Thermal and mechanical properties of the precursor polymers: Comparison of their properties with poly(amic acid)

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Abstract
The DSC thermograms of P‐PHA show a large endothermic peak at 450–550°C. As the annealing temperature increases from 250°C to 400°C, the endothermic peaks become smaller and then disappear for samples annealed above 450°C. As observed for P‐PHA, the endothermic enthalpy of PHA and PAA became smaller with an increasing annealing temperature. The cyclization onset temperature (T1) of the three precursors increases linearly with an increasing annealing temperature at a constant annealing time (30 min). Otherwise, the initial decomposition onset temperature (T2) was shown to be constant. T2 of P‐PHA, PHA, and PAA were observed in the temperature ranges of 601–603°C, 576–577°C, and 532–534°C, respectively. These TGA results confirm that all of the samples are thermally stable. Increasing the annealing temperature of the three precursor polymers significantly increases the tensile properties of the films. The tensile properties of all annealed precursors were much higher than those of the unannealed films. In contrast, the initial modulus of PAA is improved only slightly when compared with the other two polymers regardless of the heat treatment. The biaxial stresses in the PHA and PAA films were investigated by holographic interferometry. The stresses in the films were 6.85–7.61 MPa for PHA and 27.01–27.70 MPa for PAA.

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