The Effect of ␥-Irradiation on Thermal Strain of HighStrength Polyethylene Fiber at Low Temperature
Atsuhiko Yamanaka,1 Yoshinobu Izumi,2 Tooru Kitagawa,1 Takaya Terada,2 Hiroshi Hirahata,1Kimiko Ema,2 Hiroyuki Fujishiro,3 Shigehiro Nishijima2
1

Research Center, Toyobo, Co., Ltd., 2–1-1 Katata, 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 19 May 2005; accepted 22 October 2005
DOI 10.1002/app.23550
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: To understand the contribution to negative
thermal expansion by the length of the molecular chains inhigh-strength ultra-high-molecular-weight polyethylene(UHMW-PE) ﬁber, the thermal expansion coefﬁcient in therange of low temperature was investigated for high-strength
UHMW-PE ﬁber (Toyobo, Dyneema®; hereinafter abbreviated to DF), irradiated by ␥-rays (␥-rays treatment) thatinduce the molecular scission. The molecular weight of DF
decreased by ␥-ray treatment. X-ray diffraction behavior did
not change by ␥-ray treatment. The melting behavior observed by DSC showed the main chain scission of DF by␥-ray treatment. The DFs, with and without ␥-ray treatment,
expand by cooling down (negative thermal expansion). Thechange of negative thermal expansion of DF by ␥-ray treatment was small. It is suggested that negative thermal expansion does not change by only the molecular chain scission.
These results suggested that the effect of negative thermal
expansion of DF in the temperature range from 213 to 303 K
by the molecular chain scissions is small and that the lengthof extended molecular chains contributes to a negative thermal expansion a little. © 2006 Wiley Periodicals, Inc. J Appl

INTRODUCTION

It is known from X-ray studies that the thermal
expansion coefﬁcient of polymer crystals, for example,
PE in the direction of the chain axis measured, isnegative for most if not all polymers20 –22 The negative
thermal expansion is explained by “Linear Chain
Model” and so on.23–25 Therefore, highly oriented andhighly crystallized PE shows a negative thermal expansion in the oriented direction. This thermal expansion can be treated by a mechanical model composedof a crystal part showing a negative thermal expansion
and an amorphous part showing a positive thermal
expansion (Takayanagi model).22,26
The DF is a high-crystallinity and high-strength ﬁber made from ultra-high-molecular-weight PE(UHMW-PE).26 –29 As aforementioned, previous reports showed that DF has a negative thermal expansion coefﬁcient.1–3 A previous report elucidated thatthe negative thermal expansion of DF is explained by
the serial–parallel mechanical model composed of
crystal and amorphous region.3 In the case of DF, it isconsidered that the negative thermal expansion ofextended molecular chains in continuous crystal partin chain axis contributes to that of DF in the ﬁber
direction.3 Meanwhile, the effect on negative thermal
expansion of the length of extended molecular chainsby using the UHMW-PE has not been investigatedsystematically. Therefore, the relation between the

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, nuclear
fusion reactors, and so on. To apply polymeric materials to these areas, their thermal properties, and inparticular their coefﬁcient of linear expansion andtheir thermal conductivity, have been important parameters to study. For example, high-strength polyethylene (PE) ﬁber (Toyobo, Dyneema®; hereinafter
abbreviated to DF) possesses a negative coefﬁcient oflinear expansion toward the direction of the ﬁber
axis.1–3 Typical applications for these materials are ascoil bobbins4 –14 and spacers8,13,15,16 for super conductors and package of Fiber Bragg Grating for opticalﬁlter.17,18 Those materials are desired not only as thermal insulators but also as electrical insulating thermal
conductors for thermal conductivity.16,19
Correspondence to: A. Yamanaka (Atsuhiko_Yamanaka@kt.
toyobo.co.jp).
Journal of Applied Polymer Science, Vol. 102, 204 –209 (2006)© 2006 Wiley Periodicals, Inc.

Polym Sci 102: 204 –209, 2006

Key words: negative thermal expansion; thermal strain; ultra-high-molecular-weight high-strength polyethylene ﬁber;
␥-rays; scission

EFFECT OF ␥-IRRADIATION ON THERMAL STRAIN

205

negative thermal expansion and the molecular lengthof DF is investigated in this work.
It is well known that PE undergoes main chain
scission by irradiation with ␥-rays in the presence ofoxygen.31,32 In this article, we report the thermal expansion coefﬁcients of DFs affected by main chain
scission and induced by irradiation with ␥-rays, forinvestigation of the relation between the thermal expansion coefﬁcient and the length of main chain, toclarify the effect of negative thermal expansion on the
chain length of DF.
EXPERIMENTAL
SampleIn this work, thermal expansion coefﬁcients of DF andthose irradiated with ␥-rays (␥-ray treatment) were
measured in the range of low temperatures. The used
DF was Dyneema®, 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 and83.3 kGy/h. Irradiation time was 6 h. The total absorbed doses were 0.2 and 0.5 MGy, respectively.
These are hereinafter abbreviated to DF (0.2 MGy) andDF (0.5 MGy). The DF without ␥-rays treatment isabbreviated 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 ␥-ray treatment, were measured by the viscosity method, using decalin solution. The DFs, withand without ␥-ray treatment, were dissolved in decalin with a concentration of 0.0045 g/dL. The relativeviscosities at 408 K were measured by using Ubbelhode viscometers in a thermostat. This method wasreported in elsewhere.33
Crystallinity and orientation
X-ray diffraction and melting behavior by differential
scanning calorimetry (DSC) were observed, to investigate the relative change of crystallinity of DFs by␥-ray treatment. A Rigaku RU-200 (40 kV ⫻ 100 mA)
was used with X-ray diffraction on Ni-ﬁltered Cu K␣(␭ ⫽ 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
weighted at 2 mg, and the melting experiments were

Figure 1 Dependence of molecular weight of DF on absorbed dose irradiated by ␥-rays. [Color ﬁgure can beviewed in the online issue, which is available at www.
interscience.wiley.com.]made in the temperature range from 323 to 523 K at aheating rate of 1°/min.
Thermal expansion coefficients
Thermal expansion coefﬁcients of DFs were measured
by thermomechanical analysis (TMA) on tensile mode.
TMA measurements were carried on a MAC Science
TMA 400S. Samples were run in TMA in the temperature range from 213 to 303 K, at a scanning rate of
5°/min. Sample length was 20 mm. The loading was
240 g/mm2.
RESULTS AND DISCUSSION
Molecular weight
The relation between the viscosity-average molecularweight of DF and absorbed dose is shown in Figure 1.
The measured molecular weight of DF (Blank) was 2.0⫻ 106. It decreases to 3.5 ⫻ 104 by 0.2 MGy irradiationand to 2.6 ⫻ 104 by 0.5 MGy irradiation, respectively.
This result shows what the main chain scission of DF
undergoes by the ␥-ray treatment.
In the irradiation effects of PE in air, the free radicals
of alkyl type are initially produced by irradiation, 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.
Therefore, the irradiation effects of PE in air dependon the diffusion of oxygen.31,32 It is considered thatoxygen invades into the amorphous region more easily than into the crystal region in DF. Therefore, it is206

Figure 2 X-ray diffraction proﬁles of DFs irradiated andunirradiated with ␥-rays. [Color ﬁgure can be viewed in the
online issue, which is available at www.interscience.
wiley.com.]considered that the probability of main chain scission
is higher in amorphous region than in crystal region in
DF.
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 ␥-ray treatment are investigated. The width, shapes, intensities, and reﬂection
angles of those peaks do not change by ␥-ray treatment. This result suggests that the crystal size in (110)and (200) directions do not change. From the independence of X-ray behavior with ␥-ray treatment, it isconsidered that the crystal of DF is hardly destructedby ␥-rays. The crosslinking reactions dominate in thecase of irradiation of PE in the vacuum. It is reported
that X-ray diffraction proﬁle in equatorial direction
does not change the irradiation of ␥-rays, in which theabsorbed dose is less than 0.25 MGy in vacuum34 as
like the case of this work.

YAMANAKA ET AL.

In the next, the radiation effect of melting behavior isinvestigated. 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 three to two by␥-ray treatment. This change suggests the decrease ofrestraint on molecular chains in crystal region with ␥-ray
treatment. It is considered that the molecular chains in
amorphous region are extended, and are restrained byorientation in the drawing. 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.
From the X-ray diffraction in the previous section, thecrystal size is inferred to decrease not so much by ␥-ray
treatment. But it is considered that the molecular chainsin amorphous region become more movable by main
chain scission with ␥-ray treatment and that the interactions of molecular chains between the crystal and amorphous regions decrease very much. The sharpening of
the peaks in melting behavior is inferred to be caused bydecreasing those interactions by main chain scissions by
␥-rays.
The melting enthalpies (⌬H) and melting temperatures (Tm) of DF, with and without ␥-ray treatment,
are shown in Figure 4. Both of them decrease by ␥-ray
treatment even though X-ray diffraction behavior does
not change. The differences of ⌬H and Tm of DF,
between with and without ␥-ray treatment, are remarkable. On the other hand, the differences of ⌬H
and Tm are small, with an absorbed dose between 0.2
and 0.5 MGy. It is considered that the decrease of ⌬H
and Tm with ␥-ray treatment is caused by the decrease

Melting behavior
The melting behaviors of DFs observed by DSC areshown in Figure 3. DF (Blank) shows three broad
melting peaks in the temperature range from 403 to423 K. The melting behaviors of high-strength PE ﬁber
as like DF are reported in other papers.35–37 Although
the assignments of the peaks in melting process ofhigh-strength PE ﬁber are not clariﬁed, it is reported
that UHMW-PE molecular chains extended by thetensile stress show the melting behaviors35–37 as likethat of DF (Blank).

Figure 3 DSC melting curves of DFs. [Color ﬁgure can beviewed in the online issue, which is available at www.
interscience.wiley.com.]EFFECT OF ␥-IRRADIATION ON THERMAL STRAIN

207

Figure 4 Melting enthalpy (⌬H) and temperature (Tm) of
DFs. [Color ﬁgure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]of interactions described in the previous paragraph bymain chain scission. The detailed discussion about
structural analysis will be necessary to clarify themechanism of the melting behavior mentioned earlier.
The melting behavior observed by DSC also suggested
the main chain scission in DF by ␥-ray treatment.
From the results of molecular weights, X-ray diffraction, and melting behavior, main chain scissions occurred by ␥-ray treatment, with an absorbed dose of
0.2 and 0.5 MGy, though the crystal region did notchange distinctly.
Thermal expansion coefficients
Thermal strains of DFs in ﬁber direction measured by
TMA are shown in Figure 5. All of them expand bycooling down (negative thermal expansion).
We investigate the effect of irradiation with ␥-rays
to thermal strain in the following. Thermal strain does
not change by ␥-ray treatment mostly. Although thetemperature dependence of thermal strain becomesslightly straight by ␥-ray treatment, the change is verysmall. Comparing DF (0.5 MGy) with DF (0.2 MGy),
the inclination of DF (0.5 MGy) is a little smaller thanthat of DF (0.2 MGy) and those show similar behaviors. The average thermal expansion coefﬁcients of
DFs are estimated by the inclinations of the thermal
strains of DFs, shown in Figure 5, in the range from
250 to 300 K. All samples show an almost straight linein this temperature range.
The relation between the negative thermal expansion and absorbed dose of ␥-rays is discussed here. As
aforementioned, those ␥-ray treatments induced only
a decrease of molecular weight, that is to say, thereduction of length of molecular chain. Crystal structure did not change by ␥-ray treatment in the X-ray

Figure 5 Thermal strain of DFs with and without ␥-rays
treatment in ﬁber direction. [Color ﬁgure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]diffraction measurement. The average thermal expansion coefﬁcients, absorbed dose with ␥-rays, and average molecular weight of DFs, with and without
␥-rays treatment, are shown in Table I. Although negative expansion shows the tendency to decrease a little
by irradiation, that is, the molecular chain scission, thechange is small.
In the next, the relation between thermal expansion
coefﬁcient and molecular chain scission is discussed.
Some kinds of physical properties of DF in the ﬁber
direction are explained by the serial–parallel mechanical model composed of a crystal region and amorphous region (Takayanagi model). In this model, DF is
composed of the following two parts by parallel combination. One of them is the continuous crystal region(hereinafter Part I) and the other is the series combination part of crystal and amorphous region (hereinafter Part II). In previous papers,3 it is reported that
thermal expansion coefﬁcient of DF is explained by
TABLE I
Molecular Weights and Thermal Expansion Coefﬁcientsof DFsSampleAbsorbed
dose
(MGy)

Averagemolecular
weight(104)

Thermal expansion
coefﬁcient at 253–
303 K (10⫺6/K)

DF (Blank)
DF (0.2MGy)
DF (0.5MGy)

0
0.2
0.5

200
3.5
2.6

⫺12.0
⫺11.4
⫺10.5

208

YAMANAKA ET AL.

Figure 6 Schematic diagram showing the structure of DF bya mechanical model. [Color ﬁgure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]this serial–parallel mechanical model (Takayanagi
Model). The schematic diagram, showing the structureof DF by the mechanical model used in the previous
papers,3 is shown in Figure 6. The
thermal expansion coefﬁcients of DFs are estimatedby the serial–parallel mechanical model using the following formulas:␣ I ⫽ ␣c(1)

␣ II ⫽ XIIc␣c ⫹ XII␣a(2)

␣ ⫽ 关y IEI␣I⫹yIIEII␣II兴/关yIEI ⫹ yIIEII](3)

E ⫽ 关y IEI ⫹ yIIEII](4)

E I ⫽ Ec

(5)

E II ⫽ EcEc/关XIIcEa ⫹ XIIaEc兴

(6)

X IIc ⫹ XIIa ⫽ 1(7)

yI ⫹ yII ⫽ 1

In these formulas, thermal expansion parameters
are deﬁned as follows: ␣, thermal expansion coefﬁcient of DF; ␣I, thermal expansion coefﬁcient of part I;
␣II, thermal expansion coefﬁcient of part II; ␣c, thermal
expansion coefﬁcient of crystal part in chain axis; ␣a,thermal expansion coefﬁcient of amorphous region; E,
total tensile modulus of DF in ﬁber direction; EI, tensile modulus of part I; EII, tensile modulus of part II;
Ec, tensile modulus of crystal in chain direction; Ea,
tensile modulus of amorphous; xIIc, length of crystal
part in part II; xIIa, length of amorphous part in part II;

yI, width of part I; yII, width of part II. xIIc, xIIa, yI, andyII are the parameters of the mechanical model shownin Figure 6. va ⫽ xIIayII is the volume fraction of theamorphous part.
From the result of X-ray diffraction described inprevious section, it is considered that the morphology
of crystal and amorphous does not change mostly by␥-ray treatment. Therefore, it is also considered thatthe parameters of mechanical model, xIIc, xIIa, yI, andyII, do not change. Ec is reported as 235–250
GPa,20,38 – 40 315 GPa,41 and Ea is reported as 0.3 GPa.24
The reported values of Ec and Ea and the mechanicalmodel shown in Figure 6 suggest that the thermal
expansion coefﬁcient of DF in ﬁber direction is dominated by that in continuous crystal region (part I).3
The thermal expansion coefﬁcient of part I (␣I), thatis to say, that of PE crystal in chain axis (␣c) was
estimated by X-ray measurements and calculations inprevious papers.20 –26 Although the weight-averagemolecular weight of PE samples used in these works
were various values in the range from 104 to 106, theestimated crystal thermal expansion coefﬁcients inchain axis were almost equal to ⫺12 ⫻ 10⫺6/K as sameas the calculated value.20 –22 Therefore, it is consideredthat the thermal expansion coefﬁcient of crystal inchain axis (␣c) does not depend on the molecularweight, in the range from 2.6 ⫻ 104 to 2.0 ⫻ 106.
Furthermore, the reported crystal tensile modulus (Ec)does not show the remarkable dependence on molecular weight.20 –22,38 – 40 Therefore, it is considered that
␣c and Ec described in the formulas from eqs. (1)–(7)and the mechanical model do not change by decreasing molecular weight with ␥-ray treatment.
As aforementioned, it is inferred that the parameters in the mechanical model xIIc, xIIa, yI, yII, ␣c, and Ec
do not change by the ␥-ray treatment, in the range ofabsorbed dose in this experiment. And it is suggested
that thermal expansion coefﬁcient in ﬁber direction
does not change in this case and that only the molecular weight changes and the morphology of crystal
and amorphous does not change. The direct measurements of tensile modulus (Ec) and thermal expansion
coefﬁcient (␣c) of crystal will be necessary for thecertiﬁcation of aforementioned discussions.
As shown in the result of DSC, the interactions
between crystals or between the crystal and amorphous regions are considered to decrease by molecular chain scissions with ␥-ray treatment. It is expectedthat the effect of thermal strain can be observed in the
low temperature range under the glass transition temperature (Tg; about 170 K). In the low temperature
range, the contribution of amorphous to thermal expansion coefﬁcient is expected to increase with increasing the tensile modulus of amorphous region.
The measurements of thermal strain in the low temperature range will be necessary for those discussions.

EFFECT OF ␥-IRRADIATION ON THERMAL STRAIN

CONCLUSIONS
Thermal expansion coefﬁcients were measured forhigh-strength PE ﬁber (DF) with ␥-ray treatment in thetemperature range from 213 to 303 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 ␥-ray treatment, respectively. This result showed the main
chain scission of DF by the ␥-ray treatment. The
melting peaks of DF observed by DSC became
sharper by ␥-ray treatment, which suggested themain chain scission in DF by ␥-ray treatment.
2. X-ray diffraction of DF did not change by ␥-ray
treatment, which suggested that the crystal structure of DF did not change by ␥-ray treatment.
3. All DFs with and without ␥-ray treatment
showed negative thermal expansion coefﬁcients.
4. The change of negative thermal expansion of DFby ␥-ray treatment was small. It is suggested thatnegative thermal expansion does not change by
only the molecular chain scissions.
As aforementioned, it was suggested that the effect ofnegative thermal expansion of DF in the temperature
range from 213 to 303 K by the molecular chain scissions
is small and that the length of extended molecular chainscontribute to negative thermal expansion a little.
The authors thank Professor Dr. Munehisa Yasuniwa of
Fukuoka University for useful suggestions about DSC of PE.
The authors also thank Professor Dr. Masakatsu Takeo of
Kyushu University, Professor Dr. Tomoaki Takao of Sophia
University, Professor Dr. Kenji Hosoyama of the High Energy Accelerator Research Organization and Professor Dr.
Manabu Ikebe of Iwate University for useful suggestions
about the importance of thermal expansion of materials forapplied superconductivity. Thanks to Ms. Kyoko Saito of thePolymer Institute in Research Center in Toyobo, Co., Ltd. forexperimental assistance. Thanks to Dr. Yukihiro Nomura
and members in the advanced textiles R and D center of
Toyobo, Co., Ltd. for useful discussions. Thanks to the
Dyneema Department of Toyobo, Co., Ltd. for encouragement.

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