Low-Temperature Internal-Friction Study of Diluent Effect in Some Polymethacrylates

Abstract

A transverse resonance acoustic spectrometer was used to investigate the effect of diluents on the dynamic mechanical properties of poly-2-hydroxypropyl methacrylate (PHPMA) and poly-2-hydroxyethyl methacrylate (PHEMA) down to −196°C. Samples were prepared by polymerizing the monomer-diluent mixtures through ultraviolet radiation. In the dry sample of PHPMA, below the glass transition temperature, there is an internal friction peak at −90°C (gamma peak). With the introduction of 0.5 vol % water, the gamma peak broadens into a shoulder, with the concomitant appearance of a new ``diluent peak'' at −25°C. Addition of higher water content causes the internal friction curve to exhibit only one single peak, the position of which shifts to lower temperatures while the strengths of the loss peaks increase with increasing diluent concentration. Similar observations were made for other hydrogen-bonding diluents, i.e., deuterium oxide, methanol, and glycerol. For nonhydrogen-bonding diluents, chloroform and cyclohexane, the gamma peak was retained while a shoulder appears around −20°C. Isoproponal, on the other hand, produces an internal friction curve that appears to be ``borderline'' between the above two cases. For PHEMA, which has a gamma peak at −100°C in the absence of diluents, the addition of 0.5 vol % water produced an internal friction curve that possesses a doublet near −60°C. Again, at higher water concentration one observes only a single peak. When cyclohexane was used as the diluent, the doublet character persists up to 5% diluent concentration. These observations were examined in the light of the molecular motion hypothesis. A dual mechanism is suggested that attributes the new diluent peak to the composite of displaced beta and gamma processes. It is postulated that beta transition is lowered due to the plasticizing action of the diluents, while the gamma transition is elevated on account of the hindering effect of the immobilized diluent molecules. At sufficiently high diluent concentration, these two peaks merge into the single observed peak.