The Radiation Effect on Thermal Conductivity of HighStrength Ultra-High-Molecular-Weight Polyethylene Fiberby ␥-Rays
Atsuhiko Yamanaka,1 Yoshinobu Izumi,2 Tooru Kitagawa,1 Takaya Terada,2 Hideki Sugihara,1
Hiroshi Hirahata,1 Kimiko Ema,2 Hiroyuki Fujishiro,3 Shigehiro Nishijima2
1

Research Center, TOYOBO, CO., LTD., Ohtsu, Shiga 520-0292, Japan
Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan3
Department of Materials Science and Technology, Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
2

Received 10 November 2005; accepted 3 February 2006
DOI 10.1002/app.24227
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: To understand the contribution to thermal
conductivity by the length of the molecular chains in highstrengthultra-high-molecular-weight
polyethylene(UHMW-PE) ﬁber, the thermal conductivity in the range oflow temperature was investigated for high-strength
UHMW-PE ﬁber (Toyobo, Dyneema威; hereinafter abbreviated to DF) irradiated by ␥-rays (␥-rays treatment) thatinduce the molecular chains scission. The molecular weightof DF decreased by ␥-rays treatment. X-ray diffraction behavior did not change by ␥-rays treatment. The melting
behavior observed by differential scanning calorimetry
showed the main chain scission of DF by ␥-rays treatment.
Heat capacity decreased slightly by ␥-rays treatment. Thermal conductivities of DF with and without ␥-rays treatment
decreased by decreasing temperature. Thermal conductivityof DF decreased very much by ␥-rays treatment. The decrease of thermal conductivity was explained by molecular
chain scission in DF by ␥-rays treatment and it suggested thepossibility of the reduction of mean free path of phonon. The
decreasing rates of thermal conductivity by ␥-rays treatment
were smaller than those of molecular weight. Those differences were explained by the difference of probability ofchain scission between the amorphous and crystal region.
Those results suggested the contribution of the length ofextended molecular chains because of high molecularweight on the thermal conductivity of DF. © 2006 Wiley

INTRODUCTION

rials are desired not only as thermal insulators but also
as electrical insulating thermal conductors for thermal
conductivity.17,18
From previous studies concerning thermal properties of polymeric materials, it is well known that thethermal conductivity of amorphous polymers issmaller than that of metals and semiconductors.19,20
Therefore, these have principally been used as thermal
insulators. However, other reports have shown thatpolymeric crystals possess a high thermal conductivity in the direction in which the molecular chains arecovalently bonded, polyethylene crystals being an example.20 –27 Furthermore, there are highly-crystalline
and high-strength polymeric ﬁbers that possess thermal conductivity almost as high as metals; these include DF28 –30 and high-strength poly(p-phenylene
benzobisoxazole) (PBO) ﬁber.28 Practical reports havebeen published concerning the heat-radiation effects
of high-temperature superconducting tape material byhigh thermal conductivity of DF.17 It is also reported
that DF is used for conduction-cooled superconducting magnetic energy storage (SMES).18 These indicatethat the thermal conductivity of these insulators aswell as their negative expansivity are important char-

Recently, the importance of polymeric materials aselectrical insulating or structural materials has beenrecognized in those areas of cryogenic engineeringthat require super conducting technology, such as superconducting power equipment, magnetic levitatedtransportation, medical diagnosis equipment or nuclear fusion reactors, and so on. Their thermal properties, and in particular their coefﬁcient of linear expansion and their thermal conductivity, have been
important parameters to apply polymeric materials tothese areas. For example, high-strength polyethylene
ﬁber (Toyobo, Dyneema威; hereinafter abbreviated toDF) and composite materials reinforced with DF(DFRP) possess negative coefﬁcient of linear expansion toward the direction of the ﬁber axis.1– 4 Typicalapplications for these materials are coil bobbins5–15
and spacers9,14,16,17 for super conductors. Those mateCorrespondence to: A. Yamanaka (Atsuhiko_Yamanaka@
kt.toyobo.co.jp).
Journal of Applied Polymer Science, Vol. 101, 2619 –2626 (2006)© 2006 Wiley Periodicals, Inc.

Periodicals, Inc. J Appl Polym Sci 101: 2619 –2626, 2006

Key words: thermal properties; polyethylene; radiation; ﬁbers; high performance polymers

2620

YAMANAKA ET AL.

acteristics when considering their application in theﬁeld of cryogenics.
DF is a high-crystallinity and high strength ﬁber
made from ultra-high-molecular-weight polyethylene(UHMW-PE).31–34 A previous report elucidated thatthe thermal conductivity of DF was explained by theﬁber structure illustrated in the mechanical serial-parallel model (Takayanagi-model), which consists of acrystalline/amorphous structure.30 It is consideredthat the high thermal conductivity of DF is contributed by the thermal conduction along the covalentbonding in the molecular chain direction in continuous crystal region composed of extended molecular
chains.29,30 Eventhough the effect of molecular chain
length on thermal conductivity of the UHMW-PE is
unknown, the relation between the thermal conductivity and the length of molecular chains of DF is
investigated in this work.
It is well known that polyethylene undergoes main
chain scission by irradiation with ␥-rays in the presence of oxygen.35,36 In this article, we report the thermal conductivity of DFs affected by main chain scission induced by irradiation with ␥-rays for investigation of the relation between the thermal conductivity
and the length of main chain to clarify the effect onhigh thermal conductivity of chain length of DF.
EXPERIMENTAL
SampleIn this work, thermal conductivity of DF and those
irradiated with ␥-rays (␥-rays treatment) were measured in the range of low temperature. The DF used
was Dyneema威, SK-60 with 1320dtex (Toyobo). Irradiation was carried out with Co-60 ␥-rays at ISIR
(Sanken), Osaka University, at an ambient temperature and in air. The absorbed dose rates were 33.3
kGy/h and 83.3 kGy/h. Irradiation time was 6 h. The
total absorbed doses were 0.2 MGy and 0.5 MGy,
respectively. These are hereinafter abbreviated to DF(0.2 MGy) and DF (0.5 MGy). The DF without ␥-rays
treatment is abbreviated to DF (Blank).
Measurements
Molecular weight
To observe the relative decreasing of the length ofmolecular chains by irradiation with ␥-rays, the viscosity-average molecular weight of DFs with andwithout ␥-rays treatment were measured by the viscosity method, using trans-decalin solution. The DFs
with and without ␥-rays treatment were dissolved indecalin with a concentration of 0.0045 g/dL. The relative viscosities at 408 K were measured by using
Ubbelhode viscometers in the thermostat. Thismethod was reported elesewhere.37

Structure of crystal and amorphous
X-ray diffraction and melting behavior by differential
scanning calorimetry (DSC) were observed to investigate the relative change of crystal structure of DFs by␥-rays treatment. A Rigaku RU-200 (40 kV ⫻ 100 mA)
was used with X-ray diffraction on Ni-ﬁltered CuK␣(␭ ⫽ 0.1548 nm). The distributions of (110) and (200)diffraction spots in the equatorial direction were observed. DSC measurements of DFs were carried out on
a Perkin–Elmer DSC-7. An empty sample pan was
taken as the reference sample. The samples were
weighed at 2 mg, and melting experiments were madein the temperature range from 323 to 523 K at a heating rate of 1°C/min.
Heat capacity
The heat capacities of DFs were measured by DSC in
the temperature range from 210 to 280 K at a heating
rate of 10°C/min.
Thermal conductivity
Thermal conductivity (␬) was measured by a steadystate heat ﬂow method.28,29 The measurements of ␬
were carried out on the automated measuring systemwith thermal controller of a Gifford–MacMahon (GM)cycle helium refrigerator as a cryostat.38 The ﬁbersamples were prepared by bundling about 6000 monoﬁlaments with length of 25 mm. Both ends of the ﬁber
bundle were ﬁxed by adhesive STYCAST GT. One endof the bundle was attached to the cold stage of a GM
refrigerator by mechanical pressing, using indium
metal and the adhesive STYCAST GT. A small resistance heater (1 k⍀) was adhered to the other end of thebundle using GE7031varnish. The intermediate positions, where thermocouples were contacted, werebound by ﬁne (0.1 mm) Cu wire. Au (Fe: 0.07 at%)-chromel thermocouples were used as thermometers, which were adhered by GE7031 varnish. The
sample space was evacuated to below 10⫺3 Pa by anoil diffusion pump for heat insulation. The automatedmeasuring system of ␬ was operated in the temperature range from 10 to 300 K. The k was estimated by
the relation k(mW/cmK) ⫽ (Q/⌬T)(L/S), where Q is theheat ﬂow per second, ⌬T the temperature difference
between thermometers, L the distance between thethermometers, and S the cross section of the bundle.
Details of the determination of k were described elsewhere.28,29
RESULTS AND DISCUSSION
Molecular weight
The relation between the viscosity-average molecularweight of DF and absorbed dose is shown in Figure 1.

RADIATION EFFECT ON THERMAL CONDUCTIVITY OF UHMW-PE FIBER

Figure 1 Dependence of molecular weight of DF on absorbed dose irradiated by ␥-rays.

The measured molecular weight of DF(Blank) was 2.0⫻ 106. It decreased to 3.5 ⫻ 104 by 0.2 MGy irradiationand to 2.6 ⫻ 104 by 0.5 MGy irradiation, respectively.
This result shows the main chain scission of DF by the
␥-rays treatment.
In the irradiation effects of PE in air, the free radicals
of alkyl type are produced by irradiation initially, andthe radicals combine with oxygen molecules into peroxy radicals. The peroxy radicals eliminate the hydrogen from the neighboring PE molecular chains andhydroperoxides are produced. The decompositions of
the hydroperoxides induce main chain scission of PE
molecular chains. Therefore, the irradiation effects of
PE in air depend on the diffusion of oxygen.31,36 It isconsidered that oxygen invades and diffuses into theamorphous region more easily than into the crystal
region in DF. Therefore, it is considered that the probability of main chain scission is higher in amorphous
region than in crystal region in DF.

Figure 2 X-ray diffraction proﬁles of DFs irradiated andunirradiated with ␥-rays.

nate in the case of irradiation of PE in the vacuum. It
is reported that X-ray diffraction proﬁle in equatorial
direction does not change in the irradiation of ␥-rays
with absorbed dose of less than 0.25 MGy in vacuum39
like the case of this work.
Melting behavior
The melting behaviors of DFs observed by DSC areshown in Figure 3. DF(Blank) shows four broad melting peaks in the temperature range from 403 to 425 K.

X-ray diffraction measurements
X-ray diffraction proﬁles of DFs in the equatorial direction are shown in Figure 2. The assignments of thereﬂections are shown in the ﬁgure. The changes of
those reﬂection peaks by ␥-rays treatment are investigated. The width, shapes, intensities, and reﬂection
angles of those peaks do not change by ␥-rays treatment. It is suggested that the crystal size in (110) and(200) directions do not change. Crystallinities of theDFs cannot be estimated by these results of X-ray
diffraction measurements. However, from the independence of X-ray behavior with ␥-rays treatment, it isconsidered that the crystal of DF is destructed by
␥-rays not so much. The crosslinking reactions domi-

2621

Figure 3 DSC melting curves of DFs.

2622

Figure 4 Schematic diagram of molecular chains in series
combination part of crystal and amorphous regions of DFs
before and after ␥-rays treatment.

The melting behaviors of high-strength polyethylene
ﬁber as DF are reported in other articles.40 – 42 Although the assignments of the peaks in melting process of high-strength polyethylene ﬁber are not clariﬁed, it is reported that UHMW-PE molecular chains
extended by the tensile stress show the melting behaviors40 – 42 like that of DF(Blank). The schematic diagram of the molecular chains in series combination
part of crystal and amorphous regions in DF is shownin Figure 4. It is considered that the molecular chainsin amorphous of DF(Blank) are extended and are restrained by orientation in the drawing as shown in theleft side of Figure 4. And it is also considered that
there are interactions between the extended molecular
chains in crystal region and those in the surrounding
amorphous regions or the neighboring crystal regions.
Therefore, it is considered that molecular chains in
DF(Blank) behave like those in big crystal by the interactions between crystals and that those interactions
are not uniformed.
In the next, the radiation effect of melting behavior
is investigated. The melting peaks of DF (0.2 MGy)and DF (0.5 MGy) are more sharp than that of DF(Blank). And the number of peaks decreases from 4 to
2 after ␥-rays treatment. This change suggests thedecrease of restraint on molecular chains in crystal
region with ␥-rays treatment. From the X-ray diffraction studies discussed in the previous section, thecrystal size is inferred to decrease not so much by
␥-rays treatment. But it is considered that the molecular chains in amorphous region become more movable and that the interactions of molecular chains between the crystal and amorphous or neighboring crystal regions decrease very much by main chain scission

YAMANAKA ET AL.

after ␥-rays treatment, as shown in the right side of
Figure 4. Therefore, it is considered that molecular
chains in crystal regions behave more uniformly bydecreasing interactions with neighboring amorphousor crystal regions. The sharpe peaks in melting behavior are inferred to be caused by the decrease in thoseinteractions by main chain scissions after ␥-rays treatment.
The melting enthalpies (⌬H) and melting temperatures (Tm) of DF with and without ␥-rays treatment
were shown in Figure 5. Both of them decrease by
␥-rays treatment though X-ray diffraction behavior
does not change. The differences of ⌬H and Tm of DF
between with and without ␥-rays treatment are remarkable. It is possible for the ⌬H of DF(Blank) tocontain the transition energy from orthorhombic tohexagonal, the melting energy of molecular chains in
crystal, or restraint of molecular chains in crystal andamorphous-like crystal. Those are considered to be
decreased after ␥-rays treatment. Especially, the restraint of molecular chains is considered to be decreased by main chain scission. And it is also considered that the molecular chains in the crystal of DF after␥-rays treatment behave as those in small crystal thanthose in DF(Blank) by decrease of interactions between crystals. Therefore, it is considered that thedecreases of interaction between crystal regions by
main chain scission induce the decrease of Tm. The
difference between X-ray diffraction and melting behavior observed by DSC of DFs before and after ␥-rays
treatment is inferred to be caused by the behavior ofrestrained molecular chains.
On the other hand, the differences of ⌬H and Tm aresmall with absorbed dose between 0.2 MGy and 0.5
MGy. It is considered that the decrease of ⌬H and Tmwith ␥-rays treatment is caused by the decrease of

Figure 5 Melting enthalpy (D⌬) and temperature (Tm) of
DFs.

RADIATION EFFECT ON THERMAL CONDUCTIVITY OF UHMW-PE FIBER

2623

Figure 6 Temperature dependence of heat capacity of DFs
with and without ␥-rays treatment.

Figure 7 Temperature dependence of thermal conductivityof DFs with and without ␥-rays treatment.

interactions described in earlier paragraph by main
chain scission. The detailed discussion about structural analysis will be necessary to clarify the mechanism of the melting behavior above-mentioned.
With the aforementioned, the melting behavior observed by DSC also suggested the main chain scissionin DF by ␥-rays treatment.
From the results of molecular weights, X-ray diffraction and melting behavior of DFs before and after␥-rays treatment with absorbed dose of 0.2 MGy and
0.5 MGy, only main chain scissions, could be observed, though the distinct change of crystal structure
could not be observed.

aforementioned, those ␥-rays treatments induced thedecrease of molecular weight, that is to say, the reduction of length of molecular chain. The relations between the molecular weight and thermal conductivities at 200, 150, 100, and 50 K are shown in Figure 8 fordiscussion about relation between the molecular chain
length and thermal conductivity. The molecularweight of DF (0.2 MGy) and that of DF (0.5 MGy) arenot so different. Therefore, it is reasonable that thethermal conductivities of DF (0.2 MGy) and DF (0.5
MGy) are not so different. Figure 8 suggests that thechange of thermal conductivity of DF by ␥-rays treatment agrees that of molecular chain length. But moredetailed study including structural analysis will benecessary for quantitative discussion about the rela-

Heat capacity
Heat capacities of DF with and without ␥-rays treatment in the temperature range from 210 to 280 K
measured by DSC are shown in Figure 6. Heat capacity decreases slightly by ␥-rays treatment. The heat
capacities of DF (0.2 MGy) and DF (0.5 MGy) show
similar behavior. This agrees to the results of melting
behavior.
Thermal conductivity
Temperature dependences of thermal conductivities
of DFs in ﬁber direction from 10 to 260 K are shown in
Figure 7. Thermal conductivities of all samples decrease with decreasing temperature.
We investigate the effect of irradiation with ␥-rays
to thermal conductivity in the following. Thermal conductivity of DF decreases to 50% by ␥-rays treatment
at every temperature. But the difference of thermal
conductivity by absorbed dose between 0.2 MGy and
0.5 MGy is not recognized at every temperature. With

Figure 8 The relation between thermal conductivity andmolecular weight of DFs with ␥-rays treatment.

2624

YAMANAKA ET AL.

␬ ⫽ 共1/3兲 Cp ␳ v l

Figure 9 Schematic diagram showing the structure of DF
by mechanical model.

tion between the absorbed dose and thermal conductivity.
In the next section, the relation between the decrease of thermal conductivity and molecular chain
scission is discussed. Some kinds of physical properties of DF in the ﬁber direction are explained by theserial-parallel mechanical model composed of crystal
region and amorphous region (Takayanagi Model). In
this model, DF is composed of following two parts byparallel combination. One of them is continuous crystal region (hereinafter Part I) and the other is the series
combination part of crystal and amorphous region(hereinafter Part II) as shown in Figure 4. In previous
articles,3,30 it is reported that thermal conductivity andthermal expansion coefﬁcient of DF are explained bythis serial-parallel mechanical model (Takayanagi
Model). The schematic diagram showing the structureof DF by the mechanical model used in the previous
articles3,30 is shown in Figure 9. In the earlier section,
only the decrease of molecular weight could be observed as the effect of the ␥-rays treatments. Thechange of crystal structure by ␥-rays treatment could
not be observed in the X-ray diffraction. Therefore, theeffect of irradiation with ␥-rays to thermal conductivity is discussed on the assumption that the ␥-rays
treatments induce only the main chain scission andthat the morphology of mechanical model shown in
Figure 9 does not change. Thermal conduction of DF
in ﬁber direction is dominated by the thermal conduction in the continuous crystal region (Part I) shown inthis ﬁgure.30 The chain axis in the crystal regions is
considered as ﬁber axis to discuss the thermal conductivity of DF in ﬁber direction.30 Therefore, the thermal
conduction of DF is explained by the thermal conduction along the covalent bonding in the chain axis in thecontinuous crystal region (Part I).30 Thermal conductivity is shown in the following formula1 in the case ofphonon conduction.

(1)

In this formula, the parameters are deﬁned as follows:␬, thermal conductivity in the ﬁber direction; Cp, heat
capacity at constant pressure; ␳, density of DF; v,sound velocity in ﬁber direction; l, mean free path ofphonon in ﬁber direction. The main chain scission
induces the reduction of the length of molecular chain.
Therefore, it is inferred that the extended molecular
chains composing the crystal region are shortened by
the main chain scission with the ␥-rays treatment. The
reduction of the molecular chain length means thedecrease of the average length between the ends ofmolecular chains. The reduction of the length of molecular chains decreases the mean free path of phonon
in Part I in ﬁber direction, although the morphology ofcontinuous crystal region (Part I) in mechanical model
shown in Figure 9 does not change. Furthermore, it isconsidered that the decrease of mean free path ofphonon on Part I decreases the total thermal conductivity of DF.
As described in the earlier section, the heat capacityof DF decreases slightly by ␥-rays treatment. On theother hand, the decreases of thermal conductivity by␥-rays treatment are about 50%, and it can not be
explained by the slight decrease of heat capacity. Thissuggests that the decrease of thermal conductivity by
about 50% after ␥-rays treatment is caused by thedecrease of the part except the heat capacity in theformula,1 that is to say, sound velocity or mean freepath of phonon. Those suggest the possibility that thedecrease of thermal conductivity is caused by the reduction of mean free path of phonon induced by themain chain scission with ␥-rays treatment. The direct
measurements of thermal diffusivity and sound velocity will be necessary for certiﬁcation of this discussion.
Furthermore, those results suggest the contribution ofhigh molecular weight on the thermal conductivity ofDF.
The relation between the decreasing rate of thermal
conductivity and molecular chain length by ␥-rays
treatment is discussed in the following section. The
molecular weight of DF decreases to 1.7% and 1.3% by
the ␥-rays treatment with the absorbed dose of 0.2
MGy and 0.5 MGy, respectively. It is expected that the
average length of molecular chains decrease by thesimilar rate. In the assumption of the uniformity of themain chain scission in DF, the decreasing rate of thermal conductivity is expected to be the same as that ofmolecular weight, because it is considered that the
reduction of the average length of the molecular
chains contribute to the decrease of the mean free pathof phonon. But the decreasing rate of thermal conductivity by ␥-rays treatment is about 50%. It is considered that the difference of the decreasing rate between
the molecular weight and thermal conductivity is due
to nonuniformity of main chain scission in DF. The

RADIATION EFFECT ON THERMAL CONDUCTIVITY OF UHMW-PE FIBER

reason of nonuniformity of the chain scission is discussed by mechanical model composed of crystal andamorphous region in the following section.
It is known that the main chain scission of PE is
induced by the oxidation by ␥-rays and that it depends on diffusion of oxygen.36 And it is also known
that the diffusion constant of oxygen is larger in amorphous region than in crystal region usually.36 Therefore, the probability of chain scission is considered tobe higher in amorphous region than in crystal region
as shown in Figure 4. It is considered that contribution
to the thermal conduction of DF by Part II in Figure 9
is small. Therefore, it is considered that the chain
scission in amorphous region does not contribute to
the decrease of thermal conductivity of DF mostly.
The decrease of thermal conductivity is considered to
be dominated by the chain scission in crystal region inwhich the probability of chain scission is less than thatin amorphous region. Therefore, the difference of thedecreasing rate between thermal conductivity andmolecular weight by the ␥-rays treatment can be explained by the difference of probability of chain scission between the amorphous and crystal region.
The molecular chains can be oriented by the extension of the molecular chain in DF. The existence ofintermediate region between crystal and amorphous
region are suggested in some articles.43– 45 The investigation of probability of main chain scission in the
crystal and amorphous region will be necessary forquantitative discussion about thermal conductivity of
DF with ␥-rays treatment.

CONCLUSIONS
Thermal conductivity was measured for high-strength
polyethylene ﬁber (DF) with ␥-rays treatment in thetemperature range from 10 to 300 K and the following
conclusions were drawn.
1. The molecular weight of DF decreased from 2⫻ 106 to 3.5 ⫻ 104 by 0.2 MGy absorption and to
2.6 ⫻ 104 by 0.5 MGy absorption with ␥-rays
treatment, respectively. This result showed themain chain scission of DF by the ␥-rays treatment. The melting peaks of DF observed by DSC
became sharper by ␥-rays treatment and thatsuggested the main chain scission in DF by␥-rays treatment.
2. X-ray diffraction of DF did not change by ␥-rays
treatment, and the change of the crystal structureof DF by ␥-rays treatment could not be observed.
3. Heat capacity decreased slightly by ␥-rays treatment.

2625

4. Thermal conductivities of Dfs with and without
␥-rays treatment decreased with decreasing temperature.
5. Thermal conductivity of DF decreased to 50% by␥-rays treatment of absorbed dose with 0.2 MGyand 0.5 MGy.
6. The decrease of thermal conductivity of DF by␥-rays treatment was explained by the molecularmain chain scission and it suggested the possibility of the reduction of mean free path of phonon.
7. The decreasing rates of thermal conductivity by␥-rays treatment were smaller than those of molecular weight. Those differences were explainedby the difference of probability of chain scission
between the amorphous and crystal region.
With the aforementioned, thermal conductivity of
DF decreased by the main chain scissions with ␥-rays
treatment. Those results suggested the contribution of
the length of extended molecular chains because ofhigh molecular weight on the thermal conductivity ofDF.
The authors thank Professor Dr. Manabu Ikebe of Iwate
University for his useful suggestions about measurements ofthermal conductivity. The authors also thank Professor Dr.
Munehisa Yasuniwa of Fukuoka University for useful suggestions about DSC of PE. The authors thank Professor Dr.
Masakatsu Takeo of Kyushu University, Professor Dr. Tomoaki Takao of Sophia University, and Professor Dr. Kenji
Hosoyama of the High Energy Accelerator Research Organization for useful suggestions about the importance of thermal conductivity of materials for applied superconductivity.
Thanks to Ms. Kyoko Saito of the Polymer Institute in Research Center in Toyobo, Co., Ltd. for experimental assistance. Thanks to Dr. Yukihiro Nomura and members in theadvanced textiles R & D center of Toyobo, Co., Ltd. foruseful discussions. Thanks to the Dyneema Department of
Toyobo, Co., Ltd. for encouragement.

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