Chimie des Interactions Plasma Surfaces, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons (UMONS) , Place du Parc 23, 7000 Mons, Belgium.
ACS Appl Mater Interfaces. 2014 Aug 13;6(15):12395-405. doi: 10.1021/am502255p. Epub 2014 Jul 15.
Plasma polymer films (PPF) have increasing applications in many fields due to the unique combination of properties of this class of materials. Among notable features arising from the specifics of plasma polymerization synthesis, a high surface reactivity can be advantageously used when exploited carefully. It is related to the presence of free radicals generated during the deposition process through manifold molecular bond scissions in the energetic plasma environment. In ambient atmosphere, these radicals undergo autoxidation reactions resulting in undesired polymer aging. However, when the reactivity of surface radicals is preserved and they are put in direct contact with a chemical group of interest, a specific surface functionalization or grafting of polymeric chains can be achieved. Therefore, the control of the surface free radical density of a plasma polymer is crucially important for a successful grafting. The present investigation focuses on the influence of the hydrocarbon precursor type, aromatic vs aliphatic, on the generation and concentration of free radicals on the surface of the PPF. Benzene and cyclohexane were chosen as model precursors. First, in situ FTIR analysis of the plasma phase supplemented by density functional theory calculations allowed the main fragmentation routes of precursor molecules in the discharge to be identified as a function of energy input. Using nitric oxide (NO) chemical labeling in combination with X-ray photoelectron spectroscopy analysis, a quantitative evaluation of concentration of surface free radicals as a function of input power has been assessed for both precursors. Different evolutions of the surface free radical density for the benzene- and cyclohexane-based PPF, namely, a continuous increase versus stabilization to a plateau, are attributed to different plasma polymerization mechanisms and resulting structures as illustrated by PPF characterization findings. The control of surface free radical density can be achieved through the stabilization of radicals due to the proximity of incorporated aromatic rings. Aging tests highlighted the inevitable random oxidation of plasma polymers upon exposure to air and the necessity of free radical preservation for a controlled surface functionalization.
等离子体聚合物薄膜 (PPF) 由于这类材料的独特性能组合，在许多领域的应用日益广泛。在等离子体聚合合成的特殊性所产生的显著特性中，当仔细利用时，高表面反应性可以被有利地利用。这与在高能等离子体环境中通过多种分子键断裂在沉积过程中产生的自由基有关。在环境气氛中，这些自由基会发生自动氧化反应，导致聚合物老化。然而，当表面自由基的反应性得到保留并且它们与感兴趣的化学基团直接接触时，可以实现特定的表面功能化或聚合物链的接枝。因此，控制等离子体聚合物的表面自由基密度对于成功接枝至关重要。本研究重点研究了烃前体类型（芳烃与脂肪烃）对 PPF 表面自由基的生成和浓度的影响。苯和环己烷被选为模型前体。首先，通过密度泛函理论计算对等离子体相的原位傅里叶变换红外 (FTIR) 分析，确定了作为能量输入函数的前体分子在放电中的主要断裂途径。使用一氧化氮 (NO) 化学标记与 X 射线光电子能谱分析相结合，对两种前体的表面自由基浓度作为输入功率的函数进行了定量评估。基于苯和环己烷的 PPF 的表面自由基密度的不同演变，即连续增加与稳定到平台，归因于不同的等离子体聚合机制和由此产生的结构，如 PPF 特性研究结果所示。通过由于掺入的芳环的接近而稳定自由基，可以实现表面自由基密度的控制。老化测试突出了等离子体聚合物在暴露于空气中时不可避免的随机氧化，以及为了进行受控的表面功能化而必须保留自由基。
