Filled/composite polymer systems

Note that, apart from the filler particle shape and size, the molecular mass of the base polymer may also have a marked effect on the viscosity of molten composites [182,183]. The higher the MM of the matrix the less apparent are the variations of relative viscosity with varying filler content. In Fig. 2, borrowed from [183], one can see that the effect of the matrix MM on the viscosity of filled systems decreases with the increasing filler activity. In the quoted reference it has also been shown that the lg r 0 — lg (MM)W relationships for filled and unfilled systems may intersect. The more branches the polymer has, the stronger is the filler effect on its viscosity. The data for filled high- (HDPE) and low-density polyethylene (LDPE) [164,182] may serve as an example the decrease of the molecular mass of LDPE causes a more rapid increase of the relative viscosity of filled systems than in case of HDPE. When the values (MM)W and (MM)W (MM) 1 are close, the increased degree of branching results in increase of the relative viscosity of filled system [184]. 

The book contains rather complete reviews of papers published in the last 5-10 years in the USSR and abroad on various problems of filled polymers of differing nature. The discussion is centered on the physico-chemical problems of these complex systems, their structure, mechanical, rheological, dielectric and other properties in a word, important aspects of theory and technology of filled composites. We hope the topical nature of the subjects discussed and the selection of authors that appear in the book all help to throw more light on this area of science and technology. The interested reader will be able not only to appreciate the book as a source of additional literature or a snapshot of the state-of-... 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

A second type of anisotropic system is the biaxially oriented or planar random anisotropic system. This type of material is illustrated schematically in Figure 2A. Four of the five independent elastic moduli are illustrated in Figure 2B in addition there are two Poisson s ratios. Typical biaxially oriented materials are films that have been stretched in two directions by either blowing or tentering operations, rolled materials, and fiber-filled composites in which the fibers are randomly oriented in a plane. The mechanical properties of anisotropic materials arc discussed in detail in following chapters on composite materials and in sections on molecularly oriented polymers. 

All of the examples of PEMs discussed within Section 3.3 unhl now have been composed of only one polymer system without any other compounds present—be they small molecules, polymers, or solid-state materials. A wide variety of different polymer blend and composite PEMs has been made. However, in this section, only a brief overview highlighting some of the more interesting examples that have been reported in the literature will be presented. Eor discussion, these types of PEMs have been divided into three categories polymer blends, ionomer-filled porous substrates and reinforced PEMs, and composite PEMs for high-temperature operation and alternative proton conductors. 

Compomers contain no water, but rather are mainly formulated from the same components as conventional composite resins. Typically this means macromonomers, such as bis-glycidyl ether dimethacrylate (bisGMA) or its derivatives and/or urethane dimethacrylate, blended with viscosity-reducing diluents, such as triethylene glycol dimethacrylate (TEGDMA). These polymer systems are filled with non-reactive inorganic powders, for example, quartz or a silicate glass [271]. 

Polymer resins were first introduced in the early 1940s as an aesthetic alternative to repair defects in anterior teeth. Some of the first resins were unfilled polymers of methyl methacrylate. Presently, these unfilled resins have been replaced by filled composite materials that limit the problems associated with polymerization volume shrinkage, abrasion or wear resistance, mechanical properties, water sorption, solubility, and thermal expansion. Polymeric composite materials generally consist of a monomer resin, a ceramic filler, a polymerization initiator or initiating system, and a coupling agent which binds the polymer... 

Thermal expansion differences exist between the tooth and the polymer as well as between the polymer and the filler. The tooth has a thermal expansion coefficient of 11 x 10-6/°C while conventional filled composites are 2-4 times greater [63, 252], Stresses arise as a result of these differences, and a breakdown between the junction of the restoration and the cavity margin may result. The breakdown leads to subsequent leakage of oral fluids down the resulting marginal gap and the potential for further decay. Ideal materials would have nearly identical thermal expansion of resin, filler, and tooth structure. Presently, the coefficients of thermal expansion in dental restorative resins are controlled and reduced by the amount and size of the ceramic filler particles in the resin. The microfilled composites with the lower filler loading have greater coefficient of thermal expansions that can be 5-7 times that of tooth structure. Acrylic resin systems without ceramic filler have coefficients of thermal expansion that are 9 times that of tooth structure [202-204, 253],... 

An IR spectroscopy technique was developed to study the plasticiser migration from polymer compositions to the air environment. The applicability of the method was demonstrated for filled PVC compositions plasticised with di-n-butyl phthalate. Values for the effective diffusion coefficient(D) of the plasticiser were calculated from the spectroscopic data. An increase in the chalk content in a PVC composition led to a monotonic increase in D, whereas kaolin-filled compositions exhibited a more complex behaviour. The observed pattern of changes in D with varying filler content was correlated with the competing interaction of components in the system. 20 refs. (Full translation of Vys.Soed.B, 44, No.2, 2002, p.363-8)... 

Such a reactive system can be considered as a filled composition. This approach makes it possible to apply the well-known concepts of rheology of filled polymers to the. description of the i/ft) dependence. 

As discussed earlier, while the scale of the fillers is substantially different, nanocomposite materials concepts and technology are very similar to those of conventional composite materials. This is clearly demonstrated in the case of new thermosets for nonlinear optical (NLO) applications, " " where nanocomposite of liquid crystalline thermosets, IPNs, and simple filled thermosets are evaluated. Tripathy et al. discussed four different ways to prepare nonlinear optical polymers. (1) The polymer matrix is doped with NLO moieties in a guest/host system (2) In side-chain polymer systems, NLO polymers with active moieties are covalently bonded as pendant groups (3) In the main chain polymer, the chromo-phores are incorporated as parts of the main polymer backbone to enhance the temporal stability of the NLO properties and (4) Stability of the optical noninearity in sol-gel-based thermosets is related to... 

Theories of dynamics for polymer systems, which can range from neat polymers to reactive filled polymer composites, require a combined discussion of the dynamics of suspensions, polymer solutions and polymer melts. However, prior to this discussion, one should give a brief introduction to rheological terminology. (For a detailed introduction to rheological... 

Carbon black is the most widely used conducting filler in composite industry. Carbon black filled immiscible blends based on polar/polar (65), polar/nonpolar (63,66), nonpolar/nonpolar thermoplastics (67,68), plastic/rubber and rubber/mbber blends (69,70) have already been reported in the literature. The properties of carbon black filled immiscible PP/epoxy were reported recently by Li et al. (60). The blend system was interesting because one of the components is semicrystalline and the other is an amorphous polar material with different percolation thresholds. The volume resistivity of carbon black filled individual polymers is shown in Fig. 21.23. 

Yet another important aspect is the change in the fractal dimension of polymers when they are simulated on fractal rather than Euclidean lattices. This fact is also important from the practical standpoint for multicomponent polymer systems. The introduction of a dispersed filler into a polymer matrix results in structure perturbation in terms of fractal analysis, this is expressed as an increase in the fractal dimension of this structure. As shown by Novikov and co-workers [25], the particles of a dispersed filler form in the polymer matrix a skeleton which possesses fractal (in the general case, multifractal) properties and has a fractal dimension. Thus, the formation of the structure of the polymer matrix in a filled polymer takes place in a fractal rather than Euclidean space this accounts for the structure modifications of the polymer matrix in composites. 

As well as conventional composites of the type based on bisGMA and/or UDMA and filled with silicate-based filler, there are now materials available that are essentially composites in that they comprise a polymeric matrix reinforced with finely divided filler. However, either the polymer system or the filler phase is of a different chemical composition from that of conventional composite resins. Three such materials are currently available, and these are the ormocers, the siloranes and the giomers. Their details are given in Table 3.3, and their characteristics are described in the following subsections. 

The T of tan 5 for control PU corresponds to T of polymer system and occurs at 103.9°C. The T of the FR-filled PU composites increases to 105°C with 2% FR in the PU matrix. Further addition of FR at 4% and 6% loading in the PU composites indicates a shift in tan 5 at a lower temperature. The cross-linking phenomena during polymerization is disrapted by the presence of FR. The T measured by DSC is lowered than by DMA due to differences in effective frequencies of the two instruments. However, the relationship of T obtained from DMA is in agreement with that attained by DSC. 

---