Polymer Structures and Properties

1. Polymer Research Center Institute of Applied Chemistry of NCTU Polymer Structures and Properties Dr. Ying-Chieh Yen嚴英傑 Ref: INTRODUCTION TO POLYMERS 2nd edition, R. J. Young and P. A. Lovell.

2. Polymer Structures Polymer Research Center Institute of Applied Chemistry of NCTU

3. Polymer Research Center Institute of Applied Chemistry of NCTU Basic Definitions and Nomenclature • A polymer is a substance composed of molecules which have long sequences of one or more species of atoms or groups of atoms linked to each other by covalent bonds. • The words, polymers and macromolecules are used interchangeably, the latter strictly defines molecules of which the former is composed.

4. Polymer Research Center Institute of Applied Chemistry of NCTU Basic Definitions and Nomenclature • Macromolecules are formed by linking together monomer molecules through chemical reactions, the process by which this is achieved being known as polymerization.

5. Polymer Research Center Institute of Applied Chemistry of NCTU Molecular Mass and Polydispersity • Number average molecular mass ( ): • Weight average molecular mass ( ): • The ratio must be greater than unity for a polydisperse polymer and is known as the polydispersity or heterogeneity index. • A perfectly monodisperse polymer would have polydispersity = 1.00.

6. Polymer Research Center Institute of Applied Chemistry of NCTU Classification of Polymers

7. Polymer Research Center Institute of Applied Chemistry of NCTU Classification of Polymers • Thermoplastics: It often referred to just as plastics (linear or branched polymers) which can be meltedupon the application of heat. • Crystalline: Those which do crystallize invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. (Tm) • Amorphous: Many thermoplastics are completely amorphous and incapable of crystallization. (Tg) At the temperature, thermoplastics transform abruptly from the glass state (hard) to the rubbery state (soft).

8. Polymer Research Center Institute of Applied Chemistry of NCTU Classification of Polymers • Elastomers are rubbery polymers (i.e. rubbery networks) which can be stretched easily to high extensions (e.g. 3x to 10x their original dimensions) and which rapidly recover their original dimensions when the applied stress is released. • Thermosets normally are rigid materials and are network polymers in which chain motion is greatly restricted by a high degree of crosslinking.

9. Polymer Research Center Institute of Applied Chemistry of NCTU Skeletal Structures • Linear structure: A chain with two ends. • Non-linear structures: • Branched structure: Side chains, or branches, of significant length bonded to the main chain at branch points (junction points).

10. Polymer Research Center Institute of Applied Chemistry of NCTU Skeletal Structures • Non-linear structures: • Network structure (crosslinked): Polymers have three-dimensional structures in which each chain is connected to all others by a sequence of junction points and other chains.

11. Polymer Research Center Institute of Applied Chemistry of NCTU Homopolymers • The formal definition of a homopolymer is a polymer derived from one species of monomer. • However, it often is used more broadly to describe polymers whose structure can be represented by multiple repetition of a single type of repeat unit.

12. Polymer Research Center Institute of Applied Chemistry of NCTU Some Common Homopolymers

13. Polymer Research Center Institute of Applied Chemistry of NCTU Some Common Homopolymers

14. Polymer Research Center Institute of Applied Chemistry of NCTU Some Common Homopolymers

15. Polymer Research Center Institute of Applied Chemistry of NCTU Tacticity • For polymers prepared from monomers of the general structure CH2=CXY, where X and Y are two different substituent groups, there are two distinct configurational arrangements of the repeat unit.

16. Polymer Research Center Institute of Applied Chemistry of NCTU Tacticity • Isotactic: • Syndiotactic: • Atactic:

17. Polymer Research Center Institute of Applied Chemistry of NCTU Copolymers • The formal definition of a copolymer is a polymer derived from more than one species of monomer. • However, it often is used more broadly to describe polymers whose molecules contain two or more different types of repeat unit.

18. Polymer Research Center Institute of Applied Chemistry of NCTU Copolymers • Statistical copolymers: The sequential distribution of the repeat unit obeys the statistical laws. (Markovian) • Random copolymers: A special type of statistical copolymer in which the distribution of repeat units is truly random. (Older textbooks and scientific papers often use the term random copolymer to describe both random and non-random statistical copolymers.)

19. Polymer Research Center Institute of Applied Chemistry of NCTU Copolymers • Alternating copolymers: Only two different types of repeat units are arranged alternately along the polymer chain. • Block copolymers: Linear copolymers with repeat units existing only in long sequences or blocks.

20. Polymer Research Center Institute of Applied Chemistry of NCTU Copolymers • Graft copolymers: Branched polymers with the branches having different chemical structure to that of the main chain.

21. Polymer Properties Polymer Research Center Institute of Applied Chemistry of NCTU

22. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • If the melt of a non-crystallizable polymer is cooled it becomes more viscous and flows less readily. If the temperature is reduced low enough it becomes rubbery and then as the temperature is reduced further it becomes a relatively hard and elastic polymer glass. • The temperature at which the polymer undergoes the transformation from a rubber to a glass is known as the glass transition temperature, Tg.

23. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • There is a dramatic change in the properties of a polymer at glass transition temperature. For example, there is a sharp increase in the stiffness of an amorphous polymer when its temperature is reduced below Tg. • There are also abrupt changes in other physical properties such as heat capacity and thermal expansion coefficient. • There have been attempts to analyse the glass transition from a thermodynamic viewpoint.

24. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • In the first-order transition there is an abrupt change in a fundamental thermodynamic property such as enthalpy, H or volume, V, whereas in a second-order transition only the first derivative of such properties changes. • This means that during a first-order transition, such as melting, H and V will change abruptly whereas for a second-order transition changes will only be detected in properties such as heat capacity, Cp or volume thermal expansion coefficient, α which are definded as:

25. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • As both of these parameters are found to change abruptly at the glass transition temperature it would appear that it may be possible to consider the glass transition as a second-order thermodynamic transition. • At the glass transition, the molecules which are effectively frozen in position in the polymer glass become free to rotate and translate and so it is not surprising that the value of the Tgwill depend upon the physical and chemical structure of the polymer molecules.

26. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • The most important factor is chain flexibility which is governed by the nature of the chemical groups which constitute the main chain.

27. Polymer Research Center Institute of Applied Chemistry of NCTU The Glass Transition • In vinyl polymers of the type (-CH2-CHX-)n the nature of the side group (bulky and polar groups) has a profound effect upon Tg as can be seen in the table.

28. Polymer Research Center Institute of Applied Chemistry of NCTU Crystallization • Crystallization is the process whereby an ordered structure is produced from a disordered phase, usually a melt or dilute solution, and melting can be thought of as being essentially the opposite of this process. • Features: (a) Polymer crystals are usually thin and lamellar when crystallized from both dilute solution and the melt.

29. Polymer Research Center Institute of Applied Chemistry of NCTU Crystallization (b) The lamellar thickness is related to the crystallization temperature. (c) Chain folding is known to occur during crystallization. (d) The growth rates of polymer crystals are found to be highly dependent upon the crystallization temperature and molar mass of the polymer.

30. Polymer Research Center Institute of Applied Chemistry of NCTU Thermal Degradation Temperature • The thermal degradation temperature was determined from the changes in weight in relation to change in temperature using thermogravimetric analysis (TGA).

31. Polymer Research Center Institute of Applied Chemistry of NCTU Surface Free Energy Surface free energy from work : The reversible work required to create a unit surface area is related to the surface free energy of the material. Units: mJ/m2 = mN/m high surface free energy ↔ strong cohesion surface tension of liquids corresponds to surface energy of solids

32. Polymer Research Center Institute of Applied Chemistry of NCTU γS : apolar component, accounting for Liftshitz-van der Waals type interactions γS : polar component, accounting for acid-base or donor- acceptor type interactions γS+ : Lewis-acid component, electron acceptor γS- : Lewis-base component, electron donor LW AB The Theory of Surface Free Energy Three-Liquid Acid-Base Method

33. Polymer Research Center Institute of Applied Chemistry of NCTU poly(tetrafluoroethylene) PTFE Fluoropolymers and Silicones The low intermolecular forces present in fluorinated polymers have been recognized to account for the relatively low surface free energy. poly(dimethylsiloxane) PDMS polyethylene PE

34. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties • Stress: In consideration of the mechanical properties of polymers we are mainly to interested in effect of applying surface forces such as stress or pressure to the material.

35. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties • Strain: When forces are applied to a material the atoms change position in response to the force and this change is known as strain. • Young’s modulus: E of a material which for simple uniaxial extension or compression is given by E = stress/strain.

36. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties • Viscoelasticity: A distinctive feature of the mechanical behavior of polymers is the way in which their response to an applied stress or strain depends upon the rate or time period of loading. The behavior of most polymers can be though of as being somewhere between that of elastic solids and liquids.

37. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties At low temperatures and high rates of strain they display elastic behavior whereas at high temperatures and low rates of strain they behave in a viscous manner, flowing like a liquid. Polymers are therefore termed viscoelastic as they display aspects of bothviscous and elastic types of behavior.

38. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties • General time-dependent behavior: Creep experiment Relaxation experiment

39. Polymer Research Center Institute of Applied Chemistry of NCTU Mechanical Properties

40. Polymer Research Center Institute of Applied Chemistry of NCTU Stress-Strain Curve Brittle!! Ductile!! Neck occurs!!

41. The EndThanks for Your Attention