Apparent inhibition of thermal decomposition of hydrogencarbonate ion by poly(acrylic acid). The effect of molar mass and end-group functionality

A comparative study of novel scale inhibitors with commercial scale inhibitors used in seawater desalination

A theoretical investigation into thiophenic derivative cracking mechanism over acidic and cation-exchanged beta zeolites

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on Drayton gasifier slag and four poly(acrylic acid)s with number average molar masses ranging from 3.03 × 103 to 6.44 × 104. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive and flexural strengths of the cements increased with increasing molar mass of the poly(acrylic acid)s and time. The Young's modulii increased with time and were independent of poly(acrylic acid) molar mass. Fracture toughness increased with increasing molar mass of the poly(acrylic acid)s. Fracture toughness increases over an ageing time of one week and subsequently decreased over one month. Toughness increased with poly(acrylic acid) molar mass, these increases being most pronounced at higher molar mass. The toughness values decreased with time for the higher molar mass cements, which is consistent with increased crosslinking of the poly(acrylic acid) chains and reducing molecular flow at the crack tip. Plastic zone size increased with poly(acrylic acid) molar mass and decreased with time for lower molar mass cements, remained constant for intermediate molar mass cements and increased with high molar mass cements.

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangement─a unimolecular path and a hydrogen-atom-assisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

While the 1,5-sigmatropic hydrogen shift in cyclopentadiene is generally thought to be a unimolecular pericyclic reaction, Yamabe proposed a more complex bimolecular mechanism proceeding through the exo dimer of cyclopentadiene. DFT computations by Yamabe were claimed to show that the bimolecular mechanism was kinetically more favorable than the unimolecular mechanism. Reinvestigation of the unimolecular concerted mechanism and Yamabe's bimolecular mechanism with ωB97X-D and DLPNO-CCSD(T) calculations demonstrates a 25 kcal/mol preference for the unimolecular mechanism relative to the bimolecular mechanism. While Yamabe's calculations were performed with the less accurate B3LYP functional, the incorrect conclusion was the result of a different error discovered here. We have also computed corrections for tunneling that result in computed activation barriers within 1.5 kcal/mol of the experimental values.

Inhibition of homogenous formation of calcium carbonate by poly (acrylic acid). The effect of molar mass and end-group functionality

Scorpion primers can be used to detect PCR products in homogeneous solution. Their structure promotes a unimolecular probing mechanism. We compare their performance with that of the same probe sequence forced to act in a bimolecular manner. The data suggest that Scorpions indeed probe by a unimolecular mechanism which is faster and more efficient than the bimolecular mechanism. This mechanism is not dependent on enzymatic cleavage of the probe. A direct comparison between Scorpions, TaqMan and Molecular Beacons on a Roche LightCycler indicates that Scorpions perform better, particularly under fast cycling conditions. Development of a cystic fibrosis mutation detection assay shows that Scorpion primers are selective enough to detect single base mutations and give good sensitivity in all cases. Simultaneous detection of both normal and mutant alleles in a single reaction is possible by combining two Scorpions in a multiplex reaction. Such favourable properties of Scorpion primers should make the technology ideal in numerous applications.

Fluorescence spectroscopy on a series of aqueous solutions of poly(acrylic acid) containing a luminescent label showed that polymers with molar mass, Mn < 16.5 kDa did not exhibit a pH responsive conformational change, which is typical of higher molar mass poly(acrylic acid). Below this molar mass, polymers remained in an extended conformation, regardless of pH. Above this molar mass, a pH-dependent conformational change was observed. Diffusion-ordered nuclear magnetic resonance spectroscopy confirmed that low molar mass polymers did not undergo a conformational transition, although large molar mass polymers did exhibit pH-dependent diffusion.

Ultra-high molar mass polyethylene (UHMMPE) is commonly used for ballistic-resistant body armor applications due to the superior strength of the fibers fabricated from this material combined with its low density. However, polymeric materials are susceptible to thermally induced degradation during storage and use, which can reduce the high strength of these fibers, and, thus, negatively impact their ballistic resistance. The objective of this work is to advance the field of lightweight and soft UHMMPE inserts used in various types of ballistic resistant-body armor via elucidating the mechanisms of chemical degradation and evaluating this chemical degradation, as well as the corresponding physical changes, of the UHMMPE fibers upon thermal aging. This is the first comprehensive study on thermally aged UHMMPE fibers that measures their decrease in the average molar mass via high-temperature size exclusion chromatography (HT-SEC) analysis. The decrease in the molar mass was further supported by the presence of carbon-centered free radicals in the polyethylene that was detected using electron paramagnetic resonance (EPR) spectroscopy. These carbon-centered radicals result from a cascade of thermo-oxidative reactions that ultimately induce C–C ruptures along the backbone of the polymer. Changes in the crystalline morphology of the UHMMPE fibers were also observed through wide-angle X-ray diffraction (WAXS), showing an increase in the amorphous regions, which promotes oxygen diffusion into the material, specifically through these areas. This increase in the amorphous fraction of the highly oriented polyethylene fibers has a synergistic effect with the thermo-oxidative degradation processes and contributes significantly to the decrease in their molar mass.

Polyacrylic acid was synthesized in water by persulfate-initiated polymerization (solution polymerization) of glacial acrylic acid in the absence of a chain-transfer agent. The final product is odorless and colorless. Chelation for calcium ions using a calcium electrode show that our poly(acrylic acid) has a higher chelation capacity than that of existing commercial poly(acrylic acids). A design of experiments was performed to optimize the synthesis conditions to obtain poly(acrylic acid) with a high maximum chelation value. These studies also helped us to gain insight into its high chelation capacity. The chelation capacity for calcium reaches its highest values when polymerization near isothermal conditions is done ∼ 95°C with an acrylic acid concentration of ≤21 wt % and an addition time >1 h. These conditions favor higher molecular weight poly(acrylic acid) with a polydispersity ∼ 4. The dispersion properties of our poly(acrylic acid) are similar to those of the commercial ones. This dual capability of chelation and dispersion is absent in commercial chelants such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and their analogs. At pH > 7, chelation of calcium by our poly(acrylic acid) is much higher than that observed with EDTA. Characterization by NMR, Raman, FTIR, and molecular modeling are included in an attempt to understand structural features that can explain the higher chelation capacity of our atactic poly(acrylic acid).

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on four model glasses with varying reactivity and four poly(acrylic acid)s (PAA)s with number average molar masses (Mn) ranging from 3.25 × 104 to 1.08 × 105. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive strengths (σc) of the cements increased with increasing fluorine content of the glass, with increased molar mass of the PAA and with ageing time. The Young's moduli increased with time, but were lower for cements based on the fluorine free glass. Moduli values were independant of PAA molar mass. The un-notched fracture strength (σf) of the cement increased with the molar mass of the PAA and with ageing time. Glass composition did not appreciably influence the un-notched fracture strength. The fracture toughness (KIC) increased with the molar mass of the PAA and with ageing time, but reduced with increasing fluorine content of the glass. The toughness (GIC) was dependant on molar mass. The influence of molar mass was not as great as predicted by the reptation chain pull-out model for fracture. The molar mass dependence of toughness was greatest with the lower fluorine content glasses. The plastic zone size at the crack tip increased with the molar mass of the PAA. However the plastic zone size decreased with ageing time for all the cements studied and was smaller for the more reactive higher fluorine content glasses.

The influence of poly(acrylic acid), PAA molar mass and concentration on fracture toughness and toughness of glass polyalkenoate cements was investigated. Fracture toughness and toughness increased with both the molar mass of the PAA and its concentration. The fracture toughness and toughness increased dramatically with concentration for the highest molar mass PAA studied. However the increase in fracture toughness and toughness with PAA concentration was small for the lowest molar mass PAA. The influence of molar mass was greatest at the highest PAA concentration studied and least for the lowest PAA concentration. The toughness results were analysed with a reptation chain pull-out model. The greater dependance of toughness on PAA concentration for high molar mass cements can be explained by the critical molar mass for chain entanglements to form (M e) being concentration dependant and M e decreasing with increasing PAA concentration.

The influence of poly(acrylic acid) molar mass was investigated on cements formed from zinc oxide-apatite mixtures at three aging times; one, seven and 28 days. Cements based on both hydroxyapatite and fluorapatite were investigated. The compressive strength, un-notched fracture strength and fracture toughness increased markedly with poly(acrylic acid) molar mass. The fracture toughness and un-notched fracture strength increased with aging time for the two highest molar mass cements, but decreased with time for the two lowest molar mass cements. The greater chain entanglement density present in the higher molar mass cements is thought to contribute significantly to the cement stability in addition to the crosslinking of the polyacrylate chains by calcium and zinc ions. Substitution of hydroxyapatite by fluorapatite had no significant influence on the mechanical properties of the cements at aging times longer than one day.

Application of Membrane Distillation for desalting brines from thermal desalination plants

In order to ensure long-term sustainability of the reservoir, the gas industry in Qatar is faced with the challenge of reducing the volume of produced and process water (PPW) sent to disposal wells by 50% [1-3]. Recently, Qatargas initiated a project to recycle process water and thus, reduce disposal volumes using commercial advanced water treatment technologies [4]. One emerging technology, “osmotic concentration” (OC) has been identified that offers a low-energy alternative to conventional thermal or membrane volume reduction methods. Osmotic concentration is a membrane filtration process that mimics first step in a forward osmosis (FO) system. It requires a high salinity draw solution (DS) which passes on one side of a semi-permeable FO membrane while the feed passes on the other side. Water from the feed is drawn through the membrane, via natural osmosis, reducing the feed volume and increasing the volume of the draw solution. This paper summarizes the results of bench-scale volume reduction tests wit...