Modes of Molecular Motion in Low Molecular Weight Polystyrene

Abstract

The variations with temperature of the viscosity, η, and the relaxation times for various low molecular weight, unentangled polystyrenes (PS) are analyzed. We find that the temperature dependence of η does not directly reflect the behavior of the global chain modes. Depending on molecular weight and temperature, η can exhibit a stronger temperature dependence than even the local segmental modes. However, this is only a consequence of the strong temperature dependence of the recoverable compliance J s. The Rouse relaxation time, τ1, deduced from the product of the viscosity and the recoverable compliance, has the expected behavior. For example, this Rouse time for a PS of molecular weight equal to 2 kg/mol has the same temperature dependence as that for self-diffusion; hence, there is no enhancement of translational diffusion in the polymer. The same conclusion was reached by Urakawa et al. [Urakawa, O.; Swallen, S. F.; Ediger, M. D.; von Meerwall, E. D . Macromolecules 2004, 37, 1558], although the analysis leading to it was different. The conclusion is consistent with two extant explanations of the enhancement of translational diffusion in small molecular glass-formers, one which invokes spatially heterogeneous dynamics and the other ascribing the effect to the different coupling parameters for the translational and rotational correlation functions.

A review of the slow relaxation processes in the glass–rubber transition region of amorphous polymers

“…[67] Furthermore, low-molecular-weight polymers should have smaller coupling effects for lower entanglement degrees, but the transitional diffusion of polymers with low molecular weight is not enhanced. [68,69] The slower processes were rarely thought to be connected to molecular weight. Recently, Roland et al [70] found that the T g and T g normalised relaxation time of polybutadienes (PB) varied monotonically with the number-average molecular weight.…”

Dispersion of the Structural Relaxation and the Vitrification of Liquids

“…(16). This is because, in the application of the CM to polymer dynamics and viscoelasticity [25,191,202,205,206,208,209,295], the normal modes and the local segmental mode have the same primitive monomeric friction coefficient 0 ðT À1 V Àg Þ. Based on this, the breakdown of thermorheological simplicity of the viscoelastic spectrum of amorphous polymers discussed in Section III, (paragraph 4), was explained based on the difference between the coupling parameter n a and n n of the local segmental mode and the normal modes, respectively.…”

Recovery of Shear-Modified Polybutadiene Solutions

“…All rheological measurements were done at 25 °C. 32 The crossover from Newtonian flow to shear thinning behavior occurs at a shear rate given by 33 (2) This is the approximate value of the minimum shear rate necessary to ensure disentanglement by flow.…”