Linda C. Sawyer

The structure of thermotropic copolyesters

Highly oriented polymeric products have been produced over the past fifteen years by two very different processing routes; from conventional polymers processed to highly oriented extended chain structures, and from “rod-like” polymers which exhibit liquid crystalline behaviour. Gel spun polyethylene is an example of such a conventional polymer. There are three main types of liquid crystalline polymers (LCP) which have high orientation and modulus: lyotropic aramids, such as poly(ρ-phenylene terephthalamide) (PPTA); lyotropic, aromatic heterocyclic polymers, or “ordered polymers”; and the family of thermotropic aromatic copolyesters. Extensive characterization of the thermotropic copolyesters has resulted in the delineation of a fibrillar, hierarchial structural model which accounts for the structures observed in a broad range of oriented fibres, extrudates and moulded articles. Three distinct fibrillar species are observed: microfibrils that are about 50 nm, fibrils about 500 nm, and macrofibrils about 5μm, in size. Superimposed on the structural hierarchy is a defect hierarchy, defined by the regular meander of the molecular chain and a localization of defects within a microfibril at about a 50 nm periodicity. Orientational variations, layering and skin core structures, in thick specimens, are the result of local flow fields on the basic structural units during solidification. The fibrillar textures appear to be present prior to any preparation for microscopy. A wide range of specimen preparation methods, i.e. fractography, sonication, microtomy and etching, and microscopic techniques, i.e. optical, scanning and transmission electron microscopy, were applied to the characterization of the aromatic copolyesters and PPTA. Interestingly, the same basic hierarchy is observed for both the lyotropic and the thermotropic LCPs and the microfibrillar structures of all the highly oriented polymers, including polyethylene, appear quite similar.

The fibrillar hierarchy in liquid crystalline polymers

It is well known that the structure of highly oriented liquid crystalline polymers (LCPs) can be characterized by a hierarchical fibrillar structural model. Structure models were first developed for the lyotropic aramid fibres in the late 1970s and a structural model was developed for the thermotropic copolyesters in the mid-1980s. Recently, imaging techniques with higher potential capability and resolution have been applied to assess the size, shape and organization of microfibrillar structures observed in LCPs. Field emission scanning electron microscopy and scanning tunnelling microscopy permit imaging of regions from 1 nm to many micrometres. As a result, the nature of the microfibrillar hierarchy has been further clarified and the macromolecular size has been shown to be less than 2 nm. The shape of the microfibrils has been shown to be tape-like. The LCP structural model, consisting of elongated well-ordered microfibrils continues to be consistent with measured properties: high anisotropy, very high tensile modulus and strength and poor compressive properties. A more detailed structural model is proposed to describe the macromolecular microfibril size, shape and organization for comparison with polymer composition and mechanical property evaluation.

A scanning tunnelling microscope study of groove structures in polycarbonate optical discs

The groove structure in polycarbonate substrates, commonly used in the fabrication of optical discs, has been studied with the scanning tunnelling microscope. Comparative studies of the same structures were also performed using more conventional scanning electron microscopy and transmission electron microscopy techniques. These studies illustrate the ability to characterize the shape of man-made structures that are commonly recorded in these polymer-based materials. The scanning tunnelling microscope images show the superiority of this technique for detailed cross-sectional studies of the profiles of structures with typical dimensions ∼ 500 nm in width by ∼ 50 nm in depth.

Specimen preparation methods

Specimen preparation ranges from direct and simple methods to complex, time consuming and even frustrating ones. Fortunately, there are a number of simple methods which are quite adequate for some materials. For example, many particulate materials may be handled by the simple methods. This section covers a wide range of these more simple and generally direct methods which are described in broad subsections: optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) preparations. It must be emphasized that quick observation of most materials by a combination of a simple microscopy technique and direct preparation methods is often helpful in shedding light on the problem. This aids determination of the best approach to a solution. In many cases there is no one correct approach, but there may well be approaches that can save time, if they are conducted early in the study. Tradenames of products used in specimen preparation are mentioned in the text and, unless otherwise stated, these are standard materials available from the EM suppliers (Appendix V). Specific microscopes are not mentioned but microscope vendors are listed in Appendix VI.

Introduction to polymer morphology

Organic polymers are materials that are widely used in many important emerging technologies of the twentieth century. Feedstocks for synthetic polymers are petroleum, coal and natural gas, which are sources of ethylene, methane, alkenes and aromatics. Polymers are used in a wide range of everyday applications, in clothing, housing materials, appliance housings, automotive and aerospace areas and in communication. Materials science, the study of the structure and properties of materials, is applied to polymers in much the same way as it is to metals and ceramics: to understand the relationships between the manufacturing process, the structures produced and the resulting physical and mechanical properties. This chapter is an introduction to polymer morphology, which must be understood in order to develop relations between the structure and properties of these materials. An introduction by Young [1] is but one reference from the vast literature on the topic of polymer morphology. Subsequent sections and chapters have many hundreds of references cited as an aid to the interested reader. The emphasis in this text is on the elucidation of polymer morphology by microscopy techniques.

Hierarchical Structures in Liquid Crystalline Polymers

It is well known that the structure of highly oriented liquid crystalline polymers (LCPs) can be characterized by a hierarchical fibrillar structural model. Structure models were developed for the lyotropic aramid fibers and the thermotropic aromatic copolyester fibers during the last two decades showing the existence of fibrillar hierarchies. Hierarchies of structure have also been commonly observed for the biological materials. Concepts learned from the latter are useful in materials science studies today. The nature of the smallest nanostructure that aggregates, the combination of these small structures, typically microfibrils, into larger structures and the interaction of these hierarchical entities are important to understanding their behavior. The architecture of the whole of the polymer or the biological material is a further important variable as is the relation of the process with that architecture. This paper discusses details of the structure of LCPs and draws an analogy between the materials science and biological hierarchies.

New techniques in polymer microscopy

A wide range of techniques in microscopy has either appeared within the past few years, or has only recently been applied to the study of polymer systems. These new forms of microscopy are distinct from the continuing evolution of the microscope. This is currently rapid in the direction of ease of use, computer control and increased use of digital image storage and processing, even for optical microscopy [1, 2]. Some examples of these new forms of microscopy as they have been applied to polymers are included in the previous two chapters. In the case of novel types of scanning electron microscopy and high resolution transmission electron microscopy, the principles of microscope operation and image formation are described in Chapters 2 and 3, respectively.

Problem solving summary

The preceding chapters have provided a description of microscopy techniques, imaging theory and the specimen preparation methods required to investigate polymer structures. The theme of this chapter is to put all of this together within a useful framework. This framework might be a review to experienced microscopists (who likely have developed their own protocols), but it will provide useful information regarding problem solving ideas. A problem solving protocol will be developed that permits microscopy characterizations to follow an easy and short path to a solution. These characterizations will all be classified as ‘problems’ that require a solution. Problems can range from simple to complex and include, for example, determination of the phase structure in a polymer blend, the cause of failure of a composite or the complete and fundamental characterization of a new membrane, fiber, film, etc. Clearly, such problem solving will require a range of time and effort, but the protocols used to begin the characterization and to know when the problem is solved are similar overall. Generally more than one technique is required to solve problems relating to polymer morphology and thus complementary multidisciplinary techniques are important in conducting problem solving analyses. Interpretation of the images produced is of critical importance in evaluating polymer structures and thus the topic of artifacts will be included in this discussion. Finally, although structural characterizations cannot generally be accomplished without microscopy methods and techniques, there are other complementary analytical techniques that are often quite important in understanding polymer structures. The last section will be devoted to a short description of these techniques, including x-ray diffraction, thermal analysis, electron spectroscopy and others.

Fundamentals of microscopy

Microscopy is the study of the fine structure and morphology of objects with the use of a microscope. Microscopes range from optical microscopes, which resolve details on the micrometer level, to transmission electron microscopes that can resolve details less than one nanometer across. The size and visibility of the polymer structure to be characterized generally determines which instrument is to be used. For example, the size and distribution of spherulites can be observed by optical techniques, but a study of their internal structure requires electron microscopy. Combinations of the various microscopy techniques generally provide the best insight into the morphology of polymer materials [1]. Table 2.1 shows the basic properties of the different microscopes, for the purpose of comparison.