Side-chain crystallinity. III. Influence of side-chain crystallinity on the glass transition temperatures of selected copolymers incorporating n-octadecyl acrylate or vinyl stearate

The influence of side‐chain crystallinity on the glass transition temperatures of selected copolymers was investigated. The copolymers were selected, in part, from those whose crystallinity was treated in the preceding paper. These included the lower amorphous acrylate esters, such as methyl, ethyl, n ‐butyl, and 2‐ethylhexyl acrylates, together with methyl methacrylate and acrylonitrile, each copolymerized with n ‐octadecyl acrylate over the range of composition. The decline in the glass transition temperature was linear with increasing weight fraction of n ‐octadecyl acrylate for all systems in the composition range where the copolymers were essentially amorphous. The extrapolated T g for the amorphous state of poly( n ‐octadecyl acrylate), and for amorphous poly(oleyl acrylate), was close to −111°C. This coincided with a value previously obtained by an extrapolation of data on homologs. Beyond a critical fraction of octadecyl acrylate (0.3 to 0.5), developing side‐chain crystallinity in n ‐octadecyl acrylate raised the glass temperature steadily for all systems, up to a value of 17‐C, obtained for the crystalline homopolymer. Crystallinity did not develop in stiff copolymers until T g was about 30°C below the melting point of the most perfect crystals. In compositionally heterogeneous copolymers incorporating vinyl stearate, blocks of crystalline units appeared to be dispersed in a glassy matrix of amorphous co‐units. An empirical equation was derived which fitted the experimental data for random copolymers, over all composition ranges, with fair accuracy.