POLYMERS AND POLYMERIZATION

1. POLYMERS AND POLYMERIZATION By Dr Rashid Hassan Assistant Professor RIHS ISLAMABAD

2. POLYMER • Poly = Many mer = Units I.e. a material made up of many units. Properly defined as: • Chemical compound composed of large organic molecules formed by the union of many repeating small monomer units. • E.g. • Polymethyl methacrylate. • Polyethylene. • UHMWPE.

3. RESINS • A broad term used to indicate organic substances that are usually transparent or translucent and are soluble in either acetone or similar substances, but not in water. • They are named according to their chemical composition, physical structure or means for activation or curing.

4. RESINS • The word "resin" has been applied in the modern world to nearly any component of a liquid that will set into a hardlacquer or enamel-like finish. An example is nail polish, a modern product which contains "resins" that are organic compounds, but not classical plant resins. Certain "casting resins" and synthetic resins (such as epoxy resin) have also been given the name "resin" because they solidify in the same way as (some) plant resins, but synthetic resins are liquid monomers of thermosetting plastics, and do not derive from plants.

5. ROSIN • Rosin, also calledcolophonyorGreek pitch (Pix græca), is a solid form of resinobtainedfrompines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize thevolatile liquid terpene components. It is semi-transparent and varies in color from yellow to black. At room temperature rosin is brittle, but it melts at stove-top temperatures. It chiefly consists of different resin acids, especially abietic acid. • The name, colophony or colophoniaresina, comes from its origin in Colophon, an ancient Ionic city.

6. ROSIN • Uses • A cake of rosin, made for use by violinists, used here for soldering • Rosin is an ingredient in printinginks, photocopying and laser printing paper,varnishes,adhesives (glues),soap, papersizing, soda,solderingfluxes, and sealing wax. • Rosin can be used as a glazingagent in medicinesand chewing gum) • Can be used as an emulsifier in soft drinks. • Inpharmaceuticals, rosin forms an ingredient in several plasters and ointments.

7. APPLICATIONS OF RESINS IN DENTISTRY Used in variety of applications. • Dentures. (bases, liners and artificial teeth) • Cavity filling materials. (composites) • Sealants. • Impression materials. • Equipment. (mixing bowls) • Cements. (resin based)

8. GLASS TRANSITION TEMPERATURE (Tg): • The temperature at which there is a sharp increase in thermal expansion coefficient due to increased molecular mobility is called glass transition temperature. • Interatomic bonds hold the polymers in a polymeric chain. • Valence electrons are in continuous motion. • These movements form different densities along the chains. • Adjacent chains adapt their electron densities to balance the difference in densities. • These interactions causes the formation of weak interatomic forces. • When heated to Tg or a higher temperature the weak bonds are broken and chains can move freely.

9. CLASSIFICATION OF DENTAL RESINS • Synthetic resins are polymers, often called as plastics in layman language. • A plastic material is a substance that is although dimensionally stable in normal use is plastically reshaped at some stage of manufacture. • Based upon the thermal behavior dental polymers are divided into • Thermoplastic polymers. • Thermosetting polymers. • Elastomers.

10. CLASSIFICATION OF DENTAL RESINS • THERMOPLASTIC POLYMERS: • Polymeric materials made from linear and/or branched chains that soften when heated above glass transition temperature (Tg) at which the molecular motion tend to move the chains apart. • Resin can be molded and shaped at this stage and upon cooling will harden in the shape given. • On heating they can be soften again and reshaped. • This cycle can be carried out repeatedly. • They are fusible (they melt) and soluble in organic solvents.

11. CLASSIFICATION OF DENTAL RESINS • THERMOSETTING POLYMERS: • The polymeric material that undergo a chemical change and become permanently hard when heated above the glass transition temperature (Tg). • At this temperature the material begins to polymerize. • These materials do not soften on reheating to the same temperature. • They are usually cross linked polymers. • They are insoluble and infusible. • They have superior abrasion resistance, dimensional stability, impact strength and flexural properties as compared to thermoplastic polymers.

12. CLASSIFICATION OF DENTAL RESINS • ELASTOMERS: • Used as impression materials. • Elastic when set. • Show limited reversible dimensional changes.

13. REQUISITES FOR DENTAL RESINS: • Biological compatibility. • Physical properties. • Manipulation. • Aesthetic qualities. • Low cost. • Chemical stability in mouth.

14. FUNDAMENTAL NATURE OF POLYMERS • Chain length and molecular weight: • Longer the polymeric chain, greater the number of entanglements. • Longer the length, more difficult to distort the material. • Rigidity, strength and melting temperature increase with increase in chain length. • The synthetic resins polymerize randomly from local sites, depending on the ability of the local site the chain grows and the polymeric material may consist of chains that vary in length. Cont………

15. FUNDAMENTAL NATURE OF POLYMERS Chain length and molecular weight: • Thus the average value is needed to express the molecular weight. • Two types of averages are used: • Number average (Mn): Based upon average number of repeating “mer” units in a chain. 2. Weight average (Mw): Based upon the molecular weight of the average chain. Mw/Mn = Polydispersity (measure of range and distribution of chain sizes.

16. FUNDAMENTAL NATURE OF POLYMERS • Chain branching and cross linking: • Ideally polymerization should give a linear macromolecule. • Structurally polymers are often connected together in a non-linear (branched / cross linked) pattern. • Branching refers to extra arms growing out of the polymeric chain and are sites for temporary connections (entanglements) • Cross links are permanent connections.

17. FUNDAMENTAL NATURE OF POLYMERS • TYPES OF POLYMERS: (2 types) • Homopolymers: Polymers having one type of repeating “mer” units. • Copolymers: Polymers with 2 or more types of “mer “ units. • Random copolymer: • Block copolymer: • Graft or branched copolymer. Cont…….

18. FUNDAMENTAL NATURE OF POLYMERS • Random copolymer: No sequential order exists among the two or more mer units along the polymer chain. …. AABBABAAABBBBABAAABBBBABBB……. • Block copolymer: Identical monomer units occur in relatively long sequences along the main polymer. …..AAAAABBBBBBAAAABBBBBBAAABBBB…… • Graft or branched copolymer: Sequences of one type of mer unit are attached as a graft onto the backbone of the second type of mer unit. ……AAAAAAAAAAA……… B B B B B B

19. PHYSICAL PROPERTIES OF POLYMERS DEFORMATION AND RECOVERY: • Applied forces produce stresses within polymer. • Theses stresses result in either elastic strain, plastic strain or a combination of both. • Plastic deformation: Irreversible and results in a new permanent shape. • Elastic deformation: Reversible and recovers completely when the stress is removed. • Viscoelastic deformation: Combination of both elastic and plastic strain but the recovery of only elastic strain occurs when the stress is decreased.

20. SOLVATION PROPERTIES • Polymers are usually slow to dissolve. • Solubility is dependent upon the Mw. • Longer the chain (high mol weight) more slowly polymer will dissolve. • Polymers have the tendency to absorb water and swell rather than dissolving. • Crosslinking prevents chain separation and prevents dissolution of polymer.

21. ROLE OF PLASTICIZERS: • Absorbed water spread polymeric chains apart and facilitate slippage between chains, this lubricating effect is called Plasticization. • Plasticizers are added to resins to reduce their softening or fusion temperature. • Plasticizers help to partially neutralize secondary bonds. • Sometimes the plasticizer penetrate between the macromolecule and increase the interatomic spacing, such type of plasticizer is called as essential plasticizer.

22. CHEMISTRY OF POLYMERIZATION • Monomers can be joined together by one of the two types of reactions. • Addition polymerization. • Condensation or step growth polymerization. ADDITION POLYMERIZATION: Monomers are activated one at a time and add together in sequence to form a growing chain. CONDENSATION or STEP GROWTH POLYMERIZATION: The components are difunctional and all are or become reactive simultaneously. Chain grows by a stepwise linking of difunctional monomers. This reaction often but not always produces a low molecular weight byproduct. (water or alcohol)

23. ADDITION POLYMERIZATION • Most dental resins are polymerized by this mechanism. • Monomers are added sequentially to the end of the growing chain. • Addition polymerization starts from an active centre. • One monomer adds at a time to form an active growing chain. • Chain grows indefinitely until the entire monomer is used. • Additional polymerization can produce giant molecules of unlimited size. • In addition polymerization the structure of monomer is repeatedly many times to form a polymer.

24. STEPS IN ADDITION POLYMERIZATION • Four distinct stages in addition polymerization. • Induction. • Propagation. • Chain transfer. • Termination.

25. STEPS IN ADDITION POLYMERIZATION INDUCTION: • Two processes control the induction stage. • Activation and initiation. • A source of free radical is required to begin addition polymerization reaction. • Free radical is provided by a free radical producing molecule. • Activation is done by heat, chemical, ultraviolet and visible light. (heat & visible light are used commonly in dentistry) Cont……

26. STEPS IN ADDITION POLYMERIZATION INDUCTION: ETHYLENE is a simplest monomer capable of addition polymerization. • R___R+ external energy 2R . • R . + CH2=CH2 RH2C__CH2 .

27. STEPS IN ADDITION POLYMERIZATION PROPAGATION: The resulting free radical monomer now acts as a new free radical centre. It approaches another monomer to form a “dimer” Dimer becomes another free radical. RH2C__CH2 . + CH2 = CH2 RH2C_CH2_H2C_CH2. RH2C_CH2_H2C_CH2. + CH2=CH2 RH2C_(CH2_H2C)2_CH2. ……. E.t.c

28. STEPS IN ADDITION POLYMERIZATION CHAIN TRANSFER: Active free radical of growing chain is transferred to another molecule. This initiates further chain growth. Thus a new nucleus of chain growth results.

29. STEPS IN ADDITION POLYMERIZATION TERMINATION: Addition polymerization reaction can be terminated either by • Direct coupling of two free radical chain ends. OR • By exchange of one hydrogen atom from one growing chain to another. RH2C_(CH2_H2C)m_CH2.+ .H2C_(CH2_H2C)n_CH2R RH2C_(CH2_H2C)m_CH2_H2C_(CH2_H2C)n_CH2R

30. INHIBITION OF ADDITION POLYMERIZATION • Polymerization is not likely to result in a complete exhaustion of monomer. • Impurities in monomers inhibit such reaction. • Any impurity reacts with a free radical and inhibits or retards the polymerization reaction. • Any impurity can react with the activated initiator or with an activated growing chain to prevent further growth. • Small amount of hydroquinone (0.006% or less) • Oxygen also retards the polymerization reaction as it reacts with free radical.

31. STEP GROWTH POLYMERIZATION • The reaction in step growth polymerization can result from any of the chemical reaction mechanisms that join two or more molecules in producing a simple, non macromolecular structure. • Primary compound reacts with the formation of a byproduct. (water, alcohols, halogen acids and ammonia) • Due to formation of byproduct, step growth polymerization is also called as condensation polymerization. • This is the mechanism also used by the biological tissues to form proteins, carbohydrates, deoxyribonucleic acid and ribonucleic acid.

32. STEP GROWTH POLYMERIZATION • A linear chain of repeating mer units is obtained by stepwise intermolecular condensation or addition of reactive groups. HO_(Silicone)_OH+ n HO_(Silicone)_OH HO_(Silicone)_(O_ Silicone)n_ OH + n H2O • Reaction is slow because the reaction precedes in a stepwise fashion i.e. from monomer to dimer, from dimer to trimer and so forth until large polymer is formed. • Such a reaction stops until chain reaches truly grat size because as the chain grows they become less mobile and less numerous.

33. COPOLYMERIZATION • The combination of two or more chemically different monomers is called co-polymerization. • The polymer formed is called a copolymer. • Copolymerization improves physical properties. • In small quantities they can modify the adhesive properties and surface properties.

34. TYPES OF RESINS ACRYLIC RESINS: • Acrylic resins are derivatives of ethylene and contain a vinyl group. • Two acrylic resins, • one derived from acrylic acid {CH2=CHCOOH} AND • Other derived from meth acrylic acid {CH2=C(CH3)COOH} are of dental interest.

35. METHYL METHACRYLATE • At room temperature methyl methacrylate is clear, transparent liquid,. Physical properties: • Melting point = -48 °C. • Boiling point = 100.3 °C. • Molecular weight =100. • Density = 0.945 g/ml at 20 °C. • Heat of polymerization =12.9 kcal/mol.

36. POLYMETHY METHACRYLATE • Transparent, clear and extremely stable resin. Physical properties: • Knoop hardness number =18 – 20. • Tensile strength = 60 Mpa. • Density = 1.19 g/cm³. • Modulus of elasticity = 2400 Mpa. • Softening temperature = 125 °C.

37. POLYMETHY METHACRYLATE • Between softening temperature and 200 °C, depolymerization takes place. • At 450 °C, 90% polymer depolymerizes to monomer. • P.M.M.A also absorbs water by imbibition. (shows an increase in 0.5 % of weight after immersion in water for 1 week) • This phenomenon is reversible when resin is dried.

38. QUESTIONS ????