Rigid-rod Polymers Research Articles

An ab initio study of the structural and physical properties of a novel rigid-rod polymer: PIPD

In this article, we present the first ab initio calculations on the novel rigid-rod polymer PIPD using density functional techniques. The behaviour of the molecular chain under strain is studied and the chain modulus agrees excellently with experiment. Two crystal structures are considered and hydrogen bonding networks as proposed from X-ray diffraction measurements are demonstrated to exist. Negligible energy differences were found and both structures could exist at room temperature. The electronic structure reveals a π – π interaction like in graphite, resulting in a weak bonding. The interaction due to the hydrogen bonding network is smaller, however: it leads to a significantly larger bonding energy. As the interchain bonding in PIPD is approximately 3 times the bonding without the network, the improvement in the compressive strength of PIPD compared to PBO and PBT can be attributed to the hydrogen bonding network.

Hard-sphere-chain equations of state for lyotropic polymer liquid crystals

Using Parsons-type scaling, the Onsager theory for the isotropic–nematic (I–N) transition of rigid-rod lyotropic polymer liquid crystals is combined with the equation of state for hard-sphere-chain fluids of Chapman et al. and that of Hu et al. The equation of Hu et al. gives the I–N transition pressure and density in good agreement with computer simulation by Wilson and Allen for a semi-flexible hard-sphere chain consisting of seven segments. For real semi-flexible polymers, we follow the Khokhlov–Semenov theory of persistent chains that introduces chain flexibility into the Onsager theory. Using a consistent procedure to regress the equation-of-state parameters, the equations of Chapman et al. and Hu et al. are also compared with the theory of DuPré and Yang that uses the equation of Lee for hard spherocylinders. These models are compared with experiment for two binary polymer solutions containing poly(hexyl isocyanate) and another solution containing polysaccharide schizophyllan. The concentration of polymer at the I–N transition is predicted as a function of the molecular weight of polymer. All models perform similarly and show semi-quantitative agreement with experiment.

Effects of flexibility on liquid crystalline polymer behavior: The nematic broken rod

A new theory for liquid crystalline polymers is developed, and its behavior in simple shear flow is analyzed. The theory accounts for molecular flexibility by employing a microstructure consisting of two rigid rods linked by a joint with a tunable stiffness. The probability distribution function equation for the orientation of the arms of the broken rod is derived. The adaptation of the smoothed particle hydrodynamics (SPH) technique for obtaining numerical solutions to this theory is detailed. The behavior of the theory at equilibrium is derived analytically and compared with numerical results; the SPH technique is then used to obtain results in flow. It is found that in the limit of a nearly stiff joint, the model gives behavior that is very similar to that of rigid rod polymers, the only difference being a lesser tendency to tumble due to greater variation in the order parameter. For nearly free joints, the shear flow induces interesting dynamics for the transition between states with the arms outstretched and those where they are folded up (so-called “hairpins” of main-chain LCPs). © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 281–300, 1999

Enhanced electrical properties of rigid-rod polymer incorporated with electroactive triarylamino moieties

Oxidatively stable and electrically insulating rigid-rod polymer poly(p-phenylenebenzobisthiazole) (PBT), was incorporated with electroactive triarylamino (TPA) moieties, (R)TPA-PBT with R=NMe2 and Me. Copolymers or homopolymers were synthesized with various TPA fractions changing the polymers to segmentally rigid configuration, and fabricated into films. On cast films, x-ray scattering revealed that the copolymers were random copolymers, microscopy showed “template effect” from the rigid segments. Iodine activation of the incorporated polymers led consistently to more than five order-of-magnitude increases in electrical conductivity. The activated conductivity also increased with TPA fraction. A conductivity value as high as 4.1×10−4 S/cm was realized for the activated (NMe2)TPA-PBT homopolymer. However, temporal stabilities were observed for the conductivity, which seemed to depend on the polymer composition and film morphology. Spectroscopic examination detected no new electronic state generated by iodine doping. Instead, optical absorbance was drastically increased for hν greater than 2 eV with a change in film color. Both enhanced conductivity and absorbance were not exhibited by the iodine-activated PBT polymer.

New aromatic benzazole polymers, 3. Synthesis of rigid-rod benzobisazole polymers with main-chain 2,2'-bipyridine-5,5'-diyl units

SUMMARY: Rigid-rod benzobisthiazole (Bipy-PBZT), benzobisoxazole (Bipy-PBO) and benzobisimidazole (Bipy-PyBI) polymers containing 2,2'-bipyridine-5,5'-diyl moieties were synthesized from 2,2'-bipyridine-5,5'-dicarbonyl dichloride and one of the following monomers: 2,5-diamino-1,4-benzenedithiol, 4,6-diamino-1,3-benzenediol, and 2,3,5,6-tetraaminopyridine. Bipy-PyBI is the first rigid-rod benzobisimidazole polymer without any solubilizing pendants that exhibits lyotropic behavior. These polymers displayed thin-film electrical conductivity of ≈10–10–10–8 S/cm in pristine state; 10–5–10–4 S/cm upon infiltration with AgNO3; 25–42 S/cm after Ag+ → Ag0 reduction by NaBH4.

Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes

Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.

Synthesis, Electrochemistry, and Spectroscopy of Blue Platinum(II) Polyynes and Diynes.

The smallest band gap observed so far (1.77 eV) for an organometallic polymer is exhibited by the blue, rigid-rod polymer 2, which is prepared by the reaction of trans-[PtCl2 (PnBu3 )2 ] with one equivalent of 1.

Axisymmetric entry flow of semi-dilute xanthan gum solutions: prediction and experiment

Experimental observations of the flow patterns in axisymmetric entry for rigid rod molecules are presented. The features of the flow are a large secondary flow vortex whose size is relatively independent of the flowrate, a vortex which increases in size with the rigid rod polymer concentration, decreases in size as the solvent viscosity increases, and increases as the contraction ratio increases. The secondary flow vortex sizes predicted and observed for dilute and semi-dilute rigid fibre suspensions are similar to those observed for the semi-dilute rigid rod xanthan gum solutions. Also, rounding a corner in the contraction had no effect on the observed asymmetric entry flow of the rigid rod molecular solutions. Good agreement was observed between the dimensionless re-attachment length for a 0.04% w/w xanthan gum solution and the predicted secondary re-attachment length from Binding [J. Non-Newtonian Fluid Mech. 27 (1988) 173–189] which used the independently measured extensional viscosity in the prediction. The agreement between experiment and observation confirmed Binding's assertion that the fluid will show vortex enhancement if its steady shear viscosity undergoes shear thinning more rapidly than its extensional viscosity undergoes elongational thinning. Vortex enhancement for fluids with constant extensional viscosity and steady shear viscosity, as observed, agreed with predictions by Debbaut et al. [J. Non-Newtonian Fluid Mech. 29 (1988) 119–146] and Debbaut and Crochet [J. Non-Newtonian Fluid Mech. 30 (1988) 169–184]. From the data presented it would be fair to speculate that normal stresses impede vortex growth while elongational thickening increases vortex growth.

Design, synthesis and properties of a novel rigid rod polymer, PIPD or `M5': high modulus and tenacity fibres with substantial compressive strength

A rigid rod polymer with strong hydrogen bonds between the polymer chains was found in polypyridobisimidazole based on 2,3,5,6-tetraaminopyridine and 2,5-dihydroxyterephthalic acid (D. J. Sikkema and V. Lishinsky, WO 94/25506). The strong interchain forces translate into the highest compressive strength found in polymer fibres by far, and the polarity of the polymer seems to contribute to easy adhesion in advanced composite manufacture. The modulus and tenacity are as expected for a rigid rod polymer in the general class of the earlier PBZT and PBO fibres.

XRD study of the new rigid-rod polymer fibre PIPD

The new rigid-rod polymer fibre PIPD was studied using wide angle X-ray scattering. The crystal structure of as-spun PIPD fibre can be described as a two-dimensionally ordered crystal hydrate. Its equatorial X-ray diffraction pattern was indexed to a non-primitive rectangular unit cell with parameters 16.85, 3.38 Å, containing two polymer chains. Heat treatment initially (i.e. up to 300°C) leads to the loss of water molecules from the structure as was shown by high temperature X-ray diffraction. At sufficiently high temperatures three-dimensional crystalline order is developed, as evidenced by the presence of strong off-axis reflections in the diffraction pattern of the heat-treated fibre. A procedure of unit cell determination was introduced that made it possible to index the latter diffraction pattern and elucidate the three-dimensional packing. The crystal structure of heat-treated PIPD fibre has monoclinic symmetry described by the space group P2 1/ a. Its cell dimensions are 12.60, 3.48, 12.01 Å, 90.0, 108.6, 90.0° with ρ x =1.77 g cm −3. The unit cell is non-primitive ( Z=2), with chains located at the centre and corners of the rectangular projection cell describing the lateral packing. Neighbouring chains packed along the diagonals of the projection cell are shifted relative to each other in the direction of the c-axis (chain axis) by 2.0 Å. This regular c-axis shift explains the observed absence of significant meridional scattering from the lower order layer lines of the diffraction pattern. A hydrogen bonding scheme is proposed consisting of intermolecular N–H—O bonds and intramolecular O–H—N bonds. The intermolecular hydrogen bonds form a bi-directional hydrogen bonding network linking each polymer chain to its four axially shifted neighbours. Departures of the latter hydrogen bonding scheme may be present in the form of hydrogen bonded sheets. The proposed hydrogen bonding scheme is consistent with thermal expansion data provided by high temperature XRD. The monoclinic crystal structure with its bi-directional hydrogen bonding network provides a satisfactory explanation for the exceptionally good compression performance of heat-treated PIPD fibre.

Mechanical properties and structural transitions in the new rigid-rod polymer fibre PIPD (`M5') during the manufacturing process

The polymer poly{2,6-diimidazo[4,5-b:4′,5′-e]pyridinylene-1,4(2,5-dihydroxy)phenylene} (PIPD) is spun from a liquid crystalline solution in polyphosphoric acid. The as-spun fibre is a crystal hydrate, which transforms into a bidirectional hydrogen-bonded structure during heat treatment. This transition results in an increase in crystalline modulus along the chain direction due to a decrease of the cross-sectional area per chain, and in an increase in shear modulus due to stronger interaction between the chains caused by the hydrogen bonds. Heat-treated PIPD has a tensile modulus up to 360 GPa, a tensile strength well above 4 GPa, an internal shear modulus of 7 GPa, and a compressive strength of 1.7 GPa.

Chain conformation of rod-like polymers in the melt: Small-angle neutron scattering of poly(benzoyl paraphenylene)

The chain conformation of a rigid rod polymer, poly(benzoyl paraphenylene), is determined in the melt using small-angle neutron scattering. The coherent scattering cross-section from blends of partially deuterated and hydrogenous poly(benzoyl paraphenylene) agree well with ideal rod scattering for q > 0.02 Å−1, indicating that the polymer chains are highly extended. Comparison of the results to a single chain-scattering function for chains of arbitrary stiffness yield a persistence length of ca. 130 Å. Pure component scattering at the lowest scattering lengths indicate that the melt is not molecularly homogenous, but is comprised of domains, potentially reflecting localized groupings of chains with similar backbone orientation. Furthermore, this mesoscopic structure depends on the processing history of the polymer in the melt state. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2449–2459, 1998

Rigid-rod benzobisthiazole polymer with reactive 2,6-dimethylphenoxy pendent groups

Polycondensation in polyphosphoric acid of 2,5-diamino-1,4-benzenedithiol dihydrochioride with 2-(2,6-dimethylphenoxy)terephthaloyl chloride led to rigid-rod benzobisthiazole polymers with thermally reactive dimethylphenoxy pendants. Soluble polymers with intrinsic viscosities as high as 15.5 dl/g (methanesulphonic acid, 30°C) were obtained. The polymers became insoluble after heat-treatment at approximately 400°C, possibly indicative of a crosslinking reaction between the polymer chains at the pendant sites. Fibres spun from a lyotropic polyphosphoric acid solution and heat treated at 425°C exhibited a tensile modulus of 91 GPa, a tensile strength of 1.10 GPa, and a compressive strength of 400 MPa. Wide-angle X-ray scattering patterns of the polymer fibres indicated that the bulky pendants significantly enhanced the axial registration between the neighbouring rigid-rod polymer backbones.

Laser Light Scattering Study of a Rigid-Rod Polyelectrolyte

Static light scattering (SLS), dynamic light scattering (DLS), and depolarized dynamic light scattering (DDLS) were employed to study methanolic solutions of an intrinsically stiff rigid-rod polyelectrolyte consisting of a poly(p-phenylene) backbone with dodecyl side chains. The solutions of the rigid rod polymers exhibit polyelectrolyte-solution properties at low counterion concentrations of tetramethylammonium chloride (TMACl). At higher salt concentrations, the single polymer molecules and the aggregates coexist in solution. The amount of aggregates varies with the polymer concentration and the nature of the counterions. By combining SLS with DLS studies, a new way of characterizing the basic parameters (such as the weight average molecular weight and the radius of gyration of the single polymer chains and the aggregation number of the aggregates) of the rodlike polyelectrolyte polymer chains in the presence of the corresponding aggregates has been presented. DDLS studies allowed for elucidation of the...

Unusual semimetallic behavior of carbonized ion-implanted polymers

We report a comprehensive charge transport study of ion-implanted rigid rod and ladder polymers $p$-phenylenebenzobisoxazole, $p$-phenylenebenzobisthiazole, and benzimidazobenzophenanthroline. The three pristine materials are strong and stable polymers with a room temperature conductivity ${\ensuremath{\sigma}}_{\mathrm{RT}}\ensuremath{\sim}{10}^{\ensuremath{-}12}$ S/cm. After high dosage ion implantation using ${\mathrm{Kr}}^{+},$ a carbonized and conducting layer forms on the surface of the film samples with ${\ensuremath{\sigma}}_{\mathrm{RT}}>{10}^{2}$ S/cm. The experimental results suggest that this carbonized layer is semimetallic with unusual properties. The observed dc conductivity follows $\ensuremath{\sigma}(T)={\ensuremath{\sigma}}_{0}+\ensuremath{\Delta}\ensuremath{\sigma}(T),$ where $\ensuremath{\Delta}\ensuremath{\sigma}(T)$ is weakly temperature dependent and interpreted within the model of weak localization and electron-electron interaction effects. The model reveals that the interaction effect is three dimensional for the experimental temperature range (3--300 K), whereas the weak localization effect undergoes a dimensional crossover at \ensuremath{\sim}60 K from three to two-dimensions with decreasing temperature. The magnetoconductance, thermoelectric power, and microwave dielectric constant results are all in agreement with this semimetallic model. In addition, all these results consistently point to an enhanced interaction effect at low temperatures due to the reduced dimensionality of the localization effect. It is concluded that a ${\mathrm{sp}}^{2}$ rich and three-dimensional interconnected carbon network reformed upon ion implantation of the densely packed pristine polymers is responsible for the semimetallic behavior.

Optical loss in rod-like polymer thin films

Optical thin films of three rod-like polymers, a rigid-rod polymer poly(p-phenylene benzobisthiazole) (PBZT), a diadamantane-substituted PBZT (DMT-PBZT) and a bicyclo[2.2.2]octane-substituted PBZT (BCO-PBZT), at different thicknesses were prepared with the solid-state coagulation process. In contrast to PBZT, DMT-PBZT and BCO-PBZT do not have a π-conjugated backbone because of their aliphatic DMT and BCO moieties. The PBZT thin films showed an out-of-plane optical attenuation coefficient (αo) of 1300 dB cm−1 at 600 nm wavelength while the αo value of the DMT-PBZT and the BCO-PBZT thin films at 600 nm wavelength could not be determined due to their weak absorption in the visible light region. However, the in-plane optical attenuation coefficient (αi) of the BCO-PBZT thin films was determined to be about 11.4 dB cm−1 by waveguide measurement at 632 nm. By comparing the optical transmission of the PBZT thin films to that of the DMT-PBZT and the BCO-PBZT thin films, it is concluded that the optical loss in the PBZT thin films is mainly due to intrinsic absorption of the polymer.

Synthesis of new bis(acetylide)-substituted fluorene derivatives and their bimetallic and polymeric complexes

A new series of bis(acetylide) organic ligands having fluorene or fluorenone moieties in the backbone, 2,7-di(trimethylsilylethynyl)fluorene ( 1a), 2,7-di(ethynyl)fluorene ( 1b), 2,7-di(trimethylsilylethynyl)fluoren-9-one ( 2a) and 2,7-di(ethynyl)fluoren-9-one ( 2b), has been prepared. Reaction of the diterminal alkynyl fluorene derivatives 1b and 2b with two equivalents of the complex trans-[(Ph)(PEt 3) 2Pt(Cl)] in CH 2Cl 2/ iPr 2NH with catalyic CuI readily affords the diplatinum(II) bis(acetylide) complexes trans-[(Ph)(PEt 3) 2Pt-CC-R 1-CC-Pt(PEt 3) 2(Ph)] (R 1=2,7-fluorenyl 3) and trans-[(Ph)(PEt 3) 2Pt-CC-R 2-CC-Pt(PEt 3) 2(Ph)] (R 2=2,7-fluoren-9-onyl 4) in good yields. Two new rigid-rod organometallic polymers trans-[–Pt(PBu n 3) 2-CC-R 1-CC–]∞ ( 5) and trans-[–Pt(PBu n 3) 2-CC-R 2-CC–]∞ ( 6) have also been prepared in good yields, in a similar manner to their dinuclear complexes 3 and 4, by the CuI-catalysed dehydrohalogenation reaction of trans-[Pt(PBu n 3) 2Cl 2] with one equivalent of 1b or 2b, in an amine solvent, at ambient temperature. Both poly-yne polymers (colourless 5, deep red 6) exhibit good solubility in common organic solvents and readily cast into thin films. The organometallic terminal alkyne complexes trans-[Cl(dppm) 2M-CC-R 2-CCH] (M=Ru 7, Os 8), obtained by treatment of 2b with one equivalent of cis-[M(dppm) 2Cl 2] (M=Ru, Os), based on a NaPF 6/DBU method have been utilised as starting products for access to the homobimetallic trans-[Cl(dppm) 2M-CC-R 2-CC-M(dppm) 2Cl] (M=Ru 9, Os 10) and heterobimetallic trans-[Cl(dppm) 2Ru-CC-R 2-CC-Os(dppm) 2Cl] ( 11) systems by reaction with a suitable choice of cis-M(dppm) 2Cl 2 (M=Ru or Os). All the new compounds have been characterised by analytical and spectroscopic methods, and the single-crystal X-ray structure of 1b has been determined.

Conformational and rheological dynamics of semiflexible macromolecules undergoing shear flow: A nonequilibrium Brownian dynamics study

Nonequilibrium Brownian dynamics simulations (NEBD) are used to model the dynamics of semiflexible macromolecules undergoing shear flow. The mathematical model utilizes a discretized version of the Kratky–Porod wormlike (or persistent) chain as the building block, generalized to include flow. The Fokker–Planck equation resulting from such an analysis is converted to a stochastic differential equation from which the simulation algorithm for the NEBD is obtained. Various conformational and rheological quantities are monitored, under both steady-state and transient conditions, with the primary independent variable being the flexibility parameter β, the bending constant of the chain. The model qualitatively describes many of the experimentally observed effects in such systems, most notably birefringence overshoots, cessation effects, and various steady-state effects. In addition, many of the qualitative rheological features of both rigid rod (or flexible) polymers are captured as β is made very large (or small). The advantage of the NEBD over an analytical treatment is its ability to incorporate (analytically intractable) effects such as hydrodynamic interactions and its (natural) ability to obtain transient information, a facet useful in comprehending the differing dynamics of rigid and less rigid macromolecules.

Functionalizable Self-Assembling Polydiacetylenes and Their Optical Properties

The most versatile polydiacetylene system reported to date from the perspectives of systematic chemical modification and processability was obtained from γ-radiation polymerization of the alkoxycarbonylmethylurethanes of 9-(N-methyl-N-phenylamino)-5,7-nonadiyn-1-ol. These polymers self-assemble into an acentric array, and they are readily modified by functionalization of their dialkylaniline group. The modified polymers provide examples of materials with interesting second-order nonlinear optical properties and the first case of a rigid-rod polymer that exhibits photofabricated surface relief gratings.

(LnM–Cm–)x: Hypothetical organometallic polymers with metallapolyyne and metallacumulene character *

A class of (–LnM–Cm–)x hypothetical polymers, m = 2–4, conceptually derived from experimentally known analogous rigid-rod organometallic polymers and with different electron counts, obtained by varying the metal and the ligands in the MLn fragment, have been studied using extended-Huckel tight-binding calculations. The results suggest interesting conducting properties, depending on the electron count and the length of the bridging Cm chain.