Hydration and plasticization effects in cellulose acetate: molecular motion and relaxation

Abstract

This paper examines the effects of hydration and plasticization on relaxation in cellulose acetate as revealed in the collated data from a number of techniques which include NMR, differential scanning calorimetry, dynamic mechanical thermal analysis and dielectric spectroscopy, the latter over an extended range of frequency (0.1 Hz–1 GHz). Sample type and plasticizer content are disposable parameters. Interpretation is guided by extant information on the general behaviour of water in polymers. The way in which water and plasticizer contents change over the temperature range of the relaxation measurements is also monitored.

The transition map of log ν_cvs. 1000/T for hydrated cellulose acetate reveals two relaxations in addition to the five described in the literature. Current interpretations are reviewed and, in some cases, revised. The seven relaxations are, in order of decreasing temperature: (i) the α-relaxation or glass transition; (ii) the β*-relaxation which, in this study, reveals rich detail upon the removal of water; (iii) relaxation associated with the onset of mobility of loosely bound water; (iv) the β-relaxation involving cooperative motion of the polar side groups and the sugar rings in the main chain; (v) the formation of ice when significant bulk water is present in the material; (vi) the γ-relaxation which is closely identified with the glass transition of tightly bound water; and (vii) NMR evidence of methyl group relaxation. Water, when present, tends to drive the relaxations to lower temperatures.

Events at low temperatures, in the vicinity of the γ-relaxation, are especially complex and difficult to interpret. Plausible scenarios are proposed which are consistent with the available experimental information, in particular, the glass-like character of tightly bound water in host polymers.