SynopsisGelation and melting of aqueous gelatin were investigated by differential scanning calorimetry. This phenomenon can be analyzed as a conventional crystallization process assuming predetermined primary nucleation and unidirectional growth. The results were interpreted in terms of the fringed micelle model. Calculated values of the diameter of the renatured collagen fibril were found in excellent agreement with those determined previously by electron microscopy.
SynopsisThe presence of organic acid salts in bisphenol-A polycarbonate (PC) completely modifies the crystallization mechanism, the melting behavior, and the morphology of the polymer. Organic salts are not ordinary nucleating agents for PC since they react with the polymer, producing metal phenoxide chain ends. On reaction, abundant instaneous nucleation is induced. The seeds are likely to be polymer crystalline fragments preexisting in the melt. The phenoxide chain ends significantly increase the growth rate of the crystalline phase. Melting points and enthalpies of fusion are unusually high, suggesting a high degree of crystalline perfection. Thick multilamellar crystals, which are likely to contain chains in extended configuration, are observed by electron microscopy. No trace of spherulitic morphology is found. The chemical instability of PC containing ionic chain ends is also shown to seriously affect the crystallization rate, the maximum degree of crystallinity, and the melting point.
A kinetic study of the thermal degradation in the melt of bisphenol-A polycarbonate in the presence of various anhydrous alkali metal arylcarboxylates is undertaken in order to assess the validity of the reaction mechanism previously established on a model system. The influence of the temperature, the concentration and the nature of the salt is studied. The results confirm the validity of the proposed kinetic model and suggest minor modifications to the reaction mechanism.
In the presence of a sodium arylcarboxylate or arylphenoxide, bisphenol‐A polycarbonate (PC) undergoes complex chemical modifications at high temperatures. The reaction mechanism is similar to the one previously established for model systems. Initially, the salt reacts with the carbonate groups of the polymer. This lowers the number‐average molecular weight and produces ionic chain ends of the phenoxide type. A fast transesterification reaction is then induced by a continuous exchange between the phenoxide and the carbonate groups, affecting the molecular distribution until an equilibrium is attained. In the presence of CO2, the phenoxide‐terminated PC undergoes further chemical modifications (formation of phenyl salicylate and phenyl phenoxybenzoate groups) leading to progressive crosslinking of the polymer.
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