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Polymer Chemistry. The Basic Concepts - Himenz P.C.

Himenz P.C. Polymer Chemistry. The Basic Concepts - Copyright, 1984. - 736 p.
Download (direct link): polymerchemistry1984.djvu
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In this chapter we have focused attention on various aspects of
individual polymer molecules. In the next three chapters we shall examine
some properties of assemblies of polymer molecules. Our interest in these
chapters will be mostly directed toward samples of pure polymer;
assemblies of high and low molecular weight molecules-polymer solutions-
will be discussed in Part III of this book.
We shall divide our discussion of bulk polymers on the basis of the
mechanical states of these materials; that is, instead of classification
into states based on thermodynamic criteria alone, we favor a system
based on the way samples respond to deforming forces. Accordingly, we
shall discuss the viscous state in Chap. 2, the elastic and viscoelastic
states in Chap. 3, and the glassy and crystalline states in Chap. 4.
There are good reasons for choosing these mechanical states as the basis
for classification and, incidentally, for presenting this material right
at the start of this text. If polymers did not possess the mechanical
properties they have, they would, in all likelihood, still be in the
category of laboratory curiosities-or disasters. Instead, their sluggish
flow, elastic snap, plastic flexibility, and toughness in wear are all
marketable properties. Each of these properties is ultimately traceable
to the chain structure of polymer molecules. It is not the amide in its
IUPAC name that makes nylon the important material that it is; it is the
poly which is primarily responsible!
66
The Chains and Averages of Polymers
In Chap. 2, we consider polymers whose molecular weight and/or
temperature gives them liquid flow behavior. We shall begin by
quantitatively examining what we mean by viscosity and then consider how
this property depends on, say, how fast the material is stirred and on
the characteristics of the polymer itself. It should not be too
surprising that the length of the chain is the most important polymer
property in this regard. All we need to do is think of our experience
with a plate of spaghetti to get an idea of what happens when we try to
induce some motion in a mass of tangled flexible "chains."
Polymers are not alone in displaying elastic properties, but their
elasticity is unique in both origin and magnitude. For one thing, the
elastic restoring force which acts in polymers increases with increasing
temperature, whereas the opposite is true in, say, metals. Again, this is
a consequence of the chain structure of the polymer. Stretching a live
snake to a "fully extended conformation" may be problematic under the
best of conditions, but it will resist all the more if we warm it up
besides. We shall see in Chap. 3 that polymer chains behave in pretty
much the same way.
Chapter 4 is concerned with the solid forms of polymers. Crystals of
any material consist of highly ordered arrays of molecules. In view of
the assortment of variations that can occur within polymer chains, it is
not surprising that only those polymers with a high degree of chain
regularity crystallize. Even crystalliz-able polymers fall short of
complete crystallinity. Those polymers which do not crystallize, whether
partly or totally, still solidfy in the mechanical sense at a temperature
called the glass transition temperature. Below this temperature the
thermal energy available for chain motion is inadequate to allow much
relative motion between chains. Low molecular weight materials also form
glasses, but the chain structure of polymers makes this behavior one to
which polymers are particularly susceptible.
In the next group of chapters-indeed, throughout the book-we shall be
talking about a variety of properties of individual polymer molecules and
assemblies of these molecules. One concept that should be fairly clear
from the discussions of this chapter is that the word average should be
used as a modifier in almost every statement we make about polymers. This
is true in many areas of chemistry: Even an assembly of relatively simple
water molecules possesses an assortment of isotopic possibilities;
translational, rotational, vibrational, and electronic energy states; and
different extents of hydrogen bonding and ionization. Still we speak of
the water molecule as if it were an invariant species. What we are really
talking about is an average molecule. Unless we specifically wish to
underscore the statistical aspect of a statement, however, the word
average is generally implied rather than stated. So it will be in this
volume. One of the themes of this chapter is the diversity of structures-
even down to something as fundamental as molecular weight-that polymers
display. Accordingly, the word average is even more appropriate as a
modifier in polymer
Problems
67
chemistry than in chemistry as a whole. More often than not, however, the
actual word is implied rather than stated. Remembering that this is the
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