IB: Polymers

polymers are used in

• prosthesis

• restorations

• equipment

• impression materials

• denture liners

• cements

oligomers

shorter versions of polymers; approx. less than 10 units

polymerization

chemical rxn in which monomers (low MW) are converted into polymers (high MW)

polymers can exist in three forms

• linear

• branched

• crosslinked

crosslinks

permanent connections between chains

copolymers

• formed by using more than one type of monomer

• random: order of different monomers is random

• block: large number of one monomer connected to another type

fundamental features of polymers

• chain length

• chain branching

• crosslinking

the longer the polymer chain

the more temporary connections/entanglements among chains —> the more difficult it is to distort material —> the higher the rigidity, strength, and melting temp

branched and crosslinked polymers have

stronger material/mechanical properties than linear polymers

tacticity

• relative position of side groups or branches of polymer from one unit to next; affects material properties

• types: isotactic, syndiotactic, atactic

isotactic polymer

relative position of side groups/branches of polymer is always the same

syndiotactic polymer

relative position of side groups/branches of polymer is alternating

atactic polymer

side groups/branches of polymer are randomly arranged

because of repeating structure of isotactic and syndiotactic polymers

can readily form regular arrays and crystallize

crystalline region

• highly ordered

• hard and rigid

• high melting point

• transparent or translucent

amorphous region

• random coils

• soft and flexible or glassy

• low glass transition temp

• transparent

combination (semi-crystalline region)

translucent or opaque

when a crystalline polymer is polymerized

crystalline regions begin forming at center of spherulite

elastic recovery

when material is deformed by rapidly returns to the original shape

elastic recovery occurs in

amorphous regions of polymers where randomly coiled chains straighten, recoil, and return to their original size and location in material

in elastic recovery, chains uncoil but do not

slip past one another due to amorphous regions, entanglements, or crosslinks

plastic deformation

when material is deformed and molded into a new, permanent shape

plastic deformation occurs when

polymer chains slide past one another and become relocated within material, resulting in permanent deformation

plastic “flow” and elastic recovery describe

ideal materials

factors that determine type of behavior

• chain length

• number of crosslinks

• temperature

• rate of force application

elastomers always have

small degree of permanent deformation

plastics always show

small degree of elastic recovery after deformation

drawing

when a tensile load is applied, random coils in amorphous regions are straightened and chains become oriented

effect of drawing

material becomes more crystalline and stiffer in tension because the flexible, random coils become aligned chains with less freedom to move —> slippage occurs and deformation becomes plastic

polymers tend to ___ rather than ___

absorb a solvent, swell, and soften; dissolve

the longer the chain length

the slower a polymer dissolves

crosslinks prevent

complete chain separation and inhibit dissolution

amorphous polymers swell more than

crystalline polymers (amorphous → more space → more solvent enters → more swelling but crystalline tightly packed → less solvent enters → less swelling)

plasticizing

when absorbed molecules (water) push chains apart and facilitate inter-chain slippage; cause lubricating effect

thermoplastics

materials that can soften (flow or melt) when heated and can be molded

thermoplastics undergo reversible process because

no new chemical bonds are formed

thermosets

materials where heat causes polymerization and crosslinking to make new bonds that lock material into a new shape

thermosets undergo non-reversible process because

once heat is applied and material is solidified, the material does not flow or melt

glass transition temperature

change in polymer from glassy and brittle to ductile or rubbery

two types of chemical reactions to join monomers

• addition

• condensation

addition polymerization

monomers are added sequentially to another in a chain-growth reaction

steps in addition polymerization

1) activation

2) initiation

3) propagation

4) termination

activation

application of energy to cause initiator to chemically decompose and form free radicals

initiation

breaking of monomer double bonds, causing them to begin reacting with other monomers

propagation

continuation of addition of monomer to activated, growing chain

termination

two free radicals react to form stable molecule; reaction stops

accelerator

• catalyst that makes the initiator unstable at ambient temps

• usually aromatic amines in dental materials

• always found in self-curing polymer resin systems

• additional heat is not needed

inhibitor

• additive that reacts with free radicals faster than monomers

• delay initiation and allow for sufficient working time

• act as preservative to extend storage life

• typically a hydroquinone

condensation polymerization

• all monomers react simultaneously

• catalyst activates monomer and eliminates small molecules (water, HCl) from polymer chain