Rheology and rheo-optical behavior of hyperbranched, star polymers and polymer blends

By improving polymer synthesis it is possible to obtain materials with very well controlled architecture. The importance of such materials is both pragmatic and fundamental. One of such classes of inexpensive materials is hyperbranched polymers (HBP). They can be used as viscosity modifiers, additives to linear polymers in highly demanding optical applications, and at the same time they present the class of branching materials, dynamics of which appears to be very similar to those of dendrimers and symmetric stars. In our work, to get insights on the molecular level, we build rheo-optical apparatus, which allows for simultaneous measurements of stress and flow birefringence to systematically study the effect of molecular architecture on polymer viscoelasticity. In an effort to compare the response of linear and hyperbranched polymers, ‘star-like’ hyperbranched polystyrenes (HBPS) of varying branch length and number of branches have been custom-synthesized. These materials are unentangled or weakly entangled, thereby allowing us to study the effect of branch density more readily. To correlate the effect of branching, a wide range of linear polymers and symmetric stars were used in the analysis. From the molecular characterization of model polymer systems, it appears that HBPS are very compact, sphere-like molecules, and possess solution properties intermediate between dendrimers and symmetric stars.; To our knowledge, the flow birefringence measurements on highly branched polymer melts are among the first in the literature. Our results indicate that HBPS exhibit both polymeric and soft-colloidal nature and show significantly different from L-PS and symmetric stars flow behavior: (1) highly dense HBPS develop nonterminal relaxation in rheology; (2) when the stress-optic rule (SOR) holds, the stress-optical coefficient (C) of the HBPS is much lower than those of analogous linear polymers; (3) when the branch density is high, and the branch length is low, the SOR fails for these homopolymer melts. We conclude that significant increase of the form birefringence for a given amount of stress suggests that HBPS may form a ‘core-shell’ morphology, arising from strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery.; Studying L-PS/HBPS blends, we find that the relaxation of low molecular weight linear polymer speeds up the relaxation of HBPS by ‘releasing the constraint’ due to branches of HBPS. It has been found that shorter linear molecules are likely to be viewed as better solvents for HBPS, however, they partially interpenetrate and disrupt its core-shell structure. When HBPS are blended with PVME, TTS is valid what contrasts blend of PVME with L-PS, implying improved blend miscibility even on the nanoscale.