Study of the Dual Amorphous Phases in Semicrystalline Poly(ethylene terephthalate) Using the Heat Capacity Increment at the Glass Transition

Abstract

The morphology of semicrystalline polymers is increasingly interesting due to its theoretical importance, 1,2 and many models have been proposed to explain the particular properties of semicrystalline polymers. [3][4][5][6][7][8][9] The model of "extended glass transition" proposed by Struik 3,4 describes the amorphous phase in semicrystalline polymers as a distribution of subphases with different glass transition temperatures (T g ) and has been employed in our previous papers 10,11 to successfully explain the physical aging behaviors of semicrystalline poly(ethylene terephthalate) (PET). However, the model of "rigid amorphous phase" proposed by Wunderlich et al. 5-8 divides the amorphous phase in semicrystalline polymers into a mobile amorphous subphase, which contributes to and is proportional to the heat capacity increment (∆C p ) at the glass transition, and a rigid amorphous subphase, which does not contribute to the ∆C p . Besides, the concept of "rigid noncrystalline chains" proposed by Zachmann 9 separates the noncrystalline chains in semicrystalline polymers into rigid noncrystalline chains and mobile noncrystalline chains by their conformations. In the present paper, the model of "rigid amorphous phase" will be used to further study the structural changes in semicrystalline PET during isothermal crystallization from the glassy state.Amorphous PET films had a thickness of ca. 0.15 mm, a viscosity-average molecular weight of ca. 1.63 × 10 4 , a density of ca. 1.335 g cm -3 , which means that samples were wholly amorphous, and an optical birefringence of ca. 6 × 10 -4 , which means that there was no orientation. Measurements by both differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) also showed that the samples were wholly amorphous.As-received PET films were cut into equal circles with a diameter of ca. 5.7 mm. They were held in an oven under a nitrogen atmosphere at 573 ( 0.5 K (ca. 45 K above their melting point of ca. 528 K) for 5 min to completely eliminate their thermal history. Then they were quenched in air to the room temperature of ca. 298 K. It has been proved that there was no crystallization induced by this process. Subsequently, these quenched samples crystallized isothermally in an oven under a nitrogen atmosphere at various preset temperatures between 358 and 408 K for different periods of time to obtain a different degree of crystallinity (X c ). Finally, all the semicrystalline samples were taken out and stored in a desiccator before other measurements.The density of semicrystalline PET (F) was measured at 298 ( 0.1 K by using a density gradient tube filled with carbon tetrachloride and n-heptane. The density gradient tube was calibrated by suspending glass beads with known densities. X c (apparent weight percent degree of crystallinity) of the samples was calculated by taking the density of wholly amorphous samples (F a ) to be 1.335 g cm -3 and that of perfect PET crystalline lamellae (F c ) to be 1.455 g cm -3 . 12 Then Samples of ca. 5.0 mg were sealed in...

Effects of Rigid Amorphous Fraction and Lamellar Crystal Orientation on Electrical Insulation of Poly(ethylene terephthalate) Films

“…The annealing time was set as 2 h to complete the cold crystallization at both temperatures (see Figure S1 and discussion in the Supporting Information). This result was consistent with those reported before. , …”

“…This result was consistent with those reported before. 28,29 Characterization Methods and Instrumentation. A 1 H solidstate nuclear magnetic resonance (ssNMR) echo (90-τ-90-τ) experiment was conducted using a Bruker AVANCE III 300 MHz instrument equipped with a 4 mm double resonance probe.…”