Polymers PDF

CHAPTER 4 Polymers Definitions: Polymer is a macromolecule that is made up of many units (poly = many; mer = unit). Monomer: is the smallest repeated unit in the polymer chain (mono = single). Polymerization is the process of forming a polyme...

CHAPTER 4 Polymers Definitions: Polymer is a macromolecule that is made up of many units (poly = many; mer = unit). Monomer: is the smallest repeated unit in the polymer chain (mono = single). Polymerization is the process of forming a polymer from monomers Homopolymer is formed from one type of monomer Copolymer is formed when two or more different types of monomers are joined. Classification of polymers: 1. Classification according to the origin Natural Polymer Synthetic Polymer 1- Proteins They are produced industrially or in 2- Polysaccahrides e.g. agar the laboratory by chemical alginate reactions eg: acrylic resin, 2. Classification according to spatial configuration [arranged in space]: Linear Branched Cross –linked polymers 1ry bond Covalent bond -1-  3. Classification according to their thermal behaviour: Thermoplastic resin Thermoset resin - They are softened by heat and - They set by a chemical harden by cooling reaction with evolution of heat Soft Hard [irreversible reaction] [physical change, reversible reaction] Types of Polymerization Reactions: Polymerization: The chemical reaction by which polymers are prepared from monomers. Polymerization Monomer by Polymer reaction Types: 1) Condensation polymerization The reaction between two molecules to form a larger molecule with the elimination of a smaller molecule such as water as a by-product. condenseti on Monomer  Polymer  by - product polymeriza tion 2) Addition polymerization (Chain Reaction): The reaction between two molecules to give a larger molecule without the elimination of a smaller molecule (no by-product). There is no change in the composition; the structure of the monomer is only repeated many times. Addition Monomer  Polymer polymeriza tion -2-   Stages of addition polymerization: The addition polymerization reaction passes through 3 stages: 1) Activation and initiation: - The addition polymerization requires the presence of free radical. - A free radical is a compound having chemical group of an unpaired electron. This unpaired electron makes the radical very reactive. - The initiator is the substance used to produce free radicals.  Before initiation occurs, the initiator has to be activated. Activation of the initiator is done by heat or chemical compound or light. 2) Propagation: R + M "Stable monomer"  RM "active monomer" The chain reactions continue with the evolution of heat, until all the monomer is changed to polymer. This second stage "propagation" continues as the chain grows in length RM + M  RMM + M  RMMM and so on until RMn Where n is any integral number. 3) Termination: The chain reactions can be terminated either by: a. Direct coupling: This occurs when two active chains react to form single long stable chain. b. Hydrogen atom transfer: from one growing chain to another activated chains to form two stable chains. -3-  +  Factors associated with polymerization: a. Evolution of heat as the reaction is strongly exothermic. b. Reduction in volume (polymerization shrinkage). c. Presence of residual monomer because the polymerization reaction is practically never complete. Inhibition and Retardation 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. For example, the addition of a small amount of hydroquinone to the monomer will inhibit polymerization if no chemical initiator is present, and it will retard the polymerization in the presence of an initiator.  N.B. Eugenol, or large amounts of oxygen will inhibit polymerization. General properties of polymers  Generally the polymers are molecular solids where: a- Strong primary covalent bonds exist between the mers along the whole length of the polymer chain. b- Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular). -4-  N.B. These secondary Van der Waal forces (weak, polar bond) are responsible for; - Reduced strength, hardness and rigidity. - Water sorption.  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 the properties of polymers: 1- Molecular weight (MW) and Degree of polymerization (D.P.): a. Molecular weight: - The molecular weight of a polymer molecule = weight of the various mers x the number of the mers - The higher the molecular weight of the polymer, the higher the degree of polymerization. b. Degree of polymerization: - The degree of polymerization (D.P.) is defined as the total number of mers in a polymer chain. M.W. of a polymer D.P. = M.W. of a mer - Effect on the properties; The longer the polymer chain, the greater the number of entanglement (temporary connection) that can be formed among the polymer chain.  Strength, rigidity, and glass transition temperature increase with increasing chain length. -5-  2. Cross-linking: - A chemical compound with two double bonds per molecule can act as a cross-linking agent, since each C = C can react with a different chain. Effect on the properties; - 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 increase the strength, hardness, rigidity, and glass transition temperature. - It also decreases the water sorption and crazing (separation between the polymer chains leading to minute cracks in the surface). N.B. Extensive cross-linking may lead to brittleness of the polymer. 3. Copolymerization - Copolymers are polymer chains containing two or more different types of monomers. - Types; Co-polymers are of three different types: random, block and graft. Random co-polymer Block co-polymer Graft co-polymer -6-  - Effect on the properties; Copolymerization enables chemists to "tailor-make" molecules to modify the properties of the polymer. 4. Plasticizers: - They are compounds able to penetrate between randomly oriented polymer chains which decrease the secondary Van der Waal forces (intermolecular forces). - Plasticizers may be: 1- External plasticizers: It is by the addition of the plasticizer which penetrates between the polymer chains thus the polymer chains become further apart and the forces between them become less. 2- Internal plasticizers: It is accomplished by copolymerization with a suitable co-monomer in this case the plasticizer is a part of the polymer chain Effect on the properties; They reduce strength, hardness, rigidity, and glass transition temperature of the polymer (the opposite action of cross-linking agent). Plasticizers used to produce soft lining polymers. 5. Addition of inorganic fillers: Addition of inorganic filler to the polymer. Effect on the properties It increases strength, hardness, and rigidity of a polymer. Uses of polymers in dentistry: 1. Denture base materials. 2. Artificial teeth. 3. Tooth restorative materials. 4. Cements and Impression materials. 5. Crown and bridge facings. -7-