Composite bilayer films with organic compound-triggered bending properties☆

Ke Deng,Zhuang Liu,2,*,Jiaqi Hu,Wenying Liu,Lei Zhang,Rui Xie,2,Xiaojie Ju,2,Wei Wang,2,Liangyin Chu,2,*

1 School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2 State Key Laboratory of Polymer Materials Engineering,Sichuan University,Chengdu 610065,China

Keywords:Responsive materials Composite bilayer films Organic compound-triggered bending Graphene Porous polymeric membranes

ABSTRACT Organic compounds are widely used in both industry and daily life,and composite bilayer films with organic compound-triggered bending properties are promising for applications of transducers,soft robotics,and so on.Here,a universal and straightforward strategy to generate composite bilayer films with organic compoundtriggered bending properties is demonstrated.The composite bilayer films with organic compound-triggered bending properties are designed with bilayer structures,in which one layer is a porous polymeric membrane with appropriate solubility parameter that matches the value of organic solvents in order to produce prominent affinity to the solvent molecules,and the other layer is reduced graphene oxide membrane stacked on the porous polymeric membrane as an inert layer for restraining the swelling of the polymeric membrane on one side.Guided by matching the solubility parameters between solvent and polymer,a significant bending curvature of 27.3 cm-1 is obtained in acetone vapor.The results in this study will provide valuable guidance for designing and developing functional composite materials with significant organic compound-triggered bending properties.

1.Introduction

Stimuli-triggered responsive materials have been developed with various deformations such as bending[1,2],torsion[3,4]and origami[5]behaviors,and exhibit promising applications in sensitive devices[6],artificial muscles[7-10],soft robotics[11-16],energy generators[17-19],and so on.Compared with the stimuli of heat[20-24],pH[25-28],light[29-32],electricity[25,32-37],humidity[17,38-41],magnetic field[42],and so on,organic solvents or vapors are an important family of stimulants for some promising stimuli-triggered responsive materials[6,43],because organic solvents or vapors are widely used in both industry and daily life.Most of the organic solvents can volatilize easily at room temperature,such as dichloromethane,toluene and acetone,which are harmful to the organism or environment[44-46].For instance,the benzene and its derivatives such as toluene are defined as carcinogens by the World Health Organization(WHO).Besides,concentrated organic vapors are easy to explode up on meeting fire.Development of organic compound-triggered responsive materials with simple bending deformation is strongly requisite for both fundamental research and practical applications such as forewarning[47,48],detection[6,49],actuating[50-52],and energy harvesting[17-19].

Up to now,the effective approaches to fabricate organic compoundtriggered responsive materials are generating anisotropy in structures or component of the organic compound-sensitive materials,which demonstrate anisotropic volume changes,causing a desirable bending deformation[53-56].For instance,Deng et al.have developed a new type of pure poly(vinylidene fluoride)(PVDF)polymer film[57],which possesses an anisotropic spatial distribution of the α phase and β phase.The PVDF strip with 7 mm length rapidly bends to a circle with a curvature of 3.22 mm-1in response to acetone vapor.Zhao et al.have developed a poly(ionic liquid)actuator,which represents a unique synergy of an electrostatic complexation gradient and a porous architecture through the membrane to respond to acetone[6,43,58].The prepared poly(ionic liquid)actuators with 20 mm length bend to a circle with a curvature of 1.34 mm-1within 0.4 s,which actuate quite quickly.However,the bending deformation is determined by not only the bending rate but also the bending deforming degree.When the bending degree is defined as the ratio of length and radius of the circle or arch,the bending degree value of the anisotropic PVDF film and poly(ionic liquid)membrane is 22.54 and 26.80,respectively[43,57].These organic compound-triggered responsive materials perform large-shape deformation;however,these heterogeneous structure or composition in such single-layered film may need ingenious designs and rather precise operations,while simply laminating films into bilayer structures,attracts more attentions due to the easy fabrication and programmable actuations[59-63].For examples,Zhang et al.have reported actuating materials based on two highly elastic polymers PVDF and polyvinyl alcohol(PVA)[64],which are capable of fast and perpetual reshaping in response to acetone vapors.The bending degree of PVDF/PVA film is about 15.6 due to the ordered microchannel structures of the PVDF layer.Recently,Tan et al.have demonstrated graphene oxide(GO)/latex bilayer composite materials,which exhibit reversible highly bending property in response to some organic solvents,such as n-hexane,benzene,and tetrahydrofuran(THF)[60].The large curvature of 2.75 mm-1is observed in response to THF.However,the bending degree of graphene oxide(GO)/latex bilayer actuators is only 15.04 because of the imporous latex.Although a series of organic compoundtriggered responsive actuators are developed and fabricated as single or bilayer elements,the challenges to achieve significant bending behaviors of organic compound-triggered responsive systems still remain.

Herein,we demonstrate a universal and straightforward strategy to generate composite bilayer films with organic compound-triggered bending properties.A porous polymer membrane with appropriate solubility parameter that matches the value of organic solvents is selected,and a reduced graphene oxide(rGO)film is simply stacked on the porous polymer membrane by vacuum filtration as the relatively rigid surface layer for constraint.Solubility parameter reflects the cohesive energy density[65-67].The affinity between polymer and solvent is strong when their solubility parameters are similar.Driven by the affinity of polymer membrane to solvent molecules,the porous polymeric membrane is much more swelling compared with the rGO layer,which is non-responsive to organic vapor.The bending deformation comes from the force induced by the swelling of the layer of porous polymeric membrane.According to the different solubility parameters of organic solvents,the bending degree of bilayer film is distinguishable.When the solubility parameter of porous polymeric membrane matches that of organic solvents,the composite bilayer film could achieve the most significant organic compound-triggered bending behavior.A significant bending degree of 27.3 is obtained in the study,which is larger than all of the previously reported bending degrees(Table S1).The results will provide valuable guidance to design and prepare functional composite bilayer films with significant organic compound-triggered bending properties.

2.Materials and Methods

2.1.Materials

Polycarbonate(PC)membrane(pore size 200 nm or 600 nm)was purchased from Millipore.Graphene oxide was synthesized following the modified Hummers' method.Hydrazine solution(51.2 wt% in water,Kelong)and ammonia solution(25 wt%in water,Kelong)were purchased from Chengdu Kelong Chemical Reagents.The organic solvents used in experiments were all purchased from Chengdu Kelong Chemical Reagents.All other reagents were of analytical grade and used without further purification.Deionized water(18.2 MΩ at 25°C)from a Milli-Q Plus water purification system(Millipore)was used throughout the experiments.

2.2.Preparation of rGO/PC bilayer films

Reduced graphene oxide(rGO)was obtained by treating the GO with hydrazine and ammonia solution.Typically,45 μL of hydrazine solution(51.2 wt%in water,Kelong)and 550 μL of ammonia solution(25 wt%in water,Kelong)were added to the prepared homogeneous dispersion(100 mL,0.35 mg·mL-1)to synthesize rGO,with continuous stirring in room temperature for 48 h.Then,the resultant rGO was dissolved in 40 mL deionized water.The bilayer film was prepared by vacuum filtration rGO dispersions on PC membranes.The prepared bilayer films were dried in air at room temperature for 48 h.

2.3.Characterization of rGO/PC bilayer films

Morphologies of the bilayer films were observed by SEM(JSM-7500F,JEOL)at 15 kV.Dried bilayer film samples were immersed in liquid nitrogen and then fractured for cross-section observation.All samples were coated with a thin layer of gold before observation.The mechanical properties of bilayer film were measured by an electronic universal testing machine(EZ-LX,Shimadzu)using a sensor with capacity of 10 N.During the mechanical test,the sample gauge length between clamps was constant at 15 mm and the strain rate was 20 mm·min-1.Three samples were tested for each bilayer strip to get the statistical data.

2.4.Bending behaviors of bilayer films in various organic vapors

Organic solvents or mixture of organic solvent and water was put in a culture dish.Then,a piece of bilayer film strip(1 mm×10 mm)was placed 10 mm above the liquid phase of the solvent.All the actuation movements were recorded by a digital camera(Nikon D7100,frames per second:30)at 20 °C,RH 65%.The curvature(C)of bilayer film,which is expressed as the inverse of the circle radius(C=1/R,where R is the circle radius),was measured by the circle fitting techniques.Bending degree(θ)of bilayer film was calculated by the equation θ=CL0,where L0is the initial length of bilayer film,because the graphene layer is inert to the organic solvent,which would restrain the swelling of the PC layer at the interface side.For estimating the bending curvature value of the films curling irregularly,there are two curl situations to be considered.If the curl is not very serious and regular at the beginning,the circle fitting techniques is applicative.If the curl is very serious and irregular,the radius value(R)of curvature is very small.The error of bending curvature is magnified by the expression 1/R.It is negligible that the measurement error caused by curl irregularly compared with error magnified due to the calculation of 1/R.To reduce the error,we obtain the statistical data by conducting at least three experiments and measurements for each sample.

3.Results and Discussion

3.1.Design strategy

Fig.1.Schematic illustrations of PC/rGO bilayer film structure(a,b)including chemical structure of PC(c),and bending deformation of PC/rGO bilayer film in vapor(d-e).

In this paper,the composite films with organic compound-triggered properties are designed with bilayer structures,in which one layer is a porous PC membrane for absorbing the organic vapor molecules,and the other layer is a rGO film as an inert layer(Fig.1a-c).There are less interaction sites between rGO and organic compound molecules due to the absence of oxygen groups on the surface of rGO compared with GO.The rGO/PC bilayer film is fabricated by stacking rGO nanosheets on the porous PC membrane via vacuum-filtration.The polycarbonate is a semi-crystalline and linear polymer with the solubility parameter of 20.0(J·cm-3)0.5,which is commercially available and selected as an example for designing the bilayer film[68,69].Table 1 lists the solubility parameters of the common organic solvents.Due to the porous structure of the PC membrane,the organic molecules can rapidly diffuse into the PC polymer network.The acetone enables the PC membraneto swell to the largest degree because of their similar solubility parameter,while the rGO layer almost does not make any difference.That is,at the interface of the two layers,the rGO layer would restrain the swelling of the PC layer on one side,while the other side of the PC layer can freely swell.As a result,the anisotropy in the volumetric change of the PC membrane occurred,which drives the bilayer film bending to the side of the rGO layer.To obtain significant bending deformation of the bilayer film,solvent vapor molecules should induce an obvious swell of the porous polymer membrane.The affinity between polymer and solvent is strong when their solubility parameters are similar,which can cause the significant swelling of polymer in the solvent vapor.Therefore,the curvature and bending degree of bilayer film could be correlated with the solubility parameter.Thus,in the acetone environment,which has the same solubility parameter as that of PC polymer,the rGO/PC bilayer film exhibits significant bending behavior(Fig.1d,e).

Table 1 Solubility parameter and boiling point of some solvents

3.2.Effects of structures on the bending behaviors of rGO/PC bilayer films

Fig.2.(a,b)SEM images of PC membrane(a1,b1)and PC/rGO bilayer film(a2-a3,b2-b3),in which the pore size of PC membrane is 0.2 μm(a)and 0.6 μm(b)respectively.(c)SEM image of cross-section of PC/rGO bilayer film with enlarge view.(d)Young's modulus of PC/rGO bilayer films with different thicknesses of rGO layer.Scale bar is 5 μm in(a1-a2)and(b1-b2),10 μm in(a3)and(b3),0.2 μm in(c).

To obtain optimal bending deformation of the rGO/PC bilayer films,the effects of pore size of PC membrane and thicknesses of rGO films on the bending behaviors are investigated firstly.Fig.2 shows the SEM images and Young's modulus of the PC membrane and rGO/PC bilayer films.The commercial PC membranes show a good and smooth surface morphology with uniform pore sizes of 0.2 μm and 0.6 μm(Fig.2a1,b1).After stacking 1 mg rGO sheets on the surface of the PC membrane,the pores are covered(Fig.2a2,b2).Compared with the thickness of the PC membrane,the rGO layer is very thin(Fig.2a3,b3),which is about only 220 nm(Fig.2c).Furthermore,by changing the content of the rGO sheet stacking,the thickness of the rGO layer is adjustable.For example,rGO layers can be stacked with a thickness of 165 nm or 580 nm.Due to the attached rGO layer,Young's modulus of the rGO/PC bilayer film is increased by increasing the thickness of the rGO layer(Fig.2d).The connection between the inert layer rGO and the responsive layer PC is stable due to the π-π interaction and intermolecular forces between rGO and PC.

The PC layer does not deform in the atmospheric environment.However,in organic vapor rGO/PC bilayer films can generate bending behavior due to the difference of swelling degree between the PC layer and rGO layer,which drives the rGO/PC bilayer film bending to the rGO side.Fig.3 shows the ethanol vapor-triggered bending behaviors of bilayer films,which are composed of the rGO layer with different thicknesses attached on the PC membranes with different pore sizes.The tested samples are coded as Mi/j,in which i represents the thickness of the rGO layer(nm)and j represents the pore size of PC(nm).Compared with the M220/600,the M220/200bilayer film shows larger bending degree due to the large porosity of the PC layer(Fig.3a).The PC substrate with a pore size of 200 nm is more porous than that with a pore size of 600 nm(Fig.2a1,a2).The organic vapor can quickly diffuse into the PC substrate with a pore size of 200 nm and generate large swelling of the PC membrane.At the equilibrium state,the final curvature of M220/600membrane is 0.35 cm-1,which is much less than that of the M220/200bilayer film.Thus,the PC membrane with a pore size of 200 nm is selected as substrate to stack rGO layers with different thicknesses.With increasing the thickness of the rGO layer,the bending degree of the Mi/200membrane is enlarged.The thicker the rGO layer is,the stronger the restriction at the interfacial side of the PC layer.The equilibrium curvatures of M165/200,M220/200and M580/200are respectively 1.25 cm-1,1.82 cm-1and 2.67 cm-1.Due the large bending deformation,the rGO/PC bilayer film with 580 nm thickness of rGO on PC membrane with 200 nm pore size is selected for testing in the following experiments.

3.3.Bending behaviors of rGO/PC bilayer films in various organic vapors

Fig.4 shows the bending behaviors of rGO/PC bilayer films in vapors of different alcohols such as methanol,ethanol,isopropanol and 1-butanol.In the methanol vapor,the M580/200bilayer film can rapidly bend to equilibrium within 2 s(Fig.4a,Movie S1).As to the other alcohol vapor,the equilibrium time prolongs(Fig.4b-c,Movies S2-S4).The kinetics of the bending deformation is assessed by plotting the curvature of M580/200bilayer actuator against time(Fig.4e).The results show the bending curvature of the 10-mm long M580/200bilayer actuator in methanol vapor is 2.04 cm-1within 2 s.The bending rates of the M580/200bilayer actuator in different alcohol vapors are compared in Fig.4f.Because the methanol is much volatile,the maximum bending rate is 1.31 cm-1·s-1,which is biggest among the bending rates in four vapors of alcohols.Maximum bending rate is defined as the largest line slope of curvature against time.According to the increasing boiling point of the alcohols,the maximum bending rate is decreased.However,when the bilayers are exposed in different alcohol vapors,the curvatures at equilibrium state are ranging from 1.28 to 2.66 cm-1,which seems no obvious disciplines by changing alcohol vapors.Because the solubility parameters of the alcohols are larger than that of the PC membrane,the affinity of alcohols to the PC membrane is weak.The alcohol vapor is incapable to drive the M580/200bilayer actuator to achieve significant bending state.

While exposing in the organic vapor with smaller solubility parameters compared with that of alcohols,such as dichloromethane,acetone and ethyl formate,the M580/200bilayer actuator shows significant bending behavior(Fig.5,Movies S5-S7),because the solubility parameters of dichloromethane,acetone and ethyl formate are close to that of the PC membrane.Meanwhile,the dichloromethane vapor can quickly induce the M580/200bilayer film into a multiply wound coil in 1 s.While upon exposure in ethyl formate vapor within 1 s,the M580/200bilayer still does not bend one complete circle.

Further,to present the bending kinetics of the M580/200bilayer film,the curvatures in other different organic vapors as well as water vapor are summarized against time(Fig.6a,b,Movies S8-S12).In the vapor of acetone,methyl acetate,ethyl formate,ethyl acetate and dichloromethane,the M580/200bilayer actuator takes less than 15 s to reach the state of bending equilibrium(Fig.6a).It is noteworthy that the bilayer film exhibits an ultrafast actuation speed in response to dichloromethane vapor,which can bend to several loops within 0.3 s.The final curvature is up to 22.94 cm-1in dichloromethane vapor.However,the final curvature in acetone vapor is as high as 27.3 cm-1,which is largest compared with that of other organic compound vapors used in our experiments(Fig.6b).The time to achieve bending equilibrium in acetone vapor is only 7 s.While,the M580/200bilayer film barely bends in vapor of n-hexane or water,because the solubility parameter of n-hexane is smallest and that of water is highest as listed in Table 1.Among the organic compound vapors(Figs.4f and 6c),the bending rate is super-fast in dichloromethane vapor,which may have resulted from the easy volatilization of dichloromethane.The dichloromethane molecules can rapidly enter the PC membrane and then interact with PC polymeric chains.However,the final curvature of the M580/200bilayer actuator in acetone vapor is highest(Fig.6d).Acetone has strong affinity with polycarbonate due to similar solubility parameter.The strong“acetone-polycarbonate”interaction produces huge swelling of the PC layer,resulting in obvious bending behavior with a high curvature.The bending rate of the M580/200bilayer film in acetone vapor is slower than that in dichloromethane vapor due to the difference of their volatility.

Fig.3.Optical images of the bending behaviors(a)and the curvatures(b)of the Mi/jbilayer films in ethanol vapor.The tested samples are coded as Mi/j,in which i represents the thickness of rGO(nm)and j represents the pore size of PC(nm).Scale bar is 0.5 cm in(a).

Fig.4.(a-d)Optical images of bending behaviors of M580/200bilayer films in vapors of different alcohols,including methanol(a),ethanol(b),isopropanol(c)and 1-butanol(d).Scale bar is 0.5 cm.(e)Curvatures of M580/200bilayer film in vapors of different alcohols.(f)Maximum bending rate and bending degree of M580/200bilayer film in vapors of different alcohols.

We have related the bending rate to boiling point.The kinetics of bending behavior in solvent vapor is relevant with the rates of solvent volatilization and diffusion.The boiling point usually represents the volatility.The lower the boiling point at atmospheric pressure(normal boiling point)of a solvent is,the higher the volatility of solvent is at normal temperature and pressure.As shown in Fig.7a,the bending rate is decreased by increasing the normal boiling point of solvent.When the normal boiling point of organic solvent is low,the organic molecules can easily volatilize and rapidly diffuse into the porous PC membrane to form interaction with PC polymer.The bending degree of bilayer film is up to 27.3 in acetone,which is greater than that of any previous works to our best knowledge(Fig.7b).The bending degree of bilayer film is altered with the accouplements of solubility parameters between organic solvent and PC polymer.When the solubility parameter of solvent vapor is heavily deviated from the value of polycarbonate,the bending degree of the M580/200bilayer film is small.The results provide a valuable guidance to design a composite bilayer film with significant organic compound-triggered bending properties in specific organic vapors.The significant bending angle of the actuator can be achieved by matching the solubility parameters of polymer and solvent.The composite films with organic compound-triggered properties can also be used to easily“measure”unknown solubility parameter of polymer due to the linear relationship between strain and curvature/bending angle.Moreover,the concentration of the solvent solution can be detected by the curvature of bilayer film.The limitation of this material still exists as that different solvents with same solubility parameters or the solvents with different concentrations can result in the same equilibrium curvature of the composite bilayer film.

Fig.5.Optical images of M580/200bilayer films bending in vapors of dichloromethane(a),acetone(b)and ethyl formate(c).Scale bar is 0.5 cm.

Fig.6.(a,b)Curvatures of M580/200bilayer films in different organic vapors.(c,d)The maximum bending rate(c)and bending degree(d)of M580/200bilayer films in different organic vapors.

Fig.7.Maximum bending rate of M580/200bilayer film related to the boiling point of solvent(a)and the bending degree of M580/200bilayer film against solubility parameter of solvent(b).The red dash line indicates the solubility parameter of polycarbonate is 20.0(J·cm-3)0.5[69].

3.4.Bending behaviors of rGO/PC bilayer films in organic vapors of solvents with different concentrations

Bending behaviors of bilayer films in vapors of different organic solvent-water mixtures are investigated(Fig.8a).When the concentration of acetone is 5 vol%,the bilayer film starts to bend with a curvature of 1.05 cm-1at equilibrium state.When increasing the concentration of acetone in water up to 30 vol%,the bilayer film is bending to one circle.In this case,the curvature at equilibrium state is 7.88 cm-1.When the concentration of acetone is 80 vol%,the final curvature of bilayer film is 22.22 cm-1.The curvature at equilibrium state straightly increases with increasing concentration of acetone(Fig.8b).The square of correlation coefficient R2is 0.997 with a very good linear relationship.The specific curvature is corresponding to a certain concentration of acetone in water or other miscible solvents such as methanol and ethanol,and the M580/200bilayer film still can bend with different curvatures above the organic mixture.Therefore,this bilayer film may be a promising candidate for detecting organic vapor with different amounts in atmosphere.

Methylbenzene is a kind of water-insoluble liquid;it can easily form oil-in-water or water-in-oil emulsions with water.When the content of methylbenzene in water is lower than 5 vol%,the curvature of bilayer film at equilibrium state increases gradually by increasing the content of methylbenzene.While the content of methylbenzene in water is higher than 5 vol%,the curvature at equilibrium state is almost constant to～14 cm-1(Fig.8c).Because the density of methylbenzene is smaller than water,the methylbenzene drops are enriched on the top of the mixture when the content of methylbenzene in water is higher than 5 vol%.As a result,the bending curvature of the bilayer actuator above the methylbenzene/water emulsion is close to the bending curvature in vapor of pure methylbenzene solution.

4.Conclusions

In summary,composite bilayer films with significant organic compound-triggered bending that is driven by the affinity to solvent molecules are developed.When the solubility parameters between organic compound and polymer layer are similar,the polymeric films can significantly swell and the bilayer films can significantly bend with the assistance of an inert layer.The rGO/PC bilayer films show significant bending property in response to acetone vapor,because both the solubility parameters of PC polymer and acetone are 20.0(J·cm-3)0.5.The bending angle of the rGO/PC bilayer film is changed with the matched degree of the solubility parameters between organic solvent and PC polymer,while the bending rate is dependent on the volatilization and diffusion rates of organic solvent.Interestingly,the equilibrium curvature of the rGO/PC bilayer film linearly changes in organic vapors with different partial pressures,which is promising as a candidate for detecting the concentration of organic vapor.Our proposed strategy,which is guided by the relationship of solubility parameters between organic compound and polymer materials,provides a design rule for developing functional composite polymer films with significant organic compound-triggered bending properties.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.11.018.