A novel, ester-free initiator for surface-initiated free radical polymerization has been synthesized and tested. The structurally non-symmetrical azo-based initiator features a chemically stable alkane linker between the initiating group and the silane anchoring group, setting it apart from the majority of surface initiators that are linked by hydrolyzable moieties, such as esters. The novel design of the initiator is bolstered by an original synthetic approach, leading to a greater yield and a dramatic reduction in cyanide usage relative to previous methods. Here we demonstrate the capability of this novel initiator for surface-initiated free radical polymerization (SI-FRP), reverse ATRP, and RAFT, noting that Arrhenius behavior of SI-FRP differs significantly from that of FRP in the bulk. Furthermore, we show that polymer brushes formed from the novel initiator are more stable than those formed from ester-based initiators.

Amphiphilic polymer coatings were prepared by first generating surface-anchored polymer layers of poly(2-hydroxyethyl methacrylate) (PHEMA) on top of flat solid substrates followed by postpolymerization reaction on the hydroxyl terminus of HEMA’s pendent group using three classes of fluorinating agents, including organosilanes, acylchlorides, and trifluoroacetic anhydride (TFAA). The distribution of the fluorinated groups inside the polymer brushes was assessed by means of a suite of analytical probes, including contact angle, ellipsometry, infrared spectroscopy, atomic force microscopy, and near-edge x-ray absorption fine structure spectroscopy. While organosilane modifiers were found to reside primarily close to the tip of the brush, acylchlorides penetrated deep inside PHEMA thus forming random copolymers P(HEMA-co-fHEMA). The reaction of TFAA with the PHEMA brush led to the formation of amphiphilic diblocks, PHEMA-b-P(HEMA-co-fHEMA), whose bottom block comprised unmodified PHEMA and the top block was made of P(HEMA-co-fHEMA) rich in the fluorinated segments. This distribution of the fluorinated groups endowed PHEMA-b-P(HEMA-co-fHEMA) with responsive properties; while in hydrophobic environment P(HEMA-co-fHEMA) segregated to the surface, when in contact with a hydrophilic medium, PHEMA partitioned at the brush surface. The surface activity of the amphiphilic coatings was tested by studying the adsorption of fibrinogen (FIB). While some FIB adsorption occurred on most coatings, the ones made by TFAA modification of PHEMA remained relatively free of FIB.

We examine the protein distribution within an electrospun polymer nanofiber using polyvinyl alcohol and bovine serum albumin as a model system. We hypothesize that the location of the protein within the nanofiber can be controlled by carefully selecting the pH and the applied polarity of the electric field as the pH affects the net charge on the proteins. Using fluorescently labeled BSA and surface analysis, we observe that the distribution of BSA is affected by the pH of the electrospinning solution. Therefore, we further probe the relevant forces on the protein in solution during electrospinning. The role of hydrodynamic friction was assessed using glutaraldehyde and HCl as a tool to modify the viscosity of the solution during electrospinning. By varying the pH and the polarity of the applied electric field, we evaluated the effects of electrostatic forces and dielectrophoresis on the protein during fiber formation. We surmise that electrostatic forces and hydrodynamic friction are insignificant relative to dielectrophoretic forces; therefore, it is possible to separate species in a blend using polarizability contrast. A coaxial distribution of protein in the core can be obtained by electrospinning at the isoelectric point of the protein, whereas surface enrichment can be obtained when the protein carries a net charge.

Hydrophilic coatings were produced on flat hydrophobic substrates featuring n-octadecyltrichlorosilane (ODTS) and synthetic polypropylene (PP) nonwoven surfaces through the adsorption of denatured proteins. Specifically, physisorption from aqueous solutions of α-lactalbumin, lysozyme, fibrinogen, and two soy globulin proteins (glycinin and β-conglycinin) after chemical (urea) and thermal denaturation endowed the hydrophobic surfaces with amino and hydroxyl functionalities, yielding enhanced wettability. Proteins adsorbed strongly onto ODTS and PP through nonspecific interactions. The thickness of adsorbed heat-denatured proteins was adjusted by varying the pH, protein concentration in solution, and adsorption time. In addition, the stability of the immobilized protein layer was improved significantly after interfacial cross-linking with glutaraldehyde in the presence of sodium borohydride. The amino and hydroxyl groups present on the protein-modified surfaces served as reactive sites for the attachment of polymerization initiators from which polymer brushes were grown by surface-initiated atom-transfer radical polymerization of 2-hydroxyethyl methacrylate. Protein denaturation and adsorption as well as the grafting of polymeric brushes were characterized by circular dichroism, ellipsometry, contact angle, and Fourier transform infrared spectroscopy in the attenuated total reflection mode.

Recent efforts have established that thermoplastic elastomer gels (TPEGs) composed of styrenic triblock copolymers swollen with a midblock-selective solvent exhibit remarkable electromechanical properties as high-performance dielectric elastomers. This class of electroactive polymers typically requires high electric fi elds for actuation, and a shortcoming that continues to thwart the widespread commercialization of such materials in general is the need to apply mechanical prestrain prior to electroactuation to decrease fi lm thickness and, thus, the electric potential required to promote actuation. To alleviate this requirement, TPEGs consisting of acrylic triblock copolymers differing in molecular weight and composition, and swollen with a high dielectric, midblock-selective solvent are investigated. Synchrotron small-angle x-ray scattering is used to probe the nanoscale morphologies of the resultant materials, and analysis of quasi-static and cyclic tensile properties provides additional insight into both blend morphologies and electroactuation effi cacy. Actuation strains measured in the absence of mechanical prestrain exceed 100% on an area basis, and electric fi elds capable of inducing actuation are as low as ∼ 20 kV/mm. Failure occurs by either electromechanical instability or dielectric breakdown, depending on the copolymer and TPEG composition employed. The electromechanical properties of these acrylic-based TPEGs match or exceed those of skeletal muscle, in which case they constitute an attractive and unexplored alternative to existing dielectric elastomers.

A series of poly(styrene-b-dimethylsiloxane-bstyrene) (SDS) triblock copolymers with molecular weights ranging from 55 to 150 kg/mol and polydimethylsiloxane (PDMS) volume fractions ranging from 0.59 to 0.83 were used to fabricate membranes for ethanol/water separation by pervaporation. The rigid polystyrene (PS) microphase provides the membrane with structural integrity, while the rubbery PDMS microphase provides nanoscale channels for ethanol transport. We use a simple model to study the effect of morphology and PDMS volume fraction on permeabilitites of ethanol and water through the block copolymer membranes. We defined normalized permeabilities of ethanol and water to account for differences in morphology and PDMS volume fraction. We found that the normalized ethanol permeability in SDS copolymers was independent of the total polymer molecular weight. This is qualitatively different from what was previously reported for poly(styrene-b-butadiene-b-styrene) (SBS) membranes, where the normalized ethanol permeability was found to be a sensitive function of total molecular weight [J. Membr. Sci. 2011, 373, 112]. We demonstrate that this is due to differences in the Flory−Huggins interaction parameter (χ) for the two systems. When χN is less than 100 (N is the number of segments per chain), the two microphases are weakly segregated, and the presence of glassy PS segments in the transporting microphase impedes ethanol transport. When χN exceeds 100, the two microphases are strongly segregated and the glassy PS segments do not mix with the transporting phase. We compare these results with normalized ionic conductivity data previously reported for mixtures of a lithium salt and polystyrene-b-poly(ethylene oxide) (SEO). Evidence suggests that the product χN governs the transport of widely different species such as ethanol and lithium salts through block copolymer membranes. Surprisingly, the normalized permeability of water is independent of total molecular weight for both SDS and SBS block copolymers.

We describe the formation of polyampholytic block copolymer brushes and their assembly in solution. Specifically, we employ "surface-initiated" activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) sequentially to form diblock copolymer grafts comprising blocks of poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and poly(sodium methacrylate) (PNaMA) on flat impenetrable silica surfaces, i.e., SiO(x)/PNaMA-b-PDMAEMA and SiO(x)/PDMAEMA-b-PNaMA. Protonation of the PNaMA block results in formation of poly(methacrylic acid) (PMAA). We demonstrate that ARGET-ATRP of NaMA provides a convenient route to preparation of PMAA, which is an alternative method to the more traditional approach based on preparing PMAA by polymerizing tert-butyl methacrylate (tBMA) followed by cleavage of the tert-butyl group. We also discuss conformational changes of the individual polyelectrolyte blocks in solution as a function of solution pH by monitoring adsorption behavior of functionalized polystyrene spheres.

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