Theodoly O, Jacquin M, Muller P, Chhun S
Complex Fluids Laboratory, CNRS FRE 3084, France.
Langmuir. 2009 Jan 20;25(2):781-93. doi: 10.1021/la8030254.
We investigated the spontaneous adsorption properties of charged amphiphilic diblock copolymers on hydrophobic surfaces and explained the transition of behavior from depleting frozen colloids (that do not adsorb at all) to fast adsorbing macrosurfactants when the hydrophobicity of the nonsoluble block is reduced. Three copolymer families have been used with the same hydrophilic block poly(acrylic acid), a weak acid whose ionization alpha can be varied by changing the pH. The hydrophobic blocks polystyrene, PS, poly(n-butyl acrylate), PBA, and poly(diethylene glycol ethyl ether acrylate), PDEGA, have interfacial tensions with water gammacore/solvent, respectively, of 32, 20, and 3 mN/m. We were mainly interested in the regime of high ionization alpha > 0.3, where PAA chains have no affinity for hydrophobic surfaces, and we verified experimentally that micelles do not adsorb directly. With the three copolymer families we show that the adsorption kinetics at an early stage is driven by the self-assembly properties in bulk solution: adsorption is hampered for PS-b-PAA (physically/kinetically frozen micelles in solution), controlled by unimer extraction for PBA-b-PAA (nonequilibrium micelles in solution with very low CMC < 10-4 wt %), and controlled by unimer diffusion and electrostatic repulsion for PDEGA-b-PAA (micelles at equilibrium in solution with high CMC is approximately 1-5 wt %). This explains the power law dependences of adsorption with concentration as C-1 for PBA-b-PAA and C-2 for PDEGA-b-PAA. It is finally the interfacial tension with water of the nonsoluble block and not its glass transition that is the main control of bulk solution self-assembly and consequently of the adsorption kinetics properties of amphiphilic diblocks. We also proved by preparative GPC that the fraction of non-self-assembling diblock chains, which exists in all highly hydrophobic amphiphilic diblock systems, plays a negligible role in the adsorption properties. Finally, we investigated the intrinsic thermodynamic affinity between amphiphilic diblocks and hydrophobic surfaces. We show quantitatively that this affinity depends dominantly on the interfacial energies between the hydrophobic block, the surface, and water: diblocks with strongly hydrophobic nonsoluble blocks (PS, PBA) have a low affinity for weakly hydrophobic surfaces, and oppositely, diblocks with weakly hydrophobic nonsoluble block (PDEGA) have a universal affinity for hydrophobic surfaces (like small-molecule surfactants but for different physical reasons). Finally, we showed via surface rheology that when adsorption occurs anchoring is strong and irreversible for very hydrophobic diblocks (PBA-b-PAA) and weaker and (partially) reversible for less hydrophobic diblocks (PDEGA-b-PAA).
我们研究了带电两亲性二嵌段共聚物在疏水表面的自发吸附特性，并解释了当不溶性嵌段的疏水性降低时，行为从耗尽冷冻胶体（根本不吸附）到快速吸附大表面活性剂的转变。使用了三个共聚物家族，它们具有相同的亲水性嵌段聚丙烯酸，这是一种弱酸，其电离度α可以通过改变pH值来变化。疏水嵌段聚苯乙烯（PS）、聚丙烯酸正丁酯（PBA）和聚二乙二醇乙醚丙烯酸酯（PDEGA）与水的界面张力γcore/溶剂分别为32、20和3 mN/m。我们主要关注高电离度α>0.3的区域，在该区域聚丙烯酸链对疏水表面没有亲和力，并且我们通过实验验证了胶束不会直接吸附。通过这三个共聚物家族，我们表明早期的吸附动力学受本体溶液中的自组装特性驱动：对于PS-b-PAA（溶液中物理/动力学冷冻的胶束），吸附受到阻碍；对于PBA-b-PAA（溶液中临界胶束浓度非常低<10-4 wt%的非平衡胶束），吸附由单体萃取控制；对于PDEGA-b-PAA（溶液中临界胶束浓度约为1-5 wt%的平衡胶束），吸附由单体扩散和静电排斥控制。这解释了PBA-b-PAA吸附与浓度的幂律依赖关系为C-1，PDEGA-b-PAA为C-2。最终，决定本体溶液自组装以及两亲性二嵌段吸附动力学性质的主要因素是不溶性嵌段与水的界面张力，而非其玻璃化转变温度。我们还通过制备型凝胶渗透色谱法证明，在所有高度疏水的两亲性二嵌段体系中存在的非自组装二嵌段链部分，在吸附性能中起的作用可忽略不计。最后，我们研究了两亲性二嵌段与疏水表面之间的内在热力学亲和力。我们定量表明，这种亲和力主要取决于疏水嵌段、表面和水之间的界面能：具有强疏水不溶性嵌段（PS、PBA）的二嵌段对弱疏水表面的亲和力较低，相反，具有弱疏水不溶性嵌段（PDEGA）的二嵌段对疏水表面具有普遍亲和力（类似于小分子表面活性剂，但出于不同的物理原因）。最后，我们通过表面流变学表明，当发生吸附时，对于非常疏水的二嵌段（PBA-b-PAA），锚定很强且不可逆，而对于疏水性较低的二嵌段（PDEGA-b-PAA），锚定较弱且（部分）可逆。
