Division of Chemistry and Chemical Engineering, California Institute of Technology , Pasadena, California 91125, United States.
X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
ACS Nano. 2017 Nov 28;11(11):11632-11641. doi: 10.1021/acsnano.7b06664. Epub 2017 Nov 1.
Grafting density is an important structural parameter that exerts significant influences over the physical properties of architecturally complex polymers. In this report, the physical consequences of varying the grafting density (z) were studied in the context of block polymer self-assembly. Well-defined block polymers spanning the linear, comb, and bottlebrush regimes (0 ≤ z ≤ 1) were prepared via grafting-through ring-opening-metathesis polymerization. ω-Norbornenyl poly(d,l-lactide) and polystyrene macromonomers were copolymerized with discrete comonomers in different feed ratios, enabling precise control over both the grafting density and molecular weight. Small-angle X-ray scattering experiments demonstrate that these graft block polymers self-assemble into long-range-ordered lamellar structures. For 17 series of block polymers with variable z, the scaling of the lamellar period with the total backbone degree of polymerization (d* ∼ N) was studied. The scaling exponent α monotonically decreases with decreasing z and exhibits an apparent transition at z ≈ 0.2, suggesting significant changes in the chain conformations. Comparison of two block polymer systems, one that is strongly segregated for all z (System I) and one that experiences weak segregation at low z (System II), indicates that the observed trends are primarily caused by the polymer architectures, not segregation effects. A model is proposed in which the characteristic ratio (C), a proxy for the backbone stiffness, scales with N as a function of the grafting density: C ∼ N. The scaling behavior disclosed herein provides valuable insights into conformational changes with grafting density, thus introducing opportunities for block polymer and material design.
接枝密度是一个重要的结构参数，对具有复杂结构的聚合物的物理性质有很大的影响。在本报告中，我们研究了接枝密度（z）变化对嵌段聚合物自组装的物理后果。通过通过开环易位聚合接枝，制备了跨越线性、梳状和刷状区域（0 ≤ z ≤ 1）的具有明确结构的嵌段聚合物。ω-降冰片烯基聚（d，l-丙交酯）和聚苯乙烯大分子单体与离散单体在不同的进料比下共聚，能够精确控制接枝密度和分子量。小角 X 射线散射实验表明，这些接枝嵌段聚合物自组装成长程有序的层状结构。对于具有可变 z 的 17 个系列嵌段聚合物，研究了层状周期与总主链聚合度（d* ∼ N）的标度关系。标度指数 α 随 z 的减小而单调减小，并在 z ≈ 0.2 处出现明显的转变，表明链构象发生了显著变化。比较了两个嵌段聚合物体系，一个在所有 z 处都强烈分离（体系 I），另一个在低 z 处经历弱分离（体系 II），表明观察到的趋势主要是由聚合物结构引起的，而不是分离效应。提出了一个模型，其中特征比（C）作为主链刚度的代理，与 N 作为接枝密度的函数进行标度：C ∼ N。本文揭示的标度行为为接枝密度下的构象变化提供了有价值的见解，从而为嵌段聚合物和材料设计提供了机会。
