An improved general kinetic analysis of non-linear irreversible polymerisations

A method to predict average molecular weights before and after gelation for general irreversible non-linear polymerisations forming tree-like molecules is described. Recently developed numerical methods for solving two point boundary value problems are essential for the success of these calculations after gelation and open the way to eventually be able to efficiently predicting chain length distributions. Anionic and free-radical polymerisation of vinyl monomers in the presence of divinyl monomers or with transfer to polymer are taken as case studies. Comparison to experimental data and with simulation results obtained through “numerical fractionation” confirms the usefulness of current approach.     

Simultaneous interpenetrating networks of a polyurethane and poly(methyl methacrylate). I. Metastable phase diagrams

Any simultaneous interpenetrating network (SIN) synthesis contains three key events. These are gelation of polymer I, gelation of polymer II, and phase separation of polymer I from polymer II. Metastable phase diagrams of SINs are developed, in which the time occurrence of these three events is represented. A polyurethane/poly(methyl methacrylate) (PU/PMMA) system was chosen as a model. Polymerization kinetics were followed in situ for both PU and PMMA using Fourier Transform Infrared Spectroscopy (FTIR) with the aid of a heated demountable cell. Glass transitions of fully cured samples were determined by dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC). Phase separation was determined by the onset of turbidity, and gelation of the first gelling polymer was determined by the sudden resistance of the system to flow. As a result, a metastable phase diagram was constructed for the four-component SIN system (the two monomers and their respective polymers) as a tetrahedron in three dimensions with the two monomers and two polymers at the four apexes. Phase separation and gelations of the two polymers are indicated by various surfaces. These surfaces intersect at lines and curves, representing unique conditions of an SIN synthesis, e.g., simultaneous gelation of both polymers, or simultaneous phase separation and gelation of polymer I, etc. These conditions are critical in terms of the development of the SIN morphology, dividing the reaction space into specific regions. Finally, it is shown how the tetrahedron diagram helps visualize the course of the three key events during SIN synthesis, and provides direction for controlling them. © 1995 John Wiley & Sons, Inc.     

Semibatch operation and primary cyclization effects in homogeneous free-radical crosslinking copolymerizations

A theoretical analysis is presented on the use of semibatch reactors for producing non-linear copolymers through the homogeneous free-radical polymerization of mono- and divinyl-monomers. The kinetic scheme distinguishes two kinds of pendant double bonds and five kinds of polymer radicals, as it also takes into account the primary cyclization of terminal divinyl units. In spite of the fairly large number of groups and reactions, the generating function of vector chain length distribution of polymer species can still be accurately computed by a numerical implementation of the method of characteristics and thus average molecular weights before and after gelation can be reliably obtained. The influence of feed policies of the monomers, initiator and transfer agent on gelation, average chain lengths and weight fraction of sol is discussed, as well as their sensitivity to some kinetic parameters.     

Passive microrheology for measurement of gelation behavior of a kind of polymer gel P(AM‐AA‐AMPS)

One kind of polymer gel P(AM‐AA‐AMPS) was prepared by radical aqueous copolymerization, using acrylamide (AM), acrylic acid (AA) and 1‐acrylanmido‐2‐methylpropanesulfonic acid (AMPS) as monomers, N,N‐methacrylamide (MBA) as crosslinker and ammonium persulfate (APS) as initiator. The microstructure and molecular structure of the polymer gel were characterized by environmental scanning electron microscope (ESEM), infrared spectrometer (IR) and thermal gravity analysis (TGA). Main factors affecting the gelation behavior of P(AM‐AA‐AMPS) were qualitatively and quantitatively studied by multi‐speckle diffusion wave spectroscopy (MS‐DWS) technology, and the elasticity index (EI) and macroscopic viscosity index (MVI) were introduced to evaluate the elasticity and viscosity of the polymer gel. The results show that the synthesized P(AM‐AA‐AMPS) polymer gel has three‐dimensional network structure gel with thermally resistant and salts tolerant groups. The EI and MVI of solution increase abruptly during the gelation time and the two indexes tend to stabilize. Under certain conditions, with the increase of reaction temperature and concentration of monomers and initiator, the gelation time of polymer gel gets shorter and the gel strength increases; with the increase of concentration of crosslinker, the strength of polymer gels increases, while the gelation time remains almost unchanged. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43364.     

Elimination of Surface Spallation of Alumina Green Bodies Prepared by Acrylamide-Based Gelcasting via Poly(vinylpyrrolidone)

Surface spallation of gelcast alumina green bodies due to the exposure of the surface to air during gelation was successfully eliminated by adding a proper amount of poly(vinylpyrrolidone) (PVP) in aqueous acrylamide premix solution. The influences of PVP on the colloidal characteristics of alumina powder in aqueous solutions, the rheological properties of alumina slurries, the gelation process, and the properties and microstructures of the gelcast green bodies and the sintered samples were systematically investigated. Even though the addition of PVP to premix solution has slightly negative effects on the viscosity of alumina slurries, gelation and strength of green bodies, it has no obvious influence on the final structure and properties of alumina ceramics. Thus, PVP was considered an effective polymer to resolve the problem of the inhibition of gelation due to the presence of oxygen, which usually occurs in gelcasting during the formation of polymer network gel by in situ polymerization of monomers in air. The mechanism of PVP in eliminating the surface spallation problems of green bodies is also discussed.     

Mathematical modelling of enzymatically cross-linked polymer–phenol conjugates using deterministic and stochastic methods

In this study, two different modelling approaches, namely, a deterministic and a stochastic one, are developed to model the enzymatic cross-linking of polymer–phenol conjugates. A comprehensive kinetic mechanism is postulated to describe the elementary reactions in the cross-linking of polymer–phenol chains in the presence of the horseradish peroxidase (HRP)–H₂O₂ initiation system. In the first approach, a moments-based model is derived to account for the conservation of all molecular species and leading moments of the number chain length distribution (NCLD) in the reactive system. In the second approach, a stochastic Monte Carlo kinetic model is formulated to follow the time evolution of a sample of cross-linkable polymer chains and calculate the weight chain length distribution (WCLD). From the numerical solutions of both models, the dynamic evolution of the concentrations of all the reactive species, the gelation onset time, the sol and gel mass fractions as well as the number and weight average molecular weights of the cross-linkable polymer chains are calculated. The two derived models are validated using experimental kinetic measurements on the enzymatic cross-linking of tyramine-modified hyaluronic acid and carboxymethyl-chitin. It is shown that both models can accurately predict the gelation onset time of the two cross-linkable systems over a wide range of variations in HRP and H₂O₂ concentrations. Finally, the MC model predictions on the weight average number of polymer chains in the cross-linked molecules are compared to Flory's analytical solution on the tetrafunctional cross-linking of polymer chains of uniform length.     

Influence of the multi-functional epoxy monomers structure on the electro-optical properties and morphology of polymer-dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation method with nematic LC content as low as 40 wt%, and the electro-optical properties were carefully investigated. To accomplish this, the structure of multi-functional curable epoxy monomers with different composition feed ratios and the weight percentages of the two groups were examined in this study. The combined effects of heat-curable monomers’ structure on the conspicuous morphology of polymer network of PDLC films formed small holes and suitably distributed coin-like networks in both groups A and B, respectively. The detailed characteristics and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device parameter tester, UV-Vis-NIR spectrophotometer and scanning electron microscope. Meanwhile, the enhanced curing temperature effects on the alkyl chain length, short flexible chain length, and rigid chain segment containing epoxy monomers structure on the increasing morphology of polymer network as well as electro-optical properties of PDLC films were also studied. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure and composition ratio of curable epoxy monomers, and then the electro-optics of the PDLC films could be optimized, which is beneficial for decreasing the total LC content in PDLC devices.     

Prediction of mean square radius of gyration of tree-like polymers by a general kinetic approach

This paper describes a kinetic method to predict the z-average molecular mean square radius of gyration of tree-like polymers formed by irreversible reactions, assuming Gaussian chains. It is based on the population balance equations for the two-sided molecular distributions of pendant chains associated with every chemically distinguishable kind of bonds. An automated method for the solution of those equations is valid both before as well as after gelation for complex kinetic schemes. Examples of its use are presented with polycondensation systems leading to hyperbranched polymers, the anionic polymerization of mono- and divinyl monomers and a radical polymerization with terminal branching and transfer to polymer.     

Gelcasting Performance of Alumina Aqueous Suspensions with Glycerol Monoacrylate: A New Low-Toxicity Acrylic Monomer

Gelcasting has been extensively studied and applied as a near-net-shaping technique for preparing complex ceramic parts. However, the use of acrylamide as a main component is an important drawback of this method as it is a hazardous, neurotoxic compound. In this context, a new low-toxicity monomer, glycerol monoacrylate, has been synthesized and successfully used for the gelcasting of ceramic suspensions. The presence of two hydroxyl groups in a monomer molecule allows the formation of a polymer network by the monomer polymerization and cooperative hydrogen bonding without any external crosslinking agent. The stability and rheological behavior of alumina gelcasting suspensions containing the new monomer has been studied by oscillatory measurements under the controlled deformation mode. The results were compared with those obtained for commercially available monomers, 2-hydroxyethyl acrylate and diethylene glycol diacrylate used as a crosslinking agent, in which case the effect of an initiator and activator was studied. The gelcasting performance for both the commercial and the newly synthesized monomers was studied and compared, showing that the former allows a faster gelation, whereas the latter leads to more uniform microstructures and higher sintered densities of up to 99% of TD.     

GEORDNETE EPOXIDNETZWERKE AUS 4-GLYCIDYLPHENYL-4-GLYCIDYL-BENZOAT und 3- BZW. 4-AMINOPHENYL-4-AMINOBENZOAT

The introduction of mesogenic groups in main- and sidechains of epoxy thermosets results in an ordered multiphase network with cellular structure, if gelation occurs below the maximal cure temperature T_max. T_max is individual for a given combination of monomers. The multiphase network consists of relatively soft anisotropic cell nuclei and hard isotropic cell walls. If gelation occurs above T_max single phase networks are obtained. The size of the cell nuclei strongly depends on the used monomers and varies up to two orders of magnitude. The multiphase structure was found to have no impact on the tensile elastic modulus.     

Spontaneous gelation of poly(oligo-oxyethylene methacrylate)

Summary The spontaneous gelation of poly(oligo-oxyethylene methacrylate) was studied. When the solvent of the polymer solution was removed by freeze drying or evaporation under atmosphere, spontaneous gelation occurred. This gelation is caused by the radical generation and chain transfer reaction, which is evidenced by the prevention of gelation during freeze drying and evaporation under atmosphere with adding radical inhibitor into the polymer solution. The soluble residue of the crosslinked gel formed during freeze drying was found to be poly(ethylene glycol) which is the side chain of the polymer. This fact indicates that the cleavage of the side chain accompanies the gelation. The possible mechanism of the gelation is proposed with the idea that induced stress by solvent removal would cause the cleavage of the side chain and the generated radical is transferred to the other chain resulting in gelation.     

Computing thermomechanical properties of crosslinked epoxy by molecular dynamic simulations

This paper reports the use of molecular dynamics simulations to study the thermomechanical properties of an epoxy molding compound formed by curing tri/tetra-functionalized EPN1180 with Bisphenol-A. An interactive crosslinking-relaxation methodology is developed to construct the simulation cell. This crosslinking-relaxation methodology allows the construction of highly crosslinked polymer network from a given set of monomers. Based on this computational algorithm, three-dimensional simulation cells can be constructed. By using an existing polymer consistent force-field, several thermomechanical properties of the model epoxy are computed such as the curing induced shrinkage, gelation point, coefficient of thermal expansion, glass transition temperature, Young's modulus and Poisson's ratio. The dependence of these properties on crosslink density and temperature is also investigated. Simulated results are compared with existing theoretical or experimentally measured values when available. Good agreements are observed.     

Synthesis and Gelation Characteristics of Photo-Crosslinkable Star Poly(ethylene oxide-co-lactide-glycolide acrylate) Macromonomers

Viability of encapsulated cells in situ crosslinkable macromonomers depends strongly on the minimum concentration of polymerization initiators and monomers required for gelation. Novel 4-arm poly(ethylene oxide-co-lactide-glycolide acrylate) (SPELGA) macromonomers were synthesized and characterized with respect to gelation, sol fraction, degradation, and swelling in aqueous solution. SPELGA macromonomers were crosslinked in the absence of N-vinyl-2-pyrrolidone (NVP) monomer to produce a hydrogel network with a shear modulus of 27±4 kPa. The shear modulus of the gels increased by 170-fold as the macromonomer concentration was increased from 10 to 25 wt%. Sol fraction ranged between 8-18%. Addition of only 0.4 mol% NVP to the polymerization mixture increased modulus by 2.2-fold from 27±4 (no NVP) to 60±10 kPa. The higher modulus was attributed to the dilution effect of polymer chains in the sol, by delaying the onset of diffusion-controlled reaction, and cross-propagation of the growing chains with network-bound SPELGA acrylates. Degradation of SPELGA gels depended on water content and density of hydrolytically degradable ester groups.     

Synthesis and properties of prepolymers and their siloxane analogues by thiol‐ene polyaddition of limonene with dithiols

This work presents a synthesis of monomers from a natural compound (limonene) and dimethylsiloxanes with various chain lengths by hydrosilylation. Based on these monomers, prepolymers with various lengths of the siloxane and methylene parts, as well as their analogues without siloxane moieties, were obtained by thiol‐ene polyaddition with ethanedithiol, butanedithiol and hexanedithiol. Their physicochemical characteristics, namely the thermal stability, phase behaviour and intrinsic viscosity of the polymers obtained, were compared as a function of the length of the methylene spacer and the siloxane part in the polymer chain. © 2019 Society of Chemical Industry     

Kinetics of ‘living’ radical polymerizations of multifunctional monomers

The polymerizations of multifunctional monomers with an iniferter, p-xylylene bis-(N,N-diethyldithiocarbamate) (XDT), were studied with differential photocalorimetry. The iniferter was used to simulate a ‘living’ radical polymerization. The normal autoacceleration behavior that is often observed in the polymerization of multifunctional monomers was not evident in the reactions with the iniferter. The reversible reaction between the propagating polymer chain and the sulfur radical from the XDT molecule dominated the reaction and drastically decreased the rate of polymerization. Various parameters of the reaction (length between functional groups in the monomer, iniferter concentration, UV light intensity, and temperature) were investigated to determine the effect on the polymerization. The rate of polymerization and the final conversion of the polymer were increased significantly by increasing the temperature, initiator concentration and UV light intensity, and decreasing the length between functional groups. Mechanical properties of the resulting polymers were also examined. It was concluded that the presence of the iniferter during the polymerization of the multifunctional monomers had no effect on the heterogeneity of the polymer network. This was in agreement with previous modeling results.     

Preparation of ultrahigh molecular weight polyacrylonitrile and its terpolymers

The synthesis of high molecular weight (in excess of 10⁶ million Daltons) poly(acrylonitrile) and poly(acrylonitrile-co-methylacrylate-co-itaconic acid) is described. An inverse emulsion polymerization formulation with AIBN as the initiator was used. However, polymer precipitation occurred early in the polymerization. In each case, the molecular weight distribution was surprisingly narrow (M?_w/M?_n ～ 1.5). Conversion vs. time plots with monomers containing the inhibitor had the “S” shape typical of emulsion polymerizations. The terpolymer composition and molecular weight were quite uniform throughout the polymerization. With inhibitor-free monomers, the initial molecular weights were very high (～ 3 × 10⁶ Daltons), but gelation occurred at ca. 50% conversion. There was an inverse relationship between the monomer inhibitor content and the polymer molecular weight. It is suggested that the growing polymer radicals are occluded in the precipitated polymer particles and are terminated by inhibitor diffusing into the particles, accounting for the narrow molecular weight distribution. © 1995 John Wiley & Sons, Inc.     

Structural and free volume characterization of sol–gel organic–inorganic hybrids,obtained by co-condensation of two ureasilicate stoichiometric precursors

In this study two hybrid organic–inorganic ureasilicate monomers with different length of polymer segments were chosen for preparation of sol–gel material that includes two moieties blended on the molecular scale. The first monomer was obtained by crosslinking a double terminated polyoxyalkyleneamine with an isocyanate modified silicone ethoxide and the second one – by crosslinking between the isocyanate modified silicone ethoxide with an amino modified silicone ethoxide. Sol–gel route was applied for transformation of the liquid monomer blends in transparent materials with varying degree of rigidity. The prepared samples were characterized by small-angle X-ray scattering, Fourier-transform infrared spectroscopy, positron annihilation lifetime spectroscopy, and swelling experiments. The results demonstrate that the structure of the obtained materials could be tuned by a simple variation of molar fraction ratio between these two monomers. This makes it possible to obtain nanostructured materials with predictable properties.     

Aspects of crosslinking sulfonated polyacrylamides from rheological studies on their gels

Crosslinking of sulfonated polyacrylamide by zirconium requires some acrylate sites on polymer chain, and a schematic reaction is proposed to describe this process. Moreover, gelation kinetics is found to be fairly proportional to polymer acrylate site content. On the other hand, decreasing sulfonation level or solvent salt concentration is shown to result in increasing the system gelation, and an explanation of such behavior is attempted. Furthermore, we find that gelation kinetic is highly dependent on zirconium concentration in an unusual manner. So, for a given polymer sample, a maximum of zirconium concentration exists above which gelation kinetic decreases until it vanishes. This behavior is interpreted by existence of a background reaction where highly stable zirconium complexes are formed. This reaction indeed competes against the polymer cross-linking process and zirconium concentration increases. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1067–1072, 1997     

Effect of monomer type and dangling end size on polymer network synthesis

The design of novel biomaterials for applications in biological recognition, drug delivery, or diagnostics requires a judicious choice of preparation conditions and methods for the production of well‐characterized 3‐dimensional structures, preferably by benign processes. In this work, the polymerization of poly(ethylene glycol) (PEG) methacrylates was examined by kinetic gelation modeling and kinetic analysis in order to ascertain the factors affecting the resulting structure. The kinetics of the polymerization and structure of the final polymer network are strongly affected by the length of the PEG graft chain. The propagation of the polymer chains becomes increasingly diffusion limited with the incorporation of longer PEG grafts. In addition, a more heterogeneous network consisting of numerous microgel regions is produced as the length of the PEG graft is increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3506–3519, 2003     

一种新型缓凝剂的制备及其在水泥基复合材料中的应用

针对目前固井使用的缓凝剂存在的热稳定性能不好、异常胶凝现象（结块）、水泥石抗压强度减弱、水泥浆综合性能欠佳等问题,通过水溶液自由基聚合,研制了一种新型高温缓凝剂GWR-1,并对其热稳定性能进行了评价,同时,对加入4种缓凝剂GWR-1、GWH-1、HX-400、DRH-200L的水泥浆异常胶凝现象和综合性能做了比较。微观结构分析表明：各单体参与了反应,分子量分布较为均一,分子结构高温下稳定,GWR-1的热稳定性可达350℃。GWR-1在硅酸盐水泥基复合材料中应用性能测试结果表明：加入GWR-1水泥浆的稠化温度可达160℃,高温时加量不敏感,高温抗盐性能较好,加入GWR-1的水泥浆在150℃不会发生异常胶凝现象,且其硅酸盐水泥石的强度发展良好,低密度和高密度的水泥浆的综合性能优于加入其他3种缓凝剂的水泥浆。