THE SHORT-TIME REGIME OF SEGMENTAL DYNAMICS OF GLASS-FORMING POLYMERS

The short time regime (10(-13) s less than or equal to t less than or equal to 10-(11) s) of the segmental dynamics of several glass-forming polymers with different chemical microstructure and glass-transition temperature has been investigated by means of time of flight (TOF) neu tron scattering techniques. In this paper we present a summary of the main results obtained as well as the theoretical framework proposed by us to interpret these results. The main conclusion is that the segmen tal dynamics of glass-forming polymers display in the very short time range (t approximate to 2 ps) a crossover from a simple exponential be haviour towards a non-exponential regime. The first exponential decay, which is controlled by C-C rotational barriers, can be understood as a trace of the behaviour of the system in absence of the effects (corr elations, cooperativity, memory effects...) which characterise the den se supercooled liquid like state against the normal liquid state. The non-exponential regime at t > 2 ps corresponds to what is usually unde rstood as alpha and beta relaxations. Some implications of these resul ts are also discussed.