4.3 Examples of step-growth polymers and their applications

Step-growth polymers are versatile materials with diverse applications. From polyesters like PET in beverage bottles to polyamides like Kevlar in bulletproof vests, these polymers offer unique properties tailored to specific needs.

Understanding the structure-property relationships is key to optimizing polymer performance. Factors like molecular weight, crystallinity, and functional groups influence properties such as strength, flexibility, and chemical resistance, enabling the creation of materials for various industries.

Common Step-Growth Polymers

Examples of step-growth polymers

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• Polyesters formed by the reaction of diols and dicarboxylic acids or their derivatives (PET, PBT)
  • PET widely used in beverage bottles and synthetic fibers due to high strength, stiffness, and barrier properties
  • PBT used in automotive and electrical applications for its good mechanical properties and heat resistance
• Polyamides formed by the reaction of diamines and dicarboxylic acids or by the self-condensation of amino acids (nylon 6, nylon 6,6, Kevlar)
  • Nylon 6 and nylon 6,6 used in textiles, automotive parts, and packaging for their high strength, toughness, and abrasion resistance
  • Kevlar used in high-performance applications such as bulletproof vests and aerospace composites due to its exceptional strength and thermal stability
• Polyurethanes formed by the reaction of diisocyanates and polyols
  • Thermoplastic polyurethanes used in footwear, automotive parts, and medical devices for their flexibility, durability, and abrasion resistance
  • Thermoset polyurethanes used in insulation, adhesives, and coatings for their excellent thermal and chemical resistance
• Other step-growth polymers include polycarbonates, polyimides, polyureas, and epoxy resins
  • Polycarbonates used in automotive and construction applications for their impact resistance and transparency
  • Polyimides used in high-temperature applications such as aerospace and electronics due to their exceptional thermal and chemical stability

Structure-property relationships in polymers

• Molecular weight and its distribution impact mechanical properties and processability
  • Higher molecular weight generally improves strength, toughness, and viscosity
  • Narrow molecular weight distribution leads to more consistent properties and easier processing
• Degree of crystallinity affects strength, stiffness, and chemical resistance
  • Crystalline regions contribute to mechanical properties and barrier properties
  • Amorphous regions provide flexibility, impact resistance, and transparency
• Functional groups impart specific properties such as hydrophilicity, reactivity, or UV resistance
  • Hydrophilic groups (hydroxyl, carboxyl) improve moisture absorption and adhesion
  • Reactive groups (epoxy, isocyanate) enable crosslinking and functionalization
  • UV-resistant groups (benzophenone, triazine) enhance outdoor stability and weatherability
• Crosslinking improves thermal stability, solvent resistance, and mechanical properties
  • Crosslinked polymers have higher glass transition temperatures and better creep resistance
  • Excessive crosslinking can lead to brittleness and reduced processability

Synthesis and applications of specific polymers

• Nylon synthesized by polycondensation of diamines and dicarboxylic acids (nylon 6,6) or ring-opening polymerization of lactams (nylon 6)
  • Properties: high strength, toughness, abrasion resistance, and chemical resistance
  • Applications: textiles, automotive parts, packaging, and consumer goods
• PET synthesized by polycondensation of ethylene glycol and terephthalic acid or its dimethyl ester
  • Properties: high strength, stiffness, dimensional stability, barrier properties, and chemical resistance
  • Applications: beverage bottles, food packaging, synthetic fibers, and engineering plastics
• Epoxy resins synthesized by the reaction of epoxide monomers with curing agents (amines, anhydrides, phenols)
  • Properties: high strength, stiffness, chemical resistance, adhesion, and low shrinkage
  • Applications: adhesives, coatings, composites, and electronic encapsulants

Advantages vs limitations of step-growth polymers

• Textiles
  • Advantages: high strength, durability, wrinkle resistance, and easy dyeing and processing
  • Limitations: pilling, static buildup, and low moisture absorption in some cases
• Packaging
  • Advantages: good barrier properties, chemical resistance, transparency, and easy molding and thermoforming
  • Limitations: limited biodegradability and recyclability, potential for migration of monomers or additives
• Adhesives
  • Advantages: strong bonding, chemical and thermal resistance, and ability to bond dissimilar materials
  • Limitations: may require heat or pressure for curing, limited flexibility or impact resistance in some cases