Photoinduced Morphological Changes and Optical Writing in a Liquid Crystalline Polymer on the Micron and Sub-Micron Scale

Abstract

In this paper we describe our recent work on optical effects due to photoinduced molecular reorientation in a side chain liquid crystalline polymer, namely azopolymethacrylate (PMA4). We have studied several kinds of optical effects on space scales ranging from the nanoscopic to macroscopic. In particular, we have built a computer controlled microscope-based apparatus to study photoinduced effects down to the micron scale in thin and ultrathin (Langmuir Blodgett) films of our material. Furthermore we measured the morphology and the photoinduced surface deformation by Atomic Force Microscopy in the submicron scale. INTRODUCTIONSide chain liquid crystalline polymers (LCP) based on the presence in the side chain of the azobenzene moiety have been the subject of intense study in recent years [1]. Of particular interest have been the optical effects due to the photoinduced cis-trans isomerization of the azobenzene. These effects have been studied due to their potential for optical writing applications and more specifically for entirely-optical high-density memories [2]. This class of materials features a rich and complex phenomenology, which makes them very interesting also from a fundamental point of view. Their dynamics is the result of the complex interplay between several types of interactions and processes: the mesogenic potential, the conformational main chain transitions, the glass transition, the molecular trans-cis isomerization [3]. The relative weight of such processes, and the coupling between side chain and main chain dynamics, depend of course not only on temperature, but it can be also externally influenced by optical pumping with light of the appropriate wavelength, polarization and intensity [4]. Moreover, when dealing with ultrathin films, coupling interactions between substrate (imposing a planar ordering to the polymeric main chain) and PLC become relevant. This gives rise-via a purely geometric effect-to an increased stability of the glassy phase upon heating [5], and correspondingly to an increase of the typical times characterising the back-to-equilibrium relaxation process This is of relevance also for the stability of the information optically written onto such thin films of azo-polymer. The combined action of the cis-trans photoisomerization and the nematic potential lead to collective molecular reorientation, hence to very high sensitivity of the sample to the optical writing beam. However it is not clear-and the subject of current investigationhow the nematic potential determines the collective character (and hence the length scale) of the molecular reorientation, on which the minimum addressable area depends, and hence the maximum information density possibly stored per surface unit. Although the basic microscopic mechanism for the production of optically oriented molecular

Optical Input/Electrical Output Memory Elements based on a Liquid Crystalline Azobenzene Polymer

“…Interestingly, Figure b,c shows that the film morphology is unchanged when the surface is irradiated with a LED at 365 nm. The obtained results are in good agreement with the data reported in literature related to the formation of nanostructures on the surface of AZO/P­(M)­A polymers, as well as with the formation of an intermediate phase of reorganization. , In particular, the morphological rearrangement only occurs if the LC polymer is not in its glass phase at the operating temperature condition. With regard to our systems, at RT only PA-AZO1 is in its nematic LC phase ( T g = 19 °C), explaining why it undergoes a dramatic morphological reorganization when irradiated with UV–vis light. , Moreover, Angiolini et al have reported that the PA-AZO1 the isomerization from trans to cis generates a contraction of the material.…”

“…The obtained results are in good agreement with the data reported in literature related to the formation of nanostructures on the surface of AZO/P(M)A polymers, 12 as well as with the formation of an intermediate phase of reorganization. 38,39 In particular, the morphological rearrangement only occurs if the LC polymer is not in its glass phase at the operating temperature condition. With regard to our systems, at RT only PA-AZO1 is in its nematic LC phase (T g = 19 °C), explaining why it undergoes a dramatic morphological reorganization when irradiated with UV−vis light.…”

Slow dynamics in an azopolymer molecular layer studied by x-ray photon correlation spectroscopy

“…All of these issues can be studied in a specific class of materials, namely side-chain liquid crystalline polymers with an azobenzene moiety attached to the main chain via a flexible molecular spacer [3][4][5]. In polyacrylates and poly-methacrylates the pumping of the photo-isomerization transition yields several potentially useful optical effects with very high sensitivity [6][7][8]. The vibrational and diffusive dynamics can be altered by the optical pump [9,10] and the phenomenology of the glass transition has been extensively studied by calorimetric, dynamical analysis and ESR relaxation measurements [11].…”

“…In polyacrylates and poly-methacrylates the pumping of the photo-isomerization transition yields several potentially useful optical effects with very high sensitivity [6][7][8]. The vibrational and diffusive dynamics can be altered by the optical pump [9,10] and the phenomenology of the glass transition has been extensively studied by calorimetric, dynamical analysis and ESR relaxation measurements [11].…”

Looking for the glass transition in a single molecular layer on the water surface

“…This (noting that the corresponding ω-values are very similar) indicates that the trans isomer is far more rigid than the cis form. It is commonly accepted in the literature [6,19,20] that isomerization causes fluidification of azo polymers, although no quantitative direct data have been published so far. Here we have a clear observation of this fact.…”