POLYMERSGLASS TRANSITION PHENOMENATHEORY 1 MeltingandGlassTransition Phenomena Thetempatwhichpolymersbecomehard,brittle,glossyand abovewhichitissofterandflexibleisknownasglasstransition temp. e.g.PolyethylenehasTg1200CandTm1300C, Polystyrene Tg 1000C and Tm 2500C , Polysiloxane [osi(CH3)2]Tg 120 + Tm 900C. The hard and brittle state is the glossy state and soft flexible state istherubberyorviscoelasticsate.Ifviscoelasticstateisheated further,thepolymerbecomeaviscousliquidandcanflow.This sate is known as viscofluid state. Hard and brittleGlossy stateSoft and flexibleViscoelastic stateViscous liquidViscofluid stateTg Tm Temp The temperature below which the polymer is in rubbery sate and above which it is a liquid is called Tm. POLYMERSGLASS TRANSITION PHENOMENATHEORY 2 AboveBelowAbove Tm Moleculesin polymerdonot moveapartanddo nothave movementswith molecular chains. Stressproperty islostanditis brittle. Chainsegment withinmolecular chainmove internallybutthere isnomovements fromoneposition to other. Theyhave flexibilityorstress transfer property. Kineticenergyof polymericmolecule ishighenoughto causemovement withinthechain segmentaswellas tomovefromone position to another. Ithasflow character like liquid Factors affecting (Tg) :(i)Higher molecular weight greater is Tg. (ii)A greater degree of crosslinked has higher Tg. (iii)Greater flexibility, then smaller is Tg. (iv)Adding a plasticizer to polymer decreases the Tg. (v)Polymerhavingstrongerintermolecularforcesofattraction have greater Tg. POLYMERSGLASS TRANSITION PHENOMENATHEORY 3 Viscoelasticity of polymers : Low molecular weight materials are either elastic solid of viscous liquids.Solidenergywhenexternalforceisappliedfor deformation and when released it regains original shape. Inviscousliquidthereisnopropershapehencewhenexternal like solid 7 are applied it flows out irreversibly. Silicon rubber flows over of hours, fractures like solid and deforms rapidly and also bounces like elastomers.