physio- and chemo-dual crosslinking toward thermo- and ... · contents of pta did not influence the...

mater.scichina.com link.springer.com . . . . . . . . . . . . . . . . . . . . . Published online 27 March 2018 | https://doi.org/10.1007/s40843-018-9247-6Sci China Mater 2018, 61(9): 12251236

Physio- and chemo-dual crosslinking toward thermo-and photo-response of azobenzene-containing liquidcrystalline polyesterHai-Yi Zhong1,2, Li Chen1*, Xiao-Min Ding1, Rong Yang3 and Yu-Zhong Wang1*

ABSTRACT Combining the stability of chemical crosslinkingand the processability of physical crosslinking is a well-es-tablished strategy to design new materials with desirable sti-muliresponsive properties. Herein, a series of azobenzene-based thermotropic liquid crystalline polyesters were synthe-sized by introducing mesogenic dial named 4,4-bis(6-hydro-xyhexyloxy)azobenzene (BHHAB), 2-phenylsuccinic acid(PSA), and different contents of 1,2,3-propanetricarboxylicacid (PTA) as the chemical crosslinker. All these polyestersshowed good thermal stability and smectic liquid crystallinephase. Wide-angel X-ray diffraction (WAXD) and the fluor-escence emission spectra confirmed the existence of stacking interactions as the physical crosslinking in the poly-mer chains, particularly at the lower content of PTA. How-ever, when the PTA content increased, the chemicalcrosslinking changed the chain conformation, and thus theintensity of physical crosslinking slackened gradually. Com-bining the physical and chemical crosslinking, these polyestersshowed the thermoplastic processability, thermal shapememory, heat-assisted healing and photoresponsive beha-viors. Taking advantages of these features, these multiple sti-muliresponsive polymers can bring more chances for smartmaterials such as soft actuator.

Keywords: liquid crystalline polymer, crosslinking, shapememory, photo-response, self healing

INTRODUCTIONCrosslinked liquid crystalline polymers (CLCPs) are de-veloped on the basis of the liquid crystalline polymers(LCPs). Through the moderate density of crosslinking,CLCPs display elasticity either in isotropic state or in

liquid crystal state [1,2]. Attractively, as typical polymerexhibiting the ability to change its shape reversibly afterthe application of a certain external stimulus [3], CLCPscombine the properties of polymeric elastomers (entropyelasticity) with liquid crystals (self-organization) [4].Since de Gennes predicted the shape memory propertiesof CLCPs in 1975 [5], a large amount of CLCPs withdifferent liquid crystalline phases and network structureshave been synthesized and characterized, including main-chain smectic [6,7], side-chain smectic [8,9] and nematic[10,11] CLCPs, According to the different structures de-signed in the CLCPs, the materials can be responsive todifferent stimuli including heat, light, electricity and so on[1217].For shape memory polymers, the fixing phase, such as

crystalline phase, intermolecular forces in polymerchains, crosslinking, are the important factors to de-termine the constant shape of materials [1821]. As forCLCPs, the crosslinking can be regarded as the fixingphase which ensures the translation of microscopicchanges into macroscopic motions and the repeatabilityof the actuation. In a normal condition, the chemicalcrosslinking in the CLCPs plays an extremely importantrole in their shape memory behavior because of its en-ough strength and stability. However, the chemicalcrosslinking generally occurs during the synthesis processand could not be changed anymore once synthesized[22,23], which might sacrifice the processability and re-strict the practical application of the CLCPs.In recent years, scientists have paid much attention on

finding new modes of crosslinking to replace the normalchemical crosslinking such as the exchangeable covalent

1 Center for Degradable and FlameRetardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering,National Engineering Laboratory of EcoFriendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China

2 College of Pharmacy, Guangxi University of Chinese Medicine, Nanning 530299, China3 School of Materials Science and Engineering, Changzhou University, Changzhou 213000, China* Corresponding authors (emails: [email protected] (Chen L); [email protected] (Wang YZ))

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bonds or the post-crosslinking method etc. [2429],where different secondary bonds might be another an-swer, including hydrogen bond, interaction, co-ordination linkage, and so on. For example, Ikeda et al.[30] prepared azo based liquid crystalline films usinghydrogen bond as crosslinking points, obtaining the re-versible bending/unbending properties under UV/visiblelight as well. Zhang et al. [31] synthesized azobenzenemain-chain LCPs via Michael addition polymerizationand then studied the photomechanical effects of theirsupramolecular hydrogen-bonded fibers. Chen et al. [32]synthesized a side-chain LCP which showed good thermalshape memory property with the polymer chain columnsacting like physical crosslinks. In our previous studies[3338], two main-chain LCPs with inter-/intra-mole-cular interaction as crosslinking points were designedand synthesized, and the resulting good shape memorybehaviors proved that the interaction could also bean effective modes to build stimuli responsive CLCPs. Insummary, physical crosslinking can solve the processi-bility effectively caused by chemical crosslinking, butthese weak intermolecular interactions may be influencedeasily by the movement of the polymer chains in someconditions, which also restricted the application of ma-terials.Architecturally, combining the stability of chemical

crosslinking and the processibility of physical cross-linking might be a good method to endow materials withbetter comprehensive properties [39,40]. Thus, in thisstudy, a series of azobenzene-containing liquid crystallinepolyesters with slight crosslinking were synthesized asshown in Scheme 1. Herein the phenyl group of 2-phe-nylsuccinic acid (PSA) provided the interaction inthe polymer chain as the physical crosslinking points, andthe introduction of 1,2,3-propanetricarboxylic acid (PTA)gave the LCPs very slight chemical crosslinking. Thethermal shape memory and reversible photo-responsivebehaviors were investigated, revealing the relationships

and interactions between physical and chemical cross-linking. The very slight chemical crosslinking could in-crease the thermo- and photo-responsive properties ofthe polyesters but did not worsen the thermoplasticprocessability.

EXPERIMENTAL SECTION

Materials4-Nitrophenol was manufactured from East China Nor-mal University Chemical Factory (Shanghai, China).6-Chloro-1-hexanol and 2-phenylsuccinic acid (PSA)were purchased from Aladdin Industrial Corporation(Shanghai, China). 1,2,3-Propanetricarboxylic acid (PTA)was provided by Sinopharm Chemical Reagent Cor-poration (Chengdu, China). N,N-dimethylformamide(DMF, Ke Long Chemical Co. Chengdu, China) wasstored over activated 4A molecular sieves before use. Allother reagents were commercial and used without furthertreatment.

Synthesis of PBHPS-x%PTAThe monomers, including 4,4-dihydroxyazobenzene(DHAB) and 4,4-bis(6-hydroxyhexyloxy)azobenzene(BHHAB) were prepared following the procedure re-ported in the literature [36,41]. All the polyesters wereprepared following the same procedure. The typicalpolymerization procedure of PBHPS-1%PTA was as fol-lows: BHHAB (4.1454 g, 0.01 mol), PSA (1.9418 g,0.01 mol) and PTA (0.0175 g, 0.0001 mol) were loadedinto a 100-mL three-necked flask, which was equippedwith nitrogen inlet and nitrogen outlet, a mechanicalstirrer, and a distillation trap connected to a vacuum line.In addition, 0.0122 g (0.2 wt%) Zn(Ac)2 and 0.0183 g(0.3 wt%) Sb2O3 were added into the flask as catalyst.Before the reaction, the flask was processed by vacuumpumping and nitrogen protection to remove the oxygenin it. Then, the flask was placed in a 190C salt bath, the

Scheme 1 Synthetic route and chemical structures of PBHPS-x%PTA, where 100 and x denote the molar ratio with regard to PSA, not the blocklength.

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esterification reaction proceeded and was kept for about3 h. After that, a low vacuum was employed and thetemperature was preserved at 200C for another 0.5 h. Atlast, the temperature was kept at 200C with a high va-cuum of 2040 Pa for about 2.5 h. When the poly-merization was completed, the flask was cooled to roomtemperature under vacuum to obtain the target polymers.

Characterization.The swelling ratios of polyesters were measured as follow:all the samples were cut into small slices, swelled andextracted by chloroform for 24 h. The gel content (G(%))was calculated using the equation:

G mm(%) = 100%, (1)1

0

where m1 and m0 represent the mass of dried extractedsample and original sample, respectively. All the datawere averaged over three measurements.Thermogravimetric analysis (TGA) was conducted

under a nitrogen atmosphere using a Netzsch TG 209 F1instrument. The heating rate and flow rate were kept at10C min1 and 60 mL min1 respectively. In each case, a35 mg sample was examined in a temperature range of40700C.Thermal transition temperatures of the polyesters were

measured using a differential scanning calorimetry (DSCTA Q200 1747) calibrated for both heat flow and tem-perature using indium and zinc standards. About 5 mgsample was tested under a nitrogen atmosphere at a flowrate of 50 mL min1. Firstly, it was heated to 150C di-rectly for 2 min to minimize the adverse influence ofthermal history. Then the sample was cooled to 20Cand heated to 150C again at a heating rate of 10C min1.The thermogram was recorded from the first cooling andthe second heating cycle.Liquid crystalline textures of the polyester were ob-

served via a Nikon ECLIPSE LV100 POL polarizing op-tical microscopy (POM) equipped with a hot-stage. Thesample was dissolved in chloroform in a concentration of10 mg mL1 and then the solution was cast to a glass flat.After the solvent was evaporated at room temperature,the sample was further dried at 100C for 3 min, and thencooled to 40C at a cooling rate of 10C min1 for ob-servation.Wide angle X-ray diffraction (WAXD) was carried out

with a Philip X Pert X-ray diffractometer. The sampleswere firstly under an isothermal process at 50C for30 min before test.Fluorescence spectra were recorded on an F-7000

fluorescence spectrometer (Hitachi, Japan) at the fluor-escence emission wavelength of 250 nm. All the sampleswere dissolved in chloroform to obtain solutions withconcentration of 5 mg mL1 respectively, and then thesolution was cast to an optical glass flat. The spectra wererecorded after the solvent evaporated at room tempera-ture.Shape memory behaviors of the polyesters were con-

ducted with dynamic mechanical analysis (DMA, TAQ800). The sample was cut into a rectangular shape withdimensions of approximately 20 mm 5 mm 0.5 mmto test.UV-vis absorption spectra were recorded on a Varian

Cary 50 UV-Vis Spectrophotometer (Varian Co., USA).The photochemical isomerization of the polymer solutionwas investigated by irradiating it first with a 365 nm UVlamp until the photostationary state was reached and thenwith a 550 nm visible light lamp. This lamp was alsoutilized to test the bending and unbending properties ofthe samples.The heat-assisted healing process of the scratched

polyester films was carried out on the hot stage at 60Cand was observed via Nikon ECLIPSE LV100 POL POM.The healing efficiency was tested by tensile test. TwoPBHPS-2%PTA strips of a knife-scratched depth of about0.1 mm and an undamaged strip with an average thick-ness of 0.50 mm and a width of 3 mm were prepared.Then, one of the damaged strips was put into an oven for5 h at 60C. After that, all the strips were tested by IN-STORON 3366 Testing Machine under a 100 N load celland uniaxially stretched to a displacement at 5 mm min1.The healing efficiencies (R) have been assessed by theequations below [42]. Here refers to the tensile strengthand denotes the elongation at break.

R( ) = (2)healedinitial

R( ) = (3)healedinitial

RESULTS AND DISCUSSION

PolymerizationThe slightly crosslinked liquid crystalline polyestersPBHPS-x%PTA were synthesized by melting poly-condensation. When the temperature reached 190C,BHHAB, PSA and PTA melted together into a dilutesolution with small bubble. The esterification reactionwas kept for about 3 h, and during this time, nitrogen waskept flowing to protect the reaction system from thermal



oxidation and remove the by-product water. As the timewent on, the reaction system became viscous and thewater could not be taken away by nitrogen, leading thereversible polycondensation to equilibrium. Therefore,the vacuum system was introduced to remove the by-product and keep the reaction to the positive direction.Then, the polyester became more viscous. For the PBHPSand PBHPS-1%PTA, the melt adhered to the flask wallfinally, but the 2% and 3% PTA samples became an elasticsubstance adhering to the stirring pole. This part of thereaction was kept for about 2 or 3 h with a high vacuumof 2040 Pa according to the different reactive phenom-ena. Finally, the target product was obtained after coolingthe system to room temperature under high vacuum. Theswelling test was performed to evaluate the efficiency ofthe crosslinking, and the results are listed in Table 1. Asthe contents of PTA increased, the gel contents increasedas expected, meaning the density of chemical crosslinkingincreased.

Thermal properties and liquid crystalline behaviorsFig. 1 shows the TGA and DTG curves of the PBHPS-x%PTA samples, and the specific data are summarized inTable 1. All the testing polyesters had one single de-gradation process. The temperatures of 5 wt% loss (T5%,defined as the initial decomposition temperature) andTmax were nearly the same, indicating that the in-corporation of PTA caused a slight chemical crosslinking

but did not affect the thermal stability of the polyesters.Fig. 2 illustrates the DSC curves of the PBHPS-x%PTA

samples in the first cooling scan (a) and the secondheating scan (b) at a rate of 10C min1 under N2 atmo-sphere. The relevant thermal parameters and their cor-responding values are summarized in Table 2. Thesamples, except PBHPS-3%PTA, had nearly the sameglass transition temperatures (Tg) and isotropic tem-peratures (Ti) at about 22C and 70C in the first coolingprocess, which meant that the incorporation of low

Table 1 The swelling test results and TGA data of polyesters

Sample G (%) T5% (C) Tmax (C) Residue at 700C (%)

PBHPS Complete dissolution 344.0 351.6 17.8

PBHPS-1%PTA 5.92.0 343.7 353.6 15.8

PBHPS-2%PTA 12.60.6 344.2 354.9 17.0

PBHPS-3%PTA 59.49.7 348.0 355.7 17.5

Figure 1 TGA and DTG curves of liquid crystalline polyesters in N2.

Figure 2 DSC curves of the polyesters on (a) first cooling and (b) second heating at a rate of 10C min1 under N2 atmosphere.



contents of PTA did not influence the phase transitiontemperatures. However, when the content of PTA in-creased to 3%, both Tg and Ti slightly increased to 24.6Cand 76.2C, respectively. This was reasonable because therelative high density of crosslinking would district themovement of the polymer chains, thus increasing therelevant temperatures.It was noteworthy that all the polyesters with PTA

crosslinking still showed the isotropic temperatures,meaning that the crosslinking did not negatively affect themelting processability. For further confirming the ther-moplasticity of these polyesters, PBHPS-2%PTA andPBHPS-3%PTA samples with higher crosslinking densitywere chosen to undergo a 5 time cooling and heatingcycles by using DSC respectively. As shown in Fig. 3,during the multiple cooling and heating process, thetested samples exhibited the Tg and Ti, and these phasetransition temperatures were kept the same all the time,which further confirmed the slight crosslinking did notinfluence the thermoplasticity of the resulting polyesters.

Fig. 4 shows the liquid crystalline textures of PBHPSand PBHPS-x%PTA on the first cooling process, whichwere observed and captured at 40C by using a POM. Allthe slightly crosslinking samples showed the typical focalconic textures as PBHPS did [26], which implied thesmall quantity of PTA incorporated into the PBHPSchains did not change the molecular mobility and regularpacking, as illustrated in DSC test. WAXD analysis wasalso used to determine the microstructure of the polye-sters. As shown in Fig. 5a, in the small angle region,except PBHPS-3%PTA, there were two distinct peaks at6.3 (1.39 nm) and 12.5 (0.71 nm), suggesting the ex-istence of the layer structure in the liquid crystallinechain. For PBHPS-3%PTA, the peaks in the small anglerange were negligible, because the relative high content ofPTA influenced the chain folding or arrangement, thusdestroyed the orderly structure between the layers. ForPBHPS, there was a peak at the angle of 28.1 (0.32 nm)which indicated the interactions between the phenylrings exiting in the polymer chains as reported previously

Table 2 DSC data of the polyesters at a rate of 10C min1 under N2 atmosphere

SampleFirst cooling Second heating

Tg (C) Ti (C) Hi (J g1) Tg (C) Ti (C) Hi (J g

1)

PBHPS 22.3 71.5 7.1 26.9 75.8 7.0

PBHPS-1%PTA 21.9 70.5 7.6 25.6 73.8 7.6

PBHPS-2%PTA 22.6 70.0 8.0 25.2 73.3 7.9

PBHPS-3%PTA 24.6 76.2 8.4 27.7 79.2 8.4

Figure 3 DSC curves of PBHPS-2%PTA (a) and PBHPS-3%PTA (b) in five cooling and heating cycles at a rate of 10C min1 under N2 atmosphere.



[23,26]. When 1% of PTA was incorporated, peaks in thisarea still existed, impling the interactions remainedin PBHPS-1%PTA. However, the peaks at about 28 forthe polyesters PBHPS-2%PTA and PBHPS-3%PTA de-creased obviously, which meant the interactionsmight be weakened or broken. To further reveal the re-lation between the PTA (chemical crosslinking) and the interactions (physical crosslinking), fluorescencespectra test was performed to study the intensity of interactionsthe fluorescence emission means there ex-ists conjugation in the system [35,43]. As shown in Fig.5b, the intensity of the fluorescence emission spectradecreased with increasing the contents of PTA, meaningthe interactions were decreased even disappearedgradually. This process could suggest that the physicalcrosslinking was replaced by the chemical crosslinkingslowly but surely.

Thermally induced shape memory propertiesIn this part, the thermally induced shape memory prop-

erties of the polyesters were characterized by a well-es-tablished four-step thermo-mechanical cycling methodconducted via DMA. All the samples were stretched at40C firstly and then the strain was ramped to about 50%gradually (deformation). After that a rapid cooling pro-cedure was executed, during which the force was keptconstant until the temperature reached 10C. In the thirdstep, the stress was gradually released to witness the strainfixing at 10C. In the fourth step, the strain recoveryoccurred by heating to 60C at the rate of 10C min1

(recovery). According to the Equations (4) and (5), wherem, , p and N represent the strain before unloading, thestrain after unloading, the permanent (residual) strainafter heat induced recovery and the number of cycles,respectively, the fixing ration (Rf) and the recovery ratio(Rr) were calculated to assess the shape memory property.

RN NN N=

( ) ( 1)( ) ( 1) 100% (4)f

p

m p

RN N

N N=( ) ( )

( ) ( 1) 100% (5)r p

p

Typically, Fig. 6e presents the thermally induced shapememory results of PBHPS-2%PTA sample investigated bycyclic thermomechanical analysis, while those of otherswere provided in Fig. S1. The corresponding data aresummarized in Table 3. Generally, the results recordedwithin the first cycle were strongly dependent on theresidual stresses of samples resulting from processinghistory. The Rf and Rr were therefore calculated from thedata determined within the 2nd to the 4th cycle. All thesamples had good shape fixing ratios about 98%: as thetemperature came to 10C, all the polyesters were atglassy state and lost their chain mobility, and the storingelastic energy was frozen at the same time. However, therecovery ratios were significantly different with different

Figure 4 POM pictures of PBHPS (a), PBHPS-1%PTA (b), PBHPS-2%PTA (c) and PBHPS-3%PTA (d) at 40C.

Figure 5 XRD patterns of the polyesters at room temperature (a). The intensity of the fluorescence emission spectra (b) of the polyesters.



content of PTA. When the content of PTA was 1%, therecovery ratio of PBHPS-1%PTA dropped obviously toabout 85.3%. As the introduction of PTA, the chemicalcrosslinking strongly influenced the chain conformation.As a consequence, some physical crosslinking ( in-teractions) did not form effectively, and the intensity ofphysical crosslinking became weak. However, when thePTA content was relatively low, the introduced chemicalcrosslinking was not strong enough to force the deformedsample to recover, resulting in a sharply decline of therecovery ratio. As the amount of PTA increased se-quentially, the recovery ratio of the polyesters rose again.The recovery ratio of PBHPS-3%PTA reached 98.6%.With relative high content of PTA, the chemical cross-linking became more dominant than the physical cross-linking in the system, thus making the shape memoryproperties much better. Fig. 6 shows digital photographsof the recovery process of these polyesters. The sampleswere firstly stretched and then recovered in the hot stageat 60C. The results of all these polyesters shape memoryproperties were in accordance with the DMA tests.

Photo-responsive behaviorThe photo-responsive properties of all these polyesterswere investigated by UV-vis spectroscopy in DMF solu-

tion under irradiation of 365 nm light and 550 nm light,as illustrated in Fig. 7 (PBHPS-2%PTA). When irradiatedat 365 nm, the solution underwent trans to cis photo-isomerization until a photostationary state was eventuallyreached. The intensity of the * transition band around360 nm decreased with the irradiation time, whereas thatof the n* transition band around 450 nm slightly in-creased. Then the cis to trans back-isomerization processhappened when the samples were irradiated at visiblelight (=550 nm). The * transition band increased andthe n* transition band decreased back. However, thefinally recovered absorbance of the trans-isomer waslower than that before UV irradiation. This phenomenonwas reported before [31] but the reason was not clear yet.One possible explanation is that when the cis to transphotochemical back-isomerization was performed, therewas also a weak absorption from the trans-isomer at thesame wavelength, which led to an equilibrium betweenthese two isomers during the isomerization process underthe same visible light after most of the cis-isomer returnedto the trans-isomer. The other three polyesters displayedthe similar properties under the irradiation of UV lightand visible light, and the results were given in the sup-porting information (Fig. S2).The macroscopical photo-mechanical properties of all

these polyesters were then studied. The simple meltstretching method was used to get the vimineous sampleswith a relatively high order of mesogen along the stretchdirection for the test [31,37]. Fig. 8 shows the photo-induced bending and unbending behaviors of thesepolyesters upon irradiation with UV light and visiblelight, respectively. It was seen clearly that the PBHPS bentto the UV light source direction when it was exposed to365 nm UV light (15 mW cm2) from above of the sam-

Figure 6 Digital photograph of shape memory process of polyesters: (a) PBHPS, (b) PBHPS-1% PTA, (c) PBHPS-2% PTA and (d) PBHPS-3% PTA.Programmed cyclic thermomechanical curves of PBHPS-2%PTA obtained by DMA (e).

Table 3 Fixing and recovery ratio of the polyesters obtained by DMA

Sample Fixing ratio (%) Recovery ratio (%)

PBHPS 98.60.4 94.40.9

PBHPS-1%PTA 97.90.2 85.31.1

PBHPS-2%PTA 97.90.4 94.02.6

PBHPS-3%PTA 99.50.1 98.60.4



ple. However, the PBHPS sample did not revert to itsinitial straight state upon irradiation with visible light(550 nm, 15 mW cm2). This was because the aromaticstructure in the PBHPS chain slipped during the iso-merization process of the azobenzene groups under theUV light, resulting in losing the physical crosslinkingpoints in the system. As the bending process finished, akind of new stacking formed, fixing the deformedbending shape. When irradiated by the visible light, thenewly formed stacking was broken again as the cis totrans isomerization happened. However, the final align-ment of trans azobenzene mesogens were different fromthe original ones in the absent of some hard segment torestrict the movement of the polymer chain. Thus, thePBHPS sample could not recover to its original straightstate. For the sample PBHPS-1%PTA, the introduction of1% PTA did not afford enough chemical crosslinking as a

hard segment, and thus the photo-mechanical property ofPBHPS-1%PTA was similar to that of PBHPS. A sche-matic drawing of this process is shown in Fig. 9a. Whenmore PTA was incorporated to form chemical cross-linking, the reversible photo-induced bending and un-bending properties were improved. Also shown in Fig. 8,the PBHPS-2%PTA samples could bend under the UVlight irradiation, and revert to their straight state uponthe irradiation with the 550 nm visible light with a re-covery ratio of 85.6%, although the bending angles weredecreased, which could be attributed to the chemicalcrosslinking in the polymer chains. Since the chemicalcrosslinking did not response to the UV light and re-stricted the mobility of the azobenzene groups, thesecrosslinking points (fixing phase) remained during theisomerization of azobenzene groups under UV light, andthen forced the polymer chains back to the initial align-

Figure 7 UV-vis spectral changes with time for the solution of PBHPS-2%PTA in DMF at room temperature upon irradiation with 365 nm UV light(a) and upon irradiating the polymer solution at the photostationary state with 550 nm visible light (b).

Figure 8 Digital pictures of the polyester films motion in the presenceof UV light (365 nm, 15 mW cm2) and visible light (550 nm,15 mW cm2).

Figure 9 The schematic drawing of the oriented samples PBHPS (a)and PBHPS-PTA (b) during the isomerization process under the UVlight and visible light.



ment under the visible light. The schematic drawing ofthis process is shown in Fig. 9b. However, as the PTAchemical crosslinking became dominant in the polymerchains, the mechanical properties were enhanced (Fig. S3)and the movement of the polymer chains were restrictedgradually, causing the bending angles of the crosslinkedpolyesters to decrease obviously. Even the PBHPS-3%PTA sample could not bend when induced by the UVlight.

Heat-assisted healing behaviorIn our previous work [36,37], the azobenzene basedmain-chain liquid crystalline polyester PBHPS showedheat-assisted healing behavior due to the interactionin the chain as the physical crosslink. When crosslinkerPTA is introducd into PBHPS to form some slight che-mical crosslink, would the chemical crosslink influencethe healing behaviors? Firstly, the scratch healing beha-viors of the polyesters were studied by using POM. Thescratched polyester films were put on the 60C hot stageand the heat-assisted healing process was recorded by thedigital camera equipped on the POM. The snapshots werecaptured at different time. As shown in Fig. 10, thePBHPS-1%PTA sample with lower content of PTAshowed similar healing behavior as PBHPS because of therelatively strong interaction in the polymer chains.When the healing process occurred at 60C, the polyesterswere in liquid crystalline phase, making the stackingreformed at the presence of the position order of themesogenic units. When the content of PTA increased,some stacking physical crosslink did not form ef-fectively which was proved by WXAD and fluorescenceemission spectra above. Thus, the heat-assisted healingproperties of PBHPS-2%PTA and PBHPS-3%PTA de-creased. For PBHPS-2%PTA, there still existed a blurred

scratch after healing 60 min. The PBHPS-3%PTA shrankdid not heal at the temperature of 60C, proving the interaction played an important role in heat-assistedhealing behavior indirectly.Herein, the PBHPS-2%PTA was chosen to study the

healing efficiency. The healing efficiency of scratchedhealing method was measured by tensile test, as shown inFig. 10e. The tensile stress and the elongation at break ofthe original PBHPS-2%PTA was 15.5 MPa and 928.9%,respectively; and the scratched one decreased to 8.3 MPaand 107.9%, where the testing sample fractured preciselyat the scratch trace. After 5 h healing process at 60C andwithout the compression stress, the tensile stress of thehealed one recovered to 11.1 MPa and 529.8%, thus thehealing efficiency was 71.6% (for strength) and 57.0% (forelongation), respectively.

CONCLUSIONIn conclusion, a series of azobenzene-containing liquidcrystalline polyesters combining physical and chemicalcrosslinking were synthesized successfully. All of thesepolyesters showed good thermal stability and their ther-mal decomposition temperature was higher than 340C.DSC showed that all these crosslinked samples had arelative low phase transition temperatures with thermo-plastic processability. With POM and XRD results, thesepolyesters showed smectic liquid crystalline phase. Fur-thermore, there also exist strong stacking interac-tions in the polymer chains with low content of PTA asthe crosslinking agent. When PTA content increased, asillustrated in XRD and fluorescence emission spectra, thechemical crosslinking negatively influenced the originalphysical crosslinking ( interaction) among the poly-mer chains. The thermally induced shape memory be-haviors of these polyesters were evaluated by DMA: all

Figure 10 Self-healing process of the scratched polyesters observed by POM at 60C: (a) PBHPS, (b) PBHPS-1%PTA, (c) PBHPS-2%PTA and (d)PBHPS-3%PTA. Stress versus strain curves for the original, scratched and healed state of PBHPS-2%PTA (e).



these samples exhibited expected fixing ratios about 98%;however, the recovery ratios changed obviously withdifferent content of PTA. The chemical crosslinking alsoinfluenced the photo responsive performance of thesepolyesters. At the absence of chemical crosslinking, theoriented sample (PBHPS) could be bent under the irra-diation of UV light; however, since the isomerizationprocess of azobenzene groups broke the interactionsin the polymer chains, the bent sample could not revertstraight under the irradiation of visible light. At thepresence of slight chemical crosslinking, thanks to theinert response of the crosslinking to the UV light, suchproblem was solved by restricting the mobility of theazobenzene groups, then forcing the polymer chains backto the initial alignments under the visible light. As aconsequence, the reversible bending and unbending be-haviors of the slightly crosslinked polyesters wereachieved. The heat-assisted healing properties of thesepolyesters were achieved as the physical crosslinkingdominated in the chains. Therefore, this main-chainazobenzene-containing liquid crystalline polyester couldbe utilized as a kind of smart polymer and exhibits itsown value in the industry field.

Received 23 January 2018; accepted 9 March 2018;published online 27 March 2018

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Acknowledgements This work was supported by the National NaturalScience Foundation of China (51721091) and the Sichuan ProvinceYouth Science and Technology Innovation Team (2017TD0006). Theauthors would also like to thank the Analysis and Testing Center ofSichuan University for the NMR measurement.

Author contributions Zhong HY and Ding XM carried out theexperiments; Zhong HY and Chen L analyzed the data and wrote thedraft of manuscript; Chen L and Wang YZ proposed the project andcritical comments on the writing of the manuscript; Yang R providedsome additional suggestions and comments on analyzing the data. Allthe authors checked and approved the manuscript.

Conflict of interest The authors declare that they have no conflict ofinterest.

Supplementary information Shape memory tests of polyesters byDMA, UVvis spectra of PBHPS, PBHPS-1%PTA and PBHPS-3%PTAsolution in DMF and the storage modulus of these polymers tested byDMA are available in the online version of the paper.



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Hi-Yi Zhong earned his PhD degree in Polymer Chemistry and Physics (2017) and BSc degree in Chemistry (2012) fromSichuan University under the supervision of Prof. Yu-Zhong Wang. His research interest is the stimuli responsive liquidcrystalline polymers. He is currently a lecturer in the College of Pharmacy, Guangxi University of Chinese Medicine.

Li Chen is currently a full professor in the College of Chemistry, Sichuan University. He earned his PhD degree inPolymer Chemistry and Physics (2009) and MSc degree in Materials Science (2006) from Sichuan University, and BScdegree in Chemistry (2003) from Hunan University. In 2009, he joined Professor Yu-Zhong Wangs group and his currentresearch interests are focused on the synthesis, structure and properties of functional liquid crystalline polymers andflame-retardant materials.

Yu-Zhong Wang earned his PhD degree from Sichuan University in 1994, where he was promoted to a full Professor in1995. He is the Director of the National Engineering Laboratory for Eco-Friendly Polymeric Materials (Sichuan). Hisresearch interests are focused on fire-retardant and functional polymeric materials, bio-based and biodegradable poly-mers. He has authored more than 460 publications in SCI journals and issued over 110 patents. He has been awardedeleven National and Provincial Science & Technology awards. In 2015, he was selected as an Academician of ChineseAcademy of Engineering.

1,2, 1*, 1, 3, 1*

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Physio- and chemo-dual crosslinking toward thermo- and photo-response of azobenzene-containing liquid crystalline polyester INTRODUCTIONEXPERIMENTAL SECTIONMaterialsSynthesis of PBHPS- x%PTACharacterization.

RESULTS AND DISCUSSIONPolymerization Thermal properties and liquid crystalline behaviors Thermally induced shape memory propertiesPhoto-responsive behaviorHeat-assisted healing behavior

CONCLUSION

physio- and chemo-dual crosslinking toward thermo- and ... · contents of pta did not influence the...

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