Study on surface morphology and phase separation of polymer/cellulose liquid crystal
composite membranes after post-treatment
Mei Tua,b* Wanqing Hana,b Rong Zenga,b
a
Serena M. Bestc
Ruth E. Cameronc
Department of Materials Science and Engineering, Jinan University, Guangzhou, 510632, P.
R. China
b
Engineering Research Center of Artificial Organs and Materials, Ministry of Education,
Guangzhou, 510632, P. R. China
c
Department of Material Science and Metallurgy, University of Cambridge, CB2 3QZ, UK
*
Corresponding author: Mei Tu
Address: Department of Material Science and Engineering, College of Science and
Engineering, Jinan University, Huangpu Road 601, Guangzhou, 510632, P. R. China
Tel.: +86-20-85223271;
Fax: +86-20-85223271
E-mail address: tumei@jnu.edu.cn
1
Abstract: This paper explores the effects of the incorporation of liquid crystalline (LC) phases
into polyurethane with the aim of creating composites with biomimetic surfaces. The surface
morphology and phase separation structure of polyurethane(PU)/butyl hydroxypropyl
cellulose ester (BPC) composite membranes with different BPC contents and different
post-treatments were investigated by using polarized optical microscopy (POM), scattering
electron microscopy (SEM) and small angle X-ray scattering (SAXS).
For the PU/BPC composite membranes observed in their as prepared state, the surface
morphology showed a well-dispersed liquid crystal (LC) phases.
With increasing BPC
content, the LC domains tended to form quasispherical aggregates of LC-rich phase and the
size of the aggregates became larger. The LC molecules inside the domains poorly oriented
and there was little regular phase separation between LC domains and substrate material in
this case. However, the surface morphology of membranes showed obvious changes after
post-treatment.
A sharper peak emerged in the SAXS pattern for those samples with
different LC contents after heat treating and cooling under three different conditions. This
indicated that the arrangement of LC molecular chains in the domain and the phase separation
structure between substrate and LC domain had changed. The SAXS peaks were sharper and
more intense for the membranes cooled by natural cooling in oven, indicating that the LC
domains had more regular arrangement and more obvious phase separation than those in
membranes cooled to 20 ºC and to -20 ºC respectively. It is suggested that the surface
morphology of polymer/LC membranes could be controlled through adjusting LC contents
and performing post treatment under different conditions.
Keywords: polymer/liquid crystal composite membrane; surface morphology; post-treatment;
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phase separation structure
1. Introduction
It has long been recognized that the response of host organisms to biomaterals at macroscopic,
cellular and protein levels is, in most cases, closely associated with the materials’ surface
properties, and that the surface of the material has a critical influence on the biological
response [1-7]. This means that a crucial consideration is the control the surface properties to
obtain an optimum interaction between cells and biomaterials [8-10]. Considerable efforts
have thus been focused on surface engineering to endow polymer surfaces with the ability to
maintain cell normal phenotype and function [11-15] and to promote cell adhesion, deposition,
activation, growth, differentiation and secretion [16-18].
Many physiological and biochemical responses in the body rely on ordered structure [19,20].
Cell membranes have liquid crystal like characteristics and one strategy is to mimic this
structure with the design of polymer/LC biomimetic engineering surfaces.
Previous studies have shown that addition of a LC compound with low molecular weight into
a polymer substrate could improve the blood compatibility of biomaterials due to formation of
the LC domains on the material surfaces [21,22]. Furthermore, a cholesteric LC phase was
shown to be more effective than a nematic one. The distribution, size and morphology of LC
domains on the material surfaces could be adjusted through change of LC content and the
application of electric fields during the process of film preparation [23,24]. However, it was
also found that low molecular LC escapes at a rate of 0.2wt% per day in simulated body fluid
at 37˚C, which would lead to a decrease in the antithrombogenicity of the material. The
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escaped molecules may also produce a change in osmotic pressure in the serum and cause
hemolysis. To solve this problem, cellulose LCs with high molecular weight were chosen as
the dispersed phase in this work. Preliminary studies have shown that these large molecule
LCs could also form LC domains in the matrix. Furthermore, the surface morphologies of the
polymer/LC composites were affected by LC content and the conditions of film forming,
which would have effects on the protein adsorption characteristics of the biomaterial surfaces
[25-29].
In this work, a series of polyurethane/butyl hydroxypropyl cellulose ester composite
membranes with different LC content were prepared, and post-treated under three different
conditions. The surface morphology and phase separation structure of these membranes was
characterized using polarized light optical microscopy (POM), scanning electron microscopy
(SEM) and small angle X-ray scattering (SAXS) to observe the texture, shape, size and
distribution of the LC domains on the materials surfaces. The relationship between the phase
separation, LC content and post-treatment conditions is discussed.
2. Experimental section
2.1 Chemical and materials
Medical applied grade polyurethane, (PU) [Mw=270,000, containing 35% (wt) hard segment
4,4@-methylene diphenylene diisocyanate and 65%(wt) soft segment polyether], was
purchased from Yin Tesheng Co. of Guangzhou in China. Butyl hydroxypropyl cellulose ester,
(BPC) with Mw=100,000, Tg=18.4°Cand TNI= 156.3°C was prepared as described elsewhere
[30].
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2.2 Preparation of the Polymer/LC composite membranes
A series of composite membranes composed of PU and BPC were prepared by a
solution-casting method. PU and BPC were dissolved in tetrahydrofuran at room temperature
(20C), with weight ratios of the PU:BPC of 9:1, 8:2, 7:3, 6:4 and 5:5. The mixing solution
was cast onto clean glass plates and the solvent was allowed to evaporate for 72h. The
membranes were further dried under vacuum at 20C for 10 h to eliminate the residual solvent.
After the membranes were dried completely, they had a thickness of 200m.
The composite membranes were then put into an oven at 140C for 10 h, before being cooled
down under three different conditions for 12h. The cooling conditions were air cooling to 20
C, quenching to -20C and oven-cooling to room temperature.
2.3 Characterization
The phase separation structure and surface texture of the PU/BPC composite membranes were
observed using a polarized light optical microscope (XPL3230, Guangdong Silique
International Croup Maufar Co., Ltd) equipped with a digital camera system. Small angle
X-ray scattering measurements were carried out using a Nanostar camera (Bruker AXS Inc.
Madison, WI, USA), fitted with a sealed microbeam source, at 40 kV, 35 mA and filtered to
give CuKα radiation (wavelength, λX=0.154 nm). The sample-to-detector distance was
approximately 1.05 m and the entire optical path, including sample chamber, was evacuated.
The exposure time was 3600 s for all samples. The scattering vector was defined as
q=(4/)sin(/2), where  and  are the scattering angle and the wavelength of the X-ray,
respectively. The centre of the 2D-SAXS pattern was found and the scattering angle (2θ) was
calibrated using a silver behenate reference standard. The morphology of composite
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membranes were investigated using a scanning electron microscope (PHILIPSXL-30ESEM).
A gold layer was coated on the specimen surface in a sputter coater (BAL-TEC, SCD005)
prior to observation. The static contact angles of the air side of the polymer/liquid crystal
films against water were measured by using a Drop Shape Analysis System DSA100 (Kruss,
Germany) at room temperature, and the average contact angle value was calculated based on
measurements at five different positions in each sample.
3. Results and discussion
3.1 Surface texture of PU/BPC composite membranes
The surface textures of the PU/BPC composite membranes were observed using POM. Figure
1 shows that all pure PU films showed no orientation when viewed under cross polarization,
while composite membranes showed a well-dispersed birefringent texture. The samples which
had not been heat treated had LC domains dispersed throughout the PU matrix. When LC
content was larger than 20%, the size of quasispherical aggregates were observed, which
became larger with increasing of LC content (Fig.1 m,q,u) and the visual field under the POM
became brighter. However, the surface morphology of the membranes showed clear changes
after post-treatment. In general, the phase separation and texture of the LC domains became
more homogeneous, with the LCs apparently becoming more completely dispersed and
incorporated into PU matrix. The quasispherical aggregates of LC domains which occurred on
the surfaces of as prepared
composite membranes disappeared when LC content after heat
treatment.
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Fig.1. Surface texture of pure PU membrane and PU/BPC composite membranes, there are six groups of a-d, e-h, i-l,
m-p, p-t and u-x, the LC content of each group is 0%,10%,20%,30%,40%, 50% respectively; the treatment condition
for four membranes in each group is followed by as-prepared, cooling to 20°C, cooling to -20°C and natural
oven-cooled
However, the surface morphology of the oven-cooled samples cooled was different from those
cooled to 20°C and to -20°C respectively. It could be seen from Fig.1h that there were a series
of isolated uniformly distributed dots on the membrane surface when the LC content was 10%
This effect was an obvious sign of the phase separation of the LC domains and the PU matrix.
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With increasing LC content, the number of dots decreased. The surface morphologies of
membranes air-cooled to room temperature were basically similar over the different range of
LC contents, although there was a low volume fraction of dots distributed on the membrane
surfaces. For those samples prepared by cooling to -20°C, when the LC content was lower
than 40%, there was a very small number of dots dispersed on the membrane surface..
The POM observations described above indicated that the LC contents and post-treatment
conditions significantly affect the configuration of LC domains embedded into the
polymer/LC composites. Before heat-treatment, thermotropic liquid-crystal molecules
self-assembled into phases in their simplest form with orientational order, but often without
positional order[15]. The process of texture formation with the temporal evolution of a given
LC in a certain polymeric matrix was produced by competition between the liquid
crystal–solvent and liquid crystal–polymer phases (INCLUDE REFERENCE?). When
heat-treatment was performed, the LC segments rearranged and tended to become more
regular. Because of the high relaxation times of polymer LCs, their regular alignment inside
the matrix led to ordered structures and an anisotropic character [31] during the cooling
process. Hence the membranes showed multifarious textures under the POM.
3.2 Surface morphology and phase separation behavior of composite membrane
To gain further information and understanding of the microstructure of composite membrane
surfaces, SEM observation was carried out and the results are shown in Fig.2. The SEM
image revealed that LC domains with a radiating, flower-like shape appeared on the
membrane surfaces. The shape of LC domains was similar to the hexagonal HⅡ phase(a type
of texture of LC domain) reported in the literature[32]. The size of ‘flower’ became larger
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with increasing LC content, and this implied that the volume of the LC domain increased.
After post-treatment, the LC domains tended to form an interwoven net-like texture. The
above results revealed that LC content and heat-treatment impacted on the morphological
features, including both the LC domain size and shape.
Fig.2. SEM image of PU/BPC composite membranes，there are three groups of a-d, e-h, i-l, the LC content of each
group is 10%, 30%, 50% respectively; the treatment condition for four membranes in each group is followed by
as-prepared, cooling to 20°C, cooling to -20°C and natural oven-cooled
To elucidate the effects of LC content and post-treatment on phase separation behavior, SAXS
measurement was performed. In Fig.3, SAXS profiles are shown for all samples. The SAXS
curves showed that there were no clear scattering peaks for as-prepared polymer/LC
composite membranes with different LC contents (Fig.3a). However, when the membranes
were post treated using heat-treatment and cooling under the three different cooling
conditions, a new peak emerged for every sample shown in Fig.3b, c and d. The peak
appeared to be sharpest for the samples prepared by natural oven-cooling and the peak
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position shifted slightly for each cooling condition at the same LC content (Fig.3e,f). The
appearance of the sharpened peak in the SAXS pattern after post-treatment suggested the
development of a more highly ordered structure within the membranes. This implied that the
LC domains rearranged during the cooling process, but their regularity varied under the three
different cooling conditions. The SAXS curves for those membranes cooled to -20 °C
presented a broader and less intense peak than those cooled under the other conditions. It was
also found that the LC content had no systematic effect on the scattering intensity (I(q)) and
had no effect on the peak position for the samples prepared under the same treatment regime.
The different liquid crystallization behavior in the as-prepared membranes and the
post-treated ones with the three different cooling conditions may be attributed to the changes
in the arrangement and phase separation structure of the LC domains [33,34].
For the as-prepared membranes, when the LC content was lower, the LC phase was dispersed
as particles in the PU matrix.
With increasing LC content, the LC particles gradually
aggregated to form LC domains and began to phase separate from the PU.
However, the LC
molecules were poorly oriented inside these domains, and the phase separated structure was
not obvious – as indicated by the low intensity and broad peaks in the SAXS patterns. When
those membranes were heated at 140°C (close to clearing temperature of BPC) for 10 hr, and
subsequently cooled, the chains re-arranged and re-aggregated, and the LC particles in each
1400
1400
a
1000
800
b
1200
LC:10%
LC:20%
LC:30%
LC:40%
LC:50%
I(q)(arbitrary units)
I(q)(arbitrary units)
1200
Cooling at 20℃
AS-prepared
600
400
800
600
400
200
200
0
0
0
0.5
1
1.5
2
2.5
0
0.5
q/nm-1
101400
800
Cooling at -20℃
LC:10%
LC:20%
LC:30%
LC:40%
1200
y units)
ry units)
1000
c
1
1.5
2
2.5
q/nm-1
1400
1200
LC:10%
LC:20%
LC:30%
LC:40%
LC:50%
1000
1000
800
Oven cooled
d
LC:10%
LC:20%
LC:30%
LC:40%
LC:50%
Figure 3 SAXS profiles for PU/BPC composite membranes with different LC contents, a is as-prepared one, b-d are
post-treated ones followed by cooling to 20°C, cooling to -20°C and natural oven-cooling, e,f are those with the same
LC content through post-treating in different conditions
domain tended to become regular and oriented, while the surface texture of membranes
changed (showed in POM photos) and sharper peaks occurred in the SAXS patterns from the
microphase separation structure [35]. The membranes possessed contrasting surface texture
and extent of microphase separation due to the disparity of the nature of macromolecular
re-arrangement under the different cooling conditions.
3.3 Change of surface energy of composite membranes
Investigating the interaction between water and any material to be used as a biological matrix
is very important. Therefore, the static water contact angles of the composite membrane
surfaces were evaluated to explore the effect of the liquid crystal phases on the surface energy.
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It was confirmed that all of the membranes used in this study had flat surfaces and showed no
evidence of a porous structure in the bulk phase. The surface contact angles of the as prepared
and post-treated membranes with different LC contents are listed in Table 1. It was found that
the maximum values of water contact angles for all membranes were obtained when the LC
content reached 50%. In addition, the values of water contact angle for all membranes
increased after post-treatment. Indeed, the membranes prepared by natural cooling in the oven
after heating exhibited significantly higher surface contact angles as compared with the other
two membranes cooling to 20°C and to -20°C respectively. For the as-prepared membranes,
the contact angles of the composite membranes were higher than those of pure PU, the contact
angles increased with increasing of LC content. However, for the post-treated composite
membranes, only when LC content was larger than 30% were the contact angles higher than
those of pure PU prepared under the same condition.
Table1 The surface contact angles of the as-prepared and post-treated membranes with different LC content
Content of BPC(%)
Cooling conditions
As-prepared
Cooling to 20°C
Cooling to -20°C
Natural oven-cooled
0(pure PU)
80.6±5.5
84.5±2.2
87.2±2.2
95.1±3.7
10
82.2±4.5
83.8±2.2
85.0±3.8
85.3±2.6
30
86.0±2.8
87.7±5.1
88.7±1.3
95.8±4.7
50
97.5±3.5
101.4±2.0
102.1±2.5
104.3±1.5
These results revealed that introduction of LC into substrate affected the surface contact
angles, and LC contents and that post-treatment conditions also had a significant influence. In
other words, the LC phase formed on the polymer surface could influence surface energy. It is
well known, from previous observations of POM and SEM, that the LC domains on the
surfaces of composite membranes exhibited different sizes, distributions, phase separation and
surface morphology with varying LC content and post-treatment conditions, and these
consequently changed the surface energy of membrane [ADD REFERENCE]. When the LC
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content was higher than 30%, the surface contact angles of all composite membranes
increased markedly in comparison with the PU film produced under similar treatment
conditions. This suggests that the surface energy decreased. It is suggested that PU/BPC
composite membranes which have a surface liquid crystalline phase might be expected to
show effective cellular attachment[36].
4. Summary
The effects of LC content and post-treatment conditions on surface morphology and
microphase separation structure were investigated by means of POM, SEM and SAXS.
The surface morphology of the as prepared PU/BPC composite membranes, was a
well-dispersed birefringent texture under cross polarization when the LC content was less
than 20%. The LC domains tended to form quasispherical aggregates of the LC-rich phase,
and the size of the aggregates became larger with increasing LC content. No obvious
scattering peaks in the SAXS pattern were observed with any LC content, which indicated
that the arrangement of LC molecules inside the domains was in random orientation and no
regular phase separation was observed between the LC domains and the substrate material.
However, after post-treatment the surface morphology of membranes was obviously different
from the morphology before heat treatment. The LC domains dispersed more homogeneously
into the matrix and the quasispherical aggregates of LC domains that occurred on the surfaces
of pre-treated composite membranes disappeared when LC content was larger than 20% for
all post-treatment samples. An obvious peak emerged in the SAXS pattern for those samples
with different LC contents through cooling in three different conditions after heat-treatment.
The SAXS peaks were sharper and more intense for the membranes prepared by natural
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cooling in the oven, and revealed that the LC domains embedded into the matrix had a more
regular arrangement and obvious phase separation characteristics than those cooled to 20°C
and -20°C respectively.
The results of water contact angle tests showed that the contact angles of the composite
membranes were higher than those of pure PU, the contact angles increased with increasing
LC content. Moreover, the values of water contact angle for all membranes increased with
post-treatment. It revealed that the introduction of LCs into the substrate material affected the
surface contact angles, and the LC domains formed on the surfaces of the composite
membranes were altered, indicating a profound influence of LC content
and post-treatment.
The results above indicated that the surface morphology of the polymer/LC membranes could
be adjusted by changing the LC content and post treatment performed, and, consequently, that
the surface properties could be controlled with considerable potential importance for specific
biointeractions. The effects of surface morphology, phase separation and surface energy on
the protein adsorption and cell adhesion on the surfaces of composite membranes will be
investigated further.
Acknowledgement
This work was supported by National Nature Science Foundation of China(30870613 and
31040027), Scientific Research Cultivation and Innovation Funds of Jinan University
(21610607), Special Funds for Base Construction of Industry, Education and Research of
Guangdong Province(20090918). The SAXS experiments were performed in the Department
of Materials Science and Metallurgy, Cambridge, UK, and thanks are given to Dr Peter R.
Laity for his kind help for this work.
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