Macroscopic polymer properties Flashcards
What properties of solid and liquid polymers are influential to manufacturing? (3)
elastic behaviour of solids, viscous behaviour of liquid melts, thermal properties
What two material temperatures are important when reshaping polymer solids or melts?
glass transition temperature Tg, and melting temperature Tm
as a “glassy solid” what is being stretched during deformation for an amorphous polymer?
stretching due to secondary bonds
Describe the rubber plateu for an amorphous polymer
where stiffness levels off between Tg and Tm after stiffness has dropped from lossening of secondary bonds have occured deformation of the material then occurs due to molecules sliding past each other
At what point do fully crystalline polymers display a glass transition ?
they don’t
semicrystalline polymers loss stiffness at Tg the significance of this effect is determined on what?
the amount of amorphous regions the polymer possess ( the % of crystallinity)
in terms of performance where should the Tg be relative to the propossed operating conditions?
significantly above operating temperature
For processing, at what temperature is it best to reshape a polymer?
in between the Tg and Tm
How do we measure the properties a polymer will exhibit at different temperatures?
dynamic mechanical thermal analysis (DMTA)
what does dynamic mechanical thermal analysis involve?
performing mechanical tests (such as tensile, 3 point bending) on a small scale, loads are applied repeatedly within a chamber with accurate temperature control
What are the three most common tests during a DMTA ?
3 point bending, tension, compression/penetration
Decribe the three ways Tg is identified when conducting a DMTA
Onset temp, abrupt change in slop of storage modulus E’
Peak of loss modulus,using the dynamic component of E”
Peak of tan, a measure of the ratio E’ to E”
What does DSC stand for and what is it used to identify?
differential scanning calorimeter, used to identify phase changes in a material and temperatures they occur at. (id melting and crystallisation of TP, and cure TS)
what is used to measure the rate of deformation of a fluid?
Shear rate
describe a newtonian fluid
shear rate increases linearly with shear stress, constant viscosity which doesnt depend on shear rate
describe and give another name for a shear thinning fluid?
pseudoplastic, descrease in viscosity with increasing shear rate
describe and give another name for a shear thickening fluid
dilatant fluid, increasing viscosity with increasing shear rate
ranging from less accurate to more accurate, what pieces of equipment are used to catagorize shear viscosity?(2)
viscometer and rheometer
explain the difference between a low and high performance TS in terms of viscous behaviour and molecular weight of polymer chains
high performance higher molecular weight polymer chains, low performance newtonian fluid, high performance shear thinning
why does a high performance catalysed TS polymer exhibit shear thinning behaviour?
more likely to be entangled polymer chains at low shear rates, disentangling at higher shear rates
Describe the viscosity of resin for a ts as the cure reaction progresses
initially the increase is relatively slow, significantly increasing as gel point is reached
why does the viscosity of resin for a ts increase during curing?
the pre-polymetric chains are combining therefore increasing the molecular weight, therefore increasing the viscosity
how would you describe the molecular weight of TP polymer chain?
very high
apart from temperature, what is the next most important factor in determining the shear viscosity for a TP
molecular weight
explain why TP melts typically exhibit a shear thinning behaviour
in unstressed state the polymer chains are greatly entangled (increasing resistance) as they are stressed chains untangle
describe what change in viscosity and shear thinning effects you would expect from TS and TP fluids with increasing molecular weight?
both viscosity and the magnitude of shearthinning effect will increase for both
Describe a viscoelatic material
a viscoelastic material is one whose melts displays both elastic and viscous behaviour (e.g silly putty)
What main processes are influential during manufacture? (2)
heat conduction, and phase changes
Why is transient heat conduction very critical during processing?
Polymers have very low thermal conductivities (k) which significantly slows the conduction of heat through a polymer
what does the thermal diffusivity indicate on a polymer
how fast thermal energy can be transmitted through it
Why are melting and solidification important to processes in polymer manufacturing?
if an increase in density occurs during solidification part shrinkage can occur
comment on the significance on part shrinkage for Amorphous and semicrystalline TP
negligable for amorphous, significant for semicrystalline due to microstructural reorganisation
rapid increases in density after solidification for a TP is primarily due to what?
crystallisation
