Content uploaded by Takeshi Aoyagi
Author content
All content in this area was uploaded by Takeshi Aoyagi on Apr 10, 2014
Content may be subject to copyright.
Two Different Surface Properties of Regenerated
Cellulose due to Structural Anisotropy
Chihiro YAMANE,1;
y
Takeshi AOYAGI,2Mariko AGO,3Kazuishi SATO,3
Kunihiko OKAJIMA,3and Toshisada TAKAHASHI1
1Faculty of Home Economics, Kobe Women’s University, Kobe 654-8585, Japan
2Central Research Laboratory, Asahi Kasei Corporation, Shizuoka 416-8501, Japan
3Faculty of Engineering, Tokushima Bunri University, Kagawa 769-2193, Japan
(Received December 7, 2005; Accepted April 20, 2006; Published July 7, 2006)
ABSTRACT: Contact angles of water droplet on regenerated cellulose films as an index of wettability were pos-
itively correlated with the orientation of (1-10) crystal planes and crystallinity. Because hydroxyl groups of cellulose
are located at the equatorial positions of glucopyranose rings, corresponding to the surface of (1-10) crystal planes, the
hydrophilicity of the (1-10) surface is expected to be very high. It is natural, therefore, that higher planar orientation of
(1-10) planes and crystallinity lead to higher density of hydroxyl groups on the surface of regenerated cellulose films
resulting in higher wettability. In contrast, hydrogen atoms are located at the axial positions of the glucopyranose rings,
corresponding to the surface of (110) planes. Thus, the (110) surface is expected to be hydrophobic, and the surface
energy obtained by computer simulations was far lower than that of the (1-10) surface. This suggests that cellulose with
complementary properties, i.e., hydrophobicity, may be created by structural controls such as reversing the planar ori-
entation from (1-10) to (110). Although it was not possible to reverse this orientation completely, post-treatments with a
nonpolar solvent (e.g., hexane), liquid ammonia or hot glycerol can somewhat control the wettability of regenerated
cellulose films. [doi:10.1295/polymj.PJ2005187]
KEY WORDS Cellulose / Wettability / Contact Angle / Hydrophilic Surface / Hydrophobic
Surface / Planar Orientation /
Regenerated cellulose is known to be one of the
most hydrophilic polymers. The contact angle of wa-
ter droplet on typical regenerated cellulose films, such
as cellophane and cuprophane, is ca. 12, far lower
than those of widely used polymers such as poly(vinyl
alcohol) (PVA), 36; poly(methyl methacrylate), 57;
poly(vinyl acetate), 63; nylon, 70; poly(vinylidene
chloride), 80; poly(vinyl chloride), 87; poly(sty-
rene), 91; poly(ethylene), 94; poly(propylene), 95;
and poly(tetrafluoroethylene), 108.
1
Although the hy-
drophilic polymer poly(vinyl alcohol), having many
hydroxyl groups on the molecule, has a lower contact
angle (36) than the other polymers listed above, this
value is still higher than that of regenerated cellulose
films. It is also worthy to note that the contact angle
of water on starch films is 41. Why is cellulose more
wettable than PVA, which is water soluble; and
starch, which is also water soluble and even has the
same molecular formula as cellulose? It has been
reported that the high wettability of cellulose films
results from the higher density of hydroxyl groups
on a cellulose surface than on a PVA surface,
2
but this
explanation has never been discussed from a structural
point of view.
Conversely, many experimental facts show that cel-
lulose interacts strongly with hydrophobic (nonpolar)
organic solvents. For example, hexane, toluene and
dichloromethane are captured in cellulose even after
extensive vacuum drying,
3
paraffin is easily dispersed
in water with a small amount of cellulose particles,
4
and relaxation peaks of dynamic viscoelastic measure-
ments or thermally stimulated depolarized current
measurements on cellulose shift to lower temperature
in nonpolar solvents.
5,6
These findings suggest that
cellulose has a hydrophobic nature as well.
In this paper, we discuss the essence of the hydro-
philic nature of cellulose by focusing on structural
anisotropy, and then try to clarify its hydrophobic
nature also resulting inevitably from structural aniso-
tropy.
EXPERIMENTAL
Sample Preparations
Cuprophane. Cotton linter with viscosity-average
molecular weight Mv¼1:7105was dissolved in
aqueous cuprammonium hydroxide solution according
to a known procedure
7
at a cellulose concentration of
8wt%. Mvwas estimated by using eq 1, established
for cellulose/cadoxen systems.
8
½¼3:85 102Mv0:76 ð1Þ
y
To whom correspondence should be addressed (E-mail: yamane@suma.kobe-wu.ac.jp).
819
Polymer Journal, Vol. 38, No. 8, pp. 819–826 (2006)
#2006 The Society of Polymer Science, Japan
The cellulose solution was cast on a glass plate to give
a thickness of 500 mm, which was immersed gently in
water for 1 min at 25 C and regenerated by 2 wt %
aqueous sulfuric acid for 10 min, followed by washing
with water and drying in air with a given dimension.
Cellophane. A softwood sulfite pulp mainly from
white spruce (ALAPUL-T, Alaska Pulp Co., Ltd.) was
immersed in an 18 wt % aqueous NaOH solution at
25 C. An excess liquid was squeezed out until the
cellulose concentration reached 30 wt %. The squeez-
ed pulp was hydrolyzed in air to give a Mv¼4:9
104, and this was followed by reaction with carbon
disulfide. Next, a dilute aqueous NaOH solution was
poured into the wet pulp with mixing in a kneader
to prepare a cellulose solution with the following
composition: 8.5 wt % cellulose, 6.0 wt % NaOH, and
degree of CS2substitution of 0.45. This solution was
cast on a glass plate to give a thickness of 500 mm.
These were immersed gently into a coagulation bath
which contained 14 wt % H2SO4,26wt%Na
2SO4,
and 1.5 wt % ZnSO4,at50
C for 5 min. Finally, the
films were washed with water and dried in air with
a given dimension.
Films from Cellulose/Aqueous NaOH Solution.
The sulfite pulp used above was subjected to steam
explosion treatment and the resultant refined exploded
pulp with Mv¼5:2104was dissolved in aqueous
NaOH according to a previously published proce-
dure.
5
The solution with a cellulose concentration of
5 wt % and NaOH concentration of 7.6 wt % was cast
on a glass plate to give a thickness of 500 mm. This
was immersed gently in aqueous sulfuric acid with
various concentrations ranging from 20 wt % to 65
wt % at 7C for 5 min, followed by washing with
water and drying in air with a given dimension. There-
after, these films were designated with the abbrevia-
tion SK-14.
Films from Cellulose/DMAc-LiCl Solution. The
same cotton linter used for the cuprophane preparation
was dissolved at a cellulose concentration of 3 wt % in
dimethylacetamide (DMAc) containing 8 wt % of lith-
ium chloride (LiCl), according to the procedure de-
scribed by Turbak et al.
9
The cellulose/DMAc-LiCl
solution was cast on a glass plate to give a thickness
of 500 mm. This film was then immersed gently in ace-
tone and toluene for 1 min each at 25 C, followed by
washing with methanol and drying in air with a given
dimension.
Micro-Crystalline Cellulose Films. The regenerat-
ed cellulose from cuprammonium solution (Bemlise:
Asahi Kasei Co., Ltd; Mv¼1:3105) was hydro-
lyzed by 50 wt % aqueous H2SO4at 70 C for 16 h.
This removes the amorphous regions of cellulose
and greatly increases crystallinity. This micro-crystal-
line cellulose was washed repeatedly with water by
decantation of supernatant, resulting from centrifuga-
tion at 10,000 rpm for 10 min. The micro-crystalline
cellulose suspension was cast on a glass plate to give
500 mmthick films, which were dried in air. There-
after, these films were designated micro-crystalline
cellulose films.
Amorphous Cellulose Films. The same sulfite pulp
used for the cellophane preparation was dissolved in
65 wt % sulfuric acid solution at 0 C, giving a dope
with cellulose concentration of 4 wt %. The solution
was poured into water at 0 C to precipitate amor-
phous cellulose,
4
which was hydrolyzed using a 20
wt % sulfuric acid solution at 80 C for 20 min and
then washed with water. A 1.5 wt % cellulose suspen-
sion was pulverized into sub-micron particles with
an ultra high pressure homogenizer (Microfluidizer
M-110EM, Mizuho Kogyo Co., Ltd., Japan).
4
This
pulverized cellulose suspension was cast on a glass
plate to give a thickness of 500 mmand dried in air.
Thereafter, these films were designated amorphous
cellulose films.
Post-Treatments of Cellulose Films
Cellophane was treated with the following liquids
under the given conditions: cyclohexane at 25 C for
24 h, liquid ammonia at 80 C for 5 min, and hot
glycerin at 260 C for 7min. The latter two treatments
changed the cellulose polymorph from Cell II to
Cell III and Cell IV, respectively. Completion of
these crystal transitions were confirmed by wide angle
X-ray diffraction patterns.
Planar Orientation
X-ray diffraction patterns of samples were meas-
ured by the reflection method and recorded on an X-
ray diffractometer with scintillation counter (Rotaflex
Ru-200PL, Rigaku Denki Co., Ltd., Japan). Samples
were irradiated at 2¼5{35to the membrane
surface (parallel incidence), and measurements were
made at 1/min with an internal reference (SiO2;2¼
28:45). To calculate the orientation of (1-10) crystal
planes, diffraction intensities of parallel incidence
(I(1-10))at2=ca. 12,(I
ð110Þ)at2=ca. 20and
(Ið020Þ)at2=ca. 22were measured. X-ray diffrac-
tion patterns of randomly oriented samples were re-
corded by the powder method. In order to prepare
the randomly oriented samples, films were cut into
particle-like size to ameliorate the influence of crystal-
line orientation (e.g., uni-planar orientation). Diffrac-
tion intensities of the randomly oriented samples (i)
of each crystal plane were measured. It is well known
that the (1-10) crystal plane is oriented parallel to the
film surface in almost all regenerated cellulose films.
The orientation index, f(1-10) for the (1-10) plane
was evaluated by the simple method reported by
C. YAMANE et al.
820 Polym. J., Vol. 38, No. 8, 2006
Takahashi,
10
using the eq 2. The orientation index pro-
vides a relative degree of planar orientation and takes
on values from 0 (random) to 1 (parallel to surface).
f(1-10) ¼
I(1-10) Ið110Þþð200Þi(1-10)
ið110Þþð200Þ
I(1-10)
ð2Þ
Crystallinity
The X-ray diffraction patterns of the randomly ori-
ented samples were used for calculating crystallinity
index Xc.X
cwas estimated from peak areas responsi-
ble for the (1-10), (110), and (020) planes, separated
by the Lorentz-Gaussian peak separation method.
Contact Angle
Contact angles were measured by taking photo-
graphs of small droplets of water on film samples us-
ing a contact angle meter (CA-A, Kyowa Interface
Science Co., Ltd). More than five readings on differ-
ent droplets from the same sample were averaged.
The deviation of each reading from the average was
within 1. It is well known that surface morpholo-
gy greatly affects contact angle. Wenzel’s equation,
cos 0¼r cos , relates the effect of surface area to
the contact angle, where 0,and r are contact angle
of uneven surface, contact angle of smooth surface
and ratio of uneven/smooth surface area, respective-
ly.
11
According to the equation, 0is smaller than
at <90as in the case of cellulose. However, be-
cause we found all samples to be almost flat and to
have the same structures (by SEM observations in
the 500X to 10000X magnification range), we ignored
morphology effects here.
The material which contacts a polymer during the
film preparation process affects the properties of the
polymer surface.
12,13
In order to avoid this effect, we
used the film surface which was opposite to the side
contacting the glass plate during film preparation.
Immediately prior to measurement, the films were
re-dried under vacuum at 70 C for 12 h.
Viscoelastic Measurements
The viscoelastic properties (mechanical loss tan-
gent ,tan - temperature T curves) of the cellulose
films were recorded on a viscoelastic spectrometer
(Model SDM-5000, Seiko Denshi Co. Ltd., Japan)
under the following conditions: frequency of 10 Hz,
heating rate of 10 C/min, measuring interval of 1 C/
min, sample length of 20 mm, sample width of 5 mm,
initial charge of 10 g/mm2, and temperature range of
150{350 C.
Calculation of Surface Energy
Crystal models of Cell I, Cell Iand Cell II were
constructed by using atomic coordinates reported by
Nishiyama et al.
14–16
Each crystal models contained
192 glucose residues and had 20–40 A
˚in length of
each sides. Vacuum slab was attached on the crystal
plane, of which surface energy was to be calculated,
for the crystal models with crystal surface. Free ener-
gy Gsurface and Gbulk of the crystal models with and
without the crystal surface, respectively, were ob-
tained from molecular mechanics calculations with
COMPASS force-field parameters using Forcite soft-
ware (Accelrys software Inc.). Let A denote the sur-
face area of the former crystal model. The surface
energy of the crystal plane for cellulose polymorphs
(Cell I, Cell I, Cell II) was calculated from the
following eq 3. Nonbonding energy was cut off by
12.5 A
˚, Coulomb force was calculated with the Ewald
method.
¼Gsurface Gbulk
Að3Þ
RESULTS AND DISCUSSION
Effect of Structural Parameters on Wettability of Film
Surfaces
Figure 1 shows X-ray diffraction patterns of the
film sample and the randomly oriented sample from
cellulose/aqueous NaOH solution followed by the co-
agulation of 40% H2SO4(SK-14 40% in Table I). The
randomly oriented sample was prepared by cutting of
the film sample into particle-like size in order to amel-
iorate the influence of crystalline orientation (e.g.,
uni-planar orientation). An intensity of (1-10) crystal
plane, Ið1-10), of the film sample was much higher than
that of the randomly oriented sample. This means that
the (1-10) crystal plane is oriented parallel to the film
510 25
2θ / °
Intensity
(a)
(b)
(1-10) (110) (020)
15 20 30
Figure 1. X-Ray diffraction patterns of the film sample (a)
and the randomly oriented sample (b) from cellulose/aqueous
NaOH solution followed by the coagulation of 40% H2SO4.
Two Different Surface Properties of Regenerated Cellulose
Polym. J., Vol. 38, No. 8, 2006 821
surface. This kind of the orientation is generally
known in almost all regenerated cellulose films.
10
The planar orientation index f(1-10) was calculated
from the eq 2 comparing I(1-10) and Ið110Þþð020Þof both
film samples and randomly oriented samples. Crystal-
linity index Xcwas estimated from the X-ray dif-
fraction patterns of randomly oriented samples. The
diffraction peaks at 2¼28:45is from SiO2as an
internal reference.
Table I shows contact angles of water droplet on
regenerated cellulose films with various structural pa-
rameters. The contact angles tended to be low when
f(1-10) and Xcwere high. The sample with the highest
contact angle, 42, was observed for the amorphous
cellulose film. Because of its amorphous structure,
f(1-10) could not be determined for this material. It
is noteworthy that this contact angle was nearly the
same as that of starch, 41.
Figure 2 shows the relationship between f(1-10)
and water contact angles. Numerals in the figure indi-
cate Xc. The higher the f(1-10) was, the lower the con-
tact angle became. In addition, high Xcvalues yielded
significantly lower contact angles. Specifically, the
contact angle of Cuprophane, Xc¼42%, was lower
than that of SK-14 50% with Xc¼23%, whereas both
had the same value of f(1-10), namely 0.52. Contrary
to popular belief that the hydrophilicity of amorphous
form (which has high water absorbency) is much
higher than that of the crystalline form, the wettability
of regenerated cellulose, from the viewpoint of the
surface, probably depends on its crystalline properties,
as indicated by the f(1-10) and Xcvalues.
Table I also shows parameters for amorphous
region of cellulose, namely mechanical absorption
temperature Tmax, absorption intensity tan , and water
contact angles. Dynamic absorptions of regenerated
cellulose are attributed to segmental motions as fol-
lows: for the micro-Brownian motions of cellulose
main chain segments, and for the local twisting mo-
tions of main chains. The and absorptions consist
of 1and sh and 1and 2components, respectively.
sh is observed as a shoulder or maximum peak at a
lower temperature than that of the 1absorption peak.
Here, 1is assigned to the regions where intra- and
intermolecular hydrogen bonds are highly developed.
Some of the molecular chains in this region are im-
mobile, as in the crystalline region.
17
Because precise
peaks for 1absorptions in the SK-14 series could not
be detected, the effect of 1on water contact angles
is not discussed here. Except for Tmax1, there were
no relationships between amorphous parameters and
water contact angles, as shown in Table I. Figure 3
shows the effect of Tmax1on water contact angle.
In this case, the water contact angles decreased with
Table I. Contact angles and structural parameters of cellulose films
Contact 21sh 1
Sample angle f(1-10) XcTmax tan max Tmax tan max Tmax tan max Tmax tan max
/deg. /% /C/
C/
C/
C
Cupro. 12.2 0.52 42 75:90.0442 61:90.0400 169.0 0.0660 290.0 0.0900
Viscose 11.6 0.60 30 62:30.0321 40:90.0300 — — 286.7 0.0816
SK-14 20%a10.5 0.67 45 88:50.030 — — 218.8 0.0672 289.1 0.0734
40%a16.7 0.60 27 84 0.036 — — 228.1 0.0714 282.5 0.0760
50%a19.3 0.52 23 83:00.037 — — 232.8 0.0740 279.7 0.0802
60%a21.8 0.08 20 78:50.038 — — 237.5 0.0771 277.3 0.0844
DMAc/LiCl 40.5 0.09 6 70:90.0360 60:60.0390 155.0 0.0560 272.0 0.0670
Micro-Crystal- 11.5 0.75 78 — — — — — — — —
line Cellulose
Amorphous 42.0 — 0 — — — — — — — —
Cellulose
aSK-14 donate the films from cellulose/aq. NaOH solution system, number means concentration of sulfuric acid in coagulant.
0
10
20
30
40
50
0 0. 2 0. 4 0. 6 0. 8 1
6
20
23
27
42 30 45 78
f(110)
Contact angle / deg.
Figure 2. Relation between (1-10) plane orientation index
f(1-10) and contact angle for water. Numbers denote crystallinity
Xc.
C. YAMANE et al.
822 Polym. J., Vol. 38, No. 8, 2006
increasing Tmax1, as with the relation to Xcshown in
Figure 2. Though the structural region described by 1
is amorphous, the region is considered to have a crys-
tal-like structure. When Tmax1is high, activation en-
ergy of molecular relaxation and molecular density of
the region are also high. This means that the higher
Tmax1probably makes the region more similar to
the crystalline structure. Therefore, it is natural that
Tmax1had the same relation to water contact angles
as did for Xc. The question was, why did the crystal-
line or crystal-like regions affect wettability so much
more than the amorphous regions?
Figure 4 is a schematic representation of cellulose
molecules with their hydrophilic and hydrophobic
parts.
18
The equatorial direction of the glucopyranose
ring is hydrophilic because all three hydroxyl groups
on the ring are located on the equatorial positions of
the ring. In contrast, the axial direction of the ring is
hydrophobic because of hydrogen atoms of C–H
bonds being located on the axial positions of the ring.
Thus, cellulose molecules have intrinsically structural
anisotropy. If these hydrophilic parts were arranged
together in the same direction, a super hydrophilic
surface with many hydroxyl groups would be formed.
Figure 5 shows the unit cell of a cellulose II crystal
and a plane structure model. The planar structure,
which is closely related to the (1-10) crystal plane
and formed by hydrophobic interactions between
glucopyranose rings, has a hydrophilic surface. The
glucopyranose rings in the (1-10) crystal plane are
stacked with each other by hydrophobic interactions
(such as van der Waals forces) and located perpendic-
ular to the (1-10) surface. As the result, the density of
hydroxyl groups on the (1-10) surface is very high, re-
sulting in a hydrophilic surface. Because of the struc-
tural anisotropy of crystalline cellulose, higher Xcand
f(1-10) values result in a larger surface area fraction of
(1-10) planes in regenerated cellulose films, producing
the most hydrophilic polymers, being more wettable
than PVA or starch, as described above. As mentioned
before, there is no relationship between amorphous
parameters, except 1, and water contact angle. It is
understandable that isotropic structures such as sh
or 2regions, have only a minor effect on wettability.
It is widely believed that surface roughness greatly
affects water contact angles.
11
We recognize the im-
portance of surface morphology. However, the study
of bulk structure is also important as a basis for under-
standing surface phenomena. In addition, we have al-
0
10
20
30
40
50
265 270 275 280 285 290 295
Tmax
α
1/ °C
Contact angle / deg.
Figure 3. Relation between relaxation peak temperature for
1(Tmax1) and contact angle for water.
Hydrophobic part
Hydrophobic part
Hydrophilic part
Hydrophilic part
(a)
(b)
Figure 4. Hydrophobic and hydrophilic part of cellulose molecule: (a), end view of glucopyranose ring plane; (b), front view of
glucopyranose ring plane.
Two Different Surface Properties of Regenerated Cellulose
Polym. J., Vol. 38, No. 8, 2006 823
so observed our film surfaces by SEM in the range
from 500X to 10000X magnification, and found our
samples to be almost flat, with little structural varia-
tion. For these reasons, we have ignored the effects
of surface morphology here.
Hydrophobic Nature of Regenerated Cellulose
Structural anisotropy of cellulose may provide re-
generated cellulose with the opposite aspect to the
above, namely hydrophobic properties. The hydropho-
bic nature of cellulose can be deduced from the fol-
lowing experimental facts. Silicone oil and hydrocar-
bons are easily dispersed in water
4
in the presence of a
small amount of cellulose. Hydrophobic solvents such
as hexane, toluene and dichloromethane remain cap-
tured in cellulose domains even after rigorous vacuum
drying.
3
Some relaxation peaks of cellulose in ther-
mally stimulated depolarized current measurements
shift to lower temperature in nonpolar solvents such
as hexane or benzene.
5
These suggest that there must
exist hydrophobic domains in cellulose.
Figure 6 is a schematic representation of the (110)
o
o
o
OH
OH
OH
OH
OH
OH
OH
OH
o
oo
o
o
o
o
o
o
o
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
o
o
o
OH
OH
OH
OH
OH OH
OH
OH
o
o
o
OH
OH
OH
OH
OH
OH
o
o
o
o
o
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
(110)
Molecular Sheet Structure relevant to Crystal Plane
Figure 5. Schematic representation of unit cell of cellulose II and molecular sheet structure.
Hydrophobic surface
Hydrophobic surface
(110)
Over view of crystal plane
(110)
Figure 6. Schematic representation of hydrophobic surface in cellulose II.
C. YAMANE et al.
824 Polym. J., Vol. 38, No. 8, 2006
plane of cellulose II crystals. The (110) plane is quite
different from the (1-10) plane because there are al-
most no hydroxyl groups on the (110) plane but rather,
hydrogen atoms of C–H bonds. This also helps ex-
plain the hydrophobic nature and low surface energy
of the (110) plane. In order to determine the contact
angle and the surface energy of this hydrophobic sur-
face, we attempted to prepare a film with the (110)
plane-oriented parallel to the film surface. Because
of experimental difficulties in making this film, we
estimated the surface energies of each crystal face
by computer simulation.
Table II shows the surface energies of the crystal
faces of various cellulose polymorphs. Symbols d1,
d2, and d3are arbitrary and indicate crystal faces
arranged by their wider spacings. The numerals in
parentheses denote crystal indices. The crystal face
with the highest surface energy was (1-10) of Cell II,
with a value of 178 mN m1. This is consistent with
the fact that regenerated cellulose films having their
(1-10) planes oriented almost parallel to the film sur-
face have the most wettable surfaces of widely used
polymers, being more wettable than PVA and starch,
as described before. The surface energy of the (110)
plane of Cell II, 101 mN m1, was much less than that
of (1-10), supporting the expectation that the (110)
plane of Cell II is hydrophobic. If the (110) plane of
Cell II were oriented parallel to the film surface,
the resulting cellulose would be hydrophobic. It is
noteworthy that the surface energies of hydrophobic
planes of cellulose I polymorphs were quite low. In
particular, the value of 62 mN m1for Cell Iwas on-
ly one third of the value for the (1-10) plane of Cell II,
and lower than 92 mN m1found for Cell I. Cellu-
lose is in contact with many substances such as
lignin, proteins and other polysaccharides found in
wood. The difference between the surface energies
of Cell Iand Cell Imay result from various interac-
tions of cellulose with these substances during crystal-
lization or initial periods of structural formation of
cellulose.
As mentioned above, regenerated cellulose general-
ly has a uniplanar orientation of (1-10) planes parallel
to the film surface, providing remarkable hydrophilic
properties. This orientation can be explained by the
following hypothetical mechanism for the formation
of the structure of cellulose regenerated from solution.
Glucopyranose rings are stacked with each other by
hydrophobic interactions in order to reduce hydropho-
bic surface area in highly polar solvents such as aque-
ous media, providing thermodynamic stability. The
stacks form themselves into molecular sheets related
to the (1-10) plane, as shown in Figure 5, which de-
picts a hydrophilic surface (because of the many hy-
droxyl groups). Volume contraction of coagulating
cellulose solutions, shrinking perpendicular to the film
surface, gives rise to molecular sheets with anisotropic
shapes oriented parallel to the film surface, followed
by the formation of crystalline and amorphous re-
gions by aggregation of the molecular sheets by hy-
drogen bonding. As a result, hydrophilic film surfaces
with uniplanar orientation of (1-10) planes inevitably
emerge in highly polar media through the formation
of molecular sheets because of thermodynamic con-
straints. This hypothesis is supported by the following
facts. Coagulation systems with pronounced volume
contraction give high f(1-10) values.
19
The use of non-
polar solvents (e.g., toluene) as coagulants prevents
the formation of molecular sheets, giving low f(1-10)
and low crystallinity. The existence of the molecular
sheet structures necessary for this hypothesis is also
supported by Hayashi
20
and Hermans,
21
who identified
the structures (naming them Plane Lattice Structures
or Sheet-like Structures) as the basic and dominant
features of regenerated cellulose.
As mentioned before, if we could make (110)
planes oriented parallel to the film surface, hydropho-
bic cellulose could be prepared. According to our hy-
pothesis, cellulose dissolution in a nonpolar solvent
followed by the formation of (110) molecular sheets
by hydrogen bonding, instead of (1-10) planes formed
by hydrophobic interaction, could provide such a
hydrophobic structure. This is quite difficult because
only highly polar solvents which interact strongly
with the hydroxyl groups of cellulose can actually dis-
solve cellulose.
Changes in Wettability by Post-Treatments
Because of the difficulties in obtaining hydrophobic
surfaces from cellulose/polar solvents systems, we
tried to control wettability by post-treatments. Cello-
phane, whose f(1-10) is 0.6 and water contact angle
is 11.6, as shown in Table I, was subjected to several
post-treatments such as exposure to cyclohexane, liq-
uid ammonia and hot glycerin. Although cellulose II
polymorphs remained unchanged by the cyclohexane
treatment, liquid ammonia and hot glycerin treatment
transformed cellulose II into cellulose III and cellu-
lose IV, respectively. Table III shows changes in wa-
ter contact angles resulting from the post-treatments.
Table II. Surface energies of crystal faces
Surface Energy/mN m1
Polymorphs d1d2d3
Cell-I154(100) 137(010) 62(110)
Cell-I155(1-10) 155(110) 92(200)
Cell-II 178(1-10) 101(110) 110(020)
d1,d
2,d
3: arbitrary symbols of crystal faces ranged by their
wider spacings. ( ): crystal indices.
Two Different Surface Properties of Regenerated Cellulose
Polym. J., Vol. 38, No. 8, 2006 825
The cyclohexane treatment slightly increased the con-
tact angle to 14.6and decreased wettability, while
leaving f(1-10) almost unchanged. The relaxation
peak at 50 C in Thermally Stimulated Depolarized
Current analysis in the dry state shifted to lower tem-
perature (10 C) in cyclohexane.
5
This means that
some cellulose molecules or structural domains were
mobile at room temperature in the presence of cyclo-
hexane. Under these conditions, hydroxyl groups
could possibly move to the interior of the structure
from the surface, then hydrophobic planes of gluco-
pyranose rings come to the surface.
Liquid ammonia treatment converted Cell II to
Cell III and increased water contact angle remarkably,
to 39.6. This decrease in wettability must be related
to the fact that wrinkles and shrinkage in cellulose
fabrics during washing can be effectively prevented
by liquid ammonia treatment. This must also be relat-
ed to the reduction of complement activity from 90 to
50, imparting biocompatibility to artificial kidneys
made from cellulose. Complement activity, resulting
from the reaction between thioester groups in the
complements in blood and hydroxyl groups on the
cellulose surface, is thus affected by wettability.
The contact angle increased to 24.0after hot
glycerol treatment, which changed Cell II to Cell IV.
Many experimental phenomena related to hot glycerol
treatments, e.g., increase in wet strength, decrease in
water swelling of cuprammonium rayon and stable
crystal structure (Cell IV) in water, may be related
to the decrease in wettability.
The decrease in wettability by the liquid ammonia
and hot glycerin treatments may result from low di-
electric constants of these liquids, not from the crystal
transitions. The low dielectric constants of liquid
ammonia (25) and glycerol (42.5), both much lower
than that of water (80), possibly cause a change in
the direction of surface hydroxyl groups, resulting in
lower wettability.
We have found the following three key factors to
affect the wettability of cellulose. Cellulose molecules
have two inherently different structural moieties, the
hydrophilic and hydrophobic parts, located at the
equatorial and axial positions of glucopyranose rings,
respectively. Structural anisotropy of cellulose crys-
tals derived from the above-mentioned molecular ani-
sotropy gives rise to hydrophilic and hydrophobic
crystal planes, whose surface energies are calculated
to be quite different. Planar orientation of hydrophilic
planes parallel to the film surface gives cellulose one
of the most hydrophilic surfaces of known polymers.
REFERENCES
1. ‘‘Kagakubinran Kisohen II’’ 3rd ed, The Chemical Society
of Japan, Ed., 1984, p 90.
2. T. Matsunaga and Y. Ikada, in ‘‘Modification of Polymers,’’
C. E. Carraher, Jr. and M. Tsuda, Ed., ACS Symposium
Series, No. 121, 1980, pp 391–406.
3. for example, H. Staudinger, K.-H. In der Birken, and M.
Staudinger, Makromol. Chem.,9, 148 (1953).
4. H. Ono, Y. Shimaya, T. Hongo, and C. Yamane, Materials
Research Society of Japan,26, 569 (2001).
5. C. Yamane, M. Mori, and K. Okajima, Polym. J.,28, 1039
(1996).
6. R. Fujioka and S. Manabe, ’99 Pusan-Kyeongnam/Kyushu-
Seibu Joint Symposium on High Polymers (9th) and Fibers
(7th), Gyeongszng National University, Chinju, Korea,
November 2, Proceedings, P102–105 (1999).
7. W. Gibbson, L. Spencer, and R. McCall, J. Chem. Soc.,117,
479 (1920).
8. W. Brown and R. Wikstro
¨m, Eur. Polym. J.,1, 1 (1966).
9. A. F. Turbak, ‘‘Proceedings 1983 International Dissolving
and Specialty Pulps Conference,’’ Tappi, Atlanta, U.S.A.,
1983, p 105.
10. a) T. Takahashi, Sen-I Gakkaishi,25, 80 (1969).
b) T. Takahashi, Sen-I Gakkaishi,25, 122 (1969).
11. R. N. Wenzel, J. Phys. Colloid Chem.,53, 1466 (1949).
12. T. Matsunaga, Y. Ikada, and R. Kitamaru, Polym. Prepr.,
Jpn.,27, 475 (1978).
13. B. R. Ray, J. R. Anderson, and J. J. Scholz, J. Phys. Chem.,
62, 1220 (1958).
14. Y. Nishiyama, J. Sugiyama, H. Chanzy, and P. Langan,
J. Am. Chem. Soc.,125, 14300 (2003).
15. Y. Nishiyama, P. Langan, and H. Chanzy, J. Am. Chem.
Soc.,124, 9074 (2002).
16. P. Langan, Y. Nishiyama, and H. Chanzy, Biomacromole-
cules,2, 410 (2001).
17. S. Manabe, M. Iwata, and K. Kamide, Polym. J.,18,1
(1986).
18. A. Isogai, in ‘‘Seruroosu no Zairyoukagaku (Material
Science of Cellulose),’’ Toyo U. Ed., 2001, p 2.
19. T. Hongo, C. Yamane, M. Saito, and K. Okajima, Polym. J.,
28, 769 (1996).
20. J. Hayashi, J. Masuda, and Y. Watanabe, Nippon Kagaku
Kaishi,5, 948 (1974).
21. P. H. Hermans, J. Polym. Sci.,4, 145 (1949).
Table III. Effect of post-treatments on contact angle
Crystal form Treatment Contact angle
/deg.
Cell II cyclo-hexane 14.6
Cell III liquid ammonia 39.6
Cell IV high temp. glycerol 24.0
C. YAMANE et al.
826 Polym. J., Vol. 38, No. 8, 2006
