The pore size distribution and architecture in gas shales were studied using a combination of small-angle neutron scattering (SANS), mercury injection capillary pressure (MICP), and helium ion ...microscopy (HIM). SANS analysis shows that the pore size population is not a power-law distribution across many length scales, typical of sedimentary rocks, but contains an anomalous population of pores on-the-order ∼2 nm, housed primarily in the organic matter. A model is presented showing how a “foamy porosity” with such a characteristic size is a direct result of diagenetic evolution of kerogen. Cross-linking of the kerogen combined with phase separation of gas/oil, leads to arrested coarsening with a length scale set by the cross-length density. These pore populations determined by the scattering model are directly supported by HIM images. Pore connectivity determined through pore-size-to-pore-throat analysis, suggests that interpore connections are also distinct from typical sedimentary rocks. The pore/throat ratio, unlike the simple ratios predicted from sphere packing and found for clastic rocks, is nearly constant over all pore sizes. Kerogen diagenesis is a recognized source of excess internal pressure. When this pressure causes failure of the material surrounding the kerogen to create escape pathways for the phase-separated fluid, it is likely that escape pathways will connect intergranular porosity via microfractures, producing the relatively narrow aperture size distribution.
We review the current literature pertaining to the characterization of soft matter subject to flow utilizing small-angle neutron scattering, flow-SANS, with an emphasis on the simultaneous ...measurement of the rheology, Rheo-SANS. Experimental results are discussed in terms of the flow induced structure and direct connection to the bulk rheology in which we highlight the use of the contrast match method as a unique advantage to neutron scattering techniques. Finally, we discuss specific areas in each field that could benefit from focused flow-SANS experiments, and the projected evolution of specialized flow-SANS sample environments.
Display omitted
► The application of SANS to the characterization of soft matter under deformation. ► The direct measurement of flow induced structural reorganization. ► The simultaneous measure of both structure and rheology. ► The current development of state-of-the-art dynamic SANS sample environments. ► The current approaches to the data analysis of anisotropic scattering patterns.
We report an experimental study of the dynamical arrest transition for a model system consisting of octadecyl coated silica suspended in n-tetradecane from dilute to concentrated conditions spanning ...the state diagram. The dispersion’s interparticle potential is tuned by temperature affecting the brush conformation leading to a thermoreversible model system. The critical temperature for dynamical arrest, T*, is determined as a function of dispersion volume fraction by small-amplitude dynamic oscillatory shear rheology. We corroborate this transition temperature by measuring a power-law decay of the autocorrelation function and a loss of ergodicity via fiber-optic quasi-elastic light scattering. The structure at T* is measured using small-angle neutron scattering. The scattering intensity is fit to extract the interparticle pair-potential using the Ornstein–Zernike equation with the Percus–Yevick closure approximation, assuming a square-well interaction potential with a short-range interaction (1% of particle diameter). The strength of attraction is characterized using the Baxter temperature and mapped onto the adhesive hard sphere state diagram. The experiments show a continuous dynamical arrest transition line that follows the predicted dynamical percolation line until ϕ ≈ 0.41 where it subtends the predictions toward the mode coupling theory attractive-driven glass line. An alternative analysis of the phase transition through the reduced second virial coefficient B 2* shows a change in the functional dependence of B 2* on particle concentration around ϕ ≈ 0.36. We propose this signifies the location of a gel-to-glass transition. The results presented herein differ from those observed for depletion flocculated dispersion of micrometer-sized particles in polymer solutions, where dynamical arrest is a consequence of multicomponent phase separation, suggesting dynamical arrest is sensitive to the physical mechanism of attraction.
We measure the dynamical arrest transition in a model, thermoreversible, adhesive hard sphere dispersion. At low volume fractions ϕ, below the critical point, gelation occurs within the gas-liquid ...phase boundary. For ϕ slightly below and above the critical concentration, the phase boundary follows the predicted percolation transition. At high ϕ, it melds into the predicted attractive-driven glass transition. Our results demonstrate that for ϕ above ∼20% physical gelation is an extension of the attractive-driven glass line and occurs without competition for macroscopic phase separation.
We show that the morphology of polymer-based solar cells substantially changes after annealing using small angle neutron scattering. Phenyl-C61-butyric acid methyl ester (PCBM) is found reasonably ...well dispersed within the poly(3-hexylthiophene) (P3HT) rich phase after initial processing (spin coating). However, the PCBM structure coarsens after annealing, clearly evidenced by the increase in scattering intensity at a small wave vector. The change in morphology at the nanoscale is related to improved device performance and the simultaneous, contradictory, increase in photoluminescence.
The complex, nonlinear flow behavior of soft materials transcends industrial applications, smart material design and non-equilibrium thermodynamics. A long-standing, fundamental challenge in ...soft-matter science is establishing a quantitative connection between the deformation field, local microstructure and macroscopic dynamic flow properties i.e., the rheology. Here, a new experimental method is developed using simultaneous small angle neutron scattering (SANS) and nonlinear oscillatory shear rheometry to investigate the spatiotemporal microstructure evolution of a polymer-like micellar (PLM) solution. We demonstrate the novelty of nonlinear oscillatory shear experimental methods to create and interrogate metastable material states. These include a precursory state to the shear banded condition as well as a disentangled, low viscosity state with an inhomogeneous supra-molecular microstructure flowing at high shear rates. This new experimental evidence provides insight into the complexities of the shear banding phenomenon often observed in sheared complex fluids and provides valuable data for quantitatively testing non-equilibrium theory.
Changes in the crystalline and mesoscale lamellar structure during plastic deformation of semicrystalline polymers have been extensively studied by X-ray diffraction techniques. However, direct ...measurements of single chain conformations during stretching have not been realized, although they are key to fully understand the structural transitions during cold drawing and their relation with the state of uniaxial stress. We report direct measurements of molecular alignment of a semicrystalline polymer during cold drawing by combining in-situ small-angle neutron scattering (SANS) and polarized Raman spectroscopy. The sample investigated is a linear low-density polyethylene (LLDPE) with density of 918 kg/m3 and melt index of 1.0 g/10 min. A multifaceted protocol consisting of hydrogen–deuterium exchange, followed by fractionation (by molecular weight, MW) and blending of selected deuterated fractions with protonated LLDPE, was used to elucidate, via SANS measurements, the response of the different fractions to uniaxial deformation. Under tensile deformation significant chain stretching occurs in the initial elastic regime. Further plastic deformation causes additional chain stretching, but to a lesser degree, that eventually plateaus in the strain hardening regime. Concurrently, the fraction of trans conformers increases linearly, as measured by in-situ Raman spectroscopy. The total orientation, quantified using an alignment factor, is lower for the lower MW fractions. We hypothesize through simple geometric arguments that this is directly related to the probability of forming intercrystal tie chains.
Shear-induced structural transitions of a micellar cubic phase during large amplitude oscillatory shear flow is studied with time-resolved oscillatory rheological small angle neutron scattering. This ...technique allows us to resolve the structural changes within a cycle of oscillation. By applying a strain rate near the critical melting shear rate, melting and recrystallization occurs in a cyclic mode. The maximum degree of order is observed when the shear stress reaches a plateau value during the large amplitude oscillatory shear cycle, whereas melting is maximized at the strain rate wave peaks. This structural evolution confirms the cyclic mechanism of sticking and sliding of 2D hexagonal close-packed layers I. W. Hamley et al., Phys. Rev. E 58, 7620 (1998).
The role of gravity in gelation of adhesive hard spheres is studied and a critical criterion developed for homogeneous gelation within the gas-liquid binodal. We hypothesize that gelation by Brownian ...diffusion competes with phase separation enhanced by gravitational settling. This competition is characterized by the gravitational Péclet number Pe(g), which is a function of particle size, volume fraction, and gravitational acceleration. Through a systematic variation of the parameters, we observe the critical Pe(g) of ∼ 0.01 can predict the stability of gels composed of adhesive hard spheres.
Small-angle neutron scattering (SANS) is a powerful method for probing the structural properties of polymeric materials. Contrast between polymer chains can be obtained by labeling with deuterium, ...which provides an opportunity for analyzing individual chain behavior in bulk. A transition metal (Pt/Re)-catalyzed reaction in isooctane was used to exchange deuterium for hydrogen in various saturated hydrocarbon polymers, including a commercial polyethylene. We have investigated the role of two forms of molecular heterogeneity on the labeling reaction using narrow dispersity hydrogenated polybutadiene (hPBD) samples with controlled molecular weight and ethyl branch content (short chain branching). These materials were prepared by anionic polymerization, followed by catalytic hydrogenation. A monotonic increase of deuterium labeling from 65% to 84% was observed when molecular weight was increased from 4000 to 216 000. Increasing the molecular weight to 635 000, however, resulted in almost no exchange, which is possibly due to the existence of a lower critical solution temperature (LCST) in isooctane. A similar trend with molecular weight was found for an isotope-labeled commercial linear low-density polyethylene material with 2.5% butyl branches and molecular weight ranging between 1000 and 1 000 000. Variation of ethyl branches from 2 to 50 ethyl branches per 100 backbone carbons in hPBDs reduced the level of exchange from 78% to 34%, with deuterons preferentially entering the pendant methyl groups at higher levels of branching. The materials generated from this isotope exchange reaction proved to be viable materials for SANS, providing consistent single chain statistics through proper analysis strategies, which take into account the inhomogeneous distribution of deuterium along and among individual chains caused by partial labeling and the molecular weight dependence of exchange. These results suggest that for a given chain, isotope exchange occurs on the metal catalyst surface during relatively few adsorption steps.
PDF
