Macromolecular Reaction Engineering最新文献

Chitosan nanocrystals (ChsNCs) are a renewable resource attracting research interest due to their outstanding physical, chemical, and mechanical properties. ChsNCs exhibit promising potential applications as reinforcing materials in polymer-based nanocomposites. However, the hydrophilic surface of ChsNCs leads to poor dispersibility in hydrophobic polymer matrices, hindering these potential applications. In this work, the first graft modification of ChsNCs using nitroxide-mediated polymerization (NMP) is reported. ChsNCs are initially functionalized with glycidyl methacrylate to provide a reactive surface group that enables the polymer-graft modification. Polystyrene and poly (methyl methacrylate) with well-defined molecular weight polymers are first synthesized via NMP, and the polymers are then “grafted to” the ChsNCs surface, yielding polymer-graft modified ChsNCs.

A dynamic model is proposed to account for shrinkage and swelling during the photopolymerization of 1,6-hexanediol diacrylate (HDDA) with the bifunctional initiator bis-acylphosphine oxide (BAPO) in the presence of oxygen. The model is composed of 14 partial differential equations (PDEs) that are used to track changes in film thickness along with time- and spatially-varying concentrations of monomer, initiator, oxygen, pendant vinyl groups, and seven types of radicals. Shrinkage has a noticeable influence on the model predictions. For a variety of simulated photopolymerization experiments, there is ≈9% discrepancy between predicted overall vinyl-group conversions obtained from the current model with shrinkage and a previous model without. Prediction discrepancies become larger for simulated experiments involving thin films (8 µm) or low light intensities (1200 W m⁻²). In the future, it will be important to re-estimate the kinetic parameters used in the shrinkage model to obtain accurate model predictions for use in process improvement studies.

Damkohler Number (Da) analysis can identify monomers and emulsion polymerization operating regimes where the polymerization may be monomer-transport, rather than reaction-limited. In these cases, the expected monomer concentration in the growing polymer particles will be reduced due to the transport limitation. This will reduce the expected rate of polymerization, and require the design of a larger polymerization reactor for a given production rate. In heterogenous catalysis, an effectiveness factor is used to quantify the reduction in reaction rate and necessarily increase reactor size to compensate. This paper will show that it is possible to use Da (functionally equivalent to the Thiele Modulus in heterogeneous catalysis) to estimate an effectiveness factor for emulsion polymerization. Also shown is a procedure for calculating the monomer feed ratio during binary copolymerization when one must not only take into account the reactivity ratios but also the possibility that one of the monomers is monomer-transport limited. The method provides the monomer feed ratio during the semibatch phase of a binary copolymerization. This alternative to starved-feed polymerization shall result in much faster polymerization and higher polymerization kettle utility.

Poly[(n-butyl methacrylate)-co-(n-butyl acrylate)]-based core-shell latexes are prepared by emulsion polymerization with a shell copolymer glass transition temperature (T_g) of 5 °C, but differences in core copolymer wt.% (4–90) and T_g (5–25 °C), and in wt.% of diacetone acrylamide (DAAM) in the shell copolymer, which facilitates crosslinking in the percolating phase of films through addition of adipic acid dihydrazide. Analysis of samples removed from reactions, together with analysis of film cross-sections by atomic force microscopy (AFM), confirms the core-shell particle structures and honeycomb morphologies in films, with simultaneous AFM and infrared spectroscopy showing the distribution of hydrazone crosslinks. Increasing wt.% DAAM (i.e., degree of crosslinking) in the percolating phase shifts film tensile stress–strain curves towards higher stresses and lower extensions at break. For core and shell copolymer T_gs of 5 °C there is a small effect of core wt.%. At 70 and 80 wt.% core, increasing core copolymer T_g also shifts the curves towards higher stresses and lower extensions at break. Thus by combining effects of core copolymer wt.% and T_g with effects of wt.% DAAM in the shell through the soft–soft nanocomposite approach, it is possible to achieve a wide range of tensile deformation behavior in films that have quite similar overall copolymer compositions.

Fabrics consisting of cotton-core/polypyrrole (PPy)-sheath fibers (cotton/PPy fabrics) are synthesized by aqueous chemical oxidative seeded polymerization of pyrrole and are utilized as a solar evaporation device. Scanning electron microscopy studies and elemental microanalyses reveal the thickness of the PPy sheath increases from a few tens nm to ≈200 nm with an increase of pyrrole monomer concentration in the polymerization system. The temperature of cotton/PPy fabrics increases upon irradiation with artificial sunlight to ≈33–45 °C in the dry state, due to light-to-heat photothermal conversion by the PPy component. Thanks to the photothermal property of the fabrics, water impregnated within the cotton/PPy fabrics can evaporate efficiently under the irradiation of artificial sunlight. Light-induced water evaporation experiment using an artificial seawater confirms that ionic concentrations drastically decreases, indicating successful desalination.

Low-Density Polyethylene (LDPE) reactors have the potential for rupture because of thermal runaway from auto-accelerating chemistry. Pockets of unmixed, highly reactive, LDPE constituents, called hot spots, are often generated by conditions within the reactor and are the main source of thermal runaway. Because of this, there is a need to define thresholds of hot spot conditions that produce runaway. Computational Fluid Dynamics is used to study an isolated LDPE sphere with varying initial temperature, initial catalyst concentration, and volume to determine which combinations promote thermal runaway. It is found that increasing both initial temperature and initial catalyst concentration increased thermal runaway likelihood, while, counter-intuitively, hot spot volume has no effect. An LDPE runaway map is provided to quantify the combinations that result in safe reactor operation. This allows manufacturers to make more informed control actions and to determine safe reactor conditions based on local mixture composition and temperature alone.

A new design policy to synthesize nanogel molecules having desired dimensions under unperturbed state is proposed. Miniemulsion copolymerization of vinyl and divinyl monomers, both conventional free-radical polymerization and ideal living polymerization, is used to illustrate the method. For the network formation dynamics, the newly proposed model that takes into account the size and structure dependence of cross-linking/cyclization reactions is employed. The master curve relationship that indicates the maximum average dimensions for randomly cross-linked networks is used as a guideline and the network dimension is controlled by the magnitude of network maturity index NMI, which is the average number of cycle rank per primary chain. By appropriately sizing the NMI, it is possible to synthesize network polymers with dimensions equal to or greater than the maximum dimensions achievable with a homogeneous, randomly cross-linked network polymer of the same cycle rank and molecular weight. The current strategy of designing and controlling 3D size is applicable regardless of the reaction mechanism of network formation and will also be applied to the synthesis of macro-gels.

While precipitation polymerization allows the synthesis of microgels with controlled functional-group distributions, the structural development of these microgels during the polymerization process still remains unclear. In this study, microgels with different reactivity ratios between the monomer and charged co-monomer are prepared by precipitation polymerization, and the evolution of their size, thermoresponsive behavior, and surface properties during polymerization are evaluated. In particular, the surface properties of the microgels are analyzed quantitatively using the softness parameter and the surface charge density is calculated using Ohshima's equation. The results allowed describing the structural changes of microgels during precipitation polymerization well and provided design guidelines for functional microgels with controlled functional group distributions.

Poly(vinylidene fluoride) (PVDF) is among the most produced fluoropolymers, second only to polytetrafluoroethylene. Despite its popularity, the complex microstructural properties achieved during the polymerization are not well documented in the literature. In particular, available models only track the chain length distribution of the polymer, while neglecting the distribution of other important properties, affecting the final behavior of the product. In this work, a 2D kinetic model, evaluating not only the chain length but also the number of terminal double bonds (TDBs) per chain, is developed. The numerical solution of the model is achieved by fractionating the population of polymer chains into classes with a specific number of TDBs and using the method of moments for each class. The model results are compared with experimental evidences for the amount of produced polymer, moles of main chain-ends, number, and weight average molecular weight as well as full molecular weight distribution. Based on this comparison, kinetic parameters are estimated by optimization using genetic algorithm. The model reliability is finally verified using additional experimental data at different temperatures and amounts of initiator.