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

The chain packing, crystal thickness, molecular dynamics, and melting temperature of α-form isotactic polypropylene (iPP) drawn uniaxially at high temperatures of 100–150 °C were investigated using solid-state (SS) NMR and DSC. Two types of iPP samples with disordered (α₁) and relatively ordered (α₂-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 α₂ → α₁ transformations in the original α₂-rich samples over the entire deformation range at all drawing temperatures (T_ds). A sudden α₁ → α₂ transformation was found to occur in the original α₁ sample in the small e range of approximately 3–7 at T_d = 140 °C. Then, in the late stage, the newly grown α₂ structure reversely transformed into α₁ structure with further increase in e, as observed in the original α₂-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous α₂ → α₁ transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep α₁ → α₂ transformation in the smaller e range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (e ≈ 20) of the original α₂-rich and α₁ samples at T_d = 150 and 140 °C, respectively, ¹H spin diffusion verified increases in the crystal thickness in both the former (14.1 at e = 0 → 20.1 nm at e = 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_c⟩) of the helical jump for the former drastically decreased from ⟨τ_c⟩ = 52.4 ± 5.2 at e = 0 to 9.3 ± 1.8 ms at e = 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 (T_m) for the former decreased considerably from 173 °C at e = 0 to 165 °C at e = 20, whereas the melting temperature (T_m) 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 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.