Liquid Crystalline Phase Behavior and Sol Gel Transition in Aqueous

Liquid Crystalline Phase Behavior and Sol?Gel Transition in Aqueous Halloysite Nanotube Dispersions

Zhiqiang Luo,?Hongzan Song,*,?Xiaorui Feng,?Mingtao Run,?Huanhuan Cui,?Licun Wu,?Jungang Gao,?and Zhigang Wang*,?

?College of Chemistry&Environmental Science,Hebei University,Baoding,Hebei Province071002,China

?CAS Key Laboratory of Soft Matter Chemistry,Department of Polymer Science and Engineering,Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,Hefei,Anhui Province230026,China

*Supporting Information

liquid crystalline phase behavior and sol?gel transition in halloysite nanotubes(HNTs)aqueous investigated by applying polarized optical microscopy(POM),macroscopic observation,rheometer,

electron microscopy,and transmission electron microscopy.The liquid crystalline phase starts concentration of1wt%,and a full liquid crystalline phase forms at the HNT concentration of25wt

macroscopic observation.Rheological measurements indicate a typical shear?ow behavior for

concentrations above20wt%and further con?rm that the sol?gel transition occurs at the HNT Furthermore,the HNT aqueous dispersions exhibit pH-induced gelation with more intense birefringence (HCl)is added.The above?ndings shed light on the phase behaviors of diversely topological

fabrication of the long-range ordered nano-objects.

INTRODUCTION

Halloysite nanotubes(HNTs,Al2Si2O5(OH)4·n H2O),a type of newly emerging clay with a nanotubular structure,are available in abundance in many countries and have recently become the subject of research attention as a new type of material.1?11 More importantly,HNTs possess huge speci?c surface area, abundant hydroxyl groups,environmental friendliness,and biocompatibility,not only making them have potentials as additives for enhancing the mechanical performances,12?14 thermal stability,15,16and nucleating agents for crystallization of polymers17,18but also making them attractive candidates for a variety of potential applications,including controlled release of protective agents,19?22biomimetic reaction vessels,23adsorp-tion agents,24,25corrosion prevention agents,26and cata-lysts.27,28

On the other hand,dispersions of nanoparticles are widely investigated because of their uses in many industrial applications(foods,pharmaceuticals,cosmetics,paints,etc.).29 In particular,due to their high anisosymmetric structures,the anisotropic nanoparticles can form liquid crystalline phases,such as those observed for carbon nanotubes

graphene oxide(GO),38,39mineral,40?46gibbsite platelets,47,48 cellulose,and chitin nanocrystals,49?51which are recognized as key potential precursors for the?uid phase processing of particles into aligned materials with outstanding properties.52,53 However,most studies neglect the key feature of HNTs,i.e., their anisotropic shape(300?1500nm in length and15?50 nm in outer diameter,that is,the aspect ratio typically ranging between6and100).Up to now,to the best of the authors’knowledge,there are no available data concerning on the liquid crystalline behaviors of HNT aqueous dispersions.

In this work,we report that HNT aqueous dispersions show the isotropic?liquid crystalline?liquid crystalline gel phase transitions.For a complete understanding,we have investigated the liquid crystalline phase transition behaviors versus HNT concentration for HNT aqueous dispersions.We also have

Received:July25,2013

Revised:September11,2013

Published:September26,2013

investigated the in?uence of pH on the formation of liquid crystalline and gelation of HNT aqueous dispersions.Finally, the possible formation mechanism for the liquid crystalline and liquid crystalline gel has been proposed.The current work illustrates that the liquid crystalline and liquid crystalline gel characters of HNT aqueous dispersions can be enhanced with increasing HNT concentration and/or with an addition of acid. Our?ndings could facilitate the large-scale alignments of HNTs in the?uid phase,open the way to make the long-range ordered structures of HNT-based functional materials,and o?er the opportunities to uncover the complex phase transition behaviors for HNT dispersions with particular topologies.■EXPERIMENTAL SECTION

Materials.Halloysite nanotubes(HNTs)were obtained from GuangZhou Shinshi Metallurgy and Chemical Company Ltd. (Guangzhou,China).Sodium hexametaphosphate[(NaPO3)6], sodium hydroxide(NaOH),and hydrochloric acid(HCl,concentrated 37%v/v)were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai,China),which were all analytical grade reagents and were used with no further treatments.

Puri?cation of HNTs.About250g of halloysite nanotubes powder and500mL of deionized water were mixed in the?ask and stirred for 2h.Sodium hexametaphosphate[(NaPO3)6](1.25g)was added gradually under continuous stirring,and then10wt%NaOH aqueous solution was added to adjust the pH value to between8and9.The mixture was further stirred for24h and then left to stand for6h.The impurities and large bundled HNTs precipitated at the bottom of the ?ask,while individual HNTs mainly remained in supernatant.The supernatant was decanted and then centrifuged at3000rpm for5min. The supernatant was decanted again for removing much long unbundled HNTs,and then a centrifugation at7000rpm for10 min was further performed.Afterward,the obtained precipitates were repeatedly washed with deionized water and centrifuged successively for at least three centrifugation cycles until the decantate became neutral.Finally,the milky solids were obtained.The vacuum freeze-drying method was used to further enhance the solubility of HNTs and improve the stability of HNT dispersions.The?nal sample represented a white solid that could be easily crushed into powder using mortar.

Preparation of HNT Aqueous Dispersions.The puri?ed HNTs were added into deionized water under stirring in sample bottles,and the mass concentrations were measured in wt%at25°C.The HNT concentration range was from0.1to50wt%. Characterizations.The measurements of zeta potentials of the HNT dispersions were performed on DelsaNano C Particle Size and Zeta Potential Analyzer(Beckman Coulter,Inc.).The HNT aqueous dispersions were loaded in beakers and quenched in liquid nitrogen. The frozen solids were quickly transferred to the vacuum freeze-dryer and kept at?60°C until completely dried.The?u?y solids were carefully put on?at substrates precoated with carbonic glues and then were coated with platinum for scanning electron microscopy(SEM) observations in the secondary electron imaging mode by using a JEOL SEM6700operating at5kV.Transmission electron microscopy (TEM)observations on HNTs were performed by using a JEOL JEM-2200FS with an accelerating voltage of200kV.Drops of each HNT dispersion were cast and sandwiched between two glass slides to form the?lm with a thickness of about50μm,and then the polarized optical micrographs were taken by using the Olympus BX51polarized optical microscope.In order to observe the color changes and distinguish the structural changes during phase transitions,a530nm sensitive tint plate(1λ,U-TP530,Olympus,Japan)was used as a test plate compensator,which resulted in a magenta background for the taken optical micrographs.All the observations were conducted under a nitrogen atmosphere.Small-angle X-ray scattering(SAXS)measure-ments were performed by using an in-house setup with a sealed tube equipped with two parabolic multilayer mirrors(Bruker,Karlsruhe), giving a highly parallel beam(divergence about0.012°)of monochromatic Cu Kαradiation(wavelengthλ=0.154nm).The SAXS patterns were recorded with a two-dimensional gas-?lled wire detector(Bruker Hi-Star).The HNT aqueous dispersions were injected into1cm diameter disklike copper cell,which was sealed with two Kapton?lm windows and aligned perpendicular to the X-ray beam.The sample-to-detector distance was2463mm.The SAXS intensity pro?les were extracted from the isotropic two-dimensional SAXS patterns.The rheological measurements were performed on a stress-controlled rheometer(TA-AR2000EX,TA Instruments) equipped with a cone-and-plate geometry(diameter40mm;angle 1°).Before the oscillatory shear measurements,a strain sweep from0.1 to100%with a?xed frequency of6.28rad/s was performed for each dispersion to determine the linear viscoelastic regime.The chosen strains of1?10%fell well within the linear viscoelastic regime for the frequency range of0.1?100rad/s in the oscillatory shear measure-ments.The experimental temperature was mainly set at25°C.For each sweep measurement,repeat specimens are requested,and the number of repeat specimens is three in order to examine the data reproducibility.

■RESULTS AND DISCUSSION

To obtain the liquid crystalline phase for the HNT aqueous dispersions,the?rst step is to guarantee su?cient solubility/ dispersibility and stability.The oven-dry method was widely used to prepare HNTs.However,the obtained HNT samples by this method are di?cult to crush into powder and its aqueous dispersions are poorly dispersed and unstable.54Large HNT aggregates can be seen in the sample prepared by the oven-dry method(see Figure S1in the Supporting Information).On contrast,the freeze-drying method has many applications for nanoparticle technology,especially for preventing from particle aggregation and improving solubility and long-term nanoparticle stability.55Final HNT sample prepared by the freeze-drying method represents a white?u?y solid that can be easily crushed into powder using https://www.360docs.net/doc/6d5771565.html,rge HNT aggregates are not seen in the sample prepared by the freeze-drying method(Figure S2).Therefore,compared with the common oven-dry method,we applied the vacuum freeze-drying method to further enhance the dispersity of HNTs and improve the stability of HNT dispersions.TEM micrographs shown in Figure S3clearly demonstrate that HNTs prepared by the freeze-drying method have better dispersity in water than that prepared by the oven-dry method.This is because the freeze-drying method can decrease the aggregation of HNT particles and increase the speci?c surface area,resulting in the increase of solubility and stability of HNTs in water.Figure1

shows photos of the HNT dispersions in aqueous media for48 h after mixing.(We note here that no sediments appear for these dispersions for2weeks after mixing.)Interestingly,the HNT dispersions are homogeneous with no sediments observed,which indicates the su?cient dispersion and stability of HNTs in aqueous media.Noticeably,the40wt%

aqueous Figure1.Photographs of HNT dispersions with HNT concentrations of0.1,1,10,20,and40wt%.

HNT dispersion does not ?ow when turning the sample bottle upside down,indicating it becomes a gel-like sample.It is known that the dispersion stability can be expressed by the electrostatic interactions evidenced by the zeta potential measurements.54Figure 2shows the changes of zeta potential with pH for 10wt %HNT aqueous dispersions prepared by the oven-dry method and freeze-drying method,https://www.360docs.net/doc/6d5771565.html,pared with the 10wt %HNT dispersion prepared by the oven-dry method,which generally features a low zeta potential (?35.6mV)of HNTs in water,the 10wt %HNT dispersion prepared by the freeze-drying method shows a higher zeta potential (?56.3mV).The higher zeta potential for the freeze-drying HNT dispersion makes the aqueous HNT dispersion more stable.Note the di ?erences of zeta potential between the oven-dry method and freeze-drying method are a ?ected by HNT concentration,which increases with increasing HNT concentration.For example,the zeta potential changes from ?51.3to ?62.5mV for the 0.1wt %HNT dispersions.The reason may be that the freeze-drying method can reduce the aggregation of HNT particles to form larger aggregates than the oven-dry method at the same HNT concentration.In other words,the oven-dry method produces larger particles,resulting in the changes of particle movement in the electric ?eld due to the gravitational e ?ect.It is known that the liquid crystalline formation is in ?uenced by the aspect ratio and size distribution of dispersed nanoparticles.Therefore,the sizes of HNTs measured by SEM and TEM are displayed in Figure 3.The results indicate that the HNTs have an average length of 572nm with a standard deviation (σ)of 196nm,an average outer diameter of 56nm (σ=14nm),and an average lumen diameter of 21nm (σ=7nm).The corresponding average aspect ratio (mean length/mean diameter)is about 10.Such a su ?cient aspect ratio and appropriate solubility/dispersibility and stability in water for HNTs should be prerequisites for the formation of a liquid crystalline phase and lay the foundation for our detailed studies on the liquid crystalline behaviors of HNT dispersions.56It is well recognized that a direct evidence for appearance of the lyotropic liquid crystalline phase is the evolved birefringence between analyzer and polarizer upon increasing particle concentration.The polarized optical micrographs of HNT aqueous dispersions at 25°C are displayed in Figure 4.In order to clearly demonstrate the related phase transitions,the micrographs were taken with the sensitive tint plate insertion.(For comparison,the micrographs taken without using the sensitive tint plate are shown in insets.)When the HNT concentration is 0.1wt %or lower,the dispersion is isotropic (Figure 4a).At the HNT concentration of 1wt %the emergence of microscopic birefringence and threadlike textures indicates the onset of formation of a liquid crystalline phase (Figure 4b).With increasing HNT concentration,the optical textures become more compact and the birefringence becomes stronger (Figure 4c,d).As HNT concentration increases to 25wt %,the dispersion shows birefringence with intense colors,which indicates the formation of the anisotropic phase (Figure 4e).A similar phase transition behavior was observed in

our Figure 2.Changes of zeta potential with pH for 10wt %HNT

aqueous dispersions prepared by the oven-dry method and freeze-drying

method.Figure 3.(a,b)TEM images of HNTs dispersed in water,(c)SEM image of HNTs,and (d,e,f)the length,outer,and lumen diameter distributions of HNTs measured from (a,b).

previous study on the concentrated microcrystalline cellulose (MCC)/ionic liquid (1-ethyl-3-methylimidazolium acetate,EMIMAc)(MCC/EMIMAc)solutions,in which the texture was ascribed to the cholesteric phase (chiral nematic phase)formation,and the planar textures were interpreted as a nonaligned cholesteric phase associated with the formation of the lyotropic liquid crystalline solution.57,58When the HNT concentration reaches 37wt %,the texture looks more random with bright colors,and the rheological characterization at this concentration reveals that the dispersion system eventually becomes a gel (Figure 4f).A similar behavior was observed in aqueous mixtures of carbon nanotubes (CNTs),59chitin nanocrystal,49and cellulose nanocrystal.50These dispersions contain a cholesteric phase (chiral nematic phase),which disappears at the high particle concentrations.On the other hand,for the liquid crystalline HNT dispersions,macroscopic textures can be generally observed if the dispersions are placed between two crossed polarizers.The phase separation occurs with long-time standing or can be speeded up by centrifugation,and the equilibrium between di ?usion and sedimentation results in two phases with a distinct interface.These macroscopic results intuitively a ?rm the liquid crystalline phase transition of the dispersions.38,47,48We performed the macroscopic texture observation.Figure S4shows the macroscopic photographs of aqueous HNT dispersions after phase separation taken between two crossed polarizers (Figure S4A)and the plotted relationship between the volume fraction of the anisotropic phase after phase separation and HNT concentration (Figure S4B).The results in Figure S4reveal an evolution of the three phase states including isotropic,biphasic,and liquid crystalline phases with increasing HNT concentration and con ?rm that the phase transition concentration from the biphasic phase to a full liquid crystalline phase is 25wt %,which is consistent with that obtained from the polarized optical microscope observation (Figure 4).Once the liquid crystalline phase of HNT dispersions has been validated,we investigated the liquid crystalline structural information by scanning electron microscopy (SEM)and small-angle X-ray scattering (SAXS).The freeze-dried HNTs derived from the isotropic HNT dispersion (0.5wt %)show disordered structure,and no obvious HNT clusters can be observed in Figure S5a,b.For the HNTs derived from the biphasic HNT dispersion (20wt %),some ordered domains surrounded by irregular HNT tubes can be seen in Figure S5c,d.For the HNTs derived from the fully anisotropic HNT dispersion (35wt %),various oriented HNT alignments are clearly shown in Figure S5e,f.In addition,it can be seen from these SEM micrographs that the liquid crystalline phase of HNTs has positional ordering at the high HNT concentrations.Note that the HNT dispersions contain a type of cholesteric phase,nonaligned cholesteric phase,and the corresponding texture is a planar kind (Figure 4).The reason for appearance of nonaligned cholesteric phase may lie in the high viscosity or high concentration,which prevents from migration or aggregation of the liquid crystalline phase into more organized domains,resulting in some irregular structures with low orientation.Recall that the HNTs have the average length of 572nm,average outer diameter of 56nm,and average lumen diameter of 21nm.The average aspect ratio is about 10,which is smaller than other common anisotropic nanoparticles.50,59Therefore,the critical HNT concentration for the formation of liquid crystalline phase is higher than other common anisotropic nanoparticle dispersions,which results in the higher viscosity.For our HNT dispersions,the smectic phase is not observed.However,the common nematic phase can be easily observed under shear or centrifugation conditions (Figure S6).60

The SAXS pro ?les in Figure 5further provide the detailed information on the positional ordering as a function of HNT concentration for the HNT dispersions.It can be seen that all the dispersions show one broad weak SAXS peak except for 0.1wt %HNT dispersion.The reason for the appearance of one broad weak SAXS peak may be that the HNT dispersions contain nonaligned cholesteric liquid crystals,which have some irregular structures with low orientation.Furthermore,the SAXS peak position (remarked by the orange dashed line)for 1?50wt %HNT dispersions shifts to higher q value with increasing HNT concentration,indicating that the average distance d (d =2π/q )between HNT particles in planes perpendicular to the director decreases with increasing HNT concentration.60Thus,SEM and SAXS results

clearly

Figure 4.Polarized optical micrographs of

HNT dispersions

with di ?erent

HNT

concentrations

at 25

°C with the sensitive tint plate insertion:(a)

0.1,(b)1,(c)10,(d)20,(e)25,and (f)37wt %.The white scale bar represents 100μm and is applied to all the micrographs.Insets show corresponding micrographs without the sensitive tint plate insertion.The white scale bar in the top left inset represents 100μm and is applied to all insets.

demonstrate that the orientation ordering of HNTs in the dispersions are strongly dependent on the HNT concentration.Rheological methods have been widely used to study the nanoparticle (rods,plates,etc.)dispersions because they can detect the presence of internal microstructures.The micro-scopic connectivity with three-dimensional networks produced from physical interactions (van der Waals,hydrogen bonds,and electrostatic interactions)can be investigated using the rheological methods.49,61At a critical concentration of nanoparticles,the viscoelastic response of the dispersion system changes from the liquid-like to solid-like behaviors.The dynamic rheological behaviors of the HNT aqueous dispersions were examined.The changes of storage modulus,G ′,and loss modulus,G ″,as functions of angular frequency,ω,for the HNT aqueous dispersions at 25°C are shown in Figure 6.The HNT concentrations of 10,37,and 50wt %are indicated in the ?gure.The typical characteristics of these modulus ?frequency curves can be ?gured out directly.Approximately two distinct groups of curves are separated by the concentration value of 37wt %.For an ideal gel that behaves elastically,the G ′values are expected to be independent of frequency and G ′>G ″.As it can be seen from Figure 6,for the 50wt %HNT dispersion,the G ′values are always higher than G ″in the explored frequency range,showing a strong frequency independence.However,at the low concentration of 10wt %,the HNT dispersion exhibits a liquid-like behavior because G ′

concentrations lower than 20wt %,low shear plateaus appear after the initial shear thinning region.For HNT concentrations between 20and 35wt %,the HNT dispersions signify a typical shear ?ow behavior and show decreased viscosity upon shear,likely because of the deformation of the existing gel network.Moreover,for the 40and 50wt %HNT dispersions the viscosity shows almost linear decrease with increasing shear rate,which suggests the deformation of the existing networks for gelation.Note that the viscosity at the low shear rate does not change monotonically with increasing HNT concentration;that is to say,it has a steep increase up to the maximum value at the HNT concentration of 15wt %,a drop to the minimum value at the HNT concentration of 20wt %,and then a steep increase again at higher HNT concentrations.However,at the high shear rates the viscosity shows a monotonic increase with increasing HNT concentration.

A particular characteristic for the lyotropic liquid crystals is that the steady shear viscosity and dynamic complex viscosity are not equal when the shear rate equals the frequency,which indicates that the Cox ?Merz rule is not obeyed.Figure S7shows the evolutions of steady shear viscosity with shear rate and dynamic complex viscosity with frequency for 20and 50wt %HNT aqueous dispersions,respectively.For the lower HNT concentration dispersion,the biphasic dispersion shows less obvious deviation from the Cox ?Merz rule at the low shear rate/frequency but shows obvious deviation at the high shear rate/frequency (Figure S7a).Whereas for the higher HNT concentration dispersion,the liquid crystalline gel dispersion shows obvious deviation from the Cox ?Merz rule throughout the whole shear rate/frequency range (Figure S7b).For a normal polymer solution or colloidal dispersion,the viscosity at the low shear rate increases with concentration.However,for the liquid crystalline dispersions,the

viscosity

Figure 5.SAXS pro ?les of HNT dispersions with HNT concentration

ranged from 0.1to 50wt %.The spectra depict the Lorentz-corrected scattering intensity as functions of scattering vector,q (q =(4πsin θ)/

λ=2π/d ,where 2θis the scattering

angle).Figure 6.Changes of storage modulus,G ′,and loss modulus,G ″,as functions of angular frequency,ω,at 25°C for HNT aqueous dispersions with di ?erent HNT

concentrations.Figure 7.Changes of steady shear viscosity with shear rate for HNT dispersions

with di ?erent HNT concentrations.The measurements were performed at 25°C.

does not change monotonically with increasing concentration and goes through a maximum in the biphasic region.Figure 8

shows the changes of steady shear viscosity as functions of HNT concentration for HNT dispersions at the shear rates of 1,10,and 100s ?1.Note that the separation of phase transitions by vertical dashed lines in Figure 8is based on the polarized optical microscopic observation (Figure 4),macroscopic observation (Figure S4),and rheological measurements (Figures 6and 7).Clearly,the maxima are observed at the HNT concentration of 15wt %in the biphasic region at the shear rate of 1s ?1,which is due that the viscosity increases with HNT concentration as long as the dispersions are predom-inantly isotropic,whereas the increasing fraction of anisotropic domains (liquid crystalline)eventually results in a decreased resistance to shear ?ow,and thus the viscosity decreases with further increasing HNT concentration.Furthermore,the magnitude of this maximum decreases when the shear rate increases to 10s ?1.Surprisingly,no maximum can be observed at the 15wt %HNT concentration in the biphasic region at the shear rate of 100s ?1.This result may suggest that the liquid crystalline ordering is disrupted by so high shear rate,and the viscosity only shows a monotonic increase with HNT concentration.57The shear viscosity ?concentration curves for HNT dispersions can be separated into four regions corresponding to the isotropic,isotropic +liquid crystalline,liquid crystalline,and LC gel phases,as remarked by the vertical dashed lines in Figure 8.This observation is similar to some reports about the lyotropic liquid crystalline solutions or dispersions.38,61For the charged nanoparticle dispersions,the isotropic to anisotropic and the liquid crystalline to gel transitions are sensitive to pH values (acid control),and the phase transitions occur at the lower particle concentrations at the acidic condition compared with the H +-free aqueous dispersions (neutral condition).49,62In our case,drops of 1M hydrochloric acid (HCl)were added into the 15wt %HNT dispersion to obtain the pH value of 5.0.Figure 9shows the dynamic rheological behaviors of HNT aqueous dispersions under the acidic (pH =5)and neutral (pH =7)conditions.As can be seen from Figure 9,the 15wt %HNT aqueous dispersion with no addition of HCl shows a typical liquidlike behavior.However,when the dispersion added with HCl approaches the pH value of 5.0,the dispersion exhibits a strong gel-like behavior.This result indicates that the associative interactions become dominating in the system,leading to the network formation.The associative interactions can be attributed to the electrostatic screening of HNTs by the H +addition,which leads to the minimization of the repulsive forces due to the electrostatic charges on the outer surfaces of HNT nanotubes.As shown in the polarized optical micrographs of Figure S8,the H +-free 15wt %HNT aqueous dispersion exhibits relatively weak birefringence (Figure S8a),whereas the dispersion exhibits strong birefringence when the dispersion approaches the pH value of 5.0(Figure S8b).It is also worth noting that the sol ?LC gel transition is more evident under the acidic condition for the HNT aqueous dispersion.However,these particular LC gels display inhomogeneous structures with some dark nonbirefringent portions,indicating some di ?erent structural organization from the LC gel sample with the higher HNT concentration (37wt %HNTs,with no addition of HCl)shown in Figure 4f,which exhibits the birefringence in the whole volume range.Similar results have been reported in the chitin nanocrystal dispersions at di ?erent pH levels.49

On the basis of the above results,the microstructural changes in the HNT aqueous dispersions with increasing HNT concentration and addition of HCl are schematically depicted in Figure 10.HNTs have positively charged inner surfaces and negatively charged outer surfaces in water,resulting in strong repulsive electrostatic forces rather than attractive interaction,and the dilute HNT dispersions show a random distribution of HNTs (Figure 10a),whereas some ordered alignments of HNTs appear in the biphasic region,which can be explained by the Onsager theory for parallel alignments of anisotropic particles on the entropic term (Figure 10b).With further increasing HNT concentration,the completely ordered align-ments of HNTs are obtained (Figure 10c),and eventually a percolated liquid crystalline network forms as a result of strong correlation between the adjacent liquid crystalline domains (Figure 10d).Liquid crystalline network also forms when HCl is added in the HNT aqueous dispersion in the biphasic region (Figure 10e).The reason for this particular liquid crystalline network formation is that the repulsive electrostatic forces are reduced,and the HNTs can easily aggregate when adding the acid.As the pH reaches even lower values,i.e.pH 3.0,the HNT dispersions exhibit more obvious liquid crystalline phase and stronger gel-like behavior.On the contrary,as the pH

reaches

Figure 8.Changes of steady shear viscosity as functions of HNT concentration for HNT dispersions at the shear rates of 1,10,and 100s.The vertical dashed lines separate the HNT dispersions into the

di ?erent phase

regions.Figure 9.Changes

of G ′,G ″,and |η*|as functions of ωat 25°C for 15

wt %HNT aqueous dispersions under pH =5(acidic)and pH =7

(neutral)conditions.Insets show the HNT dispersion under neutral condition

(the bottom right corner)and the dispersion under acidic condition (the top left corner),corresponding to the typical ?ow and gel states,respectively.

even higher pH value,i.e.pH 9.0,the HNT dispersions show a weaker liquid crystalline phase and weaker gel-like behavior.Thus,the H +can be considered as cross-linking points between adjacent HNTs,which have the negatively charged outer surfaces.Furthermore,some HNTs can be oriented to form new parallel aggregates,leading to the formation of the liquid crystalline network structure.63,64

■CONCLUSIONS The HNT aqueous dispersions shift from isotropic toward lyotropic liquid crystalline and liquid crystalline gel phases with increasing HNT concentration.HNT aqueous dispersions exhibit the pH-induced gelation and more intense birefringence with addition of HCl.To the authors ’knowledge,this is the ?rst report on the lyotropic liquid crystalline and liquid crystalline gel phases for the HNT aqueous dispersions.These results are essential to expand the understanding on the relations between the liquid crystalline phase and sol ?gel transition.It is believed that these liquid crystalline and liquid crystalline gels can have some optical functions for the HNT long-range ordered materials and potentially provide the foundation for their biological applications,especially for fabrication of highly ordered supramolecular complex of HNTs and biopolymers (DNA,amylose,etc.).■ASSOCIATED CONTENT *Supporting Information

Figures S1?S8show SEM and TEM micrographs of HNTs prepared by the oven-dry method and the freeze-drying method,macroscopic observation on the phase transition for HNT aqueous dispersions,SEM micrographs of freeze-dried HNT foams,SEM micrograph of dried HNT sample after shear,Cox ?Merz rule for HNT aqueous dispersions,and polarized optical micrographs for 15wt %HNT aqueous dispersions at di ?erent pH values.This material is available free of charge via the Internet at https://www.360docs.net/doc/6d5771565.html,.■AUTHOR INFORMATION Corresponding Authors

*

E-mail:songhongzan@https://www.360docs.net/doc/6d5771565.html, (H.S.).

*E-mail:zgwang2@https://www.360docs.net/doc/6d5771565.html, (Z.W.).Notes

The authors declare no competing ?nancial interest.■ACKNOWLEDGMENTS

Z.W.acknowledges the ?nancial support from the National Basic Research Program of China with Grant No.2012CB025901and National Science Foundation of China with Grant No.51073145.H.S.acknowledges the ?nancial support from National Science Foundation of China with Grant No.21304029,the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No.20121301120004),the Natural Science Foundation of Hebei Province (Grant No.B2013201117),the Plan of Science Technology Research and Development of Hebei Province (Grant No.12211204),and Hebei University (Grant No.Y2011223).The authors thank Dr.Jun Zhang ’s group at Institute of Chemistry,Chinese Academy of Sciences,for the assistance on TEM measurement.■REFERENCES

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