Dupin Damien, Fujii Syuji, Armes Steven P, Reeve Paul, Baxter Steven M
Department of Chemistry, Dainton Building, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, UK.
Langmuir. 2006 Mar 28;22(7):3381-7. doi: 10.1021/la053258h.
Emulsion polymerization of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic surfactant, and a hydrophilic macromonomer, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA), at around neutral pH and 60 degrees C afforded near-monodisperse, sterically stabilized latexes at approximately 10% solids. Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied over an unusually wide range for one-shot batch syntheses. Scanning electron microscopy studies confirmed near-monodisperse spherical morphologies, with mean weight-average particle diameters ranging from 370 to 970 nm depending on the initiator, polymeric stabilizer, and surfactant concentrations. Particle sizing studies were also conducted using disk centrifuge photosedimentometry and dynamic light scattering and gave similar data. These lightly cross-linked latexes acquired cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around pH 4.1 at 1.0 wt % DVB, which is significantly lower than the pKa of 4.92 estimated for linear P2VP homopolymer by acid titration. 1H NMR and aqueous electrophoresis studies indicated that substantial swelling occurred at low pH due to protonation of the 2VP groups, while dynamic light scattering (DLS) studies indicated volumetric swelling ratios of up to 3 orders of magnitude, depending on the initial latex diameter. Systematic variation of the degree of cross-linking led to a monotonic decrease in the pKa values of the P2VP latexes (as judged by acid titration) and also the critical swelling pH (as judged by visual inspection). This was attributed to the increasingly branched nature of the P2VP chains in their swollen microgel form. Preliminary studies of the kinetics of acid-induced swelling were also conducted using the pH jump method in conjunction with a stopped-flow apparatus. These P2VP latexes swell significantly faster than P2VP latexes described in the literature and the characteristic time scales observed in the present study are much closer to those predicted by the Tanaka equation.
在二乙烯基苯(DVB)交联剂、阳离子表面活性剂和亲水性大分子单体单甲氧基封端的聚(乙二醇)甲基丙烯酸酯(PEGMA)存在下,于接近中性的pH值和60摄氏度条件下进行2-乙烯基吡啶(2VP)的乳液聚合,可得到固体含量约为10%的近单分散、空间稳定的胶乳。明智地选择合成参数,使得在一次性间歇合成中,胶乳平均直径能在异常宽的范围内变化。扫描电子显微镜研究证实了近单分散的球形形态,根据引发剂、聚合物稳定剂和表面活性剂的浓度不同,平均重均粒径范围为370至970纳米。还使用盘式离心机光沉降法和动态光散射进行了粒度研究,得到了相似的数据。正如预期的那样,这些轻度交联的胶乳在低pH值下具有阳离子微凝胶特性。在1.0 wt% DVB时,这种胶乳向微凝胶转变的临界pH值约为4.1,这明显低于通过酸滴定法估算的线性P2VP均聚物的pKa值4.92。1H NMR和水性电泳研究表明,由于2VP基团的质子化,在低pH值下会发生大量溶胀,而动态光散射(DLS)研究表明,溶胀体积比高达3个数量级,这取决于初始胶乳直径。交联度的系统变化导致P2VP胶乳的pKa值(通过酸滴定判断)以及临界溶胀pH值(通过目视检查判断)单调下降。这归因于P2VP链在其溶胀微凝胶形式中日益分支的性质。还使用pH跃变法结合停流装置对酸诱导溶胀的动力学进行了初步研究。这些P2VP胶乳的溶胀速度明显快于文献中描述的P2VP胶乳,并且本研究中观察到的特征时间尺度更接近田中方程预测的时间尺度。