IE910627A1 - Process for the preparation of thin films - Google Patents

Abstract

Prodn. of a thin layer of amphiphilic cpd(s) (I) on a substrate (II) is by producing, in a horizontal channel, an oriented stream of polar liq. (III) contg. mols. of (I) in soln. or spread on the surface, blocking the stream with a horizontal barrier at the fluid/(III) interface and opt. choosing a concn. of (I) such that a compressed layer of (I) is continually formed at the interface in front of the barrier, then moving the substrate (II) through this zone of compression; the novelty is that (I) are polyamides, pref. poly-glutamates. A thin layer of (I) contg. a lipophilic dye (IV) is produced in the same way, by adding a soln. of (IV) in an organic solvent which is immiscible with water. A monomol. layer of (I) and (IV) on a water surface is claimed; this monolayer is produced by process 1 (b) and has a dichroic ratio above 1.0, pref. above 2.0. (I) are polycondensates of different gamma-alkyl-substd. copolyglutamates, pref. copolycondensates of gamma-alkyl-L-glutamate and gamma-alkyl'-L-glutamate (with alkyl = 1-6C alkyl and alkyl' = 12-36C alkyl), esp. pref. gamma-methyl- and gamma-octadecyl-L-glutamate; (IV) has approx. rod-shaped mols., and is pref. a diazo dye, a cyanine or a carotenoid.

Description

HOECHST AKTIENGESELLSCHAFT HOE 90/F 060 Dr. DS/AP Description Process for the preparation of thin films The invention relates to a process for the preparation of 5 thin films comprising at least one amphiphilic compound, in particular polyamides, preferably polyglutamates, on a support.

In the process according to the invention, films comprising amphiphilic molecules ordered laterally in a flow duct, i.e. an imposed macroscopic order in the interface, are produced, compressed by the flow and transferred to a solid support by the Langmuir-Blodgett method.

It is known that insoluble films comprising amphiphilic molecules spread on liquid surfaces can be compressed mechanically by compressing barriers in such a manner that, when a solid support is moved through the film, depending on the type of molecule and degree of compression of the surface film, the film is transferable to this support. Multi-layer coating which is made possible thereby is known in particular as the Langmuir-Blodgett technique and is highly topical for various applications.

In this method, the compounds are in most cases dissolved in volatile organic solvents, the solution is spread out on the surface of the polar liquid (almost always water), and the solvent is then evaporated. Only in the case of insoluble molecules is it possible to produce the high degrees of compression necessary for transferring a film by moving a barrier. This is why the Langmuir-Blodgett technique is limited to compounds which are insoluble in the liquid phase.

It is extremely complicated to carry out the film transfer continuously, since in this case the preceding IE 91627 spreading also has to be carried out continuously. A continuous Langmuir-Blodgett technique by means of a mechanical rolling process has been described by A. Barraud et al. 1983 (Thin Solid Films, Vol. 99, 221).

Films comprising amphiphilic molecules which are soluble in the liquid phase cannot be transferred by customary mechanical compression in the Langmuir trough, because they escape into the subphase by desorption. Suitable organic polymers and processes for preparing thin films having defined dye concentrations, such as are also used in the process according to the invention, are described, for example, in German Offenlegungsschrift 3,724,543.0 and German Offenlegungsschrift 3,724,542.2.

Laterally ordered films have already been described in several cases (G. Duda, A. J. Schouten, T. Arndt Preparation of multilayers of preformed polymers, Thin Solid Films 159 (1988), 221 - 230). Experiments on the orientation of molecules in the water surface were carried out as early as 1989 by Miyata (S. Miyata, 4th International Conference on Langmuir-Blodgett Films, Tsukuba 1989, Preprints p. 24 - 25). In this discontinuous process, a monofilm flowing through a narrowing duct is compressed, using a special film balance having movable side walls, while the aqueous subphase is not moved.

A continuous process and an apparatus for preparing films by the Langmuir-Blodgett method which have a preferred orientation in the film plane is described in German Patent Application, file no. P 38 28 836.2, which is not a prior publication.

The object was therefore to provide a process in which both soluble and insoluble amphiphilic molecules can be concentrated and compressed continuously on the surface of the subphase, in which the monofilm is already sub35 jected to preorientation at the water surface and the IE 91627 - 3 dichroic ratio of monofilms of this type can be increased.

The process according to the invention relates to the preparation of a thin film of organic polymers having long-chain side groups, preferably polyglutamates and, if desired, simultaneously applied dye molecules on a solid support, in which an at least partially oriented film of the molecules of the amphiphilic compound is produced by compression and, if desired, prior spreading at the interface between a fluid and a polar liquid, and a solid support is moved through the film, thus transferring the film onto the support. In this process, a directional flow of the polar liquid which contains amphiphilic molecules in dissolved form or as insoluble molecules on the surface in spread-out form is produced in an approximately horizontal duct, the flow is compressed at a barrier arranged in the duct at the fluid/polar liquid interface, the flow rate and, if appropriate, the concentration of the amphiphilic molecules are selected such that a film comprising amphiphilic molecules is continuously formed in front of the barrier at the phase boundary of fluid/polar liquid by compression, and the support is moved through this compression zone.

The film transfer itself can be carried out in the process described in German Patent Application P 38 28 836.2 in a manner known per se by the discontinuous technique described by Langmuir-Blodgett. By repeated dipping and withdrawing of the support to be coated, many layers can be produced on top of each other.

In most cases, double layers (dipping and withdrawal) are transferred. However, continuous transfer is also possible. For example, a polyester wire or a polyester film can be dipped at a constant rate into the polar liquid at a suitable point outside the compression zone, its direction can be changed by means of rollers and it can be removed again from the liquid at the location of the rigid boundary layer. In this manner, it is continuously IE 91627 - 4 coated with a monomolecular film of amphiphilic molecules .

The thin film applied to the support at first represents a monomolecular film. However, by repeating the process, several layers can be applied on top of each other. As in the LB method, films comprising pure amphiphilic compounds or films comprising a mixture of amphiphilic compounds can be transferred to a support.

In the process according to the invention, the amphiphi10 lie molecules, in particular polyglutamates, align on the surface of the liquid with the molecular axis perpendicular to the flow direction, so that the monofilms are preoriented even before being applied to a solid support.

By spreading out polyglutamates and, if desired, simul15 taneously spread-out dye molecules, high dichroic ratios can be achieved, which can be made visible in linearly polarized light.

The arrangement of the dye molecules is dictated by the arrangement of the monomolecular film, so that the orientation of the monomolecular film on the water surface can be imaged by these dye molecules.

The fluid used can be a gas or a nonpolar liquid which is insoluble in the polar liquid, for example water. If the process is carried out using two liquid phases, the polar liquid can be the top or bottom phase (for example: boundary layer toluene/water containing the amphiphile γ-octadecyl L-glutamate or -y-methyl L-glutamate) . The nonpolar liquid also takes part in the directional flow.

In the process according to the invention, polyamides, preferably polyglutamates, of approximately rod-like shape are used as the amphiphilic compounds, in particular those which contain different ester groupings in the γ position, in order to obtain laterally ordered monofilms on the surface of the liquid.

IE 91627 - 5 Thus, for example, poly(7-octadecyl L-glutamates) whose octadecyl groups are replaced by n-alkyl groups having less than 18 carbon atoms or the corresponding branched alkyl radicals or hydrocarbon radicals having C-C mul5 tiple bonds are preferred. An example of such a polycondensation product is poly(γ-methyl L-glutamate-co7-octadecyl L-glutamate).

Cocondensation products of this type can be prepared by polymer-analogous reaction of polymer homocondensation products, for example by partial transesterification of poly-7-methyl L-glutamate with stearyl alcohol (cf. J. Watanabe, Y. Fukuda, R. Gehani, I. Nemalyn, Macromolecules 17 (1984), pages 1004 ff.).

Examples of suitable lipophilic dye components are the following dyes: diazo dyes, cyanines, preferably ^-carotene or any carotenecarboxylic esters, in which the relative amounts of lipophilic dye component/polyamide can be varied within wide limits.

Thus, for example, depending on the dye system selected, 0.1 to 1 mol of lipophilic dye can be used per mole of base unit of the polymer, for example 0.3 mol for 0-carotene and 0.13 mol for oil red.

In the process according to the invention, lower amounts of dye can also be used, in particular 0.2 to 0.4 mol per mole of polymer base unit, and preferably dye mixtures can also be used.

The organic polymers, if appropriate also the dye component, are, as is generally customary in the LangmuirBlodgett method, dissolved in a highly volatile organic solvent, such as, for example, methylene chloride, chloroform, toluene, benzene, ethyl acetate or hexane. The concentration should in general be between 0.01 and 1% by weight. The process is usually carried out at temperatures between 0°^T<40°C, preferably 10 and 30°C.

IE 91627 - 6 The amphiphilic compounds used can be soluble or insoluble. Soluble amphiphilic molecules are understood to mean preferably molecules having a solubility in the polar liquid which is greater than the solubility of hexadecanoic acid in water. If they are highly soluble, for example in the case of acetic acid, highly compressed films can no longer be produced. However, it is still possible to produce and transfer films which satisfy ordinary requirements. It is still possible to produce good films if the solubility of the molecules is less than that of hexanoic acid in water.

In the case of soluble amphiphilic molecules, a stationary equilibrium is formed between molecules redissolving in the subphase and molecules continuously resupplied by the flow.

The arrangement of the barrier is preferably such that essentially only the moving boundary layer carrying the film is compressed, because this allows a particularly well defined flow. No polar liquid should flow beyond the barrier.

The flow in the duct can be turbulent or laminar. However, it is favorable for it to be laminar at least shortly before the barrier. This makes it possible to achieve a defined type of flow which is essential for the uniformity of the transferred films.

An increase in the flow rate has the same effect as an increase of pressure by means of the horizontal barrier in the LB technique.

In the case of soluble amphiphilic molecules, an increase in concentration also leads to an increase in interfacial tension and thus compression.

If the organic polymers used and, if present, the dye component are insoluble in the polar liquid, they are IE 91627 - 7 added in the form of a solution in a solvent which is then spread out on the surface of the polar liquid. It is favorable to meter in the solution shortly behind the barrier (about up to 10% of the entire flow distance), because this gives the most time for the uniform distribution of the amphiphilic molecules on the surface. It is advantageous to use one of the volatile organic solvents already mentioned for spreading out the insoluble amphiphilic compound. This has the effect that, depending on the flow rate and length of the duct, the volatile solvent is completely evaporated between the spreading point and the compression point in front of the barrier. The evaporation can be accelerated by blowing in carrier gas.

If the amphiphilic compound used is soluble in the polar liquid, the location and the type of addition of the compound are not critical. In the flow duct, a uniform build-up (in the flow direction) of the boundary layer of the amphiphilic molecules takes place. This can be detected by the uniform drop in interfacial tension, which becomes a minimum at the front edge of the barrier. However, in this case, the useful range of concentration is clearly dependent on the adsorption properties of the amphiphile.

An increase in the flow rate of the polar liquid increases the pressure and accelerates the formation of rigid, transferrable films. A low flow rate requires a lengthening of the compression zone (linear section of the duct in front of the barrier) or an increase in concentration in order to reach the same pressure.

The cross section of the duct in which the flow is produced is not critical. It is possible to use, for example, a U-shaped or a V-shaped cross section. For manufacturing reasons, a rectangular cross section is preferred.

IE 91627 - 8 If the barrier has a linear form and is arranged perpendicular to the flow direction and does not take up the entire width of the duct, a rigid film is only formed along a relatively short zone. It is better if the barrier takes up the entire width of the duct. However it is particularly preferred for the barrier to be part of a U-shaped or O-shaped frame whose opening is arranged against the flow direction on the fluid/polar liquid interface. This reduces the effect of disturbances at the edge. In most cases, the O-shaped frame will be in the form of a rectangle. It can be readily used for amphiphilic molecules which are soluble in water.

In the simplest case, the flowing liquid can only be utilized once. However, it makes economic sense to collect the polar liquid behind the barrier and recycle it, in particular continuously, and use it again for film formation. It is best to recycle the polar liquid below the duct or at the side of the duct.

It is known from R. Ollenik and W. Nitsch, Bunsenges.

Phys. Chem. 85 (1981), p. 901, that in a duct flow of two mutually immiscible liquids, the interfacial flow can be compressed at any edge present. If amphiphiles are present, compressed films of variable length are formed. Since, in the apparatus described, one of the flowing liquid phases is recycled above the rigid interfacial layer, no support can be dipped into the duct flow from outside for reasons of space. Furthermore, no description of the fact that the interfacial layer observed can be transferred to solid supports is provided.

The process described in German Patent Application P 38 28 836.2 is particularly easy to carry out if only one liquid phase is used, i.e. if the fluid is a gas, in particular air.

However, it is also possible for the fluid to be a nonpolar liquid which flows in the duct in the same IE 91627 direction as the polar liquid. In this case, it is recommended to collect also the nonpolar liquid behind the barrier and recycle it in particular continuously below or at the side of the duct. If a nonpolar liquid is used as the mobile phase, it is also possible to meter in the amphiphilic molecules to the system as a solution in the nonpolar liquid.

As it turns out, the exerted pressure by the interface (definition: interfacial tension of the interface free of amphiphiles minus interfacial tension of the coated surface) increases in the flow direction and reaches its maximum value at the barrier.

According to German Patent Application P 38 28 836.2, the duct of such an apparatus should have a minimum length which is dependent on the surface properties of the amphiphile, its concentration and the flow rate. This minimum length can be easily determined by tests. The higher the adsorbability, the concentration and the flow rate, the lower the minimum length. The ratio of length of the duct to width of the duct is preferably greater than 1, in particular greater than 2.

If possible, the barrier extends only to the point of the fluid/polar liquid interface, i.e. its height should be as small as possible.

In this manner, a lateral preorientation of the monofilm on the surface of the liquid is achieved, brought about by compression of the flow by means of the liquid flowing below the monofilm. The improved orientation of the films obtained thereby can be particularly well demonstrated by simultaneous spreading of dye molecules, the arrangement of which is dictated by the arrangement of the macromolecular monofilm.

The dichroic ratio of films of this type shows a preferred orientation in linearly polarized light, while no IE 91627 - 10 preferred orientation can be observed in the conventional compression in the Langmuir trough.

Thus, chemically analogous films (for example polyglutamate/ β -carotene) show a dichroic ratio of 1 by the conventional method, while in the case according to the invention dichroic ratios of up to 2.4 can be obtained.

Furthermore, the process according to the invention makes it possible to give the molecules a different orientation by dipping and withdrawing the substrate with the area to be coated perpendicular or parallel to the flow direction and thus producing different molecular preferred orientations in the flow direction, while in the conventional method, due to the lack of order on the water surface, the arrangement is independent of the substrate orienta15 tion with respect to the direction of compression.

Flow-ordered films of this type can be transferred in a known manner to dipped supports.

The materials which can be used for supports of this type are all conventional substrates, such as, for example, metals, for example gold, platinum, nickel, tantalum, aluminum, chromium, palladium or steel, or, inter alia, plastics, such as, for example, polyester, polyvinyl chloride, polytetrafluoroethylene, and the like.

Apparatuses for controlled slow immersion and withdrawal of a solid support in a liquid, (for example filmlift) are known to one skilled in the art from the LB technique .

Laterally highly ordered molecular films of this type are suitable in particular for the manufacture of filters for optical materials, for coating semiconductors and for finishing films or textiles, but also for all other conventional areas of application, in particular those in which high optical demands are made on the materials used. 91627 The invention is illustrated in more detail by the examples which follow.

Example 1 The flow duct is operated at a throughput of 2.4 1/min. 5 This corresponds to an average flow rate of 7.2 cm/s in the middle portion of the duct.

A solution of 2 mg of poly (γ-methyl(co-7-octadecyl) Lglutamate) (degree of polymerization 958) (PMeoOLG) in 10 ml of chloroform (0.02%) was mixed with a solution of £-carotene in chloroform, the mixing ratio being 3 mol of polymer base unit/1 mol of dye.

At the duct entry 0.3 ml of the solution were spread out. The surface transport time between the duct entry and the middle portion of the duct was about 1 min, which can be considered sufficient for substantial evaporation of the chloroform.

The absorption spectrum of the dye on the water surface was measured by means of a diode array photometer via a quartz optical waveguide by reflecting the measuring beam perpendicular to a mirror situated on the bottom of the duct. A rotating polarizer at the optical waveguide outlet made it possible to characterize the dye dichroism with respect to the flow direction.

The local film pressure π at the transfer site was 23 mN/m. The compressed film was transferred to a silanized quartz disk which had been dipped into the film in a perpendicular or parallel orientation (on the disk surface) with respect to the flow direction. The transfer rate was about 2.4 cm2/min (film lifting rate: about 1 cm/min) . The number of films transferred was in each case 16, which corresponded to 4 dipping operations (2 layers per side, transfer taking place not only during dipping but also during withdrawal).

IE 91627 - 12 The π-π* band at 478 nm of ,0-carotene was measured. The dichroic ratios measured on the water surface were on average more than 1.5, in the ideal case even 2.4 (measured perpendicular and parallel to the flow direc5 tion). Slight variations of the measured values can occur as a result of inhomogeneities in the flow field.

Measurement of the dichroic ratios after transfer of the films to the substrate surface.

= Orientation perpendicular to the direction of transfer || = Orientation parallel to the direction of transfer a) Film transfer with the disk surface perpendicular to the direction of flow Dc (1/||) = 1.52 b) Film transfer with the disk surface parallel to the direction of flow Dc (||/1) = 1.53 Example 2 The same apparatus as in Example 1 was used.

A solution of 2 mg of poly(γ-methyl (co-7-octadecyl) L-glutamate) in 10 ml of chloroform (0.02%) was mixed with a solution of oil red in chloroform. The mixing ratio was 0.13 mol of oil red per mole of polymer base unit.

Owing to the slight absorption of the monofilm, no measurements could be carried out on the water surface.

The film transfer was carried out analogously to Example 1 and the dichroic ratios on the substrate surface were then measured (in the case of oil red the band at 522 nm was measured).

IE 91627 - 13 a) Film transfer with the disk surface perpendicular to the direction of flow Dc (i/||) = 1.3 b) Film transfer with the disk surface parallel to the direction of flow Dc (||/1) = 1.2 IE 91627

Claims (12)

1. A process for the preparation of a thin film comprising at least one amphiphilic compound on a support, in which a directional flow of the polar liquid which contains amphiphilic molecules in dissolved form or as insoluble molecules on the surface in spread-out form is produced in an approximately horizontal duct, the flow is compressed at a barrier arranged horizontally in the duct at the fluid/polar liquid interface, the flow rate and, if appropriate, the concentration of the amphiphilic molecules are selected such that a film comprising amphiphilic molecules is continuously formed in front of the barrier at the phase boundary of fluid/polar liquid by compression, and the support is moved through this compression zone, which comprises using polyamides, preferably polyglutamates, as the amphiphilic compounds.

2. The process as claimed in claim 1, wherein the amphiphilic compounds used are polycondensation products obtained from copolyglutamates substituted in the 7 position by different alkyls.

3. The process as claimed in claim 2, wherein the amphiphilic compound used is a copolycondensation product of 7-alkyl L-glutamate with 7-alkyl' Lglutamate, in which alkyl is an alkyl radical having 1 to 6 carbon atoms and alkyl' is an alkyl radical having 12 to 36 carbon atoms.

4. The process as claimed in claim 3, wherein the amphiphilic compound used is a copolycondensation product of 7-methyl L-glutamate with 7-octadecyl Lglutamate.

5. A process for the preparation of a thin film comprising at least one amphiphilic compound and a IE 91627 lipophilic dye or dye mixture, in which the dye is dissolved in an organic, water-immiscible solvent and is transferred to a support together with the amphiphilic compound, which process comprises 5 producing a directional flow of the polar liquid which contains the amphiphilic molecules in dissolved form or as insoluble molecules and the dye molecules on the surface in spread-out form in an approximately horizontal duct, compressing the flow 10 at a barrier arranged horizontally in the duct at the fluid/polar liquid interface, selecting the flow rate and, if appropriate, the concentration of the amphiphilic molecules and the dye component such that a film comprising amphiphilic molecules and dye 15 component is continuously formed in front of the barrier at the phase boundary of fluid/polar liquid by compression, and moving the support through this compression zone.

6. The process as claimed in claim 5, wherein the dyes 20 have an approximately rod-like shape and diazo dyes, cyanines or carotenoids are preferably used.

7. A monomolecular film comprising at least one amphiphilic compound and a dye component on a water surface, which film has been produced by the process 25 as claimed in claim 5 and has a dichroic ratio of greater than 1.0, preferably greater than 2.0.

8. A monomolecular film comprising an amphiphilic compound and a dye component as claimed in claim 7, wherein the amphiphilic compound has an approxi30 mately rod-like shape.

9. A monomolecular film comprising an amphiphilic compound and a dye component as claimed in claim 8, wherein the amphiphilic compound is a polyamide, preferably a polyglutamate. X IE 91627 10.

10.

11. . 11 .

12. 12. A process as claimed in claim 1, substantially as hereinbefore described and exemplified. A thin film comprising at least one amphiphilic compound on a support, whenever prepared by a process claimed in a preceding claim. A monomolecular film as claimed in claim 7, substantially as hereinbefore described and exemplified.