摘要
近年来,纳米球形聚电解质刷的研究得到了广泛的关注。纳米球形聚电解质刷为丰富和完善聚电解质和聚合物刷理论提供了理想的模型体系,为胶体及胶体稳定性的研究增添了新的内容,并为大分子组装理论的研究开辟了新的思路。纳米球形聚电解质刷在废水处理、生物微反应器制备、纳米金属催化剂制备,及膜催化等领域有着广阔的应用前景。
本文分别利用光引发乳液聚合和热引发乳液聚合的方法,成功地以聚丁二烯(PB)为核制备了一系列新的纳米球形聚电解质刷。该纳米球形聚电解质刷由核、壳两部分组成:核为直径320nm的聚丁二烯胶乳微球,壳为在其表面生成的长度和接枝密度可控的线性聚丙烯酸(PAA)链。研究了pH值和离子强度对纳米球形聚电解质性能的影响,并以此球形聚电解质刷为纳米反应器,通过吸附镍离子并将其还原,在聚电解质刷壳层内生成了纳米镍金属颗粒,制备出高比表面积的镍催化剂。将此纳米金属复合物应用于4-硝基苯酚催化还原反应,考察了其催化活性。主要工作如下:
(1)光引发乳液聚合法制备纳米球形聚电解质刷(photo-brush)。先在聚丁二烯(PB)胶乳颗粒的表面接枝一层光引发剂HMEM,然后利用自制紫外光反应器引发聚丙烯酸单体聚合反应,从而在PB胶乳颗粒表面原位聚合生成了PAA刷。线性PAA链可控制在0-220nm范围内。通过调节乳液的pH值和离子强度,可控制PAA链的伸缩程度,从而控制球形聚电解质刷的尺寸大小。利用强碱切断HMEM分子与PB颗粒之间的酯键,将PAA链从PB颗粒表面切除,PB核失去稳定性从乳液中浮起并在液面聚集。然后利用紫外分光光度计检测出下层清液中PAA的浓度,利用乌氏粘度计测试并计算出PAA链的分子量,结合原PB胶乳的固含率和PB核粒径的大小,从而计算出PAA刷在PB颗粒表面的接枝密度。
(2)热引发乳液聚合法制备纳米球形聚电解质刷(thermal-brush)。在PB胶乳中加入热引发剂过硫酸钾(KPS),加热至80℃。KPS受热分解引发PB颗粒表面双键产生自由基,从而引发丙烯酸单体在PB胶乳颗粒表面聚合生成PAA刷。同样,可以通过控制加入丙烯酸单体的浓度来调节PAA链的长度,此外,pH值和离子强度同样可影响PAA链的伸缩程度及球形聚电解质刷的大小。.利用修正后的Daoud-Cotton模型估算了thermal-brush的接枝密度,发现thermal-brush比photo-brush的接枝密度略高。
(3)分别用以上两种方法合成的球形聚电解质刷作为纳米反应器,吸附加入到溶液中的镍阳离子。由于静电吸附和Donnann效应,镍金属阳离子在阴离子型聚电解质刷壳层内富集。加入硼氢化钠(NaBH4)后,在无氧条件下还原镍离子,在聚电解质刷层内镍离子被还原为镍单质微粒,包裹在纳米微球的表面。制备出具有高稳定性和高比表面积的Ni-brush纳米复合物。将该Ni-brush纳米复合物作为催化剂,在水溶液体系中催化还原4-硝基苯酚,发现photo-brush负载镍纳米颗粒和thermal-brush负载镍纳米颗粒在该反应中均具有较好的催化活性,但前者比后者的催化活性更高。同时考察了催化剂含量和Ni/brush比例对表观催化反应速率常数的影响。
本文首次通过在聚丁二烯胶乳颗粒上接枝聚丙烯酸制备了新的球形聚电解质刷,丰富了球形聚电解质刷的种类。并且,第一次利用热引发乳液聚合的方法直接在聚丁二烯胶乳颗粒表面制备了聚电解质刷,大大简化了纳米球形聚电解质刷的制备方法。
In this paper, new spherical polyelectrolyte brushes consisted of a poly(butadiene) (PB) latex with diameter of 320 nm as core and chemically grafted linear poly (acrylic acid) (PAA) chains as shell were synthesized by photoemulsion polymerization and conventional emulsion polymerization respectively. The length and grafting density of PAA chains could be controlled. We studied the catalytic activity of nickel nanoparticles prepared and immobilized in spherical polyelectrolyte brushes acting as nano-reactors.
(1) SPB prepared by photo-emulsion polymerization (photo-brushes). The poly (butadiene) (PB) latex particles were coated by a thin layer of photoinitiator HMEM firstly. Then the polymerization of acrylic acid (AA) was initiated by radiation of UV light and PAA brushes were produced on the surface of PB particles. The linear PAA chains can be 0 to 220 nm by controlling the AA monomer content. The pH and ionic strength are the most important factors to affect the size of SPB, which can be determined by dynamic light scattering. The ester bond of the photoinitiator HMEM was cleaved by a strong aqueous base, so the PAA chains were removed from the surface of the particles and analyzed separately. Thus, we obtained the grafting density of PAA brushes.
(2) SPB prepared by thermal-emulsion polymerization (thermal-brushes). Adding K2S2O4(KPS) into PB latex, radicals were produced on the surface of PB particles at 80℃, which initiate the polymerization of AA. The PAA chains grafted from the surface of PB particles. The length of PAA chains can be controlled by the concentration of AA monomer. The stretching of PAA chains were affected by pH and ionic strength remarkably like photo-brushes. The improved Daoud-Cotton model was applied to estimate the grafting density of thermal-brushes, which was found to be higher than that of photo-brushes.
(3) Adding NiCl4 to photo-brush and thermal-brush latex respectively, the Ni4+ ions concentrated in polyelectrolyte brushes layer. Then Ni4+ was reduced to metallic Ni by added NaBH4, and Ni-brushes nanocomposite particles with catalytic activity were prepared. The application of Ni-brushes nanoparticles to p-nitrophenol reduction in water solution shows that both Ni-photo-brushes and Ni-thermal-brushes have catalytic activity in this reaction. And the former have a higher catalytic activity than the later.
引文
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