Liquid Silicone Rubber Vulcanizates: Network Structure ... · PDF fileLiquid Silicone Rubber...

477Polymers & Polymer Composites, Vol. 18, No. 9, 2010

Liquid Silicone Rubber Vulcanizates: Network Structure - Property Relationship and Cure Kinetics

*Correspondingauthor:E-mail:[email protected]/[email protected]. Tel:91-3222-283180,Fax:91-3222-220312†Presentaddress:IndianInstituteofTechnology,Patna-800013,India

©SmithersRapraTechnology,2010

INTRODUCTION

Silicone rubber is a special synthetic elastomer, which provides uniqueproperties such as low temperature flexibility, excellent thermal andoxidative resistance, low surfacet ens ion and good d ie lec t r i c properties1. Liquid silicone rubber(LSR)combinestheadvantagesoftheexcellentpropertiesofsiliconerubberand effective way of processing of high precision articles such as seals, electrical connectors, sealing membranes, infant products, breast implants, etc.2-4. LSRs, firstcommercialized in 1976-77 byDow Corning Corporation5, are essentially two part systems cured by hydrosilylation reaction.

Hydrosilylation reaction6-7 is the addition of a hydridosiloxane orhydridosilane group to an unsaturated carbon-carbon bond end capped vinyl group in siloxane elastomers,catalyzed by a transition metal catalyst such as Speier’s catalyst8 and Karstedt’s catalyst9-10. The two components of LSR are Part A:vinyl-end capped PDMS, platinumcatalyst, inhibitor11-14 and Part B:polymethylhydrogenosi loxane(PMHS)(Si-Hgroups).Thisreactionisexothermicinnatureandoccursevenat room temperature. Hence, LSRs are also categorized as Room Temperature Vulcanizates(RTVs).Generally, thetwopartsarekeptseparatelybeforecuringtoavoidanypartialcrosslinkingduring storage15.

Comparedtoperoxide–curedsiliconerubber, curing by hydrosilylation requiresloweramountofcrosslinker.Itislesstoxicandshowsexceptionaltoughness, tensile strength and low porosity. The position and concentration ofbothvinylandhydride(Si-Hgroups)groups along the siloxane backbonedecide the nature of the elastomeric network. Jia et al. have designed silicone rubber based on multi-objective optimization of chemicalreactions,carriedoutfiniteelementalsimulation of mechanical property and made an integrated processing-structure-property analysis on rubber in-mould vulcanization16-18. However, a detailedcorrelationbetweencrosslinkdensity and thermal and physico-mechanical properties of the various silicone rubber vulcanizates has been donehere.Also,whateverpropertieshavebeenreportedaremostlyonfilledrubber19-20 or complicated systems21-22, which preclude any understanding.


G.Rajesh1, Pradip K. Maji1, Mithun Bhattacharya1, Anusuya Choudhury1, Nabarun Roy1, Anubhav Saxena2 and Anil K. Bhowmick1*† 1RubberTechnologyCentre,IndianInstituteofTechnology,Kharagpur-721302,India2MomentivePerformanceMaterialsPrograms,GeneralElectricIndiaTechnologyCentre,WhitefieldRoad,Bangalore-560066,India

Received: 15 February 2010, Accepted: 19 July 2010

SUMMARYLiquidsiliconerubber(LSR)vulcanizateshavebeenpreparedfromvinyl-endcappedpolydimethylsiloxane(PDMS)andpolymethylhydrogenosiloxane(PMHS)inpresenceofplatinumcatalystusingsolutioncastingtechnique.Thescienceinvolvingsynthesisofusablesiliconerubberbycuringwithpolymethylhydrogenosiloxanehasbeenstudiedindetails.TheeffectsofvinylcontentofPDMSandtheratioofhydridecrosslinkertovinylconcentrationoncrosslinkdensity,mechanical,dynamic-mechanicalandthermalpropertieshavebeendiscussed.Ithasbeenfoundthatatoptimumlevelofcrosslinker,theLSRvulcanizateshowsamaximumcrosslinkdensitywithhighestimprovementinmechanicalandthermalproperties.Theamountofcrosslinkerrequired,dependsupon the number of vinyl functionality per unit length of the polymer chain and the chemical composition of thehydridecrosslinker.ThisstudymakesanovelattempttodeterminecurekineticsofPDMSbyusingFourierTransform Infrared spectroscopy. Measurements have been done at three different temperatures and found to followthefirstorderreactionwithrespecttoconsumptionofvinyl(C=C)andsilylhydridegroups(Si-H).Theactivationenergyandkineticparametershavebeendeterminedaswell.

478 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

G. Rajesh, Pradip K. Maji, Mithun Bhattacharya, Anusuya Choudhury, Nabarun Roy, Anubhav Saxena and Anil K. Bhowmick

Most of the investigations published are in the form of patents23-28. To thebestofourknowledge,literaturereferencedealingwithcurekineticsofhydrosilylation-cured silicone rubber using FTIR spectroscopy has not been reported,althoughsimilar structure–property relationship has been carried out on other polymeric systems29-33.

Theobjectiveofthepresentworkisto study the variation of vinyl content of PDMS and the ratio of hydridecrosslinker to vinyl concentrationon crosslink density, mechanical,dynamic-mechanical and thermal properties. Also, an attempt hasbeenmadetostudythecrosslinkingkineticsofsiliconerubberinabsenceof inhibitor using FTIR spectroscopy. Theamountofcrosslinkerhasbeenoptimized through mechanical and crosslink density measurements.Exponential fittings have beensuccessfully applied to present the change in the absorption of double bonds (C=C groups) and hydride(Si-H)groupsasafunctionoftime.Thekineticparameterswhichaffectthe reaction rate have also been determined. This is a part of our ongoing studies of structure-property relationship of various special purpose elastomeric materials30-34.

2. EXPERIMENTAL

2.1 MaterialsTwo vinyl terminated PDMSliquid silicones U-10 and U-65,hydride crosslinker polymethyl-hydrogenosiloxane (V430) andplatinumcatalyst(Karstedt’scatalyst)were all supplied by Momentive

Performance Materials, Bangalore,India. The respective vinyl content of U-10andU-65are0.05mmol/gand0.03mmol/gwithmolecularweights74,400and85,400determinedbyGPC(ASTM D6474-99) and viscosities10 and 65 Pa.s determined usingOstwaldviscometer(ASTMD4283-98)respectively.Thecrosslinker,V430hasahydridecontentof4.3mmol/g.The structures are given in Figure 1. Toluene and trichloroethylene (TCE) used as solvent and inhibitorrespectively, were purchased from MerckIndiaLtd,Mumbai.

2.2 Preparation of Liquid Silicone Rubber Vulcanizates ThevinylendcappedPDMS(U-10,U-65),hydridecrosslinker(V430)andplatinumcatalystweremixedinvaryingamounts, as summarized in Table 1. VinylendcappedPDMSandplatinumcatalyst with trichloroethylene were dissolved in toluene to formPartA.HydridecrosslinkerwhichformsPartB was then mixed with this usingmagneticstirrerfor15minat30°C.Theresulting sample was degassed using ultrasonication and cast into Teflon

Table 1. Formulations for Crosslinking* of U-10 and U-65

IngredientsWeight (%)

A1 B1 A2 B2 A4 B4 A6 B6

PartAU-10 100 - 100 - 100 - 100 -U-65 - 100 - 100 - 100 - 100

Ptcatalystinsolution(100ppm) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

PartBV430 1.1 1.2 2.3 1.4 4.6 2.6 7.0 4.2TCE 100 100 100 100 100 100 100 100

*A1, A2, A4, A6 and B1, B2, B4, B6 correspond to [hydride]/ [vinyl] of 1, 2, 4, 6 of U -10 and U-65 respectively

Figure 1. Structure of (a) vinyl end-capped PDMS (b) polymethylhydrogenosiloxane (V-430), where x and y are 10 (c) Karstedt’s catalyst



moulds and cured at room temperature (30 °C) for24h.The sampleswerefurtherpost-curedat100°C for6hunder vacuum to remove volatiles and solvents. The amount of V430was calculated to vary the ratio of the concentration of Si-H to vinyl. The thicknesses of sheets obtained wereintherangeof1.2-1.3mm.

2.3 Characterization2.3.1 Gel Fraction Previously weighed samples were allowedtoswellintolueneat30°Ctilltheattainmentofequilibriumswellingtime(72h,whichwasdeterminedfromtheplotofmassuptakeagainsttime).Thetestpiecesweretakenoutanddriedto a constant weight in vacuum oven at 70°C.Thegelfractionwascalculatedfrom the weight of sample before and afterswellingusingEquation(1).

Gelcontent(%)=(W2/W1)x100(1)

where, W1=initialweightofthepolymerW2=weightoftheinsolubleportionof the polymer

The volume fraction in the swollen gel(Vr)wascalculatedusingEquation(2)35.

Vr =Ds − FfAw( )ρr−1

Ds −FfAw( )ρr−1 +Asρs−1 (2)

where,Aw =weightoftestspecimen.Ds = de-swollen weight of the testspecimen.Ff = weight fraction of insolublecomponents.As = weight of absorbed solventcorrected for swelling increment.ρr =densityofrubber.ρs=densityofthesolvent

The apparent crosslink density wascalculated according to Flory-Rehner36 equationusingthevalueofVr as follows [Equation(3)]

− ln 1−Vr( ) +Vr + χVr2

= ρrMc

×Vs Vr1/3 −

Vr2

(3)

where, χ = Flory–Huggins polymer-solventinteraction parameterρr=densityofrubberM c = molecular weight betweencrosslinksVs=molarvolumeofthesolvent

The results shown are mean values of three experiments performed foreach sample. The mean error of the measurementwas±3%fortheabovemeasurements.

2.3.2 Mechanical Properties Tensile specimens were punched out from the cast sheets using ASTMDie-C.ThetestswerecarriedoutasperASTMD412-98methodinaUniversalTestingMachine(ZwickRoellZ010,Ulm, Germany) at a cross-head speed of500mm/minat25°C.Themeanerror of the measurement was ±3%for the tensile strength and modulus at 100% elongation. For elongationatbreakvalues,theerrorwas±10%.

2.3.3 Dynamic Mechanical Analysis (DMA)The dynamic mechanical properties of the samples were obtained by using DMAofTAinstruments(modelQ800).Thesamplespecimens(10mm×9mm× 0.5 mm) were analyzed in shearmodefrom-135to50°Cataconstantfrequencyof1Hz,astrainof0.1%andaheatingrateof3°C/min.Thedatawereanalyzed by TA Universal analysissoftware. Storagemodulus (G´) and losstangent(tanδ) were measured as a function of temperature for all the samples under identical conditions.

2.3.4 Thermogravimetric Analysis (TGA)Thermogravimetric analysis was doneusingPerkinElmerInstrument,

Diamond TG-DTA. The samples(3–5mg)wereheated fromambienttemperatureto700°Cinthefurnaceof the instrument under oxygenatmosphere at 100 ml/min and at aheatingrateof20°C/minandthedataof weight loss versus temperature were recorded.

2.3.5 Fourier Transform Infrared (FTIR) SpectroscopyFor determining the rate of curing reaction in terms of disappearance of absorptionsofC=CandSi-Hbonds,inhibitorwasnotadded inpartBasit may introduce fallacy in the results obtained.ThehomogenousmixtureofPartAandPartBwithoutanyinhibitorandsolventwasquicklypouredintoatransparentKBrplateanddriedforfewminutes and thereafter spectra were recorded at regular interval with a PerkinElmerFTIR-spectrophotometer(modelRX1)intherangeof4000cm-1

-400cm-1 at resolution of 4 cm-1. The decrease of nSi-H stretching absorption bandat2157cm-1 and nC=C stretching band at 1586 cm-1 were taken intoaccount for the kinetic analysis.Thebandat1944cm-1was takenasinternal standard37 for FTIR analysis. The reactions were carried out at three different temperatures, i.e. 55 °C,60°Cand75°Cusingstoichiometricconditions viz. [Si-H]/[C=C] = 4.0which was optimized from mechanical andcrosslinkdensitymeasurements.

The thermodynamic parameter of activationenergy(Ea) was calculated usingArrheniusequation(4)29

ln k = −EaRT

+ ln A (4)

where T is the temperature in absolute scale, R (=8.314 J mol-1K-1), the universalgasconstant,A, frequencyfactor, Ea, the activation energy and k,thekineticrateconstant.



3. RESULTS AND DISCUSSION

3.1GelContentandCrosslinkDensity of LSR/PMHS VulcanizatesTable 2givesthevaluesofcrosslinkdensity Mc , Vr and gelcontentofU-10andU-65vulcanizates.Fromthedataobtained it can be clearly seen that the crosslinkdensityforU-65equilibrateswhen the hydride concentration is doublethatofvinyl,whereasforU-10the saturation is observed when the hydridecontentisquadrupled.

ThisisbecauseU-65haslessernumberof vinyl groups per unit length, as compared to U-10, which requireshigher hydride concentration due to the presence of greater number of vinyl groups in the polymer chain. The incrementincrosslinkdensityvalueswith increase in [hydride] / [vinyl]ratios can be attributed to increase in ethylenelinkagesatvinylendofPDMSleadingtothreedimensionalnetworkstructures.Withtheincreaseinamountofhydridecrosslinker,thegelcontentincreases to certain level and then stabilises. It may be interesting to note that there is deviation of the theoretical ratio(1:1)of[hydride]/[vinyl],whichhasbeendiscussedinthesubsequentsections. Though, theoretically only a 1:1[hydride]/[vinyl]ratioisrequired,but it has been found that a ratio varying from 1.2:1 to 6:1 can be used forplatinumcatalystcuredvinylPDMSsystems38. The ratio entirely depends upon the chemical composition of the

vinyl-end capped silicones as well as the silicone hydride. In this paper, we have worked with two types ofvinyl silicones which are chemically similar but differ in molecular weight. Further,thehydridecrosslinkerisalsoa poly-dimethyl-co-methylhydrogen-siloxanecopolymerwithSi-Hcontentof0.43%.Boththesefactorscombineto yield the 4:1 ratio in our system.For practical purposes, the silicone hydrides are those which are designed to have one silicon-bonded hydrogen atom per silicon atom which accounts for a theoretically 1:1 ratio39. Since, the silicone hydride used here is a copolymer this accounts for its higher concentration than the theoretical one.

3.2 Mechanical and Dynamic Mechanical Properties of LSR/PMHS VulcanizatesThe mechanical properties of the vulcanizatesofU-10andU-65curedwithdifferentratiosofcrosslinkeraregiven in Figure 2a-d. It can be inferred that at the optimal hydride concentration, themosteffectivecrosslinkingoccursin the PDMS network.As shown inTable 2, hydride to vinyl concentration ratio of four and two produce the highestcrosslinkdensityforU-10andU-65,respectively.Thecorrespondingvulcanizates show maximum tensilestrength and modulus values, as well (Figure 2c).The elongation at break(Figure 2d) for U-10 and U-65 arelowest for their optimal hydride to vinyl concentration ratios of four and two,respectively.Thevaluesare150

and160%respectively.Thedecreasein elongation at break is due to thesmaller Mc(shorterchainsbetweenthecrosslinking points) which increasescrosslinkdensity40.

The storage shearmodulus (logG´)andlosstangent(tanδ) variation with temperature for U-10 and U-65 atoptimumlevelofcrosslinkeraregivenin Figure 3.An essential change instoragemodulus-temperatureprofileofthe silicone rubber can be observed by adding higher concentration of Si-H. Increase of Si-H concentration results in an increase in the log G´ value because of more efficient crosslinking. Thedynamic mechanical properties of the vulcanizates at selected temperatures are reported in Table 3. All LSRvulcanizates samples display three transitions, which can be clearly seen from tan δ versus temperature plots. Table 4 shows the details of various transitions in LSR vulcanizates.

The f i rs t t ransi t ion 41 knownas ß transition or glass transition temperature (Tg) occurs at -115 °Cto -104 °C depending on the levelof crosslinking. A4 and B2 exhibithigher glass transition temperature than A1 andB1 respectively. Increase incrosslinkdensityresultsinrestrictioninmobility of polymer chains and thereby increases the Tg. The second transition42 at -50 °C corresponds to crystallinepeak known as the temperature ofcrystallization(Tc), whereas the third43 oneat-35°Ciscausedbycrystallinemelting,knownascrystallinemeltingpoint(Tm)ofPDMS.

3.3 Thermogravimetric Analysis(TGA)ofLSR/PMHSVulcanizatesThe comparative thermal stabilities ofcuredsamplesofU-10andU-65atoptimum[hydride]/[vinyl]ratiosaregiven in Figure 4.

The characteristic degradation such asonsetdegradationtemperature(Ti) and temperature atwhichmaximum

Table2.Gelcontent,volumefractionofrubberinswollengel(Vr) and molecular weight between the crosslinks (Mc) for U-10/U-65 vulcanizatesCompound Vr Gelcontent(%) Mc (g/mol)A1 0.132 86 16,025A2 0.209 91 4963A4 0.226 95 4046A6 0.227 96 3900B1 0.142 91 13,386B2 0.219 94 4381B4 0.221 95 4317B6 0.223 96 4214



Figure 2. Stress-strain plots of (a) U-10 vulcanizates (b) U-65 vulcanizates, (c) Tensile strength and modulus at 100 % elongation vs. [hydride]/ [vinyl] ratio (b) % Elongation at break vs. [hydride]/ [vinyl] ratio

Table 3. Storage modulus and tan δ values of LSR vulcanizates at different temperaturesSamples -125 °C -50 °C 40 °C

LogG´(Pa) tan δ LogG´(Pa) tan δ LogG´(Pa) tan δA1 9.20 0.03 8.38 0.37 6.53 0.14A4 9.20 0.05 8.24 0.57 7.13 0.02B1 9.11 0.03 8.60 0.23 6.53 0.12B2 8.97 0.08 8.32 0.45 6.77 0.03

Table4.DifferenttransitionsinDMAandTGAplotsofLSRvulcanizates

Samples DMA TGAT g(°C) T c(°C) Tm(°C) Ti (°C) Tmax (°C)

A1 -117 -52 -35 395 433A4 -104 -50 -36 415 480B1 -115 -49 -34 406 356B2 -110 -45 -35 420 453



degradation(Tmax) are given in Table 4. The results unanimously indicate that thermalstabilityofthecrosslinkedLSRvulcanizates increases with increase in [hydride]/[vinyl]ratio.Inearlystagedegradation, the formation of side chain peroxidesoccurs,followedbyspittingof the polymer substituents44.

3.4 Curing Kinetics Study Using FTIR SpectroscopyFrom the c ross l ink dens i tymeasurement and analysis of physico-

mechanical properties, the ratio of [hydride]/[vinyl]wasoptimizedat4:1for complete curing of the samples. Studyofcurekineticsdealswiththerate of disappearance of the absorption peakforC=CandSi-Hfunctionalityas a function of time and temperature. WhilepreparingthesampleswehaveusedTCEas the inhibitor since thiswould lead to a controlled process of curing, eliminating problems such as solvent entrapment and formation of bubbles which might leadtodeteriorationofproperties.But

presence of inhibitor in the system will leadtoincorrectresultsinthekineticsstudy. That is the reason why addition ofTCEwasintentionallyavoided.

PDMS shows several characteristicabsorptions in the FTIR spectra. The peakswhicharecommonlyobservedare at 2962 cm-1 for C-H stretch,2157cm-1forSi-Hstretch,1586cm-1 for C=Cstretch,1402cm-1forCH2 in plane deformation for Si-CH=CH2moiety, 1257cm-1 for symmetrical deformation vibrationofCH3,1000-1100cm

-1 for broad Si-O stretch indicating high molecularweight,928cm-1 for Si-H deformation,769cm-1forCH2rockingvibration,751cm-1forSi-Cstretchingvibration45. Scheme 1 describes the basic mechanism of crosslinking ofsilicone rubber using hydrosilylation reaction46.

The crosslinking process has beenstudied using the FTIR spectroscopy. The nSi-H stretching absorption band at2157cm-1 and nC=Cat1586cm

-1 are absent from the spectra of the addition-cured silicone rubber.

The reaction between unsaturated vinyl and Si-H groups can be used to determine the degree of curing reaction asfollows[Equation(5)]assumingthatthere are no side reactions.

Conversion ρ( ) =1− At −A∞

A0 −A∞(5)

where A0 is the normalized intensity of absorption band at the initial time, At is the normalized intensity of absorbance bandatspecifiedtimeduringthecuringand A∞isthefinalnormalizedintensityat infinite time.The decrease in theintensity of nC=C and nSi-H absorbance bands can be used to monitor the crosslinkingprocess.Inordertocorrectfor the thickness during curing, theabsorbanceat1944cm-1wastakenasinternalstandard.Baselinecorrectionwasdonetoalltheabsorbancepeaks.The decrease in intensity of the 2157 cm-1 and 1586 cm-1 band was monitored for conversion during the

Figure3.Storagemodulus,logG´(Pa)curvesandtanδ curves of LSR vulcanizates

Figure4.TGAthermogramofU-10andU-65vulcanizates



reaction. Figure5a–b describes FTIR spectra of hydrosilyation cured silicone rubberat55°Ctakenatvarioustimeintervals. Similar graphs were obtained at60°Cand75°C.

Figures 6a and 6b exhibit thedata extracted from the FTIRexperiments and the correspondingSi-H disappearance curves for the hydrosilyation cured silicone rubber respectively at 55 °C, 60 °C and75 °C. Same trend is observed forthecaseofC=CpeakdisappearanceinU-10andU-65.Theexperimentalconversiondataforthereactionkineticswere obtained from above equation[Equation(5)]fromtheseplots.

3.4.1 Crosslinking KineticsTheexperimentaldataobtainedfromFTIR measurements were inspected according tovariouskineticmodels.The crosslinking degree can beexpressedbytherelation,

ρ(t) = dp0

t

∫ (6)

Differentkineticmodels47-48 have been adopted to describe the curing process

Scheme 1. Mechanism of curing of vinyl PDMS by hydrosilyation reaction

Figure 5. Change in absorption intensity with time during curing of U-10 at 55 °C (a) for Si-H bond (at 2157 cm-1) (b) for C=C bond (at 1586 cm-1)



of the thermosetting polymer based on the above empirical law.

Theratecanbeexpressedasafunctionof curing degree and temperature. Basedon these, thekineticequationcanbeexpressedasfollows;

dpdt= k(T)f(p) (7)

Andundertheisothermalconditions:

dpdt= k× exp −Ea / RT( ) f p( ) (8)

wherekistherateconstantthatdependsupon temperature, Ea is activation energy, R is universal gas constant and T is temperature in absolute scale.

For the nthorderkineticsmodelitcanbeexpressedas

dpdt= k 1− p( )n (9)

where parameter n is related to reaction order

If n =1, ln 1− p( ) = −kt+C (10)

If n = 2, 11− p( )

= kt+C (11)

whereCistheintegrationconstant.

Inthepresentwork,nthorderkineticmodel was applied to study the curingkineticsofthehydrosilylationcured silicone rubber. The above equationshavebeenapplied tobothdisappearanceofSi-HandC=Cpeakswith time and the plots for the same are shown in Figures 7 and 8. The value of rate constant and standard deviation of Si-HandC=CareshowninTable 5, respectively.

Comparing these plots it can beconcluded that disappearance of bothSi-HandC=Cabsorptionbandsfollows1storderkineticsrespectively.

Activationenergiesweredeterminedusing the Arrhenius equation fromthe plot of lnk vs 1/T. The valuesof activation energies for the disappearance of double bond and Si-Hwerefoundtobe100.72kJ/moland95.56kJ/molrespectively.

4. CONCLUSIONS

Silicone rubber vulcanizates were prepared in a conventional way using platinum catalyzed hydrosilylation reaction by varying the amounts of crosslinker. Complete curing wasobserved at [hydride]/[vinyl] ratioof 4:1 forU-10while the ratiowas2:1 forU-65.Thus, it is establishedthat the theoretical stoichiometric ratioof1:1maynotalwaysbeabletoeffect complete curing. Tremendous improvement in properties was observed for the optimized ratio for both the systems in comparison to the conventional 1:1 ratio. Inthermogravimetric studies, Ti increased by 20 °C for U-10 while the sameincreasedby14°CforU-65,signifyingincreased thermal stability compared to virginpolymerduetohighercrosslinkdensity. Similarly, the increase in Tmax forU-10was47°Cwhileitwasabout

Figure 6b. Disappearance of Si-H stretching frequency in FTIR spectrum with time at different temperatures during curing of U-10

Figure 6a. Conversion of Si-H groups with time at different temperatures during curing of U-10



Figure 7. First order plots for Si-H disappearance during curing of U-10

Figure 8. First order plots for C=C disappearance during curing of U-10

Table5.RateconstantsandfitmentparametersforSi-HabsorptionandC=Cabsorptionbandatdifferenttemperatures

Reaction order Parameter Si-H Absorption C=C Absorption55 °C 60 °C 75 °C 55 °C 60 °C 75 °C

First kx102$ 2.3 11.3 23.2 3.6 5.4 25.3R' * 0.94 0.99 0.93 0.97 0.99 0.97

Second kx102$ 12.5 23.5 21.2 15.4 35.9 82.2R' * 0.87 0.91 0.92 0.85 0.93 0.91

$ k=rate constant, * R' =regression coefficient

100°CforU-65.InDMA,Tg increased by13°CforU-10,whileforU-65itincreasedby5°C,therebysuggestingan increased number of efficient crosslinks.Mechanicalpropertiesalsoexhibited huge improvement. Therewas a tremendous increment in tensile strengthof66%forU-10and120%forU-65andmodulusat100%elongationof160%forU-10and50%forU-65.However, a drastic fall was observed for elongation at break which is inwellaccordwithincreaseincrosslinkdensity.

The crosslink kinetics was studiedforU-10systemwithstoichiometricconditions viz. [Si-H]/[C=C] = 4.0, u s i n g F T I R s p e c t r o s c o p y measurements at three different temperatures and found to follow the first order reaction with respecttoconversionofvinylgroups(C=C)and hydride groups (Si-H). Theactivation energy was determined as well and found tobe100.70kJ/moland95.60kJ/molforthedisappearanceof double bond and Si-H functionality respectively.

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