Molecular Materials, Department of Applied Physics, Aalto University (formerly Helsinki University of Technology), P.O. Box 15100, 00076 Aalto, Espoo, Finland.
Biomacromolecules. 2013 May 13;14(5):1547-54. doi: 10.1021/bm400185z. Epub 2013 Apr 5.
The classic nanocomposite approach aims at percolation of low fraction of exfoliated individual reinforcing nanoscale elements within a polymeric matrix. By contrast, many of the mechanically excellent biological nanocomposites involve self-assembled and space-filled structures of hard reinforcing and soft toughening domains, with high weight fraction of reinforcements. Here we inspect a new concept toward mimicking such structures by studying whether percolation of intercalated domains consisting of alternating rigid and reinforcing, and soft rubbery domains could allow a transition to a reinforced state. Toward that, we present the functionalization of rigid native cellulose nanocrystals (CNCs) by esterification with a dense hydrocarbon chain brush containing cross-linkable double bonds. Composite films with 0-80 wt % of such modified CNCs (mCNCs) within a poly(butadiene) (PBD) rubber matrix were prepared via cross-linking by UV-light initiated thiol-ene click reaction. Transmission electron microscopy showed structures at two length scales, where the mCNCs and PBD form domains having internal aligned self-assemblies of alternating hard mCNCs and soft PBD with periodicity of ca. 40 nm, and where additional PBD connects such domains. Increasing the weight fraction of mCNCs causes an uncommon abrupt transition from PBD-dominated soft materials to significantly reinforced mCNC-dominated mechanical properties, suggesting that the intercalated self-assembled mCNC/PBD domains percolate in PBD upon passing 30-35 wt % of mCNCs. Maximum stress of 16 MPa at mCNC fraction of 80 wt % was obtained. The mechanical properties of the composites show exceptional insensitivity to air humidity. The shown simple concept of percolative intercalated nanocomposites suggests searching for more general biomimetic compositions involving several deformation mechanisms for improved mechanical properties.
经典的纳米复合材料方法旨在在聚合物基体中渗透低分数的剥离的单个增强纳米级元素。相比之下，许多力学性能优异的生物纳米复合材料涉及到硬增强和软增韧域的自组装和空间填充结构，具有高分数的增强剂。在这里，我们通过研究由交替的刚性和增强、软橡胶域组成的插层域的渗透是否可以允许向增强状态转变，来检验模仿这种结构的新概念。为此，我们通过酯化作用对刚性天然纤维素纳米晶体（CNC）进行功能化，使其具有包含可交联双键的密集碳氢链刷。通过 UV 光引发的硫醇-烯点击反应交联，在聚丁二烯（PBD）橡胶基质中制备了含有 0-80wt%这种改性 CNC（mCNC）的复合膜。透射电子显微镜显示了两个长度尺度上的结构，其中 mCNC 和 PBD 形成具有内部对齐的交替硬 mCNC 和软 PBD 自组装的域，其周期性约为 40nm，并且在这种域之间存在额外的 PBD 连接。增加 mCNC 的重量分数会导致一个不寻常的从以 PBD 为主的软材料到以 mCNC 为主的显著增强的机械性能的突然转变，这表明在通过 30-35wt%的 mCNC 时，插层自组装的 mCNC/PBD 域在 PBD 中渗透。在 mCNC 分数为 80wt%时，获得了 16MPa 的最大应力。复合材料的力学性能对空气湿度表现出异常的不敏感性。所展示的简单渗透型纳米复合材料概念表明，需要寻找更通用的仿生组成，涉及几种变形机制，以提高机械性能。
