Polypeptide-Coated Silica Particles Dispersed in Lyotropic Liquid Crystals of the Same Polypeptide.

When a particle is introduced into a liquid crystal (LC), it distorts the LC director field, leading to new arrangements of the particles. This phenomenon is ordinarily studied using >100 nm particles and ∼2 nm mesogens. Usually the particle surface and mesogens are chemically distinct, which adds an enthalpic effect, even though the more interesting interactions are entropic. To raise the structures to the visible regime, while minimizing chemical differences between the particle surface and mesogen, silica particles coated with an α-helical polypeptide have been prepared and dispersed in lyotropic polypeptide LCs. The polypeptide is poly(γ-stearyl-α,l-glutamate) or PSLG. To make the particles easy to manipulate and easy to find, the silica core included superparamagnetic magnetite (Fe3O4) and covalently attached dye. Two methods were used to place polypeptides on these magnetic, fluorescent particles: a multistep grafting-to approach in which whole polypeptides were attached and a one-pot grafting-from approach in which the polymerization of the monomers was initiated from the particle surface. These approaches resulted in sparse and dense surface coverages, respectively. The influence of surface curvature and polypeptide molecular weight on the design of sparsely covered particles was investigated using the grafting-to approach. The aggregated grafting-from particles when freshly dispersed in a PSLG/solvent matrix disrupted the orientation of the characteristic cholesteric LC (ChLC) phase directors. In time, the hybrid particles were expelled from some domains, enabling the return of the familiar helical twist of the cholesteric mesophase. The sparsely coated grafting-to hybrid particles when inserted in the PSLG/solvent matrix assembled into stable islet-like formations that could not be disrupted even by an external magnetic field. The bulk particles aligned in chains that were easily manipulated by a magnetic field. These results indicate that polypeptide ChLCs can control and facilitate colloidal assembly of particles with matching surfaces.

Colorful Polyelectrolytes: An Atom Transfer Radical Polymerization Route to Fluorescent Polystyrene Sulfonate.

A labeled green fluorescent polystyrene sulfonate (LNaPSS) has been synthesized using atom transfer radical polymerization of a styrene sulfonate monomer with a fluorescent co-monomer, fluorescein thiocyanate-vinyl aniline. As a result this 100 % sulfonated polymer contains no hydrophobic patches along the chain backbone besides the fluorescent marker itself. The concentration of the fluorescent monomer was kept low to maintain the characteristic properties of the anionic polyelectrolyte, LNaPSS. ATRP conditions facilitated the production of polymers spanning a range of molecular weights from 35,000 to 175,000 in gram-scale batches with polydispersity indices of 1.01-1.24. Molecular weight increased with the monomer to initiator ratio. Gel permeation chromatography results show a unimodal distribution, and the polymer structure was also confirmed by (1)H NMR and FT-IR spectroscopy. Fluorescence spectroscopy confirmed covalent bonding of fluorescein isothiocyanate to the polymer, indicating that the polymer is suitable as a probe in fluorescence microscopy. To demonstrate this ability, the polymer was used to locate structural features in salt crystals formed during drying, as in the evaporation of sea mist. A second application to probe diffusion studies is also demonstrated.

Aqueous-based initiator attachment and ATRP grafting of polymer brushes from poly(methyl methacrylate) substrates.

Many polymers, such as PMMA, are very susceptible to swelling or dissolution by organic solvents. Growing covalently attached polymer brushes from these surfaces by atom-transfer radical polymerization (ATRP) is challenging because of the typical requirement of organic solvent for initiator immobilization. We report an unprecedented, aqueous-based route to graft poly(N-isopropylacrylamide), PNIPAAm, from poly(methyl methacrylate), PMMA, surfaces by ATRP, wherein the underlying PMMA is unaffected. Successful attachment of the ATRP initiator, N-hydroxysuccinimidyl-2-bromo-2-methylpropionate, on amine-bearing PMMA surfaces was confirmed by XPS. From this surface-immobilized initiator, thermoresponsive PNIPAAm brushes were grown by aqueous ATRP to yield optically transparent PNIPAAm-grafted PMMA surfaces. This procedure is valuable, as it can be applied for the aqueous-based covalent attachment of ATRP initiator on any amine-functionalized surface, with subsequent polymerization of a variety of monomers.

Thermal Response of Poly(N-isopropylacrylamide) Brushes Probed by Surface Plasmon Resonance.

The thermally induced hydration transition of surface-grafted poly(N-isopropyl acrylamide) (PNIPAAm) brushes was probed by surface plasmon resonance spectroscopy (SPR) and contact angle measurements. Data are presented for a PNIPAAm brush film with a dry thickness of ∼50 nm that was synthesized by atom radical transfer polymerization on the surface of a self-assembled monolayer on gold. SPR measurements were taken as a function of temperature in two modes: the quasi-static mode, in which the sample was equilibrated at each temperature for ∼15 min prior to measurement, and the real-time mode, in which SPR reflectivity data were collected as the sample was heated and cooled at ∼4.5 °C/min. Both types of measurement indicate that the hydration transition for the PNIPAAm brush occurs over a broad range of temperatures (∼10-40 °C). This result is in accordance with theoretical predictions that have suggested that polymer brush structures on planar surfaces do not exhibit true critical solubility transitions. Contact angle measurements revealed a discontinuity in the surface wettability at a temperature (∼32 °C) that corresponds to the dilute aqueous critical solution temperature. Taken together, these results suggest that the polymer segments in the outermost region of the brush remain highly solvated until the dilute solution lower critical solution temperature (∼32°), while densely packed, less solvated segments within the brush layer undergo dehydration and collapse over a broad range of temperatures.