WO1993009466A1

WO1993009466A1 - Nonlinear optical polymers

Info

Publication number: WO1993009466A1
Application number: PCT/GB1991/001910
Authority: WO
Inventors: Nicholas Blagden
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1991-10-31
Filing date: 1991-10-31
Publication date: 1993-05-13
Anticipated expiration: 1994-04-30

Abstract

A nonlinear optical polymer exhibiting high SHG activity and low vapour pressure consists of an ionomer of a polymeric acid and an organic base. The acid preferably comprises a polymer of an ethylenically-unsaturated carboxylic acid and is exemplified by polyacrylic acid. The base comprises an electron donor group, in particular -NO2, linked to an electron acceptor group, in particular an organic group containing a primary amino group, through a system of delocalized n-electrons such as a substituted arylene group.

Description

Nonlinear Optical Polymers

This invention relates to nonlinear optical polymers, which are materials that produce second harmonic generation (SHG) of optical wavelength electromagnetic radiation, in particular laser radiation.

Nonlinear optical materials have industrial applications in the field of optoelectronics, in particular as laser beam wavelength converters. Various polymers have been proposed in the past for use as or within such materials, since they tend to impart high speed response characteristics and strong resistance to optical damage.

Nematic liquid crystalline materials orientated in the presence of an electric field represent one form in which polymers are used as nonlinear optical materials. For example, EP-02-44288-A describes an electric field-orientated polymer consisting of a polymeric backbone such as a polyacrylate, a polymethcrylate or polysiloxane having side groups exhibiting mesogenic properties and side groups exhibiting nonlinear optical response characteristics. However, the SHG activity of nematic liquid crystal polymers tends to be too low for practical purposes.

Attempts at avoiding such liquid crystal polymers are disclosed by Miyata et al in Polymer Preprints, Japan Vol 36 No. 8, 2523 (1987) who used mixtures of p-nitroaniline in polyethylene oxide recrystallized under electric field, but the SHG activity of this material is found to decay rapidly after its formation. Further attempts are disclosed in EP-338702-A1 involving the use of blends of polypeptides with nonlinear optical (NLO) compounds. However, blends of organic materials tend to separate out over long periods, are difficult to reproduce by techniques guaranteeing consistent SHG properties, and lack high thermal stability (in particular, high melting points). Furthermore, they can exhibit high vapour pressures which is a particular problem if they contain volatile, toxic NLO compounds.

It is the object of the present invention to provide novel nonlinear optical polymers whereby the aforementioned disadvantages are overcome or at least mitigated in part.

According to the present invention, there is provided a nonlinear optical polymer comprising an ionomer of a polymeric acid and an organic base, the base having the structure

A - (system of delocalised n-electrons) - B

wherein A is an electron donor group and B is an electron acceptor group.

Polymers according to the present invention are found to possess high SHG activities, and because the base is ionically linked to a polymeric backbone the polymer as a whole possesses an intrinsically low vapour pressure.

The polymeric acid preferably comprises a polymeric carboxylic acid, and preferbly contains from 100 to 20,000 repeat units. More preferably, it comprises a polymer of an ethylenically-unsaturated carboxylic acid and most preferably it comprises polyacrylic acid, which is readily soluble over a wide range of molecular weights in appropriate solvents from which the present ionomers may be precipitated.

In order to provide the base with sufficient basicity to react with the polymeric acid to form an ionomer, A preferably comprises an organic group containing from 1 to 20 carbon atoms and at least one primary or secondary amino group.

The group B is preferably selected from -N0₂, -NO, -CN, and F ^*.

The system of delocalised n-electrons preferably comprises an optionally-substituted arylene group, and most preferably comprises an amido derivative of a phenylene group.

An example of the present invention will now be described.

Preparation of 4-N-f -(aminomethyl)piperidino)-V(N-ethanaπ.ido) nitrobenzene ("Ampen") salt of polvacrvlic acid A solution of 2-fluoro-5~nitro ethaneanilide (lOg; 50 mmol) in nitromethane (100 cm³) was added dropwise for 20-30 minutes to a mixture of -4-(aminomethyl) piperidine (6.9 g; 60 mmol) and sodium bicarbonate (-4.2 g; 50 mmol) in nitromethane (50 cm³). The latter mixture was heated under reflux for ten minutes prior to the addition of the 2-fluoro-5^_nitro ethaneanilide solution.

The resulting solution was heated under reflux for a further - minutes and then left to cool to 20°C. The reaction vessel was left at this temperature for 2-4 hours which caused the precipitation of NaF and^' unreacted sodium bicarbonate. Following careful decanting of the yellow liquid, the solvent was removed by evaporation which resulted in the precipitation of a yellow solid. The crude product was then recrystallized in a minimum volume of methanol to yield a bright yellow solid hereinafter referred to as "Ampen". Yield: 10.7g; 732- 55-6 ; H, 6.92; N, 18.3% . M.pt. 126-128°C.

Ampen (0.25 g- 0.87 mmol) was then dissolved in methanol (2.5 cm³) and slowly added to a solution of polyacrylic acid (0.25 g; 25 % ,' 2.12 mmol; Mw: 230000) in methanol (10 cm³). This represents a 1:1 reaction as each mol of polyacrylic acid contains approx. 3200 -C00H groups (3200 x 2.72 mmol = O.87 mmol). Mixing the acid and the base resulted in the immediate precipitatation of a yellow, lumpy solid.

The material of the Example was screened for its SHG activity by the following technique. Powdered samples of the material were prepared and mounted on slides in front of a 1064 nm output, pulsed Nd:Yag laser. The laser beam was focused on the slide through a neutral density filter followed by a 30 mm focal length convex lens. SHG activity was assessed by viewing the intensity of the scattered light from the samples through protective goggles which stopped the 106-4 nm fundamental frequency but passed 32 nm frequency (green) radiation. The SHG activity of each sample was compared with that of an ungraded powder sample of urea, a material which is known to possess an SHG activity several orders of magnitude greater than KDP but which has very poor crystal growth characteristics and poor thermal stability. In all cases, the powdered samples of the material prepared in accordance with the Example exhibited SHG activities significantly greater than that exhibited by the powdered urea sample.

Claims

1. Nonlinear optical polymer characterised by ionomer of a polymeric acid and an organic base, the base having the structure

A - (system of delocalized n-electrons) - B

wherein A is an electron donor group and B is an electron acceptor group.

2. Nonlinear optical polymer according to claim 1 characterised in that the polymeric acid comprise a polymer of an ethylenically-unsaturated carboxylic acid.

3. Nonlinear optical polymer according to claim 1 or claim 2 characterised in that A comprises an organic group containing from one to 20 carbon atoms and at least one primary or secondary amino group.

4. Nonlinear optical polymer according to any one of the preceding claims characterised in that B is selected from the group consisting of

-NO, -NO, -CN. and R₃N⁺-

5- Nonlinear optical polymer according to any one of the preceding claims characterised in that the system of delocalized n-electrons comprises an optionally-substituted arylene group.

6. Nonlinear optical polymer according to claim 5 characterised in that the organic base comprises 4-]ϋ-(*4-(aminomethyl) piperdino)-3~ (N-ethanamido) nitrobenzene.