Jonathan C. Sokolov

Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

A unique polyurethane (PU) elastomer containing inorganic polyhedral oligomeric silsesquioxane (POSS) molecules as molecular reinforcements in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. The mechanical properties of POSS modified polyurethane (POSS-PU) were also compared to those of polyurethane without POSS. The crystal structures of two different POSS molecules were first determined by X-ray powder diffraction analysis, yielding a rhombohedral cell with aa 11.57 A ˚, aa 95.58 for octacyclohexyl-POSS (1,3,5,7,9,11,13,15octacyclohexylpentacyclo[9.5.1.13,9.15,15.17,13] octasiloxane) and aa 11.53 A ˚, aa 95.38 for hydrido-POSS (1-[hydridodimethylsiloxy]3,5,7,9,11,13,15-heptacyclohexylpentacyclo [9.5.1.13,9.15,15.17,13] octasiloxane). WAXD results showed that reflection peaks distinct to POSS crystal diffraction were seen in POSS-modified polyurethane, which suggests that POSS molecules formed nanoscale crystals in the hard domain. During deformation, the average size of POSS crystals in POSS-PU was found to decrease while elongation-induced crystallization of the soft segments was observed at strains greater than 100%. The SAXS results showed microphase structure typical of segmented

Confinement-induced miscibility in polymer blends

The use of polymer thin films in technology is increasingly widespread—for example, as protective or lithographic surface coatings, or as active (electronic or optical) elements in device architectures. But it is difficult to generate films of polymer mixtures with homogeneous surface properties, because of the tendency of the polymers to phase-separate,. Copolymer compatibilizers can induce miscibility in polymer blends, but only with chemical components that are either close to a critical point in the phase diagram or which have an attractive interaction between them,. Instead of manipulating the chemical composition of the blend, we show here that complete mixing can be obtained in polymer blends by the physical effect of confinement in thin films. The compatibilization results from entropic inhibition of phase separation into micelles, owing to confinement. The result is an intimately mixed microemulsion with a perfectly flat surface and a two-dimensional maze-like structure with columnar domains that extend through the film.

Analysis of x-ray reflectivity data from low-contrast polymer bilayer systems using a Fourier method.

X-ray reflectivity data of polymer bilayer systems have been analyzed using a Fourier method which takes into account different limits of integration in q-space. It is demonstrated that the interfacial parameters can be determined with high accuracy although the difference in the electron density (the contrast) of the two polymers is extremely small. This method is not restricted to soft-matter thin films. It can be applied to any reflectivity data from low-contrast layer systems.

Self‐assembly of a homopolymer mixture via phase separation

We propose a mechanism for creating lateral order in thin films of a homopolymer mixture via spinodal decomposition in the presence of a laterally structured substrate surface. The system under consideration is a mixture of deuterated polystyrene and partially brominated polystyrene (PBrxS, x=0.5) spuncast onto different substrates from toluene solution. We demonstrate that periodic stripe‐like domain structures can be observed in the polymer films after spinodal decomposition on a periodic array of Cr lines on a H‐terminated Si surface. The effect vanishes when the chemistry of the underlying structure is changed, strongly indicating an interface‐driven ordering of the polymer film.

Synthesis and Characterization of Segmented Polyurethanes Containing Polyhedral Oligomeric Silsesquioxanes Nanostructured Molecules

Segmented polyurethanes based on diphenylmethane-4,4′-diisocyanate and polytetra-methylene glycol were synthesized using a mixture of polyhedral oligomeric silsesquioxane (POSS)-diol and 1,4-butanediol as chain extenders. The polymers were characterized by differential scanning calorimetry, wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS) and tensile property tests. Microphase separation between the hard and soft segment domains was observed in all the samples by SAXS. The increase of the POSS concentration was found to weaken the microphase separation between the domains and increase the T g of the soft segments. The WAXD results showed that when the POSS concentration was greater than 10 wt%, the 101 diffraction peak from the POSS crystals could be observed, which suggested the formation of POSS nanocrystals in the hard domain. The tensile property tests showed that polyurethanes containing the nanostructured POSS molecules had higher moduli, but lower maximum elongation ratios.

Focused ion beam/lift-out transmission electron microscopy cross sections of block copolymer films ordered on silicon substrates

Thin poly(styrene210-b-2-vinylpyridine 200) and poly(2-vinylpyridine94-b-styrene760-b-2-vinylpyridine94) films spun cast on silicon and annealed at 1808C for 3 days were directly cross sectioned in less than 1 h using the focused ion beam (FIB) lift-out technique. We show that with the FIB procedure, it is possible to produce cross sections that reveal structure near the silicon interface and hence the surface induced phase transitions could be examined and compared quantitatively with theoretical models. Atomic force microscopy, dynamic secondary ion mass spectrometry, and transmission electron microscopy were used to characterize the films.q 2000 Elsevier Science Ltd. All rights reserved.

Confinement-induced super strong PS/MWNT composite nanofibers

Shear Modulation Force Microscopy (SMFM) together with the Atomic Force Microscopy (AFM) based three-point bending technique were used to measure the mechanical properties of electrospun polymers and polymer nanocomposite fibers. Both techniques showed that the moduli of the fibers increased significantly with decreasing fiber diameter. We attributed this enhancement to the orientation of polymer chains which occurs during the electrospinning process. We then predicted, and confirmed experimentally, that the phenomenon scales with Rg rather than with the absolute fiber diameter and can propagate radially for large distances (~20Rg) into the fiber interior. The inclusion of nanotubes into the fibers further enhanced the orientation by introducing additional surfaces. The additional increase in modulus (more than an order of magnitude) could then be explained by the same model and scaled on a universal curve.

Substrate dependence of morphology in thin film polymer blends of polystyrene and poly(methyl methacrylate)

We studied the effects of different substrates on the development of the morphologies in thin film polymer blends in as-spun and annealed films. We have utilized thin films of blends of polystyrene and poly(methyl methacrylate) spun cast on Si, Au and Co surfaces, and produced quantitative composition maps with near edge X-ray absorption fine structure (NEXAFS) microscopy as a function of annealing time. We observed a marked dependence of the morphologies and the qualitative dynamics on the surface composition. # 2000 Society of Chemical Industry

Self-assembled monolayers of rigid thiols.

The preparation, structure, properties and applications of self-assembled monolayers (SAMs) of rigid 4-mercapto-biphenyls are briefly reviewed. The rigid character of the biphenyl moiety results in a molecular dipole moment that affects both the adsorption kinetics on gold surfaces, as well as the equilibrium structure of mixed SAMs. Due to repulsive intermolecular interaction, the Langmuir isotherm model does not fit the adsorption kinetics of these biphenyl thiols, and a new Ising model was developed to fit the kinetics data. The equilibrium structures of SAMs and mixed SAMs depend on the polarity of the solution from which they were assembled. Infrared spectroscopy suggests that biphenyl moieties in SAMs on gold have small tilt angles with respect to the surfaces normal. Wetting studies shows that surfaces of these SAMs are stable for months, thus providing stable model surfaces that can be engineered at the molecular level. Such molecular engineering is important for nucleation and growth studies. The morphology of glycine crystals grown on SAM surfaces depends on the structure of the nucleating glycine layer, which, in turn, depends on the H-bonding of these molecules with the SAM surface. Finally, the adhesion of PDMS cross-linked networks to SAM surfaces depends on the concentration of interfacial H-bonding. This non-linear relationship suggests that the polymeric nature of the elastomer results in a collective H-bonding effect.

Bulk and surface characterization of a dewetting thin film polymer bilayer

We have monitored the progression of the dewetting of a partially brominated polystyrene (PBrS) thin film on top of a polystyrene (PS) thin film with scanning transmission x-ray microscopy (STXM) as well as photoemission electron microscopy (PEEM). We mapped the projected thickness of each constituent polymer species and the total thickness of the film with STXM, while we determined the surface composition with PEEM. Our data show that the PBrS top layer becomes encapsulated during the later stages of dewetting and that atomic force microscopy topographs cannot be utilized to determine the contact angle between PBrS and PS.