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Experiment_07-Mechanical_Properties_of_Polymers -...

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1 Objective The primary objective of this study is to gain an understanding of the mechanical properties, and underlying atomic structures that cause the properties, for the three major classifications of polymers: thermoplastics, thermosets and elastomers. Abstract The mechanical properties of polymers vary significantly from polymer to polymer as a result of atomic structure and bond strength. Thermoplastic polymers are generally composed of long coiling carbon chains that are primary bonded along the chains, but are secondarily bonded between chains. Thermoplastics tend to either contain crosslinks (primary bonds between chains) or are composed of three-dimensional space networks, but thermosets tend to be comparatively expensive and can t be recycled. The tensile properties of representative samples from the three primary polymer types (thermoplastics, thermosets and elastomers) will be tested and evaluated. When a polymer is subjected to a constant strain yet realizes a decrease in stress as a function of time, it is said to be undergoing Stress Relaxation. Background P olymers ("Plastics") are high molecular weight materials that are synthesized from simple organic compounds. The simplest of these is polyethylene, -(CH 2 )- , which is obtained by polymerizing ethylene, C 2 H 4 . Ethylene is an example of a monomer (single mer); monomers are usually unsaturated with higher order covalent bonds between the carbon atoms in the molecule. In the polymerization process, the higher order bond is broken leaving a single bond that takes its place and frees an electron for further bonding. The polymerization process generally continues rapidly until the chain ends are terminated; in the case of polyethylene, OH molecules terminate the growth of the chains. Characteristic properties of polymers include low density, low strength and high mechanical damping ability. They are generally unable to withstand low or high temperatures and are therefore used primarily at ambient temperatures. Vinyl compounds are formed when one of the hydrogen atoms is replaced by another atom. If in C 2 H 4 one of the hydrogen atoms is replaced by a chlorine atom, the polymer is termed polyvinyl chloride (PVC). Similarly, when a hydrogen atom is replaced by a benzene ring (C 6 H 6 ), polystyrene is formed. In an engineering context, the combination of two or more monomeric compounds yield what we call copolymers. Consider the combination of vinyl chloride and vinyl acetate alternating along the carbon chains. Different proportions of these compounds can be mixed and polymerized to yield different copolymers with different properties. Similarly, polymer properties can also be modified by grafting, branching, cross-linking etc. and by the addition of suitable additives.
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