TEM研究苯乙烯微乳液聚合机理

用透射电镜(TEM)对苯乙烯(St)/十二烷基硫酸钠(SDS)/正戊醇/水的O/W微乳液体系在反应前、聚合过程中和反应结束后等不同时间里进行观察,并测定苯乙烯微乳液聚合前后粒径的大小,根据粒径大小的变化研究苯乙烯微乳液聚合成核机理,发现其成核位置主要是在胶束和单体微珠滴里。此外,还发现SDS/St的质量比越大,相对分子质量越低。     

表面活性单体存在下的MMA/BA乳液共聚合(Ⅱ)聚合动力学及机理

研究了表面活性单体[磺化-十二醇-烯丙基甘油-丁二酸酯钠盐(ZC-L)]的用量对MMA/BA/ZC-L乳液聚合速率和粒径的影响,用CoulterLS230型激光粒径分析仪测定聚合过程中乳液的粒径和粒径分布变化,并与MMA/BA无皂乳液聚合及十二烷基苯磺酸钠存在下的MMA/BA乳液聚合作了比较.[ZC-L]CMC时,成核机理包括均相成核和胶束成核机理,生成的粒子因吸收体系中的表面活性单体而稳定存在.     

苯乙烯微乳液种子聚合

通过观测苯乙烯微乳液种子聚合前后体系内聚合物粒子大小及其分布的变化发现,无论是γ射线还是KPS引发,聚合过程中都没有新的聚合物粒子生成。尽管聚合前体系中存在单体溶胀的胶束,但在聚合过程中这些胶束主要充当单体仓库,自己成核聚合的几率很低。由于微乳液种子聚合体系内,单体量相对较低,聚合物粒子数目很大,其聚合动力学明显不同于常规乳液种子聚合。     

结晶聚合物的辐射效应：多官能团单体对LDPE的结晶成核作用

根据x射线、电镜、DSC和凝胶分数的测试结果,讨论了多官能团单体结构对低密度聚乙烯聚集态结构的影响以及结晶度与辐射交联(或剂量)的关系。发现由于单体的存在导致低密度聚乙烯结晶度增加而结晶尺寸变小(即结晶成核效应);成核效应越大,增强辐射交联的效果也越大。添加具有高结晶成核作用的单体有利于低密度聚乙烯的辐射交联,同时也加速其结晶结构的破坏。     

结晶聚合物的辐射效应——多官能团单体对LDPE的结晶成核作用

根据x射线、电镜、DSC和凝胶分数的测试结果,讨论了多官能团单体结构对低密度聚乙烯聚集态结构的影响以及结晶度与辐射交联(或剂量)的关系。发现由于单体的存在导致低密度聚乙烯结晶度增加而结晶尺寸变小(即结晶成核效应);成核效应越大,增强辐射交联的效果也越大。添加具有高结晶成核作用的单体有利于低密度聚乙烯的辐射交联,同时也加速其结晶结构的破坏。     

细乳液自由基聚合动力学研究进展

液滴成核是细乳液聚合区别于常规乳液聚合的一个重要特征.本文在介绍细乳液聚合成核机理的基础上,重点综述了采用不同助稳定剂时,如十六醇、十六烷、聚合物助稳定剂和单体助稳定剂等,细乳液中自由基聚合的动力学特征.     

MMA/BA种子半连续乳液聚合成核过程及机理研究

引入一种不溶于水的染色剂(BL-S)作示踪剂,研究甲基丙烯酸甲酯(MMA)/丙烯酸丁酯(BA)种子半连续乳液聚合中各变量对成核过程及成核机理的影响,运用最终乳胶粒中染色剂的含量(Pdye)、最终乳胶粒子数(Npc)、胶束成核和均相成核所形成的粒子数目(Nm和Nh)等参数对聚合过程中的成核情况进行定量分析.结果发现,当引发剂浓度[I]增大时,Pdye、Nh、Nm和Nh/Npc均随之增大,同时Nm/Npc相应地减小,且Nm/Npc=-0.0262[I]+0.8833,表明均相成核随[I]的增大而增加,但胶束成核的比例减小.乳化剂浓度[E]在不同范围内对成核机理的影响不同,在[E]=0.7216×10-2mol.L-1时,体系中胶束成核和均相成核比例相等,各为50%;当[E]>0.7216×10-2mol.L-1时,随[E]增大,成核时间t1,2逐渐缩短,Nm/Npc增加,Nh/Npc减小,胶束成核所占比例大于均相成核,胶束成核逐渐上升为主要成核方式;反之当[E]<0.7216×10-2mol.L-1时,体系中胶束成核所占比例小于均相成核,均相成核为主要成核方式.当MMA的摩尔分率fMMA由0增至0.6时,Nm/Npc从80.93%下降至50%,体系中以胶束成核为主,均相成核为辅;当fMMA由0.6增至1时,Nm/Npc已降至40%,而Nh/Npc增至60%,体系中已转变成均相成核为主,胶束成核为辅.在常规和种子半连续乳液聚合中,t1,2分别为12min和6min,而Pdye变化较小,表明聚合方式只影响粒子的形成过程和成核时间的长短,对成核方式影响甚微.     

电沉积二氧化锰成核机理及其充放电性能

采用计时电流法沉积纳米MnO₂电极材料,利用Scharifker-Hills成核理论模型分析时间-电流(i-t)曲线判断了MnO₂成核机理。 对3种不同的成核方式制得的MnO₂材料进行电化学超级电容性能测试、用SEM观察了其微观形貌。 比较了不同沉积方法对沉积材料结构、电容性能的影响。 计时电流测试发现,在0.1 mol/L Mn²⁺溶液中,电势阶跃至0.365 V,初始成核符合瞬时成核机理,在0.01 mol/L Mn²⁺溶液中,电势阶跃至0.418 V,初始成核存在瞬时成核和连续成核两种不同机理,在0.5 mmol/L Mn²⁺溶液中,电势阶跃至0.515 V,初始成核则符合连续成核机理。 超级电容性能测试发现,瞬时成核下制得的MnO₂电极材料相对于另外两种成核方式得到的电极材料具有更好的电容性能,这是因为瞬时成核更易于形成多孔、纳米片(棒)状等高比表面积的沉积物,表明制备方法影响MnO₂电极材料电容性能。     

三元无皂乳液共聚合动力学及其模型的研究

以苯乙烯（ Ｓｔ） 和甲基丙烯酸甲酯（ Ｍ Ｍ Ａ） 为主单体,以丙烯酸（ Ａ Ａ） 为功能单体进行了无皂乳液批量共聚合．考察了功能单体浓度、引发剂过硫酸铵（ Ａ Ｐ Ｓ） 浓度及聚合温度对其动力学行为的影响．建立了转化率 时间关系曲线的模型函数——— Ｇａｍ ｍａ 积分函数,用它拟合了转化率 时间关系曲线,获得了聚合过程的重要特征参数,如平均成核速率（ Ｎ Ｖ） ,聚合最大速率（ Ｍ Ｖ） 和平稳期平均聚合速率（ Ａ Ｖ） 及成核结束和聚合进入完成期对应的转化率．同时对聚合速率与以上各聚合参数的关系数据进行了非线性拟合,得到了它们之间的关系式．研究发现拟合误差很小,成核结束时转化率在１５ ％ 以内,成核及聚合速率均随以上参数增大而增大,引发剂过硫酸铵在聚合过程中起决定作用．     

偏氟乙烯分散聚合的成核及初级粒径

以分散聚合的相关成核理论为基础,采用核磁共振（19F-NMR）和傅里叶红外分析（FT-IR）等分析方法研究了偏氟乙烯（VDF）分散聚合的成核机理,并确认了以低聚物成核机理为其主要形式。同时,通过实验确定了PVDF乳胶粒粒径（d）与转化率（X）、反应压力（p）、初始引发剂质量浓度（ρI,0）以及初始乳化剂质量浓度（ρS,0）有着如下关系：d∝X0.36 p1.20ρI-,00.76ρS-,00.33。在各个因素中,反应压力（或单体质量浓度）的影响程度最大。     

Microemulsion copolymerization of styrene and acrylonitrile with n‐butanol as the cosurfactant

The microemulsion copolymerization of styrene and acrylonitrile in an n‐butanol/cetyltrimethylammonium bromide/oil/water microemulsion system was studied. The solubilization sites of the two monomers were determined with an NMR technique. The results showed that the solubilization behaviors of the two monomers were quite different. Most of the styrene was solubilized in the palisade layer of the microemulsion, whereas the acrylonitrile had an equilibrium distribution in the aqueous phase and palisade layer of the microemulsion. The reactivity ratios of styrene and acrylonitrile in the microemulsion system were different from those in other media. The effect of the monomer feed composition on the copolymerization kinetics was investigated, and the mechanism of nucleation of the latex particles was examined. The experimental results showed that the copolymerization loci were changed from the microemulsion droplets to the aqueous phase when the concentration of acrylonitrile in the monomer feed reached approximately 80%; this could be further proved by the effect of the monomer feed composition on the copolymerization kinetics. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 203–216, 2005     

毛细压力法组装纳米聚合物微球的二维有序排列

通过半连续乳液聚合的方法获得了单分散性好,数均粒径为７０６９ｎｍ的苯乙烯,丙烯酸丁酯和丙烯酸（Ｓｔ／ＢＡ／ＡＡ）高分子微球,并通过毛细压力法,成功地在玻璃基体上组装成了紧密的二维有序排列点阵结构．点阵结构的形成过程可分为成核和增长两个阶段．在二维有序排列形成过程中,微粒之间的相互排斥作用可通过添加一定浓度的电解质来进行调节．组装后的二维有序排列用扫描电镜进行了观察．     

Emulsion polymerization of styrene: Nucleation studies with nonionic emulsifier

The nucleation mechanism of styrene emulsion polymerization in the presence of the nonionic surfactant Emulphogene BC-840 was investigated. It was found that the conversion-time curves of this system show two rate regions. The change in rate occurs at ca. 40% conversion. Second-stage nucleation leads to a faster rate and consequently to a bimodally dispersed latex. The particle size distribution is relatively narrow before 40% conversion. The bimodally dispersed latex shows a bimodal molecular weight distribution in gel permeation chromatography and the results from fractional “creaming” of the latex indicate that the small-size particles contain the higher-molecular-weight polymer.     

含丙烯酸的苯丙共聚乳液乳胶粒形态的电镜研究

采用透射电镜观测了苯乙烯-丙烯酸丁酯-丙烯酸乳液共聚体系的乳胶粒形态,讨论了组分含量、共聚物链柔性和乳液pH值等因素对乳胶粒形态、乳液粘度及成膜性的影响.结果表明:软单体的加入和体系pH值的提高将促进乳胶粒的粘连成膜和溶涨及部分聚合物的溶解.羧基主要富集于乳胶粒表面.     

P(AA-SA) latex particle synthesis via inverse miniemulsion polymerization-nucleation mechanism and its application in pH buffering

In this work, poly(acrylic acid-co-sodium acrylate) P(AA-SA) latex particles were prepared by inverse miniemulsion polymerization and used as a pH buffering agent for application. The polymerization was quickly initiated by a redox initiator (ammonium persulfate/sodium metabisulfite) at 0-5 degrees C. Thus the possibility of monomer dissolving in a solvent was reduced, which enhanced the degree of droplet nucleation. The effects of costabilizer and the ratio of SA/(AA+SA) in functional latex particles on the nucleation mechanism and emulsion stability were investigated. The apparent pK(a) values of the synthesized P(AA-SA) latex particles were determined by titration experiments. Their properties on pH buffering were also studied, including the pH temporal response and pH buffering ability. The results showed that sodium hydroxide, which was introduced as the costabilizer to enhance the osmotic pressure and to increase the deprotonation of acrylic acid, was effective in guaranteeing droplet nucleation predominantly. Meanwhile, the surfactant concentration was controlled to be less than its critical micelle concentration (CMC) value to avoid micellar nucleation. Furthermore, the P(AA-SA) latex particles thus synthesized were found to be an excellent material for pH buffering. The pH temporal response was very rapid and related to the crosslinking degree of the latex particles. The terminal range of pH buffering for latex particles was controllable by the ratio of SA/(AA+SA).     

Effect of hydrophilic comonomer and surfactant type on the colloidal stability and size distribution of carboxyl- and amino-functionalized polystyrene particles prepared by miniemulsion polymerization

Carboxyl and amino-functionalized polystyrene latex particles were synthesized by the miniemulsion copolymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride (AEMH). The reaction was started by using an oil-soluble initiator, such as 2,2'-azobis(2-methylbutyronitrile) (V-59). The effect of the functional monomer content and type of surfactant (non-ionic versus ionic) on the particle size and particle size distribution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). A bimodal particle size distribution was observed for functionalized latex particles prepared in the presence of the non-ionic surfactant (i.e., Lutensol AT-50) when 1 wt % of acrylic acid or 3 wt % of AEMH as a comonomer was employed. The copolymer particle nucleation was studied by using a highly hydrophobic fluorescent dye. From the obtained results, the formation of bimodal particle size distribution may be attributed to a budding-like effect, which takes place during the earlier stage of polymerization and is caused by the additional stabilizing energy originated from the ionic groups of a functional polymer. The reaction mechanism of particle formation in the presence of non-ionic and ionic surfactants has been proposed. The amount of the surface functional groups was determined from polyelectrolyte titration data.     

Synthesis of Sub-100 nm and Narrow Particle Size Distribution Cationic Latex by One-Step Emulsion Polymerization

A novel approach to synthesize narrow particle size distribution cationic latex particles based on styrene and butyl acrylate was proposed. The effect of monomer/water ratios, surfactant (cetyltrimethylammonium chloride) concentrations, and monomer compositions on the evolution of particle size, distribution, number, and morphology as a function of monomer conversion was concerned in order to confirm the optimum polymerization condition. As expected, the particle size of the ultima latex increased with monomer/water ratios and styrene contents decreased with increasing surfactant concentrations. Continuous nucleation phenomena occurred when monomer/water ratio was lesser than 30/70, resulting in a gradual increase in the number of particles in the whole polymerization process. Combined with the previous work (Colloid and Polymer Science, 2014, 292: 519–525), it was concluded that particle coagulation easily took place in cationic emulsion polymerization of styrene. Thus, the narrow particle size distribution cationic latexes with particle scale between 50 nm and 80 nm, 30 wt% solid content could be prepared in a short reaction time.     

Capillary zone electrophoresis for the characterization of latex particles

Capillary zone electrophoresis (CZE) was applied to the separation of acrylic styrene copolymer emulsion particles. Fast separations could be performed on samples containing chemically identical latex particles of different size, as well as on samples with particles of the same size but differing in chemical composition. The developed method was also used for the analysis of water soluble fractions of urethane dispersions. Additionally, the physical interaction between different particles (e.g., acrylic and urethane particles) could be studied using this method. The separation mechanism is based on the zeta potential of the particles and the relaxation effect under the applied analytical conditions.     

Pickering emulsion polymerization kinetics of styrene: Comparison of bare and surface modified SiO2 nanoparticles

A step towards the understanding of some mechanistic events occurring in the styrene Pickering emulsions, using a SiO₂ dispersion, is presented. Polymerizations at 80°C with different levels of a water soluble initiator were performed. The emulsion polymer content was ca. 15% with conversions close to 90%. With conversion and particle size measurements, the particle density was estimated for bare and surface modified SiO₂ particles. Then, the average number of radicals per particle was inferred, yielding a pseudo-bulk type polymerization. It was found that bare SiO₂ nanoparticles do not participate in the nucleation mechanism; however, they, along with the initiator, promote an enhanced oligomer coagulation. On the other hand, the hexadecyltrimethylammonium bromide modified SiO₂ nanoparticles do participate in the nucleation and coagulation mechanisms, yielding more stable and smaller poorly-covered polymer particles. This approach allowed untangling some events such as: particle nucleation, radical entry to particles, particle density, coagulation and vitreous and Trommsdorff effects.