Micropatterning of semicrystalline poly(vinylidene fluoride) (PVDF) solutions

Micropatterning of a semicrystalline poly(vinylidene fluoride) (PVDF) solution was performed by a temperature controlled capillary micromolding where the rate of solvent evaporation was controlled by substrate temperature. In order to choose proper solvents for micropatterning, we have investigated the solubility of PVDF in various organic solvents and crystal structures of the PVDF bulk films cast from the solvents. The films prepared from the polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) dominantly showed γ type crystals regardless of preparation temperature, while the films from tetrahydrofuran (THF) exhibit α type crystals and the ones from acetone and methyl ethyl ketone (MEK) show the characteristics of both α- and γ-PVDF. The quality of micropatterns and shapes of the PVDF crystals in the patterns significantly depend on solvent evaporation rates. Micropatterns of PVDF formed in DMF at 120 °C showed the best uniformity in shape. Crystals of the PVDF nucleated at the center regions of microchannels tended to be elongated with the b-axis of γ-PVDF crystals along the channels as the concentration of the solution decreased. In contrast, crystals nucleated at the corner regions of the channels had their b-axis oriented perpendicular to the channels. In line patterns with the width of 2 μm, the corner nucleated crystals were dominant and a resulting bamboo-like crystalline microstructure was observed in which the b-axis of γ-PVDF crystals, fast growth direction, is oriented normal to the microchannels. The crystal structures of the bulk films and the micropatterns were characterized by X-ray diffractometer, Fourier transform infrared spectroscope in Attenuated Total Reflection mode, Polarized Optical and Scanning Electron Microscope.     

DRDF analysis of wide-angle x-ray scattering of natural rubber

A differential radial distribution function (DRDF) of molten natural rubber (NR) was derived from its wide-angle x-ray scattering (WAXS) data. Three peaks with Bragg spacings 4.85, 2.31, and 1.23 Å corresponding, respectively, to interchain, C₁-C₃ and C₁ -C₂ distances are present in the corrected WAXS curve. The derived DRDF, which is shown to be greatly improved than the ones published so far, contains six peaks located at 1.51, 2.48, 3.98, 5.68, 10.65, and 15.25 Å. Maximum contributions of intramolecular atomic distances to the radial distribution peaks were estimated and compared with the experimental results. The comparison shows that the first three peaks are intramolecular in origin and the remaining peaks are predominantly due to intermolecular regularities. A periodicity of an approximately constant interval of 5.26 Å is found between the intermolecular peaks and is attributable to the organizations of more or less parallel chain segments in the material. This finding provided additional evidence for the presence of local lateral ordering to the extent of about 30 Å in molten-state natural rubber.     

Effects of crystal content on the mechanical behaviour of polyethylene under finite strains: Experiments and constitutive modelling

The mechanical stress-strain behaviour of polyethylene (PE) materials under finite strains is studied both experimentally and theoretically. In order to gain insight into the structure and physical properties of investigated PE materials, a series of thermal (DSC and DMTA) and microstructural (small-angle X-ray scattering and AFM) characterizations have been undertaken. The influence of crystallinity on the various features of the tensile stress-strain response is considered over a large strain range, implying thermoplastic-like to elastomer-like mechanical behaviour. A physically-based hyperelastic-viscoplastic approach was adopted to develop a pertinent model for describing the mechanical behaviour of PE materials under finite strains. The semicrystalline polymer is being treated as a heterogeneous medium, and the model is based on a two-phase representation of the microstructure. The effective contribution of the crystalline and amorphous phases to the overall intermolecular resistance to deformation is treated in a composite framework, and coupled to a molecular network resistance to stretching and chain orientation capturing the overall strain hardening response. In order to extract the individual constitutive response of crystalline and amorphous phases, a proper identification scheme based on a deterministic approach was elaborated using the tensile test data of PE materials under different strain rates. Comparisons between the constitutive model and experiments show fair agreement over a wide range of crystallinities (from 15% to 72%) and strain rates. The constitutive model is found to successfully capture the important features of the observed monotonic stress-strain response: the thermoplastic-like behaviour for high crystallinity includes a stiff initial response, a yield-like event followed by a gradual increase of strain hardening at very large strains; for the elastomer-like behaviour observed in the low crystallinity material, the strain hardening response is largely predominant. Strain recovery upon unloading increases with decreasing crystallinity: this is quantitatively well reproduced for high crystallinity materials, whereas predictions significantly deviate from experiments at low crystallinity. Model refinements are finally proposed in order to improve the ability of the constitutive equations to predict the nonlinear unloading response whatever the crystal content.     

A semicrystalline inorganic ion-exchanger for thin-layer chromatographic separation of phenolic compounds

Summary The thin-layer chromatographic behaviour of 28 phenolic compounds was studied on semicrystalline stannic tungstate layers using various solvent systems of which formic acid and n-butanol-formic acid were found to be the most useful for the separation of these compounds. Dimethyl sulphoxide was found to act as a self detector for a number of phenols. An interesting feature of the study of phenols using stannic tungstate is the novel separation of different isomeric compounds. A number of binary, ternary and quarternary separations of bioanalytical and immunophysiological importance have been achieved.     

Properties of fluoroelastomer/semicrystalline perfluoropolymer nano-blends

Properties of blends of semicrystalline perfluoropolymers in fluoroelastomers strongly depend on the size of the dispersed phase and are at the best when dispersed phase dimension is well below 0.1 μm, i.e. in the nano-scale region. This fine dispersion is obtained with an innovative mixing technology based on microemulsion polymerization. Further increase of properties can be obtained by generating chemical links between fluoroelastomer and semicrystalline fluoropolymer. Nano-blends combine the performing properties of fluoroelastomers with those of semicrystalline perfluoropolymers. For example, these nano-blends have at the same time the sealing and mechanical properties of fluoroelastomers and the exceptional thermal and chemical resistance, low permeability and low friction coefficient of semicrystalline perfluoropolymers. In addition, as dispersed phase size is below visible light wavelength, finished items made with these nano-blends are optically transparent even when they contain as much as 40 wt.% of semicrystalline perfluoropolymer.     

Single gas permeation through compositionally different zeolite NaA membranes: Observations on the intercrystalline porosity in an unconventional,semicrystalline layer

The successful application of zeolite A membranes in the industrial market has thus far been restricted to the pervaporative dehydration of solvent streams in the pharmaceutical and engineering industries. Their application in gas separation processes remains elusive, largely due to the existence of uncontrollable, intercrystalline diffusion pathways in the boundary regions of neighbouring crystals.     

Semicrystalline morphology in thin films of poly(3-hexylthiophene)

The thermodynamic phase behavior and the morphology in thin films of poly(3-hexylthiophene) (P3HT) has been studied using calorimetry, X-ray scattering, and scanning force microscopy (AFM). Around 225 °C a phase transition from the crystalline state to a layered, liquid crystalline structure occurs in regioregular P3HT, while the regiorandom counterpart material is disordered at all temperatures and displays a glass transition temperature T_g–3 °C. Regioregular P3HT is semicrystalline and forms needle or plate like crystallites which in solution cast thin films are oriented with respect to the substrate. Films produced by spin coating display a non-equilibrium structure with reduced order and orientation. Annealing of these films in the liquid crystalline state leads to the formation of a morphology similar to the one observed in solution cast films.

Thermal stability of poly(vinylidene fluoride) films pre-annealed at various temperatures

This paper investigates the relationship between the pre-annealing conditions and the thermal stability of uniaxially-drawn poly(vinylidene fluoride) (PVDF) films in order to clarify their technical limits in terms of temperatures that can be used for assembly processes and for practical applications. Specimens that are pre-annealed below their melting temperature apparently shrink in the stretch-direction when they are exposed to elevated temperatures above the pre-annealing temperature. Since the content of β-PVDF in the films decreases simultaneously with the shrinkage, their piezoelectric properties also deteriorate. In addition, there is a suggestion that the level of polarization in the remaining β-phase decreases significantly during annealing above 90-100 °C. However, the dimensions and the piezoelectric coefficients of the films remain stable during annealing below the pre-annealing temperature. Therefore, the thermal stability of PVDF films can be controlled practically by using the appropriate pre-annealing temperature. By contrast, the films were softened at 90-100 °C when the pre-annealing treatment was conducted above the melting temperature. The softening of films that are pre-annealed above the melting temperature is a different phenomenon from that observed in specimens that are pre-annealed below the melting temperature.     

Nanoindentation of high performance semicrystalline polymers: A case study on PEEK

Semicrystalline high performance polymers (HPPs) exhibit desirable thermomechanical properties which make them suitable materials in several industries. However, when their mechanical characteristics are investigated at nanoscale, the inhomogeneous nature of semicrystalline HPPs results in measurement of localized properties which might not represent the overall material response. In this paper, the nanomechanics of poly(ether-ether-ketone) (PEEK) as a tough HPP is scrutinized using the nanoindentation technique. It is demonstrated how surface properties of this polymer can affect the nanoindentation measurements and reveal significant anomalies compared to the nanoindentation of fully crystalline or amorphous solids. In addition, polishing and annealing procedures are introduced as simple techniques that can be used for eliminating the inhomogeneity of the mechanical response of PEEK. Lastly, the capability of depth sensing indentation for determining the distribution of crystalline and amorphous sub-regions within semicrystalline solids is scrutinized and critically discussed.