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

2014-05-13T00:00:00Z (GMT) by Jia Kang Toshikazu Miyoshi
The chain packing, crystal thickness, molecular dynamics, and melting temperature of α-form <i>isotactic</i> polypropylene (<i>i</i>PP) drawn uniaxially at high temperatures of 100–150 °C were investigated using solid-state (SS) NMR and DSC. Two types of <i>i</i>PP samples with disordered (α<sub>1</sub>) and relatively ordered (α<sub>2</sub>-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (<i>e</i>) of approximately 20. High-resolution <sup>13</sup>C NMR detected continuous α<sub>2</sub> → α<sub>1</sub> transformations in the original α<sub>2</sub>-rich samples over the entire deformation range at all drawing temperatures (<i>T</i><sub>d</sub>s). A sudden α<sub>1</sub> → α<sub>2</sub> transformation was found to occur in the original α<sub>1</sub> sample in the small <i>e</i> range of approximately 3–7 at <i>T</i><sub>d</sub> = 140 °C. Then, in the late stage, the newly grown α<sub>2</sub> structure reversely transformed into α<sub>1</sub> structure with further increase in <i>e</i>, as observed in the original α<sub>2</sub>-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous α<sub>2</sub> → α<sub>1</sub> transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep α<sub>1</sub> → α<sub>2</sub> transformation in the smaller <i>e</i> range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (<i>e</i> ≈ 20) of the original α<sub>2</sub>-rich and α<sub>1</sub> samples at <i>T</i><sub>d</sub> = 150 and 140 °C, respectively, <sup>1</sup>H spin diffusion verified increases in the crystal thickness in both the former (14.1 at <i>e</i> = 0 → 20.1 nm at <i>e</i> = 20) and the latter (9.2 → 17.0 nm). Centerband-only detection of exchange (CODEX) NMR at 120 °C demonstrated that the correlation time (⟨τ<sub>c</sub>⟩) of the helical jump for the former drastically decreased from ⟨τ<sub>c</sub>⟩ = 52.4 ± 5.2 at <i>e</i> = 0 to 9.3 ± 1.8 ms at <i>e</i> = 20 but slightly increased from 4.2 ± 1.3 to 7.1 ± 0.9 ms for the latter. Additionally, DSC indicated that the melting temperature (<i>T</i><sub>m</sub>) for the former decreased considerably from 173 °C at <i>e</i> = 0 to 165 °C at <i>e</i> = 20, whereas the melting temperature (<i>T</i><sub>m</sub>) remained nearly invariant at 163 °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 <i>T</i><sub>m</sub> of largely deformed <i>i</i>PP materials. The established relations among the structures, the dynamics, and the thermal properties provide a useful guide to achieving improved properties of <i>i</i>PP materials under processing.