The phase morphology of multiphase polymer blends, as well as its
evolution, is a basic issue in polymer processing area (1). The size,
shape, spatial distribution, and time evolution of the dispersed phase,
which are the focus of the morphology control, decide to a large extent
the macro-properties of the materials. Therefore, characterizing the
morphology of multiphase polymer blends has been considered as the key
to investigate the relationship of structure and properties.
Presently, the morphology characterization is mostly based on
microscopy (e.g., scanning electron microscopy (SEM), transmission
electron microscopy (TEM), and atom force microscopy (AFM)), from which
the microstructure can be observed directly. However, all the methods
mentioned above are mostly confined to off-line characterizing the
samples in the solid phase. Apart from time-consuming, the analytic
results are not always precise as the samples just come from a small
portion of the whole specimen (2). Moreover, measurements usually
require separate sample preparation (e.g., quenching the melts rapidly),
which may significantly alter the original state due to the different
thermo-physical property of each phase. Thus, research and development
of the rapid in situ characterization techniques aiming at the phase
morphology of blend melts have been deemed to be a necessary and
In recent years, optical-related technologies have been
successfully applied to polymer processing for real-time monitoring the
morphological evolution during the melt blending. Leukel et al. (3)
placed an optical microscope in an extruder die for real-time image
observation and proved that this method is effective for characterizing
the morphology of polymer blends resulting from the extrusion process.
Light scattering techniques, which are based on the interaction between
the light and the polymer melts, have been used to predict the
concentration and the particle sue of dispersed phase. A succession of
representative studies were performed by the following research groups,
i.e., Sheng et al. (4), (5), Schlatter et al. (6), and Hobbie et al (7),
(8). However, the need of sapphire optical windows and the desire of
good light transmission properties for materials greatly limited the
application of these techniques.
Ultrasonic diagnosis, as a novel method with the favorable fast,
nonintrusive, real-time and convenient characteristics, appears to be
appealing in this aspect for the measurements can be conducted in highly
concentrated or optically opaque dispersions (9). Parameters such as
ultrasonic velocity (or transit time) and attenuation can provide useful
information on the morphology of polymer blends. Experimental studies of
ultrasound signal dependence on the compatibility of polymer blends have
been widely performed in polymeric solution and solid phase. Hourston et
al. (10-12) and Sing et al. (13-15) suggested that ultrasonic velocity
varied linearly with the blend composition for miscible blends, while it
deviated from linearity if the miscibility of polymer blends decreased.
He et al. (16) used ultrasonic attenuation to investigate blend
morphology, and the results showed that attenuation depended on the size
of dispersed phase and a discontinuity of scattering attenuation was
always observed as phase inversion occurred.
Few authors have tried to study blend melts using ultrasound due to
the heavy absorption and strong interference of detection signal, as
well as the high temperature which limits the application of the
transducer. Thanks to the work of Jen's group (17), a novel
ultrasound transducer equipped with a clad metallic buffer rod can be
successfully applied to the polymer melts and recently a series of work
have been carried out. Based on the fact that the alteration in
molecular chain orientation could result in a change in the sonic
velocity of polymer melt, Li et al. (18-20) in-line investigated the
relaxation of orientation and disorientation of various polyolefins.
Villanueva et al. (21) fixed the ultrasound sensors (US) onto the
extruder exit, and real-time studied the effects of screw configuration
and clay nature on the dispersion of nanoclays in a LDPE matrix. Other
works such as in-line monitoring the composition ratio (22), the
residence time distribution (23), and the polymer degradation (24) were
also conducted. However, few attentions have been paid to the blend
morphology and the attempt to associate the acoustic parameters with the
phase morphology is rarely reported (25).
High density polyethylene (HDPE) and polyamide 6 (PA6) are
semicrystalline plastics, which have versatile industrial applications.
HDPE has excellent low temperature flexibility, low cost, and good
resistance to moisture permeation (26), while PA6 shows high strength,
favorable thermo-mechanical characteristics, and good resistance to
oxygen permeation and hydrocarbons (27). Thus, it is conceivable that
blends of HDPE and PA6 (HDPE/PA6) can synergically combine the merits of
both components, provided that the blends are appropriately
compatibilized and the corresponding phase morphology gets optimized.
In this work, the immiscible HDPE/PA6 blends with 10, 20, and 30
wt% of a dispersed PA6 phase were prepared. Different amounts of HDPE
grafted with maleic anhydride (HDPE-g-MAH), as a reactive
compatibilizer, were added to obtain the different PA6 phase size. The
morphology of the samples was firstly characterized by the conventional
SEM. Then ultrasonic diagnosis as a novel technique was performed to
investigate the influences of the PA6 concentration and the HDPE-g-MAH
content on the ultrasonic velocity and attenuation. This work aims to
explore the relationship between the ultrasonic parameters and the phase
morphology for the droplet-matrix blend melts, which lays the foundation
for the potential application of in-line ultrasonic diagnosis in the
A commercial HDPE (5000S) with a density of 0.946 g/[cm.sup.3] and
a melt How index of 1.0 g/10 min (190[degrees]C, 2.16 kg) was purchased
from Lanzhou Petrochemical Co., China. PA6 (M3400) with a density of
1.14 g/[cm.sup.3] and a flow melt index of 6 g/10 min (230[degrees]C,
2.16 kg), was obtained from Guangdong Xinhui Meida-DMS Nylon Chips Co.,
China. HDPE-g-MAH with maleic anhydride content of 0.8 wt% and a (low
melt index of 1.2 g/10 min (190[degrees]C, 2.16 kg) was supplied by
Ningbo Nengzhiguang New Materials Technology Co., China.
Compatibilized and uncompatibilized HDPE/PA6 blends were prepared
by melt mixing with an intermeshing corotating twin-screw extruder
(Haake Rheomex, PTW16/25). The temperatures of different zones were set
to 240[degrees]C, except that those of feeder and die were 190 and
235[degrees]C, respectively. The feeding and screw speed were separately
40 and 60 rpm. Prior to melt blending, the pellets of PA6 and HDPE-g-MAH
were dried in a vacuum oven at 85[degrees]C for 24 h to avoid the
effects of moisture. The extruded strands were cooled by air and then
Three composition ratios (wt/wt) of HDPE/PA6 blends were 90/10,
80/20, and 70/30. For each blend composition, the concentrations of
HDPE-g-MAH ranging from 0 to 10 wt% with respect to the HDPE/PA6 resin
Samples for the morphological analysis were prepared through
compression molding at 230[degrees]C. The phase morphology was examined
by Philips XL30FEG SEM operated at an accelerating voltage of 10 kV. To
create better contrast, samples were fractured in liquid nitrogen,
etched by formic acid to remove the PA6 particles, then sputter-coated
with Au before SEM observation.
The SEM micrographs were analyzed by the JEOL Smile View software.
At least 300 diameters were measured per sample. The number average
diameter ([D.sub.n]) was then calculated by:
[D.sub.n] = [[SIGMA].sub.i][N.sub.i][D.sub.i] /
where D, and /V, were the diameter and number of particles,
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Ultrasonic Instrumentation and Measurement Principles
Ultrasonic measurements were performed on a homemade slit die which
was fitted to the barrel exit of the capillary rheometer (RH7, Malvern).
Details of the setup were illustrated in Fig. 1. Two flush-mounted US
were diametrically opposed across the die to emit and receive
longitudinal waves with a central frequency of 5.0 MHz. Two pressure
transducers (P2, P3) with J-type thermocouples were used to measure the
melt pressure and temperature ([T.sub.m]). A wrapped-on heating jacket
with a piecewise proportion integral differential program was employed
to control the die temperature ([T.sub.die), which ensured that the
[T.sub.m] in the slit die was consistent with that in the capillary
rheometer. All the sensors were connected to the GIM1 system (PACE
Simulations) composed of an ultrasonic pulser-receiver, a sampling
digital oscilloscope, and an automated data acquisition system. And by
this device, we could simultaneously track the evolution of temperature,
pressure, and ultrasonic velocity and attenuation of the polymer melt in
the slit die. In this work, pellets were first held in the barrel of
capillary rheometer for melting, and then the melts were forced into the
slit die at a constant speed of 10 mm/min and finally stayed in the slit
die. All the measurements were performed at 230[degrees]C after enough
relaxation until the melt pressure was zero and the ultrasonic velocity
was unchanged. Each sample was taken for five measurements, and the mean
values of the sound velocity and attenuation were then obtained,
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Figure 2 showed the schematic of ultrasound propagation in the
polymer me 11 which was confined between two aligned steel buffer rods.
The longitudinal waves with amplitude of [a.sub.0] traveled to the
steel/polymer interface where part of its energy went through the molten
polymer with a thickness d = 2 mm. The acoustic wave was then reflected
back and forth between the two interfaces. This produces a series of
echo signals, [A.sub.1], [A.sub.2] ..., exiting from the second
interface and directed toward the receiving transducer (Fig. 3).
Usually, the first and second echoes were used for calculation of the
ultrasonic velocity and attenuation. By measuring the flying time
([t.sub.1] and [t.sub.2]) and the amplitude ([A.sub.1] and [A.sub.2]) of
the two successive echoes, the ultrasonic velocity (C) and attenuation
(a) in the polymer melt can be separately expressed as follows:
C = 2d/[t.sub.2] - [t.sub.1], (2)
[chi] = 20[log.sub.10]([A.sub.1] / [A.sub.2])/2d, (3)
RESULTS AND DISCUSSION
[FIGURE 5 OMITTED]
The SEM images of the 90/10, 80/20, and 70/30 HDPE/PA6 blends with
different amounts of HDPE-g-MAH are presented in Figs. 4, 5, and 6,
respectively. It can be seen that PA6 phase is dispersed in HDPE matrix
in the form of spherical particles, which is known as the typical
droplet-matrix morphology. As is expected, the phase morphology takes on
increasingly refined structure with the increasing amount of HDPE-g-MAH
due to the effect of reactive compatibilization. The statistic average
sizes of PA6 particles as a function of HDPE-g-MAH content for the three
compositions are depicted in Fig. 7. It can be seen that for all the
compositions, the particle size first decrease sharply with the increase
of the HDPE-g-MAH content, and then gradually levels off, following a
typical behavior of an emulsion curve (28). The following exponential
equation (29) provides a good estimate of the dependency of the particle
size on the HDPE-g-MAH concentration.
[FIGURE 6 OMITTED]
([D.sub.n,c] - [D.sub.n,[infinity]]) / ([D.sub.n,0] -
[D.sub.n,[infinity]]) = exp(-nc), (4)
where [D.sub.n,c] is the number average diameter for a
concentration c of HDPE-g-MAH, [D.sub.n,0] is the number average
diameter for a blend without HDPE-g-MAH, [D.sub.n,[infinity]] is a
constant representative of the limiting particle size, and n is a
constant that determines the efficiency of the HDPE-g-MAH as an
[FIGURE 7 OMITTED]
Table 1 presents the constants [D.sub.[infinity]] and n for Eq. 4.
Ii can be seen that, with the increase of PA6 concentration,
[D.sub.[infinity]] gets increased while n shows the opposite. It is
suggested that it should add more amounts of compatibilizer for the
blends with higher concentration of PA6, to ensure the same emulsifying
effect. This can be confirmed by the fact that the particle size
increases with the increasing PA6 concentration when the content of
HDPE-g-MAH is fixed, which is also shown in Fig. 7.
On the basis of the above SEM results, the relationship between the
ultrasonic parameters, i.e., ultrasonic velocity and attenuation, and
the phase morphology will be investigated in the following section.
Before we conducted ultrasonic measurements for the HDPE/PA6 blends, the
acoustic properties of neat HDPE and PA6 were first measured at
230[degrees]C, and the results were listed in Table 2. For such
homogeneous polymer melts, the differences in both the velocity and the
attenuation for HDPE and PA6 are mainly originated from the different
molecular structure. However, as for the HDPE/PA6 blends, the blend
composition and the phase size may have additional effects on the
propagation of ultrasound wave considering the presence of the
Figure 8 presents the ultrasonic velocity as a function of the
HDPE-g-MAH content for the 90/10, 80/20, and 70/30 HDPE/PA6 blends. The
velocities seem unchanged irrespective of the HDPE-g-MAH content for the
three systems. According lo the results of the SEM observation, it is
known that the particle size decreases with the increasing HDPE-g-MAH
content. Therefore, the constant velocity means that ultrasonic velocity
is insensitive to the variation of the particle size. On the other hand,
Fig. 8 also shows that there are velocity differences among the three
blend compositions, and the velocity is larger for the blends with more
concentration of PA6. Further relationship between ultrasonic velocity
and PA6 concentration are shown in Fie. 9. It can be observed that the
ultra-sonic velocity of the blends increases linearly with the PA6
concentration up to 30 wt% in our experiment. In terms of the above
discussion, it is suggested that the ultrasonic velocity could be used
to investigate the concentration of the dispersed phase, but it is
unavailable for predicting the phase size.
As it has shown that ultrasonic attenuation is more sensitive than
velocity to characterize the compatibility and morphology in solid blend
systems (16), it is expected that this sensitivity could remain in the
blend melts as well. Figure 10 shows the relationship between the
ultrasonic attenuation and the HDPE-g-MAH content for the 90/10, 80/20,
and 70/30 HDPE/PA6 blends. It should be noted that the attenuation used
here is also called the total attenuation. Contrary lo the behavior of
ultrasonic velocity, it m evident that the total attenuation shows great
dependence on the HDPE-g-MAH content for the three blend compositions.
In general, the attenuation shows an initial sharp decline with the
increasing HDPE-g-MAH content, and then gradually levels off, showing
the similar behavior as that of the particle size. Further observations
can be found that the changes of the total attenuation with the
HDPE-g-MAH content are well fitted by the exponential equation as
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
([[alpha].sub.total,c] - [[alpha].sub.total,[infinity]]) /
([[alpha].sub.total,0] - [[alpha].sub.total,[infinity]]) =
where [[alpha].sub.total,c] (is the total attenuation for a
concentration c of HDPE-g-MAH, [[alpha].sub.total,0] is the total
attenuation for a blend without HDPE-g-MAH,
[[alpha].sub.total,[infinity]] is a constant representative of the
limiting total attenuation, and [n.sub.1] is a constant that determines
the efficiency of HDPE-g-MAH to decrease the total attenuation.
[FIGURE 10 OMITTED]
Table 3 presents the constants [[alpha].sub.total,[infinity]] and
[n.sub.1] for Eq. 5. It can be found that, with the increase of PA6
concentration, [[alpha].sub.total,[infinity]] gets increased while
[n.sub.1] decreases, suggesting that the higher concentration of PA6 in
the blend compositions, the more amounts of HDPE-g-MAH should be added
to ensure the same reducing effect. This can also be confirmed in Fig.
10, which displays the fact that the total attenuation shows higher
value in the blend with more PA6 concentration when the content of
HDPE-g-MAH is fixed.
In terms of the similarity between the changes of the total
attenuation and that of the particle size with the HDPE-g-MAH content,
it is reasonable to speculate that the attenuation could indirectly
reflect the change of the particle size of the dispersed phase. However,
as mentioned above, the ultrasonic attenuation in Fig. 10 refers to the
total allenualion ([[alpha].sub.total]), which is mainly originated from
the intrinsic wave absorption ([[alpha].sub.intrinsic), as well as the
scattering effect due to the presence of the inclusion
([[alpha].sub.excess]). [[alpha].sub.intrinsic] is simply the sum of the
absorption of each individual component and can be expressed by
[[alpha].sub.intrinsic] = (1 -[PHI])[[alpha].sub.PE] +
[PHI][[alpha].sub.PA6], where [[alpha].sub.PE] and [[alpha].sub.PA6]
are, respectively, the attenuation of the pure HDPE and PA6, and [PHI]
is the weight fraction of PA6. Herein, [[alpha].sub.excess] is mainly
concerned, because it depends largely on the inclusion size. The value
of [[alpha].sub.excess] can be obtained by subtracting the intrinsic
absorption from the total attenuation, and the equation is expressed as
[[alpha].sub.excess] = [[alpha].sub.total] -
[[alpha].sub.intrinsic] = [[alpha].sub.total] - [(1-
[PHI])[[alpha].sub.PE] + [PHI][[alpha].sub.PA6]]. (6)
Figure 11 displays [[alpha].sub.excess] as a function of the
HDPE-g-MAH content for the three blend compositions. Similar to the
behavior of the total attenuation, the excess attenuation decrease
sharply with the increase of HDPE-g-MAH content, then gradually level
off. Also, the experimental data can be reasonably well described by the
following exponential equation.
([[alpha].sub.excess,c] - [[alpha].sub.excess,[infinity]]) /
([[alpha].sub.excess,0] - [[alpha].sub.excess,[infinity]]) =
where [[alpha].sub.excess,c] is the excess attenuation for a
concentration c of HDPE-g-MAH, [[alpha].sub.excess,0] is the excess
attenuation for a blend without HDPE-g-MAH,
[[alpha].sub.excess,[infinity]] is a constant representative of the
limiting excess attenuation, and [n.sub.2] is a constant that determines
the efficiency of HDPE-g-MAH to decrease the excess attenuation.
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
The constants [[alpha].sub.excess,[infinity]] and [n.sub.2] for Eq.
7 are also displayed in Table 3. Consistent with the tendency of
[[alpha].sub.excess,[infinity]] and [n.sub.1],
[[alpha].sub.excess,[infinity]] and [n.sub.2] show the increase and
decrease with the increasing concentration of PA6, respectively. Apart
from the difference for the 90/10 HDPE/PA6 blends, [n.sub.1] and
[n.sub.2] keep the same for the 80/20 and 70/30 HDPE/PA6 blends. This
might be ascribed that the contribution from the scattering effect is
not large enough for 90/10 HDPE/PA6 blends, while it plays the main role
for the other blend compositions. Moreover, if comparing the [n.sub.2]
with the n in Table 1, it is found that, for each blend composition, the
constant [n.sub.2] is not entirely consistent with but more or less
smaller than n. As we mentioned above, the constant [n.sub.2] or n
determine the efficiency of the HDPE-g-MAH to decrease the excess
attenuation or particle size, so it indicates that ultrasonic results
show some delays in reflecting the efficiency of the HDPE-g-MAH compared
with the SEM results. Possible reasons may be due to the difference in
thermal history, causing a slight difference in the particle size for
the samples used for SEM observation and ultrasonic measurement.
Although some differences in the particle size exist for the two
characterization method, it is still encouraging to seek the
relationship between the present particle size and the excess
attenuation. By combination of Figs. 7 and 11, the quantitative
relationship between [[alpha].sub.excess] and the particle size of
dispersed phase can be obtained in Fig. 12. Figure 12 shows that
[[alpha].sub.excess] increases linearly with the increase of the
dispersed particle size for all the three blend compositions. By the
linear fitting of experimental data, three equations can be obtained as
For 90/10 HDPE/PA6 blends, [[alpha].sub.excess] = 0.18 + 0.6 x
For 80/20 HDPE/PA6 blends, [[alpha].sub.excess] = 1.01 + 0.9 x
For 70/30 HDPE/PA6 blends, [[alpha].sub.excess] = 1.25 + 1.0 x
Thus, after in-line measuring [[alpha].sub.excess] of HDPE/PA6
blend melts, the particle size of the dispersed phase can be easily
calculated using these linear equations. However, it needs to note that,
the Eqs. 8-10 have not applied to other immiscible polymer blends up to
present, and more experiments need to be conducted in the future to
verify the generality of these equations.
In this work, a reliable ultrasonic method was first used to
measure the concentration and particle size of the dispersed PA6 phase
for PA6/HDPE blend melts. It showed that ultrasonic velocity was
insensitive to the particle size but varied linearly with the blend
composition in our experimental region. However, the decrease of the
ultrasonic attenuation with the addition of HDPE-g-MAH suggested that
the attenuation depended greatly on the particle size of the dispersed
phase. Similar to the evolution of the particle size, the relationship
between the excess attenuation and HDPE-g-MAH content could be
reasonably well described by the exponential decay model. Further
investigations revealed that there was a good linear relationship
between the excess attenuation and the particle size of the dispersed
phase, which made it promising for in-line monitoring the morphological
evolution by measuring the attenuation of the blend melt during polymer
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Shan Wang, (1) Congmei Lin, (2) Huimin Sun, (1) Fan Chen, (1) Jiang
Li, (1) Shaoyun Guo (1)
(1.) The State Key Laboratory of Polymer Materials Engineering,
Polymer Research Institute of Sichuan University, Chengdu 610065, China
(2.) Institute of Chemical Materials, China Academy of Engineering
Physics, Mianyang, Sichuan 621900, China
Correspondence to: Jiang Li; e-mail: firstname.lastname@example.org or Shaoyun
Guo; e-mail: email@example.com
Contract grant sponsor: National Natural Science Foundation of
China; contract grant number: 50973075.
Published online in Whiley Online Library (wileyonlinelibrary.com).
[c] 2011 Society of Plastic Engineers
TABLE 1. Fining parameters of Eq. 4.
Blend compositions (HDPE/PA6) [D.sub.n,x] n
90/10 0.72 1.0
80/20 0.96 0.63
70/30 1.2 0..51
TABLE 2. Acoustic properties of HDPE and PA6 measured at 230[degrees]
Material Velocity (m/s) Attenuation (dB/cm)
HDPE 1016 4.95
PA6 1352 3.26
TABLE 3. Filling parameters of Eqs. 5 and 7.
(HDPE/PA6) [[alpha].sub. [n.sub.1] [[alpha]. [n.sub.2]
90/10 5.39 1.2 0.60 0.94
80/20 6.27 0.53 1.66 0.53
70/30 6.59 0.46 2.15 0.46