Novel Reinforced Polymers Based on Blends of Polystyrene
and a Thermotropic Liquid Crystalline Polymer
R. A. WEISS, WANSOO HUH, and L. NICOLAIS
Department of Chemical Engineering and Polymer Program
Uniuersity of Connecticut
Storrs, Connecticut 06268
A thermotropic liquid crystalline polymer (LCP), when
added to polystyrene (PS),can function as both a processing aid and a reinforcing filler. Thermal, rheological, and
mechanical properties of the pure components and blends
containing up to 10 percent LCP are reported. The LCP
used is immiscible with PS, and when an extensional
component of flow is present during processing, the LCP
forms an elongated fibrous phase oriented in the flow
direction. This oriented phase lubricates the melt, substantially lowering the viscosity. When the processed
blend is cooled, the dispersed fibrous LCP phase is preserved in the solidified material. The LCP microfibers behave like short reinforcing fibers to improve the mechanical properties of the blend: for example, at an LCP concentration of 4.5 percent, the modulus is increased about
40 percent vs. pure PS.
INTRODUCTION
he increasing demand in recent years for
lightweight, high performance materials
has stimulated considerable research and development of reinforced polymeric systems.
While the growth in the use of high modulus
and high strength thermoset composites has
been rapid, the development of compositions
based on thermoplastic matrices has been
slowed by the difficulties involved with processing such materials. The addition of fibers to a
polymer melt results in a substantial increase
in the viscosity, making processing difficult and
energy intensive ( 1-6). In addition, it is difficult
to achieve a uniform dispersion of fibers having
the aspect ratios required for optimizing the
product properties (7).Fiber breakage that occurs during the processing of viscous melts reduces the mechanical properties of the final
product.
One possible way to circumvent the processing problems of fiber-reinforced polymers is to
develop compositions that can be processed at
elevated temperatures as homogeneous melts
from which a rigid reinforcing phase develops
when the material is cooled to the solid state.
This concept was investigated by Kardos, et al.
(8- 10) who prepared composites from amorphous polymer matrices filled with low-molecular-weight organic crystals. The low-molecular-weight compound was soluble in the poly-
T
684
mer at melt temperatures, but became insoluble
and crystallized in a needlelike form as the
temperature was lowered. Shaw and Chen (11)
studied similar systems and found that the melt
viscosity of the blends were lower than that of
the polymer alone. In the solid state, these materials had higher modulus and damping than
the unfilled polymer. More recently, Chung, et
al. (12) described the use of low-molecularweight crystalline materials as processing aids
for polymers. The low-molecular-weight additive was a solvent for the polymer above its
melting point, but phase separated into discrete
crystalline domains below its melting point. As
a result, the additive plasticized the polymer
melt, but did not affect adversely the mechanical properties of the polymer in the solid state.
Liquid crystals are fluids that may exhibit
one- or two-dimensional order (13), and the significance of liquid crystalline polymers was recently reviewed (14).During the past decade, a
substantial research effort, both industrial and
academic, has been directed toward the development of melt processable, thermotropic liquid
crystalline polymers with high stiffness and
strength. An important characteristic of thermotropic liquid crystalline polymers, hereafter
referred to as LCP, is that a degree of molecular
order is induced by the application of heat.
When LCP are injection molded or extruded,
molecular orientation occurs and the physical
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 7987, Vol. 27, NO. 9
Polymers Based on Blends of Polystyrene
properties of molded articles exhibit a high degree of anisotropy. This often leads to exceptional mechanical properties that may, in principle, approach those of metals or fiber-reinforced polymers. Often these materials are referred to as being “self-reinforcing’’(15).
The rheological behavior of LCP was recently
reviewed by Wissbrun (16). LCPs have lower
viscosities and higher relaxation times than
comparable isotropic polymer melts. Because
oriented LCPs relax very slowly, the orientation
developed in a processed melt may be retained
after solidification.
The research described herein considers the
feasibility of controlling the rheology-property
balance of polymeric composites by employing
organic reinforcements that exhibit a liquid
crystalline fluid phase. The explicit objective
was to determine whether a single organic molecule may be used both to reinforce the composition in the solid state and also to act as a
processing aid at elevated temperatures.
The goals of the research were (1) to exploit
the fluid state of the liquid crystalline polymer
at processing temperatures, and (2) to utilize
the ability to achieve a preferentially oriented
anisotropic solid reinforcing phase via cooling
the stressed melt. This was accomplished by
choosing the pairs of components so that the
polymer and the liquid crystal were immiscible
and, concomitantly, by processing within a temperature interval in which the second phase
exhibited liquid crystallinity. A s the melt is
cooled below the crystallization temperature of
the mesomorphic additive, one might expect to
form a separate elongated phase. Such a fibrous, crystalline phase may result in mechanical properties for the blend similar to those
attained by adding reinforcing fibers to the polymer. In this case, however, fiber breakage is
avoided because the reinforcement is a fluid
during processing and the fiber is formed in
situ.
RELEVANT LITERATURE
There have appeared in the open literature
relatively few papers that consider blends of
either low-molecular-weight or polymeric liquid
crystals with polymers. Most of these works,
and especially those concerned with polymerpolymer blends, have been very recent. These
are reviewed in the discussion that follows.
Blends of Low-Molecular-Weight Liquid
Crystals with Polymers
Most of the research on blends of low-molecular-weight liquid crystals, hereafter referred to
as LC, and polymers has been concerned with
their thermodynamic interactions and phase
diagrams (17-24). Applications of LC and polymer blends have been reported by Buckley, et
al. (25)and Kajiyama, et al. (26).
The patent by Buckley, et al. (25) describes
the use of LCs as processing aids for polyolefins
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, No. 9
and polyesters. The use of LCs as plasticizers
for polymers was also. reported by our laboratory (27).Kajiyama, et al. (26)studied the transport properties of composite membranes of
poly(viny1chloride) and the LC N-(4-ethoxy-benzylidene)-4’-butylaniline.They reported enhanced oxygen and nitrogen permeabilities in
the vicinity of the crystal-to-nematic transition
of the LC.
Polymer-LCP Blends
Few publications have considered polymerpolymer blends in which at least one component
exhibits liquid crystallinity. Billard, et al. (26)
studied blends of linear mesogenic polyesters
with flexible spacers ranging in molecular
weight from 9000 to 20,000. Using polarized
microscopy they constructed binary phase diagrams and demonstrated that solid, cholesteric,
and liquid solutions could be formed by polymeric enantiomers. Kimura and Porter (29)discussed the phase behavior and some mechanical properties of blends of poly(buty1eneterephthalate) (PBT)and a liquid crystalline copolyester of ethylene terephthalate and p-oxybenzoate. In a subsequent paper, Desper, et al. (30)
presented X-ray diffraction results of these
blends that suggest nematic order of the PRT.
Sevrin, et al. (31)studied the phase diagrams
of blends of a flexible chain polymer, polyethylene glycol (PEG),of varying molecular weight
with a thermotropic cholesteric liquid crystalline polymer, hydroxypropyl cellulose (HPC).
Specific attractive intermolecular interactions
led to miscibility for blends involving low-molecular-weight PEG, but the higher molecular
weight polymers were immiscible.
Joseph, et al. (32-34) studied the structure,
thermal behavior, and mechanical properties of
blends of poly(ethy1eneterephthalate) (PET)and
a liquid crystalline copolyester of 60 mole percent parahydroxybenzoic acid and 40 mole percent poly(ethy1ene terephthalate). They found
that the LCP acted as a nucleating agent for the
crystallization of PET and that the bending
modulus of a n injection-molded 50/50 blend
was four times that of pure PET. Injectionmolded samples exhibited a skin-core morphology. The LCP formed oriented rods in the skin
region and spheres in the core.
A U.S. patent issued to Imperial Chemical
Industries (35)claims the use of LCP as processing aids for polymer melts. When about 120 percent of various LCPs were added to conventional thermoplastic polymers, a significant
improvement in the melt viscosity was realized
at shear rates between 100 and 1000 s-’. No
claims were made as to the mechanical properties of such blends, though it was mentioned
that “excessive” incompatibility between the
components has a n adverse effect on the mechanical properties. Reference is also made to
a United Kingdom patent (No. 2,008,598)that
discloses the reinforcement of a flexible poly685
R . A. Weiss. W. Huh, and L. Nicolais
mer with a rigid polymer dispersed as particles
of 1 pm or less.
Recently, Siegmann, et al. (36)described polymer blends of a n amorphous polyamide and a
LCP based on 6-hydroxy-2-naphthoic acid and
p-hydroxybenzoic acid. They observed a lower
melt viscosity for the blends than for the individual polymers. The polymers were immiscible
and the LCP phase in injection-molded samples
ranged from ellipsoidal particles to a fibrillar
structure with increasing LCP content. This
two-phase morphology resulted in reinforced
compositions, though the properties were not
homogeneous throughout the specimen. A skincore morphology was obtained and the orientation of the LCP and the mechanical properties
in the skin were higher than in the core. Apicella, et al. (37)studied the recovery above Tg
of melt drawn extruded film of polystyrene and
several LCPs. A significant improvement of the
dimensional stability of drawn polystyrene was
obtained by adding small concentrations of immiscible LCP. The LCP was oriented during the
hot drawing process, and this oriented phase
acted to constrain the shrinkage of the film at
high temperature. This effect disappears above
the melting temperature of the LCP phase.
Another approach to forming composites
from blends of two polymers was proposed by
Husman, et al. (38)and Hwang, et al. (39-41).
These authors used blends of rigid rod polymers
and flexible coil polymers to form “molecular”
composites. They found that composites derived
from a flexible coil polymer, poly-2,5(6)benzimidazole (ABPBI),and a rigid rod polymer, polyparaphenylenebenzbisthiazole, (PPBT), had
properties corresponding to a composite with
fibers having the modulus of PPBT and a n aspect ratio greater than 300. The compounds
used in that work were intractable and, therefore, unprocessable by anything other than solution-casting techniques.
EXPERIMENTAL
A commercial polystyrene (PS) with a number-average molecular weight of 86,000 and a
weight-average molecular weight of 230,000 as
determined by gel permeation chromatography
(GPC)was obtained from Montedison Chemical
Co. The liquid crystalline polymer (LCP) was
kindly provided by Professor A. Sirigu of the
University of Naples (Italy) and had the following structure:
‘*-
CH3
- o ~ = N - N = c
CH3
Blends of the LCP with PS ranging from 1.54
to 10.04 weight percent LCP were prepared on
686
a 2-inch, 2-roll mill at 19O”C,mixing for about
8 min. Films of the polymer blends were prepared by compression molding. Drawn samples
were produced with a Scientific Instruments
Max-Mixing Extruder equipped with a take-up
roll. The draw ratio was varied by varying the
speed of the take-up roll.
Thermodynamic transition temperatures of
the starting polymers and the blends were measured with a Perkin Elmer Differential Scanning Calorimeter (DSC),.Model 2, equipped with
a mechanical cooling accessory and a Perkin
Elmer Thermal Analysis Data Station, Model
3600. The cell was continually flushed with
nitrogen, and all measurements were made
with a heating rate of 20 K/min.
Thermal optical analysis (TOA)was also used
to determine the thermodynamic transitions of
the liquid crystalline component of the blends.
This instrument consisted of a polarized light
microscope with a controlled light source, a
temperature-controlled sample holder, and a
photometer. Samples were heated at 4 K/min.
Changes in the intensity of transmitted light
correspond to changes in the molecular order of
the sample.
Rheological measurements were made with a
Rheometrics System-4 Mechanical Spectrometer (RMS) and a n Instron Capillary Viscometer.
The viscous and elastic properties of polymer
melts, using steady shear and oscillatory shear,
were measured. The components of the complex
viscosity (11’ and q”) and the storage and loss
moduli (G’ and G”) were determined as a function of frequency. The experimental temperatures were selected to cover the different physical states of the LCP. Steady shear viscosity
measurements at shear rates of 7.5-1900 s-’
were made with a n Instron Capillary Viscometer. Measurements were made at 220°C using a
capillary with a length and diameter of 3 inches
and .035 inch, respectively, and a n entrance
angle of 100 degrees.
Dynamic mechanical properties were measured with a Du Pont Dynamic Mechanical Analyzer (DMA), Model 98 l : the temperature range
covered was -100°C to 200°C.
Fracture surfaces of samples subjected to different stress and thermal histories were examined by scanning electron microscopy (SEM)
(SEM ARM Model 1200) and optical polarized
microscopy. Samples, 5- 10 pm thick, were microtomed parallel and perpendicular to the flow
direction in order to characterize the organization of extruded materials.
Tensile modulus and tensile strength were
measured with an Instron Universal Testing
Machine. Drawn fiber samples with diameters
of about 0.12 mm were used, and the crosshead
speed was 0.4 inch/min. All properties reported
are based on the initial cross-section area.
RESULTS AND DISCUSSION
A DSC thermogram of the neat LCP is given
in Fig. 1. Two first-order transitions are ob-
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, NO. 9
Polymers Based on Blends of Polystyrene
served above T,: a crystalline melting point at
158°C and a mesophase-to-isotropic-liquid
transition (clearing point) at 251 "C.The mesophase between 158 and 251°C was identified
as a nematic phase by X-ray diffraction.
Dynamic mechanical analysis data for the
LCP measured from -100°C to about 140°Care
given in Fig. 2. The highest temperature relaxation (tan 6 peak at about 130°C)is most likely
associated with the melting transition, and the
relaxation at about 44°C (peak in tan 6) is due
to the glass transition. This temperature is
slightly higher than the value for Tgmeasured
by DSC, 35°C;this difference is most likely due
to differences in the characteristic frequency of
the measurement. In addition to T, and T,, a
broad low temperature mechanical relaxation
was observed centered at about -33°C. Although the origin of this transition is not clear,
lam
1
i
similar low temperature relaxations have been
observed in other aromatic polyesters (42).
These have been attributed to water by some
authors and to local motions of the ester linkages by others.
DSC thermograms of three blends containing
1.54,4.53,and 10.04percent (weight)LCP are
compared with those of the neat polymers in
Fig. 3. The T, of the PS in the blends was
independent of composition and within experimental error was identical to that of the pure
PS. The clearing temperature of the LCP component was observed in all the blends. The
measured clearing temperatures (peak temperature) for all the blends were identical within
experimental error to that of the neat LCP.
These results indicate that the LCP forms a
separate phase in the blend. Enthalpies of transitions are listed in Table 1. Based on a comparison of the enthalpy change at the clearing
point of the blend with that of the LCP, it is
estimated that the solubility of the LCP in PS is
of the order of 1 percent.
Both the crystal-to-nematic ( K + N ) and nematic-to-isotropic ( N + I) transitions were also
detected by TOA (Fig. 4), but in this case
depression of the transition temperatures was
observed as the LCP concentration decreased.
The data are summarized in Table 1 . The differences between the DSC and TOA results may
be due to the slower heating rate for the TOA
I
grn mrn
31nm
340.~3
TEMPERATURE [K)
Fig. 1 . DSC thermogram of ClOc12F3 liquid crystal polymer. Heating rate was 2OKlmin.
1
TEMPERATURE (K)
-
and polyFig. 3. DSC thermograms of blends of C10C12F3
styrene. The percentages denote the weight percent LCP.
All heating rates were 2OKlmin.
tz
w
lo
e
Table 1. Thermal Transitions' of the Neat Polymers and
Blends.
LCP conc
(wt%)
lg T + N
lo7
TEMPERATURE
Fig. 2. Dynamic mechanical data, E' and tan b us. temperature, for the CloC12F3liquid crystal.
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, No. 9
0
1.54
4.53
10.04
100
DSC
AHKdb
TOA
T+I
A H u b TKA T u
93
37
158
158
0.97
250
247
247
251
0.03
0.20
0.52
6.06
145
156
159
159
228
238
242
250
'Transition temperatures are in OC, enthalpy changes are in Callg.
AH for blends is based only on the mass of LCP in the blend.
687
R . A. Weiss, W . Huh. and L. Nicolais
experiment. Both techniques employ dynamic
conditions to measure equilibrium properties.
Because a much slower heating rate was used
in the TOA, it is likely that these results are
better representative of the equilibrium conditions. Both, however, clearly indicate that these
are two-phase systems.
The data from the DSC and TOA experiments
were used to construct partial phase diagrams
(Fig. 5). Regions I11 and IV represent the twophase regions consisting of semicrystalline LCP
c
1.541 LC,.F, IPOLYSTYREM
4.532 C,.L.F, /POLYSTYRENE
dispersed in glassy P S and melt, respectively.
Region I1 is the two-phase liquid of the nematic
mesophase and PS. The curve separating regions I1 and I11 is the melting point of the LCP
phase. Region I is a single-phase isotropic liquid.
An SEM micrograph of the morphology of a
fracture surface of a compression-molded blend
containing 4.53 percent LCP is shown in Fig.
6. The LCP forms spherical domains that are
about 3-5 ym in diameter and are uniformly
dispersed in a PS matrix.
G’ and G ” master curves of the viscoelastic
response of PS and the three LCP/PS blends are
given in Fig. 7. These curves were constructed
from data collected at eight temperatures between 110 and 250°C by shifting the curves
horizontally along the frequency axis. The reference temperature was taken as 1 10°C. Superposition of the modulus-frequency data worked
surprisingly well considering that the data included samples in which the LCP component
was a solid, an anisotropic liquid, and an isotropic liquid. Although it is not clear why superposition should work so well, it may be a consequence of having a stable dispersed phase
that is not sufficiently deformed by the oscillating shear field. Thus, even at the elevated temperatures where the LCP component is a liquid,
the rheological response of the blend over the
frequency range covered is that of a polymer
melt containing filler particles.
The other observation of interest in Fig. 7 is
that the G’ and G“ data for the 1.54 percent
LCP blend are below those of PS, while the data
for higher LCP concentrations are higher than
those for PS. This is a consequence of limited
solubility of the LCP in PS. A t low LCP concentrations, the soluble LCP plasticizes the PS, thus
lowering the modulus and the viscosity. Above
the miscibility limit, the dispersed phase acts to
increase the elasticity (G’) and viscosity (G”)of
the blend.
Steady shear viscosity data at 220°C are given
-----
10.042 C,.C,.F,
/POLYSTYRENE
L
0
50
150
100
TEMPERATURE
250
200
300
&>
Fig. 4 . TOA thermograrns of CI0Cl2F3
and the LCP/polystyrene blends.
140 k
X
REGION I
X
%
-4
so0 ‘u
0IIlA TOA
X
DSC
REGION I 1
c
!
il
I-
400 ‘K
380 k
Fig. 5. Partial phase diagram of the binary system of
CloCl2F3and PS.
688
Fig. 6. SEM micrograph of the fracture surface of a
compression-molded blend containing 4.53 percent LCP.
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, No. 9
Polymers Based on Blends of Polystyrene
in Fig. 8; at this temperature the LCP forms a
nematic mesophase. At the lower shear rates,
the viscosity of the blends is greater than that
of PS. The explanation for this is most likely
the same as given above for the increase in G'
and G". That is, the LCP exists as spherical
domains in the PS melt (this is shown in the
above discussion on the morphology of the
blends), which are not deformed by the shear
field. Instead, these domains rotate and tumble,
:yl
0
I
4
-S
0
+'
* *
I
-7
-5
-3
-1
1
3
-1
1
3
LOG F . A .
lo
1
0
-9
-7
-5
-3
LOG F . A r
Fig. 7 . G' and G" master curves for P S and the blends.
Reference temperature was 1 10°C.
0
2'
CC
.F
..
TEMPERATURE
-
220
t
>
3
z 3
2
5WAR RATE 1 LSEC-'
>
Fig. 8. Viscosity us. shear rate for LCP/PS blends. Percentages denote the weight percent LCP.
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, NO. 9
which results in increased energy dissipationand, thus, a n increase in viscosity.
At the higher shear rates, measured with the
capillary viscometer, a n inversion of the viscosity data occurs. The viscosity decreases as the
LCP concentration increases, and all the blends
have lower viscosities than that of PS. This may
be a result of the high extensional forces present at the entrance of the capillary that deform
and orient the LCP domains in the flow direction
(evidence of this will be presented shortly). Because of the unusually high relaxation times of
LC polymers, the aligned domains formed in the
entry region remain deformed and aligned in
the capillary. The LCP polymer has a much
lower viscosity than PS, and in the blend the
oriented LCP phase tends to lubricate the melt.
For example, a concentration of 10 percent LCP
results in about a 40 percent reduction in viscosity. It is precisely these two phenomena, the
lower viscosity and oriented LCP domains, that
were exploited in this study to prepare selfreinforced blends with improved processability.
Microphotographs of a blend containing 4.53
percent LCP that was extruded with the capillary viscometer are shown in Fig. 9. Figure 9a
is a fracture surface and was taken with the
SEM. The LCP was highly oriented during the
extrusion and is present in the solidified material as microscopic fibers that are well aligned
in the flow direction. The fiber aspect ratios,
determined by dissolving away the PS matrix,
were of the order of 20- 150.
Figures 9b and 9c are optical microphotographs of the extruded blend cut through the
center of a n extruded fiber sample, parallel and
perpendicular to the flow direction. Again, the
high degree of orientation of the fibers is evident. It is also worth noting that the fibers are
not homogeneously distributed throughout the
sample. The LCP fibers are concentrated in the
core of the specimen; very few fibers are present
near the surface. In fact, a ring of high concentration of fibers can be observed at a position
intermediate between the surface and the center of the sample. The distribution of LCP fibers,
however, is a function of the extrusion rate. A
sample of the same material, but extruded at a
much lower flow rate is shown in Fig. 9d. In
this case, the fibers appear to be uniformly
distributed throughout the entire sample.
Skin-core morphologies of LC polymers dispersed in thermoplastic polymers have been
reported by Joseph, et al. (33)and Siegmann,
e t al. (36).They observed, however, that the
fibrous LCP phase was concentrated near the
surface of injection-molded specimens. The difference in the blend morphologies of those studies and this one may be due to the rheological
properties of the matrix resin. Joseph, et al.
used poly(ethy1ene terephthalate) and Siegmann, et al. used a polyamide. Both of these
resins behave as elastic-viscous fluids, whereas
PS is highly pseudoplastic. An analysis by Gau689
R. A. Weiss, W. Huh,and L. Nicolais
thier, et al. (43),based on the theory of Chaffey,
et al. (44), showed that the lateral migration of
deformed liquid drops in elastic-viscous fluids
is toward the tube axis, while in pseudoplastic
liquids the drops attained an equilibrium position between the tube wall and the center line.
In the latter case, a two-way migration occurs.
Drops initially close to the wall migrate toward
the center of the tube, and drops initially close
to the tube axis migrate toward the wall. When
the flow rate is increased, the inward migration
becomes dominant and the equilibrium position
moves toward the axis.
Although certainly not definitive, Mason's
theory predicts qualitatively many of the fiber
distributions observed in this study. On this
basis, a more thorough investigation of these
and similar fluid mechanical theories in the
context of LCP/polymer blends would be appropriate.
Better fiber formation and orientation of the
LCP phase may be accomplished by melt drawing the blend extrudates. Macroscopic fibers of
the blends were prepared using a laboratory
extruder. The processing temperature was
220°C and a draw ratio of 225, based on the
ratio of the fiber diameter to the die diameter,
was used. Tensile properties of these fibers are
reported in Table 2. The tensile modulus increased with increasing LCP concentration. For
example, a concentration of 10 percent LCP
yielded a 40 percent increase in the tensile modulus over that of PS. The enhancement of the
modulus is a consequence of the formation of
the microscopic LCP fibrous phase that reinforces the polymer in the same way as do conventional short fibers, e.g., glass or graphite. In
Table 2. Mechanical Properties of Drawn Fibers of the LCP/PS
Blends (Draw Ratio = 225).
Wt. Fraction Vol. Fraction Tensile Modulus' Tensile Strength'
(1Os Pa)
(W Pa)
LCP
LCP
0
.0154
,0453
.1 004
1 .o
0
.0136
.0405
,0903
1 .o
2.39
3.03(.37)
3.19(.60)
3.59 (.89)
4.46
0.79
1.03(.17)
0.81 (.07)
0.95 (.09)
1.27
* Standard deviation given in parentheses.
Fig. 9. Microphotographs of extruded samples of a blend containing 4.53 percent LCP: (a)fracture surface; [b)extruded at
1963 s-l, cut parallel to theflow direction; [c)extruded at 1963 s-', cut perpendicular to theflow direction; (d)extruded at
7.5 s-', cut perpendicular to theflow direction.
690
POLYMER ENGINEERING AND SCIENCE, MID-MAY, 1987, Vol. 27, No. 9
Polymers Based on Blends of Polystyrene
this case, the modulus of the LCP is considerably lower than that of glass or graphite, and
therefore, the level of reinforcement is lower.
One must remember, however, that at similar
concentrations the commercial fibers would increase the melt viscosity and make processing
more difficult.
The data for tensile strength are more scattered because this property is inherently more
sensitive to sample imperfections. The data
clearly show, however, that no significant reduction of the strength of PS occurs by adding
the LCP; if anything, the trend appears to be
that of a small increase in strength. This result
is especially significant in light of the poor interfacial adhesion between the LCP phase and
PS. This is evident in the fracture surface of
the blend in Fig. 9a,where it appears that the
LCP fibers have clearly pulled out of the PS
matrix.
CONCLUSIONS
It has been demonstrated that a thermotropic
liquid crystalline polymer, when added to a
thermoplastic polymer such as polystyrene, can
function both as a processing aid and a reinforcing filler. In this study, the LCP was chosen
to be immiscible in most proportions with PS.
When a n extensional component of flow is present during processing, the LCP forms a n elongated, fibrous phase that orients in the direction
of flow. This phase lubricates the melt, thus
lowering the viscosity. Because of the long relaxation times of LCP, the fibrous nature of the
dispersed phase may be preserved even in cases
where the extensional flow is followed by simple
shear flow. If the blend is solidified under conditions that preserve the microscopic LCP fibers, reinforcement of the blend may be accomplished.
ACKNOWLEDGMENTS
Support from the International Programs Division of the National Science Foundation
(Grant INT-8219761) and the Center for LCP
Research (DARPA Grant NO0014-86-K-0772)
gratefully acknowledged. We also wish to thank
Professor M. T. Shaw for many useful consultations.
REFERENCES
1. L. E. Nielsen, “Mechanical Properties of Polymers a n d
Composites,” Vol. 2,Marcel Dekker, New York (1974).
2. 1. R. Rutgers, Rheol. Acta, 2,305 (1962).
3. T. Kataoka, T.Kitano. M. Sasahara, a n d K. Nishijima,
Rheol. Acta, 17,149 (1978).
4. J. Mewis a n d A. J. B. Spavll, Adv. Colloid l n t e ~Sci.,
.
6,173 (1976).
5. T. Kitano, T. Kataoka, T. Nishimuia, a n d T. Sakai,
Rheol. Acta, 19,764 (1980).
6. M. J. Folkes, “Short Fibre Reinforced Thermoplastics,”
J o h n Wiley a n d Sons, New York (1982).
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