Film-based structural components with controlled coefficient of thermal expansion

This invention relates in general to the preparation of near zero CTE (coefficient of thermal expansion) film-based structural components from aromatic heterocyclic, molecularly oriented, or lyotropic liquid crystalline, polymers, and to the use of such near-zero CTE film based structural components for the preparation of space-based structural members, especially film-based components used in the construction of satellites, space craft, space stations, space-based mirrors, e.g., for the Strategic Defense Initiative, and the like.

FIELD OF THE INVENTION 
This invention relates in general to the preparation of controlled, 
especially near zero, CTE (coefficient of thermal expansion) film-based 
structural components from molecularly oriented lyotropic liquid 
crystalline polymers, and particularly to the use of these near-zero CTE 
film based structural components for the preparation of space-based 
structural members, especially film-based components used in the 
construction of satellites, space craft, space stations, space-based 
mirrors, e.g., for the Strategic Defense Initiative, and the like. 
BACKGROUND OF THE INVENTION 
Conventional structural components cannot meet the strength demands of 
outer space without severe weight penalties, high cost, and/or slow 
fabrication. On the other hand, near zero CTE film based structural 
components, especially composite structures made from self-reinforcing 
multiaxially molecular oriented lyotropic liquid crystalline polymers and 
secondary reinforcing materials, demonstrate excellent intrinsic and 
tailorable properties which fulfill all of the strength and weight 
requirements envisioned for space based materials. 
The multiaxially oriented film-based materials used in the construction of 
space based components are preferably prepared from rod-like 
extended-chain, aromatic--heterocyclic polymers, which are also referred 
to as lyotropic liquid crystalline polymers, and which are also referred 
to by the shorthand expression "ordered polymers." See for example, U.S. 
Pat. Nos. 4,533,692, 4,533,693, 4,533,724 to Wolfe et al., U.S. Pat. Nos. 
4,051,108, 4,207,407, and 4,377,546 to Helminiak et al., U.S. Pat. Nos. 
4,323,493 and 4,321,357 to Keske et al., U.S. Pat. No. 4,229,566 to Evers 
et al., U.S. Pat. No. 4,108,835 to Arnold et al., and U.S. Pat. No. 
4,423,202 to Choe. The disclosures of these patents, to the extent 
necessary, are hereby incorporated herein by reference. 
Molecularly oriented lyotropic liquid crystalline polymers have been under 
development for over ten years. Impressive successes have been realized in 
the synthesis of these rigid rod-like polymers, whose strong stiff 
molecules can be processed into extremely high strength, high modulus 
fibers. 
One especially preferred molecularly oriented liquid crystalline polymer of 
this type is poly paraphenylenebisbenzothiazole or PBzT. The processing of 
PBzT into fibers has been of special interest to many investigators, and 
is currently under development for large quantity production. 
On the other hand, the processing of ordered polymers such as PBzT into 
non-fiber components is still in its infancy. Most current investigative 
work is directed at attaining only film type materials. 
SUMMARY OF THE INVENTION 
The present invention is based upon the discovery that ordered polymers, 
such as PBzT, can be readily formed into near zero CTE film based 
structural components which retain a multiaxial orientation imparted 
thereto during processing, and that these rigid, high-stiffness film based 
components can exhibit near zero coefficient of thermal expansion (CTE). 
Zero CTE materials are not adversely affected by vast swings in 
temperature, e.g., from very hot to very cold in short periods of time, 
nor are they adversely affected by long term exposure to either 
temperature extreme. The near zero CTE film based components of the 
present invention thus represent a revolutionary advance in space based 
hardware, as the extreme temperatures experienced there will not affect 
the rigidity of structures containing these components. 
Another advantage of ordered polymer near zero CTE film based structures 
are their specific stiffness, which is calculated by dividing the modulus 
value for the material by its density value. This feature is important 
because thin-walled near zero CTE film based components will be limited by 
the stress required to cause local (or shell) buckling (this will be at a 
much lower stress than the material's compressive strength), and this 
stress is directly related to the modulus. 
It is known that high modulus, low density materials are required for space 
structures. Metals fall into the range of specific stiffness of 
1.1.times.10.sup.8 in. or less. Quasi-isotropic graphite fiber-reinforced 
composite materials have specific stiffness up to 2.times.10.sup.8 in., 
but are limited by thickness. 
PBzT film-based materials have the capability to exceed 2.times.10.sup.8 
in. specific modulus as a very thin quasi-isotropic material. This allows 
the formation of thin, dimensionally stable, high stiffness near zero CTE 
film based components, and sandwich-type composites. 
In one preferred embodiment, PBzT film has been fabricated into a very 
thin-walled tube (0.005 in. wall thickness) which can be used as a 
compression strut in space-based applications. Local buckling loads could 
be further raised by slightly pressurizing the tube with a gas or liquid. 
A comparable strut made with conventional graphite/epoxy composites would 
weigh about four times as much. 
Ordered polymers, such as PBzT, are electrically and thermally poor 
conductors, although these properties may be altered by modification of 
their base chemical composition. Conductivity can also be achieved by thin 
coatings, fillers or laminates. The compressive strength of PBzT fibers is 
known to be relatively low, but this may be of minor concern in space 
structures where compressive loads are limited by elastic stability before 
compressive yield stress is reached. 
The present invention is thus directed to innovative, near zero CTE film 
based structural members prepared from lyotropic liquid crystalline 
polymers, especially well suited for space based applications, having the 
following properties: 
(a) very thin gauge, e.g., from about 0.001 to 0.050 in. 
(b) very high specific stiffness, e.g., over about 1.times.10.sup.8 in., 
preferably &gt;2.times.10.sup.8. 
(c) tailorable properties including, but not limited to; strength, 
stiffness, toughness, and thermal conductivity. 
(d) near zero coefficient of thermal expansion, e.g., less than 10.sup.-6 
.degree.C.sup.-1, which is also tailorable. 
(e) high impact resistance and fracture toughness, even at cryogenic 
temperatures. 
(f) low temperature capability, e.g., to less than about 20.degree. K., 
e.g., for liquid hydrogen space craft fuels. 
(g) may be coated for hardness and survivability, or to alter thermal and 
electrical conductivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As set forth above, the present invention is based upon the discovery that 
ordered polymers, such as PBzT, can be readily formed into near zero CTE 
film based structures which retain a multiaxial orientation imparted 
thereto during processing, and that these rigid, high-stiffness near zero 
CTE film based components can exhibit near zero coefficient of thermal 
expansion (CTE). 
More particularly, the present invention is directed to the use of PBzT for 
the formation of film based structural components having a near-zero 
coefficient of thermal expansion (CTE). 
PBzT is the most technically advanced ordered polymer currently available. 
However, similar ordered polymers with superior properties, e.g., PBzO, 
PBzX, and the like, may be likewise be included within the fabrication and 
processing techniques developed for PBzT, and thus also formed into near 
zero CTE film based structural components having the necessary properties 
for space based applications. 
The rod-like molecular structure of PBzT and its related family of ordered 
polymers, yields a "self-reinforced" material which achieves the 
properties of advanced fiber reinforced composites, but without the 
drawbacks of distinct fiber and matrix components. These well-known 
drawbacks include microcracking which occurs between the fiber and the 
matrix; mismatching of properties between the components, thickness, 
constraints, and the like. Proper orientation of the molecules during 
processing affords a structural component having a near-zero thermal 
expansion by counteracting the negative CTE in the longitudinal direction 
by the positive CTE in the transverse direction. 
Table I illustrates the CTE properties of PBzT based films. 
TABLE I 
______________________________________ 
MEASURED CTE OF PBzT FILM 
Coefficient of Thermal 
Expansion, ppm/.degree.C. 
alpha.sup.11 
alpha.sup.22 
(machine (transverse 
Film Orientation direction) 
direction) 
______________________________________ 
Uniaxial tape -15 +30 
Biaxial film, primary 
-14 +4 
orientation at .+-.110 deg. 
Biaxial film, primary 
-10 -8 
orientation at .+-.100 deg. 
Biaxial film, primary 
-7 -5 
orientation at .+-.43 deg. 
______________________________________ 
Similar behavior has been observed for graphite fiber reinforced 
composites, where the negative CTE of the graphite counteracts the 
positive CTE of the matrix but these materials suffer from the drawbacks 
outlined above. 
In preferred embodiments of the present invention, PBzT dope (PBzT and 
polyphosphoric acid) is fed to a counter rotating die to make a 
pre-oriented "green sheet." This "green sheet" can then be further 
processed by molding, stretching, and the like, to form desired 
film--based structural components. The output of the counter rotating die 
is a controllable CTE tube, which can then be further treated, e.g., with 
impregnants, and the like, to modify its properties. 
The preferred counter rotating die system for use herein is described in 
detail in Harvey et al., U.S. Application Ser. No. 098,710, filed 21Sept. 
1987, the disclosure of which, to the extent necessary, is hereby 
incorporated herein, by reference. 
A secondary processing method of the present invention involves the use of 
a compression mold having two flat plates which can be raised and lowered 
as well as rotated to provide two directions of flow and shearing. A mold 
of this type is described in U.S. Pat. No. 4,614,629 to Economy et al. The 
biaxially oriented PBzT film produced by this process can then be further 
processed, if desired, into near zero CTE film based structural members. 
In yet another processing method of the present invention, a biaxial 
stretching frame can be employed to produce a carefully controlled stretch 
in a pre-conditioned sheet of PBzT ordered polymer. 
For example, a square piece of uncoagulated polymer solution is first 
extruded using the counter-rotating die. In preferred embodiments, this 
die extrudes a tube 1.5 in. in diameter by 5 in. long. The tube is slit 
longitudinally and then placed in the biaxial stretch frame. Elongation 
ratios from 2:1 to 5:1 have been applied to the extrudate to produce 
varying degrees of molecular orientation. 
CTE measurements have been run made using a Perkin-Elmer TMS-2 
thermo-mechanical analyzer. Results show that biaxially oriented ordered 
components of PBzT film have a slightly negative CTE, and that the 
magnitude of the thermal behavior depends upon direction of orientation 
relative to the test direction. 
Compression molded specimens were tested in both the radial direction and 
the tangential direction. Specimens were mounted between split copper pins 
and cycled from -170.degree. C. to 100.degree. C., at 5.degree. C./min. 
The temperature range was selected to simulate the operational 
temperatures of outer space. 
Modifications were made to the Perkin-Elmer TMS-2 system to increase its 
accuracy and performance. The system was mounted on damping pads to 
isolate all vibrational motion. Heat tape was applied to the stand to 
prevent icing of the area surrounding the linear variable differential 
transformer which is susceptible to temperature changes. 
System accuracy using the penetration probe was checked with National 
Bureau of Standards (NBS) specimens including silica, tungsten and copper. 
All calibration trials resulted in less than 5 percent error. These 
calibrations did not use the film holder grips which were used for PBzT 
film samples. Further results from the calibration tests were not used as 
correction factors on PBzT tests. Additional reliability trials were 
necessary when using the film sample probe. Since NBS does not provide 
calibration standards in film form, pure aluminum wire was obtained from 
Perkin Elmer and used to check system error with the film probe. 
Simplified modeling techniques, based on composite plate theory have shown 
that two axis, i.e., biaxial orientation, can result in zero CTE along 
only one axis. However, the +/-45 deg. biaxial film prepared herein was 
found to be isotropically negative with CTE values of -3 to -5 
ppm/.degree.C. 
High draw, nearly uniaxial film also showed potential for laminations to 
yield low CTE values. The following discussion will compare the 
relationships between CTE and angle ply molecular orientation angle. 
PBzT film produced using the preferred counter rotating die could be made 
with primary molecular orientations of from +/-3 deg. to +/-45 deg. The 
CTE of such ordered components showed strong orientation dependence. High 
draw ordered components (+/-3 deg.) were strongly negative in the machine 
direction, with high positive CTE values in the transverse direction. 
FIG. 1 is a typical output from the quartz tube dilatometer showing the 
change in specimen length with temperature for +/-11 deg. film. The 
machine direction CTE has a value of -14 ppm/.degree.C., while the CTE in 
the transverse direction is +4 ppm/.degree.C. Transverse direction tests 
show some hysteresis with thermal cycling. As the orientation angle is 
increased with respect to the machine direction (MD) the transverse 
direction (TD) CTE falls rapidly to negative values due to the overriding 
effect of the MD high modulus. These ordered components approach -7 
ppm/.degree.C. at 45 deg., where MD equals TD. 
FIG. 2 is an output for +/-43 deg. film showing nearly equal slopes in both 
the MD and TD and, therefore, isotropic CTE. 
The compression molded PBzT ordered films exhibited a variety of thermal 
expansion behaviors due to the morphological differences between 
specimens. The differences were primarily due to orientation processing 
procedures. Radial flow of the viscous polymeric solutions upon 
compression resulted in radial orientation. Subsequent rotation of the 
plates resulted in orientation of the polymer molecules in the rotation 
(tangential) direction. Isotropic positive CTE film was produced by 
combining radial flow and rotation. 
The present invention will be further illustrated by the following 
examples, which are intended merely to assist the reader in understanding 
the present invention, and are not to be construed as limitations thereof. 
EXAMPLE 1 
In preferred embodiments, the PBzT/PPA dope is extruded through the 
preferred counter rotating tube die, forming a cylinder of uncoagulated 
PBzT, biaxially oriented by shearing between the two surfaces of the die. 
The PBzT film is maintained in an uncoagulated state by keeping it free 
from moisture until it is placed in a biaxial stretching frame. The 
stretching frame is used to stretch a flattened PBzT cylinder by a 
controlled amount in two orthogonal directions, thereby producing a film 
with additional biaxial orientation. 
The next required processing step is the coagulation of the oriented film 
by spraying it with water to lock in the orientation and then the 
placement of the film in a water bath to remove the acid solvent by 
diffusion. The water-swollen oriented PBzT film is then dried. 
EXAMPLE 2 
Using the Perkin-Elmer TMS-2 instrument, samples of PBzT film produced as 
in Example 1 were tested for CTE. One sample was made with only a small 
biaxial orientation. As expected, the CTE along the machine direction was 
negative--while it is positive in the transverse direction. Machine 
direction values of CTE ranged from -0.7 ppm/.degree. K. near 100.degree. 
K. to 10 ppm/.degree. K. The transverse direction showed hysteresis 
effects with generally a positive CTE of higher absolute values than in 
this machine direction. 
As illustrated in Table II, PBzT film can have a variety of CTE values, 
depending upon the treatment during processing. At low orientation angles, 
PBzT film shows a negative CTE in the machine direction and positive CTE 
in the transverse direction. From .+-.11 to .+-.19 deg., the transverse 
CTE goes from positive to negative. For .+-.43 deg. the measured CTE is 
negative in both the machine and transverse directions. 
TABLE II 
__________________________________________________________________________ 
CTE SUMMARY 
DESCRIPTION 
ppm/.degree.C. 
DIRECTION 
TEMP. RANGE, .degree.C. 
COMMENTS 
__________________________________________________________________________ 
.+-.11.degree., Tube Die 
-14, +4 MD, TD -170 to 100 
.+-.19.degree., Tube Die 
-10, -8 MD, TD -170 to 0 
.+-.25.degree., Tube Die 
-24, -24; 4.3 
MD, TD -150 to 150 
.+-.43.degree., Tube Die 
-7, -5 MD, TD -170 to 0 
.+-.45.degree., Tube Die 
+2, -10 MD, TD -50 to 150 
.+-.45.degree., Tube Die 
-3, -9, -17 
MD, TD, 45.degree. 
-150 to 150 
Tube Die -5, +9 MD, TD -150 to 150 Sample rolled into tubes, 
Compression 
Probe Test 
Compression Die 
8, 12, 3; 
RD; AZ -170 to 0 From green roller die; no 
rotation 
14, 16 
Compression Die 
7, 20, 10; 
RD; AZ -150 to 150 From green roller die; rotation; 
see X-Ray 
Compression Die 
3.30; -1, -5 
RD, AZ -150 to 0 From green roller die; high 
rotation 
Compression Die 
29, 30; -15, 
RD; AZ -150 to 0 Extruded into die; high rotation 
-15 
Compression Die 
-6, -11; -22, 
RD; AZ -160 to 150 Incomplete acid washout 
-14; 15.2 
__________________________________________________________________________ 
Key: 
MD = machine direction 
TD = transverse to machine direction 
RD = radial direction 
AZ = azimuthal direction 
1,2,3 is an orthogonal direction set 45.degree. in plane to MD, AZ, 1, 
etc. 
EXAMPLE 3 
The best structural components were made by infiltration of the PBzT film 
with IP600 polyimide (Nat. Starch) and lamination with the high 
temperature polymer PEEK (ICI Ltd.). IP600 was used because it is a low 
viscosity solution before curing so that it rapidly impregnates the wet 
PBzT film. Also, IP600 is a high temperature (300.degree. C.) and low 
outgassing polymer. 
PEEK resin was used as an adhesive because of its high temperature 
resistance, low outgassing characteristics, good chemical and 
environmental resistance and compatibility with PBzT and polyimide 
processing conditions. In addition, PEEK has a relatively low modulus of 
7.times.10.sup.3 mPa and high CTE of +30 .times.10.sup.-6 .degree. 
C..sup.-1. CTE values of 1 to 3 ppm .degree. C..sup.-1 were achieved. 
In addition, the values were linear throughout the temperature range from 
-170.degree. to 75.degree. C. Mechanical tests, reported below, confirmed 
the good performance of the materials and directed the future steps 
necessary to achieve a truly low (10.sup.-8 .degree. C..sup.-1) CTE 
polymer-based system. 
Table III compares the CTE values of several PBzT/Epoxy laminates. The 
numerical CTE values are based upon experimental CTE measurements and the 
units, ppm/.degree. C., are omitted for brevity. 
TABLE III 
__________________________________________________________________________ 
COEFFICIENT OF THERMAL EXPANSION FOR LAMINATED PBzT MATERIALS 
MACHINE TRANSVERSE TEMPERATURE 
MATERIAL DIRECTION.sup.1 
DIRECTION.sup.2 
45.degree. DIRECTION 
RANGE .degree.C. 
COMMENTS 
__________________________________________________________________________ 
PBzT/EPOXY -10 -10 -6 -170 to 0 Angle Ply 
PBzT/EPOXY -13 +4 -6 -170 to 0 Balanced Symmetric 
PBzT/EPOXY -10 -7 -8 -170 to 0 Quasi Isotropic 
PBzT/EPOXY -15 +2 -- -170 to 0 Angle Ply 
PBzT/POLYIMIDE 
-10 +3 -- -100 to 0 Angle Ply 
Lam. PBzt/E 50% v/v 
0, 0.11; 14; 
-- -- -160 to 150 Axial Film 
Lam. PBzT/E 12; -14, -6 
-- -- -170 to 150 .+-.45.degree. Film 
(.+-.45/0/90/Ni)s 
Lam. PBzT/E/N 
5.2; 2 -- -- -170 to 150 6 PBzT: 1Ni, Vol:Vol 
(o/90/.+-.45)s 
__________________________________________________________________________ 
The angle ply laminate was layed up from .+-.45 deg. film to have molecules 
equally oriented in the 1 and 2 directions. CTE in the 1 and 2 directions 
are equal (-10). The added epoxy affected the base CTE value by raising it 
from -15 to -10. Similarly, the CTE at 45 deg. was driven to -6 from a 
value of about -10. The balanced symmetric PBzT/Epoxy laminate made from 
high draw (.+-.11.degree. ) film shows different behavior from the 
quasi-isotropic layup. 
The specimen with less PEEK (and more PBzT) has a low CTE of +1 to +3 in 
the plane. This sample was not successfully isotropic but did give the 
lowest CTE values yet reported. The sample made with thick PEEK film was 
highly isotropic. Engineering calculations were used to estimate the most 
desirable volume fractions for a zero CTE laminate of this type, which 
yield; PBzT: 52 percent; polyimide: 38 percent; and PEEK: 10 percent. 
EXAMPLE 4 
PBzT structural composites were mechanically tested to compare their 
engineering properties to those of other structural materials and to 
determine the effects of infiltrating a low molecular weight thermosetting 
polyimide in the film prior to lamination. The uninfiltrated laminates 
were approximately 50 percent PBzT and 50 percent PEEK by volume. The 
infiltrated laminates contained approximately equal volume fractions of 
PEEK, PBzT and IP600. All laminates tested were in a quasi-isotropic 
layup. 
Initial observations on the highly oriented PBzT materials showed low 
intralaminar strength as evidenced by the ability to delaminate a film 
layer by applying a pressure sensitive tape to the surface of the film and 
peeling normal to the surface. The peeling force was sufficient to remove 
fibrils from the film. 
The process of infiltration of a thermosetting polyimide was used to 
eliminate the inherent weakness of low intralaminar strength. The data 
generated from the mechanical tests of short beam shear, tensile, and 
flexure showed much improvement in material properties with infiltration 
and the necessity to lock in the continuous rigid rods with another 
material to form a structural substrate. 
EXAMPLE 5 
The addition of nickel cladding was investigated and found to alter the 
overall CTE. The quasi-isotropic specimen was made like the previously 
described laminated specimens, with the exception that nickel foil was 
laminated to the surface with epoxy. The final specimen contained 20 
percent nickel. 
CTE measurements showed positive average CTE values in the machine and 
transverse directions with values of 5 and 2, respectively. The anisotropy 
is most likely due to slightly unbalanced or unsymmetric film plies. 
Nickel foil has a CTE of +13 and a modulus of 30.times.10.sup.6 psi. The 
high modulus and positive CTE of nickel is responsible for the large 
increase in CTE of the laminate. 
TABLE IV 
______________________________________ 
Temperature 
Description ppm/.degree.C. 
Direction 
Range, .degree.C. 
______________________________________ 
PBzT structural composites/ 
5; 2 MD; TD -170 to 150 
Ni Foil 
______________________________________ 
EXAMPLE 6 
Bonded honeycomb sandwich construction has been a structural concept in the 
aerospace industry for over thirty years. The resulting high stiffness 
with low weight is ideal for large space-based structures. This interest 
culminated in the fabrication of PBzT structural composite stiffened 
honeycomb panel having a low (less than 10.sup.-6 .degree. C..sup.-1) CTE. 
Biaxially oriented PBzT film with .+-.45 deg. orientation angles was 
laminated into two-, four- and eight-ply quasi-isotropic layups. Epoxy was 
used as the adhesive. The face sheets were subsequently bonded to 1/8-in. 
cell size Nomex honeycomb from Ciba-Geigy in a laboratory press. The 
adhesive used was AmericanCyanamid's FM123-2 which is a film adhesive 
designed for honeycomb core bonding applications. Lamination pressures 
were 85 psi. 
The panel resulting from lamination of two plies of PBzT to the core showed 
good adhesion. Four-ply construction was also demonstrated. Eight-ply 
construction was successful in forming a panel with a smooth surface. 
The present invention has been described in detail, including the preferred 
embodiments thereof. However, it will be appreciated that those skilled in 
the art, upon consideration of the present disclosure, may make 
modifications and improvements on this invention and still be within the 
scope and spirit of this invention as set forth in the following claims.