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Glass Transition Temperature of Plastics

Glass Transition Temperature of Plastics. Introduction. Different types of polymers have different stiffness: Styrene and Plexiglas Rigid Glasslike Rubber and Polyethylene Soft and pliable Almost Ductile. Specific volume vs. temperature for polyvinyl acetate.

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Glass Transition Temperature of Plastics

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  1. Glass Transition Temperature of Plastics

  2. Introduction • Different types of polymers have different stiffness: • Styrene and Plexiglas • Rigid • Glasslike • Rubber and Polyethylene • Soft and pliable • Almost Ductile

  3. Specific volume vs. temperature for polyvinyl acetate

  4. Transitions in amorphous polymers

  5. Factors affecting Tg • Chain Length • Plasticizers • Chain stiffness and side groups • Copolymers • Pressure • Test conditions, length of test, and types of impurities in sample • Polymer chain factors

  6. Chain Length • As molecular weight increases, Tg also increases • As molecular weight decreases, Tg also decreases • More end groups • Easier movement of molecules • More inherent free volume in the polymer Tg = Tg∞ – C X Where: C = constant for each polymer X = degree of polymerization

  7. Plasticizers • Small organic molecules mixed into a polymer having the effect of reducing its Tg • Have the effect of increasing the free volume • Their small molecules fit in between the chains • Causes chains to be less tight amongst themselves • Plasticizers technically do not effect Tg but practically do

  8. Chain stiffness and side groups • Chains that have difficulty uncoiling have higher Tg • Chain stiffness is related to higher energy level • Bulky side groups hinder rotation

  9. Copolymers • Made from two different monomers • Tg will be some kind of average between the two monomeric materials • Assuming they were converted into polymer • Will be based roughly on the weight % of each material present in the copolymer Volume Fraction Tg = VFa x Tga + VFa x Tgb 1 = Wf1 + Wf2 Tg Tg1 Tg2

  10. Pressure • A noticeable increase in Tg occurs at several thousand psi. • Pressure pushes molecules together • Removes free volume

  11. Rate dependence of Tg

  12. Polymer chain conformation • Factors governing conformation • Potential energy hindering internal rotation • Repulsive forces and Van Der Waals Forces • Electrostatic interactions • Hydrogen bonding • Packing considerations

  13. Process Temps

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