A supramolecular nematic phase in sulfonated polyaramides

Article abstract of DOI:10.1039/b404809c

A water-soluble polyaramide sulfo-invert-PPTA surprisingly, already at a molecular weight of 10,000 g mol^-1, exhibits a nematic liquid crystalline phase in water at very low polymer concentrations, around 1-2wt%, indicating that supramo lecular assemblies of molecules are the building blocks of the nematic phase.

Full text of DOI:10.1039/b404809c

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A supramolecular nematic phase in sulfonated polyaramides
Sebastien Viale, Adam S. Best, Eduardo Mendes, Wolter F. Jager and Stephen J. Picken*
Delft University of Technology, Polymer Materials and Engineering, Dutch Polymer Institute, Julianalaan
136, 2628 BL Delft, The Netherlands; Fax: +31 15 2787415
Received (in Cambridge, UK) 30th March 2004, Accepted 10th May 2004
First published as an Advance Article on the web 9th June 2004
A water-soluble polyaramide “sulfo-invert-PPTA” surprisingly,
already at a molecular weight of 10,000 g mol21, exhibits a
nematic liquid crystalline phase in water at very low polymer
concentrations, around 1–2wt%, indicating that supramo-
lecular assemblies of molecules are the building blocks of the
nematic phase.
The poor solubility of sulfo-PPTA has been previously discussed
by Chu-Evan et al.,3 whereby the sulfonic acid group interacts with
the proton of the amide bond, reducing the solubility of the polymer
chain in solution and leading to the observed gel formation. The
enhanced solubility of sulfo-invert-PPTA as compared to sulfo-
PPTA might be explained by the presence of both the carbonyl and
sulfonic acid groups on the same monomer. It is expected that water
will interact in a much stronger manner due to a clear separation
between the sulfonic acid and the amino groups than in the case of
sulfo-PPTA. This may explain why a solution of sulfo-PPTA with
comparable molecular weight to those prepared here forms a
birefringent gel5b instead of a liquid nematic phase. The hydro-
phobic interaction of the unsubstituted aromatic ring is suspected to
contribute to the aggregation of the polymer chains.
Further characterization of the supramolecular liquid crystal
phase was achieved using X-ray scattering (XRD)¶. The 2-D data
for a 6wt% solution in water is displayed in Fig. 3, including the
equatorial reflection in log-log scale, which reveals a 21 slope at
high q values characteristic of a rod like system.
The scattering of the sample at low angles reveals that sulfo-
invert-PPTA has a substantial level of order in solution, confirmed
by the azimuthal scan (Fig. 3c) through the peak in the anisotropic
pattern: the calculated order parameter is 0.92. This value is
unusually high for such a low polymer concentration. By
comparison, a normal PPTA solution with molecular weight around
30,000 to 40,000 g mol21 forms a nematic phase at 8–10 wt%, and
at this point the order parameter only reaches a value of about
0.4.
Previous work by Kwei1 and ourselves2 report that poly-(p-
sulfophenylene-terephthalamide), or “sulfo PPTA”, forms a bi-
refringent nematic liquid crystalline gel in water. Further structural
studies provide strong evidence for the presence of supramolecular
aggregates in these gels.3,4 Similar behaviour was recently reported
for poly(p-benzamide) in DMAc/LiCl.5
We have found that by displacing the sulfonic acid group along
the polymer backbone – from the diamine to the acid ring – it is
possible to avoid gel formation in water and instead a free flowing
nematic phase is obtained. The synthesis and characterisation of
this material “sulfo-invert-PPTA” (1) will be discussed. Inter-
estingly, despite the rather low molecular weight of our samples,
lyotropic nematic behaviour is observed in water at very low
polymer concentration, around 1wt%. We propose that this is due to
the presence of supramolecular aggregates in these lyotropic
nematic solutions.
The aramid polymer PPTA is usually obtained via p-phenyl-
enediamine and terephthalic acid chloride. On the introduction of
the sulfonic acid group on the terephthalic acid (TA), the acid
chloride form could not be obtained. An alternative polymerization
of polyaramides was developed during the 1970’s, by Yamazaki et
al.,6 Higashi et al.,7 Preston et al.8 and Russo et al.9 all based on the
use of terephthalic acid and triphenylphosphite (TPP) (Scheme 1).
The polymerization conditions that have been used here were
inspired by the work of these authors.
To perform the synthesis the required 2-sulfoterephthalic acid
(sTA, 2) was readily obtained by oxidation of 2,5-dimethylbenze-
nesulfonic acid with potassium permanganate in water followed by
precipitation from solution using hydrochloric acid.†
Determination of the molecular weight (Mw) and polydispersity
index of our samples was achieved using Size Exclusion Chroma-
tography (SEC) in sulfuric acid.‡ Sulfuric acid was used as the
solvent in order to prevent aggregate formation. SEC shows that the
polymerization reaction gives materials of approximately 10,000 g
mol21 with a polydispersity index of 2.
Dissolving the sulfo-invert-PPTA powder in demineralised
water reveals nematic liquid crystal behavior at surprisingly low
polymer concentrations. A biphasic solution is found at 1wt%
where the upper 5% part is a biphasic nematic and isotropic mixture
(see Fig. 1§) and the remaining lower part is fully nematic. Fully
nematic solutions showing Schlieren textures are obtained for
solutions with polymer concentrations between 2 and 4wt%. For
higher concentrations (5 ~ 6wt%) coloured nematic textures are
obtained as shown in Fig. 2. This reveals that the system has a
substantial level of birefringence.
Fig. 1 Optical polarization micrograph of the upper part of a 1wt% “sulfo-
invert-PPTA” solution in water: nematic droplets suspended in isotropic
solvent are observed.
Fig. 2 Optical polarization micrograph of (a): 4wt% of “sulfo-invert-PPTA”
in water, showing a nematic Schlieren texture, and (b): 6wt% “sulfo-invert-
PPTA” water solution.
Scheme 1 Polymerization route of “sulfo-invert-PPTA” (1) using 2-sulfo-
terephthalic acid (sTA, 2) and p-phenyldiamine (PPD, 3).
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Assuming that qmax (Fig. 3b) is associated with the distance
between aggregates ~ 97 Å we can estimate the diameter of the
rods using a hexagonal lattice. This gives a width of 22 Å. Using a
cubic lattice does not significantly change the estimated value.
Applying the Onsager10 relation F* = 4d/L for the critical I–N
concentration for polydisperse rods, the length of the aggregates is
estimated to be about 8700 Å. Here we take the critical
concentration F*, to be about 0.01. The contour length for an
average sulfo-Invert PPTA chain is only 700 Å with a diameter of
5.4 Å. Consequently this would imply that there are over 2000
polymer molecules in the supramolecular rod-like aggregates
Since the polymer prepared here is a lyotropic system, it is of
interest to examine the effect of shear flow on the liquid crystalline
texture. Nematic solutions are well known to give rise to a variety
of interesting flow phenomena, such as flow-alignment, various
tumbling regimes and band texture formation. As a first result it is
worth noting that upon cessation of simple shear flow,11–12 applied
manually to the sample, a clear band-texture is formed as displayed
in Fig. 4.
Our results indicate the existence of a supramolecular nematic
phase in sulfo-invert-PPTA in water, probably containing a
microstructure of self-organized rod-like aggregates. Initial charac-
terisation of the nematic phase was performed using optical
polarization microscopy and X-ray scattering. Cessation of shear
flow reveals band-texture formation. These results indicate that we
have found a novel self-assembled liquid crystal polyelectrolyte.
Systematic variation of the solution parameters, like concentration,
thermal history, salt concentration etc. may provide additional
information for understanding this result. This will be developed
further in a forthcoming publication.
The authors thank Dr Anton Schotman and Bernard H. Schel,
Teijin Twaron Research Laboratory, Arnhem, The Netherlands, for
the size exclusion chromatography measurements. This work forms
part of the research program of the Dutch Polymers Polymer
Institute (DPI).
Notes and references
†
2-Sulfoterephthalic acid (2). In a 1 L reactor vessel equipped with a
mechanical stirrer and a cooler, 20.84 g (0.111 mol) of 2,5 dimethylbenze-
nesulfonic acid were dissolved in 500 ml of water, and then 82.36 g (0.52
mol) of KMnO4 were added stepwise during 5 hours. The reaction mixture
was heated at reflux during 48 hours, and then filtered to remove MnO2. HCl
was added to the filtrate and immediate precipitation of 2 occurred. The
yield was 15 g (54.5%). 1H-NMR (DMSO-d6) d 8.33 (s, 1H), 8 (dd, 1H, J
= 1.8 Hz, J = 8.1 Hz), 7.69 (d, 1H, J = 7.8 Hz). 13C-NMR (DMSO-d6) d
169, 167 (COOH), 145, 136.1, 132.62, 130.67, 130.34, 128.656.
Polymerization. In a 500 ml 3 necked flask, 10 g (40 mmol) of 2, 27.5 g
(88.7 mmol) of TPP, 20 g of LiCl and 50 ml of pyridine were dissolved in
200 ml of NMP. The solution was heated 40 °C for 15 min and then 4.35 mg
(40 mmol) of 3 added. The reaction mixture was then heated at 115 °C for
3 hours. The yellow viscous solution was precipitated in 1 L of methanol. A
yellow precipitate was obtained, filtrated and washed with methanol (500
ml) and diethyl ether (500 ml) The goldish powder was dried under vacuum
at 80 °C overnight, yielding 2.3 g (77%).Mw = 10,000 g mol21, PDI =
2.1H-NMR (DMSO-d6) d 11.32ppm (1H, s, NHCO), 10.5ppm (1H, s,
NHCO), 8.85ppm (1H, s), 8.48ppm (s, 1H), 7.5ppm (q, 4H), 7.34 ppm
(1H,s) 13C NMR (DMSO-d6): 165.26 ppm, 164.37 ppm CNO, d 144.44,
143.62, 130.69, 129.36, 128.33, 126.54, 125.89, 125.76, 120.9, 120.74,
120.03, 119.89.
‡
Size exclusion chromatography (SEC) has been performed by dissolv-
ing the sample in concentrated sulfuric acid (1 mg ml21) and separating it
using a modified Zorbax column (250 3 6.2 mm), and concentrated sulfuric
acid as the mobile phase (0.1 ml min21). A UV detector operating at 340 nm
was used for detection. From the chromatograms, the Mw values were
calculated using Cirrus version 1.1 GPC software (Polymer Labs). As
references in the SEC analysis, a Twaron (PpPTA) yarn, type 1010 and an
aramid trimer were used.
§
Optical polarization microscopy. The polymer samples were dissolved
in hot water and ultrasonic mixing at 60 °C for 2 hours and cooled down at
room temperature before further observation. A Jenapol microscope (3003
magnification) has been used to study the solutions between crossed
polarizer’s.
¶
X-ray scattering. Bruker-Nonius D8-Discover set-up with a 2D detector
has been used to perform the experiment. The sample was at 60 °C and the
sample to detector distance set at 29cm, and the incident beam wavelength
is 1.54 Å. In order to enhance the alignment of the liquid crystalline
solution, the sample was subjected to a magnetic field of approximately 4T
in the horizontal direction.
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Fig. 3 (a) XRD of a 6wt% solution at 60 °C and subjected to a magnetic field
along the horizontal direction, (b) 2-theta scan and (c) azimuthal scan along
the peak.
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Fig. 4 Band-texture formed after cessation of simple shear flow on a 6wt%
“sulfo-invert-PPTA” solution in water.
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