4. Polymer Morphology in the solid state Flashcards
Types of polymer morphologies, do they have a Tg and a Tm?
- Semi-crystalline: Have Tm, Tg (Tg<Tm)
- Amorphous: have only Tg
- Chemically crosslinked: some have Tg, none form a liquid
What is the glass transition? What is the reverse process called?
- The gradual and reversible transition in amorphous polymers/amorphous regions of semi-crystalline polymers from a hard, relatively brittle ‘glassy’ state into a viscous/rubbery state as temperature increases.
- The reverse process is ‘vitrification’
Alternate expression for glass transition temperature
Temperature at which Gibbs free energy is such that the AE for the cooperative movement of ~50 elements of the polymer is exceeded
-> molecular chains slide past each other when force applied
Material properties of polymer in relation to glass transition (Amorphous)
- Below Tg: Glass
relatively hard and transparent - Just above Tg: weak rubber
viscoelastic solid with some elasticity, but flows when pulled slowly w/ only a little force - Above Tg and Tm: liquid/melt
viscous flow
Material properties of polymer in relation to glass transition temperature (Semi-crystalline)
- Below Tg and Tm: glassy and crystalline, relatively hard but opaque
- Above Tg and below Tm: a bit stronger rubber, higher elasticity but deforms permanently when pulled hard
- Above Tg and Tm: liquid; viscous flow
Polymer backbone flexibility influence on Tg
More flexible backbone results in lower Tg, including single vs double and aromatic units
Also influenced by size of atoms
Effect of side chains on polymer backbone motility (and Tg)
There is an increase in rotational barrierswith size of (rigid) substituent
As a result, Tg increases with larger substituents
Effect of flexible side chains on polymer packing (and Tg)
- An increase in conformational flexibility results in a lower Tg
- Longer side chains also increase free volume, lowering Tg
Further considerations for influence on Tg
- Attractive forces between molecules (increase Tg)
- Chain length (higher Mw = increased Tg)
- Plasticisers
Plasticisers: describe, give an example
Embedded between polymer chains, reducing attractive interactions
Increase in free volume -> lower Tg
e.g. dibutyl pthalate
Polymer crystallisation: describe; consequence of higher Mw on this process
- Backfolding -> Stacking -> Lamella
- As this process requires huge chain rearrangement, defects occur more often with higher Mw polymers; they can never fully crystallise
- Crystal morphology and degree of crystallisation is influenced by chosen crystallisation process
Types of crystallisation
- From dilute solution: regular stacking of lamella, joined by ‘tie-molecules’, with amorphous regions in-between
80% crystallinity - From polymer melt: (‘spherulites’) from chain entanglement arises radial growth of lamellae, w/ amorphous regions in fibrils of a radiating pattern
40% crystallinity
Degree of crystallinity depends on…
-
Polymer chain structure:
Mw distribution, bulkiness of substituents, flexibility, degree and length of branching, regularity of subst. on BB (tacticity, copolymer constitution) -
Processing:
From melt or stn, cooling rate, shear or strain (fibre drawing)
Mixing Rule; how does this affect miscibility in polymers, what might make polymers miscible?
Gibbs free energy must be <0, very difficult as Mw increases since deltaS becomes incredibly small
Means that miscible polymers are v hard to realise; relies on deltaH<0, so requires specific attractive interactions
e.g. H-bond donor/acceptor polymers, ionic polymers
How to make use of immiscible polymer blends
- e.g. HIPS
- Must ensure mesoscale phase separation by inclusion of phase mediators (compatilisers) like graft polymers
