Intimate blending of binary polymer systems ... - Wiley Online Library

Intimate Blending of Binary Polymer Systems from Their Common Cyclodextrin Inclusion Compounds TAMER UYAR,1 CRISTIAN C. RUSA,1 XINGWU WANG,1 MARIANA RUSA,1 JALE HACALOGLU,2 ALAN E. TONELLI1 1

Fiber and Polymer Science Program, College of Textiles, North Carolina State University, Raleigh, North Carolina 27695-8301 2

Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey

Received 7 February 2005; revised 15 June 2005; accepted 15 June 2005 DOI: 10.1002/polb.20546 Published online in Wiley InterScience (www.interscience.wiley.com).

A procedure for the formation of intimate blends of three binary polymer systems polycarbonate (PC)/poly(methyl methacrylate) (PMMA), PC/poly(vinyl acetate) (PVAc) and PMMA/PVAc is described. PC/PMMA, PC/PVAc, and PMMA/PVAc pairs were included in c-cyclodextrin (c-CD) channels and were then simultaneously coalesced from their common c-CD inclusion compounds (ICs) to obtain intimately mixed blends. The formation of ICs between polymer pairs and c-CD were confirmed by wide-angle X-ray diffraction (WAXD), fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). It was observed [solution 1H nuclear magnetic resonance (NMR)] that the ratios of polymers in coalesced PC/ PMMA and PC/PVAc binary blends are significantly different than the starting ratios, and PC was found to be preferentially included in c-CD channels when compared with PMMA or PVAc. Physical mixtures of polymer pairs were also prepared by coprecipitation and solution casting methods for comparison. DSC, solid-state 1H NMR, thermogravimetric analysis (TGA), and direct insertion probe pyrolysis mass spectrometry (DIP-MS) data indicated that the PC/PMMA, PC/PVAc, and PMMA/ PVAc binary polymer blends were homogeneously mixed when they were coalesced from their ICs. A single, common glass transition temperature (Tg) recorded by DSC heating scans strongly suggested the presence of a homogeneous amorphous phase in the coalesced binary polymer blends, which is retained after thermal cycling to 270 8C. The physical mixture samples showed two distinct Tgs and 1H T1q values for the polymer components, which indicated phase-separated blends with domain sizes above 5 nm, while the coalesced blends exhibited uniform 1H spin-lattice relaxation values, indicating intimate blending in the coalesced samples. The TGA results of coalesced and physical binary blends of PC/PMMA and PC/PVAc reveal that in the presence of PC, the thermal stability of both PMMA and PVAc increases. Yet, the presence of PMMA and PVAc decreases the thermal stability of PC itself. DIP-MS observations suggested that the degradation mechanisms of the polymers changed in the coalesced blends, which was attributed to the presence of molecular interactions C 2005 Wiley between the well-mixed polymer components in the coalesced samples. V ABSTRACT:

Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2578–2593, 2005

Correspondence to: A. E. Tonelli (E-mail: alan_tonelli@ ncsu.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 43, 2578–2593 (2005) C 2005 Wiley Periodicals, Inc. V

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Keywords: cyclodextrin; inclusion compound; miscibility; blend; polycarbonate; poly(methyl methacrylate); poly(vinyl acetate)

INTRODUCTION The blending of two or more polymer components is a very common commercial approach to obtain new polymeric materials with improved characteristics. The conventional methods for producing polymer blends are melt blending, coprecipitation, and solvent casting.1 When two or more polymer components are blended together, it is usually likely to obtain phase-separated materials because of a small entropy of mixing and the generally unfavorable enthalpy of mixing when there is no specific interaction (hydrogen bonds, van der Waals or polar interactions, etc.) between the component polymer side groups. In the case of poor mixing, the blends exhibit properties inferior to their component polymers because of the poor degree of interfacial adhesion between the phase-separated components, which create a multiplicity of defects in the system.2 In addition to the properties of each polymer component and their compositions, the ultimate characteristics of a polymer blend depend most importantly on the miscibility of the component polymers in the blend.1–3 The preparation of miscible polymer blends depends on many factors, such as solvent, temperature, weight/molar ratio of the components, molecular weight, etc.3 Additionally, the presence of specific interactions (hydrogen bonds, van der Waals or polar interactions, etc.) between the polymer chains is usually essential to obtain miscible polymer blends.4 Significant studies have been carried out on polycarbonate (PC)/ poly(methyl methacrylate) (PMMA) and PMMA/ Poly(vinyl acetate) (PVAc)blends, while only a few studies have been reported on the PC/PVAc blend system.4–23 The effects of solvent, solubility parameter, temperature, evaporation rate of the solvent, weight composition of the polymer components, molecular weight, tacticity, and so forth on generating miscible blends of these pairs were investigated. Yet, various contradictory data/results among these studies were reported. It has been generally observed that PMMA/ PVAc blends are not miscible, though casting from certain solvents at appropriate temperatures can result in partial miscibility. However, even the examples of partially miscibile PMMA/

PVAc blends were observed to phase separate at temperatures above their Tgs. The phase behavior of PC/PMMA blends has been investigated, and certain preparation methods were found to be very effective in producing initially homogenous blends. Melt blending of PC/PMMA has been reported to yield partially miscible blends, whereas the homogeneity of blends from solvent casting techniques depends on the solvent and temperature.18–22 Hsu et al. studied the effect of solvent for PC/ PMMA blends, where the blends cast from tetrahydrofuran (THF) did not show visible phase separation, but blends cast from chloroform indicated a phase-separated structure. Conversely, opposite behavior was observed when syndiotactic PMMA was used instead of atactic PMMA. Even though PC/s-PMMA was not found to be thermodynamically miscible, it was stated that syndiotacticity favors the miscibility between PMMA and PC.18 This finding was supported by a previous report in which a single-phase blend system of s-PMMA/PC was demonstrated.19 Studies on the PC/PVAc blend system are limited in the literature. Sato et al. reported that PC forms partially miscible blends with poly (vinyl alcohol) (PVA) and partially saponified PVAc because of the existence of side-chain hydroxyl groups, which improves their miscibility by intermolecular hydrogen bonding, but PVAc was found to be immiscible with PC.23 To summarize, the homogeneity and phase behavior of the PMMA/PVAc, PC/PMMA and PC/PVAc blends showed some ambiguity because of their instability at different temperatures and in different solvent environments and show phase separation and incompatibility when they are prepared by traditional methods. Moreover, it has been revealed that the effects of solvent and temperature have played a key role in the homogeneity of their blends.5–11,18–23 Processing polymer blends via the formation of and coalescence from their common cyclodextrin (CD) inclusion compounds (ICs) eliminates the effects of both solvent and temperature. CDs, cyclic starch oligomers, are shallow truncated molecular cones consisting of 6, 7, and 8 glucose units, and are named (a-), (b-) and (c-) CDs, respectively, (see Fig. 1). Although the depth of the cavities for the three CDs is the

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Figure 1. (A) c-CD chemical structure; (B) approximate dimensions of a-CD, b-CD and c-CD; schematic representation of packing structures of (C) cage-type; (D) headto-tail channel-type crystals; and (E) cyclodextrin-inclusion compound (CD-IC) channels with included polymer guests.

˚ ), their cavity diameters are
5, 7, same (
7.9 A ˚ and 9 A, respectively.24 Because of their unusual structure, CD molecules may host both polar and nonpolar guests, which include polymers as well as small molecules.25–30 Our research group has recently reported that CDs may act as hosts in the formation of ICs with various guest polymers. Polymer-CDICs represent crystalline compounds obtained by threading of the annular CD molecules onto the guest polymer chains. The included polymers are confined to the narrow, continuous CD channels, and so are necessarily highly extended and segregated from neighboring polymers chains by the walls of the CD stacks (see Fig. 1e). We have shown that coalescence of guest polymers from their CD-IC crystals can result in a significant improvement in their bulk physical properties caused by modification of their structures, morphologies, and even conformations compared with those observed in their normally produced bulk states.28–30 Recently, we have developed a novel approach for mixing thermodynamically incompatible polymers by first obtaining a common polymer-CD-IC. Subsequently, the guest polymers are coalesced from their common CD-

IC crystals by removal of the CD host.31–35 This method has successfully produced intimate blends of poly(e-caprolactone) (PCL)/poly(L-lactic acid) (PLLA),31 PC/PMMA,32 poly(ethylene terephthalate) (PET)/poly(ethylene 2,6-naphthalate) (PEN)33 and the ternary blend of PC/ PMMA/PVAc.34 In this paper, we report the intimate mixing of binary PC/PMMA, PC/PVAc, and PMMA/PVAc blend pairs by coalescence from their common cCD-ICs, which was necessary for the formation of an intimately mixed ternary blend of PC/ PMMA/PVAc.34 The formation of ICs between cCD and the PC/PMMA, PC/PVAc, and PMMA/ PVAc pairs was successfully achieved. The coalesced binary polymer blends were obtained by removal of c-CD, and it was found that the host c-CD molecules preferentially include PC chains over the other two vinyl polymer guests.

EXPERIMENTAL Materials PC (Mw ¼ 28,800 g/mol, Mn ¼ 17,300 g/mol), PMMA (Mw ¼ 15,000 g/mol), and PVAc (Mw

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Figure 2. Schematic representation of polymer-CD-IC formation, the coalescence process and the coalesced polymer. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

¼ 12,800 g/mol) were purchased from Aldrich Co. (Milwaukee, WI), and used as received. THF (Aldrich, 99þ%), dioxane (Aldrich, 99þ%), d-chloroform (CDCl3) (Aldrich, 99.9 atom % D) were used without any purification. c-CD was purchased from Cerestar (Hammond, IN). Deionized (DI-H2O) water was used for the aqueous solution of CD and for coalescence of the ICs. The a-amylase enzyme (Clarase L-4000) was used for the removal of c-CD was obtained from Genencor International, Inc. (Rochester, NY). Formation of Polymer-c-CD-IC One c-CD molecule may host three repeat units of PMMA or PVAc or 0.66 repeat units of PC in its interior cavity.24 On this basis we would expect a PC:PMMA or PVAc feed molar ratio of 2/3:3 ¼ 1:4.5 would lead to 1:4.5 coalesced blends. However, as discussed subsequently, inclusion of PC by c-CD was preferred over PMMA and PVAc, so a 1:6 M ratio of PC:PMMA or PC:PVAc and a 1:1 M ratio of PMMA:PVAc was used for formation of the common IC. A quantity of 0.208 g (0.82 mmol) of PC and 0.490 g (4.9 mmol) of PMMA were dissolved together in 100 mL of a common solvent (dioxane) at 50 8C. An aqueous saturated solution of c-CD (3.712 g (2.86 mmol) in 10 mL DI-H2O at 50 8C) was added drop-wise to the binary polymer solution while the stirring rate was kept maximum at a temperature of 50 8C. A white, turbid solution was observed once the c-CD aqueous solution was added to the binary poly-

mer solution (Fig. 2). Subsequent to continuous stirring for three hours at 50 8C, the white suspension was cooled down to room temperature while the stirring rate was adjusted to a moderate level for another three days (higher yields of ICs were obtained after three days of stirring at room temperature). The resulting suspension was then vacuum filtered and the collected white crystals were vacuum dried at 45 8C for 24 h. The abbreviation for this inclusion compound is PC/PMMA-c-CD-IC. The same experimental procedure was applied for the formation of the other two binary ICs (abbreviated as PC/PVAc-c-CD-IC and PMMA/ PVAc-c-CD-IC). A quantity of 0.208 g (0.82 mmol) of PC and 0.421 g (4.9 mmol) of PVAc were dissolved together in dioxane (100 mL, at 50 8C), and 0.429 g (4.29 mmol) of PMMA and 0.369 g (4.29 mmol) of PVAc were dissolved together in dioxane (100 mL, at 50 8C) for the formation of PC/PVAc-c-CD-IC and PMMA/PVAcc-CD-IC, respectively. About 3.5 g of binary polymer/CD-IC crystals were obtained for each of the binary systems, or approximately a 75% yield, and it is worth mentioning that the ICs contained some small amount of free CD, which was indicated by X-ray (see below). Coalescence of Polymer Blends from Their ICs The c-CD was removed from the IC (PC/PMMA-cCD-IC, PC/PVAc-c-CD-IC, and PMMA/PVAc-c-CDIC) by washing the white powders with deionized water at 50 8C while stirring for at least 6–24 h

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until the presence of c-CD could not be detected by FTIR. The a-amylase enzyme (Clarase L-4000) was used for the removal of c-CD at pH ¼ 5 (adjusted with HCl) when necessary. The coalesced binary polymer blends, PC/PMMA, PC/ PVAc, and PMMA/PVAc, were then vacuum filtered and vacuum dried at 40 8C for 2 days. The molar composition of coalesced binary blends was identified by solution 1H NMR spectroscopy. The molar ratios of PC:PMMA, PC:PVAc and PMMA:PVAc were found to be 2.8:1, 1.8:1, and 1:1, respectively. These coalesced binary blends were abbreviated as coalesced-PC/PMMA, -PC/ PVAc and -PMMA/PVAc. Preparation of Binary Blends by Precipitation and Solvent Casting To serve as control samples, binary blends of PC/PMMA, PC/PVAc, and PMMA/PVAc were prepared by dissolution in THF at the same molar concentrations as those observed in the coalesced samples. Both solvent cast and coprecipitation methods were employed to generate the physical blends. The solvent cast films were air-dried overnight and vacuum dried further at 50 8C for another 24 h. Phase segregation in the cast films was macroscopically apparent. Infrared spectroscopic examination (FTIR microscope) of these polymer blends confirmed the macroscopic phase separation of polymer domains, which was indicated by the presence of single and/or binary polymer components for different regions of the cast films. As a consequence, binary control blends of PC/PMMA, PC/PVAc, and PMMA/PVAc were also prepared by the coprecipitation method. Binary polymer components (same molar composition as in coalesced blends) were dissolved in THF and precipitated into deionized water (nonsolvent). The precipitates were collected by vacuum filtering through the ceramic frit in a Bu¨chner funnel and were vacuum dried.

CHARACTERIZATION Wide-Angle X-ray Diffraction (WAXD) WAXD measurements were performed with a Siemens type-F X-ray diffractometer using a Ni˚ ). The filtered Cu Ka radiation source (k ¼ 1.54 A diffraction intensities were measured every 0.18 from 2h ¼ 5–308, at a rate of 2h ¼ 38/min. The

supplied voltage and current were 30 kV and 20 mA, respectively. Solution 1H Nuclear Magnetic Resonance (NMR) Coalesced and precipitated binary polymer blends were dissolved in deuterated chloroform or deuterated THF for 1H NMR analysis, which was carried out on a Mercury 300 MHz spectrometer using tetramethylsilane (TMS) as the internal standard. Solid-State 1H NMR Solid-state 1H NMR data were collected using a Bruker DSX wide-bore system with a field strength corresponding to a 1H Larmor frequency of 300.13 MHz. Radio-frequency power levels were 71 kHz for spin-locking and decoupling, corresponding to p/2 pulse widths of 3.5 ls. Data were obtained using MAS speeds of 4.5 kHz on a commercial 7-mm probe. 1H T1q measurements were made at room temperature using standard 13C cross-polarization observation experiments, in which the length of the 1H spin-lock pulse was incrementally varied prior to cross-polarization. Cross-polarization contact times were 1 ms. Depending on the amount of samples, 512–2000 scans were collected per relaxation time increment. Fourier Transform Infrared Spectroscopy (FTIR) A Nicolet 510P FTIR spectrometer was operated to acquire the infrared spectra of samples mixed into potassium bromide (KBr) and pressed into pellets. The spectra were taken over a range of 4000–400 cm1, with a resolution of 2 cm1 after 64 scans. An attached FTIR microscope was also used to directly observe the as-cast films. Differential Scanning Calorimetry (DSC) Experiments were performed with a Perkin Elmer DSC-7 under nitrogen purge gas. Indium was used as a standard for calibration. All samples were subjected to heating and cooling cycles, unless otherwise specified, consisting of 1.0 min hold at 0 8C, ramp to 270 8C at 20 8C/ min, hold at 270 8C for 1 min, and quench to 0 8C at a cooling rate of 200 8C/min. Samples were subjected to a second set of heating and cooling cycles to investigate their thermal behavior/stability.

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Figure 3. X-ray diffraction patterns of (a) as-received c-CD cage and (b) columnar c-CD channel structures. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Thermogravimetric Analysis (TGA) Thermal analyses of the samples were performed with a PerkinElmer Pyris 1 TGA instrument. The thermal decompositions of the samples were recorded between 25 and 650 8C. The heating rate was 20 8C/min and nitrogen was used as a purge gas. Direct Insertion Probe Pyrolysis Mass Spectrometry (DIP-MS) Our DIP-MS system consists of a 5973 HP quadruple mass spectrometer coupled to a JHP SIS direct insertion probe pyrolysis system. Samples (0.01 mg) were pyrolyzed in flared glass sample vials. The temperature was increased at a rate of 10 8C/min and the scan rate was 2 scans/s.

RESULTS AND DISCUSSION Wide-Angle X-ray Diffraction WAXD is a useful characterization technique for investigating the crystalline phase transition between the cage structure of as-received c-CD.7H2O (Fig. 3a) and the channel structure of c-CD (c-CDchannel) (Fig. 3b). The channel structure of c-CD, where the channels only contain water, can be obtained by precipitating cCD into acetone.36 Subsequent water removal from the channel structure by vacuum drying destabilizes the channel structure, resulting in an amorphous diffraction pattern (Fig. 4a).

Once the host CD molecules are threaded onto the polymer chains, it is expected that the final IC crystals adopt a channel-type crystal structure (Fig. 1e). To confirm the formation of a polymer-CD-IC, the crystals resulting from precipitation are vacuum-dried after filtration. This destroys any columnar c-CD with only water included in the channels, since columnar c-CD collapses into an amorphous structure upon dehydration of the crystals.36 However, in the case of polymer-c-CD-ICs, the polymer chains included in the c-CD channels preserve the channel structure. Figure 4 shows WAXD patterns for the vacuum-dried binary polymer-cCD-ICs. The reflections consistent with c-CD in the channel structure persisted after vacuum drying of the polymer-c-CD-ICs. One reflection, in particular, occurring at 2h ¼ 7.58 for polymerc-CD-IC crystals is indicative of c-CD in the channel structure and has been assigned to the 200 plane of a tetragonal lattice by Takeo and Kuge.37 Yet, the diffraction pattern of our polymer/polymer-c-CD-ICs also indicated the existence of some amorphous structure, which is due to the presence of some c-CD with only water included that collapsed into an amorphous structure upon vacuum drying. The presence of a peak at 2h ¼ 7.58 represents the first evidence that c-CD molecules formed a channel structure IC with the possible inclusion of polymer guests residing in the c-CD cavities. FTIR and DSC studies were carried out to confirm that the polymers are included in the IC channels. The presence of polymers in the IC

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Figure 4. X-ray diffraction patterns of vacuum-dried crystals (a) c-CD channel, (b) PC/PMMA-c-CD-IC, (c) PC/PVAc-c-CD-IC, and (d) PMMA/PVAc-c-CD-IC. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

samples was confirmed by FTIR, where the C¼ ¼O absorption bands from all three polymers were evident (data not shown). DSC is a useful tool to determine whether or not the polymer chains are included inside the CD cavities: thermal transitions for polymers are observed if there is any uncomplexed polymer present in the CD-ICs. None of the DSC scans of PC/PMMA-, PC/PVAc-, or PMMA/PVAc-c-CD-ICs (data not shown) exhibited glass transitions for any of the potentially uncomplexed polymers. This result reveals that polymer chains are threaded and covered by CD molecules in the polymer-CD-ICs. Solution 1H NMR Though not presented here, 1H NMR solution spectra of the coalesced PC/PMMA, PC/PVAc, and PMMA/PVAc binary blends were recorded. The molar ratios of PC, PMMA, and PVAc in the coalesced binary blends were determined by comparison of integrals of the three distinct polymer peaks at 7.2 ppm for PC, 4.8 ppm for PVAc, and 3.6 ppm for PMMA vs. TMS. The molar ratios of the coalesced PC:PMMA, PC:PVAc, and PMMA:PVAc blends were found to be 2.8:1, 1.8:1, and 1:1, respectively. Comparison with the 1:6 M ratio of the starting solutions of PC:PMMA or PC:PVAc suggests that the host c-CD molecules preferentially include PC chains over the

other two vinyl polymer guests. The tendency of PC to form an inclusion compound with c-CD is close to 17 times higher when compared with PMMA and 11 times higher when compared with PVAc. However, apparently there is no inclusion preference observed between PMMA and PVAc as shown by the equivalence between the initial and final molar ratios of these two polymers. Similar behavior has also been observed for the ternary system of these polymers, where PC chains are more likely to be included and reside in the c-CD channels than PMMA and PVAc chains.34 It has previously been reported that host c-CD molecules posses a greater inclusion affinity for more hydrophobic, rather than hydrophilic, guest polymers.38 Here, the preferential inclusion of PC may be explained by its hydrophobicity, which is higher than that of PMMA or PVAc. Other possible reasons for this behavior are still under investigation. Solubility differences between the coalesced and physical blends were observed when we tried to redissolve the binary blends in a common solvent. We were able to dissolve the coalesced binary blends only at high temperature and after long time ultrasonication, whereas the physical blends were readily redissolved at room temperature. The lower solubility of the coalesced PC/PMMA, PC/PVAc, or PMMA/PVAc binary blends indicated a higher degree of mix-

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Figure 5. DSC thermograms of (a) first heating scan and (b) second heating scan of coalesced PC/PMMA blend and (c) first heating scan of coprecipitated PC/PMMA blend. [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com.]

ing in these systems when compared with their physical blends. When the polymer blends are intimately mixed, the entropy gained from dissolution is reduced and enthalpically favorable interchain interactions maybe established. Apparently, it requires more time and higher temperatures to break the interactions between these well-mixed chains when compared with their phase-separated physical blends.

to their physical blends, with the exception of the PVAc C¼ ¼O band in the coalesced PC/PVAc blend, which is shifted from 1735 cm1 (physical blend) to 1740 cm1 (coalesced blend). Here we can suggest that PVAc chains in close proximity to PC chains in the well-mixed coalesced PC/ PVAc blend might be producing this shift in vibrational frequencies. Differential Scanning Calorimetry

Fourier Transform Infrared Spectroscopy Though not presented here, the FTIR spectra of PC/PMMA-, PC/PVAc-, and PMMA/PVAc-c-CDICs show the presence of two carbonyl bands (C¼ ¼O) at 1775 cm1, 1730 cm1, and 1735 cm1 characteristic for PC, PMMA, and PVAc, respectively. This demonstrates that all three polymer pairs are included in the binary c-CD-ICs. In general, absorption bands are shifted in the infrared spectra of blends when there are specific interactions between, or there are conformational changes in, the constituent polymers. The infrared spectra of coalesced PC/ PMMA, PC/PVAc, and PMMA/PVAc polymer blends and the physical blends were examined to identify any possible differences between them (data not shown). The vibrational bands observed for coalesced blends of PMMA/PVAc, PC/PMMA, and PC/PVAc were very similar

In general, the appearance of a single glass transition temperature (Tg) is a satisfactory verification of a homogeneous amorphous phase present in a polymer blend. However, the observation of a common glass transition temperature in a polymer blend does not necessarily mean that the homogeneous phase is in a thermodynamic equilibrium state. A polymer blend is likely to show a single Tg once the homogeneous phase is trapped, but might phase separate during heating above its Tg where phase segregation can occur because of the segmental mobility of the polymer chains. Therefore, it is necessary to apply several heating and cooling scans in DSC experiments performed on polymer blends. Retention of a single Tg after repeated heat treatments should reveal the homogeneity of the blend system. In our previous report,39 we have studied the thermal behavior of as-received and coalesced

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Figure 6. DSC thermograms of (a) first heating scan and (b) second heating scan of coalesced PC/PVAc blend and (c) first heating scan of coprecipitated PC/PVAc blend. [Color figure can be viewed in the online issue, which is available at www.interscience. wiley.com.]

homopolymers by DSC. The Tgs of the asreceived PC, PMMA, and PVAc were found to be 145, 83, and 29 8C, respectively, whereas the Tg values for coalesced PC, PMMA, and PVAc were 149, 97, and 42 8C, respectively. Figure 5 displays the DSC scans recorded for the coprecipitated physical and coalesced PC/ PMMA blends. Two glass transition temperatures (Tg) were observed for the coprecipitated PC/PMMA blend in the first DSC scan (Fig. 5c): a weak transition at 81 8C for PMMA and a Tg at 136 8C for PC domains. For polymer blends, a single Tg is indicative of a state of molecular dispersion, whereas the appearance of multiple distinct Tgs suggests the occurrence of phase separation. DSC results indicated that the physical PC/PMMA binary blend prepared by coprecipitation does not form a miscible blend. In contrast, the first scan of the coalesced PC/PMMA blend (Fig. 5a) did not show any apparent glass transition, but instead a melting of crystalline PC regions at 242 8C. It is noteworthy that PC was found to be completely amorphous in the asreceived sample and its coprecipitated PC/ PMMA blend, whereas PC chains form crystalline regions in the coalesced PC/PMMA blend. The presence of a crystalline phase in coalesced PC/PMMA and PC/PVAc blends was also confirmed by X-ray measurement (data not shown). The X-ray diffraction from these coalesced blends showed a similar pattern with the

main diffraction peak at 2h ¼
188, whereas coprecipitated physical blends exhibited a broad amorphous halo in that region. This crystalline phase may be a consequence of retention of the extended conformation required by inclusion in the narrow c-CD-IC channels for the coalesced PC, as well as their partial segregation into proximal regions of the common c-CD-IC crystals, as discussed previously,40 as well as the preferential inclusion of PC chains mentioned above. The crystalline behavior of PC was also observed for its coalesced homopolymer and its ternary PC/PMMA/PVAc blend.34,39 However, PC crystals are not reformed once the blend is heated above the melting point of PC. The second DSC scan was run for the sample after quenching (at a rate of 200 8C/min) from 270 to 0 8C. In the second heating scan (Fig. 5b), the melting peak for PC crystals disappeared and a single Tg at 126 8C was observed, indicating the presence of a single homogeneous amorphous phase in the binary blend of PC/PMMA, where these two polymer components are intimately mixed with each other. The intimate mixing between PC and PMMA could originate from intermolecular contacts and specific interactions, which generally create smaller domain sizes, thereby introducing a higher degree of compatibility. Figure 6 displays the DSC scans recorded for the coprecipitated physical and coalesced PC/

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Figure 7. DSC thermograms of (a) first heating scan (b) and second heating scan of coprecipitated PMMA/PVAc blend and (c) second heating scan of coalesced PMMA/ PVAc blend.[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

PVAc blends. The PC/PVAc blend system showed thermal behaviors similar to the PC/PMMA blend, where melting at 243 8C for PC crystals was observed in the first scan of the coalesced blend (Fig. 6a). The absence of PC melting in the first DSC scan (Fig. 6c) of the coprecipitated PC/PVAc blend indicated that both polymer components were amorphous in this blend. Additionally, the DSC scan showed two distinct Tgs at 39 8C and at 137 8C because of phase-separated PVAc and PC domains, respectively. However, the second DSC scan of the PC/PVAc blend (Fig. 6b) obtained by coalescence of PC/PVAc-c-CD-IC showed a single Tg at 133 8C, which reveals the intimate mixing of these two components. In both the coalesced PC/PMMA and PC/PVAc blends only the melting of PC crystals was observed during their first heating DSC scans, which disappeared after melting and rapid cool-

ing during their second heating DSC scans. This suggests the presence of small PC crystals in close contact with PMMA and PVAc chains, which upon melting lead to well-mixed PC/ PMMA and PC/PVAc amorphous regions that evidence single Tgs, though this behavior warrants further study. Figure 7 shows the DSC scans recorded for the coprecipitated physical and coalesced PMMA/PVAc blends. The first DSC scan of the coprecipitated PMMA/PVAc blend showed two separate Tgs at 47 8C and at 81 8C because of the presence of two segregated amorphous phases in the polymer blend. However, the Tg observed at 47 8C is higher than the Tg of pure PVAc which could be due to the partial miscibility of PMMA and PVAc domains. Previously, it was reported that at temperatures higher than the Tg of PVAc, PVAc macromolecules begin to

Table 1. T1p(1H) (in ms) of Binary PC/PMMA Blends for Protons Attached to Different PC and PMMA Carbons (13C resonances in ppm vs TMS) Peak PC (ppm)

PC PMMA Precipitated blend Coalesced blend

Peak PMMA (ppm)

150

130

120

40

30

178

52

45

5.1 – 5.2 6.8

5.5 – 5.4 7.0

5.5 – 5.4 7.6

5.2 – 5.1 6.9

5.6 – 5.2 6.7

– 14.8 – –

– 14.9 16.9 7.9

– 15.5 13.0 7.7

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Table 2. T1p(1H) (in ms) of Binary PMMA/PVAc Blends for Protons Attached to Different PMMA and PVAc Carbons (13C resonances in ppm vs TMS) Peak PMMA (ppm)

PMMA PVAc Precipitated blend Coalesced blend

Peak PVAc (ppm)

178

52

45

171

68

21

14.8 – 15.5 21.2

14.9 – 18.3 19.8

15.5 – 18.5 22.8

– 10.6 33.3 16.5

– 12.2 40.7 19.6

– 11.2 31.8 20.2

diffuse from the interphase and migrate to PVAc rich domains.9 Here, we observed the same behavior for PVAc domains in the PMMA/PVAc physical blend when it was subjected to heating–cooling cycles. The Tg is shifted from 47 to 33 8C in the second heating DSC scan, which indicates the phase separation of PVAc domains from PMMA domains. Figure 7c displays the second heating scan of the coalesced PMMA/ PVAc blend. The thermogram shows a Tg at 36 8C, indicating the presence of phase-separated PVAc domains in the blend, and a Tg at 53 8C (a value between the Tgs of the homopolymers PMMA (Tg ¼
80 8C) and PVAc (Tg ¼
35 8C)), indicating the presence of an intimately mixed phase of amorphous PMMA/PVAc chains. The retention of the Tg at 53 8C even after several heating–cooling cycles reveals the presence of intimately mixed and thermally stable PMMA/PVAc domains in the coalesced blend.

Solid-State 1H NMR The 1H spin-lattice relaxation times of the PC/ PMMA and PMMA/PVAc blend systems observed in the rotating frame (1H T1q) are presented in Tables 1 and 2, respectively. The pure polymers have their own uniform 1H T1q values, which indicates the necessary homogeneity in each pure polymer. Some 1H T1q data are missed in the blend samples because of the overlap of the peaks with the spinning side bands of PC. For the PC/PMMA blend in Table 1, the physical mixture sample shows two distinct 1H T1q values, one for each component, which indicates a phase-separated blend with domain sizes above 5 nm. The coalesced blend sample exhibits a uniform spin-lattice relaxation value, indicating the intimate blending of PC and PMMA chains in the coalesced PC/PMMA sample. On the other hand, the similar 1H T1q values observed for

pure PVAc and PMMA shown in Table 2 indicate that the spin-lattice relaxation experiment may not be an effective method to study the phase behavior of this polymer blend system. Nevertheless, the 1H T1q values shown in Table 2 suggests that the precipitation process changed the morphology of both polymers; the distinct 1H T1q values for PMMA and PVAc in the precipitated blend show the phase segregation in this sample. In the case of the coalesced PMMA/PVAc blend, the closely similar 1H T1q values suggest a certain degree of intimate blending.

Thermogravimetric Analysis Figure 8 displays the TGA curves of coalesced binary blends and physical mixtures of PC/ PMMA, PC/PVAc, and PMMA/PVAc. Thermogravimetric results of all binary blends indicated two weight losses stages. The decomposition temperatures, Tds, are at 431 and 542 8C for the coalesced PC/PMMA blend and are at 421 and 546 8C for the physical mixture (Figs. 8a and 8b). The low temperature weight losses observed for individual as-received and coalesced PMMA samples39 were not observed for both of the binary blends. The first degradation stage of coalesced PC/PMMA is higher than the main degradation stage of coalesced PMMA (423 8C), indicating an increase in thermal stability of the PMMA component. A similar behavior was also noted for the physical mixture compared with as-received PMMA (410 8C). However, the second Td detected for coalesced and physical blends of PC/PMMA is lower than the Tds of asreceived and coalesced PC samples, at 549 and 554 8C.39 The TGA curve of the coalesced PC/PVAc binary blend shows two Tds at 377 and 551 8C and that of the physical mixture of PC/PVAc

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Figure 8. TGA thermograms of (a) coalesced and (b) coprecipitated PC/PMMA blends; (c) coalesced and (d) coprecipitated PC/PVAc blends; and (e) coalesced and (f) coprecipitated PMMA/PVAc blends. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

shows two Tds at 369 and 538 8C (Figs. 8c and 8d). The first degradation stages of individual coalesced and as-received PVAc were observed at 369 and 360 8C, respectively.39 The second degradation stage for the coalesced PVAc sample was shifted to 483 8C from 464 8C, the corresponding value of the as-received sample, indicating an increase in thermal stability for the coalesced PVAc. The second degradation stages of both of the binary mixtures are significantly higher than these values. However, the second Td for coalesced and physical blends of PC/PVAc at 551 and 538 8C, respectively, are slightly lower than the Tds of coalesced and as-received PC samples. The TGA results of coalesced and physical binary blends of PC/PMMA and PC/PVAc reveal that in the presence of PC, the thermal stabilities of both PMMA and PVAc increase, while the presence of PMMA and PVAc decreases the thermal stability of the PC component. For the coalesced PMMA/PVAc blend, the two Tds are at 360 and 424 8C, very close to the Td values of the physical mixture at 360 and 422 8C and to the corresponding values for pure PVAc. The low temperature weight losses recorded for as-received and coalesced PMMA samples were not observed for PMMA/PVAc blends, as in the case of PC/PMMA blends. The second degradation steps observed at 424 and 423 8C are very close to that for the coalesced PMMA at 423 8C, but lower than the second degradation step for PVAc at 464 and 483 8C.

Direct Insertion Probe Pyrolysis Mass Spectrometry In general, DIP-MS facilitates analyses of degradation mechanisms using structural information from the thermal characterization of degradation products.41–43 However, pyrolysis mass spectra of polymers are usually very complex, as thermal degradation products further dissociate in the mass spectrometer during ionization. Also, all fragments with the same mass to charge ratio make contributions to the intensities of the same peaks in the mass spectrum. Thus, in pyrolysis MS analysis, not only the detection of a peak, but also the variation of its intensity as a function of temperature, that is, its evolution profile, is important. The trends in evolution profiles can be used to determine the source of the product or the mechanism of thermal degradation. The DIP-MS technique was applied for thermal characterization of degradation products of coalesced and physical binary blends of PC/ PMMA, PC/PVAc, and PMMA/PVAc. As the maximum attainable temperature with our present pyrolysis system is 445 8C, the highest temperature weight losses detected in TGA curves could not be studied. Thermal degradation behaviors of PC, PMMA, and PVAc have been detailed in the literature.44–49 The evolution of cyclic oligomers due to intermolecular exchange reactions and evolution of CO2 by decarboxylation of the carbonate groups forming ether bridges and by

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Figure 9. TIC curves (left) and pyrolysis mass spectra (right) recorded at the maximum of the peaks and shoulders in the TIC curves of a) PC/PMMA physical mixture, b) coalesced PC/PMMA, c) PC/PVAc physical mixture d) coalesced PC/PVAc, e) PMMA/PVAc physical mixture, and f) coalesced PMMA/ PVAc blends.

hydrolysis reactions producing phenolic end groups has been proposed for the thermal degradation of poly (bisphenol A carbonate) in the temperature range (400–500 8C) studied here.44,45 PMMA degrades mainly by depropagation initiated by a mixture of chain end and chain scission processes at elevated temperatures (380– 420 8C).46,47 The thermal degradation of PVAc occurs in two steps; in the first step, deacetylation occurs around 360 8C and in the second step, disintegration of the polyolefinic backbone occurs around 440 8C.48,49

Recent pyrolysis studies on coalesced PC, PMMA, and PVAc revealed that the total removal of c-CD could not been achieved.39 For the coalesced PC, low molecular weight PC based products started to appear in the pyrolysis mass spectra at significantly lower temperatures, although Td shifted to higher temperatures. The low temperature evolution of low molecular weight PC based products was attributed to loss of PC chain ends or units strongly interacting with c-CD. On the other hand, the temperature dependence of the total ion current

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Figure 10. Evolution profiles of C6H5 C(CH3)2 C6H4 O C6H4 C(CH3)2 C6H4 (m/z ¼ 405Da), C6H5 O COO C6H4 (m/z ¼ 213 Da), C2H4O2 (m/z ¼ 60Da), C2H3O (m/z ¼ 43 Da), and C7H7 (m/z ¼ 91Da) recorded during the pyrolysis of a) physical and b)coalseced PC/PVAc blends.

(TIC) of coalesced PMMA was quite similar to that of the as-received sample. Yet, the low temperature peaks were diminished and the high temperature peaks with maxima at 325 and 420 8C became sharper, indicating a decrease in polydispersity and removal of low molecular weight oligomers during the inclusion and washing processes. For the coalesced PVAc, crosslinking of the polyolefinic backbone with the decomposition products of c-CD involving two or more OH groups generated during the first thermal degradation stage has been proposed.39 The TIC curves of coalesced and physical binary blends of PC/PMMA, PC/PVAc, and PMMA/PVAc are shown in Figure 9. The pyrolysis mass spectra recorded at the shoulders and at the TIC maxima are also included in this figure. In general, the maxima present in the TIC curves are quite similar to the Td values in the corresponding TGA

curves. For all the coalesced binary blend samples, weak shoulders around 300–360 8C, are detected and associated with evolution of c-CD based products indicating that all the samples contained small amounts of remnant c-CD. The maxima at 435 and 430 8C in the TIC curves of coalesced and physical PC/PMMA blends, respectively, are more than 20 8C higher than the TIC maxima recorded for coalesced PMMA (Figs. 9a and 9b). The mass spectra recorded at the weak shoulder around 360 8C in the TIC curve of the coalesced sample are dominated by peaks characteristic of PC, with the C6H5OCOOC6H4 fragment peak at m/z ¼ 213 Da being the base peak. Peaks diagnostic for c-CD were also recorded in these spectra. However, PMMA based products peaks were relatively weak. Though not presented, evolution profiles of some characteristic fragments of pure PC, PMMA and c-CD were observed. The

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maxima of the evolution profiles shifted slightly to higher temperatures for the coalesced blend. The most significant change is the low temperature evolution of the C6H5-O-COO-C6H4 fragment as in the case of coalesced PC. It is clear that the thermal stability of PMMA chains increased in the presence of PC. The increase in stability is more pronounced for the coalesced sample. On the other hand, for the coalesced blend, evolution of PC based degradation products started at significantly lower temperatures. It may be concluded that presence of PC enhances the thermal stability of PMMA, while the presence of PMMA reduces the thermal stability of PC in their blends. The TIC curves of the coalesced and physical PC/PVAc blends show maxima around 370 and 380 8C, respectively, (Figs. 9c and 9d). For the coalesced sample, a shoulder around 325 8C and a second maximum on the high temperature tail at 440 8C are seen. Analyses of the pyrolysis mass spectra of the coalesced binary blend indicated evolution of c-CD around 325 8C together with some PC based fragments. Unlike the PC/ PMMA physical mixture, the evolution of PC based fragments were also shifted to low temperatures for the PC/PVAc physical blend (Figs. 9c and 9d). Furthermore, for the physical blend, evolution of fragments generated by decarboxylation of the carbonate groups forming ether bridges also occurred at lower temperatures in contrast to the PC/PMMA physical mixture. It may be that the acetate degradation product of PVAc occurring around 360 8C activates degradation of PC even at low temperatures. In Figure 10, evolution profiles of some characteristic products are shown. Compared to pure PVAc, a significant decrease in the ratio of CH3COO to CH3CO yields was noted for the physical blend supporting the reactions of CH3COO with PC chains. However, for the coalesced blend CH3COO yield was noticeably higher. Furthermore, evolution of PC based fragments shifted slightly to higher temperatures. The TIC curves of coalesced and physical blends of PMMA/PVAc are quite similar, except for the presence of a weak shoulder around 310 8C for the coalesced blend, in accordance with TGA results, and show two maxima at 360 and 430 8C (Figs. 9e and 9f). It is clear that degradation of remnant c-CD occurred around 310 8C for the coalesced sample. For the physical blend, the mass spectra recorded around 360 8C are dominated by peaks diagnostic for PVAc and those recorded around 430 8C are

dominated with peaks diagnostic for PMMA. Though the TIC curve of the coalesced blend was quite similar to that of the physical blend, the fragmentation pattern recorded in the pyrolysis mass spectra of the coalesced PMMA/PVAc blend is quite different. At high temperatures relative intensities of low molecular weight fragments were enhanced. Especially, the decrease in PMMA monomer yield was very significant for the coalesced blend. Thus, it may be concluded that the degradation mechanism of PMMA has changed in the coalesced sample, and the thermal degradation of PMMA was initiated by loss of side chains because of the interactions of PMMA with PVAc in the coalesced binary mixture instead of a depropagation mechanism as in the case of pure PMMA. Details of the pyrolysis mass spectrometry results are discussed elsewhere.50

CONCLUSIONS We have demonstrated that thermally stable and intimately mixed blends of PC/PMMA, PC/ PVAc, and PMMA/PVAc can be obtained by formation of and coalescence from their common ICs with c-CD. Preparation of these same binary blends by both solvent-casting and precipitation, on the other hand, yielded phase-segregated samples. The well-mixed binary blends achieved by coalescence from their common c-CD-ICs also exhibited thermal stabilities and thermal degradation mechanisms that were distinct from those of the pure component polymers and their binary mixtures obtained by both solvent-casting and precipitation. Thus, both the phase structures and the properties of polymer blends can be modified by processing with CDs. The authors are grateful to the National Textile Center (US Dept. of Commerce) and North Carolina State University for their financial support. The authors also thank Middle East Technical University for funding (METU Research Funds BAP 03-01-03-04), and they appreciate the DIP-MS experiments performed by Evren Aslan.

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