Linearity of heat capacity step near the onset of α glass transition in poly(n‐alkylmethacrylate)s

DSC measurements around the glass transition in the homologous series of seven poly(n‐alkylmethacrylate)s (alkyl=methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl) are presented. The step height Δcp decreases linearly with the number of n‐alkyl C‐atoms in the side chain. The usual glass step in the DSC curves, e. g. for poly(methylmethacrylate), degenerates to a bend near poly(n‐octylmethacrylate). This finding is discussed as a result of a linear relation between the caloric relaxation strength Δcp and the temperature difference from the α onset Tonset in the splitting region of poly(n‐alkylmethacrylate)s, with Δcp → 0 for T → Tonset. The number of monomeric units per cooperatively rearranging region at Tg decreases from PMMA (Nα∼20) to PnOMA (Nα<5). This demonstrates that the cooperativity at the α onset in poly(n‐alkylmethacrylate)s is very small, as expected from a fluctuation approach to the glass transition.