Effect of crystallinity on properties The morphology of most polymers is semi-crystalline. That is, they form mixtures of small crystals and amorphous material and melt over a range of temperature instead of at a single melting point. Specific volume (cc/g) 41 42 Factors that affect the melting temp. of polymer crystals Intermolecular Interactions : Enthalpy change - The enthalpy of fusion or heat of fusion is the change in enthalpy resulting from heating a given quantity of a substance to change its state from a solid to a liquid (T m ) Why? Crystallinity! G f = H f T m S f = 0 - The 'enthalpy' of fusion is a latent heat: The latent heat of fusion is the enthalpy change of any amount of substance when it melts. - The liquid phase has a higher internal energy than the solid phase: energy must be supplied to a solid in order to melt it and energy is released from a liquid when it freezes - The enthalpy of fusion is almost always a positive quantity T m = H f / S f Depends on Enthalpy change and Entropy changes 43 44
Intermolecular Interactions : Enthalpy change Intermolecular Interactions : Enthalpy change More polar T m = H f / S f 45 T m = H f / S f 46 Intermolecular Interactions : Enthalpy change Intermolecular Interactions : Enthalpy change - Nylon 66 has more hydrogen-bonding than nylon 6. - Nylon 6,6 is tighter with less openings, making it the stronger and more resistant to heat (m.p.) nylon 6,6-250 o C, nylon 6-215~220 o C 47 - Nylon 66 has less amount of amorphous region and higher crystallinity than nylon 6 48
Difference in properties Nylon 6,6 shows Intermolecular Interactions : conformational Entropy change Stiffer polymer chains have less entropy changes between ordered crystal state and unordered melt state : T m - Higher thermal properties : the preferred Nylon for temperature performance products. - Higher tensile strength in use. - Excellent abrasion resistance. T g at 65% RH ( o C) Moisture regain (21 o C, 65% RH) Water shrinkage (%, 15mins; 98 o C) Melting point ( o C) Softening point ( o C) Nylon 6 Nylon 66 60 4.0-4.5 10 215-220 170 74 3.5-4.0 5.3 250 235 49 T m = H f / S f 50 Chain flexibility and Entropy change The effect of diluents Diluents(low molecular weight materials) increase the entropy changes between crystal and melt state T m 51 52
The effect of copolymerization and M.W. Stereoregulararity Highly crystalline polymers Polypropylene Amorphous polymers: Poly(methyl methacrylate) Low M.W. number of terminal group T m Stereoregular chain structure more crystalline Entropy change T m Syndiotactic polystyrene Polyamide (Nylon 6, 66) Kevlar와 Nomex Polyketone Atactic polystyrene Polycarbonate Polyisoprene Polybutadiene 53 54 Motions of materials Low molecular weight materials Movements of materials Free volume When appropriate energy is added, molecules can move through free volume among molecules Glass Transition Temperature (T g ) 55 56
Movement of polymer chains Motion in polymers The dynamics of polymer chains Free volume A. Definition of Glass Transition Temperature. a. The temperature at which the glassy state(brittle crystalline) is changed into the rubbery state. b. The glassy state: short-range vibrational and rotational motion of atoms hard, rigid and brittle. Heating : increased kinetic energy of molecules At T g, 20-50 chain atoms are involved in the segmental movement Remember! T g has to do with free volume increase c. The rubbery state: long-range rotational motion of segments (20-50 atoms) soft and flexible. 57 58 V Restricted local motion Greater local motion Free volume Tg T Brittle glass Melt, tough polymer or other 59 60
The amorphous state - *Rheology Limited molecular motion : very short range vibration and rotation B. Change of physical properties at T g. a. Specific volume : increase free volume above T g. Segmental motion (conformational freedom) b. Enthalpy( H) change : kinetic energy of segmental motion. c. Refractive index : change of density. Above T m Below T g d. Modulus : glass rubber e. Heat conductivity : free volume. *Rheology : Science of deformation and flow 61 62 Factors affecting T g of polymers Factors affecting T g : 1. MW 1. Molecular weight 2. Chain flexibility 3. Substituents a. Steric effect b. Flexibility (plasticizing effect) c. Chain length (entanglement; side chain crystallization) d. Branching 4. Polarity (intermolecular interaction) 5. Crosslinking 6. Crystallization 7. Diluents 8. Stereochemistry 63 Ex) PS T g = 40 C for M n = 3,000 100 C for M n = 300,000 64
Factors affecting T g : 2. Chain flexibility Factors affecting T g : 2. Chain flexibility Flexible chain : low T g Rigid chain : high T g 65 66 Factors affecting T g : 3. Steric effect Factors affecting T g : 3. Steric effect Bulkier substituent Less rotational freedom higher T g 67 68
Factors affecting T g : 3. Steric effect Factors affecting T g : 4. Polarity T g with polarity of pendant group 69 70 Factors affecting T g : 5. Cross-linking Cross-linking chain movement T g Factors affecting T g : 6. Crystallization Crystallization amorphous chain movement T g 71 72
Factors affecting T g : 7. Diluents Diluents free volume (Plasticizing effect) T g Factors affecting T g : 7. Diluents Diluents free volume (Plasticizing effect) T g 73 74 Factors affecting T g : 7. Diluents Factors affecting T g : 8. Stereochemistry Diluents free volume (Plasticizing effect) T g 75 76
Kinetics, Crystallization and Glass Transition Kinetics, Crystallization and Glass Transition Observed behavior depends on: Structure Cooling rate Crystallization Kinetics (ABJK) Amorphous polymers : the more rapidly cooled polymer Glass transition appears at higher temp. (T g ) (ABDG) low molecular weight compound : transition at single temp. (T m ) (ABHI) Amorphous polymers : No discontinuity at T m the change occurs in slope over a range of temp. (~20 o C) (T g ) (ABCEFG) partly crystalline, partly amorphous polymer: crystallization begins at T m. trailing off between C & E cf. low molecular weight comp. The change in slope between EF and FG occurs at T g. HI is the glassy state, and the threshold for its appearance is T g BH is said to be supercooled 77 78 T g and T m Liquid Crystal A. Definition of liquid crystallinity a. Liquids which exhibits anisotropic behavior. b. Molecules are ordered in liquid. c. The ordered regions in the liquid are called mesophases. a state of matter intermediate between liquid and solid. Ex. Gelatin, lipid bilayers of cell membranes B. Two types of liquid crystal molecules. a. Low molecular weight liquid crystals which have been studied since 1960. b. Polymeric liquid crystals which have been studied since 1970s. 79 80