Thermal behaviour of acrylonitrile copolymers having methacrylic and itaconic acid comonomers

Abstract

The effect of comonomers, methacrylic (MAA 3–6mol%) and itaconic acid (IA 2–3mol%) on the glass transition temperature, Tg, hydration melting and thermo-oxidative stabilization of acrylonitrile copolymers has been studied by differential scanning calorimetry (DSC). While, two Tg‘s were observed, near 70 and 140°C, for PAN homopolymer, copolymers showed another baseline shift below 70°C. The melting point, Tm, of PAN and copolymer, P(AN/IA) containing 3.2mol% of IA, has been reduced to 186 and 158°C, respectively, in high-pressure DSC. The copolymers showed two distinct melting peaks in the first heating cycle, but the melt on subsequent cooling exhibited a single exotherm due to crystallization. The melting point of IA copolymer (158°C) was found to be lower than that of MAA copolymer (164°C) and PAN homopolymer (186°C). Tm further decreases with increase in the comonomer content. In the second heating cycle, observed Tm is higher than that noted in the first heating run, indicating some amount of physical transformations. The enthalpy of fusion, ΔHf decreases with the incorporation of comonomers, e.g. it is maximum for PAN homopolymer (57.8Jg−1) and minimum for P(AN/IA) copolymer (26Jg−1) having 3.2mol% of IA content. The lower entropy of fusion (ΔSf) of copolymers indicates that they are less mobile in the molten state. DSC of copolymers run in air atmosphere, at a heating rate of 10°Cmin−1, shows a broader exotherm compared to homopolymer. But, the broadening of exotherm seems to be more in P(AN/IA) polymers than P(AN/MAA) and the onset of cyclization shifts to a lower temperature as compared to PAN. The activation energy of nitrile cyclization, Ea, for PAN is found to be 30kcalmol−1. For copolymer P(AN/IA), IA=3.2mol%, Ea is 18kcalmol−1.