Goodman Diane, Kizhakkedathu Jayachandran N, Brooks Donald E
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 2B5, Canada.
Langmuir. 2004 Mar 16;20(6):2333-40. doi: 10.1021/la035843t.
Using an atomic force microscope (AFM), we have investigated the interaction forces exerted by latex particles bearing densely grafted polymer brushes consisting of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA), poly(N-isopropylacrylamide) (PNIPAM), and PMEA-b-PNIPAM in aqueous media (good solvent). The brushes were prepared by controlled surface-initiated atom transfer radical polymerization, and the hydrodynamic thicknesses were measured by dynamic light scattering. The molecular weight (Mn), grafting density (sigma), and polydispersity (PDI) of the brushes were determined by gel permeation chromatography and multiangle laser light scattering after cleaving the polymer from the latex surface by hydrolysis. Force profiles of PDMA (0.017 nm(-2) < or = sigma < or = 0.17 nm-2) and PMEA (sigma = 0.054 nm-2) brushes were purely repulsive upon compression, with forces increasing with Mn and a, as expected, due to excluded volume interactions. At a sufficiently low grafting density (sigma = 0.012 nm-2), PDMA exhibited a long-range exponentially increasing attractive force followed by repulsion upon further compression. The long-range attractive force is believed to be due to bridging between the free chain ends and the AFM tip. The PNIPAM brush exhibited a bridging force at a grafting density of 0.037 nm(-2), a value lower than the sigma needed to induce bridging in the PDMA brush. Bridging was therefore found to depend on grafting density as well as on the nature of the monomer. The grafting densities of these polymers were larger than those typically associated with bridging. Bridging interactions were used to confirm the presence of PNIPAM in a block copolymer PMEA-b-PNIPAMA brush given that the original PMEA homopolymer brush produced a purely repulsive force. The attractive force was first detected in the block copolymer brush at a separation that increased with the length of the PNIPAM block.
我们使用原子力显微镜（AFM）研究了在水性介质（良溶剂）中，由聚（N，N - 二甲基丙烯酰胺）（PDMA）、聚（甲氧基乙基丙烯酰胺）（PMEA）、聚（N - 异丙基丙烯酰胺）（PNIPAM）和PMEA - b - PNIPAM组成的带有密集接枝聚合物刷的乳胶颗粒所施加的相互作用力。这些聚合物刷通过可控的表面引发原子转移自由基聚合制备，其流体动力学厚度通过动态光散射测量。通过凝胶渗透色谱法和多角度激光光散射法，在通过水解将聚合物从乳胶表面裂解后，测定了聚合物刷的分子量（Mn）、接枝密度（σ）和多分散性（PDI）。PDMA（0.017 nm⁻² ≤ σ ≤ 0.17 nm⁻²）和PMEA（σ = 0.054 nm⁻²）刷的力曲线在压缩时完全是排斥性的，正如预期的那样，由于排除体积相互作用，力随Mn和σ增加。在足够低的接枝密度（σ = 0.012 nm⁻²）下，PDMA表现出长程指数增加的吸引力，进一步压缩时则表现为排斥力。这种长程吸引力被认为是由于自由链端与AFM针尖之间的桥连作用。PNIPAM刷在接枝密度为0.037 nm⁻²时表现出桥连力，该值低于在PDMA刷中诱导桥连所需的σ值。因此发现桥连作用既取决于接枝密度，也取决于单体的性质。这些聚合物的接枝密度大于通常与桥连相关的接枝密度。鉴于原始的PMEA均聚物刷产生的是纯排斥力，桥连相互作用被用于确认嵌段共聚物PMEA - b - PNIPAMA刷中PNIPAM的存在。在嵌段共聚物刷中首次检测到吸引力时的间距随着PNIPAM嵌段长度的增加而增大。
