Polymers- Glass Transition Flashcards
Three stages of G vs T graph for different polymers
Gradual decline is thermal expansion. Abrupt drop is relaxation process. Dominant relaxation is glass to rubber. Don’t all happen clearly in all polymers
What changes through Tg?
Dramatic change in: specific volume, modulus, heat capacity, refractory index
What is glass transition?
Not a transition in structure but one in molecular motion. Above has fast motion and below slower (molecular motions frozen). At Tg thermal energy lower than rotational energy barrier?
What is Tg defined as?
The point where the thermal expansion coefficient undergoes a discontinuity.
α=(1/V)(dV/dT)
V-T curve
Specific volume vs T. Going upwards in slight curve then follows a straight line and meets it with a corner. Shorter equilibrating time has higher curve and greater Tg. All curves for different equilibrating times meet the same straight line but at different points
Different things to measure to determine Tg
Heat capacity Refractory index Differential thermal analysis X- and β-Ray absorption Gas permeability studies
Free volume theory
Free volume Vf is unoccupied space in a sample arising from inefficient packing of disordered chains in the amorphous regions of a polymer sample.
V=Vo+Vf
V is total volume and Vo actually occupied by molecules
Free volume is a measure of the space available for the polymer to undergo rotation and translation
Effect of T on occupied volume
Linear function of T (thermal vibration)
Irrespective of whether polymer is glassy or rubbery
Effect of T on free volume
Linear vs T in rubbery state above Tg
Vf contracts with decreasing T, reaches a critical value at Tg that there is insufficient free space for large scale chain movement
Effectively constant below Tg as molecular chains immobilised
Free volume fraction formula from Simha and Boyer definition of free volume
fg=Vf(Tg)/Vg=(αr-αg)Tg=TgΔα
Vg is volume at Tg
r and g sub mean rubber and glass
They observed TgΔα=0.113 for wide range of polymers
Implies free volume fraction fg same for all polymers in the glassy state
Why can polymer have a lower environmental impact than some other materials?
Can cost less to make and with less CO2 than metal, paper and glass. Produce fewer effluents of SO2, NOx, dust during manufacture than paper (greater HCs, CO). Lower energy to make than glass bottle and gain energy from combustion
Disposal methods of plastic waste
Landfill, energy or thermal recycling (incineration), chemical recycling (back to feedstock), mechanical or material recycling
Problems with polymer recycling
Separation, identification, cleaning, transportation.
Mechanical recycling gives inferior properties.
Energy recycling has environmental concern.
Chemical recycling expensive.
Must be economic and show net environmental gain
Relationship between Vf and Tg
High Vf gives low Tg, vice versa
What is characteristic ratio?
Proportional to chain rigidity and 1/chain flexibility
How do side and skeletal groups affect Tg?
Bulky side group (like benzene) restricts rotation of bonds and raises Tg.
Rigid skeletal groups (like benzene and O and SO2 in backbone) raise Tg
How does molar mass of chains affect things?
Greater molar mass M reduces number of chain needs and Vf and increases Tg
Formula for Tg based on molecular mass M
Tg=Tg(inf)-(2ρNAθ)/αfM Tg(inf) is Tg of polymer with infinitely large M θ is Vf contribution of one chain end ρ is polymer density NA is Avogadro number αf is free volume expansivity (αr-αg)
What do polar groups affect?
Increases lateral forces and hinder molecular motion so greater Tg
How does degree of cross linking affect things?
Greater degree of crosslinking restricts molecular motion more to increase Tg
See crystallinity graph p24 week 4
Weird
Tg of random copolymers
Tg of copolymer (Tgc) lies between those of the pure homopolymers. Dependence with T upon composition can be predicted using free volume theory
Free volume fraction of copolymer
fc=fgc+(T-Tgc)αfc
How to predict Tg of random copolymers
Assume free volumes add in proportion to weight fractions and so do thermal expansion coefficients (αf)
Gibbs and Di Marzio equation predicting Tg of random copolymers
See p27 week 4
