Shape Memory: An Efficient Method to Develop the Latent

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Shape Memory: An Efficient Method to Develop the Latent Photopatterned Morphology for Elastomer in Two/Three Dimension Jing Bai and Zixing Shi* School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composite Materials, and Shanghai Key Lab of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, 800 Dungchuan Road, Minhang, Shanghai, People’s Republic of China S Supporting Information *

ABSTRACT: Shape memory behavior was applied here as a new approach for developing the latent photopatterned morphologies in two/three dimension (2D/3D) on the modified poly(styrene-block-butadiene-block-styrene) (SBS). By attaching anthracene groups onto the SBS chains, the elastomer frozen in the deformed state was photopatterned via the photodimerization of anthracene. Upon thermal treatment, shape memory process could effectively develop the latent photopatterning induced 2D−2D and 2D−3D shape transformation. Due to the reversible dimerization of anthracene, the photoinduced patterns and the shape conformation could be erased and redeveloped for multiple times.

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arbitrary-sized patterns and the reversibility of photodimerization induced cross-linking can reprogram photopatterns and shape development in multiple times. In this paper, we chose the typical thermoplastic elastomer SBS13−18 as the soft matrix since it has good thermal-elasticity for shape memory behavior. Then, anthracene was selected as a UV-light responsive group to incorporate into the backbone of the SBS for the fabrication of the UV-sensitive elastomer (USBS; Figure 1). The branching ratio of anthracene groups is approximately 10% to the double bonds on the SBS chains. The detailed synthesis process and structure analysis (Figures S1 and S2) are discussed in the Supporting Information. Anthracene groups undergo [4 + 4] photodimerization upon the UV-light of λ > 300 nm irradiation, while the formed dimers cleave to the original anthracene groups upon thermal treatment >150 °C19−22 (as shown in Figure 1). The change of mechanical properties (Figure S3 and Tables S1 and S2), inner morphologies (Figure S4), and solubility (Figure S5) of the USBS after the 365 nm UV-light irradiation and thermal

hape actuation to form the designed configuration with a specific surface topographical feature has been intensively investigated owing to its great potential application in sensors, actuators, robotics, smart systems,1,2 and other areas. Here, shape memory behavior (SMB), as a novel and effective approach, is performed on developing the morphologies of the photopatterned elastomer and this method can precisely and simultaneously program both the shape and the surface topography stimulated by thermal treatment without the presence of any solvents,1,3,4 sophisticated molding equipment,5−7 or precise physical contact.8−10 In this method, the elastomer sheet in the temporary shape (the deformed state) is transferred from isotropic to anisotropic state via the reversible photodimerization induced cross-linking with the aid of the designed photomasks. As a result, upon thermal treatment to trigger the shape memory behavior, the photopatterned elastomer does not recover to the original permanent shape, but exhibits a new programmed permanent shape in both two and three dimensions (2D/3D) via the combination of its original elasticity and the new additional photopatterning induced plasticity. Unlike the conventional photolithographic approaches that involve complex etching and developing procedures, SMB can realize the fabrication of multiple and complex patterns without any solvent or surface photoresists deposition.11,12 Furthermore, SMB can be performed on any © XXXX American Chemical Society

Received: May 31, 2017 Accepted: September 1, 2017

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DOI: 10.1021/acsmacrolett.7b00403 ACS Macro Lett. 2017, 6, 1025−1030

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ACS Macro Letters

Figure 1. Process for the fabrication of the anthracene-branched SBS(USBS) elastomer, the photodimerization of anthracene, and the thermalinduced dedimerization of anthracene dimers on the USBS chains.

Scheme 1. Process of Fabricating 2D Patterns and 3D Shape Shifting Behavior

treatment are studied and discussed in the Supporting Information. The photodimerization of anthracene can be used to form the latent cross-linked patterns with the aid of the photomasks for obtaining USBS in the temporary state with an inhomogeneous structure (Scheme 1). On the course of SMB upon thermal treatment, the shape recovery to the permanent state can develop the programmed 2D/3D shape based on its photopatterning induced heterogeneous liberated stress inplane. As the patterned sample is heated at 150 °C, the anthracene dimers are broken (as shown in Figure 1) and the

sample recovers to its original homogeneous state. Then, the designed shape is totally erased (as shown in Figure 2D). As a result, 2D/3D shape actuation can be developed and erased for many times, which is hardly achieved in the traditional photolithographic approaches.23,24 Scheme 1 further illustrates the detailed SMB for developing the photopatterned shape morphologies in 2D/3D. A USBS sample was strained at 80 °C followed by freezing the deformed USBS sample at room temperature. The fabricated USBS sheets in the deformed (temporary) state with different uniaxial strains were exposed to the 365 nm UV-light irradiation 1026

DOI: 10.1021/acsmacrolett.7b00403 ACS Macro Lett. 2017, 6, 1025−1030

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Figure 2. (A) Sample that was irradiated and stretched to 50% strain during the UV-light irradiation process before heating at 80 °C. (B) Sample in (A) heated at 80 °C for 5 min. (C) Sample in (A) heated at 80 °C for 10 min. (D) Sample in (C) heated at 150 °C for 10 min. The mask used is shown on the top left corner of the picture.

Figure 3. (A) Sample that was irradiated and stretched to 100% strain during the UV-light irradiation process. (B) Sample in (A) heated at 80 °C for 2 min. (C) Sample in (A) heated at 80 °C for 5 min. (D) Sample in (A) heated at 80 °C for 10 min. The original shape of the sample is 5 mm in width and 30 mm in length. The mask used is shown on the top left corner of the picture.

as shown in Figure S6. In the shape memory process, NU and CU regions in the sheet of USBS exhibit different shape response behaviors. NU regions of the sample recover to the original permanent state without the photoinduced crosslinking, but CU regions cannot recover to the original permanent state showing a different elasticity behavior for the chemical structure of USBS in CU region has transferred from the linear structure to the cross-linked state. As a result, the nonuniform internal stress in the sheet of USBS is quickly built upon heating to 80 °C, which finally triggers developing the latent photo-cross-linking-induced patterns on the USBS sheet in a new permanent shape. Via the comparison of these patterns on the samples with two different strains (Figure S7), it is found that the height of the patterns is increased from approximately 0.1 mm with 10% strain to approximately 0.5 mm with 50% strain and the patterns formed on the sheet of USBS with 50% strain are more detectable. Therefore, the high strain in the fabrication of deformed USBS sheets (temporary shape) can produce much clearer patterns during SMB (Figure S7).26 This phenomenon is explained with the fact that the high strain for building the temporary shape leads to a more unbalanced energy state among two different regions of USBS. We further fabricated more complex patterns with irregular features on the surface of USBS as an application of an antifake label with this method. The sample of USBS in the temporary shape was first deformed with 50% strain, and then the deformed sample was irradiated with the 365 nm UV-light covered with the photomask having our university logo for 10 min (Figure 2A). After heating the sample at 80 °C, the logo was gradually extruded on the surface and observed clearly after 10 min (Figure 2C). Much more interesting, the pattern formed on the surface was completely erased (Figure 2D) after heating the sample at 150 °C for 10 min due to the dedimerization of anthracene dimers. The patterns on the sample can be developed and erased for many times via the decross-linking induced erasing. Therefore, developing and erasing the patterns on the surface of USBS in this convenient way provide great flexibility in designing the complex patterns on the surface of soft materials.

covered with a photomask. The photomasks contain black and transparent features with different geometrical shapes. Upon the 365 nm UV-light irradiation, the light-exposed regions of the sheet covered with transparent part of the mask contained the cross-linked USBS (CU) and the light-protected regions of sheet under the black part of the mask had uncross-linked USBS (NU). With the external thermal treatment, CU and NU regions can function as the distinct different compositions to develop the 2D/3D shape via the unbalanced in-plane force from the photopatterning-induced nonuniform internal structure. The SMB performed on developing patterns and the lateral shape shifting behavior of USBS sheets has the following advantages: (1) it provides a simple and versatile strategy for patterning the soft materials with different geometric information, which determines the final structure and shape morphing manner in 2D/3D. (2) It provides an easy method to form the nonuniform structure for a USBS sheet in a reversible way and the shape of the soft material can be repaired, restored, and recycled to fabricate new configurations via the reversible dimerization of anthracene. (3) Through designing the photomasks with different patterns, the USBS can be selectively programed to achieve 2D−2D patterns or 2D−3D shapetransformation via the thermal-induced SMB. To illuminate a rationale for the particular inhomogeneous structure of USBS in the SMB process, we first used SMB to develop the 2D photoinduced patterns on the temporary shape with two different (10% and 50%) strains and various photomasks with circles, squares, and diamonds as CU regions. As shown in Figure S7 and Scheme 1, upon heating the irradiated sample at 80 °C to trigger the shape recovery behavior, the new permanent shape of USBS was developed to exhibit the well-defined circle, square and diamond patterns commensurate with the photomasks. The mechanism for this development of the photoinduced patterns by SMB is based on the heterogeneous structure and can be explained in the specialized theory reported in Anthamatten’s work.25 We have investigated the effect of the photoinduced plasticity on the elastic shape memory behavior, 1027

DOI: 10.1021/acsmacrolett.7b00403 ACS Macro Lett. 2017, 6, 1025−1030

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Figure 4. Samples stretched to 50% (A1−A3), 100% (B1−B3), and 200% (C1−C3) strain and irradiated with 365 nm UV-light for 10 min covered with different patterned photomasks having strip angles 30°, 45°, and 60°. These samples are 8 mm in width and 20 mm in length. The masks used are shown on the top left corner of the pictures.

was not observed for the USBS sheet at angle of 30° with 50% strain but these stripe patterns were clearly developed on the surface of USBS. Increasing the angle to 45° resulted in the formation of the helix structure. More compact helix structure was obtained via further increasing angle to 60°. Therefore, the morphology of 3D helix development can be controlled by strip angles. Similar results are also obtained for the USBS sheet at the same angle with different strains. As shown in Figure 4, it was observed that more compact helix was formed upon increasing the strain from 50% to 200%. Therefore, both the angle and the strain are used to tailor the structure of the helix shape. As the angle is below 45°, bigger strain (>50%) is needed to realize the 3D helix transformation. Increasing the angle has a similar effect on the helix shape with increasing the strain. Much more interesting, the 3D helix shape development can be repeatedly produced via the reversibility of the photodimerization of anthracene. Therefore, we can easily transfer the helix shape of a USBS sample from left-handed to righthanded chirality. As shown in Figure 5B, the sample transformed from a flat to the left-handed helix with −60° of strip patterns. Such helix can be easily erased to recover the original plane state upon thermal treatment at 150 °C for 10 min due to the dedimerization of anthracene dimers. When the erased sample was irradiated with the 365 nm UV-light covered with the photomask with the angle of 60°, the right-handed helix was formed as shown in Figure 5D. Similarly, the righthanded helix was erased upon thermal treatment and returned to a flat (Figure 5A or C). The shape transformation shown in Figure 5 proves that the chiral helix shape can be easily erased and rebuilt due to the reversibility of anthracene dimerization. During the shape deformed process, the photodimerization of anthracene leads to the formation of a heterogeneous structure, while in the thermal treatment process (at 150 °C), the dedimerization liberates the chains of the anthracene-branched SBS from the cross-linked network to recover to the original

Based on the results of the fabrication of 2D patterns via SMB, we further explore this novel method to develop the 3D structure transformation on a latent specific photopatterned USBS sample. To study this typical planar-to-helical shape transformation behavior, we first designed the photomasks having alternating black (1 mm in width) and transparent strips (1 mm in width). The strips are aligned along the x-direction with different angles, which are used to tailor the morphologies of helical development. After irradiation under this type of photomasks, the patterned sheet underwent the transition to a helix upon thermal treatment at 80 °C. As a typical example, Figure 3 illustrates the developing process for the planar-helix transformation of the temporary USBS having 100% strain. After heating the irradiated sample at 80 °C, the sheet of USBS (60° angle) immediately began to shrink and crimp and finally formed the helix structure after 5 min. In the meantime, the strip protruded on the surface of USBS in 2D on the course of forming the helix structure. Therefore, we can simultaneously realize the formation of 3D shape with 2D stripe patterns. Similar shape transformation behaviors are also observed in the liquid crystalline elastomers (LCE).27−29 However, the mechanism of shape transformation in USBS is quite different from that in LCE. For LCE, the shape shifting behaviors are achieved by the orientation switch between two different phase structures of LCE triggered by a temperature jump or light irridiation.27 Take the azobenzene-based polymers for example, the light-induced cis−trans isomerization reaction of azobenzene generates the reversible conformation shift at the molecular level and induces shape change of the LCE macroscopically.28,29 While the shape transformation in our system comes from inhomogeneous cross-linked structure via the photomask controlled regional dimerization. Furthermore, our method offers unprecedented flexibility in tailoring the morphologies of the helix shape via the strip angles, the strain and the reversible dimerization of anthracene. As shown in Figure 4, it was found that the helix development 1028

DOI: 10.1021/acsmacrolett.7b00403 ACS Macro Lett. 2017, 6, 1025−1030

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: + 86-21-54747445. Tel.: + 8621-54743268. ORCID

Zixing Shi: 0000-0002-7659-5050 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (No. 51641304) and the Natural Science Foundation of Shanghai (16ZR1416600) for financial support.

Figure 5. Shape transformation process of the sample in a reversible way. The original shape of the sample is 5 mm in width and 30 mm in length.

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homogeneous structure. Therefore, the 3D helix shape development of the sample can be defined repeatedly One important property of USBS should also be declared that the material exhibits the self-healing ability without ruining the patterns and the latter shape conformation (Figure S8). For anthracene groups are connected to the polymer chains via esterification, the dynamic transesterification is responsible for this self-healing process. The detailed study and discussion on the self-healing capability is shown in the Supporting Information. In summary, we synthesized anthracene-modified SBS(USBS) that can be photo-cross-linked under the 365 nm UV-light irradiation and de-cross-linked upon thermal treatment. Via the local photo-cross-linking for anthracene dimerization, the latent patterns are written on the deformed shape, and such latent patterns can be effectively developed in 2D/3D via SMB. The dedimerization of anthracene dimers makes this type of transformation totally erasable and the new reconfiguration can be performed. This work using SMB as a developing method provides a convenient way for producing thermal-responsive adaptable materials to form the designed shape in 2D/3D without using sophisticated equipment and procedures.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00403. Materials, synthesis, and characterization of the small molecular and the modified polymers, the preparation of the films, measurements, UV−vis spectra trace, and UVlight tunable mechanical properties for USBS under the UV-light irradiation and thermal treatment, the morphologies and the solubility change of the UV-sensitive elastomer upon the UV-light irradiation and thermal treatment, the photoinduced cross-linking (plasticity) and the erasing process in thermal mode during the process of shape memory behavior, photoinduced patterns on the temporary shape with two different (10% and 50%) strains using photomasks with circle, square, and diamond as CU regions. Self-healing process of the patterned sample (PDF). 1029

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