POLYMERS

Factors affecting properties of polymer 1- Degree of polymerization: - The average number of repeating units in a chain Strength of a polymer is affected by D.P. D.P chain entanglements attractive forces between macromolecules strength.

2- Molecular weight: It is the number of repeating units multiplied by their molecular weight. it may range from thousands to millions. For a polymer it is an average due to the random growth of polymer chains.

3- Glass Transition Temperature: Tg Dental polymers at room temperature are either rubbers or hard. Rubbers consist of long chain molecules that are coiled. When the material is stretched the only work done is uncoiling of the molecules.

3- Glass Transition Temperature: Tg Effect of intermolecular forces increases when temperature decreases At certain temperature (Tg) intermolecular forces become so large, inhibit uncoiling.

3- Glass Transition Temperature: Tg Transformation from a rigid material to rubber-like characteristics upon heating.

3- Glass Transition Temperature: *Above glass transition (Tg) Polymer chains are free to move, polymers become rubber like and more elastic.

3- Glass Transition Temperature: *Below glass transition temperature (Tg) Polymer chains are "frozen" chains can not slip past each other polymers become glass-like stiff, strong and brittle

4- Copolymerization: - To modify the properties of a polymer it is often use two or more different chemical monomers Co-polymerization process enabled chemists to “tailor–make” molecules of predicted properties for special applications.

4- Copolymerization: Ex: Addition of Octyl M 4- Copolymerization: Ex: Addition of Octyl M.A results in soft and flexible polymer at mouth temperatures and has been used as a soft liner for denture. Hydroxyl-ethyl M.A, increase the water sorption and wettability by saliva. as soft contact lenses and soft liners.

5- Cross-Linking: -Cross-linking is joining of linear chains by covalent bond. -In rubbers, this is called vulcanization cross-linking strength, hardness, rigidity glass transition temperature.

5- Cross-Linking: - If all the chains are cross-linked together, a three-dimensional rigid polymer formed.

5- Cross-Linking: Small amount of cross-linking limits the movement of the polymer chains. The deformation is elastic rather than plastic. The polymer may have a higher glass transition temperature. Cross-linked polymers are harder and more brittle Less surface cracking or crazing. More resistant to the action of solvents e.g. acetone and alcohols

6- Plasticizers: -They are small molecules occupy positions between the polymer chains, increasing the inter-chain distance with a reduction in the secondary intermolecular bonding

6- Plasticizers: The addition of plasticizers resulting in: Flexible and ductile polymer. Glass transition temperature

6- Plasticizers: -Chains packed tightly together -Adding a plasticizer lets the chains slide

6- Plasticizers: Types: External plasticizers: by the addition of the plasticizer which penetrates between the polymer chains thus the chains become further apart and the forces between them become less.

6- Plasticizers: Types: Internal plasticizers: by copolymerization with a suitable co-monomer in this case the plasticizer is a part of the polymer chain.

7- Polymer crystallinity: Generally, polymers are amorphous structure, but in certain polymers the chains can align themselves to form highly ordered crystalline structure.

7- Polymer crystallinity: The more the crystallinity of the polymer: The higher density, The higher strength and hardness The higher rigidity, The higher melting temperature, Tg But the lower ductility. and the lower water sorption

Addition of inorganic fillers to the polymer forms composite structure That increases: strength hardness rigidity of the polymer.

Uses of polymers in dentistry 1- Denture base materials (may be flexible) 2- Acrylic teeth for partial and complete dentures 3-Soft linings

Uses of polymers in dentistry 4- Impression materials: *E.g. -Alginates -Rubbers

Uses of polymers in dentistry 5- Composite filling materials 6-Cements *E.g - Glass ionomer -Zinc polycarboxylate

Uses of polymers in dentistry 7-Maxillofacial prostheses

Uses of polymers in dentistry 8-root canal filling materials

Uses of polymers in dentistry 9- varnish 10- fissure sealants