Stress-induced Bimodal Ordering in POSS/PCL Biodegradable Shape Memory Nanocomposites
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Stress-induced Bimodal Ordering in POSS/PCL Biodegradable Shape Memory Nanocomposites Bonifacio Alvarado-Tenorio1,2, Angel Romo-Uribe1,* and Patrick T. Mather2,* 1
Lab. de Nanopolimeros y Coloides, Instituto de Ciencias Fisicas, Universidad Nacional Autonoma de Mexico, Cuernavaca Mor. 62210, MEXICO 2 Syracuse Biomaterials Institute, Syracuse University, Syracuse NY 13244-1240, U.S.A. * To whom correspondence should be addressed: [email protected], [email protected] ABSTRACT Simultaneous wide- and small- angle X-ray scattering (WAXS-SAXS) has revealed a stress-induced bimodal orientation of POSS crystals and PCL chains, both in a constrained POSS/PCL crosslinked network architecture with shape memory properties. POSS/PCL nanocomposites with molecular weight of 2,600 g/mol exhibiting shape memory behavior were synthesized and variation of crosslinker molar ratio was used to obtain POSS/PCL networks with different crosslink density (Alvarado-Tenorio et al., Macromolecules, 44, 5682, 2011). In that study it was shown that there are POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. In this work, it will be shown that elongation at room temperature of all the networks yielded a double-induced orientation (90º and 180º) of the POSS crystals, as indicated by the 101 reflection. Moreover, it was also detected stretched-induced crystallization of the otherwise amorphous PCL chains. Furthermore, SAXS data showed long periods in the meridional and equatorial orientations of 630 Å, 90 Å and 45 Å corresponding to a lamellar nanostructure of PCL chains. The induced bimodal orientation of the POSS-PCL molecular network will be correlated with its shape memory properties. INTRODUCTION Polyhedral oligosilsesquioxane (POSS) nanocomposites with shape memory properties represent an attractive field for biomaterials application. The organic-inorganic nanoparticle POSS provides significant enhancement of thermal and mechanical properties. Recently Knight et al created a series of biodegradable hybrid polymer networks with shape memory combining covalently poly (lactide-co-glycolide) (PLGA) segments and POSS nanoparticles [3]. The segments of activation (soft segments) of Poly (d-lactide) showed a defined Tg and also phase separation of the activation segments which was independent of the concentration of POSS (hard phase). However, POSS induced a rubber elastic plateau which increased as the concentration of POSS increased. The materials were proven to be biodegradable, opening the potential for resorbable medical implants with shape memory properties. On the other hand, Lee et al [1] reported a unique POSS-PCL nanocomposite suitable for biomedical applications. The molecular architecture consisted of a “double network” which features the superposition of a covalent network with a percolative physical network. Covalent network chain features polycaprolactone (PCL) tethered on a single POSS moiety by virtue of its use as a di-functional initiator for PCL ring-opening polym
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