8.1 Glass transition temperature and factors affecting it

Glass transition temperature is a crucial property of polymers, marking the shift from rigid to flexible states. It determines how polymers behave in different conditions, affecting their applications from plastic bottles to rubber tires.

Factors like molecular weight, crosslinking, and plasticizers influence a polymer's glass transition temperature. Understanding these factors helps engineers tailor polymers for specific uses, balancing properties like strength and flexibility to meet various needs.

Glass Transition Temperature (Tg) and Its Significance

Glass transition temperature definition

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• Temperature range at which a polymer transitions from a hard, glassy state to a soft, rubbery state
  • Polymers are rigid and brittle below Tg (polystyrene at room temperature)
  • Polymers become flexible and deformable above Tg (rubber bands at room temperature)
• Crucial property in determining the application and processing conditions of polymers
  • Used below Tg for structural applications requiring rigidity and dimensional stability (plastic bottles, eyeglass lenses)
  • Used above Tg for applications requiring flexibility (elastomers like tires, adhesives like tape)
• Not a sharp transition but occurs over a temperature range
  • Midpoint of this range often reported as the Tg value (polycarbonate Tg around 145 ℃)

Factors Affecting Glass Transition Temperature

Factors affecting glass transition

• Molecular weight
  • Higher molecular weight polymers have higher Tg values
    • Longer polymer chains have more entanglements and intermolecular interactions, restricting chain mobility (ultra-high molecular weight polyethylene)
  • Lower molecular weight polymers have lower Tg values due to increased chain mobility (low molecular weight polyethylene glycol)
• Crosslinking
  • Crosslinking increases the Tg of polymers
    • Covalent bonds between polymer chains restrict chain mobility and increase rigidity (vulcanized rubber)
  • Higher crosslink density leads to higher Tg values (highly crosslinked epoxy resins)
• Plasticizers
  • Low molecular weight additives that lower the Tg of polymers
    • Increase free volume between polymer chains, enhancing chain mobility (phthalate plasticizers in PVC)
  • Higher plasticizer content results in lower Tg values (flexible PVC tubing)
• Copolymerization
  • Tg of copolymers depends on the composition and Tg values of the constituent monomers
    • Copolymers with monomers having significantly different Tg values exhibit intermediate Tg, depending on the composition (styrene-butadiene rubber)
• Substituent groups
  • Bulky side groups or pendant groups can increase Tg by hindering chain rotation and flexibility (poly(methyl methacrylate))
  • Flexible side groups can decrease Tg by increasing chain mobility (silicone rubber)

Chain flexibility and glass transition

• Polymer chain flexibility is inversely related to Tg
  • More flexible polymer chains have lower Tg values
    • Flexible chains require less energy to overcome intermolecular forces and initiate segmental motion (polyisoprene)
  • Less flexible (stiffer) polymer chains have higher Tg values
    • Rigid chains have limited rotational freedom and require more energy to initiate segmental motion (polycarbonate)
• Factors affecting chain flexibility
  • Backbone structure
    • Polymers with single carbon-carbon bonds in the backbone are more flexible than those with double bonds or aromatic rings (polyethylene vs polystyrene)
  • Side groups
    • Bulky or rigid side groups hinder chain rotation and increase Tg (poly(vinyl chloride))
    • Flexible side groups enhance chain mobility and decrease Tg (polysiloxanes)

Effects on mechanical properties

• Polymers exhibit distinct mechanical properties above and below their Tg
• Below Tg (glassy state)
  • Polymers are rigid, brittle, and have high modulus (unplasticized poly(vinyl chloride) pipes)
  • Deformation is primarily elastic, with limited plastic deformation
  • More resistant to creep and stress relaxation
• Above Tg (rubbery state)
  • Polymers are soft, flexible, and have low modulus (elastomeric shoe soles)
  • Deformation is primarily viscoelastic, with significant plastic deformation
  • More susceptible to creep and stress relaxation
• Near Tg
  • Exhibit a combination of glassy and rubbery properties
  • Mechanical properties, such as modulus and strength, change rapidly with temperature (automotive bumpers)
• Importance of considering Tg in polymer applications
  • Select polymers based on Tg and intended use temperature
    • For structural applications, use below Tg to maintain rigidity and dimensional stability (plastic gears)
    • For flexible applications, use above Tg to ensure adequate deformability and elasticity (rubber seals)