Understanding the Grafting of Telechelic Polymers on a Solid Substrate to Form Loops

Recent experimental and theoretical studies have demonstrated that relative to singly tethered chains, the presence of polymer loops at interfaces significantly improves interfacial properties such as adhesion, friction, and wettability. In the present study, a simple system was studied to examine the formation of polymeric loops on a solid surface, where the grafting of carboxylic acid terminated telechelic polystyrene from the melt to an epoxy functionalized silicon is chosen. The impact of telechelic molecular weight, grafting temperature, and surface functionality on the telechelic attachment process is studied. It was found that grafting of the telechelic to the surface at both ends to form loops is the primary product of this grafting process. Moreover, examination of the kinetics of the grafting process indicates that it is reaction controlled. Fluorescence tagging of the dangling ends of singly bound chains provides a mechanism to monitor their time evolution during grafting, and these results indicate that the grafting process is accurately described by recent Monte Carlo simulation work. The results also provide a method to control the extent of loop formation at interfaces and therefore provide an opportunity to further understand the role of the loops in the interfacial properties in multicomponent polymer systems

Illumination of Conjugated Polymer in Solution Alters Its Conformation and Thermodynamics

The importance of chain structure in conjugated polymer-based material active layers and its relation to device efficiencies in OPVs, organic field transistors, OLEDs, and other devices has been well established. However, the influence that the absorbance of the light inherent to these devices might have on the conjugated polymer structure is not well understood. Herein, we employ small-angle neutron scattering to investigate structural changes occurring in solutions of poly­(3-hexyl­thiophene-2,5-diyl) with exposure to white light. Results indicate significant decrease in both Kuhn length (<i>b</i>) and radius of gyration (<i>R</i>_g) of the polymer upon illumination, coupled with a drop in the second virial coefficient (<i>A</i>₂). We explain this phenomenon through a chain collapse model, proposing that the interaction of light with the polymer backbone alters its thermodynamic interactions with and solubility in the surrounding solvent. The presence of such an effect, which we observe in several conjugated polymers, introduces the possibility of a powerful, nondestructive, and tunable method for controlling polymer conformation in solution. This in turn opens a path to develop a broad range of new light-responsive materials, in that a variety of conjugated polymers could be used as the stimuli-responsive material. Additional implications include the identification of the importance of illumination in the reproducible fabrication of organic electronic active layers from conjugated polymer inks

Polymer nanotube nanocomposites: Correlating intermolecular interaction to ultimate properties

Polymer nanocomposite films containing 5 wt% single-walled carbon nanotubes (SWNT) or 5 wt% multi-walled carbon nanotubes (MWNT) with random copolymers of styrene and vinyl phenol were processed from dimethyl formamide solutions. Vinyl phenol mole ratio in the copolymer was 0, 10, 20, 30, and 40%. FTIR analysis indicates that the composites containing the copolymer with 20% vinyl phenol exhibit the maximum intermolecular interactions (hydrogen bonding) between the hydroxyl group of the vinyl phenol and the carbon nanotube functional groups. Tensile properties and electrical conductivity also are the highest in the samples containing the copolymer with 20% vinyl phenol. Thus, these results show that the optimization of the extent of intermolecular interactions between a polymer chain and a carbon nanotube results in an optimal increase in macroscopic properties. Moreover, the extent of intermolecular hydrogen bonding can be improved by optimizing the accessibility of the functional groups to participate in the non-covalent interaction. In this system, this optimization is realized by control of the amount of vinyl phenol in the copolymer, i.e. the copolymer composition.close353

Polymer-nanotube composites: Controlling properties by controlling interaction

Carbon nanotubes (CNT) have recently emerged as a promising class of materials that possess extraordinary mechanical and electrical properties. Dispersing nanotubes in a polymer matrix provides an effective way to exploit these extraordinary properties, however sufficient dispersion has been difficult to achieve due to strong intertube interaction. Previous work in our lab has shown that a favorable interaction such as hydrogen bonding between a copolymer and carbon nanotubes enhances dispersion of the nanotubes in the polymer matrix. The composites of copolymers are prepared with SWNT and MWNT and characterized by FTIR spectroscopy and tested for electrical conductivity and tensile properties. Electrical conductivity results show that the nanotubes in PSVPh20 exhibit enhanced dispersion. The tensile tests show maximum enhancement in tensile strength for the composite of nanotubes with PSVPh20. FTIR analysis for hydrogen bond interaction between the polymer and nanotubes are in good agreement with tensile test and electrical conductivity results

Looped Polymer Brushes Formed by Self-Assembly of Poly(2-vinylpyridine)-Polystyrene-Poly(2-vinylpyridine) Triblock Copolymers at the Solid-Fluid Interface. Kinetics of Preferential Adsorption

The kinetics of assembly of a series of poly(2-vinylpyridine)-polystyrene-poly(2-vinylpyridine) (PVP-b-PS-b-PVP) triblock copolymers from the selective solvent toluene onto a silicon surface has been studied using phase-modulated ellipsometry. The adsorbed amount and thickness have been determined independently as functions of time. Even though the adsorbed amount as a function of time follows the traditional two-step process that is typical of the self-assembly of diblock copolymerssthere is an initial fast adsorption followed by a slow buildup of the layer (brush regime) - the thickness shows an “overshoot” that corresponds to the brush regime. We attribute this phenomenon, not observed in the self-assembly of amphiphilic diblock copolymers, to having both ends of the chain tethered. The final ellipsometric thicknesses of the brush made from the triblocks are less than that expected for a single-end tethered brush made from a diblock copolymer with a buoy block of similar molecular weight. This result supports the conclusion that PVP-b-PS-b-PVP triblock copolymers adsorb mainly in a looplike conformation