Heavily branched/linear polyolefin blends: Synthesis, thermodynamics, and dynamics.

摘要

Polymer properties and applications are significantly influenced by chain branching. The objective of this dissertation is to explore the long chain branching effect on thermodynamic phase behavior and dynamic properties of branched/linear polyolefin blends.; We successfully synthesized well-defined model linear and branched polyolefins, i.e., poly(ethylene-r-ethylethylene) random copolymers with different percentage of ethylethylene units.; We have employed small angle neutron scattering (SANS) to examine comb/linear blends where the only difference is their branching architecture. We have found that the major contribution to the interaction parameter χ is entropic in origin, due to architectural effects, and architectural asymmetry alone is sufficient to induce phase separation. We also successfully predict these results qualitatively using modified mean field theory.; The next question we have studied is how various factors influence the phase behavior of comb/linear blends. These factors include heat of mixing, arm length difference and temperature. We observed that differences in short chain branching produce enthalpic contributions while long chain branching results in an excess entropy of mixing. Both effects increase the magnitude of the χ parameter, and either can induce phase separation between linear and branched polyolefins. We also found that there exists an inverse relation between χ and radius of gyration of the arm R_M, suggested by the modified mean field theory, although the suggested −3 power seems to be too strong. Temperature study of miscible comb/linear blends gave evidence that χ has almost no dependence on temperature and hence is truly entropic in origin.; Dynamic mechanical spectroscopy has been used to explore the dynamic properties of linear, branched polyolefins and their blends. We have observed that long chain branching introduced broadened and slower relaxation spectra, the longest relaxation times and viscosities seemed to correlate with the longest end-to-end linear length, the number of branches only had a small effect on the rheological properties, and the miscible comb/linear blends behaved conventionally and can be characterized by typical linear/linear mixing rules. Furthermore, the modified reptation theory for comb polymer cannot describe these model polyolefins very well when the branches are not well entangled.