Effect of crystallinity on properties Melting temperature 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 Melting temperature 42 Melting temperature 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 (Tm) - 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. Why? - 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 → Crystallinity! - The enthalpy of fusion is almost always a positive quantity Gf = Hf – Tm Sf = 0 Tm = Hf / Sf → Depends on Enthalpy change and Entropy changes 43 44 Melting temperature Melting temperature Intermolecular Interactions : Enthalpy change Intermolecular Interactions : Enthalpy change More polar → Tm = Hf / Sf 45 Melting temperature Tm = Hf / Sf 46 Melting temperature 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 - 250oC, nylon 6 - 215~220 oC 47 - Nylon 66 has less amount of amorphous region and higher crystallinity than nylon 6 48 Melting temperature Melting temperature Difference in properties Intermolecular Interactions : conformational Entropy change Nylon 6,6 shows Stiffer polymer chains have less entropy changes between ordered crystal state and unordered melt state : → Tm ↑ - Higher thermal properties : the preferred Nylon for temperature performance products. - Higher tensile strength in use. - Excellent abrasion resistance. Tg at 65% RH (oC) Moisture regain (21oC, 65% RH) Water shrinkage (%, 15mins; 98oC) Melting point (oC) Softening point (oC) Nylon 6 60 4.0-4.5 10 215-220 170 Nylon 66 74 3.5-4.0 5.3 250 235 Tm = Hf / Sf 49 Melting temperature 50 Melting temperature Chain flexibility and Entropy change The effect of diluents 51 Diluents(low molecular weight materials) increase the entropy changes between crystal and melt state → Tm ↓ 52 Melting temperature Melting temperature The effect of copolymerization and M.W. Stereoregulararity Low M.W. → number of terminal group ↑ → Tm ↓ Stereoregular chain structure → more crystalline → Entropy change ↓ → Tm ↑ 53 Motions of materials Highly crystalline polymers Amorphous polymers: Polypropylene Poly(methyl methacrylate) Syndiotactic polystyrene Atactic polystyrene Polyamide (Nylon 6, 66) Kevlar와 Nomex Polyketone Polycarbonate Polyisoprene Polybutadiene 54 Movements of materials Low molecular weight materials Free volume When appropriate energy is added, molecules can move through free volume among molecules Glass Transition Temperature (Tg) 55 56 Movement of polymer chains Glass Transition Temperature, Tg Motion in polymers – The dynamics of polymer chains A. Definition of Glass Transition Temperature. Free volume 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 Tg, 20-50 chain atoms are involved in the segmental movement → c. The rubbery state: long-range rotational motion of segments (2050 atoms) → soft and flexible. Remember! Tg has to do with free volume increase 57 Glass Transition Temperature, Tg 58 Glass Transition Temperature, Tg Greater local motion V Restricted local motion Free volume Tg Brittle glass T Melt, tough polymer or “other” 59 60 Glass Transition Temperature, Tg Glass Transition Temperature, Tg The amorphous state - *Rheology Limited molecular motion : very short range vibration and rotation B. Change of physical properties at Tg. a. Specific volume : increase free volume above Tg. Segmental motion (conformational freedom) b. Enthalpy(∆H) change : kinetic energy of segmental motion. c. Refractive index : change of density. d. Modulus : glass → rubber Above Tm e. Heat conductivity : free volume. Below Tg *Rheology : Science of deformation and flow 61 Glass Transition Temperature, Tg 62 Glass Transition Temperature, Tg Factors affecting Tg of polymers Factors affecting Tg : 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 Ex) PS 63 Tg = 40 C for Mn = 3,000 100 C for Mn = 300,000 64 Glass Transition Temperature, Tg Glass Transition Temperature, Tg Factors affecting Tg : 2. Chain flexibility Factors affecting Tg : 2. Chain flexibility Flexible chain : low Tg Rigid chain : high Tg 65 Glass Transition Temperature, Tg 66 Glass Transition Temperature, Tg Factors affecting Tg : 3. Steric effect Factors affecting Tg : 3. Steric effect Bulkier substituent → Less rotational freedom → higher Tg 67 68 Glass Transition Temperature, Tg Glass Transition Temperature, Tg Factors affecting Tg : 3. Steric effect Factors affecting Tg : 4. Polarity Tg ↑ with polarity of pendant group ↑ 69 Glass Transition Temperature, Tg 70 Glass Transition Temperature, Tg Factors affecting Tg : 5. Cross-linking Factors affecting Tg : 6. Crystallization Cross-linking ↑ → chain movement ↓ → Tg ↑ Crystallization↑ → amorphous chain movement ↓ → Tg ↑ 71 72 Glass Transition Temperature, Tg Glass Transition Temperature, Tg Factors affecting Tg : 7. Diluents Factors affecting Tg : 7. Diluents Diluents↑ → free volume ↑ (Plasticizing effect)→ Tg ↓ Diluents↑ → free volume ↑ (Plasticizing effect)→ Tg ↓ 73 Glass Transition Temperature, Tg 74 Glass Transition Temperature, Tg Factors affecting Tg : 7. Diluents Factors affecting Tg : 8. Stereochemistry Diluents↑ → free volume ↑ (Plasticizing effect)→ Tg ↓ 75 76 Kinetics, Crystallization and Glass Transition Kinetics, Crystallization and Glass Transition (ABJK) Amorphous polymers : the more rapidly cooled polymer Observed behavior depends on: Glass transition appears at higher temp. (Tg’) • Structure • Cooling rate • Crystallization Kinetics (ABDG) low molecular weight compound : transition at single temp. (Tm) (ABCEFG) partly crystalline, partly amorphous polymer: crystallization begins at Tm. trailing off between C & E cf. low molecular weight comp. (ABHI) Amorphous polymers : No discontinuity at Tm The change in slope between EF and FG occurs at Tg. the change occurs in slope over a range of temp. (~20oC) (Tg) HI is the glassy state, and the threshold for its appearance is Tg BH is said to be supercooled 77 Tg and Tm 78 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
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