05 | 2010

Polyurethanes | Elastomers
with the polyether content. One of the key parameters is the
modulus of the corresponding polyamide homopolymer.
The modulus of polyamide homopolymers increases with
the amide / methylene ratio. Indeed, a higher concentration
of amide groups will lead to higher crystallinity and hence
a more rigid material. Unfortunately, melting point of the
material will also increase, together with moisture uptake
and density. Absorption of water will have a significant impact
on mechanical properties, for instance the modulus of PA6
can be reduced by half under moist conditions. Increasing
the density does not help towards designing lighter materials
for demanding athletes. Another hurdle is that the solubility
parameter of polyamide increases with the amide / methylene
ratio, increasing the gap with the solubility parameter of the
polyether, leading to a higher enthalpy of mixing. Mixing of
the two blocks will thus be more difficult, leading to slower
polymerisation, if any.
Among the possible polyamides, the odd ones, meaning
having an odd number of carbons, do have peculiar
properties, as the positioning of the amide groups in the
chain is important for structural order and packing efficiency.
Among these polyamides, PA11 rapidly became the centre of
our attention. Indeed, being an odd polyamide, its elementary
lattice can theoretically lead to either a parallel or an antiparallel
configuration of the chains with every amide group
able to be engaged with another one through hydrogen bonds.
Depending on the cooling procedure, crystals will either have
a hexagonal arrangement or triclinic one. Usually, both are
co-existing. A very interesting phenomenon is that the triclinic
phase can change into a pseudo-hexagonal one under
thermal or mechanical stress. Thus, the amount of energy
needed to perform the crystalline transition will decrease
the energy dissipated within the material, thus explaining the
outstanding strain hardening behaviour of PA11 vs. PA12. In
the case of PA12, an anti-parallel configuration of the chains
is only observed because of the even number of carbon and
the extra twist of the chains, necessary to optimise hydrogen
bonding, leads to a γ-monoclinic structure. On a mesoscopic
scale, the two materials exhibit noticeable differences:
ringed spherulites can be observed for PA11, whereas coarse
spherulites are the typical form for PA12.
The next step was then to try and see whether it was
possible to transpose the remarkable mechanical properties
to a multi-segment block structure comprising a PA11 block
with a very low molecular mass.
Traction Modulus (MPa)
1500
1250
1000
750
500
250
0
PEBAX ® Rnew 70R53
PEBAX ® 7033
0 10 20 30 40 50 60 70 80
Figure 2 Evolution of the traction modulus of Pebax
copolymers depending on polyether content
PA12
PA11
Figure 3 Differences in the crystallite structure
between PA12 and PA11.
bioplastics MAGAZINE [05/10] Vol. 5 47