Shape memory effect in the epoxy polymer composites with aggregated filler

The specific features of the stress-induced shape memory effect are studied for composites based on various polymers and fillers, which can form aggregates and are capable of marked compaction. The appropriate deformation schemes make it possible to prepare composites with different structural states, which give various signs and ratios between longitudinal and transversal strains upon shape recovery. The criterion is proposed for estimating the ability of the composite for shape recovery with marked volume changes: the relative density of the filler defined as the ratio of powder density and compacted powder density.     

Effect of filler content on the stress relaxation behavior of fly ash/polyurea composites

In this study, fly ash/polyurea (FA/PU) composites of various fly ash volume fractions were fabricated. The time-domain stress relaxation behaviors of pure polyurea and the FA/PU composites were measured by dynamic mechanical analysis (DMA) in tension mode at various temperatures. Both temperature and volume fraction of fly ash can affect the segmental motion of polyurea as well as the interaction between fly ash hollow spheres and polyurea matrix, which constitutes the intrinsic mechanism of stress relaxation. Master curves of relaxation modulus were constructed and compared for PU and FA/PU composites. A model was proposed to relate the relaxation modulus and volume fraction of fillers based on three-parameter fractional derivative model and Mori-Tanaka model. The numerical predictions obtained from the model are found to be in good agreement with the experimental results.     

Determination of filler content for natural filler polymer composite by thermogravimetric analysis

Determination of filler content by thermogravimetric (TG) analysis is commonly utilized to investigate the effectiveness of processing methods for composite materials and to quantify the dispersion of filler within the matrix. However, the existing analysis method is not capable of accurately predicting the filler content for natural fiber composites for the case where thermal degradation of the filler and matrix occurs within similar temperature ranges. In the present study, the authors have proposed a generic equation for the determination of filler content which can be utilized for any given range of thermal degradation temperatures in natural filler polymer composites. Oil palm shell unsaturated polyester composites were selected to verify the proposed equation using the TG test with the results indicating good agreement between the estimated and experimental filler contents with a maximum error on the order of 10 %. The suggested technique provides a simple, yet generic, approach to determining the filler content of green or lignocellulose-based polymer composites by TG analysis.

     

Hyperelasticity and stress softening of filler reinforced polymer networks

Starting from an analysis of filler networking in bulk polymers, a constitutive micro-mechanical model of stress softening and hysteresis of filler reinforced polymer networks is developed. It refers to a non-affine tube model of rubber elasticity, including hydrodynamic amplification of the rubber matrix by a fraction of hard, rigid filler clusters with filler-filler bonds in the unbroken, virgin state. The filler-induced hysteresis is described by an anisotropic free energy density, considering the cyclic breakdown and re-aggregation of the residual fraction of soft filler clusters with already broken, damaged filler-filler bonds. Experimental investigations of the quasi-static stress-strain behaviour of silica and carbon black filled rubbers up to large strain agree well with adaptations found by the developed model. The microscopic material parameters obtained appear reasonable, providing information on the mean size and distribution width of filler clusters, the tensile strength of filler-filler bonds and the polymer network chain density.     

The effect of gamma-radiation on polymer composites based on thermoplastic matrices

The effect of ⁶⁰Co γ-radiation on polymer composite materials (PCMs) based on reinforcing glass cloth, polyethylene (PE), polyamide (PA) and polypropylene (PP) matrices has been studied. It has been found that PCMs based on more durable PP and PA matrices have a substantially lower radiation resistance as compared to their PE-matrix analogs. More stable carbon-reinforced plastics based on the PE matrix also have a lower radiation resistance as compared to fiberglass plastics. High-strength PE fiber, PE film, and PE-matrix composites behave in fundamentally different manners under the action of radiation.     

The effect of thermal stress on the thermal expansion coefficient and glass transition temperature of glass fiber–polymer composites

The thermal expansion coefficients of glass fiber–polymer composites were calculated applying the solid cylindrical model taking into account the interaction effects among the glass fibers. The stress and displacement in the composite model were determined as functions of the thermal stress. It was found theoretically that the deviation of the thermal expansion coefficient from the linear mixture relationship based on volume additivity appeared at around T_g + 20 K upon cooling. The thermal expansion coefficient of the composite was also found to be markedly dependent on the dispersion state of the glass fibers. An expression for the difference in the T_g of the matrix resin in the composite from that in the unloaded resin was obtained on the assumption that the volume change of the matrix resin caused by mixing was compensated by free volume expansion. The experimental results obtained by differential scanning calorimetry (DSC) measurements were found to agree well with the theoretically predicted ones.     

The effect of schungite filler on the degree of crystallinity of polypropylene and polyethylene in ternary mixed composites

The effect of schungite filler with a carbon content of 39 wt % on the degree of crystallinity of polypropylene and polyethylene in mixed ternary composites is studied. The PP-to-PE volume ratios are 80: 20 and 50: 50. With an increase in the content of the schungite filler, the fraction of the crystalline phase in PP increases, although the degree of crystallinity of PE decreases; this behavior is related to the high affinity between polypropylene and schungite filler. The surface structure of the initial and schungite-loaded PP-PE materials of various compositions is studied via AFM. The surface structure of the above composites is shown to be different, and its specific features are dependent on the ratio between polymer components and on the order in which all components are introduced into the system.     

The effect of the physicomechanical characteristics of the polymer matrix and structure of a filler on the stress-strain behavior of polymer-montmorrilonite nanocomposites

The stress-strain behavior of PE-Na⁺-montmorillonite nanocomposites is studied. The stress-strain characteristics of the composites are shown to be controlled by the structure of their nanofiller, which is formed upon melt mixing of the polymer and layered silicate (intercalated or exfoliated), the profile of the stress-strain curve of PE matrix, and the fracture mechanism (either adhesive or cohesive). In nanocomposites with a strong adhesive bonding between matrix and clay particles (under cohesive fracture), both the modulus and yield point are found to be markedly increased. In the case of adhesive fracture, mechanical characteristics are less improved due to debonding between matrix and filler results. For nanocomposites, experimental stress-strain characteristics are compared with theoretical estimates calculated according to the models proposed for predicting the characteristics of filled thermoplastic polymers. In some cases, experimental values of modulus and elongation at break appear to differ appreciably from theoretical estimates. The applied models should take into account the orientation of anisodiametric inclusions in a polymer matrix and the character of separation in the composite (adhesive or cohesive).     

Understanding the effect of filler shape induced immobilized rubber on the interfacial and mechanical strength of rubber composites

Styrene butadiene rubber (SBR) composites with silica, halloysite nanotubes (HNTs) and montmorillonite (MMT) were prepared and the interfacial and mechanical properties were compared to understand the reinforcing behaviours of these fillers based on the results of SEM, DSC, DMA, etc. Due to the formation of interparticle domain, HNTs immobilized more rubber approaching their surface than silica and MMT. Interestingly, only tightly immobilized rubber chains made contribution to the enhancement of interfacial and mechanical strength of SBR composites. This was because the tightly immobilized rubber acted as a bridge in the filler-rubber interface and induced the formation of stretched rubber chains linked filler network when the composites were loaded in tension, while loosely immobilized rubber were easy to slip off from filler surface, causing the separation between filler and bulk rubber. Therefore, silica with more tightly immobilized rubber approaching its surface showed better reinforcing effect on rubber than HNTs and MMT.     

Improved performance of isotactic polypropylene/titanium dioxide composites: Effect of processing conditions and filler content

Titanium dioxide (TiO₂) filled isotactic polypropylene (iPP) with various content of TiO₂, which was used as filler, were first single-extruded by an extruder, and then double-molded by compression molding. Scanning electron micrographs show a better adhesion between iPP and filler in the extrusion cum compression-molded samples than the extrusion-molded ones. X-ray diffraction and IR spectral studies reveal a structural change from a three-phase (α, β and γ) crystalline system of the neat iPP sample to only α-form due to inclusion of fillers. Microhardness increases rapidly and then levels off with increasing filler content and also shows variations with respect to molding conditions. A slight decrease of melting temperatures and a considerable increase of degradation temperatures of the samples with addition of filler are also observed. The dc electrical resistivity is observed to decrease with increasing TiO₂ content and temperature. Both the thermal and electrical properties are also found to affect by processing conditions. Based on these results, effect of processing conditions and filler content on changing morphologies and properties of the composites is described.     

Visual research filler network structure in polymer composites and its structure-activity relationship by fluorescent labeling and LSCM

For organic-inorganic composite materials, the spatial dispersion of inorganic fillers in the organic matrix is of great significance for designing and manufacturing high-performance composite materials. To improve the understanding of the micro-physical mechanism of the filler-reinforced polymer matrix, we studied the relationship between filler network structure and macro-mechanical properties of silicone rubber by using fluorescent labeling technology and three-dimensional (3D) visualization imaging. The experimental results showed that a good filler network structure in the polymer matrix can more effectively dissipate external mechanical energy, which generate a visible mechanical strengthening effect. Additionally, this visualization method truly reflects the macrodispersion of the filler and the evolution of the filler network structure under dynamic stress due to its non-invasive and intuitive characteristics, which provides new theoretical guidance for the design of high-performance composites.     

Effect of filler on the in situ formation of linear polymer blends

The kinetics of component formation and phase separation for the in situ preparation of aerosil-filled blends of linear PU and linear PS is studied. The introduction of a filler into the PU-PS system increases the rate of PS formation. This feature distinguishes this system from the previously studied PU-PMMA system, where the introduction of aerosil reduces the rate of PMMA formation. This effect may be associated with a decrease in the mobility of blend components that is related to introduction of the filler, which leads to more viscous systems, and with differences in conditions of the in situ polymerization of styrene and methyl methacrylate in the blend with the PU being formed. The acceleration of phase separation in the PU-PS system with an increase in the PS content may be explained by the preferential adsorption of PU on aerosil.     

The influence of the fractional composition of a filler on thermal conductivity of a polymer composition

The dependence of thermal conductivity of polymer composites based on Viksint PK-68 on the dispersion composition of such fillers as powders of silicon carbide and microdiamonds has been studied. It has been shown that the thermal conductivity of polymeric compositions depends on the gradient of fractions in the powders of fillers.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive‐polymer‐stabilized ferroelectric liquid crystals (PPS‐FLCs) was examined. The polymer/FLC samples exhibited two‐beam coupling gain coefficients of about 6～12 cm^?1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer‐stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two‐beam coupling signal was reduced significantly in the PPS‐FLC samples.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive-polymer-stabilized ferroelectric liquid crystals (PPS-FLCs) was examined. The polymer/FLC samples exhibited two-beam coupling gain coefficients of about 6∼12 cm^-1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer-stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two-beam coupling signal was reduced significantly in the PPS-FLC samples.     

Observation of magnetic field effect on polymer yield in photoinduced dispersion polymerization of styrene

Photoinduced dispersion polymerization of styrene in the presence of dibenzyl ketone as a photoinitiator, with or without magnetic field, has been followed with time. The experiments were carried out at low degrees of conversion of monomer. Since the conditions of the polymerization reaction (viscosity of the medium, reagents concentration, etc.) were approximately unchanged, the results were explained on the basis of a simplified model of dispersion polymerization. The normal growth of the polymerization rates with temperature was registered, and the activation energy of the polymerization was estimated to be about 12 kJ/mol in the temperature range from 20 to 60 °C. The magnetic field effect was best seen at low temperature. The maximum yield of the polymerization product increases by 20% when the magnetic field of 1000 G was applied at the temperature of 20 °C.     

Physicochemical properties of ocher used as polymer filler

Principal physicochemical properties of ocher were studied in order to use it as a mineral filler for polymers.