Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityGlass Transition in Photosensitive PolymersOptical pumping of the isomerization transition inpolymeric systems containing the azobenzene moietyhas received much attention in recent years, mainlybecause of applications to optical writing, potentiallydown to the nanoscale [1]. To this end, however,much fundamental study is still necessary, due tothe complexity of the systems, which feature a richphenomenology of relevance to phase transitions inliquid crystals, the glass transition, dewettingphenomena[2], the anomalous diffusional andvibrational dynamics in spatially heterogeneoussystems, and to the well known anomalies in the lowfrequency vibrational density of states (VDOS)ofglass formers. A link is generally found between thedynamical fragility m, and the Boson Peak, which inturn seems in many cases to be related to the localstructure of the glass. However it is still not clearwhat is the cause of the vibrational anomalies, oreven if there is only one general cause. It is hopedthat the study of photosensitive polymers could helpin solving this problem, in particular due to thepossibility of inducing or modifying morphology downto the nanoscale.As part of this general program, we have describedseveral effects on the macroscopic scale: changes inrheological properties probed by opticalmeasurements [3] and by direct measurements onLangmuir monolayers [4] and in bulk [5], leading tothe possibility of fast optical quenching of thematerial; relaxation times are sensitive to theapproach of the glass transition [6]. In spite of theextent of experimental work and modeling, it is stillnot clear what the actual connection is between themicroscopic phenomena related to azobenzeneisomerization and the changes in the macroscopicproperties of the system. We have studied the lowfrequency dynamics on the same material by acombination of neutron scattering techniques,showing that while at low energy, low momentum,we had a standard picture for a polymer but nophotoinduced effect, in the intermediate region ofEnergy (meV)1412108642Fig. 1.Hg OFFHg ON00 5 10 15Exchange momentum, Q (nm -1 )the BP a reduction of the v-DOS was observed uponUV illumination [7]. Therefore basic purpose of ourmost recent activity is to connect the nanoscalesingle molecule optically induced conformationalchanges of the azobenzene side chain to the abovediscussed macroscopic effects of fluidification andspatial homogenization, through the study of theeventual photoinduced changes in the low frequencyvibrational dynamics as probed by differenttechniques.As an example in the following we discuss the resultsrecently obtained by Inelastic X-Ray scattering onthe beamline ID16 of ESRF in dark and under UVillumination, with the incoming X-ray beam focuseddown to 30X100 µm 2 . The measured Q rangecovered up to the first sharp maximum in the staticstructure factor S(Q). IXS data were analyzed with astandard Damped Harmonic Oscillator model plus apurely elastic component. In the figure we show thedispersion curves as measured at T=200K in darkand under UV photoperturbation.The photoperturbation mostly affects the dispersioncurve in the high Q region, while the low Q, longrange structure of the polymer remains unaltered.The photoinduced difference becomes maximumaround 15 nm -1 , which corresponds to the first sharpmaximum in the static structure factor. This suggeststhat photoisomerization, a process that is located onthe scale of the single molecule, affects macroscopicproperties such as viscosity, via the softening of aborder zone mode which is likely to have at leastsome transverse (or localized) character. These ideashave received preliminary confirmation from lowfrequency Raman data, and a preliminary report hasbeen submitted for pubblication [8].References[1] P. Camorani, M. Labardi, M. Allegrini - Mol. Cryst.Liq. Cryst. 372, 365-372 (2001).[2] L. Cristofolini, S. Arisi, M. P. Fontana - Phys.Rev. Letters 85, 4912 (2000). L. Cristofolini, M.P.Fontana T. Berzina, P. Camorani - Mol. Cryst. Liq.Cryst. 398, 11 (2003).[3] Sanchez,-C.; Alcala,-R.; Hvilsted,-S.;Ramanujam,-P.-S. Appl.Phys.Lett. 77, 1440 (2000)P. Camorani, PhD Thesis, Univ. of Parma (2004).[4] L. Cristofolini, M.P. Fontana - PhilosophicalMagazine B 84, 1537, (2004).[5] P. Camorani, L. Cristofolini, G. Galli, M. P.Fontana - Mol. Cryst. Liq. Cryst. 375, 175 (2002).[6] L. Cristofolini, P. Cicuta, M.P. Fontana- J.Physics: Condensed Matter, 15, S1031 (2003).[7] L. Cristofolini, M.P. Fontana, M. Laus B. Frick -Phys. Rev. E, 64, 061803 (2001)[8] L. Cristofolini et al. Submitted for pubblicationAuthorsP.Camorani, L.Cristofolini, M.P.Fontana, E.PontecorvoUniversity of Parma CRS SOFT CNR-INFM.SOFT Scientific Report 2004-0660