Page 97 - Science at the nanoscale

RPS: PSP0007 - Science-at-Nanoscale
7:6
June 12, 2009
4.4. From Molecules to Supramolecules
for every new C–O covalent linkage forms between the glucose
molecules.
Mankind has learnt to mimic nature by producing a variety of
macromolecules as synthetic polymers, or plastics. Thus, small
molecules containing specific functional groups are chosen and
chemically reacted into linear or branching chains of polymers.
Common examples are nylon (polyamides) and PVC (polyvinyl
chloride) that have taken over traditional raw materials such as
fabrics, wood, concrete and clay.
Chemists and material engineers have learnt to control the poly-
merisation process to optimise the molecular properties (i.e. chain
length, branching, density, crystallinity, etc.) in order to obtain the
desired material properties. One successful example is polyethy-
lene (PE), which consists of simple –(CH 2 –CH 2 ) n – repeating units.
3
While high density polyethylene (HDPE, density ≥0.941 g/cm )
is industrially processed to have a low degree of branching and
thus has stronger intermolecular forces, low density polyethylene
3
(LDPE, density between 0.910–0.940 g/cm ) is produced with a
high degree of branching and thus the chains do not pack into well
crystalline order. Thus HDPE is suitable for applications such as
packaging as jugs, bottles and water pipes; whereas molten LDPE
has desirable flow properties for it to be processed into flexible
plastic bags and film wrap. Our ability to make synthetic poly-
mers and to control their material properties have revolutionised
the way of living in today’s world.
While scientists and engineers are good at optimising material
properties at the macroscopic scale, there is still a long way to
learn for making delicate materials or machineries at the nanome-
tre scale. Returning to Nature for a clue, we see that in complex
biopolymers such as proteins and DNA (Figs. 4.13–4.14), weak 87 ch04
intermolecular interactions are elegantly exploited to organise
simple building blocks into functional structures with multi-levels
of complexity. On one hand, the strong covalent bonding between
the linkages has ensured that the molecule remain intact dur-
ing reformation; on the other hand, the weak interactions allow
the complex structures to be constituted and re-constituted flexi-
bly. Two points are significant for this building blocks approach:
(i) information needed for constructing the complex structures are
encoded already in the building blocks, e.g. the folding of DNA is
largely determined by the sequence of base-pairs in the strands,
(ii) a large number of weak interactions provides the driving force