Relation between main and normal mode relaxations for polyisoprene studied by dielectric spectroscopy

Abstract

Broadband dielectric spectroscopy (10-2-106 Hz) is employed to study the relation between a relaxation and normal mode process for oligomeric polyisoprene samples of different molecular weights.It is shown that a relaxation and normal mode process merge above the Vogel temperature of the a relaxation.This means that the Rouse model is not applicable to the normal mode relaxation near the glass transition. Further, this result is discussed from a microscopic point of view in terms of the different temperature dependences of the two relevant length scales, the correlation length for the a relaxation 4 and the end-to-end vector of the polymer chain. A quantitative evaluation gives more evidence that there exists a characteristic length scale for the a relaxation which is in the order of magnitude of 2-3 nm a t the glass transition temperature.

Dielectric relaxation of poly(ethylenglycol)- b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers above the glass transition temperature

“…As can be observed, the normal and α modes merge at low temperature, while it is expected that the α and β modes merge in a temperaturefrequency range which is outside our experimental window (see below). This behavior is qualitatively similar to that described for PPG [9,10], and for polyisoprene [6].…”

“…This is in contrast with the results of Stünh and Stickel [26], who found values of γ that decreased with T for a polyisoprene-polystyrene copolymer. Schönhals [6] obtained molecular-weight-dependent values of γ for polyisoprene samples, which is at odds with the Rouse model predictions for type-A homopolymers. Although this conclusion has been criticized by Nicolai and Floudas [27] in a study of linear and star poly(oxypropylene) polymers, these authors report normal mode relaxations broader than the predictions of Rouse dynamics.…”

“…It has been found that for the β-relaxation γ = 1, i.e., the relaxation can be described by a Cole-Cole function. As has been discussed by Schönhals [6], and Stühn and Stickel [26] for pure type-A polymers, if the normal mode can be described by a Rouse-type dynamics, the normal mode should be characterized by δ = 1 and γ = 0.7. Adachi et al [17] have shown that for block copolymers in which type-A chains are diluted in non-type-A blocks the shape of the relaxation depends on the copolymer composition.…”

“…with a component parallel to the chain), this so-called normal mode can be studied by dielectric spectroscopy [5]. So far, most of the attention has been devoted to the study of the normal a e-mail: RGRUBIO@eucmax.sim.ucm.es mode relaxation of homopolymers, mainly using polyisoprene [5][6][7][8], and poly(propyleneglycol) (PPG) [9][10][11][12][13][14][15]. The results show that the same monomer friction coefficient is responsible for the α and the normal modes, and that Rouse-type dynamics fails to describe some aspects of the experimental results near T g [9,10].…”

Dielectric Relaxation of Type-A Polymers in Melts and Solutions

“…[17 -19, 55, 56] This difference, that might reflect a difference of the smallest motional units in these processes, is an interesting subject related to the cooperative, local dynamics. [18,55,56] However, this local dynamics is out of the scope of this article. Thus, in this article, we limit ourselves to high T (S T g ) and low x where the a relaxation is completed, except for oligo(styrene) utilized in Section 6.1 as a model material for rheo-dielectric tests.…”