Cellulose Based On Molecular Dynamics Simulation

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energiesArticleThermal Stability of Modified Insulation PaperCellulose Based on Molecular Dynamics SimulationChao Tang 1, *, Song Zhang 1 , Qian Wang 2 , Xiaobo Wang 1 and Jian Hao 3123*College of Engineering and Technology, Southwest University, Chongqing 400715, China;xbf [email protected] (S.Z.); [email protected] (X.W.)State Grid Chongqing Electric Power Co. Chongqing Electric Power Research Institute, Chongqing 401123,China; [email protected] of Power Transmission Equipment & System Security and New Technology,Chongqing University, Chongqing 400044, China; [email protected]: tangchao [email protected]; Tel.: 86-023-68251265; Fax: 86-023-68251265Academic Editor: Issouf FofanaReceived: 26 January 2017; Accepted: 17 March 2017; Published: 20 March 2017Abstract: In this paper, polysiloxane is used to modify insulation paper cellulose, and moleculardynamics methods are used to evaluate the glass transition temperature and mechanical propertiesof the paper before and after the modification. Analysis of the static mechanical performance ofthe model shows that, with increasing temperature, the elastic modulus of both the modified andunmodified cellulose models decreases gradually. However, the elastic modulus of the modifiedmodel is greater than that of the unmodified model. Using the specific volume method andcalculation of the mean square displacement of the models, the glass transition temperature ofthe modified cellulose model is found to be 48 K higher than that of the unmodified model. Finally,the changes in the mechanical properties and glass transition temperature of the model are analyzedby energy and free volume theory. The glass transition temperatures of the unmodified and modifiedcellulose models are approximately 400 K and 450 K, respectively. These results are consistentwith the conclusions obtained from the specific volume method and the calculation of the meansquare displacement. It can be concluded that the modification of insulation paper cellulose withpolysiloxane will effectively improve its thermal stability.Keywords: insulation paper; polysiloxane; glass transition temperature; mean square displacement;molecular simulation1. IntroductionThe glass transition temperature of a polymer is very important, being the turning point at whicha polymer material transitions from a glass state to a highly elastic state [1]. The main componentof oil-immersed power transformer insulation paper is natural cellulose [2], which is composed ofcrystalline and amorphous regions. The crystalline region is closely arranged and its structure is stable,meaning that its performance is relatively stable under high temperatures. In contrast, the amorphousregion is loosely and irregularly arranged, and so thermal aging usually starts from the amorphousregion [3,4]. The initial mechanical properties of cellulose insulation paper can basically meet theneeds of its applications, but after its glass transition temperature changes, its mechanical propertiesseriously decline [5], which causes the paper to fail to meet the needs of the application. The generaloperating temperature of a transformer is below 100 C, but local winding temperatures can exceed473 K under extreme conditions. At 473 K, the properties of many polymers change greatly [6]; thus,the enhancement of the glass transition temperature of cellulose insulation paper is of importantpractical significance.Energies 2017, 10, 397; s

Energies 2017, 10, 3972 of 11Polysiloxane has many advantages, such as high temperature stability, oxidation resistance, andwater repellency, and so has been used to modify the toughness and thermal stability of polymerin recent years [7–9]. Li et al. [10] used polysiloxane to graft and modify E-20 modified epoxy.Spectral and differential thermal analyses showed that the thermal stability of the modified resinwas improved significantly. Su et al. [11] used dimethyl-diethoxy-silane to modify bisphenol Aepoxy resin, and found that the tensile strength, elongation at break, and mechanical propertiesof the modified material were improved, and the glass transition temperature reached 166.07 C.Zhang et al. [12] used polysiloxane to improve the thermal stability and mechanical propertiesof resin. Guan et al. [13] used fluorine-containing silicone to modify resin, and reported that themaximum thermo-gravimetric rate temperature of the modified composite material reached 375 C.Regarding other polymers, Zong et al. [14] used polysiloxane to modify polyurethane, and found thatthe water resistance properties of the modified material were improved. Obviously, modification withpolysiloxane has a good effect on the performance of polymers, and especially has great potential forimproving their thermal stability.With the development of computer simulation technology, molecular simulation has been widelyapplied to the study of material properties [15–17]. Moreover, the results of studies combiningmolecular simulations and experiments have shown simulated and experimental values that werehighly consistent, indicating that molecular simulation of materials is feasible and effective [18,19].Molecular simulation has been widely used in researching the properties of insulation paper celluloseto enhance its thermal stability. Liao et al. [20] grafted dicyandiamide onto the C6 atoms of cellulosechains, and showed through molecular simulation that the stability of the modified cellulose chainin water and acid environment was improved. Cheng et al. [21] used inorganic particles to modifycellulose, and reported that the thermal stability of the modified cellulose was improved. However,the modification of insulation paper cellulose with polysiloxane has rarely been reported.Therefore, this paper focuses on the two important indexes of the thermal stability of celluloseinsulation paper: glass transition temperature and mechanical properties. The effect of polysiloxane onthe thermal stability of insulation paper cellulose was evaluated by a molecular dynamics simulationmethod. The internal mechanism of the effect of polysiloxane on the mechanical properties, exerciseintensity, and glass transition temperature of the cellulose was also studied.2. Model Building and Parameter Setting2.1. Model BuildingMazeau and Heux [22] used molecular simulation to study a model of the amorphous region ofcellulose chains with different degrees of polymerization (DP), of 10, 20, and 40. They found no obviousdifference in molecular conformation or physical and chemical properties. Chen et al. [23] suggestedthat, for the amorphous models of cellulose with a DP of 20, the mechanic properties obtained bymolecular simulation were consistent with the experimental data. Meanwhile, our previous researchresults [24,25] also verified the consistency of molecular simulation and experiment results whenDP is 20. Therefore, in this paper, a cellulose chain with a DP of 20 was chosen to build the model.The DP of the polysiloxane used for grafting and modification was 3. The polysiloxane is graftedonto the hydroxyl group of the C6 atom in the cellulose unit, which is shown in Figure 1. The modelfor the modified cellulose was established using the method used to construct the amorphous regionmodel proposed by Theodorou and Wsuter [26]. The target density of the model was 1.5 g/cm3 .The constructed model is shown in Figure 2.

Energies 2017, 10, 397Energies2017,2017,10,10,397397Energies3 of perpapercellulose.cellulose.FigureCellulose,DP 20DP 20Cellulose,PolysiloxanePolysiloxaneDP 3DP ewerecarriedout.The modelmodelwasstructurallythen mizationwerecarriedwasthenoptimizedthroughenergy minimizationcarriedout.out.The Themodelwas thenoptimizedthrough molecularmoleculardynamics simulation.simulation.The forceforcefieldusedin thethe structuralstructural optimizationoptimizationand molecularmolecularmolecularusedinanddynamics dynamicssimulation.The force Thefieldusedfieldin thestructuraloptimizationand consistentforcefield)[27],whichisverysuitablefor thethedynamicsPCFF consistent(polymer forcefield)consistent [27],forcefield)whichis veryforsimulationsimulationwas PCFFwas(polymerwhich[27],is verysuitablefor suitablethe structuraloptimization,carbohydrates and other organic molecules. During the structural optimization, dynamics yEnergydynamicssimulationspswereatofK,800of 50 ps werecarried ofoutof1000K, 800 K, and600minimizationwasminimizationwascarriedout afterafterevery dynamicsdynamicssimulation,and thethe everysimulation,andcarried out aftereverydynamicssimulation,andthe obtainedminimizedstructurewas minimizedusedas thestructurewas usedused asas thetheinitialconformationfor imulation.dynamicsinitial conformationforthe initialnextdynamicssimulation.Then,of Then,50ps ftertheaboveprocessing,minimization were carried out at 400 K. After the above processing, the local unreasonable ated,whichmadein theamorphousmodelwas basicallyeliminated,modelwhichwasmadethe modelstable onableequilibriumgeometryreal material, and provided a reasonable equilibrium geometry conformation for the next molecularconformationfor thethe[28].nextOnmoleculardynamicssimulation[28]. simulationOn thisthis mics simulationthis basis,the moleculardynamicswas itiontemperaturebeforeandTo determine the glass transition temperature before and after modification, a temperature tureofwasforsimulation,witheveryof 200–650K was selectedfor therangesimulation,withKKevery50K a nKdynamicsa target temperature.moleculardynamics simulationeachwas basedonsimulation ofTheeachtarget temperaturewas basedofonthetargetfully temperatureoptimized ationof100pswascarriedout,afterwhichequilibrium simulation of 100 ps was carried out, after which molecular dynamics simulation of 200 ascarriedout. Thesimulationintegral steplengthwascarried1 fs, ionevery 5000 rties3.1.relationship betweenbetween stressstress sHooke’slaw:law:The

Energies 2017, 10, 3974 of 11Energies 2017, 10, 3974 of 11σ i Cij ε j(1)σi Cij ε j(1)ε is the stress matrix, σ is the strain matrix.where C represents a 6 6 elastic coefficient matrix,First, the inverse matrix S of the C matrix is obtained, and then the effective bulk moduluswhere C represents a 6 6 elastic coefficient matrix, ε is the stress matrix, σ is the strain matrix.and shear modulus are calculated by Reuss’ average method [29]:First, the inverse matrix S of the C matrix is obtained, and then the effective bulk modulus and 1shear modulus are calculated by Reuss’ averagemethodΚ 3(a 2b) [29]:(2)K [3( a 52b)] 1G G In the formula:In the formula:(2)(3)4a 45b 3c(3)4a 4b 3c1a 1 S S 22 S 33 , S33 ),a 3(S1111 S2231b 1 (S S1212 Sb S23 SS3131 ), ,23 3311c S ). .c (S S4444 S5555 S 666633Through the relationship between the modulus of the same-phase material:Through the relationship between the modulus of the same-phase material:EE 2G2G( 11 ν) 33KK( 11 2ν2 ) , ,(4)(4)where EErepresentsthethevolumemodulus,is cmodulus,modulus,K Kis isvolumemodulus,G Gis theshearmodulus,andrepresentsPoisson’sratio.v representsPoisson’sratio.The ratioratio ofof strainstrainandandstressstressis iselasticelasticmodulusE, whichreflectsrigiditya material.modulusE, whichreflectsthe therigidityof a ofmaterial.TheThegreaterthe valueis,greaterthe greaterthe rigidityandstrongerthe strongerthe abilitythe materialtogreaterthe valueof E ofis, Ethethe rigidityand thethe abilityof theofmaterialto resistresistdeformationwillThus,be. Thus,E sformertransformer insulationdeformationwill be.E canreflectthethemacro-mechanicalpaper [30,31]. TheThe elasticelastic modulusmodulus calculatedcalculated forfor thethe modified model and unmodified model atdifferent temperaturestemperatures isis shownshown inin FigureFigure 3.3.10UnmodifiedModified 1281046 E (GPa)E (GPa)682422003004005006000700T (K)Figure3. Elasticand unmodifiedunmodified models.models.Figure 3.Elastic modulusmodulus ofof modifiedmodified andWith increasing temperature, the elastic modulus of both models gradually decreases. However,With increasing temperature, the elastic modulus of both models gradually decreases. However,the elastic modulus of modified model is larger than that of the unmodified model in the wholethe elastic modulus of modified model is larger than that of the unmodified model in the wholesimulation process. In Figure 3, E is the difference between the elastic modulus of the modifiedsimulation process. In Figure 3, E is the difference between the elastic modulus of the modifiedmodel and the elastic modulus of the unmodified model. E is positive in the whole temperaturemodel and the elastic modulus of the unmodified model. E is positive in the whole temperaturerange, which shows that the elastic modulus of the modified model is improved over that of therange, which shows that the elastic modulus of the modified model is improved over that of theunmodified model. This is because, when modifying cellulose by polysiloxane grafting, the bond

Energies 2017, 10, 397Energies 2017, 10, 3975 of 115 of 11energyof themodel.flexibleThisSi–Ointroducedon the cellulosechainis 451 kJ/mol,grafting,which isthegreaterunmodifiedis bondbecause,when modifyingcelluloseby polysiloxanebondthantheof356the C–CThe greaterbond energy,it whichis to breakthe ducedon thethecellulosechain isthe451harderkJ/mol,is thpolysiloxaneisalwayshigherthanthatofthe 356 kJ/mol of the C–C bond. The greater the bond energy, the harder it is to break the bond, so thetheunmodifiedmodelincreasingThisis meansthedeformationresistance,elastic modulusof thewithmodelmodified temperature.with polysiloxanealways thathigherthanthat of the unmodifiedmechanicalproperties,temperature.and thermal Thisstabilityof thatthe modelmodified withpolysiloxaneare enhanced.model with increasingmeansthe deformationresistance,mechanicalproperties,and thermal stability of the model modified with polysiloxane are enhanced.3.2. Glass Transition Temperature3.2. Glass Transition TemperatureAs its temperature is gradually increased, an amorphous material will undergo glass transition.As its temperatureis graduallyincreased,ana amorphousundergoglasstransition.The transitiontemperature[1] of a materialfromglass state tomateriala highlywillelasticstate isknownas theThe transitionmaterialfrom acontinuesglass statea highlyelasticstate g.ofIf athetemperatureto torise,a polymermaterialwillchangethe glasstransitionTg . Ifflowthe statetemperaturecontinuesa polymerwillfroma highlyelastictemperaturestate to a viscousat its meltingpoint,toTrise,f. Generally,the materialpropertiesofchange fromhighlybeforeelastic glassstate toa viscousAtflowstate at its highermeltingthanpoint,theTf .glassGenerally,thepolymersarea stabletransition.temperaturestransitionproperties of thepolymersare stablebefore ofglasstransition.At temperatureshigher namelymechanical strengthandthethermaltransition willtemperature,the mechanicalpolymers,namely ofmechanicalandstability,be significantlyreduced.propertiesTherefore,of theimprovementthe glassstrengthconversionthermal stability,will bepapersignificantlyTherefore, theimprovementof the glass conversionpropertiesof insulationcellulosereduced.at high temperatureis significantlyimportant.propertiesof theinsulationpapercelluloseat highis significantlyimportant.Amongmethodsusedto evaluatethetemperatureglass transitiontemperatureof polymers, the mostAmongmethodsto evaluate the reliableis theusedvolume-temperaturecurvetransitionmethod [32,33].In theofNPTensemblecommonreliableis theN,volume-temperaturecurve method[32,33]. Inthe NPT ensemble(fixedvaluesof andparticlenumberpressure P, and temperatureT), molecularsimulationwas performedvaluesparticleN, pressureP, and temperaturemolecularsimulation wasonthe ofmodelsatnumbereach selectedtemperatureto calculateT),thedensity parameter.The performedreciprocal onofthe modelsat eachselectedtemperatureto calculatedensityvolumeparameter.The temperaturereciprocal of densitydensityis thespecificvolume,thus, thefigure ofthespecificversuscan beis the specificvolume,thus, the offigurespecificbe obtained.Next,obtained.Next,the fore temperatureand after thecaninflectionpoint ofthethe intersectionlines fitting thebeforeand afterturningthe inflectionvolumespecificvolumeoftemperatureis regionsthe glasstransitionpoint. pointThus,of the specificcorrespondingtemperature is thepoint. TheThus,glassthe correspondingtemperatureis theglasstemperaturethe ransition temperaturefittingcurveistransitiontemperature.The glass transition temperature fitting curve is shown in Figure 4.shownin Figure4.UnmodifiedModifiedUnmodified curve fittingModified curve fittingSpecific volume (cm3 0700T (K)Figure 4. SpecificSpecific volume–temperature curve.TheThe glassglass transitiontransition temperaturetemperature ofof thethe unmodifiedunmodified modelmodel isis 402402 K,K, whilewhile thatthat ofof thethe sthatthemodificationimprovestheglassmodified with polysiloxane is 450 K, which means that the modification improves the glass ure of the cellulose chain by 48 K. In the Handbook of Polymers [34], the glass erature of variety of industrial cellulose is reported to be between 250 and 580 K, but this is onlythisis only Thus,a reference.the glasstransitionof temperaturesthe simulatedcelluloseanda reference.the glassThus,transitiontemperaturesthe simulated ofcelluloseand dels in this paper are credible, and the comparative analysis of the glass transition temperature oftransitiontemperatureof the modifiedunmodified model is valid.the modifiedand unmodifiedmodel is andvalid.The glass transition temperature of the modified model is higher than that of the unmodifiedmodel, which means that the process from the glassy state to highly elastic state of the modified

Energies 2017, 10, 3976 of 11TheglasstransitionEnergies2017,10, 397temperature of the modified model is higher than that of the unmodified6 of 11model, which means that the process from the glassy state to highly elastic state of the modified modelmodelis slowerthanthatunmodifiedof the unmodifiedmodel. Therefore,the modificationof celluloseshouldis slowerthan thatof themodel. Therefore,the modificationof celluloseshould resultinresultin excellentperformanceand enhancedthermal stability.excellentperformanceand enhancedthermal stability.3.3. GlassGlass TransitionTransition Temperature3.3.Temperature AnalysisAnalysisThe moremore intenseTheintense thethe chainchain movementmovement inin insulationinsulation paperpaper cellulose,cellulose, thethe worseworse itsits hermalstability.Theintensityofthechainmovementcan bebeperformance, which leads to a poor thermal stability. The intensity of the chain movement cananalyzed byby thethe meanmean squaresquare displacementdisplacement (MSD).(MSD). TheThe MSDMSD describesdescribes thethe overalloverall movementmovement ofof thetheanalyzedmolecular chainchain centroidcentroid [35],molecular[35], andand cancan bebe calculatedcalculated as:as:** (0)2 2 ,MSD rr ii ((t)) (5)MSD rri (i 0) ,(5)* * and rrii ((00)) ic positionposition vectorsvectors atat time t and initial time(t) andwhere rrii(ttime i,i,respectively, andrepresents thethe ensembleThe MSDMSD ofof thethe modifiedmodified modelmodel andand thetherespectively,and representsensemble average.average. TheunmodifiedmodelareshowninFigure5.unmodified model are shown in Figure 5.200K450K250K500K300K550K350K600K400K650K10(a)MSD (Å2)86420050100150200t MSD (Å2)6420050100150200t (ps)Figure 5.5. MSDMSD ofof (a)(a) unmodifiedunmodified modelmodel andand (b)(b) modifiedmodified modelmodel atat differentdifferent temperatures.Figuretemperatures.According todegreeof chainmovementincreasesgraduallywith degreeof chainmovementincreasesgraduallywith temperature,indicatesthat insulationpaper celluloseaffected byistemperaturetransformeroperation.In thewhichindicatesthat insulationpaperis celluloseaffected del,between 350and 400K, the350MSDtheK,chainappearsto jump,is basicallyoperation.the unmodifiedmodel,betweenandof400the MSDof thechainwhichappearsto jump,which is basically consistent with the glass transition temperature of the unmodified model of 402 K.Between 500 and 550 K, the MSD of the unmodified model appears to jump again, which maycorrespond to the transition temperature of cellulose chain from the highly elastic state to theviscous flow state. In the modified model, the first jump in the MSD of the chain between 400 and

Energies 2017, 10, 3977 of 11consistent with the glass transition temperature of the unmodified model of 402 K. Between 500and 550 K, the MSD of the unmodified model appears to jump again, which may correspond to thetransition temperature of cellulose chain from the highly elastic state to the viscous flow state. In themodified model, the first jump in the MSD of the chain between 400 and 450 K also corresponds tothe glass transition temperature (450 K) of the modified model. The MSD of the modified model doesnot appear to jump again obviously within the studied temperature range. This may be because themodification of the cellulose chain not only increased the glass transition temperature of the chain,but also increased the temperature of the highly elastic state to the viscous flow state transition. It canalso be seen that, before the glass transition, the intensity of the movement of the chain was relativelysmall, and the MSD value of the two models is less than 1 Å2 . After the glass transition, the intensityof the movement of the chain is obviously enhanced; the maximum MSD of the unmodified model isabout 10 Å2 , while that of the modified model is about 9 Å2 .The chain movement is the direct embodiment of the thermal movement ability of the insulationpaper cellulose chain. The more intense the chain movement is, the worse the mechanical performancewill be, which leads to a poor thermal stability. The thermal stability of insulation paper cellulose isimproved by grafting with polysiloxane. In oil-immersed transformers, cellulose insulation paperis wound around the copper conductor as electrical insulation. If the mechanical properties of theinsulation paper are affected by temperature and become worse, the insulation paper wrapped outsidethe copper wire will be easily damaged when the conductor coil is subject to mechanical stress,which will damage the insulation of the transformer. Therefore, the improvement of the mechanicalproperties of insulation paper also indirectly improves the insulation of the transformer, thus, ensuringits safe operation.4. Analysis and Discussion4.1. Enhancement MechanismPolysiloxane is a cross-linked polymer with Si–O–Si as the main chain, and silicon atoms connectedto other organic groups. The organic groups can be methyl or phenyl, so polysiloxane can have organicand inorganic characteristics. Polysiloxane has many excellent properties, such as heat resistance andchemical corrosion resistance. The Si–O–Si bond is the basic bond form of polysiloxane, essentiallythe same as quartz, but its side groups are connected to organic groups. The thermal stability ofinorganic compounds composed of Si–O bonds can be as high as 2073 K. When organic substituentsare introduced into the polymer, its thermal stability will decrease by 623–873 K, but its heat resistancewill still be better than that of common organic compounds. Additionally, because of its organic andinorganic characteristics, it can solve compatibility problems well. The bond energy of the Si–O bondis 451 kJ/mol, much higher than the energy of the C–C bond of about 356 kJ/mol. Thus, the thermalstability of the Si–O bond is better than that of the C–C bond. The length of the Si–O–Si bond is alsolonger (0.164 nm), and its bond angle is larger (140–180 ) [36]. The chain structure of polysiloxaneis very soft, the interaction between chains is weak, and its surface tension is low, which means thatpolysiloxane can still maintain its original performance in a wide range of temperatures. Therefore,the mechanical properties of cellulose modified with a certain amount of polysiloxane change littlewith increasing temperature, resulting in a very good thermal stability.Energy is a parameter that can best reflect the stability of the system. The energy of the systemunder a PCFF force field [6] can be expressed as:Etotal ( Einternal Ecross ) Enobond ,(6)where Einternal Ecross represents the bond energy and Enobond represents non-bond energy. Figure 6shows the overall potential energy and non-bond energy of the unmodified model and modified modelat each temperature studied in this paper.

Energies 2017, 10, 397Energies 2017, 10, 3978 of 118 of 111000Total energyNon bond energy900(a)Energy (kcal mol-1)800700390K600500400300200300400500600700T (K)800700Total energyNon bond energy(b)Energy (kcal mol-1)600500400452K3002001000200300400500600700T (K)Figure 6.6. EnergiesEnergies ofof thethe twotwo models:models: (a)(a) thethe unmodifiedunmodified model;model; andand (b)(b) thethe modifiedmodified model.model.FigureThe overall potentialpotential energyenergy andand thethe non-bondnon-bond energyenergy ofof bothboth modelsmodels increaseincrease linearly,linearly, but thegrowth raterate ofof hanthatof ofthetheoverallpotentialenergy.In rtstobecomelargerthanthenon-bondunmodified model, the overall potential energy starts to become larger than the non-bond energy atenergyat the390modifiedK. In the model,modifiedoverallenergypotentialenergystarts tobecomethan the390 K. Inthemodel,overallthepotentialstartsto becomelargerthanlargerthe non-bondnon-bondat about452 K, a difference62 K. Equation(6) thatshowsthatthewhenthe ofvalueenergy at energyabout 452K, a differenceof about of62aboutK. Equation(6) gesoverall potential energy starts increase beyond that of the non-bond energy, the value changes fromfromnegativeto positive.This indicatesthe repulsiveforce betweengreaterthan thenegativeto positive.This ind

dynamics simulation [28]. On this basis, the molecular dynamics simulation was the carried out. To determine the glass transition temperature before and after modification, a temperature range of 200-650 K was selected for the simulation, with every 50 K a target temperature. The molecular dynamics simulation of each target temperature was .