Chain Trajectory and Crystallization Mechanism of a Semicrystalline Polymer in Melt- and Solution-Grown Crystals As Studied Using ¹³C–¹³C Double-Quantum NMR

Abstract

The re-entrance sites, successive chain-folding number ⟨<i>n</i>⟩, and chain-folding fraction ⟨<i>F</i>⟩ of the chain-folding (CF) structure of ¹³C CH₃-labeled <i>isotactic</i> poly­(1-butene) (<i>i</i>PB1) with an weight-averaged molecular weight (⟨<i>M</i>_w⟩ = 37 K g/mol) in solution- and melt-grown crystals as a function of crystallization temperature (<i>T</i>_c) were determined using solid-state (SS) NMR. The solution- and melt-grown crystals possessed adjacent re-entry structures between the right- and left-handed stems along the (100) and (010) planes, which were invariant as a function of <i>T</i>_c. The adjacent re-entry structures in the former exhibited long-range order (⟨<i>n</i>⟩ ≥ 8) compared with that in the latter (⟨<i>n</i>⟩ ≥ 1.7–2). These results indicated that the concentration and entanglement of polymers play significant roles in the CF process and structural formation during the initial stage of crystallization, whereas kinetics does not. Transmission electron microscopy (TEM) revealed well-defined hexagonal and circular crystals grown from the solution state at <i>T</i>_c = 60 and ∼0 °C, respectively. The morphological and molecular-level structural data demonstrated that kinetics influences the structural formations of polymers differently at different length scales during crystallization. Moreover, SS-NMR, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) indicated that the crystallinity (χ_c) and lamellar thickness (⟨<i>l</i>_c⟩) of the melt-grown crystals are highly dependent on <i>T</i>_c, whereas in the solution-grown crystals, these parameters are independent of <i>T</i>_c. The experimental results and molecular dynamics, as reported in the literature, indicated that both χ_c and ⟨<i>l</i>_c⟩ are primarily determined by the molecular dynamics of the stems after deposition of the chains on the growth front (late process)