Self-reinforcement of alginate hydrogel via conformational controlYongjun Ahna, Hyungsup Kimb,⁎, Seung-Yeop Kwaka,c,⁎
a Department of Material Science and Engineering, Seoul National University, Seoul 08826, Republic of KoreabDepartment of Organic and Nano System Engineering, Konkuk University, Seoul 05029, Republic of Koreac Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 08826, Republic of Korea
Alginate hydrogel with high stretchability and toughness was prepared using binary solvent system consisting ofwater and glycerol. The mechanical performance was achieved via conformational control for the alginatemolecules using water/glycerol binary system. The conformational characteristics of the alginate chains inglycerol/water solution were systematically investigated by spectroscopic analysis and rheological approach.The glycerol in the solution deconstructed intra-hydrogen bonding of alginate molecules, resulting in an increaseof chain flexibility and hydrodynamic volume. The flexible chains had more chance to be entangled with theneighboring chains. The inter-molecular entanglement effectively transferred the stress and hindered crackpropagation. The study provides fundamental insight for the design of alginate hydrogel with high stretchabilityand toughness.
1. Introduction
Alginate is linear anionic polysaccharide obtained from brownseaweeds consisting of naturally derived linear copolymer of 1,4-linkedβ-D-mannuronic acid and α-L-guluronic acid units in various composi-tion and sequence. Due to its nontoxicity, unique tissue compatibilityand biodegradability, alginate has been studied extensively in tissueengineering, including regeneration of skin, cartilage, bone, liver andcardiac tissue [1–6]. To meet the requirements for each applications,alginates have been fabricated in diverse forms such as fibers, films,particles and hydrogel. Among those forms, hydrogel have outstandingmerits for biomedical applications since this structure has facility ofpore properties relating to drug release and containing water content[7,8].
Hydrogels have three-dimensional networks composed of highmolar mass polymer, water and crosslinking agent. Due to the hydro-philic network, alginate hydrogels can swell several times of intrinsicvolume from dry volume under different environmental stimuli [9–12].However, the poor geometrical stability derived from low mechanicalproperties of alginate hydrogels has limited their further biomedicalapplications. According to previous results [13,14], the alginate mole-cules have extended association with high rigidity in water, even underan infinitely high salt concentration. The rigid conformation of the al-ginate is mainly originated by the guluronic acid units, which are linkedby biaxial linkages and stabilized by intra-hydrogen bonding. The rigid
units could not form effective chain entanglements, resulting in poormechanical properties.
Recently, tremendous efforts have been devoted to improve themechanical performance of alginate hydrogels. Hydrogel/filler com-posites are widely studied for complementing their limitations.However, it is generally considered that the organic/inorganic fillersmay limit the biocompatibility [15–17]. Compared to these methods,double-network hydrogels attract much attention since the improve-ment of mechanical performance can be easily achieved by tuninginter/intra-molecular interactions and network structure using a widevariety of polymeric monomers, crosslinking agent and crosslinkingmethods [15,18–21]. Although many types of hydrogel have been de-veloped in the recent years with remarkable performance, it is still notfree for the additional chemicals required in the preparation of hy-drogels. In this point of view, a novel approach should be required toimprove mechanical properties of alginate hydrogel via overcoming theinherent limitation for the effective interaction between alginate mo-lecules. Glycerol can be considered as a candidate to control the algi-nate conformation due to its strong polarity which give flexibility of thealginate molecules [22]. The induced chain flexibility is expected tofacilitate physical interaction between alginate molecules, and thenimprove the physical properties of alginate hydrogel.
In this study, we prepared highly stretchable and tough alginatehydrogel without additional crosslinking agent and supportingpolymer. To reinforce inter-molecular interaction, intra-hydrogen
https://doi.org/10.1016/j.eurpolymj.2019.03.017Received 6 September 2018; Received in revised form 25 January 2019; Accepted 6 March 2019
⁎ Corresponding authors at: Department of Material Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea (S.-Y. Kwak).E-mail addresses: [email protected] (H. Kim), [email protected] (S.-Y. Kwak).
bonds of alginate were controlled by using binary solvent system con-sisting of water and glycerol. The approach focused on the comparisonof the alginate conformation changed by glycerol concentration as arelevant representation of physical network, and the effect of self-re-inforcement by the formed network. The effect of glycerol on con-formation of the alginate molecule and mechanical properties wasstudied using spectroscopic and rheological approaches. The compre-hensive analysis demonstrated that networking structure played im-portant roles in increasing the stretchability and the toughness.
2. Experimental
2.1. Materials and solution preparation
Sodium alginate extracted from brown algae (290,000 gmol−1,Sigma Aldrich, USA) was used as a source of solution and hydrogel. Thechemical composition of the alginate sample was determined by FT-IRmeasurements (see Supplementary Information). Glycerol (purity:99.5%) and calcium sulfate (CaSO4, purity: 99%) were purchased fromDaejung Chemicals & Metals Co. (Korea). All the reagents were used asreceived.
The binary solvent system of water/glycerol with different glycerolweight ratio was prepared by mixing these two solvents at room tem-perature. The weight ratio of water against glycerol was controlled from10/0 to 5/5. According to the glycerol concentration, the solution codewas named as G0 to G50. To confirm the effect of alginate concentration,another series of solutions was prepared with 0.07–8.16wt% alginateconcentration in pure water and water/glycerol (7/3 w/w). Sodium al-ginate was dissolved for 4 h at 50 °C using a mechanical stirrer. Afterdissolution, the solutions stood for 1 h to remove the bubbles.
2.2. Molecular characterization
Light-scattering photometer (SLS-5000HM, Otsuka Electronics Co.,Japan) was used for static light-scattering (SLS) measurements. Thephotometer has a He-Ne laser source with a wavelength of 633 nm. Thesolutions were filtered by membrane filters with a pore size of 0.6 μmand then with a pore size of 0.2 μm several times before measurements.The reciprocal reduced scattered intensity, Kc/Rθ, at scattering angle θcan be generally expressed in terms of a virial expansion as a powerseries in concentration. SLS measurements are performed for a dilutesolution, and then the virial expansion can be expressed as follows
KcR M
R q A c qn
1 1 13
2 4/w
g2 2
20
= + + =(1)
where K, c, Rθ, Mw, Rg2 , λ0, n, and A2 denote an optical constant, the
concentration, the excess Rayleigh ratio of the solution at θ, the weight-average molecular weight, the mean square radius of hydration, thewavelength of the incident beam, the refractive index of the solvent andthe second virial coefficient, respectively.
Measurements of 1H NMR spectra of 0.1% w/v alginate solutions inD2O was performed a Bruker Avnace-600 spectrometer. Before NMRmeasurement, the solutions were prepared using the 4 wt% alginatesolution consisting of water and water/alginate solution (7/3 w/w).The prepared solution was diluted with an excess of water to removeglycerol and dried under vacuum. Finally, the prepared alginate wasdissolved in D2O, and then conducted to 1H NMR measurement.
2.3. Measurements for viscoelastic properties of alginate solution
A plate-to-plate-type rheometer, RS-1 (ThermoFisher Scientific,Germany), was used for dynamic viscoelasticity and steady-flow mea-surements. Radii of the plates were 35mm, and a reservoir were
equipped to prevent drying during measurements. The dynamic vis-coelasticity measurements were performed at various temperaturesranging from 30 to 90 °C. The range of the oscillatory frequency is from10−2 to 102 Hz at each temperature. The data were reduced to thereference temperature data at 60 °C using the time-temperature super-position principle. The steady flow measurements were carried out at30 °C within the shear rate range from 10−2 to 10 s−1.
The zero-shear viscosity and relaxation time were calculated byfitting the viscosity curve using Cross model [23].
( )1 ( )n
0=+ (2)
From Eq. (2), zero-shear viscosity, η0, relaxation time, τ, and power-law exponent, n, can be obtained.
To examine the reliability of the plateau modulus, the master curvesof G′ and G″ as function of angular frequency is fitted by generalizedMaxwell model:
GG ( ) ( )[1 ( ) ]i
Ni i
i1
2
2=+= (3)
GG ( ) ( )[1 ( ) ]i
Ni i
i12=
+= (4)
where Gi and τi are the initial modulus and relaxation time corre-sponding to the ith Maxwell element in the Maxwell model. Based ongeneralized Maxwell model, plateau modulus and entanglement molarmass can be simply calculated by the following simple equation:
G GN i0 = (5)
M RTGe
N0=
(6)
where GN0, Me, ρ, R and T are plateau modulus, entanglement molarmass, density, ideal gas constant and absolute temperature, respec-tively.
2.4. Mechanical properties of alginate hydrogel
To obtain the hydrogel form, the prepared alginate solutions wereinjected into a closed Teflon mold (dog-bone shape) covered with aglass slide. The prepared solutions were immersed in excess of CaSO4slurry (1 wt%) at 50 °C to form hydrogel and remove containing gly-cerol. This procedure was repeated three times, and then finallyachieved pure alginate hydrogels.
The uniaxial tensile strength of hydrogels was measured by Instron-5545 universal testing machine. The strain rate was 2mmmin−1.
3. Results and discussion
3.1. Molecular characterization of alginate
The results from the SLS measurement are shown in Fig. 1a in theform of Debye plot for the alginate aqueous solutions with 0 and 30wt% glycerol concentration at RT. For the alginate solution without gly-cerol, the linear relation was observed in the whole c-region where themeasurements were carried out. By the linear extrapolation to c=0with the data of (Kc/Rθ)θ→0 in this c-region, thus, the values of Mw andA2 were estimated, as listed in Table 1. However, for the alginate so-lution with glycerol, the linear relation was only observed in the c-region less than 4mgml−1. Above this concentration, the linear ap-proximation broke down, and the deviation of (Kc/Rθ)θ→0 from thestraight line became gradually larger with increasing concentration.This means that the second-order term in the expression of Kc/Rθ as a
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virial expansion in powers of c cannot explain this case; this is, thealginate/water/glycerol solution above that concentration is not a di-lute solution for the scattering measurement. That concentration cor-responds to the overlap concentration, c*, at which the alginate mole-cules start to overlap each other, and the value of c* can be determinedmore definitely as 1/(Mw·A2) (=4.15mgml−1). This change of de-pendence on concentration means that alginate can efficiently interacttheir neighbor chains by introducing glycerol. The discussion on thedifferent physical interaction of alginate molecules is discussed later inrheological properties of alginate solution.
Fig. 1b and c show Zimm plots for the alginate dissolved in water orwater/glycerol solution at RT with P= 20 to estimate the value of⟨Rg2⟩z1/2. Both of the extrapolated data, (Kc/Rθ)c→0 and (Kc/Rθ)θ→0 arerepresented by filled symbols. As clearly shown in the figure, the q2-dependence of (Kc/Rθ)c→0 appeared to exhibit almost linear relation,and thus, the value of ⟨Rg2⟩z1/2 was estimated to be 67 and 91 nm forthe alginate aqueous solution without and with glycerol, respectively.This result indicates the capability that the addition of glycerol canextend the chain size of the alginate. This explanation is strongly sup-ported by the results of the concentration dependence of the viscosity ofthe solution, which are discussed later.
To confirm the chemical structure of alginate, the alginate samplesprepared from each solvent system was analyzed by 1H NMR, as shownin Fig. 2. The peaks of the alginate components, such as guluronic acid(G) and mannuronic acid (M) were assigned on the basis of the datapreviously reported in the literature [24,25]. The anomeric regions ofthe alginate samples show specific peaks of the guluronic acid anomericproton (G-1) at 5.17 ppm (peak I); guluronic acid H-5 (G-5) at 4.56 ppm(peak III); mannuronic acid anomeric proton (M-1) at 4.76 ppm (peakII) and also the C-5 of alternating blocks (GM-5) at 4.82 ppm imbricatedwith peak III. The signals and the relative area of anomeric protons canbe used for the quantitative analysis of the individual guluronic acid(FG) fraction and the M/G ratio, and these values are summarized inTable 1 (calculation was detailed in Supplementary Information).Both alginate samples from pure water and glycerol/water solutionexhibited similar fraction of guluronic acid and mannuronic acid. Thesimilarity between two samples strongly suggests that the alginateconformation is controlled without chemical modification in this study.
3.2. Glycerol dependence of rheological properties
Fig. 3a shows the shear rate dependence of steady shear viscosity of4 wt% alginate/water/glycerol solutions with 0–50wt% glycerol con-centrations. All solutions exhibit shear-thinning behavior at differentglycerol concentrations, indicating that the alginate solutions locate insemidilute region. When the glycerol concentration increased, the onsetof shear-thinning was shifted to low shear rate at which the viscositybecame larger. For uncross-linked polymers, the phenomenon of shear-thinning can be attributed primarily to intermolecular effects [26]. Thestructure of polymer network undergoes distortion by shear flow de-formation, resulting that the molecules are completely rearranged toflow direction at a certain relaxation time.
In order to confirm the effect of glycerol, the conformational change
Fig. 1. (a) Debye plot, (Kc/Rθ)θ→0 versus c, and Zimm plot, Kc/Rθ versus sin2(θ/2)+ Pc, for alginate with (b) pure water and (c) 30% of glycerol aqueous mixture atRT. K, c, Rθ and P represent optical constant, concentration, excess Rayleigh ratio at scattering angle θ and a constant (here, P=20).
Table 1Molecular characteristics of alginate in water and water/glycerol solution.
Fig. 2. 1H NMR spectra of alginate precipitated from after dissolution in wateror water/glycerol (7/3 w/w). The concentration of each solution was fixed at4 wt%.
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was observed by logarithmical relationship between relaxation timeand zero-shear viscosity, as shown in Fig. 3b. With increasing glycerolconcentration, the relaxation time was proportional to zero-shearviscosity. When the alginate solutions laid above 30wt% of glycerolconcentration, the relationship showed weak dependence. The slope isabout 0.5 while below 30wt% of the glycerol it is about 1. This meansthat the conformational effect can influence on the shear flow de-formation. According to definition of zero-shear viscosity, the viscosityis the product of the plateau modulus and the relaxation time [27]. Thesame increase ratio of viscosity and relaxation time (slope=1) is re-garded as constant plateau modulus, indicating that the each molecularshows similar conformation. However, this result shows that the re-laxation time make less contribution to the zero-shear viscosity abovethe certain glycerol concentration. Accordingly, it is expected that theincreasing viscosity is attributed significantly by plateau modulus re-lating to network structure.
Fig. 4a shows the master curves of 4 wt% alginate solutions withvarious glycerol concentration by the time-temperature superposition(TTS) principle at 30 °C. TTS was well fitted for alginate solutions withor without glycerol, meaning on the self-similarities at different hier-archical scales for alginate molecular relaxation, and maintenance ofthe solution homogeneity regardless of glycerol concentration. Thereare cross-over points between storage modulus, G′, and loss modulus,
G″, above 30 wt% of glycerol concentration, which shows the typicalbehaviors of entangled polymer solutions. As the glycerol concentrationincreased, the cross-over point was shifted toward low angular fre-quency. The cross-over point is related with the entanglement anddiffusing out of the original tube [27]. The shifting to low frequencywith increasing glycerol concentration means the slower relaxation ofthe alginate chain restricted by topological constraint.
Fig. 4b shows plateau modulus and entanglement molar mass of 4 wt% alginate/water/glycerol solutions with various glycerol concentration.As mentioned previously, we confirmed the plateau modulus is
Fig. 3. (a) Shear rate, , dependence of the steady shear visocosity, η, for 4 wt%of alginate solution in water/glycerol solutions with different glycerol con-centration ranging from 0 to 50%, and Cross model datafit curves. (b) Plot ofcharacteristic relaxation time, τ, versus zero shear viscosity, η0.
Fig. 4. (a) Dependence of storage modulus, G′, and loss modulus, G″, on re-duced angular frequency ω·aT for 4 wt% of alginate aqueous solution withvarious glycerol concentration, ranging from 0 to 50%. Solid lines represent thecurves reproduced by calculation using a generalized Maxwell model. Thesecurves are shifted vertically by a factor, B, except for the alginate solution with30% of glycerol concentration. (b) Dependence of glycerol weight fraction, Xs,on viscoelastic properties of alginate solution; plateau modulus, GN, and en-tanglement molar mass, Me.
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influenced by the addition of the glycerol above 30wt% of glycerolconcentration. The glycerol concentration is proportional to the plateaumodulus, resulting in the decreasing entanglement molar mass. This isprobably due to the deconstruction of the intra-hydrogen bonding inguluronic acid by hydroxyl group of glycerol. As reported in other re-searchers [13,14], the molecular association of alginate/water solution isdispersed in extended and rigid conformation, which significantly con-tributes the hindrance for entanglement between neighbor chains. On thecontrary, the conformation of alginate molecules is expected to be aflexible coil in water/glycerol mixture, since the glycerol moleculescleave the intra-hydrogen bonds in guluronic acid [28].
3.3. Rheological properties of alginate/water/glycerol solutions withdifferent alginate concentration
The Frequency dependences of G′ and G″ for alginate solutions inwater and glycerol were investigated to compare the dynamic behaviorof alginate solution in water. In Fig. 5a, the master curves for differentconcentrations of alginate in water are plotted against the angularfrequency at reference temperature RT. The data are shifted along thevertical axis by same method as mentioned above. Terminal flow re-gion, where slops of G′ and G″ are about 2 and 1, respectively, wasobserved for all the concentration of alginate in the sufficiently smallvalues of ω·aT as shown in Fig. 5a. The applicability of the TTS principlemeans that all the relaxation times shift by same ratio with temperaturechange [29]. The dynamic behavior of alginate solutions with 30wt%of glycerol (Fig. 5b) was similar with results of alginate solutions inpure water. However, the plateau region was observed, indicating thatthere exists entangled network. According to the rubber elasticitytheory [30], the value of the G′ at the plateau region is proportional tothe number density of cross linking. This suggests that physical con-straint became strong with an increase of alginate concentration.
Fig. 5c shows the flow activation energy, ΔEa, obtained from algi-nate solutions with or without glycerol. The value of the activationenergy increased with increasing the alginate concentration in water/glycerol mixture while the values in water were almost constant. Thisdiscrepancy is explained by the difference in the network structure
between the water solution and the water/glycerol solutions. As de-scribed above, it is revealed by the frequency dependency of G′ and G″that there exists the extended and rigid molecular associations in thealginate/water solution. The conformation of guluronic acid wouldstabilize the inter-molecular biaxial linkage between neighbor chains[13]. The translational and rotational motion of the extended molecularassociations would be restricted by their excluded volume [30], re-sulting in difficulty to entangle each other. The extended molecularassociations are preferable to flow parallel to their direction, and theflow depends on the cross sectional area, which is independent of theconcentration. The entanglement in the network for water/glycerolsolution becomes dense with increasing alginate concentration. As aresults, the concentration dependence on the value of flow activationenergy for water/glycerol solution is stronger than that for water so-lution.
The relation between plateau modulus and concentration includinga numerical factor, can be expressed as the following equation:
G KcN = (7)
where K is the plateau modulus of pure alginate in a molten state (i.e.alginate melt) and the increasing exponent depended by concentration.The plateau modulus can be estimated from concentration dependenceon the plateau modulus as shown in Fig. 5a and b. On the basis of thesevalues, the chain dimension, ⟨Rg2⟩z1/2 can be expressed by the fol-lowing equations:
G B kTP45N
2 3=(8)
p MR NA
20
=(9)
where k, T, ⟨Rg2⟩z1/2, ρ and p represent the Boltzmann constant, theabsolute temperature, the unperturbed mean-square end-to end dis-tance, polymer density and packing length, respectively. B is a constantequal to 5.65 × 10−2, which has been experimentally determined at298 K and is independent of the temperature. Considering the value of1.6 × 103 kgm3 for alginate crystal as the density of hypothetical
Fig. 5. Frequency dependence of storage modulus, G′, and loss modulus, G″, for (a) alginate/water and (b) alginate/water/glycerol solutions with different alginateconcentrations vary from 2.86 to 8.16 wt%. Glycerol concentration was fixed at 30%. All curves are shifted vertically by a factor, B, except for the solution with4.83 wt% of alginate concentration. (c) Activation energy, ΔEa, for alginate/water and alginate/water/glycerol solutions with different alginate concentrations.
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alginate melt, we estimate the ⟨Rg2⟩1/2 values of alginate in water andwater/glycerol solution to 67 and 98 nm, respectively. It is interestingthat these values are very close to the corresponding Rg values obtainedfrom SLS measurements. This correspondence strongly supports theincreasing molecule size of the alginate induced from the addition ofglycerol.
The specific viscosities, ηsp, were carried out for various con-centration of alginate in water and water/glycerol solutions to confirmthe influence of the changes in the shape of molecular associations onthe mechanical properties. The specific viscosities for alginate in waterand water/glycerol solutions were double logarithmically plottedagainst the concentration of alginate as shown in Fig. 6a. The con-centration dependence of specific viscosity changes at the logarithmicconcentrations about 0.43 (11.55mgml−1) and 0.93 wt%(4.23mgml−1) for water/glycerol and water solution, respectively,indicating an overlap concentration. The overlap concentration for al-ginate/water/glycerol solution is very comparable with the resultsobtained from SLS measurements. Based on the mean field theory [30],the overlap concentration is closely related to own molecule size. Underan assumption of the conservation of the volume of dispersed particle,overlap concentration is estimated as c* ∼ M·Rg−3, indicating that theradius of gyration is an effective factor to determine the value ofoverlap concentration. As described in Table 1, the value of radius ofgyration for an alginate in water/glycerol solution is larger than that inwater solution without change of molar mass. Consequently, the algi-nate molecule in glycerol/water solution can have more effective in-teraction with its neighbor chain.
For the higher alginate concentration, the dependence of specificviscosity in water/glycerol solution obviously exhibited another criticalpoint which can be considered as entanglement concentration. Thepoint of the alginate in water/glycerol solution appeared in lowerconcentration than that in water. The discrepancy demonstrates thatthe addition of glycerol make a significant contribution to form en-tanglement between alginate molecules.
The viscosity in alginate/water/glycerol solution exhibited the closerelation with a concentration in whole experimental range. It is gen-erally known that the dependence becomes stronger when the solventquality descends, or molecule association increases [31]. To clarify theeffect of glycerol, the concentration dependence on the relaxation timewas compared. Fig. 6b shows the relation between the concentrationand steady relaxation time, τ, determined as same way in Eq. (2). Theconcentration dependence of relaxation time τ∼c2.7 and τ∼c1.5 canbe found for alginate in water and water/glycerol solutions. Based onphysics of polymeric system, the concentration dependence on relaxa-tion time was described as relationship of τ∼ c7/3 and τ∼ c1.6 in θ- andgood solvent, respectively [27]. These results suggest that the solventsystem for alginate shifts from θ-solvent to good solvent. The change ofsolvent quality also demonstrate the size change of alginate molecule indifferent solvent system. The addition of glycerol enhances solventquality, resulting in extension of molecule size by a large excludedvolume. Accordingly, the large size of the alginate with the flexible coilstructure promotes inter-molecule interaction.
3.4. Mechanical properties of alginate hydrogel
Fig. 7 shows the stress-strain curves of alginate hydrogel orientedfrom 4wt% alginate/water/glycerol solution with 0–50% glycerolconcentration. Interestingly, the remarkable enhancement of tensilestrength and stretchability was observed above 30% glycerol con-centration at which alginate molecules started to be preferable to haveentanglement, as discussed in rheological properties. The tensilestrength and stretchability exhibited 9.9 and 5.9 times enhancement,respectively, compared to the hydrogel from pure water solution. Cor-responding to previous results [15,18–20], the introduction of a jointpoint, such as double crosslinking and ionic bonding, in a hydrogelcould make significant contribution to hinder propagation of crack andgive elastic properties. It suggests that the entanglement can substitutethose contribution, as shown in Fig. 7b. This may be considered to bevalid, considering previous results for effect of degree of entanglementon mechanical properties in hydrogels [32]. However, the improvingstretchability was limited by 30% of glycerol concentration. When theglycerol concentration was higher than the critical concentration, thedeformability of the hydrogel was decreased with steep rise of Young’smodulus. In high glycerol concentration (> 30%), each individual al-ginate chain could form entanglement within a short molar mass(Fig. 4b). The decrease of entanglement molar mass increases a numberof tie point per unit volume. Consequently, the mobility of the alginatemolecules are more restricted in the chain network, resulting in hightoughness with low deformation.
4. Conclusions
In this study, high stretchable and tough alginate hydrogel wasprepared by control of the molecule conformation using glycerol.Glycerol deconstructed the intra-hydrogen bonding of alginate in aqu-eous media, resulting in enhancement of chain flexibility. In addition,the glycerol improved the solvent quality for the alginate molecule, andextended the molecule volume. The flexible and large alginate mole-cules became favorable to form the physical interaction. The numeroustie points significantly contributed to resist external force, whichgreatly improved the mechanical properties.
Fig. 6. (a) Concentration dependence of specific viscosity, ηsp, and (b)Concentration dependence of relaxation time, τ, for alginate/water solution andalginate/water/glycerol solution with 30% of glycerol concentration at RT.
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Acknowledgements
This research did not receive any specific grant from fundingagencies in the public, commercial, or not-for-profit sectors.
Data availability
The raw/processed data required to reproduce these findings cannotbe shared at this time as the data also forms part of an ongoing study.The data will be made available on request.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.eurpolymj.2019.03.017.
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Fig. 7. (a) Representative stress-strain curves of alginate hydrogels prepared from alginate/water/glycerol solutions with different glycerol concentration, rangingfrom 0 to 50%. (b) Tensile strength and elongation at break calculated from stress-strain curves, plotted against weight fraction of glycerol. (c) Elastic modulus andtoughness plotted against weight ratio of glycerol. All measurements are an average of triplicate samples. (d) Schematic representation showing different architectureand fracture propagation. For hydrogel from water solution, crack can be easily propagated chain to chain interface. For hydrogel from water/glycerol solution,deconstruction of intra-hydrogen bonds by glycerol can allow to formation of interaction between alginate molecules. These networks efficiently transfer the loadover deformation zone.
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