Anyfantakis, M.

Butt, H.

Fytas, G.

Köhler, W.

Königer, A.

Loppinet, B.

Pispas, S.

MPG.PuRe

Versatile light actuated matter manipulation in transparent non-dilute polymer solutions

Manos Anyfantakis

Andreas Königer

Stergios Pispas

Werner Köhler

Hans-Jürgen Butt

Benoit Loppinet

George Fytas

Crossref

Soft Matter

Anyfantakis, Manos

Königer, Andreas

Pispas, Stergios

Köhler, Werner

Butt, Hans-Juergen

Loppinet, Benoit

Fytas, George

EPub Bayreuth

Dynamic Article LinksC<Soft MatterCite this: Soft Matter, 2012, 8, 2382www.rsc.org/softmatter COMMUNICATIONPublished on 23 January 2012. Downloaded by UNIVERSITAT BAYREUTH on 5/18/2020 10:46:56 AM. View Article Online / Journal Homepage / Table of Contents for this issueVersatile light actuated matter manipulation in transparent non-dilutepolymer solutionsManos Anyfantakis,*a Andreas K€oniger,b Stergios Pispas,c Werner K€ohler,b Hans-J€urgen Butt,aBenoit Loppinet*d and George FytasadeReceived 19th November 2011, Accepted 29th December 2011DOI: 10.1039/c2sm07219jA local polymer concentration enhancement upon moderately weaklaser irradiation was recently discovered in non-absorbing entangledpolymer solutions. Here, we uncover the reverse effect: depending onthe (good) solvent environment, macromolecules are either effec-tively attracted or repelled by visible laser light independently of theoptical contrast. The versatile effect, qualitatively different fromwell-known mechanisms, may lead to new ways for macromolecularmanipulation and micropatterning by optical fields.Light–matter interactions have been at the focus of scientific researchfor centuries. Recently the class of soft materials has attractedincreased attention as the utilization of light opens novel pathways tomanipulate these materials on a microscopic scale. Applied opticalfields can result in structural changes in complex fluids and lead tonovel types of optical nonlinearities. The availability of a large varietyof characteristic length scales, intermediate between atomic andmacroscopic sizes, and their associated broad temporal scales1 opennumerous possibilities. The detailed investigation of light–soft matterinteractions has just started.2 In mixtures of materials with differentpolarizabilities, various sources of optical nonlinearities3 are intro-duced. In addition to the electronic coupling inherent to the indi-vidual components, the optical properties of the medium can changelocally due to spatio-temporal variation of the local concentrationc(r,t). The response of c(r,t), coupled possibly to temperature andmolecular orientation, will therefore condition the local refractiveindex change dn and, in turn, the light propagation. Various mech-anisms, either well-understood like electrostriction4,5 or of less clearmicroscopic origin like thermo-diffusion,6,7 lead to new observationsthereby adding more pieces to the puzzle of soft matter response tolight irradiation.8,9The effect of radiation pressure on single colloidal particles and oncolloidal dispersions is well-understood and extensively used.5 Thesituation is somewhat less clear for macromolecules in solutions. AaMax-Planck Institute for Polymer Research, 55128 Mainz, Germany.E-mail: anyfanta@mpip-mainz.mpg.debPhysics Department, University of Bayreuth, 95440 Bayreuth, GermanycInstitute of Theoretical and Physical Chemistry, NHRF, Athens, GreecedInstitute of Electronic Structure and Laser, FORTH, Heraklion, Greece.E-mail: benoit@iesl.forth.greDepartment of Materials Science and Technology, University of Crete,Heraklion, Greece2382 | Soft Matter, 2012, 8, 2382–2384series of experimental investigations on aqueous solutions of variouspolymers reported aggregation or phase separation induced by theapplication of sharply focused (diffraction limited spots) near-IRlaser beams.10–13Recently, a concentration optical nonlinearity was revealed intransparent homogeneous mixtures of alkane solvents with commonpolydiene polymers [cis-1,4-polyisoprene (PI) and cis-1,4-poly-butadiene (PB)]. Irradiation by a weak red laser resulted in a refrac-tive index increase dn and the formation of polymer-rich fibers.14Thisnovel light–material coupling was shown to lead to a number ofdifferent patterns, such as optical spatial soliton15-like filaments,16multi-filament arrays due to modulational instabilities17 and multi-filament holographic gratings,18 depending on the irradiation condi-tions.19 Here, we uncover new polymer solution’s responses to visiblelight irradiation and report on the versatility of the effect whendifferent dispersion media are utilized.The response of the polydiene solutions to laser light was exploredwith a simple experimental setup (similar to the one described in ref.16) which allowed for their irradiation and simultaneous phasecontrast (PC) imaging in the direction perpendicular to the laserbeam (Fig. 1). The laser sources used were a fiber coupled diode laser(Sch€after + Kirchhoff) of wavelength l ¼ 660 nm and a DPSSlaser (Shanghai Laser Dreams), l ¼ 671 nm. The reported resultswere identical for the twowavelengths used. Amicrolens (f¼ 38mm,Sch€after +Kirchhoff) attached on the fiber or a microscope objective(f ¼ 35 mm, Melles Griot) was used to focus the laser light inthe middle of the sample cell, with focal spot diameters d in the range17–28 mm. The cells used were square borosilicate glass cuvettes withinner dimensions of 1mm (Vitrocom) and home-made flat glass cells,mounted on a (x, y) translation stage. The loading of the viscoussamples was achieved by using syringes or glass pipettes. Afterloading, the cuvettes were carefully sealed using epoxy glue, in orderto avoid solvent evaporation during the experiments. The home-made cells were sealed with a coverslip.For the phase contrast imaging, we used the white light K€ohlerillumination unit of a commonopticalmicroscope (ZeissAkioskop 2)to create a collimated beam of a diameter dz 3 mm, an (20–32)objective lens (Zeiss) and a 640 480 CCD camera (Basler/Pixelink).A filter was placed in front of the CCD to block the scattered laserlight. Images were acquired with the microscope objective lens off-focus (always above the focal plane) leading to snapshots of theintensity which can be quantitatively related to the refractive indexThis journal is ª The Royal Society of Chemistry 2012Fig. 1 A schematic of the experimental setup employed to study theresponse of the polydiene solutions to the application of visible laser light.The samples are irradiated and simultaneously imaged under an opticalmicroscope in the direction perpendicular to the laser beam.Published on 23 January 2012. Downloaded by UNIVERSITAT BAYREUTH on 5/18/2020 10:46:56 AM. View Article Onlinevariation.20 The imaged contrast is directly related to the differencebetween the local refractive index nstr and the refractive index of thesolution structure contrast ns, dn¼ nstr ns. Brighter (darker) regionsin the illuminated area indicate local increase (decrease) of the indexof refraction. Typical defocusing distances employed were Dz ¼20 mm and Dz¼ 100–1000 mm for the high contrast (i.e. Fig. 2a) andlow contrast structures (i.e. Fig. 2b–d), respectively.The samples investigated were equilibrated solutions (in theconcentration range c¼ 0.04–0.13 g ml1) of anionically polymerizedcis-1,4-polyisoprene [–(CH2–C–CH3]CH–CH2)–]N (PI) andFig. 2 The diverse light-induced micro-patterns, as imaged by phasecontrast microscopy. (a) PI/decane (Dn > 0, dn > 0, c¼ 0.0406 g ml1). (b)PI/bromonaphthalene (Dn < 0, dn > 0, c ¼ 0.0413 g ml1). (c) PB/tetra-hydronaphthalene (Dn < 0, dn < 0, c ¼ 0.0851 g ml1). (d) PI/tetrahy-drofuran (Dn > 0, dn < 0, c ¼ 0.0486 g ml1). Dn ¼ np  n0 is the polymersolution optical contrast with np and n0 being the refractive index of bulkpolymers and the neat solvents, respectively. dn ¼ nstr  ns is the lightinduced local pattern contrast with nstr, and ns denoting the refractiveindex of the illuminated (pattern) and non-illuminated (surrounding)solution.This journal is ª The Royal Society of Chemistry 2012commercial cis-1,4-polybutadiene [–(CH2–CH]CH–CH2)–]N, (PB,Polymeri Europa). The average degrees of polymerization were N ¼16 030 and N ¼ 7220, and the polydispersity index PDI ¼ 1.07 andPDI ¼ 2.5 respectively. PI and PB (refractive indices np ¼ 1.519 andnp¼ 1.52, respectively) were dissolved in different good solvents withvarying refractive index n0 (Table 1), in order to vary the so-calledpolymer solution optical contrast, Dn ¼ np  n0. All solvents werefiltered through PTFE filters of pore size 0.2 mm (Millipore) to ensuredust-free polymer solutions. All samples were stirred gently forseveral days, until homogeneous solutions were obtained.The ‘standard’ effect14 is illustrated in Fig. 2a. A positive patterncontrast (nstr > ns) is observed in samples with positive solutioncontrast, i.e.when np > n0. In this case, a polymer enriched filament isformed upon light irradiation. Polymer is attracted into the irradiatedregion and the formed bright filament extends over the whole cellthickness, displaying a constant diameter of about 10 mm. The phasecontrast image in Fig. 2b demonstrates the case of a negative polymersolution contrast (np < n0). Now the solvent has a higher refractiveindex (n0¼ 1.657) than the polymer solute and hence the observationof a positive pattern contrast (nstr > ns) implies, in this case, theformation of a polymer-depleted filament along the laser beam. Bothcases, however, are in qualitative agreement with an electrostrictionmechanism according to which it is the higher refractive index speciesbeing either the polymer (Fig. 2a) or the solvent (Fig. 2b) which isdrawn into the inhomogeneous electric field. Based solely uponelectrostrictive forces, the lower quadrants with negative patterncontrast (nstr < ns) should remain out of reach, as it would infer anenrichment of the lower refractive index species.Fig. 2c and d show experimental realizations of these two unex-pected cases of negative pattern contrast utilizing samples possessingboth negative (np < ns¼ 1.542, Fig. 2c) and positive (np > ns¼ 1.407,Fig. 2d) solution contrast. Indeed, Fig. 2c and d both show a darkstripe with low but clearly negative (nstr < ns) pattern contrast dnrevealing polymer enrichment and depletion, respectively. Interest-ingly, for the two common solvents with very close solvent refractiveindices (THF and decane) used in the solutions of Fig. 2a and d withsimilar polymer concentration, the documented optical response isreciprocal, polymer attraction into (decane) versus repulsion from(THF) the irradiated region.The polymer–solvent couples examined so far are presented inTable 1.Alongwith the signs ofDn and dn, the solvent dipolemomentm and relative static permittivity 3r are also shown.21,22 Neither of thementioned physical properties define solely the response of theTable 1 Summary of the examined solutions. The index of refraction (n0at 589 nm), the dipole moment (m) and the relative static permittivity (3r)for each solvent are shown21,22Solvent n0 m (D) 3r Dn dnHexane 1.375 0 1.89 + +THF 1.407 1.63 7.52 + Decane 1.411 0 1.99 + +Cyclohexane 1.426 0 2.02 + +Tetradecane 1.429 0 2.03 + +cis-Decalin 1.481 0 2.22 + +Bromocyclohexane 1.495 2.3 8.00 + +Toluene 1.497 0.375 2.38 + +Chlorobenzene 1.524 1.69 5.69  +1,2,3,4-Tetralin 1.541 0.22 2.77  1-Bromonaphthalene 1.657 1.55 4.77  +Soft Matter, 2012, 8, 2382–2384 | 2383Published on 23 January 2012. Downloaded by UNIVERSITAT BAYREUTH on 5/18/2020 10:46:56 AM. View Article Onlinepolydiene solutions (expressed by different pattern formations) to theapplied laser beam. It is important tonotice that noneof the polydienesolutions examined showed neutral behavior when exposed to theoptical beam (which should mean no pattern formation) under theirradiation conditions mentioned so far.Depending on the solvents, these polymer solutions respond eitherby an increase/decrease of both polymer concentration and refractiveindex independently of the solution contrast. We conclude that thesign of the polymer solution contrast Dn, which can be readilyselected, does not (solely) determine the system’s response to theoptical field. We note that both PI and PB solutions in the samesolvent displayed an identical response to light irradiation.The minute absorption of polymer and solvent in the visibleregion should exclude photo-thermal and thermophoretic effects asthe main driving forces. In fact, the experimental Soret coefficientfor a PI/hexane solution (identical behavior to the PI/decanesolution in Fig. 2a) and the PI/THF solution in Fig. 2d assumes thepositive values ST ¼ 0.27 K1 and 0.046 K1, respectively. In thepresence of a temperature gradient, the polymer solute would moveto the cold region, i.e. away from the laser beam in either case.Thus, the qualitative difference between the responses of thepolymer solutions in neutral solvents, otherwise expected to behavesimilarly (a versus d and b versus c in Fig. 2), points towardsa microscopic origin of the light–matter interaction, presumably atthe atomic/molecular rather than the mesoscopic level. Forcompleteness we want to mention that also removal of dissolvedoxygen by careful degassing did not change the effect.Moreover, the writing efficiency was found to show a strongwavelength dependence. A qualitative investigation of the laser lightwavelength effect was conducted with one PI/decane solution (c ¼0.04 g ml1) as a reference and revealed strong dependence of theformation kinetics. The response of the sample to laser light and itscharacteristic time was assessed either by the evolution of the trans-mitted beam spot or by imaging the sample under the opticalmicroscope (Fig. 1). The light-induced concentration alteration wasfound to be the fastest in the red, with formation times of the order ofa few seconds to a few minutes depending on the laser intensity (fivewavelengthswere used; l¼ 633 nm, 647 nm, 650 nm, 660 nm and 671nm). A significant slowing down was observed for shorter wave-lengths. For l ¼ 532 nm and l ¼ 488 nm, the effect was present butwith clearly slower response (up to10 times slower compared to redlaser light of similar intensity). Irradiation with a near-infrared laser(l ¼ 830 nm) did not show any change for several hours in similarconditions (P¼ 67 mW, beam size similar to the one used to observethe micropatterns in Fig. 2). The light–refractive (concentration)coupling efficiency as measured by the formation kinetics ismaximum in the red. This strong wavelength dependence possiblypoints towards a resonant mechanism.Coupling of optical fields with matter can occur through differentmechanisms. Though always related to the electronic structure, theyare described at the coarse graining level of refractive index differ-ences. In binary mixtures the sign of the light-induced concentrationchange is expected to be solely dictated by the sign of the polariz-ability or refractive index increment (dn/dc) through the electro-striction mechanism.23 Simple descriptions in terms of refractiveindices, however, oversimplify the local solute–solvent interactions.This macroscopic light–matter coupling cannot capture, even quali-tatively, the observed behavior in Fig. 2a–d, where similar polymersolution optical contrasts lead to opposite behaviors.2384 | Soft Matter, 2012, 8, 2382–2384In summary, we have shown an effect of light-induced attractionor repulsion in polydiene-based non-dilute solutions, with clearwavelength dependence on the kinetics. In particular we have pre-sented cases of micropatterns with refractive index lower than theaverage refractive index of the material, which is against an electro-strictive mechanism. Such a behavior is unexpected for transparentbinary solutions. Polydiene solutions in good solvents thereforeprovide media with a unique type of light–refractive index coupling.This new type of photonic materials may offer a rich platform for theinvestigation of light–matter interactions with an un-equaled versa-tility. Depending on the precise conditions (material and irradiation)light application can give rise to a large number of light propagationand material patterning effects. This effect could be utilized tomanipulate and pattern transparent polymer solutions at moderatelylow laser intensities.Shedding light on the specificity of these materials might unlockthe quest for other complex fluids which could present similarphenomena. Elucidation of the underlying mechanism might facili-tate the control of macromolecular manipulation, writing of threedimensional structures and the investigation of soft matter nonlinearoptics taking advantage of the unique versatility of polymers.The Greek Secretariat for Research and Technology (programPENED-03ED805), the Max Planck Society (program IMPRS forpolymermaterials), and theDFG (Bu 1556/27) are acknowledged forthe financial support. Dr Christos Mantzaridis’ help with polymersynthesis is also acknowledged.Notes and references1 R. A. L. Jones, Soft CondensedMatter, Oxford University Press, NewYork, 2002.2 C. Conti, G. Ruocco and S. Trillo, Phys. Rev. Lett., 2005, 95, 183902.3 G. I. A. Stegeman, D. N. Christodoulides and M. Segev, IEEE J. Sel.Top. Quantum Electron., 2000, 6, 1419–1426.4 A. Ehrlicher, T. Betz, B. Stuhrmann, D. Koch, V. Milner, M. G. Raizenand J. K€as, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 16024.5 A. Ashkin, Science, 1980, 210, 1081–1088.6 S.Duhr andD.Braun,Proc.Natl. Acad. Sci. U. S.A., 2006, 103, 19678–19682.7 W. K€ohler, A. Krekhov andW. Zimmermann,Adv. Polym. Sci., 2010,227, 145–198.8 C. Conti and E. Del Re, Phys. Rev. Lett., 2010, 105, 118301.9 P. J. Reece, E. M.Wright and K. Dholakia, Phys. Rev. Lett., 2007, 98,203902.10 P. Borowicz, J.-I. Hotta, K. Sasaki and H. Masuhara, J. Phys. Chem.B, 1997, 101, 5900–5904.11 J. Hofkens, J.-I. Hotta, K. Sasaki, H. Masuhara, H. Faes and F. C. DeSchryver,Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1996, 283, 165–172.12 T. A. Smith, J.-I. Hotta, K. Sasaki, H.Masuhara and Y. Itoh, J. Phys.Chem. B, 1999, 103, 1660–1663.13 Y. Tsuboi, M. Nishino, Y. Matsuo, K. Ijiro and N. Kitamura, Bull.Chem. Soc. Jpn., 2007, 80, 1926–1931.14 R. Sigel, G. Fytas, N. Vainos, S. Pispas and N. Hadjichristidis,Science, 2002, 297, 67–69.15 G. I. Stegeman and M. Segev, Science, 1999, 286, 1518–1523.16 M. Anyfantakis, B. Loppinet, G. Fytas and S. Pispas, Opt. Lett.,2008, 33, 2839–2841.17 D. Kip, M. Soljacic, M. Segev, E. Eugenieva andD. N. Christodoulides, Science, 2000, 290, 495–498.18 B. Loppinet, E. Somma, N. Vainos and G. Fytas, J. Am. Chem. Soc.,2005, 127, 9678–9679.19 M. Anyfantakis, G. Fytas, C. Mantzaridis, S. Pispas, H.-J. Butt andB. Loppinet, J. Opt., 2010, 12, 124013.20 A. Barty, K. A. Nugent, D. Paganin and A. Roberts,Opt. Lett., 1998,23, 817.21 CRCHandbook ofChemistry andPhysics, ed.D.R. Lide, CRCPress, 2009.22 Handbook of Organic Solvents, ed. D. R. Lide, CRC Press, 1995.23 Y. Tsori, F. Tournilhac and L. Leibler, Nature, 2004, 430, 544.This journal is ª The Royal Society of Chemistry 2012

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Butt, Hans-Jürgen

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Versatile light actuated matter manipulation in transparent non-dilute polymer solutions

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MPG.PuRe

EPub Bayreuth

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