Oxidation of ultra high molecular weight polyethylene (UHMWPE) part 1: Interpretation of the chemiluminescence curve recorded during thermal oxidation

The chemiluminescence curve of polyethylene differs from that of polypropylene even though the oxidation behaviour is similar. The CL curve of PE normally exhibits a double sigmoidal behaviour, whereas PP shows a single sigmoid, and its luminescence intensity is also much lower. It was found, by investigating the build-up of carbonyls and hydroperoxides by means of FTIR, that the first peak coincides with a maximum in hydroperoxide concentration and the second with the build-up of carbonyls. The intensity of the first peak is enhanced by doping the PE with DPA (9,10-diphenylanthracene), which is a chemiluminescence activator, but unaffected by doping the PE with DBA (9,10-dibromoanthracene), which is an energy acceptor. The intensity of the first peak also depends on the presence of carbonyls in the sample. From these observations it is concluded that the CL from PE is a type of activated chemiluminescence, which originates from hydroperoxide decomposition, with carbonyls acting as activator.     

Oxidation of ultra high molecular weight polyethylene (UHMWPE) part 3: Decomposition of hydroperoxides with SO2 and its effect on the chemiluminescence signal

Samples of slightly oxidised UHMWPE were treated with SO₂ to decompose hydroperoxides prior to chemiluminescence measurements. It was found that the effect was quite substantial, but it was not just due to the elimination of hydroperoxides; a stabilising effect was also manifested. This effect increased with an increasing concentration of hydroperoxides in the samples. The stabilising effect could not be seen in fresh samples or when the hydroperoxides were decomposed by heating the sample in nitrogen. In addition, a stabilising effect was also observed for samples in which the hydroperoxides were reduced by DMS. It was concluded that the stabilisation was due to the formation of H₂SO₄ in the case of SO₂ and DMSO in the case of DMS. Both these substances can act as stabilisers against oxidative degradation of polymers and were evenly distributed in the sample films after the reaction with the hydroperoxides.     

Total luminescence intensity (TLI) offers superior early oxidation detection in unstabilised polyethylene but is no better than FT-IR for stabilised polyolefins

In an earlier study, we have shown that chemiluminescence (CL) and the total luminescence intensity (TLI) method are highly sensitive to oxidation in degradable PE. In this study, stabilised PE and PP were characterised with CL in an inert (TLI) and in an oxygen atmosphere (CL-OIT) and the results were compared to those obtained by the commonly used techniques, FT-IR (carbonyl index (CI)) and thermal analysis (DSC-OIT). PE was aged at a low temperature (80 °C) and PP was aged at temperatures between 60 and 120 °C. Non-Arrhenius behaviour was observed in the oxidation of PP. This showed the importance of aging at a low temperature to obtain realistic results. TLI and CI of stabilised PP and most of the stabilised PE gave comparable results with the same sensitivity for oxidation detection. This was in contrast to our previous results for degradable PE. However, TLI of unstabilised PE showed earlier oxidation detection than CI, which agreed with our earlier results. TLI of PE had a higher sensitivity than CL-OIT, and both TLI and CI of PP were sufficiently sensitive to detect the effect of aging at different temperatures, whereas DSC-OIT was not.     

Methods for determining the spatial distribution of oxidation in ultra-high molecular-weight polyethylene prostheses

Oxidative degradation is a well-known problem for UHMWPE used in prostheses. The aim of the present study has been to find suitable techniques to study the spatial distribution of this oxidation in 8 retrieved acetabular cups. The techniques used were visual examination using an optical microscope and computer scanner, FTIR mapping, imaging chemiluminescence, and staining with SO₂ and HCl. The staining technique is based on a previous study which showed that by treating oxidized UHMWPE with SO₂ followed by heat treatment, the hydroperoxides present in the sample react with the SO₂ and discolor the sample. The intensity of this discoloring is, at low levels of oxidation, proportional to the amount of hydroperoxides and accordingly to the level of the oxidation. The same study also showed that staining a sample with hot HCl resulted in a brown discoloration which was proportional to the amount of carbonyls. It was found that the staining techniques do not give as much information about the chemical and physical changes in the material as FTIR mapping but have a great advantage in better spatial resolution of the oxidation and are also much quicker and easier to use. Imaging chemiluminescence turned out not to be a suitable method to use, compared to the other two, since it gives less information and is more difficult to interpret.When interpreting the results from the different techniques used, it was found that all cups showed the typical oxidation behavior of gamma sterilized UHMWPE. All cups but one showed substantial wear of the articulating surface but very little backside wear. Examination of the oxidation and whitening profile suggests that at least some of the oxidation must have occurred in vivo.     

Determination of spatial distribution of oxidation products in ultra-high molecular weight polyethylene by staining with sulphur dioxide or hydrochloric acid

Treating a sample of oxidised ultra-high molecular weight polyethylene (UHMWPE) with SO₂ will cause a reaction between SO₂ and hydroperoxides in the sample, forming a hydrosulphate group. During subsequent heat treatment the hydrosulphate group will leave as sulphuric acid, and a double bond will form in the polymer chain. During this step the sample turns brown. This browning has previously been used to determine the spatial distribution of oxidation in PP. The aim of this work was to apply this also to UHMWPE and to determine what causes the browning and how well it corresponds to the true distribution and concentration of hydroperoxides. The possible future use of the technique to determine the spatial distribution of oxidation in UHMWPE has also been evaluated.The true nature of the browning has been difficult to establish, but it seems to originate from a charge transfer complex between the double bonds and the sulphuric acid. A similar type of coloration develops in oxidised UHMWPE, when it is treated with hot hydrochloric acid. However, in that case the coloration is proportional to the carbonyl concentration and has been concluded to originate from the oxonium ion formed by protonation of ketones. The coloration of both SO₂/heat-treated samples and samples treated with hot hydrochloric acid can be used to give a qualitative picture of the spatial distribution of oxidation in UHMWPE. The techniques have been applied to study the heterogeneous oxidation in badly consolidated UHMWPE. It was found that the very good spatial resolution gave information about the heterogeneous oxidation that is not possible to obtain when using other techniques, such as FTIR mapping and imaging chemiluminescence.     

The effect of adding virgin material or extra stabilizer on the recyclability of polypropylene as studied by multi-cell imaging chemiluminescence and microcalorimetry

The effect of adding virgin material or new stabilizers on the recyclability of polypropylene (PP) was determined. Stabilized PP was subjected to oven ageing at 130 °C for 100, 250 or 500 h before and after upgrading with virgin material (0, 20, 50, 80 or 100%) or additional stabilizer during reprocessing. The effect of upgrading recycled PP with extra stabilizer or virgin material was determined by measuring the elongation at break, yellowness change, oxidation induction time (OIT) or total luminous intensity (TLI) by chemiluminescence (CL) techniques. Selected specimens were analysed by microcalorimetry (MC).It has been shown that upgrading recycled PP with virgin material was not effective. Adding 80% virgin material did not increase the lifetime more than adding 20% virgin material. This indicates that a small amount of recycled material can induce degradation by contamination. Adding extra stabilizers instead of virgin material was much more effective. Of all techniques used the OIT as determined with CL showed a clear difference in residual stability of differently treated materials and seems to be the most valuable analytical method to determine the recyclability of PP. A comparison between CL and MC shows that MC is sensitive enough for measuring the solid-state oxidation of stabilized PP. Slightly lower OITs were detected by MC probably associated with the static atmosphere used during the measurements. The TLI method is not suitable to determine the degradation state of to be recycled material.     

替代模板印迹-高效液相色谱测定空气PM2.5中的羰基化合物

以替代模板法制备了分子印迹固相萃取柱（MISPE），该柱能够有效去除空气PM2.5样品分析过程中过量的衍生剂2，4-二硝基苯肼（2，4-DNPH），并能够大幅度浓缩样品溶液，提高羰基化合物的富集效率和监测灵敏度，缩短采样时间。与高效液相色谱联用，实现了对空气PM2.5中的羰基化合物的快速检测。本研究解决了传统固相萃取材料选择性低，不能去除2，4-DNPH干扰的瓶颈问题。本方法用于研究广州市大学城早春季节雾霾天和非霾天羰基化合物水平及其来源，结果发现无论是灰霾天气还是正常天气，14种目标羰基化合物均被检出。研究发现，在污染物排放和天气因素的影响下，灰霾天气羰基化合物总量增高，尤其是异戊醛增长迅速，并且与丙醛浓度变化呈现高度正相关。通过相关性分析，证明除了光化学反应等自然因素，人类活动对广州羰基化合物水平的干扰严重，应引起重视。     

聚对苯二甲酰对苯二胺纤维的热老化行为与机理研究

在250～400℃空气中对自由状态下对位芳纶进行等温热老化处理,采用万能材料试验机、红外光谱法、广角X射线衍射法、声速法和特性黏数法表征了老化过程中力学性能和结构的变化.结果表明,在老化初期,由于分子链的解取向,强度随时间快速下降;随后的热分解使强度随老化时间继续降低,符合二级反应动力学模型,其热老化表观活化能为32.4kJ/mol.老化样品的强度随温度升高显著下降,但高于350℃时热交联反应变得明显,同时结晶度增大和结晶结构完善,使强度的损失速率减小.老化样品的模量随老化温度的升高而增大,低于350℃时,非晶态分子链的解取向占优,模量较未热老化样品低;升高至350℃时,结晶结构完善占优,表现在第二类晶格畸变参数降低、表观微晶尺寸增大,特别是微晶横向融合使a,b轴方向尺寸显著增大,模量明显高于未老化样品.     

Profile of oxidation in irradiated polyethylene

Following gamma irradiation in air which causes bond scission and yields large concentrations of peroxy radicals, maximum oxidation and an increase in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals generate carbonyls, mostly ketones. On the polymer surface, peroxy radicals continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer abstract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of oxygen is available to react with radiolytic radicals, so that more efficient crosslinking and a low level of hydroperoxide chain reaction occur. After long periods of time a surface maximum in carbonyl concentration is produced. Heating polyethylene in high pressures of oxygen accelerates the oxidative process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 329–339, 1998     

Determination of chromium(III) and total chromium using dual channels on glass chip with chemiluminescence detection

A simple, rapid and sensitive method for the determination of chromium(III) and total chromium using the simple dual T channels on glass chip with negative pressure pumping system and chemiluminescence (CL) detection is presented. The CL reaction was based on luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by chromium(III). Total chromium in form of chromium(III) was achieved after chromium(VI) was completely reduced by acidic sodium hydrogen sulfite. Total chromium could then be determined with the same strategy as the chromium(III). The CL reagent was composed of 1.0 × 10⁻⁴ mol/L luminol, 1.0 × 10⁻² mol/L hydrogen peroxide and 0.10 mol/L sodium bromide in 0.050 mol/L carbonate buffer (pH 11.00). The 1.0 × 10⁻² mol/L ethylenediaminetetraacetic acid was added into the sample solution in order to improve the selectivity. Chromium(III) could be detected at a notably concentration of 1.6 × 10⁻¹⁶ mol/L and a linear calibration curve was obtained from 1.0 × 10⁻¹⁵ to 1.0 × 10⁻¹³ mol/L. The sample and CL reagent consumption were only 15 and 20 μL, respectively. The analysis time was less than 1 min per sample with the precision (%R.S.D.) was 4.7%. The proposed method has been applied successfully to the analysis of river water, mineral waters, drinking waters and tap water. Its performance was verified by the analysis of certified total chromium-reference materials and by recovery measurement on spiked synthetic seawater sample.     

Direct chemiluminescence determination of cysteine in human serum using quinine-Ce(IV) system

A simple, sensitive and selective chemiluminescence (CL) method was developed for the determination of cysteine. This method is based on that the weak CL of cysteine oxidized with cerium (IV) can be greatly enhanced by quinine. The calibration curve was linear over the range 3.5×10⁻⁹-3.5×10⁻⁶ M with a detection limit of 2.5×10⁻⁹ M (S/N=3). The RSD was found to be 8.4% by 10 replicate determinations of 2.9×10⁻⁸ M cysteine. Due to high sensitivity, the proposed method can be used directly to determine the total concentration of cysteine in human serum through simply diluting the sample for a thousand fold. The obtained result was in agreement with that given by amino acid autoanalyzer. The present method does not require any separation, showing a simpler analytical characteristic. The mechanism of the CL reaction was also discussed.     

Chemiluminescence from poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) block copolymers

A detailed analysis of the chemiluminescence emission (CL) from poly(styrene-b-ethylene-co-butylene-b-styrene), SEBS, was carried out. A phenol-phosphite stabilization system based on Irgafos 168 and Irganox 1330, was studied. The kinetic analysis of the CL profile under nitrogen shows a first-order reaction for the decay of chemiluminescence. The activation energy shows different values as a function of temperature, showing that different reactions are involved in the thermal degradation of the SEBS. The CL decay rate correlates well with the amount of the phosphite, Irgafos 168, and confirms the activity of this stabilizer as radical chain-breaking antioxidant in these copolymers.The isothermal analysis of CL under oxygen allows evaluation of the oxidation state, as well as the efficiency of the antioxidants. Good correlations are found between the CL parameters and concentration of Irgafos 168. Several factors suggest that oxidation begins in the interfacial region. Spectral analysis of the chemiluminescence shows the presence of different types of hydroperoxides.Finally, the characterization of the SEBS copolymers by differential scanning calorimetry reveals an order-disorder transition, assigned to aggregates that behave as paracrystalline regions.     

Photodegradation of polyethylenes: Comparative effect of Fe and Ca-stearates as pro-oxidant additives

The effect of iron and calcium stearates on the degradation of polyethylene (LDPE and LLDPE), under natural and artificial exposure, has been studied. The activity of stearates has been evaluated by chemiluminescence and FTIR of polyethylenes. The analysis of the molecular weight changes and content of degradation products identified by GC-MS during ageing process confirmed their pro-degrading activity. Films containing stearates exhibited lower CL emission, and revealed the higher efficiency of Fe-Stearate compared to Ca-Stearate in decomposing hydroperoxides, leading to higher degradation during processing. The results were confirmed by TGA analysis, where the weight loss onset and T_max shifted to lower temperatures in polyethylenes with incorporation of Fe- and Ca-stearates compared to pure polymers.Polyethylenes were outdoor and accelerated exposed, and CL measured at different period of times. Chemiluminescence temperature-ramping tests under nitrogen showed the formation of a peroxide peak at lower temperature, and a significant increase in carbonyl index for PE containing stearates was found by FTIR. The results were supported by GC-MS, where the concentration of extracted products identified in the polyethylenes containing Fe-stearate was significant, and a much greater decrease in molecular weight was determined by GPC, which confirmed the development of degradation for polyethylenes with Fe-Stearates in comparison to pure or Ca-stearate polyethylenes.     

High throughput method for the analysis of cetrizine hydrochloride in pharmaceutical formulations and in biological fluids using a tris(2,2'-bipyridyl)ruthenium(II)-peroxydisulphate chemiluminescence system in a two-chip device

A fast, economic and sensitive chemiluminescence (CL) method has been developed for the analysis of cetrizine hydrochloride (CET) in pharmaceutical formulations and in biological fluids. The CL method is based on the oxidation of tris(2,2′-bipyridyl)ruthenium(II) (Ru (bipy)₃²⁺) by peroxydisulphate in a two-chip device. Up to 180 samples can be analysed per hour, consuming only minute quantities of reagents. Three instrumental setups were tested to find the most economical, sensitive and high throughput setup. In the first setup, a continuous flow of sample and CL reagents was used, whereas in the second setup, a fixed volume (2 μL) of (Ru (bipy)₃²⁺) was introduced into a continuous infusion of peroxydisulphate and the sample. In the third design, a fixed volume of sample (2 μL) was injected while the CL reagents were continuously infused. Compared to the first setup, a 200% signal enhancement was observed in the third setup. Various parameters that influence the CL signal intensity, including pH, flow rates and reagent concentrations, were optimized. A linear response was observed over the range of 50 μg L⁻¹ to 6400 μg L⁻¹ (R² = 0.9959) with RSD values of 1.1% (n = 15) for 1000 μg L⁻¹. The detection limit was found to be 15 μg L⁻¹ (S/N = 3). The amount of consumed sample was only 2 μL, from which the detected amount of CET was found to be 6.5 × 10⁻¹⁴ mol. This procedure was successfully applied to the analysis of CET in pharmaceutical formulations and biological fluids.     

Viscosity reduction and disentanglement in ultrahigh molecular weight polyethylene melt: Effect of blending with polypropylene and poly(ethylene glycol)

A significant reduction in melt viscosity of ultrahigh molecular weight polyethylene (UHMWPE) was obtained by blending with polypropylene (PP) and poly(ethylene glycol) (PEG). The mechanism of viscosity reduction was investigated from the view of disentanglement effect. Dynamic mechanical analysis indicated that the pseudoequilibrium modulus (E′) of UHMWPE/PP(80/20) blend in the rubbery plateau was much lower than that of UHMWPE. Accordingly, the calculated entanglement density (ν_e) of UHMWPE/PP (80/20) blend was smaller than that of UHMWPE. Further reduction in E′ and ν_e of the blend was obtained by the incorporation of 1 phr PEG. Slow DSC analysis showed that the high temperature endotherm and exotherm for UHMWPE at slow temperature ramp diminished and increased, respectively when 5 phr PEG was added. It also revealed that the entanglement level of UHMWPE decreased with the addition of a small amount of PEG.     

Surface studies of ultra‐high molecular weight polyethylene irradiated with high‐energy pulsed electron beams in air

Ultra‐high molecular weight polyethylene (UHMWPE) was irradiated in air with high‐energy (9 MeV), pulsed electron beams to doses ranging from 2.5 to 100 Mrad and subsequently heat treated at 120°C for a time period of 120 min. Surface characterization of the target side of irradiated UHMWPE samples was carried out both before and after the heat treatment by means of attenuated total reflection Fourier‐transform infrared (FTIR/ATR) spectroscopy and microhardness measurement. The obtained results provided further evidence supporting our earlier observation (Tretinnikov, O. N.; Ogata, S.; Ikada, Y. Polymer 1998, 39, 6115) that thermal decomposition of hydroperoxides formed upon irradiation of UHMWPE with high‐energy, pulsed electron beams in air leads to surface crosslinking, and the subsequent surface hardening of the irradiated polymer. Importantly, we found that this phenomenon has the highest contribution to the surface hardness enhancement of the polymer when the radiation dose is in the range of 10–30 Mrad. In addition, we found that this irradiation and subsequent heat treatment of UHMWPE in air does not lead to formation of carbonyl‐containing products unless the radiation dose exceeds 20 Mrad. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1503–1512, 1999     

Post electron-beam irradiation oxidation of orthopaedic UHMWPE

The mechanisms and products of the oxidation process induced by electron-beam irradiation of UHMWPE were thoroughly investigated in the present study. The e-beam technique has the peculiarity of dispensing the whole radiation dose in a very short time and it allows following the post-irradiation oxidation process at room temperature, which represents a major advantage for the present purpose.Thin sections of orthopaedic grade UHMWPE GUR 1050 were irradiated at doses ranging from 5 to 100 kGy in vacuum, air or pure oxygen, depending on the aim of the following analyses. After irradiation, the samples were stored in liquid nitrogen until the measurements began. Chemical and physical changes caused to the polymer by irradiation were investigated by FTIR, derivatisation techniques and spectral subtractions.As reported in the literature, secondary hydroperoxides and ketones were found to be the main oxidation products, along with carboxylic acids and alcohols. Nevertheless, in our case, the kinetics of formation of oxidation products were inconsistent with previously reported mechanisms. An alternative oxidation mechanism, including the simultaneous formation of hydroperoxides and ketones, is proposed. The thermal stability of hydroperoxides is also discussed in detail and the similarities between the post-irradiation oxidation process and the thermal oxidation process are highlighted.     

Use of 9,10-diphenylanthracene as a contrast agent in chemiluminescence imaging: The observation of spreading of oxidative degradation in thin polypropylene films

Thin films of polypropylene were doped with a chemiluminescence (CL) activator, 9,10-diphenylanthracene (DPA), and were thermally oxidised in a CL imaging apparatus to determine whether heterogeneous oxidation processes such as spreading of oxidation could be observed. The presence of DPA resulted in significantly more intense CL images compared with undoped polymer, due to the efficient chemically induced electron exchange luminescence reaction between DPA and hydroperoxides. Hence, the CL images from DPA-doped PP were used to locate the position of hydroperoxides in the oxidising polymer. For thermal oxidation at 150 and 140 °C hydroperoxides were observed to form in localised regions of the films, whilst other areas remained hydroperoxide free. As the oxidation time increased the concentration of hydroperoxides in these areas increased and they were observed to spread to the remainder of the polymer. Time-resolved line maps from the images indicated that zones with high concentration of hydroperoxides travel through the polymer during oxidation. Integrals of CL images from the thermal oxidation of DPA-doped polymers indicated that a significant degree of oxidation had occurred by the end of the “induction period” for a conventional CL-intensity oxidation-time profile. This is a likely reason why spreading of oxidation has not previously been observed for undoped PP films.     

Enhancement of on chip chemiluminescence signal intensity of tris(1,10-phenanthroline)-ruthenium(II) peroxydisulphate system for analysis of chlorpheniramine maleate in pharmaceutical formulations

The effect of detection chip geometry on chemiluminescence (CL) signal intensity of tris(1,10-phenanthroline)-ruthenium(II) peroxydisulphate system for analysis of chlorpheniramine maleate (CPM) in pharmaceutical formulations was investigated. It was observed that the design of the detection chip is very crucial and can play an important role in enhancing the CL signal intensity in this system. The CL signal intensity was enhanced 250% when a teardrop micromixer chip was used, compared to the commonly used serpentine chip geometry. The study was conducted using a multi-chip device. In this device, chip 1 was used to prepare and pump the reagent mixture, whereas chip 3 was used for pumping the sample. The two chips were connected to the teardrop chip (2) via silica capillary where detection took place. Non-linear regression curve fitting of the calibration data revealed that the calibration curves are best described by third order polynomial equation with excellent correlation coefficients (R² = 0.9998) for the concentration range 7.69 × 10⁻⁸ to 5.12 × 10⁻⁵ mol L⁻¹. A linear response is also observed over the range 7.69 × 10⁻⁸ to 1.28 × 10⁻⁵ mol L⁻¹ (R² = 0.9996) and the detection limit was found to be 5.49 × 10⁻⁸ mol L⁻¹. The device was successfully used for the analysis of CPM in tablets and a multi-component cough syrup. Results were reproducible with relative standard deviation (RSD) of 0.6-1.1%.     

Electrical and self‐heating properties of UHMWPE‐EMMA‐NiCF composite films

Conductive polymer composites (CPC) containing nickel‐coated carbon fiber (NiCF) as filler were prepared using ultra‐high molecular weight polyethylene (UHMWPE) or its mixture with ethylene‐methyl methacrylate (EMMA) as matrix by gelation/crystallization from dilute solution. The electrical conductivity, its temperature dependence, and self‐heating properties of the CPC films were investigated as a function of NiCF content and composition of matrix in details. This article reported the first successful result for getting a good positive temperature coefficient (PTC) effect with 9–10 orders of magnitude of PTC intensity for UHMWPE filled with NiCF fillers where the pure UHMWPE was used as matrix. At the same time, it was found that the drastic increase of resistivity occurred in temperature range of 120–200 °C, especially in the range of 180–200 °C, for the specimens with matrix ratio of UHMWPE and EMMA (UHMWPE/EMMA) of 1/0 and 1/1 (NiCF = 10 vol %). The SEM observation revealed to the difference between the surfaces of NiCF heated at 180 and 200 °C. Researches on the self‐heating properties of the composites indicated a very high heat transfer for this kind of CPCs. For the 1/1 composite film with 10 vol % NiCF, surface temperature (T_s) reached 125 °C within 40 s under direct electric field where the supplied voltage was only 2 V corresponding to the supplied power as 0.9 W. When the supplied voltage was enough high to make T_s beyond the melting point of UHMWPE component, the T_s and its stability of CPC films were greatly influenced by the PTC effect. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1253–1266, 2009