Chapter 1
Light-responsive Surface: Photodeformable Cross-linked Liquid-Crystalline Polymers Based on Photochemical Phase Transition

Lang Qin and Yanlei Yu

Fudan University, Department of Materials Science, 220 Handan Road, Shanghai, 200433, China

1.1 Introduction

Smart materials have drawn wide attention from physicists and chemists in recent years due to their superior properties. As a kind of novel material, smart materials have potential in applications in artificial muscles and soft actuators [1-5], biomedical systems [6], and so on. Among these smart materials, photodeformable materials promise significant roles in converting light energy into mechanical actuation. Compared to other stimulus-driven methods, such as by heat [7, 8], pressure [9], pH variations [10], electric field [11, 12], and magnetic field [13], light is a particularly ideal stimulus, since it is a clean energy and can be precisely and conveniently manipulated in terms of wavelength, intensity, and polarization direction. Besides, it is known that polymer matrix materials have many advantages, such as good flexibility, excellent corrosion resistance, high process ability, moderate mechanical strength, and light weight. Therefore, polymers that can undergo photoinduced deformation are utilized in many studies and definitely merit further investigation.

As a combination of cross-linked polymers and liquid crystals (LCs), cross-linked liquid-crystalline polymers (CLCPs) exhibit unique properties such as elasticity, anisotropy, stimuli-responsiveness, and molecular cooperation effect [14-16]. CLCPs, as three-dimensional networks, are able to undergo controllable and reversible shape change in response to an external stimulus. Since most of the CLCPs are chemically cross-linked, they are suitable to be used in a dry state. The incorporation of azobenzene chromophores into CLCPs can provide photoresponsiveness and induces a reduction in LC alignment and causes deformation upon exposure to UV light as a result of photochemical reaction of azobenzene units [16-18].

In this chapter, we mainly describe photoinduced deformation observed in azobenzene-containing CLCPs, focusing our attention on the factors affecting photodeformation, deformation forms, and light-driven soft actuators in both the macro- and microscale. The mechanism of deformation based on photochemical phase transition in CLCPs is also included. Our goal is to summarize the development of photoinduced behavior of CLCPs and provide an insight into their potential applications as light-driven devices as well as on the recent progress in this field.

1.2 Photochemical Phase Transition

De Gennes proposed the possibility of using CLCPs as artificial muscles, by taking advantage of their substantial contraction in the direction of the director axis [19]. The basic principle behind the shape variation is the conformational change of the polymer backbone at LC-isotropic phase transition [20]. The polymer chains in an anisotropic LC environment deviate from the isotropic conformation. As a consequence, the coil-dimensions parallel and perpendicular to the LC director are different. If the CLCPs lose their anisotropic properties, which results from the decrease in alignment order of an LC, an isotropic chain conformation will be adopted and the whole sample will have to change its shape. For example, if the nematic CLCP films are heated toward the nematic-isotropic phase transition temperature, the nematic order will decrease, and when the phase temperature is exceeded, the CLCPs exhibit a general contraction along the alignment direction of the mesogens, and revert to their original size by expanding if the temperature is lowered back below the phase transition temperature. There have been a number of works on thermal-induced deformation of the CLCPs based on LC-isotropic phase transition ([21-25]). However, it would be expected that if the alignment of an LC can decrease by light, then this would be accompanied by equally dramatic mechanical responses.

Cooperative motion of molecules in LC phases may be most advantageous in changing the molecular alignment by external stimuli. The alignment of the majority of LC molecules will be changed if the alignment of a small portion of LC molecules is changed in response to an external stimulus. This phenomenon illustrates that LC molecules only require a small amount of energy to change the alignment: the energy needed to induce an alignment change of only 1 mol% of the LC molecules is enough to bring about the alignment change of the whole system. In other words, a huge amplification is possible in LC systems. When a small amount of a photo-chromic molecule is added into LCs and the resulting guest/host mixture is irradiated to cause photochemical reactions of the photochromic guest molecules, an LC-isotropic phase transition of the mixtures can be induced isothermally. Ikeda et al. reported the first explicit example of a nematic-isotropic phase transition induced by trans-cis photoisomerization of a nematic LC with an azobenzene guest molecule dispersed in it [25].

Azobenzene is a well-known chromophore that has two configurations. It undergoes trans to cis photoisomerization upon exposure to UV irradiation and irradiation with visible light leads to a cis to trans back-isomerization process. Therefore, azobenzene is the most frequently used photochromic moiety in photoresponsive polymers. The rod-like trans form of the azobenzenes stabilizes the phase structure of the LC phase, whereas its bent cis isomer tends to destabilize the phase structure of the mixture. As a consequence of two different conformations, the LC-isotropic phase transition temperature (Tc) of the mixture with the cis form (Tcc) is much lower than that with the trans form (Tct). If the temperature of the sample (T) is between Tct and Tcc and the sample is irradiated to cause trans-cis photoisomerization of the azobenzene guest molecules, Tc decreases because of the increase of the cis form. When Tc becomes lower than the irradiation temperature T, LC-isotropic phase transition of the sample is induced. The sample reverts to the initial LC phase through cis-trans back-isomerization due to reversible photochromic reactions. Thus, phase transitions of LC systems can be induced isothermally and reversibly by photochemical reactions of photoresponsive guest molecules (Figure 1.1) [26].

Figure 1.1 Phase diagrams of the photochemical phase transition of azobenzene/LC systems (N, nematic; I, isotropic).

(Ikeda 2003 [26]. Reproduced with permission of Royal Society of Chemistry.)

Ikeda et al. reported the first example of the photochemical phase transition in liquid-crystalline polymer (LCP)s [27-29]. They demonstrated that by irradiation of LC polymers doped with low-molecular-weight azobenzene derivatives with UV light to give rise to trans-cis isomerization led to a nematic-isotropic phase transition; upon cis-trans back-isomerization, the LCPs reverted to the initial nematic phase. Although doping the chromophores in a matrix is most convenient, the resultant LCP systems often exhibit instabilities, such as phase separation and microcrystallization. This occurs because of the mobility of the azobenzene chromophores in the matrix and the propensity of the dipolar azobenzene units to form aggregates. In order to address the problem, higher quality LCP systems are obtained when the azobenzene moiety is covalently bound to the host polymer matrix (Figure 1.2). The azobenzene moiety plays a role both as a mesogen and a photosensitive group in azobenzene derivatives that form an LC phase.

Figure 1.2 Schematic illustration of reversible LC-isotropic photochemical phase transition.

(Wei and Yu 2012 [3]. Reproduced with permission of Chinese Physical Society.)

1.3 Photodeformation

1.3.1 Photoinduced Contraction and Expansion

Finkelmann et al. reported pioneering work on photodeformation of a monodomain nematic CLCP, which had a polysiloxane main chain and azobenzene chromophores at cross-links. The CLCP film generated a contraction by 20% upon irradiation with UV light to give rise to the trans-cis isomerization of the azobenzene moieties (Figure 1.3) [21]. It is necessary to take photomechanical effects into consideration: the subtle variation in nematic order upon trans-cis isomerization causes a significant uniaxial deformation of the LCs along the director axis when the LC molecules are strongly associated by covalent cross-linking to form a three-dimensional polymer network. The contracted elastomer thermally returned to the original state due to the cis-trans back-isomerization after stopping irradiation. Terentjev et al. prepared CLCPs with a wide range of azobenzene derivatives as photoresponsive moieties and examined the deformation behavior of CLCPs upon exposure to UV light [30, 31].

Figure 1.3 Photo-induced contraction of CLCP prepared from 1a-f. ? = 313 K, ? = 308 K, = 303 K, * = 298 K. (Inset) Recovery of the contracted CLCP at 298 K after irradiation was switched off.

(Finkelmann 1987 [14]. Reproduced with permission of Wiley.)

Keller and coworkers synthesized oriented monodomain nematic side-on CLCPs containing azobenzenes (Components 2a,b) by photopolymerization with a near-infrared photoinitiator [32]. The photopolymerization was...