THE POLYMER

Basic Terms and Definitions

A set of key terms and clear definitions is an important requisite for the understanding of any science; and this is especially true in the case of a science as new as high polymers. Since the basic terms and definitions that are the cornerstones of the "language" of high polymers are a logical starting point for the beginner, we are placing them at the beginning of the book. A little time spent now in acquiring a familiarity with these key definitions will make the science of high polymers much easier to assimilate as well as more fascinating.

The nomenclature of high polymers has been assembled in alphabetical order so that it will be convenient to refer back to specific definitions. As an understanding of the language characteristics of high polymer science is acquired, the knowledge behind myriad useful high polymer products that are playing an increasingly important role in your daily life will unfold.

Crystallite (see Fibril, Micelle)
When two or more long-chain molecules come close enough together laterally to lock in any way, the nucleus of a crystallite is born and a crystallite begins to grow. A crystallite consists of a cluster of associated long-chain molecules that make up a more or less regular structural unit, or building block, out of which a particular high polymer is constructed. In other words, it is the basic micro-unit of the geometric architecture of high polymers. The size varies over a wide range—from the order of tens to hundreds of Angstrom units (see Figure 1). Crystallites can increase or decrease in size without a significant change in their chemical make-up. The long-chain molecules in crystallites are so tightly packed that even beams of x-rays directed at them are readily diffracted. It is usually
by means of this structural characteristic that the size of crystallites is determined experimentally.

Degree of Polymerization (D.P.)
The degree of polymerization (D.P.) is the average number of repeating units in a linear macromolecule, if such a macromolecule consists of regularly repeating units; or, the average number of mers (monomeric units) per macromolecule if such a long chain molecule is built up of identical monomers. The D.P. is determined by dividing the (average) molecular weight of the monomer into the molecular weight of the macromolecule.

Elastomer (or Rubber)
Elastomer is a term that refers to non-crystalline high polymers or rubbers that have a three-dimensional space-network structure (e.g., vulcanization) which imparts stability or resistance to plastic deformation. Normally, elastomers exhibit long-range elasticity (rubber band effect) at ordinary room temperatures.

Fiber
A fiber is a thread or thread-like structure composed of fine strings or filaments of linear macromolecules that are intertwined or associated in such a manner as to give rise to an assemblage of molecules having a high ratio of length to width. The dual structure of ordered and disordered regions coupled with the orientation of these regions is known as "fiber structure."

Fibril (see Crystallite, Micelle)
A fibril consists essentially of an aggregate of micelles or crystallites that is large enough to appear like a very fine fiber under a high-power microscope. In the extreme case, of course, a fibril and a large micelle or crystallite may be considered synonymous. Normally, however, a fibril is an aggregate of many crystallites connected by long chain molecules that may run continuously be¬tween several of the component crystallites (see Figure 1).

In modern times, two main schools of thought have grown up regarding the basic nature of fibrils. On the one hand, there are those who believe that fibrils are "preformed" to a characteristic size or dimension. The other group argues that fibrils are a natural con¬sequence of the aggregation of long-chain molecules. If the formation of micelles—i.e., more or less discrete aggregates of molecules —is a reflection of an optimum thermodynamic state, then it may be said that both schools of thought are belaboring the same basic point. A third point of view is worth mentioning. It is true that the size of "micelles" or "fibrils" found after destructive chemical attack on a polymer is influenced by the severity of the Treatment. If these ultimate units of fiber or film structures reduce to finer and finer strands with increasingly severe chemical treatment, then it is conceivable, at least, that the true ultimate fibril is a single long-chain molecule.

Film
A film is a relatively thin skin, membrane, or pellicle less than 10 mils thick which usually is transparent or translucent.

Glass
In the broadest sense, a solid mass comprised of long-chain molecules that is transparent or translucent is termed a glass. In essence, glasses are considered to be supercooled liquids in which the long chain or macromolecules exhibit local regularities in structure only over relatively short ranges.

High Polymer
By convention, the term high polymer includes all materials whose chemical and physical structures depend on the arrangement in sequence of many monomers (identical or similar groups of atoms) connected by primary chemical bonds to form long chains or macromolecules. Aggregations of these long chains yield useful products which have more or less distinctive characteristics such as tensile strength, extensibility, elastic recovery, and many others.

The macromolecules that make up a high polymer consist of multiples of lower molecular weight units. The repeating units (monomers or mers) in the long chain macromolecule do not all have to be of the same size, nor do they have to possess exactly the same composition or chemical structure. Differences in composition and chemical structure in the units in the long chains of a high polymer arise from occasional branches, the presence of end groups, and other irregularities.

By way of clarifying different terms used by English speaking scientists and those in the continental European countries, it should be pointed out that the terms "high polymer" and "macromolecular substance" have the same meaning. Staudinger coined the word "macromolecule" to identify long chain molecules prior to 1930, and it has enjoyed wide usage on the Continent ever since. The term "high polymer" is more commonly used in Great Britain and the United States.

High polymer substances in general may be subdivided into several major types and classes:

Block Copolymer. In a block copolymer the repeating units con¬sist of segments or blocks of similar monomers tied together along the macromolecular chain. This differs somewhat from copolymers (see below) where the repeating unit consists of two or more different single monomers.
Branched High Polymer. This polymer is one in which the long chain molecule is not uniformly straight like a pencil, but has branches extending from its trunk as shown in Figure 2. The long-
chain molecule, despite these branches, remains unattached to other similar molecules surrounding it.

Copolymer. This is the term applied to a long-chain molecule comprised of at least two different monomers joined together in irregular sequence. A typical example of a copolymer is the textile fiber "Vinyon." This fiber consists of a series of vinyl acetate and vinyl chloride molecules repetitively joined together in a hand-in-glove arrangement. In contrast to this, a physical mixture of poly¬ethylene and polypropylene macromolecules, for example, is not a copolymer, but simply a mixture of homopolymers.
Segments of two separate "Vinyon" molecules are compared in Figure 3. Attention is called to the acetate group projecting from
the side of the molecule in the right hand formula; steric hindrance would prevent the chain segment from packing closely with the chain segment represented by the left hand formula.

Cross-linked High Polymers. These polymers are those in which the long-chain molecules — either straight or branched — have ladder-rungs or cross bridges binding them together as shown in Figure 4. In this type of high polymer, all or most of the component long chain molecules are rigidly locked to each other laterally by primary linkages (e.g., wool, Figure 4). This is different from "space-network" polymers, where the cross-bonding between chains proceeds along three-dimensional rather than by two-dimensional or lateral planes. In practice, however, the term
"cross-linked" polymer is sometimes applied to both (see Space-network High Polymer below).

Derived High Polymer. When a primary high polymer or a natural high polymer is altered chemically (so as to produce a derivative) it is called a derived high polymer.

Graft Copolymer. When a given kind of monomer is polymerized and, subsequently, another kind of monomer is polymerized onto the primary high polymer chain, a graft copolymer results.

High Polymeric (or Macromolecular) Compound. By convention, this term is more specific than the broad category of "high polymer." It is used to describe a substance consisting of a composite of long-chain molecules that are alike in composition, chemical structure, and size.
Primary High Polymer. This type of polymer is produced by the polymerization of chemically identical monomers into long chains, without subsequently altering the chemical nature of the resulting macromolecules.

Space-network High Polymer. When there are two or more reactive functional groups in the monomer or mer building block, the growth of the polymer in three dimensions is possible during the course of the polymerization. Such a process gives rise to a space-network high polymer. A good illustration is the reaction between glycerol and phthalic anhydride, which yields a three-dimensional network polymer. Other examples are thermosetting plastics such as the phenol-formaldehyde and the urea-formaldehyde types, respectively.

Stereoregular High Polymers: Atactic Polymer. When the R-groups or substituent groups are positioned on all sides of the main backbone of a long-chain molecule in a completely random manner, an "atactic" polymer results, as shown in Figure 5. Such
molecules cannot pack tightly together because of steric hindrance, and result in soft, non-crystalline and rather gummy products.

Isotactic Polymer. An isotactic polymer is one in which the R-groups or substituent groups all lie either above or below the main backbone of the long-chain molecule. Such an arrangement (sometimes referred to as stereoregular) makes possible a very highly ordered or compact high polymer, one that crystallizes readily (see Figure 5).

Syndiotactic (or Syndyotactic) Polymer. When the R-groups or substituent groups occupy positions that alternate regularly and in sequence above and below the main backbone of a long-chain molecule, a syndiotactic polymer results (see Figure 5). Such an arrangement permits relatively easy packing of the long-chain molecules and gives rise to substances whose properties lie between those of an isotactic and an atactic polymer, respectively.

Macromolecule. See discussion under High Polymer Micelle (see also Fibril, Crystallite)
A micelle is an aggregation of crystallites of colloidal dimensions that exist either in the solid state or in solution. It represents a reasonably reproducible dimension as the result of uniform chemical or mechanical treatments. It is sometimes used synonymously with the term "crystallite." It is very probable that a particular polymer micelle is an aggregation of long chains, reflecting the most stable state (thermodynamically) as the individual molecules pack from a solution or melt to form a solid product under a given environment.

Molecule (see also Monomeric Unit)
In the classic sense, a molecule is the smallest part of a substance that can exist separately and still retain a unique identity. This, in essence, defines a molecule as two or more atoms that are held together by primary or atom-to-atom bonds to produce a specific compound. On the basis of this definition, therefore, a diamond or piece of quartz might exist as a single molecule, just as could the simple gas oxygen (O2).

Monomeric Unit or Mer
All high polymers are formed by the joining together of many molecular units (as in polyethylene) or of groups of molecular units as illustrated in the case of cellulose. The monomeric unit or mer of a linear high polymer is the unit of the molecule which contains the same kinds and numbers of atoms as the real or hypothetical repeating unit.

The monomeric unit (or mer) of a linear high polymer is not, therefore, necessarily a "molecule." The repeating unit of many linear polvmers is a distinct segment of the molecular chain. The complete macromolecule (neglecting minor irregularities at the ends, branch junctions, etc.) might conceivably consist of a large number of these units, oriented in the longitudinal axis of the chain.

Polymer
A polymer in its broadest sense is a product formed by the combination of the same elements in the same proportions, but differing from the original "building blocks" in molecular weight. For example, cyanuric acid, C3N3O3H3, is a polymer of cyanic acid, CNOH, three molecules of CNOH combining to form C3N3O3H3. Similarly, paraformaldehyde (CH2O)n is a polymer of formaldehyde CH2O, in which n molecules of (CH2O) combine to give a new product or polymer (CH2O)n. (See High Polymer.)

Polymerization
The term "polymerization" refers to the process of formation of large molecules from smaller molecules, with or without the simultaneous formation of other products, such as water. (See Polymer.)

Some kinds of molecules, like ethylene (CH2=CH2), can react with themselves to form uniform long chain molecules, for example, —CH2—CH2—CH2—CH2—CH2— (polyethylene). In other cases Iwo kinds oi monomers react, forming copolymers.

Polyaddition. Polyaddition occurs when small molecules join each other, under the stimulus of a catalyst or a free radical mechanism to form linear polymers, usually without the coincident formation of by-product molecules. The formation of polyethylene from ethylene is a classic example.

Poly condensation. Polycondensation is a special type of polymerization commonly called C-polymerization. It refers to the union of monomers involving a chemical change, namely, release, or "splitting off" of simple molecules (such as H2O or NH3) coincident with the formation of macromolecules. The polycondensation of glucose by bacteria to produce long chain cellulose molecules with loss of a molecule of water is a typical example of this kind of polymerization. The formation of nylon by the polycondensation of hexamethyldiamine and adipic acid is a classic example of a synthetic high polymer produced by means of this type of polymerization.

Resin
A resin is a high polymeric compound which will not crystallize, is insoluble in water but soluble in some organic solvents, softens with heat, and may be a very viscous liquid or a solid at room temperature. Solid resins and thermoplastics are very similar, with physical state and properties at room temperature serving as the differentiating basis.

Spherulite
The term spherulite is being used in high polymer science in a sense paralleling its original definition, which refers to spherical crystalline bodies made up of radiating crystalline patterns like those found in mixtures of quartz and feldspar. A spherulite results from the aggregation of partly oriented crystallites that radiate outward from a common point to produce a pattern like butterfly wings. One characteristic of a spherulite structure is that the crystallographic axis of each of the component crystallites points outward somewhat like the spokes in a wheel.



Battista O. A,. 1958. Fundamentals Of High Polymers. Maruzen Asian Edition. Reinhold Publishing Corporation. New York