Polymer-dispersed liquid-crystal materials fabricated with frontal polymerization

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

Anisotropic frontal polymerization in a model resin-copper composite

This work investigates experimentally and numerically frontal polymerization in a thermally anisotropic system with parallel copper strips embedded in 1,6-hexanediol diacrylate resin. Both experiments and multiphysics finite element analyses reveal that the front propagation in the thermally anisotropic system is orientation-dependent, leading to variations in the front shape and the front velocity due to the different front-metal strip interaction mechanisms along and across the metal strips. The parameters entering the cure kinetics model used in this work are chosen to capture the key characteristics of the polymerization front, i.e., the front temperature and velocity. Numerical parametric analyses demonstrate that the front velocity in the directions parallel and perpendicular to the metal strips increases as the system size decreases and approaches the analytical prediction for homogenized systems. A two-dimensional homogenized model for anisotropic frontal polymerization in the metal-resin system is proposed.

Controllable Frontal Polymerization and Spontaneous Patterning Enabled by Phase-Changing Particles

Frontal polymerization provides a rapid, economic, and environmentally friendly methodology to manufacture thermoset polymers and composites. Despite its efficiency and reduced environmental impact, the manufacturing method is underutilized due to the limited fundamental understanding of its dynamic control. This work reports the control and patterning of the front propagation in a dicyclopentadiene resin by immersion of phase-changing polycaprolactone particles. Predictive and designed patterning is enabled by multiphysical numerical analyses, which reveal that the interplay between endothermic phase transition, exothermic chemical reaction, and heat exchange govern the temperature, velocity, and propagation path of the front via two different interaction regimes. To pattern the front, one can vary the size and spacing between the particles and increase the number of propagating fronts, resulting in tunable physical patterns formed due to front separation and merging near the particles. Both single- and double-frontal polymerization experiments in an open mold are performed. The results confirm the front-particle interaction mechanisms and the shapes of the patterns explored numerically. The present study offers a fundamental understanding of frontal polymerization in the presence of heat-absorbing second-phase materials and proposes a potential one-step manufacturing method for precisely patterned polymeric and composite materials without masks, molds, or printers.

Rapid frontal polymerization achieved with thermally conductive metal strips

Frontal polymerization, which involves a self-propagating polymerizing reaction front, has been considered as a rapid, energy-efficient, and environmentally friendly methodology to manufacture lightweight, high-performance thermoset polymers, and composites. Previous work has reported that the introduction of thermally conductive elements can enhance the front velocity. As follow-up research, the present work investigates this problem more systemically using both numerical and experimental approaches by investigating the front shape, front width, and heat exchange when aluminum and cooper metal strips are embedded in the resin. The study reveals that the enhancement in the front velocity is mainly due to a preheating effect associated with the conductive element. Moreover, the numerical parametric study for the system size shows that the front speed increases as the system size decreases, ultimately approaching a prediction provided by a homogenized model for polymer-metal composites.

Fast-switching optically isotropic liquid crystal nano-droplets with improved depolarization and Kerr effect by doping high k nanoparticles

We proposed and analyzed an optically isotropic nano-droplet liquid crystal (LC) doped with high k nanoparticles (NPs), exhibiting enhanced Kerr effects, which could be operated with reduced driving voltages. For enhancing the contrast ratio together with the light efficiencies, the LC droplet sizes were adjusted to be shorter than the wavelength of visible light to reduce depolarization effects by optical scattering of the LC droplets. Based on the optical analysis of the depolarization effects, the influence of the relationship between the LC droplet size and the NP doping ratio on the Kerr effect change was investigated.

Generation of a carbon dots/ammonium persulfate redox initiator couple for free radical frontal polymerization

We report a new redox initiator couple of ammonium persulfate (APS)/carbon dots (CDs) with high initiation efficiency used in frontal polymerization (FP).

Facile access to poly(DMAEMA-co-AA) hydrogels via infrared laser-ignited frontal polymerization and their polymerization in the horizontal direction

pH sensitive hydrogels are fabricated via infrared laser-ignited frontal polymerization (LIFP), and LIFP in the horizontal direction is employed towards dye adsorption.