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Synthesis and characterization of perfectly alternating polycarbonate-polydimethylsiloxane multiblock copolymers possessing controlled block lengths.

INTRODUCTION

Because of their ability to produce highly ordered and tuneable nanoscale morphologies through self-assembly, block copolymers have become a very important class of materials. Depending on compositional variables, such as monomer structure, block copolymer architecture, and block lengths, a wide variety of well-ordered, nanoscale morphologies can be obtained such as spheres, cylinders, gyroid structures, and lamella. Early development of well-defined block copolymers was accomplished using chain-growth polymerization. For example, in 1956 Szwarc demonstrated the successful block copolymerization of styrene and isoprene using anionic polymerization [1], This work eventually led to commercialization of the very important styrenic block copolymers that find application as thermoplastic elastomers and components of a variety of formulated materials including adhesives, sealants, coatings, and roofing and paving materials.

Since the pioneering efforts of Swarc et al., many other methods have been developed for the production of new block copolymers. Although particular attention was paid to chain growth mechanisms [2, 3], step-growth polymerization systems have also been used to produce block copolymers. A straight-forward method for producing block copolymers using a step-growth method involves the use of a macromonomer in a conventional step-growth polymerization. For example, polyester thermoplastic elastomers have been produced by melt copolymerization of poly(tetramethylene ether) glycol (PTMEG) with dimethyl terephthalate and 1,4-butanediol [4], For these copolymers, a multiblock copolymer is produced possessing low [T.sub.g] polyether blocks, derived from PTMEG, randomly distributed along the polymer backbone amongst higher [T.sub.g] polyester blocks of varying block length. Although this synthetic method does not allow for control of block architecture or block lengths, it has enabled the production of important commercial polymers. Likewise, polyurethane multiblock copolymers produced by a step-growth polymerization mechanism involving a low [T.sub.g] macromonomer have high commercial value as thermoplastic elastomers [5].

In addition to thermoplastic elastomers, the process of using step-growth polymerization and a macromonomer has been used to produce multiblock copolymers for application as engineering thermoplastics. For example, Pospiech et al. [6] synthesized a series of polysulfone-based multiblock copolymers based on different, highly flexible macromonomers using conventional melt condensation polymerization. Multiblock copolymers containing blocks derived from other engineering thermoplastics, such as poly(ether ether ketone) [7], polyimide [8], poly (ether sulfone) [9], and poly(phenylene oxide) [10, 11], have also been produced.

Bisphenol-A (BPA) polycarbonate, commonly referred to simply as polycarbonate (PC), is a tremendously useful engineering thermoplastic that was originally produced in high molecular weight in 1953 from the transesterification of bisphenol-A (BPA) and diphenyl carbonate [12]. Despite enormous ongoing efforts to develop new engineering thermoplastics, PC is still generally recognized as the most impact-resistant transparent engineering thermoplastic available. In fact, the high impact resistance and optical transparency of PC has been utilized in the production of bullet-proof glass [13].

Because of the high polymer chain flexibility, good thermal stability, flame resistance, weatherability, and good oxygen and nitrogen permselectivity associated with polydimethylsiloxane (PDMS), PC-PDMS multiblock copolymers have been of interest. A number of synthetic methods have been used to produce PC-PDMS multiblock copolymers. The first reports by Vaughan in the mid 1960s involved the reaction of an [alpha],[omega]-dichloroPDMS oligomer with BPA and phosgene in a pyridine/methylene chloride solution [14-16]. A disadvantage of this synthetic method is that the aryloxy silane groups formed from the reaction of BPA with [alpha],[omega]-dichloroPDMS oligomer are hydrolytically unstable [17, 18]. This issue was overcome by using hydrosilylation to produce a PDMS macromonomer with phenolic groups [19, 20], As illustrated in Fig. 1, the PDMS blocks in these multiblock copolymers are randomly distributed along the polymer backbone and the PC block lengths vary widely due to the fact that the polymerization is a statistical copolymerization. Despite the fact that the polymerization is a statistical copolymerization, Hagenaars et al. showed, by using a continuous polymer fractionation technique, that this synthetic method can produce significant variations in chemical composition between polymer chains of the same bulk sample [21]. PC-PDMS multiblock copolymers produced by this method were found to exhibit excellent impact resistance. Beach et al. [22] showed that the incorporation of 25 weight percent PDMS blocks into PC reduced the ductile brittle transition temperature from -20[degrees]C to -110[degrees]C. In addition, Okamoto [23] showed that glass-filled composites based a PC-PDMS multiblock copolymer matrix can provide higher impact strength and lower melt viscosity than glass-filled PC. Further, Nodera and Kanai [24] showed that the incorporation of PDMS into PC via block copolymerization enhances flame retardancy even at PDMS contents as low as 1.0 wt%.

Tang et al. [25] successfully produced perfectly alternating PC-PDMS multiblock copolymers using the silyamine-hydroxyl reaction to couple silylamine-terminated PDMSs with BPA-terminated PCs [26], Multiblock copolymers were produced that possessed relatively short PC blocks (3100-4200 g [mol.sup.-1]) and a wide variation in PDMS block molecular weights (1800-15,000 g [mol.sup.-1]). Over this compositional space, the physical properties of the copolymers ranged from plastic-like materials at PC contents above 50 wt%, to leathery materials at PC contents of 34 and 42 wt%, to elastomeric materials at PC contents below 30 wt%. Even at the lowest block molecular weights (PC/PDMS = 3400/1800 g [mol.sup.-1]), phase separation was observed in these materials. The strong propensity for phase separation was attributed to the large difference in solubility parameter between PC ([delta] = 9.5 [cal.sup.1/2] [cm.sup.3/2]) and PDMS ([delta] = 7.3 [cal.sup.1/2] [cm.sup.3/2]). The morphology of the perfectly alternating PC-PDMS multiblock copolymers was described as being "sponge-like" in structure regardless of composition. As pointed out by the authors, a similar morphology had been previously observed for segmented polyurethanes [27]. This initial work was followed-up by a study in which perfectly alternating PC-PDMS multiblock copolymers were produced that possessed similar PC content (i.e., ~50 wt%), but different block molecular weights [28]. Of four different multiblock copolymers produced, only the polymer with the longest PC and PDMS blocks exhibited the expected lamellar morphology [29]. The other multiblock copolymers with lower molecular weights exhibited the "sponge-like" morphology exhibited by the perfectly alternating PC-PDMS multiblock copolymers produced in the initial report [25].

Similar to the PC-PDMS multiblock copolymers produced by Vaughan from an [alpha],[omega]-dichloroPDMS oligomer, BPA, and phosgene, the alternating PC-PDMS multblock copolymers produced using the silyamine-hydroxyl reaction results in hydrolytically unstable aryloxy silane groups in the polymer backbone. Despite the fact that Noshay and Matzner [30] showed that PDMS-based multiblock copolymers possessing aryloxy silane groups show significantly better hydrolytic stability than "nonblock" copolymers possessing aryloxy silane groups, it was of interest to produce and characterize perfectly alternative PC-PDMS multiblock copolymers possessing hydrolytically stable silicon-carbon bonds between the PC and PDMS blocks. The perfectly alternating PC-PDMS multiblock copolymers produced as result of this study utilized hydrosilylation reactions to couple the PC and PDMS blocks. The study involved systematic variations in both PC block molecular weights and PDMS block molecular weights. As will be discussed, the extent of phase separation and phase morphology of the perfectly alternating PC-PDMS multiblock copolymers produced using hydrosilylation were significantly different from that of the multiblock copolymers produced using the silyaminehydroxyl reaction.

EXPERIMENTAL

Materials

Bisphenol-A [4,4'(propane-2,2diyl)diphenol], triphosgene [bis(trichloromethyl) carbonate], eugenol (4-allyl-2-methoxyphenol), triethylbenzylammonium chloride, sodium hydroxide, tetrachloroethane (TCE), and methanol were used as received from Sigma-Aldrich. Four different hydride-tenninated PDMSs (H-PDMS-H) with vendor identifications of DMS-H03, DMS-H11, DMS-H21, and DMS-H25 were used as received from Gelest. Anhydrous methylene chloride and tetrahydrofuran (THF) were used as received from VWR International. For comparison purposes, a commercially available PC-PDMS multiblock copolymer was obtained from SABIC Innovative Plastics.

Synthesis of Eugenol-Terminated PC Oligomers

The following procedure was used to synthesize a eugenol-terminated PC oligomer (E-PC-E) with a number-average molecular weight ([M.sub.n]) of 4810 g [mol.sup.-1] (5KPC): To a 1-L flask equipped with a high-speed overhead stirrer, nitrogen inlet, inlet tube for triphosgene (TPG) addition, outlet tube connected to a NaOH scrubbing solution, and a thermometer, 4.79 g of bisphenol-A (BPA; 21 mmol), 0.69 g of eugenol (4.2 mmol), and 5.04 g of NaOH (126 mmol) were dissolved in 200 mL of [H.sub.2]O and cooled below 5[degrees]C using an ice bath. TPG (2.49 g, 8.4 mmol) and TEBA (0.68 g, 3.0 mmol) were dissolved in 200 mL of anhydrous C[H.sub.2][Cl.sub.2] in a 500 mL one-neck, round-bottom flask inside a glove box. The organic phase was added to the aqueous phase under high speed stirring (900 rpm) using a syringe pump. Reaction temperature was kept below 5[degrees]C during the TPG addition. After completion of the TPG addition, high-speed stirring was continued for 90 min at a temperature below 15[degrees]C. The C[H.sub.2][Cl.sub.2] phase was separated and washed with water until the pH of the aqueous phase was 7.0. The polymer was isolated from C[H.sub.2][Cl.sub.2] by precipitation into methanol. The precipitate was isolated by filtration and dried at 80[degrees]C under vacuum. For the other two E-PC-Es used for the study, the synthesis procedure was same with the exception that 1.38 g (8.4 mmol) and 0.35 g (2.1 mmol) of eugenol were used for the production of the 3070 and 7240 g [mol.sup.-1] E-PC-E, respectively. The 3070 and 7240 g [mol.sup.-1] E-PC-E were designated 3 and 7 KPC, respectively.

Synthesis of PC-PDMS Alternating Multiblock Copolymers

Using the three E-PC-Es synthesized and the four different commercially available H-PDMS-Hs, a series of 12 different PC-PDMS alternating multiblock copolymers were prepared using hydrosilylation. The four different H-PDMS-Hs designated as 0.7KPDMS, 1.2KPDMS, 5.3KPDMS, and 12KPDMS possessed [M.sub.n]s of 740, 1190, 5300, and 12,420 g [mol.sup.-1], respectively, as determined by [sup.1]H NMR. Figure 2 provides a schematic illustration of the PC-PDMS multiblock copolymers produced. The following procedure was used to synthesize a PC-PDMS alternating multiblock copolymer possessing a PC block [M.sub.n] of 4810 and PDMS block [M.sub.n] of 1190: In a 250-mL round-bottom flask equipped with a nitrogen inlet and a condenser, 15.72 g (3.27 mmol) of the 4810 g [mol.sup.-1] E-PC-E (5KPC) was dissolved in 80 g of anhydrous TCE. To this solution, 4.28 g (3.60 mmol) of 1.2KPDMS and 7.42 g of a 0.1 wt% [P.sub.t][O.sub.2] dispersion in TCE were added and the mixture heated at 120[degrees]C under a nitrogen atmosphere for 24 h. Next, the resulting multiblock copolymer was precipitated into methanol, isolated by filtration, and dried under vacuum at 80[degrees]C for 48 h. Table 1 lists the reagents and their concentrations used for the synthesis of each of the 12 PC-PDMS alternating multiblock copolymers produced. The molecular weights and actual compositions for all 12 PC-PDMS alternating multiblock copolymers produced are listed in Table 2.

Instrumentation and Characterization Methods

Proton nuclear magnetic resonance ([sup.1]H NMR) spectra were recorded using a JEOL 400 MHz spectrometer at 25[degrees]C. CD[Cl.sub.3] was used as the lock solvent. Gel permeation chromatography (GPC) was performed using a Symyx Rapid-GPC with an evaporative light scattering detector (PL-ELS 1000). Samples for GPC were prepared in THF at a concentration of 1 mg [mL.sup.-1]. The molecular weight of the PC-PDMS multiblock copolymers were expressed relative to polystyrene standards. Thermal properties were characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC was carried out using a Q2000 modulated differential scanning calorimeter manufactured by TA Instruments. The calorimeter was calibrated with sapphire and indium standards and sample sizes were in the range of 5-10 mg. A modulation amplitude of 0.531[degrees]C and a period of 100 s was used at a heating rate of 2[degrees]C [min.sup.-1] when samples were modulated between -180 to 0[degrees]C and 25 to 180[degrees]C. TGA was conducted in air on ~10 mg samples using a TGA Q500 Thermal Analyzer from TA Instruments. The heating rate was 20[degrees]C [min.sup.-1] over a range of 25 to 800[degrees]C. Viscoelastic properties were characterized using a DMA Q800 Dynamic Mechanical Analyzer over a temperature range of -150 to 160[degrees]C. A heating rate of 5[degrees]C [min.sup.-1], strain of 0.01%, preload force of 0.01 N, and frequency of 10 Hz were used for the measurements. Samples were prepared by pressing dried powders into films at 180[degrees]C using a hot press. The films, which were ~100-[micro]m thick, were cut into 5-mm wide strips for testing. The distance between the clamps was 15 mm. Small angle X-ray scattering (SAXS) experiments were performed on compression molded samples using a Rigaku S-Max 3000 3 pinhole SAXS system, equipped with a rotating anode emitting X-ray with a wavelength of 0.154 nm (Cu K[alpha]). The sample-to-detector distance was 1.6 m, and q-range was calibrated using a silver behenate standard. Two-dimensional SAXS patterns were obtained using a fully integrated 2D multiwire, proportional counting, gas-filled detector, with an exposure time of 1 h. The SAXS data were corrected for sample thickness, sample transmission, and background scattering. All the SAXS data were analyzed using the SAXSGUI software package to obtain radically integrated SAXS intensity versus scattering vector q, where q = (4[pi]/[lambda])sin([theta]), [theta] is one half of the scattering angle and [lambda] is the wavelength of the X-ray. The profiles were vertically shifted to facilitate a comparison of peak positions.

RESULTS AND DISCUSSION

Synthesis and Characterization of Eugenol-Terminated Polycarbonate Oligomers

As illustrated in Fig. 3, the synthetic procedure for producing the perfectly alternating PC-PDMS multiblock copolymers of interest involved the use of hydrosilylation reactions to couple E-PC-Es and H-PDMS-Hs. With this process, the purity of the telechelic oligomers must be high to enable relatively high molecular weight multiblock copolymers to be produced. For PC, the production of high purity telechelic oligomers can be challenging since, depending on the polymerization process, cyclic species can be generated [31]. E-PC-Es produced using interfacial polymerization has been previously reported; however, no characterization was conducted to determine if the polymer samples contained cyclic species [32].

The process used by the authors to produce E-PC-Es involved interfacial polymerization and triphosgene as the carbonate source. Because interfacial polymerization can result in the formation of PC cyclics and only limited reports of the synthesis of PC using triphosgene were available, experiments were conducted to determine the most appropriate polymerization process to produce high purity E-PC-Es. As a result of these experiments, which are reported elsewhere, [33] a polymerization process was identified that provided E-PC-Es possessing a minimum purity of 98% as determined using matrix assisted laser desorption ionization-time of flight spectroscopy. Because of the high purity of the E-PC-Es produced for the study, the absolute number-average molecular weight ([M.sub.n]) of the oligomers could be quantified using 'H NMR. Figure 4 displays a representative [sup.1]H NMR spectrum of a E-PC-E produced for the study. From this [sup.1]H NMR spectrum, it can be seen that all of the peaks in the spectrum are nicely separated enabling integration to be readily used to quantify [M.sub.n] by comparing integration values obtained for protons associated with the eugenol end-groups to protons associated with BPA residues in the oligomer repeat units.

Synthesis and Chemical Characterization of PC-PDMS Alternating Multiblock Copolymers

The perfectly alternating PC-PDMS multiblock copolymers of interest were produced from E-PC-E and H-PDMS-H using platinum-catalyzed hydrosilylation, as shown in Fig. 3. As illustrated in Fig. 5, the progress of multiblock formation was easily monitored using [sup.1]H NMR by observing the disappearance of the hydride protons of H-PDMS-H and the vinyl protons of E-PC-E. Successful multiblock formation was also confirmed using GPC. As shown in Fig. 6, the GPC trace obtained for each PC-PDMS mulliblock copolymer was shifted to lower retention time compared to the H-PDMS-H and E-PC-E precursors. In addition, no high retention time shoulders on the GPC traces for the multiblock copolymers were observed that would suggest significant contamination by unreacted precursor oligomers. These results clearly indicate that hydrosilylation was effective for producing perfectly alternating PC-PDMS multiblock copolymers.

Thermal Properties of PC-PDMS Alternating Multiblock Copolymers

The thermal properties of the E-PC-Es, H-PDMS-Hs, and the PC-PDMS multiblock copolymers were characterized using MDSC. Figure 7 displays representative MDSC thermograms obtained over the temperature range of 40-175[degrees]C, while Fig. 8 displays representative MDSC thermograms over the temperature range of -170[degrees]C to -10[degrees]C. Over the temperature range of 40-160[degrees]C, MDSC measurements were obtained in triplicate to account for experimental error. Table 3 lists the [T.sub.g] values obtained. For E-PC-Es, [T.sub.g] was dependent on [M.sub.n] and all of the E-PC-Es possessed [T.sub.g]s well below that typically obtained for relatively high molecular weight PC. The [T.sub.g] measured by MDSC for commercially available PC with an [M.sub.n] of 21,900 g [mol.sup.-1], obtained by GPC and expressed relative to polystyrene standards, was determined to be 145[degrees]C. These results indicate that all three of the E-PC-Es possessed A/ns below the critical molecular weight required for polymer entanglement.

For PC-PDMS multiblock copolymers possessing a PDMS content below 70 wt%, the [T.sub.g] of the PC-rich phase could be identified using MDSC. For these copolymers, the [T.sub.g] associated with the PC-rich phase varied significantly with copolymer composition. Figure 9 displays [T.sub.g] data associated with the PC-rich phase as a function of the block A/ns of the copolymers. From Fig. 9, it can be seen that the presence of the short 740 g [mol.sup.-1] PDMS blocks dramatically reduced the [T.sub.g]s of the PC-rich phase compared to the pure E-PC-Es. In general, the magnitude of the reduction in PC-rich phase [T.sub.g] decreased with increasing PC block [M.sub.n], as shown in Fig. 10. This result indicates that, despite the very large difference in solubility parameter between PC ([delta] = 9.5) and PMDS ([delta] = 7.3) of 2.2, significant partial miscibility between the PC and PDMS blocks occurred and, as expected, the degree of partial miscibility decreased with increasing block molecular weight. For the multiblock copolymers possessing the highest molecular weight PC block (i.e., 7240 g [mol.sup.-1]) and PDMS blocks of 5300 g [mol.sup.-1] or higher, no reduction of the PC [T.sub.g] was observed indicating that the blocks were of sufficient molecular weight to result in complete separation of PDMS segments from the PC phase. To further illustrate the influence of block [M.sub.n]s on the degree of partial miscibility, Fig. 11 was generated which provides a comparison of multiblock copolymers with equivalent or similar PC/PDMS ratio but differing block [M.sub.n]s. As shown in the figure, copolymers with short block [M.sub.n]s consistently produced multiblock copolymers with a lower [T.sub.g] for the PC-rich phase.

As shown in Fig. 8, all of the H-PDMS-Hs used to produce the PC-PDMS multiblock copolymers exhibited a [T.sub.g] and melting endotherm. With the exception of the lowest [M.sub.n] H-PDMS-H, they also exhibited a crystallization exotherm associated with cold crystallization. Both [T.sub.g] and melting temperature were found to vary with molecular weight. With regard to [T.sub.g], it increased with increasing [M.sub.n] which, for this relatively low [M.sub.n] range, can be attributed to the greater segmental mobility of the polymer chain ends. With regard to the melting behavior, all four of the H-PDMS-Hs exhibited two melting peaks. The observation of two melting peaks for PDMS has been observed by others and attributed to the presence of two distributions of crystallites with different degrees of perfection/thickness. As discussed by Aranguren [34], the lower melting endothenn is associated with imperfect, relatively thin crystallites fonned upon cooling; while the higher temperature endothenn is associated with thicker, more perfect crystallites produced as a result of cold crystallization and crystal reorganization that occurs during the course of the MDSC heating process. For the lowest molecular weight H-PDMS-H, the melting temperatures of the endotherms were much lower than for the other three higher [M.sub.n] H-PDMS-Hs, which can be explained by the higher number of polymer chain ends that essentially serve as defects in the crystallites. For the 1.2 KPDMS, the two melting points were higher than that for the 0.7 KPDMS but lower than that for the 5.3 KPDMS, which can be attributed to an endgroup concentration effect as just discussed. Compared to the two highest [M.sub.n] PDMSs, the 1.2 K showed a significant difference in the relative area of the lower melting endotherm to the higher melting endotherm. The 1.2 KPDMS displayed a higher fraction of the lower melting, less perfect crystallites than was observed for the higher Mn PDMSs. Again, this result is a consequence of the difference in the number of polymer chain ends.

With regard to the PC-PDMS block copolymers, none of the block copolymers based on the lowest molecular weight PDMS exhibited a [T.sub.g] or crystalline phase associated with the presence of a PDMS-rich phase. This result suggests that the short 0.7 KPDMS blocks are unable to produce a separate phase of sufficient volume to produce these low temperature transitions. For the multiblock copolymers based on the 1.2 KPDMS, a very diffuse, subtle inflection in the range between -140 and -120[degrees]C can be observed indicative of a PDMS-rich phase [T.sub.g]. However, no melting transitions were observed indicating that the presence of the PC blocks and interactions between PDMS and PC chain segments inhibit PDMS crystallization. For the PC-PDMS multiblock copolymers based on the two highest [M.sub.n] PDMS blocks (i.e., 5.3 and 12 K), both a [T.sub.g] and a melting endotherm were observed, but no cold crystallization was observed for any of the multiblock copolymers. For the multiblock copolymers based on the 5.3 KPDMS, Tm increased slightly with increasing PC block [M.sub.n] and only one melting endotherm was observed with a peak temperature below that attributed to the more stable crystallites observed for the pure 5.3 KPDMS. The observation of a [T.sub.g] indicates that a separate PDMS-rich phase exists for these multiblock copolymers. In addition, the slight increase in [T.sub.m] with increasing PC block [M.sub.n] suggests less interaction of PDMS segments with PC segments which enables thicker, more perfect PDMS crystallites to be formed. For the multiblock copolymers based on the highest [M.sub.n] PDMS, the melting transition also varied systematically with PC block molecular weight. For the multiblock copolymers based on the lowest Mn PC [i.e., 3 KPC(18%)--12 KPDMS(82%)], a single melting endotherm was observed while the melting temperature for the multiblock copolymers based on 5 or 7 KPC displayed a melting temperature with a shoulder on the high temperature side of the endotherm. This result also suggests that increasing PC block molecular weight enables greater phase separation between PDMS and PC segments such that thicker, more stable PDMS crystallites can be formed.

The thermal stability of the PC-PDMS multiblock copolymers and the E-PC-E precursor oligomers in an air atmosphere were characterized using TGA. As indicated by the TGA thermograms shown in Fig. 12, all of the PC-PDMS multiblock copolymers as well as the E-PC-E oligomers exhibited a two-step degradation process in air. This same two-step degradation process in air was also observed by Ma et al. [35] for neat PC and a PC-PDMS multiblock copolymer with a random distribution of blocks. Grubbs et al. [36] conducted an extensive investigation of the thennal stability of PC-PDMS multiblock copolymers with a random distribution of blocks and attributed the first step of the degradation process to thermo-oxidative reactions involving methyl groups in the PDMS blocks and endgroups of the PC component. The second step of the degradation was believed to be primarily due to depolymerization of the PDMS component [37-39]. The most significant difference between the TGA thermograms shown in Fig. 12 is in the temperature region associated with char fonnation. For all of the E-PC-E precursor oligomers, complete volatilization occurred, while all of the PC-PDMS multiblock copolymers exhibited significant levels of char formation. In general, as listed in Table 4, the amount of remaining residue or char increased with increasing PDMS content in the block copolymer. This residue is largely comprised of oxides of silicon [40-42].

Viscoelastic Properties of PC-PDMS Alternating Multiblock Copolymers

Viscoelastic properties of films of the PC-PDMS multiblock copolymers were characterized using DMA. Figure 13 displays storage moduli and loss tangent data for all three multiblock copolymers possessing the lowest Mn PDMS blocks. Consistent with the MDSC data, all three block copolymers exhibited a Ts associated with a PCrich phase that decreased with decreasing PC block molecular weight. In addition to a reduction in the [T.sub.g] of the PC-rich phase with decreasing PC block [M.sub.n], the transition broadened with decreasing PC block [M.sub.n] consistent with a greater decrease of partial miscibility of the PC phase with PDMS segments. Further, all of the block copolymers exhibited a subtle relaxation over the temperature range extending from about 30 to 70[degrees]C. The magnitude of this transition appeared to increase with increasing PDMS block content suggesting that it may be associated with polymer chain segments located in the interphase between PC-rich and PDMS-rich phases.

Figure 14 provides a comparison of viscoelastic properties at approximately equivalent PDMS content. As shown in Fig. 14, the block copolymer based on the higher [M.sub.n] PC and PDMS blocks exhibited a higher [T.sub.g] for the PC-rich phase consistent with less partial miscibility between PC segments and PDMS segments.

Figure 15 illustrates the effect of PDMS block [M.sub.n] on viscoelastic properties. As shown in the figure, increasing the PDMS block [M.sub.n] from 1.2 to 5.3 kg [mol.sup.-1], which corresponds to an increase in PDMS content from 21 to 65 wt%, caused a dramatic change in viscoelastic properties. At 65 wt% PDMS, the storage modulus dropped by more than two orders of magnitude when the sample was heated beyond the [T.sub.g] of the PDMS-rich phase. In addition, an increase in storage modulus was observed at temperatures just above the PDMS-rich phase [T.sub.g]. This increase in storage modulus can be attributed to cold crystallization of PDMS segments. The large drop in storage modulus after reaching the PDMS-rich phase [T.sub.g] clearly indicates that the PDMS segments form the continuous phase.

For comparison purposes, a sample of a commercially available PC-PDMS multiblock copolymer produced using interfacial polymerization and possessing a random distribution of PC and PDMS blocks was obtained from SABIC Innovative Plastics and the viscoelastic properties characterized using DMA. Using [sup.1]H NMR, the PDMS content of the multiblock copolymer was determined to be 5.1 wt%, and, using GPC, the block copolymer [M.sub.n] was determined to be 27,800 g [mol.sub.-1] relative to polystyrene standards. Figure 16 displays DMA data for a high molecular weight, commercially available PC homopolymer, the commercial PC-PDMS multiblock copolymer, and two of the alternating PC-PDMS multiblock copolymers with a relatively low PDMS content (i.e. 9 and 13 wt% PDMS). From Fig. 16, it can be seen that the viscoelastic properties of the commercial PC-PDMS multiblock copolymer was similar to that of the PC homopolymer. The primary difference in viscoelastic response between the two polymers was associated with the PC [T.sub.g]. Although the temperature of the maximum of the tan d curves is the same for both polymers (i.e., 155[degrees]C), the beginning of the relaxation started at a lower temperature for the PC-PDMS multiblock copolymer, which can be attributed to the interfacial region between the PC and PDMS phases. In contrast, the two alternating PC-PDMS multiblock copolymers exhibit significant partial miscibility between the phases as indicated by the lower [T.sub.g] for the PC-rich phase and the broader temperature range associated the [T.sub.g] of the PC-rich phase.

Phase Morphology of PC-PDMS Alternating Multiblock Copolymers

Many of the applications for PC and PC-PDMS multiblock copolymers require high optical transparency. Because the difference in refractive index (RI) between PC and PDMS is quite significant (PC RI = 1.585, PDMS RI = 1.403), phase domains must be very small to prevent scattering of visible light. As illustrated in Fig. 17, most of the twelve PC-PDMS alternating copolymers produced exhibited very high optical clarity. Only those multiblock copolymers based on the highest molecular weight H-PDMS-H and sample 7KPC(57%)-5.3KPDMS(43%), exhibited haziness. This result suggests the existence of phase domains in the 10 s of nanometer scale or less.

To characterize the phase morphology of the alternating PC-PDMS multiblock copolymers, SAXS was conducted on most of the samples. In addition, for comparison purposes, the commercially available PC-PDMS multiblock copolymer with a random distribution of blocks was also characterized. The scattering results are shown in Fig. 18 and the profiles were vertically shifted to facilitate a comparison of the peak positions. Estimates of average interdomain spacings were calculated using Bragg's law and are listed in Table 5. For the pure PC homopolymer (Fig. 18a), the scattering intensity decreased monotonically with increasing q, which is consistent with a lack of structure (as expected). For the commercial PC-PDMS random multiblock copolymer (Fig. 18a), a broad peak at q = 0.21 [nm.sup.-1] was observed. A diffuse scattering behavior suggests morphological heterogeneity. The heterogeneity may be caused by a large distribution of domain sizes, differences in interdomain ordering, or by compositional variations of the components within the domains (i.e., phase mixing).

For the 3 KPC series of multiblock copolymers (Fig. 18b), 3 KPC (79%)-0.7 KPDMS (21%) showed a weak and very broad scattering peak centered near q = 0.8 [nm.sup.-1]. As the length of PDMS blocks increased, the primary peak became more distinct and shifted to lower q, indicating enhanced phase separation and an increase in the distance between phase-separated domains. For 3 KPC (70%)-1.2 KPDMS (30%), a much sharper peak was observed, which indicates a more uniform distribution of interdomain spacings. For 3 KPC (35%)-5.3 KPDMS (65%), a clear second peak [2q.sub.max]) at q = 0.7 [nm.sup.-1] and a third order peak ([3q.sub.max]) at q = 1 [nm.sup.-1] was observed. This scattering behavior suggests the presence of a highly ordered lamellar structure [43], The SAXS data obtained for the 3 KPC series of copolymers was consistent with the thermal and viscoelastic property data in that the degree of phase mixing decreased with increasing PDMS block molecular weight. For the 5 and 7 KPC series of multiblock copolymers, a similar trend was observed, which further supports that an increase in the molecular weight of PDMS blocks leads to enhanced phase separation between PC and PDMS blocks.

CONCLUSION

Perfectly alternating PC-PDMS multiblock copolymers possessing systematic variations in block lengths were successfully produced using hydrosilylation to couple preformed PC and PDMS telechelic oligomers. Based on GPC results, the high purity of the telechelic oligomer precursors enabled multiblock copolymers to be produced that were essentially free of homo-oligomer precursors. Despite the relatively large difference in solubility parameter between PC and PDMS, significant partial miscibility between the PC and PDMS blocks was observed. As expected, the degree of partial miscibility was highly dependent on block lengths with partial miscibility decreasing with increasing block lengths. With regard to phase morphology, increasing molecular weight of the PDMS block enhanced phase separation between PC and PDMS blocks. Compared to pure PC, the PC-PDMS multiblock copolymers exhibited significant char formation during thermal decomposition, as indicated by TGA, and the amount of char generally increased with increasing PDMS content.

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Partha Majumdar, (1) Andrey Chernykh, (1) Hanzhen Bao, (2) Elizabeth Crowley, (1) Mingqiang Zhang, (3) James Bahr, (1) Michael Weisz, (1) Chad Ulven, (4) Tingting Zhou, (4) Robert B. Moore, (3) Bret J. Chisholm (1,2)

(1) Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, North Dakota, 58102

(2) Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, North Dakota 58102

(3) Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061

(4) Department of Mechanical Engineering, North Dakota State University, Fargo, North Dakota 58102

Correspondence to: Bret J. Chisholm; e-mail: bret.chisholm@ndsu.edu

Contract grant sponsor: Defense Threat Reduction Agency; contract grant number: HDTRA1-09-1-0022; contract grant sponsor: National Science Foundation; contract grant number: DMR-0923107.

DOI 10.1002/pen.23708

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Reagents and their concentrations used for the production of
the array of 12 different PC-PDMS alternating multiblock copolymers.

                                                        H-PDMS-H
Block copolymer             E-PC-E Mn     E-PC-E       [M.sub.n]
designation                  (g/mole)    wt. (g)    (g [mol.sup.-1])

3KPC(79%)-0.7KPDMS(21%)        3070       15.81           740
3KPC(70%)-1,2KPDMS(30%)        3070       14.02           1190
3KPC(35%)-5.3KPDMS(65%)        3070        6.90           5300
3KPC( 18%)-12KPDMS(82%)        3070        3.67          12,420
5KPC(86%)-0.7KPDMS( 14%)       4810       17.10           740
5KPC(79%)-1,2KPDMS(21 %)       4810       15.72           1190
5KPC(46%)-5.3KPDMS(54%)        4810        9.04           5300
5KPC(26%)-12KPDMS(74%)         4810        5.21          12,420
7KPC(91 %)-0.7 KPDMS(9%)       7240       17.98           740
7KPC(85%)-1,2KPDMS( 15%)       7240       16.94           1190
7KPC(57%)-5.3KPDMS(43%)        7240       11.08           5300
7KPC(35%)-12KPDMS(65%)         7240        6.93          12,420

Block copolymer              H-PDMS-H    Wt. Pt[O.sub.2]
designation                  wt. (g)      (a) disp. (g)

3KPC(79%)-0.7KPDMS(21%)        4.19           11.69
3KPC(70%)-1,2KPDMS(30%)        5.98           10.37
3KPC(35%)-5.3KPDMS(65%)       13.10           5.10
3KPC( 18%)-12KPDMS(82%)       16.33           2.71
5KPC(86%)-0.7KPDMS( 14%)       2.90           8.07
5KPC(79%)-1,2KPDMS(21 %)       4.28           7.42
5KPC(46%)-5.3KPDMS(54%)       10.96           4.27
5KPC(26%)-12KPDMS(74%)        14.79           2.46
7KPC(91 %)-0.7 KPDMS(9%)       2.02           5.64
7KPC(85%)-1,2KPDMS( 15%)       3.06           5.31
7KPC(57%)-5.3KPDMS(43%)        8.92           3.47
7KPC(35%)-12KPDMS(65%)        13.07           2.17

Each reaction mixture also contained 80 g of TCE.

(a) 0.1 wt% Pt[O.sub.2] dispersion in TCE.

TABLE 2. Description of the PC-PDMS alternating multiblock copolymers
produced.

Sample designation          PC Block           PC            PDMS
                          [M.sub.n] (a)   content (a)    [M.sub.n] (a)

3KPC(79%)-0.7KPDMS(21%)       3070             79             740
3KPC(70%)-1,2KPDMS(30%)       3070             70            1190
3KPC(35%)-5.3KPDMS(65%)       3070             35            5300
3KPC(18%)-12KPDMS(82%)        3070             18           12,420
5KPC(86%)-0.7KPDMS(14%)       4810             86             740
5KPC(79%)-1,2KPDMS(21%)       4810             79            1190
5KPC(46%)-5.3KPDMS(54%)       4810             46            5300
5KPC(26%)-12KPDMS(74%)        4810             26           12,420
7KPC(91%)-0.7KPDMS(9%)        7240             91             740
7KPC(85%)-1,2KPDMS(15%)       7240             85            1190
7KPC(57%)-5.3KPDMS(43%)       7240             57            5300
7KPC(35%)-12KPDMS(65%)        7240             35           12,420

Sample designation            PDMS          PC-PDMS       PC-PDMS
                          content (a)    [M.sub.n] (b)    MWD (b)

3KPC(79%)-0.7KPDMS(21%)        21           32,500          1.7
3KPC(70%)-1,2KPDMS(30%)        30           33,000          1.8
3KPC(35%)-5.3KPDMS(65%)        65           43,700          1.7
3KPC(18%)-12KPDMS(82%)         82           49,100          1.8
5KPC(86%)-0.7KPDMS(14%)        14           33,500          1.7
5KPC(79%)-1,2KPDMS(21%)        21           39,200          1.6
5KPC(46%)-5.3KPDMS(54%)        54           39,800          1.6
5KPC(26%)-12KPDMS(74%)         74           43,500          1.7
7KPC(91%)-0.7KPDMS(9%)         9            42,800          1.6
7KPC(85%)-1,2KPDMS(15%)        15           45,000          1.5
7KPC(57%)-5.3KPDMS(43%)        43           36,100          1.5
7KPC(35%)-12KPDMS(65%)         65           39,300          1.5

All molecular weights are expressed as g/mole and all contents are
expressed as weight percent.

(a) Determined using [sup.1]H NMR.

(b) Determined using GPC and expressed relative to polystyrene
standards.

TABLE 3. [T.sub.g]s for the E-PC-E and H-PMDS-H precursors and the
PC-PDMS multiblock copolymers obtained from MDSC.

                                                      [T.sub.g] of
Sample                     [T.sub.g] of PC-rich         PDMS-rich
                            phase ([degrees]C)     phase ([degrees]C)

3KPC                                109                    --
3KPC(79%)-0.7KPDMS(21%)       82 [+ or -] 2.0         Not observed
3KPC(70%)-1.2KPDMS(30%)       86 [+ or -] 1.7             -136
3KPC(35%)-5.3KPDMS(65%)       98 [+ or -] 3.0             -125
3KPC(18%)-12KPDMS(82%)         Not observed               -121
5KPC                                122                    --
5KPC(86%)-0.7KPDMS(14%)      101 [+ or -] 2.1           Not obs.
5KPC(79%)-1.2KPDMS(21%)      105 [+ or -] 1.7           Not obs.
5KPC(46%)-5.3KPDMS(54%)      109 [+ or -] 1.2             -126
5KPC(26%)-12KPDMS(74%)         Not observed               -125
7KPC                                134                    --
7KPC(91 %)-0.7KPDMS(9%)      116 [+ or -] 1.4           Not obs.
7KPC(85%)-1.2KPDMS(15%)      117 [+ or -] 2.9           Not obs.
7KPC(57%)-5.3KPDMS(43%)      135 [+ or -] 2.5             -121
7KPC(35%)-12KPDMS(65%)       133 [+ or -] 3.1             -121
0.7KPDMS                            --                    -143
1.2KPDMS                            --                    -139
5.3KPDMS                            --                    -130
12KPDMS                             --                    -128

TABLE 4. Results obtained from TGA analysis using an air analysis using
an air atmosphere.

                            [T.sub.10%]    [T.sub.50%]    [T.sub.775%]
Sample designation          ([degrees]C)   ([degrees]C)   ([degrees]C)

3KPC                            430            514            0.0
3KPC(79%)-0.7KPDMS(21%)         419            468            6.3
3KPC(70%)-1,2KPDMS(30%)         434            513            8.9
3KPC(35%)-5.3KPDMS(65%)         425            512            17.8
3KPC(18%)-12KPDMS(82%)          434            483            16.1
5KPC                            427            499             0
5KPC(86%)-0.7KPDMS(14%)         428            482            6.3
5KPC(79%)-1,2KPDMS(21%)         430            479            10.0
5KPC(46%)-5.3KPDMS(54%)         404            516            26.8
5KPC(26%)-12KPDMS(74%)          409            486            24.0
7KPC                            437            483             0
7KPC(91%)-0.7KPDMS(9%)          427            469            4.4
7KPC(85%)-1.2KPDMS(15%)         429            483            7.4
7KPC(57%)-5.3KPDMS(43%)         416            513            12.9
7KPC(35%)-12KPDMS(65%)          432            507            16.0

[T.sub.10%], [T.sub.50%], and %[R.sub.775] indicate the temperature at
10% weight loss, the temperature at 50% weight loss, and the weight
percent remaining at 775[degrees]C, respectively.

TABLE 5. Interdomain spacings for select polymers.

Sample designation          [q.sub.max]       d (nm)
                           ([nm.sup.-1])
PC
Commercial PC-PDMS             0.22             29
3KPC(79%)-0.7KPDMS(21%)        0.81            7.6
3KPC(70%)-1,2KPDMS(30%)        0.60            10.5
3KPC(35%)-5.3KPDMS(65%)        0.35             18
5KPC(86%)-0.7KPDMS(14%)        0.45             14
5KPC(79%)-1,2KPDMS(21%)        0.45             14
5KPC(46%)-5.3KPDMS(54%)        0.24             26
7KPC(91%)-0.7KPDMS(9%)         0.74            8.5
7KPC(85%)-1,2KPDMS(15%)        0.44             14
7KPC(57%)-5.3KPDMS(43%)        0.24             26

Sample designation         [2q.sub.max]   [3q.sub.max]

PC
Commercial PC-PDMS              --             --
3KPC(79%)-0.7KPDMS(21%)         --             --
3KPC(70%)-1,2KPDMS(30%)         --             --
3KPC(35%)-5.3KPDMS(65%)        0.7             ~1
5KPC(86%)-0.7KPDMS(14%)         --             --
5KPC(79%)-1,2KPDMS(21%)         --             --
5KPC(46%)-5.3KPDMS(54%)        0.5             ~1
7KPC(91%)-0.7KPDMS(9%)          --             --
7KPC(85%)-1,2KPDMS(15%)         --             --
7KPC(57%)-5.3KPDMS(43%)        0.5             ~1
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Author:Majumdar, Partha; Chernykh, Andrey; Bao, Hanzhen; Crowley, Elizabeth; Zhang, Mingqiang; Bahr, James;
Publication:Polymer Engineering and Science
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Date:Jul 1, 2014
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