2.1 Classification based on origin, structure, and properties

Polymers come in two main flavors: natural and synthetic. Natural ones are made by living things, while synthetic ones are cooked up in labs. Each type has its own unique properties and uses, making them versatile materials in our world.

Polymer structures can be linear, branched, or cross-linked, affecting their properties. These structures determine how polymers behave when heated, leading to classifications like thermoplastics and thermosets. Understanding these relationships is key to harnessing polymers' potential.

Polymer Classification

Natural vs synthetic polymers

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• Natural polymers originate from living organisms
  • Produced by plants, animals, or microorganisms (cellulose, proteins, natural rubber)
  • Often have complex structures and unique properties adapted for biological functions
  • May require extraction and purification processes for industrial use
• Synthetic polymers are man-made through chemical synthesis
  • Created by polymerizing monomers using various techniques (polyethylene, nylon, polystyrene)
  • Offer a wide range of customizable properties and applications
  • Can be designed to mimic or surpass the properties of natural polymers

Types of polymer structures

• Linear polymers have monomers connected in a single continuous chain
  • Chains can be packed closely together, resulting in high density and crystallinity (high-density polyethylene HDPE, polyvinyl chloride PVC)
  • Linear structure contributes to high tensile strength and resistance to deformation
• Branched polymers feature a main chain with side chains or branches
  • Branches disrupt the close packing of chains, leading to lower density and crystallinity (low-density polyethylene LDPE, glycogen)
  • Branching increases flexibility and improves processability of the polymer
• Cross-linked polymers have chains connected by covalent bonds, forming a network
  • Cross-links restrict chain movement and provide mechanical strength and thermal stability (vulcanized rubber, epoxy resins)
  • Degree of cross-linking affects properties such as solubility, swelling, and elasticity

Thermal properties of polymers

• Thermoplastics soften and melt when heated, then harden upon cooling
  • Reversible thermal transitions allow for reshaping and recycling (polyethylene, polypropylene, polystyrene)
  • Glass transition temperature $T_g$ and melting temperature $T_m$ are key parameters
    • $T_g$: temperature at which polymer transitions from glassy to rubbery state
    • $T_m$: temperature at which crystalline regions melt
• Thermosets permanently harden after initial heating and cooling
  • Irreversible cross-linking occurs during the curing process, often initiated by heat or light (epoxy resins, phenolic resins, polyurethanes)
  • Once cured, thermosets cannot be reshaped or recycled due to the cross-linked network
  • Curing temperature and time are critical factors in the processing of thermosets

Structure-property relationships in polymers

• Molecular structure significantly influences polymer properties
  1. Linear polymers exhibit high tensile strength and crystallinity due to close packing of chains
  2. Branched polymers have lower density, crystallinity, and increased flexibility due to disrupted chain packing
  3. Cross-linked polymers display high mechanical strength, thermal stability, and insolubility resulting from the network structure
• Thermal properties and classification are interconnected
  • Thermoplastics undergo reversible thermal transitions, with $T_g$ and $T_m$ being key parameters for processing and application
  • Thermosets experience irreversible thermal transitions during curing, with the curing temperature determining the final properties
• Mechanical properties are influenced by the polymer structure and classification
  • Linear and branched polymers exhibit viscoelastic behavior, as demonstrated by stress-strain curves
  • Cross-linked polymers have high modulus and low elongation at break due to the restricted chain movement in the network