Biomedical Engineering Reference
In-Depth Information
Polypropylene
The story is slightly different for polypropylene (or according to IUPAC
polypropene
is derived from the monomer
propene
). The monomeric unit is: -.CH
3
/CH
CH
2
-.
The backbone still is -C-C- but with methyl side groups, called
chain sub-
stituents
,[-R]
[-CH
3
]. The methyl groups can be arranged all along one side of
the chain (
isotactic
) (Fig. 11.6), or only the second methyl group reaches the same
position as the first (
syndiotactic
).
3
The size of a methyl group is larger than the
distance between one and the next third carbon, so the isotactic isomer is for steric
reasons energetically less favorable. If the positions are random up and down of the
backbone, the isomer is called
atactic
.
An interesting aspect is the helical structure of syndiotactic polypropylene. One
turn in isotactic PP is made up by three successive units as shown in Fig.
11.7
.
Left- and right-winding helices are present in equal amounts because R is sym-
metric. In other polymers where R is asymmetric, helices are forced into a left- or
right-winding chain. Notice that the optical active biomolecules are all left-winding.
The amino acids added to MEM solutions, listed in Appendix D, are all the L-forms
(L-arginine, L-leucine, etc.). The influence of such a 3D conformation is reflected
in the profile of mechanical properties : the theoretical E modulus is 50 GPa, while
that of PE is 340 GPa! PE has a zig-zag chain, which is much harder to strain than
a helix. The practical E modulus of PP is 1.1-1.6 GPa. To illustrate how extreme
D
Fig. 11.6
Polypropylene
CH
3
CCH
2
H
CH
3
CCH
2
H
CH
3
CCH
2
H
left-winding
helix
Fig. 11.7
Representation of
the 3
l
of isotactic
polypropylene. White
balls: H, small black balls: C,
big white: CH
3
.
Left
: spatial
arrangement.
Right
:same
chain but not showing the
hydrogen atoms. The
positions of the methyl
groups correspond to a left
winding helix. By courtesy of
Springer [356, Fig. 22]
3
The terms are derived from the Greek words: taktikos
D
ordered; isos
D
equal; syn
D
with,
together; dio
D
two.
