The dispersion of magnetic nanoparticles (NPs) in homopolymer poly(methyl methacrylate) (PMMA) and block copolymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films is investigated by TEM and AFM. The magnetite (Fe3O4) NPs are grafted with PMMA brushes with molecular weights from M = 2.7 to 35.7 kg/mol. Whereas a uniform dispersion of NPs with the longest brush is obtained in a PMMA matrix (P = 37 and 77 kg/mol), NPs with shorter brushes are found to aggregate. This behavior is attributed to wet and dry brush theory, respectively. Upon mixing NPs with the shortest brush in PS-b-PMMA, as-cast and annealed films show a uniform dispersion at 1 wt%. However, at 10 wt%, PS-b-PMMA remains disordered upon annealing and the NPs aggregate into 22 nm domains, which is greater than the domain size of the PMMA lamellae, 18 nm. For the longest brush length, the NPs aggregate into domains that are much larger than the lamellae and are encapsulated by PS-b-PMMA which form an onion-ring morphology. Using a multi-component Flory–Huggins theory, the concentrations at which the NPs are expected to phase separate in solution are calculated and found to be in good agreement with experimental observations of aggregation.
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机译：通过TEM和AFM研究了磁性纳米颗粒（NPs）在均聚物聚甲基丙烯酸甲酯（PMMA）和嵌段共聚物聚苯乙烯-b-甲基丙烯酸甲酯（PS-b-PMMA）薄膜中的分散性。磁铁矿（Fe3O4）NPs接枝了分子量为M = 2.7至35.7 kg / mol的PMMA刷。尽管在PMMA基质（P = 37和77 kg / mol）中，使用最长刷子的NP均匀分散，但是发现使用较短刷子的NP会聚集。此行为分别归因于湿刷理论和干刷理论。将NP与最短的刷子在PS-b-PMMA中混合后，铸态和退火薄膜在1 wt％处显示出均匀的分散性。但是，在10 wt％的条件下，PS-b-PMMA在退火后仍保持无序状态，并且NP聚集到22 nm的区域中，该区域大于PMMA薄片的区域大小18 nm。对于最长的刷子长度，NP聚集到比薄片大得多的域中，并被PS-b-PMMA封装，形成洋葱环的形态。使用多组分弗洛里-哈金斯理论，可以计算出NP在溶液中预计会发生相分离的浓度，并与聚集的实验观察结果非常吻合。
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