GB2201155A - Polymer materials - Google Patents

Abstract

A polymer compound comprises units of the general structure <IMAGE> where azobenzene or stilbene derivatives may be present as in Fig. 1. This gives a preformed polymer compound that can be deposited by a Langmuir-Blodgett technique to give a comparatively thick multiple layer film. The film has high thermal stability, is mechanically tough and can be used to form optoelectronic devices such as waveguides and switches. <IMAGE>

Description

POLYMER MATERIALS This invention relates to polymer materials. It relates particularly to -polymer materials which possess- optoelectronic properties.

A technique which is gaining increasing favour for the formation of optically active films for electronic and optical devices is Langmuir-Blodgett deposition where an air/water interface is used.

Since the film material needs to be stable and robust this has prompted the investigation of polymeric materials which are generally mechanically tougher than their low molecular weight counterparts.

The formation of a polymeric L-B film can be effected in either of two ways. In a first approach, the film could be prepared from monomer material and this could then be polymerised in situ after the film had been laid down. This technique is not entirely satisfactory since the polymerisation process can cause a considerable reduction in material volume and this will produce stresses in the film which can lead to cracking. An alternative approach would be to prepare the film directly from the preformed polymer material. Some of the work already done on preformed polymers is disclosed in British Polymer Journal, Vol. 17, No. 4, 1985,- 368 (Hodge P., Khoshdel E., Tredgold R.H., Vickers A.J. and Winter C.S.). This relates particularly to copolymers of styrene and maleic anhydride.

The present invention was devised to provide new polymer materials which are capable of being deposited by the L-B technique from the preformed state and which will have useful optoelectronic properties. The films were required to have a high thermal stability and to be capable of being deposited in multiple layers.

According to the invention, there is provided a polymer compound exhibiting optoelectronic properties and which is capable of being deposited by a L-B technique, the compound comprising units of the general structure:

where the polymer is a random copolymer and the three sidegroups are fully interchangeable, where,

R = CmH2m+l m = O to 30 a = O to 3 b = O to 30 M = -CO- or -OCO- or -COO N = degree of polymerisation and R1 is defined by one of the Groups 1 to 4 as follows, Group 1 R1

TRANS or CIS n = 0 to 30

-COO-, -OCO-, -NRCO- or -CO A = -N= or-C= Y = -NO2, -CN, -H, -SO3H, (02R, -OCOR (OR, -OR, -NHR, -N'R2, -NRCOR, -NHCOR or -R Group 2 R1

= Y < A = A }Z these branches being alternative Z = -CONHR, -OR, -SR or -NR2, -N02, -CN, -H, -SO3H, -CO2R, -OCOR, -COR, -NHR, -NRCOR, -NHCOR or -R (for -NR2, the values of R may be the same or different) Group 3 R1

Group 4 R1

, (plus a suitable counter lon) = R d CH2

d = 0 or 1 G = (O-, -COO-, -OCO-, -CONH-, -OCNH-, -CONR- or -OCNR In order to produce a film which would be useful as an optical waveguide it is necessary to have a layer of a particular minimum thickness with a low attenuation coefficient in the visible and near infrared region.Films made from copolymers bearing both hydrophilic and hydrophobic side groups are particularly interesting because these will tend to spread at the air/water interface and thus may be likely to deposit as good L-B films in the Y-mode. This will permit multiple thicknesses to be deposited.

Particular classes of the polymer compound which have useful non-linear optical properties are those in which the hydrophobic side chains are formed by azobenzene or stilbene derivatives.

The invention further comprises an optical device, in which the optical element comprises multiple layers of a Langmuir-Blodgett deposited film of a preformed polymer compound and which forms a region of high refractive index, which is supported on a substrate body formed of a lower refractive index material. The resulting device which provides definition of a channel of high refractive index material will result in a two-dimensional confinement of the light which makes possible the construction of a range of planar waveguide structures. By using a photolithographic definition process, a channel waveguide structure can be produced which may have a width of a few microns.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures 1 to 6 are structural formulae of particular polymers including azobenzene or stilbene derivatives; Figure 7 is a sketch of a planar waveguide structure including a Langmuir-Blodgett deposited film of the invention, and, Figure 8 shows a channel waveguide structure including a similar film.

The preparations of the compounds of the present invention can be effected by synthetic procedures similar to those disclosed in the aforementioned publication by P. Hodge et al. Other relevant details are given in C.S Vinter et al, Thin Solid Films, 134 (1985) 49, A.J. Vickers et al, in the same journal at page 43, and R.H. Tredgold et al, J. Phys. D: Appl. Phys., 18 (1985) 1139.

In the polymer compound of Figure 1, the numeral n may have the values 2, 8. The same applies to the compound of Figure 4. In Figure 6 n may be a long or short chain.

These compounds were selected as being suitable for use as preformed polymer material for forming Langmuir-Blodgett films.

They were found capable of forming multiple layers such that comparatively thick films (in thickness greater than 0.5 micrometres) could be deposited. The films were temperature stable up to a temperature of about 300or and were observed to have a low optical attenuation (of about 10 dB.cmb In order to achieve this very low level of optical attenuation it is necessary to use a production technique which minimises the occurrence of extrinsic microscopic defects in the film. This may require use of a system with closed loop air circulation and filtration to reduce dust, and irradiation by ultraviolet light to inhibit the growth of bacteria.

Following synthesis and purification of each compound, a solution of known concentration of the material under test is made up using a suitable solvent, for example ethyl acetate.

By use of the Langmuir Trough unit it is possible to form a monomolecular layer on a ssubstrate base and this operation is capable of being carried out under accurately reproducible conditions. The organic material to form the monomolecular layer in the selected solvent is first deposited on the surface of a suitable liquid subphase (usually highly purified water), the temperature and pH of which have been correctly optimised. The liquid subphase is contained in the trough of the Langmuir unit and the accurately measured amount of organic material solution is deposited within part of the subphase surface that is capable of being confined within a movable surface barrier which defines a working area on the liquid surface. Upon evaporation of the solvent, individual molecules of the organic material are left floating on the subphase surface. By suitably moving the surface barrier, the working surface area may be compressed and the molecules can be made to form a quasi-solid layer one molecule in thickness.

The stability of the test monolayer is capable of being monitored by measuring the change in surface area with time at a preset surface pressure under given subphase conditions.

To form a multilayer of the material of the test monolayer requires the use of a dipping mechanism which is part of the Langmuir unit and which is normally housed on a gantry located above the trough. The mechanism includes a screw thread drive arrangement to which an appropriate body of a selected substrate material can be attached. Examples of common substrates are silicon wafers (n- type or p- type), glass or evaporated metals, the surfaces of which have been chemically treated to ensure cleanliness and the presence of a suitable hydrophilic or hydrophobic surface condition.

The substrate is lowered and raised through the subphasemonolayer interface such that a transfer of the monolayer material from the subphase to the substrate surface takes place. By repeating this cycle, a multilayer deposit of the organic material can be built up upon the substrate body.

The substrate body carrying a mono or multilayer deposit of the organic compound according to the invention formed an optical element having nonlinear optical properties. The element can be used to construct various optical devices as depicted in Figures 7 and 8.

Figure 7 shows a planar waveguide formed from a substrate body 1 which supports a Langmuir-Blodgett film 2 multilayer 6f one of the aforementioned compounds. The substrate 1 had a lower refractive index than that of the film 2, and the provision of a high refractive index region bounded by a lower refractive index region thus gave the necessary confinement to provide the required waveguide effect.

A typical thickness for the material of the film 2 for use in guiding visible and near infrared light was of the order of one micron.

From the planar waveguide construction, the definition of a narrow channel in the high refractive index material will result in a two-dimensional confinement of the light and make possible the construction of a range of channel waveguide structures. One such construction is shown in Figure 8 where the substrate body 1 supports a Langmuir-Blodgett deposited film 2 multilayer which by photolithographic definition has been cut away to leave a width of only a few microns.

The constructions of the planar and channel waveguide structures which have just been described can lead to other electrooptic and all-optical switching and signal processing devices which make use of the organic compounds of the invention.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims.

For instance, it is not essential that the azo, stilbene or other compounds of the invention should be deposited by the dipping bath technique that has been specifically described. In a different embodiment the technique of film assembly might use processes such as crystal growth, chemical vapour deposition etc..

Before the deposit of the relevant compound is placed on the substrate body, the body might be given an initial coating such as one of silicon nitride or one to form a reflecting surface on the body.

The layers of compound on the substrate body also might be provided with intervening layers of different substances.

Claims (6)

1. A polymer compound comprising units of the general structure:

where the polymer is a random copolymer and the three sidegroups are fully interchangeable, where,

R = CmH2m+ m = 0 to 30 a = 0 to-3 b = 0 to 30 M = CO- or -OCO- or -COO N = degree of polymerisation and R1 is defined by one of the Groups 1 to 4 as follows, Group 1 R1

TRANS or CIS n = O to 30

-COO-, -OCO-, -NRCO- or -CO A = -N= or-C= Y = -NO2, -CN, -H, -SO3H, CO2R, -OCOR, COR, OR, -NHR, -NR2, -NRCOR, -NHCOR or -R Group

2

R1 = YtA=A+Z these branches being alternative Z = CONHR, -OR, -SRor NR2, -NO2, -CN, -H, -SO3H, -CO2R, -OCOR, -COR, -NHR, -NRCOR, -NHCOR or -R (for -NR2, the values of R may be the same-or different) Group 3

Group 4

(plus a suitable counter ion) R1 = R < GACH2tSUITABLE

d = 0 or 1 G = -CO-, -COO-, -OCO-, -CONH-, -OCNH-, -CONR- or -OCNR 2. A polymer compound as claimed in Claim 1, in which the polymer has a structure as depicted in any one of Figures 1 to 6 of the accompanying drawings.

3. An optical element having nonlinear optical properties, the element comprising a substrate body supporting a layer of a polymer compound having a composition as claimed in Claim 1 or 2.

4. An optical element as claimed in Claim 3, in which the substrate body supports multiple layers of said polymer compound, the layers constituting a fill of high refractive index material.

5. An optical device, such as a planar or channel waveguide structure, including an optical element as claimed in Claim 3 or 4.

6. An optical element, substantially as hereinbefore described with reference to Figure 7 or 8 of the accompanying drawings.

6. An optical element, substantially as hereinbefore described with reference to Figure 7 or 8 of the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. A polymer compound comprising units of the general structure:

where the polymer is a random copolymer and the three sidegroups are fully interchangeable, where,

R = CmH2m+ m = 0 to 30 a = 0 to 3 b = 0 to 30 M = -CO- or -OCO- or -COO N = degree of polymerisation and ,R1 is defined by one of the Groups 1 to 3 as follows, Group 1

TRANS or CIS n = O to 30

-COO-, -OCO-, -NRCO A = -N= or-C= Y = -NO2, -CN, -H, -SO3H, -CO2R, -OCOR, or -COR, Group 2

R1 = Y+A = A Z = ~ these branches being alternative Z = -CONHR, -OR, -SR -NR2, -NO2, -NRCOR or -R (for -NR2, the values of R may be the same or different) Group 3

(plus a suitable counter ion) 2. A polymer compound as claimed in Claim 1, in which the polymer has a structure as depicted in any one of Figures 1 to 6 of the accompanying drawings.

3. An optical element having nonlinear optical properties, the element comprising a substrate body supporting a layer of a polymer compound having a composition as claimed in Claim 1 or 2.

4. An optical element as claimed in Claim 3, in which the substrate body supports multiple layers of said polymer compound, the layers constituting a film of high refractive index material.

5. An optical device, such as a planar or channel waveguide structure, including an optical element as claimed in Claim 3 or 4.