Microfibrillar Morphology Research Articles

Improvement of the properties of polyarylate reinforced with liquid crystalline polymers

AbstractBlends of polyarylate of bisphenol A, PAr, with two commercial main chain liquid crystalline polymers, Vectra A950 and Vectra B950, are studied. From dynamic viscoelastic measurements it is deduced that both systems (PAr/Vectra A950 and PAr/Vectra B950) are immiscible and scanning electron microscopy (SEM) micrographs show the presence of spherical domains of the liquid crystalline polymer when PAr constitutes the matrix. Extrusion capillary measurements reveal that, under conditions of temperature and shear rate similar to those of processing, the viscosity is reduced to approximately 10% of its value when the content of liquid crystalline polymer is only 20%. This great improvement of the rheological properties is observed in both PAr/Vectra A950 and PAr/Vectra B950 blends. The effect of draw ratio on Young's modulus for different compositions is also analyzed, pointing out the reinforcing action of both liquid crystalline polymers on polyarylate: for instance, 20% of Vectra B950 in the blend gives rise to a 700% increase of the modulus of fibres prepared at a draw ratio of 50. SEM of the extrudates reveals that the spherical domains are elongated at the entrance of the capillary giving rise to a microfibrillar morphology which is related to the excellent rheological and mechanical properties of the blends.

Morphological modification of UHMPE fibers

AbstractUHMPE fibers can display several morphological features, including skin‐core, microfibrillar and shish‐kebab structures. In this work, an attempt was made to eliminate these morphologies by treating the UHMPE in a two‐roll mill, at temperatures ranging from ambient to 145°C. Up to 130°C the tensile strength and modulus of the fiber were retained, and thermal analysis and crystallographic data showed that ordering in the fiber remained intact. The shish‐kebab and microfibrillar morphologies were greatly reduced and the fiber cross‐section was altered from circular to a flat ribbon.

Lamellar structure formation in the mixture of two cylinder‐forming block copolymers

AbstractTwo cylinder‐forming polystyrene‐block‐polybutadiene block copolymers of type A‐B with antisymmetric composition (polymer P1 with A : B = 3 : 1 (w/w) and polymer P2 with A : B = 1 : 3 (w/w)) and their binary mixture were studied by small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) in order to determine the microphase structure. The cylindrical microstructure is confirmed for both copolymers, whereas, in the mixture, both techniques have unambiguously shown that a regular lamellar structure is obtained at the polymer‐polymer composition P1 : P2 = 1 : 1 (ratio of the numbers of chains). Qualitatively, the same results are obtained for corresponding systems simulated by the cooperative motion algorithm (CMA). Direct observations of the structure as well as the cooperative structure factors determined in the strong segregation limit for the simulated systems indicated lamellar structures in the blend of composition P1 : P2 = 1 : 1, in contrast to the microfibrillar morphology of the copolymers. The simulation additionally indicated a polymer‐polymer microseparation. The experimental and simulation results are compared with theoretical predictions based on the self consistent field theory.

Treated polyethylene fibres as reinforcement for epoxy resins

Ultra-high-modulus polyethylene (UHMPE) fibres were treated in order to develop favourable surface and, possibly, microstructure characteristics. The main aim was to eliminate the microfibrillar morphology of the fibre and improve interfacial bonding between fibre/matrix so that better compressive properties can be achieved in reinforced resins. Calendering at 130 °C was performed, and the surface treatment used oxidative solutions. Adhesive bonding to epoxy matrices was highly improved in chromosulphate-treated material exceeding that of a commercial, corona-treated product, but the mechanical properties of these fibres deteriorated. Calendering did not significantly affect fibre strength and only improved adhesive bonding slightly. The use of these treated reinforcements is expected to improve the performance of composite materials, especially at low fibre volume fractions, because of their improved interfacial characteristics.

Polyethylene-polyethylene microfibrillar composites

Solid-state drawing of melt-crystallized, or gel(solution)-crystallized, polyethylene (PE) is well established as a means of producing high modulus high-strength fibres. Here, an alternative route, based on melt-processing, is reviewed and its merits are assessed. Contrary to expectation, melt processing of flexible chain polymers can directly yield oriented products with good mechanical properties, without the need for post-drawing in the solid state. The melt-processed PE can give a special microfibrillar composite morphology which results in good mechanical properties. The paper also reviews aspects of composites design by comparing these microfibrillar composites with traditional fibre composites and molecular composites.

In-Situ Synchrotron X-Ray Scattering Studies of Polybenzoxazole Fiber Spinning

AbstractInitial results are presented of in-situ wide angle x-ray scattering (WAXS) studies of structure and orientation development in the draw zone during fiber spinning of poly(p-phenylene benzobisoxazole) (PBO) in polyphosphoric acid (PPA). The diffraction patterns show strong [100] and [002] reflections. The coherence lengths found from the [100] and [002] reflections are consistent with those found by electron diffraction and suggest important elements of the microfibrillar morphology may already be present in the texture of the extrudate before coagulation. Increasing extrusion rate is found to produce little improvement in molecular orientation. Molecular orientation initially increases with increasing draw ratio from 1 to 10 but, remains constant for higher draw ratios. These results are compared to an affine deformation model during elongational flow.

Atomic force microscopy of polymer crystals. 6. Molecular imaging and study of polymorphism in poly(p-phenyleneterephthalamide) fibers

The macromolecules in extended-chain crystals of as-spun and annealed poly(p-phenylene-terephthalamide) fibers and the microfibrillar morphology were visualized by atomic force microscopy (AFM). Quantitative information about the crystal lattice on the surface of the fibrils was obtained. The positions of the phenylene groups with respect to the hydrogen-bonded sheet of the crystal lattice were visualized. Results are compared with structures deduced from X-ray diffraction measurements on one hand, and from predictions for polymorphic forms of the crystal structure calculated in computer simulation experiments on the other

The effect of deformation history on the morphology and properties of blends of polycarbonate and a thermotropic liquid crystalline polymer

AbstractThe effect of deformation history on the morphology and properties of liquid crystalline polymers (LCP) blended with polycarbonate resin was assessed. The addition of an immiscible LCP phase was found to improve the melt processability of the host thermoplastic polymer. In addition, by employing a suitable deformation history, the LCP phase may be elongated and oriented such that a microfibrillar morphology can be retained in the solid state.

Mechanical anisotropy in rolled nylon 66

AbstractThe nine independent stiffness constants Cpq for rolled nylon 66 at thickness reduction ratios λt = 2.4, 4.9, 7.6 have been measured from −40 to 50°C by an ultrasonic method at 10 MHz. Analysis of x‐ray pole figures indicates that at high λt the material has a uniplanar‐axial texture, with hydrogen‐bonded (010) planes parallel to the roll plane and the molecular chains along the roll direction. The mechanical behavior is determined not only by the alignment of molecular chains and hydrogen bonds but also by the microfibrillar morphology. On the basis of this idea the magnitudes of the tensile moduli along the three principal axes can be understood in terms of the Takayanagi model. The effects of water absorption have also been investigated.

The Crystal Solvate Phase of Heteroaromatic Rigid Polymers Yachin Cohen

ABSTRACTThis work concerns solutions of the rigid polymers: poly(p-phenylene benzobisthiazole)-PBZT, and poly(p-phenylene benzobisoxazole)-PBO, in polyphosphoric acid (PPA), and formation of a crystal solvate phase by cocrystallization of polymer and solvent. Several crystal-solvate phases have been observed in these solutions, depending on temperature and the extent of exposure to moisture. The structure of these phases is studied by X-ray diffraction. Transitions between the solvated phases involve changes in the lateral packing as well as the axial registry of parallel chains. All PBOPPA crystal solvates observed so far exhibit three-dimensional order, whereas such order appears only in the PBZT-PPA phase obtained at low moisture levels. These differences are interpreted in terms of the state of protonation of the polymer chains, whereby the highly ordered structures reflect unique associations of the protonated polycation and PPA anions. Observations by electron microscopy indicate that the transition from the highly ordered crystal solvate phase to the less ordered phase involves formation of thin microfibrils, less than 100 Å in width. These microfibrils may be the precursor of the microfibrillar morphology observed in fibers formed from solutions of the rigid polymers.

Scanning electron microscopy of oriented high density polyethylene

The microfibrillar and lamellar morphologies in cold-drawn and cold-drawn/annealed high-density polyethylene sheets were observed by means of scanning electron microscopy. Differences in contrast on fracture surfaces for cold-drawn sheet are interpreted in terms of a preferential orientation of inter-microfibrillar tie molecules in the plane of the sheet brought about by the drawing mechanism. In annealed, cold-drawn sheet, stacks of lamellae were observed which showed twinned orientations of inclined lamellae. This “roof-top” structure is interpreted in terms of shear within the individual microfibrils during micronecking, and corresponds to the well-known 4-point small-angle X-ray pattern for this type of specimen. Light etching with fuming nitric acid was necessary in order to resolve the individual lamellar texture.

An electron microscopy study of the microfibrillar structure of deformed polyethylene spherulites

AbstractA quantitative analysis of the deformation of thin spherulitic polyethylene films was undertaken at the microscopic level with the aid of a gold marking technique. The deformation microstructure which developed was further characterized through the use of a gold decoration technique. The microfibrillar morphology was found to agree quite well with Peterlin's model of the microfibril. Changes in the sample morphology as delineated by the gold decoration, support the proposed mechanisms of the transformation of the folded chain spherulitic morphology into the microfibriliar morphology and the subsequent deformation of the microfibrillai structure.

Dependence of diffusive transport on morphology of crystalline polymers

Abstract The sorption and diffusion of low molecular weight penetrants proceeds almost exclusively through the amorphous component of the semicrystalline polymer solid. The diffusive transport properties and geometrical distribution of the amorphous component are substantially modified by mechanical and thermal treatment. Deformation of spherulitic material first loosens the structure and then transforms it into a densely packed fibrous structure with a great many taut tie molecules in the amorphous component. Annealing lets the crystals grow in thickness, removes crystal defects, sharpens the boundaries between crystalline and amorphous component, and relaxes the taut tie molecules. The resulting changes of transport properties cannot be described in a satisfactory manner by crystallinity and orientation but requite a detailed consideration of morphology. The elastic tensile deformation enhances sorption and diffusion by reducing the density of amorphous component. The high anisotropy of diffusion and the drastic reduction of sorption and diffusion of fibrous material are the consequence of the microfibrillar morphology with the large fraction of highly aligned and closely packed taut tie molecules which eliminate many sorption sites, enormously reduce the diffusivity, and increase its concentration dependence. The anisotropy may be reduced during plastic deformation of the fibrous material by the increased number of interfibrillar tie molecules.

Influence of morphology on the transport properties of crystalline polymers

Abstract

Physical and fine structure properties of cotton fibers swollen with trimethylbenzylammonium hydroxide

AbstractCotton fiber was treated with aqueous trimethylbenzylammonium hydroxide (Triton B) at concentrations over the range 25%–40%. After complete removal of the swelling agent, the samples were evaluated for the extent of swelling, strength and elongation, birefringence, moisture regain, density, crystallinity, x‐ray diffraction patterns, and microfibrillar morphology. Electron‐microscopical examination and other evaluation of fine structure properties revealed that the nature of swelling is intercrystalline up to 30% concentration of Triton B, and intracrystalline beyond that. Although the swelling as measured by propanol‐2 retention after treatment with 30% Triton B is about twice as much as that of the control, the original structure remains almost unchanged except for some gain in strength and elongation and increase in moisture regain. At 32% Triton B concentration and beyond, rapid decrystallization takes place, accompanied by a fall in birefringence, density, and crystallinity index. X‐Ray analysis showed significant loss of lateral order and partial conversion of cellulose I to cellulose II at 35% and 40% Triton B concentrations. The results indicate that, used at the critical concentration of 30%, Triton B can be a useful swelling agent for cotton fibers as it opens up the fine structure of cellulose considerably without impairing any important physical properties.

Electron-Microscope Study of Cotton Treated with Inter- and Intracrystalline Swelling Agents

Electron micrographs are presented of cotton cellulose treated with aqueous solutions of the inter- and intracrystalline swelling agents, morpholine, piperidine, piperazine, ethylenediamine, and zinc chloride. The concentration of each swelling agent was such as to produce maximum recoverable swelling of cotton cellulose. Physical and fine-structure characteristics of these treated cottons, after complete removal of the swelling agent, revealed that morpholine and piperidine are intercrystalline and piperazine, ethylenediamine, and zinc chloride are intracrystalline swelling agents. Thisw as confirmed by electron-microscope determination of microfibrillar morphology and dimensions. The layer-expansion technique was used to examine these treated cottons along with samples that had been treated with sodium hydroxide and phosphoric and sulfuric acids. The electron micrographs revealed marked gradation in swollen and decrystallized cottons, which may serve as a visual scale at the microfibrillar level for cotton decrystallized to varying degrees. To confirm the extent to which the morphology of decrystallized cotton observed by the layer-expansion technique really exists in the swollen state before the removal of the swelling agent, the cottons treated with sodium hydroxide, morpholine, zinc chloride, and sulfuric acid were embedded in glycol methacrylate in the swollen state and the ultrathin cross sections of these were examined.