Dielectric relaxations in poly(hydroxyethyl acrylate): influence of the absorbed water J. L. Gbmez Ribelles, J. M. Meseguer Duefias and M. Monlebn Pradas Laboratory of Thermodynamics and Physical Chemistry, Universidad Polit&cnica de Valencia, PO Box 22012, 46071 Valencia, Spain (Received 24 July 1987; revised 18 November 1987; accepted 10 December 1987) Poly(hydroxyethyl acrylate) presents two relaxation zones, labelled ~ and ~, when it is completely dry. The temperature of the maximum of the 7 relaxation, as well as its apparent activation energy, are somewhat higher than in poly(hydroxyethyl methacrylate), a fact that could be explained by higher intermolecular interactions in the series ofpolyacrylates than in the series ofpolymethacrylates. The absorption of even slight traces of water causes a new relaxation to appear, the intensity of which increases with the content of water, while at the same time the intensity of the 7 relaxation decreases. This fact suggests the formation of an association of the water molecules with the side groups of the polymer. The characterization of the relaxation is difficult because of the high d.c. conductivity component of the permittivity. Its temperature suggests the presence of hydrogen bonds which render the main chains rigid. (Keywords: poly(hydroxyethyl acrylate); dielectric relaxation; water sorption) INTRODUCTION Poly(hydroxyethyl acrylate) is a highly hydrophilic polymer. It differs from poly(hydroxyethyl methacrylate) in not having the methyl group in the main chain. The dielectric secondary relaxations of this last polymer have been thoroughly studied, and its relaxation spectrum serves as a reference for the study of that of poly(hydroxyethyl acrylate). EXPERIMENTAL The samples were obtained by radical block polymerization at 60°C of monomer (Merck), which had previously been distilled at low pressure in order to remove the inhibitor. Azobisisobutyronitrile (0.05 Yo by weight) was employed as initiator of the polymerization. No crosslinking agent was used. After polymerization the sample was held in a vacuum at 70°C until constant weight. The same sample was employed for all the tests. Addition of water was achieved by leaving the sample in ambient conditions until it reached the weight gain corresponding to the desired water content. The sample was then isolated until the time of the measurements. A small portion of the dry sample was subjected to a thermogravimetric analysis with a Du Pont Thermogra- vimetric Analyzer 951 in nitrogen atmosphere. No loss of weight due to volatile substances (monomer or trace water) was detected at temperatures below 250°C, when degradation of the polymer starts. The measurements were performed on a General Radio capacitor bridge keeping the measuring cell in a dry atmosphere, for frequencies ranging from 60 to 100 kHz. RESULTS AND DISCUSSION The dielectric relaxation spectrum of the dry polymer shows only two relaxation zones: a secondary one, in the temperature range between -150 and 0°C, and the 0032-3861/88/061124-04503.00 © 1988 Butterworth & Co. (Publishers) Ltd. 1124 POLYMER, 1988, Vol 29, June relaxation zone, associated with the glass transition, between 35 and 58°C, for frequencies ranging from 60 Hz to 100 kHz. By analogy with the spectrum of the poly(hydroxyethyl methacrylate) (PHEMA) the secon- dary relaxation is attributed to local movements in the lateral group CH2-CH2-OH, and will be labelled 7 hereafter, following the literature concerning PHEMA 1 - 5 7 relaxation in the dry polymer Figure 1 displays the values of the imaginary part of the complex dielectric permittivity e" as a function of the frequency for temperatures ranging from - 140 to -9°C. The maxima corresponding to the 7 relaxation are somewhat shifted towards lower frequencies when compared with those of PHEMA t'5'6 or, what amounts to the same, the maxima of the curves of e" as a function of temperature are shifted towards higher temperatures. Also, the apparent activation energy found for the PHEA, 13.5 kcal mol-1, is somewhat greater than that measured for PHEMA (between 9 and 10 kcal mol - 1 (refs. 1 and 6); ref. 5 gives values of 15 to 16 kcal mol-1, but these are calculated following a different method). Something similar happens with the V relaxation of poly(propyl acrylate) 7 (PPA) and poly(butyl acrylate) a (PBA), whose apparent activation energy is 7 kcal mol - 1, when compared with that of poly(propyl methacrylate) (PPMA) and poly(butyl methacrylate) (PBMA), which have the lower value of 5.5 kcal mol- 1 (refs. 9 and 10). If the potential barrier which causes the relaxation were due only to intramolecular interactions, without any influence of the environment, it would be difficult to account for these differences. Nevertheless, studies of molecular dynamics lt,12 have shown that the intramolecular interactions can account for only about 50 Yo of the value of the apparent activation energy of the fl relaxation in poly(methyl methacrylate) (PMMA) when this last is assumed to originate in the rotation of the