polymeric Nanocomposites 2.1. Well-Defined Molecular Architectures 2.1.1. Amphiphilic POSS Telechelics. As previously discussed, POSS moieties incorporated into polymers feature eight vertex (corner) groups, generally with one (1) tethering POSS to the polymer backbone and the other seven (7) being identical in composition, owing to the preparation procedure from a single trichlorosilane (RSiCl3 ) or tri-alkoxysilane (such as RSi(OCH3 )). In nearly all cases, these vertex groups on the silicon-oxygen cage


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are linear- or cyclo-aliphatic in nature. This renders the POSS cage quite hydrophobic in nature (unlike silica) and further provides the potential to create amphiphilic molecules (and accompanying structures) when combined covalently with hydrophilic chains. As for any amphiphilic system, self-assembled microstructures can be expected in both the bulk and in solvent, especially if a solvent selective for one of the components (hydrophilic or hydrophobic) is utilized. In such a selective solvent, amphiphilic molecules can display supermolecular aggregates with shapes ranging from spheres, to rods, to lamellae. Specifically considering POSS-based amphiphilic polymers, one architectural strategy is to fix POSS molecules at the end(s) of a hydrophilic chain, yielding either a dumbbell (telechelic) or tadpole polymeric architecture. Natural choices for the hydrophilic chain used in such structures are poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO), respectively, due to their commercial availability, nonionic simplicity, and (not insignificantly) mutual solubility with POSS in such reaction media as tetrahydrofuran (THF). Knischka and co-workers102 synthesized hemi-telechelic POSS-PEO molecules by hydrosilylation of H8 Si8 O12 to allyl-functional poly(ethylene oxide) and subsequent conversion of the remaining seven vertices to ethyl groups. In solution, the hemi-telechelic molecules selfassembled into micellar and vesicular structures. The researchers further cross-linked the POSS moieties at high pH to form the silica shells of the spherical vesicles. Employing TEM and AFM microscopy, the authors observed that bimodal aggregations form in water when the pH value was elevated up to 9.3, one featuring a micellar-type structure 10 nm in diameter, the other featuring a vesicular-type aggregate 60 nm in diameter. Kim and Mather103 synthesized well-defined POSS-PEO-POSS telechelics by endcapping PEG molecules with two equivalents of a cyclohexyl-POSS monoisocyanate, yielding the structure depicted schematically in Scheme 2(e) and Fig. 1. This resulted in a dumbbell configuration and, quite surprisingly, led to physical behavior akin to triblock ABA copolymers. As shown in Fig. 1, WAXD observations provided evidence that the covalently linked POSS end-caps and PEG bridges separately formed their individual characteristic crystalline phases, driven by thermodynamic incompatibility. Based on this data, combined with thermal analysis and hot-stage polarizing optical microscopy, the authors proposed a sequence of microstructures on heating shown schematically in Scheme 4, and investigated the relationship of microstructures and rheological behavior through these transitions.104

Scheme 4. A proposed microstructure evolution of POSS telechelic, i.e. Tel10k, with the temperature increase. Reprinted from Kim, B. S.; Mather, P. T., Macromolecules 2006, 39, 9253–9260 (reference 104) with permission of American Chemical Society.


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Figure 1. Wide angle X-ray diffraction (WAXD) patterns of the as-synthesized amphiphilic POSS end-capped PEG telechelics: (a) PEG8k, (b) Tel10k, (c) Tel8k, (d) Tel3.4k, (e) Tel2k, (f) Tel1k and (g) POSS macromers, “Tel” is the abbreviation of telechelic and the number stands for the molecular weight of PEG bridge. Reprinted from Kim, B. S., Mather, P. T., Macromolecules 2006, 39, 9253–9260 (reference 104) with permission of American Chemical Society.

Within the range of the explored temperature (50◦ C < T < 180◦ C), the rheological behavior of the POSS telechelics did not obey the Time-Temperature-Superposition (TTS) principle, suggesting a temperature-dependent morphology. Above the PEG melting point, the persistence of a POSS nanocrystalline phase, acting as physical crosslinking sites, preserved solid-like rheological characters of the telechelics. In particular, G was observed to be higher than G between the PEG-phase and POSS-phase melting points. Crossing the POSS melting point, the telechelics showed a solid-liquid transition, manifested as a precipitous drop in both G and G and resembling the order-disorder transition of block copolymers. The amphiphilic nature of POSS-PEG telechelics also underlies the sensitivity to solvent polarity and the possible self-association behavior in given solutions. For example, they were found insoluble in both water and hexane, which are the selective solvents good


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for PEG and POSS, respectively. Kim and Mather105 further investigated the influence of polymer concentration and solvent polarity on viscometric properties of POSS-PEG telechelics in order to reveal the molecular association of POSS-PEG telechelics in a salt-free solution. Like pure PEG, POSS-PEG telechelics in THF showed the linear dependence of reduced viscosity on concentration, indicating a lack of self-association behavior. However, in mixed water/THF solutions, adding more water (increasing solvent polarity) led to a dramatic upturn of reduced viscosity in the very dilute regime, a polyelectrolyte-like effect, which was interpreted to indicate the formation of micelles induced by POSS-POSS associations. This hydrophobic-hydrophobic association behavior of POSS-PEG telechelics was found to vary with the POSS content and the length of PEG blocks, as well as solvent polarity. Water, a good solvent for PEG and selective non-solvent for POSS, was found to tune the intra- and intermolecular interactions of POSS-capped POSS-PEG telechelics (via a hydrophobic interaction) and the more water that was added, the more significant the association as reflected in the reduced viscosity. Recently, Lee et al.106 reported on the synthesis of isobutyl-POSS-centered poly(εcaprolactone) (PCL) polyol telechelics with varying PCL molecular weight, achieving the structure shown simply in Scheme 2(g). By end-capping the POSS-PCL telechelic polyols with acrylate groups and photocuring with the stoichiometric addition of a tetrathiol cross-linker, a family of POSS-PCL networks was achieved. Like the case of POSS-PEOPOSS telechelics, this system showed evidence for microphase separation and complex crystallization competition between PCL and POSS. It was found that only those PCL networks with high POSS loading (>34 wt%) led to POSS crystalline domains as indicated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Dynamic mechanical analysis showed two distinct rubbery plateaus in the sample with 42 wt% POSS loading, one between the glass transition temperature of amorphous PCL and the prominent POSS melting point, the other appearing above the POSS melting point and afforded by the covalent crosslinks. For samples with high POSS loading, and thus featuring POSS crystallinity, an excellent one-way shape memory response (shape fixing and recovery) was observed. This indicated that POSS crystallization on cooling of strained samples was capable of fixing a large elastic strain and thus percolated space to form a mechanically robust network capable of resisting the elastic strain energy of the covalent, PCL network. 2.1.2. Block Copolymers Incorporating POSS. Haddad and coworkers107 reported on the synthesis of AB block copolymers, poly(norbornene-POSS)-b-poly(norbornene) prepared using ring-opening metathesis polymerization. As shown in Fig. 2, the TEM images of thin sections revealed that such diblock copolymers featured prominent micro-phase separation when the POSS content was above 10 wt%. The distinct dark POSS-rich phase, due to its higher electron density, featured the strong ordering as cylinders. However, the long-range ordering of cylindrical POSS-rich micro-domain was poor, perhaps due to polydispersity or contamination with the PN homopolymer. In other work on POSS block copolymers, Pyun and co-workers108 successfully synthesized well-defined ABA triblock copolymers, poly(MA-POSS)-b-poly(n-butyl acrylate)(pBA)-b-poly(MA-POSS), employing atom transfer radical polymerization (ATRP). Transmission electron microscopy (TEM) images obtained with selective staining of POSS revealed that microphase separation did not occur at lower POSS loadings, where the degree of polymerization (DP) followed poly(MA-POSS)6 /pBA481 /poly(MA-POSS)6 . At a higher molar ratio (poly(MAPOSS)10 /pBA201 /poly(MA-POSS)10 ), strong micro-phase separation was observed but with a morphology inverse that expected: pBA cylinders were periodically distributed in a


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Figure 2. Transmission electron microscopy (TEM) images of a series of poly(norbornene)-bpoly(norbornene-POSS) di-block copolymers varying with CyPOSS(upper) and CpPOSS(below) derivatives. The POSS loading is 10, 30, 60 wt% from left to right. Reprinted from Haddad, T. S., Mather, P. T., Jeon, H. G., Chun, S. B., Phillips, S. Materials Research Society Symposium Proceedings 2000, 628, CC2.6.1–CC2.6.7 (Reference 107) with permission of Materials Research Society.

continuous poly(MA-POSS) (as shown in Fig. 3 (left)) despite it being the larger weight fraction component. Above both glass transition temperatures of the two phases, the rheological behavior showed a lack of fluidity at even the highest temperature and lowest frequency as shown in Fig. 3 (right). The slope near 1/2 at lower frequency, observed for both log G & G vs log reduced frequency, was consistent with other rheological observations for ordered block copolymers. This non-terminal rheological behavior was attributed to elasticity derived from the micro-phase separated morphology of a strongly segregated system. It was reported in the same paper108 that longer POSS lengths were not possible using ATRP and the POSS monomer utilized. More specifically, the molecular weight of the poly(MA-POSS) homopolymer synthesized by ATRP was limited to just 12 kg/mol (Mw /Mn = 1.8) or approximately 12 repeated units per chain, and this could be due to steric crowding during the polymerization of the homopolymer of POSS-methacrylate in that block, noting that a propyl spacer separated the methacryl group from the POSS moiety. Indeed, this may be the reason why very few reports exist pertaining to POSS-based block


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Figure 3. The microstructures and rheological behavior of tri-block copolymer: p(MA-POSS)10b-p(BA)210-b-p(MA-POSS)10: (left) Transmission electron microscopy (TEM) images of (a) low magnification images with a overall morphological feature, (b)–(c) high magnification micrographs showing well-defined bright pBA cylindrical phase regularly dispersed in the dark pMA-POSS continuous phase. The local hexagonal packing of the cylinders was confirmed by (d) Fourier transform of selected area from micrograph, (right) Master curves of dynamic oscillatory shear storage modulus(G ) and loss modulus(G ) for the tri-block copolymer p(MA-POSS)10-b-p(BA)210-b-p(MA-POSS)10. The reference temperature was 80◦ C. Reprinted from Pyun, J., Matyjaszewski, K., Wu, J., Kim, G.-M., Chun, S. B., Mather, P. T., Polymer 2003, 44, 2739–2750 (Reference 108) with permission of Elsevier Ltd.

copolymers containing poly(POSS) block(s). More broadly, it remains a distinct challenge to achieve POSS homopolymers with molecular weight high enough to achieve an entangled system. 2.2. Less–Defined Molecular Architectures 2.2.1. Polymer Blends Incorporating Molecular POSS. Most POSS studies have focused on POSS-based polymeric nanocomposites prepared in one of two ways: physical mixing of free (monomeric) POSS or random copolymerization. In the former case, POSS moieties can be dispersed in the polymer matrix at a level mediated by weak (van der Waals) or strong (hydrogen-bonding) interactions. In the latter case, POSS can be incorporated covalently as a pendant group along a polymer chain through homo- or copolymerization. A fundamental difference of these two approaches is that physical blending may lead to macroscopic phase separation between POSS and the polymeric host, if driven to do so thermodynamically, while this cannot occur for POSS copolymers due to covalent attachment. Practically speaking, physical mixing is an easier way to prepare POSS nanocomposites and, naturally, has been adopted by many research groups. As one prominent example, Fu and Hsiao109 studied the nanocomposites of ethylpropylene copolymer and octamethyl-substituted POSS (methyl-POSS) prepared by melt mixing. An analysis of the microstructure using WAXD revealed patterns revealed evidence for a crystalline POSS phase with distinct crystalline peaks appearing virtually unmodified


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from the pure methyl-POSS. The authors estimated the average size of methyl-POSS crystalline domains dispersed in ethyl-propylene copolymer to be 50 nm by analysis of the diffraction peak widths. The researchers further investigated the melt rheology of ethylenepropylene polymeric nanocomposites with methyl-POSS and compared this behavior with octaisobutyl-POSS (i Bu-POSS) and observed, for both cases, alteration of the low frequency (long time) behavior transition from liquid–like to solid–like, indicating a loose elastic network of crystalline POSS particles. This was further substantiated by observation of a finite yield stress that increased with increased POSS loading. In a similar vein, Joshi et al.110 investigated the relationship between morphology and rheology of methyl-POSS/HDPE blended nanocomposites. In contrast with the work of Fu and Hsiao,109 it was found that methyl-POSS can be dispersed in HDPE at the molecular level, for very low loading levels, lower than 1.0 wt%. Rheological characterization showed that POSS particles reduced the complex viscosity magnitude when the POSS content was in the range 0.25 to 0.5 wt%. Above a POSS loading of 1.0 wt%, POSS aggregation in the form of nanocrystals was witnessed. Consequently, the complex viscosity increased with POSS loading and solid-like behavior emerged at low frequencies for POSS loading >5.0 wt%, a finding attributed to the aggregation of nanocrystalline POSS domains and the formation of a physical network of such domains. Cole-Cole (η vs η ) and van Gurp-Palmen (δ vs log|G*|) plots111 allowed clear visualization of these phenomena, especially the transition to solid-like behavior above a POSS loading of 1 wt%. Zhou et al.112 examined the impact of reactive blending in octavinyl-POSS/isotactic polypropylene (iPP) blends on rheological behavior. The baseline iPP/POSS composites prepared by physical blending exhibited rheological behavior dependent on the miscibility between POSS and iPP and the resulting morphological transition. Specifically, miscible blends (POSS ≤ 2 wt%) featured lower modulus and viscosity compared with pure iPP. When the POSS loading was increased to greater than 2 wt%, the modulus and viscosity were found to increase with POSS loading and thermorheological simplicity was lost, as evidenced by non-collapsed Han (log G vs log G ),113,114 Cole-Cole (η vs η ) and van Gurp-Palmen (δ vs log|G*|) plots. It was concluded that molecularly dispersed POSS plasticized the iPP, while aggregated POSS domains lead to fluid-solid transition at higher temperature due to their interactions, consistent with other findings reported above. With the aid of the thermal radical generator, dicumyl peroxide (DCP), octavinyl-POSS molecules could be reacted with the host iPP chains. As a consequence, both G and G significantly increased, more so at higher POSS contents. The frequency dependencies of G and G monotonically decreased (flattened) with increasing POSS loading. The Han plot of the reactively blended composites deviated significantly from the expected form for entangled linear polymers, even at a quite low POSS content (0.5 wt%). These observations were ascribed to the grafted POSS’s strong anchoring effect that retarded the polymer chain relaxation at a low POSS content (≤0.5 wt%), and the formation of a POSS-iPP network at a higher POSS loadings (>0.5 wt%). Studying blends of POSS and poly(methyl methacrylate) prepared by physical mixing, Kopesky and coworkers58 found that both isobutyl-POSS and cyclohexyl-POSS can crystallize, even for loadings as low as 1 vol%. (It is noted that in POSS systems, the vol% and the wt% should be approximately the same, volumetric equation-of-state measurements (PVT) have not been conducted to prove this.) Above 1 vol%, untethered POSS crystallization within the PMMA matrix was significant enough to increase both the shear viscosity and plateau modulus, while below this level untethered POSS dispersed at a nanoscopic level and decreased the zero-shear viscosity. In order to study amorphous yet untethered POSS in


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PMMA, Kopesky and coworkers115 selected two POSS alternatives that are liquid at room temperature: octamethacryl-propyl-POSS (MA-POSS) and hydrogenated octamethacrylpropyl-POSS (hMA-POSS). For loadings below 10 vol%, WAXD analyses revealed only a single amorphous halo centered at d =6.27Å, which was attributed to interchain spacings for PMMA. For both such systems, POSS lowered the viscosity and glass transition of the blends monotonically with POSS loading. Time-Temperature-Superposition was successfully employed for all blends and, surprisingly, the Williams-Landel-Ferry (WLF) analysis indicated that the decreases in Tg and viscosity observed could not be ascribed to an increase in free volume with increasing POSS content. This argument was made on the basis of the free volume at Tg being virtually the same for all blends studied. For volume fractions of POSS greater than 20 vol%, POSS was found to aggregate in the PMMA matrix and the rheological behavior similar to the POSS-filled system was recovered. POSS moieties physically blended into a polymer matrix tend to aggregate and crystallize as nanoscale particles and only disperse molecularly at very low loadings and with favorable POSS-matrix interactions. The resulting rheological behavior is sensitive to this dispersion level. In the miscible systems, POSS incorporation decreases the zero shear viscosity and modulus.115 Conversely, POSS crystallized at the nanoscale not only increases the rubbery plateau moduli, but also engenders a secondary rubbery plateau and induces the fluid-to-solid transition. This rheological effect is attributed to strong interactions between POSS aggregated domains.58 2.2.2. Random POSS Copolymers. Besides physical mixing, POSS moieties can be introduced into a polymeric matrix through copolymerization. For such systems, one may ask: (1) How does the polymer chain configuration influence the tethered POSS dispersion and (2) How do the tethered POSS cages impact the resulting rheological behavior? Driven to answer these two questions, Romo-Uribe et al.116 studied poly(4-methylstyrene) copolymers incorporating POSS. The polymers were largely unentangled, due to synthesis limitations at that time, with degrees of polymerization ranging from 150 to 400. The WAXD patterns of the poly(4-methylstyrene) copolymers tethered by cyclopentyl (Cp) and cyclohexyl (Cy)-POSS moieties showed two distinct amorphous halos without any crystalline peaks related to POSS crystals evident for POSS weight fractions is up to 8 mol%. The two amorphous peaks were indicated to represent the polymer chain and side chain/group correlations, respectively. When POSS content increased above 16 mol%, the X-ray diffraction peaks in the POSS region sharpened, particularly for CpPOSS, and this was attributed to limited POSS aggregation. By comparison with the results on filled systems,58,115 it is clear that molecular-level dispersion is facilitated by copolymerization. Rheologically, the unentangled poly(4-methylstyrene) homopolymer showed terminal (G ∝ ω2 ) and Rouse zones (G ∝ ω1/2 ) without an intervening rubbery plateau. Copolymerization of CyPOSS at a modest level (4 mol%; 27 wt%) did not significantly alter the rheological behavior relative to the homopolymer, but increasing the loading level to 8 mol% (42 wt%) resulted in the replacement of the terminal zone with a rubbery plateau with G ∼ 103 Pa. At a still higher CyPOSS loading of 17 mol% (64 wt%), a very broad rubbery plateau spanning 4-decades in frequency was observed. Similar observations were made for the CpPOSS copolymers, though with quantitative differences. The authors suggested the mechanism of “sticky reptation”117 originally conceived for hydrogen-bonded polymer melts, hypothesizing attractive interactions between POSS groups. Because of the limited polymerization degree, the determination of an effect of POSS on the rubbery plateau was not possible. Continuing this line of research, Wu et al.118 reported the rheological properties of higher molecular weight and entangled random polystyrene-isobutyl-POSS copolymers. As


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Figure 4. The microstructures characterization of i BuPOSS-PS random copolymers: (I) TEM images of the copolymers with (a) 6 and (b) 30 wt% i BuPOSS. The dark i BuPOSS particles disperse in bright PS matrix almost with the size of a single POSS size, 1.5∼3 nm. (II) WAXS patterns of as-cast films of the copolymers with (i) 0, (ii) 6, (iii) 15, (iv) 30, (v) 50 wt% i BuPOSS and (vi) styryl i BuPOSS macromer. The copolymers are devoid of any crystalline features of styryl i BuPOSS macromer. Reprinted from Wu, J., Haddad, T. S., Kim, G. M., Mather, P. T., Macromolecules 2007, 40, 544–554 (Reference 118) with permission of American Chemical Society.

shown in Fig. 4, they found with WAXS and TEM that copolymerized iBu-POSS segments dispersed in the PS matrix at a molecular level for loadings up to 50 wt%. The copolymers were found to be thermorheologically simple, obeying time-temperature superposition (TTS) up to 50 wt% loading, indicating single phase, amorphous polymers far from any transformation event. Moreover all of the copolymers exhibited a terminal zone, a rubbery plateau, and a transition zone with increasing frequency, though the rubbery plateau was ill-defined owing to polydispersity. The temperature-dependent shift factors, aT (T) were well described by the WLF equation and analysis thereof revealed, interestingly, that the fractional free volume at Tg increased with the iBuPOSS loading, while the corresponding free volume expansivity decreased. In other words, the copolymers became increasingly less temperature-sensitive with increased POSS loading. Moreover, iBuPOSS incorporation dramatically decreased the rubbery plateau modulus in proportion to the copolymerization level, indicating a strong dilation effect of iBuPOSS on the entanglement density. The authors suggested that this dilation effect originated from the topology alteration to the host PS chain, with pendant POSS moieties acting as small, spherical branches as depicted in Fig. 5. In the context of the tube model of polymer dynamics, a length scale known as the “packing length” emerges:118−121 1/3 3 RT lp = 3.67 × 10 0 GN

(1)

where R(J·mol−1 ·K−1 ) is the universal gas constant, T(K) is absolute temperature, G0N (Pa) is the rubbery plateau modulus and the unit of constant, 3.67 × 103 , is Å3 ·m−3 ·mol−1 . The packing length, lp (Å), is the occupied volume of a chain divided by the mean square end-to-end distance, a scale proportional to the tube diameter, dt (dt = 19lp ).121 Based on their observations of monotonic decrease in G0N with increasing POSS copolymerization


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Figure 5. Schematic drawing of the influence of the pendant POSS group on PS chain topology, the resultant tube dimension, and the resultant rubbery plateau modulus as a function of i BuPOSS weight fraction. Reprinted from Wu, J., Haddad, T. S., Kim, G. M., Mather, P. T., Macromolecules 2007, 40, 544–554 (Reference 118) with permission of American Chemical Society.

level, Wu et al.118 reported that the packing length increased monotonically; i.e., POSS copolymerization dilates the tube diameter. The same researchers have undertaken further research on the role of the vertex group composition in determining viscoelastic properties of the polystyrene system, including the comparison of isobutyl (iBu), cyclohexyl (Cy), and cyclopentyl (Cp) POSS vertex groups, revealing significant differences.122 So far, we have summarized reports on the linear viscoelastic behavior of both physically mixed and copolymerized POSS nanocomposites, revealing distinct differences. The combined system, physically mixed and copolymerized, was studied by Kopesky et al.58 who investigated the rheological behavior of three categories of POSS-based polymeric nanocomposites: POSS-poly(methyl methacrylate) (PMMA) random copolymers, blends of POSS and neat PMMA, and blends of POSS and POSS-PMMA random copolymers. Like POSS-PS random copolymers,118 the storage modulus master curves of POSS-PMMA random copolymers were shifted strikingly downward relative to the pure PMMA homopolymer, indicating a lowering of the plateau modulus (and entanglement density) of POSS-PMMA copolymers. At the same POSS loading level, the copolymer incorporating iBuPOSS showed a lower plateau modulus and higher entanglement molecular weight than the CyPOSS counterpart. In contrast, the linear viscoelastic behavior of the


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Figure 6. Time-temperature-superposition (TTS) master curves of dynamic oscillatory shear (a) storage modulus (G ) and (b) loss modulus (G ) for blends of 25 wt% iBuPOSS inclusion and iBuPOSS-PMMA random copolymers with varying 0 and 30 vol% iBuPOSS. The reference temperature was 80◦ C. Reprinted from Kopesky, E. T., Haddad, T. S., Cohen, R. E., McKinley, G. H., Macromolecules 2004, 37, 8992–9004 (Reference 58) with permission of American Chemical Society.

POSS-PMMA copolymer incorporating untethered octa-isobutylPOSS(octa-iBuPOSS), as shown in Fig. 6, featured storage and loss moduli that each increased monotonically with octa-iBuPOSS content. In addition, the storage modulus featured a noticeable change in the terminal slope for samples incorporating 30 vol% octa-iBuPOSS. In contrast to the tube dilation effect for POSS-copolymers,118 this system featuring POSS nano-crystallites was well modeled (at least with respect to the plateau modulus) using the Guth-Smalllood equation for filled systems:123 G0N (ϕ) = G0N (0) [1 + 2.5ϕ + 14.1ϕ 2 ]

(2)

where G0N (φ) is the plateau modulus of pure PMMA and φ is the POSS nano-crystallite volume fraction. Actually, at a higher POSS loading level, the plateau modulus of the POSS filled PMMA remained constant and then monotonically increased at high POSS loading (greater than 5 vol%). In particular, the enhancement of CpPOSS is much larger than that of iBuPOSS due to the interaction between that R-group and polymer matrix. In contrast, the plateau modulus of iBuPOSS-filled copolymer featured a simple upturn trend for all POSS loading and the data obeyed the prediction of Guth-Smalllood equation very well. Meanwhile, the authors considered that if POSS nanocrystallites can be regarded as hard sphere fillers, the shear viscosity of POSS filled homopolymer and copolymer should satisfy the prediction of Einstein-Batchelor equation:124 η0 (φ) = η0 (0) [1 + 2.5φ + 6.2φ 2 ]

(3)

As this equation predicts, the zero shear viscosity of POSS-filled polymer blends should increase monotonically with POSS loading. Actually, the zero shear viscosity of POSS-filled PMMA homopolymer negatively diverged from the prediction of Eq. (3). This observation can be attributed to molecularly dispersed POSS inclusions that introduced additional free volume and associated plasticization, resulting in an increase of polymer chain mobility and a decrease in viscosity. However, while POSS-filled PMMA homopolymer negatively deviated from Eq. (3), the iBuPOSS-filled copolymers did exhibit a monotonic increase in zero shear viscosity. This increase was attributed to a strong thermodynamic interaction


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Figure 7. TEM micrographs of (a) pure polyurethane-urea (PUU) and (b) PUU-based composites incorporating with 10 wt% POSS. The dark hard segment particles disperse in the bright soft segment matrix. The POSS incorporation makes PUU composites feature finer morphology. Reprinted from Madbouly, S. A., Otaigbe, J. U., Nanda, A. K., Wicks, D. A., Macromolecules 2007, 40, 4982–4991 (Reference 125) with permission of American Chemical Society.

between untethered POSS and tethered POSS. The plasticizer effect played a minor role in the zero shear viscosity of POSS-filled POSS-PMMA random copolymers. Other researchers have reported on the impact of POSS on the formation of nanocrystalline domains in a polymeric host. Recently, Madbouly et al.125 found that the incorporation of diamino-POSS within the hard segments of multiblock polyurethane-urea (PUU) led to a finer nanostructure between hard and soft segments of PUU. As shown in Fig. 7, TEM observations revealed that the PUU with 10 wt% POSS exhibited a much finer nanoscale micro-phase separation than pure PUU. The authors explained that the large surface area of POSS nanoparticles created a large interaction zone with the PUU segments and concomitant higher affinity between PUU hard and soft segments. Accordingly, 10 wt% POSS incorporation made the microphase separation temperature (Tmps ) shift upward from 140◦ C, for pure PUU, to 160◦ C. Meanwhile, the rheological behavior of PUU was also significantly changed by the incorporation of diamino-POSS, particularly a significant increase in melt viscosity and zero shear viscosity with POSS incorporation. Figure 8 showed that TTS applied only temperatures below Tmps , above which G showed a strong dependence on temperature. Meanwhile, the incorporation of POSS dramatically increased the viscosity and flow activation energy of PUU-POSS nanocomposites. In a quite distinct approach, Lee et al.126 synthesized aluminum (Al)-containing POSSgrafted poly(styrene-vinyl diphenylphosphine oxide) random copolymers. Different from POSS cages covalently pendant to the polymer chain, Al-containing POSS are attached to P O by a coordination bond. This enabled the authors to investigate the dependence of rheological behavior of the random copolymers purely on POSS loading level, where the studied polymers had the same polymerization degree and molecular weight distribution. Similar to other copolymers with covalently bonded POSS cages, the relaxation time of these copolymers increased with POSS loading and the time-temperature-superposition principle was obeyed. Interestingly, the POSS-coordinated polymers also obeyed a timecomposition superposition principle as shown in Fig. 9, indicating that an increase of POSS loading at a fixed temperature simply shifts the distribution of relaxation times but does not alter the shape of the distribution. This important result suggested that other POSS


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Figure 8. Time-temperature-superposition (TTS) master curves of: (a) pure polyurethane-urea (PUU) and (b) PUU-based composites incorporating with 10 wt% POSS at reference temperature of 120◦ C. The POSS incorporation makes the microphase separation temperature (Tmps ) increase, above which the rheological behaviors of PUU-based composites obey TTS principle. Reprinted from Madbouly, S. A., Otaigbe, J. U., Nanda, A. K., Wicks, D. A., Macromolecules 2007, 40, 4982–4991 (Reference 125) with permission of American Chemical Society.

copolymers should be examined in a similar manner. Nevertheless, it was observed that the copolymer with a high POSS content exhibited a slow gelation response, violating the time-composition superposition, which the authors attributed to POSS-POSS interactions. To summarize this section, the rheological behavior of POSS-based polymeric nanocomposites depends strongly on the underlying microstructure, itself depending on interactions between the POSS moieties and between POSS and the host polymer segments. This leads to a sensitivity of microstructure and rheological properties to the vertex (R) groups at the corners of the POSS cage. As can be seen in this review thus far, the rheological studies have focused exclusively on linear viscoelastic properties of the linear random copolymers, with no reports on nonlinear rheological phenomena such as normal stresses under steady shearing, shear start-up, elongational flow, or capillary rheometry. However, knowledge of these rheological phenomena is of great practical importance for plastics processing operations. Moreover, the study of non-linear rheological behavior provides a rigorous testing platform that can clearly discern different models for polymer dynamics. Thus, a significant opportunity exists for researchers to begin a study of non-linear rheological phenomena for POSS-based polymers. Beyond bulk rheology, the rheological behavior of polymers and other complex fluids in confined geometries, such as at surfaces and interfaces, has been paid increasing attention by rheologists, especially following the advent of Fuller’s interfacial stress rheometer,127 as well as passive single particle tracking128 or active optical tweezers.129 At the same time, recent studies have indicated that trisilanol-POSS130−132 and POSS-PEO-POSS telechelics133 can form a stable monolayer at the air/water interface. While such interfacial structures


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Figure 9. Isothermal rheological behaviors of poly (styrene-co-vinyl diphenylphosphine oxide) (PSP) grafted with POSS at 170◦ C (The diphenylphosphine oxide (dPhPO) content is 7.4 mol%): (a) loss modulus (G ) and complex viscosity (η*) with varying POSS grafting, (b) Time-compositionsuperposition(TCS) mater curves of loss modulus (G ) and complex viscosity (η*) of PSPs with varying POSS content: (•, ◦) 0, (, ) 10.9, (, ) 21.7, (, ∇) 54.5 mol% of dPhPO sites attached with POSS. Solid and empty symbols stand for G and η*, respectively. Reprinted from Lee, A., Xiao, J., Feher, F. J. Macromolecules 2005, 38, 438–444 (Reference 126) with permission of American Chemical Society.

will be discussed elsewhere in Section 3.1, it should be mentioned here that a significant opportunity exists to study the interfacial rheology of such systems, heretofore largely unexamined. Such studies could provide great insight into the dynamic nature of interfacial phenomena that underpin the properties of hybrid emulsions, blends, and composites.

3. POSS Behavior at Surfaces and Interfaces 3.1. Langmuir-Blodgett (LB) Films at the Air/Water Interface In the last decade, three dimension (3-D) bulk properties of POSS-based polymeric materials have been extensively studied and feature very wide potential applications. Equally important are studies on the influence of POSS incorporation on two-dimensional (surface and interfacial) properties of polymeric materials, especially considering applications in thin films and coatings. Though comparatively less studied, several excellent contributions along these lines, particularly those from the laboratory of Prof. Esker of Virginia Tech,


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bear consideration. Deng et al.130,131,134 for the first time, revealed that incompletely condensed POSS cages, trisilanol-POSS derivatives, can form stable Langmuir-Blodgett (LB) monolayers at the air/water interface due to their amphiphilic nature. Since then, the trisilanol-POSS has been broadly employed as a model amphiphilic nanoparticle in the study of LB monolayers consisting of polymer and POSS. Hottle et al.135 reported the LB films prepared from trisilanolisobutyl-POSS/PDMS blends. The two amphiphilic components were blended to form a homogeneous monolayer at the air/water interface, at least at high surface dilution. With the increase of surface concentration, a transition from a monolayer state into multilayer structures through PDMS desorption was observed. Still further surface compression led to the collapse of the POSS moieties into multilayer domains. Additionally, it was reported that the concentration of PDMS can change the aggregation state of POSS particles in the multi-layers. For example, a decrease of the PDMS content can result in the trisilanolisobutyl-POSS forming network-like aggregation, as witnessed by Brewster-angle microscopy. Interestingly, if the PDMS was replaced by its phosphine oxide derivative, the networklike POSS aggregation structure can be converted into uniform dispersion of the so-called “particle aggregates.”136 In order to further investigate the influence of nanoparticle amphiphilicity, Hottle et al.137 incorporated non-amphiphilic octaisobutyl-POSS into PDMS, instead of amphiphilic trisilanolisobutyl-POSS. They found PDMS can dramatically reduce the aggregation degree of non-amphiphilic nanoparticles at the air/water interface, while pure octaisobutyl-POSS formed aggregates at all surface concentrations. Surface pressure isotherms and Brewster angle microscopic observations indicated that the surface morphology of the amphiphilic polymer/non-amphiphilic octaisobutyl-POSS blends featured quite strong composition dependence. For compositions with greater than 70 wt% POSS, the blends formed large rigid domains, though smaller than those from pure octaisobutylPOSS films, with a surface pressure isotherm shape that deviated from that of pure PDMS. Similar to pure octaisobutyl-POSS, the blends featured a sharp upturn in surface pressure with the decrease of the average area per monomer, while pure PDMS showed a transition to a plateau. As for blends with POSS contents between 40 wt% and 70 wt%, it was found that POSS particles formed extensive networks and the corresponding isotherms were qualitatively similar to pure PDMS. Finally, for POSS contents below 30 wt%, POSS was apparently homogeneously dispersed in the PDMS matrix (without large-scale aggregation) and no distinct difference in the surface pressure isotherm was observed in comparison to the pure PDMS. The authors proposed that the observed 2D morphologies are relevant to understanding the more complex 3-D (bulk) counterparts and thus indicate an opportunity to study the aggregation/dispersion mechanism of nanoparticle inclusion in polymer matrices. Indeed, we feel that the LB platform presents a powerful tool for such studies due to comparative simplicity and facile reversibility of structure formation via surface compression. In addition to the study of POSS-polymer blends as LB monolayers, the same POSS-based amphiphilic telechelics discussed earlier103 have been studied at the air/water interface.133 Like amphiphilic trisilanol-POSS derivatives, the amphiphilic telechelic polymers consisting of two non-surface-active components (e.g. hydrophilic PEG and hydrophobic i Bu-POSS), exhibited surface activity at the air/water interface. In particular, it was reported that the surface activity depended strongly on the molecular weight of the PEG bridge. For a PEG ≥ 8 kg/mol, the amphiphilic telechelics did not form stable monolayers, a finding attributed to the fact that, for these telechelics, the POSS end groups were not sufficiently hydrophobic to balance the large hydrophilic PEG bridge at the


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air/water interface. Shorter PEG bridges (1 kg/mol, for example) afforded excellent surface stability and allowed the transfer of Langmuir-Blodgett multilayers. X-ray reflectivity of POSS-PEG1k-POSS multilayers, grown to >60 nm in thickness, indicated Y-type LB multilayers films with double layer thickness measuring 3.52 nm. Meanwhile, observations of the surface pressure isotherm, Brewster angle microscopy, and X-ray reflectivity revealed that surface compression can induce the packing structure evolution of the amphiphilic telechelics at the air/water interface, show schematically in Fig.10. The authors proposed that the versatility in surface morphology and packing model of the inorganic-organic amphiphilic telechelics proved to be a new strategy to create nanostructured coatings. Recently, Mitsuishi et al.138 studied LB films of unique POSS-based amphiphilic random copolymers on the air/water interface, which were synthesized through free radical copolymerization employing amphiphilic N -dodecylacryamide and non-amphiphilic methacryloypropyl-POSS macromers containing seven non-reactive vertex (R) groups, trifluoropropyl (SQF) or phenyl (SQPh). The yielded amphiphilic random copolymers can form stable monolayers at air/water interface and featured high LB film deposition capacity, e.g. ∼400 layers. The multilayer LB films prepared by Y-type LB technique showed a well-defined layer structure, each layer being approximately 1.7 nm in thickness. Moreover, the POSS units homogeneously dispersed in the film. The film surface was very flat and smooth (the value of the root-mean-square is 0.4 nm at 1 µm × 1 µm). Importantly, POSS incorporation had a unique influence on the temperature dependence of the refractive index of the obtained LB films. Upon heating, the refractive index of the LB films prepared from the random copolymers with phenyl POSS increased from 1.43 (200◦ C) to 1.49 (270◦ C) Conversely, the refractive index of those prepared from pure poly(N -dodecylacryamide) decreased from 1.38 to 1.28 when the temperature increased from 200◦ C to 220◦ C. The authors attributed this phenomenon to the compact POSS packing configuration in the thin film and proposed this unique optic property for optoelectronic nanodevice applications.

Figure 10. A schematic cartoon proposed for the packing model evolution of POSS-PEG1k-POSS telechelics LB film with the increase of surface pressure (). The solid particles and solid lines stand for POSS cages and PEG chains, respectively. (A) a monolayer in a gas-like state ( ∼ 0 mN·m−1 ), (B) a liquid-expanded (LE) phase( ∼ 1 mN·m−1 ), (C) a phase with brushlike conformation of PEG chains (1 < < 5 mN·m−1 ), (D) a liquid-crystal (LC) film (5 < < 30 mN·m−1 ) with closely packed POSS cages and PEG loops, and (E) a film with multilayer POSS cage collapse ( > 30 mN·m−1 ). On the right is shown the total film thickness vs. number of LB deposited film layers for POSS-PEG1K-POSS deposited at a surface pressure of 25 mN. m−1 . The slope of d = 1.76 +/- 0.09 nm provides the thickness of a POSS-PEG1K-POSS monolayer. Reprinted from Lee, W., Ni, S. L., Deng, J. J., Kim, B. S., Satija, S. K., Mather, P. T., Esker, A. R., Macromolecules 2007, 40, 682–688 (Reference 133) with permission of American Chemical Society.

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3.2. Anti-Dewetting Effect of POSS Moieties in Polymer Thin Film Polymer thin films feature broad technological applications, including dielectric layers, biocompatible coating, microelectronics, among others. The particular problem in producing polymer thin films is how to prevent them from dewetting and breaking up on the substrate.139 Adding nano-particles into the polymeric matrix has been found to be an effective way to stabilize polymer thin film, since Barnes et al.140 for the first time discovered that the spin-coated polystyrene (PS) and polybutadiene (PB) thin films can be prevented from dewetting on the silicon wafer substrates by adding a small amount of fullerene (C60 ) nanoparticles. Furthermore, nanofillers improve the stability of polymer thin films not only on inorganic substrates,141,142 but on organic ones as well.143,144 With a well-defined size of approximately 1.5 nm, POSS moieties are also good candidates to be utilized as a stabilizer to polymer thin films. Recently, Hosaka et al.145 found the incorporation of cyclopentyl-POSS(CpPOSS) could lead to a dewetting inhibition of PS thin film on silicon wafers. After annealing at 393 K for 20 min, the PS thin film was completely dewetted. However, the holes formed on the PS-based blend thin film incorporating 10 wt%CpPOSS stopped growing and the dewetting process was inhibited. Furthermore, the dewetting area fraction of CpPOSS/PS thin film decreased with the increase of CpPOSS incorporation. Once the CpPOSS concentration was higher than 15 wt%, almost no holes formed on the substrate. The XPS analysis indicated that CpPOSS redistributed in the thin film during the annealing process. Accordingly, CpPOSS moieties were rich in two regions: one was at the surface of the film, where the Si and O concentration was 6 times as high as the theoretical value of the mixture. The other was at the film-substrate interface (the rim of the hole). The authors proposed that CpPOSS segregation at the film surface reduced the surface free energy and spreading coefficient, and its segregation at the film-substrate pinned the contact line of PS film and substrate, leading to the inhibition of dewetting. Meanwhile, they realized the POSS segregation and dispersion state were a function of vertex (R) groups surrounding the Si-O cage, which determined the interaction between POSS moieties and the polymer matrix. They selected three kinds of POSS molecules with various vertex R-groups: phenethyl (PhPOSS), fluoroalkyl (CpPOSS-Rf ) and hydroxyl (CpPOSS-2OH). The chemical structures of these POSS molecules and their influence on dewetting of PS thin films are shown in Fig. 11. The presence of phenethyl group rendered PhPOSS homogeneously dispersed in the PS thin film, resulting in a decrease of glass transition and viscosity. Consequently, the film dewetting rate was accelerated and the corresponding rupture was enhanced. Due to the presence of fluoroalkyl groups, CpPOSS-Rf was immiscible with PS and tended to strongly aggregate at the surface of the PS thin film. This reduced the surface free energy and resulted in the retardation of film dewetting on the silicon wafer substrate. Like CpPOSS-Rf , CpPOSS-2OH was also immiscible with the PS matrix. Interestingly, CpPOSS-2OH nanoparticles not only aggregated on the film surface, but also segregated at the film-substrate interface, which was assigned to the specific attraction between –OH groups of CpPOSS-2OH and silicon substrate. The rough and immobilized layer at the interface played the pinning role to inhibit the dewetting. The authors proposed that the POSS dispersion state be one of the key parameters to control the dewetting of the polymer thin film on the given substrate. Like any system with two components, the miscibility of POSS moieties and the polymer is a function of temperature and composition, which can be described by a phase diagram. Paul et al.146 studied the thin film blends of poly (tert-butyl acrylate)(PtBA) and trisilanolphenyl-POSS(TPP) and demonstrated that this blend featured lower critical


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Figure 11. Optical microscopic images of LB thin film PS-based blends with varying POSS moieties: (a) pure PS, (b) Ph-POSS/PS, (c) CpPOSS-Rf/PS and (d) CpPOSS-2OH/PS. The molecular weight of PS uesed is 2 kg/mol and POSS inclusion is 10 wt%. All of the thin films with ∼60 nm thickness were annealed at 100◦ C for 24 h under vacuum. The embedded bar is 300 µm. Reprinted from Hosaka, N., Otsuka, H., Hino, M., Takahara, A., Langmuir 2008, 24, 5766–5772 (Reference 145) with permission of American Chemical Society.

solution temperature (LCST) behavior. The critical temperature and composition were 70◦ C and 60 wt% TPP, respectively. Off the critical condition, the thin film blends with 58 wt% and 62 wt% POSS followed the nucleation and the growth mechanism of phase separation. As to the sample with 60 wt% POSS, Fast Fourier Transform (FFT) analysis of their optical microscopic images revealed the thin film blends underwent spinodal decomposition at elevated temperatures. At the early stage, the characteristic wavevector (q) followed the scaling law with time (t), e.g. q ∼ t n with n = −1/3 ∼ −1/4. At the late stage, the phase separated domains were pinned. Apparently, the dispersion and aggregation of POSS nanoparticles in the polymer thin film could be controlled by phase behavior and phase separation, leading to the controllable dewetting behavior on the substrate. Related significant studies are in progress. Paul et al.147 similarly investigated the morphological evolution of the POSS/polymer thin film bilayer, which was different from the system of the POSS/polymer blends previously reviewed. The selected trisilanolphenyl-POSS (TPP) was deposited on a PS-coated silicon wafer employing the spin-coating technique. It was found that the morphological evolution of the TPP/PS bilayer did not follow nucleation-growth nor spinodal decomposition. Annealing at high temperature initially made the TPP layer crack because of its internal tensile stress. Due to the fact that they were the nucleation sites, cracks were induced the dewetting and aggregation of TPP on the PS layer, which further induced dewetting of the lower PS layer. The two stage cracking/dewetting mechanism can be attributed to a brittle upper TPP layer that, unlike its polymer counterpart, underwent plastic deformation. Interestingly, the TPP/PS bilayer finally formed the TPP encapsulated PS droplet after completely dewetting.

3.3. Photo-Oxidative Resistance of POSS As mentioned previously, POSS cages are usually terminated by alkyl groups at eight corners of the Si-O bonded cage. Relative to the stronger Si-O bonds, C-H, C-C, and even C-Si are significantly weaker. Once POSS cages are exposed to a severe environment, including high temperature, high-energy ion beams, and oxygen plasma, only Si-O bonds


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can survive, while others degrade and form volatile organic compounds. More importantly, the survived Si-O bonds can further form a SiO2 -like surface layer on the POSS materials to prevent further etching and consumption. Due to such excellent oxidation resistance, POSS-based polymeric nanocomposites become new promising materials utilized for photooxidatively resistance materials and the next generation of lithography technology.93,148 Eon et al.93,148 investigated poly(tert-butyl methacrylate) (PtBMA)-based POSS random copolymers as the photoresist components for 193 nm lithography, a process intended for the achievement of microelectronic circuits with a minimum critical dimension around 50 nm. Employing in situ ellipsometry and in situ X-ray photoelectron spectroscopy, the authors observed the etching and thickness loss behavior of PMMA-POSS random copolymers in an oxygen plasma with a voltage bias varying from 0 to − 100 V. It was found that the copolymer etching rate decreased exponentially with plasma exposure time and that this passivation-type etching resistance increased with the increasing POSS content. A partial replacement of methacrylic acid (MA) with tBMA did not change the etching rate and its time evolution. With regard to bias voltage effect, the initial etching rate of the copolymers with high POSS loading (>60 wt%) was almost independent of bias voltage; however, the materials with lower POSS content (≤40 wt%) exhibited a significant increase in the initial etching rate as the negative bias voltage was increased from 0 V to − 100 V. Using FTIR (Fig. 12), the authors found that with increasing of oxygen plasma exposure time, the SiO-Si absorption peak (asymmetric stretching mode) shifted from 1105 cm−1 to 1050 cm−1 and at the same time the Si-CH3 peak diminished. It is known that the peaks centered at 1105 cm−1 and 1050 cm−1 are assigned to Si-O-Si in POSS and SiO2 , respectively. Thus,

Figure 12. FT-IR spectra for 100 wt% ethyl-POSS materials varying with oxygen plasma exposure time. The peaks of Si-O-Si in SiO2 and in POSS are centered at 1150 cm−1 and 1050 cm−1 , respectively. The peak of Si-CH3 is centered at 1250 cm−1 . Reprinted from Eon, D., Raballand, V., Cartry, G., Cardinaud, C., Vourdas, N., Argitis, P., Gogolides, E., J. Vac. Sci. Technol. B 2006, 24, 2678–2688 (Reference 148) with permission of American Vacuum Society.


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the oxygen plasma oxidized the POSS materials to form a SiO2 -like layer on the surface of the POSS-based random copolymers. XPS was further employed to analyze the change of content of the various elements at the surface of the POSS-based random copolymer after oxygen plasma exposure. The peak intensity integration revealed that after oxygen plasma exposure, the ratio of Si to O at the surface layer was almost 1/2, and the atomic percentage of carbon (C) was minimal in the oxide layer. Thus, FTIR and XPS data indicated that the oxygen plasma oxidized the POSS-based copolymers to form a passivating silicon dioxide layer that prevented further etching because of strikingly high bond energy of Si-O. This may be the mechanism by which POSS can improve the photo-oxidative resistance of materials. Another promising and related application for POSS-based polymers is their use for space-bodies in earth orbit.23 In space, whether low earth orbit or geosynchronous orbit, severe environmental conditions prevail: the atomic oxygen and vacuum ultraviolet radiation can make the highest performance polymers,149 such as KaptonTMand TeflonTM, degrade rapidly. The reason is that the bonds of organic molecules will undergo scission at about 4 eV, which is lower than the energy of atomic oxygen collision with about 5 eV. By comparison, the bond energy of Si-O is approximately 8 eV.49 Although the surface organic sections of the polymeric materials incorporated by POSS will be eroded by atomic oxygen, it will result in the formation of a SiO2 thin-layered network protecting the surface from further erosion of atomic oxygen.150−152 The formation of such a SiO2 -like surface layer also changes the surface energy and hydrophilicity of the materials. For POSS-based random copolymers, the hydrocarbon backbone and the alkyl groups surrounding POSS cage are typically hydrophobic. After oxygen plasma exposure, these weak bonds, including C-H, C-C, and C-Si bonds, are preferentially oxidized, resulting in the glass-like layer at the surface. Augustine et al.153 studied the effect of plasma exposure on the surface hydrophilicity of i BuPOSS-based PMMA random copolymers. They found plasma exposure not only made the thickness of i BuPOSS-PMMA thin film decrease, but also significantly reduced their contact angle with water. Moreover, the contact angle can be tuned by the ratio of oxygen to nitrogen in the plasma gas. For the copolymer with 40 wt% POSS after 100 s exposure, the oxygen plasma resulted in a contact angle of θ ∼ 41◦ , while nitrogen plasma led to a more hydrophobic surface with a contact angle of θ ∼ 57◦ . The authors suggested that the surface hydrophicility or hydrophobicity of POSS-based polymeric materials can be adjusted by the plasma condition, which is significant to control the surface chemistry and the wettability of biomedical devices.

4. Applications in Biomaterials The framework of POSS, constituted by Si-O and Si-C bonds, is similar to silicone, which is a favored option in biomaterials and first introduced into breast surgery in the 1960s due to its inert nature and low inflammatory response. Its biocompatibility can be ascribed to the foci of silicon-rich areas with increased surface free energy.154 Unlike carbon nanotubes,155 POSS moieties are non-toxic and cytocompatible.156,157 Additionally, it has been confirmed that POSS cages, as nanoscale building blocks, can be incorporated into other polymers with improved mechanical and viscoelastic properties. Thus, materials scientists are motivated to extend POSS-based polymeric nanocomposites to tissue engineering and biomedical application.

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Figure 13. Optical microscopic images of Toluidine Blue (TB)-stained human umbilical vein endothelial cells (HUVEC) on POSS-PCU: (A) at 48 hours, (B) at 6 days. The cell confluence increases with culture time. After 6 days, the cell confluence is around 80∼90%. Reprinted from Kannan, R. Y., Salacinski, H. J., Sales, K. M., Butler, P. E., Seifalian, A. M., Cell Biochem. Biophys. 2006, 45, 129–136 (Reference 158) with permission of Humana Press, Inc.

4.1. Cardiovascular Nanocomposites Kannan and Seifalian et al.81−83,158,159 introduced POSS moieties into poly(carbonate-urea) urethane (POSS-PCU), and systematically studied their cytocompatibility, antithrombogenicity, and biostability. By culturing primary human umbilical vein endothelial cells (HUVEC), they first assessed the cell viability, adhesion, and proliferation of hybrid PCU nanocomposites with 2 wt% POSS.158 The Alamar Blue assay showed that the endothelial cells were able to adhere to POSS-PCU nanocomposites within 30 minutes of contact without difference from the control cell culture plates. The PicoGreen assay also evidenced that POSS-PCU nanocomposites were able to sustain good cell proliferation for up to 14 days (even from low seeding density, i.e. 1.0 × 103 cells/cm2 ) and reach saturation by 21 days. As shown in Fig. 13 and Fig. 14, light microscopy and scanning electron microscopy (SEM) observations revealed that endothelial cells can reach confluence on the surface of POSS-PCU nanocomposites with optimal endothelial cell motility. Additionally, the authors successfully improved cytocompatability and endothelialization by ammonia (NH3 ) gas-UV surface modification160 and incorporation of bioactive peptide (Arg-GlyAsp, RGD),161 respectively. After 5 min irradiation of UV light of Xe2 *-excimer lamp at a wavelength of 172 nm in a NH3 gas, the treated nanocomposites featured a significantly

Figure 14. Scanning electron microscopic images of adsorbed human umbilical vein endothelial cells (HUVEC) morphology on POSS-PCU at 48 hours. There are a lot of cellular filopodia to form without “rounded” cells and cell retraction observed. Reprinted from Kannan, R. Y., Salacinski, H. J., Sales, K. M., Butler, P. E., Seifalian, A. M., Cell Biochem. Biophys. 2006, 45, 129–136 (Reference 158) with permission of Humana Press, Inc.


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increased cell proliferation between 3 and 8 days after HUVEC seeding as compared to the untreated ones. Due to the incorporation of RGD, the number of endothelial progenitor cells (EPC) colonies on the bioactive nanocomposites was almost four-fold larger than the untreated counterparts. The antithrombogenic properties were evaluated using thromboelastography (TEG) Fibrinogen ELISA assays and Anti-factor Xa test.83 TEG analysis showed a significant decrease in clot strength and an increase in clot lysis, although the nanocomposites did not have a significantly lower TEG amplitude value (MA) than pure PCU and the controlled PS sample. Fibrinogen ELISA assays and the Anti-factor Xa test indicated that the nanocomposites featured a lower fibrinogen adsorption, which can be seen as the main reason why the nanocomposites featured a significantly less platelet adsorption than both of pure PCU and PTFE control. It was believed that this antithrombogenic effect was also the result of the surface morphology/topology of POSS-PCU. The POSS nanocages induced a nanoscale extended surface configuration with a “mushroom/domelike” profile, which lowered fibrinogen adsorption and improved the protein repellent activity and the repulsion of platelet in the blood case as well. Biostability is one of the most important considerations for the selection of polymers for medical use. In vitro hydrolysis and oxidation tests were conducted to assess the degradative resistance of POSS-PCU.159 Strength and toughness/elasticity analysis revealed that all samples showed no significant difference in their elastic properties after a 70-day hydrolysis and oxidation test, indicating their striking biostability for all forms of in vitro degradation environments. The authors proposed that POSS nanocages chemically incorporated in PCU chains conferred a type of “protective” or “shielding effect” on the soft phase (CU), thereby preserving its elasticity and compliant properties from all forms of degradation, particularly in oxidation and hydrolysis. Although it is known that strongly basic162 and strongly acidic163 conditions can cause the opening and reaction of POSS cages, the POSSPCU backbone remained intact for all in-vitro degradation environments. After implanting the POSS-PCU samples in healthy sheep, the authors found that POSS-PCU adsorbed and inactivated the fibrinogen on their surface, leading to inflammation inhibition. In contrast, the siloxane control sample showed significant inflammation, degradation, and fibrous encapsulation.81 Simply put, both in vitro and in vivo experiments revealed that POSS incorporation enhanced the biological stability as compared with traditional PTFE and silicone biomaterials. It can be concluded that POSS-PCU may be an alternative material selection in place of poly(tetrafluoroethylene) (PTFE) and poly(ethylene terephthalate) (PET, DacronTM), for the construction of both vascular prostheses and bypass grafts. We should note that not all POSS polyurethanes are biostable; in fact, biodegradability is possible through the use of a different soft segment. Recently, Knight et al.164 found poly(lactide)(PLA)-based polyurethanes with 20.8 wt% POSS incorporation showed 60% loss in molecular weight as compared to the original one after one week incubation in a PSB buffer. This observation indicated that the biostability of POSS-based polyurethane depended on the polyurethane soft segment.

4.2. Dental Nanocomposites Methacrylate-based polymers have been extensively introduced as dental implant materials for 40 years. However, as compared with metallic and ceramic materials, the family of methacrylate-based polymeric dental implants features several clinical weaknesses to be overcome, such as volume shrinkage during polymerization and lack of strength and


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toxicity due to the residual monomers. Recent research156,165−168 showed one promising solution to these problems was to chemically incorporate POSS moieties into these polymeric materials. Gao and Culbertson168 tried three synthesis routes to incorporate methacrylatePOSS (MA-POSS) into poly(methyl methacrylate) (PMMA): (1) Copolymerization of dental monomer (methyl methacrylate, MMA) and MA-POSS, (2) Copolymerization of POSS-containing macromonomer and MMA, and (3) Synthesis of POSS-containing copolymers followed up by an in situ polymerization with a dental monomer. It was found that the incorporation of a small amount of MA-CpPOSS can efficiently reduce the shrinkage of MA-based neat resins and increase the double bond conversion in all of the three cases. More importantly, the authors found that a proper synthesis route is critical to POSS cage dispersion and mechanical properties improvement. As compared with synthesis route (1), both synthesis route (2) and (3) allowed POSS moieties to disperse better in the polymeric composites matrix, resulting in the increase of compressive strength, flexural strength, and tensile strength over a range from 20∼50%. Fong et al.166 evaluated the effects of MA-POSS content on the properties of 2,2 -bis-[4(methacryloxypropoxy)-phenyl]-propane (Bis-GMA)/tri-(ethylene glycol) dimethacrylate (TEG DMA)-based dental composites. They found that the volume shrinkage of polymerization was independent of the MA-POSS content and the double bond/monomer conversion decreased with the MA-POSS content increase. A small amount of the MAPOSS (