Stimuli Responsive, Self- Sustainable and Self-Healable

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Surfaces, Interfaces, and Applications

Stimuli Responsive, Self- Sustainable and Self-Healable Functionalized Hydrogel with Dual Gelation, Load-Bearing and Dye Absorbing Properties Muzammil Kuddushi, Sargam Rajput, Ankit Shah, Jitendra P. Mata, Vinod K. Aswal, Omar A. El Seoud, Arvind Kumar, and Naved I Malek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01129 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces Page 1 of 33

Stimuli Responsive, Self- Sustainable and Self-Healable Functionalized Hydrogel with Dual Gelation, Load-Bearing and Dye Absorbing Properties Muzammil Kuddushia, Sargam Rajputa, Ankit Shaha, Jitendra Matab, Vinod K Aswalc, Omar El Seoudd, Arvind Kumare, Naved I. Maleka* aApplied

Chemistry Department, S.V. National Institute of Technology, Surat-395007, Gujarat, India bAustralian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organization, Lucas Heights, NSW 2234, Australia cSolid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India dInstitute

of Chemistry, The University of São Paulo, P. O. Box 26077, 05513-970, São Paulo, SP, Brazil

eSalt

and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, India

ABSTRACT The motivation for designing low molecular weight gelator with self-healing characteristics originates from elegant examples in biology such as vines of the genus Aristolochia, whose internal secondary growth exhibit rapid self-healing in their stems. In the present work, we had explored the stimuli responsive dual gelation characteristics for the ester functionalized surfactant (4-(2-(hexadecyloxy)-2oxoethyl)-4-methylmorpholin-4-ium bromide, C16EMorphBr) in aqueous medium at 7.20 % (w/v) critical gel concentration and at pH 7.4. The hydrogel provides an excellent platform to study dynamic phase behaviour within a supramolecular network as it exhibits transformation from fibrillar opaque hydrogel to transparent hydrogel

upon

heating.

Molecular

interactions,

arrangement

within

the

supramolecular framework and mechanical properties of the hydrogels were characterized using FT-IR, small angle neutron scattering (SANS), rheological analysis, tensile strength test and cyclic loading-unloading test respectively. The fibrillar opaque gel has been characterized for its morphology using scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The

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self-sustained, self-healable porous fibrillar opaque xerogel was further explored for selectively absorbing anionic dye and for its load-bearing characteristics. We conclude a perspective on designing a new age gelators that can open entirely new avenues in environmental protection and wearable ‘smart’ devices. Keywords: Hydrogel, Self-assembly, Thermo-responsive dual gelation, Selfsustainability, Self-healing, dye absorption Introduction Hydrogels comprises the most important and transformable functional class of materials due to their moderate mobility, flexibility and physicochemical properties, that could be fine-tuned selectively through changing the molecular structure of the gelator and/or through various external stimulies.1-2 They are formed through selfassembling the low-molecular weight gelators (LMWGs) in aqueous medium by maintaining a delicate balance between polar and non-polar part of the gelator. The polar part interacts favorably with water molecules and enhances its solubility whereas the non-polar part tends to come closer to each other and minimize their exposure towards water (hydrophobic interaction), thus facilitating self-assembly within these molecules.3,4 The three-dimensional (3D) network structures are responsible for the hydrogel formation through various non-covalent interactions including

hydrogen

bonding,

-

stacking,

hydrophobic,

and

electrostatic

interactions. These interactions could be adjusted with (i) strategic designing of the gelator structure, (ii) medium polarity and (iii) molecular interactions within the components of the geltaor or with external additives.5 Among the spectrum of LMWGs, surfactants are most promising classes of gelator, as they offer tunable structure that offer adjustable properties and applications to the hydrogels.6,7 Surfactant-based hydrogels through entanglement of the alkyl chains forms the 3D network structure in which the water molecules get penetrated through surface as well as capillary forces. Herein, water molecules are penetrated in such a way that apparently no boundaries exist between the 3D network and water molecules.7 Fine-tuning the (i) structure of the alkyl chain, (ii) the


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binding sites and (iii) solution parameters affect this entanglement and so the interwoven 3D structure and the properties of the hydrogels.3,4 Various structural architectures such as biodegradable, natural, functional, gemini and recently studied surfactants with ionic liquid (IL) type characteristics are studied for their ability to form the hydrogels in their pure form as well as in the presence of several additives.8 The inherent ability of the surfactant based gelator to respond external stimuli, such as temperature, pH, electric field, enzymes, and light, change its characteristic properties particularly their shape and size.9-11 Gels that are responsive to the external stimuli are termed as ‘smart gels’ or ‘stimuli responsive gels’. The characteristic feature of these ‘smart gel’ is their ability to transform their character based on their application. Physical and chemical properties of the hydrogels changes abruptly under these external stimuli due to the disturbance within their short range and long range, selective and non-selective, directional and non-directional interactions involved.7 Judiciously selecting these interactions could form the hydrogels that offer stimuli responsiveness with tailor made properties and could be applicable in variety of the fields including catalytic reactions, to deliver the drugs irrespective to the hydrophobiicty, in tissue engineering as scaffolds, nanoparticles/nano-clusters synthesis as the templates, for the enhanced oil recovery, materials for biomedical applications, and in various opteo-electronic sensors to name a few.12,13 Temperature responsive hydrogels forms the most promising class within the available literature of ‘smart gels’.14-22 With increasing the temperature, water molecules that are interpenetrated within the 3D network structure gets excluded. This exclusion of water not only affects the gelator-gelator interactions but gelatorwater interactions too. This resulted in the transition of the phases from gel to sol as was observed in the protein based hydrogels with terminal leucine group,14 or gel to sol transition as was observed for peptide based gelator.17 The low viscous solution of unfolded peptide based gelator at ambient temperature through uni-molecular folding forms the hydrogel at high temperature. The nonpolar residues of the unfolded peptide get dehydrated at higher temperature and forms the hydrogel



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through hydrophobic collapse.14-17 Hydrogels with characteristic feature of gel to gel phase transformation is limited despite their technological relevance in material designing as well as in sensors and drug delivery among others. Meister et al observed temperature responsive gel to gel transformation in the symmetrical singlechain bolaamphiphiles having different polymethylene chain lengths,18 while liquid crystalline organogelators forms the hard gels at lower tempeature that were transformed into soft gel at higher temperature.19 Recently, zwitterionic pH sensitive surfactant was also investigated for the formation of hydrogel that exhibited phase transition (gel to gel) upon increasing the temperature. The phase transition thus occurred is due to the morphological differences between both phases.20 Novel surfactants with ionic liquid character was studied for the sol to gel to sol transformation under the influence of concentration and temperature in aqueous medium.21 Ester functionalized imidazolium based surfactant with ionic liquid character was recently investigated by our group for its temperature induced opaque to transparent gel transition due to the structural arrangement within its 3D network structure. The hydrogels thus formed have good dye absorption and drug encapsulation charectertics.22 Thermal stability of the hydrogel due to the negligible vapor pressure of the ionic liquids type surfactant makes them an important candidate for various applications than the hydrogels and organogels from conventional gelators and is studied in the present manuscript. Lee et al reported thermochromic ionogel with super cyclic stability (5000 heating-cooling cycles) with no detectable liquid leaching for solar modulation recently.23 Despite their superior performance, the desired properties of the stimuli responsive hydrogels often get deteriorated or are even vanished when these hydrogels are subjected to damage by any means including breaking of inter or intra molecular forces that manage their 3D network structure. This could affect the integrity of their 3D network and so their performance.7 To overcome these, novel smart hydrogels with self-healing and self-sustaining properties as of biological tissues have been developed.24-26 Majority of them are polymer based,27 rubber based,28 clay based,29 and dendrimer based.30 Deng et al designed self-healing


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hydrogels with stimuli responsiveness using host-guest interactions where

-

cyclodextrin was used as the host whereas N-isopropylacrylamide served as the guest.31 Wang and his coworkers demonstrated a novel self-healing hydrogel formed due to physical cross-linking network that exhibited self-healing, nontoxic, higher ionic conductivity, and metal adhesion characteristics.32 Li et al summarizes interesting literature dealing with the synthesis and applications of the self-healing gels. The strength and weakness of the polymeric gels and small molecular gels was further compared.7 Self-healing polymer based hydrogels, formed through noncovalent or physical interactions are available in literature but non-polymeric materials that not only have the self-healing approach but also have shape persistent free standing behavior with moldable characteristics and have ability to absorb the analysts from the medium is a challenging and fascinating task and not available in literature to date (Scopus and Web of Science Search as on 14.01.2019). In our efforts to design materials with above mentioned characteristics, we had reported several stimuli responsive supramolecular functional materials that change their structural aggregates through self-aggregation or through intermolecular interactions with external additives including drugs.33-37 To further advance our efforts in material designing with above mentioned characteristics, herein we had designed

the

morpholinium

based

ester

functionalized

surfactant,

4-(2-

(hexadecyloxy)-2-oxoethyl)-4-methylmorpholin-4-ium bromide (C16EMorphBr) with melting point of 78.53 °C. The surfactant holds the characteristic feature of ILs (arbitrarily, ILs are defined as electrolytes with m.p. I 100 0C) and have established surface active properties but yet to be explored gelation behavior at higher concentration.38 Presence of oxygen atom in the ring reduces the toxicity of the morpholinium based ILs relative to the imidazolium based ILs.39 More specifically, morpholinium based ILs is reported nontoxic to B. magna and P. subcapitata.39 Pabbathi et al 40 studied the interaction of morpholinium based ILs with calf-thymus DNA and proved that structure of DNA is retained and thermal stability of DNA is enhanced in the presence of morpholinium IL. Further, incorporating the ester functionality within the alkyl chain reduces the toxicity relative to the alkyl chain.41



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C16EMorphBr forms hydrogels at 7.20 % (w/v) critical gel concentration and at pH 7.4. Structure of the surfactant is given in Scheme 1. The formed hydrogel exhibits phase transition from an opaque to transparent through changing the temperature from 25 0C to 50 0C. The dual gelation properties are due to the changes within its supra-molecular framework from bilayer lamellar to cylindrical due to the exclusion of water molecules. This phase behavior was characterized using various states of the art physical, spectroscopic, microscopic and scattering techniques. The porous xerogel that forms after exclusion of the water molecules from the opaque gel, shows selfsustaining, self-healing, moldable and load-bearing properties. Rheology, tensile strength and compression test were performed to study the mechanical properties of the xerogel. The same xerogel is then used as an absorbent for the selective removal of anionic dyes from an aqueous solution.

[C 16EMo rph] +[Br] -

Methyl Orange (MO)

Crystal Vio let (CV)

Scheme 1. Chemical Structures of C16EMorphBr, Methyl orange (MO) dye and Crystal violet (CV) dye. RESULTS AND DISCUSSION The LMWG, C16EMorphBr was synthesized and characterized by the procedure reported elsewhere38 (Crystalline white powder; m.p. 78.53°C). For the gelation study, the pre-weighted solid C16EMorphBr was added in a glass vial and 10 ml of water was added in the vial. The mixture was then heated at 60 0C on water bath to get the transparent homogeneous solution, which was then cooled to room temperature to get the fibrillar opaque hydrogel visually, Figure 1. At ambient condition, the solution having pH 7.4 was transformed into fibrillar opaque hydrogel


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at 7.20% (w/v), which is the critical concentration for gelation (CCG). It is to be noted here that on decreasing the pH of the solution to 2.0 through addition of HCl and increasing the pH to 9.0 through addition of NaOH, no hydrogel was formed. Above CCG (>12.77% w/v), the solutions form precipitates whereas viscous gel or loose gel was obtained below 7.20% (w/v). For further study, we had selected gelator concentration of 8.00 % (w/v) and solution pH 7.4. The hydrophobic interaction between the alkyl chains and intermolecular hydrogen bonding between the ester group with the cationic head group of the surfactant and water molecules are certainly responsible for the gelation to occur as observed for the –COOH functionalized imidazolium based surfactant and confirmed through FTIR results,

vide infra.8 Cheng et al confirms the gel formation due to the intermolecular hydrogen bonding between the COOH group present in the –COOH functionalized imidazolium based surfactant through density functional theory (DFT) calculations and compared the results with the non-functionalized surfactants.8 On changing the temperature from 25 0C to 50 0C, the hydrogel undergoes unusual and unique reversible phase transition from fibrillar opaque hydrogel to transparent hydrogel.

50 50 °C °C

Figure 1.

45 45 °C °C

40 40 °C °C

35 35 °C °C 30 30 °C °C

25 25 °C °C

Dual gelation properties of hydrogel (transparent

opaque).

Turbidity study as a function of temperature and DSC analysis was performed to confirm the reversible transition of opaque hydrogel into transparent hydrogel on changing the temperature from 25 to 50 0C. As shown in Figure 2a, the phase behavior was monitored by measuring the transmittance as a function of temperature, where dramatic increase in transmittance was observed as the temperature reaches between 45.50 to 50.0 0C with visual appearance of the hydrogel turns from opaque to transparent.19,22 DSC analysis supports the transmittance data



by exhibiting an endothermic peak in the thermogram at 47.72 0C, well before the melting point of the gelator that was observed at 78.53 0C in the DSC thermogram (Figure 2b). The observed phase changes below the melting point of the gelator might be due to the disruption of the 3D network structure of the hydrogel and not due to the melting of the gelator.19,22 0.0

100

Transmittanc e (%)

-1

Heat Flo w (W g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.4

-0.8 0

So lid (78.53 C )

75

50

25

0

-1.2

Io no gel (47.72 C)

40

60

0 0

80

100

30

Temp erature ( C )

50

60

70

0

Temp erature ( C )

(a) Figure 2.

40

(b)

(a) DSC thermogram of the solid C16EMorphBr and hydrogel; (b) Turbidity of the hydrogel as a function of temperature.

It is interesting to explore the morphology of the hydrogel to understand the structural arrangement of the gelator within them. To do that, we have examined the morphology of the opaque gel through scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The opaque hydrogel contains the long, entangled fibers that are made up of tightly winded tubules and forms the 3D branched network within the opaque gel and is evidenced through SEM image (Figure 3a).42,43 Water molecules got entrapped within this 3D branched network of the gelator to form the opaque gel. In principle, water molecules are supposed to be encapsulated between the bilayers of the lamellar sheets, SANS results vide infra. The FE-SEM image of the opaque fibrillar gel (Figure 3b) confirms the formation of long and extended fibrillar networks that are formed through several entangled fibers


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with high aspect ratios of 300 m width and several micrometer lengths.44 TEM image (Figure 3c) of opaque fibrillar gel shows the long tube like structures. The wall structures of these tubes are of lamellar shaped that are composed of polar and nonpolar layers, SANS results vide infra. The AFM image confirms the 3D multisheet arrangement for spongy or semi-solid structure for fibrillar opaque gel.45 The height profile as well as the cross section analysis of the 3D view of AFM confirms that the opaque fibrillar gel consists of sheets with height of 1-20 m (Figure 3d).

50.0 m

300 m

(a)

(b)

100 nm

(c) Figure 3.

(d)

Microscopic images for fibrillar opaque gel at 25 0C (a) SEM, (b) FESEM, (c) TEM and (d) 3D AFM

To understand the phase behavior of hydrogels, we had performed the SANS analysis of the hydrogels at two temperatures, 250C and 50 0C, where the gels are visually opaque and transparent respectively. Experimental scattering data along with the fitted curves are shown in Figure 3, whereas the experimental setup and fitted parameters are presented in the experimental part of the manuscript. The



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fitting curves are represented by the solid lines, whereas the experimental data through symbols. The extracted final fit parameters are summarized in Table 1. As shown in Figure 4a, experimental data at 25 0C for the opaque gel is best fitted to lamellar type structures with the characteristic Bragg peak at lower q region. Scattering at lower q region with a characteristic q-4 dependency is observed for the opaque gel, which is due to the critical ‘surface scatter’ from the faces of large structures such as lamellar sheets. Further, the ratio of peak positions at high q (1:2) suggests the presence of lamellar structures at 25 0C. In addition to this, the dspacing of P 62.5 nm at high q values for the opaque gel indicate the greater elasticity of the gel. The poor fitting at low q region, just before the first Bragg peak for the q-4 dependency infers the presence of D2O molecules in the 3D network of the gel, may be within the lamellar sheets of the tubes. Anionic fluorinated surfactant with spherical micellar morphology transforms into the lamellar sheets through interaction with perfluorooctan-1-ol observes similar fitting pattern as in present case for the lamellar sheets.46-47 The stronger hydrophobic interaction between the alkyl chains is expected in the lamellar sheets as evidenced through the FTIR results,

vide infra. The lower values of the Caille parameter (CP), that defines the rigidity of the lamellar membranes also indicates the higher bending modulus that are associated with the flexible bilayers (Table 1). Overall, the wall of the tubes that constitutes the 3D structure of the opaque gel through encapsulating the water molecules are of lamellar shaped. The lamellar structure of the opaque gel is composed of the tubes made up of polar and nonpolar layers, FE-SEM images vide

infra. For better understanding, we had schematically illustrated the lamellar structure of the opaque gel through embedding the gelator within the FE-SEM image in Figure 4b. The experimental scattering data of the hydrogel at 50 0C was best fitted to the cylindrical like structures having greater elasticity, as evident in Figure 4a. The scattering at the low q region follows a characteristic q-1 behavior that indicates the formation of long cylindrical or worm-like micelles.48 As reported in Table 1, the cylinders are made up of the interacting spheres with 2.5 nm radius and length of


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ACS Applied Materials & Interfaces



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phases of

transparent hydrogel at 50 °C. The blue and green dotted

lines are experimental SANS data; the black and red lines were generated by curve fitting. (b) Schematic representation of the lamellar sheets in FE-SEM image. The hydrogel was subjected to the dynamic rheological analysis (strain sweep and angular frequency sweep) at 25 and 50 0C to explore the mechanical properties of the hydrogels. The visco-elasticity of the gels was studied through measuring the angular frequency sweep upto the maximum frequency of 100 rad s-1 at fixed strain of 1 % (Figure 5a). For both of the gels, the elastic modulus (G’), the ability of the gel to restore its original geometry is higher than the loss modulus (G’’) over the entire frequency range studied (0 to 100 rad s-1) with no cross-over frequency.49 Moreover, the higher values of G’ for opaque gel relative to the transparent gel indicate the available additional storage sites in the opaque gel, in the form of branching (SEM, FE-SEM, TEM and SANS results, vide supra). Similar behavior for the loss modulus (G’’) that of G’ for both of gels was observed. The absence of any cross-over frequency ( c) within the frequency range studied indicates the stable network structure of the colloidal particles within the frequency range studied. The overall observation from the frequency sweep measurements suggests the formation of soft semi solid-like gel material with good tolerance towards the external forces. To have better insights towards the viscoelasticity, both hydrogels were subjected to strain sweep rheological measurements at fixed angular frequency of 1 rad sec-1. It was observed that for the opaque gel, the values of G’ and G’’ remains constant upto 6 % strain, the critical strain level ( c) (Figure 5b).

c

for the transparent

gel, an indicative of structured gel matrix was 170 % (Figure 5c).50 Beyond c, G’ and G’’ decreases rapidly in both gels and the cross over occurred at 7.6% and 248 % strain for the fabriliar opaque and transparent gel respectively. Below

c

the hydrogel

exhibits viscoelastic behavior that hinders the flow of the gel, whereas above

c,

due

to the breaking of the 3D network structure of the gel, G’ starts decreasing for both of the gels.51 The abrupt decrease in G’ against G’’ above


c

indicate the gel–sol

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transition under high strain. Moreover, these results also suggest the maximum bearable shear stress for the gel matrix for their mechanical strength.25,50 Above results confirmed the formation of stable continuous elastic network of gels upto 7.6 % and 248 % of strain for the opaque and transparent gel respectively, which is an indication of true gel till the

c.

So we concluded that the transparent gel consists of

more stable intertwined temporally persistent network than the opaque gel, which are formed as a result of interactions due to topological constrains.51 1000

900

500

25 C

G'

50 C

0

0

50 C

G"

0

G"

0 0

G'

0

G'

25

G', G" (P a)

G', G" (P a)

25 C 50

25 0C 450

G''

0

75

0

100

2

4

6

8

10

Strain (%)

-1

Angular Freq uenc y (rad .s )

(a)

(b)

600

G'

G', G" (P a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

50 C 300

G''

0 0

100

200

300

Strain (%)

(c) Figure 5.

(a) Frequency sweep rheology data for hydrogel (8.00 % w/v) at 25 °C and 50 °C (strain 1%). (b, c) Strain sweep rheological data for hydrogel



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(8.00 % w/v) at 25 °C and 50 °C. (G = storage modulus, G = loss modulus, Angular frequency of =1 rad/s).

Driving force for the phase transition within hydrogel system could be hydrophobic interaction within alkyl chains, inter as well as intra molecular hydrogen bonding, van der Waals interactions and electrostatic interactions. To elucidate the dominant interaction, we had compared the FTIR spectra of powder C16EMorphBr with both hydrogels; opaque and transparent (Figure 6). As shown in Figure 6, the asymmetric and symmetric vibration frequencies of –CH2 group for the powder C16EMorphBr was observed at 3138 and 3015 cm-1 respectively. These vibration frequencies were red shifted to 2947, 2842 for the fibrillar opaque and 2951, 2844 cm-1 for the transparent gel. This is due to the increased conformational disorder, that is to say gauche conformation in the long alkyl chain due to the hydrophobic interactions within the alkyl chains. The difference in shift confirms that as the temperature increased, from 25 0C to 50 0C, interaction between the alkyl chains became weaker.52 The –OH stretching frequency of water shifted from 3427 to 3468 for the opaque to transparent phase transition. The hydroxyl group of water molecules got associated with the gelator strongly in opaque gel as compared to the transparent gel, shifting the stretching peak to a lower frequency, as observed for the ionic liquid based gemini surfactant.53 Further, -C=O band frequency for the C16EMorphBr powder is different from both the hydrogels, likely due to the different levels of H bonding because of the different sample environment that affect the -C=O stretching vibration.49 The C=O band of fibrillar opaque gel is also red shifted at higher temperature (1748 for opaque gel shifted to 1753 for transparent). The head group frequency for the powdered C16EMorphBr sample was shifted from 1163 cm-1 to 1147 cm-1 and 1154 cm-1 for the opaque and transparent gel respectively, indicating the intermolecular hydrogen bonding that is responsible for the gel formation. The formation of H-bond present in the ester group changes from an associated state to free-state at high temperature causing the shift in its vibration frequencies. Conclusively, among the various interactions, H-bond interactions and hydrophobic


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interactions that becomes weaker with temperature are the two major interactions responsible for the phase transition within the studied hydrogel system.22,52-53

Transmittanc e (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


So lid 25 0C Gel 50 0C Gel 1753 2842 3468 2947 2844 2951

3427 3138

3600

3200

1748

3015

2800

1775

2400

2000

1600

1200

-1

Wavenumb er (c m ) Figure 6. FT-IR spectra of Solid C16EMorphBr, opaque (25 0C) and transparent (50 0C)

gel.

Self-sustaining, self-healing and load bearing properties of xerogel. The self-sustaining nature50 of the xerogel indicates that the gel with immobilized solvent molecules are stable, intact and stand by itself without any support and without changing its shape such as cylinder, ring and rectangular. The xerogel was obtained from the removal of water from the fibrillar opaque gel. We observed that the xerogel exhibited excellent self-sustainability (Figure 7a-c), as these xerogels were tailored into cylindrical and ring shape by carefully cutting them in pieces (Figure 7f). The rigidity and load bearing ability of the xerogel was further investigated by placing Indian 1 (4.96 g/coin) and 5 (5.99 g/coin) rupee coins over them one by one (Figure 7d). It was found that the xerogel can hold the weight of 16 coins (85.54 g; 10 coins of 1 rupee and 6 coins of 5 rupees) with ~50 % reduction of height (Figure 7e), might be due the leakage of the water.



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Figure7

Self-Sustaining nature of the xerogel, rectangular box shape of the gel (ac); (d-e) Self-sustainability of the xerogel through carrying the load of sixteen coins before and after compression in the height; (f) cylindrical and ring shape of the xerogel.

The self-healing property of the gel is defined as the ability of the gels to reconstruct their non-covalent interactions after damage just like biological tissues. Figure 8 (a-d) illustrates the self-healing properties of the xerogel in conventional ‘gel block fusion’ experiment.54 The xerogel was first grown inside the hollow glass vials and then was chopped into the smaller pieces. These chopped blocks were then put in direct contact along the cut surfaces. It was observed that these chopped pieces recombined within 12 h at ambient condition without the application of any external stimuli. To have better visual appearance, we used methyl orange (MO) doped and undoped gel pieces to construct 4.5 cm long self-supporting bridge through the combination of five xerogel pieces. Experimental results suggest the good self-healing nature of the xerogel (Figure 8 (b-d)). During the formation of long 4.5 cm selfsupporting bridge, we also observed diffusion of dye molecules from doped to undoped pieces. Dynamic dissociation and re-association of the hydrogen bonds present in the xerogels are the driving force for the self-healing characteristic of the studied xerogel as observed for the poly(N-acryloyl glycinamide) based hydrogels.55 The self-healing character of the xerogel was further confirmed through the tensile test.


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Figure 8.

(a) Dye doped and undoped cylindrical pieces and (b-d) A self-sustaining bridge (4.5 cm) constructed by connecting alternative dye doped and undoped pieces.

Mechanical properties of the xerogel The xerogel was tested for its mechanical properties by the tensile tests, which were conducted on cylindrical gel samples of 20 mm width and 7 mm length. The xerogel exhibited good stretchability and lower toughness. Stretchability of the xerogel was measured through clamping the sample between the two arms of clamps, with one arm being fixed and the other is pulled at 75 mm min-1. The xerogel could be stretched firmly between the two clamps and the elongation at break was measured to be 500 ± 25 % at 600 % strain similar to poly(acrylamide) based hydrogel, for which 360 % elongation was observed at lower strain of 500%30 (Figure 9a, 9b). Various polymeric hydrogels offer higher elongation as compared to the present xerogels, e.g. elongation in the polymeric hydrogels composed of poly(acrylamidestearyl methacrylate) increased from 1800 to 5000% upon decreasing the sodium dodecyl sulfate (SDS) composition from 15 to 10% in the catanionic micellar solution of CTAB and SDS.56 Han et observed 3100 % elongation for the polydopaminepolyactylamide (PDA-PAM) based hydrogel at a loading rate of 100 mm min-1,57 whereas incorporating graphene oxide (GO) produced conductive and self-healable



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PDA-pGO-PAM hydrogel with high tensile strength (TS) and large extension ratio (ER).58 The higher crosslinking density within the studied hydrogel due to the intermolecular hydrogen bonding resulted in the increased rigidity of network and decreased elasticity.55 The tensile stress–strain curve of the xerogel showed the nonlinear inelastic mechanical behavior of the xerogel (Figure 9c). The healed samples were tested for its tensile strength measurements as a function of stress, and the self-healing efficiency was assessed through comparing the tensile strength before and after selfhealing (Figure 9c). After self-healing for 12 hours at room temperature without any external stimuli, xerogel can withstand stress of 300 % without failure at the interface. Tensile test confirmed that the fracture stress before and after 12 hours of self-healing is 61 and 48 kPa respectively with the healing efficiency of 78.68 %. For the polymeric hydrogels, the fracture stress was reported to be higher than the present system. Gulyuz et al reported that the fracture stress of the polyacrylamide based hydrogel increases from 41 to 173 kPa on increasing the initial monomer concentrations.59 Self-healing of the hydrogels based on poly(vinyl alcohol) exhibited fracture stress of 200 kPa after 48 hours of healing without external stimuli with healing efficiency of 72 %.60 The lower value of Young’s modulus for the present hydrogel, i.e. 10 ±2 kPa confirms that the xerogel is soft in nature as observed for the poly(acrylamide) based hydrogel.30 Young’s modulus or the elastic modulus, the characteristic feature of the gel to behave either soft or tough is quite higher for the polymeric gels and can be further modulated by hydrophobic association, e.g. elastic modulus for the poly(acrylamide/lauryl methacrylate) based hydrogels (40 kPa) was improved to 50 kPa through incorporating the latex particles within the hydrogel.61 The xerogel was further characterized for its mechanical properties through measuring the tensile loading–unloading tests that reveal its energy dissipation capacity from bond breakage. The tensile loading–unloading curve shows small hysteresis, unlike polymeric hydrogels where pronounced hysteresis was observed, e.g. PDA-PAM based hydrogel.57 During tensile loading–unloading tests, the residual strain was observed very small, even when the hydrogel was stretched to a strain of


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the industrial zones (Chemical structures of MO and CV are shown in Scheme 1). MO and CV were selected based on their charge, i.e. negative and positive for MO and CV respectively. Several smart materials have been tested for this purpose naming polymers, clay, various hydrogels and several ligands. The characteristics feature of these smart materials is of having hydrophobic and hydrophilic domains that have the tendency to interact both hydrophobic and hydrophilic part of the dyes to get them absorbed. Herein, the opaque fibrillar gel, due to its 3D network structure and hydrophilic head groups have both domains, and therefore, may have potential to act as dye absorbing agent. With the same motive, the xerogels of the opaque fibrillar gel was tested to absorb MO and CV dye at ambient temperature. It was observed that 7.05 mg of the MO per gm of the xerogel was absorbed after 12 h incubation time (Figure 10a), whereas CV was not absorbed within the xerogel after the same incubation time. The selective encapsulation of the dye is the result of the electrostatic and host-guest type interactions (Figure 10b). CV being the cationic charged, xerogel with cationic charge could not absorb it, while it can absorb the negatively charged dye, i.e. MO. Electrostatic interaction is certainly dominating for such absorption behavior and is proved through Fourier transform IR (FTIR) spectral analysis. The electrostatic interaction between the positively charged morpholinium head group and negatively charged sulfonate group of the MO is clearly visible through the shifting of the vibration band of the aromatic morpholinim head group from 1147 cm-1 to 1108 cm-1 (Figure 6 and 10c). Further, the frequency associated with the sulphonate group shifted from 1225 to 1215 cm-1 for the neat MO and MO loaded xerogel matrix respectively (Figure 10c); certainly electrostatic interaction is responsible for this shift. Cheng et al reported selective absorption of the anionic dyes (MO and eosin Y) within the imidazolium based supramolecular gel within H2O/DMSO binary solvent mixture.8 Xerogel obtained from the metal-organic gels,54 and metallogel 62 also exhibited selective absorbance of MO. The SEM image (Figure 10d, taken after dye absorption) clearly shows that dye molecules are entrapped in to the 3D fibrillar network of the opaque xerogel.


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0.6 0h

1.2

0h

0.4

12 h

8h

0.2

Ab so rb anc e

Ab so rb anc e

4h

12 h

0.0 300

450

600

750

0.8

0.4

0.0 400

12 h 600

800

Wavelength (nm )

Wavelength (nm)

(a)

(b)

1225

Transmittanc e (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MO MO+Gel

1108 1215

2500

2000

1500

1000 -1

Wavenumb er (c m )

(c) Figure 10

(d)

Absorption spectra of (a) Methyl Orange (MO) and (b) Crystal Violet (CV) before and after incubation, (c) FTIR spectra of neat MO and MO loaded xerogel (d) SEM image of the xerogel after absorbing MO.

CONCLUSSION In the present manuscript, ester functionalized morpholinium based surfactant was investigated for its ability to form hydrogel in water with 7.20 % of CCG at 7.4 pH. The hydrogel exhibited unique temperature responsive gel-to-gel reversible transition; opaque at 25 0C and transparent at 50 0C. Turbidity data and DSC



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analysis confirms that the phase transition was observed below the melting point of the surfactant. The opaque gel was studied for its morphology using SEM, FE-SEM, TEM and AFM analysis. The phase behavior and the structure of the walls were characterized through SANS analysis for both of the gels. It was observed that the wall of the opaque gel is made up of lamellar shaped whereas the wall of the transparent gel is of cylindrical shaped. Water molecules, that are entrapped between the bilayers of the lamellar sheets gets excluded at higher temperature resulting in the shift from the lamellar to cylindrical shaped walls of the tubes with transforming the hydrogel from opaque to transparent visually. FT-IR spectroscopy was used to confirm the entangled temporally persistent network of the fibrilliar opaque hydrogel that transform into transparent gel at higher temperature due to the change in different levels of H-bonding and hydrophobic interactions. Strain sweep and frequency sweep rheology measurements were performed to study the mechanical properties of the hydrogels. Xerogel obtained from the fibrilliar opaque hydrogel exhibited self-sustaining, self-healing, moldable, load-bearing and anion selective dye absorbing properties. The tensile and cyclic loading-unloading tests confirmed that the xerogel is soft and self-healing characteristics. The results show some potential applications of LMWGs. EXPERIMENTAL SECTION a)

General. All the chemicals used in the investigation such as 2-Bromoacetic

acid, Hexadecan-1-ol, toluene-4-sulfonic acid monohydrate and N-methylmorpholin were purchased from S. D Fine Chemicals and used without further purification. Solvents used in the present work were of highest purity. Triple distilled water was used throughout the experiment. b)

Synthesis of C16EMorphBr

C16EMorphBr was synthesized following the two step procedure; first step being formation of hexadecyl-2-bromoacetate and second step was the formation of C16EMorphBr.22,42 Briefly, in the first step, cetyl alcohol (2.43 g, 10 mmol) was heated


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at 80 0C on the magnetic stirrer with bromoacetic acid (12 mmol, 8.63 mL) in the presence of p-toluene sulphonic acid monohydrate (10%, 1 mmol, 192 mg) as catalysts for 4 hours. The progress of the reaction was monitored by TLC using mobile phase of n-hexane: Ethyl acetate (9:1). Before washing the product with 100 mL of water (10 aliquots), the reaction mass was dissolved in chloroform to remove the unreacted reactants. The reaction mass was transferred to the rotary evaporator to remove the excess water and chloroform. The dry crude product was washed with warm aqueous methanol solution with the methanol to water ratio of 98:2 and the dry hexadecyl-2bromoacetate was further dried in rotary evaporator. In the second step, equimolar amount of hexadecyl-2-bromoacetate and Nmethyl morpholin were stirred on magnetic stirrer at 90 °C for 2-3 hr. The reaction mass was washed with 10 mL aliquots of diethyl ether (10 mL) before precipitating in cold acetone. The white solid C16EMorphBr having melting point of 78.53 °C was separated, dried under vacuum for 12 hours and characterized through 1H NMR (CDCl3 as solvent and on 400 MHz Bruker Advance 300 spectrometer) to check the purity and TGA for the thermal degradation. 1H

NMR ( in ppm): 5.23 (t, 2H), 4.28-3.69 (m, 10H), 3.62 (s, 3H), 1.66 (m, 2H), 1.30

(s,26H), 0.89 (t, 3H); FT-IR using KBr pallets (Shimadzu): 3138 cm-1 (Ar (C-H)str/b), 3015 cm-1 (Aliphatic (C-H) str),1775 cm-1(Ester C=O), 1658 cm-1 (Ar (C-C)), 1480cm-1 (Ar, str/deform), 1391 cm-1 (Me (C-H), b,asym), 1271 cm-1(head group C-O, str). c)

Thermal Gravimetric Analysis

Thermo gravimetric analyzer SDT Q600 was used to study the thermal stability of the gelator. Temperature range studied was between 25

0C

to 500

0C

in N2

atmosphere at the heating rate of 10 °C/min. Tstart, the temperature at where the decomposition starts was measured to be 198 °C for the gelator. d)

UV-Visible Spectroscopy

The gel-to-gel transition through turbidity measurement was studied through measuring the transmittance of the hydrogel as a function of temperature from 25 to



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70 °C on Varian Cary 50 UV spectrophotometer. Dye absorption experiments were carried out at 25 °C at 463 nm ( e)

max

of MO) and 588 nm (

max

of CV).

Differential Scanning Calorimetry (DSC)

The phase transition between the opaque and transparent gel and melting point of the gelator was measured by performing DSC measurements on METTLER TOLEDO DSC 1 STARe instrument in triplicate under N2 atmosphere between temperature range of 30-90 0C at heating rate of 2 °C min-1. f)

Scanning Electron Microscopy (SEM)

The gold coated gel was dried at 25 0C and was analyzed for its morphology by Scanning Electron microscopy instrument (Hitachi S-3400N) operated at 15 kV. g)

Field Emission Scanning Electron Microscopy (FE-SEM)

Hitachi S-4800 Field Emission Scanning Electron Microscope (FE-SEM) operating at 5 kV was used to study the morphology of the opaque gel sample. The gel sample (2030 mg) was dried under reduced pressure, coated (90 s, 2-3 nm thickness) with platinum and then placed on the microscope cover glass. h)

Transmission Electron Microscopy (TEM)

Philips CM-200 electron microscope working at an acceleration voltage of 200 kV was used to take the images of the opaque gel sample. The gel sample that is stained with 1% sodium phosphotungstate solution was deposited on the copper grid of 200 mesh size that was coated with the carbon Formvar. i)

Atomic Force Microscopy (AFM)

Atomic Force Microscope (Park XE 100) was used to investigate the morphology of the vacuum dried (30 0C for 72 h) opaque gel sample by placing the dried gel sample on the microscopic glass. j)

FT-IR Spectroscopy


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The interaction involved in the phase transition from opaque to transparent was characterized through measuring the FT-IR spectrum of the partially dried gel samples (opaque and transparent) and solid C16EMorphBr sample on Shimadzu FTIR-8400S spectrophotometer. k)

Small angle neutron scattering (SANS)

SANS measurements for the gel samples were performed at sample temperature of 25 °C and 50 °C on Quokka (ANSTO, Australia). Data are presented as a function of the scattering vector, q by using the incident neutron wavelength, =

as

4 2

Herein, q-range of 0.005 – 0.400 Aº-1 was selected with 2 and 14 m as the sampledetector distances and 5 Aº (

/ =10%) as an incident wavelength. The experimental

data were reduced from raw counts on the 2D detector to a radially-averaged 1D scattering pattern with the assumption of radially isotropic scattering. The sensitivity of each detector pixel was calibrated by comparison of its response to a flat scatterer, and then the scattering from an empty SANS cell was subtracted. The scattering was then radially averaged (accounting for instrument configuration) to provide the intensity as a function of q. The absolute intensity scale was provided by normalizing each sample by its thickness (1 or 2 mm), and then comparing to either the scattering from an empty beam measurement. The data modelling program SAS view was used to fit the SANS data. l)

Rheology Experiment.

Physica MCR 301 Rheometer from Anton Paar was used to measure the mechanical properties of both gel samples (opaque and transparent). For the detailed experimental description, refer reference 22. m)

Tensile Strength Measurement.



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(1) Tensile tests of the xerogel were performed on a Testometric M100-1CT instrument with the loading rate of 75 mm min-1. The specimen had thickness of 2.5 mm and width of 20 mm. The gauge distance between the clamps was 7 mm. (2) Cyclic loading–unloading tests were performed by loading the xerogel under tension to strains of 200 and 500% and then unloading to zero force. Acknowledgments M. Kuddushi acknowledges financial assistance of UGC-DAE for the Collaborative Research Scheme (UDCSR/MUM/AO/CRS-M-276/2017). Naved Malek acknowledges financial assistances through Department of Science and Technology, New Delhi (SR/FT/CS-014/2010), Institute Research Grants to the Assistant Professors by SVNIT and Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 01 (2545)/11/EMR-II).

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Stimuli Responsive, Self- Sustainable and Self-Healable

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