Poly[2-(methacryloyloxy)ethylphosphorylcholine] (pMPC) brushes provide extremely low friction coefficients up to high compressions, but their use as boundary lubricating layers is limited by the challenge of surface grafting of the chains. Coating by thin layers of cross-linked pMPC hydrogels may provide an attractive alternative. Here we use a surface force balance (SFB) and other techniques to examine the effect of light cross-linking (0.1% cross-linker) on the surface interactions and frictional behavior of grafted-from pMPC brushes on mica, up to physiologically high contact pressures. Atomic force microscopy, X-ray photoelectron spectroscopy, and interferometric surface-excess measurements show little difference between the non-cross-linked (linear) pMPC brushes and the cross-linked brushes prepared under otherwise identical conditions. Normal force–distance profiles between the polymer-bearing surfaces, however, reveal a marked compaction of the unperturbed thickness of the cross-linked brushes relative to linear ones, attributed to the cross-linking which limits chain swelling. The cross-linked pMPC layers exhibit very low friction (friction coefficients of order 10^–3–10^–4, depending on sliding velocities), similar to the corresponding linear brushes and due to hydration lubrication by the highly hydrated phosphocholine monomer structure. Within the range of our parameters, however, there is a marked qualitative difference in the dependence of friction on sliding velocity v_s. While for the linear brushes friction is only very weakly v_s-dependent (over 3 orders of magnitude in v_s), due to a self-regulating brush interpenetration, for the cross-linked brushes friction increases markedly with v_s (∼v_s^1/2), an effect attributed to suppression of interpenetration by the cross-links.

中文翻译：

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交联的高润滑性磷酸胆碱聚合物刷：对表面相互作用和摩擦行为的影响

聚[2-（甲基丙烯酰氧基氧基）乙基磷酰胆碱]（pMPC）刷可提供极低的摩擦系数，直至高压缩率，但由于其表面接枝的挑战，其用作边界润滑层受到限制。交联的pMPC水凝胶薄层涂层可以提供有吸引力的选择。在这里，我们使用表面力平衡（SFB）和其他技术来检查光交联（0.1％交联剂）对从pMPC刷子移植到云母上的表面相互作用和摩擦行为的影响，直至达到生理上的高度接触压力。原子力显微镜，X射线光电子能谱和干涉表面多余测量表明，非交联（线性）pMPC刷子与在其他相同条件下制备的交联刷子之间几乎没有差异。然而，在含聚合物的表面之间的法向力-距离分布显示出交联刷相对于线性刷的未扰动厚度的显着压实，这归因于交联限制了链膨胀。交联的pMPC层显示出非常低的摩擦力（摩擦系数为10左右^–3 –10 ^–4，取决于滑动速度），类似于相应的线性电刷，并且归因于高度水合的磷胆碱单体结构的水合润滑。但是，在我们的参数范围内，摩擦对滑动速度v _s的依赖性存在明显的质量差异。尽管对于线性电刷，由于自调节电刷的互穿性，其摩擦力仅非常微弱地依赖于v _s（在v _s上超过3个数量级），而对于交联电刷，摩擦力则随v _s（〜v _s）显着增加_。^1/2），这是由于交叉链接抑制互穿而产生的效果。