In recent years, increasing interest in biodegradable polymer films has
developed mainly due to concern over the disposal of conventional synthetic
polymers derived from petroleum. Conversely, biodegradable films from
renewable sources not only are degraded readily after their disposal or
implantation but also can extent the food shelf-life and the controlled
release drug delivery, thus improving the quality of food and drug therapy,
respectively. Among various available biodegradable polymers, considerable
attention has been given to chitosan because of its unique properties.
The chitosan is a biopolymer that has received great attention in a variety
of applications because of their biodegradability and biocompatibility (Ravi
Kumaer et al., 2004; Muzzarelli and Muzzarelli,
2005). It is derived from chitin, which is the second most abundant polysaccharide
on earth next to cellulose and is available from waste products in the shellfish
industry. Because of its excellent film-forming property, chitosan can be used
effectively as a film-forming material to carry active ingredients such as mineral
or vitamin for food packaging applications (Park and Zhao,
2004; Rhim et al., 2006) and hydrophilic or
hydrophobic drugs for drug delivery applications (Senel et
al., 2000; Shu et al., 2001). The chitosan
film does not promote prolonged drug release due to the fast dissolution rate
in aqueous media. The hydrophobic drug-loaded poly(lactide-co-glycolide) microparticles
have been incorporated into chitosan films to improve prolong drug release profiles
(Perugini et al., 2003). However, these microcomposite
films exhibit microphase separation, which may be give rise to inconsistent
properties such as nanocomposite films.
Modified-spontaneous emulsification solvent diffusion method (modified-SESD
method) has been used to prepare surfactant-free nanoparticles of hydrophilic-hydrophobic
diblock copolymer (Baimark et al., 2007a). Higher
energy apparatus, such as a homogenizer or a sonicator and surfactants were
not used for this method.In the earlier study by Baimark
et al. (2007b) and Khamhan et al. (2008),
the nanocomposite chitosan-based films containing methoxy poly (ethylene glycol)-b-poly
(Îµ-caprolactone) diblock copolymer have been reported. The MPEG-b-PCL
nanoparticles were formed in chitosan aqueous solution by the modified-spontaneous
emulsification solvent diffusion method (modified-SESD method) before film casting.
In this research, the nanocomposite chitosan-based films containing methoxy
poly (ethylene glycol)-b-poly (D, L-lactide) diblock copolymer were prepared
for comparison in morphology and thermal stability. The film transparency and
moisture uptakes were also determined.
MATERIALS AND METHODS
Materials: Chitosan (90% deacetylation and molecular weight of 80 kDa)
was purchased from Seafresh Chitosan Lab Co., Ltd., (Thailand) and used without
further purification. Acetic acid (99.7%, Merck, Germany) was used as received.
Methoxy poly(ethylene glycol)-b-poly (Îµ-caprolactone) (MPEG-b-PCL)
and methoxy poly (ethylene glycol)-b-poly (D, L-lactide) (MPEG-b-PDLL)
diblock copolymers were synthesized by using MPEG with molecular weight of 5,000
g mol-1 and stannous octoate as the initiating system as described
in the earlier studies by Baimark et al. (2007a)
and Baimark et al. (2007b).
Molecular weights of the MPEG-b-PCL and MPEG-b-PDLL obtained
from curves of gel permeation chromatography were 62,300 and 73,600 g
Preparation of nanocomposite films: Nanoparticles of MPEG-b-PCL
or MPEG-b-PDLL diblock copolymers were prepared by the modified-SESD
method in chitosan solution without any surfactants. The chitosan solution
was prepared by using 1% (v/v) acetic acid aqueous solution as the solvent.
The preparation of nanocomposite films was typically described as follows.
The diblock copolymer was dissolved in 2 mL of 4/1 (v/v) acetone/ethanol
mixture. The diblock copolymer solution was then added drop-wise into
20 mL of chitosan solutions with stirring at 600 rpm. The organic solvent
was removed in fume hood for 5 h. The nanoparticles suspended in the chitosan
solution were obtained. Then, the film casting was done on Petri dish
and subsequently dried at 30°C for 72 h. The nanocomposite films with
chitosan/diblock copolymer ratios of 100/0, 90/10, 80/20 and 70/30 (w/w)
were investigated. The resulted nanocomposite film was kept in vacuo
at room temperature for a week before characterization.
Characterization of nanocomposite films: FT-IR spectroscopy was
used to characterize functional groups of the both chitosan and diblock
copolymer in nanocomposite films with air as the reference. The FT-IR
spectra of films were obtained with resolution of 4 cm-1 and
Thermal stability of the films was determined by thermogravimetry (TG)
using a TA-Instrument TG SDT Q600 thermogravimetric analyzer. For TG analysis,
10-20 mg sample was heated from 50 to 1,000°C at the heating rate
of 20°C min-1 under nitrogen atmosphere.
Morphology of film surfaces and cross-sections was investigated by Scanning
Electron Microscopy (SEM) using a JEOL JSM-6460LV SEM. The film cross-section
was obtained after cutting film with paper-scissors. Before SEM measurement,
the films were sputter coated with gold for enhancing the surface conductivity.
Film transparency was determined by UV-Vis spectroscopy using a Perkin-Elmer
Lambda 25 UV-Visible spectrophotometer at 660 nm as described in the earlier
studies by Khamhan et al. (2008).
Moisture uptake (%) of the chitosan and nanocomposite films was determined
following the method. Briefly, the sample films with 20x20 mm in size
were dried in vacuo at room temperature for a week. After weighing,
they were kept in a desiccator with 90 ± 5% relative humidity maintained
with a saturated sodium chloride solution at 30 ± 2°C. The
sample films were weighed again after keeping in the close desiccator
for 48 h. The moisture uptake (%) was calculated by Eq. 1.
Moisture uptake (%) = (Mf-Mi)/Mix100
where, Mi and Mf are the initial and final weight
(g) of the films before and after moisture absorption test, respectively.
RESULTS AND DISCUSSION
FTIR analysis: The amine, residual amide and hydroxyl groups of chitosan
can form the intermolecular hydrogen bonds with the ether and carbonyl groups
in PEG and polylactide, respectively (Zhang et al.,
2002; Chen et al., 2005). Existence of these
intermolecular bonding were investigated from FTIR spectra. The FTIR spectrum
of chitosan film in Fig. 1a shows the absorption bands at
1,654 and 1,587 cm-1 attributed to the amide carbonyl group (amide
I) and the free amino groups, respectively.
FTIR spectra of (a) chitosan, (b) 90/10, (c) 80/20,
(d) 70/30 (w/w) chitosan/MPEG-b-PCL nanocomposite and (e) MPEG-b-PCL
The FTIR spectrum of MPEG-b-PCL
in Fig. 1e shows the strong carbonyl band at 1,724 cm-1 and CH stretching vibration band at 2867 cm-1 of PCL and MPEG blocks,
respectively. The FTIR spectra of chitosan/MPEG-b-PCL nanocomposite films
are shown in Fig. 1b-d. These spectra demonstrated the both
absorption bands characteristics of chitosan and MPEG-b-PCL. As would
be expected, the intensities of carbonyl bands increased with the MPEG-b-PCL
ratio. The FTIR spectra of chitosan/MPEG-b-PDLL nanocomposite films show
similar evidence as the chitosan/MPEG-b-PCL nanocomposite films.
Film morphology: Film thicknesses of the chitosan and nanocomposite
film measured from SEM images were in the ranges of 10-50 μm.
SEM micrographs of film surfaces of (a) chitosan, (b)
70/30 (w/w) chitosan/MPEG-b-PCL nanocomposite and (c) 70/30
(w/w) chitosan/ MPEG-b-PDLL nanocomposite (bar = 1 μm)
film thicknesses increased with the diblock copolymer ratio. Figure
2 and 3 shows SEM micrographs of film surfaces and
cross-sections, respectively. The both surface and cross-section of chitosan
film were smooth appearance as shown in Fig. 2a and 3a, respectively. While the incorporation of nanoparticles
induced roughness on the both surfaces and cross-sections of the chitosan
The nanoparticles dispersed into chitosan films can be observed on the
both film surfaces and cross-sections. The nanoparticles can be clearly
detected on the film cross-sections. The nanoparticle morphology was spherical
in shape with smooth surface (Fig. 3b, c).
The nanoparticle sizes of MPEG-b-PCL and MPEG-b-PDLL were
approximately 200 and 100 nm, respectively. The number of the nanoparticles
increased when the diblock copolymer ratio was increased. This may be
expected that faster solidified rate of the MPEG-b-PCL nanoparticles
after solvent diffusion process gave the larger size nanoparticles.
In addition, the nanopores were also observed on the film surfaces and
cross-sections. This might be due to the nanoparticles were self-condensed
and nano-scopically phase separated from the chitosan film matrices during
the drying process. The film cross-section showed highly nanoporous structure
than that observed at its surface. The interconnected nanopores were approximately
300 and 50 nm in sizes for the chitosan/MPEG-b-PCL and chitosan/MPEG-b-PDLL
nanocomposite films, respectively, which indicates that these nanocomposite
films contain nanoporous structures.
Thermal stability: Thermal stability of chitosan film and nanocomposite
films was studied by TG analysis, which can be clearly determined from
differential TG (DTG) thermograms as shown in Fig. 4
and 5 for the chitosan/MPEG-b-PCL and chitosan/MPEG-b-PDLL
nanocomposite films, respectively. From DTG thermograms, the temperature
of maximum decomposition rate (Td, max) can be determined and
shown in Table 1. It was found that the chitosan, the
MPEG-b-PCL and the MPEG-b-PDLL had a main Td, max
at 296, 418 and 312°C, respectively.
The DTG thermograms of chitosan/MPEG-b-PCL nanocomposite films consisted
of the both Td, max of chitosan and MPEG-b-PCL components.
It should be noted that the both Td, max of each component of nanocomposite
films slightly shift toward higher temperature than those of chitosan film and
MPEG-b-PCL powder supporting intermolecular bonds existed between chitosan
and MPEG-b-PCL nanoparticles.
||DTG thermograms of chitosan, chitosan/MPEG-b-PCL
nanocomposite and MPEG-b-PCL films
The results indicated that chitosan and
the MPEG-b-PCL enhanced their thermal stability from intermolecular interaction
according to chitosan/MPEG-b-PCL homogeneous blends (Noi et al., 2008).
However, the chitosan/MPEG-b-PDLL nanocomposite films showed only a
single Td, max between the chitosan and the MPEG-b-PDLL in
all composite ratios. The results supported that the smaller MPEG-b-PDLL
nanoparticles are well dispersed into the chitosan film matrices.
Film transparency: The colors of chitosan and nanocomposite films
were slight yellowish and whiteness tint, respectively. The film transparency
was directly shown to their transmittances (%) at 660 nm (T660)
from UV-Vis spectrophotometry. The T660 values of films were
shown in Table 2. The transparency of chitosan film
was decreased when the nanoparticles were incorporated into the film matrices.
As the increasing diblock copolymer ratios, the film transparency decreased.
This may be due to the dispersed nanoparticles were opaque phases.
The chitosan/MPEG-b-PCL nanocomposite films showed more opaque
than the chitosan/MPEG-b-PDLL nanocomposite films in all chitosan/diblock
copolymer ratios. This could be explained that the MPEG-b-PCL chitosan
and MPEG-b-PCL nanoparticles.
||Thermal decompositions of chitosan nanocomposite films
and diblock copolymer powders
||DTG thermograms of chitosan, chitosan/MPEG-b-PDLL
nanocomposite and MPEG-b-PDLL films
|| Transparency and moisture uptakes of chitosan nanocomposite
|aMeasured from three samples and standard
deviations less than 0.5
The result nanoparticles were larger
in sizes and semi-crystalline phases, whereas the MPEG-b-PDLL nanoparticles
were smaller in sizes and amorphous phases.
Moisture uptakes: The moisture (%) uptakes of chitosan and nanocomposite
films were measured instead of water uptake (immersion in water) because
of the partial dissolution of chitosan. The moisture (%) uptakes of films
were calculated from Eq. 1 and also shown in Table
2. The moisture (%) uptake of the chitosan films was slightly decreased
when the both MPEG-b-PCL and MPEG-b-PDLL nanoparticles were
dispersed into the chitosan film suggested that the nanoparticles improved
the moisture resistant of the chitosan film. The moisture uptakes (%)
slightly decreased as the diblock copolymer ratio increased. This suggested
that the hydrophobic characteristics of diblock copolymer exhibited resistant
properties to water vapor.
However, the chitosan/MPEG-b-PCL nanocomposite films exhibited
higher moisture resistant than the chitosan/MPEG-b-PDLL nanocomposite
films in all chitosan/diblock copolymer ratios. This can be explained
that the MPEG-b-PCL shows higher hydrophobicity than the MPEG-b-PDLL
due to long chains of ethylene units in PCL blocks.
The chitosan nanocomposite films contained nanoparticles of diblock copolymers
were successfully prepared by film casting of nanoparticle suspension-chitosan
solution. The nanoparticles of MPEG-b-PCL or MPEG-b-PDLL
diblock copolymers were formed. The MPEG-b-PCL nanoparticles sizes
were smaller than the MPEG-b-PDLL. The nanopores were also observed
throughout the chitosan film matrices. The incorporating MPEG-b-PCL
nanoparticles into chitosan films enhanced thermal stability of the both
chitosan film matrices and dispersed MPEG-b-PCL nanoparticles,
whereas thermal stability of only chitosan film matrices were improved
when the MPEG-b-PDLL was used instead of MEPG-b-PCL. The
film transparency and moisture uptakes of chitosan films decreased as
the nanoparticles incorporated. These nanocomposite films might be of
interested for use in biomedical, pharmaceutical and packaging applications.
This research was supported by the Center of Excellence for Innovation
in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of