The paper in Nature Communications is here: https://go.nature.com/2HjWPxq
Engineering materials in three dimensions is mature, at atomic, meso-, and macroscale. The modular building block assembly approaches, aided by advanced computational methods, have enabled manufacturing of extended 3D-frameworks, hybrid solids reaching nowadays records in porosity or complex functionality by integrating complex organic recognition moieties. On the nano- and mesoscale, templating using liquid crystalline mesophases and nanocasting has reached unprecedented degrees of precision. Additive manufacturing and digitalization boosts current production technologies on macroscale for industry.
However, will it possible in near future to engineer materials in time, i.e. control their temporal evolution to realize truly engineered 4D materials?
I am recently intrigued by this very fundamental question, as we started to unravel a series of porous materials (metal-organic frameworks) showing colossal dynamic structural deformations as a response towards molecular stimuli. Their structural transformations traverse through metastable states apparently at different rates. Kinetically hindered transitions lead to a new counterintuitive phenomenon called negative gas adsorption (NGA) resulting in the expulsion of gas from the MOF upon external pressure increase. Such materials are quite exotic as they respond with gas pressure increase (rebound) against the outer counter-bombarding gas molecules resulting in an overall pressure amplification in a closed system. The mechanical analogue is a material that expands isotropically when you press on it (negative compressibility), while all normal materials would shrink at least in one dimension under external forces. Isn’t this a strange world? Imagine you press a button or tissue, and it bounces back once you touch it? How counterintuitive!
Form an application point of view such pressure amplifying materials could be of high interest for pneumatic control systems, pneumatic dampers, and microsystems. However, the fundamental underlying principles controlling pressure amplification are unexplored so far.
Our current publication describes a first step towards rationalization of these long lived metastable states by crystal size tuning. Only crystals above a critical edge length traverse through metastable states. Their pressure amplification (as a measure of the kinetic barrier involved) increases monotonically with crystal size. Such phenomena are only observable in highly porous materials with specific surface areas exceeding 5000 m2g-1, also referred to as adsorbents with ultrahigh specific surface area.
Clearly, this contribution is only a first step towards a better understanding of dynamics in highly porous solids. The field is open! Not only for developing families of dynamic porous solids, with controlled transformation rates and dynamics. Computational simulation of dynamic materials over several lengths and time scales is a true challenge but mandatory! Moreover, the development of advanced in situ technologies to access relevant time scales will play a pivotal role to explore 4D-materials in future. There is plenty of room in time…