Phase Transitions III Flashcards
What do non-isothermal crystallization kinetic tests give us?
give a comparative measure of:
- nucleation in the presence of a nucleating agent
- the impact of the difference in molecular structure (ie branching)
- the effect of blending with a second polymer
*great of getting board overview of all phase transitions in a material, but not great for determining kinetics of those phase transitions
What happens to crystallization as a nucleating agent is added?
The temperature at which nucleation initially occurs increases (and when melting occurs decreases) - it gets closer to Tm,e. The enthalpy for crystallization might also change
*as nucleating agents are added, it lowers the barrier to create crystals
What happens to crystallization as cooling rate changes?
As cooling rate increases, it reaches Tc (maximum of peak) later
Faster cooling time gives crystals less time to rearrange, thus causing crystallisation to occur for longer range of temperatures
Non-isothermal crystallization kinetics - isoconversional method
A model-free approach to analyzing crystallization under non-isothermal conditions
- uses Arrhenius relationship
Hoffman-Lauritzen theory
considers 2 terms: diffusion term and nucleation term
What happens to the glass transition temperature as the cooling rate changes?
Tg decreases with decreasing cooling rate and has decreased specific volume (since the cooling rate is slower, polymers have more time to rearrange into crystals)
*Tg is sensitive to thermal history, thus different cooling rates with yield different Tg values
What is physical aging and how does it impact volume or enthalpy?
over time, through small non-cooperative motions (ie. local relaxation of few chain segments), the volume slightly decreases tot he extrapolated specific volume from the liquid state (for non-relaxed and partially relaxed glassy state)
**if quenched very quickly will see very very slow/small decrease in volume (molecules mostly just rotating in place but over significant time will move towards extrapolated specific volume from liquid state)
How does DSC see glass transition state?
At temperatures below the crystal melt (or crystalization), the heat flow will experience a sharper linear region. The change in heat capacity can be read as the difference between the two parallel lines before and after the sharp drop
What is the rigid amorphous fraction?
practice found that the fraction of amorphous and fraction or crystalline did not add up to 1 (fractions found with the specific heat capacities of the 2 phases)
instead, there exists a third intermediate phase at the crystalline-amorphous interphase - this was called the rigid amorphous fraction (as opposed to the mobile amorphous fraction)
How is the RAF found?
By subtracting the amorphous (found with change in heat capacity) and crystalline (found by the change in enthalpy) from unity (remaining is the rigid amorphous phase)
