Glassy transitions in semicrystalline polymers

Abstract

AbstractConsiderable confusion exists in the literature concerning the glass temperature, Tg, of many highly crystalline polymers, in particular, linear polyethylene (PE), poly(vinyl fluoride) (PVF), poly(vinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), poly(oxymethylene) (POM), and poly(ethylene oxide) (PEO). We attempt to resolve some of this confusion first, by defining the nature of the problem; second, by developing criteria for defining the locations along a relative temperature scale (T/Tg or T/TM) of the key relaxations and transitions, Tg, T < Tg, TM, and Tαc for C‐C and CO backbone polymers free from long alkyl side groups; third, by developing a set of typical mechanical relaxation spectra for amorphous, semicrystalline, and single crystal materials of known Tg; fourth, by defining the concept of a double glass transition with values Tg(L) and Tg(U) and the optimum conditions for observing same. Finally, we apply these concepts and criteria to resolving the value of Tg for PVF and PVDF. Criteria employed include ratios of relaxation temperatures such as (T < Tg)/Tg, Tαc/TM, and Tg/TM, typical activation enthalpies for various relaxation processes, thermal expansion and heat capacity data. Two theoretical subjects are explored: the effect of crystallinity on increasing Tg and the effect of crystallinity on the relative strengths of the Tg and T < Tg processes. We also employ a “comparative anatomy” technique of comparing temperatures and related activation enthalpies of the several relaxations and transitions for a series of closely related polymers such as PE, PVF, PVDF, PTFE, Polytrifluorochloroethylene, and poly(vinyl chloride). We suggest that a new scientific discipline of interpreting the relaxation spectra of crystalline polymers may be emerging through integration of a large body of literature, the refinement of old criteria, and the development of new criteria. Even so, we are not yet able to offer a satisfactory interpretation of the relaxation spectra for POM and PEO.