Determination of Chain-Folding Structure of Isotactic Polypropylene in Melt-Grown α Crystals by 13C–13C Double Quantum NMR and Selective Isotopic Labeling

Chain Dynamics of Partially Disentangled UHMWPE around Melting Point Characterized by 1H Low-Field Solid-State NMR

Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused ¹H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. ¹H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.

Quantifying the Chain Folding in Polymer Single Crystals by Single-Molecule Force Spectroscopy

Chain folding is a motif of polymer crystallization, which is essential for determining the crystallization kinetics. However, the experimental quantification of the chain folding remains a challenge because of limited instrumental resolution. Here, we quantify chain folding in solution-grown single crystals by using atomic force microscopy (AFM)-based single-molecule force spectroscopy. The fingerprint spectrum of force-induced chain motion allows us to decipher the adjacent and nonadjacent re-entry folding with spatial resolution of subnanometers. The average fractions of adjacent re-entry folds ⟨f⟩ are in the range 91-95% for polycaprolactone, poly-l-lactic acid, and polyamide 66, which is higher than the values determined by other classical technologies. The established single-molecule method is applicable to a broad range of crystalline polymer systems with different chain conformations or compositions.

Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR

Chain-level structure of semicrystalline polymers in melt- and solution-grown crystals has been debated over the past half century. Recently, 13C⁻13C double quantum (DQ) Nuclear Magnetic Resonance (NMR) spectroscopy has been successfully applied to investigate chain-folding (CF) structure and packing structure of 13C enriched polymers after solution and melt crystallization. We review recent NMR studies for (i) packing structure, (ii) chain trajectory, (iii) conformation of the folded chains, (iv) nucleation mechanisms, (v) deformation mechanism, and (vi) molecular dynamics of semicrystalline polymers.

Chain Trajectory of Semicrystalline Polymers As Revealed by Solid-State NMR Spectroscopy

Over the last half century, a chain-folding structure of semicrystalline polymers has been debated in polymer science. Recently, ¹³C-¹³C double quantum (DQ) NMR spectroscopy combined with ¹³C selective isotope labeling has been developed to investigate re-entrance sites of the folded chains, mean values of adjacent re-entry number ⟨n⟩ and fraction ⟨F⟩ of semicrystalline polymers. This viewpoint highlights the versatile approaches of using solid-state (ss) NMR and isotope labeling for revealing (i) chain trajectory in melt- and solution-grown crystals, (ii) conformation of the folded chains in single crystals, (iii) self-folding in the early stage of crystallization, and (iv) unfolding of the folded chains under stretching.

Unfolding of Isotactic Polypropylene under Uniaxial Stretching

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, ¹³C-¹³C Double Quantum (DQ) NMR was applied to trace the structural evolution of ¹³C-labeled isotactic polypropylene (iPP) chains inside the crystallites stretched to an engineering strain (e) of 21 at 100 °C. DQ NMR based on spatial proximity of ¹³C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of iPP.

Role of Thermal History and Entanglement Related Thickness Selection in Polymer Crystallization

Using molecular dynamics simulations and primitive path analysis, we show that hot entangled polymer melts can crystallize faster with higher crystallinities and larger crystalline stem lengths, as compared to cold melts under rapid quenching conditions or during cold-crystallization. This counterintuitive phenomenon similar to the so-called Mpemba effect observed for water can be explained by the temperature dependence of entanglements. Our results demonstrate the key role of the entanglement state for crystallization properties and provide a new approach to understand the role of thermal history and to the open question of thickness selection in polymer crystallization.

Folding of Polymer Chains in the Early Stage of Crystallization

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated ¹³C-¹³C double quantum (DQ) buildup curves demonstrated that individual iPP chains adopt adjacent re-entry sequences with an average folding number ⟨n⟩ = 3-4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨F⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.

Molecular Structural Basis for Stereocomplex Formation of Polylactide Enantiomers in Dilute Solution

Poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) alternatively pack with each other and form stereocomplex crystals (SCs). The crystal habits of SCs formed in the dilute solution highly depend on the molecular weight (⟨M_w⟩). In this study, we investigated chain-folding (CF) structure for ¹³C labeled PLLA (l-PLLA) chains in SCs with PDLAs that have either high or low ⟨M_w⟩s by employing an advanced Double Quantum (DQ) NMR. It was found that the ensemble average of the successive adjacent re-entry number ⟨n⟩ for the l-PLLA chains drastically change depending on ⟨M_w⟩s of the counter PDLA chains in the SCs. It was concluded that the CF structures of l-PLLA depending on ⟨M_w⟩s of PDLA determine the crystal habits of SCs.