K. Resch1*, J. Fischer1, A. Weber1 and G. M. Wallner2

1 Polymer Competence Center Leoben GmbH, RoseggerstraBe 12, A-8700 Leoben, Austria
2 Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben, Austria

Corresponding Author, resch@pccl. at

Abstract

In this paper the optical and morphological properties of a thermotropic system with fixed domains were investigated. The optical properties and the switching were determined by UV/Vis/NIR spectrophotometry. The morphology was characterized applying Atomic Force Microscopy (AFM) and Raman microscopy. The thermotropic films exhibited a hemispheric solar transmittance of 85% in the clear state, with a diffuse fraction of 40%. The material underwent a transition from the clear to the scattering state at a temperature of 45°C. Above the switching temperature the hemispheric solar transmittance decreased to a value of 79%, with a diffuse fraction of 64%. In general the thermotropic resin was characterized by a steep and rapid switching process. The comparison of the films switching performance with the additives thermal transition determined by Differential Scanning Calorimetry revealed a good correlation. The significant increase of the diffuse transmittance along with the moderate change in hemispheric transmittance was in good agreement with average scattering particle dimensions of 0.4 to 2.5 pm ascertained by AFM phase imaging and mapping of the chemical constitution of the surface by Raman microscopy.

Keywords: thermotropic resin, UV/Vis/NIR spectroscopy, Atomic Force Microscopy, Raman microscopy

1. Introduction

Thermotropic materials that change their light transmission behaviour from highly transparent to light diffusing upon reaching a certain threshold temperature reversibly can provide overheating protection for solar thermal collectors [1]. Especially thermotropic systems with fixed domains that consist of thermotropic additives dispersed in the matrix of a curable resin possess a high potential for solar thermal applications [2,3,4]. To prevent overheating of an all-polymeric flat plate collector with twin — wall sheet glazing and black absorber thermotropic layers with switching temperatures between 55 and 60°C (thermotropic glazing) or 75 and 80°C (thermotropic absorber) as well as a solar transmittance of 85% in clear state and between 25 to 60% in opaque state are required. The overall objective of this research work is to perform a comprehensive characterization of a thermotropic system with fixed domains and to establish structure-property relationships. Optical properties and the switching characteristics are characterized by UV/Vis/NIR spectrophotometry. The films switching temperature is related to the thermal transition of the additive determined by Differential Scanning

Calorimetry. Furthermore the switching performance is compared to scattering domain size determined by Atomic Force Microscopy (AFM) in phase imaging mode and Raman microscopy.

2. Experimental