Polymeric Liquid Crystals- macromesogens

1. Polymeric Liquid Crystals-macromesogens CHM3T1 Lecture- 4 M. Manickam School of Chemistry The University of Birmingham M.Manickam@bham.ac.uk

2. Out line of This Lecture • Introduction • Structure-Property Relations • Synthesis of PLCs Strategies and Methods • Application PLCs • Final comments

3. Learning Objectives After completing this lecture you should have an understanding of and be able to demonstrate, the following terms, ideas and methods. • What are polymers? • The different types of polymerization reactions. • The different types of liquid crystal polymers. • The importance of structure-property relationship in polymers. • Synthesis of liquid crystal polymers. • Application of liquid crystal polymers.

4. What are Polymers? Polymers are substances containing a large number of structural units joined by the same type of linkage. These substances often form into a chain-like structure. Polymers in the natural world have been around since the beginning of time. Starch, cellulose, and rubber all possess polymeric properties. Man-made polymers have been studied since 1832. Today, the polymer industry has grown to be larger than the aluminium, copper and steel industries combined

5. Application of Polymers Polymers already have a range of applications that far exceeds that of any other class of materials available to man. Current application extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics. • Agriculture and Agribusiness • Medicine and Consumer Science • Industry and Sports

6. Polymerization Reactions • The chemical reaction in which high molecular mass molecules • are formed from monomers is known as polymerization. • There are two basic types of polymerization, • Chain-reaction (or addition) and • step-reaction (or condensation) polymerization.

7. Polymerization Reactions Chain-Reaction Polymerization One of the most common types of polymer reactions is chain-reaction (addition) polymerization. This type of polymerization is a three step process involving two chemical entities. The first, known simply as a monomer, can be regarded as one link in a polymer chain. It initially exists as simple units. In nearly all cases, the monomers have at least one carbon-carbon double bond. Ethylene is one example of a monomer used to make a common polymer. Ethylene

8. Chain-Reaction Polymerization The other chemical reactant is a catalyst. In chain-reaction polymerization, the catalyst can be a free-radical peroxide added in relatively low concentrations. A free-radical is a chemical component that contains a free electron that forms a covalent bond with an electron on another molecule. The formation of a free radical form an organic peroxide is shown below: With (.) representing the free electron In this chemical reaction, two free radicals have been formed from the one molecule of R2O2. Now that all the chemical components have been identified, we can begin to look at the polymerization process.

9. Step 1: Initiation The first step in the chain-reaction polymerization process, initiation, occurs when the free-radical catalyst reacts with a double bonded carbon monomer, beginning the polymer chain. The double bond breaks apart, the monomer bonds to the free radical, and the free electron is transferred to the outside carbon atom in this reaction. Polymer chain

10. Step 2: Propagation The next step in the process, propagation, is a repetitive operation in which the physical chain of the polymer is formed. The double bond of successive monomers is opened up when the monomer is reacted to the reactive polymer chain. The free electron is successively passed down the line of the chain to the outside carbon atom. This reaction is able to occur continuously because the energy in the chemical system is lowered as the chain grows. Thermodynamically speaking, the sum of the energies of the polymer is less than the sum of the energies of the individual monomers. Simply put, the single bonds in the polymeric chain are more stable than the double bonds of the monomer. Propagating Polymer chain monomer New polymer chain

11. Step 3: Termination Termination occurs when another free radical (R-O. ), left over from the original splitting of the organic peroxide, meets the end of the growing chain. This free-radical terminates the chain by linking with the last CH2. component of the polymer chain. This reaction produces a complete polymer chain. Termination can also occur when two unfinished chains bond together. Both termination types are below. Other types of termination are also possible. Leftover free radical Propagating Completed polymer chain polymer chains Propagating Completed polymer chain

12. Examples: Polymerisation Addition Polymers polymerisation Ethane gas Poly(ethylene), Solid polymerisation Methylacrylate ester Poly(methy methacrylate)

13. Second Type: Step-Reaction Polymerization Step-reaction (condensation) polymerization is another common type of polymerization. This polymerization method typically produces polymers of lower molecular weight than chain reaction and requires higher temperatures to occur. Unlike addition polymerization, step-wise reactions involve two different types of difunctional monomers or end group that react with one another, forming a chain. Condensation polymerization also produces a small molecular by-product (water, HCl etc.). Below is an example of the formation of Nylon 66, a common polymeric clothing material, involving one each of two monomers, hexamethylene diamine and adipic acid, reacting to form a dimer of Nylon 66.

14. Step-Reaction Polymerisation: Example: Nylon Condensation Reaction Hexamethylene diamine Adipic acid loss of water polymerisation Nylon 66 This polymer is known as nylon 66 because of the six carbon atoms in both the hexamethylene diamine and the adipic acid.

15. Example: Dacron or Terylene Condensation Reaction Polymerisation Loss of water Dacron or Terylene

16. Degree of Polymerization The polymerization process rarely creates polymer molecules all of which have the same number of monomers. Therefore, any sample of the polymer materials contains polymer molecules made from different numbers of monomers. To describe a polymer sample, we must state the average number of monomers in a polymer molecule (called the degree of polymerization) and state by how much the majority of the polymer molecules differ from this average number.

17. Copolymer Polymers can also be made from a chemical reaction in a mixture of two types of monomers. The result of this process is called a copolymer. If the two types of monomers (M and m) combine at random to form the polymer, a random copolymer result ( MmMMmMmmmMmMM). If the two monomers form short sequences of one type first( MMMM or mmmmm), which then combine to form the final polymer (MMMMmmmmMMMMMmmmm), a block copolymer result. Finally, if short sequence of one monomer (mmmmm) are attached as side chains to a very long sequence of the other monomer (MMMMMMMMM), a graft copolymer is formed.

18. Main Chain Liquid Crystal Polymers (MCLCPs) • Basically, there are two types of liquid crystal polymers; • Main chain liquid crystal polymers (MCLCPs) • 2. Side chain liquid crystal polymers (SCLCPs) • MCLCPs consist of repeating mesogenic (liquid crystal like) monomer • units (see below). • The monomer unit must be aniostropic and bifunctional (one function at each end) • to enable polymeristaion and the generation of mesophases. • For example, one end of a long, lath-like mesogenic unit might be a carboxylic acid • and other end might be an amine; condensation would sequentially link the • mesogenic unit together to give a liquid crystalline poly(amide) Mesogenic unit Linking unit A general template for main chain liquid crystal polymers

19. Examples of Main Chain Polymers g 65 N 135 I MCLCPs have repeating mesogenic units Flexible alternating hydrocarbon spacers Racemic form Discotic cores of polymer are separated by long flexible chains which again give the polymer a sufficiently low melting point for mesogenic behaviour. In this case, as is common in discotic systems, a hexagonal columnar mesophase is exhibited (confirmed by X-ray) The M.Wt of polymer 24,000. C 98 Dh 118 I

20. Side Chain Liquid Crystal Polymers (SCLCPs) Terminally Attached Laterally Attached Polymer backbone Spacer unit Several methylene units, with ester or ether (for attachment) Discotic mesogenic unit Calamitic mesogenic unit A general template for side chain liquid crystal polymers

21. Third Class: Combined Liquid Crystal Polymers Third class of liquid crystal polymers is called combined liquid crystal polymers These polymers, combine the features of MCLCPs and SCLCPs. Linking unit Main chain mesogenic uints spacer Side chain mesogenic uints Figure - B Figure- A A general template for combined liquid crystal polymers Side chain mesogenic units can be attached, via a spacer unit, to a mesogenic main chain either at the linking unit Figure - A or at the mesogenic unit Figure- B

22. Types of Side Chain Liquid Crystals Polymers Linking units backbone Homopolymers Side chain copolymers Spacer unit Mesogenic unit BB (backbone) e.g., siloxanes, Acrylates Methylacrylates Ethylenes Epoxides BackBone copolymers SC/BB copolymers A range of different types of SCLCPs

23. Mesogenic Unit on Mesomorphic Behaviour A template structure for possible mesogenic side chain units Typical template for some possible mesogenic units commonlyemployed in SCLCPs (m and n areusually one or two)

24. Flexible Spacers used in SCLCPs Effect of spacer length on mesomorphic behaviour The influence of the flexible spacer that is normally essential for the generation of mesophases in SCLCP is of great interest. In general, the increased ordering generated on polymerisation means that smectic phases predominate and the nematic phase is only exhibited by polymers with a short spacer and a short terminal chain.

25. Influence of Spacer Length on Mesomorphic Properties Methacrylate polymers Where the polymers without spacer units exhibit liquid crystalline phases, they are of the smectic type (a); however, a short spacer usually generates a nematic phase (b) Which gives way to the smectic phases as the spacer length increases (c and d )

26. Influence of Terminal Chain on Mesomorphic Properties R = terminal chains n = spacer Acrylate polymers

27. Mesogenic Side Chain Units Cyanobiphenyl units have commonly been incorporated into SCLCP polymers in order to generate polymers with a +ve dielectric anisotropy. Polymers 1-3 differ only in the unit which links the spacer to the mesogenic unit. Polymer 1 has a particularly high clearing point because of the enhanced polarisability, whereas Removal of the ether oxygen in polymer 2 has reduced the clearing point. The clearing point recovers by the use of an ester linkage 3 but not to the level of polymer 1 because of the kink in the structure. Glass transition temperature (Tg) relates to the polarity of the connecting unit, highest for the polar ester unit 3 and lowest for the hydrocarbon unit 2 1 g 40 SA 121 I 2 g 30 SA 81 I 3 g 45 SA 93 I

28. Length of Mesogenic Unit on Mesomorphic Properties The increased polarisability and increased molecular length in going from two to four phenyl rings considerably enhances the clearing points of these nematic polymers. The nematic phase is probably exhibited in preference to the smectic phase because the spacer and terminal chain lengths are short. Polymer become more crystalline as the mesogen length increases; again this is expected.

29. Polymer Backbone on Mesomorphic Behaviour Common, non- mesogenic polymers Natural rubber: cis-2- Methylbuta-1,3-diene Super glue: methyl α- cyanoacrylate alkenes Methyl group and X could be the point of mesogenic unit attachment Poly(nitriles) Poly(phosphazenes) Unusual polymer backbones that been used in SCLCPs

30. Polymer Backbone on Mesomorphic Behaviour Nylon 6,6: Composed of hexamethenediamine and adipic acid Natural rubber: cis-2- Methylbuta-1,3-diene alkenes Super glue: methyl α- cyano acrylate Common, non- mesogenic polymers

31. Backbone Flexibility on Mesomorphic Properties 1 2 3 The backbone flexibility dominates for three polymers (1-3) with identical mesogenic side chains but with methacrylate, acrylate and siloxane backbones, repectively. Here Tg and TN-I values fall with increasing backbone flexibility.

32. Synthetic Routes to Polymeric Mesogens The nature of liquid crystals polymers means that there are two aspects to the synthesis • Firstly, conventional synthesis to provide the monomer units. • Secondly, the polymerisation reaction that yield the desired liquid crystals polymers

33. Kevlar: Nematic phase diamine Dicarboxylic acid heat Kevlar Kevlar exhibits a namatic phase when dissolved in sulfuric acid, and extrusion in the nematic phase provides the great strength. It is well-known polymer material that is extremely strong and is used in bullet-proof vests in construction.

34. Main Chain Liquid Crystals Polymer New ester Dimethyl terephthalate transesterification 2000C 2800C Ethylene glycol Poly (ethylene terephthalate) transesterification heat 4-hydroxybenzoic acid 4-hydroxybenzoic acid units randomly within the new polymer chain to generate a MCLCPS This polymer prepared by transesterification

35. Siloxane Backbone Based LCP Siloxane backbone Alkenic moiety polysiloxanes

36. Final Comments LCPs have been the subjects of much research since their realisation nearly twenty years ago. However, no commercial application has yet been found for the more commonly encountered side chain liquid crystal polymers. However, the combination of polymeric and liquid crystal properties is very special and further research is required to exploit fully LCPs in commercially viable new technologies. MCLCPs have found application in high strength plastics for use in construction. Plastics owe their strength to the orientation of the polymer chains during the extrusion process. Polymers in a LC phase have inherently ordered chains. Accordingly, when extruded in the LC phase, polymers with extremely high strength are generated. For example, Kevlar is produced from a lyotropic liquid crystal polymer that is extremely strong and is used in many items, such as bullet-proof vests, mooring cables and car body panels. Further research into MCLCPs will provide designer polymers for a wide range of applications.