SiliconesSilicones

AndyAndy TseTse CE435CE435 IntroductionIntroduction zz OtherwiseOtherwise knownknown asas polysiloxanespolysiloxanes.. zz MadeMade upup ofof anan inorganicinorganic siliconsilicon--oxygenoxygen backbonebackbone ((……--SiSi--OO--SiSi--OO--……)) withwith twotwo sideside groupsgroups connectedconnected toto thethe SiSi.. zz MostMost popularpopular andand commoncommon typetype isis polydimethylsiloxanepolydimethylsiloxane (PDMS).(PDMS). zz ““SiliconeSilicone”” waswas mistakenlymistakenly derivedderived fromfrom ketoneketone.. PolydimethylsiloxanePolydimethylsiloxane (PDMS)(PDMS) zz InorganicInorganic SiSi--OO backbonebackbone withwith organicorganic CHCH3 sideside groups.groups.

zz ChemicalChemical formulaformula isis

(CH(CH3))3SiO[SiO(CHSiO[SiO(CH3))2]]nSi(CHSi(CH3))3. PDMSPDMS zz ““SiliconeSilicone oilsoils”” zz MolecularMolecular Structure:Structure: LinearLinear zz RepeatingRepeating Monomer:Monomer: SiO(CHSiO(CH3))2 zz PhysicalPhysical State:State: GlassyGlassy (Amorphous(Amorphous Solid)Solid) zz PolymerizationPolymerization Formations:Formations: StepStep--growth,growth, RingRing--openingopening zz PolymerizationPolymerization Method:Method: SolutionSolution PDMSPDMS z SiSi--OO backbonebackbone chainchain isis uniquelyuniquely flexible.flexible. TheThe bondbond betweenbetween SiSi andand OO cancan easilyeasily openopen andand close.close.

z HasHas nono energyenergy barrierbarrier forfor rotation.rotation. z GlassGlass--transitiontransition temperaturetemperature ((TTg)):: --127127 °°CC z MeltMelt temperaturetemperature (T(Tm):): --5454 °°CC SynthesisSynthesis ofof PDMSPDMS z Begins with the hydrolysis of dimethylchlorosilane in presence of excess water.

(CH3)2SiCl2 Æ (CH3)2Si(OH)2 → HO[- (CH3)2SiO]n-H + (CH3)2SiO]m H2O - HCl -H2O n = 2,3,4,… m = 3,4,5… linear cyclic z Linear and cyclic oligomers are obtained. The ratio of cyclic to linear compounds as well as the chain length of the linear siloxanes may be controlled by the conditions of hydrolysis. The linear oligomers are condensed and the cyclic oligomers are polymerized to form the final product. PolycondensationPolycondensation zz TheThe silanolsilanol groupsgroups inin linearlinear oligomersoligomers areare condensed,condensed, aa reactionreaction catalyzedcatalyzed byby acidsacids oror bases,bases, andand thethe bondsbonds ofof thethe siloxanesiloxane areare cleaved.cleaved. ThisThis condensationcondensation methodmethod isis essentiallyessentially aa stepstep--growthgrowth polymerization.polymerization. zz TheThe HOHO--HH isis removedremoved fromfrom thethe systemsystem andand PDMSPDMS isis formed.formed. PolymerizationPolymerization z A strong base, such as KOH, is used to break the cyclic oligomer. The initial step is the nucleophilic attack of the anion on the silicon. This shifts the pair of electrons from the Si-O bond solely to the oxygen. This break opens the bond and in effect, the ring. This process keeps repeating which creates more monomers and allows it to grow linearly. This method is essentially a ring-opening polymerization. MethodMethod ofof PolymerizationPolymerization z SinceSince thethe initialinitial processprocess inin formingforming PDMSPDMS requiresrequires water,water, itit willwill occuroccur inin aa systemsystem ofof solutionsolution polymerization.polymerization. z System:System: Monomer:Monomer: DimethylchlorosilaneDimethylchlorosilane DiluentDiluent:: WaterWater Initiator:Initiator: HClHCl Polymer:Polymer: PDMSPDMS PropertiesProperties z PDMSPDMS cancan bebe strengthenedstrengthened andand crystallizedcrystallized byby physicalphysical andand chemicalchemical crosscross--links.links. z TheThe flexibleflexible backbonebackbone becomebecome looselyloosely entangledentangled whenwhen molecularmolecular weightweight isis high,high, whichwhich resultsresults inin aa highhigh viscoelasticityviscoelasticity.. z ViscosityViscosity isis linearlylinearly proportionalproportional toto averageaverage chainchain length.length. z AtAt lowlow temperatures,temperatures, itit actsacts likelike aa viscousviscous liquid,liquid, andand atat highhigh temperatures,temperatures, itit actsacts rubbery.rubbery. PropertiesProperties z Low surface tension (20.4 mN/m) indicates low intermolecular forces. z Low intermolecular forces explain high solubility and high diffusion coefficient of gases into PDMS. z Critical surface tension of wetting (24 mN/m) gives itself the capability of wetting itself, a property that promotes good film formation and good surface covering. z High thermal stability results from the strong siloxane bond. PDMS can withstand temperatures above 200 °C. z Low-temperature changes vs. temperature z High compressibility z Hydrophobicity z DielectDielectricric properties z Physiological inertness z Energy intensive and cannot be recycled. DataData z Attributes Range Price ($/kg) 7.20 - 17.20

Density (Mg/m3) 1.1 - 2.3 Elasticity Modulus (GPa) 0.001 - 0.05 Elongation (%) 0.8 - 5.3 Fracture toughness (MPa*m1/2) 0.03 - 0.7 Yield strength (MPa) 2.4 - 5.5 Service Temperature (°C) -100 - 300 Specific Heat (J/kg*K) 1050 - 1300 Thermal conductivity (W/m*K) 0.2 - 2.55 Thermal expansion (10-6/K) 250 - 300 Energy content (MJ/kg) 175 - 190 Recycle potential low ComparisonComparison z Competing Materials: Polyurethanes, EVA z Features compared to other polymers. + cold temperature + corrosion resistant + damping + FDA approval + flame retardant + heavy + hot temperatures -- impact + resilient -- strong -- wear resistant UsesUses andand ProductsProducts z CaulkingCaulking andand sealantssealants z AdhesivesAdhesives z GreaseGrease andand otherother lubricantslubricants z DefoamingDefoaming agentsagents z DampingDamping fluidsfluids z HeatHeat transfertransfer fluidsfluids z CosmeticsCosmetics z FillerFiller fluidfluid inin breastbreast implantsimplants z SillySilly putty*putty* SillySilly PuttyPutty z UniqueUnique material.material. z PDMSPDMS isis mixedmixed withwith boricboric acidacid formingforming aa softsoft andand pilablepilable mixture.mixture. z AbleAble toto bebe moldedmolded intointo anyany shape.shape. z MaterialMaterial isis veryvery bouncy.bouncy. z GentlyGently flowsflows whenwhen pushed.pushed. z CracksCracks whenwhen hithit hardhard withwith aa hammer.hammer. z WhenWhen spreadspread overover newspapernewspaper andand pulledpulled away,away, imageimage ofof newspapernewspaper texttext isis formedformed onon putty.putty. TheThe EndEnd