Diverging and Converging Propagation of the Polymerization Front of Cyclooctadiene for Tunable Front Velocity and Patterning.

Frontal polymerization (FP) with thermochemical instabilities provides a morphogenic manufacturing method for polymeric parts by autonomously forming patterned domains with different phases, leading to mechanical properties surpassing the homogeneous counterpart. The mechanical properties largely depend on the characteristics of the patterns. Understanding and controlling the pattern features are of vital importance to FP-based manufacturing processes. In the present study, we demonstrate that the geometry and propagating route of FP in cyclooctadiene have significant impacts on the front velocity and the pattern formed by the instability. By varying the diverging angle of the propagating route, the front velocity and the wavelength of the pattern can be typically adjusted by 25% and 50%, respectively. The tuning range on the wavelength of patterns is almost twice that with controlling merely the thermal condition. The tuning effects on the front velocity and pattern features are further validated with a parametric study. The mechanism of energy balance between the reaction and diffusion and the geometry-dependent cure kinetics are elucidated with detailed finite-element analyses. The current results and findings provide a fundamental understanding of unstable FP in various geometries and can theoretically guide the development of FP-based morphogenic manufacturing methods with adjustable material properties.