CuAAC-methacrylate interpenetrating polymer network (IPN) properties modulated by visible-light photoinitiation

Abstract

Interpenetrating polymer networks (IPNs) are a class of materials with interwoven polymers that exhibit unique blended or enhanced properties useful to a variety of applications, ranging from restorative protective materials to conductive membranes and hydrophobic adhesives. The IPN formation kinetics can play a critical role in the development of the underlying morphology and in turn the properties of the material. Dual photoinitiation of copper-catalyzed azide-alkyne (CuAAC) and radical mediated methacrylate polymerization chemistries enable the manipulation of IPN microstructure and properties by controlling the kinetics of IPN formation via the intensity of the initiating light. Specifically, azide and alkyne-based polyethylene glycol monomers and tetraethylene glycol dimethacrylate (TEGDMA) were polymerized in a single pot to form IPNs and the properties were evaluated as a function of the photoinitiating light intensity. Morphological differences as a function of intensity were observed in the IPNs as determined by thermomechanical properties and phase-contrast imaging in tapping mode atomic force microscopy (AFM). At moderate intensities (20 mW/cm²) of visible light (470 nm), the TEGDMA polymerization gels first and therefore forms the underlying network scaffold. At low intensities (0.2 mW/cm²), the CuAAC polymerization can gel first. The ability to switch sequence of gelation and IPN trajectory (simultaneous vs. sequential), affords control over phase separation behavior. Thus, light not only allows for spatial and temporal control over the IPN formation but also provides control over their thermomechanical properties, representing a route for facile IPNs design, synthesis, and application.

Figure 1.

Top) Monomers used in the current study [heptaethylene glycol di(azidoethyl ether) (PEG8 diazide), 1,1,1-tris(propargyl hydroxymethyl)propane (trialkyne), and tetraethylene glycol dimethacrylate (TEGDMA)]. Bottom) Potential network topologies formed assuming a more intertwined, simultaneous mechanism (bottom, left) and a more phase separated, sequential mechanism (bottom, right).

Figure 2.. Impact of intensity on IPN kinetics

a) Conversion vs. time (min) for IPN 50–50 (50 wt.% PEG8 CuAAC (orange square)- 50 wt.% TEGDMA (purple circle)) under 470 nm irradiation at 20 mW/cm² intensity after five initial minutes in dark. b) Conversion vs. Time (min) for IPN 50–50 (CuAAC (orange square)-methacrylate (purple circle)) under 470 nm irradiation at 0.2 mW/cm² intensity after five initial minutes in dark. c) Conversion of TEGDMA vs Conversion of CuAAC at the following intensities: 0.2 (gray square), 1 (light violet circle), 2 (light blue upright triangle), 10 (dark blue downward triangle), and 20 mW/cm² (blue diamond).

Figure 3.. Overall reaction mechanism for simultaneous photo-CuAAC and methacrylate network formation.

(top) In the present 470 nm light, camphorquinone (CQ) is excited to its singlet state followed by intersystem crossing (ISC) to a triplet state, which subsequently abstracts a hydrogen from trimethylaniline (TMA) to produce an initiating radical species (Init*). The initiating species can initiate radical polymerization of methacrylate (bottom, left) or reduce copper(II) to copper(I), thus initiating the copper(I)-catalyzed azide–alkyne cycloaddition reaction (CuAAC) (bottom, right). Additionally, the two reactions are coupled through an atom transfer radical polymerization process, consisting of an equilibrium between the growing radical chain and Cu(II)Cl₂/L with the chloride capped chain and Cu(I)Cl/L.

Figure 4.. IPN composition and photoinitiation intensity effect on conversion trajectory and linear mechanical properties.

a) Conversion of methacrylate vs conversion of CuAAC at compositions of IPN 25–75 (pink square), IPN 50–50 (brown diamond), and IPN 75–25 (light blue triangle) and at intensities of 20 mW/cm² (closed shape) and 0.2 mW/cm² (open shape). b) E′ (MPa) vs. Temperature (°C) curves for pure CuAAC network (orange square), pure TEGDMA network (purple circle), and IPNs formed with the following compositions and intensities: 25–75 (pink square), 50–50 (brown diamond), 75–25 (blue triangle) and 20 mW/cm² (closed shape) and 0.2 mW/cm² (open shape)

Figure 5.. tan(δ) vs. Temperature (°C) curves

for a) IPN 25–75, b) IPN 50–50, c) IPN 75–25, with the curves of the pure CuAAC (orange square) and TEGDMA (purple square) networks in each of the plots for reference.

Figure 6.. Atomic force phase-contrast micrographs in tapping mode with the corresponding phase angle distributions below.

(Top left) Phase image and (Bottom left) phase angle distribution for IPN 50–50 synthesized at 20 mW/cm², (Top right) Phase image and (Bottom right) phase angle distribution for IPN 50–50 synthesized at 0.2 mW/cm² (all images are 10 μm × 10 μm).