Relaxation Dynamics of Poly(methyl acrylate) at Elevated Pressure

A fascinating aspect of the dynamics of supercooled liquids and polymers is the degree to which the reorientational or segmental relaxation time, τR, governs various properties. 1,2 For example, the shape of the relaxation peak, commonly reflected in the Kohlrausch stretch exponent, the state points associated with the dynamic crossover, and the dynamic correlation volume for many glass formers depend only on the relaxation time, independently of temperature and pressure. Similarly, the phase transitions of liquid crystals usually transpire at a fixed value of τR. 9 There is also a scaling relation that superposes τR measured at various temperatures and pressures, whereby any combination of temperature, T, and density, F, yielding a given value of T/F, where γ is a material constant, will have the same relaxation time. 13 Herein we examine the conformance of poly(methyl acrylate) (PMA) to these general properties. Acrylate and methacrylate polymers have been the subject of many investigations, due both to their commercial significance and to their unusual dynamics. When the pendant alkyl group in methacrylates is large, the polymers exhibit a very broad or even two glass transitions, and these materials often have unusually intense, multiple secondary relaxations. 20 Acrylates commonly serve as the backbone for liquid crystalline polymers. 24 PMA has the smallest repeat unit among acrylate polymers, which simplifies the relaxation behavior. Moreover, the effect of pressure on the fragility of PMA is among the highest reported for any polymer, which makes it an obvious choice to study the effect of pressure on relaxation properties. Despite the diminutive chain units, we find that there are two secondary relaxation processes, and the properties of the primary structural relaxation in PMA deviate from some of the general behaviors mentioned above.