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Crystallization

Crystallization. Nucleation Tendency for random tangled molecules in the melt to become aligned and form small ordered regions (nuclei) Homogeneous Heterogeneous Growth r = v t Spherulite radius v: growth rate Growth rate strongly depends on the crystallization temperature.

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Crystallization

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  1. Crystallization • Nucleation • Tendency for random tangled molecules in the melt to become aligned and form small ordered regions (nuclei) • Homogeneous • Heterogeneous • Growth • r = v t • Spherulite radius • v: growth rate • Growth rate strongly depends on the crystallization temperature

  2. Crystallization Kinetics • Polymer melt of mass W0 • Homogeneous nucleation • The rate of nucleation (# of nuclei formed per unit time and unit volume) is a constant

  3. Crystallization Kinetics • Impingement of spherulites • Reduction of overall volume of the system • Spherulites move closer to each other

  4. Molecular mechanism of crystallization Experimental Observation • Polymer crystal are usually thin and lamellar when crystallized from dilute solution and melt • Lamellar thickness and crystallization temperature, 1/DT (where DT =Tc-Ts) • Chain folding (dilute solution and maybe from melt) • Growth rate of polymer crystals highly depend on Tc and molar mass of the polymer

  5. Crystallization Kinetics secondary nucleation on a pre-existing crystal surface

  6. Melting • Characteristics of the melting behavior of polymers • Not possible to define a single melting temperature for a polymer • Depends on the specimen history, particularly the temperature of crystallization • Depends on the rate of heating

  7. Melting • Tmo concept, equilibrium melting temperature – an infinitely large crystal • In general Tm > Tc • The line Tm = Tc represent the lower limit of the melting behavior • The implication of Tmo

  8. Relation between Tm and Crystal Thickness Tm is affected by crystal thickness, and consequently the crystal thickness affects Tm • Annealing • When crytsalline polymers are heated to just below Tm there is an increase in lamellar crystal thickness • The increase of l implies an increase of Tm

  9. Factor affecting Tm • For a specific polymer system, Tm depends on molar mass and degree of branching • Chain ends and braches: impurities which depress Tm • For different polymers, chemical structure determines Tm • Stiffness of the main chain • More flexible  lower Tm • -O- or –CO-O- increase flexibility • Phenylene group increases stiffness • Polar group such as amide linkage which forms intermolecular hydrogen bonding • Type and size of any side group

  10. Volume change on Tm

  11. Re-cap of XRD

  12. Amorphous Polymers • Examples of flat-film x-ray photographs • X-ray diffraction pattern – shown the structure in amorphous polymer is random • PET and PP • Crystallize slowly • Amorphous when rapidly quenched • Crystallization can be induced by annealing the quenched polymer at an elevated temperature

  13. Glass Transition • Tg is the temperature at which the polymer undergoes the transformation from a rubber to a glass • Definition: Specific volume vs T • Depends on the cooling rate • The lower cooling rate, the lower Tg

  14. Chemical Structure vs Tg • Factors affecting Tg are similar to those for Tm: Tm depends on molar mass and degree of branching • Table 4.4 • CH2-CHX- the effect of side group • Polar group –Cl, -OH or –CN tends to raise Tg than non-polar groups • Polar interaction restricts rotational movement

  15. Tg & Tm • Volume change – Dilatometry

  16. Tab DSC and DTA

  17. Tg & Tm • Differential thermal analyzer (DMA) • Ref. such as Alumina • At const. rate of T increase • Endotherm: Tm • Small amount of sample, fast data and accuracy • Disadv: weak signal for crystalline • Heat change – Differential scanning calorimetry (DSC) • Keep temperature of sample the same as that of Reference • Monitor the current flow

  18. DSC

  19. Tg and Tm similarity Tg = 0.5 to 0.8 Tm

  20. Elastomer • Above its Tg • Have a very low degree of crystallinity • Lightly cross-linked • Example: cis-1,4-polyisoprene • Crosslinking – vulcanization by sulphur • Crystallization under stretch 300-400%

  21. Thermoplastic elastomers • Behave like elastomer without the necessity of chemical crosslink • Crosslink is physical, not chemical in nature • Thermoplastic at elevated temperature; elastomer when cooled down to ambient temperature • Reversible

  22. Thermoplastic elastomer • Thermoplastic PU elastomer: • PU: hard segment • Tends to aggregate to give ordered crystalline domain w/ possibility of forming hydrogen bonding • Can be disrupted by heating the materials to its processing temperature orby the action of a solvent • Polyol: soft segment • Above their Tg and remains amorphous at ambient • Can be moulded • Similar behavior found in segmented polyester copolymer

  23. ABA-tri-block copolymer • A: glassy • B: rubbery (polydiene such as polybutadiene or polyisoprene) • A-B incompatible on the molecular level because of their different chemical type • Flexible polydiene linked by covalent bonds to polystyrene • When heated up, PS-rish domain become disrupted and materials can be processed as a normal thermoplastic

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