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PROBLEMS AND ISSUES: RECOM MENDATIONS FOR FURTHER WORK
MACROMOLECULAR DESIGN AND SYNTHESIS
Sidechain LCPs
Although not suited for structural use, sidechain LCPs have potential as
functional materials. The anisotropic organization of pendant groups can be
intimately related to function, as, for example, in the nonphotobleachable
colored film materials derived from cholesteric sidechain polymers (Shannon,
1984~. In this case and in general, mesomorphic structure is readily frozen
into a glassy mesophase without crystallization as the polymer is cooled below
its glass transition temperature (T~.
Retention of mesophase positional alignment and dipolar orientation upon
electric field poling while cooling below To can lead to bulk noncentro-
symmetry in a mesomorphic system (Meredith et al., 1982) e Development of
properties dependent on an acentric structure, such as certain NL0 effects,
piezoelectricity, and pyroelectric~ty, is possible if the structural chemistry
of the polymer is designed properly. To the degree an anisotropic arrangement
of functional species can contribute to enhancement of these effects, a
mesogenic polymer is advantageous.
Ferroelectric behavior has been observed for smectic C sidechain LCPs
(Shibaev et al., 1984~. Electro-optical devices based on ferroelectric LCs
are known (Clark and Lagerwall, 1984~. Although usually slower in response
than low-molar-mass ferroelectric LCs, advantage can be taken of their
polymeric nature. Since surface-stabilized ferroelectric LCs are in a sense
"self-poling," a combination of ferroelectric and, for example, NL0 properties
in a single material would be attractive.
Lightly cross-linked LCPs, to date primarily sidechain LCPs, can be
elastomeric above T~ (Finkelmann et al., 1981~. These elastomers exhibit
orientation of mesogenic pendant groups upon application of stress to the
network. LCP elastomers have been envisioned as optical waveguides and as
71
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72
selective barrier materials (membranes). In situ photopolymerization of
sidechain LCPs in the mesophase has been recently demonstrated and is
perceived (Broer et al., 1988; Hoyle et al., 1988) as a processing option that
may lead to high-speed formation of LCP films and coatings. In addition,
masking techniques used with in stun photopolymerization are seen as
potentially useful for integrated optics, displays, and optical information
discs. Laser-addressed smectic sidechain LCPs have been described for
information- storage applications (Hopwood and Coles, 1985 ~ .
Historically, the synthesis of sidechain LCPs has been primarily free-
radica1 vinyl polymerization of acrylate or methacrylate monomers . Recently,
combined sidechain and mainchain LCPs have been described (Reck and Ringsdorf,
1985 ~ based on a polyester mainchain. There has been rapid development of
synthetic methodology toward introduction of new mainchains for sidechain LCPs
including polysulfones (Braun et al ., 1987 ), polyesters (Griffin et al .,
19 8 8 ) , and polyurethanes (Tanaka and Nakaya, 19 8 8 ~ Polysiloxanes (Finkelmann
and Rehage, 1980) and, more recently, polyphosphazenes (Singler et al., 1987)
have been used for reactions in which a mesogenic pendant group is grafted
onto the polymer backbone. Sidechain LCPs from these preformed polymers and
from new polymer backbones offer interesting auxiliary properties, such as
variability in T~, solubility, stability, etc. Hence, it is recommended that
attention be paid to novel functional sidechain (and mainchain) LCPs.
Opportunities for exploiting the interplay of function and chemical structure
in these anisotropic systems should be taken where their structural anisotropy
and glassy mesophase formation can lead to enhanced performance.
Thermotrop~c LCPs
Among the serious structural issues arising in LCP polyester syntheses
are the elucidation and control of the mer sequence along the polymer
backbones (Economy et al., 1989; Muhlebach et al., 1988~. At best, one can
only see dyads via high-resolution NMR in the solid state or in solution
(where soluble). These tend to confirm X-ray studies, which conclude that the
sequences are essentially random (in other than the trivial simple A-A B-B
case) (Blackwell and Biswas , 1986 ~ . Given the heterogeneous nature of
polymerizations involving free terephthalic acid as well as differences in
condensation rates, blocky sequences would be anticipated in the absence of
extensive transesterification. Clearly, the latter is an important
accompanying reaction as confirmed recently (Economy et al. , 1989 ; Jin 1989~ ;
controlled sequence distributions were shown to randomize rapidly by
interesterification on heating. To control the sequence distribution, new
polymerization techniques and/or catalysts and transesterification inhibitors
will have to be developed, as well as improved methods to measure the
resulting distr~butions. Until these are accomplished, the structure-property
relationships associated with backbone sequence and subsequent design of
possibly improved polymers will be severely hampered.
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73
Monomer Costs
The reason for the commercial and military importance of LCPs resides in
their unique properties. However, their commercial future and availability is
intimately tied to lowering their cost. In turn, this is largely determined
by the cost of the monomers required for their manufacture. The LCPs that are
most significant from a commercial and/or military viewpoint are derived
essentially from all-aromatic monomers.
The significant monomers for commercial and near-commercial thermotropic
polyesters include A-B as well as A-A and B-B types. The latter include the
dials, such as 4,4'-biphenol (in Amoco's Xydar~ and S~mitomo's Ekonol@),
hydroquinone (in ICI's SRP) and phenylhydroquinone (Du Pont), and the diacids,
including terephthalic and isophthalic acids. The significant A-B monomers
are 4-hydroxybenzoic (in most LCPs) and 6-hydroxy-2-naphthoic acids (in
Hoechst-Celanese's Spectral. Of the above, the lowest-cost monomers are the
diacids, at substantially less than $1.00/lb. Next in price are hydroquinone
(potentially) and p-hydroxybenzoic acid, at less than $2.00/lb. The most
expensive of the thermotropic LOP monomers are 4,4'-biphenol and
6-hydroxy-2-naphthoic acid, at somewhere between $3.00/lb and $10.00/lb.
Phenylhydroquinone is probably included in the latter range.
Of the lyotropic LOP monomers, terephthalic acid and its acid chloride
(equivalent) are under $.00/lb, p-phenylenediamine is in the $2.00/lb range,
the meta, pare hybrid diamines are probably in the $2.00 to $5.00/lb range,
and far and away the most expensive of the monomers, by at least an order of
magnitude, are the multifunctional monomers required for the heterocyclic PBX
LCPs.
Future Cost Challenges
The cost challenges inherent in developing future thermotropic LCPs will
be either (a) to develop higher-temperature properties utilizing the lower-
cost hydroquinone or (b) to develop low- cost syntheses for the higher-
temperature monomers or (c) to design and synthesize new low-cost aromatic
monomers. An example of a very significant breakthrough via the second
approach would be a direct coupling of phenol to produce 4,4'-biphenol in one
step rather than via the existing processes, which either sulfonates the
biphenyl followed by alkali fusion or couples 2,6-di-t-butylphenol at the 4
position, to form the 3,3' ,5,5' -tetra-t-butyl-4,4' -biphenol, and subsequent
dealkylation. It should be noted that direct coupling of phenol has been
achieved biologically but with no selectivity, all available carbon positions
coupling at random. An example of the third approach would be a similar one-
step coupling of benzoic acid to yield the 4,4'-bibenzoic acid.
As for the lyotropics, the biggest challenge is the development of lower-
cost routes to the benzobisthiazole and benzobisoxazole polymers and by lower-
cost routes to their multifunctional monomer precursors. Because of the very
high price and therefore limited availability of these monomers, any advances
in synthetic approaches would have a marked beneficial effect on their
availability and on the future of the extremely high-perfo~mance materials
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that can be made from them. Of considerable significance is the recent
announcement by Dow that it is preparing a benzobisoxazole polymer in pilot-
plant quantities.
UNDERSTANDING AND THEORY
Semiflexible LCPs
Liquid crystal formation by semiflexible polymers has been modeled by
extensions of the Onsager method alluded to in Chapter 2. The relevant
controlling parameter revealed by these theories (Doi and Edwards, 1986;
Odijk, 1986) is the ratio of the persistence length to diameter of the polyme
chain. Little experimental verification of these theories is yet in the
literature. Dramatic predictions concerning the influence of macromolecular
flexibility on the order parameter and concentration regime of biphasic
stability are contained in these newer theories. These predictions should be
tested with carefully obtained data on polymer systems that conform to the
assumptions of the developments .
No totally general models exist yet for the anticipation of liquid
crystallinity of semiflexible polymers at all concentrations. In particular,
the thermotropic behavior is only poorly understood. The primary difficulty
in developing a comprehensive picture of the thermodynamic aspects of LCPs
lies in the fact that a high degree of coupling exists between all degrees of
freedom in the system, internal and external to the polymer chains. Contrary
to the situation in polymer melts and in amorphous systems, where chains can
assume all conformations and the external constraints on a chain are
essentially isotropic, in the liquid crystalline phase the necessity for the
macromolecule to conform to the anisotropic spatial requirements of its
environment reduces drastically its freedom in conformation (internal) and
orientation (external). Chains in crystalline materials are similarly
restricted, of course, but there the reduction of fully developed degrees of
freedom for the chain is so drastic that in most cases only one conformation
and one packing arrangement can exist; as a consequence, the analysis of
crystalline systems has been mastered decades ago, and very simple rules can
be applied (e.g., Natta's "equivalence principle," which states that in a
polymer crystal the chain conformation must be a repeated sequence of local
conformations). In liquid crystalline phases the molecules in a mesomorphic
phase must adopt orientations, configurations, and a "packing'' arrangement
compatible with the weak, but not insignificant, constraints of symmetry and
density of the mesophase. In consequence, both intra- and inter-molecular
degrees of freedom, static and dynamic, are highly coupled, between molecules
as well as within individual chains. This coupling is effective not only in
thermotropic systems but also in lyo tropic ones, as long as the concentration
of polymer exceeds a critical value.
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PolvdispersitY and Blends of Polymers
If systems with more than one component are involved in phase equilibria,
one can, in general, expect an unequal distribution of the different species
between the phases; LCPs are no exception. Three interesting situations
exist: polymers not uniform with respect to molecular weight, mixtures of
flexible and rigid chains, and LCP blends. To date, the first two situations
have been addressed with some success:
· As prepared most LCPs are not uniform with respect to molecular
weight and therefore are multicomponent systems; they may fractionate,
especially in solution. The higher-average-molecular-weight species are found
in the anisotropic phase; the lower-average-molecular-weight species
accumulate in the isotropic phase. Computations have been performed for a
variety of molecular weight distributions (see Flory, 1984, and citations
therein).
· Mixtures of ideal random coils and rigid rods also fractionate
strongly (Flory, 1984), even when the chains are composed of "identical"
segments and are of identical molecular weight. The rigid species strongly
prefer the anisotropic phase.
No theoretical treatment seems to have addressed the question of the
blending of different mesogenic polymers. If both types of chains are
constitutionally uniform, of equal rigidity and sufficiently long, the
problem, to zeroth approximation, can be viewed in analogy to the blending of
random coiling chains; in both cases the configurational entropy of mixing is
very small and the enthalpy of mixing determines if blending is possible. One
would therefore expect that LCPs mix only rarely. However, in flexible
chains, miscibility can often be achieved by blending copolymers, and there is
no reason why similar effects cannot be exploited in mesogenic polymers. The
situation in LCPs is complicated, however, by the fact that the different
species probably would be of different rigidity (exhibit different persistence
lengths); one might speculate that for this reason blending would become even
rarer than in flexible polymer mixtures, but only initial attempts at
theoretically illuminating the situation has been put forward to date
(DeMeusse and Jaffe, 19881.
Thermodynamic treatments of polymer-polymer mixing and of polymer
mesophase formation have been treated by Flory (1986) and others (Flory and
Ronca, 1979; Doi and Edwards, 1986; Odijk, 19861. The application of these
concepts to LCP-containing blends is just beginning to appear in the
literature. There is no reason to expect that these concepts will not provide
an effective framework for understanding the phenomena observed in LCP-based
polymer mixing. Comparison of LCP behavior in blending with that of low-
molecular-weight LCs should also prove instructive. It is to be expected that
miscibility between LCPs will be rare, as it is with conventional polymers.
In contrast, most low-molecular-weight LCs of a given type are miscible with
each other. The in-depth understanding of the physical chemistry of LCP
blending will be an important cornerstone in defining the ultimate utility of
LCP blends and should be rigorously pursued.
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Dynamics
The dynamics of rigid rod systems have been investigated with intensity
(Chandrasekhar, 1977; Doi and Edwards, 1986), and quantitative results are
available for moderately concentrated solutions (below the transition to the
nematic state). In the bulk, similar problems exist as for flexible chains.
To date, only single polymer systems with chains of uniform molecular weight
and flexibility (usually stiff rods) have been addressed. The following is
known:
· The rotational constant in dilute (isotropic) solution of rod-like
polymers grows with chain length approximately like Dr ox ln~x) /X3 . Delis is a
very rapidly decreasing function of the aspect ratio x ; long rigid rods rotate
very slowly.
· In semidilute solutions (above the concentration where frequent
intermolecular contacts occur, but still in the isotropic regime) of rigid-
rod polymers, the rotational diffusion is attenuated by an additional
dependence of the approximate form x-2 with respect to the one in dilute
solution.
· In concentrated (i.e., anisotropic) solutions, and in the bulk
phase, no rotational diffusion effectively exists.
· The translational diffusion coefficient parallel to the rod axis is
roughly twice that perpendicular to that axis in dilute (isotropic) solution.
Both diffusion constants grow as D ~ ln~x)/x. This indicates a much less
rapid decline than in the case of the rotational diffusivity.
· For semidilute (isotropic) solutions of rigid-rod polymers, the
translational diffusion in the direction of the rod is roughly that for the
same rod in dilute solution, but translation perpendicular to the rod is
negligible.
· Very little is known about translational diffusion in the
anisotropic solutions and in the bulk phase.
Many other dynamic phenomena have been addressed; of particular interest
are the viscosity of solutions as a function of concentration (which follows
the experimentally observed cusp curve) (Hermans, 1967) and the degree of
order as a function of the concentration and external fields in anisotropic
phases. Little quantitative information is available, however, in the latter
cases.
Rheolo~v
The anisotropic orientation in melt-processed molded parts cannot be
predicted because a complete continuum theory for LOP rheology is not
available. The classical theory of nematic liquid rheology developed by
Leslie (1966, 1968) and Ericksen (1960, 1961) is inadequate for polymers
because of the simplifying assumptions in the derivation: The stress is
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assumed to be linear in the instantaneous rate of deformation, and the
entropic free energy associated with local director gradients, which leads to
an elastic "Frank" stress, is assumed to be quadratic. The theory does not
take chain flexibility into account, and magnetic field experiments on
thermotropic nematic melts (Moore and Denn, 1988) have indicated that there is
creep in the Frank stress. The theory thus stands in relation to LCPs
(particularly lyotropic solutions of very rigid molecules) in much the same
way as the Newtonian fluid to conventional flexible polymers. Much can be
gained by use of the Leslie-Ericksen theory, but results cannot be expected to
be quantitative and may even be incorrect in qualitative terms for flows well
outside the applicable range of the theory.
One important qualitative feature of LOP flow that does seem to be
contained in the Leslie-Ericksen theory because of the presence of the
entropic Frank elastic stress is the tendency of the director sometimes to
adopt an orientation that is transverse to the plane of shear (Rey and Denn,
1989; Beris and Edwards, 1990~. A continuum theory of LCPs by Doi (1981) is
incomplete in that it does not contain any elastic contribution to the stress
resulting from local director gradients; the Doi theory thus apparently cannot
predict any phenomena associated with rapid spatial changes in orientation,
although an extension of the theory that includes director gradient, has been
reported (Beris and Edwards, 1990~. Neither the Leslie-Ericksen nor the Doi
theory has been applied to the flow of fluids with domain-like textures; it is
unlikely that the Doi theory in its present form can be applied here because
of the limitations already cited.
The theological feature of LCPs that has been most exploited in
applications other than fiber formation is the very high degree of shear
thinning at all deformation rates (see Kulichikhin, 1989, and Muir and Porter,
1989, for recent reviews). It is this property that makes thermotropic melts
attractive for molding applications with complex shapes and small passages.
Qualitative models (Marrucci, 1984; Wissbrun, 1985) based on the relative
motion of domains predict shear thinning at low deformation rates, but no
adequate theory exists here either. It is possible that the presence of
microcrystals in the melt is a factor in the shape of the flow curve, as well
as long transients that are observed under conditions of apparent thermal
stability.
The self- organizing feature of LCPs is an asset or a liability , depending
on the shaping flow of interest. Predictive ability is lacking in any event,
because the rheology is not understood in a fundamental way. Simulations of
flow and orientation distributions in complex parts are unlikely to be correct
in the absence of an adequate theological theory, except in the case of a
uniformly extensional flow of sufficient strength to effect uniaxial
orientation (in which case no theory is needed). Progress in melt fabrication
of molded parts that exploit the orientability of LCPs is unlikely until
sufficient understanding of LOP behavior has been achieved to allow the
modeling of geometrical and filling schemes that will lead to orientation
distributions that approximate macroscopic isotropy and contain no rapid
changes in orientation; the latter is particularly serious because of the poor
self-adhesion of rigid polymers. It is not obvious that these problems can be
overcome by conventional melt processing.
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CONCLUSIONS
· Rigid rod theories are in reasonably good order; macromolecular
flexibility is manageable.
~ There are deficiencies in theories regarding blends of LCPs and LCPs
with flexible polymers, and consequently there are no guidelines for
experimental work in this area.
· A lack of understanding of chain dynamics and rheology of LCPs is
preventing solutions to processing problems such as the weld line, adhesion,
and orientation development in molds.
RECOMMENDATIONS
~ Theoretical modeling leading to predictable distinctions between
conventional isotropic phases and LC phases based on local primary and
secondary chain structure is necessary to understand and develop new LCPs;
intensified research should be encouraged.
· More interactive research among theoreticians and experimentalists
on aspects of blending LCPs is needed.
~ Theoretical studies of the fundamental dynamics and the rheology of
LCPs should be encouraged, with accompanying experimentation on the relation
between rheology and microstructure for the development and optimization of
processing methods. The implications of available theories of LOP rheology
should continue to be explored, but processing conclusions need to be examined
with care because of the incompleteness of the existing theories.
PROCESSING
Lvotropic LCPs
Lyotropic LCPs have been the subject of intensive investigations.
Molecular composites- a molecular dispersion of (lyotropic) rigid rod polymers
in a (conventional) polymer matrix have dominated the question of processing
rigid rod LCPs. This novel state of matter remains elusive, however. The
intricacies of retaining isolated rod-like molecules from dilute solution into
the solid state are unknown, unpublished, proprietary, or a combination of
these. Coagulants are critical to forming the molecular composite; their
effect on both polymers needs to be addressed before fabricating and
processing can be controlled.
Another problem lurking in the background concerns the retention of the
level of molecular dispersion during use of such composites.
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Aside from the intensive effort to fabricate molecular composites, there
has been one more significant area of activity in processing lyotropic LCPs
(apart from fiber production). This concerns attempts to fabricate biaxial
structures- films from PBX polymers. While there are reports of success at
Foster-Miller using proprietary schemes, this work has been limited for the
most part to bench-top scales (R. Lusignea, private communication). A major
difficulty with evaluating the potential of this technology appears to be the
severely limited availability of the PBX polymers. Pilot plant scale-up will
be necessary before a meaningful assessment of film production is possible.
However, it would appear that a viable interim solution could be
explored- namely, investigating the feasibility of fabricating biaxial
structures with commercially available lyotropic LCPs such as Ke~rlar~ aramid.
(See also the discussion of film formation in Chapter 2.)
Thermotropic LCPs
There are fundamental difficulties in traditional processing techniques
(injection molding, extrusion, rotational molding, etc.~. This may be
generally attributed to intrinsic problems associated with highly ordered
polymer melts. Self-adhesion in polymer melts, which is an important factor
in the strength of parts that contain weld lines, is governed by diffusion of
chains across interfaces. The rigid molecules of LCPs must diffuse a much
greater distance to achieve adhesion than flexib] e polymers. It is for this
reason that self - adhesion in LCPs is poor, both at weld lines and in regions
of rapid orientation change. The "weld line" problem may be insurmountable at
the molecular level for highly anisotropic melts and will require an
engineering solution based on mold design. Herein we focus on novel
processing possibilities for thermotropic LCPs.
Solid-State Forming
In studies of the extrusion of powder preforms of Xydar~ (a terpolymer of
p-hydroxybenzoic acid, terephthalic acid, and biphenol), workers at Alcoa
found (Zaidi, 1988) that defect-free rods could be cold-extruded from the
preforms only if the preform was annealed properly, the die was designed with
the correct angle, and the temperature and rate of extrusion were held in the
range that gives metal-like strain-hardening flow characteristics. This
suggests that the eons titutive behavior of solid LCPs as a function of
temperature and pressure should be a critical area of research. It is also
clear that the influence of pressure on solid-solid transitions would be an
important conjunctive area of research between chemical structure and novel
processing.
The explanation for cold-forming-induced optical clarity is still an
issue. Contributions resulting from the small size of the crystalline domains
in the formed material and the elimination of shrinkage voids have been
mentioned. In the case of the LCPs, there is some indication that cold-formed
shapes may be free of the skin-core effect and may enjoy outstanding
machinability and wear characteristics relative to melt-formed material
(Zaidi, 1988).
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From a general point of view, solid-state forming does have its
drawbacks. It often involves heavy, unconventional equipment and may require
very expensive tooling. The parts may exhibit poorer dimensional stability
than melt-formed ones, and extra allowance is often required for ''spring-back"
of the parts after removal from the dies. The severe orientation introduced
by some forming processes may result in the tendency of the product to
split already a difficulty with LCPs. Because LCPs are relatively expensive
polymers, the savings in energy and cycle time associated with solid-state
forming may not have a significant effect on the overall cost of the part.
The use of powder preforms, which appears to be a way around the
splitting problem, may hold little promise for exploitation of the ultimate
strength properties of LCPs. If this proves to be the case, the entire
combination" LCP fabricated with solid-state extrusion -becomes decidedly
unattractive. Clearly, to accrue the full advantages of the preform extrusion
technology, the constitutive behavior of solid LCPs as a function of
temperature and pressure must be understood. In addition, the influence of
pressure on solid-solid transitions would be an important conjunctive area of
research for developing the connections between chemical structure and novel
processing (Hsiao et al., 1988~.
CONCLUSIONS
LYotropics
An important objective of LCP processing technology is to develop strong
materials for ultralight primary structures that are difficult to detect by
radar and can withstand elevated temperatures. In the case of composites, the
temperature resistance and tensile properties of fibers must also be combined
with outstanding compressive properties and adhesion in fabrication schemes.
For the LCP fibers to be effective in high-performance structural composites,
they should also exhibit compressive strength that approaches the compressive
properties of current carbon fibers.
Uniaxial structures prepared from liquid crystalline precursors (melts or
solutions) exhibit outstanding tensile properties. For some time it was
believed that the liquid crystalline state of the precursor during processing
(spinning) was essential for achieving the molecular alignments and the
perfection of structure that yield almost theoretical modulus and exceptional
strengths properties far above those of fibers produced from isotropic melts
or solutions (Economy et al., 1970; Aharoni and Sibilia, 1978~. Later, it was
shown that the isotropic solutions of semirigid polymers produced at a low
rate of elongational flow in the spin-way yield relatively weak fibers as
spun. At high rates of deformation, however, the semirigid polymers yield
properties close to those of aramids.
More recently, it was shown that even very flexible polymers, such as PE
and PVA, can be converted into exceptionally strong fibers without becoming
obviously liquid crystalline during the process of converting the isotropic
solution into fiber. The quiescent liquid crystalline state is, therefore,
beneficial for the preparation of strong fibers without after drawing but it
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is not essential for all polymers. These observations must be borne in mind
in assessing processing schemes and/or potential polymers for novel
applications.
Thermotropics
Solid-state forming of LCPs is an immature area. It can be anticipated
that many of the advantages of solid-state forming found with conventional
semicrystalline polymers will accrue for LCPs. It is clear that there are
unanticipated, unique properties that can be obtained by solid-state forming
of powder preforms, but the basic science of this process as well as those
using melt-formed preforms (e.g., sheets) needs to be investigated. The
practical outcome of funding in this area would be an increase in the
availability of high-performance LOP products for use in defense-related
engineering projects. Not to be overlooked is the "knowledge" product: The
exploration of solid-state forming techniques could lead to information
concerning important phenomena such as the poor compressive strength of highly
oriented LCPs.
RECOMMENDATIONS
~ Fabrication of biaxial film using readily available lyotropic LCPs
should be investigated.
· Other routes (nonmesomorphic gels) to high-modulus fibers should be
explored.
· Mold-design studies and experiments aimed at the weld line problem
should be encouraged.
· Solid-state forming of LCPs needs to be explored sufficiently to
establish patterns allowing the performance of these novel processes to be
compared with the extensive body of information already available for
semicrystalline, random-coil polymers, as well as the definition of any
advantages unique to LCPs.
~ For electro-optical applications, electric poling and associated
fabrication processes need further study.
MECHANICAL PROPERTIES
Tensile Properties
Remote from major transitions, axial properties of solid-state filaments
formed from LCPs are excellent and approach the theoretical limit in tensile
modulus and exhibit extraordinary high tensile strength. Some typical
properties of commercial fibers were shown in Figure 3.1. Transverse
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go
It has been found that the presence of LCP may accelerate and presumably
direct the crystallization of conventional polymers (Joseph et al., 1983~.
Blending biphasic polymers, such as the PET-polyHBA copolymers, may promote
miscibility between the conventional phase of the Aphasic polymer and another
miscible conventional polymer, such as X7-G with poly~butylene terephthalate).
These phenomena may offer direction in the search for useful compatibilizing
agents for LCP-conventional polymer systems.
There are examples given in the literature (Froix, 1984) where the
presence of small quantities of LCP blended with a conventional polymer
results in mechanical properties significantly better than either component.
No explanation for this observation has been advanced. The analogy of
"introducing the lignin into a woody morphology consisting only of fibrin" is
appealing but is not consistent with emerging models of LCP structure or LCP-
conventional polymer interactions.
As sparse as the data set describing mainchain nematic LCP blends with
conventional polymers is, it is rich compared to the almost nonexistent data
on the blending of other types of LCPs sidechain polymers, flexible spacer
polymers, smectics, etc.
LCP-LCP Blends
Researchers have studied blends of thermotropic copolyesters with other
thermotropic copolyesters, examining both blends of different copolymer ratios
with identical chemistry (HBA-HNA type) and blends of different chemistries.
Recognizing that each copolymer may be viewed as a blend (chain-to-chain
variations, mer sequence variations), it was hypothesized that blending
offered the means to "engineer the distribution. " Initial results indicate
that this is the case. Through such blending the behavior of both the
mesophase and the solid state can be systematically modified. For example,
transition temperatures can be shifted and power law indices of viscosity can
be changed. These results also strongly imply that the "sequence matching"
model of the thermotropic copolyesters in the solid state is more likely
correct than the other models suggested in the literature (DeMeusse et al.,
1988~. Evidence was accumulated that LCP-LCP miscibility is not universal and
that, at least in the mesophase, the basic concepts are consistent with the
observed physical chemistry. The importance of this work is that it offers a
direction for achieving sufficient insight into the nature of LCP structure-
property relationships to design more appropriate molecules for given end
uses. Related work indicates that transesterification may be responsible for
some of these observations (J. Economy, private communication). Careful
evaluation of the data cannot rule out transesterification effects, but it
strongly suggests that transesterification is not causal in the observed
behavior.
As in the case of LCP-conventional polymer blending, little information
exists on the blending of LCPs of different inherent chain architecture or
mesophase symmetry. Recent publications show phase separation in blends of
sidechain nematics with other similar polymers or small-molecule analogues.
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It is now established that, in contrast to the behavior of low-molecular-
weight LCs, LCPs are often immiscible
Investigations into the blending of LCPs with other polymers,
conventional or LC, are in their infancy, and little is understood In detail.
The literature is sparse, highly observational in nature, and very difficult
to reproduce. Much of what is published is more an indication of what might
be than an accurate data base from which to draw conclusions. It is clear,
however , that, until the aches ion of IMP to other polymers and the rules
controlling blend morphology are understood, the field will remain highly
empirical and unlikely to yield many commercial successes . Conversely, for
all the reasons that polymer blending is an attractive route to modified
polymer products, blends containing LCPs are especially attractive.
Systematic research to understand the physical chemistry of LCP-containing
blends is likely to produce results of both commercial and scientific impact.
This is true for LCP-conventional blends and all -LCP blend systems . Although
initial work should focus on the commercial nematic polymers, other symmetries
and phases should not be ignored. The area of compatibilizers ("molecular
glue") is another concept worthy of support because of the high payback
pa tential .
Molecular Composites
To date it is unclear whether a true molecular compos ite utilizing a
molecularly dispersed LCP has been demonstrated, although materials possessing
small agglomerations of rods (diameter of structure less than 50 A' have been
produced with very high tensile properties. It is not yet established,
however, if these materials offer an advantage in tension. The concepts
underlying molecular composite physics are consistent with the concepts of
miscible blends; the materials being produced, even at the very small sizes of
rod structures observed, fit the definitions associated with immiscible LCP-
conventional polymer blends. Molecular composites can be treated with the
already established framework of polymer-polymer mixing and do not require new
concepts for accurate description. Success of molecular composites will be
strongly linked to the economics of the processes and materials employed.
Research in this area should be supported until a meaningful evaluation of
cost-performance can be performed.
CONCLUSIONS
The combination of LCPs with other materials to control the balance of
properties and improve cost-effectiveness is clearly an important technology
area for increasing the overall utility of LCPs . The problems inhibiting the
rapid development of this technology are the same as those slowing LCP
acceptance in other areas , namely :
High property anisotropy in finished parts,
Poor compressive strength,
Poor adhesion to conventional and mesogenic materials, and
High cost.
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92
This is not in contradiction to the observations that fillers and
blending can mitigate many of these effects; rather, it is an important hint
into what the underlying controlling parameters of the science must be.
RECOMMENDATIONS
From both the scientific and technological points of view, studies of the
behavior of LCPs in multicomponent systems should be strongly supported. The
following are specifically recommended:
· Support research efforts to understand and exploit the combination
of processing and fillers needed to provide a range of properties from fully
isotropic to anisotropic.
· Establish research efforts to understand and improve LOP adhesion to
common composite matrices, fillers, and reinforcements, and other conventional
and mesogenic polymers.
· Establish systematic research efforts to understand and exploit the
physical chemistry of LCP-containing polymer blends.
NONLINEAR OPTICAL PROPERTIES
In spite of the obvious advantages of organic NLO materials, there are
major obstacles for utilizing polymers. In the case of second-order NLO
applications, some obstacles are these:
· The uncertainty with respect to thermal stability of electric field-
induced ordering in harsh environments, i.e., greater than 80°C.
· The small published data base on NLO properties, physical
properties, and processing conditions for making active and passive structures
in polymer films, and the sparse literature on second-order NLO properties of
LCPs.
· The competitive advantage relative to LiNbO3.
.
The maturity of LiNbO3-based technology.
· A lack of polymers designed specifically for SHG as well as device
concepts and structures-for utilizing them.
Although considerable recent progress has been made in achieving high
nonlinear coefficients, additional work ranging from fundamental science to
new materials development remains to be done if materials in this category are
to reach their technological potential.
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93
Liquid crystallinity has been shown to lead to enhanced alignment under
certain poling conditions. The utilization of extremely high poling fields on
amorphous systems, where a substantial fraction of dielectric saturation can
be achieved, makes this less of a potential advantage. The anisotropic
optical properties of LCPs would also have to be made extremely uniform to
avoid scattering losses in bulk or wavegu~de devices.
The main benefit of liquid crystallinity may be one of imparting thermal
stability to systems once they are poled. More esoteric applications might
also exploit LCs. For example, the ferroelectric smectic C* phase, which is
inherently noncentrosymmetric, might exhibit high degrees of macroscopic
alignment and the desirable processing characteristics of a polymer. A
polymeric system exhibiting this phase may require poling or other treatments
to exhibit alignment, but the phase is inherently bistable as opposed to
metastable in the case of poled thermoplastic. The smectic C* state combined
with chromophore substituents designed for specific NLO applications would be
an extremely attractive materials option.
For third-order NLO materials, the obstacles for utilization are more
formidable:
· The state of understanding of third-order NLO properties based on
the electronic structure of the constituent molecular or polymeric species is
at a more rudimentary level, and a considerable diversity of opinion exists
among scientists regarding the fundamental origin of nonlinear responses in
these materials.
Highly delocalized electronic structures are difficult to design
into stable, easily processible polymeric structures.
point.
.
Approaches to increasing the nonresonant X(3) are unclear at this
· The stringent linear optical properties, including optical
uniformity, transparency, and low scattering losses, combined with large X(3)
required for device applications , have not yet been demonstrated, and
formidable obstacles associated with -C-H vibrational overtones in the 1.3-pm
and 1.55-pm regions remain to be addressed.
Practical polymer processing conditions for achieving high degrees
of orientation to enhance xt3) in formats suitable for waveguide formation
have not yet been demonstrated.
Organic materials, because of their large nonresonant X(3) values, may
constitute an enabling technology if progress can be made in the areas listed
above. Low-dimensional electronic structures tend to exhibit the large values
of X(3)' since all of the oscillator strength is confined to one predominant
direction in these materials. At the macroscopic level, uniaxial orientation
of a material with low-dimensional molecular constituents or chains can
increase X(3) by up to a factor of 5. Because of trade-off between n2 and a,
it will be important to achieve the factor of 5 by process ing and fabrication
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94
methods. Liquid crystallinity and other self-organizational influences and
factors may offer routes to alignment, optical uniformity, and stability.
CONCLUSIONS
~ Inorganic crystals and thin films have advantages for some NLO
applications, by virtue either of their function or the maturity of their
technological position, but some may be displaced by organic polymeric thin-
film materials because of cost and performance advantages; other applications
may be made possible by the unique properties of organic and polymeric
materials, assuming that fundamental advances can be made.
~ For second-order NLO processes, poled LCPs containing chromophores
with a large molecular hyperpolarizability may offer advantages over amorphous
polymers with respect to the degree and stability of the induced second-order
nonlinear coefficient at the expense of more complex linear optical
proper", es.
· For third-order NLO processes, LCPs offer ~ route to achieving
uniaxial alignment through processing and the associated potential for a five-
fold increase in nonlinear coefficient relative to an electronically identical
amorphous system, although considerable progress in fundamental understanding
of microscopic processes, materials design and synthesis, and processing will
be required to take advantage of this.
RECOMMENDATIONS
~ Research initiatives on organic and polymeric materials for second-
order NLO addressing the issues of stability and magnitude of alignment by
electric field poling, spontaneous or self-alignment, physical property
studies, processing, and exploratory device utility should be supported.
Priority should be given to interdisciplinary program where molecular and
polymer design and synthesis, NLO studies, and polymer physical
characterization can be combined.
· Research initiatives aimed at improving the microscopic
understanding of X(3) and testing models through structure property
investigations should be supported. Because of the rudimentary state of
theory, the data base of X(3) measurements on organic and polymeric structures
should be expanded. Synthesis and characterization of new polymeric systems
with an emphasis on increasing X(3) and achieving excellent linear optical
properties including transparency at 0.85 ~m, 1.3 am, and 1.55 Am should be
supported. Processing studies aimed at the unique requirements of wave guide
nonlinear optics should be supported. Interdisciplinary studies of a
fundamental nature as well as novel exploratory approaches should be
encouraged.
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95
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Representative terms from entire chapter:
liquid crystalline
