Polymers Flashcards
What are the four types of polymer?
Thermosets
Thermoplastics
Elastomers
Natural polymers
What are the defining characteristics of a thermoset?
High density and stiffness - lots of strong inter-chain cross-links (covalent bonds)
What are the defining characteristics of a thermoplastic?
Highly flexible, remouldable - far fewer covalent bonds between chains
What’s an elastomer?
A non-linear elastic that has few interlinks
What do the interlinks in a polymer do?
Prevent chains sliding relative to one another, and lends a polymer a higher melting point
Briefly explain stable necking
Chain alignment due to deformation causes local strengthening, causing the neck to extend rather than narrow further.
What is the memory effect?
Chains prefer to be tangled + curled up - heating them allows them to return to this form because it creates free volume around the chains.
Define a foam
A mixture of air and another material
What are the three types of crack propagation in a composite?
Pullout - crack yanks fibres from matrix
Bridging - fibres bridge crack and pull faces together
Deflection - crack runs around and between fibres
What is the standard linear model? What is it a model of?
A series spring and dashpot in parallel with a spring. Approximates creep modulus and relaxation modulus.
What is the upper elastic modulus of a composite?
The Voigt modulus - assumes fibres are aligned with loading direction (fibre + matrix strain by same amount)
What is the lower elastic modulus of a composite?
The Reus modulus - assumed fibres are perpendicular to loading direction (fibre + matrix have same stress)
