Azo Polymers

This chapter is the introduction of this book to discuss the background and terminology related to azo compounds and azo polymers. As azo groups play a critically important role in determining the functions of azo polymers, their structures and spectroscopic features are discussed first. Light absorption and electronic excitation of the aromatic azo cores are then discussed to understand the initial steps to cause the photoresponsive variations of azo polymers. Photoisomerization of azo groups, which is the most important concept for understanding the contents of this book, is briefly introduced here. Based on the understanding, azo functional structures are classified according to their spectral features and isomerization behavior. Finally, a brief retrospect of azo polymers is given to end this chapter.

Following the brief discussion in Chap. 1, the trans–cis isomerization of azo chromophores is discussed in detail in this chapter. A brief overview is given first to guide the reading, and the discussion starts from the trans/cis isomers of azobenzene and its thermal isomerization. More detailed discussions are given to understand the excited states, intermediates, and pathways during the isomerization. Some pioneering models are discussed here as the background knowledge. Then, a thorough discussion of the photoisomerization mechanism of azobenzene is presented, starting from the famous rotation vs inversion controversy. More experimental results and theoretical models are discussed together with the understanding from computer simulation. Based on the knowledge of the azobenzene isomerization, the substituted effect is further discussed in order to understand the isomerization behavior of azobenzene derivatives and other azo chromophores, which are actually used as the functional groups in most azo polymers.

This chapter discusses different ways to obtain azo polymers. Like other functional polymers, azo polymers can be synthesized by polymerization and copolymerization of monomers that contain azo functional groups or by chemical modification of a suitable precursor polymer to introduce the azo functional groups after polymerization. The chain polymerization and step polymerization are discussed as the two typical ways to obtain azo homopolymers and random copolymers. Controlled radical polymerization is introduced as a new way to synthesize azo polymers with well-controlled molecular weight and distribution. Then, the ways to obtain azo block and graft copolymers, azo dendritic and star polymers, and azo hyperbranched polymers are discussed separately in each section in detail. Finally, syntheses of azo polymers with π-conjugated backbone and azobenzene-containing polypeptide are presented as separate sections due to their unique characteristics.

This chapter is devoted to the photoinduced orientation and anisotropy of azo polymers, which are intensively investigated as a way for holographic recording and data storage based on the polarization gratings. The orientation induced by linearly polarized light endows the polymer films with dichroism and birefringence. Moreover, out-of-plane orientation and chirality can also be induced by irradiation with unpolarized light and circularly polarized light. The main types of the photoanisotropic materials include liquid crystalline azo polymers, amorphous azo polymers, amphiphilic azo polymers, azo polyelectrolytes, and azo block copolymers. The preparation, structure–property relationship, mechanism and models, and possible application in holography are presented in this chapter.

This chapter discusses another fascinating function of azo polymers, photoinduced mass transport, in detail. This photoinduced effect is unprecedented in polymers and other materials, which has been intensively studied to understand the mechanism and also for possible applications in many areas such as surface modification and patterning. The original discovery and many following investigations from materials to functions are presented in a step-by-step way. The basic concepts and background are introduced at the beginning. Then, material design and preparation, optical setup, structure–property relationship, mechanism, and models are discussed in sequence. Meanwhile, some new observations and insights are discussed to document the recent progresses in this area.

In this chapter, one very interesting function of azo polymers is discussed, which is unique for the azobenzene-containing liquid crystal polymers with light cross-linking. These azo polymer networks, which are classified as azo liquid crystal elastomers, show many interesting properties and potential applications in actuators, shape memory, and others. Similar to the above two chapters, the discussion starts from the basic concepts and background as the introduction. Then, material design and preparation, structure–property relationship, mechanism, and models are sequentially discussed. Meanwhile, several selected cases are presented to show the characteristics and potential applications of the functions.