Crystallization kinetics and morphology of polymer blends of poly(tetramethyl- p-silphenylene siloxane) fractions

Abstract

The crystallization behaviour of polymer blends or mixtures of the same system has been studied over a wide range of molecular weight and crystallization temperatures. Blends were made by mixing fractionated polymer samples. The spherulitic growth in these mixtures is dependent upon the number-average molecular weight of the system at the shorter chain lengths, but then becomes insensitive to molecular weight values when about 10 5 to 10 6 are reached. The growth rate kinetics of mixtures can be described by a kinetic model used for fractionated poly(tetramethyl- p-silphenylene siloxane) (TMPS) polymers. The crystal surface energies deduced from these rate data are molecular weight dependent as are the pre-exponential and transport factors in the rate equation. These parameters are explained in terms of the crystallite morphology. Mixtures (as well as fractions themselves) of all polymer fractions ranging from the monomer to the highest molecular weight (10 6 approximately) have similar morphological features and form negatively birefringent spherulites. Although molecular weight segregation appears to play an important role in crystallization at comparatively small undercoolings, its influence seems to be minimal at large undercoolings (close to or below the growth rate maxima). Very low molecular weight additives significantly affect the overall crystallization kinetics. Compared to the undiluted sample, mixtures so formed have lower observed melting points and glass transition temperatures. Rates of crystallization are generally facilitated by the diluent with the peak in the growth rate being displaced to lower temperatures. The growth rates for diluted over the undiluted polymer at similar undercoolings are usually larger. At high molecular weights the log of the spherulitic growth rate varies as M −1 2 n , over a considerable range, but at low molecular weight values the rate depends more strongly on M n approaching a limit of M −1.2 n as the monomeric state is approached.