Elastic modulus in the crystalline region of poly(p-phenylene terephthalamide)

Fibres from aromatic polyamides have a much higher Young's modulus than fibres from aliphatic polyamides. In order to contribute to the explanation of this observed difference we looked at one of the ultimate properties, the elastic modulus in the crystalline region in the chain direction (E_cr∥). We carried out measurements on a bundle of filaments of PRD 49 fibre, which we identified by i.r. and X-ray analyses as poly(p-phenylene terephthalamide). With the X-ray technique we determined the lattice extensions under loading and from these data the E_cr∥ was calculated. The E_cr∥ was found to be 20 × 10¹¹ dyne/cm² which is in good agreement with the calculated E_cr∥, but not very different from that of nylon-6,6. The Young's modulus was found to be 11 × 10¹¹ dyne/cm².     

Crystal lattice deformation in single poly(p-phenylene benzobisoxazole) fibres

The relationships between distortion of the crystal lattice with deformation and fibre mechanical properties are complex. Micro-focus synchrotron diffraction is an ideal tool for probing such relationships. The investigation of fibres with different processing histories enables the influence of processing to be assessed. Longitudinal and transverse crystal strain is examined for three different varieties of PBO fibre (AS, HM and HM+). Large variations are found to exist between the different fibre types, although they share the same chemical structure.Crystal modulus calculated from longitudinal crystal strain and stress is found to be lowest in the fibre with the highest tensile modulus (HM+). Deformation increases the a-axis dimension of the unit cell for PBO AS and HM whilst decreasing both the a- and b-dimensions for HM+. These effects have been attributed to molecular chain rotation. It is also observed that the unit cell angle increases with deformation and the rate of change in unit cell volume is consistent between all fibre types, regardless of processing history.     

Direct fluorination of poly(p-phenylene)

Because of its good chemical and thermal stability due to the π-delocalization along the polymer chain, poly(p-phenylene) was used as starting material for fluorination to obtain new carbon fluoride. A complete characterization of both the virgin polymer, in order to define parameters such as the chain length and the degree of crystallinity, and the fluorinated materials was performed by complementary techniques (¹⁹F, ¹H and ¹³C NMR, XRD, FT-IR and EPR). From these data, possible fluorination mechanisms are discussed. Finally, tribological properties have been also studied for the fluorinated samples and compared to those of conventional graphite fluorides.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Differential water sorption studies on Kevlar 49 and as-polymerised poly (p-phenylene terephthalamide): adsorption and desorption isotherms

We have conducted differential water vapour sorption experiments on Kevlar^TM 49 at 30°C over a series of water vapour pressures from 0 to 90% of saturation, and on the as-polymerised form of the material at 30°C, 45°C and 60°C over a series of water vapour pressures of 0-60%, 0-25% and 0-15%, respectively. The equilibrium isotherms obtained for both samples show a distinct hysteresis-type behaviour. For Kevlar^TM 49, the hysteresis loop can be divided into two regions, namely above 30%, which is indicative of the presence of microvoids, and below 30%, which suggests inclusion of water into the intimate structure of the surface layer of the polymer crystallites, in a process known as intercalation.     

Micro-flowers of poly(p-phenylene pyromelliteimide) crystals

Morphology control of poly(p-phenylene pyromelliteimide) (PPPI) crystals was examined using reaction-induced crystallization of oligomers during solution polymerization of self-polymerizable N-(4′-aminophenyl)-3-carboxyl-4-alkoxycarbonylphthalimide. Micro-flowers of the PPPI needle-like crystals were formed in which the needle-like crystals grew radially from the center part as petals. The molecules aligned regularly along the long axis of the needle-like crystal. The structure of alkoxy group in the monomer and the monomer concentration influenced the size of the needle-like crystals, and their average length and width were changeable from 640 nm to 1.69 μm and from 110 nm to 210 nm, respectively. The average thickness was 20 nm. The obtained micro-flowers possessed high crystallinity and exhibited excellent thermal stability.     

Crystallographic texturing in single poly(p-phenylene benzobisoxazole) fibres investigated using synchrotron radiation

A micro-focus synchrotron beam has been used to investigate crystallographic texturing in poly(p-phenylene benzobisoxazole) fibres. By generating diffraction patterns across single fibres, it is possible to produce profiles showing scattering intensity as a function of beam position across the fibre. A straightforward model has been developed to allow the degree of texturing to be quantified for direct comparison between fibre types. The experimental results are found to fit a radial fibrillar-texturing model, which incorporates a distribution in radial orientation about the fibre axis. Previous studies reporting the a-axis of the PBO unit cell to be aligned radially within fibrils about the fibre axis are found to be correct.The degree of radial fibrillar texturing is in the same fibre order as tensile modulus and crystalline domain orientation for PBO fibres with different processing histories. It is proposed that the degree of radial fibrillar texturing is therefore related to fibre homogeneity. An extrapolation of tensile modulus to that of a perfectly homogeneous fibre results in a value in good agreement with the PBO crystal modulus. This further supports the proposal that the degree of radial fibrillar texturing is related to fibre homogeneity.     

Synthesis of poly(p-phenylene vinylene) and derivatives via a new precursor route, the dithiocarbamate route

A new synthetic route towards poly(para-phenylene vinylene) (PPV) and its derivatives has been investigated. In this route, a bis-dithiocarbamate monomer is polymerized towards a dithiocarbamate precursor polymer by the addition of a strong base (LDA). The corresponding conjugated polymer is obtained via a heat treatment of the precursor polymer. This dithiocarbamate (DTC) precursor route strikes an optimal balance between several straightforward but sometimes troublesome precursor routes and the more complex sulphinyl precursor route yielding superior purity materials. The novelty lies in the fact that the monomer synthesis is not as difficult as compared to the sulphinyl route and this without losing the quality and accessibility of the precursor polymer. In the paper, the mechanism of the polymerization process is discussed and strong indications for radical as well as anionic mechanisms are presented.     

The direct synthesis of wholly aromatic poly(p-phenylene)s bearing sulfobenzoyl side groups as proton exchange membranes

Sulfonated poly(p-phenylene)s (SPPs) containing sulfonic acid groups in their side chains had been directly synthesized by Ni(0) catalytic coupling of sodium 3-(2,5-dichlorobenzoyl)benzenesulfonate and 2,5-dichlorobenzophenone. The synthesized copolymers possessed high molecular weights revealed by their high viscosity, and the formation of tough and flexible membranes by casting from DMAc solution. The copolymers exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendent groups. Transmission electron microscopic (TEM) analysis revealed that these side-chain type SPP membranes have a microphase-separated structure composed of hydrophilic side-chain domains and hydrophobic polyphenylene main chain domains. The proton conductivities of copolymer membranes increased with the increase of IEC and temperature, reaching values above 3.4 × 10⁻¹ S/cm at 120 °C, which are almost 2-3 times higher than that of Nafion 117 at the same measurement conditions. Consequently, these materials proved to be promising as proton exchange membranes.     

Direct fluorination of various poly(p-phenylene): Effects of the polymer synthesis and thermal post-treatment

In order to complete a preliminary work about the direct fluorination of poly(p-phenylene) (PPP), the main parameters of the process were investigated. The molecular (chain length and crystallinity) and morphological effects (B.E.T. surface, granulometry) were studied. So, two new starting samples were studied in addition to PPP synthesized by the Kovacic's method (i) a commercial PPP (Polysciences) and (ii) a pyrolyzed PPP, which is similar to an amorphous hydrogenated carbon. Moreover, an annealed PPP (Kovacic's synthesis and post-treatment at 400 °C for 36 h) was compared to the as-synthesized polymer. The reactions with F₂ gas differ significantly in accordance with the synthesis way and the post-treatment (annealing or pyrolysis). Investigations about the direct fluorination of PPPs were carried out for a better understanding of their behavior with respect to molecular fluorine. An extensive characterization was performed by complementary techniques (¹⁹F and ¹³C NMR, FT-IR, and EPR). The fluorine content in the fluorinated PPPs is evaluated by these methods and a reaction mechanism is proposed.     

Synthesis of soluble poly(9,10-dihydrophenanthrene-2,7-diyl)s. A new class of luminescent poly(p-phenylene)s with ethylene type bridges

Poly(9,10-dihydrophenanthrene-2,7-diyl)s with -OSi(R)₂(R′) groups at the 9,10-positions were synthesized by dehalogenative polycondensation of the corresponding monomers by using a zerovalent nickel complex. They showed number average molecular weights (M_n's) of 9800-69,000 and high quantum yields (62%-quantitative) in photoluminescence. Palladium catalyzed copolymerization of 2,7-dibromo-9,10-dihydrophenanthrene having -OCH₃ or -OSi(R)₂(R′) groups at the 9,10-positions with diethynyl- or diboronic-aromatic compounds also gave photoluminescent polymers with high quantum yields.     

Solution properties and kinetics of aggregation of an alkyl-substituted poly(p-phenylene)

Freshly prepared solutions of poly(2,5-di-n-dodecyl-1,4-phenylene) (PPP 12) in toluene are metastable at room temperature with regard to a process which leads to the formation of aggregates composed of up to 100 individual macromolecules. This aggregation process has an induction period of more than 10 h at room temperature. The kinetics of aggregation was investigated by making use of a fast capillary membrane osmometer. Aggregation follows an Avrami-Evans type formalism and suggests that clusters of a lyotropic liquid crystalline phase of the polymer are formed of the same type as observed in the melt. The long induction period of aggregate formation in dilute solution in toluene allows to apply conventional techniques of molar mass determination like membrane osmometry and size-exclusion chromatography (SEC). A relationship [η] = 1.94 × 10⁻³ M^0.94 was found for PPP 12 in toluene at 20 °C and a persistence length of 15.6 nm was derived applying the Bohdanecky-formalism. This gives evidence of the worm-like nature of the non-aggregated PPP 12 in dilute solution.     

Considerations of polymerization method and molecular weight for proton-conducting poly(p-phenylene) derivatives

Ni(0)-catalyzed coupling polymerization of 2,5-dichloro-4′-phenoxybenzophenone was investigated by varying the ligand and coligand, temperature, reaction time, and solvent. The weight-average molecular weight (M_w) of poly(4-phenoxybenzoyl-1,4-phenylene)s (PPBPs) could be controlled by the polymerization conditions and reached a maximum of 4.4 × 10⁵ g mol⁻¹. Sulfonated PPBPs (S-PPBPs) with various M_ws were prepared with sulfuric acid to study the effect of molecular weight on the chemical and electrical properties of PPBP-based electrolytes. The strong molecular interactions in S-PPBP provided an ion exchange capacity of 2.9 meq g⁻¹ without loss of high mechanical properties. High molecular weight S-PPBPs had more desirable properties for fuel cell applications. While the swelling ratios and hydration numbers of S-PPBPs decreased with increasing molecular weight, the mechanical strength, proton conductivity, and fuel cell performance increased. S-PPBP also showed anisotropic behavior in the swelling and proton conductivity; such behavior is caused by the rigid-rod nature and the liquid-crystal structure.     

Characterization and conformation of aromatic polyamides: poly(1,4-phenylene terephthalamide) and poly(p-benzamide) in sulphuric acid

The dilute solution properties of two aromatic polyamides, poly(1,4-phenylene terephthalamide) (PPDT) and poly(p-benzamide) (PBA) in 96% sulphuric acid, have been investigated by measurements of the intrinsic viscosity, by light scattering and by gel permeation chromatography (g.p.c.). The Mark—Houwink relation for PPDT indicates that the conformation is intermediate between a coil and a rod-like particle. The conformations of both aromatic polyamides have been determined precisely by coupling g.p.c., light scattering and viscosity and it was found that PPDT and PBA in 96% sulphuric acid are not very rigid particles. The rigidity has been characterized in terms of a worm-like chain. The persistence lengths q which evaluate the rigidity of the chain are $\text{q = 175 ± 25}\text{A}\text{?}$ for PPDT and $\text{q = 500 ± 100}\text{A}\text{?}$ for PBA has the more rigid polymer chain.     

Modelling the structural and physicomechanical properties of substituted poly(p-phenylene)s using molecular mechanical and molecular orbital methods

Conducting polymers are important technological materials that are finding increasing use in batteries and display devices. The conformation and packing of these polymers in the amorphous glassy state are poorly understood, despite the fact that they dictate their most important physical and mechanical properties. The processing of currently known conducting polymers is difficult and there is a strong incentive to increase their processability through functionalization. Developing an ability to predict the structure and structure-property relations of conducting polymers in the bulk will help with the design of new structures that combine processability with favourable electronic properties and facilitate their use in future high-technology applications. In this work, we concentrate on substituted poly(p-phenylene)s. Detailed atomistic molecular models have been developed with the help of molecular mechanics and semi-empirical quantum mechanical calculations using Cerius and MOPAC V6.0 program packages and structural, volumetric, and mechanical properties, e.g. geometrical values, densities, have been calculated by simulations on these models. The results from both methods have been compared with simulated and experimental data and conclusions have been drawn on the methodology and the approximations used. This study was used to compare with results obtained on unsubstituted poly(p-phenylene)s carried out earlier and to continue to develop our methodology for calculating structure, physical and mechanical properties that will be generally applicable to conductive polymers.     

Synthesis, electroluminescence, and photovoltaic properties of dendronized poly(p-phenylene vinylene) derivatives

Two dendronized poly(p-phenylene vinylene) (PPV) derivatives, ED-PPV and BB-PPV, have been successfully synthesized according to the Gilch route. The obtained polymers possess excellent solubility in common solvents, good thermal stability with 5% weight loss temperature of more than 340 °C. The weight-average molecular weight (M_w) and polydispersity index (PDI) of ED-PPV and BB-PPV are in the range of (1.26-2.34)×10⁵ and 1.37-1.45, respectively. Polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al devices were fabricated, and the PLEDs emitted green-yellow light. The turn-on voltages of the PLEDs based on ED-PPV and BB-PPV were approximately 4.3, and 4.5 V, respectively. The PLED devices of ED-PPV exhibited the maximum luminance of about 157 cd/m² at 10.5 V. Photovoltaic cells with the configuration of ITO/PEDOT:PSS/polymer:C₆₀ (1:1)/Al were also fabricated, and the energy conversion efficiency of the devices based on ED-PPV and BB-PPV was measured to be 0.58, and 0.014%, respectively, under the white light at 75 mW/cm².     

Green-blue luminescence dichroism of cyano-containing poly[(m-phenylene ethynylene)-alt-(p-phenylene ethynylene)] aggregates dispersed in oriented polyethylene

A new highly luminescent cyano-containing poly[(m-phenylene ethynylene)-alt-(p-phenylene ethynylene)] derivative (CN-PPE) has been prepared and blended in micro/nano-sized dispersions into linear low density polyethylene (LLDPE) films by melt-processing. CN-PPE macromolecules show pronounced tendency to form molecular aggregates in the excited state when dispersed in the polymer matrix with emission characteristics depending on the structural order of the luminescent chromophores. In CN-PPE/LLDPE films, the polymer matrix orientation induces effective anisotropic arrangements of the π-π staking interactions between luminescent guest aggregates, thereby generating a pronounced green-blue dichroic emission. The capability to easily modulate the luminescent properties of polyethylene films (from green to blue reversibly) simply by varying CN-PPE concentration or by stretching the polymer matrix, suggests various technological applications in the field of smart and intelligent films from thermoplastic materials.     

The synthesis and characterisation of poly(paraphenyleneaminosulfone)

Summary Poly(paraphenyleneaminosulphone), which contains alternating electron donating and electron withdrawing functionalities linked by a paraphenyl group, has been synthesised and characterised. The significant order observed may be due to intermolecular hydrogen bonding.     

Synthesis and properties of new sulfonated poly(p-phenylene) derivatives for proton exchange membranes. I

Several high molecular weight poly(2,5-benzophenone) derivatives were synthesized by high yield nickel-catalyzed coupling polymerization of 2,5-dichloro-4′-substituted benzophenones. The monomers were prepared by Friedel-Crafts catalyzed reaction of 2,5-dichlorobenzoyl chloride and several aromatic compounds. The resulting polymers are organosoluble and show no evidence of crystallinity by differential scanning calorimetry (DSC). The temperatures of 5% weight loss of the polymers via dynamic thermogravimetric analysis in air were above 480 °C. Sulfonation of selected polymers utilizing concentrated or fuming sulfuric acid at room temperature introduced sulfonic acid moieties to the aromatic side group. Activated fluoro aryl groups were also used to generate pendent sulfonated functionalities. The sulfonated polymers were examined for ion exchange capacities, water absorption capacities and proton conductivities. The sulfonated polymers were not good film formers, but could be demonstrated to show high values of proton conductivity in the range of 0.06-0.11 S/cm when supported on glass fabrics or via polymer blending strategies.