Polymer-nanoparticle interfacial behavior revisited: a molecular dynamics study

By tuning the polymer-filler interaction, filler size and filler loading, we use a coarse-grained model-based molecular dynamics simulation to study the polymer-filler interfacial structural (the orientations at the bond, segment and chain length scales, chain size and conformation), dynamic and stress-strain properties. Simulated results indicate that the interfacial region is composed of partial segments of different polymer chains, which is consistent with the experimental results presented by Chen et al. (Macromolecules, 2010, 43, 1076). Moreover, it is found that the interfacial region is within one single chain size (R(g)) range, irrespective of the polymer-filler interaction and the filler size, beyond which the bulk behavior appears. In the interfacial region, the orientation and dynamic behaviors are induced by the interfacial enthalpy, while the size and conformation of polymer chains near the filler are controlled by the configurational entropy. In the case of strong polymer-filler interaction (equivalent to the hydrogen bond), the innerest adsorbed polymer segments still undergo adsorption-desorption process, the transport of chain mass center in the interfacial region exhibits away from the glassy behavior, and no plastic-like yielding point appears in the stress-strain curve, which indicates that although the mobility of interfacial polymer chains is restricted, there exist no "polymer glassy layers" surrounding the filler. In addition, it is evidenced that the filler particle prefers selectively adsorbing the long polymer chains for attractive polymer-filler interaction, validating the experimental explanation of the change of the bound rubber (BR). In short, this work provides important information for further experimental and simulation studies of polymer-nanoparticle interfacial behavior.     

Performance of a phenolic antioxidant introduced by different procedures into polyethylene containing dispersed fillers

The performance of Irganox 1010 phenolic antioxidant was studied in relation to the procedure of its introduction into a stock of the polymer and dispersed filler powders. Three procedures were used for introducing the antioxidant: the antioxidant was deposited onto the surface of filler (procedure I) or polymer powder (procedure II) particles prior to mixing the components, or it was deposited after mixing the polymer and filler powders (procedure III). We studied mixtures with an inert filler (aluminum oxide) and with fillers active in polymer oxidation and regeneration of the phenolic antioxidant (dispersed zinc, zinc oxide). The film samples for the studies were prepared from powder stocks by hot pressing. The antioxidant performance was evaluated by the induction period of oxidation, which was determined from the time dependences of accumulation of carbonyl groups in the polymer structure.     

Modeling of the formation of oriented-polymer layers at filler particles in polymer nanocomposites

A theory to explain the appearance of oriented layers at the filler surface in polymer nanocomposites is proposed. The theory is based on the assumption that small oriented polymer regions have an effect on the state of neighboring regions and tend to orient polymer chains in these regions. As a result, the point-to-point transfer of this effect takes place, thereby causing the propagation of the oriented layer in the polymer nanocomposite over a considerable distance from the filler surface. The appearance of the polymer in the biaxially oriented state, which is transferred to the neighboring regions and leads to the formation of a layer with specific mechanical properties, is possible near the filler particles in this case.     

Structure and dielectric properties of a thermoplastic blend containing dispersed metal

In the present work broadband dielectric relaxation spectroscopy measurements were employed to investigate the dielectric properties of polymer composites. A polyethylene/polyoxymethylene (PE/POM) thermoplastic blend was used as a matrix, while the inclusions were iron (Fe) particles. For comparison, the two pure polymers- PE and POM- were used as a matrix, too. In the PE/POM-Fe composites, the polymer matrix is two-phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. In PE-Fe and POM-Fe composites, the filler spatial distribution is random. The behaviour of all the composites studied is described in terms of the percolation theory. The PE/POM-Fe composites, based on the PE/POM blend, demonstrate different electrical behaviour compared to that of POM-Fe and PE-Fe systems. The percolation threshold value of the PE/POM-Fe composites was found much lower than that of the other two systems. The results were related to the microstructure of the composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behaviour of the PE/POM-Fe composites by taking into account the ordered distribution of the filler particles in a binary polymer matrix. Optical microscopy photographs confirm this model.     

Mechanical properties of polymers containing fillers

The addition of fillers can significantly change the mechanical characteristics of a material. In this paper, a general, mechanistic model is established to determine the moduli, relaxation moduli, break strengths, and break strains for polymer films containing liquid and solid micro fillers. Based on rigorous continuum mechanics principles, this model considers the filler/filler interactions, incorporates the nonlinear synergistic effects of fillers, and provides accurate predictions in comparison with experimental data. The analytical model developed provides information that is not available or extremely difficult to obtain experimentally. The model can be applied to determine the filler/matrix adhesion and filler modulus using measured modulus of a filled polymer film (a filled polymer is a polymer containing fillers). It is found that the compression moduli of polymer films containing liquid fillers differ significantly from the tension moduli, especially when the volume fraction of the filler is high. The difference in compression and tension Young's moduli normalized by the tension Young's modulus is as high as 35%. The relative error in maximum pressure calculation during Hertzian contact caused by using the tension moduli is as high as 48%. The relaxation modulus of a filled polymer film is determined through inverse Laplace transforms of its composite modulus in the s‐space. For a filled polymer film containing liquid phase fillers, a closed form solution for its relaxation modulus has been obtained. It is found that the composite relaxation modulus of the filled polymer is proportional to the relaxation modulus of the matrix polymer multiplied by a factor related to the volume fraction of the liquid filler. The break strength of the filled polymer is found to be proportional to the break strength of the polymer matrix material multiplied by a power function of the modulus ratio of filled polymer to polymer matrix, R. The break strain of the filled polymer is proportional to the break strain of the polymer matrix multiplied by a power function of 1/R. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 155–172, 1999     

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. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 256–275, 2001     

Thermal properties of lignocellulosic filler-thermoplastic polymer bio-composites

Summary In the TG analysis of the bio-composites, their thermal stability was found to slightly decrease and the ash content to increase as the lignocellulosic filler loading increased. This is a logical consequence of the lower thermal stability of the lignocellulosic filler compared to that of the matrix polymer. The dispersion and interfacial adhesion between the lignocellulosic filler and thermoplastic polymer were important factors affecting the thermal stability of the composite system. In order to improve their compatibility and interfacial adhesion, the incorporation of a compatibilizing agent into the lignocellulosic material-thermoplastic polymer composites is recommended. In the TMA analysis, the thermal expansion of the composites was found to decrease with increasing filler loading and incorporating compatibilizing agent. Lignocellulosic filler is a suitable material for preventing the thermal expansion of the composite materials caused by atmospheric changes.     

Nanoparticle dispersion and aggregation in polymer nanocomposites: insights from molecular dynamics simulation

It is a great challenge to fully understand the microscopic dispersion and aggregation of nanoparticles (NPs) in polymer nanocomposites (PNCs) through experimental techniques. Here, coarse-grained molecular dynamics is adopted to study the dispersion and aggregation mechanisms of spherical NPs in polymer melts. By tuning the polymer-filler interaction in a wide range at both low and high filler loadings, we qualitatively sketch the phase behavior of the PNCs and structural spatial organization of the fillers mediated by the polymers, which emphasize that a homogeneous filler dispersion exists just at the intermediate interfacial interaction, in contrast with traditional viewpoints. The conclusion is in good agreement with the theoretically predicted results from Schweizer et al. Besides, to mimick the experimental coarsening process of NPs in polymer matrixes (ACS Nano 2008, 2, 1305), by grafting polymer chains on the filler surface, we obtain a good filler dispersion with a large interparticle distance. Considering the PNC system without the presence of chemical bonding between the NPs and the grafted polymer chains, the resulting good dispersion state is further used to investigate the effects of the temperature, polymer-filler interaction, and filler size on the filler aggregation process. It is found that the coarsening or aggregation process of the NPs is sensitive to the temperature, and the aggregation extent reaches the minimum in the case of moderate polymer-filler interaction, because in this case a good dispersion is obtained. That is to say, once the filler achieves a good dispersion in a polymer matrix, the properties of the PNCs will be improved significantly, because the coarsening process of the NPs will be delayed and the aging of the PNCs will be slowed.     

Thermogravimetry as a Method for Investigating the Thermal Stability of Polymer Composites

Thermogravimetry was used to investigate the effects of different inorganic functional fillers on the heat resistance of polymer matrices. The kinetic parameters of thermal oxidative degradation were shown to depend on the polymer, the chemical composition of the filler surface, the filler concentration, and the processing method, which determines the distribution of filler particles in the polymer matrix. Magnetic fillers (carbonyl iron, and hexaferrites of different structural types) were shown to be chemically active fillers, increasing the heat resistance of siliconorganic polymers. Their stabilizing effect is due to blocking of the end silanol groups and macroradicals by the surface of the filler and non-chain inhibition of thermal oxidative degradation. In the case of fiber-forming polymers (UHMWPE, PVOH and PAN), most magnetic fillers are chemically inert, but at concentrations of 30–50 vol% they increase the heat resistance of the composite. Addition of carbon black increased the heat resistance of the thermoplastic matrix. The dependence of the thermal degradation onset temperature on the kaolin concentration in the polyolefin matrix exhibited a maximum. Analysis of the experimental results demonstrated the operating temperature ranges for different composites, and their maximum operating temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role,effect, and influences of micro and nano‐fillers on various properties of polymer matrix composites for microelectronics: A review

Ever since the discovery of polymer composites, its potential has been anticipated for numerous applications in various fields such as microelectronics, automobiles, and industrial applications. In this paper, we review filler reinforced polymer composites for its enormous potential in microelectronic applications. The interface and compatibility between matrix and filler have a significant role in property alteration of a polymer nanocomposites. Ceramic reinforced polymeric nanocomposites are promising candidate dielectric materials for several micro‐ and nano‐electronic devices. Because of its synergistic effect like high thermal conductivity, low thermal expansion, and dielectric constant of ceramic fillers with the polymer matrix, the resultant nanocomposites have high dielectric breakdown strength. The thermal and dielectric properties are discussed in the view of filler alignment techniques and its effect on the composites. Furthermore, the effect of various surface modified filler materials in polymer matrix, concepts of network forming using filler, and benefits of filler alignment are also discussed in this work. As a whole, this review article addresses the overall view to novice researchers on various properties such as thermal and dielectric properties of polymer matrix composites and direction for future research to be carried out.     

The influence of non-rubber constituents on performance of silica reinforced natural rubber compounds

An in-rubber study of the interaction of silica with proteins present in natural rubber show that the latter compete with the silane coupling agent during the silanisation reaction; the presence of proteins makes the silane less efficient for improving dispersion and filler–polymer coupling, and thus influences the final properties of the rubber negatively. Furthermore, the protein content influences the rheological properties as well as filler–filler and filler–polymer interactions. Stress strain properties also vary with protein content, as do dynamic properties. With high amounts of proteins present in natural rubber, the interactions between proteins and silica are able to disrupt the silica–silica network and improve silica dispersion. High amounts of proteins reduce the thermal sensitivity of the filler–polymer network formation. The effect of proteins is most pronounced when no silane is used, but they are not able to replace a coupling agent.     

Molecular-dynamics simulation of model polymer nanocomposite rheology and comparison with experiment

The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results; shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction.     

Influence of interfacial adhesion and the nonequilibrium glassy structure of a polymer on the enthalpy of mixing of polystyrene-based filled composites

For composites based on polystyrene, the styrene-butadiene copolymer, and polybutadiene filled with various kinds of zinc oxide powder, the enthalpy of mixing is calculated in the entire range of filler contents on the basis of calorimetric measurements with the use of the thermochemical cycle. It is shown that, when the polymer is in excess in the composites based on polystyrene and its copolymer, the enthalpy of mixing is negative, whereas at a high content of the filler, this value is positive. The alternating-sign pattern of the concentration dependence of the enthalpy of mixing is interpreted in terms of the superposition of the negative contribution of the enthalpy of the interfacial-adhesion interaction of a polymer matrix with the filler and the positive contribution due to a gain in the level of nonequilibrium of the glassy matrix near the surface. A thermodynamic model that makes it possible to separate these contributions and to describe the experimental curves of the enthalpy of mixing for the composites is advanced.     

Polymer blends filled with carbon black: structure and electrical properties

Morphology and electrical conductivity of polymer blends based on polypropylene, polyethylene, polyoxymethylene, polyamide 12, co‐polymer ABS filled with carbon black (CB) were studied. When filler is introduced in polymer blend, three cases of filler spatial distribution can be obtained. CB can be distributed randomly within polymer matrix, can be situated in one of the polymer components or can be localized on the boundary of the polymer components. The existence of different cases of filled blends structure is defined by following reasons: thermodynamic factors (relationship between interface surface tensions of polymer‐filler γ_pf and polymer‐polymer surface tension γ_pp); kinetic factors (relationship between viscosities of polymer components); processing factors (methods of the filler introduction in the complex polymer matrix, which can enhance or depress the influence of thermodynamic and kinetic factors).     

Molecular Arrangements in Polymer‐Based Nanocomposites

Monte Carlo computer simulations have been performed for model polymers containing randomly distributed spherical filler particles of diameter 8 times the transverse diameter of the polymer chains. The partial volume of the filler has been varied from 10% to 50%. The polymer/filler interface is characterized by densely packed and ordered shells of polymer units of thickness nearly twice the diameter of the units. The chain dimensions in filled systems are always found to be slightly smaller than in the unfilled melt. The behavior of various parameters chosen such to characterize the molecular arrangements in polymer‐based nanocomposites is analyzed, leading to a general picture in which the chains are considered to be sequences of interface, bridge and loop segments. The relative abundance of segments of each type, their average length and the way they are allocated in sequence change with changing the partial volume of filler.     

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.

     

Molecular dynamics simulation for insight into microscopic mechanism of polymer reinforcement

By employing an idealized model of a polymer network and filler, we have investigated the stress-strain behavior by tuning the filler loading and polymer-filler interaction in a broad range. The simulated results indicate that there actually exists an optimal filler volume fraction (between 23% and 32%) for elastomer reinforcement with attractive polymer-filler interaction. To realize this reinforcement, the rubber-filler interaction should be slightly stronger than the rubber-rubber interaction, while excessive chemical couplings are harmful to mechanical properties. Meanwhile, our simulated results qualitatively reproduce the experimental data of Bokobza. By introducing enough chemical coupling between the rubber and the filler, an upturn in the modulus at large deformation is observed in the Mooney-Rivlin plot, attributed to the limited chain extensibility at large deformation. Particularly, the filler dispersion state in the polymer networks is also characterized in detail. It is the first demonstration via simulation that the reinforcement mechanism stems from the nanoparticle-induced chain alignment and orientation, as well as the limited extensibility of chain bridges formed between neighboring nanoparticles at large deformation. The former is influenced by the filler amount, filler size and filler-rubber interaction, and the latter becomes more obvious by strengthening the physical and chemical interactions between the rubber and the filler. Remarkably, the reason for no obvious reinforcing effect in filled glassy or semi-crystalline matrices is also demonstrated. It is expected that this preliminary study of nanoparticle-induced mechanical reinforcement will provide a solid basis for further insightful investigation of polymer reinforcement.     

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.     

Rheological analysis as a means for determining the silane crosslink network structure and content in crosslinked polymer composites

For evaluating the crosslink content of a polymer, gel content determination is a commonly used method. However, for crosslinked polymer composites containing particulate filler, the gel content may be overestimated due to partly trapping filler inside the gel portion. In this paper, parallel-plate rheology was used, together with the gel determination and FTIR measurement, for determining the silane crosslink network structure and content in crosslinked ethylene–octene copolymer composites. The effects of filler surface property on structure and content of silane crosslink are also discussed. The results show that a correlation plot between gel content, IR absorption index and crosslink density provides useful information on changes in silane network structure and properties of the crosslinked composites. The network structure formed (loose or tight network) shows a strong influence on the final tensile properties of the crosslinked products.     

Gel Polymer Electrolytes with Talc as a Natural Mineral Filler and a Biodegradable Polymer Matrix in Sodium-Ion Batteries

A gel polymer electrolyte based on poly(vinyl alcohol) (PVA) is used in sodium-ion batteries (SIBs). The use of biodegradable and water-soluble polymer potentially reduces the negative environmental impact. The other components include sodium salt (NaPF₆), sulfolane (TMS) as a plasticizer and talc. For the first time, natural and abundant talc has been used as an inert filler in a gel polymer electrolyte. The best results were obtained for moderate amounts of filler (1 and 3 wt%). Then, an increase in the conductivity, transference numbers, and thermal stability of the membranes was observed. Moreover, the presence of talc had a positive effect on the cyclability of the hard carbon electrode. The discharge capacity after 50 cycles of HC|1 % T_TMS|Na and HC|3 % T_TMS|Na was 243 and 225 mAh g⁻¹, respectively. The use of talc in gel polymer electrolytes containing sodium ions improves the safety and efficiency of SIBs.