Ordered Langmuir-Blodgett films derived from a mesogenic polymer amphiphile

Article abstract of DOI:10.1039/b003220o

The synthesis of a novel side-chain liquid crystalline polymer in which cholesterol mesogenic units are linked to a styrene backbone by a tris(ethyleneoxy) spacer is described. This amphiphilic polymer displays an unusual discontinuity in its monolayer behaviour at the air-water interface. This behaviour appears to be associated with the attainment of a closely-packed arrangement of the cholesterol units at higher values of surface pressure. Glancing angle X-ray diffraction shows that a well-ordered Y-type film is obtained if deposition is performed at a surface pressure above that of the apparent rearrangement.

Full text of DOI:10.1039/b003220o

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J O U R N A L O F
Ordered Langmuir±Blodgett ®lms derived from a mesogenic
polymer amphiphile
Ziad Ali-Adib, Peter M. Budd, Neil B. McKeown* and Kamolrat Thanapprapasr
Department of Chemistry, The University of Manchester, Manchester, UK M13 9PL.
E-mail: neil.mckeown@man.ac.uk
C H E M I S T R Y
Received 20th April 2000, Accepted 19th July 2000
First published as an Advanced Article on the web 24th August 2000
The synthesis of a novel side-chain liquid crystalline polymer in which cholesterol mesogenic units are linked to
a styrene backbone by a tris(ethyleneoxy) spacer is described. This amphiphilic polymer displays an unusual
discontinuity in its monolayer behaviour at the air±water interface. This behaviour appears to be associated
with the attainment of a closely-packed arrangement of the cholesterol units at higher values of surface
pressure. Glancing angle X-ray diffraction shows that a well-ordered Y-type ®lm is obtained if deposition is
performed at a surface pressure above that of the apparent rearrangement.
The fabrication of thin organic ®lms is an important area of
research which underpins a number of existing and emerging
technologies.1,2 Although many recent advances involve ®lms
with active functionality (e.g. electroluminescence) there is still
a requirement for highly uniform organic ®lms which can act as
robust passive layers. For example, electronic biosensors are
often designed to mimic the lipid bilayer found in cellular
membranes by embedding a receptor in a thin, defect-free
insulating layer so that a clear electronic signal is observed in
the presence of an analyte.3 In addition, uniform passive thin
®lms are of great importance as spacers in various optical and
electronic devices.4 The highly ordered ®lms prepared from
amphiphilic molecules (e.g. long-chained alkanoic acids) by the
Langmuir±Blodgett technique are particularly attractive for
this purpose, although they suffer from a severe lack of thermal
and mechanical stability. As compared to low molar mass
amphiphiles, preformed polymeric amphiphiles generally
produce LB ®lms with enhanced mechanical and thermal
resilienceÐbut at the cost of a less ordered structure.5±8
Cholesterol, a rigid component of many natural lipid
bilayers,9 continues to play an important role in the ®eld of
self-organising organic systems since the liquid crystalline state
of matter was ®rst discovered in some of its ester derivatives in
the 1880s.10 For example, it has been a component of model
membrane systems, liposomes, chiral liquid crystals and
polymeric liquid crystals.9,11±13 An interesting amphiphilic
cholesteric derivative 1, described by Decher and Ringsdorf,
was found to exhibit both thermotropic (temperature induced)
and aqueous lyotropic (solvent dependent) liquid crystal-
linity.14 It also forms micellar aggregates and an ordered
monolayer ®lm at the air±water interface. However, it cannot
be deposited as a multilayer LB ®lm. More recently, Hodge et
al. prepared LB ®lms derived from amphiphilic cholesterol-
containing polymers based on a hydrophilic poly(maleic acid)
backbone (e.g. 2).15 It was shown that reasonably well ordered
LB ®lms can be prepared from mesogen-containing poly(-
maleic acid). For example, the LB ®lm derived from
amphiphilic polymer 2 displays four orders of X-ray diffraction
corresponding to the repeat bilayer spacing.
This paper describes the self-ordering and ®lm-forming
properties of a polystyrene derivative 3 in which the repeat unit
contains a pendant mesogen, closely related to the amphiphilic
compound 1. Of particular interest is the compatibility of the
hydrophobic polymer backbone with the formation of an
ordered LB ®lm.
Experimental
Material synthesis
High resolution (500 MHz) 1H NMR spectra were recorded
using a Varian Unity 500 spectrometer. IR spectra were
recorded on an ATI Mattson Genesis Series FTIR (KBr/
Germanium beam splitter). Elemental analyses were obtained
using a Carlo Erba Instruments CHNS-O EA 108 Elemental
Analyser. Routine low resolution chemical ionisation (CI) and
electron ionisation (EI) were obtained using a Fisons Instru-
ments Trio 2000. All solvents were dried and puri®ed as
2270
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This journal is # The Royal Society of Chemistry 2000
DOI: 10.1039/b003220o

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described in Perrin and Armarego.16 Silica gel (60 Merck 9385)
was used in the separation and puri®cation of compounds by
column chromatography. All materials were heated at 100 ³C
under vacuum for 18 hours as the ®nal step of puri®cation.
Molar mass was determined using gel permeation chromato-
graphy (GPC) using 3x PL Gel-Mixed B analytical columns
(Polymer Laboratories) calibrated against polystyrene stan-
dards and using THF as eluant.
(1H, dd, J 11,18), 6.82 (2H, d, J 9), 7.29 (2H, d, J 9); m/z (EI)
621 (MzzHz), 369 (Mz2C14H19O4), 253 (Mz2C27H46).
Poly(4-[8'-cholesteryloxy-3',6'-dioxaoctyloxy]styrene) 3
A solution of 6 (0.68 g, 1.1 mmol) and AIBN (1.5 mg,
0.009 mmol) in toluene (0.5 ml) was sealed in an ampoule
under vacuum. The solution was heated at 80 ³C for 3 h. Upon
cooling, THF (10 ml) was added to the reaction mixture and
the solution added dropwise to chilled methanol (0 ³C). The
resultant solid was collected by centrifugation and puri®ed by
reprecipitation (62) from THF solution into methanol to yield
3 as a white waxy solid (0.46 g, 68%), transition temperatures;
Tg~16.7 ³C, smectic A±isotropic~135.9 ³C (Found C, 79.41;
H, 10.40%; C41H64O4 requires C, 79.18; H, 10.53%); dH
(500 MHz, CDCl3): 0.65±2.45 (46H, m, steroid nucleus and
polystyrene Hs), 3.10±3.17 (1H, m), 3.58±3.62 (4H, m), 3.66±
3.74 (4H, br m), 3.79 (2H, br s), 4.00 (2H, br s), 5.28±5.31 (1H,
m), 6.10±6.60 (4H, br m).
8-Cholesteryloxy-3,6-dioxaoctyl toluene-p-sulfonate 4
To a solution of cholesterol (6.04 g, 15.6 mmol) and triethylene
glycol di-p-tosylate (21.52 g, 46.9 mmol) in anhydrous THF
(75 ml) was added sodium hydride (0.8 g, 32 mmol) and the
mixture stirred under nitrogen for 72 h at 50 ³C. Water was
added (20 ml) and the mixture extracted with ethyl acetate
(3660 ml), washed with water (2650 ml) and dried over
anhydrous magnesium sulfate. The solvent was removed and
the crude product puri®ed by column chromatography using
an eluent of ethyl acetate±petroleum ether (1 : 5) to give 4 as a
viscous, opaque and colourless oil (4.45 g, 42%), transition
temperature; Tg~226 ³C, smectic A±isotropic~55.6 ³C
(Found C, 71.28; H, 9.71; S, 4.96%; C40H64SO6 requires C,
71.31; H, 9.59; S, 4.76%); dH (500 MHz, CDCl3): 0.65±2.42
(46H, m, steroid nucleus and Ts CH3), 3.10±3.16 (1H, m), 3.52±
3.62 (8H, m), 3.66 (2H, t, J 7), 4.15 (2H, t, J 7), 5.28±5.31 (1H,
m), 7.32 (2H, d, J 10), 7.79 (2H, d, J 10); m/z (EI) 368
(Mz2C13H19SO6), 303 (Mz2C27H46).
Liquid crystal characterisation
Differential scanning calorimetry measurements were made on
a Seiko DSC 220C machine and calibrated using an indium
standard. Optical microscopy observations were made on a
Nikon Optiphot-2 microscope with a Mettler FP80 HT Hot
Stage. Low resolution X-ray diffraction from powder samples
Ê
were recorded using Cu-Ka radiation (l~1.54 A) from a
Philips PW1130/00 generator with a nickel ®lter. The samples
were contained in glass capillaries (Hilgenberg 0.01 mm thick
glass, 1.0 mm outside diameter X-ray capillaries) and placed in
the beam in an aluminium heating block. The temperature was
regulated by a Control Techniques Process Instruments 453
Plus Thermal Controller. The diffracted X-rays were detected
with a ¯at-plate photographic camera using Agfa-Gevaert
Osray M3 X-ray ®lm. The system was calibrated using sodium
chloride.
4-(8'-Cholesteryloxy-3',6'-dioxaoctyloxy)benzaldehyde 5
To a suspension of anhydrous potassium carbonate (1.46 g) in
dry THF (40 ml) was added 4-hydroxybenzaldehyde (1.14 g,
9.3 mmol). After stirring the mixture at 20 ³C for 1 h, a solution
of 4 (4.42 g, 6.6 mmol) in THF (10 ml) was added dropwise.
The mixture was stirred at re¯ux for 72 h and then washed with
aq. NaOH solution (2 M, 25 ml) and the product extracted
with ethyl acetate (60 ml63). The organic layer was washed
with aq. NaOH (2 M, 25 ml62) and water (20 ml63) then
dried over anhydrous magnesium sulfate. On removal of the
solvent, 5 was obtained as a viscous, opaque and colourless oil
(3.96 g, 97%) which was used in the next step without further
puri®cation. dH (500 MHz, CDCl3): 0.65±2.42 (43H, m, steroid
nucleus), 3.10±3.18 (1H, m), 3.58±3.62 (4H, m), 3.68 (2H, t, J
7), 3.74 (2H, t, J 7), 3.82 (2H, t, J 7), 4.20 (2H, t, J 7), 5.28±5.31
(1H, m), 6.99 (2H, d, J 10), 7.80 (2H, d, J 10), 9.84 (1H, s); m/z
LB Film fabrication and characterisation
Isotherm behaviour was measured for monolayers, spread
from chloroform (Aldrich, HPLC grade) solution, on pure
water at pH 5.5 with no added ions using the apparatus and
procedures described previously.6 Multilayer ®lm formation
was performed on clean glass slides treated with an atmosphere
containing 1,1,1,3,3,3-hexamethyldisilazane to provide
a
hydrophobic surface. Glancing angle X-ray diffraction from
the ®lms was recorded using Cu-Ka radiation in a Philips
PW1050 X-ray Diffractometer using a rotating intensity
detector.
(EI)
(Mz2C27H46).
623 (MzzHz),
369 (Mz2C13H17O5), 255
4-(8'-Cholesteryloxy-3',6'-dioxaoctyloxy)styrene 6
Results and discussion
To a suspension of methyltriphenylphosphonium bromide
(3.40 g, 9.5 mmol) in dry THF (25 ml) was added potassium
tert-butoxide (1.08 g, 9.6 mmol). The resulting yellow suspen-
sion was added dropwise to a stirred solution of 5 (3.94 g,
6.3 mmol) over a period of 0.5 h and the reaction was stirred
for a further 3 h. Water (20 ml) was added, the THF was
removed under vacuum and the mixture extracted with ethyl
acetate (40 ml62). The organic layer was washed with water
(20 ml63), dried over anhydrous potassium carbonate and the
solvent was removed under vacuum. The crude product was
puri®ed by column chromatography using an eluent of ethyl
acetate±petroleum ether (1 : 5) to give 6 as a viscous, colourless
oil (3.30 g, 80.5%), transition temperatures; Tg~229 ³C,
smectic A±chiral nematic~18.0 ³C, chiral nematic±isotro-
pic~20.4 ³C (Found C, 79.00; H, 10.30%; C41H64O4 requires
C, 79.18; H, 10.53%); dH (500 MHz, CDCl3): 0.65±2.42 (43H,
m, steroid nucleus), 3.10±3.18 (1H, m), 3.58±3.62 (4H, m), 3.68
(2H, t, J 7), 3.74 (2H, t, J 7), 3.84 (2H, t, J 7), 4.12 (2H, t, J 7),
5.06 (1H, d, J 11), 5.28±5.31 (1H, m), 5.59 (1H, d, J 18), 6.58
Material synthesis
Polystyrene 3 was prepared using the route outlined in
Scheme 1. Previously, mesogen-containing polystyrenes have
been prepared by grafting the mesogenic unit onto preformed
poly(p-hydroxystyrene).17,18 More recently, mesogenic styrene
monomers have been prepared to determine their utility for
photo-initiated in situ polymerisation.19±21 The chosen route,
via the styrene monomer 6, was anticipated to ensure a
homogeneous sample of 3. The synthesis of 6 was achieved
using a Wittig reaction between the benzaldehyde derivative 5
and methyltriphenylphosphonium bromide, using work by
FreÂchet as precedence.22 Free radical polymerisation of 4 in
deaerated toluene solution using AIBN as initiator gave 3 in
68% yield.23 High resolution 1H NMR showed that the
repeated reprecipitation of 3 from THF into methanol had
successfully removed any unreacted monomer. GPC analysis of
3 indicated a number average molar mass of 266103 amu
J. Mater. Chem., 2000, 10, 2270±2273
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(polydispersity~2.4) relative to polystyrene standards and
con®rmed the absence of non-polymeric contaminants.
Thermal behaviour
Thermal analysis of polymer 3 by DSC reveals a reproducible
and reversible glass transition commencing at 20.3 ³C on
heating, although on the initial heating cycle this transition is
observed at 35 ³C. The latter value is consistent with the
observation that 3 is obtained as a waxy solid at room
temperature on precipitation from methanol. A very sharp ®rst
order endotherm is observed at 135.9 ³C (enthalpy of
transition~5.5 mJ mg21), on heating, which polarising optical
microscopy shows to be associated with a liquid crystal to
isotropic liquid transition. On cooling slowly from the isotropic
melt, a largely homeotropic optical texture is observed with ®ne
focal conic defects consistent with the formation of a smectic A
mesophase. A powder X-ray analysis of the mesophase (at
80 ³C) shows strong lamellar ordering with three orders of
Ê
diffraction corresponding to a smectic layer spacing of 53 A.
This ®gure is signi®cantly larger than the estimated length of
Ê
the side chain unit (y32 A) which indicates that the mesophase
possesses a bilayer structure. It is interesting to compare the
large thermal range of the liquid crystal phase of polymer 3
with the thermal behaviour of its monomer 6 which has a
narrow chiral nematic phase between 18.0 and 20.4 ³C with a
smectic A phase at lower temperatures. This represents an
extreme example of the well documented enhancement in the
thermal stability of the smectic phase caused by the ordering
effect of the polymer backbone.24
Fig. 2 Glancing angle X-ray diffractograms of Y-type LB ®lms
composed of 100 monolayers of polymer
3 deposited onto a
hydrophobic substrate: a) deposition at 30 mN m21 (relative intensity
of ®rst order peak~249 counts s21
, second order peak~43
counts s21); b) deposition at 42 mN m21 (relative intensity of ®rst
order peak~422 counts s21, second order peak~93 counts s21).
there is a reversible and highly reproducible discontinuity so
that at higher surface pressures (40±50 mN m21) the p±A
curves of all three materials are very similar. Such
a
Monolayer properties
discontinuity on a p±A isotherm is associated with either a
molecular rearrangement within the adsorbed monolayer,
resulting in a lower area per repeat unit, or the destruction
of the monolayerÐusually the latter is not reversible whereas
the monolayer of 3 only becomes irreversible at values of
surface pressures approaching the collapse pressure which is in
A small quantity of 3 was adsorbed at the air±water interface
by carefully placing a few drops of dilute chloroform solution
onto the surface of ultra-pure water contained in a Langmuir
trough and allowing the solvent to evaporate. On compression
of the adsorbed layer, the relationship between surface pressure
excess of 50 mN m21
.
(p/mN m21) and the area occupied per repeat unit (A/A ) was
Ê 2
recorded. The p±A isotherm of 3 is given in Fig. 1, together
with those of compound 1 and polymer 2 taken directly from
the literature.14,15 All three isotherms were recorded under
similar conditions (20 ³C, pH of subphase y5.5). It is apparent
that the general appearance of the p±A isotherm of 3 differs
from that of 1 and 2. Speci®cally, at low values of p the area per
repeat unit of 3 is signi®cantly greater but at p~38 mN m21
LB ®lm formation
Deposition of polymer 3 onto a hydrophobic substrate was
successful using the vertical deposition technique both on the
upward and downward motion of the substrate through the
monolayer to give a Y-type multilayer ®lm. In both directions
the transfer ratios approached unity. LB ®lms were prepared
from monolayers held at a surface pressure of 30 mN m21, a
conventional value for ®lm deposition, and at 42 mN m21Ða
value just above the unusual discontinuity in the p±A isotherm.
Both types of ®lm were highly uniform in appearance with a
gold lustre. However, glancing angle X-ray diffraction analysis
(Fig. 2) showed that there is a profound difference between the
degree of order within ®lms prepared at these two surface
Fig. 1 The surface pressure, p/mN m21 versus area per repeat unit, A/
Ê 2
A
isotherms for 1 (dashed line), 2 (thin solid line) and 3 (thick solid
line) recorded on an aqueous subphase at 20 ³C, pH~5.5, compression
speed~1 mm s21. The data for 1 were taken from ref. 14 and that of 2
from ref. 15.
Scheme 1
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pressures. Surprisingly, the ®lm with the higher degree of order
was derived from the monolayer held at 42 mN m21 which
indicates that the discontinuity in the p±A isotherm of 3 is not
associated with collapse of the monolayer. This ®lm displays at
least three orders of diffraction which correspond to a repeat
Acknowledgements
We acknowledge the ®nancial support of the National Metal
and Materials Technology Centre which is a development
agency of the Ministry of Science, Technology and Environ-
ment, Thailand (KT).
Ê
layer spacing (d) of 53.8 A. The lamellar spacing obtained from
this ®lm is almost identical to that observed from the powder
diffraction analysis of the smectic mesophase of 3. The d-
spacing is again signi®cantly larger than the estimated length of
References
Ê
the side chain unit (y32 A) and indicates a bilayer structure, as
would be expected for a Y-type LB ®lm. Some degree of
intercalation or tilting of the side-chains is inferred from the
smaller observed lamella spacing as compared to the estimated
1
2
3
4
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6
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8
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bilayer width (y64 A). In contrast, the ®lm deposited at a
surface pressure of 30 mN m21 shows only two distinct orders
Ê
of diffraction (d~52.6 A) which are only half as intense as the
corresponding re¯ections from the ®lm prepared at
42 mN m21. There is also diffuse diffraction due to scattering
from regions of ill-de®ned structure including a broad, weak
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A reasonable conclusion from these results is that the
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transition between a disordered monolayer arrangement at
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Ê 2
21
Ê 2
30 mN m21 (52 A ) than at 42 mN m
(40 A ) may be
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associated with the hydrophobic polystyrene backbone dis-
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unit at 42 mN m21 is consistent with that expected for a close
packed arrangement of the cholesterol units similar to that
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Although many pre-formed polymers have been used for the
fabrication of LB ®lms, there are far fewer examples which
show
a convincing layered structure by glancing angle
XRD. Generally, these are based on a hydrophilic polymer,
for example poly(maleic anhydride),5±8 poly(ethyleneoxy)25 or
poly(acrylamide),26 with hydrophobic side-chains. The present
work demonstrates that it is possible to obtain an ordered LB
®lm from a hydrophobic polymer containing an appropriate
side-chain. The importance of the hydrophilic oligo(ethyle-
neoxy) spacer is apparent by our failure to form a stable
monolayer at the air±water interface derived from a similar
polymer to 3 but containing an alkyl (undecyl) spacer. In
addition, it is likely that the success of 3 as a ®lm forming
material is due to the extreme hydrophobicity and excellent
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the polymer chain in stabilising the monolayer is also apparent
from the failure of monomer 6 to form a monolayer at the air±
water interface. It is possible to envisage other LB ®lm-forming
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acts as the hydrophilic group.
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