Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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750 Charged Particle and Photon Interactions with Matter

PEFChasbeen gradually commercialized for residential cogeneration systems and is expected to be

utilized in fuel cell hybrid vehicles (FCHV), while DMFC has been announced to be commercial-

ized for mobile phone batteries and notebook type PCs from several companies. In order to apply

radiation-grafted membranes to fuel cell PEM, the membranes must have higher acidic groups than

carboxylic acid, to show higher proton (ion) conductivity, which is directly related to higher fuel cell

power density and possess size stability in fuels (water and aqueous methanol) (Figure 27.18). Thus,

recently, there have been many attempts for fuel cell PEM prepared by grafting styrene derivatives

into uorinated-polymer substrates such as cross-linked PTFE (cPTFE), FEP, ETFE, and PVDF

(Figure 27.19) (Gubler et al., 2005b; Gursel et al., 2008). These membranes have been examined as

electrolyte membranes for both PEFC and DMFC, for which the membranes must be durable up to

at least 60°C for DMFC and 80°C for PEFC.

27.3.3.2.1

Radiation Grafting into Fluoropolymers

The rst radiation-induced grafting of uorinated-polymer substrates had been reported in 1959

and summarized in a review in 1962 by Chapiro (Chapiro, 1959, 1962). He reported that styrene

and MMA could be grafted into the deep inside of PTFE (Teon) lms by simultaneous irradia-

tion with a low dose rate. Since Teon substrates are not swelled in the monomer solutions at all,

the author proposed the so-called grafting front mechanism. The graft polymerization of styrene

commences at the surface region of Teon lms to give a grafting layer consisting of polystyrene.

The newly generated grafting layer, in which propagation of styrene polymerization continues,

expands and induces deformation of polymer chains of Teon. As a result, the stain between grafts

table 27.2

type

and Characteristics of Fuel Cells

FC type peFC dmFC

Fuel Hydrogen Methanol

Application Fuel

cell vehicle Mobile phone

Residential FC Note PC

Temperature 70°C–130°C r.t. –50°C

Alkali–button cell separator

Radical

Grafting

Irradiation

COOH

AAc in H

AAc-grafted PE

(Separator: 25 μm)

Irradiated PE

PE film

•

Figure 27.17 Preparation of AAc-grafted PE for bottom cell separator (roll lm).

Radiation Processing of Polymers and Its Applications 751

and substrate interfaces due to the deformation of Teon polymer chains leads to further advance

of monomers into the inner part of Teon.

Later, Hegazy et al. reported that acrylic acid could be grafted into Teon (Hegazy et al., 1981a,b).

The AAc-grafted PTFE lms with grafting degrees above 30% showed high ion conductivity (>10

−2

S/cm). They conrmed that the lms with high conductivity have a homogeneous distribution of

poly(AAc) grafts in Teon substrates. Bozzi et al. had also reported the radiation grafting of AAc

into FEP (Bozzi and Chapiro, 1988). However, there had been no reports from the viewpoint of fuel

cell

applications at that time.

27.3.3.2.2

PEM Consisting of Peruorinated-Polymer Substrates

The preparation of polymer electrolyte membranes using radiation-induced grafting and their evalu-

ation as a fuel cell membrane was reported by the group of PSI in 1993 (Gupta et al., 1993b; Rouilly

et al., 1993). Later, there have been many reports of the evaluation of PEFC (hydrogen fuel type)

and DMFC (methanol fuel type) performance of the single fuel cells using the radiation-grafted

–

Anode

Water

PEM

Cathode

Catalyst

Air

)

Hydrogen

)

Requirement for PEM:

High ion conductivity

High mechanical strength

High fuel gas barrier

•

Figure 27.18 Schematics of fuel cells.

Graft chain

Ion conducting

grafts (SO

Grafted PEM

Polymer substrate

γ-rays

Grafted film

Figure 27.19 Preparation scheme of radiation-grafted PEM for fuel cells.

752 Charged Particle and Photon Interactions with Matter

membranes. They prepared the PEM by radiation-grafting of styrene into FEP substrates with

thickness of mainly 25 and 50μm and successive sulfonation of the polystyrene grafts to obtain the

PEM consisting of poly(styrenesulfonic acid) (PSSA) grafted into FEP. They precisely investigated

the preparation conditions such as absorbed doses, grafting solvents, and the electrolyte, chemi-

cal, and thermal properties (Gupta et al., 1993a, 1994; Gupta and Scherer, 1994). The advantages

of this method, they claimed, are the low cost and the simple procedures of production processes

compared with fully uorinated Naon

membranes. Even with grafting degrees less than 20%, the

FEP-based

PEM showed a higher ion conductivity than that of Naon.

The fuel cell performances of the MEA consisting of the FEP-based PEM were also investigated

by the same group (Büchi et al., 1995a,b; Felix et al., 1995). The radiation-grafted membrane consist-

ing of PSSA with a grafting degree of 18% and divinylbenzene as a cross-linker (10wt% of mono-

mers) exhibited a fuel cell performance similar to the one while using same operation conditions

using Naon. The structure and amount effects of cross-linkers on the PEM properties were carefully

examined; they concluded that the usage of TAC with DVB improved the durability to be stable for

1400h in the PEFC operation at 80°C. They recently reported that the FEP-based PEM with 25μm

thickness with a DVB cross-linker showed a stable operation more than 2500h at 60°C–80°C.

Even though PTFE (Teon) based PEM had been prepared by the radiation-grafting of styrene

and the subsequent sulfonation of the polystyrene grafts, the PTFE-based PEM showed quite high

swelling in water and aqueous methanol, mainly because of the radiation sensitive (degradable)

property of a PTFE substrate. Consequently, it was difcult to give high conductivity by increasing

IEC (namely, with high grafting degrees) of PTFE-based PEM. On the other hand, it was reported

that PTFE lms were cross-linked by γ-rays or EB at temperatures higher than its melting point

(320°C), while below it, PTFE is well known as a typical radiation degradable polymer. After the

new ndings, two groups independently reported the PEM consisting of cross-linked PTFE (cPTFE)

substrate and PSSA grafts with cross-linkers (Figure 27.20) (Sato et al., 2003; Yamaki etal., 2003).

The cPTFE membranes showed several excellent polymer properties such as mechanical

strength, radiation resistance, heat deection temperature, and wear resistance. Thus, cPTFE lms

kept their mechanical strength after irradiation for grafting even with quite high doses, resulting in

cPTFE-based PEM with higher grafting degrees, namely, higher conductivity (σ = 2.5 S/cm) and

IEC (3.0mmol/g). The cPTFE-based PEM is dimensionally quite stable in bulk methanol and aque-

ous methanol solutions owing to the cross-linking structure. Furthermore, recently, the PEM was

revealed to have quite low permeability to methanol and water with superior proton conductivity

compared with Naon. Thus, the cPTFE-based PEM should be promising, especially in DMFC

applications (Sawada et al., 2008). The PEFC performance of the cPTFE-based PEM have been

examined. The MEA of the cPTFE-based PEM with higher grafting degrees (higher IEC) showed

a similar FC performance compared to that of Naon-based MEA. After optimization of structures

and amounts of cross-linkers, the MEA maintained stable operation for several hundred hours at

80°C

(Li et al., 2006).

Grafting

Polystyrene grafts

Grafted xPTFE

cPTFE-based

PEM

Sulfonation

Cross-linking

Cross-linked film

(cPTFE)

PTFE

Polymer chain

Cross-linking

Conducting grafts

Figure 27.20 Preparation and grafting of cPTFE by radiation technique.

Radiation Processing of Polymers and Its Applications 753

27.3.3.2.3 PEM Consisting of Partially Fluorinated-Polymer Substrates

The group of PSI had reported the preparation of PEM with ETFE as a substrate using the same

technique and compared them with the FEP-based PEM (Brack and Scherer, 1997; Gubler et al.,

2005a). The ETFE-based PEM showed similar conductivity with grafting degrees of 20%–30% and

was superior to the FEP-based PEM in terms of mechanical strength. Below 20%, the grafting in

the middle of the grafted lm was not sufcient; above 30%, the grafted lm became brittle and had

lesser mechanical strength. The ETFE-based PEM consisting of 10% DVB showed good chemical

and physical durability; the MEA consisting of the above PEM showed stable operation for 770h at

80°C

with fully humidied hydrogen fuel in a single fuel cell device.

Recently,

Yoshida et al. had precisely evaluated the effects of graft monomers and cross-linking

procedures on PEM properties (Figure 27.21) (Chen et al., 2006a,b). They clearly showed that graft

monomer structures, cross-linker structures, and radiation-induced cross-linking procedures were

important parameters for oxidative stability of the PEM (3% H

at 60°C), which was one of the

mostdistinct characteristics to evaluate PEM stability in fuel cell operation. Compared with the PSSA

grafts, the ETFE-based PEM with the co-graft polymers, poly(methyl styrene–co-t-butylstyrene

sulfonic acid), showed four times longer durability periods. They introduced 1,2-distyrylethane

(DVPE) as a exible cross-linker and the small amount of DVPE with DVB enhanced the durability

of the PEM in the oxidative condition. Furthermore, the post radiation-cross-linking after grafting

of styrene monomers, which induced multi-cross-links of substrate–substrate, graft–graft, and sub-

strate–graft, possessed three times longer durability than the PEM without the post cross-linking.

Accordingly, they concluded that the ETFE-based PEM with hydrophobic styrene monomers and

a small amount of cross-linkers with post-cross-linking process exhibit quite high DMFC perfor-

mance with high oxidative stability.

27.3.3.3 aromatic

ydrocarbon

olymers

for Fuel Cell pem

27.3.3.3.1

Poly(Ether Ether Ketone)-Based PEM

Fluorinated PEM such as Naon-type and Even graft-type PEM have drawbacks such as low

mechanical strength at higher operating temperatures and unsatisfactory gas barrier properties. In

this respect, the sulfonated form of aromatic hydrocarbon polymers, so-called “super engineering

plastics” such as poly(ether ether ketone) (PEEK), polyimide (PI), and poly(sulfone) (PSU), have

been given attention owing to their excellent mechanical properties at elevated temperature as well

as excellent barrier properties against fuels (methanol, H

) and oxygen. Thus, the aromatic polymer-

based PEM are actively investigated for hydrogen fuel type fuel cells (PEFC), which are utilized in

residential

cogeneration system fuel cells and fuel cell hybrid vehicles (FCHV).

The

super engineering plastic lms also have high chemical resistance; thus it is difcult to intro-

duce grafting monomers into the lms. Recently, we had reported that the grafting of styrene into

the PEEK lm pre-irradiated with 30kGy was accelerated in 1-propanol as a grafting solvent at 80°C

to obtain the styrene-grafted PEEK with grafting degrees more than 60% (Hasegawa etal.,2008).

Multi-cross-linking

Cross-linkers

Quaternary monomers

Hydrophobic monomers

Grafts – polymer

ETFE-based PEM

Grafts – grafts

Polymer–

polymer

Figure 27.21 Preparation of ETFE-based PEM for DMFC.

754 Charged Particle and Photon Interactions with Matter

However, the sulfonation of the polystyrene grafts with chlorosulfonic acid in a dichloroethane solu-

tion proceeded not only at the grafts layers but also at the crystalline and amorphous phases of

PEEK. In consequence, the PEEK-based PEM with conductivity of more than 0.01 S/cm are subject

to severe deformation due to higher water uptake.

To prevent the damage of a PEEK substrate during sulfonation process, the graft polymerization

of sulfo-containing styrene, ethyl 4-styrenesulfonate (E4S), into two PEEK substrates with high and

low crystallinity, h-PEEK and l-PEEK (degree of crystallinity: 32% and 11%), was examined via

preirradiation and post grafting method because the obtained grafted PEEK lms should be con-

verted to PEEK-based PEM only by hydrolysis of the sulfonic acid ester containing precursor grafts.

Graft polymerization of E4S hardly proceeded into h-PEEK, whereas, it gradually proceeded into

l-PEEK with grafting degrees of more than 50% at 80°C for 72h. The PEEK-based PEM with

0.08 S/cm and water uptake of <50% can be obtained by aqueous hydrolysis of grafted lms and

exhibited

mechanical strength (95

MPa)

of 62% of original PEEK substrates.

found that a small amount (<10 wt%) of divinylbenzene (DVB) could be introduced into

l-PEEK via thermal grafting. Surprisingly, the introduction of DVB via thermal pre-grafting

enhanced the grafting rate of E4S, which is a precursor of poly(styrenesulfonic acid), in radiation-

induced grafting; thus, the grafting degrees of DVB-PEEK reached 100% and above at 80°C for

10 h. As a consequence of the enhancement of grafting rates, the DBV-PEEK-based PEM, after

hydrolysis, showed quite high conductivity (0.12S/cm) with low water uptake (<50%) (Figure 27.22)

(Chen et al., 2008). In comparison with the commercial Naon, the obtained PEEK-based PEM pos-

sesses superior proton conductivity and mechanical property at 80°C under high relative humidity of

100 RH%. Here, the grafted layers introduced by thermal treatment enhanced the radiation-induced

grafting to give the electrolyte membrane exhibiting 1.5 times higher conductivity and 2.3times

higher

mechanical strength, compared with conventional uorinated electrolyte membranes.

The

single fuel cell device consisting of the developed electrolyte membrane maintained stable

operation over 1000 h at 95°C under relative humidity of 80% RH with pure hydrogen and oxygen

gases (Chen et al., 2009). Furthermore, under severe conditions (95°C, 40% RH), in which Naon

obviously degraded, these membranes can maintain their output voltages for more than 250h. Thus,

these membranes can be applied to fuel cell vehicles, which should contribute to solve the current

environmental problem, judging from the fact that no appreciable degradation of these membranes

occurs even under the operating conditions of lower humidity (Figure 27.23).

27.3.3.3.2

Other Aromatic Hydrocarbon PEM

Recently, we reported the preparation of polyamide-based PEM by radiation grafting of sodium

styrenesulfonate (3S) in dimethylsulfoxide (Li et al., 2009). There have been many attempts to use

a 3S monomer as a grafting monomer because of no extra treatment to obtain sulfonic acid groups

after grafting is required and also due to the relatively low cost. However, a 3S monomer is soluble

O O

PEEK

substrate

DVB

PEEK/DVB

based PEM

ETS

Scaffold

graft layer

Conducting

grafts

Thermal

grafting

Radiation

grafting

Figure 27.22 Thermal/radiation-induced two-step grafting using PEEK.

Radiation Processing of Polymers and Its Applications 755

in aqueous solvents. Thus, radiation grafting of 3S into hydrophobic polymer substrates hardly

proceeds, probably due to less swelling of the substrates in aqueous media. However, when dimeth-

ylsulfoxide is utilized as a grafting solvent, the monomer is completely dissolved in the solvent

and the polyamide lms are slightly swelled in the 50% monomer solution. An aromatic polyam-

ide, poly(m-xylene adipamide) (Nylon-MXD6), lms were pre-irradiated with 60kGy under argon

atmosphere and then immersed in DMSO at 60°C for 10h to give the 3S-grafted Nylon-MXD6 lm

with a grafting degree of 49%. The obtained PEM showed conductivity of 0.083S/cm with water

uptake

of 70%.

The

alicyclic polyimide (A-PI) lm, consisting of aromatic dianhydride and alicyclic diamines,

was utilized as a substrate of radiation-induced polymerization to prepare a PI-based PEM for fuel

cells. We successfully prepared the graft-type PEM by radiation-induced grafting of styrene into

A-PI lms and subsequent sulfonation (A-PI-PEM). The obtained A-PI-PEM has higher ion con-

ductivity and mechanical and thermal properties compared with conventional PEM (Naon) (Park

et al., 2009). However, these PEM consisting of polyamide and polyimide substrates are not stable

in aqueous media at temperatures higher than 80°C. In order to utilize the polycondensation type

aromatic polymer substrates, we must introduce less hydrolyzed structures in the main chains.

27.4 new material produCtion using

radiation-induCed

egradation

27.4.1 plant growth proMoter

Oligosaccharides obtained by radiation-induced degradation stimulate plant growth in shoot elonga-

tion and productivity. Oligosaccharides can be degraded by acid hydrolysis (Holtan et al., 2006) and

enzymatic reaction (Zhang et al., 2004). However, the plant growth promoter prepared by radiation

degradation shows higher activity than that obtained by acid hydrolysis (Luan et al., 2006). This is

because the oligosaccharide molecular weight from 1000 to 3000 is produced at a higher yield by

radiation degradation than by acid hydrolysis. This component has the highest activity against the

plant growth promoter (Matsuhashi and Kume, 1997). Oligosaccharides such as radiation-degraded

alginate,

chitosan, and carrageenan have been tested on the growth of crops and vegetables.

reFerenCes

Abrol, K., Qazi, G. N., and Ghosh, A. K. 2007. Characterization of an anion-exchange porous polypropylene

hollow

ber membrane for immobilization of

ABL

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PEEK based PEM

Operating time (h)

Voltage (V)

0.2

0.4

0.6

200 400 600 800 1000

Nafion 112

Temp.: 95°C

40% RH

80% RH

Figure 27.23 Fuel cell performance of PEEK-based PEM.

756 Charged Particle and Photon Interactions with Matter

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