Dental Bio-materials PDF

Dental Bio-materials Ass. Prof. Dr. Mohamed Abed el-Aziz Souhail Head of Dental Bio-material Department, Faculty of Dental Medicine, Al- Azhar University. POLYMERS Polymers are Plastic and rubbers Polymers are long chain molecules consisting of many (poly) repeating units (mers). A monomer: Is th...

Dental Bio-materials Ass. Prof. Dr. Mohamed Abed el-Aziz Souhail Head of Dental Bio-material Department, Faculty of Dental Medicine, Al- Azhar University. POLYMERS Polymers are Plastic and rubbers Polymers are long chain molecules consisting of many (poly) repeating units (mers). A monomer: Is the smallest repeating unit in the polymer Polymerization : Is the chemical reaction by which polymers are formed from monomers chain e.g. CLASSIFICATIONS OF POLYMERS Classifications according to Origin Thermal Spatial Polymerization reaction The origin Natural polymers Agar, cellulose DNA, proteins Natural rubber Collagen and silk A) Synthetic Polymers Produced in the laboratory by chemical reactions. e.g. Acrylic resin ,nylon and polystyrene. Originally the synthetic polymers regarded as substitutes for existing natural polymers such as rubber and silk .Nowadays a polymers can be produced and have medical use such as dialysis and oxygenator membranes and B) Natural Polymers Proteins polyamides of polypeptides polyisoprenes e.g. rubber and gutta-percha polysaccharides e.g. starch , cellulose, agar and alginate polynucleic acids such as DNA and RNA. According To Thermal Behavior Thermoplastic resin They are polymers which can be shaped by heat and after cooling they maintain their shape the reaction is reversible and can be repeated characteristics The polymer chains are bonded to each other by secondary bond. They are relatively soft and their mechanical properties are sensitive to heat. They dissolve in organic solvent thermosetting polymers They are formed into a permanent shape and set by chemical reaction. They can not be remelted and reformed into another shape but degrade or decompose up on being heated to high temperature.. Thermoset cannot be fused. The polymers form network with cross –links Thermal a) Thermosetting b) Thermoplastic (polyvinyl chloride polyethylene Nylon Polystyrene b) Thermoset polymers Harden during fabrication (irreversible or chemical reaction). Chemical Soft hard reaction Examples are cross-linked polymethylmethacrylate, silicons ,and bisphenol A-diacrylates. According to the basis of structural or spatial configuration 3 Basic Types Branched linea r copolyme r Simple polymer Has one type monomer homopolymer OR Random Block Crosslinked polym er Homo Copolymer OR Rando m graft Linear polymers) a( (simple polymer) This may be homopolymer it has one type of monomer ,or copolymer, this means that it has more than one type of monomer, Homopolymer random block copolymer, b) Branched polymers This type of polymers show higher strength hardness and less water sorption than linear polymer . Branched polymers are either homo or copolymer The branched 2copolymer may be random or graft types. Homopolymer copolymer, random C) Cross-Linking Polymer chains are cross - linked or attached by a cross - linking agent such as glycol dimethaerylate. Through primary and small degree of cross linking will limit the amount of movements of the polymer chains relative to each other. When the material is stressed the deformation Characteristic 1. 2. 3. 4. Higher strength and hardness. Higher resistance to crazing which may be due to Alcohol drinking or solvents It is the best type used for construction of denture base and artificial teeth Low water sorption :Polymerization The chemical reaction by which polymers are .prepared from monomers Condensation polymerization Occurs when two molecules react to form a large molecules with the elimination of smaller molecule )Often but not always water ( Addition polymerization Occurring when a reaction between two molecules either the same to form a homopolymer or( )dissimilar to form heteropolymer produces a large molecule without the elimination of a smaller molecule such as water Types of polymerization Most polymerization reactions are of two types condensation and addition polymerization. 1)Condensation polymerization: The reaction between two molecules to form a larger molecule with the elimination of a smaller such as water as a by product. condensation Polymer + by product Monomer polymerization Example is the polysulphide rubber impression material. Addition polymerization)2 The reaction between two molecules to give a larger molecule without the elimination of a smaller molecule (no by-product). 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), polymethacrylic acid) STAGES OF ADDITION POLYMERIZATION STAGES OF ADDITION POLYMERIZATION A Initiation Activation and initiation (by free radical mechanism) B Propagatio n C Terminatio n Direct coupling Exchange of a hydrogen atom A) Activation and initiation A free radical is a compound with an unpaired (unshared )electron (very reactive ) The symbol (c= c) represents four electrons the monomer itself become free radical Benzoyel peroxide is a substance most commonly used to generate free radicals before initiation The activation is done by light, heat or by a chemical compound such as dimethyl Three stages of addition polymerization Heat 1) Initiation Amines C6H5 COOCC6H5 Benzoyl peroxide dioxide CH3 R+ CH2 = C COOCH3 Heat Amines 2 (C6H5COO) + CO2 free radicals (R) + carbon CH3 R-CH2 – C COOCH3 Free radical + monomer monomer Free radical activated B) Propagation The chain reactions continues with the evolution of heat until all the monomer has been changed to a polymer and so on R1M +M + R1MM + M +R1MMM R1MN where n is any integral number. polymer free radical + monomer chain growing C) Termination The reaction terminated either by a) Direct coupling This occurs when two free radicals react to form a stable molecule 1- R1MN + R2MM R1MN R2MM b) Exchange of a hydrogen atoms The chain reactions can be terminated either by direct coupling or by the Chain transfer H H H H by transfer of R1Mn – C – C + R2Mm – C – C H H H H a hydrogen atom H H H H R1Mn – C = C + R2Mm – C – C - H H H H Factors associated with polymerization A) Evolution of heat as the reactions is strongly exothermic due to breaking of bonds. B) Reduction in volume. 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 activated initiator or any activated nucleus or with an activated growing chain to prevent further growth The presence of such inhibitors influences the length of the initiation period, as well as the degree of polymerization. The addition of small amount of inhibitor (Hydroquinone) to the monomer (Methyl methacrylate) will inhibit polymerization if no chemical initiator is present and it will retard the polymerization in Factors affecting the properties of polymers Many factors affect the properties of polymers 1. Chemical composition of the chain. 2. Degree of polymerization. 3. The number of branches and 4. The cross – links between polymer chains. In general Longer chains and higher molecular weight polymers characterized by (a) Increased strength, hardness Brittleness and stiffness (b) Increased resistance to creep. For example composite resins highly crosslinked matrix in which large number of strong covalent linkages between chains transforms the molecules into a rigid very high molecular weight material resulting strength, stiffness and withstand occlusal stresses. In contrast Elastomeric impression materials composed of individual coiled chains with just few cross- links this type of molecular structure permits the large scale uncoiling and recoiling of chains that gives these materials high flexibility. Two basic properties which characterize polymers are glass transition temperature (Tg) and melting temp. (Tg ) Is the temperature at which the polymer ceases to be glassy , brittle and