5 years ago

Characterization of Dilution Effect of Semidilute Polymer Solution on Intrinsic Nonlinearity Q0 via FT Rheology

Characterization of Dilution Effect of Semidilute Polymer Solution on Intrinsic Nonlinearity Q0 via FT Rheology
Seung Joon Park, Hyeong Yong Song, Kyu Hyun
Intrinsic nonlinearity Q0(ω) under medium amplitude oscillatory shear (MAOS) deformation was investigated for monodisperse polystyrene (PS) solutions at various concentrations, which were classified as unentangled or entangled solution in semidilute regime. These two types of PS solutions displayed different shapes when Q0 was plotted as a function of frequency (ω). Unentangled solutions showed increases of Q0 with frequency at the low frequency region and then plateau behavior at the high frequency region. On the contrary, entangled solutions showed an increase of Q0 before the MAOS terminal relaxation time and a subsequent decrease, which is similar to that observed for entangled linear polymer melts. The Q0(ω) curves of each group were superposed in a dimensionless coordinate (Q0/Q0,max vs De), so that transition from the plateau of Q0 to decreasing Q0 at the high frequency region might indicate the onset of entanglement in polymer solution. In particular, all unentangled solutions had the same Q0,max value (0.006) regardless of polymer concentration and molecular weight because Q0 responds to Rouse-like relaxation process only, which is featured as no interchain interaction and chain stretching. However, the Q0,max values of entangled solutions were dependent on the number of entanglements (Z). The master curve of Q0,max as a function of Z showed that Q0,max was constant at low entanglement numbers (few or virtually no entanglements) and then increased with the beginning of entanglement to approach a limiting value at high entanglement numbers, where reptation is the dominant linear relaxation process. In addition, the master curve of Q0,max as a function of Z was used to quantify the degree of tube dilation based on the dynamic tube dilution (DTD) theory. Direct comparison of the Q0,max values of semidilute solutions and melts showed that they followed the same molecular dynamics in MAOS flow like SAOS (small amplitude oscillatory shear) flow. Comparison between static and dynamic dilutions using the Q0,max master curve suggested that this curve could characterize the effective number of entanglements per backbone chain for branched polymers. Because it was confirmed again that Q0(ω) is highly sensitive to various relaxation processes, MAOS tests may provide a new means of investigating molecular dynamics.

Publisher URL: http://dx.doi.org/10.1021/acs.macromol.7b00119

DOI: 10.1021/acs.macromol.7b00119

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