Basic principle of polymer materials and properties

This document dives into the world of polymers, those supersized molecules made by linking up smaller building blocks called monomers. We'll explore different types of polymers, how they're categorized, their unique properties, and the cool things we can do with them. Polymers come in two main flavors: homopolymers and copolymers. Homopolymers are like a team wearing the same uniform - they're built from just one type of monomer. Copolymers, on the other hand, are a mix-and-match bunch, containing multiple different monomers in their structure. Finally, we'll look at some of the basic mechanical properties of polymers and how these properties can be influenced by the size and structure of the building blocks themselves.

1. BASIC PRINCIPLES OF MEMBRANE TECHNOLOGY CHAPTER-2 SURIYA MARIMUTHU

2. Polymers. Polymers are high molecular weight components built up from a number of basic units, the monomers. The number of structural units linked together to form the 'long chain molecule' is defined as the degree of polymerisation . Polymerisation of ethene to polyethylene .

3. Schematic representation of various copolymers Homopolymer- All the repeating units, the segments, are the same in polyethylene. Copolymer- The repeating units are different.

4. Stereoisomerism Vinyl polymers are characterised by -CH2-CHR-- repeating units, where the side group -R is different for different polymers. The side group R can be attached to the carbon atom in two different ways the so called D and L form. Three different arrangements may be distinguished in the polymer.

5. Chain flexibility Determined by two factors: i) the character of the main chain. ii) the presence and nature of the side chains or side groups. In many polymers (e.g. vinyl polymers) the main chain consists entirely of -c-c- bonds. A further class of polymer does not contain carbon atoms in the main chain; such polymers are called inorganic polymers. Chain flexibility is also determined by the character of the side groups, which determine to some extent whether rotation around the main chain can take place readily or whether steric hindrance occurs.

6. Molecular weight The chain length is an important parameter in determining the properties of a polymer. Polymers generally consist of a large number of chains and these do not necessarily have the same chain length. Hence there is a distribution in molecular weight.

7. Schematic drawing of an entanglement. In linear and branched polymers only secondary interaction forces act between the different chains, whereas in network polymers the various chains are bound to each other covalently. Three different types of secondary force can be considered: Dipole forces (Debye forces). Dispersion forces (or London forces). Hydrogen bonding forces.

8. State of the polymer The state of the polymer is very important relative to its mechanical, chemical, thermal and permeation properties. The state of a polymer is defined as the phase in which the polymer appears. Compared to low molecular weight compounds this is more complex with polymers. Tensile modulus E as a function of the temperature for an amorphous polymer . Glass transition temperature ( T g ) The temperature at which transition from the glassy to the rubbery state occurs.

9. Effect of polymeric structure on Tg The physical properties of a polymer are determined to a large extent by the chemical structure . The thermal motion of the polymer chains is dependent on the ability to rotate around the main chain. This is mainly determined by two factors: i) chain flexibility ii) chain interaction Glass transition temperature for various vinyl polymers containing different side groups.

10. In addition to the glass transition temperature, another important parameter, the degree of crystallinity, also determines the state of the polymer. Some polymers have very regular structural units and are therefore able to crystallise because the chains can be packed in a regular pattern. Atactic vinyl polymers are generally too irregular to allow crystallisation . Some polymers are not completely crystalline, the degree of crystallinity being far less than 100% . These polymers are called semi-crystalline and consist of an amorphous and a crystalline fraction. Two types of crystallites often found are the 'fringed micelles' and the spherulites. Cont …..

11. Schematic drawing of two types of crystallites Crystallites have a large influence not only on the mechanical properties of a polymer but also on its transport properties. In semi-crystalline polymers the glassy phase exhibits the same mechanical properties as for a completely amorphous polymer. However, in the rubbery state the mechanical properties will depend on the crystalline content of the polymer.

12. Tensile modulus of a semi-crystalline polymer as a function of the temperature. Permeabilities of nitrogen and oxygen in various polymers

13. chemical structures of polytrimethylsilylpropyne (PTMSP) and polyvinyltrimethylsilane (PVTMS). The highest permeability is found for polytrimethylsilylpropyne (PTMSP), a glassy polymer. Another glassy polymer, polyvinyltrimethylsilane (PVTMS), also shows a very high permeability The glass transition temperatures of nylon-6 and cellulose acetate are little different but because of its much higher crystallinity the permeability of nylon-6 is lower. Poly(vinyl alcohol) (PVA) also has a very low permeability because of its high crystallinity.

14. Thermal and chemical stability Ceramics have become of increasing interest as membrane materials because of their outstanding thermal and chemical stability in comparison to polymers. The definition of thermal and chemical stability is not exact and a distinguish should be made between In general, the following factors which lead to an increase in the thermal stability also increase the chemical stability: Change or loss of properties which is a reversible process and often referred to as softening. Decomposition or degradation which is an irreversible process. Those that increase T g and T m Those that increase the crystallinity.

15. Resonance structures in polyimide, polyoxadiazole and polybenzimidazole.

16. Number of thermally and chemically stable polymers

17. Mechanical properties Mechanical behaviour involves the deformation of a material under the influence of an applied force. Generally, mechanical properties are not very important in membrane processes because the membrane is held by a supporting material. However, hollow fibers and capillary membranes are self-supporting and in these cases the mechanical properties may become important, especially when high pressures are applied such as in gas separation. Generalized stress-strain diagram

18. Elastomers Thermoplastic elastomers The thermoplastic elastomers (TPE) are a very special class of materials which are characterised by the fact that the two blocks are not miscible with each other which results in phase separation in which one block constitutes the continuous phase whereas the other block exist as micro-domains within this continuous phase.

19. Polyelectrolytes This class of polymer, the polyelectrolytes, which contain ionic groups. Because of the presence of fixed charges strong interactions exist in such polymers and counterions are attracted to the fixed charges. Polyelectrolytes that contain a fIxed negatively charged group are called cation-exchange membranes because they are capable of exchanging positively charged counterions. When the fixed charged group is positive, the membrane (or polymer) can exchange negatively charged anions; such membranes are called anion-exchange membranes.

20. Membrane polymers A classification will be made between the open porous membranes , which are applied in microfiltration and ultrafiltration and the dense nonporous membranes , applied in gas separation and pervaporation. Porous membranes Porous membranes contain fixed pores, in the range of 0.1 - 10 µm for microfiltration and 2 - 100 nm for ultrafiltration.

21. Chemical structure of cellulose Chemical structure of an aliphatic polyamide (Nylon-6) Chemical structure of anaromatic polyamide (Nomex)

22. Cont ….. Chemical structure of polyetheretherketone (PEEK) and polyetherketone (PEK) Chemical structures of poly(ether sulfone) (PES)

23. Nonporous membranes Nonporous membranes are used in gas and vapour separation and pervaporation. For these processes either composite or asymmetric membranes are used. In this type of membrane the performance (permeability and selectivity) is determined by the intrinsic properties of the material. An example is the class of polyoxadiazoles and polytriazoles .

24. Inorganic membranes Four different types of inorganic materials frequently used may be distinguished: Ceramic membranes. Glass membranes. Metallic membranes (including carbon). Zeolitic membranes. Thermal stability- ceramics These polymers can be applied over temperatures ranging from 100 - 300°C. The valence electrons of the metal part are retained by the nonmetal atoms resulting in a highly stable bond and consequently these materials are highly thermally and chemically resistant. The melting points are very high and can reach values above 4000 o C.

25. Biological membranes Biological membranes or cell membranes have very complex structures because they must be able to accomplish specific functions. However, a characteristic of various cell membranes is that they contain a basic lipid bilayer structure. Schematic drawing of a lipid bilayer General chemical structure of a phospholipid The cell membrane consists of two main components: The lipid bilayer which is the backbone. The proteins take care of the specific transport functions.

26. Two types of carrier-mediated transport can occur Active transport Passive transport Three different types of passive carrier-mediated transport mechanism may be distinguished similar to transport in liquid membranes Facilitated diffusion Co-transport Counter-transport. The Simplest type of carrier-mediated transport is 'diffusion' or 'facilitated diffusion', because the protein carrier allows the solute to diffuse through the membrane. The second type of carrier-mediated transport is 'cotransport’ . The third type of carrier-mediated transport is 'counter transport'.

27. Schematic representation of Na/K transport Schematic representation of a multilamellar vesicle (MLV) or liposome and an unilamellar vesicle (ULV)