Structural reorganization of cylindrical nanoparticles triggered by polylactide stereocomplexation.

Co-crystallization of polymers with different configurations/tacticities provides access to materials with enhanced performance. The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material. Herein, we report the preparation of stereocomplex micelles from a mixture of poly(L-lactide)-b-poly(acrylic acid) and poly(D-lactide)-b-poly(acrylic acid) diblock copolymers in water via crystallization-driven self-assembly. During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders. Furthermore, mixture of cylinders with opposite homochirality in either THF/H2O mixtures or in pure water at 65 °C leads to disassembly into stereocomplexed spherical micelles. Similarly, a transition is also observed in a related PEO-b-PLLA/PEO-b-PDLA system, demonstrating wider applicability. This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.

In vitro biostability of poly(dimethyl siloxane/hexamethylene oxide)-based polyurethane/layered silicate nanocomposites.

We have prepared a number of silicone-based thermoplastic polyurethane (TPU) nanocomposites and demonstrated an enhancement of in vitro biostability against metal-ion-induced oxidation for potential use in long-term implantable medical devices. Organoclays based on both low-aspect-ratio hectorites and high-aspect-ratio fluoromicas were evaluated after being dual-modified with two quaternary alkyl ammonium salts with differing degrees of polarity. The resultant nanocomposites were tested for in vitro biostability using physiologically relevant oxidizing conditions. Subsequently, the effects of oxidative treatment on the surface degradation and bulk mechanical integrity of the nanocomposites were investigated and compared with the parent TPUs to identify nanocomposites with the most desirable features for long-term implantation. Here, we demonstrate that the low-aspect-ratio organohectorite was delaminated and well dispersed in the nanocomposites. Importantly, these factors gave rise to the enhanced oxidative stability. In addition, the mechanical properties of all nanocomposites were less adversely affected by the oxidative treatment compared to their parent TPUs. These results suggest the potential for improved mechanical integrity and biostability when suitable dual modified organoclays are incorporated in a silicone-based TPU.

Self-assembly and encoding of polymer-stabilized gold nanoparticles with surface-enhanced Raman reporter molecules.

Polymer-stabilized gold nanoparticles (AuNPs) were prepared and encoded with a range of surface-enhanced Raman reporter molecules. A range of as-synthesized polymers produced by reversible addition fragmentation chain transfer (RAFT) polymerization were demonstrated to self-assemble at the surface of AuNPs dispersed in water. The method involved the coprecipitation of polymer-gold conjugates by the addition of polymer dissolved in a water-miscible solvent to gold AuNPs dispersed in water. This method represents a simplification of the preparation of polymer-stabilized AuNPs compared with other published methods, in that the AuNPs do not need to be first transferred to an organic solvent. The process enabled the polymer stabilized AuNPs to be easily recovered by filtration or by phase transfer of the AuNPs to an organic solvent in which the RAFT polymer was soluble. The polymer-stabilized AuNPs were characterized by a range of methods including UV-visible spectrophotometry, transmission electron microscopy, thermogravimetric analysis, dynamic light scattering, and attenuated total reflection Fourier transform infrared spectroscopy. Furthermore, 1H pulsed field gradient spin echo NMR was utilized to characterize the self-diffusion of the polymer-stabilized AuNPs. Finally, we then demonstrated that these polymer-stabilized AuNPs maintained their ability to be encoded with surface-enhanced Raman spectroscopy reporter molecules.