Control of the Porous Structure of Polystyrene Particles Obtained by Nonsolvent Induced Phase Separation.

Porous polystyrene microspheres were produced by a process of nonsolvent induced phase separation (NIPS) from ternary polymer-solvent-nonsolvent (polystyrene-toluene-ethanol) systems and characterized by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) techniques. This study provides evidence for a link between the structural morphology of the porous polystyrene particles and the polystyrene concentration in the initial solutions. A reciprocal relationship between pore diameter and polymer concentration was observed for the systems with the polymer amount below the critical chain overlap concentration, C*. Above C*, this relationship breaks down. The reciprocal relationship between porosity and polymer concentration can be used to facilitate the fine control of the void size. We demonstrate that the observed reciprocal relationship between pore diameter and polymer concentration correlates well with the relative amount of nonsolvent present in the system at the onset of the phase separation process. The pore size can be reduced and, consequently, the pore surface area can be increased either by reducing the polymer concentration in the initial solution or by decreasing the polymer molecular weight in the sample composition.

2D-LC characterization of comb-shaped polymers using isotope effect.

A rigorous molecular characterization of comb-shaped polystyrene (PS) was carried out taking advantage of its molecular structure, a normal hydrogenous backbone, and deuterated side chains. Normal phase LC (NPLC) can separate the comb PS species well according to their molecular weight. Nonetheless, it cannot distinguish the backbone from the side chains and the differently structured polymers having a similar molecular weight, e.g, a single backbone comb and a coupled backbone comb with fewer side chains. In contrast to NPLC, the hydrogenous polymer is retained longer than the deuterated counterpart in reversed phase LC (RPLC). When the isotope sensitivity of RPLC is taken advantage of, the comb PS is cross fractionated by NPLC and RPLC, and a two-dimensional mapping with respect to the backbone chain length and the number of branches is fully established.

The effect of PEO length on the self-assembly of poly(ethylene oxide)-tetrapeptide conjugates prepared by "Click" chemistry.

Two series of poly(ethylene oxide)-tetrapeptide conjugates have been prepared using a "Click" reaction between an alkyne-modified tetra(phenylalanine) or tetra(valine) and various azide-terminated poly(ethylene oxide) (PEO) oligomers. Three different PEO precursors were used to prepare these conjugates, with number-average molecular weights of 350, 1200, and 1800 Da. Assembly of mPEO-F4-OEt and mPEO-V4-OEt conjugates was achieved by dialysis of a THF solution of the conjugate against water or by direct aqueous rehydration of a thin film. The PEO length has a profound effect on the outcome of the self-assembly, with the F4 conjugates giving rise to nanotubes, fibers, and wormlike micelles, respectively, as the length of the PEO block is increased. For the V4 series, the propensity to form beta-sheets dominates, and hence, the self-assembled structures are reminiscent of those formed by peptides alone, even at the longer PEO lengths. Thus, this systematic study demonstrates that the self-assembly of PEO-peptides depends on both the nature of the peptides and the relative PEO block length.

Soft hydrogels from nanotubes of poly(ethylene oxide)-tetraphenylalanine conjugates prepared by click chemistry.

A new poly(ethylene oxide)-tetraphenylalanine polymer-peptide conjugate has been prepared via a "click" reaction between an alkyne-modified peptide and an azide-terminated PEO oligomer. Self-assembled nanotubes are formed after dialysis of a THF solution of this polymer-peptide conjugate against water. The structure of these nanotubes has been probed by circular dichroism, IR, TEM, and SAXS. From these data, it is apparent that self-assembly involves the formation of antiparallel beta-sheets and pi-pi-stacking. Nanotubes are formed at concentrations between 2 and 10 mg mL(-1). Entanglement between adjacent nanotubes occurs at higher concentrations, resulting in the formation of soft hydrogels. Gel strength increases at higher polymer-peptide conjugate concentration, as expected.

The performance of poly(styrene)-block-poly(2-vinyl pyridine)-block-poly(styrene) triblock copolymers as pH-driven actuators.

Poly(styrene)--poly(2-vinyl pyridine)--poly(styrene) (PS--P2VP--PS) triblock copolymers were synthesised by anionic polymerisation. Thick films were cast from solution and their structure analysed by small angle X-ray scattering (SAXS). Longer annealing times led to more ordered structures whereas short evaporation times effectively "lock" the polymer chains in a disordered state by vitrification. Well-ordered structures not only provide an isotropic network, which reduces localised stress within the material, but are also essential for fundamental studies of soft matter because their activity on the molecular scale must be analysed and understood prior to their use in technological applications. Well-characterised PS--P2VP--PS materials have been coupled to a pH-oscillating reaction and their potential application as responsive actuators is discussed.