Physical properties of poly (n-alkyl acrylate) copolymers
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The physical properties of n-alkyl acrylate copolymers, including thermal characteristics, structure as determined by small angle X-ray scattering, and gas permeability as a function of temperature, were examined in detail and compared to the corresponding homopolymers. Two types of copolymers were examined: those with two crystalline comonomers and those with one crystalline and one non-crystalline comonomer. The crystalline / crystalline copolymers exhibit co-crystallization and, thus, for a given average side-chain length have comparable melting temperatures as the corresponding homopolymers. For a given side-chain length, the copolymers have somewhat lower heats of fusion than the corresponding homopolymers because of a reduction in crystallite size as revealed by SAXS. The crystalline / non-crystalline copolymers do not co-crystallize and experience melting point depression in which the non-crystalline comonomer does not affect the Tm and ∆Hf as much as two crystalline comonomers do. Though not entering the lattice, the non-crystalline comonomers impede the formation of perfect crystals, also reducing the crystallite size, as indicated by SAXS. This depression in crystallinity is reflected in the permeability data for the copolymers. Poly (n-alkyl acrylates) exhibit a ‘jump’ in their gas permeability at the Tm of the side-chain lengths that is mainly caused by a switch in the side-chain morphology from crystalline to amorphous upon melting. The depression in crystallinity for both types of copolymers results in a smaller permeation jump. Interestingly, copolymers containing A10, a comonomer on the border of being crystalline, experience the broadest jump peak. The jump breadth of all copolymers examined correlate with the melting endotherms for these polymers as determined by DSC. Ultimately, the melting endotherms for these copolymer systems provides an excellent tool for predicting permeability changes across the melting region.