The crystalline character of atactic poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate) in their dynamic mechanical response

Dynamic mechanical properties have been determined in atactic poly(p-biphenyl acrylate) (PPBA) and poly(p-cyclohexylphenyl acrylate) (PPCPA) in the temperature range from 80 to 540°K at frequencies in the range 10³–10⁴ Hz. The general behavior of the dynamic elastic modulus as a function of temperature shows a transition region from the glassy state at about 390°K for both polymers, a plastic region extending over a temperature interval of about 100°K, and another transition to the melt situated at 540 and 480°K for PPBA and PPCPA, respectively. The experimental data show that the mechanical behavior of both polymers strongly resembles that of crystalline polymers. The loss spectrum of PPBA shows the presence of several important maxima: one corresponding to the melting point, characterized by a very rapid increase of losses with increasing temperature (α′ relaxation), one in the glass-temperature range, characterized by a rather broad peak (α′ relaxation), and others below T_g, associated with secondary relaxation effects. The analysis of the different transitions and relaxations indicates that some of these processes can be ascribed to motions taking place in the ordered regions of the polymer. PPCPA shows a similar loss pattern; however, owing to the lower melting point the α maximum is partially submerged in the α′ relaxation associated with the melting process. Of particular interest is the γ process in the glassy state of this polymer, caused by the chair–chair transition of the cyclohexyl rings. The limited intensity of this relaxation as compared with that of most polymers containing cyclohexyl side groups, has been interpreted as due to the high ΔF associated with such a transition for cyclohexyl rings linked to phenylene groups. This leads to some interesting conclusions about the conformation of the side groups in PPCPA.