Diffusion power spectra as a window into dynamic materials architecture

Chemical recycling of commodity and specialty polymers presents a
multifaceted challenge for industrial societies. On one hand,
macromolecular architectures must be engineered to yield durable products
that, on the other hand, rapidly deconstruct to recyclable monomers under
pre-determined conditions. Polymer deconstruction is a chemical process
that requires deep understanding of molecular reactivity in heterogeneous
media, where porous material architectures evolve in both space and time.
To build this understanding, we develop herein experimental and analytical
methods describing sets of diffusive eigenmodes that exist within
time-varying, non-Euclidean boundary conditions—a situation commonly
encountered in the reactive deconstruction of polymers where chain
fragments splay, alter their local dynamics, and evolve in their
confinement of reacting media. Diffusion power spectra, discerned
experimentally by NMR, yield polymer and solvent frequency-domain velocity
autocorrelation functions that are analyzed in the context of physical
models for chemical reactions parameterized with fractal mathematics. The
results connect local motion in polymers to chemical reactivity during
acidolysis of circular elastomers.

通用与特种聚合物的化学回收，对工业社会而言是一项多维度挑战。一方面，需通过工程设计优化高分子结构以制备耐用产品；另一方面，该类产品需可在预设条件下快速解聚为可回收单体。聚合物解聚是一类化学过程，其依赖于对非均相介质中分子反应活性的深入理解——在此类介质中，多孔材料结构会随空间与时间动态演化。为构建该领域的认知体系，本文开发了一套实验与分析方法，用于描述时变非欧几里得（non-Euclidean）边界条件下存在的多组扩散本征模；这类边界条件常见于聚合物的反应解聚场景，此时链段会发生舒展、改变局部动力学特性，并在反应介质的限域环境中持续演化。通过核磁共振（NMR）实验可测得扩散功率谱，该谱能够得到聚合物与溶剂的频域速度自相关函数，后续可结合以分形数学（fractal mathematics）参数化的化学反应物理模型对其开展分析。本研究结果将聚合物的局部运动与环状弹性体酸解（acidolysis）过程中的化学反应活性关联起来。