Fully Room-Temperature Reprogrammable, Reprocessable, and Photomobile Soft Actuators from a High-Molecular-Weight Main-Chain Azobenzene Crystalline Poly(ester-amide)

Abstract

Azobenzene (azo) polymer photoactuators with full room-temperature reprogrammability, reprocessability, and photomobility are highly desirable for large-scale applications, but their development remains a daunting challenge. Herein, a strategy is first presented for fabricating such advanced photoactuators from a highmolecular-weight main-chain azo crystalline poly(ester-amide) (PEA) prepared via Michael addition polymerization. This azo PEA can be readily processed into both physically cross-linked, uniaxially oriented fibers and films with high mechanical robustness and reversible photoinduced bending/unbending at room temperature. Importantly, the presence of both amide unit-induced hydrogen bonding and crystalline domains in such films and fibers endows them with dynamic, yet stable cross-linking points, which enable their easy reprogrammability under strain at room temperature into various three-dimensional (3D) shapes (e.g., film helicoid and spiral ribbon, fiber spring) capable of showing completely different shapedependent photomobile modes. In particular, these reshaped photoactuators can maintain their accurate 3D shapes and highly reversible photoinduced motions even after being kept at 80 °C for 20 days or at 100 °C for 2 days. They can also be reprocessed and recycled from solution at room temperature. Such a multifunctional main-chain azo crystalline PEA can serve as a versatile platform for fabricating various photoactuators with desired 3D shapes and motion modes under mild ambient conditions.

Enhancing the performances of physically cross-linked photodeformable main-chain azobenzene poly(ester-amide)s via chemical structure engineering

“…In this context, it is worth mentioning that there should also exist crystalline domain-induced physical cross-linking networks in these unaxially oriented samples, as described in our previous report. 25 However, the above results indicated that they hardly made any contribution to the photomechanical effects of the studied fibers and films. It is also noteworthy here that although the UV light-induced disruption of H-bonding 68 and crystallinity 53 has been reported to be responsible for the photomobile behaviors of some main-chain azo polymers, the photoinduced azo switching proved to have negligible influence on the H-bonding and crystallinity of our photodeformable samples under the studied photoirradiation conditions (Fig.…”

“…, PEA- n -3 h or briefly PEA- n ). 25 These Michael addition polymerization systems remained homogeneous throughout the whole polymerization processes. Orange-yellow azo PEAs were obtained in high yields (90–92%, entries 1, 4 and 7 in Table 1) after the reaction mixtures were precipitated into MeOH and thoroughly washed with MeOH until no M-EA- n was detected by thin layer chromatography.…”

“…The fibers of the main-chain azo PEAs [including PEA-4- t ( t = 0.5, 1, 3, 6, and 12 h), PEA- n ( n = 2, 4, 6), and PEA-2b] were readily obtained by using the simple melt spinning method following a previously reported method. 22–26 They were demonstrated to have a homogeneous azo mesogen alignment along the fiber axes by a sharp contrast inversion every 45° upon rotating the samples with respect to the analyzer under the POM (with a crossed polarizer and an analyzer) observation (Fig. 5a and Fig.…”

“…11,12 Among the presently developed photodeformable azo polymers, physically cross-linked ones have drawn great attention because of their apparent advantages over the chemically cross-linked ones including their less complex preparation (no chemical cross-linking procedure is required) and high recyclability and reprocessability. 9,10,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Particularly, those with main-chain structures are of high interest because they can show higher mechanical robustness and enhanced photomechanical effects owing to their more rigid backbone structures and much stronger chain anisotropy. 1,4,28 Over the past years, our group has developed a series of physically cross-linked photodeformable main-chain azo polymers with supramolecular hydrogen-bonded (H-bonded) networks via Michael addition polymerization, [22][23][24][25][26] mainly because of its mild reaction conditions, high tolerance of functional groups, and rapid rates.…”

“…9,10,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Particularly, those with main-chain structures are of high interest because they can show higher mechanical robustness and enhanced photomechanical effects owing to their more rigid backbone structures and much stronger chain anisotropy. 1,4,28 Over the past years, our group has developed a series of physically cross-linked photodeformable main-chain azo polymers with supramolecular hydrogen-bonded (H-bonded) networks via Michael addition polymerization, [22][23][24][25][26] mainly because of its mild reaction conditions, high tolerance of functional groups, and rapid rates. [29][30][31] They have both ester and secondary amino groups, 22,23 both ester and amide groups [poly(ester-amide)s (PEAs)], 24,25 and both ester and urea groups [ poly(ester-urea) (PEU)] 26 in their backbones.…”