Stephen W. Bigger

Characterizing the solid-state thermal oxidation of poly(ethylene oxide) powder

Abstract The oxidative degradation of powdered poly(ethylene oxide) (PEO) resin was studied by polarized optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), solution viscometry and Fourier-transform infra-red spectroscopy (FTi.r.). Powdered PEO readily oxidizes under mild ageing conditions (60°C) owing to its large surface area, its strained crystalline lattice and the weak carbon-oxygen bonds in its backbone. As a result, its physical properties deteriorate after an induction period of about 23 days and, in the extreme case, the free-flowing powder is transformed into a soft wax. With increasing oxidation, there is also a pronounced change in the morphology of PEO from a spherulitic to an axialitic structure. This transition is due to oxidatively induced changes in molecular weight and dispersity that affect the crystallization conditions. Examination of the PEO powder by SEM shows that it has an intricate, fibrillar, surface structure, which produces a large surface area available for oxidation. The emergence of multiple d.s.c. melting peaks after oxidation indicates that a number of low-melting, low-molecular-weight fractions are formed as a result of chain scission processes.

Structural morphology and compaction of nascent high-density polyethylene produced by supported catalysts

The morphologies of three nascent high-density polyethylene (HDPE) powders, polymerized in the gas phase by different catalysts, were investigated using scanning electron microscopy (SEM). Silica-supported catalyst systems comprising TiCl4/MgCl2,bis(triphenylsilyl)chromate andbis(cyclopentadienyl)chromium were found to produce polymers with globular, nodular and worm-like microstructures, respectively. The topographies of the fluff particles are related to the compaction behaviour of the HDPE powders. Long, worm-like strands that protrude from the particles are capable of forming more extensive entanglements than the shorter, nodular structures. The entanglements are the main cause of agglomeration of the particles during their long-term bulk storage. Furthermore, the rate of thermal oxidation is influenced markedly by the polymer microstructure. The microstructure determines the surface area available for oxygen attack. High-resolution SEM combined with low-temperature plasma etching reveals that the worm-like structures consist of folded-chain lamellae that are coiled around a core of extended chains.

The effects of pigments on the photostability of polyethylene

The affect of carbon black and various colourizing pigments on the ultraviolet (UV) stability of high and low density polyethylene (HDPE and LDPE) was determined using a novel method for the analysis of oxygen uptake profiles. Samples were exposed to 0.27 Wm−2 (measured at 340 nm) UV irrdiation at 25.0±0.1° C in air at 1.0 atm. The usefulness of this method of assessment of UV stability is demonstrated. The method also enables the rapid collection of data that enable the comparison of the relative photostabilities of experimental and commercial formulations containing pigments and stabilizing additives. The results show that carbon black is an effective UV screening agent for HDPE when added at levels as low as 0.05% (wt/wt) and that increased photoprotection is achieved with increasing concentration of carbon black, up to 5% (wt/wt), above which there is no further significant increase in photostability. LDPE containing ultramarine blue pigment (Na7Al6Si6O24S3) exhibits relatively poor photostability, whereas ferric oxide (Fe2O3) and chrome orange (PbCrO4.PbO) pigments are better photostabilizers for this material. Cadmium sulphide (CdS) was found to photosensitize LDPE. A compound containing 0.10% (wt/wt) carbon black, 0.12% (wt/wt) titanium dioxide (TiO2) and 1.78% (wt/wt) phthalocyanine green (C33H2N8Cl14Cu) is an effective formulation for the stabilization of LDPE. Formulations of LDPE containing ultramarine blue-TiO2 or ferric oxidecarbon black combinations absorb heat on exposure and this may affect their photostability.

Comparative study of the structural, morphological and oxidative characteristics of high-density polyethylene and poly(ethylene oxide)

A comparative study was made of the thermo-oxidative stabilities of high-density polyethylene (HDPE) and poly(ethylene oxide) (PEO) aged in air at 90 and 60°. The PEO was a commercially available grade and two types of HDPE were produced using organo-chromium catalysts supported on a porous silica substrate. Examination of the silica by scanning electron microscopy (SEM) showed it to consist of spherically shaped particles with rough, irregular particulates adhering to their surfaces. Fractured silica particles reveal a system of voids which influences the ultimate mechanical strength of the silica and hence determines the stage at which the silica particles shatter during the polymerization process. The particle size distribution of each silica support was determined by laser light scattering. It was found that the silica support which had the higher effective surface area to weight ratio increased the reactivity and productivity of the catalyst system, thus affecting the morphological characteristics of the nascent polymer particles. The SEM examination of nascent PEO showed ductile, drawn cobweb structures. Since HDPE catalysed using a bis(triphenylsilyl)chromate shows a similar cobweb morphology and is known to have an induction period proceeding steady-state polymerization, it can be inferred that PEO polymerizes after an induction period. The rate of polymer oxidation was assessed by carbonyl index measurements obtained by Fourier transform i.r. spectroscopy. The rate of oxidation correlates with the specific surface area of the polymer, which is determined by the nascent morphology. Polarized optical microscopy showed that isothermally crystallized films of the oxidized polymers display an axialitic morphology. After oxidation, it appears that the calcium oxide residue [ca 0.8% (w/w)] in the commercial grade of PEO can act as an efficient nucleating agent for axialitic growth, when the surfaces of the residue particles are wetted by oligomeric oxidation products.

Efficiency of processing stabilizers using a micro-oxygen uptake technique

Abstract The thermooxidative stability of various low density polyethylene (LDPE) film formulations was investigated using the technique of micro oxygen uptake measurement following multiple extrusions. The results show that the micro oxygen uptake technique is more sensitive than conventional test methods. High molecular weight hindered phenolic stabilizers are more effective in reducing gel formation during polymer film production than are lower molecular weight species such as butylated hydroxytoluene (BHT). The antioxidant 2,2′-ethylidene-bis-(4,6-di-tert-butylphenol) is an effective stabilizer, but it forms a highly coloured complex with transition metal impurities. The hindered phenol/organic phosphite system, comprising a combination of 0.008% (wt/wt) octadecyl-3-(3,5-di-tert-butyl)-4-hydroxy-phenol propionate and 0.032% (wt/wt) tris-(2,4-di-tert-butyl)phenyl phosphite, is effective in suppressing the formation of coloured products but does not provide adequate thermal stability.

Temperature dependence of fluorescence from polymer-bound 2-(2′-hydroxyphenyl)-2H-benzotriazole photostabilizers

Abstract The temperature dependence of fluorescence from films of polymer-bound o-hydroxy-phenylbenzotriazole derivatives was investigated. The films exhibit a temperature-dependent red fluorescence (λmax ≈ 570 nm) ascribed to emission from the proton-transferred tautomer formed following excited state intramolecular proton transfer (ESIPT) and a temperature-insensitive blue emission (λmax ≈ 400 nm) associated with molecules in which the intramolecular hydrogen bonding is disrupted. Analysis of the fluorescence data indicates that the activation energy for the ESIPT process is negligible, whilst the temperature dependence of the red fluorescence is attributed to vibronically assisted internal conversion in the proton-transferred species.

Electronic Energy Transport in Vinyl Aromatic Polymers

Ultrafast electronic energy transport phenomena occurring in macromolecules following absorption of light were investigated using picosecond laser-based time-resolved fluorescence instrumentation and computer-aided data analysis procedures. Analysis of the fluorescence data for copolymers of 2-naphthylmethacrylate (2NMA) and 2-(2’-hydroxy-4’-methacryloxyphenyl)-2H-benzotriazole (BDHM), indicate that the dominant energy transport mechanism is a one-step Forster-type process from both monomer and excimer sites to the BDHM trap sites in the polymer chain. The trapping of excitation energy by the highly photostable BDHM moieties results in enhanced photostability of the copolymers compared to poly(2-naphthyl methacrylate) (P2NMA).

Morphology of Nascent Polyethylene

The morphologies of three nascent high density polyethylene (HDPE) powders polymerized in the gas phase, were investigated using scanning electron microscopy. Different silicasupported catalysts were used for each of the polymers. The powders have globular, nodular and worm-like microstructures. The worm-like structures are comprised of extended chain cores and an overgrowth of folded chain lamellae. This dual morphology, which displays some similarities to a shish-kebab structure, arises from the shear during crystallization.