Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces
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730 Charged Particle and Photon Interactions with Matter
acid generation is saturated at the G-value of main-chain scission or –COOC
•
H
2
–. Considering
the above-mentioned decomposition mechanism and acid yields with and without acid generators,
the acid generation in PMMA consists of two parts, namely, polymer-limited and acid-generator-
limited regions. Below 0.1 mol dm
−3
, the yield of triuoromethanesulfonic acids is limited by the
reaction of acid generators with electrons. Generally, the yield of cationic species is increased by
the addition of an electron scavenger (acid generators are typical electron scavengers), because the
recombination reaction of cationic species with electrons is suppressed. In PMMA, the ejected elec-
trons are trapped by PMMA itself. Therefore, the addition of acid generators does not noticeably
affect the proton yield. Therefore, the observed acid yield corresponded to the decomposition yield
of acid generators (Kozawa et al., 2007a). Above 0.1mol dm
−3
, the proton yield was less than the
counteranion yield produced by acid generator decomposition (Kozawa et al., 2007a). In this region,
the acid generation efciency is restricted by the deprotonation efciency of PMMA radical cations.
The deprotonation efciency of PMMA radical cations is lower than that of PHS radical cations
owing to the difference in the deprotonation mechanisms. It has also been reported that the proton
yield strongly depends on the ester group (Nakano et al., 2004b). All these facts suggest that protons
are
mainly generated by the deprotonation of side-chain (radical) cations.
26.6 summary
The next-generation lithography requires extremely strict specications for resist materials. In par-
ticular, the trade-off relationship among resolution, sensitivity, and LER is the most serious problem
in the development of next-generation materials. Because the enhancement of pattern formation
efciency is an essential solution for this problem, the understanding of the energy ow from the
exposure tool to chemical reactions induced in the materials is important for the development of
resist materials. Knowledge on radiation physics and chemistry will be a prerequisite in the elds of
the
mass production of semiconductor devices.
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27
Radiation Processing of
Polymers
and Its Applications
Masao Tamada
Japan Atomic Energy Agency
Takasaki,
Japan
Yasunari Maekawa
Japan Atomic Energy Agency
Takasaki,
Japan
27.1 brieF introduCtion to basiC ChemistryoFCross-linking,
graFting,
and d
egradation
27.1.1 radiation-induced croSS-linking
Cross-linking is the most well-known radiation effect on polymers in industrial applications such
as manufacture of rubber tire and covered electric wire. In the case of rubber tire, the cross-linking
increases the mechanical strength and reduces the tack strength as pre-cross-linking. Electric wire
covered with polyethylene and polyvinylchloride can be improved in terms of thermal resistance.
There is a linear relationship between such radiation effects and dose. After cross-linking, viscos-
ity, molecular weight, and branching increase with the degree of resulting cross-linking and these
phenomena
are induced by a strong interaction of polymer chains.
Cross-linking
occurs in the radical reaction of the neighboring molecules. The G-value of cross-
linking is estimated by a famous Charlesby–Pinner plot in which gel percentage, molecular weight,
Contents
27.1 Brief Introduction to Basic Chemistry of Cross-Linking, Grafting, and Degradation ........ 737
27.1.1 Radiation-Induced
Cross-Linking............................................................................ 737
27.1.2
Radiation-Induced
Graft Polymerization .................................................................738
27.1.3
Radiation-Induced
Degradation................................................................................738
27.2
New
Material Production Using Radiation-Induced Cross-Linking.................................... 739
27.2.1
Polymer Gels............................................................................................................. 739
27.2.1.1 Applications
of Polymer Gels.................................................................... 740
27.2.2
Biodegradable
Plastics.............................................................................................. 743
27.3
New
Material Production Using Radiation-Induced Grafting ............................................. 745
27.3.1
General
Application.................................................................................................. 745
27.3.2
Fibrous Metal-Ion Adsorbents.................................................................................. 747
27.3.3 Polymer
Electrolyte Membranes for Battery and Fuel Cells.................................... 749
27.3.3.1
AAc-Grafted
PE: Bottom Battery ............................................................. 749
27.3.3.2
Fluoropolymers
for Fuel Cell PEM............................................................ 749
27.3.3.3
Aromatic
Hydrocarbon Polymers for Fuel Cell PEM................................ 753
27.4
New Material Production Using Radiation-Induced Degradation ....................................... 755
27.4.1 Plant
Growth Promoter............................................................................................. 755
References...................................................................................................................................... 755
738 Charged Particle and Photon Interactions with Matter
and dose are measured (Charlesby and Pinner, 1959). The degree of cross-linking is affected by
molecular weight, chemical structure, and irradiation conditions such as temperature and atmo-
sphere. Though the radical number in polymers is determined by dose, the cross-linking effect
appears as gelation in proportion of a radical number in weight average molecular weight (Mahmudi
et al., 2007). Polymers having a polyethylene-like chemical structure of –(CH
2
–CH
2
)– are classi-
ed as cross-linkable polymers. Vinylidene types, –(CH
2
–CR
2
)–, belong to the group of degradable
polymers. Both cross-linking and degradation occurs in the vinyl type, –(CH
2
–CRH)–. However,
polyvinyl chloride is a typical cross-linking polymer that is utilized in the coating of electric wires.
In the presence of oxygen gas, however, degradation becomes the main reaction in polyvinylchlo-
ride. Polytetrauoroethylene (PTFE) reduces the mechanical strength by irradiation, but it can be
cross-linked at a small range of temperature 13°C higher than melting point (327°C) (Sun et al.,
1994; Oshima et al., 1995). Water-soluble polysaccharides such as carboxymethyl cellulose and
starch can be cross-linked at a high concentration of aqua solution. The radiation-induced cross-
linking of PTFE enhances several polymer properties such as mechanical strength, radiation resis-
tance,
heat deection temperature, and wear resistance.
27.1.2 radiation-induced graft polyMerization
Radiation-induced graft polymerization is a convenient method to impart a desired function into
the polymers. The barrier membrane in the button-type battery is synthesized by grafting an ionic
monomer into the polyethylene membrane. The chemical lter used in semiconductor factories
uses a brous adsorbent prepared by the grafting of acidic monomers to remove the trace ammonia
released
from the human body.
The
important factor for graft polymerization is to select a functional monomer and trunk poly-
mer and to optimize the conditions such as dose, solvent, reaction temperature, reaction time, etc.
Generally, the trunk polymer is irradiated and grafted in inert nitrogen gas. Various shapes of
trunk polymers such as fabric (Lee et al., 2008), lm (Choi and Nho, 2000), hollow ber (Saito
et al., 1999), particle, and ber are available. Grafting needs the chemical propagation reaction of
monomer on trunk polymer. There are two methods distinguished by irradiation step in radiation-
induced graft polymerization. In the rst method, mutual grafting, a trunk polymer is irradiated
in a monomer solution. Thus, the process of grafting is quite simple. However, a large amount of
homopolymer is produced since the monomer solution is irradiated at the same time. Accordingly,
the dose rate affects the monomer utilization to grafting. The grafting rate is limited by monomer
diffusion into the trunk polymer. In the second method, preirradiation grafting, the trunk poly-
mer is irradiated in advance and is then put into a monomer solution. The radiation and grafting
steps are discrete, so that lesser homopolymer is created. The total grafting yields in both grafting
methods are determined by irradiation dose. For an industrial application of grafting, therefore, the
preirradiation grafting is generally selected since the cleaning of homopolymer from grafted trunk
polymer is an extremely cumbersome process. The industrial gas adsorbent manufacturing process
has
adopted a continuous process of preirradiation grafting (Fujiwara, 2007).
27.1.3 radiation-induced degradation
Degradation occurs simultaneously during cross-linking. The general classication of cross-
linkable and degradable polymers was described in Section 27.1.1. PTFE is classied into typical
degradable polymer. The changes of the thermal properties are reviewed in the different dose rate
and irradiation atmospheres (Schierholz et al., 1999; Briskman and Tlebaev, 2007). After degrada-
tion, polytetrauoroethylene has been applied as a ne powder used as a chemically and thermally
stable lubricant agent for gaskets.
As new applications of degraded polymers are developed, polysaccharides gain importance as an
attractive material. Decomposed marine polysaccharides such as alginate and chitosan play the role
Radiation Processing of Polymers and Its Applications 739
of plant growth promoters. The radiation degradation of such polysaccharides has been investigated
from the viewpoint of solvent (Hien et al., 2000) and mechanism (Wasikiewicz et al., 2005b). The
industrialization of degraded polysaccharides has been supported by IAEA as several coordinated
research
projects (CRP) (Haji-Saeid et al., 2007).
27.2 new material produCtionusingradiation-induCed
Cross-linking
27.2.1 polyMer gelS
Polysaccharides and their derivatives were well-known degradation polymers in radiation process-
ing. These materials transformed to hydrogel by adding sodium salt (Mitsumata et al., 2003) and
polyamines (Miani et al., 2004). It was found that water-soluble polysaccharide derivatives like car-
boxymethyl cellulose (CMC) could be cross-linked when irradiated in a highly concentrated solu-
tion (paste-like condition), without any additives. CMC is a cellulose derivative that is synthesized
by reacting cellulose with chloroacetic acid in alkaline condition. The chemical structure of CMC is
a polymer of β-(1-4)-d-glucopyranose, as shown in Figure 27.1. CMC is used as a food additive, for
its properties as a viscosity modier and thickener. It is also a component of many nonfood products
such as toothpaste, laxatives, diet pills, water-based paints, and detergents. This is because it has a
high
viscosity, is nontoxic, and is generally nonallergenic.
The
kneading of CMC powders with water at concentrations higher than 10% gives a homoge-
neous paste-like state. When CMC in the paste-like state is irradiated by electron beams and γ-rays,
cross-linking
occurs and biodegradable hydrogels are formed (Wach et al., 2003). If the CMC con-
centration
is lesser than 10% and higher than 70%, degradation is preceded and no gel fraction is
obtained. This result indicates that the distances between CMC molecules are not enough for the
cross-linking reaction at concentrations less than 10%, and CMC is not homogenously distributed in
water
and some insoluble regions of polymer exists at concentrations higher than 70%.
There
are many CMC grades having different degrees of substitution (DS), since the CMC mol-
ecule has three hydroxyl parts in one unit of glucopyranose. The gel fractions of cross-linked CMC
having different DS are shown in Figure 27.2. A high degree of substitution leads to higher cross-
linking at 20% CMC aqua paste. This phenomenon implies that carboxymethyl groups are related to
radiation-induced cross-linking. When the obtained hydrogel was soaked in water, swelling reached
an equilibrium after soaking for 4h. The swelling, in grams of absorbed solvent per gram of dried
gel,
was calculated as follows:
Degree of swelling =
s d
d
( )G G
G
−
where
G
s
and G
d
are the weight of the hydrogel in a swollen and dry state, respectively. The degree
of swelling was 400 and 120 in water and NaCl solution, respectively, since ionic strength retarded
the
swelling of the gel.
OH
OH
O
O
O
OH
OH
O
CH
2
OCH
2
COONa CH
2
OCH
2
COONa
n
Figure 27.1 Chemical structure of CMC.