Physical Pharmacy 2 1 KAUSAR AHMAD KULLIYYAH OF PHARMACY, IIUM THERMAL TRANSITION OF POLYMERS

CONTENTS Glass transition temperature Crystalline melting point Factors affecting Tg Factors affecting Tm Thermal methods of polymer analysis Physical Pharmacy 22

THERMAL TRANSITIONS OF POLYMERS Completely amorphous polymer: Tg only Completely Crystalline: TmSemi-crystalline: Both Tg & Tm Physical Pharmacy 23

AMORPHOUS POLYMERS TemperatureLOW MWHIGH MW LOWGlassy solidRubbery: 1. rigid solid – rubber 2. rubber – liquid HIGHViscous fluid Physical Pharmacy 24

SEMICRYSTALLINE POLYMERS Two major types of thermal transitions: Crystalline melting point, Tm, at which the crystalline domains melt. Above this temperature, polymer is liquid. Below this temperature, polymer forms flexible crystalline solid. Tg, glass transition temperature. Below this, polymer exists as hard, rigid glassy solid. Physical Pharmacy 25

FACTORS AFFECTING TG Nature of polymer backbone. C-C and C-O bonds are flexible. Nature of groups directly attached to the backbone (due to repulsion and tendency to form H-bonding). Flexibility enhanced by methylene groups. Flexibility reduced by ring structures phenylene and amide substitution of H atoms by HC groups eg. Tg: PTFE > PE Chain flexibility, interactions & structure Physical Pharmacy 26

…CONT’…FACTORS AFFECTING TG Inclusion of double bond stiffens it but increases flexibility of adjacent bonds. Segmental rotation is affected by other chains in the same region (mainly dipole interaction and hydrogen bonding). Tg: PVC > PE (more polar). Tg: polyvinylidene < PVC (less polar due to symmetrical substitution. Hydrogen bonding also significantly increases Tg. Physical Pharmacy 27

FACTORS AFFECTING TM Same factors affect both Tg & Tm.Usually high Tg & high Tm are found together and vice versa.Asymmetrically structured polymers have higher Tm Eg PVC vs polyvinylidene High polarity and hydrogen bonding increases Tm.Molecular symmetry leads to high Tm. Eg PE has high Tm due to tightly packed crystals although Tg is low due to flexibility of the chain. Physical Pharmacy 28

VALUES OF TG & TM PolymerRepeating UnitTg (˚C)Tm (˚C) Polyethylene-CH 2 CH Natural rubber-CH 2 C(CH 3 )=CHCH Polypropylene-CH 2 CH(CH 3 ) Polyvinylchloride-CH 2 CHCl Polystyrene-CH 2 CHØ Polymethylmethacrylate-CH 2 C(CH 3 )(CO 2 CH 3 ) Physical Pharmacy 29

THERMAL ANALYSIS OF POLYMERS TechniqueQuantity measuredApplications Differential Scanning Calorimetry (DSC) Heat flowReaction kinetics, purity analysis, polymer cures Differential thermal analysis (DTA) Temperature difference Thermal stability, phase diagram Thermogravimetric analysis (TGA) massThermal stability, compositional analysis Physical Pharmacy 210

DIFFERENTIAL SCANNING CALORIMETRY (DSC) Polymer and reference samples are heated in such a way that they are kept at the same (increasing) temperature. When a thermal transition occurs thermal energy is supplied to the polymer or reference. The energy transferred is equivalent to the energy absorbed or evolved in the transition. Physical Pharmacy 211

The resulting plot of differential rate of heating Vs temperature can be studied to measure heat capacity, melt enthalpy and transition temperature. DSC PLOT Physical Pharmacy 212

CHARACTERISTICS OF A DSC CURVE Each substance has a characteristic DSC curve that can be used for qualitative identification. Endotherms generally represent physical changes while exotherms indicate crystallization, polymerization, curing, decomposition etc. The area under the peak (trough) is proportional to the heat evolved or absorbed during the reaction. Physical Pharmacy 213

APPLICATIONS OF THE DSC CURVE % Crystallinity =  Hf,x/  Hf = Heat of fusion of sample/heat of fusion of 100% crystal % Crystallinity Height of peak is proportional to rate of reaction Heat of reaction = Heat absorbed or emitted/No. of moles Rate of reaction Manifests as a change in baseline. A sharp increase is typical of Tg. Change in heat capacity Physical Pharmacy 214

MELTING POINT FROM DSC CURVE Can determine the exact melting point as the temperature at which melting is complete. The actual melting of the polymer takes place at a range of temperature. On-set Mid Physical Pharmacy 215

THERMOGRAVIMETRIC ANALYSIS TGA Measurement of the mass vs temperature.Plotted as mass vs time or mass vs temperature.Characteristics Unique shape for each substance. Temperature at which changes occur. Each plateau indicates stable state & any deviation or changes shows the presence of a reaction. Physical Pharmacy 216

TGA CURVE Physical Pharmacy 217

REFERENCES Aulton, M. E. (1988). Pharmaceutics: The Science of dosage form design. London: Churchill Livingstone. Wise, D. L. (2000). Handbook of Pharmaceutical Controlled Release Technology. New York: Marcel Dekker. Chasin, M & Langer, R (1990). Biodegradable polymers as drug delivery systems. New York: Marcel Dekker. Vyas, S. P & Khar, R. K. (2002). Targeted and controlled drug delivery. New Delhi: CBS. Physical Pharmacy 218