Polymer-plasticizer blends, glass transition

The homopolymers poly(methyl methacrylate) and poly-(ethyl methacrylate) are compatible with poly(vinylidene fluoride) when blended in the melt. True molecular com-patibility is indicated by their transparency and a single, intermediate glass transition temperature for the blends. The Tg results indicate plasticization of the glassy methacrylate polymers by amorphous poly(vinylidene fluoride). The Tg of PVdF is consistent with the variation of Tg with composition in both the PMMA-PVdF and PEMA-PVdF blends when Tg is plotted vs. volume fraction of each component. PEMA/PVdF blends are stable, amorphous systems up to at least 1 PVdF/I PEMA on a weight basis. PMMA/ blends are subject to crystallization of the PVdF component with more than 0.5 PVdF/1 PMMA by weight. This is an unexpected result. 

Mixtures of poly(vinylidene fluoride) with poly (methyl methacrylate) and with poly (ethyl methacrylate) form compatible blends. As evidence of compatibility, single glass transition temperatures are observed for the mixtures, and transparency is observed over a broad range of composition. These criteria, in combination, are acceptable evidence for true molecular intermixing (1, 19). These systems are particularly interesting in view of Bohns (1) review, in which he concludes that a compatible mixture of one crystalline polymer with any other polymer is unlikely except in the remotely possible case of mixed crystal formation. In the present case, the crystalline PVdF is effectively dissolved into the amorphous methacrylate polymer melt, and the dissolved, now amorphous, PVdF behaves as a plasticizer for the glassy methacrylate polymers. 

A conducting, polymeric film of poly(indole-5-carboxylic acid) has been employed for covalent immobilization of tyrosinase, which retains catalytic activity and catalyzes oxidation of catechol to the quinone <2006MI41>. Poly(l-vinylpyrrole), polyfl-vinylindole), and some methyl-substituted compounds of poly(l-vinylindole) are of potential interest as photorefractive materials with a relatively low glass-transition temperature and requiring a lower quantity of plasticizer in the final photorefractive blend <2001MI253>. Polymers of 5,6-dihydroxyindoles fall within the peculiar class of pigments known as eumelanins and their chemistry has been reviewed <2005AHC(89)1>. 

The results of DSC analyses of freeze-dried plum (skin and pulp at the natural proportion) presented different behaviors for each domain. At Uy, 0.75, two glass transitions (Tg) were visible (Figure 58.1a) as a deviation in base line and shifted toward lower temperatures with increasing moisture content and caused by the plasticizing effect of water (Slade and Levine, 1991). The first one, clearly visible at lower temperatures, was attributed to the glass transition of a matrix formed by sugars and water. The second one, less visible and less plasticized by water, was probably caused by macromolecules of the fruit pulp. Two Tg are normally visible in systems formed by blends of polymers (Verghoogt et al., 1994) and in edible films (Sobral et al., 2002) caused by phase separation between polymers and between proteins and plasticizers, respectively. However, Sobral et al. (2001) and Telis and Sobral (2002) also observed two Tg for persimmon and tomato, respectively, at low domain. 

The second example considers a blend formed by LDPE, with 30% crystallinity, and PVC. The polymer matrices examined are pure LDPE, the blends LDPE (80%)-PVC (20%) and LDPE (50%)-PVC (50%), and pure PVC, with toluene as the penetrant. Experimental data by Markevich etalS report solubiUty of toluene in the above blends, at the temperature of 30° C, while toluene solubiUty in pure PVC was taken from Berenst l The glassy transition temperature is equal to —25° C for LDPE and to +75° C for pure PVC. Therefore, pure PVC is a glass at 30° C however, due to the large swelling and plasticization of the polymer induced by toluene sorption, it can be seen that the sorption of toluene lowers the glass transition of PVC to temperatures below 30° C, already at relatively low toluene activities. That is also confirmed by the sorption isotherm which is concave to the concentration axis as is typical of rubbery polymers. The glass transition temperatures for the blends are estimated to be — 10°C for the 80% LDPE blend and +17° C for the 50% LDPE blend, all below the temperature of the sorption experiment. The crystalline fraction of LDPE is assumed, as is usual, not to contribute to the sorption process, therefore we consider only the amorphous fraction of LDPE in the sorption calculations based on EoS. For the sake of simplicity, we present here only the results obtained with the LF equilibrium model. 

Here the primary interest is in the plasticity of single-component glassy polymers well below the glass-transition temperature. We consider no heterogeneous blends and multi-component polymers. Some consideration of such polymers for purposes of toughening is deferred to Chapter 13. 

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