Rheological responses of polymer materials with different architectures
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Polymer rheology is a sensitive method to determine the change in the molecular chemical structure, size, architecture, and composition for polymer materials. In this dissertation, we focus on the influence of the molecular architecture and composition to the rheological responses of some polymer with unique architecture. The first part of the work is to characterize the macrocyclic polymer materials prepared by the Ring Expansion Metathesis Polymerization (REMP) method. In this work, we tried different methods to get pure ring samples, examine their viscoelastic properties and compare the results with theoretical predictions. Four kinds of polymer rings (polyoctenamer, polyammonium, dendronized and wedge ring) were synthesized using two catalysts UC5 and UC6. However, limited by the reaction mechanism of the catalyst, we still can’t get 100% pure ring polymer with current reaction system. It is found that for monomers with simple structures, the samples prepared by the two catalysts contain at least 20% of linear chain contamination. Even we change the structure of the monomer and the use the macromonomer to increase the steric hindrance to help generate ring samples. The samples still contain certain amount of linear chain contamination. The relaxation behaviors of these ring/linear mixtures and some literature reported ring samples were analyzed by the retardation spectrum. It is found that polymer rings may reptate like the linear chains. Their relaxation time scales following what is predicted by the reptation model. Linear chain contamination will increase the viscosity, radius of gyration and plateau modulus of the polymer ring samples. With linear chain contamination, the “threading effect” will slow down the relaxation of polymer rings. But when the amount of linear chain contamination is high, the mixtures still relax following the reptation model. In the second part, the rheological responses of a new kind of branched polymer with well defined side chain structure and branching space were studied. The branched polymer was prepared by the Ring Opening Metathesis Polymerization (ROMP) of norbornane based macromonomers linked with different length or composition of side groups. Due to the spacing effect of the side chains, these branched polymers are hard to get entangled. For PLA brush polymers, with increasing side chain length, similar double relaxation behaviors were found as those long chain branched polymers. The brush polymers relax following the sequence of the segmental relaxation first, then the side chain (arm) relaxation and the terminal (whole chain) relaxation. With such high molecular weight, the samples are still not well entangled. The plateau found for the relaxation of the backbone is not the rubbery plateau and is actually the inverse value of the steady state recoverable compliance. For wedge polymer with enough high backbone DP, the samples can get entangled and reptate as a common linear chain. But influenced by the huge side chain structure, the polymer exhibits a much lower rubbery and glassy modulus. The large volume fraction of side chain strongly influenced the glassy behaviors of the brush polymer. The PLA brush, wedge and dendronized polymer have the same backbone, but their glass transition behaviors are strongly affected by the steric hindrance introduced by the side chain structures.