利用小角度X光散射探討金屬奈米粒子於 高分子混成材料之階層性組裝結構

Small Angle X-Ray Scattering Study of the Hierarchical Assembly of Metallic Nanoparticles in Polymer Hybrid

林榆喬(Lin, Yu Chiao)

指導教授 ： 陳信龍

國立清華大學/工學院/化學工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00572

摘要

本研究將乙醯丙酮鈀鹽類 (Pd(acac)2)於poly(2-vinylpyridine) (P2VP) 高分子基材中還原，利用同步輻射光源進行小角度X光散射 (SAXS)， 探討金屬鈀奈米粒子 (Pd NPs) 於基材空間中的組織行為以及奈米粒子間的作用力。完全還原後所形成的複合材料階層性結構的 SAXS 圖譜，可藉由以下兩種粒子間之可能位能的加以分析: (1) 硬核排斥 (即硬球作用力)以及 (2) 硬核排斥與吸引力位能區 (即黏硬球作用力)。分析結果顯示，吸引作用力位能為不可缺少考量之因素，我們進一步發現，奈米複合材料的相形態由三層級的特徵結構所組成:奈米粒子 (一級結構)聚集形成之區域性團簇 (二級結構)，這些團簇結構進一步聚集成更大尺度的碎形(mass-fractal) 結構 (三級結構)。本研究所建構之散射函數可用於定量解析高分子複合材料之相形態結構，特別是針對奈米粒子間的交互作用力位能之探討。
我們進一步使用同步輻射光源進行時間解析之小角度X光散射量測，探究奈米複材階層性結構之形成機制。研究結果發現，從最初鈀奈米粒子的形成到鈀奈米粒子逐漸群集並聚集形成碎形結構的過程中，其結構變化依奈米粒子所佔之總體體積分率 (overall) 共分成四個階段。階段 (1)：奈米粒子間因顆粒濃度過低 (overall(1) ≤ 0.04) 而無作用力，呈現均勻分散。當 overall 隨著化學還原反應進行而超過 0.04 後，結構變化開始進入階段 (2)。此時，分散於連續相中之奈米粒子間開始產生黏硬球作用力。當 overall 持續增加至 0.135 時，結構轉變開始進入階段 (3)。此時，複合材料以 spinodal decomposition 機制而產生相分離，形成奈米粒子富相及奈米粒子貧相。當 overall 在奈米粒子富相中超過 0.2 後，結構轉變開始進入階段 (4)。此時，數個奈米粒子開始產生局部群集，該群集進而聚集而建構出碎形維度為 2.3 之碎形網絡結構。

關鍵字

小角度X光散射 ； 階層性結構 ； 奈米複材

並列摘要

Using small angle X-tray scattering (SAXS), we elucidated the spatial organization of palladium (Pd) nanoparticles (NPs) in the polymer matrix of poly(2-vinylpyridine) (P2VP) and the nature of inter-nanoparticle interactions, where the NPs were synthesized in the presence of P2VP by the reduction of palladium acetylacetonate (Pd(acac)2). The experimental SAXS profiles of the hybrids formed after the completion of the reduction were analysed on the basis of a hierarchical structure model considering the following two types of interparticle potential: (i) hard-core repulsion only (i.e., the hard-sphere interaction) and (ii) hard-core repulsion together with an attractive potential well (i.e., the sticky hard-sphere interaction). The corresponding theoretical scattering functions, which were used for analysing the experimental SAXS profiles, were developed within the context of the Percus-Yevick closure and the Ornstein-Zernike equation in the fundamental liquid theory. The analyses revealed that existence of the attractive potential well is indispensable to account for the experimental SAXS profiles. Moreover, the morphology of the hybrids was found to be characterized by a hierarchical structure with three levels, where the primary NPs (level one), formed local clusters (level two), and these clusters aggregated to build up a large-scale mass-fractal structure (level three). The scattering function developed here is of general use for quantitatively characterizing the morphological structures of polymer/NP hybrids and in particular for exploring the interaction potential of the NPs on the basis of the fundamental liquid theory.
The time-resolved SAXS experiment using synchrotron radiation was further performed to reveal the mechanism of the hierarchical structure formation. We revealed that the structural evolution from the beginning of NP formation to the establishment of a fractal structure by the clusters of Pd NPs constituted of four distinct stages governed by the overall NP volume fraction (overall). At Stage (1), the NPs were uniformly distributed with negligible interparticle interaction due to low particle concentration (overall(1) ≤ 0.04). The structural evolution advanced to Stage (2) when overall was increased above 0.04 by chemical reduction. In this case, the NPs distributed over the matrix phase experienced the sticky hard sphere (SHS) interaction, and the hybrid was still located in the one-phase regime. Stage (3) was accessed as overall continued to increase to ca. 0.135; in this case, the hybrid underwent a phase separation through spinodal decomposition, yielding a particle-richer phase and a particle-poorer phase. When the NP volume fraction in the particle-richer phase exceeded ca. 0.2, the structural evolution entered Stage (4), where several NPs assembled to form local clusters driven by depletion interaction, and the clusters further aggregated to build up a fractal network with the mass fractal dimension of ca. 2.3.

並列關鍵字

small angle X-tray scattering ； hierarchical structure ； hybrid

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