Functional Polymer Structures

Polymer micro- and nanografting is introduced in the context of different methods to locally endow polymers with selected functionalities. The second part of the chapter gives an introduction to polymer brushes, which are readily accessible using grafting techniques and have a high application potential as functional coatings. Fundamentals of their formation and conformation dependent on environmental conditions are summarized, as are functionalities provided by various brush systems.

Keywords

Inert polymers; functional polymers; polymer functionalization; polymer substrate; polymer device; structuring methods; polymer brush; polyelectrolyte

Materials with a multitude of functionalities have been introduced in our daily lives and are used without thinking much about their origin and way of production. Examples include magnetic, electrical, optical, and biological functions, and many of them are implemented in polymeric or polymer-based systems. This chapter focuses on the chemical properties and related functionalities that can be introduced to polymer systems using different methods. Main emphasis is placed on polymer brushes, which are extremely versatile and interesting for the functionalization of surfaces and which are accessible on polymers using grafting technology.

1.1 Polymer Systems: Inertness Versus Functionality

Polymers are the most promising materials for current and future applications due to their special properties: They have low densities, exhibit relatively high specific strength and flexibility, and in some cases display remarkable chemical inertness (e.g., fluoropolymers and polyaryletherketones). For many applications, the chemical inertness of polymers provides a substantial advantage. Chemically inert polymers are long-lasting and stable, resistant to weathering, and show very low sorption of water. To benefit from these properties, polymeric coatings are often used as the finish for surfaces of daily life goods, applied in order to achieve (chemical) inertness and stability.

In contrast, other classes of polymers are not inert. They may interact strongly with the environment and adopt special functions. Examples include specific interactions with molecules and ions exploited in separation and purification techniques; electrical and optical properties used in polymer solar cells, organic light emitters, and optical elements; as well as properties relevant for bio-applications, such as anti-biofouling properties and specific binding of proteins.

Adding locally defined functionality to inert surfaces is interesting for many applications. For instance, integration of small sensing elements that could—actively or passively—monitor the freshness of packed goods is of great interest to the packaging industry. With current structuring and patterning technologies, length scales can be addressed which are interesting for studying interactions with cells. Such studies are of importance for the functional design of polymeric implants. Further developments of patterning technology to reach dimensions of the size of single protein molecules are in progress, which will be beneficial for constructing ultrasensitive bioanalytical devices.

In polymer systems, the combination of properties of different components is achieved in a multitude of ways. For example, polymer layers of different functionalities are applied on a polymer substrate by spraying, casting, dip coating, or spreading with a doctor blade. Polymer coatings can be combined with layers of nonpolymeric materials applied from solutions or suspensions of the coating material or precursors thereof, or deposited via vapor phase using physical and chemical vapor deposition.

To achieve structures in deposited layers, screen printing, stamping, and ink-jet printing are very well-established and commercially used technologies. Current and future applications based on such technologies include polymer solar cells [1] and polymer electronics [2]. Polymeric optical waveguide structures on polymer substrates, which require high precision in structure definition, may for instance be produced with micromolding techniques [3]. Taking advantage of the mechanical flexibility of many polymers, developments also aim at elastomeric light-emitting devices and displays [4] or molecularly stretchable electronics [5].

Grafting techniques present an approach on the molecular level for anchoring functionalities on surfaces. The term “grafting” is used in analogy to the biological grafting of a branch from a tree or bush onto the trunk of another closely related species in order to combine the properties of the tree (e.g., the growth of high-quality fruit that in many cases cannot be grown from its own seeds) with the properties of the trunk (e.g., easy to grow and resilient against various insects). In polymer grafting, chemical links are formed between the polymer chains of one material, typically a polymer film, to the chains of another polymer.

In comparison to the methods of layer formation discussed previously, the covalent anchoring of grafted polymer layers intrinsically circumvents adhesion problems that are frequently encountered between different polymers. In addition, grafting techniques offer particular flexibility on the molecular level. A large variety of chemically different monomers and polymers may be grafted, and their strong anchoring allows further functionalization and chemical modification to widen the range of accessible functional properties of the modified polymer surface. Furthermore, polymer chains attached on one end to the surface and grafted in high density usually adopt a so-called polymer brush configuration providing interesting properties as outlined later.

Polymer micro- and nanografting combines grafting techniques with structuring. It is related to a number of fields of science and technologies, as schematically summarized in Figure 1.1. Polymer materials provide the substrates for the micro- and nanografting process, which aims at local introduction of specific properties and functionality using structuring technologies. Functionality is provided, for instance, by polymer brushes or polyelectrolytes. Aspects of functional polymer surfaces are summarized in the following section in the context of polymer brushes and are further detailed in Chapter 4. Understanding the impact of radiation on polymers in terms of radical formation after ionization and bond-breaking events is fundamental for radiation grafting, which is discussed extensively in Chapter 2. Finally, different aspects of graft polymerization reactions are important for successful formation of functional structures starting from either radiation-generated radicals or immobilized initiators, which are discussed in Chapters 2 and 3, respectively.

1.2 Polymer Brushes

Polymer brushes have attracted a tremendous amount of attention in recent years because they enable tailoring of physical, chemical, and biochemical properties of various material surfaces. These brushes are densely packed arrays of polymer chains tethered at one chain end to the surface. At high packing density and in the presence of a good solvent, the chains are forced to elongate perpendicular to the surface. Polymer brushes can be considered as extended interfaces between the polymer surface and the surrounding environment. Due to the flexibility of polymer chains, small molecules are able to penetrate and interact with the chains or with other species embedded within the brush. The term “polymer brush” is not limited to flat surfaces of solid materials; it is also applied to systems in which polymers are densely grafted at the surfaces of nanoparticles, micelles, or even chains of another polymer. The following summary focuses on brushes and brush structures grafted on solid supports.

1.2.1 Formation of Polymer Brushes on Surfaces

Polymer chains may be attached to surfaces by using “grafting-to” or “grafting-from” techniques (Figure 1.2). “Grafting-to” means that preformed polymer chains are bound at one end to the surface or to surface-bound linker molecules via chemical reactions. One intrinsic problem of grafting-to methods is that the bound polymer restricts the diffusion of further chains to the surface, often resulting in low grafting densities. In the “grafting-from” approach, the polymer chains are grown from initiators bound to the surface. Therefore, the term surface-initiated polymerization is also used for the same process [6,7]. Typical initiators for free radical polymerizations are azo- and peroxide compounds [8], which are cleaved to reactive radicals under polymerization reaction conditions. In grafting-from processes, only relatively small monomer molecules diffuse to reaction sites at the end of the growing chains; consequently, a higher density of grafted chains can be achieved.

The high polydispersity of grafted chains is one particular...