Nomex™ polyaramid
O
O
N
H
N
H
n
Discovered: 1958, by P.W. Morgan (Du Pont).
Commercialised: 1967, in fibre form, by Du Pont.
Key properties: Very good thermal resistance
(Tm = 371 deg.C, c.f. nylon 66 which melts at 260
deg.C). Essentially flameproof in fabric form. Good
resistance to organic solvents (because of
crystallinity) but attacked and hydrolysed by strong
acids and bases, especially at high temperatures.
Excellent dimensional stability. Rated for
continuous use in electrical applications (several
thousand hours) at 200 deg.C. Very much higher
resistance to ! radiation than aliphatic polyamides.
61
Nomex™ polyaramid
O
O
N
H
N
H
n
Synthesis
H3C
O2
HOOC
CH3
Mn/Co
HNO3
COOH
O2N
NO2
H2
SOCl2
ClOC
H2SO4
COCl
H2N
Ni
NH2
DMAc
O
O
N
H
N
H
n
62
Nomex™ polyaramid
O
O
N
H
N
H
n
Fibre production: Fibre-spinning does not require
isolation of polymer from the reaction solution.
The solution is partially neutralised with ammonia,
the NH4Cl filtered off, and neutralisation is completed
with CaO (the so-formed CaCl2 helps retain the
polymer in solution).
This solution is dry-spun into hot air at 200 deg.C,
the fibre wound up at ca. 100 m/min, extracted
withwater to remove the CaCl2, and finally drawn in
steam to 5 or 6 times its original length.
63
Nomex™ polyaramid
O
O
N
H
N
H
n
Crystal and molecular structure
Aromatic rings are parallel but
not coplanar. Both acid- and
amine-derived rings are tilted
some 30 degrees from the
plane of the amide
group.Networks of H-bonds link
molecules in both the a and b
directions (perpendicular to the
chain axes).
64
Nomex™ polyaramid
O
O
N
H
N
H
n
Typical applications for Nomex fibre
Heat-resistance applications: Filter bags for hotgas filtration (e.g. from steel-making plants).
Insulating paper for electric motors and
transformers. Braided tubing for wire-insulation.
Sewing thread for high-speed machine sewing.
Fire-resistance applications: Protective clothing
for foundry workers, welders, pilots, racing drivers,
and fire-fighters. Carpets, upholstery, and tents.
Dimensional-stability applications: Conveyor
belts, fire-hoses.
Permselectivity applications: Hollow fibres of
Nomex are excellent semi-permeable membranes,
and are used commercially for desalination of
sewater and brackish water.
65
Tenax™ Polyphenylene ether
Ph
O
Ph
n
Discovered: 1968 by Hay (GE), using oxidative
polymerisation of 2,6-diphenylphenol.
Commercialised: 1970, by Akzo (Holland).
Key properties: Amorphous 'as made' (Tg = 230
deg.C), but crystallises rapidly above Tg.
Tm = 480 deg.C, so polymer unprocessable once
crystallised. Amorphous polymer soluble in
chloroform and benzene.
Applications: By far biggest application is as
very high temperature adsorbent in gas
chromatography. Otherwise, applications
remarkably underdeveloped.
66
Tenax™ Polyphenylene ether
Ph
O
Ph
n
Synthesis:
Ph
Ph
O2, TMED-CuCl2
HO
O
ODCB, 85oC
n
Ph
Ph
Mechanism:
Ph
OH
Ph
Ph
OH
Cu(II)
Ph
.
O
Ph
Ph
O
Ph
H
.
Ph
OH
Ph
Cu(I)
Cu(II)
Cu(I)
H+
Ph
etc.
Ph
Ph
O
Ph
Ph
OH
O
H
Ph
+
Ph
Ph
OH
Ph
67
Tenax™ Polyphenylene ether
Ph
O
Ph
n
Crystal structure: Polymer is helical, with four monomer
residues per turn. Unit cell contains two chains, running in
opposite directions (the chain has directionality).
X-ray fibre diagram of Tenax
68
Part 3
Liquid Crystalline Polymers
69
Basic Concepts
Liquid Crystals: Crystals are fully ordered in three
dimensions over a long range. Liquids may have shortrange correlations of molecular position, but are essentially
disordered. What about long-range order in only one or two
dimensions?
Many organic compounds display phases ('liquid crystal-'
or 'meso-' phases) which have just such degrees of order.
Key feature is shape of molecule. It must be highly
anisotropic, so that the cumulative intermolecular forces
operate strongly in one or two directions, but not three.
Strong interaction
Weak interaction
Mesophases can occur lyotropically (by increasing the
concentration of a material in solution), or thermotropically
(by raising the temperature of a crystalline solid).
70
Basic Concepts
Liquid crystal polymers: Two main classes,
side-chain LC polymers and main-chain LC
polymers. Only concerned here with main-chain
types. Require an essentially rigid-rod structure.
Existence predicted by Flory in 1956. At about
the same time, Robinson and Ballard (Courtaulds)
discovered liquid crystalline solutions of synthetic
polypeptides, e.g. poly(g-benzyl-L-glutamate).
Rigid, linear, helical structure of such polymers
gives rise to lyotropic LC behaviour.
71
Basic Concepts
Rigid-rod polymers: Wide range of chemical
structures give rise to this type of polymer. Total rigidity
not necessary, though can be achieved, e.g. in
poly(p-phenylenebenzobisthiazole):-
N
S
S
N
n
Forms liquid-crystalline solutions in polyphosphoric acid
Low-energy molecular conformations should be
linear (i.e. restricted rotations needed), but significant
flexibility allowed:O
O
O
X
X
X
(X = NH or O)
O
X
72
Basic Concepts
Fibres: Characterised by anisotropy, both in dimensions
and properties. Ratio of l/d for natural fibres (cotton, flax,
wool) typically 1000 -2000. Anisotropic properties of
many fibres (strength, stiffness etc.) caused by molecular
orientation in the long-dimension of the fibre. For
conventional synthetic fibres, orientation is achieved
mechanically , by drawing:
draw direction
Perfect alignment very difficult to obtain with flexible
polymers, since cannot avoid knots and entanglements.
Consequently theoretical fibre performance not even
approached. Theoretical tensile strength depends only
on strength of covalent bond, since, when perfect
alignment achieved, intra-molecular bonds are weaker
than sum of inter-molecular forces (!)
73
Kevlar™ polyaramid
O
O
N
H
H
N
n
Discovered: 1965, by Stephanie Kwolek of
Du Pont.
Commercialised: 1971, by Du Pont.
Key Properties: Forms liquid-crystalline solutions
in organic solvents, and a liquid-crystalline
complex with sulphuric acid. Such solutions
self-orient under shear, so polymer can be spun to
give highly ordered fibres without subsequent
drawing. Kevlar fibres have extremely high
tensile strength (2.64 GPa) and modulus (127.5
GPa) when compared with aliphatic polyamides
such as Nylon 66 (0.90 and 5 GPa respectively).
Because of low density, Kevlar has highest
specific modulus of any known material. Very good
thermal stability (to 450 deg.C), and 60% of
strength is retained at 300 deg.C.
74
Kevlar™ polyaramid
O
H
N
N
H
O
n
Typical applications: The extreme tensile strength and
lightness of Kevlar® leads to applications including
knife-and bullet-proof clothing, boat hulls, racing cars,
cut-resistant gloves, fiber-optic cable-sheathing, firefighters! suits, fuel hoses, helmets, aircraft
components, radial tyres, spacecraft, bicycles, tennis
and squash racquets, golf clubs and skis.
Characteristics of fibre dominate composite-materials
properties – excellent in tension but poor in compression.
1.5
tension
1.0
COMPOSITE
STRESS (GPA)
0.5
compression
0.5
1.0
1.5
COMPOSITE STRAIN (%)
75
Kevlar™ polyaramid
O
O
N
H
H
N
n
Applications
76
Kevlar™ polyaramid
O
O
H
N
N
H
n
Synthesis: Original Du Pont process involved:COCl
NH2
+
RT
COCl
O
HMPA
O
H
N
N
H
n
NH2
Toxicity of HMPA required an alternative solvent
before commercialisation possible: NMP/CaCl2
found to be usable though less than ideal (low MW
oligomers tend to precipitate; problem overcome by
use of high-shear reactor-design).
Alternative synthesis (Higashi et al.):
COOH
NH2
+
P(OPh)3
NMP/LiCl
Py/CaCl2
100oC
COOH
NH2
O
O
N
H
H
N
n
77
Kevlar™ polyaramid
O
N
H
O
H
N
n
Mechanism of phosphorylative synthesis
COOH NH2
P(OPh)3
NMP/LiCl
Py/CaCl2
+
COOH NH2
O
O
100oC
H
N
N
N
H
n
PhOH
P
ArCOO
ArCOOH + P(OPh)3
OPh
OPh
Ar'NH2
ArCONHAr'
O
O
P
H
OPh
OPh
O
H P
Ar
NHAr'
OPh
OPh
78
Kevlar™ polyaramid
O
O
H
N
N
H
n
Fabrication: Polymer forms solid complex with
100% H2SO4 in mole ratio 1:10. Complex melts
at 70 deg.C to give a liquid crystalline phase which
can be spun into water, via an air-gap. Liquidcrystal domains become oriented during spinning:-
domain structure
spinneret
orientation
partial deorientation
air-gap
water
reorientation
precipitation
79
Kevlar™ polyaramid
O
O
N
H
H
N
n
Structure: As well as showing liquid-crystalline
behaviour in solution, Kevlar has a high degree
of 3-dimensional crystallinity in the solid state.
X-ray fibre pattern
N.B. C=O- - - H-N hydrogen bonds
80
Kevlar™ polyaramid
O
O
N
H
H
N
n
Crystal Structure:
N.B. C=O- - - H-N hydrogen bonds
81
Kevlar™ polyaramid
O
N
H
O
H
N
n
Supramolecular structure: Molecules form hydrogenbonded sheets which stack radially in the fibre. Nice
example of molecular self-organisation. Not previously
observed in synthetic fibres, and can even lead to
spontaneous growth of Kevlar fibres under gel-type
polymerisation conditions.
O
O
O
H
H
H
N
N
O
fibre
axis
O
O
O
O
O
N
N H
N H
N H
O
O
O
82
Kevlar™ polyaramid
O
N
H
O
H
N
n
Contrast between (high) tensile- and (low) compressive
modulus:
Chains are fully extended (all-trans)
so very high tensile modulus
Compressive stress
however leads to
buckling of the supramolecular sheet
structure
O
O
O
H
H
H
N
N
O
O
O
O
O
O
N
N H
N H
N H
O
O
O
83
Vectra™ thermotropic polyester
O
O
O
0.7n
O
0.3n
Discovered: 1974, by Calundann of Celanese.
Commercialised: 1985, by Hoechst-Celanese.
Key Properties: Melts at ca. 275 deg.C, to form
a liquid crystal phase which can be processed in
similar fashion to conventional thermoplastic, i.e.
by injection moulding, extrusion, or fibre-spinning.
Major differences however are 1) the mesophase
has very much lower viscosity than a conventional
polymer melt, and 2) molecular orientation occurs
under shear, so that anisotropic mouldings and
extrusions are abtained. Have very high modulus
in shear direction, but can be much weaker in other
diections. Orientation leads to fibrillar wood-like
texture. Materials are tough, with good impactresistance, and retain useful strength up to
200 deg.C. Thermochemically stable, with
continuous service temperatures up to 220 deg.C.
84
Vectra™ thermotropic polyester
O
O
O
0.7n
O
0.3n
Typical applications: Low mesophase viscosity
allows very complex and finely detailed mouldings
to be obtained (e.g. for microelectronic applications).
Transparency to microwaves, coupled with thermal
stability, means material suitable for microwave
cookware. Orientation under shear allows meltspinning of high-modulus fibre, c.f. Kevlar.
85
Vectra™ thermotropic polyester
O
O
O
O
0.7n
0.3n
Synthesis:
COOH
O
H3C
+
O
COOH
O
H3C
O
200oC, N2 purge
Clear melt
250 - 280oC
CH3COOH
Turbid dispersion
280 - 340oC
O
O
O
0.7n
O
0.3n
Opalescent polymer "melt"
86
Vectra™ thermotropic polyester
O
O
O
0.7n
O
0.3n
Q. Why a co-polymer?
A. Because the homopolymers are intractable.
The homopolymer of 4-hydroxybenzoic acid is in
fact manufactured, as 'Ekonol', but it can only be
fabricated via powder-metallurgy techniques such
as high-energy-rate forging and plasma-spraying.
O
O
PhOH
Ph
O
HO
n
O
Ekonol
'Ekonol' undergoes a thermal transition at 350
deg.C, to give an as yet incompletely characterised
phase which may or may not be liquid crystalline.
Copolymerisation with 2,6-HNA however
produces materials with a crystal-to-mesophase
transition as low as 250 deg.C (60:40, HBA:HNA)
87
Vectra™ thermotropic polyester
O
O
O
0.7n
O
0.3n
Sructure: The detailed structure of liquid crystal
polyesters is still a matter of debate. 13C NMR of
Vectra indicates the polymer-sequence is fully random,
and yet it shows a relatively high degree of crystallinity.
One possible explanation is based on formation of
non-periodic layer crystallites :-
ababbababbababaababababbbabaab
ababbaababbabaababbababaababba
aababbaababababababaabaaabbaba
abbbbabbabbabababbbaaabaabbaba
aababbabbabababaabbababbabbaab
Essentially involves matching of sequences in adjacent
chains. Limited length of matches results in low melting
point, but overall can still achieve significant crystallinity.
88
Vectra™ thermotropic polyester
O
O
O
0.7n
O
0.3n
Non-periodic layer crystallites: Very remarkable
theory, and increasing evidence appearing in the
literature, but still controversial idea. Best seen
via computational simulation:-
89
Chain Rigidity and Polymer Crystal Melting
We have seen that crystalline polymers with rather rigid and inflexible
chains (e.g. aromatic polyamides and aromatic polyesters) have
extremely high crystal melting points, generally above their
decomposition temperatures. Unless a thermotropic liquid-crystalline
phase can be obtained, e.g. by copolymerisation, such polymers
cannot be processed in the melt, and must be fabricated from solution
or by using solid-state techniques such as powder-sintering.
The close relationship between chain rigidity and polymer crystal
melting point can be explained as follows:
"Gm = "Hm - Tm"Sm,
but for a phase transition "G = 0, so that:
0 = "Hm - Tm"Sm
and therefore:
Tm = "Hm / "Sm
For a flexible chain, the entropy of melting "Sm, will be large as the
chain can take up only one conformation in the crystal but very many
in the melt state.Thus Tm will be low. For a rigid chain however, "Sm
will be small, as the number of different chain conformations available
in the melt is very small, and Tm will be very much higher. (This
assumes that "Hm, which depends mainly on intermolecular attractive
forces, is not greatly different in the two cases).
As an example, the aliphatic polyamide Nylon 6,6 melts at 280 °C,
whereas the much more rigid aromatic polyamide, Kevlar, does not
melt below its decomposition temperature of ca. 450 °C.
90