Chemical composition distribution of binary and multicomponent copolymers.

摘要

Chemical composition distribution (CCD) of copolymers describes the distribution of average comonomer content among copolymer chains. This information is very important as CCD can significantly influence physical properties of copolymers. The main goal of this study is to further our understanding of the CCD of copolymers.; The present understanding of CCD of multicomponent copolymers is very limited and there are no analytical equations capable of describing this distribution. In the present study, analytical expressions for describing the CCDs of random multicomponent copolymers are developed using a statistical approach. The results from this theoretical expression are validated with the results from Stockmayer's distribution and Monte Carlo simulations for limiting cases.; In the case of semi-crystalline binary copolymers, temperature rising elution fractionation (Tref) and crystallization analysis fractionation (Crystat) can be used for the estimation of CCD. The effects of chain microstructure and operating conditions on these techniques are investigated using a series of ethylene/alpha-olefin copolymers with well-defined microstructures. Both molecular weight and comonomer content are shown to significantly affect the shape of Crystaf profiles. Fortunately, comonomer content is the main determining factor for Crystaf peak temperature, permitting the use reliable calibration curves for this technique. A Crystaf model based on the average ethylene sequence distribution in the chains is proposed and the results from this model show very good agreement with the present and previously reported experimental data.; The typical operating conditions used in Tref and Crystaf analyses are found to lead to fractionation conditions that are far from the thermodynamic equilibrium. Consequently, crystallization kinetics can strongly influence the CCD estimated with Tref and Crystaf. Sample cocrystallization during analysis is also considered as it may interfere with the fractionation processes of both techniques. The similarity of chain crystallizabilities and fast cooling rates are found to promote cocrystallization. Cocrystallization under a certain conditions can be so strong that it can seriously affect the interpretation of the CCD measured with Crystaf and Tref.