Page 62 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 25

TABLE 2.1
Typical Properties of Straight Chain Hydrocarbons
Average
Number of Physical State at
Carbon Atoms Boiling Range ( C) Name Room Temp. Typical Uses
o
1–4 <30 Gas Gas Heating
5–10 30–180 Gasoline Liquid Automotive fuel
11–12 180–230 Kerosene Liquid Jet fuel, heating
13–17 230–300 Light gas oil Liquid Diesel fuel, heating
18–25 305–400 Heavy gas oil Viscous liquid Heating
26–50 Decomposes Wax Waxy Wax candles
50–1,000 Decomposes Tough waxy Wax coatings of food
to solid containers
1,000–5,000 Decomposes Polyethylene Solid Bottles, containers, fi lms
>5,000 Decomposes Polyethylene Solid Waste bags, ballistic wear,
fibers, automotive parts,

truck liners

the presence of amorphous regions. These amorphous regions include the portions of the chains
that interconnect the various ordered regions as well as regions resulting from the branching and
other related phenomena. Thus, synthetic polyethylene contains both crystalline regions, where
the polymer chains are arranged in ordered lines and which impart strength to the material, and
amorphous regions, where the chains are not arranged in as ordered lines and which contribute
flexibility with the combination giving a strong material, which on the basis of weight is stronger

than steel. The tensile strength/density for bulk steel is 500 while for ultrahigh molecular weight
polyethylene (UHMPE) it is about 3,800. The strength of UHMPE is recognized in many appli-
cations, including acting as one of the materials employed in the construction of many ballistic
resistant body armors (i.e., bullet-proof vests), where it acts to both blunt and distribute the energy
of incoming projectiles.
As a side note, low molecular weight polyethylene with appreciable side branching has a melting
o
range generally below 100 C whereas high molecular weight polyethylene with few branches has a
o
melting range approaching the theoretical value of about 145 C.
In general, most polymers contain some combination of crystalline and amorphous regions pro-

viding a material that has a combination of flexibility and strength.
High-density polyethylene (HDPE), formerly called low-pressure polyethylene, [H(CH CH ) )H],
2
2 n
like other alkanes [H(CH ) H], may be used to illustrate a lot of polymer structure. As in introduc-
2 n
tory organic chemistry, we can understand the properties and chemical activities of many com-
plex organic compounds if we understand their basic chemistry and geometry. HDPE, like decane
[H(CH ) H] or paraffi n [H(CH ) H], is a largely linear chain-like molecule consisting of cat-
2 about 50
2 8
enated carbon atoms bonded covalently. The carbon atoms in all alkanes, including HDPE, are
o
joined at characteristic tetrahedral bond angles of approximately 109.5 . While decane consists of
10-methylene groups, HDPE may contain more than 1,000 of these methylene units (Figure 2.1).
While we use the term normal or linear to describe nonbranched chains, we know that because of
the tetrahedral bond angles and ability for twisting that the chains are zigzag shaped with many
possible structural variations.
The distance between the carbon atoms is 1.54 Å or 0.154 nanometers (nm). The apparent zig-
zag distance between carbon atoms in a chain of many carbon atoms is 0.126 nm. Thus, the length
of an extended nonane chain is 8 units times 0.126 nm/units = 1.008 nm. For polyethylene, the
repeat unit has two methylenes so that the apparent zigzag distance is 2 × 0.126 nm or 0.252 nm for

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