Polymers PDF

 Polymers presented by Dr. Amr Sharawy Associate professor of dental materials  Polymers Millions of the polymer chains interlock and entanglement like the spaghetti structure. -Generally the mechanical properties of the polymers are Much lower tha...

 Polymers presented by Dr. Amr Sharawy Associate professor of dental materials  Polymers Millions of the polymer chains interlock and entanglement like the spaghetti structure. -Generally the mechanical properties of the polymers are Much lower than metals and ceramics. Polymer: A long chain molecule consisting of many repeating units called monomers or mers. Monomer (mer): Smallest repeating unit in a polymer. Mono = Single Mer term is derived from the Greek. Oligomer: Is a short polymer composed of less than 10 mers.  Polymerization reaction: The reaction by which the monomer units become chemically linked together to form(polymer). Homopolymer: One type of monomer Methylmethacrylate monomer PolyMethylmethacrylate polymer Copolymer: More than one type of monomer BUTYALmethacrylate & Methylmethacrylate Mers bonded together by covalent bonds within the polymer chain : Intra-molecular = covalent bond = 1ry bond Polymer chains bonded together by van deer +ve waal force due to the polarization and -ve -ve formation of dipoles along the polymer chain. +ve Intermolecular = 2ry -ve bond (polar bond) +ve -ve  Classification of Polymers 1. According to Origin: Natural polymers: e.g. a) proteins b) polysaccharides (starch) e.g. agar and alginate c) polynucleic acids (DNA) Synthetic polymers: They are produced in the laboratory by chemical reactions, e.g. Acrylic resin and elastic impression materials 2. According to their spatial configuration [arrangement in space]: 1.Linear: The monomer units are connected to one another in linear sequence.(chains are bonded by 2ry bonds) 2. Branched: In which side chains are connected to the main linear chain.(chains are bonded by 2ry bonds.). 3. Cross-linked polymers: In which the adjacent linear chains are joined one to another at various positions by covalent bonds.  3. According to Thermal Behavior Thermoplastic polymers Thermoset polymers They soften by heat when heated They undergo a and harden by cooling). chemical reaction and become permanently hard. *Upon heating, weak polar *They can not be re-melted or bonds are broken,& chains are separated, and it can be fused or reformed into other shaped. shape but degrade or decompose They are relatively soft and upon being heated to high their mechanical properties are sensitive to heat. temperature. The polymer chains are bonded *Linear Polymer chains are with primary covalent bonds. bonded to each other by (cross-linked acrylic resin). secondary bonds.  Polymerization reactions is the chemical reaction by which monomer units become chemically linked and form polymer chain Types of polymerization reactions: Condensation Addition polymerization polymerization  1. Condensation polymerization: Definition: reaction between two molecules to form a larger molecule, with the elimination of a smaller molecule (by-product)) -Change in composition takes place during polymerization. -Characterized by: 1- Elimination of Small molecular weight by-product. 2- Poor dimensional stability due to evaporation of the byproducts. Examples of condensation polymerization in dentistry are: -Polysulphides rubber impression materials Where water and lead sulfide are by-products of this reaction. -Condensation Silicones Where ethyl alcohol as by-products of this reaction.  2. Addition polymerization: Addition polymerization results in the formation of large molecules without the formation of by-products. No change in composition takes place. The structure of the monomer is repeated many times in the polymer, e.g. poly (ethylene), poly (acrylic acid), poly (methacrylic acid) and poly (methyl methacrylate). Ethylene C2H4 monomer ( H2C=CH2 ) + ( H2C=CH2 ) +( H2C=CH2)  Stages of addition polymerization: i- Activation and initiation ii- Propagation. iii- Termination. a. Activation and initiation : ***Requires the presence of free radicals. A free radical is a compound with an unpaired (unshared) electron. This unpaired electron makes the free radical very reactive. -To release the free radical initiator must be activated by either :- *Heat or *Light or *Chemical compound (dimethyl paratoluidine) R R heat or light 2R Initiator chemical compound free radical -Example of initiator is benzoyl peroxide -Addition polymerization could be inhibited by any material that reacts with the free radical When a free radical (R ) encounters a double bond → it pairs with one of the electrons leaving the other member of the pair free. Dot = single electron Now the monomer is a free radical b. Propagation: -Chain reactions should continue, with the evolution of heat until all the monomer has been changed to a polymer. -Linear growth of molecules →↑ Viscosity And so on until R1Mn. Where n is any integral number. c. Termination: The chain reactions can be terminated either by 1) Direct coupling or by the. 2) Exchange of a hydrogen atom from one growing chain to another Factors associated with polymerization: a. Evolution of heat as the reaction is strongly exothermic due to breakage of bonds. b. Reduction in volume on polymerization (polymerization shrinkage) c. Residual monomer is always left due to termination. Inhibition of polymerization: Any impurity in the monomer which can react with free radicals will inhibit or retard the polymerization reaction It can react either with: - the free radicals - any activated nucleus, - an activated growing chain to prevent further growth. Such inhibitors influences the length of the initiation period markedly, as well as the degree of polymerization.. e.g. Hydroquinone, Eugenol, large amounts of Oxygen Clinical significance: 1- The addition of small amount of inhibitor (hydroquinone) to the monomer will inhibit premature polymerization during storage increasing its shelf life. 2-Eugenol must not be placed in contact with any polymerizing material (e.g. resin composite), to avoid retardation of polymerization reaction.  General properties of polymers 1-Generally the polymers are molecular solids where: Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (interatomic) Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular). Secondary Van der Waal forces (weak, polar bond) are responsible for; -- Lower strength, hardness and rigidity. -- Higher water sorption &dissolution in organic solvent (CRAZING). 2-Polymers are characterized by being amorphous and having glass transition temperature (Tg) (it is the temperature at which the polymer start to be soft) i.e. above Tg : the polymer is soft and rubber like and below Tg : the polymer will be very rigid 3. Visco-elastic materials: -The polymers are sensitive to the rate of loading. -At low rate of loading (slow forces) they behave in a ductile manner (more permanent deformation). -At high rate of loading (rapid forces) they respond in a brittle manner (fracture at or near the proportional limit). - During loading they undergo viscoelastic behavior and creep.  Factors affecting properties of polymers 1. Molecular weight and Degree of polymerization : -Molecular weight Molecular weight of the various mers multiplied by the number of the mers = M.W of one monomer X No. of monomer units The higher the molecular weight of the polymer, → the higher the degree of polymerization -Degree of polymerization defined as the total number of mers in a polymer chain. D.P.= M.W. of a polymer/ M.W. of a mer Effect on the properties; The longer the polymer chain → the greater the number of entanglement that can be formed among the polymer chain. i.e. The more difficult to distort the polymer. ***Strength, rigidity, and glass transition temperature increase with increasing chain length. Average molecular weight Not all the chains within the polymer will have the same length. Each chain will have its own molecular weight and degree of polymerization. In general the molecular weight of a polymer is reported as the average molecular weight because the number of repeating units may vary greatly from one chain to another.  The molecular weight distribution The molecular weight distribution is the fraction of low, medium, and high molecular weight molecules in a polymer. It has an important effect on the physical and mechanical properties as the average molecular weight does. Give Reason: Two polymers- having the same chemistry and same molecular weight- may have different properties. This may be because one is formed of long chains, while the other is formed mainly of shorter chains i.e. different molecular weight distribution. 2. Crystallization: Polymers may be crystalline or amorphous. Crystalline ones have their chains arranged in a regular geometric pattern with intervening amorphous regions. Effect on the properties: The higher the crystallinity of a polymer, the higher is its tensile strength as well as its Tg  3- Cross-linking: Adjacent linear chains are joined one to another at various positions by chemical bonds (covalent bonds). The effect of cross-1inking: A small degree of cross-linking limits the amount of movement of the polymer chains relative to each other when the material is stressed. This * increases : - strength, hardness, rigidity, -resistance to the action of solvents -and - glass transition temperature. * decreases water sorption. Extensive cross-linking may lead to brittleness of the material 4. Co-polymerization: Copolymers are polymer chains containing two or more different types of monomer units. Co-polymerization processes enable chemists to "tailor-make" molecules of predicted properties for special applications. Co-polymers are of three different types: random, block and graft. Linear co-polymer random Branched co-polymer random Block co-polymer Graft co-polymer 5. Plasticizers: Def: They are compounds which are added to partially neutralize the secondary bonds or intermolecular forces that normally prevent the resin molecules from slipping past one another when the material is stressed. There are mainly two types of plasticizers: a. External plasticizers: in which the plasticizer penetrates between the polymer chains. The polymer chains become further apart and the forces between them become less. b. Internal plasticizers: plasticizer of a resin can also be accomplished by copolymerization with a suitable co-monomer. In this case the plasticizer is a part of the polymer main chain. The effect of plasticizers: *They usually reduce the strength and hardness of the resin * Reduce the softening point (glass transition temperature) ***At room temperature the main difference between rigid and flexible polymers is that the former have their glass transition temperature above room temperature whereas the latter have theirs below room temperature. (the opposite action of cross-linking agent). 6. Sensitive to the rate of loading Polymers are sensitive to the rate of loading because they are viscoelastic materials.At slow rate of loading they behave in a ductile manner(more permanent deformation). At high rates of loading they respond in a brittle manner (fracture after elastic limit). 7. Sensitive to temperature. -Polymers are characterized by being amorphous and having glass transition temperature (it is the temperature at which the polymer start to be soft)i.e. above which the polymer is soft and rubber like material, and below which the polymer will be very rigid 8. strength can be increased by: a. Increasing the degree of polymerization and thus increasing M.W. b. Cross-linking. c. Copolymerization. d. Addition of inorganic fillers to form composite structures. e.g. composite restorative resins.  Applications in dentistry 1. Denture base materials. 2. Artificial teeth. 3. Impression materials. 4. Tooth restorative materials (e.g. resin composite). 5. Cements. 6. Crown and bridge facings. 7. Dies. 8. Endodontic fillings. 9. Implants. 10. Maxillofacial prosthesis  Application of polymers in dentistry Acrylic resin polymer Restorative Rubber impression materials materials Cement Endodontic filling materials THANK YOU