Rigid-rod Molecules Research Articles

Mechanical properties of molecular composites. I. Poly (p‐phenylene terephthalamide) anion molecules dispersed in poly(4‐vinylpyridine)

Molecular composites have been prepared by dispersing rigid-rod molecules of ionically-modified poly(p-phenylene terephthalamide) (PPTA anion) in a polar poly(4-vinylpyridine) (PVP) matrix. For concentrations up to 5 wt % of the rigid-rod reinforcement, the resulting composites are transparent and possess a single glass transition temperature that increases with concentration of the PPTA anion. The mechanical properties of the molecular composites are found to increase with concentration and to attain maximum values at about 5 wt % of the PPTA anion. The enhancement in properties, and the miscibility induced between the two component polymers, is attributed to the development of specific interactions between the ionic groups of the PPTA anion and the polar units of the PVP matrix. When such interactions are not present, as in composites reinforced with non-ionic PPTA, the samples are opaque and their properties are significantly reduced compared to those of the PPTA anion/PVP composites. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2201–2209, 1999

Mechanical properties of molecular composites. I. Poly (p-phenylene terephthalamide) anion molecules dispersed in poly(4-vinylpyridine)

Molecular composites have been prepared by dispersing rigid-rod molecules of ionically-modified poly(p-phenylene terephthalamide) (PPTA anion) in a polar poly(4- vinylpyridine) (PVP) matrix. For concentrations up to 5 wt % of the rigid-rod reinforce- ment, the resulting composites are transparent and possess a single glass transition temperature that increases with concentration of the PPTA anion. The mechanical properties of the molecular composites are found to increase with concentration and to attain maximum values at about 5 wt % of the PPTA anion. The enhancement in properties, and the miscibility induced between the two component polymers, is attrib- uted to the development of specific interactions between the ionic groups of the PPTA anion and the polar units of the PVP matrix. When such interactions are not present, as in composites reinforced with non-ionic PPTA, the samples are opaque and their properties are significantly reduced compared to those of the PPTA anion/PVP composites. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2201-2209, 1999

Direct evidence for the importance of hydrophobic mismatch for cell membrane recognition

In this Letter, we describe the synthesis of amphiphilic oligo( p-phenylene)s from 31 to 44 Å length and delineate the interaction of these rigid-rod molecules with lipid bilayers using fluorescence quenching methods. The results demonstrate high importance of hydrophobic mismatch for selective cell membrane recognition by rigid-rod molecules.

Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. I. Diffusion

The dynamics of a 20 base pair oligonucleotide is studied by dynamic light scattering-photon correlation spectroscopy and depolarized Fabry–Perot interferometry. The 20 base pair oligonucleotide is a well-defined, albeit short, rigid rod molecule that serves as a model for polyelectrolyte solution dynamics. The effects of added salt on the solution rotational and translational dynamics are examined in detail as functions of the 20-mer concentration. Coupled mode theory together with counterion condensation theory gives good predictions for the effects of salt on the translational diffusion of the 20-mer at the relatively low oligonucleotide concentrations studied. Comparison of the experimental results with these theories shows that the effective charge density of the polyion in solution is approximately equal to the reciprocal of the product of the Bjerrum length and the counterion charge, νeff≅1/NλB. Calculation shows that the numerical solution of the coupled mode theory matrix gives a better fit of our measured polyion diffusion coefficients than the approximate equation derived by Lin, Lee, and Schurr. Simple approximations for the effective rod length, Leff=L+κ−1, and effective rod diameter, deff=d+κ−1, are used to model the thermodynamic-hydrodynamic interactions for charged rodlike molecules and to make predictions for the diffusion second virial coefficient as a function of added salt concentration. This alternative to the coupled mode theory also gives good agreement with experiment. The rotational diffusion constants of the oligonucleotide measured by depolarized Fabry–Perot interferometry show a slowing down of the rotation at low added salt concentrations as the oligonucleotide concentration is increased.

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.

The effect of concentration, temperature, and molecular weight on the dynamics of rigid-rod molecules in semi-dilute solutions

The dynamics of rigid-rod-like molecules are studied using rheo-optical techniques. Measurements of flow birefringence as a function of shear rate are utilized to understand the scaling behavior of rotational diffusivity with respect to concentration and temperature. The concentration scaling exponent increases with increasing concentration and the scaling laws are valid in narrow concentration windows. The Doi-Edwards (DE) scaling law Dr ∼ c−2, holds at very high concentrations (cL3 > 150). The concentration scaling exponent decreases dramatically with increasing temperature at concentrations, cL2d > 1. Scaling of rotational diffusivity, with respect to temperature and solvent viscosity in the semidilute regime, does not follow the predictions of DE theory (and related caging ideas). On the contrary, a model proposed by Fixman was found to explain both the temperature and concentration dependence of the rotational diffusivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 181–190, 1998

Toward Biomimetic Ion Channels Formed by Rigid-Rod Molecules: Length-Dependent Ion-Transport Activity of Substituted Oligo(p-Phenylene)s

Toward Biomimetic Ion Channels Formed by Rigid-Rod Molecules: Length-Dependent Ion-Transport Activity of Substituted Oligo(<i>p</i>-Phenylene)s

Mesomolecules: from molecules to materials

Preface From molecules to materials. Cascade molecules. Dendritic polynuclear metal complexes with made-to-order luminescent and redox properties. Molecular tectonics. Supramolecular assemblies from Tinkertoy rigid-rod molecules. Graphite: Flat, fibrous and spherical. Fractal index and fractal notation. Index.

In situ thermoset molecular composites

AbstractThe polymerization of rigid rod polymer precursors in a reactive matrix precursor, which is later cured in the mold, constitutes the in situ process. A poly‐azomethine (PAM) was used as the rigid rod molecule. The resin used was an epoxy. We discuss the prediction of mechanical properties using micromechanics equations for chopped fiber composites. The chemistry used to synthesize the rigid rod polymer PAM in the epoxy precursor is reviewed. Approaches to better control the cure of these epoxy systems through cure kinetics and cure rheology studies completes the thermoset in situ molecular composite process. There was a 71% increase in tensile modulus in comparison to that of the neat epoxy resin. Molecular modeling simulations and continuum mechanics are used to help understand these findings. PAM/epoxy systems were used as a matrix material in the fabrication of unidirectional glass fiber/(PAM/epoxy) structural composites. © 1994 John Wiley &amp; Sons, Inc.

Alkynylcubanes as Precursors of Rigid-Rod Molecules and Alkynylcyclooctatetraenes

Alkynylcubanes as Precursors of Rigid-Rod Molecules and Alkynylcyclooctatetraenes

High performance polymers prepared by transformation of processable polyamides

A new route to the preparation of films and fibres of intractable rigid-rod molecules is described. Aromatic polyamide systems are prepared with varying levels of alkyl-based side-chain substitution, which leads to materials with good solubility in standard solvents such as tetrahydrofuran. The chemical structure and the physical properties of such materials may be transformed by cleavage of the solubilizing side-chains in the solid state. Films prepared in this manner are insoluble and intractable.

Molecular dynamics of rigid rod polymers

Molecular dynamics (MD) calculations have been used to study the behaviour of isolated rigid rod molecules of poly( p-phenylene), poly( p-phenylene benzobisthiazole) and poly( p-phenylene benzobisoxazole). The molecular mechanics force field was initially modified to improve agreement between minimized structural geometries and available X-ray data, as well as results from semiempirical molecular orbital calculations. The MD simulations show the molecules to be surprisingly flexible, with changes in end-to-end distances as large as 16%. An examination of the energies (calculated by various methods) associated with out-of-plane bending deformation, suggests that the rigid rod polymers may in fact be even more flexible than the simulations indicate. The results provide rationalizations for the relatively short persistence lengths measured in solution and for bending observed in high-resolution electron micrographs of these materials.

Curing kinetics of liquid-crystalline epoxy resins

Abstract By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.

Dynamic melt rheology. II: Re‐examining the relationship of g′ in oscillatory rheometry to the melt elasticity

AbstractIn oscillatory rheometry, since the dynamic storage modulus (G′) represents the elastic component of the viscoelastic melt, it has been a rather common practice to associate it with the melt elasticity. Although this assumption might be valid in many instances, there can be exceptions as well. In the present study, we have chosen polypropylene/talc system to demonstrate an exception. For example, the melt elasticity decreases with increasing talc content as revealed by extrudate die swell, entrance pressure drop, and elastic compliance measurements; the opposite trend being reflected by G′. We can envision similar exceptions when stiffening agents are added to a polymer melt, e.g., rigid rod molecules. We are suggesting that G′ represents the elastic modulus or stiffness of the melt, which in certain situations might not reflect elasticity, i.e., the ability of a melt to recover from a deformation. The implication of the present manuscript is that caution be exercised when relating G′ to the melt elasticity of a polymer.

Self-assembled monolayers of parent and derivatized [n]staffane-3,3(n-1)-dithiols on polycrystalline gold electrodes

Synthesis of the terminally disubstituted rigid rod molecules, [n]staffane-3,3 (n-1) -dithiols, n = 1-4, and their singly functionalized derivatives carrying and acetyl or a pentaammineruthenium(II), [Ru(NH 3 ) 5 ] 2+ , substituent is described. Self-assembled monolayers of the neat [n]staffane derivatives and of their mixtures with n-alkyl thiols were prepared on polycrystalline gold electrodes

Rigid-rod molecules: carborods. Synthesis of tetrameric p-carboranes and the crystal structure of bis(tri-n-butylsilyl)tetra-p-carborane

Rigid-rod molecules: carborods. Synthesis of tetrameric p-carboranes and the crystal structure of bis(tri-n-butylsilyl)tetra-p-carborane

Study of the Lowest Electronic States in a Polythiophene Oligomer, α-Sexithienyl, by One and Two-Photon Spectroscopy

Abstract α-sexithienyl (T6) is a large rigid rod molecule with twelve conjugated double bonds. The electronic structure of T6 has been investigates by one-photon and two-photon spectroscopy as well as by photoconductivity. From the single crystal polarized spectra at 4.2K we observe that the lowest allowed π-π transition (λmax = 478 nm) is polarized parallel to the long molecular axis and a second intense transition at 370 nm is polarized parallel to the short in plane axis. The two-photon excitation spectra in polycristalline thin films have been investigated at 4.2 K in the spectral range between 910 and 1180 nm of the fundamental laser radiation. The intense two-photon absorption band at 544.9 nm is assigned to the 2lAg exciton band origin. The lowest “gerade” exciton level lies therefore at 898 cm−1 above the lowest one-photon allowed 11Bu exciton level. The photophysics of T6 is studied by photoconductivity and one-photon excitation spectroscopy. An efficient pathway for carrier generation open up at...

Electron microscopic observation of suprastructure constructed by inorganic rod molecule, imogolite.

Lyotropic liquid crystals formed by inorganic rigid rod molecules, imogolite, were observed directly by transmittion and scanning electron microscopies.

Morphology of High-Performance Rigid-Rod Polymer Fibers and Molecular Composites

Rigid-rod polymers such as PBO, poly(paraphenylene benzobisoxazole), Figure 1a, are now in commercial development for use as high-performance fibers and for reinforcement at the molecular level in molecular composites. Spinning of liquid crystalline polyphosphoric acid solutions of PBO, followed by washing, drying, and tension heat treatment produces fibers which have the following properties: density of 1.59 g/cm3; tensile strength of 820 kpsi; tensile modulus of 52 Mpsi; compressive strength of 50 kpsi; they are electrically insulating; they do not absorb moisture; and they are insensitive to radiation, including ultraviolet. Since the chain modulus of PBO is estimated to be 730 GPa, the high stiffness also affords the opportunity to reinforce a flexible coil polymer at the molecular level, in analogy to a chopped fiber reinforced composite. The objectives of the molecular composite concept are to eliminate the thermal expansion coefficient mismatch between the fiber and the matrix, as occurs in conventional composites, to eliminate the interface between the fiber and the matrix, and, hopefully, to obtain synergistic effects from the exceptional stiffness of the rigid-rod molecule. These expectations have been confirmed in the case of blending rigid-rod PBZT, poly(paraphenylene benzobisthiazole), Figure 1b, with stiff-chain ABPBI, poly 2,5(6) benzimidazole, Fig. 1c A film with 30% PBZT/70% ABPBI had tensile strength 190 kpsi and tensile modulus of 13 Mpsi when solution spun from a 3% methane sulfonic acid solution into a film. The modulus, as predicted by rule of mixtures, for a film with this composition and with planar isotropic orientation, should be 16 Mpsi. The experimental value is 80% of the theoretical value indicating that the concept of a molecular composite is valid.

Compressive deformation of embedded high-performance polymeric fibers

A composite sample fabrication technique was designed and demonstrated for applying direct axial compression to high-performance polymeric fibers for studies of fiber microstructure under in situ compression. Two kinds of polymers were involved in the fibers of interest. One is a polymer of rigid-rod molecule; the other is a polymer of extended coil-like molecule. A multifilament fiber bundle was aligned uniaxially and embedded in an epoxy matrix. The composite sample geometry and the Poisson's ratios were analyzed by finite-element computations that suggest that a high modulus matrix is required for inducing high-compressive load and large stress gradient along the embedded fiber. This was verified by optical microscopy on compressed composite samples and facilitated the selection of the epoxy matrix. A compressive strain of 2% was successfully applied to the polymeric fibers resulting in the fiber kink band formations, which are considered a fiber compressive failure, for microstructure studies. Our sample fabrication also indicates that fibers of rod-like polymer have a lower critical compressive strain compared to that of fibers of coil-like polymer. X-ray scattering results are also introduced to show the validity of the prescribed technique for examining the microstructure of fibers under compressive deformation.