The effect of hydrogen bonds on the dynamic mechanical properties of glassy polymethacrylates from 77[ddot]K

Abstract

The role of hydrogen bonds in the dynamic mechanical properties of a series of polymethacrylates was investigated from 77[ddot]K to their respective glass transition temperatures. An automatic, resonancetype acoustic spectrometer was designed for this purpose. It was found that the presence of hydrogen bonds increases the glassy moduli, the primary glass transition temperatures, and the low-temperature gamma transition temperatures. The internal friction peaks of hydrogen-bonding polymers are also broader. Similar effects were noted in polymers containing other types of polar bonds. A series of poly(butyl methacrylate) isomers which possess similar intermolecular cohesive bonding were also studied. With the exception of poly(butyl methacrylate), similar properties were exhibited by these isomers. These results were examined in the light of both the “volume of action” hypothesis of Hoff, Robinson, and Wilbourn and the “bond loosening” hypothesis of Andrews and Hammack. The internal friction as a function of reciprocal temperature was calculated on the basis of the standard linear solid model. Comparison with the experimental curves shows that glassy polymethacrylates possess a distribution of relaxation times. Amid the very extensive literature on the correlation of mechanical properties of polymers with their molecular parameters, the effect of intermolecular forces has by comparison received little attention. It is only recently that interest in this regard has increased. The roles of hydrogen bonds [1-3], ionic bonds [4-8], and polar bonds [9, 10] have been explored by a number of authors. In a previous paper, Tobolsky and Shen [3] have reported on the effect of hydrogen bonds on the viscoelastic properties of a number of polymethacrylates. It was found that, while the shapes of the viscoelastic master curves for the hydrogen-bonded and the non-hydrogen-bonded polymers were quite similar, the glass transition temperatures and the rubbery moduli of the former were higher. Only limited data were given for the glassy region properties, however, since the technique employed was not intended for studies at very low temperatures. For the past decade or so, there has been considerable interest in the study of the relaxation properties of polymers down to cryogenic temperatures [11]. These works employ a variety of techniques, such as nuclear magnetic resonance, dielectric, and dynamic mechanical properties studies. In this paper we shall utilize the latter method to investigate the effect of hydrogen bonds on the mechanical properties of a number of polymethacrylates from 77[ddot]K to their respective glass transition temperatures. An acoustic spectrometer was constructed which subjects the samples to transverse resonance vibrations. Polymers were so chosen that they are similar in structure, such as side-chain length, steric effects, etc., except that some possess hydrogen-bonding capability and some do not.