PLA,PP,MAPP纤维复合材料断裂行为综述

Strong synergetic effect of fibril-like nucleator and shear flowon the melt crystallization of poly(L-lactide)Piming Ma a ,⇑,Qingqing Yu a ,Tianfeng Shen a ,Weifu Dong b ,Mingqing Chen b ,⇑aThe School of Chemical and Material Engineering,Jiangnan University,1800Lihu Road,Wuxi 214122,China b The Key Laboratory of Food Colloids and Biotechnology,Ministry of Education,1800Lihu Road,Wuxi 214122,Chinaa r t i c l e i n f o Article history:Received 3November 2016Received in revised form 1December 2016Accepted 21December 2016Available online 23December 2016Keywords:Poly(L -lactide)Fibril-like nucleator Shear flow Synergistic effect Crystallizationa b s t r a c tBio-based and bio-degradable poly(L -lactide),PLLA,suffers from low crystallization rate.The synergetic effect of fibril-like nucleator (N 1,N 10-(ethane-1,2-diyl)bis(N 2-phenyloxalamide))(OXA)and shear flow on speeding up the melt crystallization of thePLLA is the first time reported in this work.The synergetic effect was investigated by usinga rheometer.Neither fibril-like OXA nor shear flow showed obvious accelerating effect at155°C on the crystallization of PLLA leading to limited crystallinity (X c )within 70–100min.In contrast,a crystallinity of more than 40%and an onset crystallization time(t onset )of less than 3min were achieved by tailoring the shear conditions.The critical shearcondition for the fast crystallization of the PLLA/OXA systems at 155°C is 0.4rad/s for 10–15s.A lower shear temperature (e.g.,150°C in comparison with 155°C)would increasethe crystallization rate much further.Mechanism study revealed that the OXA fibril net-work and a certain extent of PLLA chain orientation that induced by the shear flow whilesubsequently stabilized by the OXA fibrils are responsible for the significantly enhancednucleation and overall melt crystallization kinetics.In addition,the shear flow and fibril-like OXA nucleators not only facilitated the crystallization of PLLA a -form crystals but alsopromoted the formation of a 0-(or d )and b -form crystals.Ó2016Elsevier Ltd.All rights reserved.1.IntroductionThe bio-based and bio-compostable semi-crystalline poly(L -lactide),PLLA,has gained increasing attention these years in view of the sustainability and environmental issues that associated with traditional polymers [1,2].The excellent perfor-mances make PLLA promising as an alternative for traditional polymers [3–5].However,the end-use of PLLA articles suffers from low heat distortion temperature (HDT)due to its relatively low glass transition temperature ($60°C)and low crys-tallinity after practical processing such as injection molding.Consequently,the wide application of PLLA in both commodity and biomedical fields is restricted.It is thus of importance to achieve high crystallinity of PLLA by speeding up the crystallization.Heterogeneous nucleating agent is widely used to enhance the crystallization kinetics of PLLA by promoting the polymer chain folding at high(er)temperatures.Inorganic nucleating agents such as talc,clay,carbon nanotubes and graphene have been introduced into PLLA matrix,whereas agglomeration was often obtained due to the poor compatibility and insolubility in the PLLA melt,leading to uncontrollable shape/size and nucleation efficiency [6–8].On the other hand,organic additives /10.1016/j.eurpolymj.2016.12.0260014-3057/Ó2016Elsevier Ltd.All rights reserved.⇑Corresponding authors.E-mail addresses:p.ma@ (P.Ma),mq-chen@ (M.Chen).such as N,N-ethylene-bis(12-hydroxylstearamide)(EBH),poly(vinylidenefluoride),orotic acid,nucleobases,substituted-aryl phosphate salts(TMP-5)and N,N0,N00-tricyclohexyl-1,3,5-benzene-tricarboxylamide(TMC-328),N,N’-bis(benzoyl)hexane-dioic acid dihydrazide(TMC-306),N1,N10-(ethane-1,2-diyl)bis(N2-phenyloxalamide)(OXA)and PLA stereocomplex were also developed as nucleating agents for PLLA[9–24].Among above nucleating agents,TMC-328,TMC-306and OXA were proven with high reactivity which can self-organize into superstructures in PLLA melt to accelerate the crystallization of PLLA[18–21].However,the previous study is mainly confined to the crystallization of PLLA under static conditions and very limited or even no crystallinity was obtained under high temperature conditions(e.g.,145°C for90min)even in the presence of OXA [20–21,25].In addition,shearflow is an effective approach to increase the rate of crystallization which exists inevitably in polymer processing,e.g.,extrusion,injection and blowing molding.Theflow-induced crystallization is evidenced to take place in var-ious semi-crystalline polymers such as poly(ethylene)(PE)and poly(propylene)(PP)[26–28].Apparently,in appliedflow field,the high orientation of molecular chains is examined along theflow direction and shearflow brings a large amount of row nuclei and consequently enhance the crystallization kinetics[29,30].PLLA,as one of environmentally friendly poly-mers commercialized recently,however,is less studied in terms offlow-induced crystallization compared with polyolefin [31,32].Nucleators together with shearflow may further accelerate the crystallization of PLLA,whereas their synergetic effect is seldom reported[33].Rheometry as a helpful technique providing shear-flowfield to investigate the crystallization of polymeric materials pos-sesses several important advantages[34,35]:(i)it is more convenient and easier than other measurement methods such as small-angle light scattering(SALS),small-angle X-ray scattering(SAXS),wide-angle X-ray scattering(WAXS),and optical microscopy to track the crystallization processes of polymers,(ii)it can produce a continuous shear-flowfield to magnify the very small transformation of microstructure in the polymeric materials and(iii)it is applicable to the component where some other methods may not work,for instance,the colored system and/or thefilled composite component[36–38].In our previous works,the OXA was proven with high nucleation activity which can self-organize into needle-like tem-plates in the PLLA melt to initiate the fast crystallization of PLLA.Consequently,the half-life crystallization time(t0.5)is decreased from38.6to2.6min at135°C[20].However,the effect of OXA on the PLLA’s crystallization in aflowfield has not been reported,and the synergetic effect of the needle-like OXA and the shearflow on the rapid crystallization of PLLA is not yet clear.In this work,it is confirmed by employing rheometer,DSC and SEM that the synergistic effect of the OXA and the shearflow on accelerating the crystallization of PLLA.Thefinding on the synergistic effect may create a new approach to make PLLA-based products with high(er)crystallinity and fast(er)shaping rate.2.Experimental section2.1.MaterialsPoly(L-lactide)(PLLA,4032D)(M n=2.1Â105g molÀ1,PDI=1.7)with2wt%of D-lactide was provided by NatureWorksLLC,U.S.A.N1,N10-(ethane-1,2-diyl)bis(N2-phenyloxalamide)with a purity of98%,briefed as OXA,was synthesized in thelab-Fig.1.The room temperature polarized optical microscopy(POM)image of the OXA with an inset of its chemical structure. 222P.Ma et al./European Polymer Journal87(2017)221–230P.Ma et al./European Polymer Journal87(2017)221–230223oratory according to a method reported previously[21].The initial dimension of the OXA is10l m in length and2l m in diameter and the melting temperature is338°C.The polarized optical microscopy image with the insert chemical structure of the OXA is presented in Fig.1.2.2.Sample preparationAll PLLA and OXA were used after drying at60°C under vacuum for12h.Firstly,OXA(0.5wt%)was mixed with PLLA in a chamber of a rheometer(HAAKE Polylab-OS,Thermo Fisher Scientific,Germany)at180°C and50rpm for5min.The samples were then molded into disk-shape,with a diameter of25mm and a thickness of1.0mm under10MPa and180°C using a hot compression molding machine.The disk-shaped samples were further dried in a vacuum oven at60°C for12h before the rheological measurements.2.3.Characterization2.3.1.Rheological behaviorRheological measurements were performed by using a DHR-2rheometer(TA Instruments,USA).All samples were placed on a plate-plate configuration(25mm in diameter and a gap of1mm)to study the isothermal crystallization processes of PLLA and PLLA/OXA with and without shear,respectively.Fig.2schematically describes the experimental procedures for the shear-induced crystallization of the samples.After holding at200°C(T1)for3min the samples were exposed to a dynamic temperature sweep with-5°C/min ramp to155°C(T2),and then a shear pulse with different shear rates or shear time was applied to the melts.For the isothermal crystallization at T2an oscillatory time sweep was performed to trace the evolution of storage modulus for polymers with time until the crystallization was complete.What’s more,the applied strain and oscil-lation frequency was1%and1Hz,respectively.In this study,the temperature of155°C was chosen to investigate the syn-ergetic effect of needle-like nucleator and shearflow on the crystallization of PLLA.2.3.2.Differential Scanning Calorimetry(DSC)The crystallization behavior was studied by using differential scanning calorimetry(DSC8000,Perkin Elmer).In this work,approximately4mg of each sample was heated from0to200°C at a heating rate of10°C/min.The crystallinity of PLLA(X c)can be computed via X c¼ðD H mÀD H ccÞ=ðxÃD H0mÞÂ100%,where D H m and D H cc are melt and cold crystallization enthalpy of PLLA,respectively,x is the proportion of PLLA’s weight in the PLLA/OXA blends,while the D H0m=93.6J/g is the melting enthalpy of PLLA whose crystallinity reach100%[39,40].All tests were carried out in a nitrogen atmosphere.2.3.3.Wide Angle X-ray Diffraction(WAXD)WAXD measurements were carried out by using an X-ray diffractometer(Bruker AXS D8,Germany)equipped with a Ni-filtered Cu K a radiation source with a wavelength of1.542Å.The measurements were conducted at40kV and40mA with scan angles from5°to50°and a scan rate of3°/min.2.3.4.Scanning Electron Microscopy(SEM)Phase morphology of the PLLA/OXA sheets after shearing was observed by using a scanning electron microscope(S-4800, HITACHI,Japan).The accelerated voltage of2kV was used.Before observation,the cross sections of the sheets obtained by cryo-fracture were etched in a water-methanol(1:2by volume)solution containing0.025mol/L NaOH and subsequently coated with a thin gold layer.Shearapplications for224P.Ma et al./European Polymer Journal87(2017)221–2303.Results and discussion3.1.Synergetic effect of OXA and shearflow on the crystallization behaviors of PLLARheology is used to investigate the melt crystallization process of PLLA since it is sensitive to microstructural changes [20,35].Fig.3shows the evolution of storage modulus(G0)of the PLLA and PLLA/OXA samples during isothermal crystalliza-tion at155°C with different shear conditions.For both non-sheared and sheared PLLA,almost no increase in the G0was observed within thefirst100min.A small and smooth increase after100min is observed and ascribed to the slow crystal-lization of PLLA.The non-sheared PLLA/OXA sample shows a similar tendency,whereas the G0increases earlier(around 70min).In the case of the sheared PLLA/OXA sample,a dramatic increase in the G0occurred within5min after the shear application,demonstrating a rapid crystallization of the PLLA.Apparently,sole shear application or nucleating agent has no effect on speeding up the crystallization of the PLLA at such a high temperature of155°C.Therefore,the PLLA/OXA sample was selected for further and detailed investigation on the synergetic effect of the OXA and shearflow.3.2.Effect of shear conditions on the crystallization behaviors of the PLLA/OXA samples3.2.1.Effect of shear rateThe storage modulus(G0)varies during the shear-induced isothermal crystallization of the PLLA/OXA sample at155°C after controlled shear with varied shear rates(c=0,0.1,0.2,0.3,0.4and0.5rad/s,respectively),as revealed in Fig.4a. The shear time was tuned in order to remain the same shear angle for all the samples.The G0of non-sheared PLLA/OXA sam-ple shows no increase with time indicating no obvious crystallization of PLLA.Furthermore,an indistinctive decrease of the G0was noticed which might be due to a certain extent of degradation at such a high temperature.When afixed shear angle is applied,the G0–t curves shift to the shorter time region.As shown in Fig.4a,even a shear rate as small as0.1rad/s,the G0–t curve shows an obvious rising trend with time.With the increase in time,an evident upturn of G0is observed ascribing to the nucleation and crystallization of PLLA/OXA blend.For briefness,we define this inflection point of G0–t curves as the onset crystallization time(t onset).The t onset plots as a function of shear rate of the PLLA/OXA samples was depicted in Fig.4b. The t onset values decreased from25min to8min when the shear rate was increased from0.1to0.5rad/s,indicating the nota-ble shear-accelerated crystallization kinetics.Moreover,the slopes of the G0–t curves after the inflection point become larger with the shear rate implying a shorter half-life crystallization time(t0.5)as well.On the other hand,the t onset and the G0–t slopes of the PLLA/OXA samples leveled off when the shear rate is above0.4rad/s which is regarded as a critical shear rate (R c)for applications.The crystallized PLLA/OXA samples exhibit double endothermic peaks during heating by differential scanning calorimetry (DSC),as demonstrated in Fig.5a.The one with lower temperature is regarded as the melting temperature(T m1)of the initial crystals that generated in the rheometer[41].The T m1of the sheared PLLA/OXA samples was raised gradually from164.4°C to166.6°C with increasing the shear rates from0.1to0.5rad/s(Fig.5b).At the same time,the crystallinity(X c)was increased to more than40%when the shear rate was above0.4rad/s.The detailed thermal data are provided as supporting information(Table S1).Thus,better organization and uniformity of PLLA crystals may be formed in the presence of shear flow.aa50100150200Time (min)PLLA and PLLA/OXA samples as a function of crystallization timeP.Ma et al./European Polymer Journal87(2017)221–2302253.2.2.Effect of shear timeIt is known from the above discussion that a shear rate of0.4rad/s was strong enough to enhance the crystallization of the PLLA in the presence of OXA.Therefore,it was selected andfixed in this section to assess whether shear time(single direction rotation)is important for the crystallization of the PLLA/OXA samples.Fig.6a illustrates the evolutions of G0as a function of shearing time at155°C with a reference of non-sheared sample.The t onset as a function of shear time was pre-sented in Fig.6b.As shown in the Fig.6,the sample that pre-sheared for5s shows a steep rise of G0which is correlated with the growth of PLLA crystals.The variation of G0is attributed to thefiller effects of the crystals behaving as a suspension of growing particles in an amorphous polymeric matrix.When the shear is applied,the G0–t curves shift to the shorter time region(left side),and the t onset is reduced to less than4min with increasing the shear time up to15s,after which the t onset becomes constant.In contrast,almost no crystallization occurred for the non-sheared PLLA/OXA sample.These results indi-cate a strong acceleration of the crystallization kinetics by the shear with enough shear time.However,the shift becomes less obvious after a sufficient shear time,which implies that the microstructural orientation might reach an equilibrium state226P.Ma et al./European Polymer Journal87(2017)221–230after a critical shear period(t c).The t c value in the present system is10–15s at the shear rate of0.4rad/s,corresponding to the total shear angle of4–6rad.The crystallized samples after the rheological experiments were subjected to DSC characterization.The DSC results are similar to the samples that sheared with different rates,and thus provided as supporting information to avoid repeating, see Fig.S1and Table S2.3.2.3.Effect of shear temperatureThe overall maximum crystallization rate of PLLA under static conditions is around120°C(T max),and the overall crystal-lization rate decreases monotonically above this temperature[42,43].On the other hand,orientation and relaxation of the PLLA/OXA molecules and the OXA needle-like superstructures are both dependent on temperatures[44].Therefore,the shear temperature followed by the isothermal crystallization is important on the crystallization of the PLLA/OXA samples.Fig.7a shows the G0variation of the PLLA/OXA samples during the isothermal crystallization after a controlled shear(0.4rad/s for 10s).The t onset values derived from Fig.7a are presented in Fig.7b.No crystallization occurred for the non-sheared PLLA/OXA at155°C within70min in this work and145°C within90min in our previous work(static conditions)[20].However,PLLA/ OXA could crystallize completely within20min with a pre-shear at these high temperatures.It is also seen from Fig.7that the lower shear temperature is significant and beneficial to the crystallization of the PLLA/OXA as evidenced by(i)the enhancement in the slopes of the G0–t curves after the t onset points(Fig.7a),and(ii)the obvious reduction in t onset values (from18.7to1.4min in Fig.7b)with reducing the shear temperature from160°C to150°C(Fig.7b).In the studied temper-ature span,the viscosity of the PLLA/OXA melt gets higher with decreasing the shear temperature,thus the effective orien-tation degree of the PLLA/OXA would be higher at lower temperatures(i.e.,less relaxation effect)leading to more ordered PLLA molecules as nucleating sites.On the other hand,the chain dis-folding from the PLLA lamellae would be less dominant with decreasing the temperature.Both of these two factors are beneficial to the accelerated crystallization kinetics at the relatively low(er)temperatures.3.3.Effect of shear and OXA on the crystalline structures of the PLLAWide angle X-ray diffraction(WAXD)was carried out to illustrate the synergistic effect of the shearflow and the OXA on the crystalline structures of the PLLA.The diffraction patterns are shown in Fig.8.Hardly any crystalline diffractions are observed in the non-sheared PLLA sample due to its intrinsic low crystallization rate and crystallinity,which is in good accor-dance with the above rheological responses and literatures[45,46].The addition of OXA without shear could promote the crystallization of PLLA to a certain extent.Consequently,three different diffraction peaks at2h=16.7°,19.2°and22.6°are distinguished corresponding to the200/110,203and105planes of the PLLA a-form crystals,respectively[41,47,48].It turns out that sole OXA as a nucleator cannot make PLLA crystallize completely as evidenced by a broad amorphous diffraction peak.Intriguingly,the diffraction peaks were strongly elevated when a shearflow was applied and the broad diffraction peak vanished.According to the X-ray diffraction patterns,the shear treatment and OXA enhanced notably the growth of200/110 planes in the a-form crystals of the PLLA leading to a higher crystallinity.Meanwhile,diffraction peak shift to smaller2h positions is noticed for both200/110and203planes after applying the shearflow.A similar phenomenon was observed in PLLA/TMC-306system and the shift was ascribed to some disordered a0-form crystals in the sheared samples[49,50].A small and weak peak at2h=31.3°was detected for the sample after shearing at0.1rad/s which might be assigned to b-form crystals of the PLLA[51–53].According to literature,the b-form crystals with a left-handed31helical conformation were only formed by drawing at elevated temperatures such as175°C[54].It is speculated that a small portion of the a-form crystals may gradually transformed into a0-(or d)and/or b-form crystals with the synergetic effect of the needle-like nucleator and the shearflow at the temperature of155°C.However,the transformation from a-form to b-form wasnot stable as none b -form crystals occurred at the shear rate of 0.5rad/s (not shown here).Hence,the mechanism for the crystal form transformation upon shearing or shear-induced crystallization needs to be further explored.3.4.Effect of shear on the morphology of the PLLA/OXA samplesThe crystal morphology (SEM images)of the PLLA and PLLA/OXA are illustrated in Fig.9.The PLLA crystallized under static conditions presents typical spherulitic morphology (Fig.9a)which changed into small crystals in the presence of OXA because of the nucleating effect of the OXA which can self-organize into fibril nucleating agents in the PLLA melt (Fig.9b).The OXA fibrils were randomly distributed in the PLLA matrix in the absence of flow,as more clearly shown by 253035(015)PLLA/OXA-shear-0.1PLLA/OXA-shear-0PLLA-shear-0Fig.9.SEM images of the PLLA and PLLA/OXA samples after crystallization:(a)PLLA sample crystallized under static conditions,(b)non-sheared PLLA/OXA sample crystallized under oscillation conditions,(c)sheared PLLA/OXA sample with incomplete crystallization (around t onset )under oscillation conditions and (d)sheared PLLA/OXA sample after complete crystallization under oscillation conditions.The shear condition is 4rad/s for 5s at 155°C.P.Ma et al./European Polymer Journal 87(2017)221–230227the inset of Fig.9b.Interestingly,the self-organizedfibrils reassembled into larger needle-like superstructures in the pres-ence of shearflow(Fig.9c).The larger superstructures are formed by end-to-end joining(length)and shoulder-by-shoulder packing(diameter)of the initialfibrils,as shown by the inset of Fig.9c.The subsequent oscillation conditions facilitate the needle-like superstructures to re-disperse into small(er)fibrils and network structures(Fig.9d).Thefiner OXAfibrils net-work must provide more nucleating sites that can further contribute to the rapid crystallization of the PLLA.3.5.Mechanism discussion on the synegetic effect of the OXA and the shearflowShearflow and OXAs were demonstrated separately to be effective in promoting PLLA crystallization[20,21,25,33–35]. Interestingly,the combination of shearflow and OXA shows a distinct synergistic effect on the crystallization kinetics of the PLLA.A schematic illustration is given to acquire a general phenomenological mechanism of the fast crystallization of PLLA induced by the combination of shearflow and OXA nucleators(see Fig.10).At200°C,the OXA can only be partially dissolved in PLLA matrix[20],and the amorphous PLLA chains are in disorder state,as shown in Fig.10a.Upon cooling from the melting temperature to the set temperatures(e.g.,155°C),the dissolved part of OXA molecules can self-organize into fibrils.The original and newly formedfibrils re-assemble into needle-like superstructures via intermolecular interaction and orientate along theflow direction,as illustrated in Fig.10b.The needle-like superstructures may eventually organize intofinerfibrils under the oscillation conditions(i.e.,the conditions for isothermal crystallization in this work),as illustrated in Fig.10c/d.However,the PLLA could not crystallize effectively within the experimental time span only in the presence of the OXA at around155°C under the oscillation conditions(see Fig.3)or at above145°C under static conditions[21]because the PLLA chains are too dynamic to pack effectively within a lamellae structure.On the other hand,PLLA macromolecules would also align along theflow directions.It is generally believed that the oriented molecular chains can assemble into par-allel array and gradually develop into the precursors of primary nuclei for polymer crystallization[29,30].However,the chain orientation is accompanied by a relaxation process at the elevated temperatures,which explains the absence of crys-tallization of the sheared PLLA sample(Fig.3).It was reported that carbon nanotube could restrict the relaxation process of extended polymer chains[33].Similarly,the presence of OXAfibril network may also slow down the relaxation process of the orientated PLLA chains via intermolecular interactions,e.g.,hydrogen bonding.As a result,a certain extent of orientation of PLLA chains would remain,notably nearby the OXAfibrils(e.g.,on the surface),as schematically shown in Fig.10c.As a consequence,the synergetic effect of the shearflow and the OXA nucleators facilitates the nucleation and overall crystalliza-tion of the PLLA matrix(Fig.10d).4.ConclusionsThe crystallization behavior of PLLA with and withoutfibril-like nucleators,(N1,N10-(ethane-1,2-diyl)bis(N2-phenyloxalamide))(OXA)and shearflow are investigated at a temperature range of150–160°C by using rheometer as a key tool.The PLLA hardly crystallized within70–100min at around155°C when solefibril-like OXA or shearflow was applied.However,the nucleation and crystallization were accelerated significantly by the combination of OXA and shear flow,leading to(i)a high crystallinity of>40%,(ii)a decrease of onset crystallization time(t onset)from hours to around 3min,and(iii)an overall crystallization time of less than5min.The results indicate that the shear conditions such as shear rate,time and temperature are important factors for the crystallization control.A critical shear condition for fast crystalliza-tion of the PLLA/OXA systems is indentified as0.4rad/s for10–15s at155°C.In the range of studied temperatures,the lower temperature the higher crystallization rate.The promoted nucleation and overall crystallization kinetics of the PLLA in this work were attributed to the synergetic effect of the OXAfibril network and the shearflow induced orientation of PLLA chains around the OXAfibrils.The X-ray diffraction patterns indicated that the shearflow andfibril-like OXA nucleators not onlyenhanced the formation of a-form PLLA crystals but also a certain extent of a0-(or d)and b-form crystals.The combinationofFig.10.Schematic illustration of structure evolution of the PLLA/OXA system in the presence of shearflow showing the synergetic effect offibril-like nucleator and shearflow on the melt crystallization of the PLLA:(a)PLLA/OXA melt with random coil PLLA chains and dispersed OXA molecules and aggregates,(b)orientation of PLLA chains and OXA needle-like structures in shearflow directions,(c)the relaxation process of PLLA chains and(d) nucleation and crystallization of the PLLA from the OXAfibrils under the oscillation conditions.228P.Ma et al./European Polymer Journal87(2017)221–230P.Ma et al./European Polymer Journal87(2017)221–230229 OXA and shearflow creates a new approach to high(er)crystallization rate and crystallinity of PLLA,which might expand the application range of PLLA-based materials.NotesThe authors declare no competingfinancial interest.AcknowledgementsThis work is supported by the Natural Science Foundation of Jiangsu Province,China(BK20130147),National Natural Science Foundation of China(51573074)and the Fundamental Research Funds for the Central Universities(JUSRP51624A). 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