High temperature solid polymer

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

The concept of polymer free volume is illustrated in Figure 2.22, which shows polymer specific volume (cm3/g) as a function of temperature. At high temperatures the polymer is in the rubbery state. Because the polymer chains do not pack perfectly, some unoccupied space—free volume—exists between the polymer chains. This free volume is over and above the space normally present between molecules in a crystal lattice free volume in a rubbery polymer results from its amorphous structure. Although this free volume is only a few percent of the total volume, it is sufficient to allow some rotation of segments of the polymer backbone at high temperatures. In this sense a rubbery polymer, although solid at the macroscopic level, has some of the characteristics of a liquid. As the temperature of the polymer decreases, the free volume also decreases. At the glass transition temperature, the free volume is reduced to a point at which the... 

Stress is related to strain through constitutive equations. Metals and ceramics typically possess a direct relationship between stress and strain the elastic modulus (2) Polymers, however, may exhibit complex viscoelastic behavior, possessing characteristics of both liquids and solids (4.). Their stress-strain behavior depends on temperature, degree of cure, and thermal history the behavior is made even more complicated in curing systems since material properties change from a low molecular weight liquid to a highly crosslinked solid polymer (2). ... 

A rapid high-temperature solid state pol5mierization of crystalline thermoplastic polymers may be done under conditions of mechanically induced surface stress and friction applied to polymer particles at the incipient melt point temperatures. ... 

High-Temperature Solid-State NMR of Cross-Linked, Insoluble, and Unswellable Polymers... 

Crystallizable polymers follow a different path on cooling. At high temperatures, crystalline polymers are also viscoelastic in character. On cooling a temperature is reached at which the polymer commences to crystallize. During crystallization, there is a significant increase in density as the chains pack more efficiently in the solid state. The crystallization temperature may be much lower than the polymer melting point since the melt is readily supercooled. After the bulk of the crystallization has taken place, the polymer is a solid... 

The manufacture of thermosets begins with a fluid mixture of the two monomers. The fluid is first shaped and then polymerized, either by heating or by being mixed with an initiator. The product of the polymerization is a network of covalently bonded atoms that is a solid, even at high temperatures. When heated to high temperatures, thermoset polymers char and decompose, but they do not melt. 

Production of novel materials. Chemical engineers will design processes to produce ceramic parts for engines, high-temperature superconductors, polymer-composites for stmctural components, and specialty chemicals produced in small amounts to exacting specifications. Chemical processes will shift from the traditional area of petrochemicals to inorganic compounds, from liquids to solids, and from large scale to small scale. 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

PET suitable for bottle manufacture is produced by a modified process. Here the high-viscosity polymer melt is subjected to a rapid quenching in water to produce clear amoiphous pellets. These are further polymerised in the solid phase at temperatures just below the T . This is useful to reduce aldehyde content, since aldehyde-forming degradation reactions occur less in the lower temperature solid phase polymerisations. Aldehydes can impart a taste to beverages and it is important to keep the aldehyde content to below 2.5 p.p.m. 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

At temperatures below the glass transition, the polymers are fairly rigid and the yield strength xy is high. vxy > kT is plausible in the case of solid polymers. Eq. (6.3) can be rearranged to read ... 

High-molecular-weight polymers can easily be obtained by solid-state polymerization carried out just below the melting temperature of the polymer.2 5 6 8-28-41-42 In solid-state polymerization... 

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