State of the Theory of Iso-free Volume and Glass Formation in Amorphous Polymers

Methods of determining the free volume fraction of polymers at the glass formation temperature and the concept according to which all polymers have a universal free volume fraction at the glass point are considered critically. The concept of iso-free volume in glass formation can be regarded only as a rough approximation and the free volume fraction is a function of several variables including the flexibility of the polymeric chain and the nature of the molecular packing. The development of the theory of glass formation by amorphous polymers on the basis of the free volume concept is considered and it is shown that a free volume distribution function has to be introduced for the correct description of the process. Conclusions are drawn concerning the inapplicability of the iso-free volume concept for describing glass formation processes in amorphous polymers. The possibilities of a more adequate description of glass formation are discussed. 69 references.