Interaction of 8 MeV Electron beam with P31 <i>Bombyx mori</i> Silk Fibers

doi:10.4236/msa.2011.27112

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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.

Email: [email protected]

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:

 





Is=ncos2nd ss







 (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:





AnAn An



(2)

For a paracrystalline material, Ad(n) can be obtained,

with Gaussian strain distribution [19],









22 2

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:

  

An 1

nd diP n dinP i di



 







(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)

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

3257

2929 2345

1744

1457

1165

867

Transmittance (%)

Wavenumber (cm-1)

(a)

3500 3000 2500 2000 1500 1000

2945

3311

3201 1650

1377

1004

Transmittance (%)

Wavenum ber (cm-1)

(b)

3500 3000 2500 2000 1500 1000

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

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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