[高等教育]06 chapter 6 Mechanical property of polymer.ppt

chaptervithemechanicalpropertiesofpolymers sectionioverviewsectioniihighelasticitysectioniiiviscoelasticitysectionivultimatemechanicalbehavior yield destructionandstrength sectionioverview 1 1classificationofthemechanicalproperties1 2thebasicphysicalquantitiescharacterizingthemechanicalproperties1 3characteristicsofthemechanicalpropertiesofpolymers 1 1classificationofthemechanicalproperties themechanicalpropertiesarethebaseofexcellentphysicalpropertiesofpolymer e g onepolymerhasgoodfrictionandwear 磨耗 performance butwithpoormechanicalproperties verycrisp cannotbeusedforanti friction 减摩 material e g thepolymerforwireinsulation alsorequirescertainmechanicalproperties strengthandtoughness ifitisfoldedafewtimes itwillbebroken theneveniftheelectricalinsulationisgood butcannotbeusedaswireinsulation commonterms mechanicalbehavior refersdeformationresponsedtoanexternalforceappliedtothematerial deformationproperties mechanicalbehaviorundernon extremeconditions fractureproperties 断裂性能 mechanicalbehaviorunderextremeconditions elasticity foranidealelasticbody itselasticdeformationcanbeexpressedbyhooke slaw namely stress 应力 isdirectlyproportionaltostrain 应变 andthestrainistime independent viscosity intheexternalforce thedisplacementoccursbetweenthemolecules deformationoftheidealflowofviscousfluidcanbedescribedbynewton slaw stressisdirectlyproportionaltothestrainrate generalelasticity 普弹性 inlargestress onlyproducedsmall linearreversibledeformation itiscausedbythechangeofthebondlengthsandbondangles relatedtochangeofinternalenergyinmaterials internalenergyincreaseswhenthedeformation deformationrecovery releaseenergy worktooutside glassy crystalline polymers metals ceramicshavesuchaperformance generalelasticityalsoknownasenergyelasticity highelasticity agreatreversibledeformationcanoccurunderasmallstress causedbyinternalconformationalentropychange soalsoknownasentropyelasticity rubberwithhighelasticity staticmechanicalproperties themechanicalbehaviorofconstantstressorconstantstrain dynamicmechanicalproperties viscoelasticbehavioroftheobjectinthealternatingstress stressrelaxation inthecaseofconstantstrain stresschangewithtime creep 蠕变 undertheconstantstress thedeformationofanobjectchangesovertimestrength thestressmaterialscanwithstandtoughness 韧性 theabsorbedenergywhenmaterialsfracture 1 2thebasicphysicalquantitiescharacterizingthemechanicalproperties 1 3characteristicsofthemechanicalpropertiesofpolymers 1 polymermaterialhavethewidestmechanicalpropertieswithinalltheknownmaterial includingliquids softrubbertohardsolids avarietyofpolymersresponsesvariedwidelyformechanicalstress forexample psproductsisverybrittle brokenonaknock brittle nylonproductsaretoughanddifficulttodeformation arenoteasytobreak toughness slightlycross linkedrubberstretching itcanstretchseveraltimes releasingforcecanalmostrestitution elasticity afterdeformation clay 胶泥 remainscompletelynewshape viscosity thesediversitiesofpolymermechanicalpropertiesprovideabroadchoiceasdifferentapplications 2mostoutstandingcharcteristicofmechanicalpropertiesofpolymersisflexibilityandhighviscoelasticity thehighelasticityofpolymer duetoagreatmolecularweight polymerchainshavemanydifferentconformations andthechangesofconformationleadtopolymerchainshavetheiruniqueflexibility chainflexibilityistheessenceofpolymerhighelasticity thedifferencefromthegeneralelasticityistheconformationchange whendeformation theconformationentropydecreases increasesduringrecovery internalenergyisnotthemainroleinthehighelasticdeformation butitisamajorcauseofgeneralelasticdeformation 2 thevisco elasticpolymer referstothatthepolymermaterialnotonlyhasthegeneralcharacteristicsofelasticmaterials butalsohassomecharacteristicsofviscousfluid elasticityandviscosityareshowingsimultaneously whichisparticularcharacteristicofpolymermaterials thevisco elasticofpolymerisshowninitsoutstandingmechanicalrelaxationphenomenon inthestudyofitsmechanicalproperties therelationshipamongthestress strainandtimemustbeconsidered temperatureisalsoveryimportantfactoronmechanicalproperties describetheviscoelasticmechanicalbehaviorofpolymermaterialsmustconsiderfourparameterssimultaneously thestress strain timeandtemperature themechanicalpropertiesofpolymermaterialsontimeandtemperaturedependencearethefocustounderstanditsmechanicalproperties andalsomustpaygreatattentionfortestingandusing sectioniihighelasticity 2 1characteristicsofhighelasticity2 2thermodynamicanalysisofhighelasticityintheequilibrium2 3theusetemperatureofrubber 2 1characteristicsofhighelasticity high elasticstateisamechanicalstateuniquetopolymerbasedonsegmentmotion undercertainconditionsthroughthepolymerglasstransition toachieveit thepolymerinthehigh elasticstateexhibitsuniquemechanicalproperties highelasticity thisisaveryexcellentperformanceinapolymer rubberishighlyelasticmaterial highflexibilitycharacterizedby largeelasticdeformation upto1000 whiletheelasticdeformationofmetallicmaterialsdoesnotexceed1 elasticmodulusissmall butitincreaseswiththeabsolutetemperatureincrease andelasticmodulusofmetallicmaterialsreach anditdecreaseswiththeabsolutetemperatureincrease inthefasttension adiabatic 绝热 process thepolymertemperaturerise andmetaltemperaturedecreases ifyoustretchtherubbersheet stickitonlipsorcheeks youwillfeelheatwhentherubberelongation absorbsheatwhenretraction deformationdependsontime therubber scompressionortensionbytheexternalforce constantstress deformationalwaysevolveovertime andfinallyreachesthemaximumdeformation thephenomenoncalledcreep 蠕变 reason becausetherubberislong chainmolecules themolecularmovementmustovercomeintermolecularforcesandinternalfriction high elasticdeformationisoriginatedfrommolecularchainmovement itmaytakeafewminutes hoursorevenyearsforthemolecularchainfromoneequilibriumstatetoanotherequilibriumstatethatadaptstheexternalforce undernormalcircumstances thatissayingthedeformationalwaysoccursdelayingtotheforce sotherubberdeformationneedstime 2 2thermodynamicanalysisofhighelasticityintheequilibrium high elasticdeformationcanbedividedintoequilibriumdeformation reversible andnon equilibriumdeformation irreversible assuminghigh elasticdeformationofrubberoccurswhenstretching whenremovetheexternalforces itcanbefullyreinstated andthatistosay thedeformationisreversible sothefirstlawofthermodynamicsandthesecondlawcanbeusedtoanalyze physicalmeaning externalforceintherubber withtherubberstretching ontheonehand theinternalenergychange ontheotherhand therubberentropychanges or therubbertensionisduetotheinternalenergyandentropychangewhendeformation thisisthethermodynamicequationofrubber inexperiments istheverticalaxis tistheabscissaaxis mapping intercept slope 发现各直线外推到时均通过原点 即截距为0 77 33 11 4 固定拉伸时的张力 温度曲线 so sowhentherubberistensioned theinternalenergyalmostdoesnotchange butcausetheentropychanges thatistosay undertheexternalforce therubbermolecularchainschangefromtwisteddisorderedstatetothestraightorderlystate entropychangesfromlargetosmall ordertodisorder finalstateisanunstablesystem aftertheexternalforceisremoved itwillspontaneouslyreverttotheinitialstate rubberfromstretchedstatetotheoriginalstateistheentropyincreasingprocess spontaneousprocess alsoexplainswhythehigh elasticdeformationisreversible asintension theentropydecrease isnegative soshouldalsobenegative indicatingwhyreleaseheatinthestretchingprocess astheidealelastomertensiononlycauseentropychangeoronlytheentropychangecontributestotheidealelastomerflexibility alsoknownasentropyelasticity 熵弹性 2 3theusetemperatureofrubber higherthanacertaintemperature therubberlosesitselasticityduetoaging lessthanacertaintemperature therubberlosesitselasticityastheglasstransition howtoimprovetheheatresistanceandcoldresistanceofrubber itisveryimportanttoexpanditstemperaturerange iimproveagingresistance improvedheatresistancevulcanized 硫化 rubberhascross linkednetworkstructure unlessthemolecularchainsbreaksorcrosslinkedchainsdestruction itwouldnotflow heatresistanceofthevulcanizedrubberappearsgood however infact vulcanizedrubberat120 hasbeendifficulttomaintaintheirphysicalandmechanicalproperties 170 180 havealreadylostvalue why themainchainofrubbercontainsalargenumberofdoublebonds susceptibletooxygendamageandpyrolysis the methylenehydrogennexttothedoublebondiseasilyoxidizedanddegradedorcrosslinked inordertoimproveitsagingresistentpropertiesthatwetake1changethemainchainstructureofrubber 1 themainchaindoesnotcontaindoublebonds 2 themainchainhaslessbutylrubberwithdoublebonds isobutyleneandisoprene 3 polysulfiderubberwithsatomsonmainchain 4 polyetherrubbercontainingoatomonthemainchain 5 dimethylsiliconerubber themainchainarenon carbonatoms 2 changethestructureofsubstituentstherubberwithelectrondonorsubstituentseasilyoxidized naturalrubber styrenebutadienerubber therubberwithelectronwithdrawingsubstituentsisnoteasyoxidized chloroprenerubber 氯丁橡胶 fluorinerubber 3changethestructureofcross linkedchainsprinciples cross linkedchainswithlesssulfurhavegreatbondenergy goodheatresistance ifthecrosslinkbondisccorco bondenergyisgreater andheatresistanceisbetter thechloroprenerubbervulcanizedwithzno cross linkedchainsisthe c o c naturalrubbercross linkedbyperoxideorradiation cross linkedchainsthe c c 2lowertoavoidcrystallizationandimprovecoldresistancethereasonoflackofcoldresistanceisduetoglasstransitionorcrystallizationatlowtemperatures whichleadstotherubberbrittleandharden lossofelasticity thereasonofpolymerglasstransitionisthemoleculesclosetoeachother strengtheningtheintermolecularforces sothatthemovementofchainsegmentsarefrozen anymeasurestoincreasetheactivityofmolecularchain weakentheinteractionbetweenmoleculeswillcausedecline anymeasurestoreducecrystallizationabilityandcrystallizationrateofpolymerwillincreasetheflexibilityofthepolymer increaseresistancetocold becausecrystallizationisregulararrangementofpolymerchainsorchainssegments itwillgreatlyincreasetheinteractionforcebetweenmolecules increasethepolymerstrengthanddecreaseelasticity 1plasticizers theweakeningofintermolecularforces如氯丁胶 45 加葵二酸二丁酯 80 可使其的 62 如用磷酸三甲酚酯 64 可使其 57 plasticizationeffectnotonlyrelatestotheplasticizerstructure butalsorelatestoitself thetgofplasticizerlower theofplasticizedpolymerisalsolower attentiontothesideeffectsofplasticizersitincreasesthemolecularchainsmobility andalsocreatetheconditionsforformingcrystallinestructure sousingtheplasticizertoreduce wemustalsoconsiderthepossibilityofcrystalformation 2 thecopolymerizationpolystyrenehasalargesidebase sorotationisdifficultinthemainchain morerigid higherthanroomtemperature butthecopolymerofstyrene butadienerubberis 53 polyacrylonitrilehavepolarity sorotationdifficultinthemainchain morerigid higherthanroomtemperature usingbutadieneandacrylonitrilecopolymer thenbr 丁睛橡胶 to 42 3 reducetheabilityofpolymercrystallizationlinearpolyethylenemolecularchainisveryflexible verylow butbecauseofhighregularity socrystallization polyethylenecannotuseastherubber theintroductionofsmallernon polarmethylsubstituentstodisrupttheregularityofmolecularchainofpolyethylene thendestructtheircrystallinity whichisethyleneandpropylenecopolymerrubber 60 destroyingtheregularityofthechaintoreducetheabilityofpolymercrystallization improvingflexibility butthesideeffectsaredetrimentaltostrength sectionthreeviscoelasticity 3 1relaxationphenomenon3 2creep3 3stressrelaxation3 4hysteresis3 5mechanicalloss3 6methodofmeasuringviscoelasticity3 7viscoelasticmodel3 8relationshipofviscoelasticityandtime temperature time temperatureequivalence 3 9boltzmannprincipleofsuperposition 迭加 3 1polymermechanicalrelaxationphenomenon mechanicalrelaxation themechanicalpropertiesofpolymerschangewithtimethemostbasicare creepstressrelaxationhysteresis 滞后 mechanicalloss idealelasticbodysubjecttoexternalforces thebalancedeformationachievedinstantaneous strainisproportionaltothestress deformationistime independent idealviscousbodysubjecttoexternalforces thedeformationislineardevelopmentovertime thestrainrateisproportionaltothestress polymerdeformationrelatetotime thisrelationshipbetweentheidealelastomerandtheidealviscousbody whichmeansthatbothstrainandstrainratearerelatetothestress thereforepolymersareoftencalledasviscoelasticitymaterials 3 2creep creep atacertaintemperatureandconstantexternalforce tension pressure torsion 扭力 etc thematerialdeformationincreaseswithtimegraduallyincreasing creepprocessincludesthefollowingthreedeformations generalelasticdeformation high elasticdeformation viscousflow generalelasticdeformationpolymermaterialsunderexternalforce bondlengthsandangleswithinthemolecularchainimmediatechange deformationissmall aftertheremovingofexternalforces generalelasticdeformationimmediatelyfullyrestored independentoftime schematicdiagram high elasticdeformationaprocessofmolecularchainthroughsegmentmovementhasgraduallyextended thedeformationismuchgreaterthanthegeneralelasticdeformation deformationisexponentialrelationshipwithtime whenremoveexternalforce thehigh elasticdeformationgraduallyrestored schematicdiagram viscousflowintermolecularnon cross linkedlinearpolymer itwillproducearelativeslipbetweenthemolecules ithaslinearrelationshipwithtime aftertheremovingofexternalforces viscousdeformationcannotberestored istheirreversibledeformation schematicdiagram polymerunderexternalforce threedeformationsoccurtogether thematerials totaldeformationisasisirreversibledeformation soforthelinearpolymer afterremovingexternalforces alwaysleavesomeirreversibledeformation therelativeproportionsofthethreekindsdeformationvaryaccordingtospecificconditions themainis themainareand areallsignificant creepisrelatedtotemperaturelevelandthesizeoftheexternalforcetemperatureistoolow below orexternalforcesaretoosmall thecreepisverysmall anddeformationdevelopsveryslowly difficulttoobservecreepinashorttime temperatureistoohigh abovemuch ortoolargeexternalforces deformationdevelopsrapidly butalsodifficulttoobservethecreep temperatureaboveafew chainsunderexternalforcescanmove buttheinternalfrictionislarge canobservecreep differenttypespolymerhavedifferentcreepbehaviorslinearamorphouspolymersif canonlyseethestartpartofthecreep ifyouwanttoseeallcurvesshouldobservesomemonthsorevenyears if canonlyseethelastpartofthecreep fortestinginthevicinity youcanobserveallthecurvesinarelativelyshorttime cross linkedpolymercreepnoviscousflowpart crystallinepolymercreepnotonlyrelatetotemperature andbecauseofrecrystallizationandsoon socreepislargerthantheexpected applicationcreepbehaviorsofvariouspolymersatroomtemperatureareverydifferent understandingthisdifferenceisimportantforpracticalapplications 1 psf2 聚苯醚3 pc4 改性聚苯醚5 abs 耐热 6 pom7 尼龙8 abs 2 01 51 00 5 1 2 3 4 5 6 7 8 小时 10002000 23 时几种高聚物蠕变性能 canbeseen thepolymerwhosemainchaincontainingaromaticheterocyclicrigid chain hasgoodcreepresistance soitbecamewidelyusedengineeringplastic replacingmetalmaterialsintomachineparts materialthatcreepsseriouslyshouldtakethenecessaryremedialmeasureswhenapplication example1 hardpvchasgoodcorrosionresistance itcanbeusedforchemicalpipelines buteasytocreep sosupportingstandisneededwhenusingit example2 ptfehastheminimumcoefficientoffrictioninplastic sohavegoodself lubricatingproperties butthecreepserious itisnotuseasmechanicalparts butitisagoodsealingmaterial example3 rubberisvulcanizedforcross linkingtopreventirreversibledeformationslippagebetweenmoleculesproducedbythecreep 3 3stressrelaxation definition foralinearviscoelasticbody inthecaseofremainingthesamestrain 应变 thestressgraduallydecreaseswithtimedecay thephenomenoncalledstressrelaxation forexample stretchinganon cross linkedrubbertoacertainlength andkeepthelengthconstant astimeincreases theresilienceforceofrubberwillgraduallydecrease becausetheinsidestressslowlydecreases finallyto0 therefore usingnon crosslinkedrubberfortransmissionbelting 传动带 cannotwork stressrelaxationandcreeparetwosidesofoneproblem bothreflectedthethreekindsofmovementwithinthepolymermolecules whenthepolymerisstretchedatthebeginning inwhichmoleculesinanunbalancedconformation theygraduallytransitiontobalancedconformation thatissay segmentmovesalongthedirectionoftheexternalforceinordertoreduceoreliminateinternalstress 1 if asrubberatroomtemperature chainseasilymove internalfrictionissmall molecularreorientationalongtheexternalforceisfast internalstresswillsoondisappear relaxed andevenapproachingthedegreeofnotaware 2 if astheplasticatroomtemperature althoughthechainsundergreatstress butbecauseoffrictionislarge chainsmotionabilityisverysmall sostressrelaxationveryslow itisnoteasytoperceive 3 ifthetemperatureiscloseto neartensofdegrees stressrelaxationcanbeobservedmoreclearly suchassoftpvcwire useittotiethings atbeginning rollingverytight thenitwillgraduallybecomeloose thatistheobviousexamplesofstressrelaxation 4 onlycross linkedpolymerstressrelaxationwillnotreducetozero sincenointermolecularslip butstressrelaxationoflinearpolymerscanbereducedtozero 3 4hysteresisphenomenon asastructuralmaterial inpractice polymeroftenunderalternatingforce suchastires drivebelts gears damper 消振器 etc theyareusedunderalternatingforce thetires forexample carsintheroad whileapartofitabovegroundortouchground sufferacertainfrequencyofexternalforce itsdeformationalternatebetweenlargeandsmall forexample carwithvelocity60kmperhour forsomewhereonthetire itisequivalenttobearperiodicallyforcewith300timesperminute assumingthetirediameteris1m theperimeterwas3 14 1 speed1000m 1min 1000 3 14 300r 1min consideringastresswithperiodicalsinefunction recordchangeofthestressanddeformationofthetirealongwithtime cangetthefollowingtwowaveform 波形 curves hysteresisphenomenon polymerinalternatingforce thedeformationlagbehindthestressexplanation chainsegmentsmovementsufferinternalfriction whentheexternalforcechanges themovementofchainsegmentscannotkeepupwithchangesofexternalforces deformationlagbehindthestress sothereisaphasedifference thegreaterphasedifferenceis themoredifficultsegmentmotionis themorecannotkeeptheforcechange hysteresisphenomenonofpolymerisrelatedtoitsownchemicalstructure usuallyhysteresisphenomenonofrigidmoleculesissmall eg plastic flexiblemolecularhasserioushysteresisphenomenon suchasr