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Crosslinked polydimethylsiloxanes were exposed to corona discharges in dry air at normal pressure. Short-time solvent extraction and subsequent analysis of the extractables, by gas chromatography, mass spectrometry and size exclusion chromatography showed that, oligomers consisting mainly of cyclics with 4–9 repeating units were formed during corona exposure. The size distribution of the oligomers was independent of the crosslink density and corona exposure time. The amount of oligomers located at the surface increased, with increasing storage time, after the corona exposure in qualitative agreement with the ongoing hydrophobic recovery process. Longer extractions penetrated deeper into the samples, and, in addition to the cyclic oligomers, higher molar mass species (∼50,000gmol−1for unexposed samples) were detected. Samples exposed to corona, treated in this way, showed a broadening of the high molar mass peak towards lower molar masses.

Polydimethylsiloxanes with different crosslink densities were exposed to corona discharges or GHz air plasma and studied by contact angle measurements, X-ray photoelectron spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. The degree of surface oxidation increased with increasing exposure time with a limiting depth of 100–150nm. Surface oxidation was faster in more highly crosslinked polymers. Within the oxidised layer, a brittle, microporous silica-like layer with a minimum organic silicone content of 40% gradually developed with increasing exposure time. The strain at which the silica-like layer cracked decreased with increasing dose of corona or air plasma. The hydrophobic recovery following the corona/air plasma exposure occurred at a slow rate by diffusion of oligomers through the microporous but uncracked silica-like layer or at a much higher rate by transport of oligomers through cracks in the silica-like layer.

Blends of polypropylene and two polyketone grades with low and medium-high viscosities were prepared by melt extrusion. To obtain good compatibility, a maleic-anhydride–polypropylene copolymer was added to the blends. Polyoxypropylenediamine was added to some of the blends to further enhance compatibility. The blends were analysed with differential scanning calorimetry. In a second step, the blends were compression or injection moulded. Scanning electron microscopy, shear viscosity, density measurements and infrared spectroscopy were used to characterise the moulded blends and their oxygen permeabilities were assessed. Impact strength and hardness were measured on injection-moulded blends. It was shown that the oxygen barrier properties of polypropylene could be greatly enhanced by a small addition of primarily the low-viscosity polyketone. A content of 23.9% by volume of polyketone was sufficient to lower the permeability by 70% compared to pure polypropylene. This was because a polyketone-rich surface layer was formed during compression moulding. The incorporation of polyoxypropylenediamine had a profound effect on the morphology. The polyketone particles in this case were very small, and the absence of “pull-outs” suggested an enhanced phase adhesion between the different components. Further, the incorporation of polyoxypropylenediamine had no impact on the oxygen permeability but the impact toughness and hardness were increased and the shear viscosity was also increased in its presence. This indicated that chemical bonds were formed between polyketone, polyoxypropylenediamine and the maleic-anhydride–polypropylene copolymer. This network suppressed crystallisation of primarily the polyketone component.

AbstractA dual-layer urethane acrylate UV-cured coating is widely used to protect optical fibers because of its well-balanced mechanical properties, weathering resistance and rapid curing. The long-term mechanical behavior of fiber coatings is important for the reliability of optical fibers. Long-term exposure of UV-cured polyether urethane acrylate films was carried out in dry air and in water at elevated temperatures. Tensile testing was performed to reveal changes in mechanical properties and dynamic mechanical analysis to determine both the glass transition temperature and the crosslink density. The equilibrium swelling allowed assessment of the crosslink density. Tensile testing and strip force measurements were performed on virgin and aged optical fibers. Initially the fracture strengths of the secondary coatings increased under all aging conditions indicating post-curing reactions and the possible loss of uncrosslinked species. Aging under wet conditions led at a later stage to hydrolytic degradation of the network and to a decrease in the fracture stress. The equilibrium swelling and equilibrium modulus measurements showed good correlation with the changes in strength. The primary coatings showed a decrease in mechanical strength after only 2–4 weeks under all conditions.

Moisture transport properties were assessed by sorption and desorption measurements on hydroxyl-functional hyperbranched polyesters based on 2,2-bis(methylol) propionic acid (bis-MPA) as AB2-monomer with ethoxylated pentaerythritol. A series of these polymers with different molar masses were studied. For the first time, it is reported that sigmoidal sorption curves were successfully modelled using a time-dependent surface boundary concentration, where the relaxation time was obtained from the mechanical stress relaxation data. The zero concentration diffusivities were very small and comparable with the values of fully amorphous poly(vinyl alcohol). Both the diffusivity data and the stress relaxation data indicated that these materials were plasticized by moisture. Ethylene glycol transport properties were measured on a hexadecanoate(C16)-terminated bis-MPA hyperbranched polymer. The replacement of the hydroxyl groups with C16 paraffin tails resulted in an increase in solute diffusivity despite the fact that these polymers became semicrystalline. The shape of the desorption curves indicated that continuous micropores were present and facilitated mass transport. The moisture transport properties were also measured on a series of semicrystalline dendrimer-like star poly(ε-caprolactone(s)). The low measured densities of these highly crystalline dendrimers confirmed that the porosity, as indicated by the shape of the desorption curves, had to be discontinuous. The porosity in the (C16)-terminated and the poly(ε-caprolactone(s)) was impossible to eliminate by applying a high external pressure during crystallization. A collective view of all the results obtained confirm that the transport properties are primarily controlled by the hydroxyl group concentration.

Anisotropic networks were prepared by photoinitiated cationic bulk polymerization of aligned liquid crystalline mixtures of mono- and bifunctional vinyl ethers. The thermal reversibility of the macroscopic order of the networks was assessed by infra-red spectroscopy measuring the dichroic ratio of the oriented samples at different temperatures between 20°C and 200°C. The order parameter gradually decreased with increasing temperature and the relative reduction decreased with increasing crosslink density. For systems showing smectic mesophase transitions, the decrease in order parameter was concentrated in the transition temperature ranges. The almost complete recovery of alignment after cooling to 20°C was due to the network character of these polymers. A permanent loss in order parameter of about 15% after the first heating and cooling cycle was recorded. This was attributed to a thermally induced polymerization occurring during the first heating scan.

Infrared rheometry showed that the mesogenic groups of two smectic side-chain liquid-crystalline polymers were aligned by shear until a steady-state value was achieved after approximately 10 s. The shear experiments were carried out at temperatures 10–50°C below the isotropisation temperature. The alignment of the mesogenic groups was always perpendicular to the shear flow direction. The degree of orientation increased with increasing shear rate and the highest reported value for the Hermans orientation function was −0.35 (−0.5 corresponds to perfect perpendicular orientation; it was assumed that the mesogen-group-orientation was unixaial). The turbidity of the sheared smectic samples indicates the presence of small domains in spite of their high level of orientation. A gradual loss of mesogen group orientation occurred after cessation of shear, and full recovery of the original isotropic state was in no case achieved within 60 000 s.

A high-temperature-vulcanized polydimethylsiloxane (PDMS) elastomer has been subjected to corona discharges for different periods of time in dry air. The loss and recovery of hydrophobicity of the surface have been characterized by contact angle measurements. Immediately after exposure to corona discharges, samples showed a low surface hydrophobicity and, on storage in dry air, a continuous increase in hydrophobicity finally approaching the hydrophobicity of the unexposed material. The activation energy of the hydrophobicity recovery was two to four times greater than the activation energy of the diffusivity of low molar mass PDMS in PDMS elastomers, indicating that the diffusivity properties of the oxidized surface layer were different from that of the bulk. PDMS elastomers quenched in liquid nitrogen or subjected to small mechanical deformation ( < 1% strain) after exposure to corona discharges for 1 h or more recovered their hydrophobicity faster than untouched specimens kept under identical conditions. X-ray photoelectron spectroscopy confirmed the early formation of a silica-like surface layer, with a thickness of at least 10–12 nm. The atomic composition of the oxidized surface layer remained essentially unchanged after the first hour of corona discharges. It is suggested that the silica-like surface layer delayed the recovery of hydrophobicity by inhibiting the transport of low molar mass PDMS to the surface. It is also suggested that thermally or purely mechanically induced stresses lead to a cracking of the brittle silica-rich layer and that this in turn facilitates the transport of low molar mass PDMS to the surface and to a more rapid recovery of the hydrophobicity. Data obtained by reflection infrared spectroscopy assessing the outermost micrometer, confirmed the oxidation and the formation of hydroxyl groups at a progressively higher concentration with increasing exposure time of corona discharges.

Dielectric permittivity and loss have been measured over the frequency range 0.1–1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of smectic monomers. The polyacrylates were based on mixtures of chiral monofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and nonchiral bifunctional monomers, either with an ester group in the mesogen or with an ester group in the mesogen and a lateral nitro group, creating networks with different crosslink densities. Poled copolymers based on these monomers exhibit second-order nonlinear optical properties. Polymers based only on monomers with the chiral and nitro groups exhibited a pronounced γ process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temperature subglass process (β) associated with reorientation of the mesogen. The polymers with the bifunctional monomer without the nitro group exhibited clearly both γ and β processes. The dielectric data indicate that the ester groups in the bifunctional units undergo conventional reorientation according to the β mechanism and that the β process is suppressed in the pendant chains with the lateral nitro groups. The β process involves a coordinated torsion about two bonds in the mesogen and the swept-out volume involved in the motion becomes extensive when a lateral nitro group is present. © 1997 Elsevier Science Ltd.

Samples of approximately uniform degree of orientation based on poly(hydroxybenzoic acid (73 mol%)-co-hydroxynaphthoic acid (27 mol%)), known as Vectra A950, were obtained by sectioning injection-moulded specimens. The degree of chain orientation was assessed by X-ray scattering and infrared spectroscopy. A comparison of the data obtained by the two methods enabled the angles between the chain axis and the transition moment vectors for the infrared absorption bands at 1474 cm−1 (22°) and 890 cm−1 (90°) to be assessed. The oriented samples showed, depending on the particular thermomechanical history used, either an increase or a decrease in chain orientation on annealing at temperatures below and above the crystal melting range. Constraining factors, the application of an external force to keep the samples at constant length or the presence of stabilizing crystallites, favoured an increase in chain orientation presumably through the annihilation of disclinations. Slow and gradual heating favoured recrystallization, i.e. the transformation of pseudohexagonal crystallites to the thermally more stable orthorhombic crystallites and possibly also to crystal modification II of poly(hydroxybenzoic acid). These form new physical cross-links which prevent shrinkage of the sample. Unconstrained oriented nematics containing no stabilizing crystallites showed a rapid approach (decrease for the highly oriented samples) to a universal value for the Hermans orientation function of ∼0.3 at temperatures above 290°C. This behaviour, distinctly different from that of small-molecule nematics, shows some similarity with the behaviour of oriented flexible-chain polymers.