kinetics of graft copolymerization of acrylic acid ontocannabis - Sid

17 Pages·2011·1.26 MB·English

Available online at: http://journal.ippi.ac.ir
Kinetics of Graft Copolymerization of Acrylic Acid
onto Cannabis indica Fibre
Iranian Polymer Journal
20 (11), 2011, 913-929
Amar S Singha* and Ashvinder K Rana
Department of Chemistry, National Institute of Technology,
Hamirpur-177005, H.P., India
Received 3 June 2011; accepted 26 September 2011
A B S T R A C T
Natural fibres have received vast attention because of their combustible,
non-toxic, low cost, hydrophilic and biodegradable properties. In this study,
functionalization of cannabis indicafibre has been carried out by its grafting with
acrylic acid (AAc) using a Ce3+-HNO redox initiator in an aqueous medium. Different
3
reaction parameters such as reaction time, pH, reaction temperature, concentrations of
initiator, monomer and nitric acid were optimized to achieve the maximum percentage
of grafting (21.08%). The low graft yield obtained with acrylic acid was because of its
high affinity towards water. Asuitable mechanism to explain initiation, propagation and
termination of graft copolymerization has also been proposed. The kinetics of the graft
copolymerization of acrylic acid onto cannabis indicafibre has also been studied. It has
been found that the rate expression for graft polymerization is (R ) = k[I]0.63[AAc]1.22.
g
The overall activation energy for the graft copolymerization of AAc onto cannabis
indica fibre was found to be 15.25 kJ/mol within the temperature range of 25-45°C.
Besides kinetic study the grafted samples were also evaluated for their physicochemi-
cal properties such as swelling behaviour in different polar and non-polar solvents
along with resistance towards acid and base. Graft copolymerized samples have been
found to show less resistance towards chemicals as compared to the raw fibre.
Furthermore, the grafted samples were characterized by different techniques such as
Fourier transform infrared (FTIR), scanning electron microscope (SEM), thermogravi-
metric analysis (TGA) and X-ray diffraction (XRD) studies. Thermal stability of the fibre
Key Words:
has been found to decrease after graft copolymerization which is also supported by
activation energy (E ) values calculated both for raw and grafted fibres.
cannabis indica; a
acrylic acid;
graft copolymerization;
kinetic study;
INTRODUCTION
thermal stability.
Today, graft copolymerization of exchange property [5] and dyeabil-
vinyl monomers onto cellulosic ity with the basic dyes without
fibres is a subject of intense adversely affecting their intrinsic
studies. Aremarkable change in the properties. The products can be
physicochemical properties of the used as superabsorbents and
grafted fibres has been demonstrat- ion-exchange composites, etc.
ed by various workers [1,2]. Superabsorbents are cross-linked
Grafting of acrylic acid (AAc) onto networks of hydrophilic polymers
(*) To whom correspondence to be addressed. cellulosic fibres enhances their that can absorb water and retain
E-mail: [email protected] water absorbability [3,4], cation a high volume of aqueous fluids
Kinetics of Graft Copolymerization of Acrylic Acid ... Singha AS et al.
and thus these materials can be used in many fields Afganistan and other surrounding areas. Cannabis
such as horticulture, gel actuators, drug-delivery indicafibre (a high molecular weight polysaccharide)
system, water purifier and coal dewatering [6-8], contains reactive OH groups which are convenient for
etc. Extensive attention has been given by various graft polymerization of hydrophilic vinyl monomers
workers towards superabsorbent polymers in order to achieve superabsorbent like properties.
prepared under different conditions through graft Since not much work has been reported on the surface
copolymerization of vinyl monomers onto natural modification of cannabis indica fibre through graft
polymers [9-11]. copolymerization method [22,23], therefore it is con-
Grafting of vinyl monomers onto cellulose and its sidered worthwhile to graft copolymerize AAc onto
derivatives using UV-radiation has been reported by cannabis indica fibre using CAN as a redox initiator
Geacintov et al. [12]. The change in rayon fibres and study its different physical and chemical proper-
properties for their surface grafting with AAc by ties such as moisture resistance, swelling behaviour,
chemical reactions and γ-radiation method has been thermal stability, surface morphology, chemical
reported by Kaur et al. [13]. They reported that the resistance behaviour and percent crystallinity. The
graft copolymerization through γ-radiation method is present study also investigates the kinetics of the graft
a better method of grafting in comparison to chemical copolymerization of the acrylic acid onto cannabis
method in leading to higher thermal stability. Kinetic indicafibre. The main aim of the kinetic study was to
study of graft copolymerization of AAc and determine the relationship between the rates of
methacrylate onto starch using a ceric ion initiator has grafting with acrylic acid/ceric ammonium nitrate
been studied by Taghizadeh et al. [14]. Sun et al. concentration and to correlate some kinetic parame-
prepared and characterized the graft copolymers of ters with grafting percentage.
carboxymethyl chitosan with methacrylic acid by
FTIR and 1H NMR spectroscopy and elemental
analysis [15]. Chauhan et al. have used ceric EXPERIMENTAL
ammonium nitrate as a redox initiator for the graft
copolymerization of MMA onto the cannabis indica Cannabis indica fibres were collected from higher
fibres [16]. They have also studied the effect of reaches of Himalayan region in Himachal Pradesh.
grafting on the thermal stability of the cannabis The fibres were first washed thoroughly with 2%
indica fibres. Different authors have made kinetic detergent solution and then dried in hot air oven at
studies for the graft copolymerization of AAc onto 70°C for 24 h. The dried fibres were designated as
starch using ceric ammonium nitrate (CAN) as an untreated fibres. Then cannabis indica fibres were
initiator [17-19]. Pourjavadi et al. synthesized a novel subjected to Soxhlet extraction with acetone for 72 h
biopolymer-based superabsorbent hydrogel through at 50°C followed by washing with double distilled
chemically cross-linked graft copolymers of k- water and air drying to remove waxes and other water
carrageenan and acrylic acid. Recently Lyocell fibre soluble impurities prior to graft copolymerization.
grafted with thermosensitive poly-isopropylacryl-
amide using CAN was studied for their thermal and Materials
swelling behaviour by Carrillo et al. [20]. Khan Acrylic acid (AAc) was purified by distillation under
synthesized photo-induced graft copolymer of reduced pressure at 40°C and the middle fraction of
methacrylic acid onto natural biodegradable ligno- distillate was used for further studies after drying
cellulosic fibre and reported 42% increase in over anhydrous calcium chloride. Ceric ammonium
hydrophilic character [21]. nitrate (CAN) obtained from Merck Chemicals
Cannabis indica fibres collected from higher (Germany) was used as an initiator. Weighing of the
reaches of the Himalayan region have been found to samples was done on Libror AEG-220 (Shimadzu,
contain approximately 68% cellulose. Cannabis Japan) electronic balance. Humidity chamber of
indica is a short plant, usually less than 6 ft in height Swastika made was used to study the moisture
but rarely over 8 ft and found in India, Pakistan, absorbance behaviour of the graft copolymers.
914 Iranian Polymer Journal / Volume 20 Number 11 (2011)
Singha AS et al. Kinetics of Graft Copolymerization of Acrylic Acid ...
Synthesis of Cannabis indica-g-poly(AAc) oven at 70°C to a constant weight. The samples were
Activation of the cannabis indica fibre was done by weighed again to obtain the final weight (w ).
f
its immersion in 100 mL of distilled water at room The percentage of weight loss was determined using
temperature for 24 h prior to graft copolymerization the following formula [24]:
process. After the activation of the active sites on the
w −w
polymeric backbone the fibre samples were trans- i f
Weight loss (%)= ×100
ferred into a reaction kettle containing 100 mL of w
i
distilled water used as a solvent in the grafting
reaction. A definite amount of CAN and nitric acid Swelling Behaviour
was added to the reaction flask followed by dropwise The swelling behaviour of the raw and surface modi-
addition of monomer with continuous stirring of the fied samples of cannabis indica fibre was studied in
reaction mixture. The reaction was carried out at polar and non-polar solvents such as water, butanol,
definite reaction time and temperature. Optimum dimethylformamide and carbon tetrachloride. Dry
conditions of time, temperature, pH, CAN, nitric acid samples of grafted fibre as well as raw fibres were
and monomer concentration were worked out so as to subjected to the evaluation of swelling behaviour by
acquire maximum graft yield. Homopolymer formed immersing the known weights of the fibres in certain
during the graft copolymerization was removed by amounts of different solvents for 24 h. The samples
extraction with hot water. The grafted samples were were then removed and excess solvent was removed
then dried in hot air oven at 60°C to a constant weight. by being pressed between the folds of the filter paper.
The amount of graft added onto the fibre was The samples were weighed again to obtain the final
determined gravimetrically. The grafting percentage weight. The degree of swelling was calculated by
(P ), efficiency percentage (P ) and rate of grafting using the following relationship [24]:
g e
(R ) were calculated as per the following methods
g
w −w
[24]. f i
Swelling (%)= ×100
wg −w wi
Grafting Pg(%)= ×100
w
where w is the initial weight of the dried fibre and w
i f
wg −w is the final weight after the swelling.
Efficiency Pe(%)= ×100
w
m Moisture Absorbance Study
P ×w×1000 The moisture absorbance study of the grafted and raw
g
Rate of grafting (Rg)=100×V×t×M ×100 (mol/L.s) fibres was performed at different humidity levels
ranging from 20% to 80% in humidity chamber.
where w is the weight of raw fibre, w is the weight Percentage of humidity level during moisture
g
of grafted fibre, w is weight of monomer, V is the absorbance studies was maintained by controlling the
m
volume of the reaction mixture, t is the reaction time temperature. Each sample was subjected to a particu-
and M is the molecular weight of the monomer. lar humidity level for a fixed time interval of 2 h. The
percent moisture absorbance was studied as a function
Chemical Resistance Behaviour of weight gain and was calculated using the following
The chemical resistance of the grafted fibre was stud- formula [24]:
ied as a function of percentage weight loss of fibre
when treated with different chemicals. A known w −w
Moisture absorbance (M , %)= f i ×100
amount (wi) of the raw and grafted fibres was sepa- abs w
i
rately treated with a fixed volume of hydrochloric
acid and sodium hydroxide of different strengths for a where W is the final weight of the sample removed
f
time interval of 24 h. The fibres were then washed 2- from the humidity chamber and W is the weight of
i
3 times with distilled water and finally dried in an the dried samples.
Iranian Polymer Journal / Volume 20 Number 11 (2011) 915
Kinetics of Graft Copolymerization of Acrylic Acid ... Singha AS et al.
Infrared Spectroscopy crystallinity index (CI) were calculated using the
Infrared (IR) spectra of the graft copolymerized and following formula [25]:
the raw fibre were recorded with KBr pellets on a
I
Perkin Elmer RXI Spectrophotometer (USA). Cr(%)= C ×100
I +I
C A
Scanning Electron Microscopy
Scanning electron microscopic (SEM) studies of
I −I
raw/surface modified cannabis indica fibres were CI = C A
carried out on a Leo 435 VP electron microscope IC
Thornwood, NY). All the samples were gold plated to
become conducting. Scanning was synchronized with where I and I are the crystalline and amorphous
C A
microscopic beam for the maintenance of small size intensities at 2θ scale close to 22° and 15° angles.
over large distance relative to the specimen. All the
images were taken at a resolution of 1000X.
RESULTS AND DISCUSSION
Thermal Analysis
Thermogravimetric analysis (TGA) of the raw and Reaction Mechanism
graft copolymerized cannabis indica fibres was C , C and C hydroxyls and C-H groups are the
2 3 6
conducted on a TGAwith an auto sampling (Mettler active sites for the incorporation onto polymeric
Toledo, Switzerland) analyzer at a heating rate of chains through grafting with cellulosic fibres like
15°C/min. The thermograms were recorded over a cannabis indica fibre. Grafting reaction proceeds
temperature range of 0°C to 900°C in presence of through redox mechanism in which Ce(IV) ions are
nitrogen atmosphere at a flow rate of 20 mL/min. reduced to Ce(III) ions by the transfer of electron
from the cellulose molecules, and hence active sites
X-Ray Diffraction Studies are generated on the polymeric backbone at which
X-Ray diffraction studies were performed on a monomer radicals can attack to form graft copoly-
Brucker X-ray diffractometer (D8 Advance, mers. The breaking of C -C bonds of glucose
2 3
Germany), using CuKα (1.5418 A°) radiation, a Ni- molecules in cellulose chain results in the formation
Filter and a scintillation counter as a detector at of free radical sites on the polymeric backbone. The
40 kV and 40 mA on rotation from 5° to 60° at 2θ grafting of acrylic acid onto cannabis indica (CI)
scale. backbone is supposed to take place through the
Each sample was finely powdered into a small mechanism [26,27] shown in Scheme I.
particle size and homogeneously mixed before Ceric ions form complexes with the carbon chain
subjected to X-ray exposure. The finely powdered of polymer backbone as well as with monomer and
sample was distributed in the cavity of the sample generate free radicals (eqns 1 and 2). Monomer free
holder with the help of glass slide. The randomly radicals then result in the formation of graft copoly-
oriented powdered sample with uniform surface was mer [CIC-g-poly(AAc)] by reacting at the active sites
exposed to X-rays from all possible planes. The angle of the polymeric backbone (eqns 3 and 4). Monomer
of scattering of diffracted beam was measured with may also combine with CIC-g-poly(AAc) macro-
respect to incident beam of X-rays and the relative radical to form graft copolymer (eqn 5). While, the
intensity was obtained. ceric ions and poly(AAc) free radicals combine with
Crystallinity index (CI) was determined by using CIC-g-poly(AAc) free radicals and CH-free radicals,
the wide angle X-ray diffraction counts at 2θ angle respectively, leading into termination reactions (eqns
close to 22° and 15°. The counter reading of peak 6 and 7).
intensity close to 22° and 15° is said to represent the
crystalline and amorphous materials in cellulose, Effect of Time
respectively. The crystallinity percentage and The graft copolymerization has been recorded at
916 Iranian Polymer Journal / Volume 20 Number 11 (2011)
Singha AS et al. Kinetics of Graft Copolymerization of Acrylic Acid ...
OH
OH
OH
OH
k
O
HO O O + Ce4+ HO HO O O
HO O O
O
OH
OH Ce4+
OH
OH
CI cellulose
4+
Complex(CI cellulose-Ce )
-H
k
d
OH
OH Ce3+
+
O
HO O
H O
O
O OH
OH
Chain initiation
CI cellulose free radical (1)
O
O
'
k
4+
+ Ce
OH
OH
(2)
AAc free radical
AAc
OH
OH
OH
OH
k O
O i HO O
HO O H O
H O O
O
O
O OH + O OH
O OH
OH
HO
CI cellulose free radical
(3)
OH
Chain propagation
OH
OH
OH
OH
O
HO O k O
O H O p HO H O O
O
O
O OH HOH2C O O
OH O OH
+
HO
n HO
OH
(4)
O OH
continued
Iranian Polymer Journal / Volume 20 Number 11 (2011) 917
Kinetics of Graft Copolymerization of Acrylic Acid ... Singha AS et al.
OH
OH
OH
OH
O O
HO O H O O kp HO OH O O
O
+
HOH2C O O OH OH HOH2C O O
OH
AAc
HO
HO
O OH
O OH (5)
Chain termination
OH OH
OH
OH
O
O HO O
HO H O O k O H O
O 4+ t
+ Ce
HOH2C O O OH HOH2C O O + Ce3+
HO
HO
O OH
O OH
CIC-g-poly (AAc) (6)
CIC-g-poly(AAc) free radical
OH
OH
OH
OH O
HO O
k H O
O o O
HO O
H O
O HOH2C O O
O OH +
O
OH HO
OH
CI cellulose free radical O
O OH
OH
CIC-g-poly (AAc)
O
HO (7)
Poly(AAc)
Scheme I.Apossible mechanism for graft copolymerization of acrylic acid onto cannabis indicafibre.
different stages ranging from 60 to 180 min at fixed grafting was observed at 120 min and then decreased
concentration of ceric ammonium nitrate, AAc and again with further increase in the reaction time. This
nitric acid at 45°C (Figure 1). With increase in probably happens due to generation of increasingly
reaction time, P increased rapidly and maximum monomer radicals with extended time which interacts
g
918 Iranian Polymer Journal / Volume 20 Number 11 (2011)
Singha AS et al. Kinetics of Graft Copolymerization of Acrylic Acid ...
Cell H COOH COOH
Cell H COOHH COOH
C C CH2 + CH
C C C C
H
H H H H
H COOHH COOH
H COOHH COOH
C C C C H
C C C C
H H H H
H H H H Cell
Cell
Scheme II. Chain cleavage leads to lower degree of grafting.
with the active sites on the polymeric backbone increases the diffusion of monomer molecules onto
resulting in increased P . Beyond the optimized the polymeric backbone resulting in increased P and
g g
point, at higher reaction time, P decreases due to P . On further increase in temperature, P and P
g e g e
irreversible termination by radical coupling reactions. decreased, which may be due to increased chain
This may also be due to the mutual destruction of termination reactions between monomer molecules
growing polymeric chains [13] leading to homopoly- and grafted chains. Combination of free monomer
merization of the reaction monomer radicals and radicals at higher temperature may also result in
backbiting by the active radicals (Scheme II). decreased grafting yield [28].
Effect of Temperature Effect of Initiator
The effect of temperature on graft copolymerization The effect of initiator concentration on graft copoly-
is depicted in Figure 2. It can be seen that both P and merization of polymeric backbone has been depicted
g
P show an increasing trend up to 45°C as the in Figure 3. It can be established from the figure that
e
temperature enhancement can increase the dynamic P increases with the increase in CAN concentration
g
energy of the monomer molecules, which in turn

Figure 2. Effect of temperature on the graft copoly-
Figure 1.Effect of time on the graft copolymerization. merization.
Iranian Polymer Journal / Volume 20 Number 11 (2011) 919
Kinetics of Graft Copolymerization of Acrylic Acid ... Singha AS et al.

Figure 3. Effect of CAN concentration on the graft Figure 4. Effect of nitric acid concentration on the graft
copolymerization. copolymerization.
up to 1.82×10-2 mol.L-1 at constant concentration of However, P decreases with further increase in the
g
monomer and nitric acid, and after that P shows a nitric acid concentration beyond an optimum value
g
downward trend. This behaviour can be easily under-
HO
stood in terms of increase in CAN concentration HNO3 2 H+(aq) + NO3-(aq)
which results in more free radical active sites on the
OH
polymeric backbone, which may lead to greater P .
g
OH
However, with further increase in CAN concentration,
O
it accelerates the dissociation rate of Ce(IV), which HO O
HO O
O
reduces the concentration of CAN participating in
graft copolymerization. OH
OH
Effect of Nitric Acid Concentration H
In aqueous medium CAN exists as Ce4+, [Ce(OH)]3+
OH
and [Ce-O-Ce]6+ ions (eqns 8 and 9). Due to their OH
large size, these ions are not able to form complexes O
HO H O
with polymer backbone. However, in the presence of O
O
HO
HNO , equilibrium shifts further towards Ce4+ ions,
3
OH
therefore graft copolymerization increases with
OH
increase in nitric acid concentration.
HO Slow
2
Ce4+ +H O↔[Ce(OH)3+]+H+ (8)
2
OH
OH
2[Ce(OH)3+]↔[Ce−O−Ce]6+ +H2O (9) HO OH + HO O + H
O HO O
Figure 4 shows the effect of nitric acid concentration
OH
on grafting parameters. It can be seen that Pgincreas- OH
es significantly with the increase in nitric acid Scheme III. Hydrolysis of the cellulosic fibre due to
concentration, and Pe also follows a similar trend. increased HNO3concentration.
920 Iranian Polymer Journal / Volume 20 Number 11 (2011)
Singha AS et al. Kinetics of Graft Copolymerization of Acrylic Acid ...
increases up to pH 7, but beyond pH 7 there is a
decrease in the graft yield as depicted in Figure 5.
This may be due to the fact that at higher pH the OH
radicals are generated in basic medium which
combine with the backbone free radicals to terminate
the chain reaction (Scheme IV) [13].
Effect of MonomerConcentration
The effect of monomer concentration on graft copoly-
merization onto polymeric backbone at constant
concentrations of CAN and nitric acid at temperature
of 45°C is depicted in Figure 6. It can be established
from the figure that P increases with increase in AAc
g
concentration up to 2.91 × 10-1mol.L-1beyond which
P shows a downward trend. The above behaviour of
Figure 5.Effect of pH on the graft copolymerization. g
the monomer concentration may be explained on the
(2.88 × 10-1 mol.L-1) probably due to termination of basis of greater free radicals which reach polymeric
growing grafted chains, which has been accelerated backbone, resulting in increased Pg. The decrease in
because of formation of considerable amount of Pg beyond optimum value of AAc was probably due
[Ce4+] and [CeOH]3+[28]. Also increased concentra- to preferential homopolymerization over graft copoly-
tion of nitric acid may result in hydrolysis of the fibre merization as well as due to restricted movement of
graft (Scheme III). free radicals onto polymer backbone because of
increased viscosity. Needless to say, the increase in
Effect of pH the chain transfer to monomer molecules may be
With increase in pH of the reaction medium, P another possible reason for the diminished grafting at
g
OH
OH
O
HO O
H O
O
O O + OH
OH
OH
HO O
H
H
H OH
OH
OH
O
HO O
H O
O
O O OH + H2O+ H
OH
O
HO
H
H OH
Scheme IV.Termination of the growing grafted chains leads to lower P .
g
Iranian Polymer Journal / Volume 20 Number 11 (2011) 921