Page 79 - Carrahers_Polymer_Chemistry,_Eighth_Edition

42 Carraher’s Polymer Chemistry

While polymeric hydrocarbons have been used as illustrations for simplicity, it is important to
note that the principles discussed apply to all polymers, organic as well as inorganic, and natural as
well as synthetic, and to elastomers, plastics, and fi bers.

2.4 AMORPHOUS BULK STATE
An amorphous bulk polymer contains chains that are arranged in something less than a well-
ordered, crystalline manner. Physically, amorphous polymers exhibit a T but not a T , and do not
g m
give a clear X-ray diffraction pattern. Amorphous polymer chains have been compared to spaghetti
strands in a pot of spaghetti, but in actuality the true extent of disorder that results in an amorphous
polymer is still not fully understood.
Section 13.3 contains a discussion of a number of techniques employed in the search for the
real structure of the amorphous bulk state. Briefl y, evidence suggests that little order exists in the
amorphous state, with the order being similar to that observed with low molecular weight hydro-
carbons. There is evidence that there is some short-range order for long-range interactions and that
the chains approximate a random coil with some portions paralleling one another. In 1953, Flory
and Mark suggested a random coil model whereby the chains had conformations similar to those
present if the polymer were in a theta solvent. In 1957, Kargin suggested that amorphous polymer
chains exist as aggregates in parallel alignment. Models continue to be developed, but all contain
the elements of disorder/order suggested by Flory and Mark and the elements of order suggested
by Kargin.

2.5 POLYMER STRUCTURE–PROPERTY RELATIONSHIPS

Throughout the text we will relate polymer structure to the properties of the polymer. Polymer
properties are related not only to the chemical nature of the polymer, but also to such factors as
extent and distribution of crystallinity, distribution of polymer chain lengths, and nature and amount

of additives, such as fillers, reinforcing agents, and plasticizers, to mention a few. These factors

influence essentially all the polymeric properties to some extent, including hardness, fl ammability,

weatherability, chemical stability, biological response, comfort, flex life, moisture retention, appear-
ance, dyeability, softening point, and electrical properties.
Materials must be varied to perform the many tasks required of them in today’s society. Often they
must perform them repeatedly and in a “special” manner. We get an ideal of what materials can do
by looking at some of the behavior of giant molecules in our body. While a plastic hinge must be able
to work thousands of times, the human heart, a complex muscle largely composed of protein poly-
mers (Section 10.6), provides about 2.5 billion beats within a lifetime moving oxygen (Section 16.8)
throughout the approximately 144,000 km of the circulatory system with (some) blood vessels the
thickness of hair and delivering about 8,000 L of blood every day with little deterioration of the cell
walls. The master design allows nerve impulses to travel within the body at the rate of about 300 m/
min; again polymers are the “enabling” material that allows this rapid and precise transfer of nerve
impulses. Human bones, again largely polymeric, have a strength about five times that of steel (on a

weight basis). Genes, again polymeric (Sections 10.10 and 10.11), appear to be about 99.9% the same
between humans, with the 0.1% functioning to give individuals the variety of size, abilities, and so on,
that confer uniqueness. In the synthetic realm, we are beginning to understand and mimic the com-

plexities, strength, preciseness, and flexibility that are already present in natural polymers.

Here we will briefly deal with the chemical and physical nature of polymeric materials that per-
mit their division into three broad divisions—elastomers or rubbers, fibers, and plastics. Elastomers

are polymers possessing high chemical and/or physical cross-links. For industrial application the
“use” temperatures must be above T (to allow for ready “chain” mobility), and its normal state
g
(unextended) must be amorphous. The restoring force, after elongation, is largely due to entropy. On
release of the applied force the chains tend to return to a more random state. Gross, actual mobility

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