Consider a homopolymer consisting of rigid and flexible sequences in alternating succession. The prototype is

The degree of polymerization DP is very large. The oxyethylene units may occur in various disordered conformations or in the ordered one, ttt, with a statistical weight s* compared with unity for the disordered forms, collectively. Let γζ be the number of rigid sequences containing ζ rigid units, i.e., consisting of ζ rigid members joined by ζ-1 oxyethylene units in extended (ordered, ttt) conformations, the oxyethylene units flanking this sequence being otherwise conformed.

Theory predicts that the nematic state of a homopolymer comprising rigid and flexible sequences in alternating succession depends critically on the tendency of the flexible units to reconform to ordered extended conformations. When the relative statistical weight s* of such extended conformations is very small (very flexible), the chains undergo only a minor conformational rearrangement in the nematic state. Moreover, the clearing temperature and the orientational order of these nematic polymers are not much different from those of the monomeric liquid crystals. Hlowever, as the value of s* becomes larger than a certain critical value, an ordered (nematic) state exhibiting perfect orientational order and completely rodlike conformations becomes more stable than the state of only partial order. Hlence, an extensive conformational rearrangement is predicted to occur in nematic melts of the polymers when the flexible sequences are niot very flexible. These predictions are in good qualitative agreement with the conclusions from the thermodynamic and NMR measurements on two thermotropic polymers comprising identical rigid groups joined by flexible sequences of substantially different flexibility.

: Liquid crystalline behavior is exhibited by many amphiphilic solutions as a result of the reversible formation of anisotropic aggregates. We model such a system by combining a phenomenological description of aggregation with a lattice statistics calculation of the configurational entropy of a polydisperse collection of hard rods and plates. In this model, nematic liquid crystalline phases of axial, planar and biaxial symmetry are possible. We present a calculated phase diagram, and corresponding particle size distributions, for the case when rod and plate growth are equally favored. Regions of stability are found for axial and planar phases, but not for biaxial phases.

Experiments with monolayers of macroscopic rods indicate separation into isotropic and aligned phases at greater-than-critical concentrations. This behavior is qualitatively similar to that of rodlike liquid crystalline polymers, which in turn has been modelled successfully in threedimensions by Flory and Ronca. We have adapted their approach to predict the ordering of rods in two dimensions. A preliminary phase diagram is presented. The critical rod concentration at which an aligned phase can appear first is a decreasing function of rod axial ratio. Rods of a given axial ratio will phase separate at lower overall concentrations in two dimensions than in three.

This paper presents a unique microscopic molecular theory for backbone liquid-crystalline polymers (LCPs), side-chain LCPs, and combined LCPs in the nematic (N) and multiple smectic-A (SA) LC phases and the isotropic (I) liquid phase. There are no ad hoc or arbitrarily adjustable parameters in this theory. The agreement between the theoretical and experimental values for various thermodynamic and molecular ordering properties for existing LCPs is very good (relative deviations between 0% and less than 6.2%). This theory has been used by this author to predict and design (atom by atom, bond by bond) the first super-strong (SS) LCPs. This paper presents the design of SS mostly-rigid (MR) LCPs.

This paper presents results from a unique microscopic molecular theory for side-chain liquid-crystalline polymers (LCPs) in the nematic (N) and multiple smectic-A (SA) LC phases and the isotropic (I) liquid phase. There are no ad hoc or arbitrarily adjustable parameters in this theory. The agreement between the theoretical and experimental values for various properties (including transition temperatures and quadratic characteristic radii) is very good (relative deviations between 0% and less than 6.2%). The theoretical results also show--for the first time--that the N and I phases for these LCPs involve the packing of plate-like sections of backbones and side chains and that the local bilayer SA phase involves packing of side-chains within a plate-like section. This type of packing is predicted to be typical for side-chain LCPs. This theory can predict-- for the first time--whether the side chains of a molecule pack on the same or alternating opposite sides of the backbone and whether side chains on different molecules interdigitate (overlap) with each other.

Deuterium NMR studies have been performed on variously labeled PBLG samples in the lyotroplc liquid crystalline state. The orientational orders S of the a-helical backbone were estimated from the observed quadrupolar splittings ∆v of the N-D and Cα-D bond. The values of S decrease gradually by dilution until the upper equilibrium concentration (B-point) is reached. The concentration dependence of Scrit has been compared with those derived from standard theories. The relatlve orientation of side chains with respect to the backbone was elucidated by the rotational isomeric state analysis of the ∆v/S data observed for the γ-, γ-, and ζ-methylene, and p-benzyl deuterons. The results indicate that the steric overlaps between adjacent side chains play an important role in determining the preference of conformations in the PBLG system. A similar analysis has been extended to poly(L-glutamic acid) esters carrying a homologous series of ester residues, -C(O)O(CH2)nC6D5 with n = 2 and 3. A distinct odd-even oscillation was observed for the orientation of the terminal phenyl groups with n.

I propose that certain, stereo-regular, main and side chain polymer liquid crystals would have a giant, long-time dielectric susceptibility, χ, of unusual temperature dependence

ϰ ∼ const.. exp{+h/kBT}

with a large ultimate low temperature value χ - χo N where χo is a susceptibility characteristic of a simple dielectric fluid and N the degree of polymerisation. The response would be caused by polarising molecular defects in the nematic ordering; de Gennes hair pins for main chain and torsional defects in side chain polymers. h is the defect energy.

Experimentally χ(2) - χo(2) N is known in the non-linear optical response of PBLG, a limiting case of main chain polymers with no hair pins.

The timescale for the response of side chain mesogens is predicted to vary as τ ~ N2.

Since the late seventies, the Air Force Materials Laboratory and Air Force Office of Scientific Research has had a very energetic program in lyotropic liquid crystalline rigid-rod polymers. The objective of the multifaceted program was the attainment of mechanical properties for a structural material comparable with those currently obtained from fiber reinforced composites. This paper will describe the orgins of this program as derived from the unique aggregation properties of aromatic heterocyclic ladder polymers.

The development of a method for polycondensation of rigid-rod poly(benzazoles) in poly(phosphoric acid) (PPA) in the lyotropic mesophase has led to an understanding of the synthesis variables that determine the important liquid crystalline product characteristics. These product variables, in turn, control processibility and final fiber or film properties. This paper provides a review of the effects of polymer concentration, solvent composition, polymerization temperature, and monomer purity on two liquid crystalline product characteristics: molecular weight achieved during polymerization, and concentrated solution properties, such as rheological and phase behavior. Central to the understanding of these effects are the factors affecting the polymerization rate. The methods for controlling these synthesis variables are also reviewed. Investigation of alternatives to PPA as the polymerization solvent resulted in a method to obtain high molecular weight poly(2,5-benzoxazole) by adding 40-45 wt % of P2O5 to methanesulfonic acid.

The phenomenon of mesophase-enhanced polymerization of the rodlike polymer poly(pphenylenebenzobisthiazole) in polyphosphoric acid was investigated. When the polymer concentration was greater than 5 wt%, the reacting mixture became anisotropic at an early stage of the polymerization. The reaction rate increased significantly at the isotropic-nematic phase transition as the rods were aligned in positions more favorable for the condensation reaction to occur. At 4.5 wt% of polymer, the reacting mixture remained isotropic for up to 1500 min of reaction time. The chemo-rheological properties of the 15 wt% reaction mixture indicate the occurrence of the phase change in either the shear viscosity or the first normal stress difference measurements.

The structure of quinoline oligomers synthesized by the catalytic dehydrogenative condensation reaction of 1,2,3,4-tetrahydroquinoline was elucidated on the basis of various spectroscopic data including the 2D COSY 1H NMR. The x-ray crystal structural study of two quinoline dimers successfully isolated from the bulk material resolved the puzzle of ring conjunction positions between quinoline units of the oligomer product. A reaction mechanism is proposed. From this mechanism a delicate balance between dehydrogenation and polymerization activity of the catalyst is required to optimize the yield and the molecular weight of the product.

Although ladder polymers have been known for some time, poor solubility and low molecular weights have prevented processing and have discouraged consideration of these materials for structural and electronic applications. Poor solubility can be associated with strong Van der Waals interactions between delocalized π-electron clouds on adjacent chains. The optimum orbital overlap along the polymer backbone existent for ladder polymers has motivated reexamination of these systems for electronic applications. We have pursued alternatives to the tradiational acid-catalyzed polycondensation of underivatized monomers at elevated temperatures by reacting derivatized (e.g., with alkyl or vinylamine substituents) monomers at reduced temperature in organic solvents (e.g., DMF) to produce open-chain precursors to ladder polymers. These are converted to fully-fused ladder polymers by thermal processing either in solution or as solid state (e.g., thin film) materials. The preparation of precursors of improved solubility greatly enhances processing options and has permitted the utilization of both Langmuir-Blodgett methods and the perparation of optical quality films by casting (including spin casting). By control of thermal processing conditions, both derivatized and underivatized ladder polymers have been prepared. Our approach offers obvious advantages for the investigation of the effect of conformation and of substituents upon electronic properties. The preparation of copolymers with rigid rod and flexible chain segments incorporated in the polymer backbone has also been investigated. For such materials, polymers with fully fused ladder segments can exhibit improved solubility in DMF relative to polymers with open chain segments due to the absence of favorable hydrogen bonding stuctures in the former. Finally, we are investigating the effect of electrochemical polymerization of Langmuir-Blodgett films prepared with derivatized diaminobenzene. Such an approach clearly takes advantage of improved solubility of the derivatized monomer and improved molecular order which can be realized with the Langmuir-Blodgett method. Free radical polymerization may facilitate the realization of improved molecular weight distributions.

Research in our laboratory on lyotropic liquid crystalline polymers has provided a series of high modulus, high tenacity materials derived from benzobisazole rigid-rod polymers. A number of different structural modifications on these systems have been carried out in our laboratory. This paper will be focused on those structural variations made by maintaining the para-ordered geometry while modifying the structure with pendent groups.

A series of 2-aryl-4,7-benzothiazoledicarboxylic acids have been synthesized from the corresponding dimethyl derivatives. These derivatives contain the phenyl, and the p-bromo-, mchloro-, p-chloro-, m-methoxy-, and p-methoxyphenyl aryl substituents. The phenyl, pchlorophenyl and p-methoxyphenyl substituted diacids were polymerized with 2,5-diamino-1,4- benzenedithiol dihydrochloride to yield the corresponding poly(benzothiazole)s. The polymers were of low molecular weight (intrinsic viscosity as high as 1.7) and did not show enhanced solubility in common solvents.

N1,N4-diphenyl-1,2,4,5-tetraaminobenzene was condensed with the di-acid chlorides of terephthalic acid, isophthalic acid and 2,5-pyridine-dicarboxylic acid in polar aprotic solvent to give the poly(o-phenylamino)amides which, after cyclodehydration in polyphosphoric, acid gave three new N,N'-diphenyl substituted polybenzimidazoles (PBI). The syntheses and certain of the solution and thermal properties of the PBI's are discussed. The synthesis of three new tetraamine monomers and their related model compounds is also described.

Articulated rigid-rod polymers with 2,6-benzobisoxazole and 2,6-benzobisthiazole units in the backbone were synthesized by copolycondensation in polyphosphoric acid of 4,6-diamino-l,3-benzenediol dihydrochloride and 2,5-diamino-l,4-benzenedithiol dihydrochloride, respectively, with terephthalic acid and suitably-structured difunctional monomers. Early efforts centered on the incorporation of flexible diphenoxybenzene tructures into the polymer backbone while subsequent efforts involved the more thermooxidatively stable 3,3'-biphenyl- or 4,4'-(2,2'-bipyridyl)-structures. Under dilute reaction conditions, the copolycondensation reactions proceeded in the isotropic phase to yield polymers with inherent viscosities of less than 13dL/g (CH3 SO3 H, 25°C, 0.2g/dL). Later efforts carried out under higher concentration in the nematic state led to intrinsic viscosities as high as 20dL/g. Films which exhibited optical birefringence under crossed polars could be cast from methanesulfonic acid solutions of the polymers. Thermooxidative stability of the articulated polymers was evaluated by isothermal aging in air at 316°C and/or 371°C. Stability of the polymers was found to decrease with increased content of the flexible structures, with the loss of thermooxidative stability being more pronounced with the diphenoxybenzene structures.

Three different kinds of siloxane-containing polybenzamide copolymers have been prepared: a series of siloxane-containing segmented copolymers; a siloxane-poly(benzamide) random copolymer; and a poly(benzamide) - poly(dimethylsiloxane) block copolymer. The structures of these copolymers were confirmed using diffuse reflectance FTIR spectroscopy. Dilute solution viscometery showed them to be high polymers. The thermal stabilities of these copolymers were studied using thermal gravimetric analysis. Incorporation of siloxane units via a phenyl link was found to not affect the inherent thermal stability of the poly(benzamide) chain. The solution phase behavior of these copolymers in DMAC/LiCl was studied using an optical microscope fitted with cross-polars. The segmented copolymers and the block copolymer exhibited lyotropic behavior, which was influenced by the amount of siloxane and the overall molecular architecture. Solutions of the random copolymer were isotropic at all concentrations studied.

A novel polyimide-based heterocyclic polymer, polyimide benzobisthiazole (PIBT), was prepared from diaminophenylene benzobisthiazole (DAPBT) and pyromellitic acid dianhydride (PMDA). Fibers with tensile modulus of 220 GPa were obtained from PIBT; this modulus value is consistent with the 430 GPa modulus determined by wide-angle x-ray diffraction (WAXD) analysis. Unoriented isotropic films exhibited significantly lower modulus values in the range of 10 GPa. Block copolymers of PIBT with thermoplastic segments were synthesized in an effort to prepare molecular composites. These materials showed a dramatic decrease in melt viscosity and had modulus values lower than observed in isotropic PIBT films.

This paper reports the synthesis method for polymerizaing poly(2,6-benzothiazole) (ABPBT) and poly(2,5-benzoxazole) (ABPBO), and their dilute solution properties using low angle light scattering and viscometry. Our data indicate that the Mark-Houwink-Sakurada exponents for these extended-chain polymers are near unity, suggesting a semi-rigid conformation in dilute solutions. The solution properties agree well with the Yamakawa-Fujii worm-like chain model when persistence lengths of 130 Å and 90 Å are assumed for ABPBT and ABPBO, respectively. These values of persistence length are also in good agreement with those predicted by Flory's virtual bond model using information on the molecular geometries taken from x-ray crystallographic data on oriented fibers of ABPBT and ABPBO.