Reversible monolayer-to-crystalline phase transition in amphiphilic silsesquioxane at the air-water interface

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T12420 Silicone and Siloxane Chemistry

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Received
16 October 2014

Accepted
21 January 2015

Published
17 February 2015

Correspondence and

requests for materials

should be addressed to

[M.K.S. (milank.](mailto:milank.sanyal@saha.ac.in)

[sanyal@saha.ac.in)](mailto:milank.sanyal@saha.ac.in)

## Reversible monolayer-to-crystalline phase transition in amphiphilic silsesquioxane at the air-water interface

R. Banerjee 1, M. K. Sanyal 1, M. K. Bera 1, A. Gibaud 2, B. Lin 3 & M. Meron 3

1Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700064, India,
2LUNAM, IMMM, Faculte´ de Sciences, Universite´ du Maine, UMR 6283 CNRS, Le Mans Cedex 9, 72085 France, 3Center for
Advanced Radiation Sources, The University of Chicago, Chicago, IL 60637, USA.

We report on the counter intuitive reversible crystallisation of two-dimensional monolayer of
Trisilanolisobutyl Polyhedral Oligomeric SilSesquioxane (TBPOSS) on water surface using synchrotron
x-ray scattering measurements. Amphiphilic TBPOSS form rugged monolayers and Grazing Incidence
X-ray Scattering (GIXS) measurements reveal that the in-plane inter-particle correlation peaks,
characteristic of two-dimensional system, observed before transition is replaced by intense localized spots
after transition. The measured x-ray scattering data of the non-equilibrium crystalline phase on the
air-water interface could be explained with a model that assumes periodic stacking of the TBPOSS dimers.
These crystalline stacking relaxes upon decompression and the TBPOSS layer retains its initial monolayer
state. The existence of these crystals in compressed phase is confirmed by atomic force microscopy
measurements by lifting the materials on a solid substrate.

t is believed that the lowest energy state in materials is crystalline as all the elements except helium and vast
array of compounds equilibrate in crystalline state and once this state is achieved one requires substantial
change in temperature and/or pressure to take a material into non-crystalline state. However, reversible
# I
crystallization in the tendons in the forearms of animals when stretched and/or dried 1 and reversible structural
changes of proteins in trans-membrane proton pumping mechanism of purple membrane (bacteriorhodopsin) 2,3

have been observed. Reversible crystallization in polymers observed through small angle X-ray scattering 4 and
atomic force microscopy (AFM) 5 have revealed that a material may remain in a non-equilibrium reversible state,
trapped in a local minimum rather than in the equilibrium crystalline state which, once attained, is irreversible.
Here we report reversible crystallization of two-dimensional monolayer of Trisilanolisobutyl Polyhedral
Oligomeric SilSesquioxane (TBPOSS) on water surface and by repeating the reversible crystallization cycle
one can form well-ordered crystals of TBPOSS on water surface. The results of this study may help us to
understand the recent evidences of metastable amorphous-to-crystalline reversible phase transformations in
inorganic materials like GaSb 6 and also reversible martensitic transitions in some shape-memory alloys, which
have been actively considered as medical sensors and actuators, eco-friendly refrigerators, and energy conversion
devices 7,8 . There are also reports on reversible shape changes of molecular crystals which find widespread usage as
actuators under the influence of external stimuli like photo-irradiation 9 .The general consensus is that for such
macroscopically reversible transitions, the symmetry groups of the initial and the final phases are of paramount
importance 8 . To the best of our knowledge the quantitative structural evaluation of reversible phase transitions to
form bulk-like crystals from monolayers reported here, has not been reported earlier.

Polyhedral Oligomeric SilSesquioxane (POSS) molecules possess a cage-like structure (1–3 nm in size) and a
hybrid chemical composition (RSiO1.5)n which is intermediate between silica (SiO2) and silicones (R2SiO). R can
be hydrogen, an alkyl, alkylene, aryl, or arylene group, or an organofunctional derivative of these latter groups and
n is an even number 10 . Their well defined structures contain a stable inorganic Si-O core surrounded by groups
which can be chemically tailored to include a wide range of polarities and reactivity 11,12 . This unique property has
accelerated potential applications in space-survivable coatings 13, shape memory materials 14, semiconducting
polymers 15 and as synthetic templates for nanostructured materials 16 . POSS molecules can be incorporated into
polymer systems through blending, grafting or copolymerization and can be used as technologically important
nano-composite polymeric materials for reinforcement 17,18 whose properties can be tuned between organic
plastics and ceramics. Such techniques have successfully improved physical and chemical properties such as

super-oleophobicity 19, increased glass-transition temperatures 20,
low-k dielectric constants 21 and thermal stability 22 .

There has been considerable interest to improve on the mechanical stability, swelling behaviour and thermo-mechanical properties of hydrogels, which act as host for the POSS molecule, and
has led to extensive research work over the last couple of decades.
Enhancement of such properties to more than 300% of the original
by adding small amounts of POSS (,5%) have been reported 23 . The
augmentation in mechanical properties of hybrid materials urges
potential usage in high quality plastics and rubber.

Apart from technological applications, POSS monolayer on water
can be a very important system to understand two-dimensional
ordering in the presence of hydrophilic and hydrophobic interaction
as the functional groups of POSS can be tuned by appropriate
chemistry to change its affinity towards water. In case of TBPOSS
the Si-O-cage has both hydrophobic (-R 5 -CH2CH(CH3)2) as well
as hydrophilic (-OH) functional groups (see Fig. 1(a)) rendering an
amphiphilic property to the molecules. Functionalized POSS monolayers undergo structural transition when compressed in a Langmuir
trough 24–26 and consequently show interesting changes in their viscoelastic response and surface rheology 27,28 . However, the structural
phase transitions (bringing about appreciable in-plane as well as outof-plane restructuring) in the monolayer of the POSS itself, when
compressed on a Langmuir trough has not been investigated in
details. Having an insight into the structural behaviour (pre and post
phase-transition) should in principle be very useful for understanding the ordering and restructuring of the molecules and provide
viable routes for fabrication of ultra-thin films of POSS with fascinating physical and chemical properties.

In this paper, we report results of an in-situ Grazing Incidence Xray Scattering (GIXS) study that exhibits reversible process of formation of a crystalline phase of TBPOSS from a monolayer phase on
water surface. We evidence that under compression, the GIXS patterns exhibit very clear changes going from the existence of a single
correlation peak (characteristics of a monolayer on water surface) to
powder Debye-Scherrer rings and finally to intense well defined
Bragg spots which are oriented with respect to the water surface.
The most intriguing result is the reversibility in formation of such
patterns under repeated compression-decompression cycles showing the appearance and disappearance of crystalline order as per the
surface pressure imposed by pressure barrier of the Langmuir trough.
We have proposed here an explanation of the observed reversible
monolayer-to -crystalline phase transition based on dimerization of
molecules following an earlier study 25 . Based on Brewster angle
microscopy measurements of the TBPOSS monolayer at air water
interface, it was proposed 25 that in a collapsed phase, dimers form in

the upper layer by hydrogen bonding. The experimental results
observed here can be explained assuming that the dimers having
hydrophobic shells, arrange in crystalline stacks upon compression
on the water surface and relax back to the monolayer phase upon
withdrawal of lateral pressure. We also observed that the crystal
structure of TBPOSS on water surface differed from the one observed
in solid form by conventional powder diffraction 29 . The results are
supported by Atomic Force Microscopy (AFM) measurements on
films which were transferred to solid substrates both before and after
the formation of TBPOSS crystals. Sizes of the crystallites obtained
from GIXS measurements on the film post transition matched well
with the dimensions of the crystallites observed through AFM on the
film transferred to the substrate.

Experimental
A custom-built Teflon trough including a surface tensiometer (to measure surface
pressure), controlled by a NIMA controller box (NIMA, UK) and interfaced with a
computer data acquisition system was used for all experiments. TBPOSS (from
Hybrid Plastics, USA) was dispersed in Toluene and an aliquot with a concentration
of 0.3 mg/mL was prepared. About 80 mL of the solution was spread carefully using a
micro-syringe on the trough mounted on an anti-vibration device over the liquid
spectrometer and left undisturbed for an hour for the toluene to evaporate. The single
barrier trough was contained in an air-tight box with Kapton windows at opposite
ends for access to incident and scattered x-ray beam. The trough was purged with
Helium gas until the oxygen level in the trough environment decreased to about 1% to
ensure the removal of residual vapours of toluene as well as reduce air scattering.

The monolayer was compressed at a speed of 10 mm/min at a temperature of
20uC to record the Pressure-Area isotherm. The surface pressure rose steadily on
regulated compression of the molecules from the gaseous state to a condensed
state and a phase transition was observed around 1.45 nm 2 area per molecule for
the surface pressure of 17 mN/m (see Fig. 1(b)). The pressure reduced slightly
immediately after the transition but remained constant throughout even for further compression. When the monolayer was decompressed the pressure fell
sharply to12 mN/m, remained almost constant for a while and then decreased to
zero. In the second cycle we could observe the same behaviour of the monolayer
with almost no material loss. The transition pressure was exactly at the same area
per molecule (<1.45 nm 2 ) and pressure (<17 mN/m) even for the second and
consequent cycles (Fig. 1(b)). This suggests the TBPOSS molecules form a rugged
monolayer on the water surface due to their amphiphilic nature and are highly
reproducible over repeated isotherm cycles. The flat portion in the isotherm is
indicative of a dynamical state of molecular stacking which is primarily
responsible for the intensity fluctuation observed since the molecules move in and
out of the area irradiated by the x-ray beam.

X-ray scattering measurements (see Fig. 2(a) for schematics of experimental
geometry) were performed on the monolayer both before and after the transition
to check the in-plane as well as out-of-plane ordering of the monolayer. Grazing
Incidence X-ray Scattering (GIXS) measurements were performed at
ChemMatCARS, Sector 15, Advanced Photon Source, Argonne National
Laboratory (USA) using 10 keV (l 5 1.24 A ˚ ) x-ray beam with fixed incident
angle hi (50.1u) which is below the critical angle of water (0.123u) at this energy.
A low angle of incidence (lower than the critical angle of water) ensured a low
penetration depth of the impinging x-rays due to which the scattering information
obtained was mainly from the monolayer. The incident x-ray beam was defined by

Figure 1 | (a) Schematic molecular structure of TBPOSS, here –R denotes –isobutyl group. (b) Pressure-Area isotherm of TBPOSS monolayer on
water, recorded at 20uC for 1 st (black solid line) and 2 nd (dashed red line) compression-decompression cycles. The repeatability of the isotherms shows
that the monolayer phase is retained even after undergoing a phase transition at each cycle at the same pressure and area per molecule. Blue circle denotes
the transition pressure which is same for the 1 st as well as the 2 nd cycle

incident slits of dimension 40 mm 3 200 mm (Vertical 3 Horizontal). The scattered x-rays were detected using a Pilatus 100 K Detector (Dectris) with pixel size
of 172 3 172 mm 2 and only a horizontal detector slit of 300 mm in front of it. The
scattered intensities were then plotted as function of in-plane and out-of-plane
components (qxy, qz) of the momentum transfer vector (q) which is related to the
in-plane angle (Q) and out-of-plane angle (hf) respectively 30–33 . This particular
detector-slit configuration facilitated the detection of scattered data as a function
of hf, similar to 1D position sensitive detector (PSD) as shown in Fig. 2(a) 34 . The
two-dimensional (2D) GIXS patterns were obtained by scanning the detector
along Q. The intensity distribution along qxy and qz provides substantial
information about the in-plane and out-of-plane structures 31–33 respectively.

X-ray Reflectivity (XRR) and Grazing Incidence X-ray Off-Specular (GIXOS)
measurements were also performed but were inconclusive about any out-of-plane
structure due to a lack of electron density contrast between TBPOSS and water. Prior
to the phase transition, GIXS measurements showed a characteristic peak around a
qxy value of 0.54 A ˚ 21 (Fig. 2(b)) which is attributed to short range in-plane interparticle correlations. After the phase transition the qxy region around the correlation
peak transformed into well defined arcs resembling powder diffraction patterns
(Fig. 2(c)) along with the weak correlation peak signifying the presence of a monolayer reduced in area due to the compression and subsequent molecular stacking on
top of it. Intense spots on the arcs tend to appear and disappear intermittently. This is
a clear signature that some preferred orientation of the crystalline planes occurs as a
result of some movement of the crystallites over water surface within the coherent
scattering volume despite the fact that during the process of data acquisition the
barriers were locked after having reached the desired surface pressure. The x-ray data
were collected carefully for short duration of time and regular change of sampling
area was done to avoid beam-damage 33 . AFM measurements (Nanoscope IV, Digital
Instruments) were carried out in tapping mode using etched Si cantilever on thin
films of TBPOSS (pre and post phase transition) after transferring horizontally on
hydrophilized Si substrate.

Results and Discussion
The results of the GIXS measurements presented here clearly show
that a reversible monolayer-to-crystalline phase transition occurs at
1.45 nm 2 area per molecule for the surface pressure of 17 mN/m
(refer Fig. 2). The GIXS contour in Fig. 2(b) shows an inter-particle
correlation peak at qxy 5 0.54 A ˚ 21 for the monolayer phase. This
provides an estimate of the in-plane inter-particle separation
between the TBPOSS molecules and corresponds to a length scale
of 11.5 A ˚ which is quite close to the molecular dimension of
TBPOSS. The values of the in-plane scattered angles (Q 5 2h) for
the crystalline phase were estimated from the intensity profiles of the
GIXS plot (after transition) at qz 5 0 (Fig. 2(c)) and the corresponding dhkl values were determined by fitting the arcs assuming a triclinic
crystal structure with P � 1 symmetry. The values of 2h (or dhkl) thus
obtained were used to determine the following lattice parameters: a
5 13.824 A ˚, b 5 15.481 A ˚, c 5 14.015 A ˚, a 5 88.57u, b 5 110.41u
and c 5 109.51u. A decent agreement between the calculated and
observed dhkl values (see Table 1) is found but some of the planes
corresponding to calculated values for a particular set of h, k, l values
appeared to be missing in the data. Note that with only 5 reflections
measured with the available detector, the uniqueness of this set of
parameters is not clear. For this reason we have tried to find a set of
parameters as close as possible to the ones already published from
powder diffraction 29, i.e., a 5 14.697 A ˚, b 5 14.816 A ˚, c 5 15.667 A ˚,

Figure 2 | (a) Geometry of the scattering experiment on a liquid spectrometer having a Langmuir trough. Red arrow shows direction of compression.
Grazing Incidence X-ray Scattering (GIXS) data from monolayer of TBPOSS on water surface (b) before phase transition showing a inter-particle
correlation peak at qxy50.54 A ˚ 21 and (c) post-transition having defined Scherrer rings/arcs (corresponding to definite crystalline planes) along with
bright spots indicative of preferred orientation of the reflecting planes. The presence of a weak correlation peak in the background of the Scherrer rings
signifies the presence of a reduced area of monolayer underneath the crystalline stacking. Intensity scales have been provided alongside plots.

Table 1 | The observed* and calculated values corresponding to
different dhkl planes have been tabulated. Some of the Bragg planes
were missing in the observed GIXS data. Lattice parameters and
space group used to calculate the (h k l) values have been discussed
in the text

Observed dhkl (A ˚ ) Calculated dhkl (A ˚ ) Calculated (hkl) values

- 14.5138 0 21 0
0 1 0
13.0650 13.0648 0 0 1
12.1500 12.1502 21 0 0
1 0 0
11.4833 11.4831 1 21 0
21 1 0
11.1242 11.1242 21 0 1

- 10.2527 0 21 1

- 10.0083 21 1 1
9.2458 9.2560 0 1 1

*All measurements were recorded at 20uC.

a 5 67.70u, b 5 67.89u and c 5 71.14u. In this respect, the volume of
the unit cell of the crystals forming on water surface is 2635 A ˚ 3 while
for bulk crystal cited in literature it is 2858 A ˚ 3 . This shows that the
crystals forming on water surface are constricted in volume with
respect to bulk crystals precipitated from solution 29 . A possible
explanation for this contraction of the unit cell volume can be found
in the fact that reversibility demands non-equilibrium structure of
the amphiphilic TBPOSS and the proximity with water strains the
unit cell dimensions of the molecules when compared to the bulk
crystalline state. The formation of dimers constituting two TBPOSS

Table 2 | The values of the preferred angles for different reflecting
planes obtained from the GIXS contour. Labels correspond to spots
as in Fig. 3

Plane Labels Angles (u)

1 0 0 (i) 26.8
(ii) 23.4
(iii) 10.2
21 0 1 (iv) 34
(v) 26.1

molecules 29 is due to the hydrophobic nature of the alkyl arms and
the hydrophilic nature of the –OH groups.

The presence of intense spots at different positions on the Debye
Scherrer ring upon compression is a clear signature that the system
evolves towards crystallites that are preferentially oriented with
respect to the water surface (Fig. 3(a)). Two of the several planes
(viz. 1 0 0 & 21 0 1) were more intense than the others and were
considered for further analysis. The preferred tilt angles of the dimer
planes with respect to the water surface were estimated directly from
the position of the spots in the GIXS profile (Fig. 3) and have been
tabulated in Table 2. The crystallite size obtained from GIXS measurements is estimated by the Scherrer formula given by,

t~ k|l ð1Þ

b cos h

where t is the crystal grain size, k is a constant (0.94), l is the
wavelength (1.24 A ˚ ), h is half the angle corresponding to the intensity peak and b is the FWHM of the peak around 2h. The values were
calculated for the five prominent rods (Fig. 4(a)) by estimating the

Figure 3 | (a) GIXS measurements from monolayer of TBPOSS on water surface and the preferred angular orientation of the TBPOSS dimers for
different planes. The dotted lines are fits corresponding to different dhkl values. (b) (i) and (iii) show two angles (26.8u and 10.2u) at which the dimers
(shown within border of dashed box for only one angle) are tilted and correspond to the spots designated as (i) and (iii) respectively in the GIXS plot for
the (1 0 0) reflecting plane. The respective reflecting planes are shown in yellow shades. (c) (iv) and (v) show similar two angles (34u and 26.1u) of the
dimer tilt angle corresponding to the spots (designated as (iv) and (v) in GIXS plot) for the (21 0 1) plane

Figure 4 | (a): GIXS measurements on the monolayer, after transition in the 2 nd cycle, reveal the onset of 2D ordering in the monolayer (on top of which
the dimers form columnar stacks) due to the appearance of rods in place of spots. (b) Intensity profiles (scattered plots) along the rods (hf or qz)
and across the rods (Q or qxy) and respective Gaussian fits (red lines) to ascertain the FWHM from which the in-plane as well as out-of-plane crystal grain
size is estimated using Scherrer formula. (c): GIXS measurements showing spots on arcs after phase transition for 1 st compression cycle, no spots or arcs
for 1 st decompression cycle and spots replaced by rods after another phase transition (at the same pressure) for the 2 nd compression cycle.

FWHM (three representative profiles are shown in Fig. 4(b)) along
the rods and perpendicular to the rods. The average crystal grain size
comes out to be approximately2000 A ˚ in the in-plane direction and
100 A ˚ in the out-of-plane direction.
The isotherm was repeated and the second cycle closely followed
the first. The characteristic phase transition was also observed to
occur at the same pressure for the second cycle (Fig. 1(b)) but the
GIXS measurements showed that the bright spots over defined arcs
obtained in the first isotherm cycle (Fig. 3(a)) were replaced by Bragg
rods with almost no arcs at all in the second isotherm cycle (Fig. 4(a)).
Interestingly, the positions of the rods coincide with those of the
spots obtained in the first isotherm cycle. The preferred orientation
of the planes is responsible for the observed spots on the Scherrer

rings for 1 st cycle and rods for subsequent cycles. The presence of the
spots in the 1 st cycle is primarily due to the variance in the orientation
of the favoured crystal planes and additionally due to positional
uncertainty of the relatively loosely stacked motifs within the crystals. However, no spots or rods were observed when the monolayer
was decompressed. The rods were also short-lived like their
predecessors. This is a clear signature of increase in the in-plane
correlation length. It appears that over repeated compressiondecompression cycles, 2D ordering commences in the monolayer
after the first cycle. The sequence of events depicted in Fig. 4(c) show
clear spots over arcs in the GIXS data after phase transition in the 1 st

cycle, no features are visible when the monolayer is decompressed
and spots are replaced by rods after phase transition in the 2 nd cycle

Figure 5 | AFM images of the thin films of TBPOSS transferred on Si substrate from water surface, pre and post transition. (a) shows dendritic network
of POSS monolayer of average thickness 12 A ˚ and (b) shows crystal grains of average size,2004 A ˚ and thickness,101 A ˚ . The crystal grain size
from AFM measurements compares remarkably well with that estimated from the GIXS data using the Scherrer formula.

The AFM topographs revealed presence of a dendritic monolayer
of TBPOSS (height 12 A ˚ ) for the sample transferred before phase
transition (see Fig. 5(a)) and presence of crystallites of average lateral
size and height of 2004 A ˚ and 101 A ˚, respectively, for the sample
transferred after transition (see Fig. 5(b)). The height of the TBPOSS
monolayer (12 A ˚ ) obtained from AFM is in agreement with the inplane inter-particle distance extracted from GIXS measurements

(11.5 A ˚ ) for the film before transition. The average lateral dimensions and the height of the crystallites seen in the film post transition
through AFM match well with the in-plane and out-of-plane dimensions of the crystallites obtained from GIXS data.

Based on detailed analysis of x-ray scattering data collected before
and after the transition from a monolayer to non-equilibrium crystal
phase, we now propose a mechanism of reversible phase transition

Figure 6 | Schematic illustration of the mechanism of reversible meta-stable transition of TBPOSS molecules from monolayer phase to crystalline
phase by stacking of molecular dimers as a function of lateral pressure. The movable barrier is denoted by B. (a) TBPOSS monolayer before compression
and (b) after 1 st compression which leads to compact monolayer resulting in an inter-particle correlation peak in the GIXS profile. (c) Further
compression leads to a phase transition to crystalline phases wherein the molecular dimers form crystalline stacks over the monolayer. The preferential
alignment of the stacking planes with respect to the water surface over the monolayer is responsible for the intense spots on the Scherrer rings. These spots
are replaced by intense Bragg rods on further compression decompression cycles

observed here (shown schematically in Fig. 6(a)–(c)). In this proposed mechanism it is assumed that the monolayer-to-crystal phase
transition is preceded by dimer formation. It is to be noted here that
the concept of such dimers in TBPOSS was initially proposed 29 on the
basis of x-ray power diffraction measurements and was also used to
explain the collapse of a Langmuir monolayer 25 . We believe that once
the dimers form, they spontaneously self-assemble into nonequilibrium crystal structures on further compression. This gives rise
to the sharp transition peak in the isotherm but only one peak is
visible in the isotherm upon compression as reported earlier 25 . Below
this transition peak during compression we could only measure a
correlation peak in the GIXS measurements while above this peak, at
lower area per molecule, the GIXS suddenly showed crystalline diffraction peaks. This is a signature of the transition from a monolayer
to a dimer and then eventually to the crystalline phase as a kinetically
driven process. It was not possible in this experiment to evidence the
transient regime from a dimer to crystalline phase - future studies
using a nano-sized x-ray beam utilizing detectors with much faster
data acquisition capability may be able to decipher this kinetically
driven phase transition.

The dimers start forming unstable crystals primarily by van der
Waals forces when compressed. On decompression, the dimers
which were stacked together as crystallites came in contact with
water and the crystalline stacking disappeared. The monolayer phase
was re-established but the dimers did not completely disintegrate
into monomers. Further compression cycles forced these aligned
dimers into a more compact packing leading to better alignment of
the crystals in 2D than formed during the first compression cycles
thus justifying the existence of Bragg rods (onset of 2D ordering in
the monolayer) instead of spots in further compression/decompression cycles. This repetitive process of crystalline stacking and
unstacking of the molecules as a function of lateral compression
clearly supports the concept of meta-stable crystal formation as
schematically illustrated in Fig. 6. The crystal structure so formed
has certain preferential orientations with respect to the water surface.
The reason for these preferential orientations is still not completely
understood. Similar studies with other POSS molecules with different end groups attached have to be performed to understand the
nature of monolayer-to-crystalline phase reversibility.

Summary and Conclusion
Characteristic phase transition has been observed for TBPOSS monolayer on water surface. The isotherms in consequent cycles were reversible and in each cycle the phase transition occurred at the same area per
molecule and pressure as previous cycles. GIXS measurements have
been performed on monolayer of TBPOSS before and after transition.
GIXS data show that the TBPOSS monolayer exhibits a rare reversible
non-equilibrium crystalline phase on lateral compression. The crystal
structure obtained here using GIXS measurements is different from the
one obtained by conventional powder diffraction measurements for
TBPOSS 29 . Although the crystal structure and symmetry were the same
as reported, we observed a constricted lattice for the triclinic structure.
GIXS measurements are consistent with AFM measurements done on
films transferred both before and after the phase transition. We can
subsume that the crystalline phases observed due to application of
lateral pressure are apparently reversible meta-stable phases formed
by stacking of molecular dimers which relax to the initial monolayer
phase as soon as the lateral pressure is removed. The detailed understanding of the mechanism of stacking and relaxing of the molecular
dimers as a function of lateral pressure necessitates further investigation
by altering the alkyl and the hydroxyl groups of the POSS molecule.
This phenomenology can be extended to explain the process of transient
reversible crystallization observed in nature in various other materials.

1. Clark, J. H. Reversible crystallization in tendons and its functional significance.
Proc Natl Acad Sci U S A 14 526 (1928)

2. Dencher, N. A., Dresselhaus, D., Zaccai, G. & Bu¨ldt, G. Structural changes in
bacteriorhodopsin during proton translocation revealed by neutron diffraction.
Proc. Natl. Acad. Sci. U.S.A. 86, 7876 (1989).
3. Edman, K. et al. High resolution X-ray structure of an early intermediate in the
bacteriorhodopsin photocycle. Nature 401, 822 (1999).
4. Tanabe, Y., Strobl, G. R. & Fischer, E. W. Surface melting in melt-crystallized
polyethylene. Polymer 27, 1147 (1986).
5. Androsch, R. & Wunderlich, B. Reversible crystallization and melting at the lateral
surface of isotactic polypropylene crystals. Macromolecules 34, 5950–5960 (2001).
6. Kalkan, B., Edwards, T. G., Raoux, S. & Sen, S. Nature of metastable amorphousto-crystalline reversible phase transformations in GaSb. J. Chem. Phys. 139,
084507 (2013).
7. Song, Y., Chen, X., Dabade, V., Shield, T. W. & James, R. D. Enhanced reversibility
and unusual microstructure of a phase-transforming material. Nature 502, 85
(2013).
8. Bhattacharya, K., Sergio Conti, S., Zanzotto, G. & Zimmer, J. Crystal symmetry
and the reversibility of martensitic transformations. Nature 428, 55 (2004).
9. Kobatake, S., Takami, S., Muto, H., Ishikawa, T. & Irie, M. Rapid and reversible
shape changes of molecular crystals on photoirradiation. Nature 446, 778 (2007).
10. Baney, R. H., Itoh, M., Sakakibara, A. & Suzuki, T. Silsesquioxanes. Chem. Rev. 95,
1409–1430 (1995).
11. Lichtenhan, J. D., Vu, N. Q., Carter, J. A., Gilman, J. W. & Feher, F. J.
Silsesquioxane-Siloxane copolymers from polyhedral silsesquioxanes.
Macromolecules 26, 2141 (1993).
12. Provatas, A. et al. Silsesquioxane: Part I: A key intermediate in the building of
molecular composite materials. J. Organometallic Chem. 565, 159–164 (1998).
13. Hoflund, G. B., Gonzalez, R. I. & Phillips, S. H. In situ oxygen atom erosion study
of a polyhedral oligomeric silsesquioxane-polyurethane copolymer. J. Adhes. Sci.
Technol. 15, 1199–1211 (2001).
14. Knight, P. T., Lee, K. M., Qin, H. & Mather, P. T. Biodegradable thermoplastic
polyurethanes incorporating polyhedral oligosilsesquioxane. Biomacromolecules
9, 2458–2467 (2008).
15. Xiao, S. et al. Stabilization of semiconducting polymers with silsesquioxane. Adv.
Funct. Mater. 13, 25–29 (2003).
16. Choi, J., Kim, S. G. & Laine, R. M. Organic/Inorganic hybrid epoxy
nanocomposites from aminophenylsilsesquioxanes. Macromolecules 37, 99–109
(2004).
17. Li, Q., Hutcheson, S. A., McKenna, G. B. & Simon, S. L. Viscoelastic properties and
residual stresses in polyhedral oligomeric silsesquioxane-reinforced epoxy
matrices. J. Polym. Sci., Part B: Polym. Phys. 46, 2719–2732 (2008).
18. Fu, B. X. et al. Nanoscale reinforcement of polyhedral oligomeric silsesquioxane
(POSS) in polyurethane elastomer. J. Polym. Int. 49, 437–440 (2000).
19. Tuteja, A. et al. Designing superoleophobic surfaces. Science 318, 1618–1622
(2007).
20. Haddad, T. S. & Lichtenhan, J. D. Hybrid organic-inorganic thermoplastics:
styryl-based polyhedral oligomeric silsesquioxane polymers. Macromolecules 29,
7302–7304 (1996).
21. Leu, C.-M., Chang, Y.-T. & Wei, K.-H. Polyimide-side-chain tethered polyhedral
oligomeric silsesquioxane nanocomposites for low-dielectric film applications.
Chem. Mater. 15, 3721–3727 (2003).
22. Janowski, B. & Pielichowski, K. Thermo(oxidative) stability of novel
polyurethane/POSS nanohybrid elastomers. Thermochim. Acta 478, 51–53
(2008).
23. Eshel, H., Dahan, L., Dotan, A., Dodiuk, H. & Kenig, S. Nanotailoring of
nanocomposite hydrogels containing POSS. Polymer Bulletin 61, 257–265
(2008).
24. Deng, J. et al. Polyhedral oligomeric silsesquioxanes: A new class of amphiphiles at
the air/water interface. J. Am. Chem. Soc. 124, 15194 (2002).
25. Deng, J. et al. Polyhedral oligomeric silsesquioxanes amphiphiles: Isotherm and
brewster angle microscopy studies of trisilanolisobutyl-POSS at the air/water
interface. Langmuir 20, 109–115 (2004).
26. Deng, J., Farmer-Creely, C. E., Viers, B. D. & Esker, A. R. Unique rodlike surface
morphologies in trisilanolcyclohexyl polyhedral oligomeric silsesquioxane films.
Langmuir 20, 2527–2530 (2004).
27. Deng, J., Viers, B. D., Esker, A. R., Anseth, J. W. & Fuller, G. G. Phase behavior and
viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface.
Langmuir 21, 2375–2385 (2005).
28. Yin, W., Deng, J. & Esker, A. R. Surface rheology of trisilanolisobutyl-POSS at the
air/water interface. Langmuir 25, 7181–7184 (2009).
29. Feher, F. J., Newman, D. A. & Walzer, J. F. Silsesquioxanes as models for silica
surfaces. J. Am. Chem. Soc. 111, 1741–1748 (1989).
30. Bera, M. K. et al. Reversible buckling in monolayer of gold nanoparticles on water
surface. Europhys. Lett. 78, 56003 (2007).
31. Banerjee, R., Sanyal, M. K., Bera, M. K., Singh, A., Novak, J. & Konovalov, O.
Structural re-ordering in monolayers of gold nanoparticles during transfer from
water surface to solid substrate. Phys. Rev. E 83, 051605 (2011).
32. Tolan, M. X-Ray Scattering from Soft Matter Thin Films, (Springer, Berlin,
1999).
33. Daillant, J. & Gibaud, A. X-Ray and Neutron Reflectivity: Principles and
Applications (Springer Paris 2009)

34. Danauskas, S. M. et al. Monitoring x-ray beam damage on lipid films by an
integrated Brewster angle microscope/x-ray diffractometer. Rev. Sci. Instrum. 78,
103705 (2007).

Acknowledgments
Authors are grateful to Prof. J. Matisons, School of Chemical and Physical Sciences, Flinders
University, Adelaide 5001, Australia for providing the TBPOSS samples. R. Banerjee, M. K.
Sanyal and M. K. Bera acknowledge Department of Science and Technology India for full
financial support to perform experiments at APS, ANL, USA. A. Gibaud and M.K. Sanyal
acknowledge the support of Indo French Centre for the Promotion of Advanced Research
(CEFIPRA/IFCPAR) for exchange visits under project No. 3808-3. ChemMatCARS Sector
15 is supported by the National Science Foundation under grant number NSF/
CHE-1346572. Use of the Advanced Photon Source, an Office of Science User Facility
operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National
Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

Author contributions
R.B., M.K.S. and M.K.B. designed and performed the experiment and did the data analysis,
B.L. and M.M. supported the experiments and data analysis, A.G. provided useful
discussions and performed additional theoretical calculations. The paper was written by
R.B., M.K.S., M.K.B. and A.G. and edited by all authors.

Additional information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Banerjee, R. et al. Reversible monolayer-to-crystalline phase
transition in amphiphilic silsesquioxane at the air-water interface. Sci. Rep. 5, 8497;
DOI:10.1038/srep08497 (2015).

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