Anisotropic and heterogeneous dynamics in stretched elastomer nanocomposites.

We use X-ray photon correlation spectroscopy (XPCS) to investigate the dynamics of a stretched elastomer by means of probe particles. The particles dispersed in the elastomer were carbon black or silica aggregates classically used for elastomer reinforcement but their volume fraction is very low (φ < 10-2). We show that their dynamics is slower in the direction of the tensile strain than in the perpendicular one. For hydroxylated silica which is poorly wetted by the elastomer, there is no anisotropy. Two-time correlation functions confirm anisotropic dynamics and suggest dynamical heterogeneity already expected from the q-1 behavior of the relaxation times. The height χ* of the peak of the dynamical susceptibility, determined by the normalized variance of the instantaneous correlation function, is larger in the direction parallel to the strain than in the perpendicular one. It also appears that its q dependence changes with the morphology of the probe particle. Therefore, the heterogeneous dynamic probed by the particles is not related only to that of the strained elastomer matrix. In fact, it results from modification of the dynamics of the polymer chains near the surface of the particles and within the aggregate porosity (bound polymer). It is concluded that XPCS is a powerful method for investigating the dynamics, at a given strain, of the bound polymer-particle units which are responsible, at large volume fractions, for the reinforcement.

Bioresorbable and nonresorbable macroporous thermosensitive hydrogels prepared by cryopolymerization. Role of the cross-linking agent.

Macroporous poly( N-isopropylacrylamide) (pNIPA) gels (so-called cryogels), cross-linked with different bis-acrylic compounds, N,N'-methylenebisacrylamide (MBAAm) and dimethacrylate-tyrosine-lysine-tyrosine (DMTLT), were prepared through free-radical polymerization at subzero temperature in dioxane/water media. DMTLT is a hydrolytically degradable cross-linker with relatively hydrophobic character. The effects of different synthesis conditions, namely the concentration of monomers, the cross-linker, and the initiator in the reaction mixture, on the structure of the pNIPA-cryogels have been studied. The equilibrium swelling ratio of the DMTLT cross-linked pNIPA cryogels at temperatures below lower critical solution temperature (LCST) of pNIPA, was over ten times higher than that of the gels synthesized at room temperature from the same feed composition. The MBAAm cross-linked pNIPA cryogels synthesized in water exhibited the highest equilibrium swelling and the fastest response. The critical transition temperature, T c, was lower ( T c approximately 31 degrees C) for pNIPA-cryogels synthesized in dioxane/water media or cross-linked with DMTLT as compared to MBAAm cross-linked pNIPA cryogels synthesized in water (T c approximately 33 degrees C). Scanning electron microscopy (SEM) revealed different porous structure and pore surface morphology depending on the cross-linker (MBAAm or DMTLT) and the solvent (water or dioxane/water) used. Gels and cryogels were also characterized by SAXS, showing that the nanostructure of the samples is related to swelling.

ASAXS, SAXS and SANS investigations of vulcanized elastomers filled with carbon black.

Small-angle X-ray scattering (SAXS) performed down to small q values (q

Surface area and microporosity of carbon aerogels from gas adsorption and small- and wide-angle X-ray scattering measurements.

A carbon aerogel was obtained by carbonization of an organic aerogel prepared by sol-gel polymerization of resorcinol and formaldehyde in water. The carbon aerogel was then CO(2) activated at 800 degrees C to increase its surface area and widen its microporosity. Evolution of these parameters was followed by gas adsorption and small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) with contrast variation by using dry and wet (immersion in benzene and m-xylene) samples. For the original carbon aerogel, the surface area, S(SAXS), obtained by SAXS, is larger than that obtained by gas adsorption (S(ads)). The values become nearly the same as the degree of activation of the carbon aerogel increases. This feature is due to the widening of the narrow microporosity in the carbon aerogel as the degree of activation is increased. In addition, WAXS results show that the short-range spatial correlations into the assemblies of hydrocarbon molecules confined inside the micropores are different from those existing in the liquid phase.

Morphological investigation of chemically treated poly(ethylene terephthalate)-based activated carbons.

Complementary techniques, including low-temperature nitrogen adsorption and small-angle X-ray scattering (SAXS), are applied to detect the effects of surface functionalization on the morphology of activated carbon derived from poly(ethylene terephthalate) (PET). Scanning electron microscopy (SEM) is also employed as an auxiliary method to visualize the surface below the micron scale. The SEM images reveal a micron-sized ridgelike texture. Room temperature acid treatment makes the ridges become more pronounced, while treatment with boiling acid uncovers fiberlike structures of roughly 1 microm diameter. All samples display an apparent surface fractal dimension of Ds = 2.4 in the wave vector range 0.001-0.02 A(-1). Nitric acid at room temperature increases the surface oxygen content only by 3 at. %, while all the adsorption properties and structural parameters reported in this paper are virtually unaffected. Significant differences in the morphology at submicron scales appear only after boiling acid treatment. The resulting carbon remains highly microporous, but the loss of Brunauer-Emmett-Teller (BET) surface area from about 1150 to 304 m2/g is approximately 75%. In addition to the principal peak at around 8 A, fresh peaks appear in the polydisperse Horvath-Kawazoe (HK) pore size distribution owing to the burnoff of intervening walls. The average width of the slitlike pores calculated from the Dubinin-Radushkevich (DR) plot increases from 8.4 to 11 A. The minimum slit width where the applied probe molecules, that is, nitrogen and hexane, can enter increases from about 5 to about 5.4 A. The separation distance of the basic structural units is practically unchanged. When, however, this carbon is in contact with hexane, this distance expands from about 19 to 27 A. The swelling is consistent with the deformable nature of this sample also illustrated by the low-pressure hysteresis and the reduced helium density. Particular attention was paid to the surface areas derived from low-temperature nitrogen adsorption and X-ray measurements. Owing to the wide spatial range of the structures in these samples, estimates of the specific surface area of activated carbons can be substantially in error unless both upper and lower q ranges of the SAXS spectra are taken into account. Surface areas derived from the adsorption data either by the BET or the DR approaches were always below the values obtained by standard SAXS. As an example, the carbon sample functionalized at room temperature gave surface area values of 1114, 1293, and 1970 m2/g, respectively. The possibility that this difference is caused by inaccessible pores was excluded by contrast variation measurements with hexane.

Scattering from dilute ferrofluid suspensions in soft polymer gels.

Small angle neutron and x-ray scattering methods are used to investigate the structure of dilute suspensions of two different ferrofluid systems dispersed in soft polyacrylamide hydrogels. It is found that the particles in the fluid are fractal aggregates composed of smaller particles of radius ca. 5 nm. The fractal dimension is strongly dependent on sample, taking the value 1.7 in the first sample and 2.9 in the second sample. In the presence of a magnetic field the aggregates orient, but are restricted in both their translational and rotational freedom. The effect of the gel elasticity is treated as a hindrance to the orientation process.