Anisotropic polymerization shrinkage behaviour of liquid-crystalline diacrylates R. A. M. Hikmet*, B. H. Zwerver and D. J. Broer Philips Research Laboratories, PO Box 80000, 5600JA Eindhoven, The Netherlands (Received 22 May 1990; revised 11 October 1990; accepted 11 October 1990) Anisotropic polymerization shrinkage of liquid-crystalline acrylates has been studied. In order to investigate the changes occurring on the microscopic scale, X-ray diffraction was combined with density measurements. As a result it was shown that at high polymerization temperatures the polymerization shrinkage in the direction of the molecular orientation was higher than the shrinkage in the directions perpendicular to the molecular orientation. Subsequently the shrinkage obtained on the microscopic scale was compared with the results obtained on the macroscopic scale. The behaviour was interpreted in terms of the molecular structure and the packing density within the systems. (Keywords: liquid crystals; acrylate; erosslinking; network; X-ray diffraction; packing density) INTRODUCTION The use of low-mass liquid-crystalline (LC) acrylates has recently been successfully demonstrated in the production of anisotropic networks 1-5. These molecules possess low viscosities and they can be macroscopically oriented by a simple surface treatment, by flow or in electric and magnetic fields 6. The induced orientation can subse- quently be frozen in by isothermal photopolymerization 7 of the system. Highly crosslinked networks obtained in this way possess anisotropic thermal, mechanical, electrical and optical properties. In the present study we used a homologous series of liquid-crystalline diacrylates shown in Figure 1 in order to investigate the anisotropic polymerization shrinkage that occurs in these systems. In order to study the shrinkage occurring on the microscopic scale, X-ray diffraction measurements were combined with density measurements. These results were then compared with the anisotropic linear shrinkage observed on the macroscopic scale. EXPERIMENTAL The structure of the monomers used for this study is given in Figure I. Details of the syntheses and polym- erization of the monomers can be found in refs. 2-4. The monomers were provided with 1% w/w of a photo- initiator, ~,ct-dimethoxydeoxybenzoin (Irgacure 651, Ciba Geigy). Polymerization of the materials was initiated by u.v. radiation from a 100 W high-pressure mercury lamp (366 nm, l0 mW cm -2 ) in a nitrogen atmosphere. Den- sities in the liquid state were measured as a function of temperature from the mass and the length of samples in high-precision capillaries (0.5 mm in diameter) stored in an oven where the temperature was regulated (_+ 0.1 °C). Samples were polymerized in the liquid-crystalline state close to their isotropic transition temperature, and their densities at room temperature were measured using a * To whom correspondence should be addressed density gradient column containing potassium carbonate in water. The thermal expansion behaviour of the solid samples was measured using a Perkin Elmer TMA7 unit. X-ray diffraction patterns were recorded by a Statton camera using Ni-filtered CuK, radiation. The camera was provided with a magnetic heating cell so that the temperature of the samples could be thermostatically controlled as the molecules were uniaxially oriented under the influence of the magnetic field (6 kG). RESULTS AND DISCUSSION Volume contraction during polymerization In order to investigate volume changes occurring upon polymerization of LC acrylates, the densities of the materials were measured as a function of temperature before and after polymerization. In Figure 2a the densities of various monomers are plotted as a function of temperature. It can be seen that the density of the samples decreases with increasing number of methylene spacer groups. Density of a system is determined by the Van der Waals density Pw of the molecules as well as the empty volume within the system. One can represent the empty volume in terms of packing density K (ref. 8), which is defined as: Vw p K - - (1) M/p Pw where p is the density, Vw is the Van der Waals volume occupied by the atoms of a molecule, M is the molecular weight and Pw is the Van der Waals density. In Table 1 molecular weights of various samples are given together with their Van der Waals densities calculated according to Bondi 8. Here it can be seen that the order obtained for the Van der Waals densities of the molecules is the same as the order found in Figure 2a at a given temperature. This indicates that at a given temperature 0032 3861/92/010089q37 () 1992 Butterworth-Heinemann Ltd. POLYMER, 1992, Volume 33, Number 1 89