In the previous chapters the main fields of application of the microhardness technique in polymer physics have been highlighted. The emphasis has been mostly on solving structural problems, looking for relationships between the structures of polymers and their properties (initially mechanical ones) or on studying the factors which determine the microhardness behaviour of various polymeric systems.

This chapter presents selected examples of the application of microhardness measurement for the characterization of polymeric materials after various physical treatments.

These include: the effect of the processing conditions on the mechanical properties of synthetic and natural polymers, the characterization of ion-implanted polymer surfaces, the study of mechanical changes in polymer implants after wear, the influence of coatings on surface properties, weatherability characterization of polymers, etc.

Effect on microhardness of processing conditions of polymers

The mechanical and physical properties of moulded parts, particularly those made of thermoplastics, do not depend only on the chemical constitution of the material and its properties. The processing conditions also exercise a considerable influence (Wilkinson & Ryan, 1999). Properties such as strength, toughness, hardness may vary to a greater or lesser extent in the same material, or can be selectively varied by choosing a particular processing technique. Those factors that determine the quality of a part are frequently not apparent externally, but are reflected by the internal structure of the part.

Introduction to crystalline polymers

Polymer crystallization

Under special conditions, amorphous polymers may partially crystallize and become semicrystalline materials. The coexistence of crystalline and amorphous regions is observed in most synthetic polymers that are not obtained by polymerization in the solid state of crystalline monomers or cyclic oligomers. The amorphous state is often called the liquid state, although it may differ quite appreciably from a normal low-molecular-weight liquid. The dominance of the long, randomly coiled, linear macromolecules with the enormous anisotropy of their force field and the presence of crystals strongly affect the behaviour of the chains in the amorphous component. Virtually all amorphous chain sections have one end (cilia) or both ends (folds, tie molecules connecting two different crystals) fixed in the crystal lattice. Hence, the properties of the polymer chains in the surface layer between the crystals and the amorphous layers are substantially different from those in truly amorphous materials without any crystals. Chains of flexible polymers crystallize if they are sufficiently regular and not too hampered in the liquid state by entanglements. Thus a regular, polydisperse, medium-molecular-weight (< 100,000), highly regular PE, without side chains, crystallizes very easily.

In a phase transformation such as crystallization, the two basic processes are the initiation or nucleation process by which a new phase is initiated within a parent phase, and the subsequent growth of the new phase at the expense of the previous one (Gedde, 1995).

Polymorphic transitions in crystalline polymers

When polymers crystallize from the melt or solution, the crystalline regions may exhibit various types of polymorphic modification, depending on the cooling rate, evaporation rate of solvent, temperature and other conditions. These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications have been reported (Tashiro & Tadokoro, 1987). (See also Chapter 4.)

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkern, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways et al., 1975; Yokouchi et al., 1976) and its block copolymers with poly (tetramethylene oxide) (PTMO) (Tashiro et al., 1986), PEO (Takahashi et al., 1973; Tashiro & Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka & Ishikawa, 1968), PVF2 (Lando et al., 1966; Hasegawa et al., 1972), polypivalolactone (Prud'homme & Marchessault, 1974), keratin (Astbury & Woods, 1933; Hearle et al., 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin.

Hardness in materials science and engineering

For about a century engineers and metallurgists have been measuring the hardness of metals as a means of assessing their general mechanical properties. How can one define the hardness of a material? An interesting remark in this respect was made by O'Neil (1967) in his introductory essay on the hardness of metals and alloys. He wisely pointed out that hardness, ‘like the storminess of the seas, is easily appreciated but not readily measured’.

In general hardness implies resistance to local surface deformation against indentation (Tabor, 1951). If we accept the practical conclusion that a hard body is one that is unyielding to the touch, it is at once evident that steel is harder than rubber. If, however, we think of hardness as the ability of a body to resist permanent deformation, a substance such as rubber would appear to be harder than most metals. This is because the range over which rubber can deform elastically is very much larger than that of metals. Indeed with rubber-like materials the elastic properties play a very important part in the assessment of hardness. With metals, however, the position is different, for although the elastic moduli are large, the range over which metals deform elastically is relatively small. Consequently, when metals are deformed or indented (as when we attempt to estimate their hardness) the deformation is predominantly outside the elastic range and often involves considerable plastic or permanent deformation.

Carrier transport in semiconductor devices is complicated by the rapid spatial and temporal variations that often occur. For large devices, the low- and high-field transport theory developed in previous chapters is directly applicable. Such devices can be analyzed by drift–diffusion equations with field-dependent mobilities and diffusion coefficients. Transport in small devices, however, differs qualitatively from that in bulk semiconductors because the carrier distribution function is no longer determined by the local electric field. Since transport is nonlocal in both space and time, conventional drift–diffusion equations do not apply, but new possibilities for enhancing device performance arise.

In this chapter, we explain why the drift–diffusion equation loses validity for small devices and describe some important features of carrier transport in the presence of rapidly varying fields. The objective is to gain an intuitive understanding of carrier transport in modern devices such as small bipolar and field-effect transitors. To identify the kinds of transport problems that need to be addressed, we begin by describing a generic transistor. We then examine carrier transport under several specific situations that occur in modern devices and explain why the drift–diffusion equation often loses validity. Finally, we briefly examine device simulation to indicate how the transport equations are formulated for numerical solution, so that nonlocal transport can be simulated for realistic devices.

A simple, conceptual model for a transistor consists of a carrier injector, a carrier collector, and a control region that meters the flow of carriers out of the source (Fig. 8.1).

Introduction

Only a handful of physical observables (bond length, force constant, dipole moment, polarizability) can be uniquely identified and associated with the individual bonds that bind atoms together into molecules and crystals. Over the years, each of these observables has played a pivotal role in advancing our understanding of the chemical bond. Of the four, bond length is not only the single most characteristic property of a bond, but also it is by far the most accessible and easy to measure.

Introduction

There is an increasing demand for understanding various properties of oxide melts, not only from metallurgical and petrological points of view but also from a new perspective of the crystal-growth technique through molten states. This has encouraged number of measurements of density, viscosity, surface tension, and electrical conductivity of oxide melts. To clarify their characteristic properties, a knowledge of the atomic-scale structures of oxide melts is essential, and particularly x-ray diffraction has been one of the most popular methods for the in situ structural analysis of materials since the first liquid result made by Debye in 1915 [1]. Recently, several modern x-ray-diffraction techniques have been developed for determining the fine local structure of oxides in both the molten and the glassy states. This chapter provides an extended discussion of the structure of oxide melts at high temperatures obtained by several advanced diffraction techniques.

Introduction

Partitioning of elements between minerals and melt (or fluid) is one of the most important factors in determining the elemental distribution in the Earth during its evolution. To understand geochemical processes by means of elemental distributions recorded in rocks, the partition coefficients of major and trace elements between minerals and melt (or fluid) have been measured (e.g., Ref. [1]). However, much less effort has been applied to understanding the partitioning behavior from a microscopic perspective.

Introduction

Predicting the structures and properties of materials from a knowledge of their chemical composition has been a long-standing problem of materials science [1]. Thanks to the recent development of computational science approaches, it is often not as difficult to predict stable structure and elctronic properties of even unknown materials theoretically, if we know or assume a rough arrangement of the constituent atoms in the material. It is much more difficult, however, to know how to make it and how stable it is compared with other unknown structures beyond our imagination, as we need overall knowledge of the potential-energy surface in the multidimensional configuration space to do so. An exceptionally simple and hopeful situation for such theoretical designs of materials can be found in the low-temperature compression of crystals.

Some crystals compressed at low (room) temperature undergo structural transformation without atomic diffusion (martensitic transformation), resulting in metastable structures inaccessible at thermal equilibrium or by rapid quenching of high-temperature/pressure phases.