Supramolecular photoresponsive polyurethane with movable crosslinks based on photoisomerization of azobenzene

Light‐driven actuators are widely used for smart devices such as soft robots. One of the main challenges for actuators is achieving rapid responsiveness, in addition to ensuring favorable mechanical properties. Herein, we focused on photoresponsive polyurethane (CD‐Azo‐PU) based on controlling the crystallization of the hard segments in polyurethane (PU) by complexation between azobenzene (Azo) and cyclodextrins (CDs). CD‐Azo‐PU incorporated polyurethane as the main chain and a 1:2 inclusion complex between Azo and γCD as a movable crosslink point. Upon ultraviolet light (UV, λ = 365 nm) irradiation, the photoresponsiveness of CD‐Azo‐PU bent toward the light source (defined as positive), while that of the linear Azo polyurethane (Azo‐LPU) without peracetylated γ‐cyclodextrin diol (TAcγCD‐diOH) as a movable crosslinker bent in the direction opposite the light source. The bending rates were determined to be 0.25°/s for CD‐Azo‐PU and 0.083°/s for Azo‐LPU, indicating that the bending rate for CD‐Azo‐PU was faster than that for Azo‐LPU. By incorporating movable crosslinks into CD‐Azo‐PU, we successfully achieved specific photoresponsive actuation with an enhanced rate.


INTRODUCTION
Soft robots, capable of working in extreme and hazardous environments such as the deep sea, [1] mining, [2] and disaster scenarios, [3] have gained significant scientific attention as a safer substitute for human intervention.One approach to developing soft robots is the utilization of stimuli-responsive materials, [4] which can change their shape, stiffness, and other properties through internal structural changes in response to stimuli, such as moisture, [5,6] , temperature, [7] electricity, [8][9][10] and light. [11,12]Photostimuli are particularly intriguing due to the salient features of light: remote and contactless triggering with ease of spatial, temporal, directional, and intensity control. [13]Photoinduced deformation at macroscopic scales has been observed for various polymer systems containing photochromic moieties, This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.© 2023 The Authors.[16][17][18][19] Azo-containing polymers are capable of deforming shapes due to changes in the conformation of polymeric chains based on photoinduced regulation of the orientational order, [20] free volume (physical aging), [21] crystallinity, [22,23] and crosslinking points. [24]mong these, the regulation of the crosslinking points particularly based on supramolecular chemistry could be a feasible way to control the bending behavior of photoresponsive materials because crosslinking points formed by noncovalent interactions, such as hydrogen bonding, [25][26][27][28][29][30] coordination bonding, [31] hydrophobic forces, [32,33] π-π interactions [34][35][36] electrostatic effects, [37] and host-guest complexes, [38][39][40][41][42][43][44] easily respond to stimuli due to their weak binding energies. [45]yclodextrin (CD) is a representative molecule used in host-guest chemistry.CD has a cavity to accommodate guest molecules, enabling the formation of reversible inclusion complexes with appropriate guest molecules.Our prior work has primarily focused on utilizing host-guest complexes, specifically CD and photoresponsive guest molecules (Azo or stilbene), to produce novel photoresponsive materials.This approach enables the creation of reversible and movable crosslinks, which regulates the responsiveness through control of the density and locations of crosslinking points, respectively.The expansion-contraction and selective assembly of hydrogels, [46] sol-gel transitions [47] and photoresponsive actuation of xerogels [48][49][50] have been achieved.In a similar manner, actuation in nylon (polyamide) has also been achieved by incorporating a complex of γCD and Azo as a movable crosslinker. [51]However, actuation in crystalline polymers exhibit low responsive deformation due to their high crystallinity. [52]Here, we focused on polyurethane (PU) because the crystallinity or physical crosslinking density in PU can be readily modulated by controlling the composition of soft and hard segments.The versatile design of photoresponsive PU based on host-guest chemistry is used to enhance the photomechanical properties, thereby strengthening its utility in robust soft robotics applications.
Herein, we successfully prepared photoresponsive polymeric materials (CD-Azo-PU) with a 1:2 complex composed of γ-cyclodextrin (γCD) and Azo as movable crosslinks in the PU backbone based on supramolecular chemistry.Introducing a small amount of γCD as a movable crosslinker enabled an adjustable bending direction and increased bending rate by regulating the Azo distribution within the hard segment domains in the CD-Azo-PU material with enhanced flexibility.This method provided a highly tunable approach to design photoresponsive materials for soft robotics, which could facilitate the development of multifunctional photoresponsive materials.

Preparation of photoresponsive PUs
Figure 1A shows the chemical structures of photoresponsive PU materials with movable crosslinks; these were prepared by a two-step polymerization method (Scheme S1-2).We selected γ-cyclodextrin (γCD) as a movable crosslinker because γCD formed a 1:2 inclusion complex with trans-Azo. [53,54]Photoisomerization to cis-Azo causes the dissociation of the 1:2 complex between γCD and trans-Azo; therefore, we controlled the complexation of γCD and Azo by photostimuli.The PU materials were prepared by two steps method.Firstly, the excess hexamethylene diisocyanate (HDI) and dibutyltin-diacetate (DBTDA) were added to a dichloromethane (DCM) solution of TAcγCD diol monomer (TAcγCD-diOH), [55] Azo diamine (Azo-diAm), [48] and poly(tetrahydrofuran) (PTHF: M n = 1,000)).The mixing and polyaddition reaction produced isocyanate-terminated chains.Secondly, the isocyanate-terminated chains were elongated after the addition of propane-1,3-diol (PDO) as a diol chain extender; PDO was added at 25 • C and the solution was allowed to stand for 24 h.The DCM polymer solution was poured into a mold and dried at room temperature overnight to obtain the photoresponsive PU films (CD-Azo-PU).Linear Azo PU (Azo-LPU; Figure 1B) was also prepared by a similar two-step polymerization method (Scheme S1-S2).The 1 H nuclear magnetic resonance (NMR) measurements of CD-Azo-PU and Azo-LPU showed that the mol% values of γCD and Azo among all repeating units were approximately 2 and 4, respectively (Figure S1-S2).
In the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra, the isocyanate absorption peak at 2270 cm −1 disappeared, and the absorption peaks at approximately 1683 and 1722 cm −1 , attributed to the carbonyl (C═O) groups, appeared after polymerization for all PU materials.(Figure S3).Comparable molecular weights of CD-Azo-PU (M n = 49,000) and Azo-LPU (M n = 72,000) were confirmed by gel permeation chromatography (GPC) (Figure S4 and Table S1).

Photoresponsive properties of CD-Azo-PU and Azo-LPU
Figure 2 and Supporting Movie S1, S2, S3, and S4 demonstrate the photoresponsiveness of the CD-Azo-PU and Azo-LPU films in relation to actuation upon ultraviolet (UV) light irradiation with irradiation intensities at 0.76 or 70 mW/cm 2 .These irradiations resulted in negligible changes in the temperature of the surface not affecting the actuation (Figures S5 and S6). Figure 2A depicts the actuation of CD-Azo-PU and Azo-LPU in response to photostimuli.Photoresponsive actuation of CD-Azo-PU and Azo-LPU was evaluated in terms of the flexion angle (θ).The positive direction of θ was designated as bending toward the light source.Upon UV light irradiation, CD-Azo-PU bent toward the light source within 5 min (Figure 2B).Conversely, Azo-LPU bent away from the light source.
Figure 2C displays the flexion angles of CD-Azo-PU and Azo-LPU during a 5-min period under UV irradiation, followed by a 5-min period in the dark.CD-Azo-PU films of various thicknesses (27-164 μm) were irradiated at a radiation intensity of 70 or 0.76 mW/cm 2 to optimize bending performance.Increasing irradiation intensity improved the maximum bending angle (from 5.0 • to 5.6 • ) of CD-Azo-PU films (approximately 70 μm), attributing to the faster photoisomerization of the Azo moiety.CD-Azo-PU films with a thickness of 49 μm exhibited the highest maximum flexion angle of 10.0 • by UV irradiation (70 mW/cm 2 ).This outcome demonstrated that photoisomerization of Azo units solely occurred at the film surface, serving as the driving force for bending (Figure 2D).Excessive thickness hindered bending due to a high volume of nonexposed areas consuming addi-tional energy.Moreover, too thin films exhibited ineffective bending performance, as the limited volume of nonexposed areas induced strain contributing to length change instead of facilitating bending in CD-Azo-PU.In addition, CD-Azo-PU (approximately 50 μm) showed a higher maximum flexion angle of 10.0 • compared to that of −3.5 • shown by Azo-LPU (approximately 50 μm) irradiated by UV irradiation at the intensity of 70 mW/cm 2 .
Figure 2E shows the initial bending rate (R 0 ) of CD-Azo-PU and Azo-LPU films.R 0 was calculated from the slope of the flexion degree versus irradiation time within the first 5 or 20 s (Figure S7).CD-Azo-PU (49 μm, 70 mW/cm 2 ) displayed a higher initial bending rate R 0 of 0.25 • /s compared to the R 0 value of 0.083 • /s displayed by Azo-LPU (51 μm, 70 mW/cm 2 ).CD-Azo-PU (40 μm, 70 mW/cm 2 ) exhibited the highest initial bending rate R 0 of 0.84 • /s, while the maximum bending angle of 7.1 • is lower than that of 10 • , exhibited by CD-Azo-PU (49 μm, 70 mW/cm 2 ).This result indicated that the appropriate thin sample would bend faster to its maximum bending angle, which supported the bending mechanism discussed above.Moreover, CD-Azo-PU exhibited a larger maximum θ of 10.0 • compared to that of 2.0 • for our previous work, [51] the photoresponsive nylon with a 1:2 complex composed of γCD and Azo as movable crosslinks (Nylon-γCD-Azo) (Figure 2F).
To investigate the relationship between the isomerization rate and bending rate, we used UV-visible (Vis) spectroscopy to monitor the photoisomerization of the Azo group in both CD-Azo-PU and Azo-LPU (Figure S8∼S9).Irradiation with UV light (λ = 365 nm) decreased the intensity of the ππ* transition band of the Azo group within 80 min, and the n-π* transition band appeared, indicating that the Azo unit exhibited photoisomerization and photoinduced deformation.Interestingly, the pseudo-first-order plots of the photoisomerization of the Azo units indicated comparable trans-to-cis isomerization rates of k 1 = 9.0 × 10 −4 s −1 (CD-Azo-PU) and k 2 = 7.0 × 10 −4 s −1 (Azo-LPU) (Figure S8-S9 and Table S2).These findings indicated that CD-Azo-PU exhibited a faster bending response despite having a comparable isomerization rate compared to Azo-LPU.Accordingly, the fast-bending rate exhibited by CD-Azo-PU could be attributed to the structural properties of polymeric chains with γCD as a movable crosslinker.

Photoresponsive hydrogen bonding change in CD-Azo-PU and Azo-LPU observed by in situ ATR-FTIR testing
Hydrogen bonding in CD-Azo-PU and Azo-LPU was expected to change in response to photoisomerization of the Azo moiety.To further investigate the underlying mechanism responsible for the enhanced photoresponsiveness and the opposite bending behavior of CD-Azo-PU, ATR-FTIR spectroscopy was conducted to probe hydrogen bonds of the C═O groups in both CD-Azo-PU and Azo-LPU.
When C═O groups in PUs form hydrogen bonds, a potential shift in their infrared absorption peak could occur.This C═O stretching region contained three distinct contributions: free C═O groups at 1719 cm −1 , disordered hydrogen-bonded C═O groups at 1698 cm −1 , and ordered hydrogen-bonded C═O groups at 1682 cm −1 (Figure 3(A-C)).57][58] In PU, the shorter diol and Azo diamine chains combined with the isocyanate; in this case, PDO, HDI, Azo are considered the "hard segment", which could contribute to the formation of the hydrogen bonds in PUs.The long diol chain segment, exemplified here by PTHF, is often referred to as the "soft segment". [58]Photoisomerization of the Azo moiety localized within hard segments could influence the photoresponsiveness of CD-Azo-PU and Azo-LPU.
Figure 3D, E show the ATR-FTIR spectra of CD-Azo-PU and Azo-LPU in the 1600 to 1800 cm −1 range before and after a 5-min UV light irradiation; the spectra were normalized to the C─H stretching bond peak near 2850 cm −1 .Before irradiation, Azo-LPU demonstrated a higher degree of ordered hydrogen bonds compared to CD-Azo-PU.trans-Azo moieties in Azo-LPU tended to stack due to the suitable symmetry, facilitating the formation of the ordered hydrogen bonds.On the other hand, the γCD units in CD-Azo-PU form the inclusion complexes with trans-Azo moieties. [54]he complexed Azo with γCD in CD-Azo-PU is incapable of faciliating effective hydrogen bonds, resulting in a decrease in the ordered hydrogen bonds, as our previous studies have shown that the γCD as a movable crosslinker in the PU material disturbed the formation of the ordered and disordered hydrogen bonds due to the bulky size of γCD. [55]igure 3F,G display changes in peak intensities of free, disordered, and ordered hydrogen-bonded C═O groups before and after UV irradiation at room temperature for 5 min, where red and green bars signify increases and decreases in absorbance, respectively.In CD-Azo-PU, the peak intensities of the ordered hydrogen-bonded C═O groups decreased by approximately 20%, while those of the disordered hydrogenbonded and free C═O groups increased by approximately 10% after 5 min of UV irradiation.A similar but more notable trend was observed in Azo-LPU; the peak intensities of ordered hydrogen-bonded C═O groups decreased by nearly 40%, and the peak intensities of disordered hydrogenbonded and free C═O groups increased by 30% after 5 min of UV exposure.
These results indicated that the Azo moiety existing within ordered hydrogen bonds in CD-Azo-PU and Azo-LPU could trigger dissociation of hydrogen bonding through photoisomerization from trans-Azo to cis-Azo due to changes in the structures and molecular symmetry and molecular symmetry of Azo molecules. [14,15]The change in the photoresponsive hydrogen bonding in CD-Azo-PU was less significant after a 5-min exposure to UV light irradiation compared to Azo-LPU.The observed outcome can be attributed to the trans-Azo moiety complexed with γCD [54] in CD-Azo-PU.These results indicated that the Azo moiety existing within ordered hydrogen bonds in CD-Azo-PU and Azo-LPU could trigger dissociation of hydrogen bonding through photoisomerization from trans-Azo to cis-Azo due to alterations in changes in the structures and and molecular symmetry of Azo molecules. [14,15]The change in the photoresponsive hydrogen bonding in CD-Azo-PU was less significant after a 5-min exposure to UV light irradiation compared to Azo-LPU.This is attributed to the dissociation of γCD-Azo complex in CD-Azo-PU, releasing the cis-Azo from the γCD cavity.The released cis-Azo maintained an appropriate distance from the bulky γCD and facilitated reformation of hydrogen bonds in CD-Azo-PU, which mitigated the dissociation of the ordered hydrogen bonds after UV irradiation.A similar tendency is

Sample T g1
T g2 T m1 T m2 ΔH m (Enthalpy)  S10).Both CD-Azo-PU and Azo-LPU exhibited a reduction in storage modulus G′ and loss modulus G″ by UV irradiation.However, the decrease in G′ and G″ of CD-Azo-PU was less pronounced than that observed in Azo-LPU.These results were well in accordance with the behaviors of CD-Azo-PU and Azo-LPU observed in the ATR-FTIR spectroscopy.Consequently, the extent of the photoinduced changes in hydrogen bonding in CD-Azo-PU was smaller than that observed in Azo-LPU.

Photoresponsive crystallinity change in CD-Azo-PU and Azo-LPU observed by differential scanning calorimetry measurements
Differential scanning calorimetry (DSC) was conducted to investigate changes in the crystallinity of CD-Azo-PU and Azo-LPU upon exposure to UV irradiation.Crystallization in PUs may occur when the hard segment possesses suitable symmetry and arranges itself into the crystalline domains. [58]he enthalpy of melting (ΔH m ) was directly measured by DSC through the integration of the area under the endothermic peak in the thermogram.ΔH m demonstrated distinct melting peaks of partially crystallized hard segments for the PU materials.This structural arrangement potentially regu-lates both the rate and direction of bending in CD-Azo-PU and Azo-LPU.
Figure 4A shows the DSC thermogram from the first scan for CD-Azo-PU and Azo-LPU before and after 5 min of UV light irradiation.The glass transition temperature (T g ), melting temperature (T m ), and ΔH m derived from the DSC thermograms are summarized in Table 1.After exposure to UV light irradiation for 5 min, the ΔH m value for CD-Azo-PU showed a substantial increase of 42.9%, rising from 9.8 to 14.0 J g −1 .In contrast, the ΔH m for Azo-LPU experienced a decrease of 15.3%, dropping from 17.7 to 15.0 J g −1 (Figure 4B).The photoisomerization of Azo aggregated in hard segments in Azo-LPU from the trans to cis state promoted the dissociation of hydrogen bonding, further disturbing the formation of crystalline domains in Azo-LPU.The wide-angle X-ray diffractions (WAXS) and small-angle X-ray diffractions (SAXS) profiles supported the decrease in the crystal of Azo-LPU (Figures S11 and S12).This result was in good agreement with the ATR-FTIR results discussed above, which indicated that Azo-LPU bent away from the light source due to a decrease in crystallinity upon exposure to UV light irradiation, leading to an expansion of its exposed surface.
However, the crystallinity of CD-Azo-PU increased, and dissociation of hydrogen bond was inhibited upon UV irradiation (Figure 3A,D).This result was attributed to the part of the trans-Azo moiety complexed with γCD in CD-Azo-PU that exited from the γCD cavity through photoisomerization F I G U R E 5 Proposed mechanism of the photoresponsive behavior of (A) CD-Azo-polyurethane (PU) and (B) Azo-LPU.Upon UV irradiation, trans-Azo isomerized to cis-Azo, and the Azo moiety existing in the hard segment dissociated, leading to expansion of the film in the case of Azo-LPU.In contrast, γCD formed a complex with the trans-Azo and slides along the polymeric chains, leading to contraction of the film in the case of CD-Azo-PU.
from trans-Azo to cis-Azo induced the increased crystallinity and minor hydrogen bond dissociation.Subsequently the cis-Azo rearranged within the well-organized hard segment in the crystalline domains in CD-Azo-PU, ultimately increasing the crystallinity.Additionally, the observed increase in the T m in CD-Azo-PU, from 63.3 to 67.4 • C, supported the enhanced crystallinity after UV light irradiation.WAXS and SAXS profiles supported the re-formation of crystal by the released cis-Azo from the γCD cavity (Figures S11 and S12).These results demonstrated that CD-Azo-PU bent toward the light source because of increased crystallinity after UV irradiation, resulting in a contraction of its exposed surface.

Correlation between the crystallinity and bending rate
We have discussed the bending direction.Herein, we compared the crystallinity of CD-Azo-PU and Azo-LPU before UV irradiation to investigate the correlation between crystallinity and bending rate.
Before UV irradiation, CD-Azo-PU had two T m values at 63.3 and 82.0 • C, with a ΔH m of 9.8 J g −1 , while Azo-LPU had a higher T m at 98.4 • C and a higher ΔH m of 17.7 J g −1 (Figure 4A).The lower T m and ΔH m of CD-Azo-PU could be attributed to the reduced degree of hard segments organized in the crystalline domains due to the inclusion of γCD with Azo and the large size of TAcγCD itself, which inhibited the formation of hydrogen bonds. [55]These results indicated that the polymeric chains in CD-Azo-PU were able to move more freely than those in Azo-LPU.Consequently, CD-Azo-PU with low crystallinity showed faster bending kinetics than Azo-LPU with high crystallinity, even with comparable isomerization rates (Figure S8∼S9).

Proposed mechanism of photoresponsive behavior
The different bending behaviors of CD-Azo-PU and Azo-LPU after UV light irradiation can be clarified by the proposed internal structural changes, as shown in Figure 5.The UV light, being unable to permeate the opposite side, is absorbed solely on the surface of the photoresponsive PUs. [46]Consequently, CD-Azo-PU and Azo-LPU bend in response to UV light due to either the expansion or contraction of the irradiated surface, while the volume of the nonexposed areas remains constant.The resultant strain between the exposed and unexposed areas induces the bending behavior of CD-Azo-PU and Azo-LPU.Bending toward the incident light indicates surface contraction, while bending away indicates surface expansion.
Specifically, in CD-Azo-PU, UV light irradiation causes the Azo units to rearrange within well-packed hard segments in the crystalline domain in CD-Azo-PU after exiting from γCD complexes, thereby enhancing the crystallinity of CD-Azo-PU (Figure 5A).The crystallinity detected by DSC appears as the melting of the well-organized hard segment of PU.An increase in the well-organized hard segments causes denser physical crosslinks and contraction of irradiated surfaces.Furthermore, the end-to-end distance of the Azo molecules decreases through isomerization from transto cis-form, resulting in the surface contraction of CD-Azo-PU and subsequent bending toward the incident light.The faster bending rates of CD-Azo-PU compared to Azo-LPU are attributed to higher mobility of the polymer chains with reduced presence of hydrogen bonding and lower crystallinity due to the bulky γCD as the movable crosslinker.
In contrast, for Azo-LPU, after UV irradiation, the aggregation of the Azo units dissociates, leading to considerable dissociation of the ordered hydrogen-bonded C═O groups and a reduction in the crystallinity of Azo-LPU (Figure 5B).Similarly, a decrease in the well-organized hard segments causes looser physical crosslinks and expansion of the irradiated surfaces.Thus, the irradiated surface of Azo-LPU expands and bends away from the incident light.

CONCLUSION
We investigated the photoinduced deformation of two Azocontaining PU materials, namely, CD-Azo-PU with Azo and γCD serving as movable crosslinking points and the linear Azo-LPU.We demonstrated that introducing γCD as a movable crosslink in CD-Azo-PU changed the bending direction and caused an increased bending rate in response to UV irradiation.Upon exposure to UV light, the photoisomerization of Azo from the trans to cis state in Azo-LPU could trigger the dissociation of hydrogen bonding and disturb the formation of the crystalline domains, leading to a decrease in crystallinity.In contrast, in CD-Azo-PU, the dissociation of hydrogen bonds was less pronounced due to the complexation of the trans-Azo moieties with γCD.Instead, the part of Azo that was initially complexed with γCD exited from the γCD cavity upon UV irradiation, rearranged within the well-organized hard segments, and contributed to an increased crystallinity.The differential responses in the crystalline domains of CD-Azo-PU and Azo-LPU upon UV irradiation led to contrasting bending behavior.For CD-Azo-PU, the increased crystallinity caused the UV-exposed surface to contract, resulting in film bending toward the light source.Conversely, the decrease in crystallinity in Azo-LPU expanded the exposed surface, causing the film to bend away from light.
Our study provided insights into the different bending mechanisms of photoresponsive PU materials.The findings indicated that the bending direction and rate in response to UV irradiation could be controlled by manipulating the crystallinity of the PU materials through photoisomerization of the Azo moieties and their supramolecular interactions with γCD in the system.This knowledge could be used to design and develop new photoresponsive materials with tailored photoresponsive properties for various applications, including light-driven actuators and soft robotics.

A C K N O W L E D G M E N T S
This research was funded by Scientific Research on Innovative Area JP19H05714 and JP19H05721 from MEXT of Japan, the Core Research for Evolutional Science and Technology (CREST) program JPMJCR22L4 and JST COI-NEXT program JPMJPF2218, the establishment of university fellowships toward the creation of science technology innovation JPMJFS2125, Iketani Science and Technology Foundation (0341026-A, 0351026-A), the Asahi Glass Foundation, and the Yazaki Memorial Foundation for Science.The authors would also like to thank the Analytical Instrument Faculty of Graduate School of Science, Osaka University, for supporting the NMR and DSC measurements.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Chemical structures of the polyurethane (PU) materials.(A) CD-Azo-PU and (B) Azo-LPU.

F I G U R E 2
Photoresponsive properties of polyurethane materials containing Azo. (A) Schematic illustrations for evaluating the photoresponsiveness of the polyurethane materials.The photoresponsiveness was evaluated in terms of the flexion angle (θ).(B) Snapshots of the light irradiation tests (0.76 mW/cm 2 ) of CD-Azo-PU (20 mm × 5 mm × 70 μm) and Azo-LPU (20 mm × 5 mm × 70 μm).(C) Flexion angles of CD-Azo-PU and Azo-LPU (27∼164 μm) during ultraviolet (UV) light irradiation (0.76 and 70 mW/cm 2 ) for 5 min.and subsequently placed in the dark for 5 min.(D) The limited photoisomerization in the irradiation surface induced the bending due to the contraction of the irradiated surface in CD-Azo-PU.(E) Bending rate of CD-Azo-PU and Azo-LPU during UV irradiation calculated based on their initial bending behavior.(F) Maximum flexion angle of CD-Azo-PU and Azo-LPU.

F I G U R E 3
Proposed structure of the (A) ordered hydrogen-bonded region, (B) disordered hydrogen-bonded region and (C) free C═O region.Partial attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of (D) CD-Azo-PU and (E) Azo-LPU showing hydrogen bonding behaviors before and after UV irradiation for 5 min.The absorbance change of the ordered hydrogen-bonded C═O, disordered hydrogen-bonded C═O and free C═O of (F) CD-Azo-PU and (G) Azo-LPU before and after UV irradiation for 5 min.

F
I G U R E 4 (A) Differential scanning calorimetry (DSC) curves of the 1st scan for CD-Azo-PU and Azo-LPU before and after UV light irradiation for 5 min.The temperature range was from −100 to 150 • C, and the heating rate was 10 • C/min.(B) Change in the enthalpy of melting (ΔH m ) of CD-Azo-PU and Azo-LPU before and after UV light irradiation.TA B L E 1 T g , T m , and ΔH m of the 1st scan for the photoresponsive PU materials before and after UV light irradiation.