Highly Water-Soluble Thermally Responsive Poly(thiophene)-Based Brushes

Thermoresponsive luminescent electrospun fibers prepared from poly(DMAEMA-co-SA-co-StFl) multifunctional random copolymers

We report novel thermoresponsive electrospun (ES) fibers prepared from multifunctional random copolymers of poly((2-(dimethylamino)ethyl methacrylate)-co-(stearyl acrylate) -co-(9,9-dihexyl-2-(4-vinylpenyl)-9H-fluorene)) (poly(DMAEMA-co-SA-co-StFl)). The moieties of DMAEMA, SA, and StFl were designed to exhibit the thermoresponsive, physical cross-linking, and fluorescent functionality, respectively. The effects of the copolymer compositions on the morphology and photoluminescence of the prepared ES fibers were explored. The prepared P4 copolymer with the DMAEMA/SA/StFl mole ratio of 92/3/5 showed the lower critical solution temperature (LCST) of 32.5 °C. A significant temperature-dependent swelling and de-swelling behavior was found in the P4 ES fibers, which had the diameter of 753 ± 174 nm and 5-10 nm StFl aggregated domain. Accompanied with volume-changing on the P4 ES fibers, a reversible photoluminescence (PL) quenching was also observed during the heating and cooling cycle between 40 and 60 °C. Such reversible switching of the "on-off" PL intensity was probably due to the light absorption ability of the StFl moiety, resulted from the extended/compact structural transformation on the PDMAEMA moiety. Furthermore, the high surface/volume ratio of the ES fibers led a much better temperature response compared with the corresponding spin-coated film. The present study demonstrated that the ES fibers prepared from multifunctional copolymers exhibited the thermoreversible variation on both volume and photoluminescence intensity.

Stimuli-responsive epoxy coatings

The design, formulation, and characterization of new epoxy coatings with built-in chemical and threshold temperature sensors are reported. The materials were prepared by dissolving a chromogenic, fluorescent oligo(p-phenylenevinylene) dye into a cross-linked epoxy resin by reacting monomer/cross-linker/dye mixtures at 180-200 degrees C and quenching the cured polymer to below its glass transition temperature (T(g)). Subjecting these kinetically trapped, thermodynamically unstable molecularly mixed epoxy/dye blends to temperatures above T(g) leads to aggregation of the dye molecules and causes permanent and pronounced changes of their optical absorption and fluorescence properties. Exposure of the materials to selected chemical stimuli, e.g., water, acid, base, and several organic compounds, also causes plasticization of the polymer matrix and leads to irreversible aggregation of dye molecules, concomitant with the pronounced fluorescence and absorption color change. This response is well described by standard kinetic models and can be controlled via the chemical structure and cross-link density of the resin and the dye content.

Monte Carlo simulation and molecular theory of tethered polyelectrolytes

We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.

Structure–LCST relationships for end-functionalized water-soluble polymers: an “accelerated” approach to phase behaviour studies

A novel "high throughput" technique for LCST measurement was developed which is able to identify the effect of subtle changes in end group composition on the aqueous phase behaviour of water-soluble poly(2-(dimethylamino)ethyl methacrylate).

Aggregation-Mediated Optical Properties of pH-Responsive Anionic Conjugated Polyelectrolytes

Conjugated polyelectrolyte copolymers containing 2,1,3-benzothiadiazole- (BT) and oligo(ethylene oxide)-substituted fluorene and phenylene units have been designed and synthesized. The phenylene pendent groups also have carboxylic acid functionalities, which allow probing the effect of pH on optical properties. The BT content in the backbone can be regulated at the synthesis stage. Dynamic light scattering studies show that polymers aggregate in water at low pH. Increased interchain contacts give rise to a lowering of the photoluminescence (PL) efficiency via self-quenching when the BT units are absent and increased levels of FRET from the phenylene-fluorene segments to BT. Furthermore, the PL efficiency of BT increases in the aggregated structures. Examination of solvent effects indicates that the increased BT efficiencies are likely due to decreased contact with water. The changes in PL efficiencies are reversible, showing that the aggregates are dynamic and not kinetically constrained.