Identifying the Transient Cooperative Processes behind the Dynamic Behavior of Sheared Polymer Melts

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Polymer melts exhibit a rich dynamic behavior when exposed to intense shear fields. For example, amorphous polymers display a shear thinning behavior, while crystallizable polymers form correlated segments within the flow-oriented structures. However, the role of such correlations on the dynamic behavior of crystallizable polymer melts is highly unexplored. In this context, we have recently shown the manifestations of such correlated segments on the visco-elastic behavior of sheared polymer melts through the dewetting technique. We have compared the dynamics of atactic polystyrene (aPS, amorphous) and isotactic polystyrene (iPS, crystallizable) films above the melting point (Tm). For aPS films, the apparent viscosity (hf) deduced from the dewetting experiments was expectedly smaller than the zero shear bulk viscosity (hbulk). Surprisingly, for iPS films hf was always greater than hbulk, even at about 50K above the melting temperature, with hf/hbulk following an Arrhenius behavior. The corresponding activation energy of 160±10 kJ/mol suggested a cooperative motion of segments, which were aligned and agglomerated by the fast dewetting process. Since the temperatures are above Tm, the observed cooperativity can only be transient. It is the aim of the current proposal to build on this premise by performing systematic dewetting, oscillatory rheometry (measure of the macroscopic viscosity) and time resolved in-situ X-ray scattering experiments (local structure and dynamics) for identifying the origin of transient cooperative processes in crystallizable polymers and examining their influence on the viscoelastic properties of polymer melts. To this end, polymers with different chemical structures and molecular weights will be sheared and probed in-situ using the complimentary techniques to provide a connection between the local dynamics (heterogeneities) and its macroscopic consequences. Support from the theoretical front is assured with the help of our external collaborator Dr. Semenov, Strasbourg. In addition, we expect that our studies will indicate novel strategies and pathways for tuning the properties of polymer melts.