Molecularly oriented synthetic resin composition

Abstract: A molecularly oriented synthetic resin composition whose coefficient of linear thermal expansion in two mutually perpendicular direction, below the glass-transition temperature, is at least a factor of 3 smaller than in a third direction extending perpendicularly to the first two directions, is manufactured by curing a liquid-crystalline curable synthetic resin composition. The curable synthetic resin composition comprises a chiral dopant, leading to the helicoidal order of the molymer molecules, after which the synthetic resin composition is cured so as to form a polymer network. The materials thus obtained are suitable for use as claddings on materials having a coefficient of thermal expansion which is smaller than that of most synthetic resins.

Description: The invention relates to a molecularly oriented synthetic resin composition and a cladding manufactured from a molecularly oriented synthetic resin composition. The invention also relates to a method of manufacturing a molecularly oriented synthetic resin composition by curing a liquid-crystalline curable synthetic resin composition. The expression "molecularly oriented" is to be understood to mean herein that anisotropic molecules having a preferred direction are present in the material concerned. In an unoriented synthetic resin the polymer molecules generally have the shape of an isotropic cluster. A molecularly oriented synthetic resin composition and a method of manufacturing thereof by curing a liquid-crystalline curable synthetic resin composition are described in United States Patent specification US 4,733,941. The oriented synthetic resin composition is provided as a cladding on an optical fibre for telecommunication purposes. The molecules in the synthetic resin composition are oriented in the longitudinal direction of the fibre and subsequently interconnected in a network by cross-linking. Owing to this, the material has a much larger rigidity in the longitudinal direction than in the radial directions. On the other hand, the coefficient of linear thermal expansion in the longitudinal direction is much smaller (at certain temperatures up to a factor of 10) than in the radial directions extending perpendicularly thereto. Synthetic resin compositions are used as claddings on many different, whether or not flat, substrates, for example, on semiconductor devices and optical and electronic components. However, the problem which occurs is that the coefficient of linear thermal expansion of synthetic resins is one order of magnitude larger than that of most other solids such as metals, ceramics, glass and semiconductor materials. Shrinkage and expansion as a result of variations in temperature may cause the cladding to crack or to become detached from the substrate. Besides, undesirable thermal stresses also occur. One of the objects of the invention is to provide a synthetic resin composition which, when applied in a cladding, exhibits a satisfactory thermal adaptation to the substrate. A further object of the invention is to provide a synthetic resin composition having particular, temperature-independent optical properties, for example, for use in a device by means of which a plane of polarization can be rotated or for use in a colour filter. Another object of the invention is to provide an effective method of manufacturing such a synthetic resin composition and of providing and fixing the orientation in the specific direction(s). According to the invention, this object is achieved by an oriented synthetic resin composition whose coefficient of linear thermal expansion in two mutually perpendicular directions, below the glass-transition temperature, is at least a factor of 3 smaller than in a third direction extending perpendicularly to the first two directions, said synthetic resin composition comprising a polymer network having a helicoidal order, said network comprising liquid crystalline compounds which contain at least two polymerized acrylate ester groups, epoxide groups, vinyl ether groups or thiolene groups. It has been found, that in such a case the difference in coefficient of expansion above the glass-transition temperature is even considerably larger than below the glass-transition temperature. The coefficient of expansion may even assume negative values in the above-mentioned two mutually perpendicular directions. In this case, thermal shrinkage or expansion is very small over a large temperature range. When the synthetic resin composition is used in a cladding, according to the invention, the coefficient of linear thermal expansion, below the glass-transition temperature, in two mutually perpendicular directions parallel to the plane of the cladding is at least a factor of 3 smaller than in a third direction extending perpendicularly to the plane of the cladding. The invention is based on the insight that when it is impossible to influence the coefficient of expansion of synthetic resin composition to a substantial degree, the coefficient of expansion can be influenced in a number of specific directions by means of a molecular orientation. Moreover, use is made of the possibility to fix a molecular orientation by cross-lining polymer molecules to form a network. The difference with the known cladding on an optical fibre consists in that in the case of the known cladding a uniaxial orientation is provided, for example, by means of a flow of the material, so that a small coefficient of expansion is obtained in only one direction. For use of the cladding on many, substantially flat, substrates this does not solve the shrinkage and expansion problems. By providing and fixing a helicoidal order, the coefficient of linear expansion is reduced in two directions relative to the non-oriented state. As the volume expansion coefficient of the synthetic resin composition hardly changes by the orientation, the coefficient of expansion in the third direction increases. The latter aspect is less important for use of this composition in a cladding on a substantially flat substrate, but the coefficients of linear expansion in the plane of the layer are better adapted to the substrate. The object of providing a method of manufacturing an oriented synthetic resin composition is achieved according to the invention in that a liquid-crystalline curable synthetic resin composition is selected which comprises a chiral dopant and liquid crystalline compounds which contain at least two polymerisable acrylate ester groups, epoxide groups, vinyl ether groups or thiolene groups, the molecules of the curable synthetic resin composition being helicoidally ordered, after which the synthetic resin composition is cured so as to form a polymer network. In a preferred embodiment of the method according to the invention, the curable synthetic resin composition comprises liquid-crystalline monomers or oligomers which are composed of compounds having two or more acrylate-ester groups. If desirable, the curable synthetic resin composition may comprise a mixture of various oligomeric compounds. The synthetic resin composition may further comprise one or more other suitable components such as, for example, catalysts, (light-sensitive) initiators, stabilizers, co-reacting monomers and surface-active compounds. A non-polymerizable liquid-crystalline material, for example up to a quantity of 50% by weight, may alternatively be added to change the optical properties of the material. Instead of acrylate compounds, epoxides, vinyl ethers and thiolene compounds may alternatively and satisfactorily be used as the liquid-crystalline monomer. In a particular embodiment of the method according to the invention, the helicoidal order is obtained by arranging the synthetic resin composition between two substrates, each of the surface of the substrates contacting the curable synthetic resin composition being rubbed in one direction, for example, by means of a velvet cloth. This causes the molecules of the liquid-crystalline compounds, in particular the mesogenic groups present therein, to orient themselves in the direction of rubbing at the surfaces. Preferably, the surfaces of the substrates are composed of polyimide. Suitable compounds which can be cured in the oriented state, for example after they have been provided on a rubbed substrate, are described in United States Patent Specification US 4,758,447. A method of manufacturing such compounds is described in European Patent Application EP-A1-0 261 712. As a rapid curing of the synthetic resin composition is desirable, the curing process is initiated, preferably, by means of actinic radiation. Actinic radiation is to be understood to mean herein radiation using light, in particular UV light, X-rays, gamma rays or radiation using high-energy particles such as electrons or ions. The invention will be explained in greater detail by means of an exemplary embodiment and with reference to the accompanying drawing, in which Figure 1 shows the structural formula of liquid-crystalline compounds having two acrylate-ester groups, which can suitably be used according to the invention, Figure 2 shows the structural formula of a suitable chiral dopant, Figure 3 shows the thermal expansion of an oriented material according to the invention, and Figure 4 shows, for comparison, the thermal expansion of an isotropic material which is not according to the invention. Exemplary embodiment. A starting mixture for a curable synthetic resin composition is manufactured from 80 parts by weight of a first diacrylate compound A, as shown in Figure 1, where R is a methyl group, 20 parts by weight of a second diacrylate compound B, as shown in Figure 1, where R is a hydrogen atom, and 1 part by weight of a light-sensitive initiator, in the present example 2,2-dimethoxy-2-phenyl-acetophenone, commercially available from Ciba-Geigy under the trade name Irgacure (R) 651. A method of manufacturing said diacrylate compounds is described in European Patent Application EP-A1-0 261 712. The starting mixture additionally comprises 100 ppm of a stabilizer, for example hydroquinone monomethyl ether. The starting mixture which comprises 80 parts by weight of A and 20 parts by weight of B is a eutectic composition having a melting point of 80°C. Above this temperature there exists a nematic phase which changes into an isotropic phase at a temperature of 121°C. The mixture is used between these two temperatures, both the viscosity and the molecular order being highest at the lowest temperatures. According to this example, different quantities of a chiral dopant are added to said starting mixture, for example left-handed 4-(4-hexyloxy-benzoyloxy)-benzoic acid-2-octyl-ester, see Figure 2, which is commercially available from Merck under number S811. The chiral dopant itself does not have to exhibit liquid-crystalline properties and may be a co-polymerizing monomer as well as a non-polymerizable compound. To obtain a twisted nematic order in the polymer, it is sufficient for the compound to be chiral, for example through the fact that the compound contains an asymmetrical carbon atom. The chiral dopant may be left-handed as well as right-handed. A layer of the material obtained in this manner has a nematic order, with a helical structure, also termed cholesteric order. The natural pitch of the mixture depends on the quantity of chiral dopant added and is approximately 28 µm at 0.5 mol.% and approximately 2.5 µm at 6 mol.%. The natural pitch depends to a small extent on the temperature and is 2.17 µm at 80°C, 2.31 µm at 100°C and 2.35 µm at 113°C, when 6.67 mol.% of the chiral dopant is used. At the said quantity, the transition to the isotropic phase takes place at a temperature of 114°C. According to this example, the curable synthetic resin composition is cured by photopolymerization through exposure to ultraviolet light from a short-arc mercury lamp having a radiation intensity on the surface to be cured of 2 to 5mW/cm² for 3 minutes at 100°C. The orientation is fixed in the curing process, the overall rotation angle of the layer remaining constant. For this reason, the variation in pitch during curing depends only on the change in layer thickness caused by polymerization shrinkage. The rotation angle is a measure of the number of revolutions in the molecular spiral in the cholesteric nematic phase. The rotation angle of the polymer film thus obtained exhibits no temperature dependence in the temperature range from room temperature to 250°C. This proves the complete impedance of molecular re-orientation as a result of the network of polymer molecules formed by cross-links. As the monomers contain two acrylate-ester groups per molecule, cross-linking is so strong that substantially no movement is possible in the rigid parts of the liquid-crystalline molecules. Figure 3 shows the change in length σ in three mutually perpendicular directions as a function of the temperature T. σ is the relative change in length with respect to the reference length at 25°C. The direction of thickness of the polymer film is z, the directions x and y extend in the plane of the polymer film. In the z-direction the coefficient of thermal expansion below the glass-transition temperature (approximately 90°C) is approximately 2 to 4 x 10⁻⁴/°C. Above said temperature the coefficient of thermal expansion increases to approximately 7 x 10⁻⁴/°C. In the x- and y-directions the coefficient of thermal expansion below the glass-transition temperature is less than 5 x 10⁻⁵/°C. At the glass-transition temperature the coefficient of expansion decreases and even becomes negative above said temperature. As a result hereof, there are hardly any changes in length in the x- and y-directions in the temperature range from 25° to 140°C. The shrinkage occurring when the temperature increases to above the glass-transition temperature is reversible. For comparison, Figure 4 shows the change in length of an isotropic material which is similar (having the same composition) to the material according to the invention. The coefficient of expansion is the same in all directions. The coefficient of expansion is approximately 10⁻⁴/°C at a temperature below the glass-transition temperature, at a temperature above the glass-transition temperature it increases to approximately 4 x 10⁻⁴/°C. By virtue of the above-described thermal properties, the material according to the invention is very suitable for use as a cladding material, for example, on semiconductor devices and other materials having a coefficient of expansion which is smaller than that of most synthetic resins. A further advantage for optical use of the polymer film obtained is the small temperature dependence of the rotation angle in comparison with, for example, linear cholesteric polymers having chiral groups in side chains. Within certain limits, the pitch can be influenced by curing the synthetic resin composition between two substrates. Preferably, flat substrates of polyimide or glass having a cladding of polyimide are used. The polyimide surface is rubbed uniaxially, for example, using a velvet cloth. The pitch depends on the distance between the two substrates, in this example, for example, 6 µm, and the angle between the two directions of rubbing the polyimide surface. The number of revolutions of the molecular spiral adjusts itself such that the pitch obtained does not differ too much from the natural pitch. A particular optical device is obtained by manufacturing a layer having a rotation angle across the entire layer thickness of exactly 90°. In such a device the plane of polarization of transmitted light is rotated through 90° when the condition is met that dΔn/λ is larger than 0.87. In this division, d is the layer thickness, Δn is the double refraction and λ is the predominant or average wavelength of the transmitted light. It is alternatively possible to obtain a wavelength filter by manufacturing a layer whose pitch is equal to the wavelength of the light which, in such a case, is reflected at the layer, whereas light having other wavelengths is transmitted.

Claims: A molecularly oriented synthetic resin composition, characterized in that the coefficient of linear thermal expansion in two mutually perpendicular directions, below the glass-transition temperature, is at least a factor of 3 smaller than in a third direction extending perpendicularly to the first two directions, said synthetic resin composition comprising a polymer network having a helicoidal order, said network comprising liquid crystalline compounds which contain at least two polymerized acrylate ester groups, epoxide groups, vinyl ether groups or thiolene groups. A cladding of a molecularly oriented synthetic resin composition, characterized in that a synthetic resin composition according to claim 1 is used, whereby the third direction extends perpendicularly to the plane of the cladding. A method of manufacturing a molecularly oriented synthetic resin composition, as claimed in Claim 1 or 2, by curing a liquid-crystalline curable synthetic resin composition, characterized in that the curable synthetic resin composition contains a chiral dopant and liquid crystalline compounds which contain at least two polymerisable acrylate ester groups, vinyl ether groups or thiolene groups, the molecules of the curable synthetic resin composition being ordered helicoidally, after which the synthetic resin composition is cured so as to form a polymer network. A method as claimed in Claim 3, characterized in that the curable synthetic resin composition comprises liquid-crystalline monomers or oligomers, which consist of compounds having two or more acrylate-ester groups. A method as claimed in Claim 3 or 4, characterized in that the helicoidal order is obtained by arranging the synthetic resin composition between two substrates, each of the surfaces of the substrates contacting the curable synthetic resin composition being rubbed in one direction. A method as claimed in any one of the Claims 3 up to and including 5, characterized in that the synthetic resin composition is cured by means of actinic radiation.

Inventors: BROER DIRK JAN, HEYNDERICKS INGRID EMILIENNE J, MOL GRIETJE NEELTJE, BROER, DIRK JAN, HEYNDERICKS, INGRID EMILIENNE JOANNA RITA, MOL, GRIETJE NEELTJE
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: C09K19
IPC codes: C08J5, C08F20, C08L101, G02B6, G02F1, C08L33, C08J3, G02B5, G02B1, C09K19, C08F299