湘大高分子专业英语Charpter-3-3

Charpter3-33.3.PolymerSolution3.3.1IntroductionForagivenpolymer,therearesolventsthatdissolvethepolymerwellandsolventsthatdonotdissolvethepolymer.Theformersolventsarecalled“goodsolvents”andthelatter“nonsolvents”.Tablelistsatypicalgoodsolventandanonsolventforpolystyrene,poly(methylmethacrylate),andpoly(ethyleneglycol).PolymerHandbookhasalonglistofsolventsandnonsolventsformanypolymers.Theconcentrationofthepolymerinthegoodsolventcanbeashighas100%,yetthesolutionremainsclearanduniform.Addinganonsolventtothesolutioncausesthepolymertoprecipitate,ifthenonsolventmixeswiththegoodsolvent.Asolventwithanintermediatequalitydissolvesthepolymertosomeextent.TableGoodSolventsandNonsolventsforSomePolymers3.3.聚合物解决方案3.3.1介绍对于给定的聚合物，有溶剂能够溶解聚合物以及溶剂和不溶解该聚合物。前者的溶剂称为“良溶剂”和后者“非溶剂”。表中列出了典型的良溶剂和不良溶剂的聚苯乙烯，聚（甲基丙烯酸甲酯）和聚（乙二醇）。聚合物手册有溶剂和非溶剂对许多聚合物的一个长长的清单。在良好溶剂中的聚合物的浓度可以是高达100％，但该解决方案仍然是清晰和均匀的。加入非溶剂的溶液使聚合物沉淀，如果与良溶剂的非溶剂混合。具有中间质量的溶剂溶解该聚合物在一定程度上。表良好溶剂和非溶剂的一些聚合物Likelow-molecular-weightsolutes,apolymerdissolvesinasolventwhensolvationlowersthefreeenergy.Agoodsolventlowersthefreeenergysubstantially.Anonsolventincreasesthefreeenergy.Amorphouspolymers(transparentinthesolidstate;tobeprecise,itisnotasolidbutratherasupercooledliquid)areusuallyeasytodissolveinthegoodsolvent.Incontrast,crystallineandsemicrystallinepolymers(opaqueinthesolidstate)aresometimesnoteasytodissolve.Withinacrystallite,polymerchainsarefoldedintoaregular,thermodynamicallystablearrangement.Itisnoteasytounfoldthechainfromtheself-lockedstateintoadisorderedstateinsolutionevenifthelatterstateisthermodynamicallymorestable.Heatingmayhelpthedissolutionbecauseitfacilitatestheunfolding.Oncedissolved,polymerchainstakearandom-coilconformationunlessthechainisrigid.如低分子量溶质中，聚合物溶解于溶剂中时，溶剂化降低的自由能。一个良好的溶剂会降低自由能大增。一个非溶剂增加的自由能。无定形聚合物（透明固体状态;准确地说，它不是固体而是过冷液体）通常容易在良好溶剂中溶解。与此相反，结晶和半结晶聚合物（不透明固体状态），有时不容易溶解。在一个晶粒，聚合物链折叠成有规律的，热力学稳定的安排。它不容易从自锁定状态展开的链成无序状态，在溶液中，即使后者状态在热力学上更加稳定。加热可帮助溶解，因为它有利于展开。一旦溶解后，聚合物链采取无规卷曲构象，除非该链是刚性的。3.3.2Flory-HugginsMean-FieldTheoryLatticeChainModelDissolutionofapolymerintoasolventlowersthefreeenergyofthepolymer–solventsystemwhentheenthalpydecreasesbydissolutionor,ifitdoesnot,whentheproductofthetemperatureandtheentropyofmixingisgreaterthantheenthalpyofmixing.Miscibilityismuchlowerinpolymer–solventsystemsbecauseaddingsolventmoleculestothepolymerdoesnotincreasetheentropyasmuchasitdoestothelow-molecular-weightsolutes.Solventsthatdissolveagivenpolymerareoftenlimitedtothosethatpreferentiallysurroundthepolymerchain.Wewilllearninthissectionhowsmalltheentropygainisinthepolymer–solventmixture.Wewillalsolearnwhatphenomenacharacteristicofpolymersolutionsareexpected.3.3.2弗洛里-哈金斯平均场理论格子链模型溶解的聚合物在溶剂降低了聚合物-溶剂体系的自由能时的焓通过溶解减小，或者，如果不是这样，当混合的温度和熵的乘积大于混合焓。混溶性低得多的聚合物-溶剂系统，因为加入溶剂分子与聚合物不增加熵一样，因为它不与低分子量溶质。溶剂能够溶解在给定的聚合物往往局限于那些优先包围聚合物链。我们将学习本节中的熵增益是在聚合物-溶剂混合物有多小。我们还将学习什么现象聚合物溶液的特性的预期。Miscibilityofthepolymerwithagivensolventiswellexplainedinthemeanfieldtheory.Thetheoryisanextensionofthelatticefluidtheoryoriginallydevelopedtoexplainthemiscibilityoftwolow-molecular-weightliquids.Florypioneeredtheapplicationofthemean-fieldlatticefluidtheorytopolymersolutions.ThesimplestversionofthislatticechaintheoryisgenerallyreferredtoasFigureLatticemodelforpolymersolution.Graysitesareoccupiedbypolymerchains,andwhitesitesareoccupiedbysolventmolecules.与给定溶剂中的聚合物的混溶性在meanfield理论很好的解释。理论是格子流体理论的延伸最初开发解释的两种低分子量的液体混溶性。弗洛里开创了平均场格子流体理论应用到聚合物溶液。这个晶格链理论的最简单版本通常被称为图格子模型聚合物溶液。灰点是由聚合物链中占据，白点是由溶剂分子占据。Flory–Hugginsmean-fieldtheory.Asimilarmean-fieldtheorysuccessfullydescribesthermodynamicsofpolymerblendsand,withsomemodifications,diblockcopolymersandtheirblendswithhomopolymers.Themean-fieldtheoryforthepolymersolutioncomparesthefreeenergyofthepolymer–solventsystembeforemixingandthefreeenergyaftermixing.Weconsiderasimplesituation:thepolymerisamonodispersehomopolymerandisinanamorphousstateorinaliquidstate(melt)beforemixing.TheFlory–Hugginstheoryusesthelatticemodeltoarrangethepolymerchainsandsolvents.Thesystemconsistsofnsitesites.Eachsitecanbeoccupiedbyeitheramonomerofthepolymerorasolventmolecule(themonomerandthesolventmoleculeoccupiesthesamevolume).AlinearpolymerchainoccupiesNsitesonastringofN–1bonds.弗洛里-哈金斯平均场理论。类似的平均场理论成功地描述了聚合物共混物的热力学，并与一些修改，二嵌段共聚物和均聚物与它们的共混物。平均场理论的聚合物溶液混合后，混合和自由能之前比较对聚合物-溶剂体系的自由能。我们考虑一个简单的情况：该聚合物是单分散的均聚物，并且是无定形状态或处于液体状态（熔融物）混合之前。在弗洛里-哈金斯理论使用的点阵模型来安排聚合物链和溶剂。该系统由nsite的网站。每个站点可以通过聚合物的任一单体或溶剂分子（被占用的单体和溶剂分子占据相同的体积）。线性聚合物链上占据的N-1个键的字符串N个地点。Thereisnopreferenceinthedirectionthenextbondtakeswhenapolymerchainislaidontothelatticesites(flexible).PolymerchainsconsistingofNmonomersarelaidontoemptysitesonebyoneuntilthereareatotalnPchains.Then,theunoccupiedsitesarefilledwithsolventmolecules.ThevolumefractionΦofthepolymerisrelatedtonPbyandthenumberofthesolventmoleculesnSisgivenbyBeforemixing,thepolymeroccupiesthevolumeofnPNvsiteandthesolventoccupiesthevolumeofnSvsite,wherevsiteisthevolumepersite.Thetotalvolumensitevsitedoesnotchangeuponmixing(incompressible).Thus,theenthalpyofmixingΔHmixintheconstant-pressureprocessisequaltothechangeintheintern