Identifying the Transient Cooperative Processes behind the Dynamic Behavior of Sheared Polymer Melts
Final Report Abstract
The aim of my original proposal was to identify the transient cooperative processes underlying the dynamic behavior of sheared melts of crystallizable polymers by performing systematic dewetting, oscillating rheology and X-ray scattering experiments. The main idea was, therefore, to subject polymers to different shear rates, which may allow us to understand cooperative processes leading to significant changes in the viscoelastic properties of polymers. We primarily relied on dewetting - the retraction of a polymer film from an unfavorable surface - to shear polymers in controlled conditions. We obtained polymer films via spin coating, which involved complex flow conditions. For instance, the rapid solvent evaporation during spin coating often freezes polymers in nonequilibrium conformations, manifesting into the storage of a huge amount of residual stresses. Intriguingly, experiments performed at elevated temperatures revealed that films of identical thickness exhibited orders of magnitude variations in residual stresses and their corresponding relaxation times when prepared at different rotation rates from solutions of appropriate concentrations. Such large stresses dominated the dewetting behavior of polymer films. In addition, stress relaxation times were orders of magnitude longer than the reptation time, which is the longest relaxation time of equilibrated polymers. These intriguing observations clearly suggest the need for extending and, possibly, introducing new concepts in polymer physics. Thus, we focused on understanding the microscopic reasons underlying the preparation (complex flow) induced nonequilibrium behavior of polymer films, instead of the originally proposed studies on simple shear flows. Through our systematic dewetting experiments, and theoretical support from Dr. Semenov, we identified a dimensionless preparation parameter ℘ - the ratio of equilibration and preparation time - controlling the preparation-induced deviations in the behavior of polymer films of identical and variable thicknesses. We revealed scaling relations between ℘ and the amount of preparationinduced residual stresses, the corresponding relaxation time, and the probability of film rupture. Such quantitative processing-property relations allowed us to reveal that rapid spin coating conditions may induce molecular correlations that are mediated via topological constraints. Such correlations might be the reason underlying the observed stress relaxation times that are orders of magnitude longer than the reptation time. Interestingly, the preparation-induced molecular correlations allowed significant control over the viscoelastic properties, in comparison with the equilibrium properties obtained from oscillating rheometry. In addition, we observed orders of magnitude control over the crystal nucleation density at a given quench depth even after significantly long annealing above the melting temperature, reflecting the importance of long-living molecular correlations and the transient clusters of monomers in crystallization kinetics. We are optimistic that the molecular-level understanding of processing-induced nonequilibrium behavior of polymers developed via our model experiments may, in general, be applicable for polymers in various nonequilibrium conditions.
Publications
- Time allowed for equilibration quantifies the preparation induced nonequilibrium behavior of polymer films, ACS Macro. Lett. 6 (11), 1296-1300 (2017)
S. Chandran, R. Handa, M. Kchaou, S. Al Akhrass, A. N. Semenov and G. Reiter
(See online at https://doi.org/10.1021/acsmacrolett.7b00815) - Controlling polymer crystallization kinetics by sample history, Macromol. Chem. Phys. 219, 1700315 (2018)
P. Poudel, S. Chandran, S. Majumder and G. Reiter
(See online at https://doi.org/10.1002/macp.201700315) - Tuning relaxation dynamics and mechanical properties of polymer films of identical thickness, Phys. Rev. E 97, 032507 (2018)
M. Kchaou, P. Alcouffe, S. Chandran, P. Cassagnau, G. Reiter and S. Al Akhrass
(See online at https://doi.org/10.1103/PhysRevE.97.032507) - Dewetting rheology for determining viscoelastic properties of nonequilibrated thin polymer films, Macromolecules 52 (20), 7894-7903 (2019)
A. Mulama, S. Chandran, K. Roumpos, A. O. Oduor and G. Reiter
(See online at https://doi.org/10.1021/acs.macromol.9b01384) - Processing pathways decide polymer properties at the molecular level, Macromolecules 52 (19), 7146-7156 (2019)
S. Chandran, J. Baschnagel, D. Cangialosi, K. Fukao, E. Glynos, L. M. C. Janssen, M. Müller, M. Muthukumar, U. Steiner, J. Xu, S. Napolitano and G. Reiter
(See online at https://doi.org/10.1021/acs.macromol.9b01195) - Segmental rearrangements relax stresses in nonequilibrated polymer films, ACS Macro. Lett. 8 (6), 646-650 (2019)
S. Chandran and G. Reiter
(See online at https://doi.org/10.1021/acsmacrolett.9b00116)
