Preferential adsorption of poly(2-vinylpyridine)-deuterated polystyrene-poly(2-vinylpyridine) (PVP-dPS-PVP) triblock copolymers from toluene onto silicon leads to the formation of dPS loops tethered by the PVP end blocks. Using neutron reflectometry, we have determined the segment density profiles of these looped polymer brushes in toluene, a good solvent for the dPS block, and in cyclohexane at 20 degrees C (poor solvent), 32 degrees C, (near-Theta solvent), and 50 degrees C (marginal solvent). While the swelling behavior qualitatively agrees with that observed for singly grafted brushes, there are interesting differences in the local structural details: In a good solvent, the segment density profiles are composed of an inner parabolic region and a long, extended tail. In cyclohexane, the profiles are described by exponential decays. We ascribe these features to a novel polydispersity effect that arises due to tethering the PS loops by both ends. The results also show that the less dense layers undergo more significant changes in swollen height as solvent quality is changed and that the looped brushes of different molecular weight, asymmetry, and tethering density adhere to scaling relationships derived for lightly cross-linked polymer gels.

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.

Submitted for the MAR07 Meeting of The American Physical Society Grafting of Telechelic Polymers onto Functionalized Substrate in Polymeric Matrices RUJUL MEHTA, ZHENYU HUANG, HAINING JI, Department of Chemistry, University of Tennessee, Knoxville, TN 37996, JIMMY MAYS, MARK D. DADMUN, Department of Chemistry, University of Tennessee, Knoxville, TN 37996; Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 — We have investigated the grafting of polymer chains on an inorganic surface by reaction of the functional end-groups on the polymer chains. Specifically, polystyrene (PS) terminated at one or both ends with carboxylic acid groups is grafted onto silica modified with epoxy groups by spin coating a thin film of a blend of reactive and non-reactive PS onto the silica wafer and promoting the grafting reaction. This generates a model system to study the impact of the volume fraction of reactive polymer and the chain length of both reactive and matrix polymers on the resultant grafted brush. The interfacial excess isotherms are correlated to a theoretical model proposed by Shull, which describes the end adsorption of polymers at polymer/substrate interfaces in brushes based on scaling arguments and self-consistent field theory. Comparison is made between the characteristics of brushes formed from PS chains that are reactive at one or both ends. Rujul Mehta Department of Chemistry, University of Tennessee, Knoxville, TN 37996 Date submitted: 22 Nov 2006 Electronic form version 1.4

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 copolymers-there 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.

The irreversible adsorption of telechelic polymer chains from solution and melts onto solid substrates has been studied using the bond fluctuation Monte Carlo model. Complex brush formation kinetics dominated by diffusion of chains to the substrate at short times (diffusion-limited regime or DLR) and by penetration of chains through the maturing brush at longer times (penetration-limited regime or PLR) were observed. During the entire adsorption process, the rate of chain adsorption decreases monotonically with time. In the DLR, characterized by a maximum in the concentration of singly bound chains and a rapidly increasing fraction of doubly bound chains (loops), this decrease is due primarily to the depletion of free chains near the substrate and the formation of concentration gradients of free (nonadsorbed) chains in the bulk solution. The DLR and PLR are separated by an intermediate regime during which the brush becomes dominated by doubly bound chains and both penetration of the maturing brush and diffusion of chains to the brush surface play a role in determining the kinetics of brush growth. The PLR is characterized by steep gradients of free chains within the growing brush and the disappearance of concentration gradients for free chains in the bulk solution. In the PLR, the concentration of singly bound chains is low and decreases slowly while surface coverage and the fraction of doubly bound chains increase slowly. The rates of adsorption of new chains and the formation of loops in the PLR slow dramatically with increasing surface coverage and increasing chain length and less dramatically with decreasing bulk concentration.