Thermal Testing Flashcards
What is Tg?
The glass transition temperature is the point at which the torsion angle can change, lowering the torsion tension by increasing torsion angle. This is a thermally activated process
At lower temperatures the viscosity of the polymer is higher, so more elastic strain energy is dissipated, at Tg nearly all energy is dissipated, and below Tg there is not enough energy for a conformational change to occur (becomes like glass)
What is an Amorphous polymer?
Amorphous polymers have their atoms held together in a loose, unpredictable structure. They are said to have “no long range order”.
They have a Tg, below which they are brittle and glassy solids, above which they are highly viscous liquids.
What is a Crystalline polymer?
Crystalline polymers form orderly “stacks” of folded chains called lamellae, which bring “long-range order” to the polymer.
They have a Tm
What is a Semi-Crystalline polymer?
Have both a Tg and a Tm. Below Tg a semi-crystalline polymer is a glassy solid, between Tg and Tm it is a crystalline solid, and above Tm it is a viscous liqiuid.
Tg is different for each polymer but most are above Tg at room temperature. Rubbers have a Tm above room temp, and a Tg below room temp
Describe how Chain length effects Tg
A polymer with shorter chains will have a lower Tg. Longer chains have higher Tg
Each chain end has some free volume associated with it (allows for internal movement; bond rotation, bending, stretching). shorter chains will have more chain ends per unit volume and therefore more free volume
Describe how Chain flexibility effects Tg
Polymers with a flexible backbone have a lower Tg (lower activation energy for conformational change). They are more likely to have a random coil formation
Describe how different side groups effect Tg
Larger side groups can hinder bond rotation, so this increases Tg.
Polar groups have a greater effect on Tg (Cl, CN, OH) increasing it
Describe how Branching effects Tg
Polymers with more branches have more chain ends thus more free volume (lower Tg), but branches hinder rotation (higher Tg). Tg may rise or fall due to this
Crosslinking increases Tg as it reduces mobility, and also effects macroscopic viscosity
Describe how Plasticisers effect Tg
Small molecules like esters that are added to the polymer increase chain mobility by spacing out the chains, reducing Tg
Describe how Time effects Tg
The properties of an amorphous polymer above Tg can change over time;
Very short loading times: Polymer can still be glassy because there is not enough time for the chains to move.
Intermediate times: The polymer chains uncoil and recoil (rubbery) between entanglements.
Very long times: Chains move past each other permanently, so polymer behaves as a viscous liquid
What is the Dielectric constant?
If a varying electric field is applied to a polymeric material, any polar groups will align with the field. Below Tg, rotation of the bonds is not possible so the permittivity (polarizability) will be low with a big increase around Tg
At higher temperatures the increased thermal vibrations cause the permittivity to drop again. If the frequency of the field is increased, the polar groups have less time to align, so the glass transition occurs at a higher temperature
Describe Differential Scanning Calorimetry (DSC)
DSC analyses the heat flow in a material.
Each chamber (one with the sample and one with a reference of only solvent) is heated so their temperatures are always equal and can be compared.
The enthalpy of a polymer increases as the temp increases, but with a change in slope at time Tg. This increase in heat flow is caused by the polymer having a higher heat capacity above it’s Tg. The Tg is read as the midpoint of the sudden increase in heat flow
Describe HyperDSC
Normal DSC ramp rates are 20-40°C/min, whereas hyperDSC can ramp up to 500°C/min
Useful when looking at pharmaceutical polymorphs as at lower heating rates, some materials experience re-crystallisation during melting or decompose after melting
Describe NanoDSC
Can be used to investigate the stability of proteins, nucleic acids, lipids and other macromolecules.
The information obtained provides valuable insights into the factors that stabilise these biomolecules (hydrophobic interactions, hydrogen bonding, conformational entropy and physical environment)
