Background and Perspective

D.R. Paul,     Department of Chemical Engineering, University of Texas, Austin, Texas

Publisher Summary

The field of polymer science and technology has undergone an enormous expansion over the past several decades primarily through chemical diversity. First, there was the development of new polymers from a seemingly endless variety of monomers. Next, random copolymerization was used as an effective technique for tailoring or modifying polymers. Later, more controlled block-and-graft copolymerization was introduced. Though the list of new concepts in polymer synthesis has not been exhausted, it has become clear that new chemical structures or organizations are not always needed to meet new needs or to solve old problems. The successful implementation of the concept of physically blending two or more existing polymers to obtain new products or for problem solving requires different knowledge and techniques than that used to develop new polymers. This chapter provides an overview of polymer blends. The thermodynamics of polymer-polymer mixtures or composites is one of the most important fundamental elements because it plays a major role in the molecular state of dispersion, the morphology of two phase mixtures, the adhesion between phases, and consequently influences most properties and applications. The chapter also highlights the possible phase and transition behavior in polymer blends. Blends with two phases can be organized into a variety of morphologies. Many properties and uses of a blend depend critically on the nature of this arrangement of the two phases. The purpose of polymer blending is to achieve commercially viable products through either unique properties or lower cost than some other means might provide. Commercial products are based on miscible blends such as polystyrene-poly (phenylene oxide), PVF2–PMMA, PVF2–PEMA, and PVC-nitrile rubber and immiscible blends such as rubber blends in tires, impact-modified plastics, and coextruded film and fibers.

The field of polymer science and technology has undergone an enormous expansion over the last several decades primarily through chemical diversity. First, there was the development of new polymers from a seemingly endless variety of monomers. Next, random copolymerization was used as an effective technique for tailoring or modifying polymers. Later, more controlled block-and-graft copolymerization was introduced. The list of new concepts in polymer synthesis has not been exhausted. However, it has become clear that new chemical structures or organizations are not always needed to meet new needs or to solve old problems.

The concept of physically blending two or more existing polymers to obtain new products or for problem solving has not been developed as fully as the chemical approach but is now attracting widespread interest and commercial utilization. The successful implementation of this concept requires different knowledge and techniques than that used to develop new polymers. It is the purpose of this book to bring together the pertinent principles needed to implement and advance this more physical approach to polymer products. In the first part of this book, these principles are discussed in general or scientific terms; in the latter part they are illustrated by particular systems or with reference to specific applications.

The purpose of this introductory chapter is to develop some fundamental background pertinent to the early chapters and to give a perspective that to some degree will tie together the diverse subjects and viewpoints presented by the various authors throughout this book.

TERMINOLOGY

Polymer blends are often referred to by the contraction “polyblends” and sometimes as “alloys” to borrow a term from metallurgy. Various restricted definitions might be offered for any of these or other terms; however, the boundaries of what is intended are invariably imprecise, and the terms are not used with the same meaning by everyone. No attempt will be made here to adopt any rigid terminology, and the concept of blending will be discussed in the broadest possible manner. This book covers materials or products made by combining two or more polymers through processing steps into random or structured arrangements and includes geometries that might be regarded as polymer–polymer composites (see, e.g., Volume 2, Chapter 15 and parts of Chapter 16). We do not include, obviously, separately processed polymer items that are subsequently assembled into finished products. Block-and-graft copolymers share many common features and purposes as blends, but these materials, which generally differ from blends by only a few chemical bonds, are not included here except when they are components of blends (see Volume 2, Chapter 12 and 18).

The thermodynamics of polymer–polymer mixtures or composites is one of the most important fundamental elements since it plays a major role in the molecular state of dispersion, the morphology of two phase mixtures, the adhesion between phases, and consequently influences most properties and applications. Because of this, the early chapters including this one are devoted mainly to this subject. Like many aspects of polymer blends, an impediment to understanding the thermodynamics of blends has been a lack of suitable experimental techniques and theories; however, recent activities have made a start toward removing these deficiencies as will be seen here and in subsequent chapters. One of the first, but not the only, thermodynamic questions concerns the equilibrium miscibility or solubility of two polymeric components in a blend.

Very often the term “compatibility” is used synonymously with miscibility. However, in materials technology compatibility is a more general term with a wider diversity of meanings and implications, which in the extreme might result in two materials being classified as incompatible because they are miscible. In a strict technological sense, compatibility is often used to describe whether a desired or beneficial result occurs when two materials are combined together. If one is concerned with identifying polymeric plasticizers, then complete miscibility is desired, whereas, for rubbery impact modifiers of glassy plastics complete miscibility is not desirable. Generally speaking, most polymer pairs are not miscible (but more are miscible than was recognized only a few years ago) and by this terminology are incompatible. It is very likely that the use of the word incompatible as part of this general rule has been an unfortunate psychological impediment to commercial development of polymer blends because of the accompanying implication that poor results are inevitable when incompatible materials are combined. For many purposes, miscibility in polymer blends is neither a requirement nor desirable; however, adhesion between the components frequently is. In a fundamental sense, however, adhesion, interfacial energies, and miscibility are all interrelated thermodynamically in a complex way to the interaction forces between the two polymers.

Miscibility in polymer–polymer mixtures has been the subject of considerable discussion and debate in the literature. Frequently, the concern is over the size of the phases or domains implied by a particular observation; or, is mixing on a molecular or segmental scale (see e.g., [1, 2])? Interestingly, these questions are almost never raised about solutions of low molecular weight compounds, but apparently they arise naturally for macromolecules. Similar concerns existed many years ago about solutions of polymers in low molecular weight solvents and only disappeared when appropriate thermodynamic theories and experimental data appeared [3] which demonstrated that these solutions were not unusual or unique once the conformations and large size of the polymer chains were correctly considered.

Miscibility in every case is best understood, and ultimately can only be defined, in thermodynamic terms rather than through attempts that place overdue emphasis on details at the molecular or segmental level. Until recently, techniques for examining the thermodynamics of miscible polymer–polymer mixtures critically and unambiguously were extremely limited. The neutron scattering results that are now beginning to appear [4–6] seem to fill this need for conceptual clarification and quantitative results. In the experiments of interest here, one polymer is “dissolved” in a different polymer (the two may be regarded as solute and solvent), and the “solutions” are studied via neutron scattering in a manner analogous to classical light scattering of polymer in a solvent. Preferably, one of the polymers is deuterated, but this is of no fundamental concern here. Scattered intensities have been measured as a function of “solute” concentration and scattering angle and subsequently analyzed by the familiar Zimm plot used in treating light scattering from dilute polymer solutions. Interestingly, this analysis gives the correct molecular weight for the solute polymer, which is confirmation that these “solutions” are classical ones having miscibility in a true thermodynamic sense. Furthermore, the radius of gyration of the “solute” polymer was found to be of the size one expects in bulk or dilute solution and has a similar molecular weight dependence.

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