Interaction of Poly L-Lactide and Tungsten Disulfide Nanotubes Studied by in Situ X-ray Scattering during Expansion of PLLA/WS2NT Nanocomposite Tubes.

Lison Rocher,Tiziana Di Luccio,Alla Zak,Julia A Kornfield,Alex B Lennon,Gary H Menary,Riccardo Miscioscia,Giovanni De Filippo,Fulvia Villani,Andrew S Ylitalo,Giuseppe Pandolfi

doi:10.3390/polym13111764

Abstract

In situ synchrotron X-ray scattering was used to reveal the transient microstructure of poly(L-lactide) (PLLA)/tungsten disulfide inorganic nanotubes (WS2NTs) nanocomposites. This microstructure is formed during the blow molding process (“tube expansion”) of an extruded polymer tube, an important step in the manufacturing of PLLA-based bioresorbable vascular scaffolds (BVS). A fundamental understanding of how such a microstructure develops during processing is relevant to two unmet needs in PLLA-based BVS: increasing strength to enable thinner devices and improving radiopacity to enable imaging during implantation. Here, we focus on how the flow generated during tube expansion affects the orientation of the WS2NTs and the formation of polymer crystals by comparing neat PLLA and nanocomposite tubes under different expansion conditions. Surprisingly, the WS2NTs remain oriented along the extrusion direction despite significant strain in the transverse direction while the PLLA crystals (c-axis) form along the circumferential direction of the tube. Although WS2NTs promote the nucleation of PLLA crystals in nanocomposite tubes, crystallization proceeds with largely the same orientation as in neat PLLA tubes. We suggest that the reason for the unusual independence of the orientations of the nanotubes and polymer crystals stems from the favorable interaction between PLLA and WS2NTs. This favorable interaction leads WS2NTs to disperse well in PLLA and strongly orient along the axis of the PLLA tube during extrusion. As a consequence, the nanotubes are aligned orthogonally to the circumferential stretching direction, which appears to decouple the orientations of PLLA crystals and WS2NTs.

Highlights

Dispersing nanoparticles in a polymer matrix can significantly enhance the chemical and physical properties of the polymer
We present the strain during tube expansion (Equation (1)) calculated from the measured outer diameter (OD) and the inferred inner diameter (ID) (Equation (3))
The annealing temperatures Tann, as measured by the IR sensor, range from 50 ◦C (~55 ◦C in the tube), the lowest temperature that permitted deformation, to 80 ◦C (~100 ◦C in the tube), at which quiescent crystallization becomes significant for PLLA [64]

Summary

Introduction

Dispersing nanoparticles in a polymer matrix can significantly enhance the chemical and physical properties of the polymer. This potential for polymer nanocomposites to outperform neat polymers has motivated decades of growth in academic research and the nanocomposite industry [1,2,3,4]. Bulk performance of a polymer nanocomposite is strongly dependent on the ability of the nanoparticles to disperse homogeneously in the polymer matrix, which is determined by the interaction between the nanoparticle and the polymer [5,6,7,8]. Dispersion remains a key challenge for graphene and carbon nanotubes (CNT) because the van der Waals interaction between nanoparticles leads to agglomerated bundles [9,10]. A better control over desired properties can be achieved through the manufacturing process [12,13,14] or modification of the microstructure at the interface [15] without adding further components to the composite

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