THERMAL TECHNIQUES- DSC, TGA

THERMAL TECHNIQUES THE DEVELOPING TECHNIQUES SEEING NOWDAYS INCLUDING TGA, DSC, GLASS TRANSITION, FACTORS AFFECTING GLASS TRANSITION, TEMPERATURE

1. thermal techniques 1

2. introduction The term thermal analysis incorporates those techniques in which some physical parameter of the system is determined and/or recorded as a function of temperature . Thermal analysis has been used to determine the physical and chemical properties of polymers, electronic circuit board, geological materials and coals . Differential scanning calorimetry (DSC) is one of the thermo-analytical techniques . A calorimeter measures the heat into or out of a sample. A differential calorimeter measures the heat of sample relative to a reference. 2

3. DSC is a technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. Only a few mg of material are required to run the analysis.DSC is the most often used thermal analysis method, primarily because of its speed, simplicity, and availability . 3

4. Principle When a sample undergoes a physical transformation such as a phase transition , more or less heat will need to flow to it than to the reference , to maintain both at the same temp. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. For e.g.as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temp. At the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes. (Such as crystallization) less heat is required to raise the sample temp. By observing the difference in heat flow between the sample and reference, DSC is able to measure the amount of heat absorbs or release during such transition . 4

5. Thermal transition of polymer Completely amorphous polymer: Tg only Completely crystalline ; Tm Semi crystalline : both Tg and Tm 5

6. AMORPHOUS POLYMER Temperature Low molecular weight High MW LOW Glassy Solid Rubbery: Rigid solid-rubber HIGH Viscous fluid 6

7. Semi crystalline polymer Two major types of thermal transition: 1.Crystalline mp, Tm , at which The crystalline domains melt. Above this temperature polymer is liquid. Below this temperature, polymer forms flexible solid. 2.Tg glass transition temperature. Below this, polymer exists as hard, rigid glassy solid. 7

8. Specific volume–temperature curves for a semicrystalline polymer. (A) Liquid region; (B) viscous liquid with some elastic response; (C) rubbery region; (D) glassy region; (E) crystallites in a rubbery matrix; (F) crystallites in a glassy matrix The Glass Transition 8

9. Relative thermal responses of simple molecules (a) and polymers (b). 9

10. Factors Affecting the Glass Transition Temperature 1. Chain Flexibility Long-chain aliphatic groups — ether and ester linkages — enhance chain flexibility, while rigid groups like cyclic structures stiffen the backbone. 10

11. Bulky side groups that are stiff and close to the backbone cause steric hindrance, decrease chain mobility, and hence raise Tg 11

12. 2. Geometric Factors • Polymers that have symmetrical structure have lower temp Tg than those with asymmetric structures. • The additional groups can only be accommodated in a conformation with a “loose” structure. The increased free volume results in a lower Tg . 12

13. Another geometric factor affecting Tg is cis –trans configuration. Double bonds in the cis form reduce the energy barrier for rotation of adjacent bonds, “soften” the chain, and hence reduce Tg . 13

14. 3.Interchain Attractive Forces • The presence of strong intermolecular bonds in a polymer chain will significantly increase Tg . • The steric effects of the pendant groups in series (CH3, – Cl , and –CN) their polarity increases. Consequently, Tg is increased in the order shown in the table 14

15. 4.Copolymerization. Copolymerization is a process where in unlike molecules join together in random sequences or alternating sequences. 15

16. 5. plasticizers A plasticizer is a liquid that is added to a material (usually a elastomer )making that material softer, more flexible (by decreasing the glass-transition temperature Tg of the polymer), and easier to process . The amount of plasticizer added to a polymer varies depending on the effect required. A small addition of plasticizer may be made to improve the workability of the polymer melt. This contrasts with large additions made with the specific intention of completely transforming the properties of the product. 16

17. THERMAL TRANSITIONS IN POLYMERS Figure shows the variation of Tm for a homologous series of various types of polymers. With polyethylene as a reference, we observe that: • The melting points approach that of polyethylene as the spacing between polar groups increases. • For the same number of chain atoms in the repeat unit, polyureas , polyamides, and polyurethanes have higher melting points than polyethylene, while polyesters have lower The Melting Temperature 1.Intermolecular bonding 17

18. 2.Chain Flexibility Polymers with rigid chains would be expected to have higher melting points than those with flexible molecules. This is because, on melting, polymers with stiff backbones have lower conformational entropy changes than those with flexible backbones. Chain flexibility is enhanced by the presence of such groups as –O– and –(CO·O)– and by increasing the length of (–CH2–) units in the main chain. Insertion of polar groups and rings restricts the rotation of the backbone and consequently reduces conformational changes of the backbone, as illustrated by the following polymers 18

19. 19

20. instrumentation There are four different types of DSC instrument • Heat flux DSC • Power compensated DSC • Modulated DSC • Hyper DSC 20

21. 1.Heat flux DSC In heat flux DSC, the difference in heat flow into the sample and reference is measured while the sample temperature is changed at the constant rate . The main assembly of the DSC cell is enclosed in a cylindrical silver heating block, which dissipates heat to the specimens via a chromel disc which is attached to the silver block. 21

22. The disk has two raised platforms on which the sample and reference pans are placed. A chromel disk and connecting wire are attached to the underside of each platform, and the resulting chromel-alumel and thermocouples are used to determine the differential temperatures of interest. Alumel wires attached to the chromel discs provide the chromel-alumel junctions for independently measuring the sample and reference temperature. A separate thermocouple embedded in the silver block serves a temperature controller for the programmed heating cycle. And inert gas is passed through the cell at a constant flow rate of about 40 ml/min. 22

23. 2.Power compensation DSC In power compensation DSC, the temperatures of the sample and reference are kept equal to each other while both temperatures are increased or decreased linearly. In power compensation DSC two heating units are used. These heating units are quite small, allowing for rapid rates of heating, cooling and equilibrium. The heating units are embedded in a large temperature controlled heat sink. Figure : power compensation DSC 23

24. The sample and reference holders have platinum resistance thermometers to continuously monitor the temperature of the materials. The instrument records the power difference needed to maintain the sample and reference at the same temperature. Power compensated DSC has lower sensitivity than heat flux DSC, but its response time is more rapid. This makes power compensated DSC well suited for kinetics studies in which fast equilibrations to new temperature settings are needed. It is also capable of higher resolution than heat flux DSC . 24

25. There is direct measurements of radiation flow occur under a light source. This way the degradation of material can also be observed. Temperature range of measurement is up to 400 °C with time constant of only 1.5 s or lower. Sample masses are around 20 mg. Crucibles of different volumes are made mostly of aluminium . 25

26. 3.Modulated DSC Modulated DSC uses the same heating and cell arrangement as the heat – flux DSC method. it is a new technique introduced in 1993 . The main advantage of this technique is the separation of overlapping events in the DSC scans. The DSC signal is split into two components: reflecting no reversible events and reversible events. MDSC is a valuable extension of conventional DSC. Its applicability is recognized for precise determination of the temperature of glass transition. It has been applied for the determination of glass transition of Hydroxypropylmethylcellulose films and for the study of amorphous lactose as well as some glassy drugs . 26

27. 4. Hyper DSC This is high resolution of DSC or new type of power compensating DSC provides the best results for an analysis of melting and crystallisation of metals or detection of glass transition temperature ( Tg ) in medications. Their ability to perform valid heat flow measurements with fast linear controlled rates (up to 500 K/min) especially by cooling. The benefits of such devices are increased sensitivity at higher rates . It has a great sensitivity also at a heating rate of 500 K/min with 1 mg of sample material. This technique is especially proper for the pharmaceutics industry for testing medicaments at different temperatures where fast heating rates are necessary to avoid other unwanted reactions etc . 27