Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 30– 36
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Journal of Photochemistry and Photobiology A:Chemistry
journa l h om epa ge: www.elsev ier .com/ locate / jphotochem
FM study of laser-induced crater formation in films ofzobenzene-containing photochromic nematic polymer andholesteric mixture
lexey Bobrovskya,∗, Konstantin Mochalovb, Anton Chistyakovb, Vladimir Oleinikovb,alery Shibaeva
Faculty of Chemistry, Moscow State University, Leninskie Gory, 119991 Moscow, RussiaShemyakin & Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 117871 Moscow, Russia
r t i c l e i n f o
rticle history:eceived 25 June 2013eceived in revised form6 September 2013ccepted 19 October 2013vailable online 31 October 2013
eywords:C polymers
a b s t r a c t
For the first time, AFM study of laser induced surface deformation and crater (holes) formation in liquidcrystalline (LC) azobenzene-containing photochromic polymer systems was performed. Special set upincluding combination of polarizing optical, atomic force microscopies and a possibility of irradiation witha highly focused laser beam (532 nm) was used for these investigations. Films of the nematic azobenzene-containing polyacrylate polymer and the cholesteric mixture prepared by doping of this polymer with thechiral dopant were investigated. Focused laser beam irradiation results in the crater formation formed dueto a mass-transfer outside center of beam. Depth of these craters lies in the range of tens of nanometersand increases by prolongation of irradiation time. It is found that this phenomenon takes place only for
zobenzene–Z isomerizationass-transfer
hotoorientation
thick (5–10 �m) films, whereas these phenomena do not take place for the thin spin-coated films. Fornematic and cholesteric films the rate of the crater formation, their depth and diameter are the same; thatmeans the chirality of the systems has no influence on kinetics and the depth of photoinduced craters.It is shown, that for the nonaligned LC-films no preferable directed mass transport nearby photoinducedholes was found, but in the case of the uniaxially aligned nematic polymer films the mass-transportoccurs only in direction along LC-director and does not depend on the laser polarization direction.
. Introduction
Photoinduced mass-transfer phenomena in polymer and low-olar-mass materials attract a great attention due to the promising
otential for application in holography, optoelectronics [1–7],anodevices, microlasers fabrication [8], etc. There are a largeumber of papers devoted to the so-called surface relief gratingSRG) formation produced by exposure of films of azobenzene-ased substances by two interfering polarized laser beams [1–16].t is noteworthy, that despite to many experimental investiga-ions the exact mechanism of mass-transfer in such systemss still completely unknown. Moreover, there is less numberf papers describing the effects of single beam irradiation onzobenzene-containing systems [17–23]; all these papers areevoted to amorphous polymers. For example, in several recent
apers [20–23] the interesting phenomenon of spontaneous peri-dic surface relief under single beam irradiation was investigatednd possibility of surface patterns control was demonstrated. Fewer
papers are devoted to the irradiation using single focused laserbeams. It was shown that focused laser beam irradiation of amor-phous azobenzene-based polymers results in surface deformationand directional mass-transport and hole (crater) formation [17,18].Strong polarization dependence and anisotropy of material flowwere also observed. An efficiency of mass-transport was maximalin the direction along the laser beam polarization. The mechanismof the observed surface deformation and SRG was explained in theframework of the optical field gradient force model [18].
Another still unexplored task is study of mass-transferphenomena in partially ordered media, namely, in liquid crystalline(LC) systems including polymers. Many publications [2,24–29]describe photoorientation processes in azobenzene-containing LC-polymers, whereas the peculiarities of mass-transport phenomenaare still the open area for investigations.
In our recent paper, using a novel experimental method com-bining of polarizing optical microscopy (POM) and atomic forcemicroscopy (AFM), the surface topography and optical properties
of chiral-photochromic LC systems were studied [30]. Exact cor-relations between the features of surface topography in films ofmixture of cholesteric cyclosiloxanes with azobenzene-containingdopant and POM images were found. It was shown, that a lot of
valleys” having spiral superstructure in the slowly cooled sam-les are abundant in the samples. On the contrary, the quenchedlms were characterized by the spiral “hills” structure. Polar-
zed light action (laser 532 nm) on the cholesteric mixturelms leads to the joint azobenzene and mesogenic groups uni-xial orientation in direction perpendicular to the polarizationlane of laser beam and a formation of partially aligned sur-ace structure features (“hills” instead “valleys” for slowly cooledlms). The observed photooptical effects are associated with–Z, Z–E isomerization cycles of azobenzene groups, anisotropicooperative photoinduced rotational diffusion and directionalooperative mass-transport of chromophores and mesogens in thelms.
It is noteworthy, that we did not find any indication of cratersormation in such systems. It is conceivable, that the studied LC
ixture contains of only small amount of azobenzene dopant10 wt%).
The present paper describes the first detailed study of pho-oinduced craters formation under single laser beam action inC polymers films forming two different LC phases (nematic andholesteric) with very high azobenzene fragments content (up to00%). As objects for investigations we have selected azobenzene-ontaining nematic polyacrylate PAzo4M and cholesteric mixturef the polymer with chiral-photochromic dopant Sorb.
1. Nematic polymer PAzo4M
COOO N
N CN
CH2 C(CH3)n
2. Cholesteric mixture PAzo4M + Sorb
O
OH H
O
O
O
O
CH3O
S
This polymer was selected for investigations because we haveecently performed the detailed study of photoorientation pro-esses in its thin spin-coated films [31]. The polymer forms onlyematic phase with clearing temperature Tcl ∼ 158–159 ◦C. Glassransition temperature is about ∼58 ◦C. Polarized UV and visibleight irradiation of this polymer leads to an orientation of azoben-ene groups perpendicular to polarization plane with high resultingichroism (D ∼ 0.5) increasing after LC state formation at annealingD ∼ 0.8) [31].
Chiral-photochromic dopant Sorb possesses high helical twist-ng power and capable for E–Z photoisomerizing under UV-lightrradiation [5,32]. UV-sensitivity of this substance allows oneo manipulate its helical twisting power and to modify theitch of cholesteric helix providing a possibility to study influ-nce of this parameter on photooptical properties and surfaceopography. Mixture of PAzo4M with 4.5 wt% of Sorb is charac-erized by cholesteric phase formation with clearing temperatureround 145–146 ◦C and selective light reflection in near IR range�max ∼ 900 nm after slow cooling).
The main goal of this work is a comparative study of photoin-uced topography changes in nematic and cholesteric polymerlms having different thickness and mesogens (chromophores)lignment in order to elucidate the influence of these different
Photobiology A: Chemistry 275 (2014) 30– 36 31
3
4.5%
factors (type of mesophase, film thickness, mesogens orientation)on surface relief features. As we mentioned above, previous papersdevoted to study of single beam laser action only on amorphouspolymer films. We are for the first time focused our research on liq-uid crystalline polymer systems and perform comparison of craterformation in two different mesophases, nematic and cholesteric.
2. Experimental
2.1. Materials
Polymer PAzo4M was synthesized by a radical polymerizationof corresponding acrylic monomer [24] in benzene solution in thepresence of 2 wt% (with respect to monomer) of AIBN. After 3 daysstorage at 65 ◦C the solvent was evaporated and solid product waswashed several times by boiling ethanol. Yield of polymerizationwas ∼70%. Such relatively low yield is explained by compet-ing radical transfer reaction promoted by azobenzene fragment.Molecular masses and polydispersity of polymer as determined byGPC chromatography using instrument “Knauer” are Mw ∼ 19,500,Mw/Mn ∼ 2.1.
Chiral photochromic dopant Sorb was synthesized according toour previous paper [32]. Mixture PAzo4M + Sorb was prepared by
dissolving the components in chloroform followed by slow evapo-ration and drying in vacuum at ∼120 ◦C.
2.2. Samples preparation
The films of nematic polymer and cholesteric mixture havingfree surfaces were prepared as follows. Small amount of the poly-mer sample was placed between two glass plates, heated up to150 ◦C (for polymer) or 140 ◦C (for mixture) and subjected to sheardeformation in order to obtain a planar orientation. After 30 min ofannealing at the same temperature two glass substrates were sepa-rated by shearing. As a result, two glass plates covered by polymericfilm were obtained and annealed additionally during ∼30 min fol-lowed by slowly cooling down to room temperature with the rate of1◦/min using Mettler hot-stage. Nematic polymer displays marbledor schlieren texture. In cholesteric mixtures films a planar align-ment induced by shearing is preserved, but with the presence ofdense “network” of so-called oily streaks (see Figs. 2 and S3).
For the preparation of uniaxially aligned polymer films pho-toalignment method was applied [33]. For this purpose glasssubstrates spin-coated with poly[1-[4-(3-carboxy-4-hydroxy-phenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt
3 y and Photobiology A: Chemistry 275 (2014) 30– 36
(Bi∼tSvs
c
2
fM(s
r
2c
s
w(
f
D
wtr
2i
SAfs
dAcSiflstSbs
ut
Lmc
40003000200010000
0.0
0.1
0.2
0.3
0.4
PAzo4M+SorbPAzo4M
Dic
hroi
sm a
t 355
nm
Time / s
PAzo4M anneale d PAzo4M+Sorb annealed
2 A. Bobrovsky et al. / Journal of Photochemistr
Aldrich) solution in chlorophorm (2 mg/mL) were used [34].efore film preparation substrates were irradiated with polar-
zed polychromatic light of mercury lamp (DRSh-350, 20 min,15 mW/cm2). In order to achieve the best LC-director orientation
he polymer PAzo4M films were annealed at 150 ◦C overnight.uch annealing temperature was selected in order to minimize theiscosity of the sample but, nevertheless, preserving mesophasetructure (ca. 10 ◦C below isotropization temperature).
Thin amorphousized films (∼100 nm) were obtained by spin-oating from chlorophorm solution.
.3. Phase behavior and selective light reflection study
The polarizing optical microscope investigations were per-ormed using LOMO P-112 polarizing microscope equipped by
ettler TA-400 heating stage. Differential scanning calorimetryDSC) was performed by Perkin Elmer DSC-7 thermal analyzer (acanning rate of 10 K/min).
For selective light reflection study transmittance spectra wereecorded by Hitachi U3400 UV-Vis-NIR spectrophotometer.
.4. Photoorientation processes and photoinduced dichroismalculation
Photoorientation process study was performed using greenemiconductor laser (532 nm, ∼9 mW, beam diameter ∼2 mm).
The linearly polarized spectra of the film samples were studiedith a TIDAS spectrometer (J&M) equipped with rotating polarizer
Glan–Taylor prism controlled by computer program).The dichroism values, D, of the polymer films were calculated
rom the spectra using the following Eq. (1):
= A|| − A⊥A|| + A⊥
(1)
here A|| and A⊥ are the absorbance parallel and perpendicularo the preferred azobenzene chromophore orientation direction,espectively.
.5. Experimental setup for simultaneous AFM and POMnvestigations
The main idea of our new type of experimental setup (seecheme S1 in Supporting Information) is a combination of theFM scanning system, upright optical microscope, optical table
or inverted optical microscopy and cross-polarized illuminationystem [30].
The samples (1) prepared as described above were placedirectly on the top of scanning XY-piezostage (SmartSPMTM,IST-NT) mounted into the XY-positioned table for inverted opti-al microscopy (2) with open optical axis. The AFM-head (3,martAFMTM, AIST-NT) was mounted onto the same optical tablen the manner that allows the placement of tip of AFM-probe inew micrometers vicinity to the optical axis. The spot of the greenaser (4, 532 nm, LCM-S-111, LASER-EXPORT Co. Ltd.) using forample irradiation was adjusted to the same spatial area throughhe deflectometer mirror of the AFM-head as it is shown on thecheme S1 – dashed box. Light intensity is 0.063 mW as measuredy LaserMate-Q (Coherent) intensity meter; beam is focused intopot with diameter ca. 30 �m.
All optical images were obtained with Optem Zoom 125Cpright microscope (5) adjusted to the overall optical axis passedhrough the same point as the AFM-probe tip.
The cross-polarized illumination system consists of the ACE®
ight Source (6), the homemade condenser lens system (7), poly-er linear polarizing film (as a polarizer) (8) placed directly on
ondenser at arbitrary and fixed angle and similar film as analyzer
Fig. 1. Kinetics of dichroism growth under irradiation of the fresh and annealedspin-coated films of the polymer PAzo4M and the mixture PAzo4M + Sorb by 532 nmlaser (∼10 mW, beam diameter ∼2 mm). Films were annealed at 70 ◦C overnight.
(9) placed into rotatable CCD/microscope coupler (10). Angle of theanalyzer film was adjusted with rotation of CCD/microscopecoupler up to achieving of darkest field as microscopicimage.
3. Results and discussion
3.1. Surface topography and photoorientation processes in thinspin-coated films of nematic polymer and cholesteric mixture
Firstly, let us consider the surface topography and photoori-entation processes in the thin films of polymer and mixture(100–200 nm) prepared by spin-coating method. The films pre-pared by this technique are amorphous and optically isotropic dueto the fast solvent evaporation preventing the LC phase formation.
Irradiation of the films by polarized green light even with rela-tively low intensity results in an appearance of optical anisotropy(Fig. S1) and significant growth of dichroism (Fig. 1). These effectsare related to photoinduced orientation of azobenzene chro-mophores perpendicular to the polarization plane of excitationlight. It is noteworthy that photoorientation takes place despite tothe very low films absorbance at 532 nm (A ∼ 0.01, Fig. S1a).
Considering kinetics of dichroism growth it should be pointedout that the rate of process is almost the same for both nematicpolymer and cholesteric mixture (Fig. 1). However, annealing ofthe films and LC state formation decreases rate and maximal val-ues of dichroism because multidomain LC phase formation underannealing results in partial suppression of photoorientation pro-cess.
AFM investigations of thin films of polymer and mixture beforeand after focused laser beam irradiation does not reveal anychanges in surface topography (Fig. S2). Only light action havingvery strong intensity (0.85 mW, 5 min) induces irregular interfer-ence patterning probably due to the self-diffraction of laser light(Fig. S2).
3.2. Photoinduced surface topography changes of thick films(10–20 �m) of polymer and mixture
Thick polymer films (10–20 �m) were prepared by melting ofpolymer and mixture between glass substrates (see Section 2 forthe details). Nematic polymer film has no preferred orientation ofmesogens (chromophores) and characterized by a schlieren texture
A. Bobrovsky et al. / Journal of Photochemistry and
Fig. 2. POM photo of the nonaligned film of the polymer PAzo4M before (a) andafter 4 min of irradiation (b) (532 nm, 0.063 mW).
Fig. 3. AFM scans of the nonaligned film of the polymer PAzo4M before and after 532 nm irrAFM image was obtained by subtraction of AFM scan after 4 min of irradiation with AFM
Fig. 4. AFM scans of the mixture PAzo4M + Sorb film before (a) and after 2 min of irradiatof irradiation with AFM scan before irradiation.
Photobiology A: Chemistry 275 (2014) 30– 36 33
typical for nematic phase (Fig. 2) [35]. Under preparation conditioncholesteric mixture film spontaneously formed planar texture withselective light reflection in near IR range (∼900 nm) and consists ofdense “network” of defects (so-called oily steaks, Fig. S3).
Figs. 3 and 4 demonstrate AFM-scans of prepared polymer films.We have performed a comparison of the surface roughness for thenematic and cholesteric polymer systems by calculation of differ-ent roughness parameters. As seen from obtained values listed inTable 1, the amplitude of surface height variations for nematic sam-ples is about two times higher than for cholesteric ones, whereasaverage roughness as well as RMS dispersion even three timeshigher. Perhaps, helical structure of cholesteric mixture with a def-inite value of the pitch and the planar orientation of mesogensassure the uniform thickness of the sample predetermining smooth
surface topography. It is completely unexpected, that roughness ofaligned films of nematic polymer is higher than for non-alignedones.
adiation (0.063 mW) before (a) and after irradiation during 4 min (b). (c) Subtractedscan before irradiation.
ion (b). (c) Subtracted AFM image obtained by subtraction of AFM scan after 2 min
34 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 30– 36
Table 1Parameters characterizing the roughness of polymer films surface.
a Maximal height of the surface relief obtained from whole array of the data (aftersubtraction of first-order plane, i.e. minimal height Zmin ≡ 0).
b Arithmetic mean deviations of values of height Zij in each scan (with number ofpoints Ni × Nj = 512 × 512) from the arithmetic mean of height 〈Z〉.
c A standard deviation of Zij values from 〈Z〉.
0 10 20 30 40 50 60
-12 0
-10 0
-80
-60
-40
-20
0
20
Z / n
m
X / μm
Along polari zation:1 2 4 min
Fig. 5. Time evolution of the cross-sections of the subtracted AFM scans of thenonaligned film of the polymer PAzo4M during 532 nm irradiation (0.063 mW).(
(dtsion“aoit
palsvda
ti(woi
s
43210
0
20
40
60
80
100
120
PAzo4M PAzo4M+SorbH
ole
dept
h / n
m
Time / mi n
chromophores excitation, followed by random motion of excitedchromophores along molecular axis. This angularly dependent ran-dom motion eventually induces chromophores transfer outsidehigh light intensity region.
Irradiation time is shown in the figure.)
Comparison of POM and AFM images of nematic polymerFigs. 2 and 3) and cholesteric mixture (Figs. S3 and 4) filmso not reveal any correlation between surface topography andexture defects observed by POM. Therefore, contrary to previouslytudied cholesteric cyclosiloxanes-based mixtures [30], defectsnside films and LC-director distribution have no distinct influencen surface topography. In addition, for cholesteric films we didot find any evidence of circular double-spiral domains (“hills” orvalleys”) as was observed in our previous paper [30]. This fact is inccordance with our recent paper devoted to the study of influencef cholesteric helix pitch on surface topography [36]. We showedn this paper that for cholesteric materials with helix pitch largerhan ∼700 nm circular domains formation does not take place.
Similar to the thin spin-coated films the irradiation witholarized green light of thick films leads to photoorientation ofzobenzene chromophores. Unfortunately, strong absorbance andight scattering (see absorbance spectra in Fig. S5) do not allowtudying kinetics of this process and determination of dichroismalues. Nevertheless, polarizing optical microscopy (POM) imageefinitely demonstrates alignment of chromophores in the irradi-ted zone for polymer (Fig. 2) and mixture (Fig. S3).
AFM scans of the irradiated area shown in Fig. 2 demonstratehat focused laser beam irradiation results in holes formationnduced due to mass-transfer outside center of beam trackFigs. 3 and S5–S7). Qualitatively and quantitatively similar resultsere obtained for cholesteric mixture films (Figs. 4 and S8). Depth
f these holes lies in range of tens of nanometers gradually increas-
ng by increase of irradiation time (Figs. 5 and 6).
The possible origins of the crater formation in LC polymerystems can be explained as follows. High intensity laser beam
Fig. 6. Time evolution of the cross-sections of the subtracted AFM scans of thepolymer PAzo4M and the mixture PAzo4M + Sorb films during 532 nm irradiation(0.063 mW). (Irradiation time is shown in the figure.)
induces softening of polymer film and its so-called photofluidiza-tion [17,18]. Viscosity of polymer film depends on the local lightintensity and it is smallest in the center of the beam. Photofluidiza-tion causes some fluctuations of polymer macromolecules motionand inhomogeneity of the film thickness. As the result of theseprocesses the directional migration of polymer material outside thecrater center and formation of the circular swell (or hill) of ejectedpolymer material take place.
It should be pointed out that comparison of our work with arelated study of amorphous polymers by Tripathy et al. is warran-ted [17]. Optical force model is also available in our case at firstapproximation. Effect of surface deformation and crater formationis related to the enhancement of polymer chains mobility due tothe effective fast cycles of E–Z–E photoisomerization of azoben-zene chromophores. As was shown before [17,18], possible thermaleffects play negligible role in this phenomenon.
In [3,37] similar models of photoinduced crater formation weredescribed. In these models authors consider the angularly selective
Fig. 7. POM photo of the aligned film of the polymer PAzo4M before (a) and after2 min of irradiation (b) (532 nm, 0.063 mW). Polarization plane of the laser is alongLC-director. White square in (b) shows AFM-scanned area presented in (a).
A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 275 (2014) 30– 36 35
Fig. 8. Normalized AFM images of the aligned polymer PAzo4M film irradiated with laser polarized along (a) and perpendicular to the alignment direction (b). E-vector ofe of theb polar
atpmaot
igdcomdtaopdmbaaeti
damental Research (11-03-01046a, 12-03-00553-a, 13-04-00168
lectric field of light, n–LC-director. (c and d) Corresponding cross-section profiles
oth cases (c and d) the cross-sections were made parallel and perpendicular to the
As was mentioned in Section 1, for previously studiedmorphous azobenzene-containing polymers anisotropy of mass-ransfer was observed as unidirectional increase in film height alongolarization of excitation light resulting in anisotropy of polymeraterial ejection outside the craters. In the case of LC polymer
nd mixture we did not find any correlation between a directionf polarization of the excitation light and the orientation of massransport.
Nevertheless, the special features were found for the uniax-ally oriented films of the nematic polymer prepared by usinglass substrate with photoalignment layers (see Section 2 for theetails of the film preparation). Laser irradiation of such films formsontrast spot in POM image (Fig. 7) which is seen more clearlyn the homogeneously colored background than for the multido-ain films (compare with Fig. 2). For uniaxially aligned films the
efinite anisotropy of a surface deformation was found. Contraryo amorphous polymer systems direction of the mass transportlways coincides with LC-director of the films and does not dependsn light polarization axis (Fig. 8). Such unexpected behavior isrobably associated with a low degree of chromophores photoin-uced reorientation induced by the polarized light action that isuch lower than initial LC polymer orientation predetermined
y photoalignment during film preparation. This hypothesis is ingreement with data described above for thin spin-coated films:
n annealing and LC phase formation reduce the rate of photoori-ntation and value of photoinduced dichroism (Fig. 1). Due tohe anisotropy of LC-polymer viscosity the rate of mass-transfers higher along the LC-director that predetermines the direction
holes obtained by irradiation of uniaxially aligned film of the polymer PAzo4M. Inization plane of laser.
of chromophores random motion and polymer material ejectionalong the alignment axis of the mesogens.
4. Conclusions
Investigations of photoinduced surface changes in the nematicazobenzene-containing polymethacrylate and the correspondingcholesteric mixture show specific features of surface properties ofsuch systems related to the peculiarities of laser-induced craterformation. Irradiation with focused laser beam results in crater for-mation in only relatively thick films (5–10 �m), whereas for thethin spin-coated films these phenomena do not take place. Chiral-ity has no influence on the kinetics of formation and depth of thephotoinduced holes. For the nonaligned polymer films anisotropyof mass transport was not found, but in the case of uniaxiallyaligned nematic polymer mass-transport occurs only in directionalong LC-director and does not depend on polarization direction oflaser.
Acknowledgements
This research was supported by the Russian Foundation of Fun-
and 13-03-00648) and the Ministry of Higher Education and Scienceof the Russian Federation (grant no. 11.G34.31.0050). We expressour thanks to Dr. S. Kostromine and Dr. A. Stakhanov for providingmonomer Azo4M.
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6 A. Bobrovsky et al. / Journal of Photochemistr
ppendix A. Supplementary data
Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.jphotochem.013.10.009.
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