Low-frequency thermomechanical spectrometry of polymeric materials: Tactic poly(methyl methacrylates)



A computerized and automated torsional pendulum has been used to characterize amorphous poly(methyl methacrylates) at about 1 Hz in the temperature sequence 473° → 93° → 473°K. The effects of thermal prehistory, temperature cycling, water content, and tacticity are demonstrated. In particular, a comparison of the out-of-phase shear modulus (G″) versus temperature for “syndiotactic,” “atactic,” and “isotactic” polymer specimens shows that the intensity of the glassy-state β loss peak decreases with increasing isotactic content while the temperature of its location remains the same. Extrapolation suggests that completely isotactic polymer would not display a β loss peak. The shape and location of the G″ data at low temperatures indicate that the basic mechanism of the β process is the same for the three polymer samples and support the validity of the extrapolation. The effect of tacticity is reflected also in the glass transition region; the isotactic sample has its Tg about 65°C lower with greater intensity than the syndiotactic polymer.