ABA-Type Liquid Crystalline Triblock Copolymers via Nitroxide-Mediated Radical Polymerization: Design, Synthesis, and Morphologies LONG-CHENG GAO, QI-WEI PAN, CHAO WANG, YI YI, XIAO-FANG CHEN, XING-HE FAN, QI-FENG ZHOU Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Received 10 July 2007; accepted 6 August 2007 DOI: 10.1002/pola.22348 Published online in Wiley InterScience (www.interscience.wiley.com).
We have designed and synthesized rod–coil–rod triblock copolymers of controlled molecular weight by two-step nitroxide-mediated radical polymerization, where the rod part consists of ‘‘mesogen-jacketed liquid crystalline polymer’’ (MJLCP). The MJLCP segment examined in our studies is poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (MPCS) while the coil part is polyisoprene (PI). Characterization of the triblock copolymers by GPC, 1H and 13C NMR spectroscopies, TGA, DSC confirmed that the triblock copolymers were comprised of microphase-separated low Tg amorphous PI and high Tg PMPCS blocks. Analysis of POM and 1D, 2DWAXD demonstrated that the triblock copolymers formed nematic liquid crystal phase. Morphological studies using TEM indicated the sample formed lamellar strucC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5949–5956, 2007 ture. V Keywords: block copolymers; liquid-crystalline polymers (LCP); living polymerization; nitroxide-mediated radical polymerization; rod–coil triblock copolymers; wide angle X-ray diffraction (WAXD) ABSTRACT:
INTRODUCTION Mesogen-jacketed liquid crystalline polymers (MJLCPs) have been attracting increasing interests since Zhou et al.1 first proposed the concept in the late 1980s. Unlike traditional side-chain liquid crystalline polymers (SCLCPs), whose liquid crystalline (LC) mesogens are attached to the polymer backbones through flexible segments, such as methylene groups,2 which decouple the interactions between main chain and mesogens, MJLCPs have the mesogenic pendants connected laterally to the backbones with no or very short spacers. As a result, the mesoCorrespondence to: X.-H. Fan (E-mail:
[email protected] or Q.-F. Zhou (E-mail:
[email protected]) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 5949–5956 (2007) C 2007 Wiley Periodicals, Inc. V
genic units would form a dense ‘‘jacket’’ around each chain backbone because of their high population.3 So MJLCPs distinguish themselves from other LC polymers by having chemical structures similar to conventional SCLCPs but chain properties similar to rigid or semirigid mainchain liquid crystalline polymers (MCLCPs). Most MCLCPs are prepared from condensation polymerization,4 whereas MJLCPs are synthesized from free radical polymerization. In particular, living free radical polymerization (LFRP) technique brings to well-defined structures of MJLCPs. These are mostly based on 2,5-disubstituted styrenic LC monomers.5 Because of the rigidity of MJLCPs, they always play a role of rod block when covalently attached to a flexible segment, such as polystyrene, forming rod–coil block copolymers.6 Rod– coil block copolymers are expected to induce 5949
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microphase separation, because of chemical incompatibility and physical stretching of the dissimilar blocks. Ordered morphologies such as lamellar, perforated lamellar, cylindrical, zigzag architectures have been observed.7 Rod–coil block copolymers offer the opportunity for engineering materials with novel properties and functions, and copolymers comprising rod-like MJLCPs and flexible blocks attract our great interest. Yi et al.6(f) synthesized PMPCS-b-PnBA-bPMPCS by two-step atom transfer radical polymerization (ATRP). It exhibits features of TPE, and due to the stable nematic phase, the storage modulus (G’) did not drop sharply when the temperature (200 8C) was much higher than Tg. On the other hand, the mechanical properties were not better than those of SBS. Problem exists that PnBA may not be the best candidate for flexible block. Considered rubbery polydienes are widely used in industry and academic fields for their excellent properties; they are good choice for flexible blocks. When the rubbery blocks are anchored on both sides by poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS), rod–coil triblock copolymers are formed. We chose polyisoprene (PI) and PMPCS as the building block. The controlled polymerization of 1,3-dienes, especially block copolymerization with other monomers has been almost focused on anionic procedures,8 but it does not work for MPCS. As mentioned earlier, MPCS is compatible with LFRP, like NMRP6(a,b) and ATRP.6(c–e) Reports provided ways of living radical polymerization of isoprene.9 On the basis of these observations, here we report the design and synthesis of ABA triblock copolymers, PMPCS-b-PI-b-PMPCS. PI capped by nitroxide at both terminals works as the macroinitiator of MPCS. The widely separated glass transition temperature (Tg) provides a broad range of service temperature. On combination with the LC properties of PMPCS blocks, they are expected to have behaviors of phase separation, and further increase the mechanical properties.
EXPERIMENTAL Materials Chlorobenzene (Analytical purity, Beijing Chemical Reagents) was treated with powdered CaH2 and distilled before use. LC monomer, 5-bis[(4methoxyphenyl)oxycarbonyl]-styrene (MPCS), was
synthesized from vinylterephthalic acid and 4-methoxyphenol via phosphorylation reaction as previously described.5(d) Its chemical formula is presented in Figure 1. Measurements Molecular Weights and Molecular-Weight Distributions The molecular weights (MWs) and molecular weight distributions (MWDs) of all polymers were measured by gel permeation chromatography (GPC) with a Waters 2410 instrument equipped with three Waters l-Styragel columns ˚ ). THF was the mobile phase (103, 104, and 105 A at the flow rate of 1.0 mL/min at 35 8C. The calibration curve was obtained by PS standards. The MWs of triblock copolymers were estimated from their nuclear magnetic resonance (NMR) spectroscopy performed on a Bruker ARX 400 MHz spectrometer using CDCl3 as solvent and tetramethylsilane as the reference, based on the GPC MWs of the macroinitiators. Thermal Analysis Glass transitions of polymers were investigated by differential scanning calorimetry (DSC) on a TA Instruments Q100 in a temperature range from 90 to 200 8C at a heating rate of 10 8C/min under continuous nitrogen flow. n-Octane (mp 56.76 8C) and indium (mp 156.78 8C) were used to calibrate the instrument. The average sample size was about 4 mg, and the nitrogen flow rate was 50 mL/min. A TA Instruments SDT 2960 was used for thermogravimetric analysis of block copolymer samples. Generally, about 3 mg of sample was heated from 30 to 600 8C at a heating rate of 10 8C/min under nitrogen atmosphere. Liquid Crystallinity Studies A Leitz Laborlux 12 polarizing optical microscope (POM) with a Leitz hot stage was used to characterize the LC behaviors of the triblock copolymers. 1D Wide-angle X-ray diffraction (WAXD) examination was conducted with a Bruker GADDS D8 Discover X-ray diffractometer at elevated temperatures. Samples were scanned in a 2h range between 28 and 308. 2D WAXD fiber patterns were obtained using a Bruker D8 Discover diffractometer with GADDS as a 2D detector. Calibration was conJournal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
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Figure 1. Synthesis procedure of the copolymer. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
ducted using silicon powder and silver behenate. Samples were fiber-made on the hot stage, and the point-focused X-ray beam was aligned both perpendicular and parallel to the mechanical shearing direction. The 2D diffraction patterns were recorded in a transmission mode at both room temperature and 180 8C.
under vacuum. The tube was placed in an oil bath at 125 8C for 3 h to give white powder. The product was continuously extracted with hexane in a Soxhlet apparatus overnight. The purified triblock copolymers were filtered and dried in vacuo. The result product had MW of 75k, and MWD of 1.63 by GPC.
Morphology
RESULTS AND DISCUSSION
The microphase separation of sample tri-2 was observed by TEM. Sample was annealed at 160 8C under vacuum for 24 h, and embedded into epoxy. Ultrathin sections were cut from a thin film on a ultramicrotome. To enhance the electron density contrast between the LC and PI phases, the sections were stained by OsO4 vapor at room temperature for 15 min. TEM studies were performed on a Hitachi H-800 electron microscope.
Synthesis The end-functionalized poly(1,3-dienes), especially for further block copolymerization with other monomers, has been almost focused on anionic procedures. However, the monomers to be blocked should be available in the following conditions. When MPCS was added into the carbanion
General Synthetic Procedures Synthesis of PI Macroinitiator The detailed synthetic procedure and the chemical characterization of PI macroinitiator are reported elsewhere.10 PI used here had the MWs of 48k and the MWDs was 1.64. Synthesis of Triblock Copolymer PMPCS-b-PI-bPMPCS Taking Tri-1 as an example, 55.7 mg PI (Mn,GPC ¼ 48k, MWD ¼ 1.64) was dissolved in 2.0 mL of chlorobenzene. And then MPCS (50.8 mg, 0.125 mmol) was added. The mixture was degassed by three freeze–pump–thaw cycles, and sealed Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
Figure 2. GPC curves of Tri-1, 2, 3 and PI macroinitiator. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
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Table 1. Molecular Weight, MWD, Composition, and Thermal Data of the Triblock Copolymers
Samples
MW GPCa
MWD
Mn Calcd.b
PMPCSb (wt %)
fPMPCSc
Tg1d (8C)
Tg2e (8C)
Liquid Crystallinityf
Tri-1 Tri-2 Tri-3
75k 81k 106k
1.63 1.65 1.59
60k-48k-60k 70k-48k-70k 94k-48k-94k
71.4 74.5 79.6
0.64 0.67 0.73
57.8 58.0 62.6
123.4 122.4 122.8
Yes Yes Yes
a
Calibrated by PS standard. Calibrated according to equation: wt % (PMPCS) ¼ [I( OCH3) 3 404/6]/{I( OCH3) 3 404/6 þ [I(1,2-)/3 þ I(3,4-)/2 þ I(1,4-)] 3 68} from 1H NMR analysis. c Volume fraction calibrated according to equation: fPMPCS ¼ (wt %/1.28) / [(wt %/1.28) þ (1 wt %)/0.90]. d Tg of PI block by DSC. e Tg of PI block by DSC. f Determined by POM. b
solution, many byproducts appeared. On the other hand, PMPCS was synthesized from radical polymerization procedures, especially ATRP and NMRP. Hawker and coworkers developed a novel nitroxide, 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxyl (TIPNO), which was proved to be a versatile mediator for living free radical polymerization.9 It could control 1,3-dienes polymerization, and also MPCS. We synthesized a novel alkoxyamine comprising double TIPNOs [see Fig. 1(1,2)] as the initiator of isoprene polymerization; the MWs could be very large and MWDs
relatively small. Then the a,x-TINPO-terminated PI initiated MPCS. Figure 2 shows the GPC curves of Tri-1, 2, 3 from the same PI macroinitiator and the original PI. Compared with the starting PI macroinitiator, a visible unimodal shift to higher MW is seen in the figure. The detailed data are listed in the Table 1. 1H NMR measurements were employed to confirm the structures and to calculate the MWs and compositions of the copolymers. The representative 1H NMR spectrum is shown in Figure 3(a). The characteristic peaks of the PI block are located
Figure 3. (a) Microstructure and 1H NMR spectrum of Tri-2 in CDCl3; (b) NMR of Tri-2 in CDCl3.
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nematic to isotropic transitions of the samples were observed from the DSC curves. This is related to the properties of PMPCS. Reported studies have demonstrated that the LC property of PMPCS depends on its MW and when its MW
Figure 4. Set of DSC thermal diagrams during the second heating of Tri-1, 2, 3, PI macroinitiator, and PMPCS. Heat rate is 10 8C/min (under N2 atmosphere). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com.]
in the range of d from 4.6 to 5.9 ppm, which are the responses of the various unsaturated bonds derived from isoprene, and the double peaks between 3.0 and 3.8 ppm attribute to the methoxy protons of PMPCS blocks. The 13C NMR spectrum shown in Figure 3(b) also proves the successful block copolymerization. The total MWs calculated from 1H NMR measurements are much larger than the GPC measurements. This reflects that the polymer takes a compact molecular shape in solution and the hydrodynamic volume of the copolymer is smaller compared with that of the linear PS standard with the same MW.
Thermal and LC Behaviors Thermal behaviors of the copolymers were examined by TGA and DSC. The thermal stabilities of the copolymers were investigated with TGA in a nitrogen stream. The copolymers have relatively good thermal stability and their 5% loss weight temperatures were as high as 350 8C. Two distinct glass transitions were clearly detected by DSC for all samples (see Fig. 4), which indicated the immiscibility of the corresponding blocks. The signal around 60 8C is identified as Tg of the PI block and the other around 122 8C as Tg of the PMPCS block. Tgs of samples are given in Table 1. LC textures can be seen by POM. Figure 5 shows the nematic LC textures of Tri-1, 2, 3 at 180 8C. However, no Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
Figure 5. LC textures (3200) of samples Tri-1 (a), Tri-2 (b), and Tri-3 (c) at 190 8C. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
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Figure 6. Set of WAXD powder patterns of as-casting film during the first heating route (a) and first cooling route (b) for sample Tri-2.
calibrated from 1H NMR is beyond 20,000, a LC phase is formed above its Tg, but no clearing point is detected until decomposition. This rule applies to other block copolymers containing PMPCS.11 The MWs of PMPCS blocks here were much higher than 20,000, and they had dominant weight content. The mesophase of the block copolymers was analyzed by WAXD at elevated temperatures. Figure 6 shows two sets of WAXD powder patterns of as-casting film of Tri-2 during the first
heating route (a) and the first cooling route (b). When the temperature is below 150 8C, only wide scattering halo is observed at the low angle local. As the temperature increased above 160 8C, a sharp and intense scattering peak appears on the right shoulder of the halo, that is to say, ordered structure is formed. The scattering angle is about 5.68, which corresponds to an interchain packing distance of 1.6 nm. On the other hand, the shape of higher angle scattering profile remains the same, with a slight
Figure 7. 2D WAXD fiber patters of Tri-2 obtained at room temperature. The X-ray incident beam is perpendicular (a) and parallel (b) to the fiber axis. Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
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symmetric and asymmetric volume fractions in rod–coil diblocks.7(a,b) Typical TEM image of Tri2 is shown in Figure 8. Alternating black and white layers are easily discernible. The black layers are ascribed to the PI blocks, and the white to PMPCS blocks. Further investigations of the block copolymers are in progress.
CONCLUSIONS Figure 8. TEM images of sample Tri-2. The black areas are PI blocks stained by OsO4.
shift of the angle position, related to the thermal expansion in the sample.12 The broad halo at 2h ¼ 19.68 reveals short-range orders along the backbone. The d spacing, about 0.48 nm, represents the projection of two repeating units along the chain axis, according to the previous studies.12 When the temperature decreases, the peak at lower angle stands all the time, corresponding to the ordered LC structure without being damaged. This can also be confirmed by POM. For further identification of the LC phase, 2D WAXD experiments of the samples were carried out. Parts (a) and (b) shown in Figure 7 are 2D WAXD patterns of oriented Tri-2, with the X-ray incident beam perpendicular to the axis and normal of the fiber, respectively. A couple of strong diffraction arcs on the equator can be seen, indicating that ordered structures have developed perpendicularly to the fiber axis on the nanoscale. Meanwhile, along the axis of the fiber, only ring pattern can be seen. Therefore, the LC phase is a nematic phase.
Bulk Morphology As mentioned earlier, the block copolymers selfassemble various ordered structures in bulk. Transmission electron microscopy (TEM) was used to observe the phase separated morphologies of the copolymers. Because of the low electronic density contrast between the constitutive blocks, OsO4 was used to selectively stain the PI block for the existence of the double bonds. Lamellar structures were observed for all the samples, because the interaction between the rods is relatively strong and the rods are nearly parallel to each other to form LC phase. Generally, layerlike morphology is preferentially formed for both Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola
The results show that rod–coil–rod triblock copolymers of controlled MW can be synthesized by two-step nitroxide-mediated radical polymerization using difunctional PI macroinitiator capped by nitroxide at both terminals. Copolymers of high MWs and relatively low MWDs were confirmed by GPC, 1H NMR, and 13C NMR. TGA studies indicate good thermal stability, and widely separated glass transitions detected by DSC, which are about 60 8C and 122 8C, respectively. POM and 1D-, 2D-WAXD demonstrate the copolymers have nematic LC properties. Microphase separation is proved by TEM. Lamellar structures are formed in the bulk of the samples. We gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant Nos. 20474004, 20574002, and 20634010).
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