Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 29
Weight fraction
M n M w
10 2 10 3 10 4 10 5 10 6 10 7
Molecular mass
1.22 Molecular mass distribution for high-density polyethylene.
The longer molecules cannot readily do this, but the shorter molecules can
penetrate the pores. Thus, the larger the molecule, the less time it spends
inside the gel and the sooner it flows through the column. On their way down
the column, the larger molecules get ahead of the smaller ones, and by the
time the whole column is traversed the molecular masses are sufficiently
separated for their distribution to be measured as illustrated in Fig. 1.22.
Successive samples of solution are collected and the weight fraction of polymer
in each sample is estimated from measurements of its refractive index. The
apparatus is calibrated by measuring the time for a solution of known molecular
weight to traverse the column.
The simple linear chain of polyethylene may have its chemical constitution
modified in order to produce materials with different properties. By replacing
one or two H atoms of the monomer by a side-group or radical, the vinyl
group of polymers is formed: —(CH 2 —CXY) n —. If X is H and Y is Cl,
polyvinyl chloride is produced, CH 3 substitution for Y produces polypropylene
and C 6 H 5 gives polystyrene. If X is CH 3 and Y is COOCH 3 , polymethyl
methacrylate (PMMA) is produced. These substitutions make the monomer
molecule asymmetrical so the polymer chain now can be formed in several
ways:
(i) An isotactic linear polymer has all of the side group on the same side
of the chain (Fig. 1.23(a)).
(ii) A syndiotactic linear polymer has the side group alternating regularly
on either side of the chain (Fig. 1.23(b)).
(iii) If the side groups alternate randomly, it is termed an atactic polymer
(Fig. 1.23(c)).