Coordination between yttrium ions and amide groups of polyamide 6 and the crystalline behavior of polyamide 6/yttrium composites

Article abstract of DOI:10.1016/j.molstruc.2012.04.041

Different amounts of yttrium ions were introduced into polyamide 6 (PA6) matrix by solution casting process. Structure, morphology and properties of the obtained PA6/Y³⁺ composite films were investigated by using FT-IR spectroscopy, Raman spectroscopy, scanning electron microscope (SEM), polarized optical microscope (POM) and differential scanning calorimeter (DSC) methods. Yttrium ions show strong coordination ability and their complexation with amide groups of PA6 can be reflected by the appearance of new bands in the amide A and amide I regions in FT-IR and Raman spectra. Furthermore, the FT-IR and Raman spectra of the PA6/Y³⁺ composite show that the resultant chain conformations of the amide groups in the composite films are twisted from the ideal trans conformation. The DSC results reveal that Y³⁺ ions cause a significant reduction of the melting point of PA6. In addition, the existence of Y³⁺ prevents the crystallization of molten PA6/Y³⁺ composite films during the cooling process. Moreover, the PA6/Y³⁺ composite can convert into γ phase PA6 or α phase PA6 when different solvents are used to remove Y³⁺ ions and induce crystallization of PA6.

Full text of DOI:10.1016/j.molstruc.2012.04.041

Journal of Molecular Structure 1021 (2012) 63–69
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Coordination between yttrium ions and amide groups of polyamide 6
and the crystalline behavior of polyamide 6/yttrium composites
d
Shaoxuan Liu a, Chengfeng Zhang a,b,c, Yuhai Liu a,b,c, Ying Zhao b, Yizhuang Xu a, , Yukihiro Ozaki ,
⇑
Jinguang Wu a
a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, PR China
b Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
c Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
d Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Sanda, Hyogo 669-1337, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Different amounts of yttrium ions were introduced into polyamide 6 (PA6) matrix by solution casting
process. Structure, morphology and properties of the obtained PA6/Y3+ composite films were investigated
by using FT-IR spectroscopy, Raman spectroscopy, scanning electron microscope (SEM), polarized optical
microscope (POM) and differential scanning calorimeter (DSC) methods. Yttrium ions show strong coor-
dination ability and their complexation with amide groups of PA6 can be reflected by the appearance of
new bands in the amide A and amide I regions in FT-IR and Raman spectra. Furthermore, the FT-IR and
Raman spectra of the PA6/Y3+ composite show that the resultant chain conformations of the amide
groups in the composite films are twisted from the ideal trans conformation. The DSC results reveal that
Y3+ ions cause a significant reduction of the melting point of PA6. In addition, the existence of Y3+ pre-
vents the crystallization of molten PA6/Y3+ composite films during the cooling process. Moreover, the
Received 11 December 2011
Received in revised form 16 April 2012
Accepted 16 April 2012
Available online 23 April 2012
Keywords:
Polyamide 6
Yttrium ions
FT-IR
Raman spectroscopy
Crystallization
Composite
PA6/Y3+ composite can convert into
c phase PA6 or a phase PA6 when different solvents are used to
remove Y3+ ions and induce crystallization of PA6.
Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
interactions between PA6/PA66 and different metal complexes,
such as lithium, cooper and rare earths, were investigated by FT-
IR spectroscopy, Raman spectroscopy, thermal analysis, X-ray dif-
fraction, extended X-ray absorption fine structure (EXAFS), and
near-field scanning optical microscopy (NSOM) [12–17]. The re-
sults have shown that the interactions between amide groups
and metal complexes can change the hydrogen bonding network,
decrease the melting temperature of PA6 and bring about micro-
phase separations.
Polyamide, abbreviated PA, which was first synthesized more
than 70 years ago, has been widely used as engineering plastics, fi-
bers and films [1]. As an important structural feature, polyamides
possess amide groups. It has been reported that amide groups
show considerable ability to coordinate with metal ions, thereby
providing a new way to introduce metal complexes into the PA
matrix [2–17]. Such a coordination effect is useful to modify the
structure and improve the performance of polyamides in various
aspects. For instance, Roberts and Jenekhe [2,3] and Kotek et al.
[4,5] have demonstrated that polyamides can coordinate with
strong Lewis acids such as GaCl3. As a result, the hydrogen bonds
among amide groups are completely disrupted. Ciferri, Ward and
their coworkers [6–10] have reported that LiCl and LiBr could dras-
tically reduce the crystallization rate of PA. After drawing and
annealing the PA6/lithium halide complex fibers, Young’s modulus
of the fibers can be improved significantly. Gupta et al. [11] used
PA6/GaCl3 complexes to produce porous fibers by using electros-
pinning followed by removal of GaCl3. In our previous work,
In recent years, a variety of new materials based on the partic-
ular electronic structure and physical and chemical properties of
rare earth elements have been developed [18]. We notice that rare
earth ions possess three positive charges and multiple coordina-
tion sites, and thus rare earth ions exhibit remarkable interactions
with the amide groups of PA. In our previous work [15], we found
feed-back enhancement mechanism that enhances signals of elec-
4
4
tronic Raman bands from I9/2 to I11/2 manifold of neodymium
complexes by using 1064 nm laser. As a result, an electronic Raman
band of Nd3+ can be used as a probe to reflect a change in coordi-
nation sphere around Nd3+. We used the electronic Raman band to
study an interaction between Nd3+ and PA6 and polyvinylpyrroli-
done. Nd3+ exhibits considerable ability to coordination with
amide group since significant variations could be observed in the
⇑
Corresponding author. Fax: +86 10 62751708.
E-mail address: xyz@pku.edu.cn (Y. Xu).
0022-2860/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.04.041

64
S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
electronic Raman bands. In another work [16], we carried out a
preliminary investigation on PA6 and YCl3 by using temperature
variable FT-IR and thermal analysis. In the melting process, the
amide A band at 3294 cmꢀ1 in nylon–lanthanide salt systems
exhibits a lower wavenumber shift by 60 cmꢀ1 rather than a higher
wavenumber shift, and significant wavenumber variations in
amide I and II bands are observed.
total reflectance (ATR) accessory. To obtain high signal-to noise ra-
tio, 32 scans were co-added for each spectrum.
Raman measurements were performed under ambient condi-
tions using a Jobin-Yvon HR800 micro-Raman spectrometer and
a helium–neon laser (633 nm, 1.96 eV excitation) with the back-
scattering configuration.
All the above results have demonstrated that rare earth ions
manifest strong coordination ability to amide groups. Based upon
the above results, we have introduced an additive containing rare
earth elements into the fiber spinning process to produce nylon fi-
bers [19]. It has been found that the additive containing rare earth
complexes helps to decrease spinning temperature and improve
spinnability of PA6 fibers [20]. As a result, fine denier PA6 fibers
were successfully manufactured in a series of pilot plant experi-
ments (denier is a unit of measure for the linear mass density of fi-
bers. If the mass of a fiber whose length is 9000 m is 1.0 g, the
linear mass density of the fiber is 1.0 denier). However, the indus-
trial experiments revealed that deep and comprehensive under-
standing is necessary on variations of physical–chemical
properties of PA6 under the influence of rare earth metal ions.
Thus, we revisit the PA6 and YCl3 systems.
The present study further investigates the interactions between
PA6 and yttrium chloride and the related phase transition behav-
iors by using FT-IR, Raman, DSC, polarized optical microscope
and SEM methods. The results show that Y3+ can coordinate with
the amide group of PA6 in a reversible manner. A new finding in
the present study is that the coordination brings about remarkable
influence on the conformation of the polyamide chain and crystal-
line behavior of PA6.
2.3. Thermal analysis
Thermal analysis of the films was performed on a Q100 differ-
ential scanning calorimeter of TA Corporation. A nitrogen flux
(50 mL minꢀ1) was used as purge gas for the furnace during the
experiment. The samples were heated to 260 °C at a heating rate
of 10 °C/min and then cooled down to 40 °C at a cooling rate of
10 °C/min.
2.4. Polarized microscope with a hot stage
Polarized optical microscopic observation was used to register
crystalline changes in terms of transmitted light during heating
on a Leica DMLP polar microscope with a hot stage. PA6 and
PA6/Y3+ composite films were placed between two glass slides
and were heated to 260 °C at a heating rate of 10 °C/min. During
the heating process, the micrographs of the sample under the
polarized microscope were recorded by using a digital camera.
All the parameters of microscope and digital camera were invariant
during the heating process. The recorded digital micrographs were
converted into black–white mode. Under this mode, each micro-
graph is a matrix composed of 763 ꢁ 577 pixels and each pixel is
characterized by an integer whose value is between 0 and 255 to
show how bright the pixel is. The larger the value is, the brighter
a pixel is. Therefore, we use the summation of the brightness of
all the pixels to characterize the intensity of transmitted light.
The calculation was performed by using a program for MATLAB
(The Math Works Inc.) written in our lab.
2. Experimental
2.1. Materials
PA6 pellets with
a
relative viscosity of 2.45 (Mr = 2.65 ꢁ
104 g/mol) were purchased from Baling Petrochemical Co. Ltd
(Yueyang, China).
2.5. Scanning electron microscopy (SEM)
Yttrium oxide (Y2O3) with a purity of 99.99% (Shanghai Yuelong
Chemical Factory) was used to prepare yttrium chloride by dissolv-
ing Y2O3 in concentrated HCl.
The aqueous solution of yttrium chloride was carefully heated
to remove water and HCl so that yttrium chloride can be crystal-
lized from the solution. To prevent the hydrolysis of Y3+ ions during
the heating process, HCl was added into to the yttrium chloride
solution to keep the pH value of the solution below 5.
The morphology of the films was observed on a scanning elec-
tron microscope (JEOL JSM-6700F) at an acceleration voltage of
5.0 kV. The surfaces of the samples were coated with a conductive
platinum layer (This thickness of the layer is about 5 nm) before
observation.
3. Results and discussion
PA6 and yttrium chloride were dissolved in formic acid sepa-
rately, and then mixed together with various proportions to pre-
pare PA6/Y3+ solutions. For each solution, the concentration of
PA6 always remained as 10 wt%, while the amount of yttrium chlo-
ride varied according to different stoichiometric ratios between Y3+
ions and amide groups of PA6 (nY:namide = 0.10:1, 0.14:1, 0.18:1,
0.25:1). PA6/Y3+ composite films were prepared by casting each
solution onto a silicon substrate followed by evaporation of the sol-
vent under vacuum at room temperature overnight. These compos-
ite films were denoted as PA-Y-1-x, where x is the stoichiometric
ratio of Y3+ to amide groups (x = 0.10, 0.14, 0.18, 0.25). For compar-
ison, pure PA6 film (PA-Y-1-0, x = 0) was also obtained by the same
method. These films were then kept in a desiccator for 3–5 day be-
fore further characterization.
3.1. FT-IR and Raman studies
Fig. 1 shows FT-IR spectra of a pure PA6 film and PA-Y-1-0.10,
PA-Y-0.14, PA-Y-0.18, and PA-Y-1-0.25 films in the 1700–
1480 cmꢀ1 region. In the spectrum of pure PA6 film, the amide I
and II bands are located at about 1635 and 1540 cmꢀ1, respec-
tively. A new band appears around 1602 cmꢀ1 for the PA6/Y3+ com-
posite films. As the content of Y3+ increases, the intensity of this
new band increases and the band becomes a dominant peak in this
region, while the intensities of amide I and II bands decrease signif-
icantly. For the PA-Y-1-0.25 film, the amide I and II bands strongly
diminish. According to our previous work [16], the 1602 cmꢀ1
band is assigned to the C@O stretching band of amide groups that
are coordinated with Y3+ (we denote this band as amide I0 band). In
comparison with the amide I band of pure PA6, the amide I0 band
exhibits a significant lower wavenumber shift. Upon the coordina-
tion, Y3+ ion has a strong electron-withdrawing effect on the oxy-
gen atom of carbonyl group. As a result, the bond strength of the
2.2. FT-IR and Raman spectroscopy
FT-IR spectra were measured at a spectral resolution of 4 cmꢀ1
on a Nicolet iN10 MX spectrometer equipped with an attenuated

S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
65
1600
1602
1083
1586
e
e
1608
d
d
c
1608
1635
1540
c
b
a
b
1130
1080
1638
1471
1485
1065
1700
1650
1600
1550
1500
Wavenumbers (cm-1)
a
Fig. 1. FT-IR spectra in the 1700–1480 cmꢀ1 region of a pure PA6 film and PA6/Y3+
composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18
and (e) PA-Y-1-0.25.
1600
1400
1200
1000
Raman Shift (cm-1)
Fig. 2. Raman spectra in the 1700–800 cmꢀ1 region of a pure PA6 film and PA6/Y3+
composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18
and (e) PA-Y-1-0.25.
carbonyl groups decreases, leading to the lower wavenumber shift
of amide I band.
To get a comprehensive understanding on the spectral behavior
of amide I band of PA6 upon coordination with Y3+, Raman spectra
of PA6/Y3+ composites and pure PA6 were measured and illustrated
in Fig. 2. The amide I band for the pure PA6 film can be observed at
1638 cmꢀ1 in the Raman spectrum, quite close to the amide I band
at 1635 cmꢀ1 in the corresponding FT-IR spectrum. Different from
FT-IR spectra of PA6, the amide II band is absent in the Raman
spectra. When Y3+ ions are involved, a new Raman band appears
around 1586 cmꢀ1. The intensity of the new band increases as
the content of Y3+ increases. Thus, a plausible assignment of the
1586 cmꢀ1 band is that the band is the amide I0 band that is also
3234
e
related to the amide groups that coordinate with Y3+
.
However, a large discrepancy on the peak positions of the amide
I0 band between the FT-IR spectra (1602 cmꢀ1) and the Raman
spectra (1586 cmꢀ1) should be explained. The absence of the amide
II in the Raman spectra of PA6/Y3+ may be one reason, but this is
not enough. The 1602 cmꢀ1 band in FT-IR spectra and the
1586 cmꢀ1 band are quite broad, indicating that the amide groups
that coordinate with Y3+ occur in a variety of states. At least two
coordination states of amide groups can be observed in vibrationꢀa1l
spectra, one state is represent by an amide I0 band at ca. 1605 cm
and another state is characterized by an amide I0 band at ca.
1585 cmꢀ1. The two coordinating states exhibit different sensitiv-
ity in FT-IR and Raman spectra. As a results, the coordinating state
of amide group characterized by the 1605 cmꢀ1 exhibits high sen-
sitivity in FT-IR spectra and shows strong band in FT-IR spectrum.
On the other hand, the coordinating state of amide group charac-
terized by the 1585 cmꢀ1 exhibits higher sensitivity in Raman
spectrum and shows strong band in Raman spectrum. Results of
FT-IR and Raman spectra indicate the coordination with Y3+ brings
about a significant spectral variation on the amide I band of PA6.
Fig. 3 shows the NAH stretching band region of the pure PA film
and the PA-Y-1-0.10, PA-Y-0.14, PA-Y-1-0.18 and PA-Y-1-0.25
films. A sharp band at 3294 cmꢀ1 in the spectra can be assigned
d
3294
c
b
a
3600
3400
3200
3000
2800
Wavenumbers (cm-1)
Fig. 3. FT-IR spectra in the 3600–2800 cmꢀ1 region of a pure PA6 film and PA6/Y3+
composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18
and (e) PA-Y-1-0.25.
to the NAH stretching mode of the amide group in PAs (amide
A). In most cases, the NAH groups of PAs form hydrogen bonds
with C@O groups from the vicinal PA chain [12]. The variation in

66
S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
the amide A band frequency is useful to explore the variation in the
hydrogen bonds in PA systems since the peak position of amide A
directly reflects the strength of the hydrogen bonds [2,3,21]. The
sharp amide A band at 3294 cmꢀ1 for the pure PA6 film in Fig. 3a
is characteristic of hydrogen-bonded polyamides in a crystalline
region [22]. For the PA-Y-1-0.10 and PA-Y-1-0.14 films, the sharp
amide A band is still observed around 3294 cmꢀ1. In addition, a
broad shoulder at a lower wavenumber side becomes stronger.
For the PA-Y-1-0.18 film, the broad shoulder changes into a broad
peak, locating at around 3234 cmꢀ1. For the PA-Y-1-0.25 film, the
sharp amide A band around 3294 cmꢀ1 disappears completely,
while the broad peak at 3234 cmꢀ1 dominates in the amide A re-
gion. We assign the 3234 cmꢀ1 band to the NAH stretching mode
of amide groups coordinated with Y3+. In comparison with the
amide A band of pure PA6 film, the 3234 cmꢀ1 band exhibits a
60 cmꢀ1 lower wavenumber shift. The amide A band undergoes a
lower wavenumber shift rather than a higher wavenumber shift
in the PA6/Y3+ composite films, indicating that the hydrogen bonds
become stronger as Y3+ coordinates with the amide groups. A rea-
sonable explanation for this is that both the hydrogen bonds and
the coordinate bonds coexist in this system. In our previous work
[12], we proposed a six-member ring model to explain the spectral
and thermal behaviors of PA66/lithium salt system. This six-mem-
ber ring model is still accessible in the PA6/Y3+ system (Scheme 1).
The stable six-member ring may favor stronger hydrogen bonds. In
our previous work on the PA6/Li+ system, the amide A band only
underwent a 50 cmꢀ1 lower wavenumber shift. That is to say, the
PA6/Y3+ brought about a larger lower wavenumber shift on the
amide A band. This result suggests that the hydrogen bonding in
the PA6/Y3+ system is stronger.
1439
e
d
c
b
1463
1477
1416
1438
a
1500
1450
1400
1350
1300
Wavenumbers (cm-1)
As the Y3+ ions strongly coordinate with the amide groups
of PA6, the chain conformations of PA6, especially for the
ACH2ACONHACH2A segment are also greatly affected. Fig. 4
depicts the CH2 scissoring vibration region of the FT-IR spectra,
where a group of bands can be observed for the pure PA6 film,
locating at 1477, 1463, 1438 and 1416 cmꢀ1. Among these bands,
the occurrence of the 1438 cmꢀ1 band indicates a rotational
deviation of the ACH2ACONHACH2A group from the ideal trans
conformation [23]. The intensity of the 1438 cmꢀ1 peak is rela-
tively low in the spectra of the pure PA6 film. However, when
the Y3+ ions coordinate with amide groups, the relative intensity
of the 1438 cmꢀ1 band increases dramatically and becomes domi-
nant in this region. The above results indicate that a twist of
ACH2ACONHACH2A group of PA6 from ideal trans conformation
occurs upon the coordination between Y3+ and the amide groups.
To our knowledge, no similar results are reported in the literatures
before.
Fig. 4. FT-IR spectra in the 1500–1150 cmꢀ1 region of a pure PA6 film and PA6/Y3+
composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18
and (e) PA-Y-1-0.25.
(1471 cmꢀ1, 1485 cmꢀ1), which are indicative of the trans amide
groups [24,25]. The intensities of the two shoulder peaks decrease
for the PA-Y-1-0.10 and PA-Y-0.14 films and disappear for the PA-
Y-1-0.18 and PA-Y-1-0.25 films. In the CAC stretching region, the
1065 cmꢀ1 peak and the 1130 cmꢀ1 peak are usually indicative of
an all-trans CC backbone conformation while the 1080 cmꢀ1 peak
is attributed to the presence of gauche conformation [25,26].
For the pure PA6 film, a strong peak locating at 1130 cmꢀ1 can
be observed. As the content of Y3+ increases, the intensities of
the 1130 and 1065 cmꢀ1 peaks decrease, while the intensity of
the 1080 cmꢀ1 gradually increases. For the PA-Y-1-0.18 and
PA-Y-1-0.25 films, both the 1065 and 1130 cmꢀ1 peaks are too
weak to be observed. The above spectral changes show that the
coordination between Y3+ ions and amide group of PA6 brings
about a significant conformational variation. The trans conforma-
tions in the pure PA6 film are completely destroyed when the
content of Y3+ ions is high enough. In addition, preliminary inves-
tigation on the second derivative spectra of PA6 and PA6/Y3+ com-
posite films was investigated. Significant variations were also
observed in the CAH stretching region (please see the Supporting
information).
On the other hand, the Raman spectra (Fig. 2) are quite sensitive
to the chain conformation. In the CANAH bending region (1440–
1490 cmꢀ1), the pure PA6 film exhibits two shoulder peaks
six-member ring
H
In the 1150–900 cmꢀ1 region of the FT-IR spectra (Fig. 5), infor-
N
mation concerning the crystallization behaviors of the PA6/Y3+
N
Y
C
O
H
O
composite films may be obtained.
a
crystalline phase can be ob-
phase
C
tained for the pure PA6 film as indicated by the typical
a
characteristic peaks around 959, 930 and 928 cmꢀ1 in the FT-IR
spectra [27,28]. When the content of Y3+ is relatively small as in
the case of the PA-Y-1-0.10 film, an amorphous peak at 984 cmꢀ1
[29] becomes obvious besides those
a
phase crystalline bands. As
phase
the content of Y3+ keeps increasing, the 959 cmꢀ1 band of
a
Scheme 1. Six-member ring model of the Y3+ ions and amide groups of PA6.

S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
67
25
20
15
e
d
c
10
983 959
930
b
a
5
0
0
50
100
150
200
250
Temperature/oC
1100
1050
1000
950
900
Wavenumbers (cm-1)
PA-pure
PA-Y-1-0.10
PA-Y-1-0.14
PA-Y-1-0.18
PA-Y-1-0.25
Fig. 5. FT-IR spectra in the 1150–900 cmꢀ1 region of a pure PA6 film and PA6/Y3+
composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18
and (e) PA-Y-1-0.25.
Fig. 7. The transmitted light intensity of the recorded electronic micrographs by
polarized optical microscope. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d)
PA-Y-1-0.18 and (e) PA-Y-1-0.25.
e
d
c
Table 1
The data in the melting process of DSC and polarized optical microscopic (POM)
observation experiments.
Melting
point
in DSC (°C)
Enthalpy in
DSC (J/g)
Completely
melting
point in DSC (°C)
Ending
point
in POM (°C)
PA-pure
215.4
200.0
195.0
191.2
100.8
77.4
59.4
27.4
14.1
68.6
226
208
206
202
140
228
208
206
200
115
b
a
PA-Y-1-0.10
PA-Y-1-0.14
PA-Y-1-0.18
PA-Y-1-0.25
PA-Y-1-0.25 sample, an endothermic peak at 100.8 °C with a broad
background can be observed. The enthalpy is about 68.3 J/g. We
suggest that the reason for the large enthalpy of this endothermic
peak is as follows: the melting of PA6 crystallites and removal of
adsorpted water take place simultaneously.
50
100
150
200
250
300
Temperature/oC
Fig. 6. DSC curves of PA6 and PA6/Y3+ composite films during the heating process.
(a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18 and (e) PA-Y-1-
0.25.
Fig. 7 shows the variation of the intensity of the transmitted
light as a function of temperature. Part of PA6 crystallites were de-
stroyed and changed into isotropic molten upon elevation of the
temperature. As a result, the intensities of the transmitted light de-
crease. When the PA6 crystallites are completely molten, the inten-
sities of the transmitted light reach a minimum value and do not
change as the temperature increases further. We denote the lowest
temperature at which the intensity of the transmitted light reach
minimum as the ending points. For the pure PA6 film, PA6-Y-1-
0.10, PA-Y-1-0.14 and PA-Y-1-0.18 samples, the ending points from
polarized microscope are roughly the same as the values of com-
pletely melting points in the DSC experiments (Table 1). Further-
more, the curves of the intensity of the transmitted light versus
temperature demonstrate that intensity of transmitted light of
the PA6-Y-1-0.14 starts to decrease at 140 °C and the intensity of
transmitted light of the PA-Y-1-0.18 sample starts to decrease at
90 °C. These results confirm the observation of additional DSC peak
around 136 °C and 105 °C in the DSC experiment. For the PA-Y-1-
0.25 sample, the intensity of transmitted light reaches minimum
at 105 °C. The results confirm that the endothermic peak at
100.8 °C is related to the melting of PA6 crystallites.
crystalline gradually decreases and becomes a very weak shoulder
as can be seen for the PA-Y-1-0.14 and PA-Y-0.18 films. For the PA-
Y-1-0.25 film, the characteristic peaks for
not be observed any more.
a phase crystalline can-
3.2. DSC studies
The melting behaviors of the PA6/Y3+ composite films and the
pure PA6 film were recorded by using the DSC technique as illus-
trated in Fig. 6. As for the pure PA6 film, the main melting peak ap-
pears around 215.4 °C and the melting peak end at ca. 226 °C (this
point is denoted as completely melting point in the following part)
in the heating process. The completely melting points for the PA-Y-
1-0.10, PA-Y-1-0.14 and PA-Y-1-0.18 films are ca. 208 °C, 206 °C
and 202 °C. In addition, the melting peaks become broad and the
melting enthalpies decrease as the contents of Y3+ increase. More-
over, additional broad peaks around 136 °C and 105 °C appear in
the DSC curves of PA-Y-1-0.14 and PA-Y-1-0.18 sample. For
The PA6 and PA6/Y3+ composite films were cooled down to
40 °C after being heated to 260 °C. A crystallization peak appears

68
S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
at 181 °C in the cooling process for the pure PA6 film, while no
crystallization peaks are observable in the PA6/Y3+ composite
films, indicating that Y3+ prevents PA6 from undergoing crystalliza-
tion (Fig. 8). According to the FT-IR and Raman spectra, strong
coordination between Y3+ and amide group of PA6 plays a labile
cross linking role that binds different PA6 segments together. As
a result, the movement of PA6 in the PA6/Y3+ melt is constricted.
Thus, the crystallization of PA6 is retarded or even prohibited.
3.3. Morphology crystalization studies
The morphology of the PA6/Y3+ composite films was investi-
gated by using SEM (Fig. 9). For the pure PA6 film, a typical spher-
ulite structure with clear boundaries is shown in its SEM image in
Fig. 9a. When Y3+ ions are involved, some amide groups of PA6
chain participate in the coordination with Y3+ ions. Consequently,
the chain mobility is hindered due to the labile crosslink by the
metal ions and the crystallization of PA6 is depressed to different
extent. The boundary among different spherulites becomes vague
for the PA-Y-1-0.10 film (Fig. 9b). For the PA-Y-0.14, PA-Y-1-0.18
and PA-Y-1-0.25 films, neither spherulite-like feature nor crystal-
line lamellar structure can be observed (Fig. 9c–9e). The above
results are in good agreement with the FT-IR results.
e
d
c
b
PA6 usually exhibits peculiar polymorphism phenomenon. For
example, PA6 can crystallize into
a or c form in which hydrogen
a
bonding between antiparallel or parallel polymer chains are
formed, respectively [29–33]. The introduction of Y3+ ions into
the PA6 matrix will change the hydrogen bonding structure and
the conformation of the polymer chains. Our following experi-
ments demonstrate that the amorphous PA6/Y3+ composite is an
interesting precursor of crystalline PA6, since the phase transition
behaviors of the amorphous PA6/Y3+ composite can be controlled
by using different solvents, such as water or ethanol, to remove
50
100
150
200
250
300
Temperature/oC
Fig. 8. DSC curves of melt PA6 and PA6/Y3+ composite films during the cooling
process. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18 and (e)
PA-Y-1-0.25.
Fig. 9. SEM images of the pure PA6 film and PA6/Y3+ composite films. (a) Pure PA6 film (b) PA-Y-1-0.10 (c) PA-Y-1-0.14 (d) PA-Y-1-0.18 and (e) PA-Y-1-0.25.

S. Liu et al. / Journal of Molecular Structure 1021 (2012) 63–69
69
in different crystalline forms. In summary, coordination interaction
between PA6 and Y3+ ions offers a promise way to process PA6 by
tailoring its physical and chemical properties.
959
930
c
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grant Nos. 50973003, 50673005, 50403026 and
20671007), National High-tech R&D Program of China (863 Pro-
gram) of MOST (No. 2010AA03A406) and A Program of Advanced
Technology Institute, Peking University. We are grateful for the
kind help of Jinyong Wang and Prof. Yan Li (College of Chemistry
and Molecular Engineering, Peking University) on the Raman
experiments.
976
b
a
984
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
04.041.
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c phase PA6 in water or a phase PA6 in
ethanol. Thus, we can use different solvents to remove Y3+ ions
from the composite films and induce crystallization of pure PA6