Skin-Care
The outstanding mechanical properties of an en-
gineering liquid crystalline polymer (LCP) can be
attributed to the self-reinforcement effect due to its
rigid rod-like molecular structure [1-4]. High
strength and high stiffness liquid crystalline polymer
fibres, e.g. Kevlar and Vectran have been developed
and commercialized [5, 6]. Recently there has been
increasing interest in blending thermotropic liquid
crystalline polymers with conventional isotropic
polymers. Liquid crystalline polymer fibre reinforce-
ment can be formed in situ in an isotropic polymer
matrix via a melt blending process [7-26].
The
structure and morphology of the in situ composites
may be controlled by varying the processing
conditions and theological history of the blends.
Blends of LCPs with conventional polyesters [7, 8],
nylon [9], polycarbonate [10-13], polystyrene
[11, 14], polypropylene [15, 16], polysulphone [17],
poly[ether imide] [18] and poly[phenylene sulphide]
[19] have been studied. For extrusion-blended
materials, the structure and mechanical properties
were found to be closely related to the extrusion
conditions, in particular the draw-down ratio. Most
researchers have attempted to explain the changes in
mechanical properties in terms of the morphology of
the LCP phase in the blends, with the most widely
used techniques to study the morphology being
scanning electron microscopy (SEM) and X-ray
diffraction (XRD) [7, 12].
Transmission electron microscopy (TEM) has been
widely used to study the crystalline structure of
liquid crystalline fibres [4, 6, 20, 21] and the mor-
phology of multiphase polymer systems [22]. Analy-
sis has revealed detailed information on the
crystalline structure of aramid and thermotropic
liquid crystalline polyester fibres and the develop-
ment of skin-core morphology of such fibres [4, 21 ].
These findings were related to the properties and
sample processing conditions.
The work reported in this letter is concerned with
the application of the TEM technique to the analysis
of blends of a liquid crystalline polymer and
polycarbonate. The microstructure and morphology
0261-8028 © 1996 Chapman & Hall
of the blends, in particular the development of in
situ LCP fibrils in the polycarbonate matrix during
melt extrusion, was examined. TEM analyses have
indicated that there is a differentiation in the fibril
morphology at the skin and core region of the
extrudates. It is understood that these fibrils are
ultimately responsible for the reinforcement effects
in such in situ composites. Hence the molecular
ordering and geometric morphology of these fibrils
affect significantly the mechanical properties, such
as the strength and stiffness of the blends.
The blending of a main chain thermotropic liquid
crystalline polymer (Vectra B950, Hoechst Celanese)
and a bisphenol-A polycarbonate (Lexan 151,
General Electric) was carried out using a co-rotating
twin screw extruder, the processing details having
been described elsewhere [12]. Blends of two
different compositions (10wt% and 30wt% of
LCP in polycarbonate) were prepared in the present
study and the draw-down ratio was kept constant at
about 12. The extrudates obtained, typically about
1 mm in diameter, were embedded in a cold-curing
Spur resin of suitable hardness. Ultra-thin sections
(thickness <100 nm) of the blends were cut using a
diamond knife on a Reichert Jung Ultracut-E
ultramicrotome. The cutting was carried out long-
itudinally perpendicular to the direction of drawing
with ultra-thin sections from both the outer skin and
the inner core region being prepared
.
The ultra-thin samples of these blends were
examined using a 200kV Hitachi 8100 STEM
operated under TEM mode. A low electron beam
dose was used to avoid beam damage on the
polymers. Bright field images of the in situ
composites were obtained without any staining
processes. Selected area diffraction patterns were
obtained from areas of 500 run in diameter.
Fig. 1 shows the TEM bright field images obtained
from a blend containing 10wt% LCP. TEM
micrographs were taken of the ultra-thin samples
sectioned from the skin and the core regions of the
extrudate. A distinct two-phase structure was ob-
served in the blend with the LCP phase dispersed
The outstanding mechanical properties of an en-
gineering liquid crystalline polymer (LCP) can be
attributed to the self-reinforcement effect due to its
rigid rod-like molecular structure [1-4]. High
strength and high stiffness liquid crystalline polymer
fibres, e.g. Kevlar and Vectran have been developed
and commercialized [5, 6]. Recently there has been
increasing interest in blending thermotropic liquid
crystalline polymers with conventional isotropic
polymers. Liquid crystalline polymer fibre reinforce-
ment can be formed in situ in an isotropic polymer
matrix via a melt blending process [7-26].
The
structure and morphology of the in situ composites
may be controlled by varying the processing
conditions and theological history of the blends.
Blends of LCPs with conventional polyesters [7, 8],
nylon [9], polycarbonate [10-13], polystyrene
[11, 14], polypropylene [15, 16], polysulphone [17],
poly[ether imide] [18] and poly[phenylene sulphide]
[19] have been studied. For extrusion-blended
materials, the structure and mechanical properties
were found to be closely related to the extrusion
conditions, in particular the draw-down ratio. Most
researchers have attempted to explain the changes in
mechanical properties in terms of the morphology of
the LCP phase in the blends, with the most widely
used techniques to study the morphology being
scanning electron microscopy (SEM) and X-ray
diffraction (XRD) [7, 12].
Transmission electron microscopy (TEM) has been
widely used to study the crystalline structure of
liquid crystalline fibres [4, 6, 20, 21] and the mor-
phology of multiphase polymer systems [22]. Analy-
sis has revealed detailed information on the
crystalline structure of aramid and thermotropic
liquid crystalline polyester fibres and the develop-
ment of skin-core morphology of such fibres [4, 21 ].
These findings were related to the properties and
sample processing conditions.
The work reported in this letter is concerned with
the application of the TEM technique to the analysis
of blends of a liquid crystalline polymer and
polycarbonate. The microstructure and morphology
0261-8028 © 1996 Chapman & Hall
of the blends, in particular the development of in
situ LCP fibrils in the polycarbonate matrix during
melt extrusion, was examined. TEM analyses have
indicated that there is a differentiation in the fibril
morphology at the skin and core region of the
extrudates. It is understood that these fibrils are
ultimately responsible for the reinforcement effects
in such in situ composites. Hence the molecular
ordering and geometric morphology of these fibrils
affect significantly the mechanical properties, such
as the strength and stiffness of the blends.
The blending of a main chain thermotropic liquid
crystalline polymer (Vectra B950, Hoechst Celanese)
and a bisphenol-A polycarbonate (Lexan 151,
General Electric) was carried out using a co-rotating
twin screw extruder, the processing details having
been described elsewhere [12]. Blends of two
different compositions (10wt% and 30wt% of
LCP in polycarbonate) were prepared in the present
study and the draw-down ratio was kept constant at
about 12. The extrudates obtained, typically about
1 mm in diameter, were embedded in a cold-curing
Spur resin of suitable hardness. Ultra-thin sections
(thickness <100 nm) of the blends were cut using a
diamond knife on a Reichert Jung Ultracut-E
ultramicrotome. The cutting was carried out long-
itudinally perpendicular to the direction of drawing
with ultra-thin sections from both the outer skin and
the inner core region being prepared
The ultra-thin samples of these blends were
examined using a 200kV Hitachi 8100 STEM
operated under TEM mode. A low electron beam
dose was used to avoid beam damage on the
polymers. Bright field images of the in situ
composites were obtained without any staining
processes. Selected area diffraction patterns were
obtained from areas of 500 run in diameter.
Fig. 1 shows the TEM bright field images obtained
from a blend containing 10wt% LCP. TEM
micrographs were taken of the ultra-thin samples
sectioned from the skin and the core regions of the
extrudate. A distinct two-phase structure was ob-
served in the blend with the LCP phase dispersed
No comments:
Post a Comment