Department of Polymer Science

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Chemical Communication

Received 13th November 2015

Attachment of irreversible thermochromic groups onto cellulose

M. A. Thomson

The ability of polydiacetylene (PDA)-based compounds to change

colour upon heating was investigated in a range of solvents.

Eicosa-9,11-diyne was polymerized via self-assembly upon UV

irradiation and an irreversible colour change from blue to red was

observed at approximately 45 °C. The thermochromic behaviour of

10,12-pentacosadiynoic acid was also investigated and exhibited

an irreversible colour change from blue to red at 70 °C. A 50/50

wt.% mixture of eicosa-9,11-diyne and 10,12-pentacosadiynoic

acid exhibited an irreversible colour change from blue to red

between 50 and 60 ° C.

A diacetylene is a compound which contains two conjugated triple bonds, allowing for various structural changes upon external stimu-lus

1 (such as changes in temperature, ionic interaction, electrical

fields or microbial interaction). Upon ultraviolet irradiation, the diacetylene monomers undergo a self-assembly mechanism to form polydiacetylenes (PDAs)

2. The ability to observe the colour change

of the PDA from blue to red with the naked eye is the property which makes the diacetylenes so versatile

3,4. An industrial applica-

tion of the PDAs is as an “intelligent” label5 which changes colour

irreversibly to indicate the thermal history of stored food products. Eicosa-9,11-diyne (ECD) was synthesized using Hay

6 coupling of

1-decyne (Scheme 1)1, which was ordered from Sigma-Aldrich®.

Characterization of the monomer and polymer was done via solu-tion

1H NMR and

13C NMR, and FTIR. 10,12-pentacosadiynoic acid

(PCDA) was ordered from Sigma-Aldrich® and characterized with FTIR. Solutions of each monomer (0.3 wt.%) were made using a range of solvents and applied to α-cellulosic filter paper ordered from Whatman®.

The solvent was allowed to evaporate in a fume hood at room temperature overnight. The filter paper was placed in a UV box (254 nm, 10 W) for 1 minute. After irradiation the filter paper was heat-ed on a heating plate and the temperature at which colour change from blue to red occurred was recorded. Once the thermochromic temperature of each monomer was determined, a 50/50 wt.% mix-ture of ECD (0.015 g) and PCDA (0.015 g) in DCM (10 mL) was pre-pared in attempt to observe an intermediate thermochromic tem-perature.The solvent effects of a range of classic organic solvents on the thermochromism of the monomers was investigated.

Both monomers were dissolved in a range of solvents, applied onto filter paper and the solvent was allowed to evaporate. The colour change of the filter paper before and after solvent evaporation was recorded. Each DA monomer was incorporated into a poly(vinyl alcohol) (PVA) film with a PVA:DA ratio of 1:0.3 (v/v %) in 10 mL DCM. The films which were cast were then irradiated with UV light for 1 minute. *Supplementary experimental procedures can be found in the addi-tional experimental appendix.

Wang et al.1 proposed the polymerization mechanism (Scheme

2) via self-assembly of the DA monomer units, which sees the change from conjugated triple bond structure to a double-triple bond conjugation in the backbone.

The polymerization of each monomer was attempted in both solid state and in solution with varying solvents. It was observed that the self-assembly mechanism was only possible in the solid state when deposited onto filter paper, due to the fact that in solu-tion the monomer molecules might have been too dispersed and isolated for self-assembly to occur

5. In the solid state, the monomer

units were close enough to one another to allow the self-assembly mechanism to occur. Therefore the solutions did not exhibit ther-mochromism after UV irradiation. Each monomer in solution was characterized using solution NMR and FTIR before and after UV irradiation and no structural change was observed, therefore polymerization was unsuccessful in solution.

The monomer in the solid state self-assembled on cellulosic fil-ter paper upon UV irradiation. The dried monomer was white on the surface of the filter paper and then after UV exposure a colour change to blue was observed, indicating that polymerization had occurred (Fig. 1). *(NMR and FTIR data can be found in the supplementary appendix).

Department of polymer science, University of Stellenbosch, Private bag X1, Matieland, 7602

Scheme 1: Formation of eicosa-9,11-diyne via Hay6 coupling of 1-decyne

UV

h𝑣

Fig. 1. Colour change from white to blue of eicosa-9,11,-diyne after UV irradiation

for 1 minute.

UV

h𝑣

Scheme 2: Polymerization of eicosa-9,11-diyne via UV irradiation1.

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Fig. 4. Colour transition of each PDA from before and after heating past the thermochromic point in a range of

classic organic solvents

Table 1 shows the thermochromic temperatures of ECD and

PCDA which were observed at approximately 45 °C and 70 °C, re-spectively. The thermochromic temperature of ECD was approxi-mate and exists as a range of temperatures between 45 °C and 55 °C due to the fact that the conversion of the Hay coupling

6 is ques-

tionable. The greater the conversion of the synthesis, the more uniform the thermochromic temperature of the polymer. Fig. 2a shows the colour transition temperature for the ECD polymer, while Fig. 2b shows PCDA exhibited a specific thermochromic tempera-ture of 70 °C.

Once the thermochromic temperature of each monomer had been identified, a mixture of the two monomers could be prepared in an attempt to program the temperature at which the polymer would change temperature. Fig. 3 shows that the 50/50 wt.% mix-ture of each monomer in DCM exhibited partial thermochromism at approximately 50 °C, and full colour change at 60 °C. This suggests that either the separate constituents of the mixture changed colour at their respective temperatures, or an intermediate thermo-chromic temperature was obtained in between that of each mono-mer. The temperature range in which colour change occurred was relatively large, suggesting that the mixture is heterogeneous and the temperature range is merely the overlapping of the two mono-mers’ separate colour transitions. A possible solution would be to produce a graft copolymer of ECD and PCDA, which would improve the compatibility between the two phases - and thus a single inter-mediate thermochromic temperature could be obtained rather than the broad range which was observed.

It was observed that solvent effects caused colour change of the monomers (i.e. the monomers were solvatochromic

7). Before the

evaporation of the solvent, the ECD and PCDA were green and red, respectively. Once the solvent had evaporated completely, the monomers were both white. After irradiation the change in solvent caused a change in the shade of blue of the polymer which forms on the filter paper, indicating that certain solvents allowed for the self-assembly mechanism better than others.

Fig. 4 shows the characteristic colour change from blue to red upon heating in a range of classic organic solvents. It was observed that ECDA exhibited lower solubility (even after sonication) in the range of classic solvents when compared to PCDA. This was evident by the degree of aggregation of the ECD in the solvent, resulting in clumping of solid monomer on the filter paper. Furthermore, it was observed that the PCDA samples exhibited more saturated/intense shades of blue and red than the ECD samples. This could possibly be due to the fact that the PCDA molecules contained polar carboxylic acid moieties, whereas the ECD molecules were apolar. This al-lowed the PCDA molecules to anchor onto the cellulose filter paper via hydrogen bonding between the carboxylic acid and alcohol moi-eties. Films casted of PVA/DA monomer were exposed to UV radia-

tion and the thermochromic properties observed. The PCDA-incorporated film exhibited colour change after UV exposure from clear to blue (Fig. 5), confirming that polymerization had occurred in the film state. The film changed temperature at 70 °C, which was expected as the thermochromic temperature of PCDA on filter pa-per was also 70 °C.

PVA/ECD films did not change colour upon UV exposure, indi-cating that polymerization did not occur. This could again be at-tributed to its apolar nature, leaving it unreacted on the PVA sur-face and thus inhibiting the self-assembly mechanism. Whereas the PCDA molecules could experience hydrogen bonding with the PVA, providing sites for self-assembly to occur.

Fig. 2. Colour transition of the DA monomers from blue to red (a) eicosa-9,11-diyne with a

thermochromic temperature range between 45 °C and 55 °C, (b) 10,12-pentacosadiynoic

acid with a thermochromic temperature of 70 °C.

Table 1: Thermochromic temperatures of monomers eicosa-9,11-diyne and 10,12-

pentacosadiynoic acid

Fig. 3. Colour transition of 50 wt.% eicosa-9,11-diyne/10,12-pentacosadiynoic

acid. Partial colour change at 50 °C and complete colour change at 60 °C.

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In order to incorporate ECD into a film which would then exhibit thermochromic properties, the DA monomer must be functional-ized before the polymerization step. The addition of a reactive group (i.e. carboxylic acid) would improve the compatibility be-tween the DA and the film, thereby allowing the self-assembly mechanism to occur. This proves that the production of an “intelli-gent” label is possible simply by the incorporation of a DA monomer into a polymer film, provided that the DA monomer can attach onto the film substrate to allow the self-assembly mechanism to occur.

In conclusion, the polymerization of diacetylenes in the solid state was possible via UV irradiation, producing the polydiacety-lenes (PDAs) with the characteristic of changing colour from blue to red once a specific thermochromic temperature was surpassed. A 50:50 mixture of PCDA and ECD was prepared, which exhibited a broad thermochromic transition range suggesting that the mole-cules were not compatible enough to exhibit a single intermediate thermochromic point in-between that of each monomer. The DA monomers were solvatochromic, with a difference in colour when in solution compared to in the solid state. PCDA exhibited strong-er/more intense colour when compared to ECD due to its strong interaction with the alcohol groups of the cellulose. Films were casted of each DA, of which only PCDA underwent polymerization after UV irradiation. The use of diacetylenes as sensory molecules is feasible, provided that compatibility exists between the DA and the substrate on which it is anchored.

Notes and references

* Text marked with an asterisk refers to information which can be found in the supplementary information appendix.

[1] Wang, M., Wang, F., Wang, Y., Zhang, W. & Chen, X. Polydiacetylene-based sensor for highly sensitive and selective Pb2+ detection. Dye. Pigment. 120, 307–313 (2015).

[2] Bloor, D. Topochemical Polymerization: Diynes. Compr. Polym. Sci. Suppl. 5, 233–249 (1989).

[3] Sun, X., Chen, T., Huang, S., Li, L. & Peng, H. Chromatic polydiacetylene with novel sensitivity. Chem. Soc. Rev. 39, 4244–57 (2010).

[4] Chanakul, A., Traiphol, N. & Traiphol, R. Controlling the

Reversible Thermochromism of Polydiacetylene/zinc Oxide Nanocomposites by Varying Alkyl Chain Length. J. Colloid Interface Sci. 389, 106–114 (2013).

[5] Rougeau, L., Picq, D., Rastello, M. & Frantz, Y. New irreversible thermochromic polydiacetylenes. Tetrahedron 64, 9430–9436 (2008).

[6] Hay, A. Oxidative Coupling of Acetylenes. II1. J. Org. Chem. 3–4 (1962). doi:10.1021/jo01056a511

[7] Lee, J. et al. A protective layer approach to solvatochromic sensors. Nat. Commun. 4, 2461 (2013).

Fig. 5. Polymerization of a 1:0.3 (v/v %) PVA/PCDA film via UV irradiation and colour transition from blue to red at 70 °C.

UV ΔH