Temperature Indicators Based on Polydiacetylene Compounds

The invention is further described by reference to the accompanying drawings, wherein:

FIGS. 1a, 1b, and 1c are differential scanning calorimeter diagrams of monomers;

FIGS. 2a, 2b, and 2c are absorption spectra of polydiacetylene compounds recorded over a period of irradiation time;

FIGS. 3a and 3b are differential scanning calorimeter diagrams of monomer and polymer; and

FIGS. 4a, 4b, and 4c are absorption spectra of polydiacetylene compounds taken at various times.

The temperature indicators of the invention are advantageously deposited on a chemically inert, solid substrate in the form of a thin film. The substrate, which must be inert not only towards the polydiacetylene compound but also towards the perishable product, can be, for example, paper, glass, fabric, quartz, a metal such as aluminum, or a polymer such as polyethylene.

The temperature indicators of the invention can also advantageously be incorporated as additives or as a mixture in a thermoplastic which is chemically inert towards the polydiacetylene compound.

The thermoplastic can be a polymer compatible with olefins such as polyolefins, for example polyethylene. Mixing is effected in the polymer in the molten state, after which the thin film obtained on cooling is subjected to photopolymerization in order to form the polydiacetylene compound.

The temperature indicator can consist either of a single polydiacetylene compound or of a carefully chosen mixture of such compounds, according to the temperature which it is desired to detect.

The symmetrical diacetylenic monomers of formula (1) above can be obtained by oxidative coupling of the alkyne by copper salts in accordance with the process described by EGLINGTON et al. in J. Chem. Soc., 889 (1959). This process consists in heating an alkyne in the presence of a copper salt, for example copper acetate, in a pyridine/methanol/ether mixture, at a temperature of between bout 30.degree. and 35.degree. C., under a gentle stream of pure oxygen, for about 48 to 72 h. The ether and methanol are then removed, for example by evaporation under reduced pressure, after removal of the excess oxygen. The solution is taken up by fresh anhydrous pyridine, for 48 to 72 h, at between 40.degree. and 50.degree. C. under reflux. The pyridine is then removed and the residual mixture is acidified. After extraction with ether, an oily product is obtained which crystallizes in the cold. This oily product is then purified by chromatography in accordance with known methods.

As examples of starting alkynes, it is possible to use commercially available products without further purification, such as, for example, the products known under the tradenames EGA 24,442-2 (hex-1-yne), EGA 24,446-5 (oct-1-yne), EGA 24,438-4 (dec-1-yne) and EGA 24,440-6 (dodec-1-yne), marketed by EGA-ALDRICH. These products can also be synthesized. For example, the C.sub.16 alkyne can be synthesized via sodium acetylide in liquid ammonia, which is subsequently substituted by tetradecyl bromide in dimethylformamide in accordance with the process described by W. Ziegenbein et al., Chemistry of Acetylene, H. C. Viehe, p. 187, published by Marcel Dekker NY (1969).

It is also possible to use the process involving dehydrobromination of the corresponding 1,2-dibromide, this process being well known to those skilled in the art.

The unsymmetrical diacetylenic monomers of formula (1) above can also be obtained by oxidative coupling in accordance with the process described above, using a mixture of two monoacetylenic hydrocarbons of different chain lengths in appropriate proportions.

The polymerization of the diacetylenic monomers of formula (I) is advantageously performed by the irradiation, with high-energy radiation, for example UV radiation (254 nm UV lamp), of a thin crystalline film of a monomer or monomer mixture.

The absorption characteristics of the polydiacetylene compound formed vary according to the polymerization time. For relatively short times (about 30 s), the polydiacetylene compound obtained shows a principal absorption peak at about 650 nm. When the irradiation time is increased, the spectrum moves towards the short wavelengths, the dominant shade of the product remaining blue.

Although all the polydiacetylene compounds obtained after different irradiation times are suitable for the purposes of the invention, it is preferable to use polydiacetylene compounds which have a relatively low degree of polymerization, i.e. the compounds having an absorption peak at about 580-650 nm.

The polydiacetylene compound obtained by irradiating a single diacetylenic monomer exhibits a sudden color change in a narrow temperature range, without modification of the degree of polymerization. As indicated previously, the transition temperature depends on the number of carbons in the paraffinic chain. In the case of the symmetrical alkyldiacetylenes, the transitions are spaced about twenty degrees apart for the series C.sub.16, C.sub.20, C.sub.24, C.sub.28, C.sub.32. The polymers obtained by irradiating a mixture of monomers of different alkyl chain lengths retain the color change properties of the homopolymer, but the transition regions are generally between the temperatures of each of the monomers, making it possible to replace the discrete range of transition temperatures by a quasi-continuous range. It will be noted that the mixtures generally have eutectics.

Moreover, it should be noted that the monomer present in the polydiacetylene compound can also be polymerized by further irradiation in the crystalline state. The absorption spectrum obtained will then be superimposed on the irreversible red spectrum.

In this case, the color indicator according to the invention must therefore be protected from polymerization-initiating radiation by any appropriate means.

The invention will now be described in greater detail by the non-limiting illustrative examples below.

In these examples, the purity of the monomers was checked by gas chromatography, mass spectrometry and NMR. The main residual impurity detected is the starting alkyne, its concentration being of the order of 2% or less in each case.

Dodec-1-yne (EGA 24,440-6) (1.2 g) was heated in the presence of copper acetate (4 g) in a mixture of pyridine (100 cm.sup.3) , methanol 300 cm.sup.3) and ether (400 cm.sup.3), for 72 h, at 30.degree.-32.degree. C., in the presence of a gentle stream of pure oxygen. The ether and methanol were then evaporated off under reduced pressure after the excess dissolved oxygen had been removed by the passage of argon, in the presence of 1 g of CuCl. The solution was taken up by fresh anhydrous pyridine (400 cm.sup.3) for 72 h at 47.degree. C. (reflux). Part of the pyridine was then evaporated off under reduced pressure, after which the mixture was acidified with a 10% aqueous solution of HCl. After extraction with ether and evaporation, an oily liquid was obtained which crystallized in the cold. The oil was chromatographed in CHCl.sub.3 on SiO.sub.2 to give an oil which was transparent at room temperature (0.877 g, y=73%). The product was checked by NMR, gas chromatography (purity 97%) and mass spectroscopy (peak 330).

By repeating the procedure of Example 1, the C.sub.12, C.sub.16, C.sub.20, C.sub.22, C.sub.28 and C.sub.32 diacetylenes were obtained from the following alkynes: hex-1-yne, oct-1-yne, non-1-yne, dec-1-yne, undec-1-yne, tetradec-1-yne and hexadec-1-yne respectively; the melting points (or transition temperatures) of these diacetylenes, measured by differential scanning microcalorimetry (DSC), are indicated in Table I below.

The monomers obtained according to Examples 1 or 2 were mixed in the liquid state. The transition temperatures of these mixtures were measured by differential scanning calorimetry (DSC). The transition temperature diagrams for these mixtures are shown in the attached FIGS. 1a, 1b and 1c, on which the temperature in degrees Celsius has been plotted on the ordinate and the composition of the monomer mixture on the abscissa. The scanning rate of the microcalorimeter was 5.degree. C./min.

FIGS. 1a, 1b and 1c respectively represent the following mixtures:

Eutectics were observed for the C.sub.16 -C.sub.20 and C.sub.20 -C.sub.24 mixtures, for 18% of C.sub.20 and 21% of C.sub.24 respectively. The transition temperatures of these eutectics are of the order of -30.degree. C. and -2.degree. C. respectively.

Oct-1-yne and dec-1-yne were subjected to oxidative coupling in accordance with the procedure described above (Example 1). The reaction yielded a mixture of three monomers Careful choice of the proportions of the starting alkynes made it possible to obtain a mixture formed essentially of two constituents.

A mixture of 1.65 g of oct-1-yne and 4.14 g of dec-1-yne gave 4.89 g of a compound consisting of 4% of hexadeca-7,9-diyne, 33% of octadeca-7,9-diyne and 63% of eicosa-9,11-diyne.

The transition temperatures of this mixture are given in Table 11 below (mixture no. 1).

The constituents of the mixture were separated by preparative chromatography, this separation yielding three fractions (stationary phase SiO.sub.2 --C.sub.8, eluent acetonitrile). Gas chromatographic analysis of the useful fraction gave the following composition: C.sub.18(8-10) 95%, C.sub.20(10-10) 2%, C.sub.16(8-8) 1.5%. Mass spectrometric analysis, M=246 (NH.sub.3 chemical ionization, peaks observed: M+1, M+18), corresponds to octadeca-7,9-diyne (C18(8-10))

The transition temperature is measured by differential scanning microcalorimetry (DSC). The transition takes place by a single peak situated at -26.5.degree. C.

Mixtures no. 2, 3, 4 and 5, consisting of two or three different diacetylenes, are obtained by repeating the procedure described in Example 4.1 with the following pairs of alkynes: pent-1-yne/tetradec-1-yne, dec-1-yne/dodec-1-yne, oct-1-yne/tetradec-1-yne and dec-1-yne/tetradec-1-yne. The characteristics of these mixtures are collated in Table II.

The unsymmetrical diacetylene fraction was separated from these mixtures by a method identical to that used for C.sub.18(8-10). The purity of the monomers, as checked by gas chromatography, is as follows: C.sub.19(5-14) 87%, C.sub.22(10-12) 96%, C.sub.22(8-14) 92%, C.sub.24(10-14) 96%. The transition temperature of these unsymmetrical diacetylenes is indicated in Table III below.

The mixtures described above can be used as such for polymerization, without separation.

The monomers or monomer mixtures obtained according to Examples 1 to 4 above were subjected to photopolymerization with a low-power UV lamp of wavelength 254 nm (flux at 15 cm: 260 .mu.W/cm.sup.2), the monomers being in the form of a thin film in the crystalline state.

The polymerizabilities were measured on samples of the pure diacetylenes. The most photochemically reactive pure diacetylenes are eicosa-9,11-diyne, C.sub.20(10-10), tetracosa-11,13-diyne, C.sub.24(12-12), docosa-9,11-diyne, C.sub.22(10-12), and tetracosa-9,11-diyne, C.sub.24(10-14). In Table II, the most photochemically reactive mixtures of diacetylenes (symmetrical +unsymmetrical) are mixtures no. 1, 3 and 5.

The absorption spectra of the polydiacetylene compounds were recorded as a function of the irradiation time of the monomer sample crystallized beforehand in the form of a thin film.

These spectra are shown in the attached FIGS. 2a, 2b and 2c, which respectively represent the polydiacetylene compounds obtained from the following monomers:

and from a C.sub.16(8-8) /C.sub.20(10-10) mixture containing 80% of C.sub.20(10-10) .sup.(FIG. 2 c).

The legend for FIGS. 2a, 2b and 2c is indicated in Table IV below; the times indicated in this table are the photopolymerization times at the temperatures given in the "compound" column.

The results obtained show that the polydiacetylene compounds are transparent in the near IR from 750 nm. It is noted that these compounds have a blue shade which is the more distinct, the shorter the photopolymerization time.

The transition temperatures of the monomer and the blue polymer were measured for C.sub.24(12-12) by differential scanning calorimetry. The results obtained are reported in FIGS. 3a (C.sub.24(12-12) diacetylene compound or C.sub.24 DA) and 3b (C.sub.24(12-12) polydiacetylene compound or C.sub.24 PDA) and show that there is a similarity between the two fusion diagrams.

Different transmission spectra were recorded successively at increasing temperatures. The polymers were found to change progressively from blue to red. The polymers behave similarly in all cases, this color change always being irreversible.

By way of example, FIGS. 4a, 4b and 4c show the change in the absorption spectra of the polydiacetylene compounds prepared in Example 5 from the C.sub.20 diacetylenic monomer (FIG. 4a) or from the C.sub.16(8-8) /C.sub.20(10-10) mixture (FIG. 4b and 4c), as a function of increasing temperatures (-9.degree. C. to +3.5.degree. C. for C.sub.20 and -40.degree. C. to -2.degree. C. for the C.sub.16(8-8) /C.sub.20(10-10) mixture--solid curves).

The return to low temperature after fusion gives the compounds a red type of absorption spectrum (discontinuous curves, T=-24.degree. C. for C.sub.20 and T=-23.degree. C. for C.sub.16(8-8) /C.sub.20(10-10)). The irreversibility is thus noted.

Furthermore, the observed color change is persistent. In fact, tests showed that, for red C.sub.24(12-12) or C.sub.20(10-10) polymers on different types of substrate (paper, glass, quartz), kept at temperatures of about -20.degree. C. for periods ranging up to 3 months, no notable color change took place.

Thermochromism is observed on mixtures no. 1, 3 and 5 in Table 11. Observation is carried out with the aid of a thermomicroscopic analysis system; the monomer or mixture is deposited in a sapphire dish, permitting UV polymerization and microscopic observation.

Mixture no. 1 is polymerized at -45.degree. C. (UV 254 nm, 5 cm) for a period of 10 min. It is blue-mauve in color. The start of the transformation is noted at -15.degree. C., when it becomes dark red. This transition extends up to -9.degree. C., when it becomes orange-red.

Mixture no. 3 is polymerized at -25.degree. C. (UV 254 nm, 5 cm) for a period of 5 min. It is dark blue to black in color, depending on the thickness. The transition starts at -7.degree. C., when the polymer is dark red. It becomes orange-red at -4.degree. C. and the transformation is complete at +2.degree. C.

Mixture no. 5 is polymerized at -20.degree. C. (UV 254 nm, 5 cm) for a period of 1 min. It is navy blue in color and has a shiny appearance. The transition starts at about -8.degree. C., when the color becomes mauve. It becomes red at -3.degree. C. and the transition is complete at +5.degree. C. with an orange-red color.

2.5 mg of C.sub.24(12-12) monomeric diacetylene are incorporated into 2.7 mg of molten polyethylene at 145.degree. C. (molecular weight 21,000). A thin film is obtained on cooling, which is then brought to +5.degree. C. in order to crystallize the incorporated diacetylene.

The film is then subjected to UV irradiation at 254 nm for 3 min. The film becomes an intense blue. Heating of the film is observed by a thermomicroscopic system from 5.degree. to 25.degree. C. at a rate of 2.degree. C. per minute. The color change from blue to mauve starts at 12.degree. C. and the irreversible red color appears from 16.degree. C., the maximum of the heat of fusion peak for C.sub.24(12-12) being situated at 20.5.degree. C.