XRD and Physicochemical Evaluation of Hibiscus sabdariffa Cellulose-Butyl Acrylate-co-vinyl Monomer Graft
Utilizing the renewable waste biomass to procure advanced materials has been
the aim of research. Various reaction parameters were optimized for the graft
co-polymerization of Hibiscus sabdariffa cellulose fiber using binary
vinyl monomeric mixture. The graft co-polymers thus obtained were characterized
by XRD technique and the results received the supporting evidence by various
other advanced analytical techniques. The percentage crystallinity and crystallinity
index were found to decrease with increase in grafting while a reduction in
moisture absorption was observed. There was an increase in physico-chemico-thermal
resistance in the graft copolymers. These graft copolymer can be used in various
Received: July 10, 2012;
Accepted: September 13, 2012;
Published: October 04, 2012
Nowadays, whole world has focused its attention towards renewable and sustainable
resources because of environment and health concerns. The development of new
polymer, advanced materials from renewable raw materials in comparison to artificial
fibers has been increased during last few years (Chauhan
and Kaith, 2012; Aan et al., 2011; Abd
El-Hady, 2011; Abd El-Hady et al., 2011;
Abdi et al., 2010; Raja and
Thilagavathi, 2011; Issaoui et al., 2011;
Das et al., 2011; Rocco,
2011; Adedayo, 2012). Natural fibers like pine needle,
flax, jute and ramie are the most suitable reinforcement materials in industries
such as automobile, packaging and construction materials. Low cost, easily availability
and required properties (viz. low density, effective mechanical properties)
makes them attractive in place of glass, carbon and other synthetic fibers.
The constituents of natural fibers are cellulose, hemicelluloses, lignin and
pectin with a small quantity of the extractives. The properties of bio-fibers
mainly depend upon their origin, age, climatic conditions and extraction techniques.
The presence of hydroxyl groups (polar group) in various constituents of lingo-cellulosic
fiber reduces the utilities in many applications. In order to improve it adhesion,
various techniques like graft copolymerization, chemical treatment (viz. mercerization,
acetylation, benzoylation etc.) and treatment with various coupling agent can
be used. Graft copolymerization is an efficient technique to impart desirable
properties to backbone polymers (Kaith et al., 2003,
Various workers have carried out the graft copolymerization onto different
cellulosic backbone using vinyl monomers through various chemical and radiation
techniques (Kaith et al., 2003, 2004;
Singha et al., 2004; Bavan
and Kumar, 2010; Zhu et al., 2008). Sharma
and Daruwalla (1977) reported grafting of Methyl Acrylate (MA) individually
and in binary mixtures onto cotton yarn by emulsion technique initiated by ceric
ions. Okieimen and Idehen (1987) have carried out graft
copolymerization of MA onto cellulose and thiolated holocellulose and reported
decrease in incorporation of graft in latter. Graft copolymerization can be
initiated by using chemical, ionic and radical initiator systems. Among these
initiation systems the chemical initiation by grafting involving oxidizing agents
such as potassium permanganate, potassium bromate, ceric ammonium nitrate, ozone
hydroxyl radicals etc are promising from economic point of view. Different workers
have studied the kinetics of graft copolymerization of vinyl monomers onto the
starch and synthetic copolymers but few attempts seems to have been made to
study kinetics of graft copolymerization of vinyl monomers onto natural fibers
(Monier et al., 2010; Sabaa
and Mokhtar, 2002; Eromosele and Hamagadu, 1993;
Mishra and Tripathy, 1981; Taghizadeh
and Khosravy, 2003).
Hibiscus sabdariffa (Roselle) has attained dominance as a jute substitute
and all attempts are being made to extend its cultivation in areas that are
unfavorable for jute cultivation. As the fiber is virgin so it was selected
to explore the scope of its viability as advanced material.
MATERIAL AND METHODS
Plant materials: H. sabdariffa was refluxed with acetone for
72 h. Monomers [Butyl Acrylate (BA) as principal monomer and its binary mixtures
with Methyl Acrylate (MA), Acrylic Acid (AA) and Acrylamide (Am)] were used
as received. Weighing was carried-out on Libror AEG-220 (Shimadzu) electronic
balance. LEO Electron microscope (S.No.435-25-20) and Perkin Elmer instrument
were used for the SEM and IR analysis. X-ray diffraction studies were performed
on Bruker-D8 Advance. Thermo Gravimetric Analysis (TGA) and Differential
Thermal Analysis studies (DTA) were conducted in air on Thermal Analyzer (LINSEIS,
L81-11) at a heating rate of 10°C min-1.
Grafting: Graft copolymerization of binary vinyl monomer mixture on
to H. sabdariffa was done using butyl acrylate as a principal monomer
in combination with the secondary monomers under optimized conditions as per
the method reported earlier (Kaith et al., 2003,
2004; Singha et al., 2004):
where, Wi and Wf are the weights of the original fiber
and functionalized fiber, respectively.
Fourier transformer infrared spectroscopy (FTIR) and scanning electron
microscopy (SEM): IR spectra of the ungrafted and the grafted fiber were
taken with KBr pellets on Perkin Elmer Spectrophotometer. Since, the cellulose
has non conducting behavior so it was gold plated to have an impact. Scanning
was synchronized with microscopic beam in order to maintain the small size over
large distance relative to the specimen. The resulting images has a great depth
of the field. A remarkable three-dimensional appearance with high resolution
was obtained on LEO Electron microscope (S.No.435-25-20).
X-ray diffraction studies of graft copolymers (XRD): X-ray diffraction
studies were performed under ambient conditions, on Bruker using Cu Kα
(1.5418 Å) radiation, Ni-filter and scintillation counter at 40 kV and
40 mA on rotation between 13-25° at 2θ-scale at 1 sec. step size and
increment of 0.01° with 0.5° or 1.0 mm of divergent and anti-scattering
slit. The small particle size of each sample of H. sabdariffa-g-poly
(Hs-g-poly) (BA-co-AA, BA-co-MA and BA-co-AAm)and un-grafted fiber was made.
Each sample was homogeneously mixed prior to subjecting it for X-ray diffractometry.
The randomly oriented powdered sample with a uniform surface was exposed to
X-rays from all the possible planes of the sample and then measuring the scattering
angle of the diffracted X-rays with respect to the angle of the incident beam.
The continuous scans were taken and different d-spacings (Å) and
relative Intensities (I) were obtained. The counter reading of highest peak
intensity near 22.68° represents crystalline material and the peak near
15.00° in the halo-pattern corresponds to the amorphous material in cellulose.
Degree of crystallinity and crystallinity index were calculated as per the following
method (Mwaikambo and Ansell, 2002; Chauhan
and Singh, 2011):
where, Cr (%) is percentage of crystallinity, CI is crystallinity index and
I22.60, I15.00 are peak intensities of crystalline and
amorphous content at 2θ-scale close to 22.68 and 15.00°, respectively.
Moisture absorption study: Moisture absorbance studies at various
relative humidity levels were carried-out as per the method reported earlier.
Moisture absorbance percentage was found out by placing a known weight (Wi)
of dry grafted and ungrafted samples in a humidity chamber for about two hours
and then the final weight (Wf) of the samples exposed to different
relative humidities ranging from 30-90% were taken. The % moisture absorbance
was calculated from the increase in initial weight in the following manner (Kaith
et al., 2003, 2004; Singha
et al., 2004; Chauhan and Singh, 2011; Chauhan
and Kaith, 2012):
Acid and base resistance: Acid and base resistance studies were carried-out
as per the method reported earlier. Acid and base resistance was studied by
placing a known weight (Wi) of dry grafted and ungrafted samples
in fixed volume (50 mL) of 1 N HCl and 1 N NaOH and the final weights (Wf)
of the samples were noted down after 72 h (Kaith et al.,
2003, 2004; Singha et al.,
2004; Chauhan and Singh, 2011; Chauhan
and Kaith, 2012):
RESULTS AND DISCUSSION
Ceric ion forms complex with the cellulose through C-2 and C-3 hydroxyl groups
of the anhydroglucose unit. Transfer of the electron from the cellulose molecule
to Ce (IV) would follow, leading to its reduction to Ce(III), breakage of -OH
bonds at C-2 and C-3 bond and the formation of the free radical sites where
the monomeric chains get grafted. Graft yield and homo-polymer formation have
been found to be the functions of both the monomer and initiator concentration
(Chauhan and Singh, 2011; Chauhan
and Kaith, 2012).
Optimization of the reaction parameters: During the graft copolymerization
of BA (as a principal monomer) onto H. sabdariffa fiber, the different
optimized reaction parameters to obtain the maximum graft yield (66.80%) were:
temperature (°C ), 35; time (min), 120; CAN (mol L-1), 2.27x10-4;
HNO3 (mol L-1), 1.46x10-3; BA (mol L-1
), 1.40x10-3 and pH, 7.0.
Effect of the binary monomer mixtures on percentage grafting: Graft
copolymerization of different binary mixtures: BA+MA, BA+AA and BA+AAm using
BA as a principal monomer under optimized reaction conditions showed 44.95%,
24.95% and 12.80% grafting, respectively (Table 1). Butyl
acrylate as a principal monomer resulted in high Pg (66.80%) due to its chemical
reactivity, high rate of propagation (Kp), high rate of propagation
to termination (Kp/Kt) and high radical transfer rate
constant (Cm). However, methyl acrylate as a comonomer due to relatively
similar nature and properties has lesser diffusibility into the reaction medium
that decreases its Pg. AA as a comonomer due to high solubility in the reaction
medium forms hydrogen bond that results in decreased Pg. In case of AAm as a
comonomer with butyl acrylate, the change in free radical transfer rate constant,
polarity of the monomer mixture and overall composition affects the flocculation
property of the monomer leading to significant decrease in Pg. However, many
other factors also determine the graft yield like the type of fiber, swelling,
number of active sites, the nature and amount of the solvent and temperature
of polymerization strongly influence the reactivity ratios. In absence of monomer
rich phase, the diluents will compete with the monomers for adsorption sites.
The amount of adsorption will depend upon the total amount of surface area present
and this in turn, is dependent upon the rate of stirring. Physical factors like
mixing efficiency determines the melt temperature, the pressure, the rheological
properties, solubility of the initiator and the monomer.
|| Graft copolymerization of different binary vinyl monomeric
mixture onto H. sabdariffa fiber
|Pg: Percentage grafting, BA, MA, AA, Am are monomers used
Elevated temperature favors the degradation, reduces the initiator half life,
modifies the rate or specificity of the reaction, influences the solubility
and rheological parameters (Fried, 2005; Brandrup
and Immergut, 1975; Ham, 1964).
Characterization of graft-copolymers
Fourier transformer-infrared spectroscopy: H. sabdariffa fiber
when analyzed by FTIR depicted a broad peak at 3422.0 cm-1 (bound-OH
groups) and at 2924.8, 1454.0 and 1055.0 cm-1 (-CH2, C-C
and C-O stretching, respectively). However, in case of H. sabdariffa-g-poly(BA-co-MA):
sharp peak at 1737.8 cm-1 due to >C = O stretch of methyl acrylate
while in Hs-g-poly(BA-co-AA) a peak at 2928.6 cm-1 (due to OH stretch
of COOH gp.) was observed, whereas H. sabdariffa-g-poly(BA-co-AAm) exhibited
peaks at 3442.0 cm-1 (-NH group of AAm) in addition to the >C
= O group of butyl acrylate in all the cases.
Scanning electron microscopy: The cellulosic fiber lying at a distance
in raw sample (Fig. 1) started forming bundles in graft co-polymers
i.e. Hs-g-poly(BA-co-MA), Hs-g-poly(BA-co-AA) and Hs-g-poly(BA-co-AAm) and a
distinct morphological differentiation between backbone and graft copolymers
was observed (Fig. 2-4).
|| SEM of Hs-g-poly(BA-co-MA)
|| SEM of Hs-g-poly(BA-co-AA)
|| SEM of Hs-g-poly(BA-co-Am)
|| Percentage crystallinity (Cr) and crystallinity index (CI)
of the grafted and raw H. sabdariffa fiber
|Pg: Percentage grafting
X-ray diffraction studies of graft copolymers: It is evident from the
results that there was decline in Cr (%) and CI, Fig. 5 clearly
shows that the crystallinity and crystalline index of the fiber decreases from
ungrafted to highest grafted. The results obtained are briefly tabulated in
Table 2. The Cr (%) and CI from 77.20, 0.70 in raw fiber decreases
to 72.81, 0.62 in Hs-g-poly(BA-co-MA) with Pg: 44.75. Crystallinity index gives
a quantitative measure of the orientation of the cellulose crystals in the fibers.
A lower crystalline index in case of graft co-polymers means poor order of arrangement
of cellulose crystals with respect to the fiber axis. This is due to dis-orientation
of the cellulose crystalline lattice to the fiber axis during grafting.
||XRD overlay pattern of the raw H. sabdariffa and its
graft copolymers with binary vinyl monomeric mixtures
Since, the incorporation of monomer monomer moiety in the backbone impairs
the natural crystallinity of the fiber, therefore, graft copolymerization of
vinyl monomers onto H. sabdariffa fiber resulted in impaired crystallinity
and increased the amorphous region of the fiber. Thus, with increase in percentage
grafting, the percentage crystallinity and crystallinity index decreased along-with
reduction in stiffness and hardness. The Cr (%) and CI of Hs raw (77.20;
0.70) gets reduced (72.81; 0.62) in Hs-g-poly(BA-co-MA), respectively (Chauhan
and Kaith, 2012; Chauhan and Singh, 2011).
Thermogravimetric and differential thermal analysis (TG-DTA) of graft co-polymers:
The sample, at the heating rate of 10°C per min was scanned for 60 min up
to 600°C. Thermo-gravimetric analysis of raw H. sabdariffa fiber
and its graft co-polymers was carried-out as a function of weight loss versus
temperature. The degradation occurs in various forms like deacetylation, dehydration,
decarboxylation and chain scissions. The thermograms of grafted fiber depicted
two phase decomposition, the first stage signifies the breakdown of cellulose
and it shifts to higher temperature as compared to raw fiber while the second
stage was related to the degradation of the grafted poly vinyl chain. The shift
to higher temperature could be accounted due to increase in the covalent bonds
in the graft copolymers varying with the Pg. In case of H. sabdariffa
fiber, the major weight loss occurs in the first stage due to cellulosic decomposition
followed by the oxidation of the char. In case of raw fiber, both Initial Decomposition
Temperature (IDT) and Final Decomposition Temperature (FDT) are lower i.e.,
225.7°C and 463.0°C, respectively, as compared to those of graft co-polymers.
Thus, thermal resistance of the backbone could be increased through graft copolymerization
with different vinyl monomeric mixtures (Table 3) (Ouajai
and Shanks, 2005; Princi et al., 2005).
Further, TGA studies have strongly been supported by the DTA evaluation pattern
(Table 3). The thermogram of the H. sabdariffa fiber
have shown two major exothermic peaks at 327.9°C (18 μV) and 422.7°C
(14 μV). However, in case of Hs-g-poly(BA-co-MA), Hs-g-poly(BA-co-AA) and
Hs-g-poly(MA-co-AAm) the major exothermic peaks were observed at elevated temperature
with higher energy release that again endorses the higher thermal stability
of the graft copolymers as compared to the raw H. sabdariffa.
|| Thermo-gravimetric and differential thermal analysis of Hibiscus
sabdariffa and its graft co-polymers
|Pg: Percentage grafting, IDT and FDT: Initial and final decomposition
|| Chemical resistance and moisture absorbance studies of graft
copolymers vis-α-vis back bone
|Pg: Percentage grafting
The first transition minor peak revealed the dehydration, adsorption and oxidation
from the semi-crystalline host and the two major peaks signifies the fusion
and the irreversible dissociation of the crystallites. Moreover, most of the
hydroxyl groups of the native form have been replaced by the covalent bonds
through incorporation of poly (vinyl) chains into backbone thereby leading to
higher thermal stability (Ouajai and Shanks, 2005; Princi
et al., 2005).
Physical and chemical properties of the graft co-polymers
Moisture absorbance behavior: It is evident from the Table
4 that the moisture absorbance behavior has been greatly affected by graft
copolymerization this was due to incorporation of the hydrophobic organic moieties
replacing the free hydroxyl groups at C-2, C-3 and C-6. Thus, with increase
in Pg, there was decrease in the percent moisture absorbance i.e., Hs-g-poly(BA-co-MA)<Hs-g-poly(BA-co-AA)<Hs-g-poly(BA-co-Am)
(Kaith et al., 2003, 2004;
Singha et al., 2004).
Chemical resistance: Graft co-polymerization imparts resistance to the
natural fiber against the alkaline and acidic medium. It has been observed that
increase in Pg enhanced the chemical resistance. This could be due to the fact
that incorporation of hydrophobic moieties through graft copolymerization in
natural fiber decreased the chemical sensitivity for acid-base and resulted
in higher chemical stability. The chemical resistance decreased in the following
order Hs-g-poly(BA-co-MA)>Hs-g-poly(BA-co-AA)>Hs-g-poly(BA-co-Am) (Table
4) (Kaith et al., 2003, 2004;
Singha et al., 2004).
Graft co-polymerization of butyl acrylate and its binary mixtures with methyl
acrylate, acrylic acid and acrylamide resulted in the incorporation of desired
features in the H. sabdariffa fiber while sustaining its inherent characteristics.
Graft copolymerization could yield the end product with enhanced moisture and
chemical resistance along with higher thermal stability. Moreover, a complete
morphological transformation was observed on graft copolymerization that was
important technologically as seen in the XRD results. The XRD results were supported
by characterization and justified by the evaluation of the physicochemical behavior
of the grafted fiber. The graft copolymers thus obtained are the better means
for the utilization of the waste biomass in the advancement of technology.
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