2 E¡ectofChemicalStructureon PolymerProperties 2.1INTRODUCTION Inthepreviouschapter,wediscusseddifferentwaysofclassifyingpolymersand observedthattheirmolecularstructureplaysamajorroleindeterminingtheir physicalproperties.Wheneverwewishtomanufactureanobject,wechoosethe materialofconstructionsothatitcanmeetdesignrequirements.Thelatter includetemperatureofoperation,materialrigidity,toughness,creepbehavior,and recoveryofdeformation.WehavealreadyseeninChapter1thatagivenpolymer canrangeallthewayfromaviscousliquid(forlinearlow-molecular-weight chains)toaninsolublehardgel(fornetworkchains),dependingonhowitwas synthesized.Therefore,polymerscanbeseentobeversatilematerialsthatoffer immensescopetopolymerscientistsandengineerswhoareonthelookoutfor newmaterialswithimprovedproperties.Inthischapter,wefirsthighlightsomeof theimportantpropertiesofpolymersandthendiscussthemanyapplications. 2.2EFFECTOFTEMPERATUREONPOLYMERS [1^4] Wehaveobservedearlierthatsolidpolymerstendtoformorderedregions,such asspherulites(seeChapter11forcompletedetails);thesearetermedcrystalline polymers. Polymers that have no crystals at all are called amorphous. A real 45 Copyright © 2003 Marcel Dekker, Inc. polymer is never completely crystalline, and the extent of crystallization is characterized by the percentage of crystallinity. A typi cal amorphous polymer, such as polystyrene or polymethyl meth- acrylate, can exist in several states, depending on its molecular weight and the temperature. In Figure 2.1, we have shown the interplay of these two variables and compared the resulting behavior with that of a material with moderate crystallinity. An amorphous polymer at low temperatures is a hard glass y material which, when heated, melts into a viscous liquid. However, before melting, it goes through a rubbery state. The temperature at which a hard glassy polymer becomes FIGURE 2.1 Influence of molecular weight and temperature on the physical state of polymers. 46 Chapter 2 Copyright © 2003 Marcel Dekker, Inc. arubberymaterialiscalledtheglasstransitiontemperature,T g (seeChapter12 forthedefinitionofT g intermsofchangesinthermodynamicandmechanical properties;thereexistsasufficientlysharptransition,asseeninFig.2.1a).There is a diffuse transition zone between the rubbery and liquid states for crystalline polymers; the temperature at which this occurs is called the flow temperature, T f . As the molecular weight of the polymer increases, we observe from Figure 2.1 that both T g and T f increase. Finally, the diffuse transition of the rubber to the liquid state is specific to polymeric systems and is not observed for low- molecular-weight species such as water, ethanol, and so forth, for which we have a sharp melting point between solid and liquid states. In this section, only the effect of chain structure on T g is examined—other factorswillbediscussedinChapters10–12.Inordertounderstandthevarious transitions for polymeric systems, we observe that a molecule can have all or some of the following four categories of motion: 1. Translational motion of the entire molecule 2. Long cooperative wriggling motion of 40–50 CÀC bonds of the molecule, permitting flexing and uncoiling 3. Short cooperative motion of five to six CÀC bonds of the molecule 4. Vibration of carbon atoms in the polymer molecule The glass transition temperature, T g , is the temperature below which the translational as well as long and short cooperative wriggling motions are frozen. In the rubbery state, only the first kind of motion is frozen. The polymers that have their T g values less than room temperature would be rubbery in nature, such as neoprene, polyisobutylene, or butyl rubbers. The factors that affect the glass transition temperatures are described in the following subsections. 2.2.1 Chain Flexibility It is generally held that polymer chains having ÀCÀCÀ or ÀCÀOÀ bonds are flexible, whereas the presence of a phenyl ring or a d ouble bond has a marked stiffening effect. For comparison, let us consider the basis polymer as poly- ethylene. It is a high-molecular-weight alkane that is manufactured in several ways; a common way is to polymerize ethylene at high pressure throu gh the radical polymerization technique. The polymer thus formed has short-chain as well as long-chain branches, which have been explained to occur through the ‘‘backbiting’’ transfer mechanism. The short-chain branches (normally butyl) are formed as follows: Chemical Structure on Polymer Properties 47 Copyright © 2003 Marcel Dekker, Inc. and the long-chain branches are formed through the transfer reaction at any random point of the backbone as The polymer has a T g of about À20 C and is a tough material at room temperature. We now compare polyethylene terephthalate with polyethylene. The former has a phenyl group on every repeat unit and, as a result, has stiffer chains (and, hence, higher T g ) compared to polyethylene. 1,4-Polybutadiene has a double bond on the backbone and similarly has a higher T g . The flexibility of the polymer chain is dependent on the free space v f available for rotation. If v is the specific volume of the polymer and v s is the volume when it is solidly packed, then v f is nothing but the difference between the two (v À v s ). If the free space v f is reduced by the presence of large substituents, as in polyethylene terephthalates, the T g value goes up, as observed earlier. 2.2.2 Interaction Between Polymers Polymer molecules interact with each other because of secondary bondings due to dipole forces, induction forces, and=or hydrogen bonds. The dipole forces arise when there are polar substituents on the polymer chain, as, for example, in polyvinyl chloride (PVC). Because of the substituent chlorine, the T g value of PVC is considerably higher than that of polyethylene. Sometimes, forces are also induced due to the ionic nature of substituents (as in polyacrylonitrile, for example). The cyanide substituents of two nearby chains can form ionic bonds as follows: Hydrogen bonding has a similar effect on T g . There is an amide (ÀCONHÀ) group in nylon 6, and it contributes to interchain hydrogen- bonding, increasing the glass transition temperature compared to polyethylene. In polytetrafluoroethy- 48 Chapter 2 Copyright © 2003 Marcel Dekker, Inc. lene, there are van der Waals interaction forces between fluorine atoms and, as a result, it cannot be melted: Even though the energy required to overcome a single secondary-force interaction is small, there are so many such secondary forces in the material that it is impossible to melt it without degrading the polymer. 2.2.3 Molecular Weight of Polymers Polymers of low molecular weight have a greater number of chain ends in a given volume compared to those of high molecular weight. Because chain ends are less restrained, they have a greater mobility at a given temperature. This results in a lower T g value, as has been amply confirmed experimentally. The molecular- weight dependence of the glass transition temperature has been correlated by T g ¼ T 1 g À K m n ð2:2:5Þ where T 1 g is the T g value of a fictitious sample of the same polymer of infinite molecular weight and m n is the number-average chain length of the material of interest. K is a positive constant that depends on the nature of the material. 2.2.4 Nature of Primary Bondings The glass transition temperature of copolymers usually lies between the T g values of the two homopolymers (say, T g 1 and T g2 ) and is normally correlated through 1 T g ¼ w 1 T g1 þ ð1 À w 1 Þ T g 2 ð2:2:6Þ where w 1 is the weight fraction of one of the monomers present in the copolymer of interest. With block copolymers, sometimes a transition corresponding to each block is observed, which means that, experimentally, the copolymer exhibits two T g values corresponding to each block. We have already observed that, depending on specific requirements, one synthesizes branch copolymers. At times, the long branches may get entangled with each other, thus further restraining molecular motions. As a result of this, Eq. (2.2.6) is not obeyed and the T g of the polymer is expected to be higher. If the polymer is cross-linked, the segmental mobility is further restricted, thus giving a higher T g . On increasing the degree of cross- linking, the glass transition temperature is found to increase. The discussion up to now has been restricted to amorphous polymers. Figure 2.1b shows the temperature–molecular weight relation for crystalline Chemical Structure on Polymer Properties 49 Copyright © 2003 Marcel Dekker, Inc. polymers.Ithasalreadybeenobservedthatthesepolymerstendtodevelop crystallinezonescalled‘‘spherulites.’’Acrystallinepolymerdiffersfromthe amorphousoneinthattheformerexistsinanadditionalflexiblecrystallinestate beforeitbeginstobehavelikearubberymaterial.Onfurtherheating,itis convertedintoaviscousliquidatthemeltingpointT m .Thisbehaviorshouldbe contrastedwiththatofanamorphouspolymer,whichhasaflowtemperatureT f andnomeltingpoint. Theabilityofapolymericmaterialtocrystallizedependsontheregularity ofitsbackbone.RecallfromChapter1that,dependingonhowitispolymerized, apolymericmaterialcouldhaveatactic,isotactic,orsyndiotacticconfigurations. Inthelattertwo,thesubstituentsoftheolefinicmonomertendtodistribute aroundthebackboneofthemoleculeinaspecificway.Asaresult(andasfound insyndiotacticandisotacticpolypropylene),thepolymeriscrystallineandgivesa usefulthermoplasticthatcanwithstandhighertemperatures.Atacticpolymersare usuallyamorphous,suchasatacticpolypropylene.Theonlyoccasionwhenan atacticmaterialcancrystallizeiswhentheattachedfunctionalgroupsareofasize similartotheasymmetriccarbon.Anexampleofthiscaseispolyvinylalcohol,in whichthehydroxylgroupissmallenoughtopackinthecrystallattice. Commercially,polyvinylalcohol(PVA1c)ismanufacturedthroughhydrolysis ofpolyvinylacetate.ThecommonlyavailablePVA1cisalwayssoldwiththe percentagealcoholcontent(about80%)specified.Theacetategroupsarelarge, andbecauseoftheseresidualgroups,thecrystallinityofPVA1cisconsiderably reduced. Itisnowwellestablishedthatanythingthatreducestheregularityofthe backbonereducesthecrystallinity.Randomcopolymerization,introductionof irregularfunctionalgroups,andchainbranchingsallleadtoreductioninthe crystallinecontentofthepolymer.Forexample,polyethyleneandpolypropylene arebothcrystallinehomopolymers,whereastheirrandomcopolymerisamor- phousrubberymaterial.Inseveralapplications,polyethyleneispartiallychlori- nated,butduetothepresenceofrandomchlorinegroups,theresultantpolymer becomesrubberyinnature.Finally,wehavepointedoutinEqs.(2.2.1)and (2.2.2)thattheformationofshortbutylaswellaslongrandombranchesoccurs inthehigh-pressureprocessofpolyethylene.Ithasbeenconfirmedexperimen- tallythatshortbutylbranchesoccurmorefrequentlyandareresponsiblefor considerablyreducedcrystallinitycomparedtostraight-chainpolyethylenemanu- facturedthroughtheuseofaZiegler–Nattacatalyst. 2.3ADDITIVESFORPLASTICS Aftercommercialpolymersaremanufacturedinbulk,variousadditivesare incorporatedinordertomakethemsuitableforspecificenduses.Theseadditives 50Chapter2 Copyright © 2003 Marcel Dekker, Inc. have a profound effect on the final properties, some of which are listed for polyvinyl chloride in Box 2.1. PVC is used in rigid pipings, conveyor belts, vinyl floorings, footballs, domestic insulating tapes, baby pads, and so forth. The required property variation for a given application is achieved by controlling the amount of these additives. Some of these are discussed as follows in the context of design of materials for a specific end use. Plasticizers are high-boiling-point liquids (and sometimes solids) that, when mixed with polymers, give a softer and more flexible material. Box 2.1 gives dioctyl phthalate as a common plasticizer for PVC. On its addition, the polymer (which is a hard, rigid solid at room temperature) becomes a rubberlike Box 2.1 Various Additives to Polyvinyl Chloride Commercial polymer Largely amorphous, slightly branched with monomers joined in head-to-tail sequence. Lubricant Prevents sticking of compounds to processing equipment. Calcium or lead stearate forms a thin liquid film between the polymer and equipment. In addition, internal lubricants are used, which lower the melt viscosity to improve the flow of material. These are montan wax, glyceryl monostearate, cetyl palmitate, or aluminum stearate. Filler Reduces cost, increases hardness, reduces tackiness, and improves electrical insulation and hot deformation resistance. Materials used are china clay for electrical insulation and, for other works, calcium carbonate, talc, magnesium carbonate, barium sulfate, silicas and silicates, and asbestos. Miscellaneous additives Semicompatible rubbery material as impact modifier; antimony oxide for fire retardancy; dioctyl phthalate as plasticizer; quaternary ammonium compounds as antistatic agents; polyethylene glycol as viscosity depressant in PVC paste application; lead sulfate for high heat stability, long-term aging stability, and good insulation characteristics. Chemical Structure on Polymer Properties 51 Copyright © 2003 Marcel Dekker, Inc. material.Aplasticizerissupposedtobea‘‘goodsolvent’’forthepolymer;in ordertoshowhowitworks,wepresentthefollowingphysicalpictureof dissolution.Inasolventwithoutapolymer,everymoleculeissurroundedby molecules(say,zinnumber)ofitsownkind.Eachoftheseznearestneighbors interactswiththemoleculeunderconsiderationwithaninteractionpotentialE 11 . Asimilarpotential,E 22 ,describestheenergyofinteractionbetweenanytwo nonbondedpolymersubunits.AsshowninFigure2.2,theprocessofdissolution consistsofbreakingonesolvent–solventbondandoneinteractivebondbetween twononbondedpolymersubunitsandsubsequentlyformingtwopolymer–solvent interactivebonds.WedefineE 12 astheinteractionenergybetweenapolymer subunitandsolventmolecule.Thedissolutionofpolymerinagivensolvent dependsonthemagnitudesofE 11 ,E 22 ,andE 12 .Thequantitiesknownas solubilityparameters,d 11 andd 22 ,arerelatedtotheseenergies.Theirexact relationswillbediscussedinChapter9.Itissufficientforthepresentdiscussion toknowthatthesecanbeexperimentallydetermined;theirvaluesarecompiledin PolymerHandbook[4]. Wehavealreadyobservedthataplasticizershouldberegardedasagood solventforthepolymer,whichmeansthatthesolubilityparameterd 11 forthe formermustbeclose(¼d 22 )tothatforthelatter.Thisprincipleservesasaguide forselectingaplasticizerforagivenpolymer.Forexample,unvulcanizednatural rubberhavingd 22 equalto16.5dissolvesintoluene(d 11 ¼18:2)butdoesnot dissolveinethanol(d 11 ¼26).Ifasolventhavingaverydifferentsolubility parameterismixedwiththepolymer,itwouldnotmixonthemolecularlevel. Instead,therewouldberegionsofthesolventdispersedinthepolymermatrixthat wouldbeincompatiblewitheachother. Fillersareusuallysolidadditivesthatareincorporatedintothepolymerto modifyitsphysical(particularlymechanical)properties.Thefillerscommonly usedforPVCaregiveninBox2.1.Ithasbeenfoundthatparticlesizeofthefiller hasagreateffectonthestrengthofthepolymer:Thefinertheparticlesare,the FIGURE2.2Schematicdiagramoftheprocessofpolymerdissolution. 52 Chapter 2 Copyright © 2003 Marcel Dekker, Inc. higherthehardnessandmodulus.Anotherfactorthatplaysamajorrolein determiningthefinalpropertyofthepolymeristhechemicalnatureofthe surface.Mineralfillerssuchascalciumcarbonateandtitaniumdioxidepowder oftenhavepolarfunctionalgroups(e.g.,hydroxylgroups)onthesurface.To improvethewettingproperties,theyaresometimestreatedwithachemicalcalled acouplingagent. Couplingagentsarechemicalsthatareusedtotreatthesurfaceoffillers. Thesechemicalsnormallyhavetwoparts:onethatcombineswiththesurface chemicallyandanotherthatiscompatiblewiththepolymer.Oneexampleisthe treatmentofcalciumcarbonatefillerwithstearicacid.Theacidgroupofthelatter reactswiththesurface,whereasthealiphaticchainsticksoutofthesurfaceandis compatiblewiththepolymermatrix.Inthesameway,ifcarbonblackistobe usedasafiller,itisfirstmixedwithbenzoylperoxideinalcoholat45 Cforat least50handsubsequentlydriedinvacuumat11 C[5].Thisactivatedcarbon hasbeenidentifiedashavingCÀOHbonds,whichcanleadtopolymerizationof vinylmonomers.Thepolymerthusformedischemicallyboundtothefillerand wouldthuspromotethecompatibilizationofthefillerwiththepolymermatrix. Mostofthefillersareinorganicinnature,andthesurfaceareaperunitvolume increaseswithsizereduction.Thenumberofsiteswherepolymerchainscanbe boundincreases,and,consequently,compatibilityimprovesforsmallparticles. Forinorganicfillers,silanesalsoserveascommoncouplingagents.Some ofthesearegiveninTable2.1.Themechanismofthereactionconsistsoftwo steps; in the first one, the silane ester moiety is hydrolyzed to give ðC 2 H 5 OÞ 3 ÀSiÀðCH 2 Þ 3 ÀNH 2 þ 3H 2 O À! ð OHÞ 3 ÀSiÀðCH 2 Þ 3 ÀNH 2 þ C 2 H 5 OH ð2:3:1Þ These subsequently react with various OH groups of the surface, Sur-(OH) 3 : Silane coupling agents can have one to three of these bonds, and one would ideally like to have all of them reacted. The reaction of OH groups on Si is a competitive one; because of steric factors, not all of them can undergo reaction. The net effect of the reaction in Eq. (2.3.2) is to give chemically bonded silane molecules on the surface of glass or alumina particles. The amine group now Chemical Structure on Polymer Properties 53 Copyright © 2003 Marcel Dekker, Inc. bound to the surface is a reactive one and can easily react with an acid or an aldehyde g roup situated on a polymer molecule. Recently, Goddart et al. [6] reported a polyvinyl alcohol–copper(II) initiat- ing system, which can produce branched polymers on surfaces. The initiating system is prepared by dissolving polyvinyl alcohol in water that already contains copper nitrate (or copper chloride). The calcium carbonate filler is dipped into the solution and dried. If this is used for polymerization of an olefin (say, styrene), it would form a polymer that adheres to the particles, ultimately encapsulating them. The mechanical properties of calcium-carbonate-filled polystyrene have been found to depend strongly on filler–matrix compatibility, which is consider- ably improved by this encapsulation. TABLE 2.1 Silane Coupling Agents Name Formula g-Aminopropyl triethoxy silane g-Chloropropyl triethoxy silane g-Cyanopropyl trimethoxy silane g-Glycidoxypropyl trimethoxy silane g-Mercaptopropyl trimethoxy silane g-Methacryloxypropyl trimethoxy silane Some Silanization Procedures Using g-aminopropyl triethoxy silane Glass. One gram of glass beads is added to 5 mL of 10 solution of the coupling agent at pH 5 (adjusted with acetic acid). The reaction is run for 2 h at 80 C. The silanized glass beads are then washed and dried at 120 Cinanovenfor2h. Alumina One gram of alumina is added to 5 mL of the coupling agent in toluene. The reaction mixture is refluxed for about 2 h. Alumina is washed with toluene, then with acetone, and finally dried in oven at 120 C for 2 h. Using g-mercaptopropyl trimethoxy silane Glass. One gram of porous glass is added to 5 mL of 10 solution of the coupling agent at pH 5 (adjusted with 6 N HCl). The mixture is heated to reflux for 2 h. The glass beads are washed with pH 5 solutions, followed by water, and ultimately dried in an oven for 2 h at 120 C. 54 Chapter 2 Copyright © 2003 Marcel Dekker, Inc. [...]... latter is obtained by reacting cellulose [XÀ(OH )3] with sodium hydroxide as follows: !XÀðONa 3 þ 3H2 O XÀðOH 3 þ 3NaOHÀ ð2:5:2Þ which is further reacted with sodium salt of chloroacetic acid (ClÀCH2COONa), as follows: !XÀ½OCH2 COONa 3 þ NaCl XÀ½ONa 3 þ 3ClCH2 COONaÀ ð2:5 :3 Commercial grades of CMC are physiologically inert and usually have a degree of substitution between 0.5 and 0.85 CMC is mainly... dispersion of a copolymer of 2-ethyl hexyl acrylate, vinyl acetate, and acrylic acid in water Copyright © 20 03 Marcel Dekker, Inc Structure Copyright © 20 03 Marcel Dekker, Inc Silicone rubbers Polyacrylates Polyvinyl acetals Polyvinyl acetate and its copolymers Nitrile rubber R: methyl or phenyl M ¼ 500–600 Pressure-Sensitive Adhesives Water emulsions of copolymer of 2-ethyl acrylate (35 2 parts), vinyl... on Polymer Properties 55 Polymers also require protection against the effect of light, heat, and oxygen in the air In view of this, polymers are mixed with antioxidants and stabilizers in low concentrations (normally less than 1%) If the material does not have these compounds, a polymer molecule Mn of chain length n interacts with light (particularly the ultraviolet portion of the light) to produce polymer. .. groups introduced into the polymers are carboxylic or sulfonate groups The following are the two general routes of their synthesis: 1 2 Copolymerization of a low level of functionalized monomers with the comonomer Direct functionalization of an already formed polymer Because of the special properties imparted to this new material, called an ionomer, it has been the subject of vigorous research in recent... , as follows: hn Mn À Pn ! ð2 :3: 3Þ The polymer radicals thus produced interact with oxygen to form alkyl peroxy radicals (Pn1 ÀO2 ) that can abstract hydrogen of the neighboring molecules in various ways, as shown in the mechanism of the auto-oxidation process of Table 2.2 The formation of hydroperoxide in step C of the sequence of reactions is the most important source of initiating radicals In practice,... degradation occurs around 290– 30 0 C After homolysis of polymer chains, the macroradicals depropagate, giving a monomer with 100% yield Polystyrene Between 200 C and 30 0 C, the molecular weight of the polymer falls, with no evolution of volatile products This suggests that polymers first undergo homolysis, giving macroradicals, which later undergo disproportionation Above 30 0 C, polystyrene gives a... copolymer gel of styrene and divinyl benzene (DVB), and the general-purpose resin contains about 8–12% of the latter As the DVB content is reduced, the degree of crosslinking reduces, and at around 0.25% DVB, the polymeric gel swells strongly to give a soft, gelatinous material As DVB is increased (at about 25%), the polymer swells negligibly and is a mechanically tough material The copolymer beads of. .. molecules randomly distributed on them Some of the common copolymers and their important properties are given in Box 2 .3 Polymer blends are physical mixtures of two or more polymers and are commercially prepared by mechanical mixing, which is achieved through screw compounders and extruders In these mixtures, different polymers tend to separate (instead of mixing uniformly) into two or more distinct... amine, which react with it as Copyright © 20 03 Marcel Dekker, Inc Chemical Structure on Polymer Properties 65 Experiments have shown that the rubbery nature of the polymer can be attributed to the polyol ‘‘soft’’ segments It has also been found that increasing the ‘‘size’’ of R contributed by the chain extenders tends to reduce the rubbery nature of the polymer The urethane rubber is found to have... introduced through the first synthetic route by Copyright © 20 03 Marcel Dekker, Inc Chemical Structure on Polymer Properties 69 Box 2 .3 Some Commercial Copolymers Ethylene–vinyl acetate copolymer (EVA) Vinyl acetate is about 10–15 surface gloss, and melt adhesive properties of EVA Ethylene–acrylic acid copolymer Acrylic acid content varies between 1 and 10 polymer When treated with sodium methoxide or magnesium . without degrading the polymer. 2.2 .3 Molecular Weight of Polymers Polymers of low molecular weight have a greater number of chain ends in a given volume compared to those of high molecular weight Inc. higherthehardnessandmodulus.Anotherfactorthatplaysamajorrolein determiningthefinalpropertyofthepolymeristhechemicalnatureofthe surface.Mineralfillerssuchascalciumcarbonateandtitaniumdioxidepowder oftenhavepolarfunctionalgroups(e.g.,hydroxylgroups)onthesurface.To improvethewettingproperties,theyaresometimestreatedwithachemicalcalled acouplingagent. Couplingagentsarechemicalsthatareusedtotreatthesurfaceoffillers. Thesechemicalsnormallyhavetwoparts:onethatcombineswiththesurface chemicallyandanotherthatiscompatiblewiththepolymer.Oneexampleisthe treatmentofcalciumcarbonatefillerwithstearicacid.Theacidgroupofthelatter reactswiththesurface,whereasthealiphaticchainsticksoutofthesurfaceandis compatiblewiththepolymermatrix.Inthesameway,ifcarbonblackistobe usedasafiller,itisfirstmixedwithbenzoylperoxideinalcoholat45 Cforat least50handsubsequentlydriedinvacuumat11 C[5].Thisactivatedcarbon hasbeenidentifiedashavingCÀOHbonds,whichcanleadtopolymerizationof vinylmonomers.Thepolymerthusformedischemicallyboundtothefillerand wouldthuspromotethecompatibilizationofthefillerwiththepolymermatrix. Mostofthefillersareinorganicinnature,andthesurfaceareaperunitvolume increaseswithsizereduction.Thenumberofsiteswherepolymerchainscanbe boundincreases,and,consequently,compatibilityimprovesforsmallparticles. Forinorganicfillers,silanesalsoserveascommoncouplingagents.Some ofthesearegiveninTable2.1.Themechanismofthereactionconsistsoftwo steps; in the first one, the silane ester moiety is hydrolyzed to give ðC 2 H 5 OÞ 3 ÀSiÀðCH 2 Þ 3 ÀNH 2 þ 3H 2 O À! ð OHÞ 3 ÀSiÀðCH 2 Þ 3 ÀNH 2 þ C 2 H 5 OH ð2 :3: 1Þ These. compounds, a polymer molecule M n of chain length n interacts with light (particularly the ultraviolet portion of the light) to produce polymer radicals P n , as follows: M n À! hn P n ð2 :3: 3Þ The polymer