Thermochromic polydiacetylene from nano aggregate of diacetylene lipids with varying alkyl chain lengths

Four series of diynoic acid, monoamide, diamide and unsymmetrical diamide lipids containing various lengths of methylene spacer (m = 2, 3 and 4) between the diyne and the head groups and number of methylene units (n = 6 and 9) in their hydrophobic tails are synthesized. After sonication in water, the diynoic acid and monoamide lipids with m = 4 can be dispersed in water while all diamide and unsymmetrical diamide lipids can be dispersed in water very well. All successfully dispersed diacetylene sols were photopolymerized by UV irradiation at 254 nm to give the corresponding polydiacetylene (PDA) sols. The color transitions from blue to red upon heating of the PDA sols from 20-90 °C are photographically recorded and monitored by UV-vis absorption spectroscopy. The diynoic acid and monoamide exhibited irreversible thermochromism while the diamide and unsymmetrical diamide exhibited reversible thermochromism. The color transition temperatures of diamide PDA sols can be tuned between 10-70 °C by the variation of m and n values. The increase of n value enhances the thermal stability resulting in higher color transition temperature. For m value, both the length and odd/even number affect the thermal stability of the PDAs. The lower or the even m values tend to reduce the thermal sensibility and thus increase the color transition temperature while the higher or the odd m values pose the opposite effects. The degree of reversibility, stability and morphology of these PDAs are also investigated.