Photoresponsive Block Copolymer Photonic Gels with Widely Tunable Photosensitivity by Counter-Ions

Polyelectrolyte hydrogels exhibit unique stimuli responsive physical properties,1–7 and which have been widely utilized for many engineering applications including actuators, drug delivery and sensors.8–13 Because the degree of charge dissociation in polyelectrolytes is dependent on the types of ion-pairs, the physical properties of polyelectrolyte hydrogels such as conductivity, elasticity and swelling ratio are affected by counter-ions as well as external stimuli.14–17 Recently, there have been increasing research efforts of combining hydrogels with other functional materials to extend their potential applications. For example, the photonic crystals comprising hydrogels exhibit unique tunable photonic band gaps in response to the various stimuli including temperature, pH, ionic strength, solvent compositions and electric field.10–13, 18–25

Herein, we report photoresponsive block copolymer photonic gel films exhibiting strong reflective multicolors in the visible regime in response to near-UV radiation, and their controllable photosensitivity by counter-anions. Our approach utilizes the volume transition of hydrogels induced by photocrosslinking in 1D block copolymer photonic crystal structures. Previously, we have demonstrated that block copolymer lamellar structures are useful for creating tunable photonic crystals.26–30 For instance, the lamellae assembled from polystyrene-b-quaternized poly(2-vinyl pyridine) (PS-b-QP2VP) exhibited strong reflective colors when the QP2VP layers were selectively swollen with solvents. In this case, photonic stop bands (PSBs) varied with the extent of swelling, and which can be controlled by crosslinking density and the species of counter-anions pairing with pyridinium groups. We recently found that PS-b-QP2VP block copolymer photonic gels are responsive to UV, and their photosensitivity is highly dependent on the species of the counter-anions pairing with pyridinium. Utilizing our findings, we have demonstrated that multicolor micropatterns can be created on the photonic gels by using the conventional photolithography, and in this case the created patterns can be repeatedly fixated and reactivated by sequential ion-exchange processes.

The block copolymer photonic gel films were prepared as previously described.26–29 Briefly, in-plane oriented lamellar films were first prepared by spin casting PS₅₇-b-P2VP₅₇ solution (Number-average molecular weight (M_n) of each block is 57 × 10³ g/mol and 57 × 10³ g/mol for PS and P2VP respectively, PDI = 1.08, 6 wt% in propylene glycol monomethyl ether acetate) onto glass followed by annealing the films in chloroform vapor at 50 °C for 2 days. The thickness of the films was controlled to ∼1 μm. The P2VP blocks were then quaternized using iodomethane in hexane at 50 °C for 24 hours. Finally, the counter anions pairing with pyridinium were exchanged with a variety of anions by immersing the films into various aqueous salt solutions. The as-prepared photonic gels exhibited strong reflective colors in aqueous solution. All spectra of photonic gels were taken after swelling them with DI water. Exposure of UV was carried on a stepper machine (MDA-400M, Midas System, Korea) having energy maximum at 365 nm.

The as-prepared photonic gel films originally exhibited a red color. Upon exposure to 365 nm UV, the films gradually turned to green, from green to blue and eventually became transparent as increasing the exposure energy (Figure1). The photonic gel films were almost insensitive to the light with λ > 395 nm.31 The blue-shift of PSBs is attributed to the photo-crosslinking of QP2VP layers by UV.32, 33 As previously reported,26 crosslinking constrains swelling of the QP2VP hydrogel layers, and accordingly induces shift of PSBs to the shorter wavelength. Our photonic gels showed extremely large optical changes in response to weak dose of UV irradiation. For example, the photonic gel films containing phenoxide or acetate ions showed PSB shifts by Δλ_peak = –437 and –271 nm, respectively, with irradiation of 10.3 mJ/cm² of UV at 365 nm. We think that such large changes are because of the unique volume transition of the swollen hydrogels by crosslinking. Utilizing swelling data and Peppas-Merrill equation,34 we calculated the theoretical crosslinking density change of QP2VP layers as PSBs moved to the shorter wavelength by photocrosslinking. The swelling ratio of QP2VP layers (which is expressed in terms of the volume fraction of water in a QP2VP layer, ) was first calculated by transfer matrix method (TMM) based on the measured PSBs.26, 35 For the photonic gel films containing acetate ions, TMM calculations showed that the QP2VP hydrogel layers were originally highly swollen ( = 0.938) and thus small change of the swelling ratio ( = –0.056, –6.7%) can induce the significantly large shift of PSBs (Δλ_peak = –271 nm, –42%). Such small change of the swelling ratio can be easily achieved by minimum extent of crosslinking. Our calculation using Peppas-Merrill equation shows that crosslinking of only 1% of pyridinium groups is required for inducing –6.7% of the swelling ratio and accordingly moving PSBs by –42%. Comparing with the physical property changes of typical polymer gels by crosslinking,36–38 our system showed significantly large optical change by photocrosslinking. These results suggest that our photonic gels are highly effective for transducing photochemical changes into optical signals.

Figure 1. Optical responses of the PS-b-QP2VP photonic gel film containing acetate ions with irradiation of 365 nm UV. UV-VIS transmission spectra of the photonic gel film and the corresponding photographs were taken at different exposure energy (from right to left: 0, 1.7, 3.4 and 10.3 mJ/cm²). The sample size is 1 cm × 1.2 cm.

The photosensitivity of photonic gel films is highly dependent on the species of counter-anions pairing with the pyridinium groups of QP2VP layers. To test the effects of counter-anions, photonic gel films containing various counter-anions were prepared by simple ion-exchange method. The changes of PSBs for each film were measured as a function of the exposure energy. As shown in Figure2, all of the photonic gel films containing different counter-anions exhibited different extent of PSB shifts and different photosensitivity. For example, the films containing acetate or phenoxide anions showed steep transition curves and large shifts of PSBs, while the films containing bromide or iodide anions showed almost flat transition curves and very small shifts of PSBs at the same experimental condition. The anion-dependent photosensitivity of the photonic gels implies that the photocrosslinking of QP2VP gel layers is inhibited or facilitated by counter-anions. Since most of photocrosslinking is accompanied by the formation of radicals on the backbone or the side chain of polymers, the photocrosslinking of QP2VP can be facilitated or inhibited by the counter-anions which affect the efficiency of radical formation or radical quenching.39, 40 For example, radicals can be induced more efficiently on polymer chains by using anions containing photosensitive moieties such as carbonyl group.41, 42 As shown in Scheme S1, photo-reactive anions generate the reactive radicals effectively upon irradiation of UV, and then which can be quickly propagated to polymer chains by the well-known hydrogen abstraction process.39, 41, 43 Hence it is expected that the chance of reactive radical formation on polymer chains by UV irradiation is increased by such hydrogen abstraction process, and which accordingly increase the efficiency of photocrosslinking. For this reason, acetate and phenoxide ions can facilitate photocrosslinking by inducing reactive radicals on the QP2VP polymer chains via hydrogen abstraction.42, 44–49 In this case, photocrosslinking can be further modulated by tailoring the reactivity of anions for hydrogen abstraction. For instance, it is expected that radical generation will be retarded by introducing strong electron withdrawing groups to the acetate anions since they destabilize radicals, and which accordingly affect directly photocrosslinking of polymer chains.41, 50 We confirmed that the photosensitivity decreased significantly when the films were modified with trifluoroacetate instead of acetate (Figure 2a and Figure 2c). Similarly, photocrosslinking can also be retarded by radical inhibitors such as large halogen anions (Scheme S2). In this case, the photosensitivity decreased monotonically in the order of fluoride > chloride > bromide > iodide because the inhibition power is proportional to the size of halogen anions (Figure 2f).

Figure 2. Anion dependent photosensitivity of photonic gels. Variation in λ_max with the exposure energy of 365 nm UV for the photonic gel films containing (a) acetate, (b) phenoxide, (c) trifluoroacetate, (d) bromide and (e) iodide. (f) Bar graph of PSB shifts for the photonic gel films containing various anions upon irradiation of 365 nm UV with the energy of 10.3 mJ/cm².

The anion dependent photosensitivity can be utilized for creating and fixating multicolor micro patterns on the photonic gels (Figure3 and Figure4). As shown in Figure 3, we demonstrated that multicolor patterns can be easily created by conventional photolithography techniques when the photonic gels were modified with highly photosensitive phenoxide ions, and then the resultant patterns can be fixated by modifying them with photo-insensitive iodide ions. The fixated films can be reactivated by exchanging iodide with phenoxide ions again. There was no significant difference in photosensitivity between the original films modified with phenoxide and the reactivated films after fixation process. The resolution of the pattern was as low as 8 μm (Figure 4a). The color of micropatterns was able to be controlled by varying the exposure energy. The multicolor check pattern shown in Figure 4c was created by double exposure of line pattern (Figure 4b) with rotating a photomask at 90°. Similarly, the letters and squares exhibiting different colors in Figure 4d were created by using a photomask having gradient pattern density.

Figure 3. Fixation and reactivation of photonic patterns by ion-exchange.

Figure 4. Multicolor micropatterns by photolithography on the photonic gel films modified with acetate anions. (a) An isolated line pattern with 8 μm resolution. (b) Multiline pattern with resolution of line/space = 50 μm/80 μm. (c) Check pattern created by double exposure of line pattern with rotating a photomask at 90°. The exposure energy for the areas exhibiting red, green and blue is 0, 1.8, and 2 × 1.8 mJ/cm² respectively. (d) Photonic patterns created using a photomask having gradient pattern density.

In summary, the photosensitive photonic crystal structures exhibiting strong reflective multicolors in visible regime have been demonstrated using PS-b-QP2VP block copolymer photonic gels. Since they were highly swollen, the photonic gels exhibited very large PSB shifts against the weak exposure energy of UV. Furthermore, it was demonstrated that the photosensitivity of photonic gels can be tuned by varying counter-anions. We anticipated that our findings are useful for understanding the volume transition of polyelectrolyte hydrogels by photocrosslinking, and applicable for many fields of science and engineering. Especially, the features of our photonic gels such as high photosensitivity to UVA (λ = 320–400 nm) causing skin carcinogenesis and tunable photosensitivity by simple ion-exchange are promising for the colorimetric UV radiation sensors.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology(2010-0002358) and the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea (Grant number: K2010-00295, FY 2011).