Analysis of small-angle scattering of suspensions of organically modified montmorillonite: Implications to phase behavior of polymer nanocomposites

Martini Coarse-Grained Model for Clay-Polymer Nanocomposites

We have developed a coarse-grained (CG) model of a polymer-clay system consisting of organically modified montmorillonite (oMMT) nanoclay as the nanoparticle in accordance with the MARTINI force field. We have used mechanical properties and cleavage free energy of the clay particle to respectively parameterize bonded and nonbonded interaction parameters for an oMMT clay particle, where intergallery Na⁺ ions are replaced by tetramethylammonium (TMA) ions. The mechanical properties were determined from the slope of the stress-strain curve and cleavage free energy was determined by allowing for full surface reconstruction corresponding to a slow equilibrium cleavage process. Individual dispersive and polar contributions to oMMT cleavage energy were used for determination of appropriate MARTINI bead types for the CG oMMT sheet. The self-consistency of the developed MARTINIFF parameters for the TMA-montmorillonite-polymer system was verified by comparing estimates for select structural, thermodynamic, and dynamic properties obtained in all-atomistic simulations with that obtained in CG simulations. We have determined the influence of clay particles on properties of three polymer melts (polyethylene, polypropylene, and polystyrene) at two temperatures to establish transferability of the developed parameters. We have also shown that the effect of clay-polymer interactions on structure-property relationships in the polymer-clay nanocomposite system is well captured by Rosenfeld's excess entropy scaling.

In Operando Monitoring of Dynamic Recovery in 3D-Printed Thermoset Nanocomposites by XPCS

Extrusion-based additive manufacturing methods, such as direct-write of carbon fiber-reinforced epoxy inks, have become an attractive route toward development of structural composites in recent years, because of emerging techniques such as big area additive manufacturing. The development of improved materials for these methods has been a major focus area; however, an understanding of the effects of the printing process on the structural and dynamic recovery in printed materials remains largely unexplored. The goal of this work is to capture multiscale and temporal morphology and dynamics within thermosetting composite inks to determine the parameters during the printing process that influence the recovery of the printed material. Herein, we use X-ray photon correlation spectroscopy in small-angle scattering geometry to reveal both morphology and recovery dynamics of a nanoparticle (layered-silicate Cloisite 30B) in a thermoset epoxy resin (EPON 826) during the printing process in real time. Our results show that the dynamics of the layered silicate particles during recovery are anisotropic and slow down to behavior which is characteristic of aging in colloidal clay suspensions around  t_age ≈ 12 s. The dynamics and alignment of the particles during recovery were tempo-spatially mapped, and the recovery post printing was shown to be strongly influenced by the deposition onto the build plate in addition to the extrusion through the print head. Our in operando results provide insight into the parameters that must be considered when optimizing materials and methods for precisely tailored local properties during 3D printing.

Revealing the interface in polymer nanocomposites

The morphological characterization of polymer nanocomposites over multiple length scales is a fundamental challenge. Here, we report a technique for high-throughput monitoring of interface and dispersion in polymer nanocomposites based on Förster resonance energy transfer (FRET). Nanofibrillated cellulose (NFC), fluorescently labeled with 5-(4,6-dichlorotriazinyl)-aminofluorescein (FL) and dispersed into polyethylene (PE) doped with Coumarin 30 (C30), is used as a model system to assess the ability of FRET to evaluate the effect of processing on NFC dispersion in PE. The level of energy transfer and its standard deviation, measured by fluorescence spectroscopy and laser scanning confocal microscopy (LSCM), are exploited to monitor the extent of interface formation and composite homogeneity, respectively. FRET algorithms are used to generate color-coded images for a real-space observation of energy transfer efficiency. These images reveal interface formation at a nanoscale while probing a macroscale area that is large enough to be representative of the entire sample. The unique ability of this technique to simultaneously provide orientation/spatial information at a macroscale and nanoscale features, encoded in the FRET signal, provides a new powerful tool for structure-property-processing investigation in polymer nanocomposites.

Large-Scale Morphology of Dispersed Layered Silicates

Ultra small angle x-ray scattering is used to probe the morphology of highly dispersed montmorillonite (MMT) in water and polyamide-66. In water the scattered intensity, I(q) shows a q-2 dependence for q > 0.01 Å-1, where q is the magnitude of the scattering vector. This is as expected for a two dimensional sheet-like object. On larger scales (smaller q) mass-fractal character is evident up to the radius-of-gyration of the individual scattering entities. The scattering profile is interpreted using a semi-flexible sheet model in which flat, disk-like entities of radius = 80 Å (an areal persistence length) are fractally distributed on large scales with a mass fractal dimension of 2.65. These size scales correspond to a scattering entity comprised of one or a few crumpled sheets. No evidence of inter-particle correlations is found at concentrations below the gel point. In polyamide-66 loaded with organically modified MMT long-range fractal behavior is also observed but with larger fractal dimension.

In vitro fibroblast response to polyurethane organosilicate nanocomposites

The term nanocomposite refers to organic:inorganic composites where one phase, typically the inorganic phase, has dimensions on the nanoscale. Several authors have noted the potential benefit of biomedical application of nanocomposite technology, and have suggested using quaternary ammonium compounds (QAC) as an organic modification to enhance dispersion of nanoparticles within polymer matrices. This study aimed to examine fibroblast responses in vitro to a range of nanocomposites using different organic modifiers. Composite materials were prepared from a polyether urethane (PEU) and various unmodified and organically modified montmorillonite (MMT) nanoparticles. QAC and amino undecanoic acid (AUA) modified-MMT were added to PEU at loadings ranging from approximately 1 to 15 wt %. Composites with organically modified QAC and AUA particles displayed partially exfoliated and intercalated silicate morphology, respectively. Nanocomposites showed increases in ultimate tensile properties for materials with lower QACMMT loadings. However QAC was shown to significantly inhibit cell growth following release from PEU-QACMMT under extraction conditions mimicking those of the physiological environment. Materials containing silicate modified using AUA were cytocompatible. The results of this study suggest that QAC may be unsuitable as organic modifiers for nanoparticles destined for biomedical use. Alternative modifiers based on AUA confer equivalent dispersion and are of low toxicity.

Structure and rheology of organoclay suspensions

We have characterized a montmorillonite-based organoclay dispersed in three different nonaqueous solvents using a combination of x-ray scattering, small-angle neutron scattering (SANS), and ultrasmall angle neutron scattering (USANS), together with rheological measurements. Consistent with these measurements, we present a structural model for the incompletely dispersed clay as consisting of randomly oriented tactoids made of partially overlapping clay sheets, with transverse dimensions of several microns. Intersheet correlation peaks are visible in x-ray scattering, and quantitatively fit by our model structure factor. SANS and USANS together show a power law of about -3 over a wide range of wave numbers below the intersheet correlation peak. Our model relates this power law to a power law distribution of the number of locally overlapping layers in a tactoid. The rheology data show that both storage and loss moduli, as well as yield stress, scale with a power law in volume fraction of about three. Equating the gel onset composition with the overlap of randomly oriented tactoids and taking into account the large transverse dimensions of the tactoids, we predict the gel point to be at or below 0.006 volume fraction organoclay. This is consistent with the rheology data.

Adsorption on hydrophobized surfaces: clusters and self-organization

The arrangement of liquid molecules on surfaces bristling with alkyl chains is deduced from adsorption studies, X-ray powder diffraction data, and microcalorimetric measurements of swelling-type layered materials, especially clay minerals. Small polar molecules such as water, ethanol, formamide, dimethylsulfoxide, and aromatic compounds are clustered between the alkyl chains pointing away from the surface. The energetic contribution related to the movement of the alkyl chains from direct contact with the surface atoms into upright positions is decisive. The importance of the interactions between the liquid molecules on the structure of the adsorption layer is clearly indicated by the changes of the adsorption layer thickness by salt addition. Thermodynamic data are obtained from surface excess adsorption isotherms from binary liquid mixtures combined with microcalorimetric measurements. Long-chain adsorptives such as long-chain alcohols interact with the surface alkyl chains by forming stable bimolecular films. These films undergo a series of higher-order phase transitions into kink- and gauche-block structures as the consequence of rotational isomerization of the alkyl chains. Such transitions are considered elementary processes in self-assembling films (layer-by-layer deposition, fuzzy films, Langmuir-Blodgett technique), and lipid membranes.

Swelling behavior of organoclays in styrene and exfoliation in nanocomposites

Organoclays were prepared by cationic exchange of montmorillonite (MMT) with three commercial surfactants: octadecylamine (ODA), hexadecyltrimethylammonium bromide (HTAB), benzalkonium chloride (BAC), and an in-house synthesized surfactant, vinylbenzylalkyldimethylammonium chloride (VDAC). The swelling behaviors of organoclays in styrene were compared by wettability and rheology measurements. VDAC-MMT had the slowest capillary rise rate in styrene. The Washburn equation was found not suitable for the swellable organoclay powders. All organoclay-styrene and organoclay-toluene mixtures demonstrated yield stress. However, VDAC-MMT-styrene and VDAC-MMT-toluene mixtures showed significant higher yield stresses than the other organoclay mixtures. The wettability and rheology measurements indicate that VDAC-MMT bearing a styryl functional group has the strongest interaction with styrene monomer and toluene. In order to further evaluate the extent of swelling and the microstructure, polystyrene (PS)-clay nanocomposites were prepared by in situ polymerization. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicate that ODA and HTAB-MMT led to intercalated PS-clay nanocomposites, while VDAC-MMT formed exfoliated PS-clay nanocomposites.