ORIGINAL CONTRIBUTION

Gel formation and photoactive propertiesof azobenzene-containing polymerin liquid crystal mixture

Alexey Bobrovsky & Valery Shibaev & Vera Hamplova &

Miroslav Kaspar & Milada Glogarova

Received: 7 June 2010 /Accepted: 6 July 2010 /Published online: 28 July 2010# Springer-Verlag 2010

Abstract For the first time, a photochromic azobenzene-containing liquid crystalline (LC) acrylic polymer was usedfor gelation of low-molar-mass nematic mixture (LMNM).Dissolution of LC polymer in amount of only 2.5 wt.% inLMNM at 120°C (isotropic state) followed by coolingdown results in formation of the solid-like photochromicLC gel. Gelation is associated with a phase separation andformation of microsized LC polymer domains, which forma physical “network” containing encapsulated nematic host.Textural changes of mixture during gel formation wereanalyzed, and absorbance spectra were measured. A specialattention was paid to the kinetic study of photoinduced E-Zand Z-E isomerization of azobenzene side groups ofpolymer in gel. It was shown that ultraviolet (UV)-irradiation and E-Z isomerization processes are accompa-nied by disruption of H-aggregates of azobenzene moietiesand partial dissolution of polymer.

Keywords Gel . Azobenzene-containing polymer .

Photoisomerization . Nematic phase

Introduction

Gels formed by different classes of gelators have emerged asimportant elastic soft materials over the years due to their wide

range of applications. There are a large number of papersincluding reviews devoted to the study of different kinds ofgels [1–4]. The synthesis and preparation of gel-formingsystems are directed toward the development of materialswith novel field-responsive properties, i.e. smart materials.

Among these systems self-assembled LC gels are a verypromising type of materials [5–12]. They are usuallyobtained by addition of a small amount of low-molar-mass substance forming nanoscaled fibrous aggregates intoLC matrix. The interaction of the microfibers or nanofiberswith the LC host can impart interesting properties to thistype of LC material. Among different types of LC gelsphoto- and electro-active systems are of a special interestbecause of the unique possibility to manipulate theirproperties, structure, and ordering using light or electricfield. As an example, in papers [7, 11], it has beendemonstrated how light irradiation can be utilized forreversibly induced gel–sol transition due to E-Z isomeriza-tion of azobenzene fragment of gelator.

It is noteworthy that in all papers concerning responsive LCgels, low-molar-mass compounds are used as the gelatoragents. Another large area of investigations deals with highlycross-linked three-dimensional LC networks usually obtainedby photopolymerization of 1–5% of bifunctional monomers inLCmixtures [13–22]. In the latter case, gelation is completelyirreversible and is used for stabilization of LC moleculesorientation and improvement of electrooptical performance ofelectrooptic devices (so-called polymer-stabilized LC sys-tems). There are only few papers covering investigations ofhydrogels based on non-cross-linked side-chain carboxyl-containing LC polymers swelled in water [23, 24].

In addition, it should be pointed out that several yearsago, one of us (V.S.) with other coworkers observed thegel formation for a series of comb-shaped acrylic andmethacrylic polymers in aliphatic alcohols and hydro-

A. Bobrovsky (*) :V. ShibaevFaculty of Chemistry, Moscow State University,Leninskie Gory,Moscow 119992, Russiae-mail: bbrvsky@yahoo.com

V. Hamplova :M. Kaspar :M. GlogarovaInstitute of Physics, Academy of Sciences of the Czech Republic,Prague 8 182 21, Czech Republic

Colloid Polym Sci (2010) 288:1375–1384DOI 10.1007/s00396-010-2264-0

carbons [25, 26]. Poly-n-acrylates and poly-n-methacrylateswith long side groups with n=16, 18, 22 easily formed gelsin high homologs of alcohols such as n-octanol, dodecanol,as well as n-hydrocarbons starting with n-decan up to n-hexadecan. It should be emphasized that gel formation wasobserved at the very low polymer concentration 0.3–0.5%.Taking into account this unusual feature of comb-shapedpolymers in this work, we used more complicated structureof one of polyacrylic LC polymer of comb-shaped structurecontaining photochromic (azobenzene) and chiral groups,having in mind to study the gel formation and photoopticalproperties of the polymer in LC media.

On the other hand, the study of LC photochromicpolymers is quite large growing area of investigations.Great potential of photoinduced control of supramolecularstructure, optical and mechanical properties was demon-strated in a number of papers [27–31]. Thus, the use of LCphotochromic polymers as the photoswitchable gelatoragents presents an interesting but still unexplored task forresearchers. In this connection, in the present paper, wehave studied, for the first time, LC gels formed byincorporation of chiral photochromic side chain LCpolymer PAzo as a gelator in low-molar-mass liquidcrystals:

CH2 CH

COO (CH2)10 O

O N

N OCH2CH*CH2CH3

CH3

OPAzo

This polymer consists of polyacrylic backbone, spacer with10 methylene units, azobenzene chromophore and terminalchiral group. Long aromatic fragment and long spacer allowone to obtain crystalline and LC phases with high clearingtemperature. These properties play a crucial role for self-organization and gel formation. Existence of chiral fragment isimportant for an appearance of a chirooptical photoresponse,which we will separately consider in an appropriate paper.

Mesomorphic properties of the polymer were studied byus using polarizing optical microscopy, differential scan-ning calorimetry (DSC), and X-ray scattering [32]:

Cr 118 (5.8) SmC* 145 (-) SmA* 154 (0.6) N* 194 (2.3)I (enthalpies of the phase transitions are presented inparentheses)

The main goals of this work are to study the textural andspectral changes of mixture during the gel formation. Wespecially focused on the E-Z and Z-E isomerizationprocesses of azobenzene fragments of polymer in gel andtheir influence on gel properties including comparison withphotooptical properties of azobenzene monomer solutions.

Experimental part

Materials

As LC matrices, three nematic mixtures—MLC6816,1 E48,and BL087 (Merck)—have been tested. However, gel

formation occurred only for mixture of cyclohexanederivatives, MLC, and all photooptical measurements wereperformed using this LC mixture which has clearingtemperature 76–77°C.

Acrylic azobenzene-containing monomer was synthesizedaccording to the synthetic procedure presented in Scheme 1.

Preparation of 4-(10-acryloyloxydecyl)oxybenzoic acid (3)

4-Hydroxyacetophenone [0.1 M (13.6 g)] was alkylated by0.3 M (90 g) 1.10-dibromodecane and 5.6 g potassiumhydroxide in water/dioxane solution. The vigorously stirredsolution was heated under reflux for 3 days. Dioxane wasremoved by evaporation under reduced pressure and theresidue was diluted by water and extracted twice with100 ml of n-hexane to remove of excess of the dibromo-decane.

Acid (1) was obtained by oxidation with NaBrO indioxane by usual method. The crude product was crystal-lized from ethanol, dried, and converted into iodine (2) byboiling in sodium iodine/acetone mixture for 6 h. Finally,the product was dried in vacuum at 60°C.

Dry potassium acrylate (0.1 M) was dissolved in hotdimethylsulfoxide, and after cooling down, 0.05 M ofiododecyloxybenzoic acid (2) and the mixture was stirredfor several days at room temperature. Then the reactionmixture was poured into water, the precipitate wasfiltered by suction and washed by dilute (5%) hydro-chloric acid and water. Acid (3) was finally crystallizedfrom acetone.1 This nematic mixture will be briefly designated as MLC.

1376 Colloid Polym Sci (2010) 288:1375–1384

1H-Nuclear magnetic resonance (NMR) of (3), CDCl3,300 MHz:

8.07d (2H, ortho to -COOH); 6.92d (2H, ortho to –OR);6.40 and 5.81dd (1 + 1H, CH2=); 6.15dd (1H, =CHCOO);4.18t (2H, COOCH2); 4.02t (2H, CH2OAr); 1.3–1.8 m(16H,CH2).

Preparation of 4-(2-methylbutyl)oxy-4´-hydroxyazobenzene (5)

0.1 M (15.1 g) of 2-methylbutylbromide, 0.1 M (15.1 g)of p-acetamidophenol and 5.6 g of potassium hydroxidewas heated under reflux in dioxane/water (1:1) solutionfor 24 h. Then the mixture was evaporated to drynessand extracted by chloroform twice. After removing thesolvent the residuum was boiled for 4 h in 100 ml of20% sulphuric acid. Then 100 ml of water was addedand the suspension was left to stand during for the nightat room temperature. Amine salt (4) was filtered bysuction and crystallized from water. The diazotation anddiazo-coupling with phenol were carried out by usualmethod. The azo dye (5) was crystallized from ethanol anddried in vacuo.

1H-NMR of (5), CDCl3, 300 MHz:7.82dd (4H, ortho to –N); 7.00d (2H, ortho to –OR);

6.92d (2H, ortho to –OH); 3.83 m (2H,CH2OAr); 1.9 m

(1H, CH*); 1.60 and 1.30 m (2H, CH3CH2); 1.02d (3H,CH3C*); 0.98t (3H,CH3).

Preparation of final product, monomer A10HAzo

The final product was obtained by condensation of the acid(3) and azo dye (5) in dichloromethane/tetrahydrofuranesolution in presence of dicyclohexylcarbodiimide as acondensation agent and dimethylaminopyridine as a cata-lyst. The crude product was purified by column chroma-tography under day light protection. Silica gel (0.063–0.100mm, Merck) was used as a stationary phase using a mixture(99.8: 0.2) of dichloromethane and acetone as an eluent.Product was crystallized twice from ethanol. Structure ofthe final product was confirmed by 1H-NMR (300 MHz,Varian). The chemical purity of materials was checked byhigh-pressure liquid chromatography (HPLC) using a silicagel column (Biosphere Si 100-5 μm, 4×250, Watrex) witha mixture of 99.9% of toluene and 0.1% of methanol as aneluent and detection of the eluting products by a UV-visible(Vis) detector (λ=290 nm). The chemical purity was foundbetter than 99% under these conditions.

1H-NMR of monomer, CDCl3, 300 MHz:8.18d (2H, ortho to –COO); 7.95dd (4H, ortho to –N =

N–); 7.34d (2H, ortho to –OCO); 7.00dd (4H, orthoto –OR); 6.40 and 5.82dd (1+1H, CH2= ); 6.15 m

Scheme 1 Synthesis scheme forazobenzene-containingmonomer

Colloid Polym Sci (2010) 288:1375–1384 1377

(1H, =CH–COO); 4.14t (2H, COOCH2); 4.03t (2H,CH2OAr); 3.8–3.9 m (2H, CH2C*); 1.3–1.8 m (19H,CH2, CH); 1.02d (3H, CH3C*); 0.98t (3H, CH3).

Polymerization

LC chiral-photochromic polymer was synthesized by aradical polymerization of corresponding acrylic monomerin benzene solution in the presence of 2% of azo-bis-isobutyronitrile (AIBN). After 3 days storage at 65°C, thesolvent was evaporated and solid product was washedseveral times by boiling ethanol. Average molecular mass(Mw) of polymer reaches 12,400, polydispersity (Mw/Mn)equals to ~1.5, as determined by gel permeation chroma-tography using instrument “Knauer”

Phase behavior and selective light reflection

The phase transition temperatures of the polymer weredetected by DSC with a Perkin-Elmer DSC-7 thermalanalyzer (a scanning rate of 10 K min-1).

The polarizing microscope investigations were per-formed using a Mettler TA-400 thermal analyzer and aLOMO P-112 polarizing microscope.

Photooptical investigations

For study of photooptical properties samples of gel and LCmixture with monomer were prepared between two glass orquartz plates. For preparation of uniaxially aligned LC gelsamples the above plates were spin-coated by polyvinylalcohol solution followed by unidirectional rubbing bycloth. Thickness of these cells was fixed by 10 μm Teflonspacers. Cells were filled at 120°C corresponding to thehomogeneous isotropic melt.

Photochemical investigations were performed using aspecial optical set up equipped with a DRSh-250 ultra highpressure mercury lamp. To prevent heating of the samplesdue to the IR irradiation of the lamp, a water filter was

introduced in the optical scheme. To obtain the plane-parallellight beam, a quartz lens was applied. During the irradiation,a constant temperature was maintained using a Mettler FP-80heating unit. Using the filters a light with the wavelengths of365 and 436 nm were selected. The intensity of light wasmeasured by LaserMate-Q (Coherent) intensity meter.

Spectral measurements were performed using UnicamUV-500 UV-Vis spectrophotometer.

The linearly polarized spectra of the aligned sampleswere studied with a TIDAS spectrometer (J&M) equippedwith rotating polarizer (Glan-Taylor prism controlled bycomputer program).

The dichroism values, D, of the polymer films werecalculated from the spectra using Eq. 1:

D ¼ ðAk � A?Þ=ðAk þ A?Þ ð1Þ

where Ak is the absorbance at the preferred chromophoreorientation direction; A⊥ is the absorbance perpendicular tothis direction.

Results and discussion

LC gel formation

Photochromic LC gel samples were prepared by dissolvingof polymer in nematic mixture MLC well above theclearing temperature (at 120°C) and subsequent coolingdown. An examination of LC mixtures containing differentamount of polymer in nematic mixture showed that itsminimum concentration, required for gel formation, is equalto 2.5 wt.%. Lower concentration of polymer does notinduce formation of a “solid” nonfluid gel. Figure 1a, bshows photos of mixture containing 2.5 wt.% of polymerjust after heating up to 120°C (a) followed by cooling downand keeping at room temperature during 5 min (b). Photo inFig. 1b clearly demonstrates formation of a “solid”, hardlydeformable and opaque gel.

Fig. 1 Photos demonstratingmixture of MLC with polymerPAzo just after heating up to120°C a followed by coolingdown and keeping at roomtemperature for gelation during5 min b

1378 Colloid Polym Sci (2010) 288:1375–1384

Just after cooling down from the isotropic homogeneousstate, rather mobile nematic schlieren-like texture is formedwith homeotropic black regions (Fig. 2a). In addition, thetexture consists of a number of disclinations, which aretypical for a nematic phase with a long helix pitch [33].After about 2 min, strongly birefringent microsized par-ticles filling in almost all fields of the sample in opticalmicroscope are growing up (Fig. 2b). On the base ofpolarizing optical microscopy data, one can propose thatgelation process takes place due to formation of physical“network” during microphase separation of polymer andLC mixture (Fig. 3).

At the same time, acrylic monomer A10HAZO*, beingan analog of the side group of the polymer, does not formgel in the same MLC mixture. The monomer is soluble inthe LC mixture.

In our opinion, the basic reasons, which cause gelformation, are associated with two factors. First of all, asmentioned above, the process of phase separation takesplace. This process is probably accompanied by simulta-neously crystallization of the side branches belonging to thedifferent macromolecules of LC polymer. In condition ofphase separation, the intermolecular interaction of the sidegroups of comb-shaped mesogenic macromolecules resultsin self-organization of mesogenic fragments and theirsubsequent crystallization.

Such specific “physical cross-linking” of the wholevolume of the initial homogeneous solution during itscooling leads to gelation of the polymer–liquid crystalsystem. In spite of the low concentration of polymer (only afew percents), all volume of the polymer gel turns out to betranspierced by the polymer network represented of thesmall-size crystallites formed from the interpenetratingmesogenic side groups. As far as the gel is formed veryfast just after cooling down of isotropic phase and allsamples at observation in polarizing optical microscopeconsists of the birefringent regions, one can suggest thatmolecules of LC mixture MLC are also incorporated intothis network.

Most likely, the mesogenic side groups of polymerstimulate the processes of their joint aggregation with themolecules of MLC resulting in the gelation process. Inother words, side branches of macromolecules “draw” theMLC molecules in network structure diminishing thepossibility of the homeotropic structure formation, whichis partially formed just after cooling the sample from anisotropic melt. Comparison of Fig. 2a and b clearly shows adecrease of the sizes of the homeotropic (black) regions.Combination of the phase separation of polymer and MLC,network formation due to the joint participation of macro-molecules and molecules of MLC in the process of gelformation practically exclude the cholesteric mesophaseformation, which might be formed owing to the presence ofthe terminal chiral fragments in the side groups of polymer.

LC molecules

Aggregated azobenzenegroups

UV Vis, Δ

Fig. 3 Schematic representation of polymer-rich microphase structureof physical gel after cooling of solution PAzo in MLC

50 μm

baFig. 2 Texture of mixture: ajust after cooling from isotropicstate (120°C) to 20°C; b thesame region of the sample after2 min at 20°C

Colloid Polym Sci (2010) 288:1375–1384 1379

Interestingly, that introduction of polymer in nematicmixtures based on cyanobiphenyls (E48, BL087) does notlead to the gel formation. Cooling down to room temper-ature allows one to obtain very viscous but still fluidliquids. The reason for such differences can be explained bynoticeable mutual solubility of LC mixture components inpolymer phase that most likely is accompanied byplasticizing of polymer microphase. Moreover, partialdissolution of cyanbiphenyl molecules in polymer micro-phase may induce disruption of crystalline phase that isreduced rigidity of physical “polymer network”.

Clearing temperature (Tcl) of the obtained gel is 78–81°C,which practically coincides with Tcl of pure LC hostMLC. Above this temperature range, small birefringentareas of LC polymer are still observed until 90°C. Attemperatures above 90°C, the mixture becomes completelyhomogeneous.

Figure 4a shows spectral changes during gel formationafter fast cooling of the mixture from 120 to 20°C. Alreadyafter 2 min, absorbance peak corresponding to azobenzenechromophores becomes wider due to appearance of H-aggregates containing parallel stacks of azobenzenechromophores absorbing at lower wavelength range at ca.315–330 nm [34–36] (Fig. 3).

We have plotted kinetic curves of absorbance changes atwavelengths corresponding to a non-aggregated state of thechromophores (360 nm) and mostly to the aggregated ones(320 nm; Fig. 4b). As seen from the kinetics curves,absorbance at 320 nm gradually increases, whereas at360 nm, it decreases. Overall, a shape of the kinetic curvesis monotonous, but the small peaks at time ~30 s arepresented probably due to I–N phase transition and increasein light scattering. Aggregation is almost completed 10 minafter cooling down. Gelation and formation of H-aggregatesare completely reversible and heating results in completerecovery of initial spectra of isotropic solution.

λ

λ

a

b

Fig. 5 Spectral changes under 365-nm irradiation (3.5 mW cm-2): aLC gel; b solution of monomer (2%) in MLC

λ

a

b

Fig. 4 a Spectral changes in the solution of PAzo in MLC during gelformation after fast cooling from 120 to 20°C; b the correspondingkinetics of absorbance changes for aggregated (320 nm) and non-aggregated (360 nm) chromophores

1380 Colloid Polym Sci (2010) 288:1375–1384

Photooptical properties of LC gels

Let us compare the photooptical properties of photochromicgel with solution of corresponding acrylic monomer (whichis analog of side groups of the polymer) in the same LCmixture. It is noteworthy that monomer does not form geland is soluble in MLC, at least, at concentration below 3wt.%. Figure 5a, b clearly shows that spectral changesunder UV irradiation in the case of gel are completelydifferent and more complicated as compared to themonomer solution. The spectral changes for the monomerare typical for E-Z isomerization of azobenzene derivatives(Fig. 5b). The presence of isosbestic point (at 427 nm) formonomer solution (Fig. 5b) indicates an occurrence of onlyone photoinduced process, whereas it is absent for gelindicating more complex nature of spectral changes.

Rate of the photoinduced spectral changes for gel issignificantly lower (Fig. 6), which can be explained bysterical constrains inside polymer “physical network”(microphase) decreasing the rate of isomerization. Anotherfeature of spectral changes is complicated kinetic curveshape in the case of the gel. This phenomenon is caused, atleast, by two simultaneous and competing photoinducedprocesses, namely, E-Z isomerization and disruption ofH-aggregates.

Subsequent irradiation of the gel with visible light(436 nm) induces back Z-E isomerization, i.e. decrease ofphotostationary concentration of Z-isomer. Qualitatively,spectral changes in this case are similar to the solution, butthe isosbestic points are still poorly resolved (Fig. 7a). Suchsimilarity indicates that rate of H-aggregates formationduring back Z-E isomerization is lower, and the remainingZ-isomer strongly prohibits aggregates formation. Figure 7b

λ

λ

a

b

Fig. 7 a Spectral changes in photochromic LC gel under 436-nmirradiation (1.7 mW cm-2); gel was irradiated before by 365-nm light.Insert shows spectral region near “isosbestic point”. b Spectra of freshsample just after gelation and after consequent irradiation by 365-nmand then 436-nm light

50 μm

Fig. 8 Microphoto of photochromic LC gel textures after 10 min of365-nm irradiation

Fig. 6 Kinetics of absorbance changes during 365-nm irradiation foraggregated (320 nm) and nonaggregated (360 nm) chromophores inphotochromic LC gel and solution of monomer (2%) in MLC

Colloid Polym Sci (2010) 288:1375–1384 1381

100 μm

ba

dc

Fig. 9 Textures of photochromicLC gel in planarly oriented cell(orientation direction shown asarrow on left side; crossesdemonstrates directions ofpolarizer and analyzer. a, bBefore UV irradiation; c, d justafter UV irradiation (365 nm,10 min). LC alignment wasachieved using polyvinyl alcoholunidirectionally rubbed layerspin-coated on glass substrates

⊥

||

λ

⊥

||

λ

a b

c d

Fig. 10 Spectra of polarizedabsorbance of photochromic LCgel a before and b after UVirradiation (365 nm, 10 min); ccorresponding polar plots at320 nm; d kinetics of dichroismdecrease during UV irradiationcalculated for two wavelengths(for gel and for solution ofcorresponding acrylic monomer(2%) in MLC)

1382 Colloid Polym Sci (2010) 288:1375–1384

allows one to compare spectrum of fresh sample just aftergelation and after consequent irradiation by 365-nm andthen by 436-nm light. Maximum absorbance after twoirradiation cycles strongly shifted to long-wavelengthregion (354 nm) and almost coincided with position ofmaximum of monomer solution (355 nm, see Fig. 5).

UV and following visible light irradiation results also intextural changes. Figure 8 shows texture of gel after 10-minirradiation by 365 nm and 10 min by 436-nm wave light.As mentioned above, the fresh gel exhibits the greyschlieren-like texture (Fig. 2b). UV irradiation does notchange type of texture; nevertheless, it transforms schlierentexture with homeotropic regions to bright highly coloredone with an absence of the homeotropic areas (Fig. 8).Probably, this effect can be associated with partial dissolu-tion of polymer in LC mixture and tendency to realizehelical twisting due to presence of chiral fragments inpolymer side groups. Subsequent visible light irradiationresults only in slight changes in colors.

Preparation and study of cells containing uniaxiallyaligned photochromic LC gel

Most electrooptic applications require cells with well-aligned liquid crystal material inside. For this reason, wehave studied methods of aligning the LC gel. Cells withunidirectionally rubbed polyvinyl alcohol-coated glasseswere filled by LC azopolymer mixture in the isotropicphase and then cooled down. Figure 9 shows that goodorientation of LC matrix is achieved, whereas the degree oforientation of side chains in polymer microphase is worse,because rotation of the sample with respect to crossedpolarizer and analyzer almost does not change extinction. Itis noteworthy that UV irradiation results in partial dissolu-tion of polymer in LC mixture, which is manifested bydecrease of polymer domains size (Fig. 9b, c).

We have estimated the degree of orientation of azoben-zene side groups of gel in aligned cells before and after UVirradiation. Absorbance of polarized light parallel to thealignment direction is higher in respect to the perpendicularone (Fig. 10a).This is an indication of noticeable degree oforientation of the azobenzene chromophores of polymer.UV irradiation decreases the degree of orientation(Fig. 10b, c).

We have compared degree of orientation of chromo-phores for gel and “model” solution of the monomer inMLC (Fig. 10d). It should be pointed out that calculateddichroism value for monomer is higher than for azobenzeneside fragments in polymer microphase. Moreover, UVirradiation induces strong decrease in dichroism for gel,but for monomer, decrease is negligible (Fig. 10d). Suchdifference can be explained, taking into account that in themonomer solution, only E-isomer of azobenzene chromo-

phore has impact to the dichroism value, because Z-isomerhas bent shape. The observed high dichroism for monomersolution even after E-Z isomerization remains almostunchanged because 10–20% of monomer molecules inE-form in photostationary mixture of isomers are stilloriented high enough. Bent-shaped Z-isomer does notinfluence strongly the LC order of dilute solution. Contraryto the monomer case, E-Z isomerization in polymermicrophase of gel destroys partially LC order and reducesvalue of dichroism.

Conclusions

In conclusion, for the first time, a possibility of using LCazobenzene-containing side chain polymer as gelator forlow-molar-mass LC mixture was demonstrated. Kinetics ofgel formation and aggregation phenomena was studied indetail. It was shown, that light action allows one tomanipulate optical properties of gels such as absorbanceand linear dichroism.

Acknowledgements This research was supported by the RussianFoundation of Fundamental Research (08-03-00481, 09-03-12234-ofi-m),program COST-D35, projects GAASCR IAA100100911, CSF 202/09/0047 and OC10006 project supported by the Ministry of Education Youthand Sports of the Czech Republic.

References

1. Banerjee S, Das RK, Maitra U (2009) J Mater Chem 19:6649–6687

2. Sangeetha NM, Maitra U (2005) Chem Soc Rev 34:821–8363. Carretti E, Dei L, Weiss RG (2005) Soft Mater 1:174. Terech P, Weiss RG (1997) Chem Rev 97:3133–31595. Bhattacharya S, Samanta SK (2009) Langmuir 25:8378–83816. Zhao Y-L, Stoddart JF (2009) Langmuir 25:8442–84467. Kato T, Hirai Y, Nakaso S, Moriyama M (2007) Chem Soc Rev

36:1845–21288. Tong X, Chung JW, Park SY, Zhao Y (2009) Langmuir 25:8532–

85379. Mizoshita N, Suzuki Y, Hanabusa K, Kato T (2005) Adv Mater

17:692–69610. Müller M, Schöpf W, Rehberg I, Timme A, Lattermann G (2007)

Phys Rev E 76:06170111. Deindörfer P, Eremin A, Stannarius R, Davisa R, Zentel R (2006)

Soft Matter 2:693–69812. Kato T, Tanabe K (2009) Chem Lett 38:634–63913. Urayama K, Arai YO, Takigawa T (2004) Macromolecules

37:6161–616914. Guillard H, Sixou P, Reboil L, Perichaud A (2001) Polymer

42:975315. Lee SN, Sprunt S, Chien LC (2001) Liq Cryst 28:63716. Kang K, Chien LC, Sprunt S (2002) Liq Crys 29:917. Behrens U, Kitzerow H-S (1994) Pol Adv Techn 5:43318. Beckel ER, Natarajan LV, Tondiglia VP, Sutherland RL, Bunning

TJ (2007) Liq Cryst 34:1151

Colloid Polym Sci (2010) 288:1375–1384 1383

19. Chen J, Morris SM, Wilkinson TD, Coles HJ (2007) Appl PhysLett 91:121118

20. Hikmet RAM, Kemperman H (1998) Nature 392:47621. Guo J, Sun J, Li K, Cao H, Yang H (2008) Liq Cryst 35:8722. Bobrovsky A, Shibaev V (2009) J Mater Chem 19:366–37223. Kaneko T, Yamaoka K, Osada Y, Gong JP (2004) Macro-

molecules 37:5385–538824. Yamaoka K, Kaneko T, Gong JP, Osada Y (2003) Langmuir

19:8134–813625. Shibaev VP, Tal’rose RV, Plate NA (1971) Vysokomol Soedin B13:426. Tal’rose R, Shibaev VP, Plate NA (1974) J Polym Sci Polym

Symp 44:3527. Shibaev V, Bobrovsky A, Boiko N (2003) Prog Polym Sci 28:72928. Yu Y, Nakano M, Ikeda T (2003) Nature 425:145

29. Ikeda T, Nakano M, Yu Y, Tsutsumi O, Kanazawa A (2003) AdvMater 15:201

30. Ikeda T, Mamiya J-I, Yu Y (2007) Angew Chem Int Ed 46:506–28

31. Corbetta D, Warner M (2009) Liq Cryst 36:1263–128032. Bobrovsky A, Shibaev V, Hamplova V, Bubnov A, Kaspar M,

Glogarova M, in preparation33. Dierking I (2003) Textures of liquid crystals. Wiley-VCH,

Weinheim34. Kasha M, Rawls HR, El-Bayoumi MA (1965) Pure Appl Chem

11:37135. Bobrovsky A, Shibaev V, Hamplova V, Kaspar M, Glogarova M

(2009) Monat Chem 140:789–79936. Kuiper JM, Engberts JBFN (2004) Langmuir 20:1152–1160

1384 Colloid Polym Sci (2010) 288:1375–1384