Journal of Biomaterials and Nanobiotechnology, 2012, 3, 431-439
http://dx.doi.org/10.4236/jbnb.2012.34043 Published Online October 2012 (http://www.SciRP.org/journal/jbnb)
431
Preparation of Porous Poly(3-Hydroxybutyrate) Films by
Water-Droplet Templating
Anna Bergstrand1*, Helene Andersson2, Johanna Cramby1, Kristin Sott2, A n et t e Lars s o n1
1Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden; 2Structure and Mate-
rial Design, The Swedish Institute for Food and Biotechnology, Göteborg, Sweden.
Received July 5th, 2012; revised August 24th, 2012; accepted September 3rd, 2012
ABSTRACT
Porous resorbable implants are of great interest since they may deliver bioactives or drugs, facilitate the transport of
body fluids or degradation products and provide a favorable environment for cell attachment and growth. In this work
we report on a method using concentrated emulsions to template interconnected solid foam materials and to produce
highly porous poly(3-hydroxybutyrate) (PHB) materials. Porous PHB films were cast made from water-in-oil template
emulsions including Span 80 and lithium sulphate. The films were characterized by SEM-EDX and DMA. The water
uptake of the films was recorded in order to determine the fraction water available pores. The results show that the ad-
dition of lithium sulphate allows a fine tuning of the film morphology with respect to porosity and interconnected po-
rous structure. The film porosity was determined to 51% ± 3%, 52% ± 3% and 45% ± 3% for the films made with 0%,
2.9% and 14.3% lithium sulphate in the template emulsion, respectively. The fraction water available pores was sig-
nificantly lower, 11% ±3%, 38% ±12% and 48% ± 7% for films with 0%, 2.9% and 14.3% litium sulphate respectively.
Differences in fraction water available pores and total porosity for the films reflects the film morphology and differ-
ences in pore interconnection.
Keywords: Biodegradable; Emulsion Templating; Porous; Implant
1. Introduction
Biodegradable polymers have received much attention in
the past decades due to their potential as implant materi-
als for controlled drug release or tissue engineering. The
implants act as a drug delivery device or as a temporary
support for transplantation of cells and tissue. The bio-
degradable polymer implant is being integrated and in
some cases resorbed in the body upon hydrolytic and
enzymatic degradation and there is no need for surgical
removal as it is for some non-degradable implant materi-
als. Depending of the function of the material, the biode-
gradable implant may be dense and non-porous or porous
with a low-density. Dense materials may be used for
nerve guides and vascular stents and porous materials for
injectable micro particles and scaffolds for tissue engi-
neering. Porous materials can be produced by salt par-
ticulate leaching [1-3], phase separation [4] or micro
structuring techniques [5,6]. The pore size and degree of
porosity varies, but generally these techniques result in
materials with macro porous structures of sizes from 10 -
500 μm and they include several preparation steps which
make them labor intensive and time-consuming.
A simple and straight forward method is to use emul-
sions in order to template porous structures in materials.
The emulsions may be a water-in-oil (w/o) emulsion type,
having water droplets as the internal dispersed phase and
oil as the external and continuous phase or an oil-in-wa-
ter (o/w) emulsion (with oil as the dispersed droplet
phase and water as the continuous phase). Concentrated
emulsions also referred to as high internal phase emul-
sions (HIPE) can be used to create highly porous materi-
als by emulsion templating. The HIPE has typically an
internal phase volume of more than 74%. Commonly, the
external non-droplet phase is converted into a solid
polymer and the water droplets template porous struc-
tures during solvent evaporation. This produces a highly
porous and permeable material, so called solid foam. The
spherical cavities created by the water droplets are com-
monly defined as “voids” or “cells”. In this work we will
use the term void. Under favorable conditions, small in-
terconnected pores are formed between adjacent emul-
sion droplets. These interconnecting pores between each
void and its neighbors are referred to as “windows”.
Only a few reports have been made on the production
of highly porous biodegradable polymer materials by
*Corresponding author.

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 432
emulsion templating. They may be prepared by cross-
linking of monomers in the continuous phase of a HIPE
(forming a so called polyHIPE) [7,8] or simply solidifi-
cation of the continuous phase of an emulsion by solvent
evaporation forming a solid foam. In the present study,
we report on w/o emulsion templating in combination
with solvent casting to fabricate highly porous and per-
meable poly(3-hydroxybutyrate) (PHB) foams. PHB is a
naturally occurring polyester. When implanted in the
body it is degraded into the monomer 3-hydroxybutyric
acid, a normal mammalian metabolite [9,10]. The poly-
mer is highly crystalline and hydrophobic in nature and
degrades in a very slow rate compared to most biode-
gradable polyesters. PHB is developed as an implant ma-
terial for the regeneration and redirection of nerve cells
[11,12] and as a biodegradable gastrointestinal patch [13].
A highly porous and permeable PHB material with an
interconnected pore structure could potentially be used as
a resorbable implant platform or scaffold, resistant to
mechanical deformation for a prolonged time in a wet
environment. The polymer platform offer possibilities to
achieve controlled release of drugs. An interconnection
between pores enables the transport of acidic degradation
products out from the interior region. It could also enable
the flow of nutrients and oxygen through the material
which could be essential for cellular adhesion and growth.
To the authors’ knowledge, a porous PHB material
created by emulsion templating has only been reported in
one previous study [14]. In that study a microemulsion
was used to create highly porous PHB materials. The
usage of microemulsions to produce porous materials has
advantages, but an aspect to take into account is that mi-
croemulsions usually contain large amounts of emulsifier.
An alternative approach to make porous PHB material is
to use a regular emulsion to create water-droplet tem-
plates. In the present study we use a low amount of
emulsifier to create a w/o emulsion. The solvent is effi-
ciently driven off to increase the portion internal phase to
form a concentrated emulsion and bring the emulsion
droplets sufficiently close to create windows and voids
during solidification a process very much equivalent to
pore formation utilizing HIPE templates (See Figure 1).
Lithium sulphate is used to stabilize the emulsion and to
tune the emulsion droplet sizes and the resulting film
morphology.
2. Materials and Methods
2.1. Materials
Poly-[(R)-3-hydroxybutyric acid] (PHB), Chloroform (pu-
rity 99.0% - 99.4%, stabilized with 1% ethanol), sorbitan
monooleate (Span 80) and lithium sulphate monohydrate
were purchased from Sigma-Aldrich, (Germany) and used
as received.
2.2. Preparation of PHB Template Emulsions
and PHB Solution
A w/o template emulsion was prepared from a 7% (w/v)
PHB solution. The PHB is dissolved in the organic phase
and constitutes the continuous phase. Water is dispersed
in the oil phase to produce an emulsion with a water-
to-oil ratio of roughly 1/10. In a typical procedure mak-
ing the emulsion, 560 mg PHB was dissolved in 8 ml of
chloroform at 58˚C, under vigorous stirring for 30 - 45
minutes until a clear solution was obtained. The lithium
sulphate concentration in the water phase was 0%, 2.9%
and 14.3% (w/w), based on the polymer weight. The poly-
mer solution was cooled to room temperature before ad-
dition of 100 µl of 1.5% (w/v) Span80 in chloroform fol-
lowed by stirring for a few minutes. Aqueous solutions
of 2.9 % and 14.3% (w/v) Li2SO4 in MilliQ water were
prepared. Pure MilliQ water was used as the water phase
for 0% Li2SO4 emulsion. 800 µl of the aqueous solution
with or without lithium sulphate was slowly injected to
the PHB solution with a Hamilton syringe during homo-
genization with a disperser (IKA DI18, Brazil) for 5 min
at 22000 rpm, on iced water. For pure PHB films a 7%
(w/v) PHB solution was prepared by dissolving 560 mg
PHB in 8 ml chloroform at 58˚C, during vigorous stirring
for at least 30 min, without homogenization.
2.3. Visualization of Template Emulsions
The droplet sizes of the emulsions were visualized with a
Solvent casting of water-in-oil emulsion Formation of high water phase emulsion
and fixation of water droplets
Solid foam formation by evaporation of
solvent remains
Figure 1. Schematic picture of the preparation of porous PHB material by combining emulsion templating and solvent cast-
ng. i

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 433
light microscope (Nikon, Microphot-FXA) equipped with
a Plan 40 × 0.7 differential interference contrast, DIC, oil
objective. The images were acquired using a 8 bit RGB
color camera (Altran 20, Soft imaging system).
2.4. Casting of Films from Template Emulsions
and Pure PHB Solutions
Films were prepared by solvent casting of PHB solutions
into glass petri dishes. About 7 ml of the prepared emul-
sions or pure PHB solutions were poured into petri dishes
with a diameter of 6 cm. Tryouts were made to establish
the optimal drying conditions and solvent evaporation
rate that rendered the best quality of the resulting film.
The solvent evaporation rate during film formation was
monitored and determined by gravimetric analysis. Typi-
cally, the petri dish was covered with a glass lid leaving
an opening of about 1 mm between the dish and lid dur-
ing film formation to achieve the best film quality. The
resulting films were left to dry for about 36 hours at 22˚C
in a foiume cupboard and subsequently in a vacuum oven,
at 40˚C for 24 hours, in order to evaporate the remains of
chloroform. The film thickness films was measured by a
micrometer gauge (Mitutoyo, Japan) and determined from
an average of the thickness at five positions of each sam-
ple. The range of thickness means was 145 - 175 µm.
2.5. Film Morphology and Elemental Analysis
The dry films were cut into sections, repeatedly sub-
merged into liquid nitrogen for a few minutes and freeze
fractured. The films were sputter-coated with gold and
visualized by in-line and horizontal detection mode using
a field emission scanning electron microscope (SEM),
(Leo Ultra 55 FEG SEM), at a magnification of 5000 at 3
kV. The location of sulphur in the films, originating from
lithium sulfate in water phase of the template emulsion,
was established by elemental analysis using energy dis-
persive x-ray (EDX), (Inca Oxford Instruments), analysis
on selected areas on the film surface.
2.6. Water Uptake and Porosity in Films
The water uptake of the films was determined gravimet-
rically. The experiments were carried out on triplicate
samples (1.7 × 1.7 cm) from 4 different types of film
preparations. The film samples were submerged in phos-
phate buffered saline (PBS) buffer, pH 7.4 at 37˚C during
magnetic stirring. The film samples were removed at pre-
determined time points, excess PBS was carefully wiped
off with a Kleenex, and finally the samples were weighed
and afterwards re-submerged in PBS. In order to deter-
mine the apparent porosity, circular samples of Ø 5 mm
were punched, weighed and the thickness measured by a
micrometer gauge (Mitutoyo, Japan). The apparent den-
sity was calculated from the weight and dimensions for
each of the porous PHB films (n = 3).
2.7. Dynamic Mechanical Analysis on Films
Submerged in Water
The mechanical properties of PHB films prior to and
after submersion in water were measured using a Rheo-
metrics RSAII (Rheometrics Scientific, Piscataway, USA)
dynamic mechanical analysis (DMA) apparatus equipped
with an in-house designed submersion cell [15]. Film
strips were prepared to a width of 4 mm using a razor-
edged punch. The thickness was determined as the aver-
age of three measurements and the effective initial sam-
ple length in the DMA was approximately 22 mm. The
films were mounted in the DMA apparatus and a small
static force was applied to ensure proper stretching of the
sample. After 7 min from start, 40 ml of PBS solution
adjusted to a temperature of 22˚C was added. Measure-
ments were performed in a strain-controlled stretching
mode with a static force. The amplitude of the dynamic
strain was set to 0.03% and the static force was set to
exceed the amplitude of the harmonic dynamic force by
20%. The complex modulus (E*) and the loss factor (tanδ)
were calculated from the force response and the dimen-
sions of the sample as recorded during the measurement.
Equilibrium tanδ and E* values were taken as the average
of the plateau values prior to and after PBS buffer addi-
tion.
3. Results and Discussion
3.1. Template Emulsion Morphology and
Stability
As the film porous structure is derived from the template
emulsion the emulsion preparation conditions, the droplet
size, and droplet distance will strongly influence the
morphology of the final film material. The template
emulsions were studied by light microscopy and pictures
were taken about 15 minutes after emulsion preparation.
Figure 2 shows representative light microscopy pictures
of template emulsions prepared with: (a) no Li2SO4 and
(b) 2.9% Li2SO4 in the water phase. The pictures show
that the water droplets are manifold and that there is a
great variation in droplet size. As clearly can be seen, the
template emulsion including litium sulphate (Figure 2(b))
mainly contain droplets sizes of 0.5 - 3 µm. A few larger
droplet species can occasionally be observed but the
main population is in the mentioned size range. The size
distribution is reasonably narrow and the droplets are
located quite tight in the initial emulsion. The emulsion
without Li2SO4 (Figure 2(a)) in the water phase displays
sizes between 0.5 and 7 µm and the emulsion shows
greater polydispersity in droplet size compared to all

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 434
(a) (b)
Figure 2. Light microscope pictures of template emulsions prepared with: (a) No Li2SO4 and (b) 2.9% Li2SO4 in the water
phase. The pictures were taken about 15 minutes after emulsion preparation. The scale bar is 10 µm.
emulsions with litium sulphate included in the water
droplets.
The emulsion droplet size is an important parameter
which governs the kinetic stability of an emulsion. It is
well established that an emulsion containing uniform and
small droplets is the most favorable one in terms of sta-
bility. The small droplet sizes reduce the tendency of the
emulsion to undergo coalescence and/or Ostwald ripen-
ing. Ostwald ripening is a process where larger emulsion
droplets grow on the expense of smaller species, due to
diffusion of internal phase molecules through the con-
tinuous phase. The outcome of Ostwald ripening is pro-
gressive coarsening of the emulsion, coalescence events
and eventually emulsions break-down. Emulsions con-
taining litium sulphate all display smaller droplet sizes
and a narrow size distribution which is favorable in terms
of emulsion stability. The emulsion stability test (results
not shown) demonstrations that the emulsion stability
was greatly influenced by the Li2SO4 content. It was
found that emulsions including litium sulphate are stabile
for 24 hours or longer. The Li2SO4 depleted emulsions
have separated after 30 minutes which is observed as a
creaming layer atop in the vial. The litium sulphate
clearly has a stabilizing effect on the emulsion by gov-
erning the droplet sizes. There are examples in literature
of the stabilizing effect of electrolytes in the water phase
of w/o emulsions [16,17]. Increased electrolyte concen-
tration decreases the tendency for Ostwald ripening and
increase the emulsion stability. The addition of electro-
lytes to the water phase decreases the transport of inter-
nal phase into the external phase, which results in slower
Oswald ripening.
3.2. Morphology of PHB Films
The PHB films were produced from solvent casting of
PHB solution and w/o template emulsions and then cha-
racterized by SEM. The result is displayed in Figure 3.
The pictures (a)-(c) show the cross sections and surfaces
(d)-(f) of films cast from emulsions with varying Li2SO4
content. The films made from PHB solutions are non-
porous and dense (see Figures 3(g) and (h)). The mor-
phology of the films produced from the emulsion tem-
plates varies in appearance. The interior of the film lack-
ing Li2SO4 in the water phase of the template emulsion
show non-spherical, slightly elongated voids whose sizes
ranging from about 1 - 6 µm. The polymer walls are dif-
fering in thickness and a few small windows of about 0.5
µm are visible. The upper film surface is rough (see
Figure 3(d)) with only a few small pores of 0.5 µm. The
appearance of the upper surface of film (Figure 3(d))
made from the emulsion template without Li2SO4 sug-
gests that there has been a phase separation during the
film casting procedure resulting in skin formation on the
upper surface.
Cross sections of the films made from emulsion tem-
plates with 2.9% and 14.3 % Li2SO4 in the water phase
show a more uniform void size of approximately 1 - 2
µm, with several interconnecting windows of about 0.5
µm. The polymer walls are thin and of reasonably even
thickness. The upper film surfaces show some variation
in film morphology but mainly display pores of ap-
proximately the same sizes and there is no significant
skin formation on the film. The results show that the wa-
ter droplet sizes of the emulsion templates (sizes 0.5 - 3
µm) is mostly translated into the resulting films and that
a solid foam material with neighboring voids is formed.
The SEM pictures also show the presence of intercon-
nected windows between adjacent voids. The intercon-
nected windows are created in an analogue manner to the
solidification of foam materials from HIPE, where the

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 435
(a) (b) (c)
(d) (e) (f)
(g) (h)
Figure 3. SEM micrographs of surfaces and freeze fractured cross sections of non-porous PHB films and porous PHB films
made from w/o emulsion templates with varying amount of Li2SO4 in the water phase. The pictures show the cross sections
(a)-(c) and upper surface (d)-(f) of films with 0% Li2SO4, 2.9% Li2SO4 and 14.3% Li2SO4 (from left to the right) and the up-
per surface (g) and cross section (h) of a PHB film.
high concentration of inner phase and tight packing of
emulsion droplets gives rise to window formation. Here
the highly volatile organic solvent of the template emul-
sion is rapidly driven off during film casting rendering
the emulsion into a highly concentrated emulsion. This
leads to an increase in solution viscosity and a fixation of
the water droplets in the polymer matrix during film
casting. The formation of a highly concentrated emulsion
enables the formation of windows between adjacent wa-
ter droplets during the film casting.
The solvent evaporation and film formation must be
made in a controlled manner. A too slow evaporation rate
may induce phase separation of the emulsion during the
film casting whereas a too quick evaporation on the other
hand leads to a curly and deformed film. It is therefore
necessary to monitor and optimize the drying process in
order to avoid phase separation. Several try-outs were
made in order to produce films of required quality. The
mass loss during film drying shown in Figure 4 is pre-
sented to demonstrate the range of conditions that were
tested to achieve different solvent evaporation rates and
to produce films of different qualities. The evaporation
rate was too quick for films cast with half open lid and
too slow for films cast with totally closed lid resulting in
films of bad qualities. The films cast in a fume hood
with a gap of approximately 1 mm between the petri
dish and a glass lid, in order to make a solvent saturated
microclimate and to control the evaporation rate, were

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 436
Figure 4. Solvent evaporation rate of the PHB film casting.
found to be of good quality. At 120 minutes, 50% of the
solvent has evaporated and a highly concentrated emul-
sion is present. During these conditions, the droplets
sizes of the template emulsion were translated into the
casted film during film casting and films were produced
in a reproducible manner. Considering the different film
morphologies, it is apparent that Li2SO4 is a vital com-
ponent of the emulsion template in order to obtain films
with controlled structures and uniform sizes. Also the
film casting procedure with the solvent evaporation is a
key step in order to produce reproducible films of desired
quality.
3.4. Location of Li2SO4 in the Porous PHB Films
as Studied by SEM-EDX
SEM-EDX was made in order to establish the location of
Li2SO4 after the film formation. SEM-EDX were made
on two areas of equal size, taken on the flat surface and
deep in a pore of the upper surface of porous PHB films
(See Figure 5). The upper EDX spectrum show the pres-
ence of sulphur peaks in the interior of the pore. No sul-
phur peak is visible when EDX analysis was made on the
flat upper surface (see the lower EDX spectra in Figure
5). From these results it can be concluded that Li2SO4 is
located mainly inside the pores, probably lining the pore
wall. Since no sulphur peak was recorded from the area
on the flat surface, then there is probably no litium sul-
phate residing in the polymer matrix. The Li2SO4 salt
precipitation which is lining the pore walls might influ-
ence the material properties of the porous film.
3.5. Mechanical Properties of PHB Films before
and after Water Exposure
Dynamic mechanical analysis, DMA, was used to exam-
ine if the mechanical properties of the porous material
were retained after water exposure. The complex mo-
dulus, , is considered as a measure of rigidity and
mirrors the ability of a material to resist deformation
when it is exposed to stress. The deformation of a mate-
rial may be a combination of an elastic and a viscous
portion. The viscous and the elastic portion are described
by the loss modulus and storage modulus
*
E

E


E

,
respectively. As the loss factor, tan

, is the ratio be-
tween E

 and E

an ideal elastic solid would have no
viscous contribution and tan

would be zero. The
complex modulus and tan

before and after water ex-
posure were determined for the different films, see Table
1. The percent change after water exposure of the com-
plex modulus


*
E was calculated according to:
**
*
*100
dw
d
EE
EE


 (1)
where and are the plateau values for the com-
plex modulus of the dry and water exposed films, respec-
tively. Similarily, the percent change of the loss factor
*
d
E*
w
E


tan

 after water exposure was determined from the
plateau values of the loss factor from the dry, tan d

, and
wet film, tan w

, using the following equation:
tan tan
tan 100
tan
dw
d




 (2)
In the dry state the non-porous PHB film had an
of 1.9 GPa, which is considerable larger than the porous
films with values of 0.7 - 1.2 GPa. This decrease in
is quite reasonable since porous films normally have
a lower mechanical strength compared to the corre-
sponding dense material [18]. The DMA measurements
on the porous PHB films in the dry condition show that
the increased with increasing litium sulphate con-
tent in the template emulsion (Table 1). This is probably
an effect of the spatial location of litium sulphate, lining
the interior of the pores as indicated in Figure 5, rein-
forcing the structure and thereby increasing the value of
for these materials. However, since litium sulphate
is a water soluble additive it is readily washed out upon
water contact leading to a decrease in between 12%
and 35% after water exposure, which can be seen in Ta -
ble 1.
*
E
*
E
*
E
*
E
*
E
*
E
The values of the loss factor, tan

, in Table 1 are
very low, indicating that both the porous and non-porous
PHB materials behave as an elastic solid material in the
dry state. Submerging the films in water could theoreti-
cally have a plasticizing effect on the material due to
water penetration into the material and this should then
increase the loss factor. The non-porous PHB film is
more or less unaffected by water exposure in comparison
to the emulsion prepared films which have tan


(percent change after exposure to water) values all above
40% (Table 1). From this one could argue that water acts
as a plasticizer for the emulsion prepared films, but not
for the pure PHB film. However, the loss factor is still
very low and the increase in tan

could be due to that
the dissolved additives migrates to the water phase,
hich results in a somewhat more flexible material. w

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 437
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Full Scale 105 cts Cursor: 3.490 (1 cts) keV
O
Electron Image 1
10 μm
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Full Scale 276 cts Cursor: 1.603 (11 cts) keV
Electron Image 1
10 μm
Figure 5. SEM-EDX on two areas of equal size taken on the flat surface and in a pore of the porous PHB films. The upper
spectrum shows sulphur peaks w hich originates from the counter ion of lithium. Please note that the scale on the lower graph
is enlarged compared to the upper graph.
Table 1. The complex modulus, *
d
E
, and loss factor, , of the dry non-porous PHB films and dry porous PHB films
made from emulsion templates, and percent change after water exposure determined according to Equations (1) and (2). Val-
ues within parentheses indicate min/max deviation from the average of (n = 2) measurements.
tan d δ
*
d
E tand

*
E tan


(GPa) (%) (%)
PHB 1.9 (0.096) 0.036 (0.0010) 3.59 (0.1) 1.8 (0.7)
PHB emulsion 0% Li2SO4 0.7 (0.008) 0.024 (0.0002) –12.1 (0.7) 41.7 (2.6)
PHB emulsion 2.86 % Li2SO4 0.8 (0.004) 0.023 (0.0003) –16.5 (0.3) 58.2 (1.7)
PHB emulsion 14.3 % Li2SO4 1.2 (0.006) 0.029 (0.0006) –35.3 (0.8) 72.2 (1.3)
1a
t


  (3)
3.6. Water Uptake and Film Porosity
The water uptake of a porous polymer material may ei-
ther take place in the pore structures only, as penetration
of water in the polymer matrix or as a combination of
both, depending on the time scale, the hydrophilic nature
of the polymer material and the pore interconnectivity.
The porosity was determined to 52% ± 3% and 45% ±
3% for the films with 2.9% and 14.3% litium sulphate in
the template emulsion, respectively. The film made from
template emulsion without the litium sulphate showed to
have 51% ± 3% porosity. The total film porosity (Φ), see Equation (3), was de-
termined by calculating the apparent density of the film

a

and comparing it to the true density

t

of PHB
(1.22 g·cm–3) [19]. The a

was derived from the weight
and dimensions of the porous PHB films.
The degree of water uptake is an important factor since
it reflects the wettability of the film interior but more
important it reflects the void interconnectivity of the
porous material. Figure 6 shows a plot of the water

Preparation of Porous Poly(3-Hydroxybutyrate) Films by Water-Droplet Templating 438
0
10
20
30
40
50
60
70
80
90
100
0 204060801001
Water absorption (%)
Time (min)
20
PHB 0 % Li2.86 % Li14.3 % Li
Figure 6. Plot of the water absorption capacity of PHB films
made either by w/o emulsion te mplates with varying amount
of Li2SO4 in the water phase, or PHB only.
absorption capacity of porous and non-porous PHB films
during 120 minutes. The water uptake is calculated as
follows:
100
wo
o
WW
AW


(4)
where A is absorbed water amount, o W is the initial
weight of the film and Ww is the weight of the wetted film.
Initially there is fast water absorption for the porous
PHB films made from emulsion templates. At 120 min-
utes the curves level out to reach a value of 15%, 65%
and 70% water absorption for the films made from emul-
sion templates with 0%, 2.9% and 14.3% Li2SO4. The
water uptake of the non-porous PHB film is on a minor
level, as expected. The higher the amount of Li2SO4 in
the template emulsion, the higher the water uptake and
the rate of water intrusion into the PHB film. The ini-
tially fast water absorption of films made from template
emulsions including litium sulphate may be a result from
capillary forces arising from the greater extent of small
voids and connections between the voids compared to
films made from lithium deficient template emulsions. It
could also be due to the presence of Li2SO4 which is lin-
ing the interior of the pore walls rendering the material
hydrophilic in character, hence increasing the wettability
of the otherwise hydrophobic polymer.
Considering the hydrophobic nature of PHB it is rea-
sonable to believe that the water uptake of the porous
PHB films is to a major extent dependent on the pore
structure, the number of voids and interconnecting win-
dows. The water uptake of the films can be used to cal-
culate the portion of water available pores for each film
type by simply correcting the film weights for the density
difference between water and the polymer. Films pro-
duced from template emulsions including litium sulphate
have comparable volume fractions of water available
pores, 38% ± 12% and 48% ± 7% for films with 2.9%
and 14.3% litium sulphate respectively. The morphology
of the films also appear very similar as observed from
SEM images (Figure 3). However, the film made from
lithium deficient template emulsions show a substantially
lower fraction of water available pores of 11% ± 3% only.
The total porosity was considerably higher, 51% ± 3%.
This difference probably reflects the film morphology
(See Figure 3) that shows irregular and thick polymer
walls between the voids, leading to less interconnection
between the voids. If there are pores which are not inter-
connected there may be areas which are not reached by
the water intrusion. The total porosity is more or less the
same for all film types but the fraction water available
pores differs. It is significantly less for the film made
from litium sulphate deficient emulsion template which
indicates lower pore interconnectivity due to different
film morphology.
4. Conclusion
Highly porous biodegradable PHB films were prepared
by combining basic w/o emulsion assembly with solvent
casting to achieve concentrated emulsions suitable for
templating interconnected solid foam materials. The
films have a tunable morphology with a resulting porous
and interconnected structure. Differences in fraction wa-
ter available pores and total porosity for the films made
from emulsion templates reflects the film morphology
and differences in interconnection between the voids.
5. Acknowledgements
We sincerely thank Anders Mårtensson at the Depart-
ment of Chemical and Biological Engineering for help
with the SEM micrographs. Financial support was kindly
provided by the Vinn Excellence Centre SuMo Biomate-
rials (Supermolecular Biomaterials—structure dynamics
and properties). Additional funding from Chalmers Area
of Advance—Materials Science is gratefully acknow-
ledged.
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