Stiff polymers stiffness expansion

Abstract Polymers can be cooled down to ultralow temperatures by embedding them in a gas whose temperature is lowered by laser and evaporation cooling. In this regime their fluctuations show quantum behavior. This is calculated here for the second and fourth moments of the end-to-end distribution in the large stiffness expansion. The result should soon be measurable. 

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

Glass-reinforced grades of SAN exhibit a modulus several times that of the unfilled polymer and, as with other glass-filled polymers, a reduced coefficient of thermal expansion and lower moulding shrinkage. The materials are thus of interest on account of their high stiffness and dimensional stability. 

Polymers show, in general, a considerably higher thermal expansion than other materials. Figure 8.3 gives an overall survey. Roughly speaking, a relation exists between the coefficient of thermal expansion and the reciprocal stiffness. 

Very often particles are blended into polymers, in thermoplasts as well as in thermosets and in synthetic rubbers. This is done for various reasons the aim may be stiffness, strength, hardness, softening temperature, a reduction of shrinkage in processing, reduction of thermal expansion or electric resistance, or, simply, to reduce the price of the material. The fillers used are wood flour, carbon black, glass powder, chalk, quartz powder, mica, molybdene sulphide, various metal oxides, etc. etc. 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

Expansion of plastics into new and replacement markets promises a high rate of growth. Twenty five years ago suitable applications had to be found for the synthetic polymers manufactured at that time. Today s inventors visualize the need for certain materials. They prepare these materials and develop a feasible manufacturing process since macromolecules can be designed for a specific end use. Synthetic polymers can be made with flexible or stiff chains and tailored in length. 

Polymers consist of very long, for the most part, C-based chains to which other organic atoms (for example C, H, N, Cl, F) and molecules are attached. The bonding within the chains is strong, directional, and covalent, while the bonding between chains is relatively weak. Thus, the properties of polymers as a class are dictated by the weaker bonds, and consequently they possess lower melting points, higher thermal expansion coefficients, and lower stiffnesses than most metals or ceramics. 

Engineering polymers are often filled with glass fibres or other filler types to improve certain mechanical properties, like stiffness and thermal expansion. The thermal expansion of polyketone1 samples filled with different filler materials... 

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