The ability to form blends of polymers offers the opportunity of creating a new class of materials with enhanced properties. In addition to the polymer components, recent advances in nanoengineering have resulted in the development of nanosized inorganic particles that can be used to improve the properties of the blend, such as the flammability and the mechanical properties. While traditional methods using copolymer compatibilizers have been used to strengthen polymer blends, here, we show that the inorganic nanosized filler additive can also serve as a compatibilizer as it can localize to the interface between the polymers. We use experimental and theoretical studies to show the fundamental mechanisms by which inorganic fillers with large aspect ratio and at least one-dimension in the nanometer range, can act as non-specific compatibilizers for polymer blends. We examine a series of nanosized fillers, ranging from nanotubes to nanoclays (with varying aspect ratios) in a model polystyrene (PS)/poly(methylmethacyralate) (PMMA) blend. Using a number of experimental techniques such as transmission electron microscopy (TEM), scanning tunneling X-ray microscopy (STXM), and atomic force microscopy (AFM) we postulate that the mechanism of compatibilization occurs as a result of the fillers forming in situ grafts with the immiscible polymers. We also use theoretical studies to show that the aspect ratio and the bending energy of the fillers play a key role in the compatibilization process. Our results indicate that the compatibilization is a general phenomenon, which should occur with all large aspect ratio nanofiller additives to polymer blends. Copyright (C) 2010 John Wiley & Sons, Ltd.

In order to increase the separation rate of surface electrophoresis while preserving the resolution for large DNA chains, e.g., genomic DNA, the mobility and diffusion of Lambda DNA chains adsorbed on flat silicon substrate under an applied electric field, as a function of migration distance, ionic strength, and field intensity, were studied using laser fluorescence microscope. The mobility was found to follow a power law with the field intensity beyond a certain threshold. The detected DNA peak width was shown to be constant with migration distance, slightly smaller with stronger field intensity, but significantly decreased with higher ionic strength. The molecular dynamics simulation demonstrated that the peak width was strongly related with the conformation of DNA chains adsorbed onto surface. The results also implied that there was no diffusion of DNA during migration on surface. Therefore, the Nernst–Einstein relation is not valid in the surface electrophoresis and the separation rate could be improved without losing resolution by decreasing separation distance, increasing buffer concentration, and field intensity. The results indicate the fast separation of genomic DNA chains by surface electrophoresis is possible.

Strong dependence of the crystal orientation, morphology, and melting temperature (Tm) on the substrate is observed in the semicrystalline polyethylene thin films. The Tm decreases with the film thickness decrease when the film is thinner than a certain critical thickness, and the magnitude of the depression increases with increasing surface interaction. We attribute the large Tm depression to the decrease in the overall free energy on melting, which is caused by the substrate attraction force to the chains that competes against the interchain force which drives the chains to crystallization.

We use molecular dynamics simulations to study the effect of surfactants on the mechanism of drop deformation in shear flows. Our results show deviations from fluid mechanics predictions, in both the high and the low surfactant concentration limits. We find that these deviations are a result of the local conformation of the surfactant layer which mediates the stress transfer across the interface. We show that the ability of the surfactant to affect the stress transfer across the interface is a result of the interplay between the architecture of the surfactant and the surface coverage.

Dynamic secondary ion mass spectrometry was used to investigate the chain mobility of polystyrene (MW ranging from 4.3 to 957 kg/mol) at the free surface. The data show that the diffusion coefficient was reduced relative to the bulk value within a distance, d < or = 4R(g), from the surface and scaled as 1/N(2.5) at fixed d. These results are in excellent agreement with self-consistent field calculations of the surface segmental distribution and provide the first direct confirmation of various theoretical models that predict asymmetric segmental fluctuation which arises from surface induced orientation of polymer chains.

We use a self-consistent mean-field theory which accounts for polymer semiflexibility to study the effects of semiflexible copolymers localized at the interface between a flexible and a semiflexible homopolymer. Our studies show that, in contrast to copolymers located at the interface between flexible polymers, entropic effects tend to dominate the behavior of the copolymers at semiflexible polymer interfaces. We find that the reduction in interfacial tension caused by the copolymer is a nonmonotonic function of the rigidity of the copolymer and the amount of copolymer added to the system.

Molecular dynamics simulations of polymers reinforced with nanoscopic filler particles reveal the mechanisms by which nanofillers improve the toughness of the material. We find that the mobility of the nanofiller particle, rather than its surface area, controls its ability to dissipate energy. Our results show similarities between the toughening mechanisms observed in polymer nanocomposites and those postulated for biological structural materials such as spider silk and abalone adhesive.

We use molecular dynamics simulations to study the mechanism by which a flat, homogeneous surface can serve as an electrophoretic separation medium for DNA. We find that the mobility of DNA on the surface is a function of the conformation of the adsorbed DNA molecule, and that this mobility is controlled by the attraction between the DNA and the surface. Our results will provide guidelines for the fabrication of surfaces that can be used to separate DNA in a wide size range.

The interfacial properties of a homopolymer (polybutadiene (PB)) and a terpolymer (brominated poly(isobutylene-co-p-methylstyrene) (BIMS)) are reported. Neutron reflectivity was used to study the interfacial structure. The results were complemented by scanning transmission X-ray microscopy and atomic force microscopy, which were used to probe the morphology of these binary blends. Our results show that the interfacial behavior of these elastomeric blends is a direct function of the BIMS chemical composition. The interfacial width decreased with increasing bromide functionality. At levels below 8mol%, the para-methylstyrene concentration had a less pronounced effect on the compatibility and interfacial characteristics. We also studied the effect of styrene butadiene random copolymers on the miscibility of the PB/BIMS blends. The results showed that styrene-butadiene rubber (SBR) was not fully miscible with BIMS and PB on an individual basis, but addition of relatively small amount of SBR enhances the compatibilization of the PB/BIMS interface. Self-consistent field (SCF) modeling was used to determine the optimum copolymer composition. The calculations are consistent with the experimental results.

We have used neutron reflectivity to measure the concentration profiles of polystyrenesulfonated acid (PSSA(x)) films with three different degrees of sulfonation (x = 3.4%, 12.8%, and 27.0%) in water, CCl4, and a mixture of the two solvents. The data show that; except for the x = 3.4% films where CCl4 is a good solvent, the largest degree of swelling occurred in the mixed solvent. Contrast matching the water to the polymer layer enabled us to profile the CCl4. concentration. The results showed that CCl4 and water were mixed within the polymer film in a ratio of 1:2 and 1:4 for 12.8 mol % and 27.0 mol % PSSA, respectively. Self-consistent-field calculations indicated that the number of adsorbed sulfonated blocks scales linearly with the degree of sulfonation. Using the interaction parameters between the PS and SA blocks obtained by fitting to the data in pure solvents, excellent agreement is obtained for the profiles of the polymer and the solvent mixtures for all values of x.

We present experimental and self-consistent field (SCF) modeling results on partially hydrolyzed poly(vinyl acetate) (PVA) chains adsorbed on poly(vinyl chloride) (PVC) latex particles to determine the relationship between the structure of PVA and the steric force between the colloidal particles with adsorbed PVA. By measuring the surface pressure-area isotherms for a PVA-coated PVC latex monolayer at the air/water interface with the Langmuir-Blodgett surface balance, we found that both an increase in chain length and a decrease in degree of hydrolysis increase the repulsive energy between colloidal particles. Surface pressure-area isotherms also reveal that temperatures ranging from 20 to 50 degreesC have little effect on the repulsive energy between the colloidal particles. We also used this method to analyze the effect of PVA mixtures on colloidal stability. SCF modeling was used to analyze our experimental results. Our results showed excellent agreement between the SCF modeling results and the experimental results.

A two-dimensional self-consistent-field lattice model incorporating the rotational isomeric state scheme to account for chain rigidity was developed to study the phase diagram of rod-coil diblock copolymer melts. Several morphologies such as lamella, zigzag lamella, elliptical cross-sectional cylinders, and hexagonally packed cylinders were observed for rod-coil diblock copolymers. A free energy analysis suggests that the zigzag and elliptical cross-sectional cylindrical structures are metastable. The effect of decreasing the flexibility of the coil segment of a rod-coil diblock copolymer was determined. Our calculation showed that as the flexible block of the copolymer becomes more rigid, part of the cylindrical domain in the phase diagram is replaced by lamella structures.

A novel approach of fabricating 2-D arrays of SiO2 beads on a Si surface using the Langmuir-Blodgett (LB) technique is reported. The corrugated surfaces were tested as a separation media for surface electrophoresis of DNA molecules. The measured electrophoretic mobility for ?-DNA is only 20% slower than previously measured on a flat Si wafer. This indicates that the separation mechanism is due to surface friction rather than biased reptation as reported by Tinland in the three dimensional Silica bead matrix where the mobility is two orders of magnitude smaller.

We report a new approach for performing DNA electrophoresis. Using experimental studies and molecular dynamics simulations, we show that a perfectly flat silicon wafer, without any surface features, can be used to fractionate DNA in free solution. We determine that the ability of a flat surface to separate DNA molecules results from the local friction between the surface and the adsorbed DNA segments. We control this friction by coating the Si surface with silane monolayer films and show that it is possible to systematically change the size range of DNA that can be separated.

We use numerical self consistent field calculations to determine the effect confinement has on the critical micelle concentration for diblock copolymers. Our studies show that increasing the confinement increases the free energy of micelle formation. As the confinement increases, the micelle undergoes a decrease in its size, until finally the entropic losses associated with packing the copolymer into a smaller micelle become too large and the micelle structure becomes unstable. Our results have important implications when diblock copolymers are used as compatibilizers in thin films, as our studies indicate that micelle formation will be suppressed in thin films.

The morphology of incompatible polymer blends are often stabilized by the addition of block copolymers that ideally will localize to the polymer-polymer interface and reduce the interfacial tension. However, the effectiveness of adding copolymer is significantly reduced by the tendency of the diblock to form micelles that become trapped within one of the phases. Recent theoretical and experimental results show that using compatibilizers in confined physical geometry will reduce the configurational entropy of the diblock and make it energetically more favorable for the diblock to locate to the interface [1]. Dynamics studies with Scanning Transmission X-ray Microscopy (STXM) show two regimes in the dynamical process, where growth regimes are characterized by growth exponents, alpha, where R(t) similar to R-alphat. The first growth regime consists of round micelle-like domain formation and relatively fast growth (alpha=2/3) of these structures within the PS layer. The second regime results in a relatively stable bicontinuous domain formation with slow growth (alpha=1/20).