The glass transition temperature of filled polymers and its effect on their physical properties

The glass transition temperature, dynamic shear moduli, and bulk viscosities of Phenoxy PKHH (a thermoplastic polymer made from bisphenol-A and epichlorohydrin) filled with glass beads and Attapulgite clay were investigated. The glass temperature of the polymer increased with increasing filler concentration and with increasing specific surface area of the filler. The data were interpreted by assuming that interactions between filler particles and the polymer matrix reduce molecular mobility and flexibility of the polymer chains in the vicinity of the interfaces. From the measured moduli and the viscosities of the filled and unfilled materials, the modulus reinforcement ratio in the glassy state and the relative viscosity in the viscous state were obtained as functions of the filler type and concentration. The relative modulus for the glass bead composite system follows the Kerner equation, while the clay-filled systems exhibit slightly greater reinforcement. The relative viscosities are strongly temperature dependent and do not follow conventional viscosity predictions for suspensions. It is suggested that the filler has a twofold effect on the viscosity of the composite materials; one is due to its mechanical presence and the other is due to modifications of part of the polymer matrix caused by interaction. Using the WLF equation to express all modifications of the matrix, one can isolate a purely mechanical contribution to the viscosity reinforcement. This mechanical part is approximately bounded by the theoretical predictions of Kerner,³² Mooney, ³⁶ and Brodnyan,⁴¹ for suspension viscosities.     

A study on composites of Nylon-6 with hollow glass microspheres

Summary The preparation of nylon-6/hollow glass microspheres composites by the RIM procedure, leading to a new material with reduced density and increased stiffness, is described. The shear moduli of these composites show a linear dependency on the filler concentration. By means of a modified Kerner equation the shear moduli of the various glass spheres were calculated. A linear dependency between the shear modulus of a glass sphere and its wall thickness is demonstrated. A direct method for the determination of the shear moduli of those glass spheres having a density lower than the matrix material is presented.     

Model filled polymers: The effect of particle size on the rheology of filled poly(methyl methacrylate) composites

The effect of size of crosslinked monodisperse spherical polymer particles on the steady shear and dynamic rheology of filled poly(methyl methacrylate) (PMMA) composites was studied for PMMA and polystyrene (PS) particles in the range from 0.1 to 1.3 micron particle size. For PMMA matrices filled with crosslinked PS particles, reduction in filler size increases non‐Newtonian behavior. Particle size effects on the rheology of filled PMMA were much less pronounced for PMMA filler. The rate of growth of steady shear viscosity with aging time was much larger for PMMA filled with PS particles than with PMMA particles. The apparent yield stress of filled PMMA composites was estimated from Casson plots. The yield stress was negligible for PMMA filler but increased with decreasing particle size for PS filler. We suggest that PS particles are rejected by the PMMA matrix and form clusters, causing large enhancements in viscosity and moduli. Polym. Eng. Sci. 44:452–462, 2004. © 2004 Society of Plastics Engineers.     

Mechanical behavior of reinforced polyurethane foams

The mechanical behavior of three-phase reinforced polyurethane (PU) foam composites was investigated. Chopped-glass fibers, glass beads, and graphite powder were used as reinforcing materials. Emperimental results indicated that chopped-glass fibers enhance the foam mechanical properties in tension, while glass beads and graphite powder tend to improve the mechanical properties in compression. Microscopical observations revealed that the reinforcing filler location is within the cell walls acting as a matrix reinforcement. A modified Kerner equation, based on a model that assumes the superposition of a porous matrix and a rigid particulate filler, was compared with measured elastic moduli of the three-phase composite foams.     

Structure and dynamics of polyethylene/clay films

The relationships between structure and rheology of polyethylene/clay hybrid composite blown films were investigated through rheological tests both in shear and elongational flow. Two polymer matrices (low density polyethylene, LDPE and linear low density polyethylene, LLDPE) with different relaxation kinetics were used. Independently from the matrix, morphological analyses (TEM, XRD, and SEM) indicate that the hybrid structures are similarly constituted of delaminated platelets or tactoids having a relevant degree of orientation along the draw direction. This strongly affects the rheological behavior of materials. However, despite the similarities emerged from morphological analyses, both shear (steady shear and oscillatory) and elongation measurements show a negligible effect upon the rheology of LDPE‐based nanohybrids. Conversely, relevant increases of shear viscosity, dynamic moduli and melt strength of LLDPE‐based nanohybrids have been detected. The effects of homopolymer relaxation kinetics have been investigated by means of stress relaxation tests. The results obtained seem to be consistent with the existence of a roughly bimodal population of dynamical species: a matrix component behaving like the homopolymer, and a fraction interacting with the filler, whose rheological behavior is controlled by the particles and their interactions with the polymer. Mechanical properties of hybrid films were also investigated. Differently from what happens in the melt state, the solid‐state properties mainly depend on the filler amount. The relative increases of tensile modulus and melt strength are of the same order of magnitude for both the matrices used, indirectly confirming the similarities in hybrids structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4749–4758, 2006     

Effects of polymer molecular weight and filler particle size on flow behavior of wood polymer composites

The influence of polymer matrix molecular weight and filler particle size on rheological properties and extrudate distortions of metallocene polyethylene (mPE)/wood flour (WF) composites has been investigated by rotational and capillary rheometers. It was found that at low shear rates smaller filler particles provide higher shear viscosity than the larger sized filler. At high shear rates and WF loadings above 30 wt%, the effect of particle size on the melt flow properties becomes negligible. The relative increase of the storage modulus with decreasing particle size is more pronounced in the case of low molecular weight polymer matrix than that in higher molecular weight polyethylene based composites. The wood filled polyethylenes exhibit extrudate surface defects, which are complex function of the shear rate, polymer matrix molecular weight, and filler particle size. Increasing the shear rate results in pressure oscillations and spurt‐flow. It was also observed that the evolution of the extrudate surface tearing is strongly dependent on the pressure during a single pressure oscillation cycle in the spurt flow regime. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Rheological behavior of low molecular weight polystyrene composites containing monodisperse crosslinked polystyrene beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of crosslinked polystyrene beads and low molecular weight polystyrene matrix, were measured in a cone-and-plate rheometer at different temperatures. Viscosities and dynamic moduli were found to be very sensitive to filler loading and measurement temperature. Steady shear viscosities of 30% and 40% loaded low molecular polystyrene composites showed a power-law behavior over the entire range of shear rates. Storage and loss moduli were initially linear with frequency on double logarithmic plots, with limiting slopes of 0.3 and 0.1. At high concentration of filler particles, they showed a flat plateau at low frequencies, indicating that these systems exhibit a yield behavior. A 20% PS composite loaded with beads of high crosslink densities resulted in poor dispersion of beads as a result of poor dispersion of particles. PS beads 1.16 μm in diameter showed a higher viscosity. It is due to the apparent increase in loading resulting from broken particles. At low measurement temperature, filler effects were suppressed by high viscosity matrix and showed a similar rheological behavior to high molecular weight by PS matrix. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix.     

The rheology of filled polymers

The flow properties of polymer melts containing fillers of various shapes and sizes have been examined. If there is no failure of either the filler or polymer in the solid state, then the modulus enhancement for randomly distributed filler is equal to the melt viscosity enhancement under medium shear stress conditions (10⁴ Nm^?2) in simple shear flow or in oscillatory shear flow. Submicron-size fillers, in particular, can form weak structures in the melt that greatly increase the low shear rate viscosity without changing the modulus of the solid proportionately. The highly pseudo-plastic nature of polymer melts at shear stresses of 10⁶ Nm^?2 means that, even without orientation of filler particles toward the flow direction, the viscosity enhancement is less than at lower shear stresses.     

Effect of ground fluororubber vulcanizate powder on the properties of fluororubber compound

Fluororubber vulcanizate powder (FVP) obtained from fluororubber based on tetrafluoroethylene/propylene/vinylidene fluoride terpolymer by mechanical grinding exists in a highly aggregated chain‐like structure. X‐ray photoelectron spectros‐copy (XPS) and infrared (IR) spectroscopy studies show that there are no chemical change on the rubber surface following mechanical grinding of the fluororubber vulcanizate after heat aging at 200°C for 10 days. The incorporation of FVP as a filler in the fluororubber compound results in a marginal increase of Mooney viscosity, Mooney scorch time and shear viscosity. While tensile strength, modulus and hardness marginally increase on addition of FVP into the fluororubber compound, tear strength decreases. Rhemetric studies show that FVP alone is susceptible to further crosslinking in the presence of a curing agent. Dynamic mechanical spectra reveal that the glass to rubber transition temperature shifts higher by the addition of FVP into the fluororubber compounds. Atomic force microscopy (AFM) images show uniform dispersion of FVP particles into the rubber matrix.     

Effect of the particle size on the viscoelastic properties of filled polyethylene

Composites of surface treated and non-treated colloidal calcium carbonate and high-density polyethylene with different filler loading were prepared. Their viscoelastic properties were studied by dynamic strain sweep and small-amplitude oscillatory shear, and compared to those of the corresponding composites of micron-sized calcite. The specific surface area of the filler enormously increases as the average particle diameter becomes smaller than 600 nm, leading to a strong tendency to agglomeration (soft flocks) and aggregation (hard clusters that need attrition to be disintegrated). In nanocomposites, more and stronger filler clusters are formed than in microcomposites due to the large contact area between the particles. The clusters have different shapes and maximum packing than the nearly spherical primary particles, thus enhance the moduli and viscosity of the composites. The obtained results indicate that the higher moduli and viscosity of the nanocomposites is not a direct consequence of the particle size but is due to the presence of more agglomerates and aggregates. Clusters that are local structures and do not represent a space-filling filler network enhance the moduli in the low frequency region more than at high frequencies and increase the storage more than the loss modulus. The presence of strong local structures in the nanocomposites leads to weak log moduli-log frequency dependence in the low frequency (terminal) region. Polymer adsorption on the particles' surface results in a transient filler-polymer network and slow dynamics of the bound polymer, which contribute to the moduli of the complex fluid. The sum of all these factors leads to gradual increase in moduli and to a shift of the crossover frequency to lower values. Above a certain filler volume fraction, the composite responds as a viscoelastic solid (storage modulus>loss modulus over the whole frequency range and both moduli are frequency independent in the terminal zone of the log-log plot).     

Nonlinear stress relaxation of silica filled solution‐polymerized styrene–butadiene rubber compounds

The stress relaxation of silica (SiO₂) filled solution‐polymerized styrene–butadiene rubber (SSBR) has been investigated at shear strains located in the nonlinear viscoelastic regions. When the characteristic separability times are exceeded, the nonlinear shear relaxation modulus can be factorized into separate strain‐ and time‐dependent functions. Moreover, the shear strain dependence of the damping function becomes strong with an increase in the SiO₂ volume fraction. On the other hand, a strain amplification factor related to nondeformable SiO₂ particles can be applied to account for the local strain of the rubbery matrix. Furthermore, it is believed that the damping function is a function of the localized deformation of the rubbery matrix independent of the SiO₂ content. The fact that the time–strain separability holds for both the unfilled SSBR and the filled compound indicates that the nonlinear relaxation is dominated by the rubbery matrix, and this implies that the presence of the particles can hardly qualitatively modify the dynamics of the polymer. It is thought that the filler–rubber interaction induces a coexistence of the filler network with the entanglement network of the rubbery phase, both being responsible for the nonlinear relaxation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dynamic mechanical behavior of poly(2-hydroxyethyl methacrylate)–glass beads composites

The dynamic relaxation behavior of a model two-phase system, poly(2-hydroxyethyl methacrylate)–glass beads, was studied by means of a freely oscillating torsional pendulum. The effect of the filler content on the storage and loss moduli of the composites could be described in terms of the modified Kerner equation in complex form. At temperatures below the glass transition temperature of the matrix, the agreement between experimental and theoretical data was satisfactory after correction for thermally induced stress due to different thermal expansion coefficients of matrix and filler. In the presence of filler, the capacity of the matrix to store and dissipate energy increases, but the character of molecular motions underlying the dispersions observed is preserved because the temperature of the dispersions remains unchanged. The effect of water on the dynamic relaxation behavior of composites is primarily reflected in changes in the shape of the temperature dependence of the dissipating capacity of the matrix. The data allow the conclusion to be drawn that the chain mobility at the interphase boundary does not decrease and that no additional frictional mechanisms appear.     

Model filled polymers. VII: Flow behavior of polymers containing monodisperse crosslinked polymeric beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of combinations of crosslinked beads and matrices of polystyrene (PS) and polymethacrylates (PMA), are measured in a cone and plate rheometer. Viscosities and moduli were very sensitive to chemical composition. Crosslinked beads of identical composition to the matrix exhibited the lowest viscosity enhancements at low shear rates and the lowest moduli in dynamic mechanical analysis. The effects of bead concentration on rheological behavior were compared for PS and PMMA beads in a PMMA matrix. PMMA beads produce small effects, whereas PS beads yield highly non-Newtonian systems in PMMA, showing a yield stress of 1100 Pa at 30 wt% filler loading and dynamic moduli independent of frequency. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix. Beads identical in composition to the matrix yield uniform dispersions. We propose that uniform and stable bead dispersions exhibit the lowest viscosity and moduli. Beads that cluster in the matrix, such as PS beads in PMMA, exhibit highly non-Newtonian behavior.     

Interparticle and particle-matrix interactions in polyethylene reinforcement and viscoelasticity

Composites of surface treated and untreated non-colloidal CaCO₃ particles and high-density polyethylene (HDPE) with different filler loading (0-30 vol%) were prepared. Their viscoelastic properties were studied by dynamic strain sweep and small amplitude oscillatory shear and correlated to the particle-particle and particle-matrix interactions. The results gave insight into the mechanism of polymer reinforcement by solid inclusions and the factors that lead to the often observed solid-like response in the terminal zone. With increasing filler volume fraction, the particles tend to agglomerate and build clusters (local structures) that can be disintegrated by shearing. Up to 30 vol% no evidence for a space-filling particle network could be found. The presence of clusters increases the viscosity, the moduli and the viscoelastic non-linearity of the composites. Coating the filler surface by a stearic acid monolayer reduces its tendency to agglomerate as well as the adhesion between the particles and the polymer, leading to lower viscosity and interfacial slippage with increasing strain amplitude. Solid inclusions increase the storage modulus more than the loss modulus, hence decrease the material damping. The hydrodynamic reinforcement is frequency independent and dominates at high frequency. Polymer adsorption on the particles surface results in a transient filler-polymer network, which together with the topological restraints exerted by the inclusions on the polymer chain reptation leads to slow relaxation. These slow relaxation processes are sensitive to the oscillation frequency and strongly contribute to the polymer reinforcement at low frequencies. Agglomerates differ in shape and packing from the nearly spherical primary particles, and exert strong restraints on the polymer chain relaxation, hence offer an additional contribution to the composite's moduli. The sum of these effects results in higher moduli and a shift of the crossover (liquid-like to solid-like) frequency to lower values with increasing filler volume fraction. They also lead to the often-observed tendency towards a solid-like response in the terminal zone before a space-filling filler network is formed. Hydrodynamic and micromechanical models can only predict the hydrodynamic reinforcement, provided that the polymer strongly adheres to the inclusions.     

Effect of synthesis variables on viscoelastic properties of elastomers filled with carbonyl iron powder

This work studies the influence of synthesis variables on the lineal viscoelastic properties of elastomers filled with soft magnetic particles. Three matrices [natural rubber (NR), high-temperature vulcanising silicone rubber (HTV-SR), and room-temperature vulcanising (RTV-SR)] and three volumetric particle contents (0%, 15%, and 30%) were studied. Anisotropic samples were synthesised with a softer matrix to obtain a larger magnetorheological (MR) effect, and the variation of their properties under an external magnetic field was examined. All samples were characterised within the lineal viscoelastic (LVE) region using a rheometer, because the MR effect is larger within this region. The influence of the matrix, particle content, and pre-structure on the viscoelastic properties of the synthesised samples was studied. The storage and loss modulus increased with the frequency owing to the viscoelastic behaviour of an elastomer in the rubbery phase. Both moduli also increased with the filler content. The influence of the filler is dependent on the matrix, and the maximum variation was seen in the NR-based samples. However, the maximum MR effect was seen in the samples with a softer matrix, and the effect was enhanced in the anisotropic samples. In this work, the MR effect on the loss modulus was studied, and the tendencies were found to be similar to those of the storage modulus. The main contribution of this work is that all dynamic behaviour results were comparable because all synthesis variables and characterisation conditions were identical. Therefore, how the particle content, frequency, and magnetic field affects each matrix can be studied.     

Modulus evaluation of particulate composites using generalized viscosity model for solutions with suspended particles

The theoretical relationship between the shear modulus of a particulate reinforced composite and the viscosity of a solution with suspended particles was first proposed by Goodier. Since that time several partially successful attempts have been made in the literature to derive equations to describe the available relative shear modulus–particulate concentration data. Recently a new generalized suspension viscosity equation appeared in the literature which for the first time addresses the detailed effects of particle size, particle size distribution, and packing fraction. This new viscosity equation was applied to available modulus literature on particulate composites in this study. Four significant particulate composite modulus derivations in the literature were all shown in this study to yield the same theoretical “intrinsic modulus” of a particulate composite. The generalized viscosity–modulus equation yielded an excellent fit of the shear modulus–particulate concentration data of both Smallwood and Nielsen using a variable intrinsic modulus. Some fillers predicted the Einstein limiting value of the intrinsic modulus while other fillers yielded intrinsic modulus values that were either larger or smaller than this value. The intrinsic modulus for carbon black in rubber was much larger than Einstein's predicted value. However, intrinsic modulus values smaller than Einstein's prediction were obtained at temperatures below the glass transition temperature of the matrix. Unfortunately, the previously obtained direct relationship between the particle interaction coefficient and particulate size for suspension viscosities with a constant intrinsic viscosity was not obtaind for shear modulus—particulate concentration data using a variable intrinsic modulus. © 1994 John Wiley & Sons, Inc.     

Stress–strain behavior of SAN/glass bead composites above the glass transition temperature

Relaxation and stress–strain behavior of SAN–glass bead composites are studied above the glass transition temperature. The strain imposed on the polymeric matrix of the composite is defined as ?_p = ?_c/(1 ? ?^?). Stress relaxation data for the filled polymer which is independent of strain can be calculated by multiplying the relaxation modulus (at a certain strain) by (1 + ?_p). Stress–strain curves at constant strain rate and for different concentrations of the filler can be shifted to form a master curve independent of filler content if the tensile stress is plotted versus ?^p. The relaxation modulus increases with increasing the filler concentration and can be predicted by a modified Kerner equation at 110°C.     

Melt rheology of ionomer filled with methyl methacrylate–grafted perlite

The shear viscosity, the shear compliance, and their shear rate dependence were determined by a Weissenberg rheogoniometer, and the effect of the grafted poly(methyl methacrylate) chains on the intensification of the interaction at the interface between the ionomer matrix and the filler was discussed. Results were as follows: (1) The relative viscosity of the ionomer filled with MMA-grafted perlite to the matrix ionomer and the yield stress increased with increase in the volume fraction of perlite, and these behaviors were more remarkable in the case of the perlite with larger quantity of grafted PMMA. (2) The effective thickness of the immobilized matrix layer on the filler surface in the Ziegel equation and the crowding factor in the Mooney equation showed larger values in the case of the filled systems of MMA-grafted perlite than in the case of the unmodified perlite. (3) At the same total volume fraction which was the sum of the quantities of the perlite and the grafted PMMA, the relative viscosity and the crowding factor showed respectively a maximum with the quantity of grafted PMMA. (4) The shear compliance of these filled systems decreased with perlite content. A little effect of the amount of grafted PMMA on the compliance was observed at the same volume fraction of perlite. According to these rheological properties, it could be concluded that the grafted PMMA chains were effective in increasing the interaction between the ionomer matrix and the perlite at their interface, particularly in the lower shear rate region.     

Toughening of syndiotactic polypropylene with chalk

We hypothesized that polymer crystal anisotropy is advantageous for toughening of polymer composites involving easy slip network of oriented crystalline layers around filler particles. To this end, composites of syndiotactic polypropylene (sPP) with high concentration of submicrometer calcium carbonate particles were prepared and examined because usual sPP crystals exhibit high packing anisotropy. The specific orientation of sPP lamellae around chalk grains was found, which is supposed to facilitate the plastic deformation of polymer matrices. The compression molded bars of the composite exhibited markedly higher Izod impact strength than those of neat sPP. Toughening was even enhanced in the injection molded composite, for which 4.5‐fold increase in the impact strength was achieved. Injection‐induced orientation of the disordered form I sPP crystals was enhanced in the composite. The injection molded tensile specimens exhibited also a good drawability. Debonding at chalk–sPP interface occurred both during the impact and tensile tests facilitating the plastic deformation of sPP matrix. Chalk did not have any significant influence on the thermal properties of the composites but it affected the rheological behavior, increasing the loss and storage moduli, and the viscosity. Highly filled sPP composite exhibited solid‐like behavior in a molten state with the storage modulus exceeding the loss modulus in the entire frequency range. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43651.     

Linear‐nonlinear dichotomy of the rheological response of particle‐filled polymers

Novel nanoparticles, polymer‐particle coupling agents, and functionalized polymers are being developed to enhance the performance of particle‐reinforced polymer systems such as advanced rubber compounds for automobile tires. Understanding the complex rheological behavior of rubber is critical to providing insights into both processability and end‐use properties. One unique aspect of the rheology of filled elastomers is that the incorporation of particles introduces a hysteretic softening (Payne effect) at small dynamic strains. This study demonstrates that this nonlinear viscoelastic behavior needs to be considered when attempting to correlate steady shear response (Mooney viscosity) to oscillatory shear measurements from test equipment such as the Rubber Process Analyzer (RPA). While a wide array of unfilled gum elastomers show good correlation between Mooney viscosity and dynamic torque from the RPA at all of the strain amplitudes used, rubber compounds containing silica and carbon black particles only exhibit good agreement between the two measures of processability when the oscillatory strain amplitude is high enough to sufficiently break up the filler network. Other features of the filler network and its influence on nonlinear rheology are considered in this investigation, including the effects of polymer–filler interactions on filler flocculation and the use of Fourier transform rheometry to illustrate the “linear‐nonlinear dichotomy” of the Payne effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40818.