Dielectric relaxation of interfacial polarizable molecules in chitosan ice-hydrogel materials

Accepted Manuscript Dielectric relaxation of interfacial polarizable molecules in chitosan ice-hydrogel materials Y.Q. Li, C.X. Zhang, P. Jia, Y. Zhang, L. Lin, Z.B. Yan, X.H. Zhou, J.–M. Liu PII:

S2352-8478(17)30084-9

DOI:

10.1016/j.jmat.2017.12.005

Reference:

JMAT 116

To appear in:

Journal of Materiomics

Received Date: 19 September 2017 Revised Date:

28 November 2017

Accepted Date: 20 December 2017

Please cite this article as: Li YQ, Zhang CX, Jia P, Zhang Y, Lin L, Yan ZB, Zhou XH, Liu J–M, Dielectric relaxation of interfacial polarizable molecules in chitosan ice-hydrogel materials, Journal of Materiomics (2018), doi: 10.1016/j.jmat.2017.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Revised manuscript submitted to Journal of Materiomics (Ref. #: JMAT_2017_48)

Dielectric relaxation of interfacial polarizable molecules in chitosan ice-hydrogel

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materials

Y. Q. Li1, C. X. Zhang1, P. Jia1, Y. Zhang1, L. Lin1, Z. B. Yan1, X. H. Zhou1, and J. –M. Liu1,2 1

Laboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China

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[Abstract] The functionalities of hydrogel-based smart materials are highly related to the electrostatic interactions and molecular polarization associated with the polymer networks and encapsulated water droplets, and therefore the dielectric responses of the polarizable molecules in the polymer, water, and polymer-water interfaces are particularly attractive, where the properties of polymer-water interfacial molecules remain elusive. Different from

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extensive dielectric relaxation spectroscopy studies on polymer hydrogel solutions, in this work we investigate the dielectric response of chitosan hydrogels below the water solidifying point (ice-hydrogels) so that the contribution of chitosan-water interfacial molecules can be

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isolated. It is revealed that the chitosan-water interfacial polarizable molecules have slow dielectric relaxation but large polarization compared with the chitosan chains and water molecules, and the dielectric relaxations beyond ~ 104 Hz are substantially weak. The thermal

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activation energy of the dielectric 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 interactions and interfacial polarizable molecules, informative to understand the microstructure-property relationships of chitosan-based hydrogel materials.

Keywords:

chitosan hydrogels, dielectric relaxation, polymer-water interfacial polarizable molecules, thermal activation energy.

1

ACCEPTED MANUSCRIPT I. Introduction Hydrogels consist of highly cross-bonded network of hydrophilic polymer chains and such a network acts as a framework in which the spatially confined water molecules (liquid or ice water droplets) are embedded [1, 2]. The spatial configuration of these polymer chains can

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be highly variable, allowing the structure to be flexible, and the loaded water content can be high up to 90% [3]. Here, the water content is expressed as water mass divided by total mass of hydrogels. Given various combinations of different polymer chains which may be neural or synthetic and water solutions with solvable matters, many kinds of hydrogels have been

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synthesized for a broad range of applications in biochemical, biomedical, and daily life among many others [4-7]. In particular, because of the fact that hydrogels are main

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compositions of animal bodies, a number of biomedical applications represent the main driving forces for researches on hydrogels as a class of biomaterials [8-10]. On one hand, the biological hydrogels should possess both structural robustness and mechanical flexibility, and accommodate a set of bio-functionalities such as sensing/actuating (responding), reinforcing/regulating, and self-healing etc, by transporting various biological substances via

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reaction, diffusion, and convection roadmaps [11-14]. On the other hand, conventional synthetic hydrogels inevitably have weakness of brittleness besides the low stretchability, and essential challenges remain elusive so far [15-17]. The functionalities and technical details of

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the electrical, magnetic, optical, mechanical, and environmental responses remain relatively less understood. Even though these challenges exist, a series of electronic devices based on hydrogel matrices (substrates) have been proposed and developed for various biomedical

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applications in the context of flexible electronics [18, 19]. Without doubt, a full-scale understanding of these functionalities from various aspects is of significance in a huge number of cases.

In fact, the physicochemical characteristics and microstructural origins for these various functionalities of hydrogels have been attracting attention for decades [20-24]. Naturally, these functionalities can be partially understood from their responses to external stimuli. From the viewpoint of microstructure, a basic hydrogel unit is schematically shown in Fig.1, taking the chitosan-based hydrogel as an example [25,26]. The polymer chain is shown in Fig.1(a) and (b) where the edge hydroxyl and/or carboxyl groups are hydrogen-bonded with 2

ACCEPTED MANUSCRIPT surrounding water molecules. These chains are cross-linked upon synthesis details to form spatial polymer chains network, as shown in Fig.1(c) for a guide of eyes. Basically, the microstructure is complicated and has several characteristics such as no characteristic scale, good structural flexibility, and chemical uncertainties etc [27, 28]. So far available

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investigations have been carried out via different approaches, each focusing on some specific aspects tightly connected with one property of the complicated microstructure. The popular techniques employed include the chromatography, optical spectroscopy, viscosimetry, and dielectric relaxation spectroscopy (DRS) etc [29-32].

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

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Yalcin et al [33] investigated the DRS of methylene blue-doped hydrogels and its dependence of the doping concentration. The dielectric relaxation and interaction between the polymer chains and movable charged ions have been discussed. On the other hand, the magnetic nano-particles mixed hydrogels and the network microstructures have been studied by Campanella et al [34], focusing on the dynamic relaxation behaviors using the DRS. The

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microscopic behaviors of the polymer relaxation in polymer hydrogels and the influence of water content were investigated using DRS by Einfeldt et al [35]. The dynamic mobility of polyurethane was discussed based on the DRS studies on the dielectric relaxation and

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thermodynamic analysis, focusing on the mechanisms of relaxation [36]. McCrystal et al [37] addressed that characterisation of water behavior in cellulose ether polymers using the low-frequency DRS. Liu et al [38] found the remarkable variation of morphology of the

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alginate hydrogel upon the addition of Ca2+ ions. Since the polymer-chains and water molecules are both polarizable, and their dielectric permittivity (complex dielectric constants ε = ε′ - iε″) as a function of temperature (T) and ac electro-signal frequency (f) would carry information on the static conformation of polymer macromolecules, their side-groups and chain motions, and polyion-counterion interactions among much more physics. In fact, the DRS is a well-known technique to study the static structure and dynamic response of hydrogels, as described in Ref. [39, 40], while this work simply follows this well-known consensus. In spite of an amount of movable charges in hydrogel solution, one may discuss these ingredients of physics in the framework of 3

ACCEPTED MANUSCRIPT polarizable molecules in order to make the problem simplified. To our best understanding at a very preliminary level, the possible polarization mechanism for the interfacial layers is more or less relevant with the electric field driven motion of these charged ions in the hydrogels. Certainly, driven by the ac electric field, these ions move along or opposite to the field

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direction. However, they can’t penetrate into the polymer phase but aggregating near the polymer-water interfaces, leading to the polarizable interfacial molecules. It is understood that the interfacial polarization depends on the charge mobility, local spatial electric field, and details of the microstructures. In the present system, the water is frozen into ice phase, and the

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charge motion is substantially prohibited, allowing us to connect the dielectric relaxation with the response of the polarizable interfacial molecules. First, as mentioned above, both active

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polymer (e.g. chitosan) and water molecules are electrically polarizable. Second, these polarizable molecules may be roughly classified into polymer dipoles (pch), water dipoles (pw), and polymer-water interfacial dipoles (pint), as indicated in Fig.1(d). Driven by electric field, these dipoles or polarizable molecules may move, rotate, and change their moments, making it difficult to extract information on any of them from the others in terms of dielectric

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relaxations [33, 34]. Surely, the relaxations of the polymer and water polarizable molecules pch and pw can be studied separately in pure polymer and water samples, an understanding of the relaxation of interfacial polarizable molecules (dipoles pint) has to be done with hydrogels

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themselves. Without doubt, any information on these dipoles (pint) in real hydrogels should be one of the keys to understand the interfacial properties between polymer and water molecules, while these properties are the core issues of fundamental researches on hydrogel materials

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[41-43].

Unfortunately, for hydrogels with liquid water, the electrode polarization (EP) effect can be so serious [34,44-46], in particularly in the ultralow-f range (e.g. f < 10 Hz). The EP effect is mainly induced by the large ionic conduction in hydrogels which contributes to serious charge aggregation near the electrodes. This charge aggregation makes giant dielectric response due to the charge screening on the electrodes in the ultra-low frequencies. As a side consequence, the EP effect makes the DRS signals from the polymer network and water polarizable molecules largely submerged into the background of the ionic conduction and it becomes extremely large if the polymer molecules are easily protonated [33]. Although 4

ACCEPTED MANUSCRIPT several models for extracting the intrinsic dielectric responses by excluding the EP effect were proposed [29, 47, 48], it is yet a tough issue to obtain reliable data from measured DRS signals. To overcome this problem, one may turn to alternative strategies to track the DRS of hydrogels. One strategy is to target hydrogels with solid water (ice) droplets instead of liquid

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water droplets, i.e. the hydrogels in a finite T-range below the solidifying point Tw of water droplets in the hydrogels. Hereafter, we call the hydrogels with liquid water and solid water (ice) as water-hydrogels and ice-hydrogels respectively. In this case, the polymer chains (pch) in the water- and ice-hydrogels should not show big difference in terms of the dielectric

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response, while the water molecules (pw) would be essentially frozen, losing largely their contributions to the dielectric relaxation. Certainly, as T << Tw, all the polarizable molecules

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will be frozen. However, within a finite T-range, those polarizable 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 of interfacial polarizable molecules (dipoles pint). Two issues will be addressed. First, we discuss the dielectric responses of the hydrogels upon the

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cooling (water solidification) sequences, and try to probe the features associated with the dipoles pint. Second, we estimate the thermal activation knowledge on the relaxation of

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dipoles pint, providing data for discussing the polymer-water interactions at the interfaces.

II. Experimental details 2.1. Sample preparation

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In our experiments, we chose chitosan-based hydrogels as the object of present investigations not only because they are highly favorable carriers for a variety of therapeutic agents but also for the microstructural relaxation in terms of physiochemical property [49, 50]. The samples were prepared following the standard procedure [51]. As an example of illustration, 0.6 ml dropper amount of acetic acid was added to 59.4 ml of de-ionized water to form acetic acid solution of concentration 1%. Then 1.0 g chitosan powder was added into the acetic acid solution with continuous stirring until complete 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 10 ml deionized water and then 2.0 ml 5

ACCEPTED MANUSCRIPT ammonium persulfate solution was added into the chitosan solution. Here, ammonium sulfate is used as initiator to make free radical reaction. After half an hour, 14.4 g of acrylic acid and 0.2 g of N, N'-Methylenebis (acrylamide) were added to the chitosan solution. N, N'-Methylenebis (acrylamide) was used as a crosslinking agent, so that the chitosan and

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acrylic acid could produce cross-linking effect. After sufficiently stirring for half an hour, the solution was static at 60 °C for four hours. Finally the hydrogel was slowly cooled down to room temperature. The as-generated water-hydrogel samples were washed repeatedly using deionized water to remove those non-reacted impurity before being loaded into closed box

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filled with deionized water for preservation.

For a comparison purpose, we also prepared a pure and dense chitosan plate from the

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dried hydrogels by pressing so that the holes inside the hydrogels can be removed. The same Au electrodes were deposited for dielectric measurements of the dry chitosan plate capacitors, from which the dielectric constants were evaluated. For pure water, extensive measurements on liquid water and solid ice were reported [52-54] and no repeated measurement will be done

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here.

2.2. Structural characterizations

For the microstructural characterization, we employed the environmental scanning

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electron microscope (ESEM) (Quanta 200) to image the morphology of the dried hydrogels. For preparing the dried samples, the water-hydrogels were first cooled down to liquid nitrogen temperature. In this sequence, the cooling was run in a cryo-generator chamber with

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sufficiently wet ambient to avoid water evaporation during the cooling. Then the chamber was evacuated slowly to draw-out water droplets in the sample without distorting the polymer chains network. Finally, the section of the hydrogel was sprayed with a thin layer of gold film to prevent charge accumulation, which was transferred into the ESEM sample stage for imaging. The as-prepared water-hydrogels at room temperature were characterized using the Fourier transform infrared spectroscopy (FTIR) with the NEXUS870 instrument, in order to detect the efficient 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 6

ACCEPTED MANUSCRIPT blending and tested at room temperature [51]. The covered wavelength range was from 4000 cm-1 to 400 cm-1 with a detecting resolution of 2.0 cm-1. The presented data are the averaging of four cycles of probed data. The differential scanning calorimetry (DSC) analysis was performed with the NETZSCH DSC-200 F3 instrument. The heating/cooling rate was 20

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K/min and the flowing nitrogen ambient flux of 40ml/min was used with the temperature range covering -150oC to 550 oC.

2.3. Electrical measurements

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For the dielectric measurements, the water-hydrogels were cut into thin plates of 4.0 mm × 4.0 mm × 1.0 mm in dimensions. The Au foils of 4.0 mm × 4.0 mm × 0.1 mm in dimensions,

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which were flexible sufficiently, were used as the top and bottom electrodes, to form the plate-like capacitors. The capacitors were then clamped using rubber band (used for fixing the Au-foils) for subsequent electrical measurements. Finally, the plate capacitors were sealed in a plastic bag in order to reduce the evaporation of water. It was noted that each sample was touched to the stage of the cryo-generator for cooling, and the chamber was filled with

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wet-ambient in order to avoid remarkable water evaporation during the measurement, noting that in any case a small amount of water evaporation was inevitable. The cooling rate was 0.5 K/min, which is sufficiently slow to avoid the big difference between the sample core and

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stage of the cryo-generator even though water has big latent heat. The dielectric measurements were performed using the HP 4294A impedance analyzer covering a frequency range from 40 Hz to 106 Hz. The ac-voltage signal of 0.5 V in amplitude

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was used, and a variation of the amplitude down to 0.1 V did not show remarkable influence on the DRS.

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

h = (m − mdry ) / mdry = mw / mdry where m is the hydrogel mass, mdry is the mass of the xerogel, and mw is the water mass for each sample. In our experiments, a set of samples with h varying from 0.19 to 3.00 were 7

ACCEPTED MANUSCRIPT synthesized and our attention has been paid to the samples with h ~ 1.0 as representative examples, noting that the electrical measurements on those samples with big h (e.g. > 2.0) were hard due to too much water content induced difficulties. The results presented below

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were mainly taken from the sample h = 0.93.

III. Results and discussions 3.1. Microstructures

We first characterized the microstructures. The ESEM images of the well-dried samples

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are presented in Fig.2 at different magnifications for illustrating more clearly the microstructural details. The well cross-linked chitosan network structure where the holes are

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filled with water before the measurements can be clearly identified. It is seen that the water droplets were well caged in the polymer chains network, while the typical droplet size is ~ 4.0 µm. The sample shows highly uniform distribution of water droplets with narrow size difference. Therefore, the as-prepared hydrogels can be viewed as composites where sphere-like water droplets are embedded in the chitosan polymer matrix.

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The measured DSC curves of the sample in the heating and cooling sequences are presented in Fig.3(a) and the melting and solidifying features can be clearly displayed. Because the heating and cooling rates were 20 K/min, the endothermal peak during the ice

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droplet melting is broad/diffusive and slightly higher than the ice point. Here, we believe that the broadness of the endothermal peak should be connected with wide distribution of crystal size in the samples, while the exothermic peak during the water solidification is much sharper.

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The solidifying point Tw was ~ 10 K below the ice point. The melting and solidifying points may not be accurate 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 dielectric measurement, while either the DSC measurement or the dielectric measurement has the same heating/cooling rates (20 K/min and 0.5 K/min respectively). Surely, the finite-size effect due to the water droplets of ~ 4.0 µm here can’t explain such big difference from the ice point (0 oC) since the finite-size effect induced shifting of the solidifying point toward the low-T side should be less than 2.0 K. Therefore, the differences between the measured melting/solidifying points and the ice point are resulted from the fact 8

ACCEPTED MANUSCRIPT that they are the first-order phase transitions which is seriously dependent of the cooling rate. We also present the measured FTIR curve in Fig.3(b), where the data for the pure chitosan (CS) and acetic acid (AA) are inserted for comparison. It is seen that all the features in the hydrogel sample can be properly assigned by comparing them with those from the pure

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chitosan and acetic acid, indicating that the as-prepared sample does not contain any impurity matters. In details, for chitosan (CS) itself, the stretching vibration modes at 3100 ~ 3500 cm-1 for N-H and O-H, at 2919 cm-1 from C-H , at 1660 cm-1 from C=O, at 1379 cm-1 from C-N, and at 1074 cm-1 from C-O-C, and the bending vibration mode at 1579 cm-1 from N-H, were

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identified [55]. After the graft copolymerization of chitosan into hydrogels, one observed the disappearance of the mode at 1597 cm-1 from N-H groups and serious suppression of the

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modes at 1660 cm-1 and 1074 cm-1 from C=O and C-O-C groups [56]. Instead, additional stretching vibration modes at 1540 cm-1 and 1410 cm-1 from the COO- groups and the absorption valley at 1720 cm-1 from –COOH groups of acetic acid (AA) were detected [51]. These characteristics indicate that acetic acid (AA) was grafted onto chitosan chains to form chitosan grafted acrylic hydrogel (CS-g-PAA).These features indicate that the acetic acid (AA)

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was grafted with the chitosan chains, a character for chitosan hydrogels (CS-g-PAA).

3.2. Dielectric relaxation spectroscopy

Before presenting the DRS data on the hydrogel samples, we present the measured

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dielectric real part ε′(T) of pure solid chitosan as 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 at T

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= 273 K as a function of f are plotted in Fig.4(a) and the data are taken from Ref. [54]. The dielectric real part ε′(T) at T = 273 K is ~ 87 for water in the f-range from 10 Hz to 1000 MHz, and ~ 90 for ice below 1.0 kHz and ~ 5 above 100 kHz. These values should be slightly smaller at T < Tw. For pure chitosan, the dielectric real part ε′(T) below the ice point tends to be ~ 10 when frequency f is beyond ~100 Hz. It should be mentioned here that for the ice-hydrogels, the dielectric responses include contributions from the chitosan network, ice, and chitosan-water interfaces. Given the fact that the hydrogels can be viewed as composites where the sphere-like water droplets are embedded in the chitosan polymer matrix, one understands that the dielectric constant of the composite would be in between the values of 9

ACCEPTED MANUSCRIPT water (ice) and chitosan, depending on the volume fraction of water (ice). It should be mentioned that there is indeed a rubber-to-glass transition for chitosan and this transition temperature appears at 476 K, well above the water-ice point, while our studies focus on the DRS below the ice point. Therefore, we have not considered this issue in our work. While the

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hydrogels do consist of chitosan and water but it is reasonable to believe that such a rubber-glass transition temperature will not be relevant with the phenomena studied here. The measured dielectric real and imaginary parts ε′(T) and ε″(T) for our hydrogels as a function of T respectively during the cooling sequence are plotted in Fig.5(a) and (b) with

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several chosen values of frequency f. In spite of the possible difference in the EP effect on the DRS, our measured data show no remarkable difference in the data between the heating and

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cooling sequences except the shifting of the solidifying point from the melting point. We are mainly addressing the data as T is lower than but very close to Tw ~ 262 K, while the dielectric responses of all the polarizable molecules would be essentially frozen at T << Tw. Several characters of the measured DRS data deserve for highlighting here, and we discuss mainly the dielectric real part ε′(T) at different frequencies. First, one observes two distinct

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T-regions of the ε′(T), separated by Tw which is f-independent over the whole f-range covered here. In the T > Tw region, the data show the DRS behaviors of water-hydrogels including the EP effect [34, 46]. As addressed earlier, hereafter no more discussion on the data of the

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water-hydrogels will be given since it is tough to exclude reliably the EP effect and it has been investigated extensively [47, 48]. Second, in the T < Tw region, the DRS reflects the dielectric relaxation of the ice-hydrogels which has been much less addressed so far. Roughly, the

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conventional dielectric T-dependence is identified, manifesting the gradual and monotonous decreases of both ε′(T) and ε″(T) with decreasing T. Third, the strong frequency dispersion (f-dispersion) below Tw has been found, indicating a gradual decrease of both ε′(T) and ε″(T) with increasing f. Fourth and more importantly, noting the logarithmic scale of the ε′-axis and ε″-axis, a wide and transitional T-region (∆T ~ 30 K) sandwiched by the T-plateau above Tw and the T-plateau far below Tw (T << Tw, here T < 220 K) is identified, suggesting an additional but much weaker dielectric relaxation mechanism unavailable to pure water, ice, and chitosan. It will be shown that this relaxation is associated with the interfacial polarizable molecules between ice droplets and chitosan network, to be discussed below, while the 10

ACCEPTED MANUSCRIPT chitosan chains and water droplets bind with each other at the interface to form the interfacial units (molecules), which are polarizable electrically and this seems to be a common understanding in this community. This is also the basis for DRS studies of hydrogels. It is interesting to compare these values with our ε′(T) data. First, the low-f ε′(T) below T ~ 230 K tends to be a constant which is ~ 90 at f = 100 Hz (Fig.5(a)), well consistent with the

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value of pure ice in the low-f range [54]. Second, the high-f ε′(T) below T ~ 230 K becomes also T-independent and the constant value at f = 1.0 MHz 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′(T)

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between Tw and 230 K (a specific T-window) must be dominantly contributed from the interfacial polarizable molecules. Within this T-window, the contributions from chitosan

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chains and ice droplets can be negligible because their dielectric constants are small. On the other hand, this T-window does allow a platform on which we can evaluate the dielectric relaxation of the interfacial polarizable molecules (pint), which is however hard to evaluate from the DRS of water-hydrogels.

The dielectric imaginary part ε″(T) at different f over 100 Hz to 1.0 MHz, as shown in

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Fig.5(b), show similar T-dependences and f-dispersions as the real part does, and no details will be given anymore. The difference lies in that the ε″(T) decreases monotonously with

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decreasing T instead of approaching a constant at low T.

3.3. Activation energy of the interfacial polarizable molecules Now, we pay attention to the dielectric loss tanδ(T) = ε″(T)/ε′(T), and the results are

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plotted in Fig.5(c). Again we don’t discuss the data above Tw due to the dominant EP effect but focusing on those below Tw. Due to the slightly different T-dependences of ε′(T) and ε″(T), a clear peak of tanδ(T) at Tmax within the T-window below Tw is identified. The peak is broad at low frequency. This peak broadness is not strange and in fact recent works on hydrogels in the low-temperature range with solid water also observed the similar phenomena [34, 36]. On one hand, it was found that some hydrogels may experience the so-called βSW relaxation which is attributed to the water molecules in interaction with hydroxypropyl group on the polymer side chain. On the other hand, such a broad dielectric loss peak may also be due to the α relaxation of ice. The coexistence of the β relaxation from the polymer chains and α 11

ACCEPTED MANUSCRIPT relaxation from ice makes the peak broad. However, the peak is broad becomes narrower with increasing f. The peak shifts simultaneously towards Tw with increasing 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

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contributed from the interfacial polarizable molecules (pint) rather than those from chitosan network and ice droplets (pch and pw), the peak in the dielectric loss is essentially associated with the thermal activation process of the pint. Therefore, the dependence of this peak position Tmax on frequency f may be described by the well-known Arrhenius equation [57]:

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f = f 0 ⋅ exp(− Ea / k BTmax )

where f0 is the prefactor, kB is the Boltzmann constant, and Ea is the activation energy. In spite

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of absence of sufficient evidence, one assumes that the Arrhenius equation is still applicable for the dielectric relaxation of the ice-hydrogels. The evaluated data covering the f-range from 40 Hz 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 reported energy for chitosan (chitosan films). For

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instance, the reported values of Ea are ~ 49 kJ/mol and ~ 48 kJ/mol for chitosan [58, 59], and the averaged value is roughly ~ 0.50 eV. For the dielectric relaxation of ice around the ice point or below, the reported activation energy is ~ 0.60 eV [54,60]. Certainly, this larger

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activation energy means that the interfacial polarizable molecules as electric dipoles are more rigid than the chitosan chains and ice molecules. The dielectric relaxation of the interfacial

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polarizable molecules is thus slower than the chitosan chains and ice molecules. This is the reason why the peak position approaches Tw once frequency f reaches 104 Hz. On the other hand, our results already revealed that the measured dielectric constants in this T-window (~ 30 K) right below Tw are larger than those of chitosan and ice. This fact indicates that the polarization of the interfacial molecules must be larger than those of chitosan chains and ice molecules. It should be mentioned that this is the first time to obtain the thermal activation energy for the interfacial polarizable molecules in chitosan-based hydrogels, which represents a basic parameter to understand the physiochemical properties of chitosan hydrogels. However, at 12

ACCEPTED MANUSCRIPT this stage no quantitative estimation of these molecular polarizations can be possible.

3.4. Discussion By investigating carefully the dielectric relaxation below Tw, we argue that the dielectric

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relaxation of the interfacial polarizable molecules in the chitosan ice-hydrogels has been successfully extracted. Nevertheless, two issues remain to be discussed. First, one needs to clarify whether the EP effect in the ice-hydrogels is insignificant, as argued earlier. Second, what is the apparent dielectric relaxation right below Tw, i.e. at T → Tw-0? To approach the

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two issues, we focus on the dielectric constant at T → Tw-0, and address ε′(T → Tw-0), ε″(T → Tw-0), and δ(T → Tw-0) of the ice-hydrogels. The measured ε′(T → Tw-0), ε″(T → Tw-0), and

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tanδ(T → Tw-0) as a function of f respectively are plotted in Fig.5(e) and Fig.5(f), respectively.

Earlier works on water-hydrogels revealed that the EP effect is dominant in low-f range (< 100 Hz) but still significant until f ~ 104 Hz [33, 34]. This effect can be roughly tracked by plotting ε′(f) and ε″(f) as a function of f respectively in the form of 1/f 2, i.e. ε′(f) ~ 1/f m and

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ε″(f) ~ 1/f n. The measured ε′(T → Tw-0) and ε″(T → Tw-0) do fit the inverse power-law relations against frequency f, but the power exponents for ε′(f) and ε″(f) are m ~ 1.4 and n ~ 1.0, smaller than 2.0, indicating that the EP effect at T = Tw-0 in the ice-hydrogels is indeed

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insignificant. This is reasonable due to the water solidification at Tw which seriously suppresses the ionic conductivity in water. A quantitative evaluation of this EP effect in the ice-hydrogels is not our care in this work.

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On the other hand, one can look at the dielectric loss tanδ(f) (not tanδ(T)) at T → Tw-0. A clear single-peaked dependence of tanδ(f) is shown, with the peak position at fmax ~ 104 Hz or less, consistent with the ε′(f) and ε″(f) behaviors. This fmax corresponds to a characteristic time ~ 0.1 ms, below which the electric dipoles at Tw will be dynamically frozen. This time scale is also similar to the characteristic time for the interfacial polarizable molecules in the ice-hydrogels. This confirms that the interfacial polarizable molecules do make contribution to the apparent dielectric responses of the ice-hydrogels. To this stage, we have successfully demonstrated that the chitosan-water interfacial molecules do contribute to the dielectric relaxation, and evaluated the activation energy of 13

ACCEPTED MANUSCRIPT these interfacial polarizable molecules which is ~ 0.93 eV or ~ 89.73 kJ/mol which does not allow dipole relaxation faster than 0.1 ms. Nevertheless, it should be mentioned that the chitosan hydrogels, no matter they are water-hydrogels or ice-hydrogels, are complicated in chemical bonding and microstructure, and the dielectric relaxation can’t be described simply

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by the Debye model. Besides the three kinds of permanent electric dipoles discussed here, those movable charged ions may constitute instant electric dipoles in response to the electric stimuli. The core point of the present work is to maximally isolate the contributions from the interfacial molecules from others by solidifying the hydrogel, while a model approach of the

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dielectric relaxation remains to be done in future.

Finally, we have to mention what are the interfacial polarizable molecules. Are they from

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the chemically bonded units between water and chitosan or bridged by acetic acid. These issues deserve for future investigation using advanced techniques and theoretical calculations.

IV. Conclusion

In conclusion, we have measured the dielectric constant of chitosan ice-hydrogels over

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the frequency from 101~106 Hz in order to investigate the dielectric relaxation of the chitosan-water interfacial polarizable molecules. It is revealed that the dielectric responses of the hydrogel with ice droplets in a finite T-window (~ 30 K) below the water solidifying point

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mainly come from the interfacial polarizable molecules, while the dielectric responses from both the ice molecules and chitosan chains are weak in this T-window. The thermal activation energy for the interfacial polarizable molecules has been estimated to be ~ 0.93 eV (~ 89.73

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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 work suggests that the chitosan-water interfacial polarizable molecules have stronger dielectric rigidness and larger polarization than the chitosan chains and water molecules.

Acknowledgement: This work was financially supported from the National Key Research Program of China (Grant Nos. 2016YFA0300101 and 2015CB654602), and the National Natural Science Foundation of China (Grant Nos. 51431006 and 51721001). 14

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Figures & Figure Captions

Figure 1. A sketch of chitosan macro molecular structure (a) and a chitosan chain surrounded

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by counterions (b). The microscopic morphology of chitosan hydrogels is schematically drawn in (c) just for a guide of eyes. (d) The polarizable chitosan macromolecules, chitosan-water interfacial polarizable molecules, and water (ice) molecules are viewed as electric dipoles (pch, pint, and pw), and the dielectric responses of chitosan hydrogels are treated as the responses of these dipoles. Here Tw is the solidifying point of water (droplets) in hydrogels.

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Figure 2. The SEM images at three different magnifications for the as-prepared chitosan hydrogel sample (h = 0.93), where the water droplets filled in the holes were evaporated in the

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cryogenic environment. The statistically uniform porous structure is identified and the typical size of the holes is ~ 4.0 µm. The hydrogel structure can be well viewed as a composite where

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sphere-like water droplets are embedded in the matrix of chitosan polymer.

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(a)

-3 -6

Tw

h = 0.93

-9 -100

-80

-60

-40

-20

0

T ( C)

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CS-g-PAA

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3.5

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Intensity (arb. unit)

o

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0

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DSC (mW/mg)

3

2.0

x

1.5 3

1.0

0.5

-1

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Wave-number ( 10 , cm )

Figure 3. (a) The measured DSC curves in the cooling and then heating cycle at a rate of 20 K/min, with arrows indicating the cycle. The measured FTIR spectra for pure chitosan (CS), acetic acid (AA), and the as-prepared hydrogels (CS-g-PAA) are presented in (b), see text for details.

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100

T = 273 K (a) water

60 40 20

ice

0

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ε′

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101 102 103 104 105 106 107 108 109 50 (b) 200K 40 270K 275K 30 Pure chitosan 20

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ε′

80

0

102

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10 103

104

105

106

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f (Hz)

Figure 4. The dielectric real part ε′( f ) for water and ice at 273 K (a), where the data are taken

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from Ref. [54], and for dry chitosan powder at three different temperatures (b).

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f (Hz)

106

251 25k

630 160k

(a)

= 89.4 kJ/mol 8

4

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ε′

10

Ea = 0.93 eV

105 10

1.6k 1.0M

6

103 102

4

(d)

101

3.8

4.0

4.2

ln( f ) (ln( Hz ))

100 10k

4.4

-1

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ε″

103 10

ε′

(e)

1

Tw

20 0

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(c)

Tm

ax

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tanδ

60

102 103 104 105 106

105 104 103 102 101

f (Hz)

10-1 80

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ε″

105

ε′, ε″

(b)

220 230 240 250 260 270 280

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T (K)

102

(f)

101

fmax 102 103 104 105 106

100

tanδ (T→Tw-0)

10

1000/Tmax (K )

7

f (Hz)

Figure 5. (a) ~ (c): The measured dielectric real part ε′, imaginary part ε″, and dielectric loss tanδ = ε″/ε′ at several frequencies as a function of temperature T for the as-prepared hydrogel sample (h = 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 evaluated frequency f .vs. Tmax relation from which the thermal activation energy Ea is evaluated via the Arrhenius law. (e) and (f): The evaluated ε′, ε″, and tanδ at T → Tw -0 as a function of frequency f.

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ACCEPTED MANUSCRIPT In this work, we investigate the dielectric relaxation spectroscopy of the chitosan ice-hydrogels, which demonstrates several novelties as highlighted below: (1) An experimental strategy to investigate the dielectric properties of chitosan ice-hydrogels has been developed.

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(2) The dielectric response of chitosan hydrogels below the water solidifying point (ice-hydrogels) has been measured so that the properties of the chitosan-water interfacial molecules can be partially probed.

(3) The thermal activation energy for the relaxation of chitosan-ice interfacial polarizable

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molecules has been evaluated.

ACCEPTED MANUSCRIPT Mr. Yongqiang Li, Graduate student of Nanjing University, E-mail: [email protected] Mr. Chenxiao Zhang: Graduate student of Nanjing University, E-mail: [email protected] Mr. Ping Jia: Graduate student of Nanjing University, E-mail: [email protected] Yuan Zhang: Graduate student of Nanjing University, E-mail: [email protected]

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Dr. Lin Lin: Senior research staff of Nanjing University, E-mail: [email protected] Dr. Zhibo Yan: Associate Professor of Nanjing University, E-mail: [email protected]

Corresponding author: Jun-Ming Liu

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Mr. Xiaohui Zhou: Senior engineer of Nanjing University, E-mail: [email protected]

Jun-Ming Liu: Professor of Nanjing University, Nanjing 210093, China

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Phone: 86-25-83596595, E-mail: [email protected]

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Mr. Yongqiang Li

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Mr. Ping Jia

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Mr. Chengxiao Zhang

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Mr. Yuan Zhang

Dr. Lin Lin

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Mr. Xiaohui Zhou

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Dr. Zhibo Yan

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Dr. Junming Liu