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Competition between phase separation and crystallization in polyolefin blends
In this thesis, the crystallization and melting of blends of high and low density homogeneous ethylene-1-octene copolymers with appended long chain branches have been investigated in real time by means of time-resolved SALS under cross-polarized and parallel-polarized optical alignments using a charge-coupled device camera (CCD) system, simultaneous small angle-X-ray and wide angle X-ray measurements using synchrotron radiation and differential scanning calorimetry (DSC). For the highest density material studied, our data show that in the case of crystallization at low supercooling, spherulitic growth (primary crystallization) occurs first while the apparent degree of crystallinity is less than 2%. Over 90% of the crystallinity develops after the primary crystallization process. When the average branch content of the blend is 14 branches per 1000 carbons complete spherulites are observed. The internal spherulite disorder is unchanged relative to that obtained for the high density component. When the average branch content of the blend is increased to 58 branches per 1000 carbons, the crystallization rate is faster than that of the moderate increase but the morphology suggested from SALS is consistent with incomplete spherulites. For this case, our data suggest that the domain sizes resulting from the incipient melt phase separation is the likely cause of the accelerated crystal growth. We propose that the incomplete spherulites formed may be a consequence of the competition between amorphous phase separation in the residual melt and crystallization. The results of a numerical study investigating the dynamics of spinodal decomposition in blends of linear Gaussian chains in three dimensions were used to evaluate the effect of branch content on the phase separation kinetics for mixtures of linear Gaussian chains and branched chains. The phase separation kinetics in the branched systems are identical to that of linear mixtures with larger domain sizes. Hence, it may not be possible to detect differences in the time dependence of the structure factor and domain size from scattering measurements if one varies the branch content of the blend. Blends of linear and branched polymers can be treated as blends of linear Gaussian chains even when the branch content is very high.
Akpalu, Yvonne A, "Competition between phase separation and crystallization in polyolefin blends" (1998). Doctoral Dissertations Available from Proquest. AAI9823713.