1-s2.0-S0014305713001912-main
Preparation and electrical sensitive behavior of poly
(N-vinylpyrrolidone-co-acrylic acid)hydrogel with ?exible chain
nature
Shuping Jin a ,?,Jianxiong Gu b ,Yujun Shi b ,Kerang Shao a ,Xinghai Yu a ,Guoren Yue a
a
Key Laboratory of Hexi Corridor Resources Utilization of Gansu Universities,Chemistry and Chemical Engineering,Hexi University,Zhangye 734000,People’s Republic of China b
Physics and Electrical and Mechanical Engineering,Hexi University,Zhangye 734000,People’s Republic of China
a r t i c l e i n f o Article history:
Received 9January 2013
Received in revised form 19April 2013Accepted 23April 2013
Available online 3May 2013Keywords:
Electrical sensitivity Solid-state NMR
Poly (N-vinylpyrrolidone)Poly (acrylic acid)
a b s t r a c t
On the basis of the existing knowledge of reactions and properties of poly (N-vinylpyrroli-done)(PVP)and poly (acrylic acid)(PAA),a novel pH-and electrical-sensitive poly (N-vinylpyrrolidone-co-acrylic acid)[poly (NVP-co-AA)]hydrogel was formulated and pre-pared from linear poly (NVP-co-AA).Gelation reaction was initiated by potassium persul-fate (KPS)used as a radical initiator,and the mechanism of crosslinking reaction was investigated by FTIR and solid-state CP/MAS NMR techniques.The morphology of hydrogel with ?exible chain nature and large free volume was veri?ed by measurement of scanning electron microscope.The swelling behavior and bending phenomenon of the hydrogel were investigated.In the electric ?eld,the gel bent toward cathode independently of the pH of buffer solution,applied voltage,crosslinking extent and polyion content in the back-bone of gel.But the de?ection and speed of the bending depended on them.Bending of poly (NVP-co-AA)gel was reinterpreted by bending theory of polyelectrolyte gel based upon the change of osmotic pressure,which is due to the difference of ion concentration between the sides facing anode and cathode.
1.Introduction
Polymer hydrogels,which are three-dimensional poly-meric networks formed from highly hydrophilic monomers rendered insoluble by hydrogen bond,electrostatic or covalent crosslinking [1],imbibe large amounts of water.Their high water content contributing to biocompatibility and soft elastomeric nature could ensure minimal mechan-ical and frictional irritation to surrounding tissues,as well as the low interfacial tension between the gel surface and
the aqueous surrounding ?uids contributes to a reduction in protein adsorption and hence bio fouling and cell adhe-sion onto the gel [2–4].
Polymer hydrogel is a ‘‘soft material’’capable of chang-ing its volume and shape in response to environmental stimuli,such as an applied electric ?eld,stress,light or changes in the temperature and pH value [5,6].An electric ?eld as a stimulus has advantages such as the availability of equipment,which allows precise control of the magni-tude of current,duration of electric pulses,and intervals between pulses,etc.[3].There have been a number of re-ports on electrically induced phenomena in charged poly-mer networks.Tanaka et al.[7]reported that a hydrolyzed acrylamide gel (approximately 20%of amide groups were converted to carboxyl groups)collapsed in an acetone/water (50vol%)binary mixture on application of an electric ?eld.Irie [8]reported bending behavior of
0014-3057ó2013The Authors.Published by Elsevier Ltd.https://www.sodocs.net/doc/ed10279067.html,/10.1016/j.eurpolymj.2013.04.022
?Corresponding author.Tel.:+8609368280760;fax:+860936
8276670.
E-mail address:zjxjsp@https://www.sodocs.net/doc/ed10279067.html, (S.Jin).
Open access under CC BY-NC-ND license .
acrylamide gel exposed to UV irradiation in an electric ?eld.Wu et al.[9]reported the synthesis,structure and electric?eld sensitivity of polyacrylate/polyaniline(PAA/ PANI)and poly(2-acrylamido-2-methyl propylsulfonic acid-acrylic acid)/polyaniline[P(AMPSAA)/PANI]conduct-ing hydrogels with an interpenetrating polymer network (IPN)structure.Osada et al.[10]reported a model of an electrically driven‘‘muscle’’,made of water-swollen syn-thetic polymer gel,weakly crosslinked poly(2-acrylami-do-2-methyl propane)sulphonic acid(PAMPS),which had motility when immersed in water.
With good biostability,biocompatibility,the sensitivity of hydrogels to electrical voltage or pH make gels promis-ing materials for a broad range of applications for biologi-cal engineering,such as arti?cial muscles,drug delivery, and biosensors and bioactuators in biomicroelectro mechanical systems(BioMEMS)[11–15].
Hydrogels based on poly(N-vinylpyrrolidone)(PVP) have been applied successfully as local dressings on wound treatments,such as burns,skin’s ulceration and postopera-tive dressings[16]or controls release system for drug delivery[17,18].Recently,many of work did make the study of hydrogels based on complexation between PVP and PAA[19–22].Crosslinked complexes of PVP-PAA were prepared from a mixture of NVP,AA and ethyleneglycol dimethacrylates[19]or a mixture of NVP and AA[20].In Ali’s study[21],chelating poly(N-vinylpyrrolidone/acrylic acid)copolymer hydrogels were prepared from PVP with certain molecular weight and AA by radiation-induced copolymerization.However,PVP dissolved in pure water does not exhibit a phase transition either at temperatures below the boiling point of water[19]or at pH[22].Also, electric?eld sensitivity property of gel based on PVP has been rarely reported.
In this report,poly(N-vinylpyrrolidone-co-acrylic acid) [poly(NVP-co-AA)]hydrogel composed of?exible uncross-linked poly(acrylic acid)segments was prepared by a gela-tion process of linear statistical copolymer poly(NVP-co-AA).The reversible bending behavior of the gel induced by electric current in buffer solution was demonstrated, and a mechanism based on osmotic pressure theory for the bending phenomenon was proposed once again.
2.Experimental
2.1.Materials
N-vinylpyrrolidone(NVP)(98%,ACROS)and acrylic acid(AA)(A.P.grade)were distilled under reduced pressure to remove the stabilizer prior to use.2,20-Azo-bis-iso-butyronitrile(AIBN)and N,N0-methylene-bis-acryl-amide(NNMBA)(C.P.grade)were recrystallized from95% ethanol just before to use.Potassium persulfate(KPS) was recrystallized from distilled water.The other reagents were A.P.grade and used without further puri?cation.
2.2.Preparation of poly(N-vinylpyrrolidone-co-acrylic acid) hydrogel
Firstly,linear poly(N-vinylpyrrolidone-co-acrylic acid) [poly(NVP-co-AA)]was prepared by free radical copolymerization using AIBN as an initiator.Amounts of 15.5250g NVP(0.13986mol),5.25g AA(0.0729mol.x, the molar ratio of NVP unit to AA unit,is about1.92), and0.1560g AIBN(1.0?10à3mol)were dissolved in 20mL anhydrous ethanol.A different mass value of NaOH (0.5833,0.8750,1.1666and1.7499g)was used to modu-late the degree of neutralization(20%,30%,40%and60%) of AA,respectively.After thorough mixing,the copolymer-ization reaction was maintained for2h in a70°C water bath,and then the product poly(NVP-co-AA)was puri?ed by multiple dissolutions(?3)in distilled water followed by precipitation into acetone.Then,poly(NVP-co-AA)was dried under vacuum at room temperature to constant weight.
Subsequently,2.6500g linear poly(NVP-co-AA),certain amounts of KPS and NNMBA(mass percentage of NNMBA to KPS is10wt%)were totally dissolved into15.00mL dis-tilled water.The solution was poured into a glass model, and the model was sealed with sealant tape and placed in a60°C oven to initiate the crosslinking reaction of poly N-vinylpyrrolidone chain segments.Different mass per-centages of KPS to NVP(MR=W KPS/W NVP?100%=26.8, 33.5and40.2wt%)were used to modulate the crosslinking extent.The reaction was performed for5h,and then the product hydrogels were cut into strips with dimension of 11mm?1mm?1mm.
2.3.Preparation of Britton–Robinson buffer solutions with different pH
Britton–Robinson(B–R)buffer solution was prepared from0.4M acetic acid,0.4M phosphoric acid,and0.4M boric acid that were adjusted to different pH values with 1M NaOH or1M phosphoric acid.Sodium chlorine was used to adjust ionic strength of solution to0.0456M;the pH of solution was determined by pHS-3B model pH meter.
2.4.Spectroscopic analysis
FTIR spectra of the?nal dried gels were recorded on a Thermo Nicolet Nexus670FTIR spectrometer at a resolu-tion of4cmà1.Solution NMR spectrum for linear PVP was recorded at room temperature(in D2O)by using an AVANCE600NMR Spectrometer.Solid-state NMR experi-ment was carried out at ambient temperature on a Bruker Avance400MHz wide cavity solid-state NMR spectrome-ter operating at a13C resonance frequency of400.13MHz. Samples were analyzed under cross-polarization/magic-angle spinning(CP/MAS)conditions.The chemical shifts of all13C spectra were externally referenced to the carbon sig-nal of TMS.The linear PVP was obtained by a free radical polymerization of NVP using AIBN as an initiator in distilled water.The product was puri?ed by multiple dissolutions (?3)in distilled water followed by precipitation into acetone,then was dried under vacuum at room temperature to constant weight.The sample used for solid-state NMR measurement was obtained through a crosslinking reaction of linear PVP induced by KPS as mentioned above but with-out NNMBA.To remove the unreacted KPS,the hydrogel was extracted using Soxhlet extractor for72h in deionized water.
1872S.Jin et al./European Polymer Journal49(2013)1871–1880
2.5.Determination of exact composition
The composition of linear poly(NVP-co-AA)was calcu-lated by comparing the peak areas of the carbonyl group of PVP with the carboxylic acid and acrylate groups on the PAA chain shown in the FTIR spectrum as follows:
x?A NVP
A AAtA Sodium AA
where x is the molar ratio of NVP unit to AA unit;A NVP,A AA and A Sodium AA are the areas of the carbonyl group of PVP, the carboxylic acid and acrylate groups on the PAA chain, respectively.The area was calculated through Origin8.0 after picking multiple peaks from a curve to?t Lorentz peak functions.
2.6.SEM measurement
Poly(NVP-co-AA)hydrogel with crosslinking extent MR=33.5%was swollen completely in distilled water at room temperature,and then was freeze-dried under vac-uum for15h with LABCONCO freeze dry system to avoid the collapse of porous structure after accelerated freezing by liquid nitrogen.The cross sectional morphology of the xerogel was determined using a scanning electron micro-scope,JSM-5600LV SEM.
2.7.Swelling studies
Poly(NVP-co-AA)gel with different crosslinking ex-tents were allowed to swell in different pH buffer solutions under various electric?elds.After the excess solution on the surface of the hydrogel strip had been removed with ?lter paper,the length of swollen samples was measured. The equilibrium swelling ratio was determined as follows: Swelling ratio?eL sàL dT=L d
where L s is the length of the samples in swollen state,L d is 11mm,respectively.
2.8.Measurement of bending angle of poly(NVP-co-AA) hydrogel in an electric?eld
A schematic diagram of the equipment used for study-ing the electrical response of hydrogel is shown in Fig.1 [23].All hydrogel strips were11mm long,about1mm wide and1mm thick.Two parallel carbon electrodes, 50mm apart,were immersed in the B–R buffer solutions with different pH and the hydrogel strip under investiga-tion was mounted centrally between them.Upon applica-tion of a dc electric?eld,the de?ection is expressed in terms of the degree of bending,h,measured by reading the angle of deviation from the vertical position,and the sign of the value of de?ection is positive when the hydro-gel bent toward the right and negative when it bends to-ward the left.The bending behavior was recorded with a digital camera(Canon,Japan).3.Results and discussion
3.1.Formulation and characterization
It is found that the responsive behavior of hydrogels im-mersed in a bath solution under an externally applied elec-tric?eld are affected by many parameters,including the ?xed charge density,exterior solution concentration,ionic valence,externally applied voltage,and so forth[6,24].The ionic conduction,which is a transport process occurring under an applied electric?eld,can be rationalized in a dependence of hydrogels responsive behaviors upon the ionic mobility.Andreas Killis et al.have demonstrated that there exists a direct relationship between the ion conduc-tion and viscoelastic properties of the networks,and the diffusion of ions is dependent on free volume[25].In this report,formulation of a new poly(NVP-co-AA)hydrogel with?exible chain nature and large free volume was done from existing knowledge of reactions and properties of poly(N-vinylpyrrolidone)(PVP).At60°C,KPS would decompose into the sulfate anion radical.It is well known that the anion radical can not only react with p bond of vi-nyl monomer through an addition process,but also capture the active tert-hydrogen atom to generate free radical. Compared with PAA,PVP does easily form free radical, which is stabilized by an electron-donating resonance ef-fect from nitrogen atom but PAA cannot,it was experimen-tally proved in this work.Therefore,the crosslink reaction among PVP chain segments has performed ef?ciently through bimolecular termination of free radical or initiat-ing NNMBA to form covalent crosslinks.The gelation reac-tion and the structure of poly(NVP-co-AA)hydrogel,which is composed of crosslinked PVP network and uncrosslinked PAA chain segments,can be represented by Scheme1.
The changes in the PVP chains before and after gelation are studied by solution and CP/MAS solid-state NMR spec-troscopy.The results are shown in Fig.2a for linear PVP dissolved in D2O and Fig.2b for pure PVP xerogel cross-linked by KPS inducing,respectively.Assignments of the peaks are facilitated by references26and27.Thus,for lin-ear PVP before gelation,as shown in Fig.2a,the side-chain pyrrolidone ring’s methylene carbons are assigned to d17.60(4CH2),d31.34(5CH2),and d42.61(3CH2)ppm(see numbering in Fig.2a),respectively.The main chain meth-ylene carbon(1CH2)signal is assigned to the weak peak around d17.60ppm overlaid with the peak of4CH2,while the main chain methine carbon(2CH)is located at 44ppm.The peak at177.48ppm is attributed to the car-bonyl carbon in the pyrrolidone ring of PVP.As for the peaks appearing at30.21and215.40ppm may be attrib-uted to the residual acetone,which was used as precipita-tor during the puri?cation process of linear PVP.
As can be seen in Fig.2b for PVP xerogel,a new peak is clearly observed near81.79ppm in the gel spectrum, which is a signal from the quaternary carbon atom(C?) groups in the newly-formed cross bond.The change is excellently consistent with the fact that the2CH group partly converted into quaternary carbon atom C?groups in the main chain.The carbonyl carbon peak at 215.40ppm disappeared after crosslinking.This may be attributed to the puri?cation process of PVP hydrogel
S.Jin et al./European Polymer Journal49(2013)1871–18801873
by Soxhlet extractor with distilled water as extraction solvent.
The FTIR spectra of linear poly (NVP-co-AA)before gela-tion and poly (NVP-co-AA)gel with molar ratio of NVP to AA 2:1(feed ratio)after crosslinking were carried out.The results are shown in Fig.3a and b,respectively.We know that the carbonyl group of PVP exhibits a stretching vibration peak between 1650and 1680cm à1and the group of carboxylic acid on the PAA chain exhibits a peak at approximate 1750cm à1from the literatures [19].When the carbonyl group forms intermolecular hydrogen bond,there is a negative shift exhibited in the FTIR spectrum.In this work,the carbonyl group exhibits a peak at 1654.02for poly (NVP-co-AA)before gelation but a peak at 1634.07cm à1for the polymeric gel,this negative shift from 1654.02to 1634.07cm à1signify that a stronger inter-molecular interaction after gelation than that before.Strong shoulder appearing at about 1713.85cm à1,which may be corresponded to stretching vibration of the car-bonyl group of carboxylic acid on the PAA segments,fur-ther illustrated that intermolecular hydrogen bond did occur.In sum,FTIR spectroscopic analysis surprisingly showed no signi?cant changes in the polymer before and after crosslinking by KPS inducing except for the changes in the intensity.Implying that no new groups formed dur-ing the process of gelation in which the predominant change is the formation of carbon–carbon cross bonds.
The composition of linear poly (NVP-co-AA),x ,is about 1.05.This was determined by comparing the area of the stretching vibration peak of the carbonyl group of PVP at 1654.02with that of carboxylic acid group at about 1713.85cm à1and the asymmetrically stretching vibration of carboxylate ion appearing at about 1571.75cm à1on the PAA chain.The area was calculated automatically through Origin 8.0after picking multiple peaks from a curve to ?t Lorentz peak function as shown in Fig.4.The difference of x between 1.92determined from feed ratio of NVP to AA units and 1.05calculated from FTIR may be ascribed to the different active of NVP and AA.For example,it is educed that the reactivity ratio of NVP and sodium AA is 0.808and 3.296respectively,by the mathematical method
of Fineman Ross,Kelen-Tüdo
ˇs and YBR,via the aid of test results of element analysis [26].
Cross sectional morphology of poly (NVP-co-AA)hydro-gel with crosslinking extents MR =33.5%is shown in Fig.5after swelling completely in distilled water.From this we can see that the hydrogel exhibits certain uniform and por-ous three-dimensional network structure.This may be ow-ing to the effective crosslinking bond triggered by KPS.From these we would easy to understand that the elastic nature and large free volume displayed by poly (NVP-co-AA)hydrogel,which would be favorable for the movement of polymer chain segments and migration of charged parti-cles in the inner of the hydrogel [27].
K 2S 2O 860o C,5h
SO 4-
N HOOC
O
H x
y N
O O
N
HOOC
O
H
HOOC
H
y x x x
H
COOH +
y Fig.1.Schematic diagram for testing the bending behavior of hydrogels.
N
O
N O
n
n
134
5
6*
*
1,4
2,3
5
6
b
a
5
26
1,4
3
N n O
1
2
3
4
5
6NMR spectrum of linear PVP dissolved in D 2O at room temperature (a),and solid-state 13C NMR spectrum of crosslinked PVP initiated temperature.
35003000250020001500
1000500
Wavenumbers (cm -1
)
linear poly (NVP-co-AA) poly (NVP-co-AA) gel
a
b
spectra of linear poly (NVP-co-AA)solid before gelation (a),(NVP-co-AA)gel after KPS initiate crosslinking (b).
1900
18001700
1600T r a n s m i t t a n c e (%)
Wavenumbers (cm -1
)
Fig.4.Fitting peaks (Lorentz)obtained from Origin 8.0Peak Square is 0.99532).
S.Jin et al./European Polymer Journal 49(2013)1871–18801875
3.2.Swelling behavior of poly (NVP-co-AA)hydrogel in the electric ?eld
Fig.6shows the variation of equilibrium swelling de-gree (SW eq )of poly (NVP-co-AA)hydrogel with different crosslinking extents bathing in different pH B–R buffer solutions.The experiments were measured at room tem-perature when an electric ?eld was applied.From this ?g-ure we can see that:
(1)The swelling of poly (NVP-co-AA)hydrogel is largely
dependent on the pH of bathing and the hydrogel shows a higher value of SW eq at pH 7.96and 10.88than that of at pH 2.87and 6.09,no matter what the crosslinking extent and voltage are.This behav-ior is typical property of pH-sensitive hydrogel [22].The lower level of swelling degree may be con-sidered to be due to the higher crosslinking density stemming from the formation of hydrogen bond among polymer chain segments.While the higher one at high pH is due to the dissociation of carbox-ylic acid groups,results in destruction of hydrogen bond and decrease of crosslinking density,the latter maximizes the free space that is favorable for swell-ing.Apart from this,the charge repulsion along PAA chain also results in the extension of polymer chains and an increase of SW eq at high pH.
(2)Exposure voltages applied during the measurement
have also a great in?uence on the swelling.The increase in the applied voltage enhances the swell-ing.The swelling behavior of hydrogel in the electric ?eld can be explained by Flory’s theory of osmotic pressure [28,29].When an electric ?eld is applied,the counter ions of the polyions in the inner of hydrogel and the free ions in the solution move toward their counter-electrodes.At the same time,the electro neutrality property of the gel body must be hold.This would result in an ionic gradient within the hydrogel along the direction of the electric ?eld.The ionic concentrations inside and outside the hydrogel are thereby different at the beginning of the swelling process,it turned out that the osmotic pressure originates in which and contributes to
the
The SEM micrograph of the poly (NVP-co-AA)hydrogels with MR =33.5%after swelling in distilled 4.04.55.05.5o (m m /m m )
b
0.00.51.01.52.02.53.03.54.04.55.0E q u i l i b r i u m s w e l l i n g r a t i o (m m /m m )
pH=2.87pH=6.09pH=7.96pH=10.88
a
S.Jin et al./European Polymer Journal49(2013)1871–18801877 penetration of the solvent molecules,so hydrogel Array swells.It is understood easily that the higher applied
voltage would lead to a higher number and migra-
tion rate of ion,and consequently the degree of
swelling increase.
(3)Another important factor in?uencing the swelling
behavior of the poly(NVP-co-AA)hydrogel is cross-
linking extent of the poly(NVP-co-AA)regulated by
the mass percentages of KPS to NVP unit(MR).The
curves shown in Fig.6a–c indicate an optimum
quantity of MR,which is33.5%.When MR is less
than33.5%,the number of effective net-chain in
the inner of hydrogel will be insuf?cient.On the con-
trary,a higher MR than33.5%does increase the
crosslinking density;the latter impedes the segmen-
tal motion,and contributes to a lower equilibrium
swelling degree.
3.3.Bending behavior of poly(NVP-co-AA)hydrogel in the
Bending phenomena of poly(NVP-co-AA)hydrogel with MR=26.8%and ND=40%bathing in pH7.96buffer solution when a constant voltage
swelling under same condition.So the hydrogel shows higher SW eq,bending angle and response speed at pH 7.96and10.88than that at pH2.87and6.09when the ap-plied voltage is up to20V and30V.This indicates that the bending is one of the swelling behaviors.
The effect of the extent of crosslinking on the bending behavior of poly(NVP-co-AA)hydrogel is shown in Fig.10.The bending angle and response speed?rst in-crease considerably as the crosslinking extent up to MR 33.5%from26.8%,and then decrease slightly when one continue to up to MR40.2%.Optimum mechanical proper-ties given by an optimum crosslinking extent could explain the changes in the degree of de?ection shown in this?g-ure.Shiga and Kurauchi have used a three-point mechani-cal bending model to explain the bending behavior of gels [28,29].That is,when hydrogels bend under electric stim-ulus,the osmotic pressure difference between the anode and the cathode sides,D p,is balanced by the stress caused by the strain on the polymers.Therefore,in a three-point bending test,D p is equal to the maximum tensile stress r.
D p?r?6DEY=L2
where E is Young’s modulus,Y is the amount of the de?ec-tion(the distance between the ends of the polymer gel be-fore and after bending),D is the thickness,and L is the length of the polymer gel before bending.When a constant value of electric?eld is applied to the hydrogel,D p is con-stant.And if the length and thickness of the hydrogel are also kept constant,the bending de?ection would be in in-verse proportion to its Young’s modulus[23].It is found that the crosslinking extent of MR33.5%should give an optimum value of Young’s modulus in the current test according to the discussions above.In addition,monomer unit molar ratio of NVP to AA should also in?uence the mechanical properties.These were not evaluated at pres-ent and would be discussed in detail in further work.
In Shiga and Kurauchi’s study,the direction of deforma-tion depended on the concentration of polyion,–COOà,in the gel.The gel with a small amount of polyion(molar ratio of NaOH to AA was20%in feed)shrank from the anode side of it,while the gel with a large amount of polyion(molar ratios were60%and100%in feed)swelled.The gel,content of polyion was40%modulated by molar ratio of NaOH to AA,swelled and shrank continuously[28].But the gel free of polyion was not in?uenced by the electric?eld.Both ends of the gel should move toward the cathode if a long and thin gel was stimulated by applying the?eld to it. The gel gained weight while it was bending.As mentioned above,this indicates that the bending is one of the swelling behaviors.In the present work,the great in?uence of con-centration of polyion de?ned by the neutralization degree of AA monomer on the response to the electric stimulation was evaluated.The bending angle and response speed of poly(NVP-co-AA)hydrogel bathing in pH10.88buffer solution under a cyclically varying electric?eld from30 toà30V are shown in Fig.11.The data shows that the bending phenomena are not in agreement with that re-ported in literature[28].What depended on the concentra-tion of polyion is the bending angle and response time but not the direction of https://www.sodocs.net/doc/ed10279067.html,rger amounts of poly-ion,molar ratio of NaOH to AA is40%in feed,improves both the bending angle and the response speed as com-pared with that of less amounts of polyion,molar ratios are20%and30%in feed.However,it is worth noting that the further increase in polyion content reduces electroac-tive property for hydrogel with degree of neutralization of AA of60%.This may be due to the decrease in reactivity ratio of sodium AA to NVP as an increase in neutralization degree of AA comonomer,which inevitably leads to a reduction of the polyion content in the hydrogel.
3.4.Proposing mechanism of bending
Under the in?uence of an electric?eld,electro respon-sive hydrogels generally swell,deswell or bend,depending on the shape of the gel and its position relative to the elec-trodes.Swelling and deswelling occur when the hydrogel lies perpendicular to the electrodes,whereas bending oc-curs when the main axis of the gel lies in parallel with (but does not touch)the electrodes[28,30],where a side of gel swells and the other shrinks.
The deformation and the swelling behavior of polyelec-trolyte gel related to the application of electric?elds can be
1878S.Jin et al./European Polymer Journal49(2013)1871–1880
explained by Flory’s theory of the osmotic pressure,p, which originated in the ion concentration difference be-tween the inside and outside of the gel subjected to a dc electric?eld.When p increases,the gel swells,on the con-trary the gel shrinks.As the poly(NVP-co-AA)gel strip with negatively charged polyion is placed parallelly be-tween a pair of electrodes(Fig.1)and a constant direct cur-rent is applied,the positively charged free ions migrate towards the cathode,whereas the polyion remains immo-bile.The counter ions in the inner of gel would not move out of it because of the electrostatic interaction between polyion and free cation,and for maintaining the electro-neutrality property of gel.Thus a concentration gradient of freely mobile cation is established and the concentration on the side facing negative electrode becomes larger than that on the opposite side as shown schematically in Scheme2a and b.So the gel network on the cathode side is cationic.Simultaneously,the free ions in the solution also move toward their counter electrodes.Some of nega-tively charged ions can therefore bind to gel’s surface fac-ing the cathode,then,electric double layer will be induced at the interface between the gel and solution.As the results,the osmotic pressure of the positive electrode side,p1,becomes larger than that of the negative electrode side,p2,with time as shown in Scheme2b.Consequently, the side of gel facing the anode swells easily as compared with the cathode side,and the gel would bend toward cathode.
Another explanation for the bending behavior of the poly(NVP-co-AA)gel in the electric?eld cannot be ig-nored.For polyanionic(NVP-co-AA)hydrogel,as we dis-cussed above,a concentration gradient of freely mobile cation is established and the concentration on the negative electrode side larger than that on the positive side,so neg-atively charged groups on the polymer chain are balanced by the positively charged ions.This means that the charge repulsion along the PAANa block can also be screened effectively,namely,coupling-coupling attraction increases on the side facing the cathode,accompanied by a decrease of electrostatic repulsion.But,they repel each other on the anode side.This makes the hydrogel shrinks on the cathode side and swells on the anode side and it thereby bends toward the cathode side.
4.Conclusions
In this study we have developed a synthetic strategy to fabricate poly(NVP-co-AA)hydrogel by a gelation reaction of linear statistical copolymer of NVP and AA.The cross-linking process is triggered by using KPS as a radical initi-ator.The novel hydrogel presents porous and?exible chain structure consisting of crosslinked PVP network and uncrosslinked PAA segments,which is expected to be use-ful for the improvement of electric?eld sensitivity.When the poly(NVP-co-AA)hydrogel bathing in buffer solutions with different pH is subjected to an electric?eld,the hydrogel bends toward the cathode.The bending angle and bending speed of the hydrogel increase with increas-ing pH of buffer solution and applied voltage.The degree of neutralization of AA and crosslinking extent of PVP net-work also considerably affects the bending behavior. Therefore,the hydrogel can be useful for sensors,actua-tors,switches,or drug delivery systems.But,the in?uence of monomer unit molar ratio of AA to NVP on the response to the electric stimulation,the mechanical properties and the structure of the poly(NVP-co-AA)gel should be dis-cussed in detail in the further.
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