Systematic calorimetric study of glass transition in the homologous seriesof 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 scan ning calorimetry DSC, and partly by temperature modulated DSC, TMDSC. The r esults are compared with dielectric and heat capacity spectroscopy so far a s available. The DSC thermograms are evaluated by a modified Narayanaswamy model MNM that permits a point-by-point determination of the underlying equ ilibrium trace of the dynamic glass transition in an Arrhenius diagram. The MNM parameters and their variations due to differently given Vogel tempera tures are discussed and compared with literature values for polyvinylacetat e 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 P nAMA series dramatic changes of x outside the Delta x uncertainty are obser ved near the crossover region. The results are discussed with respect to co operativity, fragility, vitrification rate, and further characteristics of the glass transition. (C) 1999 Elsevier Science B.V. All rights reserved.