Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Physicochemical surface properties of bacterial cellulose/polymethacrylate
nanocomposites: an approach by inverse gas chromatography
Marisa Fariaa, Carla Vilelab,⁎⁎, Armando J.D. Silvestreb, Bhanumathyamma Deepac,d,
Matic Resnike, Carmen S.R. Freireb, Nereida Cordeiroa,f,⁎
a LB3, Faculty of Exact Science and Engineering, University of Madeira, 9000-390 Funchal, Portugal
b CICECO – Aveiro Institute of Materials, Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193, Aveiro, Portugal
c Department of Chemistry, C.M.S. College, Kottayam, 686001, Kerala, India
dDepartment of Chemistry, Bishop Moore College, Mavelikara 690101, Kerala, India
e Jožef Stefan Institute, IJS, Department of Surface Engineering and Optoelectronics, Ljubljana, Slovenia
f CIIMAR - Interdisciplinary Centre of Marine and Environmental Research, University of Porto, 4450-208, Porto, Portugal
A R T I C L E I N F O
Keywords:
Bacterial cellulose
Nanocomposites
Poly(glycidyl methacrylate)
Poly(poly(ethylene glycol) methacrylate)
Surface properties
Inverse gas chromatography
A B S T R A C T
Nanocomposites of poly(glycidyl methacrylate) and bacterial cellulose (BC), or poly(poly(ethylene glycol) me-
thacrylate) and BC were produced via the in-situ polymerization of methacrylic monomers, inside the BC 3D
network. The nanocomposites surface properties were evaluated by inverse gas chromatography (IGC). The
dispersive component of surface energy ( sd) varied between 35.64 - 83.05mJm−2 at 25 °C. The surface of the
different nanocomposites has a predominant basic character (Kb/Ka= 4.20-4.31). Higher specific interactions
with polar probes were found for the nanocomposite bearing pendant epoxide groups, that apart from the low
surface area (SBET =0.83m2 g−1) and monolayer capacity (nm =2.18 μmol g−1), exhibits a high value of sd
(88.19mJm−2 at 20 °C). These results confirm the potential of IGC to differentiate between nanocomposites
with different surface functional groups and to predict their potential interactions with living tissues, body fluids
and other materials.
1. Introduction
The design of functional, nanostructured and high-performance
composite materials based on bacterial cellulose (BC), i.e. an extra-
cellular form of nanocellulose produced by non-pathogenic bacteria
that presents remarkable properties, is of utmost importance for myriad
applications particularly in the biomedical and pharmaceutical fields
(Silvestre, Freire, & Neto, 2014; Ullah, Santos et al., 2016; Ullah, Wahid
et al., 2016). One of the methodologies that is gaining a lot of attention
to prepare BC-based nanocomposites, due to its simplicity, is the so-
called in-situ polymerization of functional monomers within the BC
porous network that originates materials with interesting properties for
application as, for instance, stimuli-responsive nanocomposites for drug
delivery (Saïdi, Vilela, Oliveira, Silvestre, & Freire, 2017), antimicrobial
hydrogels (Figueiredo et al., 2015) and ion exchange membranes for
fuel cells (Gadim et al., 2014). This approach has been used in parti-
cular with acrylic or methacrylic monomers (Amin, Ahmad, Halib, &
Ahmad, 2012; Buyanov, Gofman, Revel’skaya, Khripunov, &
Tkachenko, 2010; Figueiredo et al., 2013, 2015; Hobzova, Duskova-
Smrckova, Michalek, Karpushkin, & Gatenholm, 2012; Saïdi et al.,
2017; Vilela, Gadim, Silvestre, Freire, & Figueiredo, 2016, 2017), but
also with other monomer types (Mashkour, Rahimnejad, & Mashkour,
2016; Gadim et al., 2014). The incorporation of these polymers within
the BC porous structure generates nanostructured materials with dif-
ferent mechanical, thermal, viscoelastic, optical and surface properties.
The latter properties are particularly relevant in the context of com-
posite materials whose surface will interact with, for example, living
tissues, body fluids or water contaminants, where adsorption and/or
adhesion processes dictate their suitability for specific applications
(Mohammadi-Jam & Waters, 2014).
Inverse gas chromatography (IGC) is one of the most used techni-
ques to examine the physicochemical surface properties of non-volatile
solid materials in the form of films, fibres and powders with both
amorphous and crystalline nature, and different morphologies (Lapčík
et al., 2016; Mohammadi-Jam & Waters, 2014). The simultaneous
measurement of physical and chemical properties by IGC allows to
https://doi.org/10.1016/j.carbpol.2018.10.110
Received 17 June 2018; Received in revised form 24 October 2018; Accepted 29 October 2018
⁎ Corresponding author at: LB3, Faculty of Exact Science and Engineering, University of Madeira, 9000-390 Funchal, Portugal.
⁎⁎ Corresponding author.
E-mail addresses: cvilela@ua.pt (C. Vilela), ncordeiro@staff.uma.pt (N. Cordeiro).
Carbohydrate Polymers 206 (2019) 86–93
Available online 31 October 2018
0144-8617/ © 2018 Elsevier Ltd. All rights reserved.
T
envisage the surface behaviour of a given sample in contact with other
materials, as well as to determine how the fabrication method can in-
fluence the surface properties (Mohammadi-Jam & Waters, 2014). As
an illustrative example, Beaumont et al. demonstrated that the drying
method (freeze-drying and supercritical drying) applied to cellulose II
gel (TENCEL®) influenced the surface properties of the obtained cellu-
lose particles, with the supercritical drying yielding aerogels of higher
surface area (Beaumont et al., 2017).
The IGC tool has already made its debut in the evaluation of the
surface properties of BC, its derivatives and composite materials
(Alonso, Faria, Mohammadkazemi, et al., 2018; Alonso, Faria, Ferreira,
& Cordeiro, 2018; Castro et al., 2015; Mohammadkazemi, Aguiar, &
Cordeiro, 2017). These studies enabled the assessment of the surface
physicochemical changes endowed by chemical (e.g. in-situ glyoxaliza-
tion of BC (Castro et al., 2015) or physical (e.g. oxidative polymeriza-
tion of aniline inside the porous network and surface of BC (Alonso,
Faria, Mohammadkazemi, et al., 2018; Alonso, Faria, Ferreira, et al.,
2018) modification in terms of surface energy, polarity and acid-base
behaviour because of the interactions with polar and non-polar probes.
In this context and following our interest on natural-based materials
and the assessment of their surface properties, the purpose of the pre-
sent work is to gain a thorough understanding of the surface properties
of BC-based nanocomposites with different surface groups by using IGC
analysis. Nanocomposite materials were prepared via the in-situ free
radical polymerization of methacrylic monomers, containing either
epoxide or hydroxyl pendent groups, namely poly(glycidyl methacry-
late) (PGMA) or poly(poly(ethylene glycol) methacrylate) (PPEGMA),
respectively, inside the BC porous structure, to evaluate the influence of
different surface groups on specific interactions, namely surface energy,
surface area, monolayer capacity and acid-base properties.
2. Materials and methods
2.1. Chemicals and materials
Bacterial cellulose (BC) was biosynthesized in the form of wet
membranes using the Gluconacetobacter sacchari bacterial strain
(Trovatti, Serafim, Freire, Silvestre, & Neto, 2011). Briefly, the BC
membranes were produced by growing the G. sacchari microorganisms
in static conditions at 30 °C during 48 h in HS (Hestrin-Schramm) liquid
medium (20 g L−1 glucose, 5 g L−1 peptone, 5 g L−1 yeast extract,
2.7 g L−1 Na2HPO4 and 1.15 g L−1 citric acid, pH 5). After incubation,
the BC membranes were separated from the media, treated with 0.5M
NaOH (90 °C during 30min) to eliminate the attached cells, washed
with distilled water until neutral pH, whitened with a 1% hypochlorite
solution and washed with distilled water.
Poly(ethylene glycol) methacrylate (PEGMA, average Mn 500, con-
taining 900 ppm monomethyl ether hydroquinone as inhibitor), gly-
cidyl methacrylate (GMA, 97%, with 100 ppm of monomethyl ether
hydroquinone as inhibitor), N,N’-methylenebisacrylamide (MBA, ≥
99.5%) and ammonium persulphate (APS) (98%) were purchased from
Sigma-Aldrich and used as received. The chemical reagents used for IGC
measurements (non-polar and polar molecules,> 99% purity, chro-
matography grade) were also purchased from Sigma–Aldrich. Methane
(reference probe,> 99.99%) and helium (carrier gas,> 99.99%) were
supplied by Air Liquid Company.
2.2. Preparation of BC-based nanocomposites
The in-situ free radical polymerization of GMA inside the BC net-
work was adapted from the procedure described elsewhere (Figueiredo
et al., 2015). First, wet BC membranes were weighed and about 60% of
their water content was drained. Then, two distinct aqueous reaction
mixtures were prepared: one containing GMA in a ratio of 1:2 (wBC/
wGMA) and 0.5% APS (w/w relative to monomer), and another con-
taining the same amounts of monomer and radical initiator, and 20%
MBA (w/w relative to monomer). The drained membranes and reaction
mixtures were both purged with N2 for 30min. Each reaction mixture
was added to the BC membranes and left for 1 h at room temperature
for complete incorporation inside the BC network. The polymerization
reaction took place in an oil bath for 6 h at 60 °C. The obtained nano-
composites (Table 1) were washed with distilled water and dried at
40 °C.
The in-situ free radical polymerization of PEGMA inside the BC
network was carried out according to the procedure described above
but with a BC to monomer ratio of 1:20 (wBC/wPEGMA) and 0.5% APS
(w/w relative to monomer).
2.3. Water uptake capacity
The water uptake (WU) of the nanocomposites was determined by
immersing the samples (1×1 cm2) in distilled water at room tem-
perature with a minimum of three replicas. The weight increase was
periodically measured for a period of 48 h. For each measurement, the
samples were taken out of the water, their wet surfaces immediately
wiped dry with filter paper, weighed, and then re-immersed. The water
uptake was calculated using the equation:
= ×WU W W
w
(%) ( ) 100%w 0
0
where, Ww is the sample weight after immersion in water and W0 is the
initial weight of dry sample.
2.4. X-ray diffraction
The X-ray diffraction (XRD) measurements were carried out with a
Phillips X’pert MPD diffractometer using Cu Kα radiation (λ=1.541 Å)
with a scan rate of 0.05° s−1(in 2θ scale). The peaks were deconvoluted
using Pearson VII peak functions (Peakfit software) for crystallinity
index, IC ,:
= ×I I
I
1 100 %C am
002
where I002 is the maximum peak intensity at 2θ around 22°, re-
presenting the crystalline region of cellulose, and Iam is the minimum
peak intensity at 2θ around 18°, representing the amorphous region of
cellulose (Jiang & Hsieh, 2014).
2.5. Attenuated total reflectance – Fourier transform infrared spectroscopy
The infrared spectra were obtained by Attenuated total reflection-
Fourier transform infrared spectroscopy (ATR-FTIR), using a Perkin
Elmer Spectrum Two coupled with a Diamond ATR accessory
Table 1
Composition in weight fraction of the prepared nanocomposites in the dry state.
Nanocomposites Nominal composition Measured composition
wmonomer/wBC wMBA/wmonomer wpolymer/wBC wBC/wTotal wpolymer/wTotal
PGMA/BC 2.0 0.0 1.56 0.39 0.61
PGMA-MBA/BC 2.0 0.2 2.03 0.33 0.67
PPEGMA/BC 20.0 0.0 6.69 0.13 0.87
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
87
(DurasamplIR II, Smiths Detection, UK). 32 scans were acquired in the
range of 4000–700 cm−1, with a wavenumber resolution of 4 cm−1.
2.6. Scanning electron microscopy and energy-dispersive X-ray spectroscopy
Scanning electron microscopy (SEM) images of the sample surfaces
were obtained in a HR-FESEM SU-70 Hitachi equipment, operating at
4.0 kV in the field emission mode. Samples were deposited on a steel
plate and coated with carbon before the analysis. The semi-quantitative
elemental chemical compositions of the samples were determined by
energy-dispersive X-ray spectroscopy (EDS). Semi-quantitative analyses
(wt.%) were carried out for carbon, oxygen and nitrogen, and the C/O
ratio was obtained. EDS experiments were conducted at an accelerated
voltage of 5 kV in a Hitachi SU 8090 equipment.
2.7. Atomic force microscopy
The morphological features of the samples were analysed through
atomic force microscopy (AFM; Solver PRO, NT-MDT, Russia) in the
tapping mode in air. The samples were scanned with the standard Si
(silicon) cantilever, with a constant force of 22 Nm−1, and at a re-
sonance frequency of 325 kHz (tip radius: 10 nm and tip length: 95 μm).
Surface roughness (Ra) was measured from representative images on
5× 5 μm2 areas and the scan rate was set to 1.3 Hz. Average surface
roughness and standard deviation were calculated from at least 3 dif-
ferent measurements on different areas of the same sample.
2.8. Inverse gas chromatography
Inverse gas chromatography (IGC) measurements were carried out
on a commercial inverse gas chromatograph (iGC, Surface
Measurements Systems, London, UK) equipped with flame ionization
(FID) and thermal conductivity (TCD) detectors. The iGC system is fully
automatic with SMS iGC Controller v1.8 control software. Data were
analysed using iGC Standard v1.3 and Advanced Analysis Software
v1.25.
The samples were packed in standard glass silanized (dimethyldi-
chlorosilane; Repelcote BDH, UK) columns with 0.2 cm internal dia-
meter and 30 cm in length by vertical tapping, for about 2 h. The col-
umns with the samples were conditioned for 8 h. After conditioning,
pulse injections were carried out with a 0.25mL gas loop. Four n-al-
kanes (heptane, octane, nonane and decane) were used for measure-
ments of the dispersive component of surface energy at 20, 25 and
30 °C. Four polar probes (tetrahydrofuran (THF), dichloromethane
(DCM), acetonitrile (ACN) and ethyl acetate (ETOAc)) were used to
determine the specific component of surface energy and acid-base
surface characteristics (KA and KB) at 25 °C. The isotherm experiments
were undertaken with n-octane, THF and DCM at 25 °C. All experiments
were carried out at 0% relative humidity with a helium flow rate of
10mlmin−1, at least in duplicate, producing a reproducibility error of
less than 4%.
The theoretical analysis and calculation of the surface energy
component, acid-base character and isotherm measurements is thor-
oughly described in the relevant literature (Alonso, Faria, Ferreira,
et al., 2018; Cordeiro, Gouveia, Moraes, & Amico, 2011; Lapčík et al.,
2016; Mohammadi-Jam & Waters, 2014).
2.9. Multivariate statistical analysis
Cluster analysis was performed by MINITAB 17 Statistical Software.
The hierarchically agglomerative clustering was used to evaluate and
achieve the similarity between IGC, AFM and EDS data through the
linkage distance (Square Euclidean distance), using the Ward linkage.
The dispersive component of surface free energy ( sd), adsorption po-
tential (Amax) of n-octane, acid-base character (K K/b a) ratio, specific
component of surface free energy ( Gssp), surface area (SBET), roughness
(Ra) and C O/ ratio were the variables introduced for the agglomerative
method.
3. Results and discussion
In the present study, three BC-based nanocomposite materials were
prepared via the in-situ free radical polymerization of a methacrylic
monomer, namely GMA and PEGMA (respectively bearing an epoxide
and hydroxyl pendent group), inside the BC three-dimensional network.
The reaction schemes of GMA polymerization in the presence and ab-
sence of cross-linker, as well as the polymerization of PEGMA are
summarized in Fig. 1. GMA was selected because it is a versatile and
low-cost commercial monomer used to obtain a hydrophobic homo-
polymer but also sophisticated (co)polymers that can be further func-
tionalized through nucleophilic epoxy ring opening reactions
(Muzammil, Khan, & Stuparu, 2017), which contributes to widen their
range of application. On the other hand, PEGMA was chosen due to its
amphiphilic nature resulting from the water-soluble side chain and the
hydrophobic methacrylate group that originates (co)polymers with in-
teresting properties (Bozkurt & Karadedeli, 2007; Guzmán, Nava,
Vazquez-Arenas, & Cardoso, 2017). Only PGMA was synthesized in the
presence or absence of cross-linker to assess the influence of using a
cross-linked and non-cross-linked matrix on the surface properties of
the corresponding nanocomposites.
The whitish and opaque PGMA/BC nanocomposites are composed
of 61 wt.% (PGMA/BC) and 67 wt.% (PGMA-MBA/BC) of PGMA, while
the yellowish and translucent PPEGMA/BC nanocomposite contains
87wt.% of PPEGMA (Table 1). The EDS data (Table 2) showed that the
C/O ratio increased around 36% and 34% for PGMA/BC (C/O=4.50)
and PGMA-MBA/BC (C/O=4.41), respectively, but decreased about
12% for PPEGMA/BC (C/O=2.90), when compared with the C/O ratio
of the native BC (C/O=3.30). These results bear out the incorporation
of PGMA, cross-linked PGMA and PPEGMA into the BC three-
Fig. 1. Radical polymerization of GMA in the (a) absence and (b) presence of
cross-linker (MBA), and (c) radical polymerization of PEGMA.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
88
dimensional network. Furthermore, PGMA/BC and PGMA-MBA/BC are
both hydrophobic nanocomposites with a low water uptake capacity of
33 and 31%, respectively, whereas BC and PPEGMA/BC are hydrophilic
with water uptake values of 138 and 432% (Table 2).
Regarding the crystallinity index (Ic, Table 2) of the nanocompo-
sites, it increased with the increasing content of BC (Ic= 65 – 79%
(Pecoraro, Manzani, Messaddeq, & Ribeiro, 2008)) from 39.4% for
PPEGMA/BC (13wt.% of BC) to 56.5% for PGMA-MBA/BC (33wt.% of
BC) and 69.0% for PGMA/BC (39wt.% of BC). This was expected given
that both PGMA (cross-linked and non-cross-linked) and PPEGMA are
amorphous polymers. The crystalline or amorphous nature of the ma-
terials is one of the parameters that can influence the IGC measure-
ments and, hence, the surface properties (Gamelas, 2013; Planinšek &
Buckton, 2003).
Fig. 2 shows the ATR-FTIR spectra of pure BC, and PGMA/BC,
PGMA-MBA/BC and PPEGMA/BC nanocomposites. The FTIR spectrum
of BC displays the typical absorption bands of cellulose at around
3340 cm−1 (OeH stretching), 2900 cm−1 (CeH stretching), 1310 cm−1
(OeH bending) and 1030 cm−1 (CeO stretching) (Pecoraro et al.,
2008). The presence in the spectra of PGMA/BC and PGMA-MBA/BC of
the absorption bands at 1724 cm−1 assigned to the C]O stretching
vibration, and 1252, 906 and 844 cm−1 attributable to the CeOeC
epoxide ring stretching (Tapeinos et al., 2013; Yang, Xu, Pispas, &
Zhang, 2013; Yuan et al., 2017), supports the inclusion of PGMA inside
the BC network. The existence of the absorption bands at 1720 cm−1
(C]O stretching vibration of the ester moiety) (Guzmán et al., 2017) in
the spectrum of PPEGMA/BC nanocomposite, corroborates the in-
corporation of PPEGMA into the BC three-dimensional structure.
3.1. Morphology and topography of the BC-based nanocomposites
The BC-based nanocomposites were characterized by SEM and AFM
to acquire information about their morphology and topography, which
is also important for the evaluation of their potentialities for specific
applications. Fig. 3 shows the SEM micrographs of the surface of pure
BC and PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites.
The three-dimensional nanofibrillar network characteristic of the BC
surface morphology is not clearly visible on the surface of the three
nanocomposites because the nanofibrils are imbedded in the respective
polymer matrix (i.e. PGMA, cross-linked PGMA and PPEGMA). This is
further validated by the AFM 2D topography images displayed in Fig. 4,
which show the defined fibril network of BC and the change in surface
texture with the incorporation of PGMA, cross-linked PGMA and
PPEGMA polymers. Furthermore, the nanocomposites exhibit different
surface roughness (Ra, arithmetic average roughness) depending on the
polymer matrix, as depicted in Table 2. The pure BC presents a surface
roughness of 37.7 ± 1.3 nm, which is higher than the value
(Rq= 25.2 ± 3.6 nm, root mean-square roughness) recently reported
for an oven-dried BC sample (Alonso, Faria, Mohammadkazemi, et al.,
2018), but much lower than the value (321 nm) obtained by for a
freeze-dried BC sample (Jia et al., 2017). The incorporation of 61 wt.%
of PGMA and 87wt.% of PPEGMA inside the BC porous structure pro-
moted a decrease of about 8.5 and 29% of the surface roughness to
34.5 ± 16.6 nm and 26.8 ± 0.1 nm, respectively, when compared to
pure BC. In contrast, the nanocomposite composed of cross-linked
PGMA (67wt.%) presents a higher a Ra value (51.4 ± 2.1 nm) than BC
and PGMA/BC nanocomposite This last result demonstrates that use of
a non-cross-linked or cross-linked polymer clearly influences the
properties of the ensuing nanocomposite because of the level of chain
entanglement. These differences in surface roughness suggest that the
surface properties of the nanocomposites will be different, as discussed
in the paragraphs below.
3.2. Surface properties of the BC-based nanocomposites by IGC
The surface properties of the three BC-based nanocomposites were
evaluated using IGC. Several physicochemical properties, namely sur-
face area (SBET), monolayer capacity (nm), dispersive component of
surface energy ( sd), Lewis acidic (Ka) and basic (Kb) constants, were
determined to understand the energetic interactions at the surface of
native BC and BC-based nanocomposites (Table 3). The heterogeneity
profile (Fig. 5) and the specific free energy of adsorption ( Gssp) (Fig. 6)
were also investigated.
According to Table 3, pure BC shows a BET specific surface area of
3.94m2 g−1 and a monolayer capacity of 10.38 μmol g−1, which are
Table 2
EDS elemental composition, water uptake capacity (WU), crystallinity index (Ic)
and average surface roughness (Ra) of BC, PGMA/BC, PGMA-MBA/BC and
PPEGMA/BC.
Nanocomposites C O C/O WU / % Ic / % Ra / nm
BC 75.37 23.05 3.30 138 95.0 37.7 ± 1.3
PGMA/BC 80.70 17.90 4.50 33 69.0 34.5 ± 16.6
PGMA-MBA/BC 72.37 16.41 4.41 31 56.5 51.4 ± 2.1
PPEGMA/BC 73.14 25.23 2.90 432 39.4 26.8 ± 0.1
Fig. 2. ATR-FTIR spectra of BC, PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
89
Fig. 3. SEM micrographs of the surface of pure BC, and PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites.
Fig. 4. AFM 2D images (5× 5 μm2) of pure BC, and PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
90
higher than the values obtained for BC films at 20 °C, SBET =1.94
m2 g−1 and nm =0.005mmol g−1 (Castro et al., 2015), but lower than
the SBET value (4.59m2 g−1 at 25 °C) recently reported for an oven-
dried BC (Alonso, Faria, Ferreira, et al., 2018). Furthermore, the na-
nocomposite containing the cross-linked version of PGMA polymer (i.e.
PGMA-MBA/BC) presents values of SBET and nm lower than those of BC,
whereas the inclusion of a polymer bearing hydroxyl pendent groups
inside the BC network originated a nanocomposite (i.e. PPEGMA/BC)
with SBET and nm values that are three times higher than the ones ob-
tained for pure BC.
The SBET increase can be due to the particle size decrease or rug-
osities increase in the material surface. As the obtained Rq values were
not correlated with SBET, the molecular changes, above referred, were
not detected by AFM analysis. As IGC uses small molecules (n-octane) as
probes, which can be inserted in the structural heterogeneities (na-
norugosities) at the material, it´s possible to detect changes at a mo-
lecular level. Thus, the SBET increase was correlated to the pendent
chain/groups inside the BC network (nanorugosities increase), and the
SBET decrease was correlated to the cross-linked polymer which de-
crease the pendent chain/groups (nanorugosities decrease).
These BET isotherm measurements can be correlated with the het-
erogeneity of the surface; thus, Fig. 5 shows the heterogeneity profiles
of BC and the three nanocomposites at 25 °C using n-octane and di-
chloromethane as non-polar and polar probes, respectively. For the n-
octane probe (Fig. 5a), PPEGMA/BC presents the lowest adsorption
potential (Amax= 7.38 kJ mol−1), followed by BC
(Amax= 9.15 kJmol−1), PGMA/BC (Amax= 12.6 kJmol−1) and PGMA-
MBA/BC (Amax= 12.6 and 14.8 kJmol−1). While PPEGMA/BC nano-
composite exhibits a surface with high number of active sites but less
energetic, PGMA-MBA/BC has a surface with more energetic dispersive
active sites. The less pronounced energetic heterogeneity of PPEGMA/
BC can explain the higher SBET value of this nanocomposite
(12.59m2 g−1, Table 3).
In the case of the dichloromethane (acid) probe (Fig. 5b), there is a
different tendency with BC exhibiting the lowest adsorption potential,
followed by PPEGMA/BC, PGMA-MBA/BC and PGMA/BC. The differ-
ences in the energetic profiles is a clear indication of the variances in
polarity among nanocomposites.
The dispersive component of surface energy ( sd), which is caused by
non-polar long-range interactions (London forces), decreased with
temperature rise from 20 to 30 °C for both pure BC and the three na-
nocomposites. These negative values of the temperature dependence of
the surface energy (Table 3) are associated with the exothermic nature
of the adsorption process and to the chain freedom. Thus, the lowest of
/ Ts
d obtained to the nanocomposites indicate the polymethacrylate
incorporation into the BC.
The sd values obtained for BC ranged from 47.22 ± 0.28mJm−2
at 30 °C to 58.34 ± 0.28mJm−2 at 20 °C. Although these values are
Table 3
IGC surface physicochemical properties of BC, and PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites.
BC PGMA/BC PGMA-MBA/BC PPEGMA/BC
SBET /m2 g−1 3.94 6.20 0.83 12.59
nm /μmol g−1 10.38 16.35 2.18 33.17
s
d /mJ m−2 20 °C 58.34 ± 0.28 59.05 ± 0.08 88.19 ± 0.08 35.46 ± 0.11
25 °C 52.19 ± 1.86 55.32 ± 0.15 83.05 ± 0.16 35.64 ± 0.36
30 °C 47.22 ± 0.28 51.90 ± 0.08 78.25 ± 0.08 36.16 ± 0.11
/ Ts
d /mJm−2 K−1 −1.11 −0.99 −0.71 −0.07
Ka 0.10 0.10 0.13 0.08
Kb 0.23 0.42 0.56 0.34
K K/b a 2.27 4.20 4.31 4.25
SBET : surface area; nm: monolayer capacity; sd: dispersive surface energy; Ka: acidic constant; Kb: basic constant.
Fig. 5. Heterogeneity profile with (a) n-octane and (b) dichloromethane from
BC, PGMA/BC, PGMA-MBA/BC and PPEGMA/BC nanocomposites at 25 °C.
Fig. 6. Specific free energy of adsorption ( Gssp) of BC, PGMA/BC, PGMA-MBA/
BC and PPEGMA/BC nanocomposites, with polar probes: THF – tetra-
hydrofuran, DCM – dichloromethane, EtOAc – ethyl acetate and ACN – acet-
onitrile, at 25 °C.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
91
higher than those obtained in previous studies for oven-dried BC,
namely 39.64mJm−2 at 20 °C (Castro et al., 2015), 39.21mJm−2 at
25 °C (Mohammadkazemi et al., 2017) and 37.7 ± 0.9mJm−2 at 25 °C
(Alonso, Faria, Mohammadkazemi, et al., 2018), Pommet et al. reported
a sd value of 61mJm−2 (Pommet et al., 2008). These values may be
due to the: (i) BC extrusion differ between strains of cellulose producing
bacteria; and/or (ii) the different synthesis conditions influence the
cellulose chains arrangement and, consequently, the cellulose type
produced, which influence the interaction between the BC surfaces and
the probes. Nevertheless, nanofibrillated cellulose (NFC), which is an-
other nanoscale form of cellulose, possesses comparable values of
s
d =32.4 – 49.5mJm−2 at 25 °C (Deepa et al., 2015) and sd =41.7 –
51.5mJm−2 at 40 °C (Gamelas, Pedrosa, Lourenço, & Ferreira, 2015).
Despite the slight increase of the dispersive component of surface
energy of PGMA/BC nanocomposite (55.32 ± 0.15mJm−2 at 25 °C)
when compared to BC, the nanocomposite containing the cross-linked
version of PGMA polymer promoted an increase of about 59% at 25 °C
(83.05 ± 0.16mJm−2, Table 3). This augment indicates that the in-
corporation of a polymer bearing epoxide pendent groups inside the BC
network originated nanocomposites with hydrophobic surfaces, which
is in line with a small water uptake capacity (Table 2). Furthermore, the
Amax peak shift towards higher energies in the heterogeneity profile
(Fig. 5a) also points to a higher hydrophobicity due to the energy in-
crease of the non-polar active sites. On the contrary, the nanocomposite
containing the polymer bearing hydroxyl pendent groups (i.e.
PPEGMA/BC) presents the lowest sd values (35.64 ± 0.36mJm−2 at
25 °C), which is indicative of a more hydrophilic surface than BC, as in
fact demonstrated by its very high water uptake capacity (ca. 430%,
Table 2) and less energetic non-polar active sites (Fig. 5a).
The acid-base nature of the surface of the three BC-based nano-
composites was evaluated based on their interactions with different
polar probes, namely tetrahydrofuran, dichloromethane, ethyl acetate
and acetonitrile. It is important to bear in mind that (i) the polarity of
the selected probes increases in the following order: dichloromethane
< tetrahydrofuran < ethyl acetate < acetonitrile, and (ii) di-
chloromethane is an acid probe, tetrahydrofuran is a base probe, ethyl
acetate is an amphoteric probe predominantly basic, and acetonitrile is
an amphoteric probe (Gamelas, 2013). The specific free energy of ad-
sorption and the Lewis acid-base constants were determined, and the
corresponding data is presented in Fig. 6 and Table 3.
The Gssp values of BC slightly increased in order of ethyl acetate
(6.76 kJmol−1)< dichloromethane (7.54 kJmol−1)< acetonitrile
(8.32 kJmol−1)< tetrahydrofuran (9.08 kJmol−1), indicating a sur-
face amphoteric behaviour but with a predominantly acidic character
(Fig. 6) as discussed in literature (Alonso, Faria, Ferreira, et al., 2018).
A general increase in Gssp is immediately visible in Fig. 6 for all the
probes after the inclusion of polymers bearing epoxide pendent groups
inside the BC porous network, i.e. PGMA/BC and PGMA-MBA/BC na-
nocomposites, which confirms the increase of polar groups on the
surface of these nanocomposites. Moreover, the amphoteric acetonitrile
probe displays a stronger interaction with the PGMA/BC and PGMA-
MBA/BC nanocomposites reaching the maximum Gssp values of 17.80
and 23.23 kJmol−1, respectively, that is indicative of acidic (electron
acceptor) and basic (electron donor) character on their surfaces.
PPEGMA/BC on the other hand is the nanocomposite displaying the
lowest interaction with the polar probes, except for the di-
chloromethane probe where it reaches a maximum Gssp value of
11.01 kJmol−1. This observation is supported by the energetic profile
obtained from the acid probe (dichloromethane) depicted in Fig. 5b and
can be ascribed to a lower concentration of polar surface groups. Fur-
thermore, the higher Gssp value of PPEGMA/BC with the acid probe
points to a surface with a more basic than acidic character.
Regarding the Lewis acid-base constants, the Ka and Kb values of
pure BC were 0.10 and 0.23, respectively, confirming that the surface of
BC is predominantly basic (Kb/Ka= 2.27, Table 3). The surface basicity
exhibited by BC is higher than the values found in previous studies
which reported Kb/Ka ratios of 0.55 (Castro et al., 2015), 0.75 (Alonso,
Faria, Ferreira, et al., 2018) and 1.83 (Mohammadkazemi et al., 2017),
but lower than the results reported by Pommet et al., with a Ka of 0.11,
Kb of 0.41 and Kb/Ka of 3.73 (Pommet et al., 2008). This is probably
also a result of small differences in surface morphology and cellulose
structural groups orientation, that affect the interaction between the BC
surfaces and the probes. Although the Ka constant of the three nano-
composites did not suffer a substantial change when compared with BC,
the basic character is quite different among the materials. In fact, the
ratio between the basic and acidic surface constants increased from
2.27 for BC to 4.20 for PGMA/BC, 4.25 for PPEGMA/BC and 4.31 for
PGMA-MBA/BC (Table 3). The augment of the basicity ratio of about 85
– 90% validates the enhancement of the Lewis basic character of the
nanocomposites surface when compared to BC, which is a direct result
of the inclusion of basic groups. Therefore, PGMA-MBA/BC is the na-
nocomposite possessing the higher concentrations of electron-donating
surface functional groups. These results correlate well with the specific
free energy of adsorption data discussed above (Fig. 6).
3.3. Cluster analysis
Overall the three nanocomposite materials exhibit distinct struc-
tural, morphological and surface properties as discussed above.
Nevertheless, and to confirm possible similarities among them, a hier-
archically agglomerative clustering analysis was carried out by gradu-
ally merging single variables into extensive groups according to their
features similarities. The dispersive component of surface free energy
( sd), adsorption potential (Amax) of n-octane, acid-base character
(K K/b a) ratio, specific component of surface free energy ( Gssp), surface
area (SBET), surface roughness (Ra) and C O/ ratio were the selected
variables for the agglomerative method. Fig. 7 depicts the dendrogram
of the above-mentioned variables, where it is possible to observe three
clusters. The first cluster presented a similarity of about 95% between
both samples (BC and PGMA/BC), whereas the second cluster
(PPEGMA/BC) showed a similarity of ca. 79% with respect to the first
cluster. In turn, the third and last cluster (PGMA-MBA/BC) exhibited a
low similarity (close to 17%) with the remaining nanocomposites.
The hierarchical clustering suggests that the incorporation of 61 wt.
% of a polymer bearing epoxide pendent groups (i.e. PGMA) inside the
BC porous network originates a nanocomposite (PGMA/BC) with quite
similar properties to BC, whereas the inclusion of 87 wt.% of a polymer
containing hydroxyl pendent groups (i.e. PPEGMA) yields a nano-
composite (PPEGMA/BC) with a lower level of similarity towards pure
BC and PGMA/BC. Conversely, the nanocomposite composed of cross-
linked PGMA and BC (i.e. PGMA-MBA/BC) exhibits significant dissim-
ilarities with the other two nanocomposites as well as with pure BC. In
fact, the incorporation of 67 wt.% of cross-linked PGMA into the BC
Fig. 7. Dendrogram of pure BC and PGMA/BC, PGMA-MBA/BC and PPEGMA/
BC nanocomposites using ICG parameters.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
92
three-dimensional network originated the most divergent results in
terms of: sd, Amax, Gssp and Ra, as clearly discussed in the above
paragraphs.
In this perspective, IGC proved once again to be an excellent tool for
surface characterization of BC and its nanocomposites. The evaluation
of the surface behaviour of the three nanocomposites in contact with
polar and non-polar materials can contribute to foresee the potential
applications of these materials. For example, while the PGMA-MBA/BC
nanocomposite can be used as a functional hydrophobic material with
an amphoteric (but predominantly basic) surface character for appli-
cations where the surface area is not a decisive factor, the PPEGMA/BC
nanocomposite can be applied as a functional hydrophilic material for
applications requiring high surface area and basic surface properties.
4. Conclusions
Nanocomposites consisting of either poly(glycidyl methacrylate),
cross-linked poly(glycidyl methacrylate) or poly(poly(ethylene glycol)
methacrylate) supported on the BC three-dimensional porous structure
were successfully prepared and characterized regarding their physico-
chemical surface properties by IGC. The dispersive component of sur-
face energy ( sd) varied between 35.64 - 83.05mJ m−2 at 25 °C. Overall,
the surfaces of the nanocomposites were rendered with a predominant
basic character (Kb/Ka of 4.20–4.31). The nanocomposite composed of
a cross-linked polymer bearing epoxide pendant groups exhibits the
higher specific interactions with polar probes, and despite the reduced
surface area (SBET =0.83 m2 g-1) and monolayer capacity (nm =2.18
μmol g-1), displays a high value of sd (88.19 ± 0.08mJ m−2 at 20 °C)
and an amphoteric (but predominantly basic) surface character. In view
of these results, the potential of IGC to differentiate between the na-
nocomposite materials developed in the present study was confirmed,
which enables the prediction of their interactions with other materials,
and consequently assess their suitability for specific applications.
Acknowledgments
The authors acknowledge the National Program for Scientific
Equipment Renewal, POCI 2010, for sponsoring the IGC work (FEDER
and Foundation for the Science and Technology), and project CICECO –
Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref.
UID/CTM/50011/ 2013), financed by national funds through the FCT/
MEC and when appropriate co-financed by FEDER under the PT2020
Partnership Agreement. The Portuguese Foundation for Science and
Technology (FCT) is also acknowledged for the post-doctoral grant to C.
Vilela (SFRH/BPD/84168/2012), and research contract under
Investigador FCT to C.S.R. Freire (IF/01407/2012).
References
Alonso, E., Faria, M., Ferreira, A., & Cordeiro, N. (2018). Influence of the matrix and
polymerization methods on the synthesis of BC/PANi nanocomposites: An IGC study.
Cellulose, 25(4), 2343–2354.
Alonso, E., Faria, M., Mohammadkazemi, F., Resnik, M., Ferreira, A., & Cordeiro, N.
(2018). Conductive bacterial cellulose-polyaniline blends: Influence of the matrix and
synthesis conditions. Carbohydrate Polymers, 183, 254–262.
Amin, M. C. I. M., Ahmad, N., Halib, N., & Ahmad, I. (2012). Synthesis and character-
ization of thermo- and pH-responsive bacterial cellulose/acrylic acid hydrogels for
drug delivery. Carbohydrate Polymers, 88(2), 465–473.
Beaumont, M., Kondor, A., Plappert, S., Mitterer, C., Opietnik, M., Potthast, A., et al.
(2017). Surface properties and porosity of highly porous, nanostructured cellulose II
particles. Cellulose, 24(1), 435–440.
Bozkurt, A., & Karadedeli, B. (2007). Copolymers of 4(5)-vinylimidazole and ethyle-
neglycol methacrylate phosphate: Synthesis and proton conductivity properties.
Reactive & Functional Polymers, 67, 348–354.
Buyanov, A. L., Gofman, I. V., Revel’skaya, L. G., Khripunov, A. K., & Tkachenko, A. A.
(2010). Anisotropic swelling and mechanical behavior of composite bacterial cellu-
lose-poly(acrylamide or acrylamide-sodium acrylate) hydrogels. Journal of the
Mechanical Behavior of Biomedical Materials, 3(1), 102–111.
Castro, C., Cordeiro, N., Faria, M., Zuluaga, R., Putaux, J. L., Filpponen, I., et al. (2015).
In-situ glyoxalization during biosynthesis of bacterial cellulose. Carbohydrate
Polymers, 126, 32–39.
Cordeiro, N., Gouveia, C., Moraes, A. G. O., & Amico, S. C. (2011). Natural fibers char-
acterization by inverse gas chromatography. Carbohydrate Polymers, 84(1), 110–117.
Deepa, B., Abraham, E., Cordeiro, N., Mozetic, M., Mathew, A. P., Oksman, K., et al.
(2015). Utilization of various lignocellulosic biomass for the production of nano-
cellulose: a comparative study. Cellulose, 22(2), 1075–1090.
Figueiredo, A. G. P. R., Figueiredo, A. R. P., Alonso-Varona, A., Fernandes, S. C. M.,
Palomares, T., Rubio-Azpeitia, E., et al. (2013). Biocompatible bacterial cellulose-
poly(2-hydroxyethyl methacrylate) nanocomposite films. BioMed Research
International, 698141, 1–14.
Figueiredo, A. R. P., Figueiredo, A. G. P. R., Silva, N. H. C. S., Barros-Timmons, A.,
Almeida, A., Silvestre, A. J. D., et al. (2015). Antimicrobial bacterial cellulose na-
nocomposites prepared by in situ polymerization of 2-aminoethyl methacrylate.
Carbohydrate Polymers, 123, 443–453.
Gadim, T. D. O., Figueiredo, A. G. P. R., Rosero-Navarro, N. C., Vilela, C., Gamelas, J. A.
F., Barros-Timmons, A., et al. (2014). Nanostructured bacterial cellulose-poly(4-
styrene sulfonic acid) composite membranes with high storage modulus and protonic
conductivity. ACS Applied Materials & Interfaces, 6(10), 7864–7875.
Gamelas, J. A. F. (2013). The surface properties of cellulose and lignocellulosic materials
assessed by inverse gas chromatography: a review. Cellulose, 20(6), 2675–2693.
Gamelas, J. A. F., Pedrosa, J., Lourenço, A. F., & Ferreira, P. J. (2015). Surface properties
of distinct nanofibrillated celluloses assessed by inverse gas chromatography. Colloids
and Surfaces A, Physicochemical and Engineering Aspects, 469, 36–41.
Guzmán, G., Nava, D. P., Vazquez-Arenas, J., & Cardoso, J. (2017). Design of a zwitterion
polymer electrolyte based on poly[poly (ethylene glycol) methacrylate]: The effect of
sulfobetaine group on thermal properties and ionic conduction. Macromolecular
Symposia, 374, 1600136.
Hobzova, R., Duskova-Smrckova, M., Michalek, J., Karpushkin, E., & Gatenholm, P.
(2012). Methacrylate hydrogels reinforced with bacterial cellulose. Polymer
International, 61(7), 1193–1201.
Jia, Y., Wang, X., Huo, M., Zhai, X., Li, F., & Zhong, C. (2017). Preparation and char-
acterization of a novel bacterial cellulose/chitosan bio-hydrogel. Nanomaterials and
Nanotechnology, 7, 1–8.
Jiang, F., & Hsieh, Y.-L. (2014). Super water absorbing and shape memory nanocellulose
aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing–thawing.
Journal of Materials Chemistry A, 2(2), 350–359.
Lapčík, L., Otyepka, M., Otyepková, E., Lapčíková, B., Gabriel, R., Gavenda, A., et al.
(2016). Surface heterogeneity: Information from inverse gas chromatography and
application to model pharmaceutical substances. Current Opinion in Colloid & Interface
Science, 24, 64–71.
Mashkour, M., Rahimnejad, M., & Mashkour, M. (2016). Bacterial cellulose-polyaniline
nano-biocomposite: A porous media hydrogel bioanode enhancing the performance
of microbial fuel cell. Journal of Power Sources, 325, 322–328.
Mohammadi-Jam, S., & Waters, K. E. (2014). Inverse gas chromatography applications: A
review. Advances in Colloid and Interface Science, 212, 21–44.
Mohammadkazemi, F., Aguiar, R., & Cordeiro, N. (2017). Improvement of bagasse fi-
ber–cement composites by addition of bacterial nanocellulose: an inverse gas chro-
matography study. Cellulose, 24(4), 1803–1814.
Muzammil, E. M., Khan, A., & Stuparu, M. C. (2017). Post-polymerization modification
reactions of poly(glycidyl methacrylate)s. RSC Advances, 7(88), 55874–55884.
Pecoraro, E., Manzani, D., Messaddeq, Y., & Ribeiro, S. J. L. (2008). Bacterial cellulose
from Glucanacetobacter xylinus: Preparation, properties and applications. In M. N.
Belgacem, & A. Gandini (Eds.). Monomers, polymers and composites from renewable
resources (pp. 369–383). Amsterdam: Elsevier.
Planinšek, O., & Buckton, G. (2003). Inverse gas chromatography: Considerations about
appropriate use for amorphous and crystalline powders. Journal of Pharmaceutical
Sciences, 92(6), 1286–1294.
Pommet, M., Juntaro, J., Heng, J. Y. Y., Mantalaris, A., Lee, A. F., Wilson, K., et al. (2008).
Surface modification of natural fibers using bacteria: depositing bacterial cellulose
onto natural fibers to create hierarchical fiber reinforced nanocomposites.
Biomacromolecules, 9(6), 1643–1651.
Saïdi, L., Vilela, C., Oliveira, H., Silvestre, A. J. D., & Freire, C. S. R. (2017). Poly(N-
methacryloyl glycine)/nanocellulose composites as pH-sensitive systems for con-
trolled release of diclofenac. Carbohydrate Polymers, 169, 357–365.
Silvestre, A. J. D., Freire, C. S. R., & Neto, C. P. (2014). Do bacterial cellulose membranes
have potential in drug-delivery systems? Expert Opinion on Drug Delivery, 11(7),
1113–1124.
Tapeinos, C., Efthimiadou, E. K., Boukos, N., Charitidis, C. A., Koklioti, M., & Kordas, G.
(2013). Microspheres as therapeutic delivery agents: Synthesis and biological eva-
luation of pH responsiveness. Journal of Materials Chemistry B, 1(2), 194–203.
Trovatti, E., Serafim, L. S., Freire, C. S. R., Silvestre, A. J. D., & Neto, C. P. (2011).
Gluconacetobacter sacchari: An efficient bacterial cellulose cell-factory. Carbohydrate
Polymers, 86(3), 1417–1420.
Ullah, H., Santos, H. A., & Khan, T. (2016). Applications of bacterial cellulose in food,
cosmetics and drug delivery. Cellulose, 23(4), 2291–2314.
Ullah, H., Wahid, F., Santos, H. A., & Khan, T. (2016). Advances in biomedical and
pharmaceutical applications of functional bacterial cellulose-based nanocomposites.
Carbohydrate Polymers, 150, 330–352.
Vilela, C., Gadim, T. D. O., Silvestre, A. J. D., Freire, C. S. R., & Figueiredo, F. M. L.
(2016). Nanocellulose/poly(methacryloyloxyethyl phosphate) composites as proton
separator materials. Cellulose, 23(6), 3677–3689.
Vilela, C., Sousa, N., Pinto, R. J. B., Silvestre, A. J. D., Figueiredo, F. M. L., & Freire, C. S.
R. (2017). Exploiting poly(ionic liquids) and nanocellulose for the development of
bio-based anion-exchange membranes. Biomass & Bioenergy, 100, 116–125.
Yang, H., Xu, J., Pispas, S., & Zhang, G. (2013). One-step synthesis of hyperbranched
biodegradable polymer. RSC Advances, 3(19), 6853–6858.
Yuan, S., Zhang, J., Yang, Z., Tang, S., Liang, B., & Pehkonen, S. O. (2017). Click func-
tionalization of poly(glycidyl methacrylate) microspheres with triazole-4-carboxylic
acid for the effective adsorption of Pb(II) ions. New Journal of Chemistry, 41(14),
6475–6488.
M. Faria et al. Carbohydrate Polymers 206 (2019) 86–93
93