Novel synthesis methods have made it possible to make polymers with well-defined composition, tacticity, architecture and molecular weight. This provides a tremendous flexibility to engineer products that have tailored properties at the molecular and nanostructural levels. However, quantifying and relating responses at these length scales to macroscopic properties is still a challenging task. We use a combination of rheology, FTIR and Raman spectroscopy, Polarimetry, Wide-angle X-ray diffraction, DSC, NMR, AFM and other experimental tools to gain insights to responses at these length scales. Our group is investigating the role of polymer architecture and tacticity on the rheology and orientation behavior. This research is done in collaboration with 3M scientists Jeff Cernohous, Shivshankar Venkataramani, Gaddam Babu and Allen Siedle. Polymer Architecture: Linear, symmetric star, and hyperbranched Polymers KEY ASPECTS: Star-like, model, hyperbranched polystyrenes (HBPS) with well-defined, unentangled branches have been synthesized When the branch density is moderate (>~20), there is a breakdown in stress-optical rule even in the homopolymer melt. When the branch density is high (~50), there is a complete breakdown of the stress-optical rule, perhaps attributable from the core-shell structure exhibited by the homopolymer The unique blending behavior of these HBPS with PVME and other polymers are being investigated. Collaborators: Jeff Cernohous (3M), Shivshankar Venkataramani (3M), Gaddam Babu (3M) For more details please see the publications and the presentations: ‘Unusual contributions of molecular architecture to rheology and flow birefringence in hyperbranched polystyrene blends’, S. Kharchenko, R. M. Kannan, J. Cernohous, S. Venkataramani, G. Babu, J. Poly. Sci (Poly.Phys), 39, 2562 (2001) [ pdf full text ] Role of architecture on the conformation, rheology, and orientation behavior of linear, star, and hyperbranched polymer melts: 1. Synthesis and Molecular Characterization’ by S. Kharchenko, R. M. Kannan, J.Cernohous, and S. Venkataramani, submitted to Macromolecules (under review) ‘Role of architecture on the conformation, rheology, and orientation behavior of linear, star, and hyperbranched polymer melts: 2. Linear viscoelasticity and flow birefringence’ by S. Kharchenko and R. M. Kannan, submitted to Macromolecules (under review) Synthesis of polymers with branched architectures and the study of the effect of architecture on the flow behavior has been the subject of keen interest over the last few decades. The branching topology may vary from the relatively simple cases of comb-like, star-like, and H-like polymers, to the more complex cases of hyperbranched polymers and dendrimers. Since the flow and the processing behavior are intimately affected by architecture, a systematic understanding of the role of architecture on the rheological and orientation behavior is vital. We explore the role of branching, especially branch density, on the conformation and rheology and orientation behavior of homopolymers and blends. We have synthesized (anionic synthesis) and (characterized triple detection SEC3) a series of model hyperbranched polystyrenes (HBPS) with branch functionality, f ranging from approximately 15 to 55, and branch molecular weight Mbr of 5, 10, 20 and 50 kg/mole. Compared to linear polymers and symmetric stars with 3 and 8 arms of the same total molecular weight, these polymers exhibit considerably lower radius of gyration, Mark-Houwink exponent and intrinsic viscosity. The hydrodynamic radii of HBPS of the highest branch density (f»50) are only about half the corresponding values of linear polymers of the same molecular weight, whereas this ratio is equal to 0.8 for symmetric stars. Our measurements suggest that HBPS have a very compact shape, with high segmental density, and a star-like architecture. HBPS with branches shorter than the characteristic critical molecular weight exhibit a linear dependence of zero-shear viscosity on molecular weight, suggesting a lack of entanglement, and further indicating the star-like nature of the HBPS. The role of architecture, especially the branch density, on the rheological and orientation behavior is investigated using simultaneous, quantitative stress and flow birefringence measurements in the melt state. Linear PS and 8-arm symmetric PS stars follow the stress-optical rule (SOR) over a wide dynamic range, with the stress-optical coefficient (C) governed by the arm molecular weight. When the branch density is ‘moderate’ (greater than ~20 arms), there is only a hint of non-terminal behavior in the viscoelastic moduli, while the C drops by 30% compared to a star with 8 arms of comparable length. When the branch density is ‘high’ (~50 arms), and the arms are unentangled, the non-terminal behavior in G* is clearly apparent, and there is a dramatic breakdown in the stress-optical rule in these homopolymer melts. The quantitative birefringence measurements suggest that the ‘excess’ birefringence may be due to the ‘form’ contributions from the core-shell structure. Such a structure may be formed by the preferential radial stretching of the chain segments near the core, as suggested by other studies on hyperstars. For comparable chain density, the core would be bigger than the shell when the arm length is smaller. Therefore, the 5K-HBPS exhibits a more severe breakdown compared to the 10K-HBPS. Effect of Molecular Architecture on the: a) Magnitude of the SOC; b) In-phase and Out-of-phase components of the stress-optical coefficeint for 10K-R-HBPS (Reference Temperature = 150ºC). Different shades on the symbol represent data taken at different temperatures. If the SOC were plotted for a linear homopolymer PS, it would be a flat line. However, in a star-like hyperbranched, homopolymer PS, the SOC fails! The magnitude of C is much higher than would be expected from a PS melt. This, and other data suggest the soft-colloidal nature of these polymers! Primary Researchers: Ajay Kulkarni     Designed by Bobby Bellair