Polymer Dispersed Liquid Crystals as Optical Storage Materials
- PDF / 2,100,499 Bytes
- 10 Pages / 417.6 x 639 pts Page_size
- 59 Downloads / 204 Views
Polymer Dispersed Liquid Crystals (PDLC) are composite materials made by polymers and liquid crystals, which show at the same time the properties of both materials [I]. They usually appear as a dispersion of liquid crystal droplets embedded in a polymeric matrix. Different preparation methods can be used, among them the PIPS technique (Polymerisation Induced Phase Separation) allows us to use PDLCs for optical recording. Two approaches are possible. In the first one, UV (or visible light) curable polymers are needed so that phase separation and consequent droplets formation occurs by light irradiation. In this case, permanent optical recording can be obtained during the irradiation process. In the second approach epoxy resins are exploited as polymeric matrix and optical recording is performed on already cured samples. Examples concerning the exploitation of these two different PIPS techniques for optical recording in PDLCs, will be presented and discussed. OPTICAL RECORDING BY UV CURING It is well known that the morphology of UV curable PDLCs is closely connected to the curing parameters such as UV intensity and exposure time [2]. Curing 65 Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
intensity and time determine the speed of polymerisation of the matrix and the degree of cure of the final sample, which in turn directly affect the size, shape and distribution of the liquid crystal domains. On this basis, it is clear that optical recording on UV curable PDLCs is possible through modulation of the morphology during the curing process. In particular, binary images can be easily obtained, by placing masks in front of the UV curing source. In this way, the spatial distribution of the curing intensity reproduces the image on the mask and a selective curing occurs, which fixes the image oa the PDLC. On the other hand, it is worth noting that conventional UV lamps are not suitable for this purpose, leading to broad edges and low quality images because of the large dimensions of the light cone impinging on the mask and then on the sample. In order to achieve higher quality images with useful spatial resolution, a highly collimated light beam is needed; thus the best source seems to be the UV line of a laser.
We have exploited the laser curing technique to write permanent binary images in PDLCs made by the UV curable mixture NOA65+E7, by using the UV lines of an Ar+ ion laser ( X = 333-363 nm) as curing source. Samples have been fixed on a horizontal plane and irradiated by the laser beam after its passage through a mask reproducing the logo of "Istituto Nazionale per la Fisica della Materia" (INFM). A lens and a diaphragm have been used to adjust the beam spot size on the sample. The following table summarises the experimental conditions:
Mixture
50% NOA65+50% E7
UV Intensity Curing Time
I=(5-500)mW/cm2
Thickness
23[tm
Substrates
ITOconductive glasses
10 seconds
All the samples have been irradiated immediately after preparation in order to avoid any spontaneous curing reaction. Details
Data Loading...
