Melting-Induced Evolution of Morphology, Entanglement Density, and Ultradrawability of Solution-Crystallized Ultrahigh-Molecular-Weight Polyethylene

Abstract

The melting-induced change in density of physical network junctions, which are formed by chain entanglements and network junctions due to anchoring of chain segments to crystals, is studied by 1 H NMR T 2 relaxometry for solution-and melt-crystallized ultra-high molecular weight polyethylene (UHMWPE) -sc-UH and mc-UH, respectively. The NMR results are complemented by real-time synchrotron WAXS and SAXS analyses to extract the sizes of the crystalline lamellae and inter-crystalline domains. Below the melting temperature, the network of physical junctions is denser in the amorphous phase of mc-UH than the one in sc-UH owing to lower entanglement density and smaller number of physical junctions from polymer crystals in sc-UH. However, the difference in the total density of physical junctions between mc-UH and sc-UH films decreases with decreasing crystallinity during melting. At the end of the melting trajectory, at vanishing crystallinity, the volume-average entanglement density, as characterized by the NMR method, is approximately the same in sc-and mc-UH. This indicates that the entanglement density in sc-UH films increases during melting owing to fast buildup of local chain entanglements. These entanglements are formed by segments of the same chain, neighboring chains, or the both due to a displacement of chain fragments upon lamellar thickening and due to the so-" p " that occurs locally in the amorphous domains. The increase in the entanglement density in sc-UH is additionally confirmed by solid-state drawability of sc-UH films that were annealed in the melting region but below the end of melting. The maximum draw ratio decreases and the drawing stress increases with increasing annealing temperature.

Gaussian and Non-Gaussian Distributions of Fracture Properties in Tensile Stretching of High-Density Polyethylene

“…54 The FID curves of HDPE samples with different microstructures are given in Figure 18. Furthermore, a linear combination of Abragamian, Gaussian, and exponential functions gives the best fit of T 2 decay at 70 °C for HDPE systems 54,64 to the annealed (e.g., 15.5 wt %) and isothermally crystallized (e.g., 14.3 wt %) ones for HDPE-151K. The molecular mobility in the entangled amorphous network as indicated by the T 2 am relaxation time, however, is affected by the thermal treatment conditions only slightly.…”

“…The FID curves of HDPE samples with different microstructures are given in Figure . Furthermore, a linear combination of Abragamian, Gaussian, and exponential functions gives the best fit of T 2 decay at 70 °C for HDPE systems , where A (0) index represents the amplitude of the relaxation component for each phase domain, and the mean α value of 0.165 was utilized to deconvolute the T 2 decay into relaxation components from the different phases. The three relaxation components were attributed to the crystalline phase ( T 2 cr relaxation time), the crystalline–amorphous interface ( T 2 in relaxation time), and the amorphous network chain fraction ( T 2 am relaxation time).…”

Optimal Entanglement of Polymers Promotes Formation of Highly Oriented Fibers

“…Using our theory, we estimated the optimal N/N e ratio to be around 10 2 . On the other hand, experimental studies suggest [8,12] that the equilibrium N/N e ratio for ultrahigh molecular weight polyethylene (UHMWPE) is around 3 × 10 3 (for the polymers with M w ≈ 5 × 10 6 g/mol). Homogeneous polymerization with simultaneous crystallization was able to reduce the entanglement density approximately 3-fold in the synthesized nascent polymer having such molecular weight [8].…”

“…Methods of obtaining oriented PE fibers include solution(gel)-spinning [4,5], electrospinning [6,7], and the recently developed solvent-free techniques [8,9]. The popular hypothesis suggests that the presence of interchain entanglements, which appear due to uncrossablility of chains, limits stretching of a polymer sample [5,[9][10][11][12]. Different techniques decrease the density of entanglements in different ways.…”

Optimal Entanglement of Polymers Promotes the Formation of Highly Oriented Fibers

“…Methods of obtaining oriented PE fibers include solution­(gel)-spinning, , electrospinning, , and the recently developed solvent-free techniques. , The popular hypothesis suggests that the presence of interchain entanglements, which appear because of the uncrossablility of the chains, limits the stretching of a polymer sample. ,− Different techniques decrease the density of entanglements in different ways. During solution­(gel)-spinning, for example, the entanglement density is reduced by placing the low-concentrated polymer material in a good solvent and drying it afterward to obtain low-entangled films or strands .…”