Figure 1.
Conceptual design of patterning with frontal polymerization. (a) Complex patterns in nature that enable functional properties: (i) microstructure of a glass sea sponge spicule (adapted with permission from ref (1). Copyright 2005 Science), (ii) textured surface of a fingertip (photograph provided by Travis Ross, Imaging Technology Group, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana–Champaign), (iii) adult emperor angel fish (adapted with permission from Florent Charpin, Copyright 2020, reefguide.org), and (iv) spots of a cheetah (adapted with permission from Brian Jones, Copyright 2018, flickr.com). (b) Equation 1: coupled reaction and thermal transport (only diffusion considered for simplicity) inherent to frontal polymerization, where T, α, and λ represent the temperature, extent of reaction, and thermal diffusivity of the resin, respectively. Equation 2: ratio of power density generated by reaction (PR) and spread by thermal transport (PT) during frontal polymerization. (c) Computed thermal profiles of frontal polymerization with different values of φ. The inner and outer radii of the circular area are 0.5 mm and 5 mm, respectively. (d) Feedback mechanism for spontaneous patterning during frontal polymerization via competition between thermal transport and chemical reaction. Using the heat generated by the reaction (Hr), thermal transport spontaneously heats unreacted monomer, activating the initiator toward polymerization. Once the polymerization reaction consumes the available monomer within the activated zone, the rate of heat release decreases, inhibiting further activation. Competition between reaction and transport generates thermal patterns that are exploited for material property variations. (e, f) Frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (e) and 1,5-cyclooctadiene (f) by a thermally activated ruthenium catalyst (Grubbs 2nd generation, GC2) inhibited by tributyl phosphite.