Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306, USA.
Langmuir. 2012 Apr 17;28(15):6348-55. doi: 10.1021/la300482x. Epub 2012 Apr 5.
Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly(diallyldimethylammonium), PDADMA, and poly(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping, and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface.
原子力显微镜（AFM）和聚电解质多层（PEMU）的纳米压痕技术，由聚二烯丙基二甲基氯化铵（PDADMA）和聚苯乙烯磺酸盐（PSS）组成，为其表面形态和生长机制提供了新的见解。表面模量的强烈奇偶交替揭示了完全水合的多层末端为聚阳离子 PDADMA 时，对外（抗衡离子平衡）电荷补偿更大。这些模量的波动表明，与以前提出的相比，层状生长机制更加不对称。通过改变压入速率突出了 PEMU 的粘弹性特性，这些特性可能会影响细胞反应，并通过外推至零压入速率来最小化，此时表面和体相平衡模量相当。使用力映射以高分辨率探测表面组成的变化，发现表面均匀，没有相分离的证据。使用 AFM 比较终止于 PSS 和 PDADMA 的湿膜和干膜，发现 PDADMA 终止的 PEMU 对水的溶胀大得多，在干燥条件下表面粗糙度特征塌陷。动态和静态接触角测量表明，玻璃态 PSS 表面的重排较少。
