Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500AE Enschede, The Netherlands.
ACS Appl Mater Interfaces. 2017 Nov 1;9(43):37929-37940. doi: 10.1021/acsami.7b11248. Epub 2017 Oct 23.
Core-shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO-philicity. The successful synthesis of core-shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core-shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.
核壳纳米粒子由二氧化硅作为核和表面接枝的聚二甲基硅氧烷（PDMS）作为壳组成，具有不同的直径，被用作异相成核剂来获得 CO 吹塑聚甲基丙烯酸甲酯（PMMA）纳米复合泡沫。选择 PDMS 作为壳材料，因为它具有低表面能和高 CO 亲合性。成功合成核壳纳米粒子通过傅里叶变换红外光谱、热重分析和透射电子显微镜得到证实。通过扫描电子显微镜确定 PMMA 微纳米多孔材料的泡孔尺寸和泡孔密度。与未改性的二氧化硅相比，使用核壳纳米粒子显著提高了 PMMA 的成核效率。观察到的最高成核效率对于直径为 80nm 的纳米粒子约为 0.5。考虑到线张力效应，讨论了核壳纳米粒子的成核效率与粒径的关系。在细胞壁界面处，直径小于 40nm 的粒子完全被聚合物基质包裹，对应的线张力值约为 0.42nN。这种线张力显著增加了异相成核的能垒，从而降低了成核效率。在批量发泡前将 CO 饱和压力提高到 300bar 导致线张力长度增加。我们观察到在 CO 饱和至 300bar 后发泡的异相成核效率降低，这归因于在这种情况下均相成核变得更有利，而牺牲了异相成核。总的来说，研究表明必须考虑线张力对异相泡沫成核自由能垒的贡献，以理解粘弹性材料的发泡。这一发现强调需要新的策略，包括使用设计核化粒子来提高泡沫成核效率。
