Rheological Study On Mesophase Separation And Crystallization Of Olefin Mulitiblock Copolymer

Chain shuttling polymerization enables an efficient production of ethylene-octene multiblock copolymers(OBCs). These OBCs are statistically coupled linear multiblock copolymers of alternating soft segments(high octene content) and hard segments(low octene content). The soft segment is amorphous blocks having a low glass temperature, while the hard segment is semicrystalline blocks having a high melting point. This new olefin-based block architecture imparts excellent elastomeric properties at higher temperatures than conventional random copolymer of same densities. Most previous studies on OBC have focused on the polymer with high octene content, but the OBCs with low octene content are more practically used as thermoplastic elastomer. These work had been focused on the relationship between mesophase separation and crystallization of OBCs. However, the direct inspection of the melt structure for OBCs is only carried out by Park et al. using rheology as a main tool. So far, the relationships between the melt phase behaviors, the crystallization behaviors and the rheological properties for OBCs remain to be further understood. In this thesis, several OBCs with low ΔC8 and low Mw were selectively chosen to investigate their rheological behaviors during the melt phase transitions and the crystallization, and the correlation between the phase behaviors and the rheological properties had been established. The main investigation and the conclusion in this thesis are summarized as follows.(1) The chain structure, thermal properties, crystallization morphology and the melt structure of OBCs had been characterized by 13 C NMR, differential scanning calorimetric(DSC), polarized optical microscopy(POM) small-angle X-ray scattering(SAXS) and atomic force microscopic(AFM). The first order Markovian statistical model was adopted to determine the hard block content and the soft block content, and then the octene content in hard and soft block were calculated. With the increase of hard block content and molecular weight, the crystallization behavior are found to be different from that in random copolymer, which can be correlated to the mesophase separation in OBC melts. The occurrence of mesophase separation in OBCs systems with low octene content is further justified by AFM characterization.(2) The melt phase behaviors of OBCs with different block structure had been investigated by rheology. Three rheological methods, namely the deviation of the scaling dependence of zero shear viscosity on molecular weight, the terminal behavior and the failure of time temperature superposition(TTS), and two-dimensional rheological correlation spectrum, are used to reveal the mesophase separation with increasing sensitivity. The occurrence of mesophase separation transitions(MST) without longrange order were observed in such low octene content and low molecular weight OBC systems, with much lower degree of segregation than the theoretical predictions in diblock copolymers. The phase diagram for the multiblock OBCs had been constructed. The extent of mesophase separation is further justified by its effect on subsequent nonisothermal crystallization behaviors.(3) Based on the liquid-solid transition theory, the physical gelation process of OBCs during early stages of crystallization are monitored by the cyclic frequency sweep at low undercooling to understand the effect of mesophase separation on the gel network formation mechanism. According to winter’s framework, the critical gel point for OBCs had been determined at various temperatures. It was demonstrated that the critical gel properties at low undercoolings for OBCs deviated from the traditional linear relationship in homogeneous systems that the log of gel strength with relaxation exponent can form a linear master curve, suggesting that the contribution of mesophase separation on critical gel behaviors can be deduced individually from the curve. The relaxation time determined for the gel network of OBCs can reflect the aggregation state of gel network formed from OBCs with different melt structure. The crystallinity at the gel point and the crystallization morphology had been investigated to illustrate the different crystallization pathway for OBCs. Then, the gel network formation mechanism had been proposed for OBCs. For weakly segregated OBCs, the minimum small crystallines can be easily reached and enough to act as junctions linking together several neighboring chains to form a spanning network. For strongly segregated OBCs, the aggregation of the crystalline in the isolated microdomains must approach a certain amount and sufficiently interconnected, thus, generating a relatively hard gel to link the surrounding soft blocks.(4) The isothermal crystallization of OBCs at large undercoolings had been investigated by the multiwave dynamic sweep to investigate the effect of mesophase separation on the aggregation structure of the crystallines detected within high frequency range. According to the strain rate amplification factor da and the stress amplification factorsa, a two-step shifting(horizontal shift with factor and vertical shift with factor) is used to construct the dynamic modulus master curve during isothermal crystallization. The vertical shift factor had been adopted to determine the crystallization fractions with the isothermal crystallization time, which had been verified by the DSC measurements. Then, the aggregation contributions of the crystallines can be readily determined from the difference between the moduli at a crystallization time and the hydrodynamic contributions(the moduli in the beginning of crystallization). We can establish a correlation between the crystallinity and the aggregation contributions of the crystallines. With the same crystallinity, the weakly segregated OBCs had a higher aggregation contributions to the dynamic moduli than that in strongly segregated OBCs, which are in consistent with the different crystallization pathway of OBCs with different extent of mesophase separation.(5) The chain structure of the random ethylene/α- olefin random copolymer and the multiblock OBCs had been investigated by the thermal fraction method. It was found that OBCs had a narrow lamella thickness distribution and crystallizable ethylene sequence length distribution. The multiple melting behaviors of OBCs had been investigated by DSC during the heating scans after the isothermal crystallization at various temperatures. Three melting peaks had been observed, which had been found to be correlated to the melting of the double lamella population with different lamella thickness or thermal stability, and the melting-recrystallization-remelting events. In addition, it was found that he strongly segregated OBC system had a higher equilibrium melting point than that in weakly segregated OBC system. According to the calculated short soft blocks content dissolved in the hard block microdomains, the weakly segregated OBC system had a much higher content of soft blocks content dissolved in the hard block microdomains leading to the much lower equilibrium melting point.