Download The Study of structures of silk fibers grafted with hexafluorobutyl

2006 International Forum on Textile Science and Engineering for Doctoral Candidates
The Study of structures of silk fibers grafted with
hexafluorobutyl mathacrylate
ZHANG Jun, CHEN Guo-qiang*
(Institute of Material Engineering, Soochow Uinversity, Suzhou Jiangsu 215021, China)
Abstract The relationship between the graft yield and the effect of hexafluorobutyl mathacrylate
graft treatment on the structural changes of the silk fibers was studied on the basis of the results of
scanning electron micrograph photographs(SEM), infrared spectroscopy (IR), Raman spectrum,
wide-angle X-ray diffraction patterns(WAXD), nuclear magnetic resonance(NMR) and amion acid
analysis. The results showed that the crystalline regions of grafted fibers were hardly affected and the
fiber fission occurred on the cross sections of grafted fibers. The surface of fibers was covered with a
high polymer film. The Raman spectrum showed there was little change in the conformation of
grafted fibers which mainly remained β-sheet conformation. The IR of the grafted silk fibers showed
new absorption of bands occurred which belonged to the stretching- vibration-absorption-peak bands
of VC=O and VC-F of aliphatic ester species. The CF3—,—CF2— and —CFH— structures of grafted
silk macromolecule were verified in the nuclear magnetic resonance spectrum(NMR). The amion acid
analysis indicated fluoride monomers were inclined to graft with TYR, ARG and GLU of silk fibers.
Keywords: silk fiber; graft; hexafluorobutyl mathacrylate; structure
In recent years, some modern testing techniques are applied to show the structural
changes of ungrafted and grafted silks, such as ZHANG You-zhu who used X-ray diffraction
patterns, X-ray dispersion energy spectrum and infrared spectroscopy(IR) to show the
changes of configurations and properties of ungrafted fibroin films and grafted fibroin films
using acrylate as monomer[1], LIU Jian-hong who used infrared spectroscopy(IR) and Raman
spectrum to study the conformation changes of the grafting of methyl methacrylate onto
fibroin fibers in the absence of initiators[2], Kawahara who used scanning electron micrograph
photographs and Raman spectrum to study structure of silk fibers grafted with
Methacryamide(Mma) [3], Tsukada who used scanning electron micrograph photographs, infrared
spectroscopy(IR), X-ray diffraction patterns and
* Corresponding author：ZHANG Jun, The Study of structures of silk fibers grafted with hexafluorobutyl
mathacrylate, Institute of Material Engineering, Soochow Uinversity,email:[email protected]
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
birefringence(△n) to show the changes of configuration structure and aggregation state of silk
fabrics grafted with methyl methacrylate and methyl propylacidamide respectively, as well as
someone who used the amion acid analysis to study the structural changes of silk fabrics
grafted with epoxy resin and diethylene glycol dimethacrylate respectively, et al[4,5,6]. In this
article, the configuration and aggregation state structure of silk fibers grafted with
hexafluorobutyl mathacrylate was studied on the basis of the results of scanning electron
micrograph photographs(SEM), infrared spectroscopy (IR), Raman spectrum, wide-angle
X-ray diffraction patterns(WAXD), nuclear magnetic resonance(NMR) and amion acid
analysis. In addition, the locations and characteristics of graft crosslinking of monomer onto
silk fibers was also discussed.
1 Experimental materials and methods
1.1
Experimental materials
Scoured silk fibers(21/22 dtex, the rate of degumming 25%) was used as grafting
substrate, purchased from ZheJiang HaoYunLai printing and dyeing Company Ltd.
1.2 Chemical reagents
hexafluorobutyl mathacrylate (Actyflon-G02, XueJia Fluoride and Silicon Chemicals
Company Ltd), potassium persulfate (KPS), acetone, formic acid，alkyl hydroxidealkyl
fullfluoro octanesulphuracylammonium (DF-10), emulsifier( span 20, tween 80), all above of
chemicals were of laboratory-reagent grade. Polyoxyethylenealiphatic alcohol ether was of
industrial grade.
1.3 Grafting procedure
Firstly, emulsified water solution containing different required components was prepared,
that is, the concentration of monomer(100%, owf), DF-10(15%, owm), span 20 (10%, owm)
and tween 80 (10%, owm). The solution was stirred at the high speed of 10000～11000 r/min
for 30 min, then put the dried and weighed ungrafted silk fibers into the reaction system at
necessary pH value(3, adjusted with acetic acid) and the material-to-liquor ratio of 1:50 was
maintained. The temperature was gradually raised to the desired value (70℃)in 30min and
then maintained constant for required reaction time(2 h). At the end of the reaction silk fibers
was extracted for 8 h at 70℃ with extractor containing a large amount of acetone in order to
remove the homopolymers of the surface of silk fibers, then rinsed with distilled water and
dried at 105℃ for 2h. Samples were placed in a dehumidifier over silica gel for 24 h before
measurements. Finally the samples were weighed and measured. The grafting yield was
calculated according to the following formula:
Gy = (m1 - m0) /m0×100%
where m1 is the weight of the grafted sample; m0, the weight of original sample.
1.4 Measuring methods
1.4.1 Scanning electron micrograph photographs(SEM) The grafted fibers were mounted
on specimen holders using electron conduction tape. The samples were coated with gold in an
ion sputter coater (S-570, Hitachi, Japan) in a low vacuum. The observation was made in a
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
Hitachi, S-570, electron microscope at an accelerating potential of 15 kV.
1.4.2 Infrared spectroscopy(IR)
The FTIR spectra of the grafted fiber using the KBr disk
technique were recorded with a NICOLET5700 FTIR spectrophotometer under dried air in
the spectral region of 4000～500 cm-1.
1.4.3 Raman spectrum
The samples were cut into powders and then analyzed on the
Raman spectrum meter(HR800, JY Company, France).
1.4.4 X-ray diffraction patterns(WAXD) The samples were cut into powders and then
tested on the model D/Max-ⅢC (Japan) instrument. The laboratory conditions are as follows:
CuKα（λ=0.1542nm），equivalent electricity current(30mA), equivalent voltage(40 kV), the
scanning speed (2(°)/min).
1.4.5 Nuclear magnetic resonance(NMR)
The samples were cut into powders and then
analyzed on the solid nuclear magnetic resonance meter(Braker Company,Germany).
1.4.6 Amino acid analysis
The grafted silk were hydrolyzed by heating them in 6N
hydrochloric acid at 110℃ for 24h. During the hydrolysis treatment, contents of silk in the
hydrochloric acid solution were kept constant. An undissolved residue of the silk was filtered
off from the hydrolysates. The amino acid contents in the hydrolysates were determined by
using an amino acid analysis(L=835-50, Hitachi Ltd.).
2 Results and discussions
2. 1 the surface, lengthwise and cross-sectional configurations of grafted silk fibers
2.1. 1 the surface configurations of grafted silk fibers
AS is shown in figure 1, the letters a,b,c and d were represented as different surface
configurations electron micrograph photographs of silk fibers with diverse graft yield, and
with the increase of graft yield, the surface of fibers became tighter relatively, therefore, the
gas permeability of fabrics decreased with the increase of graft yield(shown in table 1).The
reason was that the increase of graft yield would lead to the increase of monomers grafted
with the silk macromolecule, which added to side chains of silk macromolecule, as a result,
the chain of macromolecule became lengthened, while the grafting reaction mainly occurred
inside the amorphous region of fibers which would fill in the amorphous region and make it
tighter. In addition, another reason was that the appearance of longitudinal shrinkage and the
increase of latitudinal density occurred during grafting process for silk fibers.
Table 1 the effect of different graft yield on the gas permeability of silk fibers（unit：cc/cm2/sec）
The type of fabrics
Graft yield (%)
The amount of ventilation
Ungrafted
grafted with hexafluorobutyl mathacrylate
0
91.80
9.36
99.23
3
38.78
63.69
52.45
59.84
2006 International Forum on Textile Science and Engineering for Doctoral Candidates
a ungrafted
b 9.36%
c 38.78%
d 52.45%
Figure 1 surface configurations SEM photographs of silk fibers with diverse graft yield
（magnification:200）
2.1. 2 the lengthwise configuration of silk fibers
a ungrafted
b 9.36%
c 38.78%
d 52.45%
Figure 2 lengthwise configurations SEM photographs of silk fibers with diverse graft yield
（magnification: 1.00K）
AS is observed in figure 2, compared with ungrafted common silk fibers，the surface of
silk fiber with graft yield of 9.36% (seen from figure 2, b) was smooth and changed hardly.
While the presence few granules appeared on the surface of silk fiber with graft yield of
38.78%(seen from figure 2, c) [7], and with the increase of graft yield the granules became
more which almost formed a film completely covered with the surface of fibers(seen from
figure 2, d). Because this high polymer film can withstand heat-resistance water treatment and
extraction for a long time in the presence of active surfactants, hence it was tightly combined
depending on physical reaction and chemical bondings.
2.1. 3 the cross-sectional configuration of silk fibers
a ungrafted
b 9.36%
c 38.78%
d 52.45%
Figure 3 cross-sectional configurations SEM photographs of silk fibers with diverse graft yield
（magnification: 5.00K）
AS is shown in figure 3, after being grafted with monomer，the cross-section of fibers
became wider and be provided with more fullness of handle, at the same time, the diameter
of fibers became bigger and with the increase of graft yield the appearance of separating
fibers on the surface became more evident(seen from figure 3, b,c,d). This split was probably
due to soluble swollen, fission and stripping effects of fibers during grafting process.
Therefore, silk fibers possessed obvious characteristic of tiny cavities and the cross-section
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
of fibers exhibited reticulation distribution. Moreover，the reaction of fissions would also
increase tiny cavities of interior of fibers, enhancing water absorbency, strengthening the
hindering properties of air and further improving the warm-keeping property of fabrics.
2. 2 the measuring analysis of Infrared spectroscopy(IR)
b
Transmittance(%)
a
1747.7
1068.1
c
891.6
640.0
839.8
621.1
719.6
684.0
1406.7
1384.4
1289.8
1753.2
1449.4
3300.2
1267.2
1701.9
1658.7
4000
3500
3000
1104.3
1167.3
1188.7
1233.9
2500
2000
1516.3
1500
1000
500
-1
Wavenumbers(cm )
a：ungrafted ；b：graft yield 29.21％；c：graft yield 54.85％
Figure 4 Infrared spectroscopy(IR) of silk fibers with diverse graft yield
AS is seen from figure 4, the characteristic absorption of bands appeared in the IR of
grafted and ungrafted fibroin fibers, such as acylammonium Ⅰ (VC=O1658cm-1),
acylammonium Ⅱ ( δ N-H1516cm-1), acylammonium Ⅲ (VC-N1230 cm-1 ～ 1261 cm-1) ，
acylammoniumⅤ(δN-H 684cm-1～698 cm-1) [8,9], which showed that the grafted silk still took
on the β-sheet chain structure of fibroin. The new absorption of bands appeared at the wave
bands of 1747 cm-1～1753 cm-1 and 1289 cm-1～1302 cm-1 respectively in the IR of grafted
silk fibers, which belonged to the stretching- vibration-absorption-peak bands of VC=O and
VC-F of aliphatic esters. Furthermore, the new absorption of 1189 cm-1 bands appeared in the
IR of grafted silk fibers, simultaneously, the absorption of 1261 cm-1 bands in the IR of
ungrafted silk fibers shifted to lower wavenumbers probably caused by the -CH2-CH2- bond
of the grafting of monomers of crylic acid species onto silk fibers[10]. In addition, the
outer-surface distortion stretching- vibration-absorption-peakδN-H of acylammoniumⅤ of
ungrafted silk fibers also shifted to lower wavenumbers（684.0 cm-1）after being grafted. A
probable reason was that when the monomers of crylic acid species grafted onto silk fibers the
grafting point was at -N- atom that reduced the combination of hydrogen bond of N-O atoms.
The combinations of hydrogen bond made the stretching- vibration-absorption-peak shift to
lower wavenumbers and the bending stretching- vibration-absorption-peakδ N-H shift to
higher wavenumbers. At the same time，the new absorption of bands of the IR of grafted silk
fibers appeared in fingerprint area, probably owing to the outer-surface bending stretchingvibration-absorption-peakδC-H of -CH2- and -CH2-CH2- groups produced by the grafting
reaction.
2. 3 the measuring analysis of Raman spectrum
Raman spectrum was applied to the study of configuration and conformation of polymers
and copolymers, which is an effective tool for studying multi-peptide and the second-structure
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
of proteins. We could observe the Raman spectrums of ungrafted and grafted fibers from
figure 5 to figure 7.
Figure 5 the Raman spectrum of ungrafted fiber
Figure 7
Figure 6 the Raman spectrum of grafted fiber
(The graft yield was 8.63 %)
the Raman spectrum of grafted fiber (The graft yield was 31.55 %)
The fibroins took on three conformations under different conditions, viz., irregular clew,
α -helix, β -sheet. The different conformations of fibroins possessed characteristic
absorption spectrum bands in their Raman spectrums and could change with the variations of
heat-treating conditions, stress and chemical reaction conditions. The different characteristic
absorption peaks that occurred in figure 5, figure 6 and figure 7 were illustrated in table 2.
Table 2 the measuring analysis of Raman spectrum of ungrafted and grafted fibers
The characteristic absorption peaks (cm-1)
ungrafted
Graft yield (8.63%)
Graft yield (29.21%)
1676 or 1630(acylammonium I)(β-sheet)
1667.58
1665.82
1669.35
1615(acylammonium II)(α-helix / irregular clew)
1618.07
1616.31
1614.540
1230 (acylammonium III)( β-sheet)
1225.54
1229.08
1230.84
1267.98
1266.21
1271.51
642.04
643.81
645.58
1450.10
1450.10
1453.63
1319.25
none
none
none
none
weak
very weak
1400.59
1400.59
1252 or 1270 (acylammonium III)
(α-helix / irregular clew)
650(acylammoniumⅤ)(α-helix / irregular clew)
1450 (CH
distortion vibration)
1319.25(C-N stretching vibration of second-level
amine)
1754.22(C=O stretching vibration of ester group)
1400.59(including C-F stretching vibration)
The data of table 2 suggested that ungrafted fibroin fibers mainly took onβ-sheet
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
conformation (1667.58 cm-1) including little irregular clew and α -helix conformation
(1618.07 cm-1, 1267.98 cm-1, 642.04 cm-1). After be chemically modified, the fibers still
mainly assumed β-sheet conformation (1665.82cm-1 or 1669.35 cm-1) [11]and the intensities
of absorption peaks were sharply weakened, while the irregular clew and α -helix
conformation increased with the increase of graft yield and the intensities of absorption peaks
were evidently strengthened, which made the orientation degree of macromolecules of
fibroins decline[12]. The main reason for this phenomenon was that the introduction of
molecules of crylic acid monomers onto the chains of fibroin macromolecules interfered with
nearness of macromolecule chains and made the arrangements of macromolecule chains
become loose, disorderly and curled.
In addition，as was known from Raman spectrum, there was an absorption peak (1319.25
-1
cm ) in the spectrums of ungrafted fibroins. But after being grafted, this peak disappeared,
while according to the Amino acid analysis testing reports of grafted silks, the contents of
basic amino acid (ARG) and hydroxyl amino acid (TYR) relatively reduced more.
Accordingly it could be concluded that the absorption peak at 1319.25 cm-1 should be the C-N
stretching- vibration absorption peak of second-level amine ( R’NHR). Moreover, with the
increase of graft yield, a weak absorption peak gradually appeared at 1754.22 cm-1, which
could be infered to be C=O stretching- vibration absorption peak of ester group produced
during graft reactions. For further observation of Raman spectrum, the original absorption
peak at 1400.59 cm-1 in the spectrum of ungrafted fibers enhanced obviously with the increase
of graft yield. The reason was probable that after being grafted the existence of C-F
stretching- vibration absorption peak further strengthened the original absorption peak
intensity at 1400.59 cm-1.
2. 4 the measuring analysis of X-ray diffraction patterns
c
Relative Intensity
b
a
10
20
30
40
2θ/(°)
a：ungrafted；b：graft yield (8.63％)；c：graft yield (29.21％)
figure 8 the X-ray diffraction patterns of grafted fibers with different graft yield
As was obviously seen from figure 8, the main X-ray diffraction peaks of ungrafted and
grafted fibers all appeared at 20.05°, which was the characteristic peak of fibroins withβ
-sheet structure. The X-ray diffraction peaks of grafted fibers with different graft yield were
similar to ungrafted fibers. The testing results indicated that the crystalline regions of grafted
fibers were hardly affected, which showed that the grafting reaction mainly occurred in the
amorphous areas of silk fibers[13]. Moreover, a small and wide diffraction peak also appeared
at 10°in the form of shoulder peak . This peak was more obvious at the higher graft yield
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
and it could be inferred that the peak was connected with the graftiong copolymerization of
monomers and interior macromolecules of fibers.
2.5 Amino acid analysis
According to table 3, after being grafted, the relative percentage of some Amino acids all
decreased to different degrees, such as ASP, GLU, THR, LYS, HIS, ARG, TYR and so on,
especially the latter two, While the relative percentage of GLY increased, which showed that
some Amino acids of crystalline regions,for example GLY, possessed lower reactive activity
and didn’t get involved in grafting reaction. The reaction mainly occurred on the reactive
groups of some Amino acids of amorphous areas, such as TYR, ARG, GLU, etc. The hydroxyl,
amido, carboxyl and imine groups of these Amino acids could act as the center of reactive
activities under different conditions. The grafting polymerization began with the remnant
groups of Amino acids as starting points[14], which didn’t occur on the peptide backbone chain
of protein molecules but on the αcarbon atoms of adjacent locations of polar groups of side
chains. F or TYR, owing to electron conjugation effect, the center of reaction transferred to
theαcarbon atoms of benzene ring and the activation energy of produced free radicals was
minimal and stable, hence this kind of Amino acid was consumed more.
Table 3 the relative percentage of Amino acids of silk fibers after or before being grafted
the relative percentage of Amino acids(%)
Ungrafted
Graft yield(8.63%)
Graft yield(54.85%)
ASP
THR
SER
GLU
GLY
ALA
CYS
VAL
MET
ILE
LEU
TYR
PHE
LYS
NH3
HIS
ARG
PRO
TOTAL
2.40
1.05
12.49
2.08
33.35
29.46
0.50
2.77
0.14
0.85
0.65
10.63
1.18
0.66
0.36
0.26
0.88
0.27
100
2.37
1.03
12.51
2.07
34.00
29.45
0.47
2.76
0.17
0.84
0.61
10.49
1.18
0.59
0.39
0.24
0.51
0.33
100
2.36
0.99
12.55
2.02
34.60
29.10
0.51
2.75
0.16
0.82
0.58
10.03
1.16
0.74
0.42
0.24
0.53
0.43
100
2.6 the measuring analysis of Nuclear magnetic resonance
In common cases, the magnetic sensitivity of non-proton nucleus is inferior to that of
proton nucleus. Moreover, the natural fullness of non-proton magnetic nucleus of most
compounds is also inferior to that of proton nucleus and the phenomenon of self-spinning
coupling is not easy to occur. The above two factors make the Signal-to-Noise of non-proton
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
-215.71
-122.51
-78.16
nuclear magnetic resonance spectrum very low. Therefore, when the used instrument is
similar to the instrument for measuring proton spectrum, the spectrum peak of non-proton
nuclear is very small and even can’t be seen. Consequently, the silk samples with higher graft
yield were chosen for measuring.
Sample’1 19F
20KHZ
80
40
0
-40
-80
-120
-160
-200
-240
-280
-320
(ppm)
Figure 9 the nuclear magnetic resonance spectrum (19F) of grafted fibers (graft yield, 54.85%)
In figure 9, the chemical shift of the 19F nucleus was 300ppm or so and there were three
group peaks in spectrum, and the chemical shifts of three group peaks were -78.16ppm,
-215.71ppm and -122.51ppm respectively. But the absorption peak at chemical shift of
-122.51ppm was very weak just because there were three different chemical surroundings for
F nucleus in the molecular structure of monomer. In individual surrounding, the spectrum
peaks of various F nucleuses we observed were different owing to the difference of electron
screen. But the obvious fissions of each group peak didn’t appear mainly because the
self-spinning coupling of non-proton magnetic nuclear was not easy to occur and could hardly
affect chemical shift mutually. In accordance with the molecular structure of monomer and the
peak area and chemical shift of each absorption peak group, it could be concluded that the
chemical shifts of -78.16ppm, -215.71ppm and -122.51ppm represented the structures of CF3
— , — CF2 — and —CFH — of macromolecules of grafted fibers respectively, which
indicated the monomers has reacted with the active side-chain groups of the backbone chains
of silk macromolecules.
3． Conclusions
After being grafted with hexafluorobutyl mathacrylate, the surface configuration of silk
fibers became tighter and the cross-section became wider, even leading to the appearance of
separating fibers with higher graft yield, in addition, a highpolymer thin film was completely
covered with the surface of fibers. By the measuring analysis of infrared spectroscopy and
Raman spectrum, there was little change in the conformation of grafted fibers which mainly
remained β-sheet conformation. But in the IR of the grafted silk fibers new absorption peaks
occurred which were caused by the stretching vibrations of VC=O and VC-F of aliphatic ester
species. The grafting reaction mainly occurred in the amorphous areas and the crystalline
regions were hardly affected. The CF3—,—CF2— and —CFH—sturcture of grafted silk
macromolecule was verified in the NMR spectrum. The amion acid analysis indicated fluoride
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2006 International Forum on Textile Science and Engineering for Doctoral Candidates
monomers was apt to graft with TYR ,ARG, GLU of silk fibers.
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