New PDLC materials obtained from dispersion of LC under microgravity

• Type: Summary of Presentation in an International Conference
• Date: 2010

Authors

M.R. Matos (Author) FCUP; J.C. Leitao (Author) FCUP; R.M. Andre (Author) FCUP; R. Zambujal (Author) FCUP; Carla Carmelo Rosa (Author) FCUP; P. Simeao Carvalho (Author) FCUP; Thomas Podgorski (Author) Other

Conference proceedings International

Title: COSPAR 2010

COSPAR 10 - 38th scientific assembly of the committee on space research

Bremen, Alemanha, 18 a 25 de Julho de 2010

Scientific classification

FOS: Natural sciences > Physical sciences

Other information

Abstract (EN): In this work we present results obtained in different types of PDLC films, produced by the Photo-initiated Polymerization Induced Phase Separation (P-PIPS) technique, performed under microgravity conditions at ZARM Drop Tower. We show a comparison of the electro-optical properties between samples produced under microgravity and normal gravity conditions, as well as for homogeneous and heterogeneous P-PIPS polymerization processes. Polymer Dispersed Liquid Crystals (PDLCs) consist of micron-size droplets of Liquid Crystal (LC) dispersed in a polymer matrix. This mixture combines the properties of polymers, such as mechanical support and resistance to external mechanical disturbances, with the electromagnetic properties of Liquid Crystals, namely high electric, magnetic and optical anisotropies [1]. By applying a specific electric or magnetic field the PDLC film optical anisotropy can be changed. Among other properties of PDLC films, the size, shape and distribution of LC droplets are fundamental factors for its electro-optical properties such as light scattering, optical hysteresis, dielectric response, and most important: the reorientation field and switching speed [2]. These properties are, in turn, key to the quality and performance of PDLC based devices, such as LCDs and smart-windows, and are affected by some P-PIPS parameters [4]. The importance of studying homogeneous PDLCs in microgravity is mainly motivated by the fact that when the mixture is still in the SOL phase the polymer molecule clusters are free to move and form domains that will be filled with LC. The polymer chains start to grow when irradiated by UV-light, and consequently segregate the LC molecules, forming a matrix surrounding the discrete LC domains and inducing different morphologies depending on the polymerization conditions. An important transport process on Earth that induces distortions on droplets is convection which leads to droplet collisions and coalescence. However, in microgravity the major transport process is diffusion, leading to a more homogeneous droplet distribution and less distorted droplets, also contributing to larger droplets. Therefore, when PDLCs are produced in a 0 g environment, samples possessing less interconnected and more uniformly dispersed domains, with bigger and more spherical droplets are expected [3]. This may lead to better light scattering and lower reorientation fields. The microgravity environment was achieved at ZARM Drop Tower in November 2009 where several PDLC samples were polymerized under microgravity. Samples were previously prepared with the photo-polymer NOA-81 from Norland Products® and three different LCs at room temperature: a nematic, a chiral nematic, and a chiral smectic C. For the same mixture of LC and polymer three different mass ratios were used. Furthermore, some samples with induced inhomogeneities were produced in order to have only specific and controlled areas with 0 g polimerization. The characterization work - which includes light transmission and optical contrast, reorientation fields, response times and dielectric constant measurements - is currently being done in the lab and fresh results will be presented in the meeting.

Language: English

Type (Professor's evaluation): Scientific