Dynamics of Low Mass Molecules Dissolved in Polymers

Abstract

We studied dielectric and viscoelastic relaxation behavior for blends of low mass compounds (LM) and polymers. Specifically the polymers used are polystyrene (PS) and poly(vinylethylene) (PVE), and the LMs are alkyl-cyanobenzene (nCBz, n = 0,5), alkyl-cyanobiphenyl (nCB, n = 0,1,3,5,7), and alkyl-cyanoterphenyl (nCT, n = 5), where n represents the number of carbons in the normal alkyl groups of the LMs. The objective of this study is to clarify the effect of the sizes of LM molecules and those of polymer segments on the cooperative dynamics between those two dynamical units in the miscible state. Due to large dipole moment of the cyano-group, only the LM motions were observed by dielectric measurements even for blends with low LM content such as 5 %. On the other hand, viscoelastic terminal relaxation reflects the polymer component dynamics. By performing both dielectric and viscoelastic measurements we found the followings. (1) Rotational motion of the smallest LM, i.e., 0CBz, decoupled with the segmental motion of PS, however the other LMs with larger sizes than 0CBz exhibited cooperative dynamics with the segmental motion. (2) Below the glass transition temperature of LM / polymer mixtures, fast relaxation process (β relaxation) corresponding to the spatially restricted motion of LMs in the glassy state was observed. However for the mixtures of 5CT(5 wt%) / PS and 5CB(5 wt%) / PVE, b relaxation did not appear. This result suggests that the local wiggling motion of LM (β relaxation) is highly suppressed when the LM size becomes large and comparable with the segmental size of a polymer.