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OCR for page 49
-
PROPERTIES OF LIQUID CRYSTALLINE POLYMERS:
CURRENT AND DESIRABLE CHARACTERISTICS
The properties of LCPs are dependent on their final physical forms and
their modes of processing. This chapter is divided into sections dealing with
structural and functional properties. These are further partitioned into
subsections dealing with final forms (fibers, films, and processing), function
(electro-optic), and stability of LCPs.
STRUCTURAL PROPERTIES
Fibers
Conventional fibers are prepared either by extrusion of a polymer
solution into an evaporating gaseous stream (dry spinning) or into a
precipitating liquid medium (wet spinning) or by extrusion of a molten polymer
into a cooler gaseous environment (melt spinning). (In wet spinning of
lyotropic LOP solutions, the extruded solution usually passes through an air
(gas) gap before entering the coagulation bath. ~ The cross-section of the
spin line is attenuated during the spinning process and the molecular and/or
supr~molecular orientation in the resulting morphological hierarchy in the
filament (see Figure 2.13) is further increased by stretching (drawing) at
temperatures higher than the glass transition temperature. Ultimately the
molecular orientation in microfibrils composed of high molecular weight chains
is responsible for the desirable unidirectional high tensile strengths and
O , ~_
. . .
_ ~ _ ~ _ cat ~
moduli of fibers. Not only is it costly to achieve high orientation via the
various spinning and processing steps, but the extent of molecular orientation
is severely limited by molecular entanglements and by premature
crystallization resulting in a morphology wherein conventional polymer chains
have a more random conformation i.e., extensive chain folding and variably
oriented chains between crystalline lamellae. Accordingly, even after
annealing to extend the crystal (care crystal) component. the classical hard
fibers (e.g., ._ ~..~ ., ,___ ~
limited by this basically two-phase (crystal + amorphous) morphology to values
on the order of 5 to 15 gpd (equivalent to about 400 to 1400 MPa) and tensile
moduli on the order of 20 to 250 gpd (about l.5 to 22 GPa).
, %, ,
nv1 on ~ and nr,1 are c Marc ~ ~ i ~ i ~
~ . ~
tensile strengths that are
49
OCR for page 50
50
The LCPs, on the other hand, with their rigid, linear chains close-
packed in the fluid nematic domains , orient easily in the converging flow
field of the spinneret and refine their parallel orientation in the
extensional flow field of the spin line, emerging in a highly oriented
morphology (extended-chain crystal habit; see Figure 2.13~. The orientational
relaxation times are so long in the melt that solid fibers are readily formed
with minimal departure of the molecular orientation from that in the fluid LC
state. Annealing increases the already high orientation, resulting in
exceptionally high fiber tensile strengths and moduli. For example, Du Pont's
Ke~lar~ exhibits tensile strengths of 2.4 to 3 GPa and tensile moduti of about
65 to 145 GPa. Since Kevlar~ Is the most prevalent fiber derived from an LCP,
an extensive tabulation of its properties Is given in Table 3.1.
The temperature stability of LCPs is dramatic and may be appreciated by
comparisons with ultra-high strength conventional polymers. In recent years,
extremely large tensile values have been attained by "gel-sp~nning" of
polyethylene (PE), e.g., SpectraG (Allied-S~gnal). However, because of the
inherent molecular flexibility of PE (facile transitions among gauche and
trans-isomers) and weak intermolecular dispersion forces, its melting point is
low (about 145°C). Present and potentially commercial LCP fibers by contrast
exhibit melting points that range from 280 to well over 400°C (decomposition) .
These high values are accompanied by retention of useful mechanical properties
at temperatures well above those demonstrated by gel-spun PK. Table 3.2
summarizes the properties of a number of commercial and potentially commercial
LCP fibers; selected Kevlar~ data and those of carbon and glass are included
to facilitate comparisons.
In addition to high thermal stability, LCP fibers, in common with other
organic materials, have low densities relative to such inorganics as glass and
metals. This translates into very favorable tensile strength-stiffness-mass
characteristics (Figure 3.1), particularly significant for composites and, in
turn, for use in aircraft and space applications. Note especially the very
high specific strengths and specific moduli of the lyotropic rigid rod LCPs
such as PBZT and PBO fiber.
The very high orientations of the LCP (and gel-spun) fibers are
accompanied by poor compressive strength. To date, the only practical way to
overcome this serious problem for composites has been to employ mixtures of
the LCP fibers with graphite fibers. (Dow has recently reported that a
developmental PBO fiber may exhibit better compressive strength (H. Ledbetter,
presentation to the committee).
OCR for page 51
51
TABLE 3.1 Properties of Keviar~ Aramid Fibers Derived from Lyotropic LCPs
Property
Kevlar~ 49 Keviar~ 149
Dens i ty
Processing
Yarn tensiJe strength*
I.44g/cc (0.052 ~b/i n3) ~ . 47g/cc (0.053 ~b/i n3
Air-gap wet spinning
and heat treatment
23 gp] (2.92 GPa) IS gpd (2.29 GPa)
420 x 103 pSi 340 x 103 pSi
Yarn elongation 2.5% 1.45%
Yarn initial modulus
900 gpd (124 GPa) Ill0 gpd (141 GPa)
16.5 x lo6 psi 21 x lo6 psi
Yarn secant modulus 915 gpd (~16 GPa) 1230 gpd (156 GPa)
to I% eJongation
Epoxy strand tensi~e 525 x 103 pSi 500 x 103 pSi
strength** (3.62 GPa) (3.45 GPa)
Epoxy strand eJongation 2.9% I.9%
Epoxy strand modu~us 18-19 x lo6 psi 25-26 x 106 psi
(124-131 GPa) (172-179 GPa)
Refracti ve i ndex
Fiber axis
2.0
1.6
Equilibrium moisture
regain (25°C, 65% R.H.) 4.3%
l-~.5%
Yarn creep rate/]og 0.020% (load 41% 0.011% (load 58%
time change ultimate T. Str.) u1timate T. Str.]
Fi ber coeffi ci ent of
thermal expansion
(longitudinal) m/m/°C -4.25 x lo~6 -~.96 x lo~6
OCR for page 52
52
TABLE 3.1 Properties of Kevlar~ Aramid Fibers Derived from Lyotropic LCPs
~ continued)
Property Keviar~ 49 Kevlar~ 149
Dielectric constant, 4.14 at lo6 Hz3.90 at lD6 Hz
K (Fabric/Fiberite~ 934
Epoxy Laminate)
Electron radiation 100% of strength &
(0-200 megarads) modulus retained
Shrinkage (permanent) < 0.09% after 250°C- |
exposure
Flammability Self-extinguishing
Limiting oxygen index 29
Thermal decomposition 500°C (930°F)
temperature (TGA)
Long-term use 160°C (320°F)
temperature in air
Tensile properties at > 80% of room temp. > 90% of modulus
IS0°C (355°F) strength & modulus retained
retained
Chemical resistance, Resistant to most More chemically
bare yarns solvents and chemicals resistant than Keviar~
but can be degraded by 49
strong acids, bases,
and steam
Ultraviolet stability Degrades, but degree
depends on material
thickness since Kevlar~
aramid is a strong
absorber and is se~f
screening. In
composites, strength
loss not observed.
OCR for page 53
53
TABLE 3.l Properties of Kevlar~ Aramid Fibers Derived from Lyotrop~c LCPs
(continued)
Property Kevlar~ 49 Kev~ar~ 149
Product i on pi ant Levi arm i n genera]:
capacity:
Richmond, Virginia 45 million ib/yr
Maybown, Ireland 15 million lb/yr
*Yarn properties determined on 10'' twisted yarns (ASTM-D885~.
**Strand properties determined on untwisted epoxy impregnated yarn
(ASTM-D2343).
Source:
· Chem. and Eng. News, July 11, 1988, p. 15.
· Du Pont Data Manual for Kevlar~ 49 Aramid, May 1986.
· Du Pont Publication E-95612, 6/87, Kevlar~ 149, Properties and
Characteristics of Yarn and Composites.
· Riewald, P. G. 1988. Advanced Textile Materials Conference, Clemson
University,
Greenville, S.C., April 5-6.
Riewald, P. G., A. K. Dhingra, and T. S. Chern. 1987. ICCM and ECCM, Sixth
International and Second European Conference on Composite Materials, July
20-24, 1987, Vol. 5, pp. 5362-5370, F. L. Matthews, N. C. R. Buskell, J. M.
Hodgkinson, and J. Morton, eds. New York:
Elsevier Applied Science.
OCR for page 54
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OCR for page 56
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Kevlar 49
Em Liquid Crystalline Polymer
~ I Carbon Fiber
E~3 Ceramic Fiber
it 3 5-
_
~ 30- .
=0 2 5 i
· S Got
¢' 1 5- .
1 0- ~ Glass
l ~-
eJ O 5- . · Smel
v: - 1~ ~
o o 0 5 1 0 1.5 2.0 2.5 3.0 3.s
Specific Tensile Modulus - 10 N m.l kg
-
~ -1
,
~ High Steng~
Gel Spurt PE
Ihermo~pic LCPs
·Ul~ra~rn PE
Silicon Carbide
~ ~ 1 P3BgX RL°CP,
Boron ~H~ghModulu~
Ultrahigh Modulus
FIGURE 3.1 Specific tensile strength versus specific tensile modulus showing
LCPs in context with other materials.
OCR for page 57
S7
Injection Molding
The properties of injection molded bars of commercially available melt-
processible LCPs as reported in the trade literature are given in Table 3.3.
ICI's Victrex~ SRP-1 is believed to be a copolymer of hydroquinone (HQY,
isophthalic acid (IA), and p-hydroxybenzoic acid (HBA). Hoechst-Celanese's
Vectra~ A series is a copolymer of HBA and 6-hydroxy-2-naphthoic acid (HNA)
(approximately 73/27 HBA/HNA); Victrex~ SRP-2 is believed to be similar.
Amoco Performance Plastic's Xydar~ SRT-300 is a terpolymer of HBA, 4-4'-
biphenol (BP), and terephthalic acid (TA). Xydar~ G-330 and Sumitomo's
Ekonol~ 6000 are similar and probably also contain small amounts of IA; the
former is glass filled. Other melt-processible polymers have been announced
by Du Pont and Granmont (Montedison); the latter is commercializing technology
acquired from Owens-Corning Fiberglas Corporation. The Eastman Chemicals
copolymer based on PET and HBA, originally introduced as X7G, is also
available commercially in Japan. Other commercial announcements in Europe and
Japan are expected.
The various LCPs differ primarily in their high-temperature characteris-
tics. Therefore, a tabulation on the basis of temperature is useful (Table
3.4~. Xydar~ test pieces (based on SRT-300) demonstrate heat distortion
temperatures under load (264 psi) in the 300 to 355°C range. An Underwriters'
Laboratory rating of 240°F for continuous electrical service is reported, with
excursions to over 600°F permitted. The polymer exhibits melting points of
390 to 420°C and demonstrates excellent oxidative thermal stability at high
temperatures. LCPs useful in such high temperature ranges are known as Type
III LCPs.
Type II LCPs demonstrate melting points some 100 to 150°C lower than Type
III LCPs. This translates into lower use-temperature ranges. The Vectra~ and
Victrex~ SRP polymers fall into this category. Because of the lower melting
points, the Type II LCPs are processible at lower temperatures. This is
appealing for manufacturing molded parts that do not require the very-high-
temperature properties associated with type III.
Type I LCP properties are dominated by low glass transition temperatures
and little or no crystallinity, and they demonstrate much lower use tempera-
tures as a result. The X7G is such a polymer.
All commercial melt-processible LCPs demonstrate a number of common
characteristics. Tensile and flexural moduli of unfilled injection-molded
test bars, at room temperature, are in the 1 to 3 Mpsi range. Such moduli are
characteristic of glass-filled semicrystalline polymers; hence, these LCPs are
often described as "self-reinforcedt' polymers. These high moduli decrease as
the temperature is raised; the higher the melting point of the polymer, the
greater is the temperature range over which properties remain useful.
OCR for page 58
58
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OCR for page 59
59
TABLE 3.4 Classification of LCPs According to Thermal Behavior
Polymer
Type ~ i mi ted by
Xydar~ (SRT-300)
spectral
X7G
III Tm = 420°C
II Tm = 280°C
I T = 80-100°C
. ~
Both moduli and breaking strengths (10,000 to 40,000 psi) of bars depend
of the test specimen as well as the LOP type. This gauge effect
with more flow-direction orienta
on the gauge
arises from a marked skin-core morphology, ..~
tion located in the skin. Accordingly, the thinner the part, the greater the
strength and modulus; the effect is illustrated for Xydar~ in Table 3.5.
TABLE 3.5 Effect of Gauge on Flexural Strength and Modulus (ASTM Test Bars)
XYdar9 SRT-300 (Unfilled) XYdar49 (50% Mineral)
Gauge Strength Modul us Strength Modul us
(inches) (103 psi) (10 psi) (103 pSi) (106 pSi)
0.125 19.7 1.81 15.3 1.57
0.0625 23.8 1.98 17.1 1.78
0.0313 29.6 3.11 18.0 1.80
Source: Duska, J. 1986.
Plastics Engineering (December).
OCR for page 60
60
The combination of fountain flow (affecting the skin), complex flow in
the core, and long relaxation times results in properties that can be very
different in the flow and cross-flow directions. Anisotropy is, of course,
responsible for the desirable strength and stiffness of fibers. In molded
parts, however, it is usually undesirable. Both this anisotropy and the skin-
core effects might be considerably mitigated by proper choice and level of
fillers, resulting in more evenly distributed physical properties. However,
the choice and compounding of fillers into the various polymers is more an art
than a science.
The LCP domain structure also results in very low viscosities and
substantial shear-thinning. This viscosity behavior is basically what
distinguishes these materials from other filled high-performance polymers.
Thus, although some fiber reinforced non-LCP aromatic polymers may match many
of the physical properties of LCPs, the latter are preferred for filling long,
thin mold sections. Such low viscosities also permit heavy filler loadings.
This not only helps to smooth out the uneven directional properties referred
to above, but it can also lower the cost of the relatively expens ive base
polymers. Indeed, thermotropic LCP manufacturers do not recommend neat
polymers (except for extrusion) but, rather prefer to develop compounded
resins containing minerals - e.g., talc, glass fibers, and carbon fibers. It
should be noted that LCP viscosities are sensitive to thermal and shear
histories. It has been reported that processing temperatures can sometimes be
reduced by first raising, then lowering the temperature to the processing
temperature. Similarly, "pre-shearing" may lower the viscosity.
The all-aromatic structures of Xydar~ and Vectors (and presumable
Victrex~ SRP) translates into high resistance to burning. A particularly
dramatic demonstration involved playing an intense 2000 °F torch on a plaque of
Xydar~ without burn-through and with minimum transfer of heat, under
conditions that cut through a s imilar size aluminum panel . Such behavior ,
coupled with very low smoke generation, points to utility in military and
commercial aircraft.
One should not overlook the excellent resistance of these polymers to
most solvents. As with all polyesters, they are subject to hydrolysis, but
only under severe conditions. This much greater resistance to hydrolysis,
relative to such conventional polyesters as PET, is also a consequence of the
rigid nature of the LCP molecular structures and the accompanying highly
oriented, dense skin. (This is related, no doubt, to the reported excellent
vapor barrier properties discussed below.) It appears that the higher the
melting point of the LCP base polymer, the greater is the resistance to
solvents and chemical attack. For example, Xydar~ SRT-300 is not affected by
pentafluorophenol, whereas Vectra~ will dissolve or swell markedly.
Another important consequence of the high degree of molecular alignment
is a very low linear coefficient of thermal expansion in the direction of flow
with values approaching those of ceramics. Related to this are very low
shrinkages on molding. Proper choice and levels of fillers even out
properties and minimize warpage. The low viscosities, coupled with the low
shrinkages and Anyplace coefficients of expansion, allow for the molding of
dimens tonally s table, high- precis ion parts .
OCR for page 61
61
High dielectric strengths make for excellent insulating properties.
High-temperature resistance is required to stand up to the demands of wave and
vapor-phase soldering, IR reflow, and burn-in testing. These are
characteristic of aromatic LCPs, and consequently these polymers (usually with
fillers) are prime candidates for electrical and electronic applications,
including electronic and optical connectors, chip carriers and sockets for
integrated circuit mounting and testing. The continuing advances in
miniaturization and automation in assembling electronic arrays will require
new materials, and all indications are that there will be increased use of
LCPs in these future applications.
Finally, one area that is a significant problem in injection molding of
LCPs is that of poor weld strength. When two polymer flow fronts meet, the
strength of the resulting joint is lower than that of the bulk. This results
from poor molecular interpenetration and incomplete entanglement of the
molecules. In the case of the LCPs, becaus.e of the rigidity of the molecules,
the decrease in strength is particularly severe. Of the various stratagems
employed to compensate for this defect, the most promising approaches involve
optimizing mold and gate designs.
In summary, the LCP manufacturers encourage the use of their resins in
electrical and electronic, aviation and other transportation, chemical
processing, fiber optics, and aerospace applications. Starting from a low
volume base, LCP consumption has been predicted to grow at least 15 to 20
percent a year, far outpacing the growth rate of conventional engineering
resins.
Extrusion
_
Extrusion as a mode of processing LCPs to produce film and sheet has not
been exploited to any great extent because of difficulties in producing
uniform gauges and properties in practical widths. Markedly unbalanced
properties, i.e., weak transverse properties relative to the machine
direction result from the high orientation in the flow direction. The charac-
teristically low elongation of LCP melts makes it difficult to draw biaxial
films or to blow films to the ratios required for useful balanced properties.
Nonetheless, a few extrusion-grade LCPs are offered, primarily among the Type
II thermotropic polymers and recently films and sheets of Vectra~ have been
introduced. PBZT films from lyotropic dopes with tensile strengths and moduli
of 80 to 100 Kpsi (0.55 to 0.69 GPa) and 30 Mpsi (207 GPa), respectively, have
been reported (R. Lusignea, presentation to the committee).
There is significant motivation for the development of LCP in sheet form
for thermoforming. The excellent resistance to burning, when coupled with
light weight and high stiffness, make LCPs potentially very attractive for use
in aircraft. Another incentive for developing LCP films lies in the
increasingly attractive barrier properties that are being uncovered. Table
3.6 lists some values for LCPs versus more conventional polymers.
OCR for page 62
62
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OCR for page 63
63
FUNCTIONAL PROPERTIES
Desirable Electro-Outical Pronerties
There are several categories of second- and third-order nonlinear optical
(NLO) processes that must be considered to delineate desired properties and
compare the potential advantages (or disadvantages) of LCPs relative to
alternative polymeric or inorganic materials. In some cases a clear advantage
is perceived for the organic and polymeric materials, whereas, for others,
various classes of inorganic materials either suffice or are superior. In
general, liquid crystallinity provides anistropy in the medium that can lead
to larger nonlinear coefficients.
Table 3.7 lists general goals that are perceived to be important for
electro-optical applications (Stegeman and Lytel, 19899. For some
applications active stabilization and feedback might be used where properties
fall short of requirements.
TABLE 3.7 Important Requirements for NLO Applications
. <0.~% drift in x`2' and n for 5 to TO years
.
EJectro-optic coefficient equal to or greater than LiNbO3
Sufficient optical clarity for O.] to 0.5 db/cm attenuation at 0.85 ~m,
I.3 ~m, and 1.55 him
Low dispersion in the optical and microwave regions
<0.~% variation in thickness
High thermal conductivity
For second-harmonic-generation applications, some additional require-
ments are needed; these are listed in Table 3.8 (Stegeman and Lytel, 1989~.
These requirements should be viewed as general and will vary for specific
applications. They are biased toward thin-film optical devices as opposed to
bulk devices, for which many single-crystalline materials exist. For serial
processing of information' a premium is put on turn-off time (~) of the
nonlinear response for high throughput, low absorption coefficient for low
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64
TABLE 3.8 Requirements for SHG Applications
Low refractive index dispersion between ~ and 2
X(2) > ]0-7 esu/cm4
No 2-photon absorption
Low optical absorption at fundamental and harmonic frequencies
Photochemi Cal stabi ~ i ty
attenuation and thermal amicability, and high nonlinear coefficient . A list of
requirements that would permit development of all - optical serial switching
devices is given in Table 3.9 (Stegeman and Lytel, 19899.
TABLE 3.9 Requirements for Optical Serial Switching Devices
.
2 > ~o-14 M2/W
High device throughput aL < 0.2 for 80% throughput (a is adsorption
and scattering losses and ~ is interaction lengthy at 1.55 him and
approximately 0.]
Low-temperature dependence of n and n2
+ 100 A dimensional stability
Processible into waveguides
High damage threshold > 10 GW/cm2
n2 relaxation time < lD-42 sec
Figure of merit W = AnSa~/~) approximately 0.5 to 2.0 (device
dependent), where Kansan is the saturation value of the nonlinear
refractive index, ~ is the absorption coefficient, and ~ is the
wavelength
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65
Currently Available Electro-Outical Properties
The predominantly used electro-optical material for integrated optics
applications is LiNbO3. It has excellent electro-optical properties and can
be fabricated into high-quality waveguided devices with both active and
passive components for modulation, switching, and information-processing
applications . Methods for manufacturing, device engineering, and fabrication
are mature. Excellent progress has been made in device packaging and in
hybrid technologies for electrically driving the devices. Another relatively
new NLO material that is both more efficient than LiNbO3 and is free from many
of its deficiencies is potassium titanyl phosphate (KTP) (Bierlein and
Vanherzeele, 1989~.
In spite of many years of effort, however, LiNbO3-based integrated optic
technology has not become widespread and pervasive. There are several reasons
for this. First is the cost of manufacturing high-quality, impurity-free
substrates for processing into devices. Although device-grade material is
readily available today, impurity-controlled optical damage was a problem for
many years. Second, the predominant wave guide formation technology involves
high-temperature diffusion of titanium into LiNb03, and minor differences in
one of a number of processing parameters can have a major impact on device
performance. Third, LiNbO3 is not readily integrated with silicon- or GaAs-
based technology. Because of the predominance of silicon in the electronics
industry and the promise of III-V compound semiconductors for performing both
optical and electronic functions, considerable attention may be shifting
toward these technologies in the future. Even though KTP appears to be an
excellent NLO and electro-optical material it will continue to be incompatible
with silicon or GaAs unless progress is made in growing thin films of the
material.
It is noteworthy that, with relatively modest research efforts, poled
polymeric materials have been shown to exhibit electro-optic coefficients
similar to LiNbO3 with low losses of approximately 0.8 db/cm at O.83 ~m.
Polymers have a tremendous potential advantage relative to LiNbO3 in that they
can be spin-coated or cast, which significantly increases their chances of
being compatible with integration on silicon and GaAs substrates. Preliminary
demonstrations of waveguide formation by poling with patterned electrodes look
promising for some types of devices, and GHz responses have been measured in
traveling-wave devices (Carney and Hutcheson, 1987~. Reports of gradual
deterioration of induced alignment have been a major source of concern for
poled polymers (Ye et al., 1988), as is transparency in the important 1.3 Am
and 1.55 Am regions because of -C-H vibrational overtones.
For second-harmonic-generation applications, the situation with respect
to materials is much more complex. The major drive for this technology, aside
from spectroscopic applications, is the conversion of 825-nm output of laser
diodes to 412 nm for optical memory applications. Proton-diffused LiNbO3
waveguides have been shown to convert significant amounts of 1060-nm light to
533 nm, but with very poor beam quality (see New Scientist, 1988~. A variety
of intracavity second-harmonic-generation and frequency mixing devices have
been demonstrated with miniature KTP crystals in conjunction with diode laser-
pumped miniature Nd:YAG rods, and these are now commercially available.
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66
Because of the superior properties of KTP, including a large xf2) of 3 x 10-8
esu, excellent transparency, and linear optical properties, this material
shows great promise for frequency doubling and mixing applications. However,
the low nonlinear coefficient requires high-power diode lasers (Risk et al.,
1988).
Very little has been reported in the literature to date on poled polymers
designed for harmonic generation to 412 nm. A major problem is the trade-off
between transparency of a nonlinear chromophore in the visible region of the
spectrum and the magnitude of its nonlinear coefficient. A second problem is
the nature of the xf2' tensor for materials with electric field polar
symmetry, which makes phase matching (matching the phase velocity of
fundamental and harmonic waves) difficult in bulk devices. Sophisticated
waveguide structures using polymers might permit this. The main reasons for
considering polymeric materials for this class of applications are the
potential for high nonlinear coefficients, possibly an order of magnitude
larger than RIP, and the potential for integration with diode laser light
sources.
In discussing material properties for third-order nonlinear effects, it
is convenient to follow the distinction made earlier between those used In
devices for parallel processing and serial processing.
In parallel processing, two-dimensional arrays of pixel-like elements are
likely to be switched between on and off states in response to input and
control optical signals. Various devices have been proposed and investigated
for this purpose, ranging from nonlinear Fabry-Perot cavities that act as
bistable devices to thin-film semiconductor heterostructure devices made of
multiple quantum well (MQW) material. One example of the latter is a self-
electro-optic device (SEED) that acts as a light-triggered switch (Miller et
al., 1984~. Because throughput through such devices is highly parallel, they
are capable of handling large amounts of information. A premium is put on
optical power requirements in such devices, as opposed to the switching rate
of the device. In serial processing, bandwidth is determined by the number of
switching operations per second. All optical devices with subpicosecond
response times have a clear advantage relative to electronic devices, whose
response times are several orders of magnitude longer for this type of
application (Stegeman and Lytel, 1989~.
The light-intensity-dependent refractive index n2, response time I,
absorption coefficient a, saturation value of the refractive index Ansa~, and
FOM W = An/ for a variety of materials are shown in Table 3.10 (Stegeman
et al., 1987~. Where NR appears in the table, the measurement was made under
nominally nonresonant conditions. This does not imply, however, that ~ is
negligible. Values of bonsai were not reported for a number of the polymers,
but if one assumes they are similar to those for PTS away from and near
resonance, several observations can be made. First, n2 for GaAlAs MOW devices
on resonance is extremely large relative to PTS on resonance. The power per
bit to switch a bistable device is 6 to 7 orders of magnitude less for the MQW
than for the polymer. Since the switching rate is adequate for highly
parallel computational architectures, it is clear that organic systems of the
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Representative terms from entire chapter:
lcp fibers
67
TABLE 3.10 NLO Materials Properties (Inorganic and Organic)
Waveguide Material n2' M2/W I, sec a, cm~: Any' W
MQW-GaA]As
On resonance -10-8 10-8 104 0.1 0.1
Off resonance _lo-~2 10-8 30 -2 x 10-3 ~0 9
CdSSe-doped glass -10-~4 1o-:i 3 5 x 10-5 -0.3
1
PTS
On resonance 2 x 1O-is ~2 x 10-~2
Off resonance
lo-16
104
<0.03 x 10-~2 102
Poly-4-BCMU 6.4 x 10-~6 <10-~3 NR
1
PBZT lo-17
68
polymers for all optical serial switching operations. The second requirement
for the class of devices listed in Table 3.9 is critical for device operation
and will be extremely difficult to achieve in polymeric materials. A trade-
off exists between this requirement and the magnitude of n2 as long as ~ is
sufficiently small, i.e., less than 0.1. A gain of two orders of magnitude in
nonresonant n2 would result in a shorter interaction length and smaller
switching power and would ease the constraints on device design and motivate
considerable device-related research on such materials.
ENVIRONMENTAL STABILITY
The environmental stability and durability of LCP-derived structures,
devices, and products is a broad and complex subject, especially when certain
factors unique to military materiel are considered. These environmental
factors fall into two major categories: those associated with the natural
environment and those imposed by the end use or application environment. In
the former category, temperature and moisture are the most significant, with
W radiation and environmental pollutants such as the oxides of sulfur and
nitrogen sometimes playing a role. In the latter, temperature may be a
consideration, but application-specific factors such as operational fluids,
including fuels, lubricants, and de-leers, must also be considered. Of
special concern to the military is the need to withstand the effects of
chemical warfare (COO) liquids for both short- and long-term exposures.
The nature of the application will dictate the degree to which
environmental exposure is experienced. For example, consider an LCP-based
component in an optical computer operating In an air-conditioned, temperature-
controlled communications center, as contrasted with a Kevlar~ aramid-based
composite aircraft structural element exposed to an entire spectrum of
environments: from the tropics to the arctic, from ground level to 40,000
feet, etc. The standards for durability are clearly different for these two
cases.
The chemical and physical nature of the polymer will also affect response
to environmental exposure. Polymers that contain hydrolytically susceptible
linkages, such as polyesters and polyamides, are at greater risk in hot and
wet conditions, particularly in the presence of catalytic species such as
acids or bases, than are hydrocarbon or fluorocarbon polymers. Similarly,
amorphous polymers are usually more prone to hydrolysis than are
semicrystalline or crystalline materials because of the reduced diffusion of
water in the latter.
Resistance to CW liquids can be viewed as a specialized aspect of
durability peculiar to military materiel. Those liquids fall into two
categories: chemical agents and decontaminants. For the toxic agents, the
major concern is that a material may absorb a quantity of the agent and,
through diffusion, rerelease it at a time and place such as a maintenance
facility where its presence is not anticipated. The consequences for un-
protected personnel could be catastrophic.
69
In the case of decontaminants, the issue is the degradation of physical
and mechanical properties, both short- and long-term. This is especially true
for the most common decontaminant, DS-2, an extremely caustic mixture of
sodium hydroxide, diethylenetriamine, and methyl cellosolve. This aggressive
fluid has already been shown to attack a variety of organic materials. For
instance, immersion in DS-2 leads to the complete dissolution of polycarbonate
(Lee and Vanselow, 1987), while a glass/polyester composite gained over 6
percent in weight during extended exposure (Wentworth, 1986~. In light of
this, it seems likely that certain classes of LCPs, most notably the
thermotropic polyesters and the aramids, will be adversely affected by
exposure to CW liquids.
Although environmental exposure, whether natural or applications-related,
can lead to deterioration of LCP-derived articles and structures, this fact
should not be regarded as an indication that LCPs are unsuitable. It is
clear, however, that during the materials selection process careful
consideration must be given to the range of environments to which the item
will be exposed and that the material must be evaluated for its response to
that range of environments. In some cases, such data may already exist in the
literature. The environmental response of Kevlar~ aramid, for example, is
well documented (Morgan and Allred, in press). Where appropriate, this
includes an assessment of the effect of CW liquids. Even in those cases where
an unacceptable effect is observed, it is likely that adequate protection,
probably in the form of coatings, can be provided to reduce the effect to
acceptable levels. Indeed, there are reports that the thermotropic polyesters
themselves exhibit exceptional barrier properties (Chiou and Paul, 1987) and
may be viable candidates as protective coatings in their own right.
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Bierlein, J. D. and H. Vanherzeele. 1989. Potassium titanyl phosphate-
propert~es and new applications. J. Opt. Sac. Am. B644~:622-633.
Carney, J. K. and L. D. Hutcheson. 1987. P. 229 in Integrated Optical
Circuits and Components, L. D. Hutcheson, ed. New York: Marcel Dekker.
Chiou, J. S. and D. R. Paul. 1987. Gas transport in a thermotropic liquid-
crystalline polyester. J. Polym. Sci. Phys. Ed. 25:1699.
Lee, L. H. and J. J. Vanselow. 1987. Chemical Degradation and Stress
Cracking of Polycarbonate in DS-2. MTL TR 87-46 (September).
Miller, D. A. B., D. S. Chemla, T. C. Damen, T. H. Wood, C. A. Burrus, A.
Gossard, and W. Wiegmann. 1984. Optical-level shifter and self-
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effect device. Optics Lett. 9~129:567-569.
Morgan, R. J. and R. E. Allred. In press. Aramids. In Encyclopedia of
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New Scientist. 1988. 21:168. See also Electronics' July 1988, p. 36.
Riewald, P. G. 1988. Advanced Textile Materials Conference. Clemson
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W. Lenth. 1988. Generation of blue-light by intracavity frequency
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Stegeman, G. I., and R. Lytel. 1989. Nonlinear Optical Effects in Organic
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Ye, C., N. Minami, T. J. Marks, J. Yang, and G. K. Wong. 1988.
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