Topic 2 Flashcards
Describe polymers in the solid state
Crystalline regions; amorphous regions; rarely fully crystalline
What is Tg?
Glass transition temperature (temperature at which a material undergoes a transition from a glassy to a rubbery state (or vice versa)). When the glass temperature has been reached, the stiffness stays the same for a while until the temperature exceeds Tm and the material melts
What is Tm?
Melting transition temperature (or crystalline melting temperature); S –> L
What is an elastomer/rubber?
Material with the mechanical (or material) property that it can undergo much more elastic deformation under stress than most materials and still return to its previous size without permanent deformation. Materials are generally soft - sometimes crosslinked.
What is the Tg for useful rubbers?
< 0 degrees C
What is plastic/glass?
Polymers in the glassy state are stiff immobile materials that do not readily undergo elastic deformation. Plastic is a material capable of being shaped through plastic flow by application of deforming forces. Usually a high MW organic material
What is the Tg for useful plastics?
> 60 degrees C (i.e. doesn’t deform at room temperature)
Describe the transitions for polymers with different morphology
Amorphous - only Tg (unable to crystallise); completely crystalline - only Tm; semi crystalline - both (Tg and Tm)
True or false - “melted” polymeric materials are always above their Tg.
True - a polymer is defined as being melted when it is above its Tg.
What types of transition are Tg and Tm?
Tg - endothermic, 2nd order; Tm - endothermic, 1st order
Why does a rubber band get hot when stretched?
Due to crystallisation
What is the rate of change in the molecular weight of a step growth polymer at lower conversion and higher conversion respectively?
Decreases and then increases
How can you determine the Tg point on a graph?
Change in slope occurs
How does free volume affect Tg?
Free volume = the space in a solid/liquid which is not occupied by polymer molecules. In a liquid, free volume is high so conformations can change readily. Reduction in temperature reduces amount of thermal energy. As temperature is lowered, free volume is lowered until there is not enough free volume to allow for molecular translation/rotation. This temperature is the Tg as the polymer is frozen
What factors affect Tg?
Chain flexibility/stiffness; sterics; tacticity; molecular architecture (MW, branching, crosslinking); polarity; symmetry; rate of heating/cooling
How does stiffness affect Tg?
Stiff chains = higher Tg as more energy needed to allow them to move past each other and achieve rubbery state. Chain stiffness is increased when a ring is incorporated into the backbone (reduces rotation and no of conformations)
How does sterics affect Tg?
Size of R important. Bigger R/side group = higher Tg
How does MW affect Tg?
Initial linear relationship between MW and Tg but then levels off i.e. dependence only for low MW/short chain polymers
How does symmetry affect Tg?
Increase in symmetry reduces Tg. Free volume effect. Harder to pack, larger free volume so less energy to create free volume for chain rotation (higher free volume reduces Tg)
How does polarity affect Tg?
Increased polarity increases Tg. Increased amount of polar bonding, chains held tighter together so need more energy to break to rubbery state
How does branching affect Tg?
Low levels reduces Tg due to changes in free volume (branches lead to less end groups). High levels = Tg raises as same effect as side chains in restricting chain mobility
How does crosslinking affect Tg?
Reduces specific volume of polymer and hence free volume is reduced so Tg is raised as molecular motions are more difficult
How does MW affect Tg?
Key assumption that there is more free volume associated with a chain end than a segment in the middle part of a chain. Shorter polymer = more ends
What happens at the melting point?
Liquid and solid phases are in thermodynamic eqm
