Effect of Molecular Weight on Lateral Microphase Separation of Mixed Homopolymer Brushes Grafted on Silica Particles

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• 作者：Chunhui Bao ; Saide Tang ; Roger A. E. Wright ; Ping Tang ; Feng Qiu ; Lei Zhu ; Bin Zhao
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：October 14, 2014
• 年：2014
• 卷：47
• 期：19
• 页码：6824-6835
• 全文大小：987K
• ISSN：1520-5835

文摘

We report a systematic study on the effect of molecular weight (MW) on microphase separation of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes grafted on 172 nm silica particles. The brushes were synthesized by sequential surface-initiated atom transfer radical polymerization (ATRP) of tBA at 75 掳C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 掳C from silica particles functionalized with an asymmetric difunctional initiator bearing an ATRP initiator and an NMRP alkoxyamine. The MWs of the two polymers in each sample were controlled to be similar to each other. A series of mixed brush samples with different average MWs, from 13.8 to 33.1 kDa, but comparable overall grafting densities were made, and their microphase separation was studied by transmission electron microscopy (TEM). The TEM samples were prepared by drop-casting the dispersions of mixed brush-grafted particles in CHCl₃ and also in water (stabilized by a surfactant) and staining the brushes with RuO₄. While CHCl₃ is a good solvent for both PtBA and PS, making the brushes spread out on the carbon films of TEM grids, water is a nonselective poor solvent for the two polymers, causing the brushes to collapse uniformly on silica core. All samples exhibited lateral microphase separation, forming nearly bicontinuous rippled nanopatterns. The average ripple wavelength D increased with increasing the MW. For samples directly cast from chloroform, D scaled with MW^0.70 in the studied MW range. For uniformly collapsed mixed brushes cast from water, D was proportional to MW^0.56. The latter is close to the theoretical prediction of D 鈭?MW^0.5 for perfect Y-shaped brushes in the melt and in nonselective poor solvents. We further compared the phase separation of mixed PtBA/PS brushes grafted on silica particles and PtBA-b-PS diblock copolymers and found that the microphase separation of mixed brushes was weaker than that of diblock copolymers.