Materials Sciences and Applicatio n, 2011, 2, 827-833
doi:10.4236/msa.2011.27112 Published Online July 2011 (http://www.SciRP.org/journal/msa)
Interaction of 8 MeV Electron Beam with P31
Bombyx mori Silk Fibers
Sangappa Halabhavi1*, Sangappa Asha1, Puttanna Parameswar2, Rudrappa Somashekar2,
Sanjeev Ganesh3
1Department of Studies in Physics, Mangalore University, Mangalagangotri, India; 2Department of Studies in Physics, University of
Mysore, Mysore, India; 3Microtron Center, Mangalore University, Mangalagangotri, India.
Received December 2nd, 2010; revised March 21st, 2011; accepted May 21st, 2011.
ABSTRACT
The 8 MeV electron radiation-induced changes in physical and thermal properties in P31 (Bombyx mori) silk fibers
were investigated and has been correlated with the applied radiation doses. Irradiation of fiber samples were carried
out in dry air at room temperature using an electron beam accelerator for varied radiation doses in the range of 0 - 100
kGy. Physical properties of the irradiated silk fibers were studied using XRD, FT-IR and thermogravimetric analysis
(TGA) and compared with unirradiated fiber sample. Interesting results are discussed in this report.
Keywords: Fiber, Irradiation, Physical, Thermal Properties
1. Introduction
For several centuries, silk of Bomyx mori has been used
as a textile fiber [1] and there is rigorgitated research in
recent times to produce application oriented silk fibers.
These changes can be brought about by genetically
modifying the cocoons or by induced modification
brought about with the high energetic particle interaction
with silk fibers. These changes are quantified interms of
microstructural parameters of silk fibers. These parame-
ters and their magnitude influence the property and
strength of the fibers. Such studies have not been carried
out except for the chemical effects on these fibers [2-7].
Okuyama’s group [8] have reported crystal structure for
silk-I and silk-II fibers. Somashekar’s group [9,10] have
reported the effect of degumming and dye processing on
the microstructural parameters in pure Mysore silk, nistari,
NB7 and NB18 silk fibers. Sangappa et al. have studied
microstructural parameters in nistari, Cnichi, Hosa Mysore
and pure Mysore silk fibers [11,12]. Takeshita and group
[13] have studied the effect of electron beam irradiation on
silk fibers. Effects of gamma irradiation on biodegradation
of Bombyx mori silk fibers have been carried out by Ko-
jthung’s group [14]. Recently we have reported the
changes in polymers like HPMC due to irradiation.
At the microscopic level, the polymer degradation is
characterized by macromolecular chain splitting, creation
of low mass fragments, production of free radicals, oxi-
dation and cross-linking. These effects the macroscopic
properties like mechanical strength, color, electrical
conductivity and so on [15]. The resulting change in the
properties of the polymer may extend the range of appli-
cations for the material [16]. The study of the micro-
structural parameters, chemical and thermal properties are
of great importance, especially when the polymer proper-
ties are being modified by ionizing radiation. In this paper
we have investigated the microstructuralline changes
(physical), structural changes and thermal stability of the
P31 Bombyx mori silk fiber by X-ray diffraction study
(WAXS), FT-IR spectra analysis and thermogravimetric
(TGA) analysis after electron beam (EB) irradiation.
2. Experimental
2.1. Sample Preparation
For our study we have used raw P31 silk fiber belonging
to Bombyx mori family which comes under the classifi-
cation Multivoltine on the basis of shape, color, denier
and life cycle of the fibers/cocoons. Cocoons were col-
lected from the germ plasma stock of the Department of
Sericulture, University of Mysore India, which were then
cooked in boiling water (100˚C) for 2 min. to soften the
sericin and transferred to water bath at 65˚C for 2 min.
Then the cocoons were reeled in warm water with the

Interaction of 8 MeV Electron beam with P31 Bombyx mori Silk Fibers 828
help of mono cocoon reeling equipment EPPROUVITE.
The characteristic features of these fibers are that they
are white in color with an average filament length of 350
meter and denier being in the range 1.8 - 2.0. These fi-
bers were mounted on rectangular frame in just taut con-
dition which does not involve any mechanical stretching
of fibers. The whole process, starting from reeling to
mounting of fibers, does not involve any type of me-
chanical deformation.
2.2. Electron Beam Irradiation
Irradiation of P31 silk fibers were done at Microtron
Center, Mangalore University using the electron beams
(by lanthanum hexa fluorite source). The monochromatic
beam is made to fall on samples kept at particular
distance and the following beam features given in Table
1. The dose delivered to different samples is measured by
keeping alanine dosimeter with sample during irradiation.
The samples were subjected to various integral doses,
which were accumulated in steps, where the irradiation
doses were conducted at 0, 25, 50, 75 and 100 kGy.
2.3. Polymer Characterization
P31 Bombyx mori silk fibers (raw) and 8 MeV electrons
beam irradiated were characterized by different tech-
niques such as X-ray diffraction (XRD), Fourier trans-
forms IR (FT-IR) spectroscopy and thermal technique,
like thermogravimetric analysis (TGA) was used.
2.3.1. X-Ray Diffraction Measurements
The XRD diffractograms of the fiber samples were re-
corded using a Rigaku Miniflex-II X-ray diffractometer
with Ni filtered, CuKα radiation of wavelength λ =
1.5406 Å, with a graphite monochromator. The scattered
beam was focused on a detector. The specifications used
for the recordings were 40 kV, 30 mA. The samples were
scanned in the 2θ range 10˚ - 50˚ with a scanning speed
and step size of 1˚/min and 0.01˚ respectively and dif-
fraction scans are given in Figure 1.
Table 1. Specifications of the electron beam accelerator and
irradiation conditions.
1. Beam energy 8 MeV
2. Beam current 20 mA
3. Pulse repetition rate 50 Hz
4. Pulse width 2.2 µs
5. Distance source to sample 30 cm
6. Time of exposure 25 min
7. Dose range 0 - 100 kGy
8. Atmosphere air
9. Temperature 24˚C
10 15 20 25 30 35 40
100 kGy
50 kGy
25 kGy
0 kGy
2 theta
Intensity (a.u)
Figure 1. XRD scans of pure and 8 MeV electron irradiated
polymer samples.
2.3.2. FT-IR spectra
Fourier transform infrared (FT-IR) spectra of the virgin
and 8 MeV EB irradiated P31 Bombyx mori silk fiber
samples was recorded in transmission mode Thermo Nico-
let, Avatar 370, FTIR spectrophotometer having a resolu-
tion 4 cm-1 in the wave number range 500 - 4000 cm–1.
2.3.3. Thermogravimetric Spectra
The samples were weighed in microbalance and crimped
in aluminum pans. Thermogravimetric analysis (TGA) of
these samples was done by Perkin Elmer Thermal Analy-
sis system with nitrogen as flushing gas. The temperature
range scanned was 25˚C - 800˚C at a predetermined rate
of 10˚C/min. The thermogram, i.e., a plot of weight per-
cent as a function of temperature, was used to study the
variation in the thermal stability of the polymer. Samples
of 1.565 - 7.623 mg were used for the measurement.
3. Theory
Microstructural parameters such as crystal size (N) and
lattice strain (g in %) are usually determined by employ-
ing Fourier method of Warren and Averbach [17], and
Warren [18]. The intensity of a profile in the direction
joining the origin to the center of the reflection can be
expanded in terms of Fourier cosine series:
   
o
Is=ncos2nd ss
n
A



 (1)
where the coefficients of the harmonics A(n) are func-
tions of the size of the crystallite and the disorder of the
lattice. Here, s is sin (θ)/(), so being the value of s at the
peak of a profile, n is the harmonic order of coefficient,
and d is the lattice spacing. The Fourier coefficients can
be expressed as:


sd
AnAn An

(2)
For a paracrystalline material, Ad(n) can be obtained,
with Gaussian strain distribution [19],




22 2
d
Anexp2mn g (3)

Interaction of 8 MeV Electron beam with P31 Bombyx mori Silk Fibers 829
where “m” is the order of the reflection and g = (d/d) is
the lattice strain. Normally one also defines mean square
strain 2, which is given by g2/n. This mean square
strain is dependent on n, whereas not g [20,21]. For a
probability distribution of column lengths P(i), we have:
  
s
00
An 1
nn
nd diP n dinP i di
DD

 



(4)
where D = Ndhkl is the crystallite size and “i” is the
number of unit cells in a column. In the presence of two
order of reflections from the same set of Bragg planes,
Warren and Averbach [17,18] have shown a method of
obtaining the crystal size (N) and lattice strain (g in %).
But in polymer it is very rare to find multiple reflections.
So, to determine the finer details of microstructure, we
approximate the size profile by simple analytical function
for P(i) by considering only the asymmetric functions.
Another advantage of this method is that the distribution
function differs along different directions. Whereas, a
single size distribution function that is used for the whole
pattern fitting, which we feel, may be inadequate to de-
scribe polymer diffraction patterns [20-22]. Here it is
emphasized that the Fourier method of profile analysis
(single order method used hare) is quite reliable one as
per the recent survey and results of round Robin test
conducted by IUCr [23]. In fact, for refinement, we have
also considered the effect of back-ground by introducing
a parameter [see for details regarding the effect of back-
ground on the microcrystalline parameters]. For the sake
of completeness, we reproduce the following equations
which are used in the computation of microstructural
parameters.
(a) (b)
(c) (d)
Figure 2. (a-d) Experimental and simulated intensity pro-
files of X-ray reflection of Silk fiber samples obtained with
Exponential column length distribution function.
and experimental profiles for 8 MeV electron irradiated
and pure fiber sample for Bragg’s (110) reflection. The
simulated profile was obtained with the using appropriate
model parameters. This procedure was followed for all
the other treated samples at different radiation doses. The
computed microcrystalline parameters such as crystallite
size <N>(number of unit cells), lattice strain g in %, the
width of the crystallite size distribution (α) and the stan-
dard deviation are given in Table 2.
The Exponential Distribution
It is assumed that there are no columns containing
fewer than p unit cells and those with more decay expo-
nentially. Thus, we have [24],
P(i) = { 0 ; if p  i { exp {–(i – p)} ; if p  i (5) For the pure sample the surface weighted crystallite
size is 30.30 Å, in the case of 25 kGy irradiated fiber
sample it is 27.13 Å, in 50 kGy EB irradiated fiber sam-
ple surface weighted crystallite size is 29.56 Å, and in
the case of 100 kGy the size is 30.88 Å. When the poly-
mer is subjected to the ionizing radiation like EB irradia-
tion mainly two processes occurs, 1) Chain scissioning 2)
Cross-linking. Initially these two processes simultane-
ously occur at lower doses and later the cross-linking
dominates. From the wide angle X-ray scattering
(WAXS) study of electron irradiated P31 silk fiber
(Bombyx mori) samples, we have observed a significant
change in the values of microstructural parameters
changes after high energy electron irradiation. This
causes the cross-linking of small polymer units leading to
where  = 1/(N – p) Substituting this in Equation (4), we
get:
As(n) = { A(0) (1 – n / N) ; if n  p {A(0) {exp[–(n –
p)]} / (N); if n  p (6)
where  is the width of the distribution function, “i” is
the number of unit cells in a column, n is the harmonic
number, p is the smallest number of unit cells in a col-
umn and N, the number of unit cells counted in a direc-
tion perpendicular to the (hkl) Bragg plane.
4. Results and Discussion
4.1. X-Ray Profile Analysis Study
Figures 2(a-d) show the comparison between simulated

Interaction of 8 MeV Electron beam with P31 Bombyx mori Silk Fibers 830
Table 2. Microstructural parameters of Electron irradiated polymer samples computed by Exponential distribution func tion.
Sample <N> g in % α* Ds (Å) delta
0 kGy 7.01 ± 0.34 0.5 ± 0.02 0.013 30.35 0.049
25 kGy 7.01 ± 0.34 0.5 ± 0.02 0.012 27.13 0.042
50 kGy 6.90 ± 0.34 0.5 ± 0.02 0.013 29.56 0.049
100kGy 7.11 ± 0.43 0.1 ± 0.01 0.003 30.88 0.061
the formation of a rigid 3-dimensional network. Increas-
ing crystallite size with increasing irradiation dose shows
that there is an increase in sample tensile strength of the
fiber [25]. This aspect suggests that for 100 kGy integral
dose irradiated fiber, there is an increase in formation of
cross-linking bonds between inter polymer chains than
unirradiated one. Also, it essentially implies, the effi-
ciency of cross-linking in these fibers is improved to a
large extent by means of electron irradiation. We have
observed that the lattice strain and its variation for vari-
ous values of the radiation doses (kGy) in polymer sam-
ples are very small and this may be due to inherent model
dependent factor.
4.2. FT-IR Spectral Analysis
The FT-IR absorbance spectra for the virgin and the 8
MeV electron irradiated P31 Bombyx mori silk fiber
samples are shown in Figures 3(a-c). Silk fibroin con-
formation was often investigated using FT-IR spectros-
copy since the FT-IR spectrum represents typical absorp-
tion bands sensitive to the molecular conformation. The
FT-IR spectrum of pure silk fiber showed strong ab-
sorption bands of amide-І, which is useful for the analy-
sis of the secondary structure of proteins and is mainly
related with the C=O stretching, and it occurs in the
range of 1700 - 1750 cm–1( Figure 3(a) ). Amide-ІІ,
which falls in 1450 - 1480 cm–1 range, is related with the
N-H bonding and C-H stretching vibration. Amide-ІІІ
occurs in the range of 1160 - 1210 cm–1 and it results
from in phase combinations of C-N stretching and C=O
bending vibration [26]. 50 kGy irradiated fiber sample
shows some absorption bands. Characteristics absorption
bands assigned to the peptide bonds (-CONH-) that
originates known as amide-I amide-II and amide-III
which are not observed in the higher dose (100 kGy)
electron irradiated samples. With an increasing the irra-
diation dosage there are no characteristic absorption
bands when comparing the pure and lower dose irradi-
ated samples. This may be due to cross linking of the
fiber. This is also supported by XRD and TGA analysis.
4.3. Thermogravimetric Analysis Study
The thermograms obtained for the raw and the irradiated
P31 silk fibers are shown in Figure 4. The thermograms
of both the raw and 8 MeV electrons irradiated P31 silk
fiber showed three distinct regions. For the virgin silk
fiber first region, starting from room temperature up to
218˚C the weight loss is due to water vaporization (dry-
ing). The weight change was not significant and the sam-
ple was thermally stable. In the second, rather narrow
region from 218˚C to 398˚C the fiber experienced a great
weight loss, because of the thermal decomposition.
About 65% of the sample decomposed into volatiles.
After 398˚C the fiber was slowly decomposed and about
90 of the sample decomposed into volatiles [27]. In the
case of electron beam irradiated fiber the thermograms
showed into three distinct regions. In the 50 kGy EB ir-
radiated P31 silk fiber, the first region, starting from
room temperature up to 237˚C the weight loss is due to
water vaporization. The weight change is not significant
and the irradiated fiber is thermally stable. In the second
region from 237˚C to 419˚C, the fiber experienced a
great weight loss because of the thermal decomposition.
About 60% of the sample decomposed into volatiles. At
600˚C about 74% decomposed and at 800˚C about 80%
decomposed.
In the 100 kGy dose irradiated fiber sample also
showed the three regions and at 400˚C, about 54% of the
sample decomposed. At 600˚C about 66.5% decomposed,
at 800˚C about 70% (Table 3) decomposed into volatiles.
From this study it is clear that as the irradiation dose in-
creases the cross-linking of the polymer increases and
thermal stability of the irradiated samples increases. The
weight loss decreases as irradiation dose increases.
5. Conclusions
From the wide angle X-ray scattering (WAXS) study of
electron irradiated P31 silk fiber (Bombyx mori) samples,
we have observed a significant change in the values of
micro structural parameters occurs. This is because of the
cross-linking of small polymer units leading to the for-
mation of a rigid 3-dimensional network. FT-IR study of
8MeV electron irradiated polymer fibers undergoes
structural modifications.
From the TGA thermograms, we noticed the following:
1) an increasing irradiation dose resulted in an increase in
the thermal stability of silk fibers. 2) For all the investi-
gated samples, there was almost 10% - 15% weight loss
due to water vaporization and decomposition temperature
increases with increasing dose, this is because of the

Interaction of 8 MeV Electron beam with P31 Bombyx mori Silk Fibers 831
3500 3000 2500 2000 1500 1000
40
50
60
70
80
3257
2929 2345
1744
1457
1165
867
Transmittance (%)
Wavenumber (cm-1)
(a)
3500 3000 2500 2000 1500 1000
50
52
54
56
58
60
62
64
2945
3311
3201 1650
1377
1004
Transmittance (%)
Wavenum ber (cm-1)
(b)
3500 3000 2500 2000 1500 1000
40
50
60
70
80
90
100
1615 1513
1397
1225
1041
Transmittance (%)
Wavenumber (cm-1 )
(c)
Figure 3. FT-IR spectra’s (a) virgin (b) 50 kGy (c) 100 kGy EB irradiated samples.
Table 3. Temperature of decomposition at different weight loss (%) of P31 Silk fiber before and after EB irradiation.
Temperature (˚C) Irradiation
Dose (kGy) 100 200 300 400 500 600 700 800
0 10.45 10.77 32.07 62.64 72.96 77.75 82.75 87.39
50 09.39 10.24 28.55 61.24 68.91 74.24 77.75 80.09
100 07.79 08.64 26.42 54.64 68.91 66.25 68.91 69.98

Interaction of 8 MeV Electron beam with P31 Bombyx mori Silk Fibers 832
100 200 300 400 500 600 700 800
0
20
40
60
80
100
100 kGy
50 kGy
0 kGy
Weight loss (%)
Temperature (oC)
Figure 4. TGA thermograms of (a) virgin (b) 50 kGy (c) 100
kGy EB irradiated samples.
cross-linking of P31 silk fiber due to EB irradiation.
6. Acknowledgements
The authors are thankful to University Grant Commis-
sion, New Delhi, Govt of India, for providing financial
assistance through a project F. No. 33-14/2007 (SR). The
authors are also thankful to STIC Cochin University for
extending the experimental facilities.
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