Molecular Dynamics of a Polymer in Mixed Solvent: Atactic Polystyrene in a Mixture of Cyclohexane and N,N-Dimethylformamide

Polymer grafting to single-walled carbon nanotubes: effect of chain length on solubility, graft density and mechanical properties of macroscopic structures

Single-walled carbon nanotubes are grafted with polystyrene chains employing a graft-to protocol. Thermogravimetric analysis allows calculation of the grafted chain density and average interchain separation on the nanotube surface as a function of molecular weight. The separation scales with molecular weight as a power law with an exponent of ca. 0.588, showing the grafted chains to be in a swollen random walk conformation. This implies that chain packing is controlled by coil size in solution. In addition, the dispersed concentration of functionalized nanotubes scales with the size of the steric potential barrier that prevents aggregation of polymer functionalized nanotubes. It is also shown that the molecular weight of the grafted chains significantly affects the mechanical properties of nanotube films.

A cascade biodegradable polymer based on alternating cyclization and elimination reactions

Polymers that depolymerize by a cascade of intramolecular reactions in response to the removal of a stabilizing end-cap can allow for an unprecedented degree of control over the polymer degradation process. Described here is the development of polymers comprising N,N'-dimethylethylenediamine and 4-hydroxybenzyl alcohol linked by carbamate linkages. The polycarbamate backbone is stable in aqueous solution, but removal of a protective end-cap from the amine terminus allows the diamine to cyclize, forming N,N'-dimethylimidazolidinone and releasing the phenol, which undergoes a 1,6-elimination followed by the release of CO(2) to reveal the next amine to continue the cascade. These polymers therefore degrade by alternating cyclization and elimination reactions. First, a tert-butylcarbamate (Boc) group was introduced as a cleavable end-cap, and the degradation kinetics and mechanism were studied by (1)H nuclear magnetic resonance (NMR) spectroscopy and size exclusion chromatography. Next, to demonstrate the degradability of these polymers under biologically relevant conditions, poly(ethylene oxide) was introduced as an end-cap via an ester linkage, to provide an amphiphilic block copolymer. This copolymer was found to assemble into cascade degradable nanoparticles that were capable of encapsulating and subsequently releasing a fluorescent dye in aqueous solution. This new class of polymers therefore provides highly promising materials that can be used for the development of medical devices, drug delivery vehicles, and tissue engineering scaffolds with unique biodegradation properties.

Water and Polymer Dynamics in Chemically Cross-Linked Hydrogels of Poly(vinyl alcohol): A Molecular Dynamics Simulation Study

A topologically extended model of a chemically cross-linked hydrogel of poly(vinyl alcohol) (PVA) at high hydration degree has been developed for a molecular dynamics simulation with atomic detail at 323 K. The analysis of the 5 ns trajectory discloses structural and dynamic aspects of polymer solvation and elucidates the water hydrogen bonding and diffusion in the network. The features of local polymer dynamics indicate that PVA mobility is not affected by structural constraints of chemical junctions at the investigated cross-linking density, with a prevailing dumping effect due to water interaction. Simulation results are validated by a favorable comparison with findings of an incoherent quasi-elastic neutron scattering study of the same hydrogel system.