Phase Transitions in UHMWPE Fiber Compacts Studied by in situ Synchrotron Microbeam WAXS

Abstract

Summary: The temperature dependence of the structure of either cross‐linked or non‐cross‐linked ultra‐high‐molecular‐weight polyethylene (UHMWPE) fiber compacts was studied by synchrotron microbeam wide‐angle X‐ray scattering (WAXS), focusing on the fiber‐fiber interface. The phase transition sequence is: melting of the monoclinic phase in the fiber skin, which was completed by 135 °C; melting of the unconstrained orthorhombic phase, by 152 °C; melting of the constrained orthorhombic phase and a orthorhombic‐hexagonal phase transition until 157 °C; and gradual melting of the hexagonal phase, up to 165 °C. Cross‐linking provides additional thermal stabilization.Histograms of the azimuthally averaged X‐ray intensity as a function of temperature for cross‐linked ultra‐high‐molecular‐weight polyethylene fiber compacts molded at 145 °C.imageHistograms of the azimuthally averaged X‐ray intensity as a function of temperature for cross‐linked ultra‐high‐molecular‐weight polyethylene fiber compacts molded at 145 °C.

On the β Transition in High Density Polyethylene: the Effect of Transcrystallinity

“…These two phenomena result from the melting of the monoclinic phase in the fiber skin at 130 8C and its un-orientated recrystallization to form a folded-chain lamellar structure. [13] Finally, Figure 1c presents a typical spherulitic phase, showing the lamellae grown from a nucleation center and the meeting zone of a number of large spherulites.…”

New Insights into Lamellar Twisting in Transcrystalline Polyethylene

“…The secondwasthediscovery,achievedbyinsitumicrodiffraction studies of the fiber skin of compacted UHMWPE, of melting of the monoclinic phase while leaving intact the orthorhombic fiber core. [16] Upon cooling, the fiber skin re-crystallizes to form an unoriented lamellar structure with evidence of cocrystallization with the matrix. [4] This last observation, as well as more recent X-ray diffraction data, [17] has put into question the validity of our previous premise that the epitaxial nucleation of the transcrystalline lamellae occurs such that their c-axes are aligned parallel to the axis of the fiber.…”

Harnessing the Melting Peculiarities of Ultra‐High Molecular Weight Polyethylene Fibers for the Processing of Compacted Fiber Composites

“…[15,16] This phase was also observed in cross-linked UHMWPE fiber compacts and UHMWPE fiber compacts that were not crosslinked, both of which showed a mixture of constrained and unconstrained PE. [17] In previous investigations, we showed that under hydrostatic compression of extended chain UHMWPE fibers, the mesomorphic hexagonal phase exists even at relatively low pressure of about 10 MPa and is stable within a reasonably wide temperature-time domain: from 180 to 255 8C for more than 30 min. In addition, we reported that the transition between the orthorhombic and the hexagonal phases is highly relevant to the compaction process, [18,19] in a sense that it promotes good bonding of the fibers while preserving the essential fiber structure.…”

“…In addition, we reported that the transition between the orthorhombic and the hexagonal phases is highly relevant to the compaction process, [18,19] in a sense that it promotes good bonding of the fibers while preserving the essential fiber structure. This phenomenon of fibers surviving the compaction process without major structural changes was also observed using synchrotron X-ray diffraction by Ratner et al [17] In this study, we focused on the engineering aspects of the processing schemes for fiber compaction into monolithic composites. We interpret X-ray diffraction data in terms of a pseudo-phase diagram, to suggest how the unique crystallinity behavior of oriented UHMWPE can be used for a stable compaction process.…”