recent res. devel. polymer science, 12(2014): 107-128 … · functional polymers using radiation...

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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.


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.


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,


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•


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:


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.


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


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.


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


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.


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


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


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


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


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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