Systematic calorimetric study of glass transition in the homologous series of poly(n-alkyl methacrylate)s:: Narayanaswamy parameters in the crossover region

The glass transition in poly(n-alkyl methacrylate)s from C=1 (methyl, PMMA) over 2, 3, 4, 5, 6, 7, to C=9 (nonyl) is investigated by differential scanning calorimetry DSC, and partly by temperature modulated DSC, TMDSC. The results are compared with dielectric and heat capacity spectroscopy so far as available. The DSC thermograms are evaluated by a modified Narayanaswamy model MNM that permits a point-by-point determination of the underlying equilibrium trace of the dynamic glass transition in an Arrhenius diagram. The MNM parameters and their variations due to differently given Vogel temperatures are discussed and compared with literature values for polyvinylacetate PVAC and polystyrene PS. It seems not possible at present to determine a definite Narayanaswamy mixing parameter x for a substance as such, e.g. for PVAC and PS, the uncertainty is about Delta x=+/-0.2. For the homologous PnAMA series dramatic changes of x outside the Delta x uncertainty are observed near the crossover region. The results are discussed with respect to cooperativity, fragility, vitrification rate, and further characteristics of the glass transition. (C) 1999 Elsevier Science B.V. All rights reserved.