Kinetics of colloidal alloy crystallization of binary mixtures of monodispersed polystyrene and/or colloidal silica spheres having different sizes and densities in microgravity using aircraft

 Crystal growth rates in colloidal alloy crystallization of binary mixtures of monodispersed polystyrene and/or silica spheres having different sizes and densities are studied in microgravity by parabolic flights of an aircraft. The crystal growth rates are obtained by time-resolved reflection spectroscopy with a continuous circulating-type stopped-flow-cell system. The growth rates of alloy crystallization increase substantially in microgravity up to about 1.7 times those in normal gravity, which is in contrast to the retarding microgravity effect on the crystallization of single-component spheres. The disappearance of the segregation effect in microgravity is the main cause for the enhancing effect. The absence of convection of the suspension and the lack of downward sedimentation of colloidal spheres are also important. Received: 19 July 1999/Accepted in revised form: 1 September 1999     

Microgravity experiments on colloidal crystallization of silica spheres in the presence of sodium chloride

 Rate coefficients (k) in the colloidal crystallization of monodispersed silica spheres in the presence of sodium chloride are studied in microgravity achieved by parabolic flights of an aircraft. Time-resolved reflection spectroscopy is made with a continuous circulating-type stopped-flow cell system. The k values decrease as the salt concentration increases both at 0 and 1 G and those in microgravity are smaller than those in normal gravity by 16% (maximum), especially in water and in the presence of a small amount of the salt lower than 2 × 10⁻⁶ mol/l. The rates in flight at 1 G are larger by 15% (maximum) compared with those at 1 G on the ground. The k values obtained at 0 G, 1 G in flight and 1 G on the ground agree excellently with each other for the suspensions with 3 × 10⁻⁶ and 4 × 10⁻⁶ mol/l sodium chloride. Disappearance of the downward diffusion of spheres and no convection of the suspensions are important for retardation in microgravity. Received: 20 January 2000 Accepted: 9 March 2000     

Interfacial polycondensation of nylon-6,6 in microgravity as studied by free-fall experiments

Nylon-6,6 vesicles are synthesized by the interfacial polycondensation of hexamethylenediamine and adipoyl chloride in microgravity, and their size and shape are compared with those formed in normal gravity. Two methods are applied for the nylon syntheses. First, a carbon tetrachloride solution of adipoyl chloride (AC) is sprayed into an aqueous solution of hexamethylenediamine (HD) in a reaction column from the nozzle at the bottom of the column, and inversely HD solution is sprayed into the AC solution column. Nylon vesicles containing the solvent inside are formed. In the second method, both of the reaction solutions are sprayed in an air column and the reaction proceeds by the collision of both mists. For both methods, the reaction is followed by the transmitted-light intensity of the reaction mixture. Furthermore, the products are analyzed by microscopic observation. The nylon vesicles were not completely spherical in most cases, even if formed in microgravity. However, the size and the aspect ratio of the vesicles in microgravity are always smaller than those in normal gravity.     

沉降对不同粒径聚苯乙烯胶乳球异向聚集的影响

R.Folkersma等报道了在微重力环境下2μm聚苯乙烯(PS)胶乳球的异向聚集速率有明显增大的结果，本文作者之一孙祉伟等的实验结果却与此有很大差异。为此作者在孙祉伟等的实验基础上对1,2,3μmPS以及1μm+2μmPS胶乳球混和体系的相对聚集速率进行了研究。作者对原有实验装置进行了改进，并验证了改进后的实验装置的可行性。用密度匹配法实现模拟微重力条件，用快聚集过程中浊度随时间的变化表示相对聚集速率。结果表明，重力引起的沉降对所研究体系聚集速率的影响是很小的，在实验误差范围内可以忽略。作者认为与Stein等结果显著不同的原因是二者使用的样品表面性质不同以及实验方法的差异。     

微重力下聚合物材料制备的研究进展

阐述了聚合过程对重力的敏感性和微重力下的聚合机理,介绍了微重力条件下聚合过程的一些研究成果,同时对未来的空间微重力条件下聚合物材料的发展前景进行了分析.     

Rotational diffusion of anisotropic-shaped colloidal particles in microgravity as studied by free-fall experiments

Rotational relaxation times (τ) and diffusion coefficients of ellipsoidal colloids of tungstic acid are studied in aqueous suspension in microgravity by free-fall experiments. τ values are evaluated from the relaxation traces of the optical transmittance of the suspension using the stopped-flow technique. Experimental errors at 0 G are small compared with those at 1 G, which is ascribed to lack of the movement of impurities in suspension such as quite small dust and bubbles and the convection of the suspension in microgravity. The limiting slopes of the relaxation curves in the plots of the transmittance against time at 1 G depend on the flow direction of the suspension in the flow cell, whereas those at 0 G remain zero irrespective of the flow direction. For more reliable diffusion coefficients are obtained in microgravity; however, the diffusion coefficients themselves are quite insensitive to gravity. Received: 10 November 1998 Accepted in revised form: 22 December 1998     

Seed polymerization of tetraethyl orthosilicate in the presence of colloidal silica spheres

Kinetic analyses were made for the seed polymerization of tetraethyl orthosilicate (TEOS) in the presence of colloidal silica sphere seeds by turbidity and dynamic light scattering (DLS) measurements. Transmission electron microscopy (TEM) of the spheres formed was also used. TEOS is polymerized exclusively on the surfaces of the seed spheres, their sizes ranging from 29 to 184 nm in diameter. The sphere size versus time and the cube root of the absorbance versus time from DLS and turbidity measurements agree well, especially in the beginning of the reaction. The seed polymerization starts immediately on the addition of seed spheres, though the polymerization in the absence of the seeds proceeds after a certain induction time ranging several tens of seconds to several minutes. The polymerization rates of the reaction increase when the size and/or the concentration of the seed spheres increases. The thickness of the TEOS layers formed on the seed surfaces increases as the seed size increases; this is confirmed by the TEM pictures. These results are consistent with the polymerization mechanism of the formation of small preliminary particles followed by their coalescence on the surfaces of seeds to the final large spheres coated with silica layers. Received: 25 January 2001 Accepted: 30 May 2001     

Rotational diffusion of tungstic acid colloids in microgravity as studied by aircraft experiments

 Rotational relaxation times (τ) of anisotropic tungstic acid colloids (3.24 μm in major axis) in aqueous suspension are measured in microgravity (0G), normal gravity (1G) and at 2G. The measurements at 0G and 2G are achieved by parabolic and circular flights, respectively. The limiting slopes of the relaxation curves in the plots of the transmitted light intensity against time are close to zero at 0G irrespective of the flow directions in the flow cell, whereas those at 1G and especially at 2G depend on the flow direction by the convection of the suspension and particle sedimentation. Experimental errors at the τ values at 0G are small compared with those at 1G and 2G, which is ascribed to the lack of movement of impurities in the suspension such as quite small air bubbles, which cannot be recognized with the naked eye, and the convection of the suspension in microgravity. More reliable rotational relaxation times are obtained in microgravity; however, the relaxation times themselves are quite insensitive to gravity. Theτ values observed are larger than those calculated from the particle size, which indicates the important contribution of the electrical double layers formed around the colloidal particles. Received: 22 February 2001 Accepted: 13 June 2001     

Formation of inhomogeneities in polyacrylamide gel in the course of frontal polymerization

The deformation of gas bubbles in the course of photoinduced polymerization of polyacrylamide gel in monomer solution under microgravity conditions has been studied experimentally. The use of optical methods allows one to identify the specific features of changes in the shape of gas bubbles and to correlate these changes with different stages of polymerization. At the initial stage of the polymerization reaction, the development of gel inhomogeneities is caused by reflection and scattering of the initiating light and, correspondingly, by different rates of gel formation near the surface of a gas bubble. After its contact with the polymerization front, further deformation of the bubble is caused by the inhomogeneous development of the reaction in the formed gel. Using the interference method, the characteristic dimensions of structural inhomogeneities in the formed gel induced in the vicinity of the transparent inclusion are measured.     

Seed polymerization of tetraethyl orthosilicate in the presence of anionic and cationic polystyrene colloidal spheres

Kinetic analyses are made for the seed polymerization of tetraethyl orthosilicate (TEOS) in the presence of anionic and cationic polystyrene colloidal sphere seeds by turbidity and dynamic light-scattering measurements. Transmission-electron microscopy pictures of the spheres formed are also used. The seed polymerization of TEOS is difficult to take place on the surface of anionic polystyrene spheres (44–212 nm in diameter). On the other hand, the reaction proceeds easily on the cationic polystyrene spheres. Hairy and soft surfaces of polystyrene spheres will disturb the seed polymerization. Furthermore, the electrostatic attraction between the anionic hydrolytic products of TEOS molecules and cationic polystyrene spheres plays an important role for the seed polymerization.     

Structure and dynamics of short chain molecules in disordered porous materials: a molecular dynamics simulation study

The static and dynamic properties of short polymer chains in disordered materials are studied using discontinuous molecular dynamics simulations. The polymers are modeled as chains of hard spheres and the matrix is a collection of fixed hard spheres. The simulations show that the chain size is a nonmonotonic function of the matrix concentration for all polymer concentrations. The dependence of polymer diffusion D on the degree of polymerization N becomes stronger as the matrix concentration is increased. At high matrix concentrations we observe a decoupling between translational and rotational diffusion, i.e., the rotational relaxation time becomes very large but the translational diffusion is not affected significantly. We attribute this to the trapping of a small number of polymers. Under these conditions the polymer chains diffuse via a hopping mechanism.     

Hollow spheres of aromatic polyamide prepared by reaction‐induced phase separation

Hollow spheres of aromatic polyamide are obtained by the reaction‐induced phase separation during polymerization of 5‐hydroxyisophthalic acid and 1,4‐phenylene diamine in an aromatic solvent at a concentration of 1–2% at 320 °C without stirring. The hollow sphere has a dimple hole and the diameters of the hollow spheres are 3–4 μm. The droplets are initially generated via liquid–liquid phase separation and then rigid cross‐linked network structure formed the rigid skin layer on the surface of the droplets. The solidification of the droplets occurred owing to the further polymerization in them with maintaining the morphology to form the hollow spheres. The hollow spheres exhibit outstanding thermal stability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

High-conversion polymerization. IV. A definition of the onset of the gel effect

The gel effect in free radical polymerization of vinyl monomers has been recognized as the result of the increased viscosity of the reaction solution of polymer in monomer, which causes a decrease in the rate of the termination reaction. This effect manifests itself as an increase in the rate of polymerization over that rate to be expected in its absence. Definition of the onset of the gel effect has become necessary for several purposes. Previously, it has been common to define the onset phenomenologically, i.e., in terms of the increase in the rate of polymerization. It is proposed here that the onset of the gel effect is best defined on a fundamental basis, i.e., as occurring at that conversion at which the rate of segmental diffusion of the polymeric radicals equals the rate of their translational diffusion. Experimental evidence is presented that shows that the small minima predicted by this definition do exist for both rates and degrees of polymerization. Measurements of the viscosities of solutions of polymers in their monomers suggest that the polymer concentrations at which the “chain-entanglement” phenomena are observed are the same as those for the onset of the gel effect for styrene, methyl methacrylate, and butyl methacrylate.     

Crystallization kinetics of thermosensitive colloids probed by transmission spectroscopy

The kinetics of crystallization of poly-N-isopropylacrylamide (PNIPAM) particles has been investigated using the UV-visible transmission spectroscopy. Since the particle size decreases with the increase in temperature, microgel dispersions of different volume fractions have been obtained by varying the temperature of a single sample. It is found that the rates of the change in crystallinity, the average crystallite size, and the number density of crystallites at the most rapid stage over a certain time interval at various temperatures can be described by the power-law relations. At 19 degrees C, the PNIPAM system behaves as a hard sphere system under microgravity. The hard sphere theory based on Monte Carlo simulation has been used as a reference point to compare with conventional hard spheres, soft spheres, and PNIPAM spheres.     

分散聚合法制备PVP微球的研究

以N-乙烯基吡咯烷酮(NVP)为初始单体,乙酸乙酯为分散介质,采用分散聚合法制备了分散性能良好、粒径为3～4μm的聚乙烯基吡咯烷酮(PVP)微球.考察了单体、分散剂及引发剂浓度对PVP微球的粒径、单体转化率及分子量的影响,并对PVP的结构和性能进行研究.结果表明,单体浓度增加,PVP微球粒径和分子量增大,单体转化率升高;分散剂浓度增加,PVP微球粒径变小,分子量增大,单体转化率升高;引发剂浓度增加,PVP微球粒径变大,分子量减小,单体转化率升高.与溶液聚合法相比,分散聚合法制备的PVP分子量较小且具有一定的结晶性.     

Kinetic study of the formation reaction of colloidal silica spheres by transmitted-light-intensity and dynamic light-scattering measurements

Kinetic analyses of the formation reaction of colloidal silica spheres which are synthesized from ethyl silicate (EtSi), ammonia and a trace of water in ethanol are made by the transmitted-light-intensity and dynamic light-scattering methods. Sphere size versus time profiles from the two methods agree well especially at the beginning of the reaction. The polymerization starts after a certain induction time (t _i) ranging from several tens of seconds to several minutes. t _i increases as the concentrations of NH₃, EtSi and/or H₂O decrease. The apparent rates of the reaction, v ^′ are estimated from the reciprocal periods between the intersections of the linear line with the initial and final horizontal lines in the cube root of the absorbance versus time plots. Log v ^′ increases linearly with slopes of 1, 2 and 0.5 as the logarithms of the concentrations of EtSi, NH₃ and/or H₂O increase, respectively. These results are consistent with the assumption proposed earlier that the polymerization mechanism of the formation of the small preliminary particles is followed by their coalescence to form large silica spheres. Received: 10 November 1998 Accepted in revised form: 12 January 1999     

Consideration on formation mechanism of aromatic polyamide hollow spheres prepared by reaction‐Induced phase separation

Hollow spheres of poly(1,4‐phenylene‐5‐hydroxyisophthalamide) had been obtained by the reaction‐induced phase separation during polymerization of 5‐hydroxyisophthalic acid and 1,4‐phenylene diamine in an aromatic solvent. In this study, formation mechanism of the hollow spheres was considered from the view points of eliminated small molecules, polymer structure, and cross‐linking reaction. With respect to the eliminated small molecules, water was the most desirable to form gas‐bubbles in droplets compared with methanol and acetic acid, due to the insolubility into the polymerization system. Rigid polymer structure was also needed to prepare hollow spheres owing to the rapid solidification of droplets. Further, phenolic hydroxyl group in 5‐hydroxy‐1,3‐phenylene moiety caused the ester‐amide exchange reaction to form cross‐linked skin layer in the droplets. The efficient formation of the skin layer was important to encapsulate gas‐bubbles in the droplets, resulting in the formation of hollow structure. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1966–1974     

Synthesis of nanometer-scale polymeric structures on surfaces from template assisted admicellar polymerization: a comparative study with protein adsorption

A novel method for the formation of nanometer-scale polymer structures via template assisted admicellar polymerization (TAAP) is described. Admicellar polymerization uses a surfactant layer adsorbed on a surface to localize monomer to the surface prior to polymerization of the monomer. Nanostructures are formed by restricting adsorption to the uncovered sites of an already-templated surface, in this case to the interstitial sites between adsorbed latex spheres. Unlike most other process that form polymer nanostructures, polymer dimensions can be significantly smaller than the interstitial size because of sphere-surfactant interactions. Protein adsorption in the interstitial sites of colloidal arrays was also studied for three different proteins, and the results were compared with those obtained via admicellar polymerization.     

Kinetic modeling of controlled living microemulsion polymerizations that use reversible addition–fragmentation chain transfer

Reversible addition–fragmentation chain transfer (RAFT) polymerization is a useful technique for the formation of polymers with controlled architectures and molecular weights. However, when used in the polymerization of microemulsions, RAFT agents are only able to control the polymer molecular weight only at high RAFT concentrations. Here, a kinetic model describing RAFT microemulsion polymerizations is derived that predicts the reaction rates, molecular weight polydispersities, and particle size. The model predicts that at low RAFT concentrations, the RAFT agent will be consumed early in the reaction and that this will result in uncontrolled polymerization in particles nucleated late in the reaction. The higher molecular weight polydispersity that is observed in RAFT microemulsion polymerizations is the result of this uncontrolled polymerization. The model also predicts a shift in the conversion at which the maximum reaction rate occurs and a decrease in the particle size with increasing RAFT concentration. Both of these trends are also consistent with those observed experimentally. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6055–6070, 2006     

Semi-continuous Heterophase Polymerization of n-Butyl Methacrylate: Effect of Monomer Feeding Rate

The effect of monomer feeding rate on particle size, molar masses, glass transition and tacticity of poly(n-butyl methacrylate) (PBMA) nanoparticles synthesized by semi-continuous heterophase polymerization under monomer-starved conditions is reported. Three feed rates were examined. Highly monomer-starved conditions at the two slower addition rates were confirmed by the low amount of residual monomer throughout the reaction and by the fact that the instantaneous polymerization and feeding rates became similar at later stages of the reaction. Under these conditions, polymer particles in the nanometer range (30 to 35 nm) were obtained. Glass transition temperatures are substantially higher than those reported for commercial PBMA. Polymers tacticity was determined by ¹³C-NMR spectroscopy. NMR measurements confirm that the syndiotactic content of the PBMA synthesized here is larger than those of the commercial ones made by free-radical polymerization. Molar masses are much lower than those expected from termination by chain transfer to monomer, which is the typical termination mechanism in microemulsion polymerization.