Andy H. Tsou

Structure and dynamics of carbon black-filled elastomers

The linear and nonlinear melt viscoelastic properties for a series of carbon black-filled polymer composites were studied. Complementary tapping-mode atomic force microscopy (AFM) studies were used to examine the dispersion and structural correlations of the filler particles in these composites. The low-frequency dependence of the linear viscoelastic moduli gradually changes from liquidlike behavior for the unfilled polymer to pseudosolid character for composites with more than 9 vol % carbon black filler. The plateau modulus, inferred from the linear viscoelastic response, exhibits a somewhat discontinuous change at about 9 vol % filler. On the basis of the linear viscoelastic response, we postulate that the carbon black filler forms a continuous percolated network structure beyond 9 vol % filler, considerably lower than that expected from theoretical calculations for overlapping spheres and ellipsoids. We suggest that the lower threshold for percolation is due to the polymer mediation of the filler structure, resulting from the low functionality of the polymer and, consequently, few strong polymer–filler interactions, allowing for long loops and tails that can either bridge filler particles or entangle with one another. Furthermore, the strain amplitude for the transition from linear behavior to nonlinear behavior of the modulus for the composites with greater than 9 vol % filler is independent of frequency, and this critical strain amplitude decreases with increasing filler concentration. Complementary AFM measurements suggest a well-dispersed carbon black structure with the nearest neighbor distance showing a discontinuous decrease at about 9 vol % filler, again consistent with the formation of a filler network structure beyond 9 vol % carbon black.

Mapping nanoscale elasticity and dissipation using dual frequency contact resonance AFM.

We report on a technique that simultaneously quantifies the contact stiffness and dissipation of an AFM cantilever in contact with a surface, which can ultimately be used for quantitative nanomechanical characterization of surfaces. The method is based on measuring the contact resonance frequency using dual AC resonance tracking (DART), where the amplitude and phase of the cantilever response are monitored at two frequencies on either side of the contact resonance. By modelling the tip-sample contact as a driven damped harmonic oscillator, the four measured quantities (two amplitudes and two phases) allow the four model parameters, namely, drive amplitude, drive phase, resonance frequency and quality factor, to be calculated. These mechanical parameters can in turn be used to make quantitative statements about localized sample properties. We apply the method to study the electromechanical coupling coefficients in ferroelectric materials and the storage and loss moduli in viscoelastic materials.

Phase transformation in quenched mesomorphic isotactic polypropylene

Abstract The transformation of mesomorphic phase to α-monoclinic crystal phase in quenched isotactic polypropylene has been investigated by TEM, DSC and time-resolved SAXS and WAXD methods. It is found that even though the initial appearance of the cluster structure in mesomorphic i-PP seems to support the model of a multi-step process for polymer crystallization, results indicate that the transformation is not spontaneous. In the cluster domains, a significant fraction of chain segments must undergo a reorganization process in order to establish the correct registration of helical hands for crystallization. In addition, a fraction of the ordered chains with correct registration of helical hands should also serve as primary nuclei to initiate crystallization. However, the entire cluster domains should not be considered as ‘precursors’. The growth process via secondary nucleation eventually transforms the cluster structure to the lamellar structure.

Viscoelastic Property Mapping with Contact Resonance Force Microscopy

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Orientation‐induced crystallization in isotactic polypropylene melt by shear deformation

Development of orientation-induced precursor structures (nuclei) prior to crystallization in isotactic polypropylene melt under shear flow was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and rheo-optical techniques. SAXS patterns at 165 °C immediately after shear (rate = 60 s -1 , t s = 5 s) showed emergence of equatorial streaks due to oriented structures (microfibrils or shish) parallel to the flow direction and of meridional maxima due to growth of the oriented layer-like structures (kebabs) perpendicular to the flow. SAXS patterns at later times (t = 60 min after shear) indicated that the induced oriented structures were stable above the nominal melting point of iPP. DSC thermograms of sheared iPP samples confirmed the presence of two populations of crystalline fractions; one at 164 °C (corresponding to the normal melting point) and the other at 179 °C (corresponding to melting of oriented crystalline structures). Time-resolved optical micrography of sheared iPP melt (rate = 10 s -1 , t s = 60 s, T = 148 °C) provided further information on orientation-induced morphology at the microscopic scale. The optical micrographs showed growth of highly elongated micron size fibril structures (threads) immediately after shear and additional spherulities nucleated on the fibrils at the later stages. Results from SAXS and rheo-optical studies suggest that a stable scaffold (network) of nuclei, consisting of shear-induced microfibrillar structures along the flow direction superimposed by layered structures perpendicular to the flow direction, form in polymer melt prior to the occurance of primary crystallization. The scaffold dictates the final morphological features in polymer.

Strain-induced molecular orientation and crystallization in natural and synthetic rubbers under uniaxial deformation by in-situ synchrotron X-ray study

Abstract In-situ synchrotron wide-angle X-ray diffraction (WAXD) studies and simultaneous measurements of stress and strain during uniaxial stretching of various vulcanized rubbers were carried out (at room temperature and 0°C) to reveal the strain-induced molecular orientation and crystallization relationships. Rubbers evaluated included natural rubber (NR), synthetic poly-isoprene rubber (IR), poly-cis-1,4-butadiene rubber (BR) and butyl rubber (IIR). Some universal features were observed in these systems: (i) At high strains (> 5.0), the majority of the chains (up to 50 ≈ 75%) in natural and synthetic rubbers remained in the un-oriented amorphous state with only a small amount of crystalline fraction formed (10–20%). The rest of the chains were in the oriented amorphous state. (ii) During deformation, the oriented amorphous chains acted as precursors to strain-induced crystallization. A network of micro-fibrillar crystallites is formed within the closely populated vulcanization points, leading to the e...

Dispersion of Layered Organosilicates in Isobutylene-Based Elastomers

Abstract Dispersion morphologies of polymer-layered silicate nanocomposites based on either brominated poly(isobutylene-co-para-methylstyrene), BIMSM, or brominated poly(isobutylene-co-isoprene), BIIR, and an organosilicate, dimethylditallow ammonium-exchanged montmorillonite, Cloisite™ 6A, with and without N660 carbon black fillers were examined using SAXS, WAXS, AFM, and TEM. These compounds were prepared using an internal mixer and cured for property measurements. Due to the observed partial orientation of organosilicates and their possible heterogeneous intercalation, degrees of exfoliation and dispersion of organosilicates in BIMSM and BIIR were unable to be characterized and quantified simply by TEM, AFM, or SAXS alone. Instead, using the projected aspect ratio of organosilicates in BIMSM or BIIR, extracted from Gusev-Lusti equation based on measured permeability ratios, was found to provide a relative measure of their dispersion state. Since better dispersion, higher planar orientation, and/or incr...

Synchrotron X-Ray Studies of Vulcanized Rubbers and Thermoplastic Elastomers

Synchrotron X-ray diffraction technique has revealed strain-induced crystallization and molecular orientation in vulcanized rubbers and thermoplastic elastomers (TPE) during deformation in real time. The stress-strain curves and wide angle X-ray diffraction (WAXD) patterns in vulcanized rubbers and TPE were measured simultaneously. In-situ WAXD patterns were taken not only at different strains during uniaxial deformation but also at different temperatures at a constant strain. Results lead to several new insights. (i) Strain-induced crystallization is a common phenomenon in vulcanized rubbers, except SBR (styrene-butadiene rubber), and in TPE (with crystalline hard segments). (ii) Strain-induced crystallization decreases the stress and increases the elongation in the strained rubber. (iii) The hybrid structure of chemical networks and strain-induced crystallites is responsible to the tensile strength and elongation at break for both systems. (iiii) Some original crystal fraction (hard segment domain) in TPE is destroyed. During deformation, strain-induced crystallization increases with strain. Upon retraction even to stress zero, the majority of oriented strain-induced crystallites remains in tack with preferred orientation.

Unique Behavior of Hydrocarbon Resin Tackifier on Unaged and Aged Tack of Brominated Isobutylene-co-p-methylstyrene (BIMS) Rubber

The effect of hydrocarbon resin tackifier on autohesion of brominated isobutylene-co-p-methylstyrene (BIMS) rubber was investigated by a 180° peel test. The tack strength increased with resin (tackifier) loading up to 10 phr beyond which it dropped. At 10 phr resin concentration, the tackifier enhanced the self-bond formation by enhancing the chain mobility across the interface and, in turn, by providing greater separation resistance from the diffused chains. These conclusions were derived from measured maximum tensile stress, compression creep and viscoelastic properties of the rubber–resin mixtures. The tack strength of neat BIMS rubber showed t ½ dependence with respect to increasing contact time (t). On the other hand, the tack strength of the resin loaded sample showed t ¼ time dependence on contact time. Although the results from dynamic mechanical analysis (DMA) studies suggested good compatibility between the blend components, morphological studies revealed migration of the tackifier to the rubber surface. Upon aging at 100°C for 36 h, the values of tack strength of both the neat BIMS rubber and the BIMS/tackifier blends significantly decreased. The reduced tack strength of the aged samples was attributed to the changes in compression creep, maximum tensile stress and viscoelastic properties of the samples after aging. Upon aging, the resin loaded samples exhibited excessive blooming of the tackifier onto the rubber surface.

Filler distribution and domain size of elastomer compounds by solid-state NMR and AFM

Abstract Solid-state NMR methods were used to characterize the filler distribution in rubber blends and domain size of thermoplastic vulcanizates (TPV). Correlation between filler content and magic...