Network structure filled rubbers

P. Pal, S. De. Studies of polymer-filled interaction, network structure, physical properties, and fracture of silica-and-clay-fUled EPDM rubber in the presence of a silane coupling agent. Rubb. Chem. TechnoL, 56,737-773,1983. 

D-TEM gave 3D images of nano-filler dispersion in NR, which clearly indicated aggregates and agglomerates of carbon black leading to a kind of network structure in NR vulcanizates. That is, filled rubbers may have double networks, one of rubber by covalent bonding and the other of nanofiller by physical interaction. The revealed 3D network structure was in conformity with many physical properties, e.g., percolation behavior of electron conductivity. 

This article reviews recent developments in polymer thermomechanics both in theory and experiment. The first section is concerned with theories of thermomechanics of polymers both in rubbery and solid (glassy and crystalline) states with special emphasis on relationships following from the thermomechanical equations of state. In the second section, some of the methods of thermomechanical measurements are briefly described. The third section deals with the thermomechanics of molecular networks and rubberlike materials including such technically important materials as filled rubbers and block and graft copolymers. Some recent data on thermomechanical behaviour of bioelastomers are also described. In the fourth section, thermomechanics of solid polymers both in undrawn and drawn states are discussed with a special focus on the molecular and structural interpretation of thermomechanical experiments. The concluding remarks stress the progress in the understanding of the thermomechanical properties of polymers. 

Network Structure in Filled Rubbers - Rubber-Filler Interface and the Structure of the Physical Network... 

A good understanding of the structure of the network in filled rubbers is of great importance, because the rubber s elastic properties are determined primarily by the density of chemical and physical network junctions and their ability to fluctuate. The following types of network junctions occur in filled rubbers ... 

This behavior can be understood if a superimposed kinetic aggregation process of primary carbon black aggregates in the rubber matrix is considered that alters the local structure of the percolation network. A corresponding model for the percolation behavior of carbon black filled rubbers that includes kinetic aggregation effects is developed in [22], where the filler concentrations and c are replaced by effective concentrations. In a simplified approach, not considering dispersion effects, the effective filler concentration is given by ... 

Equation (70) predicts a power law behavior G cp3-5 for the elastic modulus. Thereby, the exponent (3 + d ) / (3 - df) 3.5 reflects the characteristic structure of the fractal heterogeneity of the filler network, i.e., the CCA-clusters. The predicted power law behavior at higher filler concentrations is confirmed by the experimental results shown in Fig. 15, where the small strain storage modulus of a variety of carbon black filled rubbers is plotted against carbon black loading in a double logarithmic manner. It also agrees with older experimental data obtained by Payne [1] as shown in [63,64]. 

Solid state NMR offers several advantages for the investigation of filled rubbers since molecular properties of elastomer chains can be measured selectively by NMR e>q)eriments. The method is very sensitive to the molecular scale heterogeneity in a sample. The network structure which is composed of chemical, physical and topological junctions can also be andyzed by NMR relaxation experiments [11,12,14,15],... 

This paper is devoted to the study of a part of the complex phenomena of reinforcement, namely the behavior of the host elastomer in the presence of filler particles. The results of solid state NMR experiments and some other methods for filled PDMS are reviewed. The short-range dynamic phenomena that occur near the filler surface are discussed for PDMS samples filled with hydrophilic and hydrophobic Aerosils. This information is used for the characterization of adsorption interactions between siloxane chains and the Aerosil surface. Possible relations between mechanical properties of filled silicon rubbers on the one hand and the network structure and molecular motions at flie PDMS-Aerosil interface on the other hand are discussed as well. 

The elastic properties of rubbers are primarily governed by the density of netw ork junctions and their ability to fluctuate [35]. Therefore, knowledge of the network structure composed of chemical, adsorption and topological junctions in filled elastomers as well as their relative weight is of a great interest. The H T2 NMR relaxation experiment is a well established method for the quantitative determination of the network structure in the elastomer matrix outside the adsorption layer [14, 36]. The method is especially attractive for the analysis of the network structure in filled elastomers since filler particles are "invisible" in this experiment due to the low fraction of protons at the Aerosil surface as compared with those in the host matrix. 

Considerable effort has been spent to explain the effect of reinforcement of elastomers by active fillers. Apparently, several factors contribute to the property improvements for filled elastomers such as, e.g., elastomer-filler and filler-filler interactions, aggregation of filler particles, network structure composed of different types of junctions, an increase of the intrinsic chain deformation in the elastomer matrix compared with that of macroscopic strain and some others factors [39-44]. The author does not pretend to provide a comprehensive explanation of the effect of reinforcement. One way of looking at the reinforcement phenomenon is given below. An attempt is made to find qualitative relations between some mechanical properties of filled PDMS on the one hand and properties of the host matrix, i.e., chain dynamics in the adsorption layer and network structure in the elastomer phase outside the adsorption layer, on the other hand. The influence of filler-filler interactions is also of importance for the improvement of mechanical properties of silicon rubbers (especially at low deformation), but is not included in the present paper. 

It is certain that the relaxation behavior of filled rubbers at large strains involves numerous complications beyond the phenomena of linear viscoelasticity in unfilled amorphous polymers. Breakdown of filler structure, strain amplification, failure of the polymer-filler bond, scission of highly extended network chains and changes in network chain configuration probably all play important roles in certain ranges of time, strain rate, and temperature. A clear understanding of the interplay of these effects is not yet at hand. 

Mohamed et al. [149] evaluated the use of several types of sulfosuccinate anionic surfactants in the dispersion of MWCNTs in NR latex matrices. Sodium l,5-dioxo-l,5-bis(3-phenylpropoxy)-3-((3-phenylpropoxy)carbonyl) pentane-2-sul-fonate showed the best dispersion capabihty and improved the electrical conductivity of the resulted composites. These results have significant implications in the development of new materials for aerospace applications because the filler s dispersiou directly influences the properties of the final material. Jo et al. [150] obtained pristine MWCNt-Ti02 nanoparticles filled with NR-CllR and epoxidized NR-CUR, concluding that the second blend proved higher thermal conductivity because the epoxy branches in ENR and the functionalized MWCNT form a stronger network. Conductivity in CNTs reinforced with rubber-based blends can be improved when reaching a critical concentration of the filler known as the percolation threshold, when a continuous network structure is formed. Thankappan Nair et al. [151] discussed the percolation mechanism in MWCNT-polypropylene-NR blends. 

Filled rubbers form a complex network of cross-linked chains connected to surface-active particles such as carbon black or amorphous silica (see Carbon Black). Here we will only indicate the structural features of importance in unfilled cross-linked elastomers. Two breakdown mechanisms are conceivable the initiation and growth of a cavity in a moderately strained matrix and the accelerating, cooperative rupture of interconnected, highly loaded network chains. The second mechanism is more important imder conditions, which permit the largest breaking elongation Xbmax to be attained (29). In that case, the quantity >-bmax is expected to be proportional to the inverse square root of the cross-link density Vg in fact, an increase of A,bmax with to is found experimentally for a... 

The nonlinear viscoelastic behavior of filled vulcanizates is somewhat different from that of filled compounds, since the chemical crosslink network of the rubber matrix is formed and, the physical rubber-filler networks and filler-filler networks are enhanced during curing at a relatively high temperature [7]. Speaking from a broad sense, filled vulcanizates can be viewed as a double network structure in which the nanoparticles supplement the inherent viscoelasticity of crosslink rubbers with additional physical network junctions. 

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