What is a Polymer or What are Plastics?

Paul Flory [1] states that "…perhaps the most significant structural characteristic of a long polymer chain… (is) its capacity to assume an enormous array of configurations."  This means that a polymer is really defined by a dynamic feature that is closely associated with thermal motion of covalently or ionically, or even "metallically", bonded molecules.  Then the definition of a polymer, as one of the three branches of Materials Science is unique in that it does not depend on static atomic features [2], but rather on a dynamic molecular feature.  In fact, we can consider that chain molecules of pure sulfur display most of the structural and dynamic features that we associate with polymers and plastics and can therefore be classified as polymers. 

The definition of a polymer is inherently confusing mainly due to the background and lexicon obtained from metallurgy, chemistry and to a lesser extent physics.  For instance, Flory (above) attempts to describe structure, the quintessential static property, using the phrase "capacity to assume", meaning a thermally equilibrated dynamic state, e. g. entropy.  We can go as far as to state outright that in the absence of thermal motion, that is at absolute 0, polymers, in the sense describe by Paul Flory, do not exist.  (This is certainly not true for ceramics and metals for instance.)  Further, since the "capacity to assume varied configurations" relies on spatial degrees of freedom, a polymer, by nature, must have a lower mass-fractal dimension compared to the space in which it is embedded.  That is, a flexible chain that does not fill space can assume different configurations, that is has a non-zero configurational entropy, and is therefore a candidate to be a polymer, while a rod, which has no configurational entropy, or a protein in its native state, with a fixed configuration, are not candidates to be polymers.  The importance of a molecules-ability to explore configurational space is vital for most of the properties we associate with plastics and polymers.  The reason for this will be explained in this course.

Since a "structure defined by dynamics" is of pivotal concern to an understanding of what a polymer is, we must consider dynamic properties of polymers as the most important feature to be understood and described.  Unfortunately, dynamic properties are some of the more difficult and least visited of academic topics especially in the undergraduate curriculum.  While modulus and strength are generally considered at an early stage of education, rheology and especially dynamic rheology are less common subjects.  Molecular dynamics is even less touched upon even in graduate education.

Further, polymers never occur in the purely crystalline state and usually display fairly complex morphologies that are related to their "structure defined by dynamics" as well as the logistics of packing chain molecules in crystalline motifs.  These make an understanding of the morphology of polymers more complicated than metals and ceramics, though a much richer subject area.  Properties and structure are related to a much wider range of size scales (structural levels) in polymers compared to metals and ceramics.  A semi-crystalline polymer displays vital structural features on the atomic, nanometer, colloidal, and micron scales simultaneously and the mechanical response of this material is largely determined by the interaction between these different length scales in a dynamic sense. 

The industrial importance of polymers is based on inexpensive processing, low cost materials with robust and stable mechanical and chemical properties.  Polymers are ideal for packaging due to the flexibility of processing and are widely seen as a route to lower cost in the automotive, electronics and aerospace industries.  Use of plastics in the automotive industry has been continuously and dramatically growing for the past 30 years and is one of the main mechanisms for enhancement of fuel economy in vehicles.  Hand-in-hand with expanded use of plastics as an alternative to metals, ceramics and wood products, is a growing awareness of the environmental consequences of oil based economies and waste management.  Though largely overlooked in the post-Clinton United States, most of the industrialized world have implemented variants of cradle-to-grave manufacturing regulations aimed at curbing solid waste (partly plastics) and non-degradable materials (largely plastics).  In most parts of the world recycling of plastics has become a business opportunity, though to a much lesser extent in the US.  Plastics are compatible with cradle-to-grave manufacturing but only if additives are controlled.  We should consider plastics and plastics manufacturing from the perspective of profitability but considering the total cost of a product to the larger society if we are to consider ourselves honest business professionals.  Nonetheless, plastics provide unique, high demand properties and non-degradable materials have the largely overlooked advantage that they do not contribute to chemical damage to the environment, a claim that an certainly not be made by paper products.  Consider, for instance, the tire.  No material alternative exists for the function of a tire and the development of the automotive industry was largely tied to the development of rubber chemistry in the 1930's and 1940's as well as engineering technology for mass production in semi-continuous processes of large rubber articles.  Although rubber is non-degradable and non-recyclable, it has not been replaced in any of the industrialized nations of the world simply because there is no alternative to organic elastomeric materials for tires.

A similar argument can be made for the development of radar in world war II and the electronics industry in the 1950's and 1960's with the development of synthetic schemes for polyethylene and other polyolefins as well as polymer processing unit operations for wire coating.  There is no alternative for plastic wire insulation and none of the modern electronic devices could have been developed prior to the development of polyolefins and the plastics extruder. 

We can make similar arguments for a wide variety of industries and technologies. 

1)  Principles of Polymer Chemistry,  Flory PJ, (1953).

2)  "The atoms in ceramic materials are held together by a chemical bond. The two most common chemical bonds for ceramic materials are covalent and ionic. For metals, the chemical bond is called the metallic bond. The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. That is why, generally speaking, metals are ductile and ceramics are brittle.", American Ceramics Society web page tutorial, http://www.ceramics.org/acers5/acers/aboutceramics.asp?id=acers#Definition.