Insights on Water Dynamics in the Hygromorphic Phenomenon of

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Insights on Water Dynamics in the Hygromorphic Phenomenon of Biopolymer Films Santhosh Mathesan, Amrita Rath, and Pijush Ghosh J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00980 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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The Journal of Physical Chemistry

Insights on Water Dynamics in the Hygromorphic Phenomenon of Biopolymer Films Santhosh Mathesan a; Amrita Rath a; Pijush Ghosh a* a

Nanomechanics and Nanomaterials Laboratory, Department of Applied Mechanics & Soft Matter Center, Indian Institute of Technology Madras, Chennai - 600 036, India.

ABSTRACT Water responsive biopolymer thin films with engineered matrix characteristics can accomplish desirable shape changing properties such as self-folding. Self-folding response of thin film is experimentally characterized by its total folding time and rate of folding. Here, atomistic simulation is employed to investigate the molecular mechanism responsible for modified self-folding behaviour observed in nanoparticle reinforced chitosan films. The nanocomposite system is solvated with water content varying from 10 % to 100% of total mass of the system. The free volume available for diffusion of water molecules is affected by the flexibility of glycosidic linkages present in chitosan chains. The increase in mobility of water molecules with increase in water content decides the rate of folding. A separate molecular system is modelled with confined region between nanoparticles densified with chitosan chains and water molecules. The thickness of confined region is determined from the critical distance of influence of nanoparticles on water molecules. The adsorption of water on nanoparticle surface and relaxation of chitosan chains are responsible for increased total folding time with nanoparticle concentration. This simulation study, complemented with experimental observations provides a useful insight into the designing of actuators and sensors based on the phenomenon of hygromorphism.

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1. INTRODUCTION Most of the biological systems in nature are water responsive materials with a unique molecular structure to perform specific biological phenomena to survive in different ecological conditions. Moisture responsive phenomenon is adopted in design of various mechanical actuators, soft grippers and humidity sensors1–3. However, incorporation of moisture responsive phenomenon in biomedical applications require specific materials which are biocompatible and biodegradable such as biopolymers/hydrogels4. The main drawback of biopolymers is lack of adequate mechanical properties to be used as an actuator or biosensor. The mechanical properties of biopolymers can be enhanced by either introducing nanofillers into polymer matrix or by forming cross-links between the polymer chains5,6. Self-folding is one of the prominent responses observed in water responsive biopolymers. The diffusion of water molecules maintains the film in a differentially stressed state due to inhomogeneous swelling within the biopolymer film giving rise to folding behavior7. Therefore, the top layer of biopolymer film with less or negligible water content is under compressive strain while the bottom layer with higher water content is under tensile strain. The diffusive nature of water molecules are modified due to the mechanical strain incorporated during self-folding phenomenon, confinement effect of nanoparticles in nanocomposite films and mechanical properties of biopolymer films. However, it is observed that the mechanical properties of biopolymers decrease with water content, primarily due to loss of interactions between polymer chains8. Therefore, self-folding is a complex interplay between diffusion and mechanical properties of biopolymer films. In recent years, researchers have focused on design and synthesis of various water responsive films with self-folding capability for specific applications9. However, the molecular origin which leads to self-folding behaviour is yet to be fully explored. There is a need for a complete understanding of the molecular level driving mechanism for effective utilization of the biopolymers in futuristic applications. This will lead to an efficient design and development of ACS Paragon Plus Environment

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biopolymers with tailor-made properties. The comprehensive understanding is highly challenging as the self-folding phenomenon is a consequence of various processes that occur at different length and time scales. Here, systematic experiments and detailed atomistic simulations were performed to correlate the macroscopic self-folding behaviour with atomistic interactions observed in hydrated biopolymer nanocomposites. The self-folding behaviour is experimentally characterized by means of response rate and total folding time. Zhao et.al have developed a polymer actuator driven by solvent sorption with a higher response rate in the presence of pores10. Alexander et.al have reported a hygromorphic composite bilayer made of active/passive layers and observed that the response rate of composite bilayer depends on the diffusion coefficient of active PVA layers11. Thus, diffusion characteristics of biopolymers determine the response rate, which is a prime factor in the design of water responsive biopolymer actuators12,13. Molecular Dynamics (MD) simulations have been recently used to probe into the water dynamics in biopolymer nanocomposites. However, the current challenge lies in utilizing MD simulation in better understanding of the diffusion mechanism and its connectivity with hygromorphic transformations such as self-folding in the presence of nanoparticles. For instance, Zhao et. al have reported the formation of water layers with specific orientation around hydrophilic hydroxyapatite nanoparticle surface via physical interactions, which restricts the mobility of water molecules present in first hydration layer14.

Bai et.al have reported the

dependence of diffusion behaviour of water molecules in PVDF/silica nanocomposite on particle size, particle concentration, mobility of PVDF chains and fractional free volume in the membrane15. The water molecules tend to have different affinities toward different functional groups present in the polymer matrix. Also, the diffusion coefficient of water molecules increases with water content and it is explained in context of free water and bound water16–19. However, the impact of diffusion of water molecules on the self-folding properties of hydrophilic nanoparticle reinforced biopolymer films is still unclear which will be elucidated here. ACS Paragon Plus Environment

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In this work, the diffusion dynamics of water molecules in (a) the confined space between nanoparticles and (b) the biopolymer/nanocomposite matrix with different water content, is used to address the mechanisms responsible for macroscopic folding behaviour. Chitosan, a biocompatible and biodegradable biopolymer is selected to represent a material exhibiting biomimetics self-folding behavior20,21. Hydroxyapatite nanoparticle is reinforced in the chitosan biopolymer matrix to obtain significant improvement in the mechanical properties6. Hydroxyapatite is a hydrophilic bioceramic material found in mammalian bones and teeth22.This manuscript describes the experimental characterization of self-folding phenomenon, effect of flexibility of chains on fractional accessible free volume available, confinement effect on water molecules which affects variation of total folding time and effect of different water content in biopolymer matrix which affects the response rate of CS and CSHAP films. 2. METHODS 2.1. SAMPLE PREPARATION Chitosan powder (Degree of deacetylation > 90%, viscosity-(100-200) cps, medium molecular weight), were supplied from SRL Pvt. Ltd. (India). Hydroxyapatite nanopowder, < 200 nm size (BET), Glutaraldehyde (25% aqueous solution), and Acetic acid, > 99.7 % were purchased from Sigma Aldrich (India, Bangalore). Pure CS, CS with 5% HAP (v/v) and CS with 20% HAP (v/v) films were prepared by solvent casting method as described in our previous work5. They are name as CS, CSHAP_5 and CSHAP_20 respectively. 2.2. FILM PREPARATION FOR FOLDING The films with 80 µm thickness were cut in the dimensions of 0.15 cm x 7 cm. The films were kept in between two glass slides at ~ 60°C for 2 hours to remove the residual stress and ensure flatness of the films before use. The cut samples were placed on top of water droplets touching only the bottom surface of the film as shown in Figure 1(a). The experimental setup used to capture the self-folding phenomenon is given in supplementary information (page no. S1). ACS Paragon Plus Environment

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2.3. RESPONSIVE CHARACTERISTICS OF SELF-FOLDING FILMS 2.3.1. TOTAL FOLDING TIME AND RATE OF FOLDING The folding of the film is accomplished within a certain time span defined by total folding time. It is the time interval between the first contact of the film with water till both ends of the film come in contact during the folding process. During self-folding phenomenon, the tip of the film undergoes a gradual change in its angle measured with the change in time. The ratio of the change in the angle and time is defined as the rate of the folding as shown in Figure 9 (a). 2.3.2. KINETICS OF SWELLING The swelling behavior of three types of chitosan films is studied by gravimetric method. The films are cut into disk shape of approximate weight 3 mg and immersed in the water bath. The weight of the swollen films after removal from water bath is measured at successive time intervals till it reaches the equilibrium swelling condition. The degree of swelling (S), diffusion coefficient (D) of water in films were determined as explained elsewhere23. 2.4. MOLECULAR DYNAMICS SIMULATIOS 2.4.1. MODELLING PROCEDURE Molecular

dynamics

simulations

were

performed

on

pristine

chitosan

and

chitosan/hydroxyapatite nanocomposite in the presence of water molecules. The pristine CS consists of five chitosan chains with a chain length of 25 monomer units. The chains were packed in a box via PACKMOL24. The nanocomposite system consists of three chitosan chains and Hydroxyapatite nanoparticle with density close to pure chitosan system. Hydroxyapatite (HAP) crystal has a hexagonal unit cell with space group P63/m. The hydration degree in pristine chitosan and chitosan nanocomposite systems were modified by varying the number of water molecules i.e. water content (10%, 30%, 50%, 70%, 100%) in proportion with the mass of chitosan system. TIP3P-Modified water model was used to simulate the hydrated conditions25. Chitosan nanocomposite systems with two different inter-particle spacing were modeled. The confined space between nanoparticles surfaces are 8Å and 12Å. The confined space was filled ACS Paragon Plus Environment

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with two chitosan chains and water molecules to match the density obtained from 100% hydrated chitosan system. 2.4.2. SIMULATION PROCEDURE LAMMPS was employed to perform Molecular Dynamics simulations26. The molecular systems were parameterized using CHARMM force field27. Periodic boundary conditions were enabled and a cutoff of 10 Å was used for both columbic and van der Waals interactions. Time step of 0.5fs was adopted in MD simulations. The data was recorded from the 10 ns simulation time for post processing. All simulations were run for 5 times and the average information is provided here. VMD was used to view the trajectory information28. The comprehensive details of the simulation setup are provided in supplementary information (page no. S2,S3). 3. RESULTS AND DISCUSSION 3.1. SELF-FOLDING OF BIOPOLYMER FILM Chitosan film shows self-folding behavior when its bottom surface is exposed to water as shown in Figure 1 (c), (d) & (e). The self-folding behaviour in response to water is due to differential swelling resulting in a strain gradient across the thickness of a film. The differential swelling is achieved due to varying water content across the thickness of film as shown in Figure 1(b). The swelling in turn is a result of the diffusion of the water molecules in the polymer matrix. Thus, the interaction of the water molecules with the matrix system affects the overall folding behavior, which is reflected in the total folding time and rate of folding of the film. The total folding time for CS, CSHAP_5 and CSHAP_20 film is calculated as 22.4 ± 0.9 s, 26.87 ± 2.57 s and 30.03 ± 0.28 s respectively. It is observed that the water content and the presence of nanoparticles in chitosan matrix are responsible for swelling characteristics and overall folding behavior of chitosan films. A detailed study on swelling characteristics and diffusion coefficient in CS and CSHAP films were performed. The experimental video showing self-folding of CS film is provided in Supplementary video (V_1).


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Figure 1. Schematic representation of (a) chitosan film placed on water surface (b) varying water content and diffusion across the thickness of a nanoparticle reinforced chitosan film. Images from experiment showing (c) initial folding (d) intermediate folding (e) final folding. 3.2. DIFFUSION COEFFICIENT (D) The equilibrium swelling percentage of pristine CS film is observed to be maximum followed by CSHAP_5 and CSHAP_20 films. A similar trend is observed in case of diffusion coefficient of water molecules as mentioned in Table 1. The atomistic mechanisms responsible for reduced diffusion coefficient in the presence of HAP nanoparticle is addressed with the aid of MD simulations. Table 1.Experimental values of ‘S’ and ‘D’ of CS, CSHAP_5 and CSHAP_20 films.

Film Type

S (%)

D (cm2/s)

Pure chitosan (CS) Hydroxyapatite chitosan (CSHAP_5) Hydroxyapatite chitosan (CSHAP_20)

10171.15

0.71 ± 0.18 X 10-5

4371.6

0.34 ± 0.13 X 10-5

161.54

0.23 ± 0.0 X 10-5


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The diffusion coefficient (D) of water molecules from MD simulation is calculated from the slope of the plot between mean square displacement (MSD) and time obtained from Einstein relation as mentioned in Equation. 1.

= →

〈 〉

(1)

where n represents dimensionality of the system , r(t) and r(0) are position vectors of oxygen present in water molecules at time t and 0, respectively, averaged over all the water molecules and simulation time. Mean square displacement of water molecules is tracked over a period of 10ns. The initial part of the plot up to 500ps is not considered while determining the diffusion coefficient because of the inertial effects29. The CSHAP system addressed in this section is comparable to biopolymer nanocomposite with uniform dispersion of nanoparticles (Single particle study). From MD results, the diffusion coefficient of water in pristine CS and CSHAP system is determined as 0.854 ± 0.03 x 10-5 cm2/s and 0.767 ± 0.04 x10-5 cm2/s respectively. The diffusion coefficient of water in chitosan obtained from MD simulations and experiments (Table 1) are close to the ‘D’ values reported in the literature30. It is observed that the diffusion coefficient of water molecules in the presence of HAP particles is less compared to pure CS system. The diffusion of water molecules is influenced by the accessible free volume and availability of hydrophilic reactive sites in the polymer nanocomposite matrix. Hence, before addressing the interaction mechanism responsible for diffusion process, it is necessary to investigate the free volume available for penetrate molecules. 3.3. FREE VOLUME The free volume is created in amorphous polymer matrix due to incomplete packing of polymer chains and due to lack of crystallinity. The presence of free volume and its distribution in polymer matrix plays a major role in diffusion process which is further dependent on the size of the solvent molecule. Estimation of fractional accessible free volume (FAV) is carried out by ACS Paragon Plus Environment

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representing polymer chains as hard spheres with van der Waals radii as described elsewhere31. Here, water molecules are modeled as sphere of radius of 1.7 Å.

Figure 2. Variation of Diffusion Coefficient (D) of water over time for CS & CSHAP system. The variation of diffusion coefficient of water molecules in CS system as shown in Figure 2 is attributed to varying FAV as mentioned in Table 2. The increase in value of D is due to higher FAV in chitosan system. Interestingly, in case of CSHAP system, the fluctuations in FAV values do not influence the drop in diffusion coefficient of water molecules over time as indicated in Figure 2. Table 2. Fractional accessible volume for CS/water and CSHAP system at different simulation time. FAV System 3 ns

4 ns

5 ns

6 ns

7ns

8ns

9ns

10ns

CS/Water

0.38197

0.38175

0.39117 0.39518 0.3733

CSHAP/Water

0.384

0.3812

0.36531 0.36604 0.37092 0.36422 0.36047 0.34456

0.38331 0.39514 0.37994

The water molecules in the vicinity of HAP surfaces are termed as “bound water”. The water molecules which are far away from the influence of HAP surfaces are referred as “free water” (Refer Figure 1). Diffusion of water molecules assists in desorption of chitosan chains from HAP surfaces. This results in the increase in number of bound water molecules and reduction in free water. The free water has a significant contribution to diffusion coefficient ACS Paragon Plus Environment

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rather than the bound water which have lesser mobility. The reduction in free water decreases the diffusion coefficient irrespective of change in FAV. However, the significant influence of FAV could be experienced when most of the reactive sites in nanoparticles get occupied. As understandable, the conformational behavior of polymer chains is the main source of variation in FAV in a polymer matrix and thus necessary to be studied in detail. 3.4. CONFORMATIONAL ANALYSIS The free volume distribution is dependent on the conformational flexibility available for the glycosidic linkages in polysaccharides. The main chain conformation torsion angles are phi (φ) and psi (ψ) which explains the relative orientation of glycosyl residues in the same. The conformation of primary hydroxymethyl group at 6-position is given by chi (χ) as shown in Figure 3.

Figure 3. Chemical structure of chitosan system with dihedral angles phi (φ), psi (ψ) and chi (χ). Hydrogen bond formation between water molecules and hydroxyl methyl/amine group. Number 1- 6 represent the carbon position in the ring. O3 in OH group at 3-position tend to form hydrogen bond with O5 to maintain the helicity of chitosan films. (Color code, carbon-yellow, oxygen-blue, nitrogen-gray, hydrogen-white, oxygen in water-red. Hydrogen bond is shown in red dotted lines).


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The presence of amine group (2-position), hydroxyl (3-position) and hydroxymethyl groups (6-position) are responsible for inter/intra molecular forces within chitosan network structure32. The hydroxymethyl group located at the periphery of polymer chains assists in the formation of inter-molecular hydrogen bonds. Nevertheless, it should be stated that various combinations of inter and intra hydrogen bonds are possible based on the adjacent reactive sites. Kirschner et.al suggested that water plays a crucial role in disrupting the hydrogen bond network within the carbohydrates33. The variations of torsion angles (φ, ψ and χ) in chitosan chains are responsible for evolution of FAV. Here, the effect of water on main chain conformation angle phi present in CS and CSHAP systems is investigated. The chitosan structure used in this simulation is amorphous in nature, where the CS chains are held together via entanglements and inter/intra hydrogen bonds.

The relative

orientation of glycosyl residues is analyzed by estimating the conformation angle phi. The frequency of distribution of phi (φ) observed in four systems is shown in Figure 4 (a) & (b) . In CS and CS/Water systems, the dihedral angle phi varies from -180° to 0°, similar to the results obtained by Franca et.al32.

However, in case of a pure CS system, the frequency of phi

population is more between -180° to -120°. The biased behaviour observed in the CS system is due to the entanglements which restrict the flexibility of glycosyl residues. Presence of hydrophilic interactions between CS and water incorporates localized chain relaxation in CS matrix which leads to uniform dihedral distribution. The localized chain relaxation modifies the free volume which in turn affects the diffusion of water molecules in CS matrix.


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Figure 4. Variation of main chain conformation angle (phi) in (a) CS and CS/water system, (b) CSHAP and CSHAP/Water system. Diffusion of water molecules in CS and CSHAP matrix redistributes the dihedral angle phi due to the formation of hydrogen bonds. In case of CSHAP system, it is observed that the dihedral distribution varies from -180° to 180° due to the adsorption of chitosan chains on the planar HAP surfaces. However, majority of angles lie within -180° to 0°, due to the chains being away from HAP surfaces. Here, the water molecules are attracted towards the HAP surface via electrostatic attraction and hydrogen bond formation14. Some of the chitosan chain segments are desorbed from HAP surfaces due to diffusion of water molecules, which is reflected in the reduction of dihedrals closer to -180° and 180° as shown in Figure 4 (b). The flexibility of desorbed chitosan chain segments is more compared to those adhered to the HAP surfaces which modify the FAV in CSHAP matrix.

Figure 5. Schematic representation of (a) the inaccessible free volume between two chitosan chains, (b) diffusion of water molecule between two chains. The change in dihedral angle phi ACS Paragon Plus Environment

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disturbs the hydrogen bond (O6--HO3) which increases the fractional accessible free volume. (Color code: Blue, Brown, Green- van der Waals radius of water molecules, Oxygen and Hydrogen respectively. A specific interaction from simulation is adopted to address the role of dihedral angle (phi) in modifying the FAV as shown in Figure 5 (a) (Snapshot from simulation is shown in Supplementary Information, page no. S4). Here, chitosan chain segments (1 and 2) interact via hydrogen bond formation (O6--HO3). The van der Waals radius of oxygen (RO) is 1.5 Å and Hydrogen (RH) is 1.2 Å. A water molecule is considered to be a particle of radius (RW) 1.7Å34. Under normal energy condition, if a water molecule has to diffuse through the chitosan chain segments (1 and 2), it requires a minimum space which can accommodate a particle of radius RW. Due to the formation of hydrogen bond, the space between O6 and HO3 remains inaccessible for the water molecule to diffuse through the chains. This is termed as inaccessible free volume as mentioned in Figure 5 (a). Absence of hydrogen bond (O6--HO3) in system 2 is due to the change in dihedral angle phi. Thus, it facilitates the enhancement of FAV allowing the water to diffuse through chitosan chain segments as shown in Figure 5(b). Hence, diffusion of water molecules is primarily influenced by FAV which is modified by the change in main chain conformation angles due to the chitosan/water interactions. However, diffusion behaviour of water molecules in CSHAP matrix is predominantly influenced by the adsorption sites in HAP. The water molecules are attracted towards HAP surface via electrostatic interactions and hydrogen bond formation which leads to the formation of hydration layer on the HAP surface. The resistance offered by hydrophilic sites in HAP is reflected in the diffusion coefficient of water molecules which in turn affects the total folding time of nanoparticle reinforced biopolymer films. The total folding time of biopolymer films increases with HAP concentration. Therefore, it is essential to investigate the water dynamics at various concentrations of nanofillers which leads to increased total folding time. 3.5. INTERPARTICLE STUDY ACS Paragon Plus Environment

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In a biopolymer nanocomposite system, inter-particle spacing plays a major role in determining the overall macroscopic properties of nanocomposites35–37. Inter-particle spacing decreases with percentage loading and it can be applied to assess the dispersion behavior of nanoparticles in a polymer matrix. Here, the effect of inter-particle spacing on the diffusion of water molecules in biopolymer nanocomposites is assessed. Influence zone of the nanoparticle is defined as the region around nanoparticle beyond which a nanoparticle does not have significant influence on polymer or water molecules. It is indicated as dotted lines around the nanoparticles as shown in Figure 6. In case of higher percentage of filler loading, it is expected to have three possible arrangements of nanoparticles based on the influence zone. They are, (I) the influence zones of two nanoparticles overlap each other, (II) the influence zones of nanoparticles are close to each other but do not overlap and (III) one nanoparticle is far away from the influence zone of another nanoparticle. Since, the case (III) is already discussed in single particle study; the other two arrangements of nanoparticles are analyzed here. The inter-particle spacing is selected primarily based on the radius of the solvation shell obtained from the single particle study. To simulate case (I) and (II) as mentioned above, the nanoparticles are separated by a distance of d1 and d2 respectively as shown in Figure 6.


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Figure 6. Schematic representation of nanocomposite with two inter-particle spacing (d1 and d2).

Case I represents a situation in nanoparticle reinforced biopolymer matrix where the

influence zones of nanoparticles overlaps each other. In case II situation, the influence zones are close to each other but do not overlap. Radial Distribution Function (RDF) plot between Ca2+ ion in HAP nanoparticle and oxygen in water obtained from single particle study indicates a sharp peak at 2.45 Å and a second broad peak, which ends at 4.25Å.This corresponds to first and second solvation layers respectively. The inter-particle spacing of 8Å(d1) denoted as case-I, corresponds to both the minimum separation distance with chitosan chains in-between the HAP surfaces and the overlapping of influence zones. The inter-particle spacing of 12Å(d2) denoted as case-II, corresponds to a system that favors the segmental motion of chitosan chains as well as diffusion of water molecules. 3.5.1. DIFFUSION OF WATER IN CONFINED SPACE BETWEEN NANOPARTICLES The confined space between HAP nanoparticles is divided into three to four solvation layers of thickness approximately 2.5Å-3Å, to understand the influence of confined surface on water molecules as shown in Figure 7 (a). The diffusion coefficient values of water molecules in ACS Paragon Plus Environment

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all three directions (X, Y & Z) for case-I and case-II are mentioned in Table 3. Here, X-direction in axes convention is along the inter-particle spacing as shown in Figure 6. In the first layer, as indicated in Table 3, the mobility of water molecules along X direction is very less compared to Y and Z direction. The water molecules are not likely to be allowed to move from the first layer to second layer due to strong physical interactions between water molecules and HAP surface. Thus, the mobility of water molecules along X direction is very less compared to other directions. However, the diffusion of water in the first layer is more favorable in Y & Zdirection. This is because of the water molecules which are allowed to swap the reactive sites (Ca2+ and PO43- ions) on the HAP surface in YZ plane. The above mechanism is true for water molecules in the first layer of both the systems (case-I and case-II). Table 3. Diffusion coefficient of water molecules for inter-particle spacing of 8Å (case-I) and 12Å (case-II) in all three dimensions. D x 10-5 (cm2/s) Layer

Separation distance (8Å)

Separation distance (12Å)

X

Y

Z

X

Y

Z

First

0.015

0.045

0.042

0.043

0.057

0.136

Second (Intermediate)

0.061

0.315

0.184

0.137

0.430

0.782

The intermediate water in case-I tend to have reduced mobility due to the influence from both the nanoparticles. This results in the formation of water bridge between surface-1 and surface-2 as shown in Figure 7 (b). In case-II system, the intermediate water molecules present at the end of influence zones are significantly influenced by chitosan chains rather than the water molecules in first solvation layer. Thus, it has higher mobility compared to intermediate water in case-I.


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Figure 7. (a) Schematic representation of different water layers for case-I and case-II (b) Snapshot from simulation showing water bridge between two nanoparticles with inter-particle spacing of 8Å (d1). The water bridge is responsible for reduced mobility of water molecules in case-I system. The relaxation of polymer chains has an important role in determining the diffusion of water molecules. In case-I system, the chitosan chains are strongly adhered to HAP surfaces whose segmental motion is restricted. Therefore, the free volume distribution remains unaltered and it does not assist in diffusion of water molecules. In case II system, due to availability of space there is an increase in segmental motion of chitosan chains, which is confirmed from mean square displacement of chitosan chains. The confinement effect of HAP nanoparticles on water molecules can be used to explain the increase in total folding time with HAP concentration. 3.5.2. EFFECT OF HAP CONCENTRATION ON TOTAL FOLDING TIME AND CRITICAL WATER CONTENT The time necessary for complete or total folding increases in the following order i.e., CS < CSHAP_5 < CSHAP_20. This is due to decrease in diffusion of water molecules which decreases with nanoparticle concentration. The minimum amount of water absorbed by biopolymer films to exhibit complete folding is termed as critical water content which is determined from experiments. It is calculated as 136.03±6.07 %, 85.51±1.56% and 96.21±2.40 for CS, CSHAP_5 and CSHAP_20 films respectively. Irrespective of the critical water content, the three films are qualitatively expected to have same strain gradient across the thickness to perform complete folding process. In case of CSHAP_5 films the differential swelling necessary ACS Paragon Plus Environment

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to induce strain gradient across the thickness of film is achieved with lesser water content. This is due to dominance of hydration of HAP nanoparticles in bottom layer, with increased swelling compared to top layer. However, at higher percentage of filler loading the water molecules get trapped in the confined space between two HAP nanoparticles in CSHAP matrix. Therefore, CSHAP_20 films require higher critical water content compared to CSHAP_5 films to achieve significant swelling in the bottom layer necessary for the folding phenomenon. Thus, interparticle spacing i.e. nanoparticle concentration plays a major role in controlling the diffusion of water molecules in biopolymer nanocomposite system. 3.5.3. HYDROGEN BOND DYNAMICS The diffusion behaviour of water in confined space is influenced by the formation and dissociation of hydrogen bonds within water molecules and water molecules/nanoparticles which affects the self-folding behaviour. The dynamic nature of hydrogen bonds with different acceptor /donor pair can be analyzed using an intermittent hydrogen bond correlation function as mentioned in Equation 2. = 〈

∑ ℎ ℎ + 〉 ∑ ℎ

(2) where hij is one when a particular pair of acceptor/donor meets the geometric criteria for hydrogen bond formation at time t and otherwise, hij is zero. The intermittent correlation function C(t) explains the probability that the hydrogen bonded molecules at time t=0 remains hydrogen bonded at time t irrespective of a possible breaking and formation at any intermediate time. Following this, the obtained data is fitted with a multi exponential function to obtain the dynamics of hydrogen bond and the lifetime of hydrogen bonds38. The intermittent hydrogen bond dynamics, C(t) over time is shown in Figure 8 (b).


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Figure 8. (a) Schematic representation of hydrogen bond formation considered for calculating the hydrogen bond correlation, (b) intermittent hydrogen bond correlation functions. (O- Oxygen in water molecules, OP- Oxygen in PO43- ions, L1-first layer, L2-second layer). In the first layer, the intermittent hydrogen bond correlation function is calculated for oxygen (O) in water molecules and oxygen (OP) in PO43- ions as shown in Figure 8 (a). It is observed that, the hydrogen bond (O_OP) tends to sustain throughout the simulation time with a higher hydrogen bond lifetime in both cases as shown in Figure 8 (b). The correlation function C(t) determined for hydrogen bonds between water molecules (O_O_L1) decays slowly which indicates stable hydrogen bond network in the first layer of case-I and case-II. However, the relaxation of C(t)for hydrogen bonds within intermediate water molecules (O_O_L2_12Å) of case-II system is faster due to the reduced influence of nanoparticle on the diffusion of water molecules when compared to case-I system. Therefore, the correlation function decays faster with increase in the inter-particle spacing i.e, filler concentration. Hence, the hydrogen bonds within the water molecules are unstable, which enhances the mobility of water molecules in the confined space as mentioned in Table 3. Experimentally, it is observed that the diffusion coefficient of water molecules decreases with increase in nanoparticle concentration as reflected in self-folding behaviour. In addition to these observations, it is necessary to describe the


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diffusion behaviour in biopolymers with different water content to address mechanism responsible for rate of change of folding angle in the presence of nanoparticles. 3.6. BIOPOLYMER WITH DIFFERENT WATER CONTENT Biopolymers exhibit self-folding behaviour due to inhomogeneous swelling within the polymer matrix which creates a water gradient across the thickness as shown in Figure 1(b). As mentioned earlier, folding behaviour of biopolymers is characterized from total folding time and rate of change of folding angle. During the self-folding phenomenon, the angle between the tip of the film and x-axis increases gradually with time. The rate of change of folding angle (dθ/dt) for pure CS, CSHAP_5 and CSHAP_20 is shown in Figure 9(c). In case of CS films, dθ/dt increases till the end of folding, whereas for CSHAP films it increases to a peak value and then it decreases. The variation of dθ/dt and the difference observed between CS and CSHAP are primarily due to varying water content across the thickness of film as shown in Figure 9 (b). This motivated us to perform the MD simulations to observe the behaviour of chitosan systems with different water content.

Figure 9. (a) Schematic to represent angle theta, (b) Chitosan film is split into different layers with different water content. ‘i’ varies from 1 to n, Li represents ith layer, Wi represents water content in ith layer, (c) Variation of rate of change of folding angle for CS, CSHAP_5 and CSHAP_20 films. The water content in different layers of the film is responsible for varying rate of change of folding angle. ACS Paragon Plus Environment

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During the self-folding phenomenon, the hydrophilic sites present in the CS film play a significant role in attracting the water molecules towards it. The water molecules try to occupy the available reactive sites present at the bottom layer of CS films (i=1). This is followed by continuous diffusion of water molecules towards the top layer of CS film. It can be assumed that the entire film can be split into infinitesimal layers (Li) and each layer will be occupied with different water content (Wi) as shown in Figure 9 (b). The diffusion behaviour of water in layer Li with specific water content Wi is assessed applying Molecular Dynamics Simulations.

Figure 10. Variation of diffusion coefficient of water molecules in CS and CSHAP system as function of water content. 3.6.1. VARIATION OF DIFFUSION COEFFICIENT WITH WATER CONTENT In order to study the diffusion characteristics of water in CS and CSHAP systems, each system is solvated with different water content. The diffusion coefficient of water molecules as a function of water content in CS and CSHAP systems is shown in Figure 10. It is observed that the diffusion coefficient increases as water content increases for both CS and CSHAP systems. The variation of diffusion coefficient values obtained for different water content can be used to address the mechanism responsible for variation in dθ/dt. As soon as the film is placed in water medium, the water molecules get adsorbed to the layer Li-1. These water molecules diffuse through layer Li-1 hydrating the reactive sites via hydrogen bond formation. They are termed as bound water and represented as black dots in Figure 9 (b). The mobility of bound water is much ACS Paragon Plus Environment

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lesser compared to the free water molecules that diffuse to the adjacent layer Li which is indicated as red dots in Figure 9 (b).The number of water molecules penetrating layer Li-1 increases over folding time. Therefore, the water content Wi-1 present in layer Li-1 is more than Wi in layer Li. Since the diffusion coefficient of water molecules increases with water content, layer Li-1 is expected to undergo higher swelling compared to layer Li. The difference in swelling behaviour observed between Li-1 and Li assists in self-folding of the CS film and induces an outof-plane curvature in the film. Thus, swelling of each infinitesimal chitosan layers is reflected in the increase in dθ/dt over folding time as indicated in Figure 9 (c). The diffusion mechanism addressed here is applicable for any two consecutive biopolymer layers with different water content. The process of hydrating the reactive sites and increase in number of free water molecules continues until all the layers get completely saturated. The absolute number of water molecules in each layers (Li) influences the total diffusion coefficient, finally affecting the folding behaviour of films. However, the absolute number of water molecules in each layer of CSHAP films is modified due to the presence of nanoparticles. This results in altered behaviour of dθ/dt with respect to CS films as shown in Figure 9 (c). Radius of gyration is applied to understand the swelling behaviour of chitosan system with different water content. The radius of gyration increases with water content due to the increase in number of free water molecules. Thus, each of the biopolymer layers (Li) with specific water content (Wi) tend to have different swelling behaviors which are finally demonstrated as self-folding. 4. CONCLUSIONS In this work, the molecular mechanisms responsible for self-folding behaviour of biopolymer film in the presence of nanoparticles were addressed with the aid of experiments and systematic molecular dynamics simulations. Experiments were performed to characterize the self-folding behaviour of biopolymer films in the presence of nanoparticles in terms of total folding time and response rate. Also, the swelling kinetics of biopolymer films was systematically investigated.

The total folding time was observed to be increased with ACS Paragon Plus Environment

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nanoparticle concentration in biopolymer films. It was further noticed that the diffusion coefficient of water molecules is reduced in the presence of nanoparticles in biopolymer matrix. Diffusion coefficient of water molecules in biopolymer matrix determined from MD simulations was found in good agreement with the experimental results. Fractional accessible free volume in biopolymer matrix determines the diffusion behaviour of water molecules. The conformational flexibility of biopolymer chains was found to have significant influence on fractional accessible free volume. The presence of hydrogen bonds and electrostatic interactions between water molecules and adsorption sites of hydroxyapatite are responsible for reduced mobility of water molecules in nanoparticle reinforced biopolymer matrix.

The effect of

diffusion dynamics of water molecules in confined space between nanoparticles on the total folding time of nanoparticle reinforced biopolymer films was analyzed.

At low confined

spacing, the water molecules remain entrapped within the confined surfaces through formation of hydrogen bonds. The dynamic behaviour of hydrogen bonds in confined space was inferred by calculating hydrogen bond correlation function. The diffusion behaviour of water molecules in biopolymer matrix with different water content is responsible for varied response rate in the presence of nanoparticles. 5. SUPPORTING INFORMATION The experimental setup used to capture self-folding phenomenon, video demonstrating selffolding of CS film, modelling/simulation details and snapshot from simulation to address the influence of dihedral angle (phi) on FAV. 6. AUTHOR INFORMATION Corresponding author *E-mail: [email protected] Phone: +91-44-2257-4060


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7. ACKNOWLEDGEMENT The authors gratefully acknowledge P.G. Senapathy Center for Computing Resource, IIT Madras for providing the computational resources. The authors declare no competing financial interests. 8. REFERENCES (1)

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Table of Contents


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Figure 1. Schematic representation of (a) chitosan film placed on water surface (b) varying water content and diffusion across the thickness of a nanoparticle reinforced chitosan film. Images from experiment showing (c) initial folding (d) intermediate folding (e) final folding. self-folding



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Variation of Diffusion Coefficient (D) of water over time for CS & CSHAP system


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Figure 3. Chemical structure of chitosan system with dihedral angles phi (φ), psi (ψ) and chi (χ). Hydrogen bond formation between water molecules and hydroxyl methyl/amine group. . Number 1- 6 represent the carbon position in the ring. O3 in OH group at 3-position tend to form hydrogen bond with O5 to maintain the helicity of chitosan films. (Color code, carbon-yellow, oxygen-blue, nitrogen-gray, hydrogen-white, oxygen in water-red. Hydrogen bond is shown in red dotted lines). glycosidic linkages



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Figure 4. Variation of main chain conformation angle (phi) in (a) CS and CS/water system, (b) CSHAP and CSHAP/Water system. Diffusion of water molecules in CS and CSHAP matrix redistributes the dihedral angle phi due to the formation of hydrogen bonds. Variation of main chain confor 61x23mm (300 x 300 DPI)


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Figure 5. Schematic representation of (a) the inaccessible free volume between two chitosan chains, (b) diffusion of water molecule between two chains. The change in dihedral angle phi disturbs the hydrogen bond (O6--HO3) which increases the fractional accessible free volume. (Color code: Blue, Brown, Greenvan der Waals radius of water molecules, Oxygen and Hydrogen respectively. inaccessible free volume



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Figure 6. Schematic representation of nanocomposite with two inter-particle spacing (d1 and d2). Case I represents a situation in nanoparticle reinforced biopolymer matrix where the influence zones of nanoparticles overlaps each other. In case II situation, the influence zones are close to each other but do not overlap. inter-particle spacing


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Figure 7. (a) Schematic representation of different water layers for case-I and case-II (b) Snapshot from simulation showing water bridge between two nanoparticles with inter-particle spacing of 8Å (d1). The water bridge is responsible for reduced mobility of water molecules in case-I system. water bridge



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Figure 8. (a) Schematic representation of hydrogen bond formation considered for calculating the hydrogen bond correlation, (b) intermittent hydrogen bond correlation functions. (O- Oxygen in water molecules, OPOxygen in PO43- ions, L1-first layer, L2-second layer). intermittent hydrogen bond cor


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Figure 9. (a) Schematic to represent angle theta, (b) Chitosan film is split into different layers with different water content. ‘i’ varies from 1 to n, Li represents ith layer, Wi represents water content in ith layer, (c) Variation of rate of change of folding angle for CS, CSHAP_5 and CSHAP_20 films. The water content in different layers of the film is responsible for varying rate of change of folding angle. rate of folding



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Figure 10. Variation of diffusion coefficient of water molecules in CS and CSHAP system as function of water content water content


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33x14mm (300 x 300 DPI)


Insights on Water Dynamics in the Hygromorphic Phenomenon of

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