Phase Transitions I Flashcards
Why do we care about polymer crystallinity?
Impacts:
- mechanical properties
- thermal stability and durability
- optical properties
- permeability
- processing
Crystalline structure what is the difference between extended chain and folded chain?
Extended chain is multiple polymers perfectly lined up together
folded chain is a single polymer folded in on itself
*folder chain is much more likely
What are the 3 things needed to form crystals?
- right place - need building blocks to move to crystal growing places
- right orientation - building blocks need to be in right position
- driving force (thermodynamics) - how much do the blocks want to be solid rather than liquid (mostly depends on temp)
*In metals/inorganics the crystalline process is very rapid below melting point
*but polymer’s long chains make it more difficult for them to orientate in the right place (entanglements slow it down) (right place and right orientation is hard)
What are the different crystalline structure models?
- adjacent re-entry (no chain connectivity) - not used much
- switchboard (gets chains that jump and merge into other lamella - forms bridges between lamella)
- fringed micelle (current model) - many pockets of relative order surrounded by mass of disorder
What is degree of crystallinity?
Kc - as kc increases, the resulting material is harder to deform but looses ductility thus becoming more brittle
(Kc is just what volume fraction is crystal form)
How does melting temperature change with crystallinity?
As larger crystals grow with less defects, get a higher melting point
What is crystal polymorphism?
Most crystallizable chains can pack into several distinct configurations or species of crystals (may change properties) - selection of polymorph typically depends on temperature
ex. alpha vs gamma polymorphism
- alpha is stronger because the chains are parallel while gamma the bonds are offset
How does crystal growth and nucleation change the crystal size?
- greater nuclei density leads to smaller crystallite sizes (small crystals run into each other)
- Growth»_space; nucleation: leads to larger crystals
- nucleation»_space; growth: leads to smaller crystals
How can lamellar thickness be increased?
thickness depends on temperature and time or formation. With sufficient time, lamellae reorganize and thicken to increase crystal perfection
*thicker lamellar lead to fewer defects and thus a high melting Temp
Why to polymers crystallize?
Crystallization begins when the spontaneous formation of a critical nucleus with radius rc occurs
How homogeneous nucleation - chains are moved at random with Brownian motion until maybe reach a radius of Rc
the critical radius occurs when the total energy balance reaches a maximum - balance of free energy gained through volume reduction and free energy cost at interface. When energy gained through volume reduction starts to be greater than energy cost at interface, nucleation occurs
When does nucleation occur?
A driving force is necessary for crystallization to occur, basically no longer wants to be a liquids.
rc occurs between Tm and Tm,e where Tm,e is the equilibrium melting point - the melting point for a lamellae of infinite thickness
(since melting point increases with crystallinity, this is basically if entire polymer was a crystal)
How does the critical radius change with temperature?
It decreases with temperature. As temperature decreases, polymers want to be solid more and so the energy gained through volume reduction increases compared to interface energy cost
What is the critical radius a function of?
Gs and Gn (sum of these two) (energy of volume reduction and energy of interface)
surface tension is function of area and interfacial tension. volume energy is a function of area and enthalpy of fusion and temperature
What is homogenous vs heterogeneous nucleation?
homogeneous - spontaneous random formation of cluster larger than critical radius (dominant mechanism with high supercooling)
heterogeneous - crystal is nucleated by an existing surface or particle melt which lowers the energy barrier to nuclei formation (generally dominant mechanism with low supercooling - higher temps)
When is the JMAK model valid?
only for modelling primary crystallization regime
and assumes isothermal formation
Assumes:
- nucleation occurs randomly and homogeneously
- growth rate, G, is both constant and independent of degree of conversion
- isotropic, 3D growth (growth outwardly radially and evenly)
