Melt Temperature Terms Links

Flory (1956a,b) considered the shrinkage of collagen in terms of statistical mechanics and concluded that thermal shrinkage is a phase transition similar to melting. The elevation of the melting temperature with increasing cross-link density was treated theoretically. 

At temperatures in excess of in Fig. 8.2, the not very well-defined melting temperature, these polymers are completely amorphous and show many of the features characteristic of liquids, the condition being termed viscofluid. Rapid cooling of the polymer from state A (Fig. 8.2) causes it to follow the path ABC, whether it it cross-linked or not. The polymer is amorphous at all stages along ABC, shows little or no change in specific volume at and when the temperature is below it is essentially glassy. 

The. differences in the properties of polymers go back to the chemical configurations. In simple terms, thermoplastics can be molded because they are iong-chain molecules that slip if pushed or pulled, especially at higher temperatures. Thermosets are cross-linked, so the long chains stay put under stress, strain, or heat. They dont melt, and they cant be molded once they set. The fibers get their flexibility and strength when the polymer molecules align during filament formation. 

Figures 9.1-9.3 illustrate these interconnected relationships.13 Figure 9.1 defines some of the terms used in this chapter. Small molecules are species with molecular weights below about 1,000. They are volatile at temperatures below say 200 100 °C. Clusters are oligomers derived from covalently linked small molecules. They have a lower volatility than small molecules and, if large enough, can be shaped by melting or by solvent evaporation methods. Linear polymers can be simple chain structures or may consist of rings linked together. In either case they are usually non-volatile and easily fabricated. Cross-linked systems can be produced from polymers or from clusters. The final ceramic may be amorphous or crystalline.

Unlike thermoplastics, which are simply melted, thermoset resins chemically react from low-viscosity liquids to solid materials during processing, a process termed curing. Structurally, thermosets differ from thermoplastics because of the presence of cross-links in the former, which means that thermosets cannot be reshaped or recycled once the chemical reaction occurs. One advantage of thermosets vs. thermoplastics is that wetting the filler becomes much easier with a low-viscosity material. By far the most common thermoset composite is automobile tires, which consist of a polymer made from styrene and butadiene monomers and carbon-black filler. The actual recipe used is much more complicated, and can include other monomers or polymers, as well as other fillers. In the absence of filler, the cured resin is rubbery at room temperature, which makes tires a... 

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