POLYMERS AND POLYMERIZATION

POLYMERS AND POLYMERIZATIONByDr Rashid HassanAssistant ProfessorRIHSISLAMABAD

11Monday, May 28, 2012Science of Dental Materials Department Dental Section W.M.D.C AbbottabadPOLYMERPoly = Manymer = UnitsI.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.2RESINSA 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.

3RESINSThe word "resin" has been applied in the modern world to nearly any component of a liquid that will set into a hardlacquerorenamel-like finish. An example isnail polish, a modern product which contains "resins" that are organic compounds, but not classical plant resins. Certain "casting resins" andsynthetic resins(such asepoxy 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 ofthermosetting plastics, and do not derive from plants.4ROSINRosin, also calledcolophonyorGreek pitch(Pix grca), is a solid form ofresinobtained frompinesand some otherplants, mostlyconifers, produced by heating fresh liquid resin to vaporize thevolatileliquidterpenecomponents. 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 differentresin acids, especiallyabietic acid.The name, colophony orcolophonia resina, comes from its origin inColophon, an ancientIoniccity.

5ROSINUsesA cake of rosin, made for use by violinists, used here forsolderingRosin is an ingredient in printinginks, photocopying and laser printing paper,varnishes,adhesives(glues),soap,papersizing,soda,solderingfluxes, andsealing wax.Rosin can be used as aglazing agentinmedicinesand chewing gum)Can be used as anemulsifierinsoft drinks.Inpharmaceuticals, rosin forms an ingredient in several plasters and ointments.6APPLICATIONS OF RESINS IN DENTISTRYUsed in variety of applications.Dentures. (bases, liners and artificial teeth)Cavity filling materials. (composites)Sealants.Impression materials.Equipment. (mixing bowls)Cements. (resin based)7GLASS 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.

8CLASSIFICATION OF DENTAL RESINSSynthetic 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 intoThermoplastic polymers.Thermosetting polymers.Elastomers.

9CLASSIFICATION OF DENTAL RESINSTHERMOPLASTIC 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.10CLASSIFICATION OF DENTAL RESINSTHERMOSETTING 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.

11CLASSIFICATION OF DENTAL RESINSELASTOMERS:Used as impression materials.Elastic when set.Show limited reversible dimensional changes.12REQUISITES FOR DENTAL RESINS:Biological compatibility.Physical properties.Manipulation.Aesthetic qualities.Low cost.Chemical stability in mouth.13FUNDAMENTAL NATURE OF POLYMERSChain 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.14ContFUNDAMENTAL NATURE OF POLYMERSChain 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.15FUNDAMENTAL NATURE OF POLYMERSChain 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.16FUNDAMENTAL NATURE OF POLYMERSTYPES 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.17Cont.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...AAAAABBBBBBAAAABBBBBBAAABBBBGraft 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.AAAAAAAAAAABBBBBB18FUNDAMENTAL NATURE OF POLYMERSPHYSICAL PROPERTIES OF POLYMERSDEFORMATION 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.19SOLVATION PROPERTIESPolymers 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.

20ROLE 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.21CHEMISTRY OF POLYMERIZATIONMonomers 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 togetherin sequence to form a growing chain.CONDENSATION or STEP GROWTH POLYMERIZATION:The components are difunctional and all are or become reactivesimultaneously. Chain grows by a stepwise linking of difunctionalmonomers. This reaction often but not always produces a lowmolecular weight byproduct. (water or alcohol)22ADDITION POLYMERIZATIONMost 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.23STEPS IN ADDITION POLYMERIZATIONFour distinct stages in addition polymerization.Induction.Propagation.Chain transfer.Termination.24STEPS IN ADDITION POLYMERIZATIONINDUCTION: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)25ContSTEPS IN ADDITION POLYMERIZATIONINDUCTION:ETHYLENE is a simplest monomer capable of additionpolymerization.R___R+ external energy2R .R . + CH2=CH2RH2C__CH2 .26STEPS IN ADDITION POLYMERIZATIONPROPAGATION:The resulting free radical monomer now acts as a newfree 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.c27STEPS IN ADDITION POLYMERIZATIONCHAIN TRANSFER:Active free radical of growing chain is transferred toanother molecule.This initiates further chain growth.Thus a new nucleus of chain growth results.

28STEPS IN ADDITION POLYMERIZATIONTERMINATION:Addition polymerization reaction can be terminatedeither byDirect coupling of two free radical chain ends.ORBy 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_CH2R29INHIBITION OF ADDITION POLYMERIZATIONPolymerization 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.30STEP GROWTH POLYMERIZATIONThe 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.

31STEP GROWTH POLYMERIZATIONA linear chain of repeating mer units is obtained by stepwise intermolecular condensation or addition of reactive groups.HO_(Silicone)_OH+ n HO_(Silicone)_OHHO_(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.32COPOLYMERIZATIONThe 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.33TYPES OF RESINSACRYLIC 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.

34METHYL METHACRYLATEAt 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.35POLYMETHY METHACRYLATETransparent, clear and extremely stable resin.Physical properties:Knoop hardness number =18 20.Tensile strength = 60 Mpa.Density = 1.19 g/cm.Modulus of elasticity = 2400 Mpa.Softening temperature = 125 C.

36POLYMETHY METHACRYLATEBetween 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.37QUESTIONS ????38