Polymers.PDF

 ‫كلية طب الفم واألسنان‬ ‫رؤية الكلية‬ ‫تتطلع الكلية أن تكون في مصاف المؤسسات التعليمية المعترف بها إقليمياً وعالمياً من خالل برامج تعليمية متطورة‬.‫وأبحاث تطبيقية م...

 ‫كلية طب الفم واألسنان‬ ‫رؤية الكلية‬ ‫تتطلع الكلية أن تكون في مصاف المؤسسات التعليمية المعترف بها إقليمياً وعالمياً من خالل برامج تعليمية متطورة‬.‫وأبحاث تطبيقية مبتكرة وتنمية مجتمعية مستدامة‬ The Faculty aspires to be a recognized educational institution, regionally and internationally, by providing advanced educational programs, innovative applied research, and sustainable community development. ‫رسالة الكلية‬ ‫ ذو كفاءة معرفية وتطبيقية من خالل برامج تعليمية‬،‫إعداد طبيب أسنان ملتزم بالقيم االنسانية واألخالق المهنية‬ ‫ كما تلتزم الكلية بإعداد بحوث تطبيقية‬.‫متطورة تتوافق مع االحتياجات الفعلية لسوق العمل المحلي والعالمي‬.‫متوافقة مع االستراتيجيات القومية وكذلك تقديم خدمة مجتمعية مستدامة وفقاً لمعايير الجودة العالمية‬ The mission is to prepare knowledgeable and well-trained dentists committed to human values and professional ethics, by developing advanced educational programs that correspond to the actual needs of the local and global labor market. The Faculty is also committed to preparing applied research in line with national strategies, as well as providing sustainable community service following international quality standards. 1. Denture base materials. 2. Artificial teeth. 3. Tooth colored restorative materials. 4. Cements. 5. Crown and bridge facings.  Polymers are long chain molecules consisting of many repeating units. The monomer is the smallest repeating unit in the polymer chain. a) Monomer : The smallest repeat unit in the polymer b) Polymers : Macromolecules formed by linking of repeating units through covalent bonds in the main backbone c) Homopolymer A polymer made of one type of monomer repeat unit, e.g. poly(methyl methacrylate) d) Copolymer: formed of two or more different monomers chain.  e) Oligomer A short polymer chain composed of 2-10 mers. The chemical reaction by which polymers are formed from monomers.  I. According to Origin II. According to arrangement in space III. According to Thermal Behavior VI. According to polymerization reaction 1) Natural Polymers: ▪ Proteins ▪ Polyisoprenes (gutta percha) ▪ Polysaccharides (agar, alginate) ▪ Polynucleic acids (DNA) 2) Synthetic Polymers: Produced in the laboratory by chemical reactions: ▪ Acrylic resin and elastic impression materials ▪ Vinyl polymers which are polymers of C2H4 1. Linear: The structural units are connected to one another in a linear sequence.  The bond between the polymer chains it is a weak physical bond (Van der Waal forces)  They have low mechanical properties, high water sorption, low Tg, low thermal and mechanical resistance 2. Branched: Side branch chains are connected to the main ones. 3. Cross-linked Polymers: The adjacent linear chains are joined to one another at various positions by covalent bonds. The linear polymers may be joined or bridged with covalent bonds to form a cross - linked structure. -it has high mechanical properties, glass transition temperature and decreased water sorption ,high thermal and chemical resistance.  Thermoplastic Polymers Thermosetting Polymers Shaped by heating and maintain Formed into a permanent shape their shape after cooling and set by a chemical reaction. physical change,reversible reaction Irreversible reaction The polymer chains are bonded The polymer chains form a to each other by secondary network with cross-links between bonds. them (1ry covalent bonds). They can not be re melted and Relativelysoft, their mechanical reformed into other shape but properties are sensitive to heat degrade or decompose upon being heated to high temp. Dissolve in organic solvents. e.g. non cross – linked PMMA Polymer Synthesis: Polymerization; the chemical reaction by which monomer units become chemically linked and form polymer. Mechanisms of Polymerization: Most polymerization reactions are of 2 types: Condensation & addition polymerization  Polymers IV. Acc to Polymerization reaction A. Condensation B. Addition Definition: ❑ Reaction between 2 molecules → larger molecule, + (by-product), of low molecular wt e.g. H2O ❑ Change in composition ❑ Slow process ❑ Early termination (low mol wt.) Dental Examples: Polysulfides & Silicones : ❖ Synthetic rubber impression materials ❖ Low MW paste High MW material by-product +PbO2 ❖ Definition : Reaction between 2 molecules → 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. ❖ Examples: Poly (ethylene), poly (acrylic acid), poly (methacrylic acid) & poly (methyl methacrylate).  Addition polymerization Free radical Ring opening  Polymers Stages of AdditionPolymerization: 1- Activation & Initiation (chemical, heat, light) 2- Propagation 3- Termination Direct coupling H atom transfer 2 free radicals react From a growing to form stable chain to another molecule Initiator Free radical Monomer Free radical Free Radical (R ) A compound with an unpaired (unshared) electron. This unpaired electron makes the radical highly reactive. Dot = single electron. 2R (Free Radical) R=R (Initiator) Activator Addition polymerization could be inhibited by any material that reacts with the free radical Benzoyl peroxide , Diketone: (as initiator) is the substance most commonly employed to generate free radicals. Activation of the initiator can be done by chemicals, heat, microwaves or electromagnetic waves as ultraviolet rays and the blue visible light. In this way the peroxide decomposes giving up free radicals to initiate the polymerization reaction. ▪ When a free radical (R ) encounters a double bond, it pairs with one of the electrons leaving the other member of the pair free. The monomer itself becomes a free radical Chain reactions should continue with the evolution of heat, until all the monomer units are changed into a polymer R1Mn Propagation continues as the chain grows in length n is any integral number.  The chain reactions can be terminated either by : Direct coupling or Transfer of a hydrogen atom from one growing chain to another Direct Coupling N.B. Under normal conditions and due to termination, the polymerization is incomplete with a residual monomer always left. A. Evolution of heat The reaction is strongly exothermic due to breakage of bonds. B. Reduction in volume, (polymerization shrinkage) C. Residual monomer(reaction is never complete  Addition polymerization Free radical Ring opening  In ring opening the terminal reactive groups in the monomers are rings. The reactive terminal rings open under the influence of a cationic initiator Whenever the ring is opened, the cation function remains attached→→ lengthening the chain and forming the polymer.  Adv of materials polymerized by ring opening polymerization:  1-less polymerization shrinkage  2-less heat evolved Examples  Polyether rubber impression material  Siloranes (low shrinkage composite) 1- impurities in the monomer → can react with free radicals → inhibit or retard the polymerization reaction?? Can react either with the activated initiator (R*) or any activated nucleus (RM*),, or with an activated growing chain(RM n *) to prevent further growth. ▪ Inhibitors influence the length of the initiation period markedly, as well as the degree of polymerization ▪ Examples: Hydroquinone, Eugenol and oxygen inhibit polymerization of composites  The addition of a small amount of hydroquinone to the monomer will inhibit polymerization if no chemical initiator is present.(Shelf life)  It will definitely retard the polymerization in the presence of an initiator. (Working time)  polymers are molecular solids where:  Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (intramolecular)  Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular).  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 which the polymer is soft and rubber like material, and below which the polymer will be very rigid) Factors Affecting Properties of Polymers  Polymers 1- mol wt & degree of polymerisation 2- Cross-linking 3- Co-polymerisation 4- Plasticizers 5- Spatial structure 6- Rate of loading 7- Temperature 8- Addition of fillers 1) Degree of Polymerization: The degree of polymerization measures the number of mer units in the polymer molecule 2) Molecular Weight: M.W. of a polymer = M.W. of the various mers x no of mers The higher the molecular weight of the polymer→ the higher the degree of polymerization D.P.= M.W. of a polymer / M.W. of a mer  Polymers of large molecules are stronger and more resistant to thermal and mechanical stresses than those composed of small molecules ??? due to more entanglement of chains and more bonds.  Strength , rigidity and Tg increase with increasing chain length Entanglement !!! Growth of polymer chains is a random process; Some chains grow faster than others and some are terminated before others Not all chains within the polymer will have the same length. Each chain will have its own molecular weight and degree of polymerization. Molecular weight of a polymer is reported as an average molecular weight ?? The number of repeat units may vary greatly from one chain to another. Molecular weight distribution is the fraction of low ,medium and high molecular weight molecules in a polymer Two polymers having the same chemistry and average molecular weight may have different properties ?? One is formed of long large chains while the other is formed mainly of shorter chains i.e. different molecular weight distribution. A B ❖ Copolymers are polymer chains made of two or more different types of monomer units. ❖ Co-polymerization processes enable chemists to "tailor-make" molecules of predicted properties for special applications. Adjacent linear chains are joined one to another at various positions by chemical bonds (covalent bonds). A small degree of cross-linking limits the amount of movement of the polymer chains relative to each other when the material is stressed ➜➜ Higher strength & hardness, Tg Increased resistance to the action of solvents Decreased water sorption,crazing N.B Extensive cross-linking ➜➜ embrittlement of the polymer 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: low volatile substances that are added to polymers. In this case, plasticizer molecules interact with polymer chains, but are not chemically attached to them by primary bonds and can, therefore, be lost. These plasticizers penetrate 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: lower its Tg, reduce the strength and hardness temperature and rigidity. (the opposite action of cross-linking agent). If sufficient plasticizer is added , a polymer which is normally rigid and hard at room temperature can have its Tg lowered to below room temperature, thus making it flexible and rubbery.(soft liners) 6. Addition of inorganic filler ----→ if added to polymer → composite structure Effect on properties: strength, hardness, and rigidity of a polymer. 7- Rate of loading: Polymers are sensitive to rate of loading as they are viscoelastic materials At slow rate of loading, they behave in a ductile manner. i.e. more permanent deformation. At high rates of loading, they respond in a brittle manner. Dental polymers exist at room temperature either as rubbers (elastomers) or hard and amorphous (organic glasses). Rubbers (Elastomers) consist of long chain molecules that are coiled and in random thermal motion. The specific feature of a rubbery polymer is that when the material is stretched the only work done is in uncoiling the molecules. Thus such materials are easy to deform, the deformation being largely reversible. At RT., Dental polymers exist either as  Rubbers (elastomers)  Hard and amorphous (organic glasses). Def: The temp. at which the polymer starts to soften. Below Tg → The polymer is very rigid, hard and glass like. Above Tg → The polymer will change rapidly to a viscous, soft and rubber like. Main difference between rigid and flexible polymers at room temp.: ▪ Rigid polymers have their Tg above room temp. ▪ Flexible polymers have their Tg below room temp. This principle is used to produce acrylic soft liners