Photoinduced Reversible Solid-to-Liquid Transitions and Directional Photofluidization of Azobenzene-containing Polymers

Photoinduced reversible liquefaction and solidification of polymers enable processing and healing of polymers with light. Some azobenzene-containing polymers (azopolymers) exhibit two types of photoinduced liquefaction properties: photoinduced reversible solid-to-liquid transition and directional photofluidization. For the first type, light switches the glass transition temperature (Tg) values of azopolymers and induces reversible solid-to-liquid transitions. For the second type, polarized light guides solid azopolymers to flow along the polarization direction. Here, we compare the two types of photoliquefaction and discuss their mechanisms. Recent progresses and applications based on photoliquefaction of azopolymers are also highlighted.


INTRODUCTION
Solid-to-liquid transition is essential for polymer processing and repairing. In general, the solid-to-liquid transition of polymers is achieved by heating above glass transition temperature (T g ) values, where polymers change from hard glass states to soft rubber states. If the solid-liquid conversion of polymers is achieved using a non-thermal stimulus, not only could polymer reprocessing be achieved at room temperature, but various applications could be realized by utilizing different properties of the two switchable states of polymers.
Over the past decades, there have been many studies on directional photofluidization of azopolymers, [28] while photoinduced reversible solid-to-liquid transition is a recently developed research field. When the earliest relevant research appeared, [14] a minireview pointed out there are two types of photoliquefaction for azopolymers. [29] In this feature article, we discuss the azobenzene types, photoisomerization processes, light wavelengths, flow conditions, liquid presence conditions and mechanisms of the two types of photoliquefaction (Scheme 1), and introduce their latest development and applications.

COMPARISON OF TWO TYPES OF PHOTOINDUCED LIQUEFACTION
Azopolymers with photoinduced reversible solid-to-liquid transitions have azobenzene type chromophores (azobenzene derivatives are divided into azobenzene-type, aminoazobenzene-type, and pseudo-stilbene-type in an early review [30] ). Trans azopolymers have T g values above room temperature, while cis azopolymers have T g values below room temperature. These azopolymers are similar to other model polymers with trans and cis side chains. [31] In the process of liquefaction, ultraviolet (UV) irradiation induces trans-to-cis isomerization and liquefaction, while visible light induces cis-to-trans isomerization and solidification. After photoliquefaction, as long as the content of cis azpolymers is high, azopolymers remain liquid even when light is turned off. [15,29] Different from reversible solid-to-liquid transitions, the mechanism for directional photofluidization is not clear up to now. The corresponding azopolymers mostly contain pseudostilbene type azo chromphores. In addition, some azopolymers containing azobenzene-type and aminoazobenzenetype azo chromphores also have the property of directional photofluidization. [1] In most cases, polarized light induces trans-cis-trans cycles, and guides azopolymers to flow along the direction of polarization. The azopolymers only show fluid property under light irradiation. [15,29]

PHOTOINDUCED REVERSIBLE SOLID-TO-LIQUID TRANSITIONS
The study of photoinduced reversible solid-to-liquid transitions of azopolymers should be traced back to the small molecule derivatives of azobenzene. In 1937, Hartley discovered that cis and trans azobenzene isomers had different melting points. [32] In 2012, Akiyama and Yoshida, and Okui and Han, respectively, observed photoinduced solid-to-liquid transition of azobenzene-containing small molecules at room temperature. [33,34] After that, more azobenzene small molecules with similar characters   were reported. [35−37] Because light can change melting points of small molecules, we hypothesized that light may switch T g values of azopolymers. In 2017, we synthesized a typical azopolymer P1 with photoliquefaction property and demonstrated the photoliquefaction mechanism of P1 ( Fig. 1). [14] We take P1 as an example to show photoinduced reversible solid-to-liquid transitions. Azopolymer P1 contains azobenzene-type chromophores on its side chains. The azobenzene groups underwent trans-to-cis isomerization via UV irradiation, and cis-to-trans isomerization via visible light irradiation. We demonstrated that the T g of trans-azopolymer P1 (48 °C, measured by differential scanning calorimetry) was higher than room temperature and it was solid, while the T g of cis polymer (−10 °C) was lower than room temperature and it was liquid. Therefore, P1 underwent reversible solid-to-liquid transitions upon alternating irradiation of UV and visible light. The transformation of azopolymer P1 from solid powder to liquid droplet was observed using optical microscopy ( Fig. 1b). The force-piezo position curves obtained with a spin-coated film of P1 showed reversible signs of a hard surface and a soft surface (Figs. 1c and 1d). The study of photoswitchable T g provided a new strategy for the application development of photoresponsive polymer. Based on photoinduced reversible solid-to-liquid transitions and photoswitch-able T g , we further explored a variety of new functions and applications of azopolymers.
The flow property of liquefied cis P1 enables the repairing of damaged hard coatings. Scratches on a P1 film were repeatedly healed upon locally and alternately irradiated with UV light and visible light. As a result, the damaged part was converted to the liquid state and the scratch was healed due to capillary flow of the polymer (Fig. 2a). Similarly, the surface roughness of patterns could be effectively reduced by the reversible solid-to-liquid transitions (Fig. 2b). [14] Especially, trans P1 has an adhesion strength up to 1.02 MPa (substrates: quartz, glued area: 0.375 cm 2 ). After photoliquefaction, the adhesion strength of cis P1 decreased to 0.08 MPa. [20] So, P1 with switchable adhesion can be used for transfer printing (Fig. 2c), [14] and reversible adhesives (Figs. 3a and 3b). Related studies of reversible adhesion of azopolymers have been reported in some works, [21−23] and summarized in a recent review. [38] Photo-controlled adhesive P1 was suitable for various kinds of substrates and underwater conditions. Azopolymers with large adhesion force transition are expected to meet the requirement of industrial application. In addition, entangled linear P1 was used as healable and reprocessable photoactuators (Fig. 3c). The photoinduced reversible solidto-liquid transitions contributed to the photoinduced bend-  ing of the nonaligned azopolymer films. [24] As a side-chain azobenzene polymer, structure of P1 presented a typical design reference. Azopolymers could retain the solid-to-liquid transition property after properly changing the length of spacer group and tail group. [21,39] Recently, different research groups have studied the properties and applications of these side-chain azopolymers. P2 also showed photoliquefaction (Figs. 4a and 4b). The thermal conductivity of P2 changed after liquefaction. [40] In addition, precise control of the micro-nano structures was achieved by utilizing the high spatial resolution of light. Yu's group presented a facile fabrication method of multiple nanopatterns in azopolymer P3 film by combining athermal nanoimprint lithography and photolithography. [3] The key of multiple nanopatterning lay in the phototunable mechanical properties of the azopolymer upon photoirradiation with different wavelengths (Figs. 4c and 4d). Chen et al. demonstrated the light-induced nanowetting method for preparing azopolymer nanoarrays with well controlled patterns using designed photomasks. [27] In addition to the typical side chain azobenzene homopolymers, some other types of azopolymers also exhibited photoinduced solid-to-liquid transition properties, including side chain copolymers [23,25] and main chain polymer. [41] Kihara and co-workers reported on an ABA-type triblock copolymer P4 with azopolymer termini. [23] This ABA-type azopolymer had reversible adhesive properties controlled by light (Figs. 5a and 5b). It had a higher flexibility than the corresponding homo-azopolymer. Yu's group developed a photocontrollable flexible microtube consisting of a copolymer P5 inner layer and outer flexible supporting layer. [25] Benefiting from photoswitchable T g , the damaged P5 layer of microtube was healed with light ( Figs. 5c and 5d). With the combination of athermal and high spatiotemporal resolution of light irradiation, photoinduced reversible solid-to-liquid transition at room temperature is a new frontier field in the study of stimuli-responsive materials.

DIRECTIONAL PHOTOFLUIDIZATION
Directional photofluidization has been studied for more than 15 years, which has been summarized by some reviews. [1,28,42] In this section, we will focus on the characteristics of liquid-like behaviors in directional photofluidization. We summarize its developments, new progresses, and differences with photoinduced reversible solid-to-liquid transitions.
The first two interrelated reports about directional photofluidization are photoinduced SRGs on azopolymers. [43,44] SRGs are created upon exposure of interference laser beams on azopolymer films. These works do not specify the flow of azopolymers, but set a precedent. In 2005, two pioneering works demonstrated the fluid-like properties of azopolymers under light irradiation. Wang and co-workers found that colloidal spheres of the azopolymer P6 stretched after being exposed to interfering polarized Ar + laser beams (Figs. 6a−6e). [45] Karageorgiev et al. reported a light-induced isothermal transition of azopolymer P7 film from an isotropic solid to an anisotropic liquid state (Figs. 6f−6h). In both works, the flow direction of the azopolymers was along the light polarization direction. [46]  After the discovery of directional photofluidization, it has attracted a lot of attention by researchers. Some groups utilized this property to develop micro and nanostructures. [47−51] Some found that polarization-controlled three-beam interference could produce micro-ellipsoidal cap arrays on P8 films. [52,53] The arrays deformed along the polarization direction of the incident light (Figs. 7a and 7b). Lee and co-workers developed photofluidization lithography based on directional photofluidization. [54−56] Combined with micro-moulding in capillaries, large-area, high resolution nanostructures were generated on P9 film with the aid of directional photofluidization (Figs. 7c and 7d). In addition, Lee et al. also made use of polarized light to achieve the repairing of wearable electrical conductors. [57] In recent years, some more complex microstructural techniques have been developed. For example, Lin and co-workers fabricated breath figure arrays of azopolymers [58,59] and found that polarized light with different directions converted P10 arrays into different structures (Figs. 7e−7j).
Although directional photofluidization has been studied by researchers for more than 15 years, the mechanism of photoliquefaction is still under debate. Some underlying mechanisms have been summarized in a previous review, [1] but none of the proposed mechanisms could explain the existing phenomena perfectly. Recently, some new opinions have been presented. Saphiannikova et al. disagreed with the concept of photofluidization and proposed the view that anisotropic light-induced stress contributed to the structure changes of azopolymers. [60] This mechanism cannot explain some photofluidization when the films are uniformly illuminated whit circularly-polarized or non-polarized (lamp) light. [57] Pellerin and co-workers put forward a new mechanism: [61] upon visible light irradiation, P11 underwent repeated transcis-trans isomerization (Fig. 8). Such a molecular motion is equivalent to the increase of local environment temperature of the azo groups (some simulation approaches reproduced relatively well the behavior observed using the real azobenzene molecule [62−64] ). When the local temperature exceeded T g of trans-P11, photofluidization occurred. However, why the flow direction is polarization-dependent is difficult to understand. Another possible mechanism is that trans-cis-trans cycling could decrease T g of azopolymers. [65] Some works showed that repeated trans-cis-trans cycles cause a decrease of certain mechanical properties upon irradiation, [66−68] and "photosoften" the polymer matrix and enhance polymer chain mobility. [69] During irradiation, the population of cisisomers in pseudo-stylbene type azopolymers rose, and softening was observed under linearly polarized, circularly polarized and nonpolarized visible illumination. [1] Decreased T g may contribute to flow/softening of azopolymers and directional fluidization is correlated with the light polarization. However, the T g values of pseudo-stilbene type azopolymers under irradiation are difficult to measure because the short half-lives of cis pseudo-stilbene azo groups and insufficient light penetration hinder the measurement of T g . In a word, the mechanism of directional photofluidization needs further exploration.

CONCLUSIONS
In summary, recent progresses on two types of photoliquefaction of azopolymers are reviewed. We presented their differences in azobenzene type, photoisomerization processes, light wavelength, flow condition, the conditions for liquefaction and mechanisms. We also highlighted their applications. Regarding the future perspective of photoliquefaction of azopolymers, there are several notable challenges and questions: (1) How to clarify the mechanism of directional photofluidization? If the two types of liquefaction have similarity in mechanism (trans-  to-cis isomerism and trans-cis-trans cycling both decrease T g values of azopolymers and induce liquefaction), how to monitor the T g of pseudo-stilbene type azopolymers during rapid isomerism? (2) Whether the applications of these azopolymers could be expanded by combining with other functional materials? Composites made up of azopolymers and nanoscale materials could still have photoliquefaction properties. (3) Moreover, could we design new photoliquefied polymers based on other photoresponsive groups or design other forms of stimuli-liquefied polymers? Besides azobenzene, spiropyran and diarylethene also have photoisomerization properties. Polymers with these groups are likely to show similar photoliquefaction behaviors.