Sebastian Koltzenburg

Current Trends in Polymer Science

In this chapter some developments are discussed that have caught our attention over the past few years for a variety of reasons. The selection is, of course, subjective, and no attempt is made to suggest whether these developments are likely to become widespread technology or remain laboratory curiosities. Nevertheless, we believe the examples presented are interesting in their originality and show some of the directions being followed in current polymer science.

The Basics of Plastics Processing

With the exception of the functional polymers discussed in Chap. 19, polymers are typically used as solid materials for the widest variety of different objects, from plastic bags to prostheses. To fulfill each requirement, one needs to take the material, composed of macromolecules that have been produced in solution, bulk, or suspension, and give it form and precise geometry depending on the demands of the intended application. The exact conditions of normal use to which the object is exposed determine the choice of the chemistry of the material, for example, thermal, mechanical, and chemical properties; the actual use determines the geometry of the article. Many different processing technologies for shaping polymeric materials have been developed over the past few decades. However, all forms cannot be achieved with all processes, and all polymers cannot be processed using all techniques. This is why it is exceptionally important for a materials scientist to have a detailed knowledge of the interplay between shape, material, and processing technique. This chapter provides a short, introductory overview of the most important aspects of the basic concepts of the thermoforming of polymers. For further details that are beyond the scope of this book, the interested reader is referred to more comprehensive textbooks (Kaiser 2007; Michaeli 2010).

Polymers in Solid State

The vast majority of all polymers produced in the world are utilized in their solid form—that is, as a classic material. Thus, a discussion of the solid-state properties of polymers, their morphology, and the impact of this and their properties on their applications is an essential part of this book.

Important Polymers Produced by Chain-Growth Polymerization

The technically most important polymers and copolymers produced by chain-growth polymerization are discussed in this chapter; those produced via step-growth polymerization are discussed in Sect. 8.5.

Step-Growth Polymerization

The synthetic processes of producing polymers from their monomers can be divided into step-growth and chain-growth polymerization. These two polymer formation reactions (Figs. 8.1 and 8.2) are fundamentally different in their mechanisms, intermediate products, the way the molar mass increases as a function of monomer conversion, and the activation energy of their elementary steps.

Ring-Opening Polymerization

Ring-opening polymerization (ROP) is an important method of polymerization. It differs from radical polymerization (Chap. 9), ionic polymerization (Chap. 10), and step-growth polymerization (Chap. 8). No low-molar-mass by-products are formed, except during the polymerization of Leuchs’ anhydride (Figs. 12.41 and 12.42). Furthermore, the driving force derived from the transformation of C=C double bonds into C–C single bonds, which offsets the loss of entropy during polymerization, is not available. A general feature of ROP is that the monomers are rings of varying size. Depending on the size and type of ring. the ability to polymerize varies. Thus, small rings (three-, four-, or five-membered rings) can be polymerized because of the ring strain released when they are open. As an example, the enthalpy associated with the ring strain of oxirane is 116 kJ/mol. The release of enthalpy is also the driving force for the polymerization of seven- and eight-membered lactones and lactams, even though the ring strain is only about 16 kJ/mol for these monomers. Unstrained six-membered rings often do not polymerize via ROP. By contrast, the ROP of disulfides, silicones, and carbonates can be ascribed to the increase in the entropy that occurs during the polymerization of these monomers. This increase in entropy is based on the increase in the degrees of freedom of rotation gained when rings are transformed into open chains.

Polymers in Solution

Polymers, especially when compared with the monomers from which they are built, have a number of special properties. For example, polymers such as starch and polypropylene oxide are much less soluble in water than their monomers, glucose and propylene oxide. Another observation is that many polymers absorb solvents or water without themselves dissolving. Thus, cotton socks, for instance, absorb water without disintegrating when they are washed in a washing machine. To explain and to be able to describe such properties, this chapter is devoted to a description of the polymeric chain structure and the consequences thereof for polymers in solution. Furthermore, the thermodynamics of polymer solutions are discussed and compared with those of small molecules to develop an understanding of the differences in solubility mentioned above.

Polymers as Materials

Polymers as materials appear in myriad forms. Everyone is familiar with floor coverings made of polyvinyl chloride (PVC) and with Plexiglas windows (polymethyl methacrylate), and the latter’s particularly successful version: the roof-top of the Munich Olympic Stadium. Many are equally familiar with the strengthening of polymers by compounding them with glass fiber. Polymers are also increasingly being used in medical applications, for instance as bone and organ prostheses. One can easily imagine that these must meet completely different requirements than, for example, an ordinary PVC tube in a chemical laboratory. These few examples amply demonstrate how diversified and partially contradictory the requirements for a material in its specific application are, and that an ideal material for all applications cannot exist.

Chemistry with Polymers

Many macromolecules still contain chemically reactive groups even after polymerization has taken place; under certain conditions, macromolecules can also be reactive chemicals. They do, however, have some special characteristics compared with reagents with a low molar mass. This chapter describes such reactive macromolecules and their peculiari ties.

Liquid Crystalline Polymers

In this chapter, liquid crystalline polymers are defined, methods for their characterization are described, and some examples of liquid crystalline polymers are discussed.