Glass transition temperature (tg)

Brief intro about crystalline and amorphous structures,
glass transition temperature,
free volume theory of glass transition temperature,
factors effecting glass transition temperature etc.

1. Glass Transition Temperature ( T g ) Presented by Devansh Gupta M.Sc Polymer Science Semester 2

2. Contents Brief Information About Crystallinity Glass Transition Temperature ( Tg ) Free Volume Theory For Tg Factors Influencing Glass Transition Temperature ( Tg ) Sources 1

3. Crystallinity One of the significant characteristics of polymers is crystallinity , or the degree of structural order in a polymer. When the macromolecular chains of a polymer sample are arranged in an orderly fashion, it is known as a crystalline polymer. When the chains are not arranged in ordered crystals and are disordered, even though they are in solid state, the polymer is identified as amorphous. In most cases, there are no fully crystalline polymers; therefore, we have semi-crystalline polymers, which are composed of both amorphous and crystalline regions. This is why the same sample of a polymer can have both a glass transition temperature and a melting temperature. 2

4. 3 Crystalline Ordered Amorphous Random Semi-crystalline Consists of both Crystallinity Crystalline Region Amorphous Region

5. Glass Transition Temperature Glass transition temperature is a temperature at which the polymer experiences the transition from the glassy state to the rubbery state. Glassy state is hard & brittle state of material which is consist of short-range vibrational & rotational motion of atoms in polymer chain, while Rubbery state is soft & flexible state of material which is a long-range rotational motion of polymer chain segments. 4 Glassy State Hard & Brittle Rubbery State Soft & Flexible T g

6. Some polymers are used above their glass transition temperature, and some are used below. Hard plastics like polystyrene and poly methyl methacrylate are used below their glass transition temperature; that is in their glassy state. Their T g ’s are well above room temperature. Elastomers like polyisoprene and polyisobutylene are used above their T g ’s , that is in the rubbery state, where they are soft & flexible. 5

7. Heating Through T g Leads To Following Break down of Van Der Waals Forces. Onset of large scale molecular motion. Polymer goes from glassy/rigid to rubbery behaviour. Upper service temperature in amorphous polymers. 6

8. Free Volume Theory One of the most useful approaches to analysing the glass transition temperature of polymer is to use the concept of Free Volume. The free volume is the space in a solid or liquid sample that is not occupied by molecules, that is the ‘empty space’ between molecules. Free volume is high in liquid state than solid, so molecular motion is able to take place relatively easy because the unoccupied volume allows the molecules to move. The theory was originally developed for amorphous polymers and the glass-transition in those polymers. 7

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10. But semi-crystalline polymers also consist of amorphous regions, so this theory can also be applied to semi-crystalline polymers. An amorphous polymer can be considered to be made up of occupied volume and free volume. As the temperature is changed, the free volume and the occupied volume both will change. As the temperature of the melt is lowered, the free volume will be reduced until eventually there will not be enough free volume to allow molecular motion or transition to take place. 9

11. 10 T V T g Restricted local motion Greater local motion Free volume Brittle and glassy Soft and Flexible

12. Schematic illustration of the total, free, and occupied volume 11

13. 12 The total sample volume V therefore consists of volume occupied by molecules V and free volume V f such that V= V f +V o At any given temperature, the fraction of the free volume is Around T g and above T g , the fraction of free volume can be expressed as, Where f g is the fraction of free volume at T g and α f is an expansion coefficient for the fraction free volume. α f is approximately α m – α g , or the difference between the thermal expansion coefficients of the polymer above and below T g .  α m stands for melt α g stands for glass Where, the approximation is based on V f << V . 

14. Factors Influencing Glass Transition Temperature From the previous discussion we know that at the glass transition temperature there is a large scale cooperative movement of chain segments. Therefore it is expected that any structural features or externally imposed conditions that influence chain mobility will also affect the value of Tg . 13

15. Some of these factors are shown below. Chain Flexibility & Rigidity Steric Effects  Effect of Intermolecular Forces Copolymerization Cross linking & Crystallinity Plasticizer 14

16. 1. Chain Flexibility & Rigidity As Tg  depends on the ability of a chain to undergo internal rotations, we expect chain flexibility to be associated with low glass transitions. For Example, Poly(dimethyl siloxane ) is an extremely flexible polymer due to the large separation between the methyl substituted silicon atoms. As compared to other polymeric materials, poly(dimethyl siloxane ) has the lowest glass transition temperature ( Tg  = -123.15°C) 15 -93.15°C -67.15°C 89.85°C 79.85°C n

17. As shown in previous slide, polymers that contain −CH2−CH2− sequences and ether linkages in the main-chain have relatively easy internal rotations and therefore low Tg  values. While substitution of ethylene groups with p- phenylene units leads to increased chain rigidity and high glass transition temperature. 16

18. 2. Steric Effects The presence of bulky side groups hinders rotation of the backbone atoms due to steric hindrance, and therefore results in an increase in T g . The magnitude of this effect depends on the size of the side groups. This is illustrated in the following Table for vinyl polymers having the general structure, —[CH 2  — CHX ]— 17 -93.15°C -20.15°C 99.85°C 134.85°C

19. 3. Effect of Intermolecular Forces The presence of polar side groups leads to strong intermolecular attractive interactions between chains which hinders molecular motion thus causing an increase in Glass transition temperature. This effect is illustrated in the following table for the polymers of type −[CH 2 −CHX ]− 18 -20.15°C 80.85°C 84.85°C

20. 4. Copolymerization It is possible to alter the glass transition of a homo polymer by copolymerisation with a second monomer. If the two homo polymers prepared from the monomers have different T g s , then it is reasonable to expect that their random copolymer should have a glass transition which is intermediate between the  T g s of the homo polymers. This is observed experimentally. The glass transition of a random copolymer is related to the  T g s of the homo polymers, T g1  and T g2 , as follows Where w 1  is the weight fraction of homo polymer 1 and w 2 is the weight fraction of homo polymer 2. 19 1/T g  = w 1 /T g1  + w 2 /T g2 *

21. 5. Cross-linking & Crystallinity Both cross-linking and  crystallinity  cause an increase of the glass transition temperature. It is very easy to explain why cross-linking increases T g  since the presence of covalent bonding between chains reduces molecular freedom and thus the free volume. Similarly, the presence of crystalline regions in an semi-crystalline material restricts the mobility of the disordered amorphous regions; thus the glass transition temperature increases which is totally depends on the percentage of crystallinity . 20

22. 6. Plasticizer Sometimes, a polymer has a high  Tg than our requirement. To tackle this proble we just mix something in it called a plasticizer. Plasticizers are small molecules which will get in between the polymer chains, and space them out from each other. Thus the free volume will increase. When this happens they can slide past each other more easily. When they slide past each other more easily, they can move around at lower temperatures than they would without the plasticizer. By this way, the  Tg of a polymer can be lowered, to make a polymer more applicable, and easier to work with. 21

23. Sources Practical Polymer Analysis By T.R. Crompton (595-629) Polymer Chemistry - The Basic Concepts By Paul C. Hiemenz (199) Polymer Physics By ULF W. Gedde (77-95) Text Book Of Polymer Science By Fred W. Billmeyer (320) Polymer Science By V.R. Gowariker (113-130) 22