Dr.Reem Gamal

Dr.Reem Gamal

POLYMERS

Definition:- Polymers A monomer Polymerization reaction

CLASSIFICATIONS OF POLYMERS

Classifications according to

Molecular characteristics

Stress strain

behavior

mechanical response at

elevated temperature

Origin

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1)According to origin A) 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.

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A) Synthetic Polymers Acrylic resin Rubber impression

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2)According to molecular characteristics

According to molecular characteristics

Molecular chemistry size shape Spatial

structure

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-Molecular chemistry: 1. Homopolymer 2. Copolymer 3. Terpolymer Copolymer may be:- • Random • Alternating • Block • Graft Block and graft polymer show improve in impact strength.

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Random copolymer Alternating copolymer Block copolymer Graft copolymer Dr.Reem Gamal

According to molecular shape:- Straight Bent Twisted kinked

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According to molecular structure (spatial structure)

-Linear -Branched -Cross-linked

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-Network

2

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3- According to mechanical response at elevated temperature

a) Thermosetting b) Thermoplastic

a)Thermoplastic They are polymers which can be shaped by heat and after cooling they maintain their shape the reaction is reversible and can be repeated

heat cooling Hared soft hared

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Characteristics Thermoplastic polymers:- • The polymer chains are bonded to each other by secondary bond. • They soften by heating harden by cooling in reversible repeated

process why? Secondary bond forces diminished by heat so relative movement

of adjacent chain happened but over heating may lead to break the primary covenant bond in the chain it self

• They are relatively soft and ductile and their mechanical properties are

sensitive to heat. • They dissolve in organic solvent

e.g linear polymers polyvinyl acrylate, polyethylene, polyester Linear PMMA, non-crosslinked PMMA

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Harden during fabrication (irreversible or chemical reaction). Heat lead to increase cross linking Chemical Soft hard reaction Examples:- cross-linked polymethylmethacrylate, silicons , and bisphenol-dimethacrylates.

b) Thermoset polymers

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Thermosetting polymers •They are formed into a permanent shape and set by chemical reaction. •The polymers chain form cross –links between them by heat (primary covalent bond).

•They can not be remelted and reformed into another shape but degrade or decompose up on being heated to high temperature..(can not be fused)

•They are harder stronger more brittle than thermoplastic polymers e.g Most network and cross linked polymers

Characteristics

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3)According to stress strain behavior

Brittle. Plastic polymer. Elastomers.

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Dental application

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Polymerization

1-condensation polymer ization Monomer with functional groups such as amide, urethane or sulfide join together with elimination of smaller molecule (often but not always water) (Step growth polymerization). Example is the polysulfide rubber impression material water and lead sulfide are by product in this reaction

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2)Addition polymerization

The reaction between two monomer unites (have C=C) give a larger molecule without the elimination of a smaller molecule (no by-product). No change in composition takes place E.g:- Poly (ethylene), poly (acrylic acid), polymethacrylic acid) poly (methacrylate).

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STAGES OF ADDITION POLYMERIZATION

Initiation

Activation and initiation (by free radical mechanism)

STAGES OF ADDITION POLYMERIZATION

Termination Propagation

C B A

Direct coupling

Exchange of a hydrogen atom

Chain transfer Dr.Reem Gamal

Three stages of addition polymerization Heat

1) Initiation C6H5 COOCC6H5 2 (C6H5COO)* + CO2

intiator Benzoyl peroxide free radicals (R*) + carbon dioxide CH3 CH3

R*+ CH2 = C R-CH2 – C* COOCH3 COOCH3

Free radical + monomer Free radical activated monomer (R*) (M) (RM*)

Heat

Amines Activator

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The activation is done by -Physical • light, ultraviolet, laser • Heat -Chemical such as tertiary amine (dimethyl paratoluidine) or mercaptant.

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B) Propagation

RM* +M RMM* +M RMMM* RMN where N is any integral number. polymer free radical + monomer growing chain Actually some free monomers remain unreacted and become trapped

within the polymer WHY? due to Rapid rate of reaction and fast increase of viscosity. These unreacted (Residual) monomer usually lead to reduce stability,

physical, mechanical and biological properties of the final polymer Dr.Reem Gamal

C) Termination

The reaction terminated either by

a) Direct coupling This occurs when two free radicals react to form a stable molecule 1- R1MN* + R2MN* R1MNNMR2

b) Exchange of a hydrogen atoms one hydrogen atom transfers from one growing chain to

another.

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hydrogen atom exchange H H H H by Exchange 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 Dr.Reem Gamal

c) Chain transfer H H H H by transfer of R1Mn – C – C *+ R2Mm – C – C – H H H H H a hydrogen atom

H H H H R1Mn – C– C – H + R2Mm – C – C* H H H H

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Factors associated with polymerization

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A) Evolution of heat as the reactions is strongly exothermic why? due to breaking of bonds C=C. B) Reduction in volume (Polymer ization shr inkage) why? due to covalent bond between monomer occupy less total volume than polymer (when the monomer next to one another in liquid state attracted by secondary boned) . C) Presence of residual monomer

Polymer additives 1. Plasticizers 2. Stabilizers and inhibitors 3. Cross-linking agents 4. colorants

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Plasticizers Type : - - External plasticizer Liquids are able to penetrate between the

chains of polymer. - Internal plasticizer ( comonomer accomplished by

copolymerization) Effect on the polymer :- 1. Improve flexibility, ductility and toughness of polymers. 2. They reduce glass transition temperature and fusion temperature

of polymer.it is used in producing soft lining materials 3. Soften the material reduce strength and hardness

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Stabilizers and inhibitors

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-It can react either with the activated initiator or any activated nucleus (decrease rate of initiation) Or - It react with an activated growing chain to prevent further

growth (increase rate of termination and decrease molecular weight)

-Eg hydroquinone, eugenol & large amount of O2 its added to monomer during storage to prevent polymerization.

CROSS-LINKING AGENTS Cross linking:- Is a chemical (primary) bond formed between

polymer chains. (called vulcanization In rubber ) Cross linking can achieved by non reversible

chemical reaction or by addition of cross linking agent at room T

eg glycol dimethaerylate N.B:-It is the best type used for construction of

denture base and artificial teeth

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Characters of cross linking polymers • The deformation is elastic rather than plastic more

brittle (extensive cross linking lead to brittle material).

• The polymer have high glass transition temperature. • The polymer is stronger and harder • more resistance to the action of solvents .( Higher

resistance to crazing which may be due to Alcohol drinking or solvents)

• Low water sorption

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Colorants: Dyes or pigments Molecules in dye dissolve and become part

of the molecular structure of the polymer (usually organic)

Pigments are usually fillers of very small

size (usually inorganic) don’t dissolve remain as a separate phase its transparent or opaque. (more stable, durable, permanent)

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Polymer properties

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1. Molecular weight and degree of polymerization 2. Polymer crystallinity 3. melting and glass transition temperature 4. mechanical properties 5. viscoelastic creep 6. crazing

Molecular weight and degree of polymerization:

They have extremely high molecular weight from thousand to million.

The molecular weight of polymer = MW of various mers X the total

number of mers in the molecule.(Degree of polymerization) It is usually expressed by the average molecular weight because

molecules may differ in its length (no. of repeating unites) due to different types of termination.(tow polymer may have the same M.W put differ in the properties)

Average degree of polymerization =total no. of structural unites (mers) total no. of molecules = M.W of a polymer M.W of a mer

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The molecular weight and degree of polymerization have

high influence on the polymer properties High molecular weight polymers usually have 1. better mechanical properties,(stronger more resistance to

mechanical stresses 2. better resistance to solvents 3. High softening or melting temperature.

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Polymer crystallinity-2 Polymers are Usually partial crystalline (short range order) due its

high molecular weigh and complexity of the polymer. Factors affect crystallinity of polymers:- • Molecular chemistry and Chain configuration (crystallinity is not favored

in complex polymers) • Side chains interfere with crystallinity • Co-Polymerization interfere with crystallinity • Network polymers tend to be amorphous • High molecular weight polymer decrease chance of crystallinity (move to

order) • Additives interfere with crystallinity (plasticizer) Characters of Crystalline polymers usually have • High density, • High strength • High melting temperature (resistance to soft by T) • More resistant to dissolution

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Melting and glass transition -3temperature

Glass transition temperature (Tg): is the temperature at which the amorphous or partially amorphous material transform from rigid (solid) state to rubber like or viscous state.( no liquefaction occurs at this temperature)

NB: • Elastomers which are polymers behaving like rubber at room

temperature have their (Tg) below the room temperature. • High MW polymers have high Tg • Factors restrict molecular motion cause increase Tg • Plasticizer decrease Tg

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Mechanical properties;-4 Mechanical properties are usually inferior to that of metals and ceramics. Mechanical properties depend on: 1. rate of deformation (strain rate dependent) slow rate of loading they

behave in ductile manner(more permanent deformation),high rate of loading they respond in a brittle manner)

2. Temperature 3. chemical nature of the environment 4. degree of crystallinity 5. cross-linking 6. molecular weight of polymer 7. degree of polymerization

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Viscoelastic creep-5 Susceptibility of polymer to creep decrease as the degree

of crestallinity increase

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Crazing-6 Crazes are fine cracks (usually 5μm ) that form at

highly mechanical stressed regions, or due to thermal stress or due to solvent action.

They usually propagate perpendicular to the tensile

stress axis forming cracks which further propagate and lead to fracture

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Factors affecting the properties of polymers

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Many factors affect the properties of polymers 1. Chemical composition of the chain. copolymerization 2. Degree of polymerization, and molecular weight 3. The cross – links between polymer chains 4. Spatial structure (branched linear) 5. Plasticizers 6. Crystallinity 7. Rate of loading 8. Temperature 9. Fillers

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