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Review Article
Recent Res. Devel. Polymer Science, 12(2014): 107-128 ISBN: 978-81-7895-611-4
6. Advances in the development of functional
polymers using radiation induced emulsion
polymerization
Mohamed Mahmoud Nasef1,2, Masao Tamada3, Noriaki Seko3 and Ebrahim Abouzari-Lotf1
1Institute of Hydrogen Economy, International City Campus, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia; 2Malaysia–Japan International Institute of Technology, Universiti
Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysia; 3Evironmental Polymer Group, Environmental and Industrial Materials Research Division, Quantum Beam Science
Directorate, Japan Atomic Energy Agency, 1233 Watanuki Takasaki, Gunma, 370-1292, Japan
Abstract. In this chapter the latest progress in the preparation of functional polymers using radiation induced emulsion polymerization technique is presented. A brief encounter for basic fundamentals, advantages, methods and parameters of radiation- induced emulsion polymerization is presented. A number of potential applications of the obtained functional polymers such as ionic adsorbents, chelating agents, polymer catalysts and polymers with controlled biodegradability are reviewed with special attention given to fast emerging environmental applications.
1. Introduction
The interest in using emulsion polymerization is growing due to their
widespread applications in the production of functional and specialty polymers.
Correspondence/Reprint request: Dr. Mohamed Mahmoud Nasef, Institute of Hydrogen Economy, International City
Campus, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
E-mail: [email protected], [email protected]
Mohamed Mahmoud Nasef et al. 108
Functional and specialty polymers are materials that possess advanced properties suitable for energy, biomedical and environmentally related areas such as stimuli-response, chelating adsorbents, polymer electrolytes, diagnostic tests, drug-delivery systems, synthetic rubbers, paints, and adhesives applications [1, 2]. This unique synthesis process which proceeds in a heterogeneous system was initially employed in industrial scale during World War II for fabricating synthetic rubbers from styrene and butadiene [3]. Currently emulsion polymerization accounts for the world production of more than 107 tons of polymers per year [1, 4]. Emulsion polymerization is a general term, which covers several processes including conventional, inverse, mini-, micro-, and nano emulsion polymerizarions [1, 5]. Normally, the conventional emulsion polymerization involves emulsification of the relatively hydrophobic monomer in water by proper emulsifier, followed by the initiation reaction either with chemical or radiation methods. Both water and oil soluble initiators (e.g. sodium persulfate and 2-2′-azobisisobutyronitrile (AIBN)) have been used to initiate the polymerization reactions [2]. The polymerization is initiated by activation of initiators (e.g. by increasing the temperature) and formation of radicals. Therefore, the required conditions for producing the radicals control the polymerization temperature and subsequently influence the polymer properties. In radiation induced emulsion polymerization (RIEP), the reaction can initiated by either ultrasonic irradiation [6, 7] or high energy radiation in form of electromagnetic waves such as γ–rays obtained from radioactive sources such as Co-60 or Cs-37 [8].
RIEP can be used to prepare variety of polymeric materials including
homopolymers, copolymers, and graft copolymers as shown in the
schematic diagram given in Fig. 1. Thus, RIEP involves hompolymerization,
Figure 1. Different polymeric materials obtained by radiation induced emulsion
polymerization.
Advances in the development of functional polymers using radiation induced emulsion polymerization 109
copolymerization or grafting copolymerization reactions depending on the
starting reactants. Particularly, graft copolymerization of a monomer onto
polymer substrates offers kinetic and technological advantages over
conventional methods [9, 10]. The substitution of chemical initiators or
catalysts with high energy radiation in RIEP simplifies the reaction and
leaves no detrimental residues in polymer mixture. This makes the
polymers synthesized by RIEP attractive for bio-related applications due to
the absence of impurities [9]. RIEP can be also used to produce high
molecular weight polymers at high reaction rates with better temperature
control during polymerization due to the more rapid heat transfer in the
emulsion.
Among RIEP reactions, graft copolymerization have attracted more
attention recently due to its merits and potential in modifying the chemical
and physical properties of preformed polymers without altering their
inherent properties. Various combinations of functional and nonfunctional
monomers together with commercial polymers in different forms (films,
beads, fabrics and fibers) are available to serve as polymer substrates for
functional copolymers [11]. Radiation induced emulsion graft
copolymerization (RIEGC) has been found to provide means to control the
composition of the obtained functional graft copolymer, reduce the
monomer consumption, reduce irradiation dose and improve the economy
of the reaction when compared with other grafting methods [12]. In
addition, using water as a dispersion medium provides nonflammable,
inexpensive, nontoxic and odorless systems.
There have been several review articles in various journals and chapters
in books on radiation induced graft copolymerization of various
monomer/polymer combinations and their applications. The coverage of
such reviews was dedicated to graft copolymers obtained by RIGC in
solvents and their various applications [8, 13-21]. The technological
process for industrialization of RIEP was briefly reviewed with the focus
on batch process for industrial-scale production and the advantages were
described in comparison with chemically initiated process [22]. However,
there are no review texts on the use of RIEP for preparation of functional
polymers and the latest progress in their applications.
The objective of this chapter is to provide an overview for the latest
progress in the use of RIEP for preparation of functional polymers. A brief
outline for basic fundamentals, advantages, methods and parameters are
presented. A number of potential applications in different fields are
reviewed with an attention given to fast emerging environmental
applications.
Mohamed Mahmoud Nasef et al. 110
2. Radiation induced emulsion polymerization
Radiation induced polymerization is a reaction that involves formation of
polymer, copolymer or graft copolymer by exposure to high energy radiation,
which generate radicals in the system that allow the reaction to proceed. This
reaction can be performed in different media including bulk, solution, and
emulsion as depicted in the schematic diagram shown in Fig. 2.
Polymerization or grafting onto polymers in the emulsion media is a method
for carrying out the reaction in a disperse system generally containing
hydrophobic monomer and water as a dispersion medium. The stability of the
emulsion system containing the monomer is maintained using a surfactant.
The surfactant plays an important role in bringing monomers into contact
with the substrate and generally acts as an emulsifier which stabilizes the
emulsion medium during the reaction. In addition to forming the micelles to
solubilize the water-insoluble monomers, the surfactant also decreases the
surface tension at the monomer-water interface.
RIEP can also be carried out in an inverse emulsion polymerization with
hydrophilic monomers [23, 24]. For example, aqueous solution of
hydrophilic monomers such as N-isopropylacrylamide, acrylamide, acrylic
acid, and salt of acrylic acid can be emulsified in nonpolar organic solvents
such as xylene or paraffin. This method is commonly used in surface
modification of films and preparation of polymeric microcapsules [23].
Emulsion polymerization reaction proceeds in three stages involving
particle nucleation, particle growth, and termination [25, 26]. In particle
nucleation, only one of 100-1000 micelles can capture the free radicals and
Figure 2. Schematic representation of radiation induced polymerization in various
media.
Advances in the development of functional polymers using radiation induced emulsion polymerization 111
becomes latex particle. The particle nucleation is followed by the
polymerization reaction, which takes place in the micelles. Thus, the micelles
can be regarded as living micro reactors, which facilitates the reaction. Based
on the Smith-Ewart theory [26] the surfactant and initiator concentrations or
irradiation dose (in RIEP) and time which controls the amounts of radicals
are the most important factors that controls the number of latex particles
nucleated per unit volume of water. One of the obvious advantages of RIEP
over conventional polymerization methods is the independency of radical
formation on the temperature. Such independency allows the polymerization
initiation at low temperature and in viscous media.
When the emulsion is used to obtain a homopolymer, the emulsion
mixture containing monomer, water and surfactant is irradiated by γ-rays
leading to formation of radicals such as H• and OH• in the water phase in
addition to monomer radicals all of which can participate in initiation of
polymerization of monomers migrated inside the micelles. Unlikely, when
irradiated polymer is incorporated in water emulsifier mixture, the reaction
takes place in surface of the polymer and proceeds inward forming side chain
grafts on polymer substrate.
In the growth step, the chains inside the micelles continue to grow until
monomer supply or free radical is exhausted. According to the Smith-Ewart
theory, at constant temperature, polymerization rate depends on the type and
concentrations of surfactant and monomer and dose rate. The following
equation is widely used to determine the rate of polymerization (Rp):
where, kp is the rate constant of the propagation step, [M]p is the monomer
concentration in the particles, n is free radicals average number in particle, Np
is number of latex particles nucleated per unit volume of water and NA is the
Avogadro number. More details on the kinetics and mechanisms can be
found elsewhere [2, 27, 28].
2.1. Homo- and copolymerization
Emulsion polymerization is among the most important methods for
obtaining variety of commercialized homo- and copolymers. This method
produces high molecular weight polymers by using the minimum amount of
volatile organic compounds in safe and environmental friendly process [29].
Most of the produced water born polymers by emulsion are industrially
important mainly in the producing synthetic rubbers, paints, adhesives,
coatings, nonwoven tissues, biomaterials and high-tech products. Industrially,
Mohamed Mahmoud Nasef et al. 112
vinyl acetate (VAc) based homo- and copolymers are account for more than
28% of the total waterborne synthetic latex, which are mainly used as low
cost and durable component in coatings and adhesives [30, 31].
As discussed earlier, producing these materials by irradiation-induced
method bring additional advantages including the yield, purity and control over
polymer properties such as molecular weight. For example, the comparison
between conversion percentage of poly(vinyl acetate) prepared by RIEP and
bulk revealed that the conversion percentage in emulsion is almost four times
higher than in bulk [32]. In addition, the polymer obtained by emulsion shows
considerable higher molecular weight and glass transition temperature.
Radiation-initiation is a complex procedure which the consequence of
ioniziation and excitation depends on the physical state and composition of
material and type of radiation source [33]. Upon irradiation, some primary
products formed due to the breaking of indiscriminate chemical bond. These
primary products may either react with other materials or recombine to
produce even more secondary products [33]. In initiation of RIEP, water is a
very important component. Pure water molecules undergo radiolysis to yield
solvated electrons (e-(aq)), protons and hydroxyl radicals [34]. The undergoing
reactions in the radiolysis of water are shown below [33]:
H2O + γ → e- + H2O+
H2O + γ → H2O*
H2O+ rapidly react with water (~10-14 s) to produce hydroniums and hydroxyl
radicals.
H2O+ → H3O
+ + OH•
The exact nature of the water in the excited state (H2O*) remains uncertain
but it seems likely that radicals will be produced on decomposition of excited
state within 10-14-10-13 s.
H2O* → OH•+ H•
H2O* → e- + H2O
•+
In the absence of reactive species electrons are rapidly (~10-11 s) solvated:
e- + nH2O → e-(aq)
Solvated electrons, which are strongly reducing species, are rapidly react
with protons at any reasonable pH:
e-(aq) + H+ → H•
Advances in the development of functional polymers using radiation induced emulsion polymerization 113
In the presence of monomers, hydroxyl radicals and free electrons can add to the
π-electrons of double bonds and start the free radical polymerization reaction.
OH•+ CH2=CRX → CH2OH-•CRX
e- + CH2=CRX + H+ → CH3-•CRX
RIEP has been applied in obtaining various polymers such as
poly(vinylacetate) [32], polystyrene [35-39], poly(vinylpyrrolidone) [24, 40],
poly(butylacrylate) [41] and poly(methylmethacrylate) [42]. It has been also
used to prepare copolymers with improved properties including
divinylbenzene crosslinked polystyrene [43], butylacrylate/acrylonitrile [44],
and 2-methacryloyloxyethyl) trimethyl ammonium chloride/acrylamide [45]
starting from their corresponding monomers in water using gamma rays in
presence of various surfactants. Table 1 shows a summary of some previous
studies on the use of radiation induced emulsion polymerization to prepare
polymers and copolymers.
Table 1. Previous studies on preparation of polymers and copolymers by radiation
induced emulsion polymerization.
2.2. Graft copolymerization
Graft copolymerization is a process in which side chain grafts originated
from a monomer are attached to the main chain of a polymer backbone by
covalent bonds to form a branched copolymer. A graft copolymer can be
represented as follows:
Mohamed Mahmoud Nasef et al. 114
where, Bn and Bm are the side chain grafts originated from the monomer B
onto the main polymer chain. The extent of polymerization in Bn and Bm
grafts is called the degree of grafting (DG) and can be gravimetrically
determined through the weight increase. The grafting reaction takes place as a
result of formation of active sites on the polymer backbone, which can be
initiated by various means including chemical initiator [46], thermo-
mechanical initiation (melt grafting) [47], plasma treatment [48], ultraviolet
light radiation [49], and high energy radiation [11]. Unlike plasma and
photoinitiated grafting, RIGC is capable of achieving bulk modification and
has been widely investigated for preparation of various ion exchange and
adsorbent polymers [8].
RIGC is an effective method for modifying the properties of a
pre-existing polymer by imparting new functionalities without compromising
the desired properties of the parent polymer [14]. This method is used to
introduce polar groups to the bulk or surface of non-polar polymers of
various morphologies (fiber, film, fabrics and beads) to bring about
modification in wettability, adhesion, printability, metallization, anti-fog
properties, anti-statics properties, and biocompatibility. Thus, it has been
used to develop various forms of functional polymers suitable for medical,
biological, environmental, chemical and solid state applications [8] [15].
In the RIGC method, the reaction usually follows free radical
polymerization mechanism and can be initiated with various high
energy radiation in form of electromagnetic waves such γ–rays obtained from
Figure 3. Schematic representation of radiation induced graft copolymerization
methods.
Advances in the development of functional polymers using radiation induced emulsion polymerization 115
radioactive sources such as Co-60 or Cs-37. However, Co-60 is more
advantageous and has been widely used due to higher energy emission (1.25
MeV compared to 0.66 MeV for Cs-137), easier preparation, lower cost and
shorter half-life (5.27 year) [50]. Particulate radiation such as electron beam
(EB) can be also used to initiate grafting but at a shorter time (few minutes)
and lower penetration levels compared to γ–rays [51].
RIGC can be performed using two main methods as shown in the
schematic representation in Fig. 3. This includes (i) direct (simultaneous)
irradiation and (ii) preirradiation. In the direct irradiation, both the polymer
and the monomer are irradiated together after the grafting solution is flushed
with N2 or evacuated using freeze thaw cycles. An inhibitor is often added to
the solution to suppress the homopolymerization in the liquid phase when the
reactive monomer is grafted. Alternatively, in the preirradiation method, the
polymer is irradiated first in the absence of monomer and then brought into
contact with a monomer solution to initiate the reaction. As can be seen in
Fig. 3, this irradiation can be either in an inert atmosphere or in the presence
of air. In the irradiation in oxygen or air atmosphere, which usually refer as a
peroxidation grafting method, peroxide and hydroperoxide radicals formed
upon irradiation. Grafting reaction is then initiated by thermal decomposition
of the trapped radicals in the polymer backbone in presence of monomer
units under controlled conditions.
2.2.1. Radiation induced emulsion graft copolymerization
RIEGC is a chemical reaction similar in principles with RIGC but differs
in the components of the grafting mixture. Particularly, the monomer in
RIEGC is diluted with water as a dispersion medium and emulsified by the
addition of a small amount of a surfactant. Although the use of solvent in
RIGC facilitates the swelling of the base polymer and monomer diffusion
leading to high levels of grafting, the stronger demand for environmentally
friendly procedures where water is used as an alternative solvent makes
RIEGC more appealing and sustainable [52]. Compared to RIGC in solvents,
RIEGC have the advantage of: 1) high efficiency, 2) low absorbed dose, 3)
using water as dispersion medium which allows easily recycling in industrial
scales, 4) possibility to use green surfactants, and 5) high sustainability and
environmental friendly. In addition, under properly chosen conditions of
emulsion, the low viscosity of the emulsion along with the lower possibility
of termination step allows higher polymerization rates and high molecular
weight polymers which is not readily accessible in solution or bulk
polymerization reactions [12, 53]. Besides these advantages, the products
synthesized by irradiation are beneficial to bio-application because
Mohamed Mahmoud Nasef et al. 116
contamination with chemical initiators is avoided and the final products are
sterilized [9].
A comparison between radiation induced graft copolymerization in a
solvent and that in emulsion is presented in Table 2.
Table 2. Comparison between RIGCs in solvent and that in emulsion.
Parameters Radiation grafting in solvent Radiation grafting in emulsion
Grafting medium Solvent Water
Surfactant No surfactant Surfactant is added
Irradiation dose High dose Low dose
Monomer concentration High Low
Residue/waste Waste production Waste can be recyled easily
Figure 4. Reaction mechanisms for preparation of various ionic copolymers by
RIEGP of GMA followed by variety of chemical treatments.
A typical example to highlight the advantages of RIEGC is the grafting
of glycidylmethacrylate (GMA) onto various polymer substrates to produce
grafted precursors that can be converted to variety of functional materials
suitable for wide spectrum of applications. In RIEGC reaction, GMA is
diluted with water and is emulsified by adding a small amount of surfactant
such as Tween 20 in presence of polymers substrates such as PE fibers or
fabrics [54, 55]. This method provides interesting advantages in terms of
elimination of solvents, irradiation dose reduction and less monomer
consumption leading to an improvement in the economy of the process.
Advances in the development of functional polymers using radiation induced emulsion polymerization 117
Fig. 4 depicts mechanisms for preparation of various ionic copolymers by
RIEGP of GMA and subsequent possible functionalization reactions.
The use of RIEGC in preparation of graft copolymers allows imparting
favourable properties by modification of existing natural and synthetic
polymers. In that regard, various grafting systems such as styrene/butadiene
latex [56], 4-chloromethylstyrene/kenaf fiber [57], methyl methacrylate/jute
fibers [58, 59], GMA/polyethylene (PE) fibers [53, 60], GMA/nonwoven
cotton fabrics [61], various vinyl monomers/polyurethane [62],
methylmethacrylic acid/waterborne polyurethane [63], styrene/waterborne
polyurethane [64], vinylacetate/poly(3-hydroxybutyrate) film [55], N-vinyl
pyrrolidone/polystyrene [9], GMA/poly(vinylidine fluoride) [65],
octafluoropentyl acrylate/silk fibers [66], GMA/PE beads [12],
acrylonitril/PE non-woven fabric [67], and 4-chloromethylstyrene [68] have
been reported in literature. In addition, grafting onto nonwoven fabric
material composed of both natural and synthetic polymers [69] as well as
co-grafting of monomer mixtures have been considered [70].
2.2.2. Factors affecting radiation induced emulsion graft
copolymerization
There are many parameters which strongly affect RIEGC process.
Variation of these parameters affects the degree of grafting in the resulting
copolymers and this provides means to closely control the compositions and
the properties of the functional copolymers. A combination of optimum
parameters has to be adapted to achieve successful grafting reactions towards
obtaining desired composition [57, 71].
Reaction parameters that have to be controlled can be classified into:
1) those directly related to irradiation source and 2) others related to the
grafting mixture [8]. The former include nature of radiation, dose and dose
rates whereas the latter includes monomer type and concentration, storage
time, grafting atmosphere, temperature, radical storage time and reaction
time. The influence of these parameters on the radiation-induced grafting has
been the subject of detailed investigations in various groups [18, 72]. In
addition, emulsion related parameters including the type (anionic, cationic,
nonionic and amphoteric) and concentration of surfactant, stability of
emulsion mixture, and micelle size were found to play important role on
grafting. For instance in the radiation-induced emulsion graft polymerization
of 4-chloromethylstyrene on kenaf fiber, it was observed that the (i) micelle
diameter gradually change upon changing both temperature and storage time,
(ii) reducing the micelles size from 500 nm to 250 nm leading to the
emulsion stability, and (iii) the larger the micelle size results in higher degree
Mohamed Mahmoud Nasef et al. 118
of grafting [57]. The effects of these parameters on the degree of grafting
could be cumulative and interdependent [58]. More details on the effect of
each one of these parameters on the degree of grafting in a variety of grafting
system were reported in various occasions [8].
3. Applications of polymers obtained by radiation induced
emulsion graft polymerization
Like their counterparts prepared by conventional radiation grafting, graft
copolymers obtained by RIEGC are proposed for various applications with
advantages of being prepared using less monomer consumption and low
irradiation doses i.e. they are more economical. The diversity of
functionalities that can be incorporated in the grafted polymers rendered them
suitable for a variety of applications. However, separation applications
remain the main field with functional graft copolymers. Particularly,
decontamination of the waste water from pollutants is in the heart of
separation applications of such materials.
3.1. Removal of pollutants from waste water
Amine-type adsorbents prepared by RIEGP of GMA onto PE fibers
followed amination with three chemical agent; diethyltriamine (DETA),
triethyltriamine (TETA) and ethylenediamine (EDA) were tested for
adsorption of Pb2+ and Ni2+ from waste water. The distribution coefficient of
Pb2+ was found to be 104 in all amine-type adsorbents and that of Ni2+
decreased in the order of DETA-type, EDA-type and TETA-type. The EDA-
type adsorbent also showed the highest selectivity against U ion and the
distribution coefficient was 2.0 × 106 [53].
Another amine-containing adsorbent for heavy metal adsorption was
prepared by radiation emulsion grafting of 4-hydroxybutyl acrylate
glycidylether (4-HB) onto polyethylene/polypropylene nonwoven fabric
followed by amination with EDA. A typical adsorbent has a DG of 135% and
amine group density of 2.8 mmol/g. The adsorbents showed adsorption metal
ions in the order of Cu2+> Pb2+> Zn2+ >Ni2+>Li+. It also exhibited not only
better mechanical property but also higher adsorption capacity of Cu2+ and
Pb2+ compared to similar amine-type adsorbent based on grafted GMA [73].
Column and batch mode studies were performed for 4-HB and GMA grafted
onto PE absorbents functionalized with various amine derivatives of
diethylamine, EDA, DETA, and TETA [74]. By using EDA-based adsorbent,
good and similar sorption behaviour for Cu2+ and Pb2+ was observed for
individual ions. However very low selectivity was resulted for mixed ions.
Advances in the development of functional polymers using radiation induced emulsion polymerization 119
Upon irradiation-induced crosslinking of amine functionalized adsorbent,
higher sorption capacity for Cu2+ and almost no capacity for Pb2+ was
obtained. By changing the anion in the solution, similar results were found
which indicates the independency of the selectivity improvement from the
anion in the solution. This increases in selectivity upon crosslinking can be
explained by the decreases in the distance of the chelating groups after
crosslinking. Since the Pb2+ radius (0.120 nm) is much larger than Cu2+
(0.072 nm), reducing the distances between the chelating groups make the
materials more favourable for chelating smaller ions.
METOLATE® is a recently commercialized heavy metal adsorbent based
on the RIEGP of GMA onto PE nonwoven fabric followed by amination [75].
This high performance fibrous absorbent was designed for the removal of Ni
and Cu ions from strongly corrosive alkaline solution which used in the
etching process of the surface of Si wafer. Due to the corrosive nature of
medium, other commercially available absorbent resins are not proper for this
application and usually shrink in such strong alkaline solutions. The
absorbent with the DG of 120% remove Ni ions from the concentration of
100 ppb in 48 wt% NaOH solution to less than 1 ppb. In the case of Cu ions
with the concentration of 10 ppb in 48 wt% KOH, less than 0.5 ppb remains
after treatment with absorbent which are satisfied for the requirement. In
addition, 30 higher lifetimes and possibility to use in almost 20 times higher
flow rate than commercial resin was reported.
A chelating adsorbent for removal of boron was prepared with RIEGP of
GMA onto PE fibers followed by treatment with N-glucamine. The obtained
adsorbent showed 4 times higher breakthrough capacity and almost 10 times
faster boron adsorption than conventional commercial glucamine resin [76].
Similar adsorbent precursors were prepared by grafting of GMA onto
poly(vinylidene fluoride) powder and acrylonitrile and GMA onto PE
nonwoven fabric using preirradiation-induced emulsion graft polymerization
[65, 67, 77]. Grafted GMA onto PE non-woven fabric functionalized with
aminated compounds such EDA, DETA, TETA and tetraethylenepentamine
were tested for the removal of copper and uranium ions from solution. For
the contaminated water with concentration up to 1000 ppb, at least 90% of
copper and 60% of uranium was removed by such absorbents [77].
Similar amine adsorbent with biodegradable nature was prepared by
grafting of GMA onto nonwoven cotton fabric followed by treatment with
EDA or DETA and tested for Hg2+ adsorption. The resulting amine-type
adsorbents showed distribution coefficients of Hg of 1.9 × 105 and 1.0 × 105
for EDA-and DETA-type adsorbents, respectively, on batch basis. The
EDA-type adsorbent removed mercury ion from 1.8 ppm Hg2+ solution at a
space velocity of 100h-1, which corresponds to 16,000 times the volume of
Mohamed Mahmoud Nasef et al. 120
the packed adsorbent when tested on column basis. The adsorbed Hg2+ on the
EDA-type adsorbent could be completely eluted by 1 M HCl solution. A
microbial oxidative degradation test revealed that the EDA-type adsorbent is
biodegradable. This suggest that the obtained adsorbent is a potential
environment friendly candidate for Hg2+ adsorption [61].
Arsenic adsorbent was prepared by RIEGP of 2-hydroxyethyl
methacrylate phosphoric acid monomer onto a nonwoven cotton fabric
followed with chemical modification by zirconium Zr(IV) solution [78]. The
phosphorous oxoacid derivatives have strong affinity against Zr(IV) to form a
complex of phosphate-Zr at a high concentration of nitric acid and this
complex displays great adsorption capacity of As(V). In a pilot scale, the
adsorbent was successfully prepared with a DG of 118% at a monomer
concentration of 5% for 1 h at 40 °C. In the batch mode adsorption studies
with 1 ppm of As(V), the adsorption capacity of 0.02 mmol/g-adsorbent was
achieved.
In the most recent study, metal ion adsorbent for removal of the Cu2+ and
Ni2+ ions in aqueous solutions was developed based on nonwoven fabric
material composed of both natural and synthetic polymers [69]. A
preirradiated mixture of abaca, which is a product of some farming regions in
the Philippines, and polyester nonwoven fabric was grafted with GMA using
emulsion grafting technique and finally modified by reaction with EDA. An
absorbent with the DG of 150% and an amine group density of 2.7 mmol/g
showed almost 4 times higher adsorption capacity for Cu2+ than Ni2+. In a
comparison with a commercial Diaion WA20 resin, which is also an amine
functional material, this adsorbent showed greater Ni2+ and Cu2+ adsorption
capacity and rate.
3.2. Controlled biodegradability
Polymeric substances with a good combination of durability and
degradability are highly interesting especially for packaging and disposable
applications [79]. In this regard, RIEGC was used to achieve the stability of
biodegradable polymers. For instance, VAc was grafted onto biodegradable
poly(3-hydroxybutyrate) (PHB) film. Grafting reactions were carried out in
VAc/water/surfactant emulsion, VAc/water, and VAc/methanol systems. For
emulsion grafting, Nonion L-4 was found to be the optimum surfactant with
respect to the stability of a single emulsion layer. The emulsion with a 10:1
(w/w) ratio of VAc to surfactant yielded the highest DG: 230%. The grafting
efficiency in the emulsion and the water and methanol solvents were
evaluated. The results indicated that the grafting efficiency of the emulsion
was 100 times more than that of VAc/methanol when the same 2 wt % VAc
Advances in the development of functional polymers using radiation induced emulsion polymerization 121
was used in the grafting reaction [55]. The biodegradation of PHB is
depressed by grafting of VAc and upon disposal it could be easily recover by
hydrolysis (treatment with NaOH).
3.3. Biodiesel production
A polymer catalyst for converting Triolein to biodiesel by
transesterification reaction was prepared by RIEGP of 4-chloromethylstyrene
onto PE nonwoven fabric. The monomer emulsion containing Tween 20 and
deionized water was reacted with PE irradiated at 100 kGy under inert
atmosphere at 40 °C. Quaternary ammonium groups were introduced to the
grafted PE fabric by a treatment with 0.5 M trimethylamine (TMA) at 50 °C
followed by a treatment with 1 M NaOH to replace Cl- with OH-. Grafting
levels up to 340% and 3.6 mmol-TMA/g-catalyst were reached. The obtained
catalyst achieved a conversion of 95% when used for transesterification of
triglycerides and ethanol after 4 h at 50 °C. The catalyst demonstrates strong
potential in the acceleration of biodiesel production [68].
3.4. Other applications
Modification of Silk fibers with octafluoropentyl acrylate through the
vinyl bonds of acryloyloxyethyl isocyanate is reported to improve the fibers
quality [66]. The thermal stability of the modified silk fibers was improved
due to the introduction of fluoroacrylate. The fibers also displayed water
repellence compared to pristine silk fibers.
PU synthesis by polycondensation reaction in emulsion were used to
prepare PU-g-polyvinyl copolymer by an in-situ graft copolymerization with
various vinyl monomers induced by γ-ray radiation [62]. Such hybrid
materials can be potentially used in various industries, especially in the
coating and paint productions. In comparison to simply mixing the PU and
vinyl polymer, more homogeneity and better film quality was results by
RIEGC.
4. Applications of some polymers and copolymers obtained
by radiation induced emulsion polymerization
Polymeric nanocapsules from polar monomers; N-vinylpyrrolidone
(NVP), styrene (St) and SDS surfactant with a liquid core of dodecane were
prepared by γ-ray initiated miniemulsion polymerization. The grafting
reaction between polystyrene and poly(N-vinylpyrrolidine) increases the
hydrophilicity of the polymer and reduces the interfacial tension between
Mohamed Mahmoud Nasef et al. 122
polymer and water, which leads to successful synthesis of nanocapsules. The
increase in NVP/St ratios, SDS amount and dodecane/monomer ratios is
favourable to obtain nanocapsules with a liquid core. The type of the
surfactant shows a great influence on the particle morphology. The release
study indicates good permeability of the synthesized nanoparticles [9].
Ag-loaded polystyrene (PS-Ag) composite nanoparticles were prepared
by two different approaches (two step and single step processes). In the two
step process, the PS nano-particles were synthesized by RIEP of styrene
followed by loading Ag nanoparticles while in the one step method, Ag-PS
nanoparticles were synthesized in a single step. The two-step method results
in loading of Ag particles on the surface of the PS particles, while the one-
step method distributes the Ag particles within the PS particles. When coated
on a cloth, the PS-Ag nanoparticles produced by the one-step method showed
an excellent antimicrobial activity. A cloth coated with those particles shows
a high antimicrobial efficiency of 99.9% against two bacteria, Staphylococcus
Aureus and Klebsiella Pneumoniase [38].
The developments of coatings which do not require any organic solvent
have been in demand for social needs. The emulsion made by RIEP is
considered to meet possibly this requirement, but it has not been utilized well
in the application to water paint. Obviously, the technology used for the
preparation of emulsion-type paint should be improved to meet the recent
requirements either in the performance characteristics or in the environmental
field. Although the emulsion-type paints have been developed and sold
commercially; however, their antipollution and water-resistance properties
are far from satisfactory [80].
5. Concluding remarks
RIEP is an advantageous technique for preparation of various functional
copolymers, graft copolymers and homopolymers with a flexibility to control
the composition of the obtained materials. Particularly, RIEP provides a
considerable reduction in monomer consumption and irradiation dose in
addition of using water as an alternative green solvent when compared with
conventional methods. Functional graft copolymers obtained by RIEGC have
been used in various applications including ionic adsorbents, chelating
agents, catalysts and polymers with controlled biodegradability mainly for
environmental remedy. In this regard, the ability of RIEGC to conduct graft
copolymerization at low dose allows the modification of natural polymers
and fine structure such as nano-fibers and converting them to cheap and
highly selective adsorbents. Copolymers and homopolymers obtained by
RIEP can be used for various applications such as making core-shell
Advances in the development of functional polymers using radiation induced emulsion polymerization 123
structures, encapsulated materials, paints and antibacterial polymers.
Intensive work is needed to develop new functional materials and to bring
more researched materials from laboratories to industrial scale application.
Acknowledgement
The author wishes to acknowledge the financial support by the Malaysian
Ministry of Higher Education under FRGS mechanism (Vote no. 4F035).
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