Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation

Abstract

Frontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.

Conversion Mapping by Raman Microscopy and Impact of Slice Overlap in Additive Manufacturing

“…While vat-based photocured systems offer the broadest range of optimized chemistries to obtain specific properties required for an application, consistent with the notion of mass customization, a number of fundamental limitations must also be overcome, including the limited range of available photocurable monomers. − One of the biggest challenges for these systems is the interrelationship between the cure additives and the print parameters and that often the optimum combination is different for every formulation, meaning that the inevitable balance between resolution, mechanical properties, and print speed must be optimized for every formulation and additive combination. Given the need for relatively large volumes of resin needed to fill the vat in conventional off the shelf printers, the combination of initiator, additives including fillers, dyes, UV absorbers, and the chemistry of the monomers means significant trial and error is required, which is especially difficult if some of the additives are expensive, dangerous, or difficult to obtain. − As an example, because of the inherent decrease in light intensity with depth, associated with the chemistry of initiation followed by propagation and chain growth, conversion is not constant with depth. For a 3D printing application this results in one of three scenarios: (i) the thickness of the resin layer to be added (slice thickness) is larger than the depth of cure, resulting in uncured monomer between layers; (ii) the slice thickness is much smaller than depth of cure, resulting in the z resolution being significantly greater than the slice thickness and potentially features being deposited in regions that are undesirable; and (iii) the slice thickness is minimally less than the cure depth providing some “overlap” and resulting in optimal resolution.…”

Phase separation in swelling and deswelling hydrogels with a free boundary

“…The transport of solvent into and out of the network leads to the swelling and drying of the hydrogel, thereby introducing large deformations of the polymer network. Since hydrogels are omnipresent in nature, in innumerable biological processes, but also in many smart soft-matter as well as medical applications, there have been a large number of theoretical and experimental studies aiming at understanding the dynamic behavior and pattern formation during swelling and drying processes [1][2][3][4][5][6][7][8][9]. Fundamental phenomena include the formation of a core-shell structure for the swelling of beads [1,10], or the appearance of wrinkling instabilities, such as those described in the seminal work by Tanaka et al [11] and others [1,12].…”

Photoinitiating systems and kinetics of frontal photopolymerization processes – the prospects for efficient preparation of composites and thick 3D structures

“…2). 17 During further polymerisation, the volume of the solution decreases, and the volume of solid phase is shrinking. The layers above them press the newly formed layers, and the next ones are formed slower and slower due to the hindered flow of light radiation until the polymerisation and recombination of radicals are extinguished, which is the end of the process.…”

“…51 Besides Pojman, there are also others reports of thiol-ene FPP systems. [15][16][17][55][56][57][58][59] The advantage of thiol-ene systems lies in rapid polymerisation with minimal oxygen inhibition. In addition, the depth of cure is larger than for acrylic monomers.…”