Page 61 - Carrahers_Polymer_Chemistry,_Eighth_Edition

24 Carraher’s Polymer Chemistry

depending on the nature of the polymer, the treatment, and the function. Examples of secondary
structures appear in Figure 2.13.
The tertiary structure describes the shaping or folding of the polymer. Examples of this are given
in Figures 2.16b, 2.17a, and 2.17c.
Finally, the quaternary structure represents the overall shape of groups of the tertiary structures,
where the tertiary structures may be similar or different. Examples are found in Figures 2.15, 2.16a,
and 2.17b.

2.1 STEREOCHEMISTRY OF POLYMERS
The terms “memory” and “to remember” are similar and used by polymer chemists in similar, but

different ways. The first use of the terms “memory” and “to remember” involves reversible changes
in the polymer structure usually associated with stress–strain deformation of a rubber material where
the dislodged, moved polymer segments are connected to one another through chemical and physical
cross-links so that once the particular stress–strain is removed the polymer returns to its original,
prestress–strain arrangement of the particular polymer segments. Thus, the polymer “remembers” its
initial segmental arrangement and returns to it through the guiding of the cross-links.
The second use involves nonreversible changes of polymer segments and whole chain move-
ments also brought about through stress–strain actions or other means to effect nonreversible
changes. These changes include any synthetic chain and segmental orientations as well as post-
synthesis changes including fabrication effects. These changes involve “permanent” differences
in chain and segmental orientation, and in some ways these changes represent the total history of
the polymer materials from inception (synthesis) through the moment when a particular property
or behavior is measured. These irreversible or nonreversible changes occur with both cross-linked
and noncross-linked materials and are largely responsible for the change in polymer property as
the material moves from being synthesized, processed, fabricated, and used in what ever capacity it

finds itself. Thus, the polymeric material “remembers” its history with respect to changes and forces
that influence chain and segmental chain changes. The ability of polymers to “remember” and have

a “memory” is a direct consequence of their size.
We can get an idea of the influence of size in looking at the series of hydrocarbons as the num-

ber of carbon atoms increases. For hydrocarbons composed of low numbers of carbons, such as
methane, ethane, propane, and butane, the materials are gases at room temperature. For the next
grouping such as hexane and octane (Table 2.1) the materials are liquids. The individual hydro-
carbon chains are held together by dispersion forces that are a sum of the individual methylene
and end group forces. There is a gradual increase in boiling point and total dispersion forces for
the individual chains until the materials become waxy solids such as that found in bee waxes and
in birthday candles. Here, the total dispersion forces are sufficient to be greater than individual

carbon–carbon bond strength so the chains decompose before their evaporation. These linear

chains are sufficiently long to be crystalline waxy solids but not sufficiently long to allow the

chains to interconnect various crystalline groupings. Thus, they are brittle solids with little phys-
ical strength. As the chains increase in length the chain lengths are fi nally sufficient to give tough

solids we call linear polyethylene. It is interesting to note that many rocks and diamonds are very

strong but brittle because they exhibit essentially no flexibility. Single, almost completely linear,
polyethylene crystals can be grown; these are very strong but brittle. But, most linear polyethyl-

ene chains gain strength and some flexibility from the chains being sufficiently long to connect
the various crystalline domains into larger groupings. This connecting allows applied forces to
be distributed throughout the surrounding areas on a microlevel. Linear polyethylene generally
contains some portions that are not completely regular or crystalline. These regions are referred

to as amorphous and introduce into the polyethylene flexibility with sufficient free volume to
allow some segmental mobility. Since most linear polyethylene are not completely linear but
contain some branching, this branching prevents complete ordering of the chains, contributing to

9/14/2010 3:35:54 PM
K10478.indb 24 9/14/2010 3:35:54 PM
K10478.indb 24