Liquid Crystalline Polymers (1990) / Chapter Skim
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2 BACKGROUND
2 BACKGROUND
Pages 13-48
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From page 13... ...
, however, restrict internal degrees of freedom in the polymers such that the mainchain is extended in space along an almost linear trajectory. Extended polymer chains or chain segments can, through excluded volume interactions, lead to long-range orientational ordering of the macromolecules- liquid crystallinity in concentrated solution or in the melt.
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From page 14... ...
polymer fluid.
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From page 15... ...
(In low-molar-mass LCs bulk fluidity in the smectic structure involves the layers gliding past one another; such a transport mechanism would be sharply attenuated in polymeric analogs, wherein a single semiflexible chain traverses more than one layer; smectics formed from rigid rod polymers with the layer spacing equal to the rod length might exist.)
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From page 16... ...
16 FIGURE 2.2 An absence of translational order in the idealized nematic. FIGURE 2.3 Smectic stratification (lateral registration)
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FIGURE 2.4 Helicoidal cholesteric structure in a mainchain LOP.
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From page 18... ...
The polymerization is usually conducted in the melt, although in some cases the use of an inert suspending medium is reported. The reaction in the melt is carried out either to completion or first to low molecular-weight-oligomer followed by solid state polymerization to high molecular weight.
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From page 19... ...
19 Typical copolyester LCPs are shown in Figure 2.6. Patented Aromatic LOP Polyesters Amoco (Carbons)
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From page 20... ...
Lyotropic LC polyamides, depending on their composition, may be spun directly from their reaction media, or they may be isolated, redissolved, and spun from solutions containing strongly interacting acids, such as sulfuric acid, oleum, etc. The use of a phosphorylation method for the preparation of aromatic polyamides involves the direct condensation of aromatic amino acids or aromatic diamines with aromatic diacids in the presence of an aryl phosphite and organic base.
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From page 21... ...
(PBO) were initially prepared at the Air Force Materials Laboratory at Wright-Patterson Air Force Base (see Wolfe, 1988, for a review)
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From page 22... ...
Sidechain Thermotropic LCPs While most of the interest in LCPs is focused on mainchain a-important class of LCPs contains the mesogenic groups as an appendage (meso~enic core) on the polymer sidechain LCPs (see Attard and Williams, .
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From page 23... ...
litOgOlit ~ HO _~OH ID \_~oL IS ( 0~) 4 L ~ FIGURE 2.11 Preparation of sidechain LCPs by polyesterification.
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From page 24... ...
These ordered arrays of columns present an interesting and unique state of order in the LC and solid state of organic polymers. Sidechain Lvotropic LCPs For completeness we note that there are examples of sidechain lyotropic LCPs wherein the mesogenic unit is ~mphiphilic (Finklemann, 1987~.
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From page 25... ...
Interacting rigid rod-like chains behave somewhat differently, but this difference becomes gradually insignificant with increasing molecular weights (Flory and Ronca, 1979~. Semiflexible chains are highly extended molecules with significant flexibility that nevertheless impart, unlike their flexible random coiling counterparts, the potential for the formation of anisotropic phases to their solutions and melts.
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From page 26... ...
Terminology derived from semi-crystalline polymers - degree of liquid crystallinity - is sometimes used to describe such heterogeneity. However, this terminology is complicated.
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From page 27... ...
Rod-like polymers, or chains with very limited flexibility and a moreor-less linear trajectory, pervade much more of space than their flexible random traj ectory counterparts; i.e., they have much larger radii of gyration. The radius of gyration for a random coil with molecular weight M varies as M° ~ at the theta point and as M06 in a good solvent; it is proportional to M for a rigid rod.
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From page 28... ...
Uniaxial solid fibers dominate the commercial utilization of lyotropic LCPs. For example, spinning lyo tropic solutions can result in fibers with very high strength and very high modulus and with nearly perfect orientation without subsequent mechanical processing, i.e., without stretching the extruded and solidified fibers.
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From page 29... ...
Films produced from anisotropic PPTA-sulfuric acid solutions (by wet or dry jet-wet extrusion with uniaxial drawdown) exhibit polymer orientation in the machine direction and highly anisotropic mechanical properties.
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From page 30... ...
MECHANICAL PROPERTIES Solid State Morpholo~v The focus herein is on fibers because morphological data on more complex geometries are incomplete. When one tries to analyze the molecular criteria for a perfectly aligned uniaxial system, it is not directly applicable to fibers because of a fiber's fundamental morphological characteristics.
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From page 31... ...
e Process Products Deformation Schematic l old drawing tFapbeesr,s, extension rods Bending channel s Pure shear 4~- ~ Cold extrusion Rods, Uniaxial ~| tapes extension ~\q ~ . - ~ - ~ - ~ N~ Hydrostatic Rods, Uniaxial ~ _ | extrusion tapes extension ~| .
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;= Upsetti ng Knobs, Compressi on (cold heating) nails Matched-die Various Pure shear (stamp)
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From page 33... ...
Although there are many ways to produce fibers, the final products always exhibit a well-developed fibrillar substructure. Moreover, this tendency toward fibrillation appears to be exacerbated in fiber derived from LCPs, even though extended-chain crystal habits as opposed to thin lamellar crystallites (see inserts in Figure 2.13)
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From page 34... ...
Sam core - macrofibrils 4~O.S~m skin extended LOP chain crystal habit 50~ OSBy_ ~1 / microfibrits semi.- \ crystalline lamellae polymer chain FIGURE 2.13 Hierarchical morphology in fibers.
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From page 35... ...
Consequently, a polymer molecule designed for optimal performance in tension will generally exhibit low compressive strength, and vice versa. When the molecular criteria are taken into consideration, the following ranking of fibers with respect to their increasing compressive strength potential is obtained: PE < thermotropic polyesters < rigid rod polymers (PBZT, PBO)
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From page 36... ...
The first two issues preclude a straightforward determination of the compressive strength for fibers, and the latter from composites. It has been observed that even with the most effective treatments, the fracture path in PE fiber composites never traverses the fibers as is the case in Kevlar~ fiber composites, but follows the fiber-matrix interface.
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From page 37... ...
The evaluation predicts that a critical region will exist wherein there is a single isotropic phase consisting of rods randomly dispersed in the coils. This region is very narrow in its stability boundaries, and the retention of this structure in the solid state depends on "beating the kinetics." Hence, if phase separation can be avoided on solidification, given the extraordinarily high mechanical properties of the individual macromolecular rods, products (including fibers)
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From page 38... ...
In the former a d.c. electric field is applied to a medium, which responds by altering its refractive index in proportion to the applied field.
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From page 39... ...
Here the velocity of the microwave pulse Vm = C/E~ must match that of light, Ve = C/n, in the medium over the interaction length required for a given amount of phase retardation to occur (DeMartino et al., 1987~. From these considerations a large refractive index would favor phase retardation.
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From page 40... ...
The first is analogous to the Pockels effect, where the refractive index change is quadratically dependent on the applied field, which can be at d.c. or optical frequencies.
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From page 41... ...
The first of these involves the use of simple spatial patterns combined with the switching behavior of nonlinear etalon devices to perform computational functions; these devices are simply miniature resonant cavities where the thickness, refractive index, and reflectivity of the internal surfaces are chosen to provide a destructive interference condition and therefore low transmission through the device. The nonlinear contribution to the refractive index of the medium, as illustrated by the equations below, causes the transmission characteristics to be light-intensity-dependent and capable of exhibiting bistable behavior.
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From page 42... ...
In waveguides, long interaction lengths can be used to generate phase retardations needed for switching applications so that the premium for materials is put on the speed of the nonlinear response and the optical losses in the material caused by linear optical processes. The decay times of nonlinear responses in semiconductors and quantum well structures, as well as the response time in photorefractive materials, are in general too slow for this type of application.
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From page 43... ...
P 65 in Nonlinear Optical Properties of Polymers: Materials Research Society Proceedings, Vol.
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From page 44... ...
1987. Liquid crystalline polymers.
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From page 45... ...
1982. Optical and nonlinear optical characterization of molecularly doped thermotropic liquidcrystalline polymers.
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From page 46... ...
P 53 in Nonlinear Optical Properties of Polymers: Materials Research Society Proceedings, Vol.
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From page 47... ...
P 173 in Nonlinear Optical Properties of Organic Molecules and Crystals, Vol.
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