Two Chain-Packing Transformations and Their Effects on the Molecular Dynamics and Thermal Properties of 伪-Form Isotactic Poly(propylene) under Hot Drawing: A Solid-State NMR Study

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• 作者：Jia Kang ; Toshikazu Miyoshi
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：May 13, 2014
• 年：2014
• 卷：47
• 期：9
• 页码：2993-3004
• 全文大小：676K
• 年卷期：v.47,no.9(May 13, 2014)
• ISSN：1520-5835

文摘

The chain packing, crystal thickness, molecular dynamics, and melting temperature of 伪-form isotactic polypropylene (iPP) drawn uniaxially at high temperatures of 100鈥?50 掳C were investigated using solid-state (SS) NMR and DSC. Two types of iPP samples with disordered (伪₁) and relatively ordered (伪₂-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (e) of approximately 20. High-resolution ¹³C NMR detected continuous 伪₂ 鈫?伪₁ transformations in the original 伪₂-rich samples over the entire deformation range at all drawing temperatures (T_ds). A sudden 伪₁ 鈫?伪₂ transformation was found to occur in the original 伪₁ sample in the small e range of approximately 3鈥? at T_d = 140 掳C. Then, in the late stage, the newly grown 伪₂ structure reversely transformed into 伪₁ structure with further increase in e, as observed in the original 伪₂-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous 伪₂ 鈫?伪₁ transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep 伪₁ 鈫?伪₂ transformation in the smaller e range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (e 鈮?20) of the original 伪₂-rich and 伪₁ samples at T_d = 150 and 140 掳C, respectively, ¹H spin diffusion verified increases in the crystal thickness in both the former (14.1 at e = 0 鈫?20.1 nm at e = 20) and the latter (9.2 鈫?17.0 nm). Centerband-only detection of exchange (CODEX) NMR at 120 掳C demonstrated that the correlation time (蟿_c) of the helical jump for the former drastically decreased from 蟿_c = 52.4 卤 5.2 at e = 0 to 9.3 卤 1.8 ms at e = 20 but slightly increased from 4.2 卤 1.3 to 7.1 卤 0.9 ms for the latter. Additionally, DSC indicated that the melting temperature (T_m) for the former decreased considerably from 173 掳C at e = 0 to 165 掳C at e = 20, whereas the melting temperature (T_m) remained nearly invariant at 163 掳C for the latter. On the basis of these findings, we conclude that the local packing structure plays a crucial role in determining the molecular dynamics of the stems and T_m of largely deformed iPP materials. The established relations among the structures, the dynamics, and the thermal properties provide a useful guide to achieving improved properties of iPP materials under processing.