Poly crystallization from dimethyl

Also the polymorphic behavior of s-PS can be altered by blending, in particular with poly-2,6-dimethyl-l,4-phenylene oxide (PPO), both for the case of crystallization from the melt [104] and for the case of crystallization from the quenched amorphous phase [105]. 

The morphology and mechanical properties of PVA were studied using atactic poly(vinyl alcohol) (at-PVA) dry gel films prepared by crystallization from solutions in dimethyl sulfoxide and water mixtures. The NMR measurements suggested changes in the hydrogen bonds. ... 

Silphenylene-Siloxane Copolymers.—The thermal properties vctsus structure for poly(tetramethyl-p-silphenylensesiloxane) and (tetramethyl-p-silphenylene/di-methylsiloxane) block copolymers have been compared. The homopolymer has a m.p. of 160°C, heat of fusion of 54.4 J/g and Tg of —20°C. The Tg of the copolymer varies monotonically with inojeased dimethylsiloxane content, from — 20 to —123 °C. Data have been reported on the crystallization kinetics and morphology of blends of fractionated poly(tetramethyl-p-silphenylenesiloxanes). The chemical degradation of poly(tetramethyl-/ -silphenylenesiloxane/dimethyl-siloxane) block copolymers by HF has been reported. In 48% HF at 30 °C, preferential attack occurs at the Si—O bond, particularly those of the MejSi—O non-crystalline components, in copolymers containing 15, 35, and 52% poly-dimethylsiloxane. - Further data have been reported on the crystal structure and fold conformation of poly(tetramethyl-/>-silphenylenesiloxane)s, obtained from X-ray diffraction studies. ... 

Thermoreversible gelation, as a consequence of crystallization from dilute solutions of random copolymers has been observed in a variety of mixtures. These include, among others, poly(vinyl chloride) in dioctyl phthalate,(55) poly-(acrylonitrile) in dimethyl formamide,(56) nitrocellulose in ethyl alcohol,(272) methyl cellulose in water,(273) ethylene copolymers,(274) syndiotactic isotactic and atactic poly(styrene),(275-279) and random copolymers of ethylene terephtha-late with isophthalate.(280) Flory and Garrett (281) have shown that the classical thermoreversible gelation system, gelatin in water, is the result of a crystal-liquid transformation. The gelation or dissolution can be treated as a first-order phase transition. 

The effects of molecular orientation on the crystallization and polymorphic behavior of SPS and SPS/poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) blends were studied with wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry [37]. The oriented amorphous films of SPS and SPS/ PPO blends were crystallized under constraint at crystallization temperatures ranging from 140 to 240 °C. The degree of crystallinity was lower in the cold-crystallized oriented film than in the cold-crystallized isotropic film. It was inferred that the oriented mesophase was obtained in drawn films of SPS and that the crystallization of SPS was suppressed in that phase. The WAXD measurements showed that the crystal phase was more ordered in SPS/PPO blend than in pure SPS under the same annealing conditions. It was principally due to the decrease in the mesophase content. The crystal forms were found to be dependent on the crystallization temperature, blend composition, and... 

Other polymers, as typified by poly(3,3-dimethyl thietane), show maxima in the overall crystallization rate.(218) The data can be analyzed in a manner comparable to that described for spherulite growth rates and the conclusions are the same. Whether a transition from Regime II to III is discerned depends on the values taken for the Vogel constants. 

An excellent example of the role played by liquid-liquid phase separation in the ensuing crystallization is found in blends with syndiotactic poly(styrene).(77) Measurements of the glass temperature in mixtures with poly(2,6-dimethyl-l,4-diphenylene oxide) (PPO) indicate that the components are miscible in all proportions in the melt. However, mixtures of syndiotactic poly(styrene) with poly(vinyl methyl ether) represent partially miscible blends. When the poly(vinyl methyl ether) content exceeds 20% by weight, the melt separates into two liquid phases, one rich in syndiotactic poly(styrene), the other in poly(vinyl methyl ether). Thus, the two blends have a common crystallizing component. However, in one the crystallization takes place from a homogeneous melt in the other from one that is phase separated. The different melt structures profoundly affect the crystaflization kinetics. This can be seen when a comparison is made between the crystallization kinetics of syndiotactic poly(styrene) from a homogeneous or phase separated melt. 

Fig. 13.26 Plot of log G against crystallization temperature for isotactic poly (styrene) crystallizing from a 0.1% dimethyl phthalate solution. (From Tanzawa (66))...

An analysis of the overall crystallization rate with composition requires that the comparison be made either at constant undercooling or at one of the nucleation temperature quantities, T / T AT or T /T(AT). This requirement is essential because of the importance of nucleation to the crystallization process. The overall crystallization kinetics of a variety of polymer-diluent systems have been reported. Many different relations between the overall crystallization rate and composition have been observed. For example, as is shown in Fig. 13.17 there is a continuous decrease in the crystallization rate with dilution for linear polyethylene-a-chloronaphthalene mixtures.(42) The results for poly(trans-1,4-isoprene) in methyl oleate follow a similar pattem.(80) In contrast, the rates for poly(dimethyl siloxane) crystallizing from toluene, at compositions V2 = 0.32 to 0.79, are the same at all undercoolings, but are faster than that of the pure polymer.(78) Another example is found with poly(ethylene oxide)-diphenyl ether mixtures.(77) In this case the crystallization rates for the pure polymer and composition = 0.92 to 0.51 are the same. However, the rates for the more dilute mixtures, V2 = 0.04 and 0.30 are lower. For poly(decamethylene adipate)-dimethyl formamide mixture the rates for the pure polymer and V2 = 0.80 are the same.(77) The mixture of isotactic poly(propylene) with dotricontane shows interesting behavior.(81) At all undercoolings studied, the crystallization rate initially decreases with dilution, reaches a minimum in the range V2 — 0.7 (a maximum in ti/2) and then slowly increases with further dilution, up to V2 = 0.10. 

Polymers that show a rate maximum with respect to temperature in the pure state do so also when crystallizing from diluent mixtures.(42a,67,88) Two examples are shown in Figs. 13.32 and 13.33 for isotactic poly(styrene) crystallizing from ether benzophenone or dimethyl phthalate respectively.(42a,67) Characteristically, the addition of the diluent causes a shift of the crystallization range to lower temperatures. A similar effect was observed with bisphenol-A poly(carbonate).(88) In addition, the growth rate maximum increases with the initial addition of diluent. This phenomenon is observed up to about 20% diluent in the case of benzophenone (Fig. 13.32) and about 50% with dimethyl phthalate (Fig. 13.33). A similar pattern is also indicated for the poly(carbonate)-diluent mixture.(89) With further additions of diluent there is a continuous decrease in the growth maxima up to very dilute... 

Fig. 13.33 Plot of spherulite growth rates of isotactic poly(styrene) crystallizing from its mixtures with dimethyl phthalate against the crystallization temperature. Weight percent polymer o pure polymer A 50% V 30% 0.1%. (From Miyamoto et al. (67))...

Gels can be made from a number of polymers, including poly[acrylamide-5 fflf-(acrylic acid)], poly(vinyl acetate), poly(dimethyl sUoxane), and polyiso-cyanurates. The poly[acrylamide-5 tor-(acrylic acid)] hydrogels can be made to swell up to 20,000 times, v/v. Thermoreversible gels can be prepared in organic solvents from polyethylene and i-polystyrene, both of which crystallize on cooling but go back in solution on heating. 

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