A Sacrificial Route for Soft Porous Polymers Synthesized via Frontal Photo-Polymerization

Abstract

Within the very large range of porous polymers and a related immense scope of applications, we investigate here a specific route to design soft porous polymers with controlled porosity: we use aqueous-based formulations of oligomers with mineral particles which are solidified into a hydrogel upon photo-polymerization; the embedded particles are then chemically etched and the hydrogel is dried to end up with a soft porous polymeric scaffold with micron-scale porosity. Morphological and physical features of the porous polymers are measured and we demonstrate that the porosity of the final material is primarily determined by the amount of initially dispersed sacrificial particles. In addition, the liquid formulations we use to start with are convenient for a variety of material forming techniques such as microfluidics, embossing, etc., which lead to many different morphologies (monoliths, spherical particles, patterned substrates) based on the same initial material.

Keywords: hydrogels; microfluidics; photo-polymerization; porous polymers; sacrificial particles.

Figure 1

Left: (a) chemical formula of poly(vinyl alcohol) (PVA) and pictures of formulations containing calcite and PVA (concentration given above the image); (b) chemical formula of poly(sodium 4-styrenesulfonate) (PSS) and pictures of formulations containing calcite and PSS. In both cases, the mass fraction of calcite is 2%. Right: size distribution of CaCO₃ powders in water with PSS or PVA (2% w/w of particles in a 2 g·L⁻¹ PVA or PSS solution).

Figure 2

Flow curves of different PSS-stabilized formulations where the volume fraction of calcite is noticed in the figure. Some yield-stress develops between 0.1 and 0.2 calcite volume fraction. The straight lines describe a purely Newtonian fluid whereas above 0.2 volume fraction of calcite, the fluid obeys a Herschel–Buckley behavior ($\sigma ={\sigma }_y+K{\dot{\gamma }}^{\alpha }$). Insert: Yield stress ${\sigma }_y$ that develops above 0.1–0.2 volume fraction of calcite. The solid line is a quadratic fit (a guide) that suggests that the critical yield stress occurs at $\approx 0.15$.

Figure 3

Left: sketch of the FPP geometry where a thick slab containing a formulation [resin + photo-initiator (A) + particle (B)] is exposed to a collimated UV-light and thickness-dependent profile of photo-conversion of the resin develops in depth (inspired by Ref. [11], where $\phi$ denotes the degree of monomer conversion, and ${\phi }_g$ the gel point that occurs upon a given cross-link value). Right: Cured thickness $z_f$ during frontal polymerization of the PEG-DA formulations against UV dose for several cases with no calcite (open symbols, circle for no degassing, square for degassed polymer) or with different volume fractions $\varphi$ of calcite but no degassing ($\varphi =5\times {10}^{-3},1\times {10}^{-2},5\times {10}^{-2},0.1,0.2,0.3$, black, blue, red, green, yellow, cyan symbols respectively). Insert: slope of the front propagation curve $\mu {\left(\varphi \right)}^{-1}$ normalized by the one of the bare resin; the line is a model not discussed here.

Figure 4

(a) monoliths prepared with volume fractions of calcite between 0 and 0.3, observed here after photo-polymerization, calcite removal by acidic dissolution, and drying; (b) SEM views of cut samples of the same monoliths for two magnifications; (c) close up of a porous polymer with 0.3 porosity.

Figure 5

Left: Calculated densities of porous samples dried under ambient conditions (red symbols) or in an oven (blue symbols) as a function of the initial volume fraction of CaCO${}_3$. The lines are calculated using only the density ${\rho }_0$ of the bulk polymer: $\rho ={\rho }_0(1-\varphi )$. Right: Porosity ${\Phi }_M$ as a function of initial volume fraction $\varphi$ of CaCO₃ obtained from image analysis (Section 3.2.2.2, black symbols) or density measurements (Section 3.2.2.1, blue and red symbols for oven or air-dried samples); the black line represents the expected porosity assuming an isotropic drying.

Figure 6

Top: sound speed of a bare polymer ($\varphi =0$) as a function of relative humidity (RH). Main: sound speed a function of the porosity in the polymer. The open symbols refer to as the nominal volume fraction of calcite in the initial formulation; the black solid symbols show the porosity estimated from image analysis; the red solid symbols show the porosity estimated from density measurements. The gray region highlights the high porosity zone where all measurements match. The solid line is derived from the theory of effective media given by Equation (2) with no fitting parameter. Insert: close-up at high porosity.

Figure 7

Batch particles produced by manual emulsification followed by UV-curing. (A) low density of calcite (before and after removal of calcite, left and right respectively where the calcite particles show as bright spots); (B) high density of calcite after its removal.

Figure 8

Left: schematic principle view of the microfluidic setup used to produced particles. Right: SEM views of the particles after calcite dissolution and drying, and histogram of size reduction after drying.

Figure 9

Sketch of the soft embossing process with a PDMS stamp designed with micro-cavities, here cylindrical cavities on a square lattice. The stamp squeezes the formulation in the cavities and may lead to well defined patterns on a substrate after UV-curing and acidic treatment. The pictures show a small part of the patterned substrate and a magnification on one cylinder, and also the logo of our lab (LOF) soft-embossed the same way.