polymers lab final.pptx

POLYMERS Dr. Hala Bahgat Dr. Dina Mostafa Polymers Dr. Hala Bahgat Dr. Dina Mostafa  Poly mers Many The smallest repeated units Dr. Hala Bahgat Dr. Dina Mostafa Polym...

POLYMERS Dr. Hala Bahgat Dr. Dina Mostafa Polymers Dr. Hala Bahgat Dr. Dina Mostafa  Poly mers Many The smallest repeated units Dr. Hala Bahgat Dr. Dina Mostafa Polymer: A long chain organic molecule consisting of many repeating units called monomers or mers. Monomer (mer): Smallest repeating unit in a polymer. Mono = Single Oligomer: Is a short polymer composed of 10 mers. Dr. Hala Bahgat Dr. Dina Mostafa Intramolecular = covalent bond = 1ry bond Intermolecular = 2ry bond Dr. Hala Bahgat Dr. Dina Mostafa Polymerization reaction: The reaction by which the monomer units become chemically linked together to form high molecular weight molecule (polymer). Homopolymer: One type of monomer Copolymer: More than one type Dr. Hala Bahgat Dr. Dina Mostafa Classification According to: I. The origin Natural Synthetic Proteins Bakelite Polysaccharides Nylon DNA - RNA PMMA Polyisoperine Polyvinyl chloride Dr. Hala Bahgat Dr. Dina Mostafa  II. Spatial configuration Linear Cross linked Branched Dr. Hala Bahgat Dr. Dina Mostafa  III. The thermal behaviour Thermoplasts Thermosets - It is a physical reaction - It is a chemical reaction - The polymer chains are - They can not be remelted bonded to each other by and reformed into another secondary bonds. shape but degrade or decompose upon being - They are relatively soft heated to high and their mechanical temperature. properties are sensitive to - High mechanical heat. properties - e.g. - Acrylic resin Impression Dr. Hala Bahgat compound Dr. Dina Mostafa  Preparation of polymers Setting reactions A- Condensation B- Addition polymerization polymerization Dr. Hala Bahgat Dr. Dina Mostafa a) Condensation Polymerization [step reaction]: Characterized by: 1- Stepwise reaction. 2- Usually consists of more than one monomer species. 3- Small molecular weight due to by – product elimination. 4- Slow reaction and short polymer chains. Dr. Hala Bahgat Dr. Dina Mostafa Clinical significance: Polysulfide Rubber Impression materials & condensation silicone Dr. Hala Bahgat Dr. Dina Mostafa b) Addition Polymerization: Characterized by: 1. Faster reaction. 2. No by - product. 3. High molecular weight. Dr. Hala Bahgat Dr. Dina Mostafa i- Activation & Initiation: By initiator like benzoyl peroxide to release free radical. Initiator must be activated to release the free radicals. Free Heat radical compound Initiator having Light chemical group of an unpaired Chemicals electron Dr. Hala Bahgat Dr. Dina Mostafa ii- Propagation: Linear growth of molecules. Shifting of free radical to the end of the growing chain. Dr. Hala Bahgat Dr. Dina Mostafa  iii. Termination: A. Direct coupling H H H H HH HH R1Mn C-C + Mm C-C R1Mn C-C-C-C Mm H H H H HH HH Dr. Hala Bahgat Dr. Dina Mostafa  b. Hydrogen Atom Transfer H H H H HH HH R1Mn C-C + Mm C-C R1Mn C=C + Mm-C-C-H H H H H H HH Dr. Hala Bahgat Dr. Dina Mostafa Factors associated with polymerization reactions: 1. Evolution of heat. 2. Shrinkage. 3. Residual monomer. Dr. Hala Bahgat Dr. Dina Mostafa 2- Ring Opening Mechanism: The terminal reactive groups in the monomers are Rings. Dr. Hala Bahgat Dr. Dina Mostafa  Ring opening mechanism Examples Polyether rubber impression material Siloranes (low shrinkage composite) Dr. Hala Bahgat Dr. Dina Mostafa  Inhibition of Polymerization - Any substance, which can react with free radical will inhibit or retard the polymerization reaction e.g. Hydroquinone, oxygen and eugenol. - The presence of such substance decreases the rate of initiation reaction as well as increases the rate of termination. Dr. Hala Bahgat Dr. Dina Mostafa Clinical significance: 1- The addition of small amount of inhibitor (hydroquinone) to the monomer will inhibit premature polymerization during storage increasing its shelf life. Dr. Hala Bahgat Dr. Dina Mostafa 2- Polymerization should not be conducted in open air but in oxygen deficient environment to avoid retardation of polymerization reaction. Dr. Hala Bahgat Dr. Dina Mostafa Dental examples: Avoid air thinning of the bonding agent. Use of a matrix during curing of the composite restorative material. Dr. Hala Bahgat Dr. Dina Mostafa 3- Eugenol must not be placed in contact with any polymerizing material (e.g. resin composite), to avoid retardation of polymerization reaction. Dr. Hala Bahgat Dr. Dina Mostafa Polymer Properties Dr. Hala Bahgat Dr. Dina Mostafa Generally the polymers are molecular solids where: Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (intra- molecular) Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular). Dr. Hala Bahgat Dr. Dina Mostafa  Secondary Van der Waal forces (weak, polar bond) are Soft responsible for: Rubbe ry 1. Reduced strength, hardness and rigidity. Flexibl e 2. Higher water sorption. Polymers are characterized by being amorphous Tg Rigid and having glass transition temperature (It is the Hard temperature at which the polymer start to be soft) Glass- like Dr. Hala Bahgat Dr. Dina Mostafa  Polymer properties 1- Molecular weight and degree of polymerization: M.W of a polymer = M.W of mers X No. of mers Degree of polymerization depends upon Total No. of mers and the total No. of molecules in a polymer D.P. =M.W. of polymer / M.W. of monomer ↑M.W. and ↑D.P.↑ → ↑ strength, ↑ stiffness,↑ softening and melting temperature. Dr. Hala Bahgat Dr. Dina Mostafa  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 is the fraction of low, medium, and high molecular weight molecules in a polymer. It also has an important effect on the physical and mechanical properties. Dr. Hala Bahgat Dr. Dina Mostafa 2- Cross linking: Polymer chains are bonded by primary bonds Effects: 1- Limits the amount of movement of the polymer chains relative to each other when the material is stressed. 2- Extensive cross-linking may lead to brittleness of the polymer. Dr. Hala Bahgat Dr. Dina Mostafa 3- Copolymerization:Polymer chain containing more than one type of mer. Random Block Graft Effects: 1- Tailor-making 2- Block & graft improve the impact strength properties. Dr. Hala Bahgat Dr. Dina Mostafa 4- Plasticizers: Internal External Plasticizers Plasticizers Effects: 1- Partially neutralize the 2ry bond. 2- Facilitating the slippage of chains over each other. 3- Lower the mechanical Properties of the polymer. e.g. Soft liners Dr. Hala Bahgat Dr. Dina Mostafa 5- Melting and glass transition phenomenon (TG): Amorphous polymers ∆ Softening ∆ Liquid (rigid solids) TG (rubber like) Polymer Polymer Temp (a) (b) Still solid (i.e. just behavior Room change of Temp solid and no state transformatio n) Cross linking → ↑ TG → rigid and brittle polymer Plasticizers → ↓ TG Dr. Hala Bahgat Dr. Dina Mostafa 6- Mechanical properties: Polymers < metals and ceramics due to ↑ 2ry bonds. ↓ E and tensile, impact and fatigue strengths. ↑ Mol. Weight ↑ Cross linking ↑ Mechanical properties ↑ Degree of polymerization Dr. Hala Bahgat Dr. Dina Mostafa 7- Viscoelastic creep: Polymers are strain – rate sensitive materials. The rate of loading The material behaves in a brittle manner The rate of loading The material behaves in a ductile manner ↑ Cross linkage ↓ Creep ↑Crystallinity Dr. Hala Bahgat Dr. Dina Mostafa 8. Addition of inorganic fillers: Effects: Mechanical Properties Dr. Hala Bahgat Dr. Dina Mostafa Applications in dentistry Dr. Hala Bahgat Dr. Dina Mostafa Dr. Hala Bahgat Dr. Dina Mostafa Dr. Hala Bahgat Dr. Dina Mostafa