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- 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
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).
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
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.
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.
54 t ~n ~ C~ - 1 CD o a, :~ ._ ._ - c o ._ 0 - o c o , I, ~C U] ~·K . o s~ COoc ~ oO E o _ ~S ~1 ~, o ·_4 a' E o V, o u) 0- o ~, C~ C~ ,J CO ¢ E~ CO _ o - o (D - u ~O-~ 0 _ t~> t ~e ~er - er ~ o _ ~ cr) _ ~ ~o oo o o ~ ,_ l' - , C`t o a, oo Ln ~ t C~ C~t ~ C~t ~ I o r ~C`t O CD ~ ~t ~ C~t C~t ~n ·_ _ e o ~ O CD · - C~, ~, I tD 0D 0 ,'o~ c~' tn ~_ o o · C~ 0 tn _ ·_ ~ o o U~ ~ - ~ ~ o I C~ _ o . ~ C~t ~ (D -a' o ,= o ~ ~ L~ C~ ~ - , I I r~ o ,~, o · _ ~ ~ ~S - , ~- tD (D {D 0 ~O · O - , o ,_ · ~ _ o ~ - o c~' q" ~r ~tD- ~<.D -~ a, _ ._ E ~S a, 2 ~ - > y C~ tntn 0 ~n o ~_ . _ . _ O er _ <- C~ O CD ,s, C'.tU ~o . ~ ~ ~ a,· 0 tD- C~, ~ C~C ~=, _ _ ._ ~n o tn ~tD t' t_ - .t C~ ~· C~ · ~ C~t ~ ~t' 0, C.D 4e ._ o7 Q == c c) cn U) ~=: ,~ CL == C == C~ ~== <= C~ o { ·- _ ~ C o O ~ - 4= C ~ ~ - ~ O ~r . tn . - tn - o I_ o ·_ , r~ · ~O C~ ~t U ~_ _ C~.t _ _ _ _ . ~ ._ ^4~ ~ _ ~s L ~- ~V D C t_ _ l~a ;~ O O- ._ _ _ ~,~ ~ ._ . ._ O O _ ~ vl - ... It'-O · O C~tO ~ V 1~ - 3;t o z C z a' o z ~ _ <~ O ~ ~ ~n ~O ~C ~Q) ~ ~ O t' . tD ~O ~· ~ Q - ~D - _ · · · O O ~ x C~ _ _ _ _ ~o Z V - - C~ · tn ~t C ~._ ._ C C ~a, Ct C:) (D tD 1 1 o o _ _ . X X r_ ~ . . C ~er 1 1 ._ L _} ._ o ~: I E _ ._ a ~4~ 4= . _ _ ~s a~ ~ ~E · o' · _ ;~ L s~ - , ~ 0> ._ t~ 01 1 _ O O · - ~r C ._ o Q ~ c ._ s . - o o ~ _ ~ - C X C ~ ~ ._ ._ - ~ . _ ,~ .- ._ ~ Q) O C ~ _ _
55 · ~ _ A ~ ~ a'-0 ~ ' - ~ ~ in ~ u o ~U ° £ - ~ ~ A, o >, U ~. .w ~ ~ ~'U U "A ~. ~· ~ o ~ So ~ ~ · - U lo. o~ ~c, ,= . ·_I H Cow << ~' - ~' - ~U a)~ u ~lo, a) u' so ' - ~ ~ ~, C)~ 0 41) ~- ,_ ^ O ~ ~_m ~ v a) 0 v tt~] · , ~ ~ ~ 5- ~ . O ~ ~ ~ O ~· ~ ~ ~ ~ O _ ·,] , U] ~ Vet AS · - - I e ' ~ ~ ~ 3~ ~ U' ~ ~ Z·- ¢ . . - V, o U ~ ~· = - ~= us a) 5- 0 oo, U ~ ~ o~ ~ o ~ ~o C) ~ Cut_ ~ U o ~ ~ ~ X U ° - ~ . - , - o ~. ~U) ~ ~ O ~ , ~,- ~ U ~ _ ~ '- ~U U ~ ~. U bC ~ ~ . - O ~ ~, - ~ ~}_, U) U ~U . - · (l,} u~ U E~ U ~ ~ ~_ O U ~ bC O ~ ~ ~. - ~ ,' _ o ~ O P ~. - O ~- ,_! ,_! L tn s ~c ~0 bO ~ ~ ~ 0 ~ ~ ~, - ~U ~ 0 := ° ¢ ~ V £ u, 1 a) ~v ~ . · ~ ~l ~ ~ ~ ~· a' 163 ~ · · o ~CO E~ ~n . - ~. 1 E~ ~ ~. - ~¢ ~ _ . - u a' ~ o ~ ~_ .= ~ ~. - ~ ~ ~· U~ s~ o ~ ~. ~· · o o~ ~ ~· ~ ~ ~_ V ~V a) ,= i ~ u ~- ~- ~a U ~ ~U~ ~ < ~11 ~· o ~ ~ ~ . ~3 u, ~o ~V~ ~3 o ~ _ a) a) ~ ~ ~0.- ~. - ~ . ¢ ~ ~ Z o E ~ ~
56 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.
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.
58 ~ o ^, X CC o - ._ 1 CO CO - U] E~ V) ¢ o 1 o u C) a' o ~Q ~ P o. s~ C~ C~ ¢ o C~ CO a' o a = ._ ~ o s~ _ ~ C~ =, ,'= X ~ o 4 a, 1 o C~ C~ ~ 1 0 C~ o tY C~ C~ C~ m0 CD _ _ CD o r) ~_ . C ~CO C~ . U ~C~ C~ ~o _ ~ o _ _ C~ o o C~ tD 0 o ~ 1 =5 ~ ^= X ~ ~ o 4~ o ~ o, a, I .3: X 4~ 1 C~ ~ ._ ~y ~X Q) _ 1 ~ C~ -C~ E ~5 z C~ CS~_ oo _ .. . . __ r ~co O _ C ~CD V C~ o CDC ~CD CD _ · ·~ _C ~O_ C~C~· CD COO ~O C ~C ~_ ~V ~ ~CD 1_ U~ .. __ {,D ~ 00 c ~c~ ~ 0 r~ CD- . . · . · · · - - C') C ~oc' C~ ~4 _ o o o C ~CO u, ~ CO CD ~ ~ c~ o - - - o oo u~ 0 0 a) ~ ~ c~ _ 0 0 CD _ C ~{D C ~O _- ~- -- O O ~c~ . . - - ~ c~ 0 c~ - CD . - ~4 - u~ c~ oD . . LO ~c~ u~ o O ~ c~ o o · . · · · · . ac) _ ~ 0 0 0 0 0 c~ I_{D C"C~ C~_O · · . . . . . - - ~o oo o ~ c~ ~ - - ~ ~ o . . · · · · - c~ c~ - - o o o o) c~ c~ c~ -oo u~ ~- c~ ~OCD O C ~U~CD cO o ~ c ~Ln ~c~ O OCD 0 0~ c ~- OCD C~ - 0 c~ o - cO ao 0 O u~ o c ~o ~o ~D c~ o . O CD - . o co o ~D c~ o ° cn C~O a) CD 1_ ~C ~U~ - ~ - - - · · - ~· ~o. x Q _ ~5 ~·~5 ·- a Y - ·- E Q ~4 ~_ ~: - ·- ~_ . _ ~C' -_ v ~CL. ·- C.) 4 ~- ~ a,~ u~ ·- ~Q y co · - O ~ ~ . co 4 ~- =- -- =d - ·~ - aB~- ~a~ -_ ~O C ~ CO ~ >~ X t1) E - ~ ~- 4= ·- ~o ~ coo ~4 ~a ~:,0 ~- ~ 3 ~·- E ~CS) CO ·- ~ ~ o ~ ~X ~ o ~- ~0 0 ·- ~ x- ~Q-- o c~ - E co ~- Q 4~ co ~ ~ ~ 4 ~ ~_ C', =. - -- O ~ Q) a) 0 4 ~ ~tn ·- c~- co - ~- ~ - - F: CO O · ~ ~- ,~ =. ~- - ~ ~ ._ _ - E too oo o o oo ·- tD ~ E ~ - ~ - o ~o X X <~ ~) O LC~ O tJ) O u ~C ~ ~- ~ E a ~Q ~ct _ ~ c~ c~ ~ ~- ~s a~ 33 E o ~E 0) a~ c) tu - - ~0) a~ 4 ~0 ~ Q c ~·- - > ~ ~s c~ 4= Q ~ _ J ~ -_ _ . _
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).
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 .
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.
62 CQ · - a) o ~4 a) o A o U) o a) · - sit Cal o o CO · - o . A: En . . . ~1_ - ~ o o. ~o o O . . U) _I O 0 1 0 1 1 1 1 11 1 1 cn So= I n5 >, O . e loo.= a, 1 1 1 1 1 A) 1 t_ 1 1 1 1 >0 ~ ~ r I · O r . O ·_ ~1 In ~LO ._ 1 ~_ C~J O · ~ t_ 1 1 1 1 1 1 1 1 1 0 1 0 en an o ~ O O ~C~ C ~I_ O · ~e e e _. O 0 1 0 'I 1 1 0 1 1 1 1 1 . - ~5 De ~e I t~ ~- 1 1 1 ~1 1 ~ ~0 1 1O ~ o a~ 0 e a' _ · c~ ~ E c ~co ~o. ^o ~0m C~O O ~ O 00 1 00 1 0 1 L~ o ~ I ~O-O`~ - OCOO I ~ a, t_ 0 1-0 ~-O ~-O ~-O ~-O _~ ~_ ~_~ _ ~_ O a) 0 · a, =m 0 ~o a, O I ~ O m.- XC~ ~ r ~ ~: cO a) ~) Q ~_ 1 0 ._ ~ ~·_ ~LLJ ~ S~ S~ ~ t_ ~1 ~ X · _ . I : ~S~ 3 ~ a) ~5 0 0 m== s~ _' co tq R~ o ~: a) ~Q c) ~: . . a) c' s~ o v,
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
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
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.
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
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 <lo-13 NR Polythiophene 8.5 x 10-16 <lo-12 6 x 103 Phthalocyanine 6.4 x 10-~5 <lo-~2 6.9 x 104 >10-3 >0. 15 SHOT glass 1o~20 <10-13 10-5 >10-7 >20 SiO2, Pb-doped 10-~8 - - - PTS type cannot compete for these applications. On the other hand, the dimensionless quantity W discounts the power requirement for a switching operation and emphasizes throughput i.e., the amount of refractive index change that can be achieved relative to the amount of light dissipated in the material. It is independent of the power and length of interaction required for switching. This parameter is much more applicable to the operation of waveguided devices likely to be used in serial switching operations. It is noteworthy that PTS is close in value to the MQW structures. Its response time is many orders of magnitude shorter, making it much more suitable for high-speed applications . The value of W = 20 for glass is impressive but misleading, since it reflects the extremely low ~ of glass. Extremely long path lengths and high powers would be required to operate devices made from this material. The large nonresonant n2, combined with
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. REFERENCES 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- linearized optical modulator using a quantum-well self-electro-optic effect device. Optics Lett. 9~129:567-569. Morgan, R. J. and R. E. Allred. In press. Aramids. In Encyclopedia of Composites, S. M. Lee, ed. New York: VCH Publishers, Inc.
70 New Scientist. 1988. 21:168. See also Electronics' July 1988, p. 36. 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 . Risk, W. P., J. C. Baumert, G. C. Bjorklund, F. M. Schellenberg, and W. Lenth. 1988. Generation of blue-light by intracavity frequency mixing of the laser and primp radiation of a miniature neodymium-yttrium aluminum garnet laser. Appl. Phys. Lett. 52~2~:85-87. Stegeman, G. I., and R. Lytel. 1989. Nonlinear Optical Effects in Organic Polymers. P. 379 in NATO ASI Series, F. Kazjar, J. Messier, P. N. Prasad, and D. R. Ulrich, eds. Dordrecht, Holland: Kluwer. Stegeman, G. S., R. Zanoni, and C. R. Seaton. 1987. P. 53 in Nonlinear Optical Properties of Polymers: Materials Research Society Proceedings , Vol. 109, A. J. Heeger, J. Ornstein, and D. R. Ulrich, eds. Pittsburgh: Materials Research Society. Wentworth, S. E. 1986. Preliminary Evaluation of the Effect of Chemical Warfare Liquids and Simulants on Selected Organic Matrix Composites. MTL TR 86-21 (May). Ye, C., N. Minami, T. J. Marks, J. Yang, and G. K. Wong. 1988. Persistent, efficient frequency doubling by poled annealed films of a chromophore- functionalized poly~para-hydroxys tyrene ~ . Macromolecules 21(9):2899-2901.
