Chapter 15: Characteristics, Applications, and Processing of Polymers

The mechanical response of polymers varies significantly across three primary categories: brittle fracture behavior similar to ceramics, plastic deformation comparable to metals, and elastomeric behavior characterized by large reversible strains under low stress. Polymer properties exhibit strong dependence on temperature, strain rate, and environmental factors, generally showing decreased stiffness and strength but increased ductility with rising temperature. Viscoelasticity represents a defining characteristic of polymers, where materials simultaneously exhibit time-dependent viscous flow and elastic recovery, quantified through the relaxation modulus. The chapter introduces critical concepts including creep and stress relaxation to explain how polymers deform progressively under sustained loads. Fracture behavior encompasses both brittle and ductile mechanisms influenced by temperature, loading rate, and structural defects, with crazing—a localized plastic deformation process generating fibrils and microvoids—preceding fracture in certain thermoplastics and contributing to toughness enhancement. Semicrystalline polymers deform through reorientation of tie chains and crystalline lamellae during drawing, thereby improving stiffness and tensile strength, while elastomers achieve stretch and recovery through entropic effects of coiled crosslinked chain networks. Vulcanization, the sulfur-mediated crosslinking of rubber, substantially improves elasticity, tensile strength, and service life. Thermal transitions including crystallization, melting, and the glass transition establish service temperature boundaries. Crystallization occurs through nucleation and growth mechanisms producing spherulite microstructures, while the glass transition temperature marks the transition between rigid glassy and rubbery states, controlled by chain stiffness, substituent groups, molecular polarity, and crosslink density. Major polymer applications span plastics including polyethylene and polyvinyl chloride, elastomers such as natural rubber and synthetic variants, high-performance fibers, protective coatings, adhesives, packaging films, foams, and biomedical implants. Advanced materials including ultrahigh-molecular-weight polyethylene, liquid crystal polymers, and thermoplastic elastomers expand engineering possibilities. Polymer synthesis pathways comprise addition polymerization through initiation-propagation-termination mechanisms and condensation polymerization producing by-products. Processing encompasses compression molding, injection molding, extrusion, blow molding, fiber spinning, film casting, and emerging three-dimensional printing technologies enabling rapid prototyping and biomedical fabrication.