Polymer Mortar Assisted Self-Assembly of Nanocrystalline Polydiacetylene Bricks Showing Reversible Thermochromism

When properly packed in the solid state, diacetylene monomers can be topochemically polymerized into polydiacetylene by UV, γ, or strong X-ray irradiation. Usually, colorless diacetylene crystals turn into blue polydiacetylene crystals, which often undergo an irreversible “blue-to-red” colorimetric transition when environmental stimuli are applied. Although this unique transition has found many applications as colorimetric sensors, its irreversibility has prevented polydiacetylenes from being repeatedly used. To achieve reversible colorimetric transition, chemical modifications of diacetylene monomers have been pursued. For example, multiple hydrogen-bonding, strong aromatic, or ionic interactions were introduced into diacetylenes by chemical modifications to enhance the bonding among the head groups in side chains. Meanwhile, these diacetylene monomer had to be assembled into well-defined physical forms, including vesicles, nanotubes, and Langmuir–Blodgett or solid thin films, to achieve the reversibility. Recently, inorganic nanomaterials were used as nanoscale substrates to achieve reversible thermochromism for otherwise irreversible polydiacetylenes. For example, in organic/inorganic hybrids based on mesoporous silica and layered double hydroxide, the head groups in polydiacetylene side chains were bound, either covalently or noncovalently, to the inorganic matrices, and reversible thermochromism was realized. Despite the above successful examples, the fundamental physics of reversible colorimetric transitions and their achievability for polydiacetylenes have not yet been well-understood and still desire further study. Once the physics of reversibility is understood, we will be able to design better polydiacetylenes as versatile sensor materials. For example, using polymer blends to achieve colorimetric reversibility for polydiacetylenes appears attractive because of the flexibility and easy processability of polymeric materials. However, no reversible thermochromism has been realized for irreversible polydiacetylenes in polymer blends so far. In this work, we successfully achieved reversible thermochromism in a self-assembled poly(vinylpyrrolidone) (PVP)/poly(10,12-pentacosadiynoic acid) (PDA, an irreversible polydiacetylene in the pure form) blend via hydrogen bonding in aqueous suspensions and solid films. In particular, hierarchical self-assemblies were observed, and the “bricks and mortar” morphology (crystallized DA bilayers being the bricks and PVP being the mortar) was found to be responsible for the reversible thermochromism, as shown in Scheme 1. From this study, we conclude that the physical constraint exerted on every PDA bilayers by the PVP mortar substantially assisted reversible conformational transitions in the PDA main chain, which was believed to be an important reason for the reversible thermochromism. A method for preparing noncovalently connected micelles (NCCM) was used to fabricate PVP/DA (the monomer of PDA) nanosuspension in aqueous solution. Both DA and PVP are commercially available. Typically, DA was dissolved in ethanol at 1.25 mg/mL and PVP in water at 5.0 mg/mL. 0.8 mL of DA ethanol solution was added at a rate of 0.05 mL/min into 10 mL of PVP aqueous solution. Shortly, DA aggregated because of its poor solubility in the aqueous solution. Note that macroscopic DA precipitates were effectively prevented by the PVP chains gathering around the DA aggregates via hydrogen bonding between carboxyl groups in DA and carbonyl groups in PVP. (The hydrogen bonding between carboxyl groups and carbonyl groups of PVP in water or other solvents has been widely reported.) As a result, a PVP/DA nanoaggregate suspension was formed (see S1 and S2 in Supporting Information). In a control experiment, reversible thermochromism could not be achieved, when PVP and PDA were mixed using a common solvent such as ethanol. When exposed to 254 nm UV light, the as-prepared suspension did not change its color, indicating lack of proper packing of DA molecules in the nanoaggregates. The above suspension was then dried in a beaker at 30, 50, 65, and 85 °C to obtain colorless PVP/DA films (see S1 in Supporting Information). For simplicity, the films obtained by drying at 30, 50, 65, and 85 °C are denoted as films A–D. Films A, B, and C could be topochemically polymerized after exposure to 254 nm UV irradiation at room temperature (RT) for 30 min, and polymerized films displayed a blue color, whereas film D did not polymerize at all. DSC results (see S3 in Supporting Information) showed melting points of films A and B at 52 and 54 °C, respectively, lower than that of pure DA crystals (Tm,0 ) 62–63 °C). However, film C presented a melting peak at ca. 75 °C, remarkably higher than Tm,0. Film D had the lowest melting point at 47 °C, suggesting a poor crystalline packing inappropriate for topochemical polymerization. Among all polymerized films, only the polymerized film C showed reversible thermochromism between 25 and 120 °C (Figure 1-A1). Polymerized films A and B underwent an irreversible blue-to-red transition upon heating. Reversible thermochromism of polymerized film C was clearly demonstrated by UV–vis measurements between 25 and 85 °C (the highest temperature allowed by the instrument) in Figure 1-A2. At 25 °C, the film had a maximum absorption at 638 nm, responsible for the blue color. Upon heating, the maximum absorption continuously shifted to lower wavelengths, and the absorption intensity at 638 nm gradually decreased. At 85 °C, the 638 nm absorption completely disappeared, and the absorption at 586 nm increased to a maximum, with a shoulder at 540 nm. Upon cooling, the absorption spectrum changed inversely. The complete thermochromic reversibility between 25 and 85 °C was demonstrated by the fact that the spectrum at 25 °C after the heating–cooling cycle almost overlapped with that before the thermal cycle (Figure 1-A2) and also confirmed by the parameter of “colorimetric response” (CR) in different * Corresponding authors. E-mail: [email protected] (L.Z.); chendy@ fudan.edu.cn (D.C.). † Fudan University. ‡ University of Connecticut. 2299 Macromolecules 2008, 41, 2299-2303