Metastable structures, interplay of sequential deformations, and interactions between tungsten disulfide nanotubes and poly(L-lactide) studied by in situ X-ray scattering

Abstract

Polymer crystallization, particularly near the glass transition, exhibits strong nonlinearities and prolonged metastability that enable fabrication of devices with complex hierarchal structure from nm to mm. A fascinating example arises in the production of bioresorbable scaffolds (BRS) from poly(L-lactide) (PLLA), in which a sequence of processes (extrusion, stretch-blow molding and crimping) create diverse semicrystalline morphologies, side-by-side within a span of a hundred microns (Figure 1). To discover how these structures form, we need to examine transient structure under conditions that mimic manufacturing processes. An apparatus that enables scattering measurements during the stretch-blow molding step, called "tube expansion" imposes a nearly constant-width elongation as it converts an extruded "preform" into an "expanded tube". To increase the range of accessible properties of PLLA-based BRS, we use this apparatus to examine inorganic nanotubes as potential reinforcing agents that also enhance radiopacity, relevant to clinical applications. Understanding how their microstructure develops during processing is relevant to increasing strength to enable thinner devices and improving radiopacity to enable imaging during implantation. Consistent with the premise of this MS, in-situ X-ray scattering reveals unanticipated phenomena in the transient microstructure of PLLA/WS2NTs nanocomposites during "tube expansion" (Figure 2).

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