Polymers chain hardening

Diselenium dichloride acts as a solvent for selenium. Similarly disulphur dichloride is a solvent for sulphur and also many other covalent compounds, such as iodine. S Clj attacks rubber in such a way that sulphur atoms are introduced into the polymer chains of the rubber, so hardening it. This product is known as vulcanised rubber. The structure of these dichlorides is given below ... 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

Epoxy resins represent a class of step-growth polymer familiar to anyone who has used epoxy to glue together a broken object. An epoxy resin consists of two components a fluid prepolymer composed of short polymer chains with reactive epoxides on each end, and a hardener, usually a diamine or triamine that ring opens the epoxides and cross-links the chains together. 

Epoxy resin (Section 30.6E) A step-growth polymer formed from a fluid prepolymer and a hardener that cross-links polymer chains together. 

Weight loss and hardening of polymer Loss of compression set resistance and some polymer volatihsation Formation of a trace amount of acidity Formation of lead mercaptide segment in the polymer chain when PbOj is used as a curing agent... 

Two possible reasons for the decrease in (m ) with thickness were considered. The first reason is the hardening due to polymer chain confinement in a thin film. 

If a polymer becomes soft when it is heated (and then hardens again when cooled), it s known as a thermoplastic. The temperature at which the polymer softens depends on what it s made of and on the size of the polymer chains. Thermoplastics are easy to recycle because they can be remolded when they re hot. 

One method of classifying plastics is by their response to heat. Thermoplasts, also known as thermoplastic polymers, soften and liquefy on heating and harden again when cooled. The process is reversible and can be repeated. On heating, the weak secondary bonds between polymer chains are broken, which facilitates relative movement between the chains. If the molten polymer is further heated until the primary covalent bonds also break, degradation of the thermoplast follows. Thermoplastic polymers are linear or exhibit branching with flexible chains and include polyethylene, polystyrene and polypropylene (Figure 4.10). 

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