Polymer Blends and Alloys

George P. Simon

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781351423618
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Distinguishing among blends, alloys and other types of combinations, clarifying terminology and presenting data on new processes and materials, this work present up-to-date and effective compounding techniques for polymers. It offers extensive analyses on the challenging questions that surround miscibility, compatibility, dynamic processing, interaction/phase behaviour, and computer simulations for predicting behaviours of polymer mixture and interaction.

PART I

COMPATIBILIZATION AND MISCIBILITY

1

Compatibilization of Polymer Blends^*

Rudolph D. Deanin and Margaret A. Manion

University of Massachusetts—Lowell, Lowell, Massachusetts

I. Introduction

A. History and Commercial Importance of Polymer Blends

Polymer blending has a long commercial history in the rubber, coatings, and ad-hesives industries, and it entered the plastics industry a half century ago. Since then it has been growing so rapidly that it is becoming an increasingly important portion of the entire plastics industry, 20–40% of the total plastics market by various estimates. With growing understanding and command of the science and engineering involved, it may well continue to offer increasing contributions to plastics and the other polymer industries.

B. Miscibility vs. Compatibility

When plastics processors first tried to blend polymers with each other, they were shocked to find that most pairs of plastics were immiscible and had very poor properties. Plastics chemists reasoned that they must develop miscible homogeneous systems to achieve useful properties. They tended to use the terms miscibility and compatibility interchangeably. They did discover a number of polymer pairs that were completely miscible to give a homogeneous single phase, with properties proportional to the ratio of the two polymers in the blend, and several of these were of commercial importance. They also illuminated the thermodynamics of polymer miscibility and immiscibility and delineated the factors that favored miscibility or immiscibility. But meanwhile, practical plastics technologists were developing a much larger number of polymer blends that were immiscible but very useful, combining some of the best practical properties of each polymer in the blend; they tended to use the term compatible for these. Unfortunately most of the authors in the field continued to use the terms miscible and compatible rather casually and indiscriminately, so that much of the literature is ambiguous or confusing.

In the present review, the term miscible will be used to describe polymer blends that have theoretical thermodynamic miscibility down to the segmental level; the term compatible will be used to describe polymer blends that have useful practical properties, regardless of whether they are theoretically miscible or immiscible.

C. Factors in Miscibility and Immiscibility

A number of specific features may contribute to miscibility/immiscibility of polymer blends (1). These may be listed in order of commercial importance:

1. Polarity

Polymers that are similar in structure or, more generally, similar in polarity are less likely to repel each other and more likely to form miscible blends (2,3). Diverging polarities generally produce immiscibility.

2. Specific Group Attraction

Polymers that are drawn to each other by hydrogen bonding, acid-base, charge-transfer, ion-dipole, donor-acceptor adducts, or transition metal complexes are less common, but when such attractions occur they are very likely to produce miscibility (1,4, 5 and 6).

3. Molecular Weight

Lower molecular weight permits greater randomization on mixing and therefore greater gain of entropy, which favors miscibility (7). More surprisingly, polymers of similar molecular weights are more miscible, while polymers of very different molecular weights may be immiscible, even if they both have the same composition.

4. Ratio

Even though two polymers appear immiscible at a fairly equal ratio, it is quite possible that a small amount of one polymer may be soluble in a large amount of the other polymer, as understood in conventional phase rule. This consideration is extremely important in natural compatibility, as will be explained later.

5. Crystallinity

When a polymer crystallizes, it already forms a two-phase system, with important consequences for practical compatibility. In a polymer blend, when a polymer crystallizes, this adds another phase to the system. If both polymers in a blend crystallize, they will usually form two separate crystalline phases; it is quite rare for the two polymers to cocrystallize in a single crystalline phase (12).

D. Practical Compatibility in Polymer Blends

Polymer blends have commercial importance because they offer properties, or a balance of properties, not available in a single polymer. These properties depend very much on the microstructure of the blend.

1. Homogeneous Blends

When two polymers are completely miscible down to the segmental level, they form a single homogeneous phase, and properties are generally proportional to the ratio of the two polymers in the blend. This gives the compounder quick and economical control over the balance of properties for different applications. Major examples are in the coatings, adhesives, and rubber industries. In the plastics industry, the major example is blends of polyphenylene ether with polystyrene (8,9).

2. Phase Rule, Morphology, and Phase Stability

When two polymers are immiscible, phase rule explains quantitatively the extent to which they separate and the extent to which each phase is actually—not pure polymer A and pure polymer B, but rather—a solution of B in A and a solution of A in B. It is extremely important to remember this distinction in trying to understand the practical properties of such two-phase systems. Generally the major phase will form the continuous matrix and control most properties, while the minor phase will form dispersed microdomains and contribute certain specialized properties to the blend. Another factor is rheology: the less viscous phase tends to form the continuous matrix (even if it is present in rather minor amount!), while the more viscous phase tends to form the dispersed domains.

The structure of the dispersed domains is referred to as the morphology. The simplest shape of a dispersed domain, which is trying to minimize its surface energy, is spherical, and most dispersed domains appear in this form. Generally, increasing attraction between phases tends to decrease the size of the spheres and increase practical compatibility. With increasing concentration of the minor phase, the dispersed domains may tend to become rodlike; and at fairly equal concentrations, the two phases may become lamellar. Another factor in morphology is shear flow during melt processing, which tends to elongate spherical domains into platelike or fibrillar form. Such nonspherical morphology is generally believed to have important effects on practical properties.

Kinetics of phase separation and morphology formation are slowed by the entanglement of large polymer molecules and the high viscosity of s...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication Page
• Contents
• Preface
• Contributor
• Part I Compatibilization and Miscibility
• Part II Characterization
• Part III Morphology
• Part IV Recent Developments
• Index