Laboratory accelerated and natural weathering of styrene-ethylene-butylene-styrene (SEBS) block copolymer

Indoor accelerated and outdoor weathering of polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) was studied by infrared spectroscopy. Accelerated conditions involved simultaneous exposure of specimens to ultraviolet-visible radiation between 295 nm and 450 nm and each of four temperature/relative humidity (RH) environments, i.e., (a) 30 °C ± 1 °C at <1% RH, (b) 30 °C ± 1 °C at 80% RH, (c) 55 °C ± 1 °C at <1% RH, and (d) 55 °C ± 1 °C at 80% RH. Outdoor exposure was conducted in Gaithersburg, MD, in two different time periods. Similar photooxidative mechanisms were operative under all conditions. In the case of indoor accelerated exposure, the rate of photooxidation was found to depend strongly on temperature. Unlike the exposure at 55 °C, moisture-assisted photooxidation was insignificant at 30 °C. A quantitative study on the synergistic effect of environmental stressors revealed that the degrading effect of combined temperature and moisture on photooxidation was greater than the sum of the two effects exerted independently. Outdoor weathered specimens exhibited significantly slower photooxidation. Acceleration of photooxidation ranged from 2.5 to 10 times in comparison to the outdoor exposure, depending on the indoor accelerated conditions.     

Evaluating aging of coatings and sealants: Mechanisms

Photodegradation mechanisms of a model coating/sealant system based upon styrene-butadiene-styrene triblock copolymer have been investigated. The model system was exposed to ultraviolet-visible radiation using an integrating sphere-based exposure chamber at 30 °C and <1% relative humidity. Chemical degradation was monitored by transmission Fourier transform infrared (FTIR) spectroscopy and changes in mechanical properties via dynamic mechanical thermal analysis (DMTA). It was found that cross-linking was the dominant degradation process, causing substantial increases in storage modulus and glass transition temperature of the butadiene units with increasing exposure. The styrene units were relatively stable as indicated by the lack of significant changes in the FTIR bands or in the glass transition temperature associated with the styrene units. The present study clearly has shown that chemorheological investigations provide useful tools to characterize the mechanisms of environmental attack of polymers. The ability to investigate both chemical and mechanical-rheological properties provides the opportunity to make modifications to the material and, hence, improve its chemical and mechanical-rheological properties.     

Nylon 6.6 accelerating aging studies: II. Long-term thermal-oxidative and hydrolysis results

Long-term (greater than 5 year exposures), low-temperature (as low as 37 °C) accelerated oven aging results were obtained for Nylon 6.6 fibers under thermo-oxidative conditions (air aging with an oxygen partial pressure of 13.2 cmHg in Albuquerque). To assess the importance of humidity on aging, experiments were also conducted under a combination of 100% RH plus 13.2 cmHg of oxygen partial pressure at temperatures ranging from 138 °C to 64 °C plus an additional experiment at 70% RH and 80 °C. The low-temperature tensile strength results showed that the Arrhenius activation energy under the pure oxidative degradation conditions dropped from ∼96 kJ/mol above ∼100 °C-∼30 kJ/mol below this temperature, indicative of a transition in the oxidative chemistry at low temperatures. Earlier work by our group on the same material concluded that hydrolytic degradation effects dominated oxidation effects at higher aging temperatures. However, the current long-term, low-temperature comparisons lead to the conclusion that humidity is not an important aging factor below ∼50 °C. By extrapolating time-temperature superposed oxidative degradation data using the low-temperature activation energy, we obtain predictions at 21 °C. At this temperature, we estimate that a tensile strength loss of 50% takes on the order of 70 years. The 21 °C predictions are shown to be reasonably consistent with long-term (up to 38 year) ambient results on similar Nylon materials removed from field-aged parachutes. Although the estimated average exposure temperature varies from parachute to parachute, the highest average temperature is estimated to be on the order of 21 °C.     

Effect of moisture on copolymer fibers based on 5-amino-2-(p-aminophenyl)-benzimidazole

In recent years, there has been concern in the soft body armor community that copolymer fibers based on 5-amino-2-(p-aminophenyl)-benzimidazole can release hydrochloric acid, which is present in these fibers as a by-product of the manufacturing process. The presence of acids could potentially be detrimental to other fibers that might come in contact with these materials. In an effort to examine this issue, a study was designed to investigate the release of acid in different environments from these fibers. During the first phase of the study, fibers were exposed to water and pH decreases were observed. While immersed in deionized water, two of the fiber samples studied released a sufficient amount of acid to drop the pH of the solution from approximately pH 6.0 to approximately pH 3.0 in less than 10 d at room temperature. Further ion-selective electrode studies of chloride ion released from these fibers indicated that hydrochloric acid may not be the species responsible for this pH reduction. In a second phase of the investigation, fibers were exposed to water vapor in an elevated temperature environment (conditions were 65 °C, 80% RH). While the pH reduction released by the water vapor exposure was substantially less than observed in the submersion phase, a reduction in the yarn tensile strength of some of the fibers was observed during this phase of the study. In a third phase, fibers were exposed in a dry oven (less than 5% RH) at 65 °C. Almost no pH reduction or strength reduction was observed. Molecular spectroscopy was also performed to better understand the effect of elevated temperature and moisture environments on these fibers.     

A capillary water retention effect to improve medium-temperature fuel cell performance

We demonstrate that small and narrow hydrophilic conducting domain morphology in sulfonated aromatic membranes leads to much better fuel cell performance at medium temperature and low humidity conditions than those with larger hydrophilic domains. A comparison of three types of sulfonated poly(arylene ether sulfone)s (SPAES) with random, block, and graft architecture indicates that small hydrophilic domain sizes (< 5 nm) appear to be important in supporting water retention under low relative humidity (RH) conditions intended for medium temperature (> 100 °C) fuel cell applications. The graft copolymer outperformed both a random copolymer and multiblock copolymer at 120 °C and 35% RH fuel cell operating conditions due to capillary condensation of water within the 3–5 nm hydrophilic domains.     

Accelerated ageing to study the degradation of cellulose nitrate museum artefacts

Cellulose nitrate is susceptible to hydrolysis as well as loss of plasticiser when left in a humid atmosphere. A comparison of the ageing behaviour of cellulose nitrate samples prepared from cotton linters was used to simulate the artefacts studied in a previous study. Certain artefacts were also subjected to accelerated ageing at 12%, 55% and 75% relative humidity at 70 °C. The rate of degradation was observed to vary with the RH, indicating the connection between the absorption of moisture and the hydrolysis process. The effect of varying the sulphate level on the rate of hydrolysis was studied using concentrations similar to those detected in artefacts. The study was carried out using <0.1 mg g⁻¹, 2 mg g⁻¹ and 5 mg g⁻¹ of sulphate and distinct differences were observed which is consistent with the conclusions drawn for a study of a number of artefacts. The observed rates of degradation are consistent with previous studies on cellulose nitrate. The degradation was studied using a combination of infrared spectroscopy, ion chromatography and gel permeation chromatography. The analysis was complemented by a study of the weight changes which occur during ageing.     

Effect of humidity and solvent vapor phase on cellulose esters films

In the present study the effect of relative humidity (RH) during spin-coating process on the structural characteristics of cellulose acetate (CA), cellulose acetate phthalate (C-A-P), cellulose acetate butyrate (CAB) and carboxymethyl cellulose acetate butyrate (CMCAB) films was investigated by means of atomic force microscopy (AFM), ellipsometry and contact angle measurements. All polymer solutions were prepared in tetrahydrofuran (THF), which is a good solvent for all cellulose esters, and used for spin-coating at RH of (35 ± 5)%, (55 ± 5)% or (75 ± 5)%. The structural features were correlated with the molecular characteristics of each cellulose ester and with the balance between surface energies of water and THF and interface energy between water and THF. CA, CAB, CMCAB and C-A-P films spin-coated at RH of (55 ± 5)% were exposed to THF vapor during 3, 6, 9, 60 and 720 min. The structural changes on the cellulose esters films due to THF vapor exposition were monitored by means of AFM and ellipsometry. THF vapor enabled the mobility of cellulose esters chains, causing considerable changes in the film morphology. In the case of CA films, which are thermodynamically unstable, dewetting was observed after 6 min exposure to THF vapor. On the other hand, porous structures observed for C-A-P, CAB and CMCAB turned smooth and homogeneous after only 3 min exposure to THF vapor.     

Compression set of thermoplastic polyurethane under different thermal-mechanical-moisture conditions

Elastomeric materials are used in the manufacture of structural dampeners due to their high damping coefficient and ease of production. However, elastomers, and in particular thermoplastic polyurethanes (TPU), are susceptible to degradation from environmental conditions. Samples of TPU were investigated, in terms of their mechanical properties, under the influence of four factors; time (up to 10 weeks thermal exposure), temperature (20-80 °C), strain (10% and 25%) and moisture (pre-soak/testing in water). Compression, hardness and compression set tests were used to determine the major contributors to the degradation process. It was found that pure thermal loading at 70 °C for 10 weeks did not result in any changes in material properties, other than an initial drying phase causing an increase in hardness of 2-3 Shore D. The compression set values were found to be heavily dependent on the test temperature, with a significant increase in compression set being seen between 70 and 80 °C. The presence of water (introduced by testing in water) acted as a plasticiser and resulted in a larger amount of compression set, than testing in the absence of water. The level of compression set was shown to be insensitive to the strain level. Overall, it was found, for the conditions tested, that temperature was the major driving force behind the compression set of the TPU material.     

Proton conductivity of nanoporous anatase xerogels prepared by a particulate sol–gel method

Nanoporous anatase xerogels were prepared via particulate sol–gel processes. The calcined xerogels were mesoporous, with a BET surface area of 121 m²/g, an average pore diameter of 5.8 nm and a pore volume of 0.236 cc/g. Proton conductivity of the membranes was measured as a function of temperature and relative humidity (RH). When anatase membranes are treated at pH 1.5, the proton conductivity increased in the whole range of temperature and RH. It indicates that the surface site density (number of water molecules per square nanometer) of these materials has a strong effect on conductivity. The proton conductivity of the studied anatase xerogels followed an Arrhenius-like dependence on the temperature (from room temperature to 90°C), in both treated and untreated membranes. A sigmoidal dependence of the conductivity on the RH was observed with the greatest increase noted between 58% and 81% RH in both treated and untreated anatase membranes. The highest value of proton conductivity was found to be 0.015 S/cm at 90°C and 81% RH, for treated anatase ceramic membranes. An increase in the conductivity could be achieved by means of longer times of treatment. According to the activation energy values, proton migration in this kind of materials could be dominated by the Grotthuss mechanism in the whole range of RH. The similar values of proton conductivity, lower cost and higher hydrophilicity of these membranes make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). Presented at the conference Solid State Chemistry 2004, September 13–17, Prague, Czech Republic     

Sample temperatures during outdoor and laboratory weathering exposures

Effective exposure temperatures (T_eff) in Arizona were calculated from hourly or 10-min parsed irradiation data along with ambient, black panel, and sample temperatures. The T_eff represents a constant temperature that creates the same amount of photodegradation as the naturally varying temperature and provides a benchmark temperature for making lifetime predictions from accelerated laboratory exposures. The annual ambient and black panel T_eff at a Wittmann, Arizona site were 30 °C and 42 °C, respectively, assuming that the photodegradation has an activation energy (E_a) of 21 kJ/mol (5 kcal/mol). T_eff was only weakly dependent on E_a over the range of 10-40 kJ/mol (3-10 kcal/mol). Samples exposed as van sunroofs were found to have T_eff that were offset from the black panel temperatures by a constant amount for the entire year. Thus, measurements of sample and black panel need to be made for only a few weeks to determine the offset and give the annual sample T_eff if the annual black panel T_eff is known. Light-colored samples probably are better compared with the ambient temperatures. Sample temperatures in xenon arc exposures usually are higher than the outdoor T_eff, so Arrhenius temperature corrections need to be carried out to relate accelerated to outdoor exposures. Temperatures in xenon arc exposure tests often correspond more closely to maximum temperatures that samples might encounter for only a few hours per year.     

The highly crosslinked dimethacrylic/divinylbenzene copolymers

The thermal stability of the ionic liquids (ILs) 1-n-butyl-3-methylimidazolium bromide, [BMIM]Br, and 1-n-octyl-3-methylimidazolium bromide, [OMIM]Br, was evaluated through thermogravimetry (TG). Long-term isothermal TG studies revealed that both of these ILs exhibit appreciable decomposition even at temperatures significantly lower than the onset decomposition temperature, previously determined from fast scan TG experiments. The long-term TG studies of both the ILs showed linear mass loss as a function of time at each temperature of 10 °C interval in the range 533–573 K over a period of 10 h. The kinetics of isothermal decomposition of ILs was analyzed using pseudo-zero-order rate expression. The activation energies for the isothermal decomposition of [BMIM]Br and [OMIM]Br under nitrogen atmosphere are 219.86 and 212.50 kJ mol⁻¹, respectively. The moisture absorption kinetics of these ILs at 25 °C and 30% relative humidity (RH) and at 85 °C and 85% RH were also studied. Water uptake of ILs exposed at 25 °C/30%RH follows a simple saturation behavior in agreement with Weibull model while that at 85 °C/85%RH fortuitously fit into the Henderson–Pabis model.     

稀土Ce掺杂对ZnO结构和光催化性能的影响

采用共沉淀-焙烧法合成了一系列不同含量的稀土Ce掺杂的ZnO光催化剂. 利用傅里叶变换红外(FT-IR)光谱、粉末X射线衍射(XRD)、扫描电镜(SEM)、紫外-可见(UV-Vis)光谱、光致发光(PL)谱等技术对所制备的光催化剂进行了系列表征. 以酸性橙II脱色降解为模型反应, 考察了掺杂不同含量的铈及不同焙烧温度对ZnO的物理结构和光催化脱色性能的影响. 结果表明: 掺入质量分数(w)为2%的铈可以明显改善氧化锌表面状态, 有利于产生更多的表面羟基; 同时可以抑制光生电子与光生空穴(e^-/h⁺)的复合, 显著提高光催化脱色活性和光催化稳定性; 焙烧温度对光催化剂的晶体结构、表面性能和光催化活性产生较大影响, 500 °C的焙烧处理使样品的结晶度较高, 同时催化剂颗粒粒径较细, 表面具有丰富的羟基. 但过高的焙烧温度(600-800 °C)将导致催化剂的物理结构发生恶化, 降低光催化性能.     

Curing conditions of polyarylacetylene prepolymers to obtain thermally resistant materials

Prepolymers of polyarylacetylene (PAA) were synthesized from 1,4-diethynylbenzene using nickel catalyst (C20, C25, and C30) or by direct thermal polymerization (T48). Their curing behaviors were investigated in detail to determine the proper curing conditions that lead to high char yields in cured PAA resins. Dynamic and isothermal differential scanning calorimetry (DSC) measurements were employed to investigate the curing conditions of the prepolymers. Dynamic DSC study reveals that exothermic heat starts at about 120 °C, reaches to a maximum at 210 °C, and ends around 300 °C. Moreover, step isothermal DSC investigation (at 120, 160, 200, 250, and 300 °C; 1 h for each temperature) shows that the major curing occurs at 160 °C, 200 °C and 250 °C, with more than 85% of the acetylene groups reacted. Using this step-curing conditions, very high thermal resistance is realized on C30, with thermal decomposition temperature (at 10% weight loss) and char yield (at 800 °C) being 686 °C and 86%, respectively. Current results indicate that highly thermal resistant PAA resins are obtainable using step curing of PAA prepolymers synthesized by Ni-catalyzed reaction.     

Preparation and properties of thermally stable polyimides derived from asymmetric trifluoromethylated aromatic diamines and various dianhydrides

A novel method for the preparation of an asymmetric fluorinated aromatic diamine, 3,4′-bis(4-amino-2-trifluoromethylphenoxy)-benzophenone was investigated. This new diamine containing trifluoromethyl side group was synthesized from the nucleophilic substitution reaction of 2-chloro-5-nitrobenzotrifluoride and 3,4′-dihydroxybenzo phenone in the presence of potassium carbonate, followed by catalytic reduction with SnCl₂·6H₂O and concentrated hydrochloric acid. This novel diamine was used to react with different commercially available aromatic tetracarboxylic dianhydrides to prepare polyimides via thermal or chemical imidization. The polyimide properties such as inherent viscosity, solubility, thermal and surface properties were investigated to illustrate the contribution of the trifluoromethyl group and the asymmetry structure of the polyimide. The polyimides obtained had good thermal stability and the glass transition temperature values ranged from 225 to 267 °C. All of these novel polyimides held 10% weight loss at the temperature above 543 °C in air and left more than 47% residue even at 800 °C in nitrogen. The inherent viscosities of the obtained polyimides were above 0.73 dL/g and were easily dissolved in both polar, aprotic solvents and some low-boiling-point solvents. Moreover, these PI films had dielectric constants of 2.94-3.53 (1 kHz), with moisture absorption in the range of 0.07-0.34 wt%. In comparison of the PIs (5) series with the analogous symmetric PIs (6) series based on 4,4′-bis(4-amino-2-trifluoromethylphenoxy)-benzophenone, the (5) series revealed better solubility, low dielectric constant and moisture absorption.     

Crack initiation and evolution in vulcanized natural rubber under high temperature fatigue

The nanoscaled crack initiation and evolution of natural rubber under high temperature (85 °C) and small strain amplitude (strain maximum α = 1) fatigue condition were investigated. It was shown by scanning electron microscopy (SEM) images that cracks and cavities with dimensions in nanoscale in the NR matrix appear during the high temperature fatigue. FTIR study indicated that thermal oxidation effect leads to the crosslinking structure destruction. According to the combined analysis of SEM, energy-dispersive X-ray (EDX) spectrometer and small angle X-ray scattering investigations, it was deduced that the destruction of crosslinking structure mainly locates in the vicinity of the ZnS particles with a diameter of 20.2 nm. The ZnS particles are generated as a byproduct in the vulcanization process. Further, the real-time SAXS analysis revealed that the cracks are primarily initiated at relative higher strains (0.7<α < 1) in the region of ZnS aggregations and larger cavities are derived from the enlargement of the cracks.     

Novel fluorinated polyimides derived from 9,9-bis(4-amino-3,5-difluorophenyl)fluorene and aromatic dianhydrides

Novel fluorinated polyimides (PIs) were prepared from 9,9-bis(4-amino-3,5-difluorophenyl)fluorene with three aromatic dianhydrides via a one-step high-temperature polycondensation procedure. These obtained PIs showed excellent solubility and could be readily soluble in a variety of organic solvents such as NMP, DMAc, DMF, CHCl₃, CH₂Cl₂ and THF. All the PIs could afford flexible and strong films with low dielectric constants (2.62-2.79 at 1 MHz) and low moisture absorptions (0.18-0.41%). Thin films of these PIs exhibited high optical transparency and light color, with the cutoff wavelength at 341-355 nm and transmittance higher than 80% at 450 nm. Meanwhile, these PIs possessed eminent thermal stability, with decomposition temperatures (T_d) above 570 °C in both air and nitrogen atmospheres and glass transition temperatures (T_g) beyond 376 °C. Moreover, these fluorinated PI films showed low surface free energy and hydro-oleophobic character. The contact angles on the films for water and glycerol were in the range of 102.3-107.9° and 94.0-100.3°, respectively. In comparison with the analogous PI non-containing fluorine group, these fluorinated PIs showed better solubility, higher optical transparency, lower dielectric constants and lower surface free energy.     

Thermal analysis of the water uptake by hydrocolloids

Water uptakes by the hydrocolloids gelatin, pectin and sodium carboxymethylcellulose (NaCMC) and their mixtures, following storage at 20°C and 93% relative humidity (RH), were investigated using differential scanning calorimetry. An increase in water uptake was observed for each sample on storage. The majority of the moisture sorption occurred in the initial 2 h. Only non-freezable, bound water was present in gelatin, pectin and gelatin/pectin or NaCMC/pectin/gelatin mixtures during storage for 28 h. Loosely bound freezable and non-freezable, bound water were detected in NaCMC stored for 2 h. Water in mixtures of NaCMC/gelatin or NaCMC/pectin was predominately loosely bound freezable and non-freezable bound. The order of water uptake was gelatin<pectin/gelatin<pectin<NaCMC/pectin/gelatin<NaCMC/gelatin<NaCMC/pectin<NaCMC.     

3D printed titanium micro-bore columns containing polymer monoliths for reversed-phase liquid chromatography

The potential of 3D selective laser melting (SLM) technology to produce compact, temperature and pressure stable titanium alloy chromatographic columns is explored. A micro bore channel (0.9 mm I.D. × 600 mm long) was produced within a 5 × 30 × 30 mm titanium alloy (Ti–6Al–4V) cuboid, in form of a double handed spiral. A poly(butyl methacrylate-co-ethyleneglycoldimethacrylate) (BuMA-co-EDMA) monolithic stationary phase was thermally polymerised within the channel for application in reversed-phase high-performance liquid chromatography. The prepared monolithic column was applied to the liquid chromatographic separation of intact proteins and peptides. Peak capacities of 69–76 (for 6–8 proteins respectively) were observed during isothermal separation of proteins at 44 °C which were further increased to 73–77 using a thermal step gradient with programmed temperature from 60 °C to 35 °C using an in-house built direct-contact heater/cooler platform based upon matching sized Peltier thermoelectric modules. Rapid temperature gradients were possible due to direct-contact between the planar metal column and the Peltier module, and the high thermal conductivity of the titanium column as compared to a similar stainless steel printed column. The separation of peptides released from a digestion of E.coli was also achieved in less than 35 min with ca. 40 distinguishable peaks at 210 nm.     

The influence of biotic and abiotic factors on the rate of degradation of poly(lactic) acid (PLA) coupons buried in compost and soil

Poly(lactic) acid (PLA) is a compostable biopolymer and has been commercialised for the for the manufacture of short-shelf life products. As a result, increasing amounts of PLA are entering waste management systems and the environment; however, the degradation mechanism is unclear. While hydrolysis of the polymer occurs abiotically at elevated temperature in the presence of water, potential catalytic role for microbes in this process is yet to be established. In this study, we examined the degradation of PLA coupons from commercial packaging at a range of temperatures (25°, 37°, 45°, 50° and 55 °C) in soil and compost and compared with the degradation rates in sterile aqueous conditions by measuring loss of tensile strength and molecular weight (Mw). In addition, in order to assess the possible influence of abiotic soluble factors in compost and soil on degradation of PLA, degradation rates in microorganism-rich compost and soil were compared with sterile compost and soil extract at 50 °C. Temperature was determined to be the key parameter in PLA degradation and degradation rates in microorganism-rich compost and soil were faster than in sterile water at temperatures 45° and 50 °C determined by tensile strength and Mw loss. Furthermore, all tensile strength was lost faster after 30 and 36 days in microorganism-rich compost and soil, respectively, than in sterile compost and soil extract, 57 and 54 days, respectively at 50 °C. Significantly more Mw, 68% and 64%, was lost in compost and soil, respectively than in compost extract, Mw, 53%; and in soil extract, 57%. Therefore, degradation rates were faster in microorganism-rich compost and soil than in sterile compost and soil extract, which contained the abiotic soluble factors of compost and soil at 50 °C. These comparative studies support a direct role for microorganisms in PLA degradation at elevated temperatures in humid environments. No change in tensile strength or Mw was observed either 25° or 37 °C after 1 year suggesting that accumulation of PLA in the environment may cause future pollution issues.     

Investigation on spontaneous ignition of two kinds of organic material with water

A systematic investigation was performed to elucidate the cause of spontaneous ignition of Refuse Derived Fuel (RDF) and Meat Bone Meal (MBM). Heat generation in both RDF and MBM with addition of water liquid and vapor at room temperature was determined by isothermal calorimetry. Compared with water liquid, the heat of wetting by sorption of water vapor at 80% relative humidity and 25 °C was larger, which can raise the temperature of RDF and MBM more than 30 and 56 °C, respectively. Heat generation due to fermentation occurred and the temperature of RDF and MBM reached or exceeded 80 °C after 5 days for RDF and 4 days for MBM at 100% RH. The spontaneous ignition for RDF and MBM results from heat of wetting and fermentation at room temperature and a further exothermic reaction at higher temperature.