Holewinski Adam, Sakwa-Novak Miles A, Carrillo Jan-Michael Y, Potter Matthew E, Ellebracht Nathan, Rother Gernot, Sumpter Bobby G, Jones Christopher W
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology , Atlanta, Georgia 30332, United States.
Chemical and Biological Engineering, University of Colorado , Boulder, Colorado 80309, United States.
J Phys Chem B. 2017 Jul 13;121(27):6721-6731. doi: 10.1021/acs.jpcb.7b04106. Epub 2017 Jun 23.
Composite gas sorbents, formed from an active polymer phase and a porous support, are promising materials for the separation of acid gases from a variety of gas streams. Significant changes in sorption performance (capacity, rate, stability etc.) can be achieved by tuning the properties of the polymer and the nature of interactions between polymer and support. Here we utilize quasielastic neutron scattering (QENS) and coarse-grained molecular dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), both in bulk form and when supported in a mesoporous silica framework. The polymer chain dynamics (rotational and translational diffusion) are characterized using two neutron backscattering spectrometers that have overlapping time scales, ranging from picoseconds to a few nanoseconds. Two modes of motion are detected for the PEI molecule in QENS. At low energy transfers, a "slow process" on the time scale of ∼200 ps is found and attributed to jump-mediated, center-of-mass diffusion. A second, "fast process" at ∼20 ps scale is also found and is attributed to a locally confined, jump-diffusion. Characteristic data (time scale and spectral weight) of these processes are compared to those characterized by MD, and reasonable agreement is found. For the nanopore-confined PEI, we observe a significant reduction in the time scale of polymer motion as compared to the bulk. The impacts of silica surface functionalization and of polymer fill fraction in the silica pores (controlling the portion of polymer molecules in contact with the pore walls), are both studied in detail. Hydrophobic functionalization of the silica leads to an increase of the PEI mobility above that in native silanol-terminated silica, but the dynamics are still slower than those in bulk PEI. Sorbents with faster PEI dynamics are also found to be more efficient for CO capture, possibly because sorption sites are more accessible than those in systems with slower PEI dynamics. Thus, this work supports the existence of a link between the affinity of the support for PEI and the accessibility of active sorbent functional groups.
由活性聚合物相和多孔载体形成的复合气体吸附剂，是从各种气流中分离酸性气体的有前景的材料。通过调整聚合物的性质以及聚合物与载体之间相互作用的性质，可以实现吸附性能（容量、速率、稳定性等）的显著变化。在此，我们利用准弹性中子散射（QENS）和粗粒度分子动力学（MD）模拟来表征此类材料中最常报道的聚合物聚乙二胺（PEI）的动态行为，包括其本体形式以及负载在介孔二氧化硅骨架中的情况。使用两台具有重叠时间尺度（从皮秒到几纳秒）的中子背散射光谱仪来表征聚合物链动力学（旋转和平动扩散）。在QENS中检测到PEI分子的两种运动模式。在低能量转移时，发现了一个时间尺度约为200皮秒的“慢过程”，并归因于跳跃介导的质心扩散。还发现了一个约20皮秒尺度的第二个“快过程”，并归因于局部受限的跳跃扩散。将这些过程的特征数据（时间尺度和光谱权重）与MD表征的数据进行比较，发现了合理的一致性。对于纳米孔限制的PEI，我们观察到与本体相比，聚合物运动的时间尺度显著减小。详细研究了二氧化硅表面功能化以及聚合物在二氧化硅孔中的填充分数（控制与孔壁接触的聚合物分子部分）的影响。二氧化硅的疏水功能化导致PEI迁移率高于天然硅醇封端的二氧化硅中的迁移率，但动力学仍然比本体PEI中的慢。还发现具有更快PEI动力学的吸附剂对CO捕获更有效，这可能是因为吸附位点比PEI动力学较慢的系统中的吸附位点更容易接近。因此，这项工作支持了载体对PEI的亲和力与活性吸附剂官能团的可及性之间存在联系。
