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J Materiomics 4 (2018) 35e43

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J Materiomics

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Dielectric relaxation of interfacial polarizable molecules in chitosanice-hydrogel materials

Y.Q. Li a, C.X. Zhang a, P. Jia a, Y. Zhang a, L. Lin a, Z.B. Yan a, X.H. Zhou a, J.eM. Liu a, b, *

a Laboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing 210093, Chinab Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China

a r t i c l e i n f o

Article history:Received 19 September 2017Received in revised form28 November 2017Accepted 20 December 2017Available online 21 December 2017

Keywords:Chitosan hydrogelsDielectric relaxationPolymer-water interfacial polarizablemoleculesThermal activation energy

* Corresponding author. Nanjing University, NanjinE-mail addresses: yongqgl@126.com (Y.Q. Li), 10276

jiap.phy@gmail.com (P. Jia), zhangyuank2010@163.cedu.cn (L. Lin), zbyan@nju.edu.cn (Z.B. Yan), xhzhouliujm@nju.edu.cn (J.eM. Liu).

Peer review under responsibility of The Chinese C

https://doi.org/10.1016/j.jmat.2017.12.0052352-8478/© 2018 The Chinese Ceramic Society. Pcreativecommons.org/licenses/by-nc-nd/4.0/).

a b s t r a c t

The functionalities of hydrogel-based smart materials are highly related to the electrostatic interactionsand molecular polarization associated with the polymer networks and encapsulated water droplets, andtherefore the dielectric responses of the polarizable molecules in the polymer, water, and polymer-waterinterfaces are particularly attractive, where the properties of polymer-water interfacial molecules remainelusive. Different from extensive dielectric relaxation spectroscopy studies on polymer hydrogel solu-tions, in this work we investigate the dielectric response of chitosan hydrogels below the water solidi-fying point (ice-hydrogels) so that the contribution of chitosan-water interfacial molecules can beisolated. It is revealed that the chitosan-water interfacial polarizable molecules have slow dielectricrelaxation but large polarization compared with the chitosan chains and water molecules, and thedielectric relaxations beyond ~104 Hz are substantially weak. The thermal activation energy of thedielectric relaxation for these interfacial polarizable molecules can be as large as 0.93 eV, i.e. 89.73 kJ/mol. The present work provides a platform for characterizing the polymer-water electrostatic in-teractions and interfacial polarizable molecules, informative to understand the microstructure-propertyrelationships of chitosan-based hydrogel materials.© 2018 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article

under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Hydrogels consist of highly cross-bonded network of hydro-philic polymer chains and such a network acts as a framework inwhich the spatially confined water molecules (liquid or ice waterdroplets) are embedded [1,2]. The spatial configuration of thesepolymer chains can be highly variable, allowing the structure to beflexible, and the loaded water content can be high up to 90% [3].Here, the water content is expressed as water mass divided by totalmass of hydrogels. Given various combinations of different polymerchains which may be neural or synthetic and water solutions withsolvable matters, many kinds of hydrogels have been synthesizedfor a broad range of applications in biochemical, biomedical, anddaily life among many others [4e7]. In particular, because of the

g 210093, China.24605@qq.com (C.X. Zhang),

om (Y. Zhang), npj-qm@nju.pld@nju.edu.cn (X.H. Zhou),

eramic Society.

roduction and hosting by Elsevie

fact that hydrogels are main compositions of animal bodies, anumber of biomedical applications represent the main drivingforces for researches on hydrogels as a class of biomaterials [8e10].On one hand, the biological hydrogels should possess both struc-tural robustness and mechanical flexibility, and accommodate a setof bio-functionalities such as sensing/actuating (responding),reinforcing/regulating, and self-healing etc, by transporting variousbiological substances via reaction, diffusion, and convection road-maps [11e14]. On the other hand, conventional synthetic hydrogelsinevitably have weakness of brittleness besides the low stretch-ability, and essential challenges remain elusive so far [15e17]. Thefunctionalities and technical details of the electrical, magnetic,optical, mechanical, and environmental responses remain rela-tively less understood. Even though these challenges exist, a seriesof electronic devices based on hydrogel matrices (substrates) havebeen proposed and developed for various biomedical applicationsin the context of flexible electronics [18,19]. Without doubt, a full-scale understanding of these functionalities from various aspects isof significance in a huge number of cases.

In fact, the physicochemical characteristics and microstructuralorigins for these various functionalities of hydrogels have been

r B.V. This is an open access article under the CC BY-NC-ND license (http://

Y.Q. Li et al. / J Materiomics 4 (2018) 35e4336

attracting attention for decades [20e24]. Naturally, these func-tionalities can be partially understood from their responses toexternal stimuli. From the viewpoint of microstructure, a basichydrogel unit is schematically shown in Fig. 1, taking the chitosan-based hydrogel as an example [25,26]. The polymer chain isshown in Fig. 1(a) and (b) where the edge hydroxyl and/or carboxylgroups are hydrogen-bonded with surrounding water molecules.These chains are cross-linked upon synthesis details to form spatialpolymer chains network, as shown in Fig. 1(c) for a guide of eyes.Basically, the microstructure is complicated and has several char-acteristics such as no characteristic scale, good structural flexibility,and chemical uncertainties etc [27,28]. So far available in-vestigations have been carried out via different approaches, eachfocusing on some specific aspects tightly connected with oneproperty of the complicatedmicrostructure. The popular techniquesemployed include the chromatography, optical spectroscopy, vis-cosimetry, and dielectric relaxation spectroscopy (DRS) etc [29e32].

Here, our attention is paid to the DRS studies of hydrogels,which has been extensively recognized for understanding themicrostructure and relaxation of hydrogels. For examples, Yalcin

Fig. 1. A sketch of chitosan macro molecular structure (a) and a chitosan chain surroundeddrawn in (c) just for a guide of eyes. (d) The polarizable chitosan macromolecules, chitosan-wdipoles (pch, pint, and pw), and the dielectric responses of chitosan hydrogels are treated ashydrogels.

et al. [33] investigated the DRS of methylene blue-doped hydrogelsand its dependence of the doping concentration. The dielectricrelaxation and interaction between the polymer chains andmovable charged ions have been discussed. On the other hand, themagnetic nano-particles mixed hydrogels and the network micro-structures have been studied by Campanella et al. [34], focusing onthe dynamic relaxation behaviors using the DRS. The microscopicbehaviors of the polymer relaxation in polymer hydrogels and theinfluence of water content were investigated using DRS by Einfeldtet al. [35]. The dynamic mobility of polyurethane was discussedbased on the DRS studies on the dielectric relaxation and ther-modynamic analysis, focusing on the mechanisms of relaxation[36]. McCrystal et al. [37] addressed that characterization of waterbehavior in cellulose ether polymers using the low-frequency DRS.Liu et al. [38] found the remarkable variation of morphology of thealginate hydrogel upon the addition of Ca2þ ions.

Since the polymer-chains and water molecules are both polar-izable, and their dielectric permittivity (complex dielectric con-stants ε¼ ε

0 - iε00) as a function of temperature (T) and ac electro-signal frequency (f) would carry information on the static

by counterions (b). The microscopic morphology of chitosan hydrogels is schematicallyater interfacial polarizable molecules, and water (ice) molecules are viewed as electricthe responses of these dipoles. Here Tw is the solidifying point of water (droplets) in

Y.Q. Li et al. / J Materiomics 4 (2018) 35e43 37

conformation of polymer macromolecules, their side-groups andchain motions, and polyion-counterion interactions among muchmore physics. In fact, the DRS is a well-known technique to studythe static structure and dynamic response of hydrogels, asdescribed in Refs. [39,40], while this work simply follows this well-known consensus. In spite of an amount of movable charges inhydrogel solution, one may discuss these ingredients of physics inthe framework of polarizable molecules in order to make theproblem simplified. To our best understanding at a very pre-liminary level, the possible polarization mechanism for the inter-facial layers is more or less relevant with the electric field drivenmotion of these charged ions in the hydrogels. Certainly, driven bythe ac electric field, these ions move along or opposite to the fielddirection. However, they can't penetrate into the polymer phase butaggregating near the polymer-water interfaces, leading to thepolarizable interfacial molecules. It is understood that the interfa-cial polarization depends on the charge mobility, local spatialelectric field, and details of the microstructures. In the presentsystem, the water is frozen into ice phase, and the charge motion issubstantially prohibited, allowing us to connect the dielectricrelaxation with the response of the polarizable interfacial mole-cules. First, as mentioned above, both active polymer (e.g. chitosan)and water molecules are electrically polarizable. Second, thesepolarizable molecules may be roughly classified into polymer di-poles (pch), water dipoles (pw), and polymer-water interfacial di-poles (pint), as indicated in Fig. 1(d). Driven by electric field, thesedipoles or polarizable molecules may move, rotate, and changetheir moments, making it difficult to extract information on any ofthem from the others in terms of dielectric relaxations [33,34].Surely, the relaxations of the polymer and water polarizable mol-ecules pch and pw can be studied separately in pure polymer andwater samples, an understanding of the relaxation of interfacialpolarizable molecules (dipoles pint) has to be done with hydrogelsthemselves. Without doubt, any information on these dipoles (pint)in real hydrogels should be one of the keys to understand theinterfacial properties between polymer and water molecules, whilethese properties are the core issues of fundamental researches onhydrogel materials [41e43].

Unfortunately, for hydrogels with liquid water, the electrodepolarization (EP) effect can be so serious [34,44e46], in particularlyin the ultralow-f range (e.g. f< 10Hz). The EP effect is mainlyinduced by the large ionic conduction in hydrogels which con-tributes to serious charge aggregation near the electrodes. Thischarge aggregation makes giant dielectric response due to thecharge screening on the electrodes in the ultra-low frequencies. Asa side consequence, the EP effect makes the DRS signals from thepolymer network and water polarizable molecules largely sub-merged into the background of the ionic conduction and it becomesextremely large if the polymermolecules are easily protonated [33].Although several models for extracting the intrinsic dielectric re-sponses by excluding the EP effect were proposed [29,47,48], it isyet a tough issue to obtain reliable data frommeasured DRS signals.To overcome this problem, one may turn to alternative strategies totrack the DRS of hydrogels. One strategy is to target hydrogels withsolid water (ice) droplets instead of liquid water droplets, i.e. thehydrogels in a finite T-range below the solidifying point Tw of waterdroplets in the hydrogels. Hereafter, we call the hydrogels withliquid water and solid water (ice) as water-hydrogels and ice-hydrogels respectively. In this case, the polymer chains (pch) inthe water- and ice-hydrogels should not show big difference interms of the dielectric response, while the water molecules (pw)would be essentially frozen, losing largely their contributions to thedielectric relaxation. Certainly, as T « Tw, all the polarizable mole-cules will be frozen. However, within a finite T-range, those polar-izable molecules (pint) on the polymer-water interfaces may be

possibly sufficiently active to be detected from the DRS.Along this line, in this work, we shall investigate the DRS of the

chitosan ice-hydrogels, focusing on the dielectric relaxation ofinterfacial polarizable molecules (dipoles pint). Two issues will beaddressed. First, we discuss the dielectric responses of the hydro-gels upon the cooling (water solidification) sequences, and try toprobe the features associated with the dipoles pint. Second, we es-timate the thermal activation knowledge on the relaxation of di-poles pint, providing data for discussing the polymer-waterinteractions at the interfaces.

2. Experimental details

2.1. Sample preparation

In our experiments, we chose chitosan-based hydrogels as theobject of present investigations not only because they are highlyfavorable carriers for a variety of therapeutic agents but also for themicrostructural relaxation in terms of physiochemical property[49,50]. The samples were prepared following the standard pro-cedure [51]. As an example of illustration, 0.6ml dropper amount ofacetic acid was added to 59.4ml of de-ionized water to form aceticacid solution of concentration 1%. Then 1.0 g chitosan powder wasadded into the acetic acid solution with continuous stirring untilcomplete dissolving of the chitosan, forming the chitosan solution.Then the chitosan solution was kept stirred at 60 �C. Subsequently,1.0 g ammonium persulfate powder was dissolved into 10mldeionized water and then 2.0ml ammonium persulfate solutionwas added into the chitosan solution. Here, ammonium sulfate isused as initiator to make free radical reaction. After half an hour,14.4 g of acrylic acid and 0.2 g of N, N0-Methylenebis (acrylamide)were added to the chitosan solution. N, N0-Methylenebis (acryl-amide) was used as a crosslinking agent, so that the chitosan andacrylic acid could produce cross-linking effect. After sufficientlystirring for half an hour, the solution was static at 60 �C for fourhours. Finally the hydrogel was slowly cooled down to room tem-perature. The as-generated water-hydrogel samples were washedrepeatedly using deionized water to remove those non-reactedimpurity before being loaded into closed box filled with deion-ized water for preservation.

For a comparison purpose, we also prepared a pure and densechitosan plate from the dried hydrogels by pressing so that theholes inside the hydrogels can be removed. The same Au electrodeswere deposited for dielectric measurements of the dry chitosanplate capacitors, from which the dielectric constants were evalu-ated. For pure water, extensive measurements on liquid water andsolid ice were reported [52e54] and no repeated measurement willbe done here.

2.2. Structural characterizations

For the microstructural characterization, we employed theenvironmental scanning electron microscope (ESEM) (Quanta 200)to image the morphology of the dried hydrogels. For preparing thedried samples, thewater-hydrogels were first cooled down to liquidnitrogen temperature. In this sequence, the cooling was run in acryo-generator chamber with sufficiently wet ambient to avoidwater evaporation during the cooling. Then the chamber wasevacuated slowly to draw-out water droplets in the sample withoutdistorting the polymer chains network. Finally, the section of thehydrogel was sprayed with a thin layer of gold film to preventcharge accumulation, which was transferred into the ESEM samplestage for imaging.

The as-prepared water-hydrogels at room temperature werecharacterized using the Fourier transform infrared spectroscopy

Y.Q. Li et al. / J Materiomics 4 (2018) 35e4338

(FTIR) with the NEXUS870 instrument, in order to detect the effi-cient chemical bonding between chitosan chains and acrylic acid[51]. The dried samples were ground into powders in a mortar,which were prepared by KBr milling and blending and tested atroom temperature [51]. The covered wavelength range was from4000 cm�1 to 400 cm�1 with a detecting resolution of 2.0 cm�1. Thepresented data are the averaging of four cycles of probed data. Thedifferential scanning calorimetry (DSC) analysis was performedwith the NETZSCHDSC-200 F3 instrument. The heating/cooling ratewas 20 K/min and the flowing nitrogen ambient flux of 40ml/minwas used with the temperature range covering �150 �C to 550 �C.

Fig. 3. (a) The measured DSC curves in the cooling and then heating cycle at a rate of20 K/min, with arrows indicating the cycle. The measured FTIR spectra for pure chi-tosan (CS), acetic acid (AA), and the as-prepared hydrogels (CS-g-PAA) are presented in(b), see text for details.

2.3. Electrical measurements

For the dielectric measurements, the water-hydrogels were cutinto thin plates of 4.0mm� 4.0mm� 1.0mm in dimensions. TheAu foils of 4.0mm� 4.0mm� 0.1mm in dimensions, which wereflexible sufficiently, were used as the top and bottom electrodes, toform the plate-like capacitors. The capacitors were then clampedusing rubber band (used for fixing the Au-foils) for subsequentelectrical measurements. Finally, the plate capacitors were sealedin a plastic bag in order to reduce the evaporation of water. It wasnoted that each sample was touched to the stage of the cryo-generator for cooling, and the chamber was filled with wet-ambient in order to avoid remarkable water evaporation duringthe measurement, noting that in any case a small amount of waterevaporation was inevitable. The cooling rate was 0.5 K/min, whichis sufficiently slow to avoid the big difference between the samplecore and stage of the cryo-generator even though water has biglatent heat.

The dielectric measurements were performed using the HP4294A impedance analyzer covering a frequency range from 40Hzto 106 Hz. The ac-voltage signal of 0.5 V in amplitude was used, anda variation of the amplitude down to 0.1 V did not show remarkableinfluence on the DRS.

It should be mentioned that all the hydrogel samples weresynthesized in the identical condition and the water content inthese samples is the only variable parameter. The hydration factor his calculated on the dry basis via

h ¼�m�mdry

�.mdry ¼ mw

.mdry

where m is the hydrogel mass, mdry is the mass of the xerogel, andmw is the water mass for each sample. In our experiments, a set ofsamples with h varying from 0.19 to 3.00 were synthesized and ourattention has been paid to the samples with h ~1.0 as representativeexamples, noting that the electrical measurements on those sam-ples with big h (e.g.> 2.0) were hard due to toomuch water contentinduced difficulties. The results presented belowweremainly taken

Fig. 2. The SEM images at three different magnifications for the as-prepared chitosan hydrothe cryogenic environment. The statistically uniform porous structure is identified and thecomposite where sphere-like water droplets are embedded in the matrix of chitosan polym

from the sample h¼ 0.93.

3. Results and discussions

3.1. Microstructures

We first characterized the microstructures. The ESEM images ofthe well-dried samples are presented in Fig. 2 at different magni-fications for illustrating more clearly the microstructural details.The well cross-linked chitosan network structure where the holesare filled with water before the measurements can be clearlyidentified. It is seen that the water droplets were well caged in thepolymer chains network, while the typical droplet size is ~4.0 mm.The sample shows highly uniform distribution of water dropletswith narrow size difference. Therefore, the as-prepared hydrogelscan be viewed as composites where sphere-like water droplets areembedded in the chitosan polymer matrix.

The measured DSC curves of the sample in the heating andcooling sequences are presented in Fig. 3(a) and the melting and

gel sample (h¼ 0.93), where the water droplets filled in the holes were evaporated intypical size of the holes is ~4.0 mm. The hydrogel structure can be well viewed as aer.

Fig. 4. The dielectric real part ε0(f) for water and ice at 273 K (a), where the data aretaken from Ref. [54], and for dry chitosan powder at three different temperatures (b).

Y.Q. Li et al. / J Materiomics 4 (2018) 35e43 39

solidifying features can be clearly displayed. Because the heatingand cooling rates were 20 K/min, the endothermal peak during theice droplet melting is broad/diffusive and slightly higher than theice point. Here, we believe that the broadness of the endothermalpeak should be connected with wide distribution of crystal size inthe samples, while the exothermic peak during the water solidifi-cation is much sharper. The solidifying point Tw was ~10 K belowthe ice point. The melting and solidifying points may not be accu-rate due to the difference in the heating/cooling rates. Here the“difference” means the temperature difference in the heating/cooling rate between the DSC measurement and the dielectricmeasurement, while either the DSC measurement or the dielectricmeasurement has the same heating/cooling rates (20 K/min and0.5 K/min respectively). Surely, the finite-size effect due to thewater droplets of ~4.0 mm here can't explain such big differencefrom the ice point (0 �C) since the finite-size effect induced shiftingof the solidifying point toward the low-T side should be less than2.0 K. Therefore, the differences between the measured melting/solidifying points and the ice point are resulted from the fact thatthey are the first-order phase transitions which is seriouslydependent of the cooling rate.

We also present the measured FTIR curve in Fig. 3(b), where thedata for the pure chitosan (CS) and acetic acid (AA) are inserted forcomparison. It is seen that all the features in the hydrogel samplecan be properly assigned by comparing them with those from thepure chitosan and acetic acid, indicating that the as-preparedsample does not contain any impurity matters. In details, for chi-tosan (CS) itself, the stretching vibrationmodes at 3100e3500 cm�1

for N-H and O-H, at 2919 cm�1 from C-H, at 1660 cm�1 from C¼O, at1379 cm�1 fromC-N, and at 1074 cm�1 from C-O-C, and the bendingvibration mode at 1579 cm�1 from N-H, were identified [55]. Afterthe graft copolymerization of chitosan into hydrogels, one observedthe disappearance of the mode at 1597 cm�1 from N-H groups andserious suppression of themodes at 1660 cm�1 and 1074 cm�1 fromC¼O and C-O-C groups [56]. Instead, additional stretching vibrationmodes at 1540 cm�1 and 1410 cm�1 from the COO- groups and theabsorption valley at 1720 cm�1 from eCOOH groups of acetic acid(AA) were detected [51]. These characteristics indicate that aceticacid (AA) was grafted onto chitosan chains to form chitosan graftedacrylic hydrogel (CS-g-PAA). These features indicate that the aceticacid (AA) was grafted with the chitosan chains, a character for chi-tosan hydrogels (CS-g-PAA).

3.2. Dielectric relaxation spectroscopy

Before presenting the DRS data on the hydrogel samples, wepresent the measured dielectric real part ε0(T) of pure solid chitosanas a function of frequency f at 200 K, 270 K, and 275 K respectively,as shown in Fig. 4(b). As a comparison, the data of water and ice atT¼ 273 K as a function of f are plotted in Fig. 4(a) and the data aretaken from Ref. [54]. The dielectric real part ε0(T) at T¼ 273 K is ~87for water in the f-range from 10Hz to 1000MHz, and ~90 for icebelow 1.0 kHz and ~5 above 100 kHz. These values should beslightly smaller at T< Tw. For pure chitosan, the dielectric real partε0(T) below the ice point tends to be ~10 when frequency f is beyond~100 Hz. It should bementioned here that for the ice-hydrogels, thedielectric responses include contributions from the chitosannetwork, ice, and chitosan-water interfaces. Given the fact that thehydrogels can be viewed as composites where the sphere-likewater droplets are embedded in the chitosan polymer matrix, oneunderstands that the dielectric constant of the composite would bein between the values of water (ice) and chitosan, depending on thevolume fraction of water (ice). It should be mentioned that there isindeed a rubber-to-glass transition for chitosan and this transitiontemperature appears at 476 K, well above thewater-ice point, while

our studies focus on the DRS below the ice point. Therefore, wehave not considered this issue in our work. While the hydrogels doconsist of chitosan and water but it is reasonable to believe thatsuch a rubber-glass transition temperature will not be relevantwith the phenomena studied here.

The measured dielectric real and imaginary parts ε0(T) and ε00(T)

for our hydrogels as a function of T respectively during the coolingsequence are plotted in Fig. 5(a) and (b) with several chosen valuesof frequency f. In spite of the possible difference in the EP effect onthe DRS, our measured data show no remarkable difference in thedata between the heating and cooling sequences except the shiftingof the solidifying point from the melting point.

We are mainly addressing the data as T is lower than but veryclose to Tw ~262 K, while the dielectric responses of all the polar-izable molecules would be essentially frozen at T « Tw. Severalcharacters of the measured DRS data deserve for highlighting here,and we discuss mainly the dielectric real part ε

0(T) at differentfrequencies. First, one observes two distinct T-regions of the ε

0(T),separated by Tw which is f-independent over the whole f-rangecovered here. In the T> Tw region, the data show the DRS behaviorsof water-hydrogels including the EP effect [34,46]. As addressedearlier, hereafter no more discussion on the data of the water-hydrogels will be given since it is tough to exclude reliably the EPeffect and it has been investigated extensively [47,48]. Second, inthe T< Tw region, the DRS reflects the dielectric relaxation of theice-hydrogels which has been much less addressed so far. Roughly,the conventional dielectric T-dependence is identified, manifestingthe gradual and monotonous decreases of both ε

0(T) and ε00(T) with

decreasing T. Third, the strong frequency dispersion (f-dispersion)below Tw has been found, indicating a gradual decrease of both ε

0(T)and ε

00(T) with increasing f. Fourth and more importantly, notingthe logarithmic scale of the ε

0-axis and ε00-axis, a wide and transi-

tional T-region (DT ~ 30 K) sandwiched by the T-plateau above Twand the T-plateau far below Tw (T « Tw, here T< 220 K) is identified,suggesting an additional but much weaker dielectric relaxationmechanism unavailable to pure water, ice, and chitosan. It will beshown that this relaxation is associated with the interfacial polar-izable molecules between ice droplets and chitosan network, to be

Fig. 5. (a) ~ (c): The measured dielectric real part ε0 , imaginary part ε00 , and dielectric loss tand¼ ε00/ε0 at several frequencies as a function of temperature T for the as-prepared

hydrogel sample (h¼ 0.93) in the cooling sequence from 330 K. Here Tw is the solidifying point of water droplets in the sample and Tw marks the peak position. (d): The evalu-ated frequency f. vs. Tmax relation fromwhich the thermal activation energy Ea is evaluated via the Arrhenius law. (e) and (f): The evaluated ε

0 , ε00 , and tand at T/ Tw �0 as a functionof frequency f.

Y.Q. Li et al. / J Materiomics 4 (2018) 35e4340

discussed below, while the chitosan chains and water droplets bindwith each other at the interface to form the interfacial units(molecules), which are polarizable electrically and this seems to bea common understanding in this community. This is also the basisfor DRS studies of hydrogels.

It is interesting to compare these values with our ε0(T) data. First,the low-f ε0(T) below T ~ 230 K tends to be a constant which is ~90 atf¼ 100Hz (Fig. 5(a)), well consistent with the value of pure ice inthe low-f range [54]. Second, the high-f ε0(T) below T ~ 230 K be-comes also T-independent and the constant value at f¼ 1.0MHz is~9, consistent with the value of pure ice in the high-f range [54].Therefore, it is safe to conclude that the measured ε

0(T) between Twand 230 K (a specific T-window) must be dominantly contributed

from the interfacial polarizable molecules. Within this T-window,the contributions from chitosan chains and ice droplets can benegligible because their dielectric constants are small. On the otherhand, this T-window does allow a platform on which we canevaluate the dielectric relaxation of the interfacial polarizablemolecules (pint), which is however hard to evaluate from the DRS ofwater-hydrogels.

The dielectric imaginary part ε00(T) at different f over 100 Hz to1.0MHz, as shown in Fig. 5(b), show similar T-dependences and f-dispersions as the real part does, and no details will be givenanymore. The difference lies in that the ε

00(T) decreases monoto-nouslywith decreasing T instead of approaching a constant at low T.

Y.Q. Li et al. / J Materiomics 4 (2018) 35e43 41

3.3. Activation energy of the interfacial polarizable molecules

Now, we pay attention to the dielectric loss tand(T)¼ ε00(T)/ε0(T),

and the results are plotted in Fig. 5(c). Again we don't discuss thedata above Tw due to the dominant EP effect but focusing on thosebelow Tw. Due to the slightly different T-dependences of ε0(T) andε00(T), a clear peak of tand(T) at Tmax within the T-window below Twis identified. The peak is broad at low frequency. This peak broad-ness is not strange and in fact recent works on hydrogels in the low-temperature range with solid water also observed the similarphenomena [34,36]. On one hand, it was found that some hydrogelsmay experience the so-called bSW relaxation which is attributed tothe water molecules in interaction with hydroxypropyl group onthe polymer side chain. On the other hand, such a broad dielectricloss peak may also be due to the a relaxation of ice. The coexistenceof the b relaxation from the polymer chains and a relaxation fromice makes the peak broad. However, the peak becomes narrowerwith increasing f. The peak shifts simultaneously towards Tw withincreasing f until f ~10 kHz beyond which no peak can be identified.

Since it was argued that the dielectric relaxation within this T-window is mainly contributed from the interfacial polarizablemolecules (pint) rather than those from chitosan network and icedroplets (pch and pw), the peak in the dielectric loss is essentiallyassociatedwith the thermal activation process of the pint. Therefore,the dependence of this peak position Tmax on frequency f may bedescribed by the well-known Arrhenius equation [57]:

f ¼ f0,expð � Ea=kBTmaxÞ

where f0 is the prefactor, kB is the Boltzmann constant, and Ea is theactivation energy. In spite of absence of sufficient evidence, oneassumes that the Arrhenius equation is still applicable for thedielectric relaxation of the ice-hydrogels. The evaluated datacovering the f-range from 40Hz to 10 kHz are plotted in Fig. 5(d)where the linear ln(f) ~ 1/Tmax relation is shown.

The extracted activation energy Ea is ~0.93 eV or ~89.73 kJ/mol.It is noted that this activation energy is larger than earlier reportedenergy for chitosan (chitosan films). For instance, the reportedvalues of Ea are ~49 kJ/mol and ~48 kJ/mol for chitosan [58,59], andthe averaged value is roughly ~ 0.50 eV. For the dielectric relaxationof ice around the ice point or below, the reported activation energyis ~0.60 eV [54,60]. Certainly, this larger activation energy meansthat the interfacial polarizable molecules as electric dipoles aremore rigid than the chitosan chains and ice molecules. Thedielectric relaxation of the interfacial polarizable molecules is thusslower than the chitosan chains and ice molecules. This is thereason why the peak position approaches Tw once frequency freaches 104 Hz. On the other hand, our results already revealed thatthe measured dielectric constants in this T-window (~30 K) rightbelow Tw are larger than those of chitosan and ice. This fact in-dicates that the polarization of the interfacial molecules must belarger than those of chitosan chains and ice molecules.

It should be mentioned that this is the first time to obtain thethermal activation energy for the interfacial polarizable moleculesin chitosan-based hydrogels, which represents a basic parameter tounderstand the physiochemical properties of chitosan hydrogels.However, at this stage no quantitative estimation of these molec-ular polarizations can be possible.

3.4. Discussion

By investigating carefully the dielectric relaxation below Tw, weargue that the dielectric relaxation of the interfacial polarizablemolecules in the chitosan ice-hydrogels has been successfullyextracted.Nevertheless, two issues remain tobediscussed. First, one

needs to clarify whether the EP effect in the ice-hydrogels is insig-nificant, as argued earlier. Second, what is the apparent dielectricrelaxation right below Tw, i.e. at T/ Tw-0? To approach the two is-sues, we focus on the dielectric constant at T / Tw-0, and addressε0(T/ Tw-0), ε00(T/ Tw-0), and d(T/ Tw-0) of the ice-hydrogels. Themeasured ε

0(T / Tw-0), ε00(T / Tw-0), and tand(T / Tw-0) as afunction of f respectively are plotted in Fig. 5(e) and (f), respectively.

Earlier works on water-hydrogels revealed that the EP effect isdominant in low-f range (<100Hz) but still significant untilf ~ 104 Hz [33,34]. This effect can be roughly tracked by plotting ε

0(f)and ε

00(f) as a function of f respectively in the form of 1/f2, i.e.ε0(f) ~ 1/f m and ε

00(f) ~ 1/f n. The measured ε0(T / Tw-0) and ε

00(T /

Tw-0) do fit the inverse power-law relations against frequency f, butthe power exponents for ε

0(f) and ε00(f) are m ~ 1.4 and n ~ 1.0,

smaller than 2.0, indicating that the EP effect at T¼ Tw-0 in the ice-hydrogels is indeed insignificant. This is reasonable due to thewater solidification at Tw which seriously suppresses the ionicconductivity in water. A quantitative evaluation of this EP effect inthe ice-hydrogels is not our care in this work.

On the other hand, one can look at the dielectric loss tand(f) (nottand(T)) at T/ Tw-0. A clear single-peaked dependence of tand(f) isshown, with the peak position at fmax ~ 104 Hz or less, consistentwith the ε

0(f) and ε00(f) behaviors. This fmax corresponds to a char-

acteristic time ~0.1ms, below which the electric dipoles at Tw willbe dynamically frozen. This time scale is also similar to the char-acteristic time for the interfacial polarizable molecules in the ice-hydrogels. This confirms that the interfacial polarizable moleculesdo make contribution to the apparent dielectric responses of theice-hydrogels.

To this stage, we have successfully demonstrated that thechitosan-water interfacial molecules do contribute to the dielectricrelaxation, and evaluated the activation energy of these interfacialpolarizable molecules which is ~0.93 eV or ~89.73 kJ/mol whichdoes not allow dipole relaxation faster than 0.1ms. Nevertheless, itshould be mentioned that the chitosan hydrogels, no matter theyare water-hydrogels or ice-hydrogels, are complicated in chemicalbonding and microstructure, and the dielectric relaxation can't bedescribed simply by the Debye model. Besides the three kinds ofpermanent electric dipoles discussed here, those movable chargedions may constitute instant electric dipoles in response to theelectric stimuli. The core point of the present work is to maximallyisolate the contributions from the interfacial molecules from othersby solidifying the hydrogel, while a model approach of the dielec-tric relaxation remains to be done in future.

Finally, we have to mention what are the interfacial polarizablemolecules. Are they from the chemically bonded units betweenwater and chitosan or bridged by acetic acid. These issues deservefor future investigation using advanced techniques and theoreticalcalculations.

4. Conclusion

In conclusion, we have measured the dielectric constant ofchitosan ice-hydrogels over the frequency from 101~106 Hz in orderto investigate the dielectric relaxation of the chitosan-waterinterfacial polarizable molecules. It is revealed that the dielectricresponses of the hydrogel with ice droplets in a finite T-window(~30 K) below the water solidifying point mainly come from theinterfacial polarizable molecules, while the dielectric responsesfrom both the ice molecules and chitosan chains are weak in this T-window. The thermal activation energy for the interfacial polariz-able molecules has been estimated to be ~0.93 eV (~89.73 kJ/mol),which is larger than the activation energy of ice molecules(~0.60 eV) and that of chitosan chains (~0.50 eV). The present worksuggests that the chitosan-water interfacial polarizable molecules

Y.Q. Li et al. / J Materiomics 4 (2018) 35e4342

have stronger dielectric rigidness and larger polarization than thechitosan chains and water molecules.

Acknowledgement

This work was financially supported from the National KeyResearch Program of China (Grant Nos. 2016YFA0300101 and2015CB654602), and the National Natural Science Foundation ofChina (Grant Nos. 51431006 and 51721001).

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Y.Q. Li et al. / J Materiomics 4 (2018) 35e43 43

Mr. Yongqiang Li, Graduate student of Nanjing University.

Mr. Chenxiao Zhang: Graduate student of Nanjing Uni-versity.

Mr. Ping Jia: Graduate student of Nanjing University.

Yuan Zhang: Graduate student of Nanjing University.

Dr. Lin Lin: Senior research staff of Nanjing University.

Dr. Zhibo Yan: Associate Professor of Nanjing University.

Mr. Xiaohui Zhou: Senior engineer of Nanjing University.

Jun-Ming Liu: Professor of Nanjing University, Nanjing210093, China.