Mechanistic insights into the formation of polyion complex aggregates from cationic thermoresponsive diblock copolymers.

HYPOTHESIS: The formation of polyion complexes (PICs) comprising thermoresponsive polymers is intended to result in the formation of aggregates that undergo significant structural changes with temperature. Moreover the observed modifications might be critically affected by polymer structure and PICs composition. EXPERIMENTS: Different block copolymers based on cationic poly(3-acrylamidopropyltrimethylammonium chloride) and thermoresponsive poly(N-isopropylacrylamide) were synthesized by aqueous RAFT/MADIX polymerization at room temperature. Addition of poly(acrylic acid) in a controlled fashion led to the formation of PICs aggregates. The structural changes induced by temperature were characterized by differential scanning calorimetry, Nuclear Magnetic Resonance spectroscopy and scattering methods. FINDINGS: Thermoresponsive PICs undergo significant structural changes when increasing temperature above the cloud point of the thermoresponsive block. The reversibility of these phenomena depends strongly on the structural parameters of the block copolymers and on PICs composition.

Liquid crystalline polymer-Co nanorod hybrids: structural analysis and response to a magnetic field.

This work deals with the structural analysis of side-chain liquid crystalline polysiloxanes, doped with magnetic cobalt nanorods, and their orientational properties under a magnetic field. These new materials exhibit the original combination of orientational behavior and ferromagnetic properties at room temperature. Here we show that, within the liquid crystal polymer matrix, the cobalt nanorods self-assemble in bundles made of nanorod rows packed in a 2-dimensional hexagonal lattice. This structure accounts for the magnetic properties of the composites. The magnetic and orientational properties are discussed with respect to the nature of the polymer matrix.

Raster microdiffraction with synchrotron radiation of hydrated biopolymers with nanometre step-resolution: case study of starch granules.

X-ray radiation damage propagation is explored for hydrated starch granules in order to reduce the step resolution in raster-microdiffraction experiments to the nanometre range. Radiation damage was induced by synchrotron radiation microbeams of 5, 1 and 0.3 µm size with ∼0.1 nm wavelength in B-type potato, Canna edulis and Phajus grandifolius starch granules. A total loss of crystallinity of granules immersed in water was found at a dose of ∼1.3 photons nm(-3). The temperature dependence of radiation damage suggests that primary radiation damage prevails up to about 120 K while secondary radiation damage becomes effective at higher temperatures. Primary radiation damage remains confined to the beam track at 100 K. Propagation of radiation damage beyond the beam track at room temperature is assumed to be due to reactive species generated principally by water radiolysis induced by photoelectrons. By careful dose selection during data collection, raster scans with 500 nm step-resolution could be performed for granules immersed in water.

Correlation between structure and rheology of a model colloidal glass.

The microstructure and rheological properties of a model colloidal system was probed in the vicinity of the glass transition by small-angle and ultra small-angle x-ray scattering, dynamic light scattering (DLS) and bulk rheology. The volume fraction of the particles was deduced by modeling the structure factor and the absolute scattered intensity in a self-consistent way. The glass transition (phi(G)) was identified from the frequency dependence of the shear moduli in the linear regime. The experimentally observed behavior was then compared with the viscoelastic properties derived from mode-coupling theory (MCT) using the experimental structure factor as input to the theory. The ensemble-averaged intermediate scattering functions from DLS measurements were also compared with those calculated from the MCT and reasonable agreement was obtained.