Fluorocarbons in the global environment: a review of the important interactions with atmospheric chemistry and physics

Fluorocarbon impact on ozone depletion is reviewed together with the efficacy of the Montreal Protocol in acting to correct the imbalance between stratospheric ozone production and destruction. The Protocol is also helping to reduce global warming: CFCs are shown to be currently the largest fluorocarbon contributors to climate change. Relative contributions to climate change from CFCs and their HFC substitutes are discussed, together with the consequences of control of minor greenhouse gases on an environmental impact which is dominated by carbon dioxide emissions. Both the potencies of the materials for environmental change and their concentrations in the atmosphere are important and are considered here.Trifluoroacetic acid, a minor product of atmospheric decomposition of some HCFCs and HFCs and of the pyrolysis of fluoropolymers, has been shown to be uniformly distributed in seawater to a depth of over 4000 m and so is natural, although the actual source has yet to be identified.The Montreal Protocol is only one example of action to reduce undesirable impact from fluorocarbons. Other, less universal, actions include abatement of fluoroform greenhouse gas emissions from HCFC manufacturing, process changes to eliminate trifluoromethylsulfur pentafluoride emissions from electrochemical fluorination and ceasing manufacture of perfluorooctanyl sulfonate compounds due to their persistence in human tissue.     

An analytical solution of a newly proposed peak broadening equation and extension of polydispersities to higher molecular weight averages in size exclusion chromatography (SEC)

Summary Herein is reported an analytical solution to the peak broadening or peak dispersion/flattening equation based on the recently proposed Instrumental Spreading Shape Function and its application to correction for imperfect resolution (inadequate peak separation and/or excessive peak broadening) for higher molecular weight averages. The relationship of these higher MW averages with the familiar Weight Average and number average molecular weights is also discussed. Criteria for perfect resolution are specified and a true molecular weight calibration curve is accordingly defined.     

Synthesis and near infrared luminescence of a tetrametallic Zn2Yb2 architecture from a trinuclear Zn3L2 Schiff base complex

Near infrared luminescence is observed in tetrametallic [Zn2Yb2L2(mu-OH)2Cl4].2MeCN which is obtained from the Zn3 Schiff-base complex [Zn3L2(NO3)2].MeOH, (H2L =N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine).     

Poly(N-isopropylacrylamide). I. Interactions with sodium dodecyl sulfate measured by conductivity

Conductometric titration of poly(N-isopropylacrylamide) (polyNIPAM) with sodium dodecyl sulfate (SDS) gave two apparent transitions labeled C1 and C2. The C1 transition was independent of polyNIPAM concentration in the 0.05–0.3 wt % range, whereas C2 was proportional to the polymer concentration. C1 corresponded to the onset of binding of surfactant with polymer. Arguments based on a simple mass action model for micellization are presented to show that C2, the second transition, is not due to any simple explanation such as being the point above which only free micelles are formed with surfactant addition. The cloud point of polyNIPAM increased with the amount of bound surfactant. This was attributed to electrostatic contribution of bound sulfate groups to the increased solubility of polyNIPAM. © 1993 John Wiley & Sons, Inc.     

Modelling free-radical copolymerization kinetics—evaluation of the pseudo-kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching

The full moment equations and equations using pseudo-kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number- and weight-average molecular weights (M̄_n and M̄_w) were compared with those found using the pseudo-kinetic rate constant method (PKRCM). The results show that the weight-average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ₁^˙ and φ₂^˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄_w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄_w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄_w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo-kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo-kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.     

Modelling free-radical copolymerization kinetics, 3. Molecular weight calculations for copolymers with crosslinking

A comprehensive model for molecular weight calculations of free-radical crosslinking copolymerizations was developed using the pseudo-kinetic rate constants and the method of moments. The moments of copolymer chain distributions are defined in such a way so that the molecular weight averages of crosslinking copolymers can be calculated using the moments. The present model is based on a general crosslinking copolymerization scheme, accounting for chain transfer to small molecules and polymer, bimolecular termination, and crosslinking reactions. The influence of crosslinking reactions on molecular weight development is discussed. The effects of the reactivity of pendant double bonds on the moments development were further demonstrated using model simulations. The simulations results suggest that the higher-order molecular weight averages are very sensitive to the reactivity of pendant double bonds. It was found that chain transfer to polymer affects the gelation point significantly. The radical fractions must be calculated accounting for chain transfer reactions in addition to propagations in order to properly evaluate pseudo-kinetic rate constants. The present model was used to predict kinetic behavior and molecular weight development of styrene/m-divinylbenzene and styrene/ethylene dimethacrylate free-radical crosslinking copolymerizations in benzene solution at 60°C. It was found that the present model is in excellent agreement with the experimental data published in the literature. Model predictions and experimental data show that the reactivity of pendant double bonds is much lower than that of vinyl and divinyl monomers. The simulation results suggest that the assumption of the same reactivity of functional groups is likely not valid for many free-radical crosslinking copolymerizations. The present model based on a kinetics approach can be used to predict molecular weight development for vinyl/divinyl free-radical crosslinking copolymerizations and to estimate kinetic parameters in the pre-gelation period.     

Effect of reactor residence time distribution on the size distribution of polymer particles made with heterogeneous Ziegler-Natta and supported metallocene catalysts. A generic mathematical model

A generic mathematical model for analyzing the effect of ideal and non-ideal reactor residence time distributions on the size distribution of polymer particles produced with heterogeneous Ziegler-Natta and supported metallocene catalysts was developed. It was shown that the residence time distribution in polymerization reactors can have a significant effect on the size distribution of polymer particles and this can lead to imperfect replication of the catalyst particle size distribution.     

High‐throughput characterization of pattern formation in symmetric diblock copolymer films

Surface‐pattern formation in thin block copolymer films was investigated by utilizing a high‐throughput methodology to validate the combinatorial measurement approach and to demonstrate the value of the combinatorial method for scientific investigation. We constructed measurement libraries from images of subregions of block copolymer films having gradients in film thickness and a range of molecular mass, M. A single gradient film covers a wide range of film morphologies and contains information equivalent to a large number of measurements of films having a fixed thickness, h. Notably, the scale of the surface patterns is generally much larger than the molecular dimensions so that the interpretation of the patterns is more subtle than ordering in bulk block copolymer materials, and there is no predictive theory of this type of surface‐pattern formation. We observed a succession of surface patterns that repeat across the film with increasing h [extended smooth regions, regions containing circular islands, labyrinthine (“spinodal”) patterns, holes, and smooth regions again]. The extended smooth regions and the labyrinthine patterns appear to be novel features revealed by our combinatorial study, and these patterns occurred as bands of h that were quantized by integral multiples of the bulk lamellar period, L_o. The magnitude of the height gradient influenced the width of the bands, and the smooth regions occupied an increasing fraction of the film‐surface area with an increasing film gradient. The average size of the spinodal patterns, λ, was found to scale as λ ～ L or λ ～ M^?1.65 and reached a limiting size at long annealing times. The hole and island features had a size comparable to λ, and their size likewise decreased with increasing M. The smooth regions were attributed to an increase in the surface‐chain density in the outer brushlike block copolymer layer with increasing h, and the scaling of λ with M was interpreted in terms of the increasing surface elasticity with M. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2141–2158, 2001     

Crystal structures of [WI2(CO)3{Ph2P(CH2) n PPh2}] (n = 2 or 3)

[WI₂(CO)₃{Ph₂P(CH₂)₂PPh₂}] (1) crystallizes out in the monoclinic space group P2₁/n, with a = 13.852(7) Å, b = 14.789(19) Å, c = 14.915(19) Å, = 102.86(1)°, Z = 4. [WI₂(CO)₃{Ph₂P(CH₂)₃PPh₂}] (2) crystallizes out in the monoclinic space group P2₁/n, with a = 10.499(15) Å, b = 14.58(2) Å, c = 20.75(3) Å, = 103.59(1)°, Z = 4. Both structures show the metal in a seven-coordinate environment with a carbonyl in the unique capping position, two further carbonyls and a phosphorus in the capped face, and two iodides and the second phosphorus in the uncapped face.     

Effect of temperature on the morphology and kinetics of surface pattern formation in thin block copolymer films

Hole formation and growth on the top layer of thin symmetric diblock copolymer films, forming an ordered lamellar structure parallel to the solid substrate (silicon wafer) within these films, is investigated as a function of time (t), temperature (T), and film thickness (l), using a high-throughput experimental technique. The kinetics of this surface pattern formation process is interpreted in terms of a first-order reaction model with a time-dependent rate constant determined uniquely by the short-time diffusive growth kinetics characteristic of this type of ordering process. On the basis of this model, we conclude that the average hole size, lambda(h), approaches a steady-state value, lambda(h)(t-->infinity) identical with lambda(h,infinity)(T), after long annealing times. The observed change in lambda(h,infinity)(T) with temperature is consistent with a reduction of the surface elasticity (Helfrich elastic constant) of the outer block copolymer layer with increasing temperature. We also find that the time constant, tau(T), characterizing the rate at which lambda(h)(t) approaches lambda(h,infinity)(T), first decreases and then increases with increasing temperature. This temperature variation of tau(T) is attributed to two basic competing effects that influence the rate of ordering in block copolymer materials: the reduction in molecular mobility at low temperatures associated with glass formation and a slowing of the rate of ordering due to fluctuation effects associated with an approach to the block copolymer film disordering temperature (T(d)) from below.