Advanced Photon Source

A U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences national synchrotron x-ray research facility

Publications from year 2003

Interfacial modification for controlling silica-polysiloxane interactions and bonding in some elastomeric composites. Silica reinforcing fillers were generated using the sol-gel approach and their surfaces were modified using either a vinyl alkoxysilane (to provide permanent bonding to a host poly(dimethylsiloxane) elastomer), or a hemiacetal ester (to provide bonding that could be thermally ruptured). The surface areas of the fillers were measured by nitrogen absorption, and their morphologies, interfacial structures, and crosslinking to the elastomer were characterized by ultra small angle and small angle X-ray scattering, scanning electron microcopy, and Fourier-transform infrared spectroscopy. Increasing the amount of the silane coupling agent decreased silica domain sizes, with corresponding increases in surface area and stronger filler-polymer interactions. Vinyltrimethoxysilane had a larger effect than vinylmethyldimethoxysilane. The tensile moduli, tensile strengths, and degrees of toughness of the composites were determined using stress-strain measurements in elongation, both under near-equilibrium conditions and under continuous extension. The permanently bonded modifications gave composites with improved mechanical properties. Rupturing the hemiacetal ester bonding at 120-150degreesC gave some increase in ultimate stress, maximum elongation, and toughness, thus underscoring the importance of interactions or 'networking' between filler particles.

B.T.N. Vu, J.E. Mark and D.W. Schaefer. Cited: Composite Interfaces, 2003, 10 (4-5), p 451-473.

Agglomerate formation during drying. The evolution of agglomerate structure during drying of particles from suspension has been studied for a nanocrystalline Y2O3 (8% mol fraction)-stabilized ZrO2 powder. Agglomerates in drying and dried suspensions were examined at the smallest size scales (1 nm to 1 mum) using ultra-small angle x-ray scattering (USAXS) and at the largest size scales (100 nm to 10 mum) using scanning electron microscopy. The results were correlated with the degree of particle dissolution in each suspension (measured by flame absorption spectroscopy of the suspension filtrate) and the zeta potential of the particles in suspension prior to drying. Results show that large agglomerates readily form across a pH range from 2 to 9. The fact that Y+3 ion dissolution varies by over four orders of magnitude in this range leads to the conclusion that there is little direct correlation between the degree of Y dissolution and agglomeration in this system (Zr ion dissolution was below the detection limit at all pH values studied). The observation of large agglomerates well before the introduction of air-water interfaces into the drying mass likewise leads to the conclusion that capillary forces are not essential to agglomerate formation. Instead, agglomerates appear to form as a direct consequence of increasing suspension concentration. Zeta potential also plays a role. Specifically, there was a notable change in agglomerate morphology as the isoelectric point was approached, at approximately pH 8. Here USAXS shows the particles in suspension to have a layered interior structure, with small primary particles aggregated in sheets to form each blocky particle. This is in contrast to the more rounded agglomerates formed away from the isoelectric point, which appear to be composed of the same primary particles arranged in chainlike structures. USAXS of powders from the dried suspensions confirms that the structures seen after drying are the same as those present in suspension. The two structural morphologies are attributed to diffusion-limited. (sheets) versus reaction-limited (chains) aggregation, respectively.

A. Vertanessian, A. Allen and M.J. Mayo. Cited: Journal of Materials Research, 2003, 18 (2), Feb, p 495-506.

Effect of drying on the structure and dispersion of precipitated silica. We use a combination of light scattering and ultra-small-angle X-ray scattering (USAXS) to establish the effect of drying on the structure of reinforcing silica. The data show that drying substantially changes the morphology on two different length-scales corresponding to aggregates and agglomerates. The rather gentle process of oven drying leads to deeply interpenetrated aggregates that are clustered into agglomerates that exceed 100 mum in radius of gyration. Spray drying, on the other hand, produces agglomerates in the 20 mum range, made up of aggregates that are weakly interpenetrated. On incorporation in organic rubber, the fillers look structurally identical as far as aggregate morphology is concerned. These results, along with light scattering data on sonicated samples, show that oven-dried powders are made up of robust agglomerates. Spray-dried agglomerates are less interpenetrated and substantially more fragile, making them easier to disperse in rubber by mechanical means. This observation was confirmed by the measuring the modulus and elongation at break measurements of silica filled rubber.

C.N. Suryawanshi, P. Pakdel and D.W. Schaefer. Cited: Journal of Applied Crystallography, 2003, 36 Jun, p 573-577.

Viscoelasticity and rheology of depletion flocculated gels and fluids. The flow properties of high volume fraction hard sphere colloid-polymer suspensions are studied as a function of polymer concentration, depletion attraction range, and solvent quality up to, and well beyond, the gelation boundary. As the gel boundary is approached, the shear viscosity tends to diverge in a critical power law manner at a polymer concentration that is a function of the polymer radius of gyration and solvency condition. The shear viscosity for different polymer size suspensions can be collapsed onto a master curve motivated by mode coupling theory (MCT). The low frequency elastic modulus grows rapidly with increasing depletion attraction near the gel boundary, but becomes a dramatically weaker function of polymer concentration as the gel state is more deeply entered. A simplified version of MCT with accurate, no adjustable parameter polymer reference interaction site model (PRISM) theory structural input has been applied to predict the gelation boundaries and elastic shear moduli. The calculated gel lines are in semiquantitative agreement with experiment at high volume fractions, but increasingly deviate upon particle dilution. Calculations of the dependence of the gel elastic shear moduli on particle-polymer size asymmetry and scaled polymer concentration are in excellent agreement with experiment, and deep in the gel follow a power law dependence on polymer concentration. Quantitatively, MCT-PRISM elastic moduli are higher than experiment by a nearly constant large factor. This discrepancy is suggested to be due to the heterogeneous nature of the gel structure which small angle scattering experiments show consists of dense clusters and voids of characteristic length scales similar to4-7 particle diameters. A simple idea for correcting the particle level MCT modulus by employing cluster network concepts is proposed. (C) 2003 American Institute of Physics.

S.A. Shah, Y.L. Chen, K.S. Schweizer and C.F. Zukoski. Cited: Journal of Chemical Physics, 2003, 119 (16), Oct 22, p 8747-8760.

Microstructure of dense colloid-polymer suspensions and gels. A systematic experimental study of polymer-induced changes of the collective structure of model hard-sphere nanocolloids in the fluid and gel states has been carried out using ultra-small-angle x-ray scattering. The focus is on small, non-adsorbing polymer depletants where a direct transition from the homogeneous fluid phase to a nonequilibrium gel state occurs with increasing polymer additions. As the polymer concentration is increased in the homogeneous fluid phase, the low angle concentration fluctuations monotonically increase, the characteristic interparticle separation decreases and tends to saturate, and the intensity of the cage order peak varies in a non-monotonic manner. These equilibrium structural changes depend in a systematic fashion on colloid volume fraction and polymer-colloid size asymmetry, and are in near quantitative agreement with the parameter-free polymer reference interaction site model theory calculations. By combining the accurate equilibrium theory with experimental observations, the loss of ergodicity and nonequilibrium structure formation in the gel state can be deduced. Abrupt departures between theory and experiment on the -2-3 particle diameter and greater length scales are observed as the gel boundary is traversed. The liquid-like local cage structure is arrested. Intermediate scale fluctuations are suppressed suggesting the formation of small, compact clusters. Large amplitude, Porod-like fluctuations emerge on large length scales due to quenched heterogeneities which are analysed using a random two-phase composite model. By combining the results of all the scattering experiments and theoretical calculations a qualitative real space picture of the gel microstructure is constructed, and its mechanical consequences are qualitatively discussed.

S.A. Shah, Y.L. Chen, S. Ramakrishnan, K.S. Schweizer and C.F. Zukoski. Cited: Journal of Physics-Condensed Matter, 2003, 15 (27), Jul 16, p 4751-4778.

Morphology of dispersed carbon single-walled nanotubes. Using scattering methods, we determine the morphology of carbon nanotube suspensions over length scales from I nm to 50 mum. We find no evidence of rod-like character at any length. Rather, a network structure of aggregated tubes, similar to that seen in dry samples, is found. These observations have significant implications regarding the use of single-walled nanotubes as a composite reinforcing filler since the network structure has significantly lower modulus than fully dispersed tubes. We also show that it is possible to isolate a rod-like fraction from the aggregated suspension using intense sonication, providing a potential route to fully dispersed nanotubes. (C) 2003 Elsevier Science B.V. All rights reserved.

D.W. Schaefer, J. Zhao, J.M. Brown, D.P. Anderson and D.W. Tomlin. Cited: Chemical Physics Letters, 2003, 375 (3-4), Jul 3, p 369-375.

Structure and dispersion of carbon nanotubes. Small-angle light scattering and ultra small-angle X-ray scattering are used to assess the morphology of single-walled ( SWNTs) and multi-walled carbon nanotubes (MWNTs). For MWNTs, a power-law scattered-intensity profile with a slope of - 1.08 is consistent with the rod-like morphology. For SWNTs, however, scattering profiles characteristic of rod-like morphology are not observed on any length-scale from 1 nm to 50 mum. Rather, disordered objects are found that we identify as a network of carbon "ropes" enmeshed with polyelectrolyte dispersants. The effectiveness of polyelectrolyte dispersants is assessed using small-angle light scattering in conjunction with exposure to ultrasound. In the presence of an anionic polyelectrolyte, sonication can assist dispersion of both SWNTs and MWNTs. In the presence of a cationic agent, however, sonication can induce aggregation. SWNTs respond differently to ultrasound depending on whether residual synthesis catalyst is present. Four dispersants are studied, of which sodium polystyrene sulfonate is the most effective and polyallylamine hydrochloride is the least effective.

D. Schaefer, J.M. Brown, D.P. Anderson, J. Zhao, K. Chokalingam, D. Tomlin and J. Ilavsky. Cited: Journal of Applied Crystallography, 2003, 36 Jun, p 553-557.

Scattering studies of the structure of colloid-polymer suspensions and gels. Depletion-driven changes in the structure of hard-sphere particles (radius R) mixed with a nonadsorbing polymer (radius of gyration R-g) dissolved in good (athermal) and ideal (theta) solvents are systematically studied. Colloidal structure factors, S(q), are determined using slit-smeared and pinhole-collimated ultrasmall-angle X-ray scattering and small-angle neutron scattering. A comparison of the structure factors extracted from the three methods demonstrates the validity of the available desmearing algorithms. Polymer additives alter the colloidal structure more for larger particle volume fractions (phi(c)) and smaller size asymmetry ratios R-g/R. At fixed phi(c) similar to0.40 and R-g/R = 0.06, increasing the reduced polymer concentration (c(p)/c(p)*) results in a monotonic shift to higher wavevectors of the location of the first peak in the structure factor, q*, and a nonmonotonic variation of the cage order parameter, S(q*), in a nearly solvent quality independent manner. Local structural correlations arrest as the gel state is entered. The reduced polymer concentration required for gelation is smaller in athermal solvents compared to its theta analogue and, in both cases, is well below the fluid-fluid spinodal boundaries. Comparisons between the measured structure factors and no-adjustable-parameter predictions of the polymer reference interaction site model theory shows near quantitative agreement over all wavevectors. When the gel phase is entered, strong differences between the theory and the experiment emerge, indicating the nonequilibrium nature of structural correlations in the nonergodic gel. Relative to equilibrium expectations, enhanced (reduced) fluctuations occur at small (intermediate) wavevectors. The combined experimental and theoretical results suggest that neither long wavelength fluctuations nor the local cage structure are the primary origin of the gelation transition.

S.A.S.S. Ramakrishnan, Y.L. Chen, K.S. Schweizer and C.F. Zukoski. Cited: Langmuir, 2003, 19 (12), Jun 10, p 5128-5136.

Preparation and characterization of some unusually transparent poly(dimethylsiloxane) nanocomposites. A technique was developed for preparing poly(dimethylsiloxane) nanocomposites having unusually high transparencies as quantitatively judged by ultraviolet-visible spectroscopy. The method was the in situ generation of silica particles by a two-step sol-gel procedure in which the required water of hydrolysis was simply absorbed from the air, and the catalyst was generated in situ from a tin salt. Electron microscopy showed that the phase-separated silica domains were very small (30-50 nm in diameter) and well dispersed, as expected from the transparency of the composites. Stress-strain measurements in tension indicated that the particles provide very good reinforcement. Ultra-small-angle X-ray scattering data showed that the domain morphology depends strongly on catalyst, but weakly on loading level. (C) 2003 Wiley Periodicals, Inc.

G.S. Rajan, G.S. Sur, J.E. Mark, D.W. Schaefer and G. Beaucage. Cited: Journal of Polymer Science Part B-Polymer Physics, 2003, 41 (16), Aug 15, p 1897-1901.

Size, volume fraction, and nucleation of Stober silica nanoparticles. Si-29 NMR, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) are used to monitor the synthesis of silica nanoparticles from the base-catalyzed hydrolysis of TEOS in methanol and ethanol. The reactions are conducted at a [TEOS] = 0.5 M, low concentrations of ammonia ([NH3] = 0.01-0.1 M) and [H2O] = 1.1-4.4 M to resolve the initial size of the first nuclei and to follow their structural evolution. It is found that after an induction period where there is a buildup of singly hydrolyzed monomer, the first nuclei are fractal and open in structure. Interestingly, the nuclei are twice as large in ethanol (R-g approximate to 8 nm) as those in methanol (R-g approximate to 4 nm). The data suggest that the difference in primary particle size is possibly caused by a higher supersaturation ratio of the singly hydrolyzed monomer in methanol than in ethanol if it is assumed that the surface energy of the first nuclei is the same in methanol and ethanol. The particle number concentration and the volume fraction of the silica particles are calculated independently from the SAXS, DLS, and Si-29 NMR results. Finally, the rate of nucleation is obtained from the particle number concentrations. (C) 2003 Elsevier Inc. All rights reserved.

D.L. Green, J.S. Lin, Y.F. Lam, M.Z.C. Hu, D.W. Schaefer and M.T. Harris. Cited: Journal of Colloid and Interface Science, 2003, 266 (2), Oct 15, p 346-358.

Natural ground water colloids from the USGS J-13 Well in Nye county, NV: a study using SAXS and TEM. We report results from ultra small angle x-ray scattering (USAXS) and transmission electron microscopy (TEM) of dilute silicate colloids that occur naturally in groundwater from the USGS J-13 well, located near the Yucca Mountain Site in Nevada. We also examined a separate sample of this groundwater that had been treated by heating to 90 degrees C in contact with crushed Topopah Spring Tuff from the Yucca Mountain. The USAXS measurements were done at the UNICAT undulator beamline at the Advanced Photon Source at the Argonne National Laboratory. Power-law plots (scattering intensity verses momentum transfer) were fitted to the USAXS data. Colloids in the untreated J-13 groundwater were shown to have a fractal dimension of nearly 3, whereas colloids in the treated groundwater ("EJ-13") have a dimensionality of approximately 2.4 over a length scale of approximately 3 to 300 nm. Similar power-law plots with dimension 3 characterized concurrent SAXS measurements from aqueous suspensions of Na-montmorillonite and NIST Brick Clay (NBS-67). We attribute these results to the sheet silicate layered structure of the clay colloids present in J-13 well water, montmorillonite, and "brick clay" systems. The differences between EJ-13 and as-received J-13 are perhaps owing to exchange of calcium for sodium with the tuff. Radionuclide incorporation into, adsorption onto, or ion exchange with existing groundwater colloids may promote colloidal transport of radionuclides in groundwater. Such radionuclide-bearing colloids could thereby increase the concentrations of actinides in groundwater and enhance migration into human-accessible aquifers. Our results demonstrate the first application of USAXS to study the physical nature of such groundwater colloids, and represent perhaps one of the most dilute systems ever studied by small angle scatering.

J.A. Fortner, C.J. Mertz, S.F. Wolf and P.R. Jemian. Cited: Scientific Basis for Nuclear Waste Management Xxvi, 2003, 757

Recent developments in the characterization of anisotropic void populations in thermal barrier coatings using ultra-small angle x-ray scattering. Thermal barrier coatings (TBC) have complex void microstructures which control their in-service properties. In the research reported here, ultra-small-angle x-ray scattering (USAXS) has been used to characterize anisotropic void populations in TBC's. A new analysis method has been developed for characterizing the voids in three dimensions. The anisotropy in the microstructure at a given length scale associated with a value of the scattering vector, \Q\ gives rise to changes in scattered intensity as a function of the sample orientation. By measuring the scattered intensity at fixed \Q\ as the sample is rotated about the beam, the distribution of the scattering population(s) at this length scale is determined. Using an anisotropic modeling method, the void orientation distribution, void size distribution, shape, volume fraction information is determined for up to four different void populations. This paper illustrates the use of three-dimensional anisotropic USAXS modeling for the analysis of scattering from voids in TBC microstructures deposited by electron beam physical vapor deposition.

T.A. Dobbins, A.J. Allen, J. Ilavsky, G.G. Long, P.R. Jemian, A. Kulkarni and H. Herman. Cited: 27th International Cocoa Beach Conference on Advanced Ceramics and Composites: A, 2003, 24

Improving the fatigue properties of poly(methyl methacrylate) orthopaedic cement containing radiopacifier nanoparticles. For over 40 years, poly(methyl methacrylate) (PMMA) based bone cement has been widely used for fixation of total joint replacement prostheses. Such cements usually comprise two components: (1) liquid MMA monomer and (2) a powder component containing prepolyrnerized 50-100 mum diameter beads of PMMA, along with 1 mum diameter radiopacifier particles of barium sulfate or zirconium oxide and benzoyl peroxide reaction initiator. Upon mixing the two components, the liquid MMA polymerizes and fuses the prepolymerized beads, and the cement slowly hardens in the patient over a period of approximately 30 minutes. The 3 volume percent of radiopacifier particles are necessary to enable orthopaedic surgeons to monitor fatigue fracture of cement in the patient using x-ray radiographs. A major problem associated with these particles is that their incomplete dispersion leads to the presence of large defects, ultimately decreasing the fatigue properties of the cement. In this study, we replaced the 0.5-3 mum size barium sulfate particles with an identical quantity of 50-100 run size barium sulfate particles. The dispersion and size distribution of barium sulfate particles in PMMA cement was characterized using low voltage scanning electron microscopy and ultra-small angle x-ray scattering performed at the UNICAT beamline of the Advanced Photon Source, Argonne National Laboratory, Argonne , IL . The fatigue life of notched dogbone shaped cement specimens was measured by subjecting them to cyclic loading. The nanocomposite cement had a fatigue life of over twice that of the microcomposite cement. LVSEM of fracture surfaces showed evidence of plastic deformation in the regions containing the nanoparticle fillers which resulted in it's higher fracture toughness.

A. Bellare, A.H. Gomoll, W. Fitz, R.D. Scott and T.S. Thornhill. Cited: Thermec'2003, Pts 1-5, 2003, 426-4 p 3133-3138.

3D Hierarchical orientation in polymer-clay nanocomposite films. Organically modified clay was used as reinforcement for HDPE using maleated polyethylene (PEMA) as a compatibilizer. The effect of compatibilizer concentration on the orientation of various structural features in the polymer-layered silicate nanocomposite (PLSN) system was studied using two-dimensional (2D) small angle X-ray scattering (SAXS) and 2D wide-angle X-ray scattering (WARS). The dispersion (repeat period) and three-dimensional (3D) orientations of six different structural features were easily identified:
(a) clay clusters/tactoids (0.12 mum),
(b) modified clay (002) (24-31 Angstrom),
(c) unmodified clay (002) (13 Angstrom),
(d) clay (110) and (020) planes normal to (b) and (c),
(e) polymer crystalline lamellae (001) (190-260 Angstrom), and
(f) polymer unit cell (110) and (200) planes.
A 3D study of the relative orientation of this hierarchical morphology was carried out by measuring three scattering projections for each sample. Quantitative data on the orientation of these structural units in the nanocomposite film is determined through calculation of the major axis direction cosines and through a ternary, direction-cosine plot. Surprisingly, it is the unmodified clay which shows the most intimate relationship with the polymer crystalline lamellae in terms of orientation. Association between clay and polymer lamellae may be related to an observed increase in lamellar thickness in the composite films. Orientation relationships also reveal that the modified clay is associated with large-scale tactoid structures. (C) 2002 Elsevier Science Ltd. All rights reserved.

A. Bafna, G. Beaucage, F. Mirabella and S. Mehta. Cited: Polymer, 2003, 44 (4), Feb, p 1103-1115.