Maximum Achievable Particle Size in Emulsion Polymerization: Modeling of Large Particle Sizes

A simplified model for particle formation in emulsion polymerization (comprising aqueous‐phase propagation to degrees of polymerization which may enter a pre‐existing particle and/or form new particles by homogeneous or micellar nucleation, coupled with the aqueous‐phase and intra‐particle kinetics of oligomeric radicals) is formulated to provide a model suitable for the simulation of systems containing large‐sized particles. The model is particularly useful to explore conditions for growth of large particles while avoiding secondary particle formation. Applied to the Interval II emulsion polymerization of styrene with persulfate initiator at 50°C, it is found that there is an effective maximum particle size that can be achieved if the formation of new particles is to be avoided. The parameter space of initiator concentration, particle number concentration and particle radius is mapped to show a “catastrophe” surface at the onset of new nucleation. Advanced visualization techniques are used to interpret the large number of simulations in the series, showing a maximum achievable particle diameter of around 5 μm.     

Importance of Radical Transfer in Precipitation Polymerization: The Case of Vinyl Chloride Suspension Polymerization

The importance of radical transfer between the reactive phases in precipitation polymerization processes is investigated with the vinyl chloride suspension polymerization as an example. A two‐film model that accounts for a mass transfer resistance in both the monomer‐rich and the polymer‐rich phase is constructed and applied. Equilibrium is assumed at the interphase boundary. Based on model calculations using intrinsic rate coefficients obtained by regression to experimental data the effect of accounting for radical transfer between the reactive phases on the simulated monomer conversion and total moments of the molar mass distribution is demonstrated. It is found that the effect of radical transfer between the reactive phases is most pronounced at low polymerization temperatures.

     

非离子型可聚合聚氨酯/苯乙烯的超浓乳液聚合

研究了以非离子型可聚合聚氨酯(PUAG)和苯乙烯(St)为混合单体的超浓乳液聚合, 并且考察了n(NCO)/n(OH)摩尔比、复合乳化剂体系质量浓度[E]、不同乳化剂的种类、引发剂质量浓度[I]、单体体积分数(或分散相体积分数, 也称内相比Φ)、聚合温度等因素对聚合稳定性、动力学的影响. 同时结合光相关光谱(PCS)测定了聚合物乳胶粒子大小和粒径分布, 用透射电子显微镜(TEM)观察了粒子形态, 结果表明: 当n(NCO)/n(OH)＝2∶1, T＝328 K, Φ＝80.39%, [I]＝0.8% g/g (PUAG-St), [E]＝0.22 g/mL H₂O, m(MS-1)/m(CA)＝2∶1, PVA＝0.01 g/mL H₂O时, 超浓乳液不仅有较好的聚合稳定性和较快的聚合速率, 而且粒径小分布均匀. 同时, 在此条件下的表观动力学表达式和表观活化能分别确定为R_p＝k[I]^0.50[E]^0.73[M]^0.54和 E_a＝29.7 kJ/mol. 热失重分析(TGA)进一步表明: 调节PUAG的含量可以达到对聚苯乙烯的改性, 提高聚苯乙烯的热稳定性.     

Molecular Watchmaking: ab initio Emulsion Polymerization by RAFT-controlled Self-assembly

Controlled radical polymerization using RAFT has the potential to make polymers with virtually any desired molecular architecture. For this to be implemented on an industrial scale, it must be performed by polymerization in disperse media. However, simply adding a RAFT agent to a conventional emulsion polymerization recipe leads to a loss of molecular weight control and formation of coagulum, probably because of nucleation in droplets, which is normally an unlikely phenomenon in emulsion polymerizations. Recently, a method has been devised for implementing RAFT in ab initio emulsion polymerization that avoids droplets in the particle formation stage. The molecular weight distribution of the polymer thus formed shows that molecular weight control is maintained throughout the polymerization. A model is developed to predict the particle size formed in this new type of emulsion polymerization. The new methodology enables synthesis of novel dispersions where molecular architecture can be precisely controlled, such as structured core-shell particles.     

Quantum Chemical Investigation of Secondary Reactions in Poly(vinyl chloride) Free‐Radical Polymerization

Free‐radical polymerization of vinyl chloride is investigated computationally with special attention to the secondary reactions involving mid‐chain radicals (MCRs). Namely, the rate constants of backbiting, chain scission, chain transfer, and propagation reactions are evaluated using a density functional theory method. The rate coefficients of such reactions are estimated taking into account the position of the radical along the chain as well as its distance from the chain‐end. In particular 1:5, 5:1, and 5:9 backbiting are the most relevant secondary reactions, followed by the slower propagation of MCRs. Finally, a kinetic model of suspension polymerization including the investigated reactions is developed, in order to determine their impact on the quality of the final polymer.

     

Styrene Emulsion Polymerization above the CMC: New Evidence on Particle Nucleation by means of AFFFF

Summary: The number (N) and size distribution of particles (PSD) of a styrene emulsion polymerization above the CMC were studied by means of asymmetric flow‐field flow fractionation (AF⁴). Bimodal PSDs were obtained, suggesting that coagulation of the primary particles is not as extensive as would be expected, according to the coagulative mechanism. AF⁴ allowed it to be demonstrated that N is constant during interval II, and that the resolution limit of other particle sizing techniques can lead to erroneous mechanistic inferences, from the evolution of N.

Particle size distribution measured at low conversion for the emulsion polymerization of styrene, obtained by AF⁴ and DLS. The initial surfactant (S₀), initiator (I₀) and monomer (M₀) concentrations are indicated in the figure.     

Particle Nucleation and Growth in the Emulsion Polymerization of Styrene: Effect of Monomer/Water Ratio and Electrolyte Concentration

This work is an extension of previous research results reported by our team (Colloid and Polymer Science 2013, 291: 2385-2398), where large scale and high solid content latexes of poly(n-butyl acrylate) were obtained with the particle coagulation method induced by the electrolyte. However, how to prepare controlled particle size distribution polymer latex has not been studied. Thus, in this study, the effect of the monomer/water ratios and electrolyte concentrations on particle formation and growth methods were studied by following the tracks of the evolutions of particle size, number and distribution as a function of reaction time or conversion. Experimental results showed that the length of time that particle nucleation occurred increased with increasing monomer charged for the systems without electrolyte. A point worthy of attention here is that homogeneous nucleation may occur at high monomer concentrations (30/70, 40/60). However, electrolyte added could be made the nucleation mechanism shift from micellar/homogeneous nucleation to micelle /coagulation nucleation. As a result, the final particle size distribution can be controlled by adding an appropriate electrolyte to regulate the nucleation mechanism. Spherical and uniformly sized particles could be obtained when electrolyte concentration is between 0.2 wt% and 0.4 wt% for water at the high monomer/water ratio (40/60). The effects of electrolyte concentration on nucleation mechanism mainly were expressed by decreasing the solubility of the monomer and interparticle potential, and then preventing homogeneous nucleation and enhancing particle coagulation.     

Potentials and Limitations of Automated Parallel Emulsion Polymerization

The application of automated parallel synthesizer robots for the investigation of polymerization processes is of major interest at present. In this contribution we describe the application of the emulsion polymerization of styrene and vinyl acetate. The preparations of emulsions and latexes were investigated in detail and compared to “conventional” stirred tank reactors. In particular the influence of the vortex mixing as well as the limitations regarding solid content and reactor fouling are addressed.

     

The Influence of Xanthate‐Based Transfer Agents on Styrene Emulsion Polymerization: Mathematical Modeling and Model Validation

A mathematical model was developed to account for the evolution of polymer product attributes in the emulsion polymerization of styrene. The effects of transfer agent, surfactant, initiator and temperature were investigated. Polymerization rate, and particle size decreased with increasing concentration of the transfer agent. The polymerization rate increased with increasing surfactant and initiator concentrations, while an increase in temperature led to a decrease of molecular weight but an increase of polymerization rate and particle size. Chain extension was successfully achieved in the presence of our RAFT agent. The model predictions compared well with our experimental results.

     

Suspended Emulsion Copolymerization of Acrylonitrile with Methyl Acrylate: Effects of Reaction Parameters on the Polymerization

Acrylonitrile/methyl acrylate copolymers were synthesized by suspended emulsion polymerization with water as dispersed phase and monomers as continuous phase, potassium peroxydisulphate (KPS) as initiator, Span-80 as emulsifier, and poly(vinyl alcohol) (PVA) as suspending agent. Effects of reaction parameters such as water/monomer mass ratio, concentration of initiator, polymerization temperature and agitation rate on polymerization conversion and the particle size distribution of acrylonitrile/methyl acrylate copolymers were studied. It was found that polymerization conversion increased with an increase of water/monomer mass ratio, concentration of initiator and polymerization temperature, while the agitation rate had no significant effect on the polymerization conversion. Particle size distribution became narrower with an increase of water/monomer mass ratio and agitation rate. Under the same initiator concentration and polymerization temperature, particle size distribution became wider along with polymerization time. The differential scanning calorimetry (DSC) results indicated that the peak temperature of the copolymers decreased with increasing MA content.     

Effect of Microwave Activation on Polymerization Rate and Molecular Weight Development in Emulsion Polymerization of Methyl Methacrylate

Polymerization rate and molecular weight development experimental results for the emulsion polymerization of methyl methacrylate (MMA), in the presence of potassium persulphate (PPS) as initiator, and activated with a 50 W microwave source, are reported. The polymerization kinetics of the microwave activation experiment (MA) was compared against a traditional conductive heating (CH) polymerization reaction. The number average molecular weights, M_n, of the polymer samples obtained with microwave activation were significantly higher than those obtained from conductive heating. These high values of M_n were obtained from the beginning of the polymerization reaction. Polydispersity index (PDI) values in the range of 1.18 to 1.83 were obtained in the microwave irradiated samples. These values are lower than those produced by conventional emulsion polymerization of styrene and other vinyl monomers, and resemble the PDI values obtained in controlled‐radical polymerization processes. Polymer particles of submicron size (60 to 100 nm) were obtained.     

原子转移自由基悬浮聚合制备PVC-g-PMMA共聚物

原子转移自由基悬浮聚合制备PVC-g-PMMA共聚物;聚氯乙稀;甲基丙烯酸甲酯;原子转移自由基悬浮聚合     

稀土掺杂PVC的紫外及微波交联研究

聚氯乙烯 (ＰＶＣ)是我国目前产量最大 ,应用面最广的高分子原材料。但其耐热性、稳定性及抗老化性都较差。我们考虑到使用稀土掺杂以改进其相关性能 ,同时掺入了少量从水玻璃中提取出来的活性硅醇到ＰＶＣ中以增加其柔性 ,探索了紫外辐射交联和微波辐射交联对ＰＶＣ机械力学性能的影响。本文采用ＰＶＣ流延膜来进行辐射交联并测定样品的力学性能。结果发现 ,稀土掺杂的结果使得样品的扯断伸长率大幅度提高 ,拉伸强度上升显著。而添加活性硅醇后ＰＶＣ流延膜的扯断伸长率进一步提高 ,拉伸强度则下降 ,柔性得以提高。将氯化钕溶于 95 %的乙醇…     

Small‐Particle High‐Solid‐Content Bimodal Latexes: Highly Crosslinked Small Particles as Pseudo‐Inert Nanofillers

High‐solid‐content low‐viscosity bimodal latexes with very small particles (<200 nm) are obtained using two emulsion polymerization approaches. In the first approach, the PSD is controlled by using highly crosslinked nanolatex particles (<30 nm) as seeds for the small mode to minimize their growth during the growth of the large mode. These particles are shown to grow at a volumetric rate that is 25 times lower than the larger particles. Latexes with 57 wt% solids content and viscosities <1400 mPa · s at 20 s^?1 are produced. The second approach is based on the re‐nucleation of the small particles by addition of a shot of surfactant in the presence of large particles to produce bimodal latexes. Latexes with 60 wt% solid content with viscosity of 490 mPa · s at 20 s^?1 are produced using this approach.

     

A Density Functional Theory Study of Poly (vinyl chloride) (PVC) Free Radical Polymerization

Summary: Quantum chemistry was applied to the free radical polymerization of Vinyl Chloride with the aim of elucidating the reaction kinetics and especially the formation of structural defects and low molecular weight polymer. The radical reactions were studied using the Density Functional Theory. All calculations were performed with B3LYP functionals and in particular the 6-31G(d,p) basis set was selected to evaluate the exchange and correlation energies. The computational method was first validated by predicting the rate constant of the propagation step and comparing the calculated values to experimental ones. Then intramolecular chain transfer, β-scission and branching reactions were also investigated, due to their direct connection with the production of defects in the growing chains. A comparison of the evaluated kinetic constants of such secondary reactions with other computational evaluations and experimental data was finally made.     

Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC

This research work has concerned a study on toughness of PVC/natural rubber (NR) blends compatibilized with epoxidized natural rubber (ENR). The aim of this work was to investigate the effect of degree of epoxidation on morphology and mechanical properties of the blends. Epoxidized natural rubber with a variety of epoxidation contents were prepared by reacting the NR latex with formic acid and hydrogen peroxide at various chemical contents. Chemical structure and epoxidation content of epoxidized natural rubber were evaluated by FTIR and ¹H-NMR techniques. After that, three grades of ENR with epoxidation contents of 15, 25 and 42 % (by mole) were further used for blending with PVC and NR in an internal mixer at 60 rpm and at 170 °C. From tensile and impact tests, it was found that tensile elongation and impact strength of the materials remarkably increased with degree of epoxidation. On the other hand, tensile strength and modulus of the materials rarely changed with the epoxidation content. An increase in toughness of the blends with epoxidation content was related to a better molecular interaction between PVC and ENR as suggested by torque-time curves of the materials.     

Molecular orientation behavior of poly(vinyl chloride) as influenced by the nanoparticles and plasticizer during uniaxial film stretching in the rubbery stage

The structural evolution during uniaxial stretching of poly(vinyl chloride) films was studied using our real time spectral birefringence stretching machine. The effect of clay loading and the amount of plasticizer as well as the rate effects on the birefringence development and true mechanical response are presented with a final model summarizing the molecular phenomena during stretching. Mechano‐optical studies revealed that birefringence correlated with mechanical response (stress, strain, work) nonlinearly. This was primarily attributed to the preexisting strong network of largely amorphous chains connected via small crystallites that act as physical crosslinking points. These crystallites are not easily destroyed during the high‐speed stretching process as evidenced from the birefringence–true strain curves along with the X‐ray crystallinity measurements. At high speeds, the amorphous chains do not have enough time to relax and hence attain higher orientation levels. The crystallites, however, orient more efficiently when stretched at slow speeds. Apparently, some relaxation of the surrounding amorphous chains helps rotate the crystallites in the stretching direction. Overall birefringence is higher at high stretching speeds for a given true strain value. When the nanoparticles are incorporated, the orientation levels are increased significantly for both the crystalline and amorphous phases. Nanoplatelets increase the continuity of the network because they have strong interaction with the amorphous chains and/or crystallites. This in turn helps transfer the local stresses to the attached chains and increase the orientation levels of the chains. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 724–742, 2005     

丙烯酸树脂水溶胶的乳液法合成

描述了通过乳液聚合方法合成丙烯酸树脂水溶胶的合成方法，研究了分子量、亲水基团(羧基)含量对水溶胶粒径的影响，分析了在乳液转变成水溶胶的过程中，体系粘度、透光率、表面张力以及粒子形态的变化。