Fillers amorphous polymers

Pure amorphous polymers, being homogeneous materials, are transparent. Atactic polystyrene is a good example. The crystalline syndiotactic form is not transparent. Alack of transparency does not necessarily indicate crystallinity, however. It can also be caused by inorganic fillers, pigments, gas bubbles (as in a foam), a second polymer phase, etc. 

Polymers with differing morphologies respond differentiy to fillers (qv) and reinforcements. In crystalline resins, heat distortion temperature (HDT) increases as the aspect ratio and amount of filler and reinforcement are increased. In fact, glass reinforcement can result in the HDT approaching the melting point. Amorphous polymers are much less affected. Addition of fillers, however, intermpts amorphous polymer molecules physical interactions, and certain properties, such as impact strength, are reduced. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

The coefficient of linear expansion of unfilled polymers is approximately 10 X 10 5 cm/cm K. These values are reduced by the presence of fillers or reinforcements. The thermal conductivity of the polymers is about 5 X 10 4 cal/sec cm K. These values are increased by the incorporation of metal flake fillers. The specific heat is about 0.4 cal/g K, and these values are slightly lower for crystalline polymers than for amorphous polymers. 

In spite of the often large contribution of secondary filler aggregation effects, measurements of the time-temperature dependence of the linear viscoelastic functions of carbon filled rubbers can be treated by conventional methods applying to unfilled amorphous polymers. Thus time or frequency vs. temperature reductions based on the Williams-Landel-Ferry (WLF) equation (162) are generally successful, although usually some additional scatter in the data is observed with filled rubbers. The constants C and C2 in the WLF equation... 

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. 

The grafting of organic and inorganic compounds and of metalloorgan-ic complexes onto the surface of amorphous silica is a subject of intense research in the former Soviet Union. As is well known, modified silicas are widely used in sorption, catalysis, and chromatography, as fillers for polymers, and as thickening agents in dispersed media. 

Noryl phenylene ether-based resins are relatively resistant to burning, and their burn resistance can be increased by judicious compounding. They may be modified with glass and other mineral fillers. Because of low moisture absorption, dimensional stability, and ability to be used over a wide temperature range, Noryl phenylene ether-based resins are especially adaptable to metallizing. However, like most amorphous polymers, they show poor solvent resistance. 

The discussion above has been limited to amorphous polymers. However, if the polymer is semicrystalline, the dotted line in Figure 1.19 is followed. Since the crystalline regions in the polymer matrix tend to behave as a filler phase and also as a type of physical cross-link between the chains, the height of the plateau (i.e., the modulus) will be governed by the degree of crystallinity]... 

Thiokols are amorphous polymers which do not crystallize when stretched and hence reinforcing fillers, such as carbon black, must be added to obtain relatively high tensile strengths. Thiokol may be... 

As crystalline materials melt, their appearance transforms from opaque to transparent because the ordered structure is lost Highly amorphous polymers, including acryhcs, polycarbonate, and polystyrene do not form crystals, so are transparent (Figure 4.6). An exception is crystalline polyester poly (ethylene terephthalate) used in fizzy drinks botdes, which is transparent because its crystals are too small to interfere with hght waves. Fillers and additives usually decrease the light transmission of a plastic by scattering incident light. 

Perfectly transparent, light-fast color filters and UV absorbers can be obtained by combining metal clusters of coin metals (silver, gold, etc.) with optical polymers [i.e., amorphous polymers with a visible refractive index close to 1.5, such as polystyrene, poly(methyl methacrylate), or polycarbonate]. The high extinction coefficients that characterize the surface plasmon absorption of these metals allows intensive coloration at very low filling factors, and the nanoscopic filler size makes possible the realization of ultrathin color filters [Carotenuto, 2001 Zheng et al., 2001]. 

Amorphous polymers are inherently nonwarping molding resins. Only occasionally are fillers such as milled glass or glass beads added to amorphous materials, because they reduce shrinkage anisotropically. 

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