Article pubs.acs.org/Macromolecules
A Dual-Cure, Solid-State Photoresist Combining a Thermoreversible Diels−Alder Network and a Chain Growth Acrylate Network Gayla J. Berg, Tao Gong, Christopher R. Fenoli, and Christopher N. Bowman* Department of Chemical and Biological Engineering, University of Colorado, UCB 596, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: A Diels−Alder (DA) network containing dissolved multiacrylate monomers is demonstrated as a novel two-stage reactive polymer network, with a potential application in self-supporting stereolithography. Initially, a thermoreversible Diels−Alder “scaffold” network is formed, containing unreacted acrylate monomers and photoinitiator. During photopatterning with light at 15 mW/cm2 from a 365 nm source for 16 s of exposure at either ambient temperature or 70 °C, both acrylates and unreacted maleimides polymerize to form a permanent, covalently cross-linked network structure that simultaneously maintains the thermoreversible characteristics afforded by the underlying DA network. Light exposure of a DA network containing between 25 and 50 wt % acrylate monomer resulted in a sharp increase in cross-link density and a 60 °C jump in glass transition temperature of the material. As a result of the temperature-dependent DA equilibrium, the temperature of the film during light exposure has dramatic effects on the resulting acrylate conversion (as measured by FT-IR) and mechanical behavior (as measured by DMA) of the complex dynamic network structure. For example, despite the irreversible acrylate network, the rubbery modulus of the material decreases above the glass transition temperature due to the presence of the dynamic thermosensitive DA network. The shape of the modulus curve was also affected by the ratio of DA monomers to acrylate monomers; higher DA monomer content resulted in greater temperature sensitivity of the rubbery modulus in light-exposed films. 3D structures with feature sizes ranging from 50 to 500 μm were produced in geometries such as stacked rectangles and “logpile” structures. In the unexposed regions, free acrylate and maleimide groups were shown to tolerate temperatures as high as 120 °C with no premature gel formation observed. Removal of unexposed material during the development step was achieved at 120 °C, where the Diels−Alder equilibrium shifts toward the furan and maleimide reactants and the network depolymerizes. Finally, a process was developed for the fabrication of 3D microstructures via layer-by-layer photopatterning. The process is highly repeatable and results in complete elimination of unexposed regions. Additionally, excess quantities of the unexposed mixture may be stored at 4 °C for at least several weeks and then reused by heating to 120 °C to fully depolymerize the DA network, subsequently using the liquid mixture to make films.
■
INTRODUCTION Because of their versatility and excellent mechanical properties, polymer networks are frequently used in a number of areas of research, including shape memory polymers, biological and dental materials, tissue culture scaffolds, drug delivery, selfhealing materials, and many others.1−4 Thus, there is a constant need to precisely tune material behavior and properties to suit a particular purpose. Because of their reliability under a wide range of conditions, click reactions, as introduced by Kolb, Sharpless, and Finn in 2001, are often a natural choice when designing networks with a variety of exciting new capabilities.5 Click reactions have been used to attach various functional groups and bioactive molecules to hydrogels;6,7 they have also been used to couple linear polymer chains with high functional group conversions.8 Step-growth polymer networks based on high-conversion click reactions are of particular interest, as they tend to be more homogeneous with well-defined molecular structures.9−13 Further combining click chemistry with photo© 2014 American Chemical Society
initiation provides precise spatiotemporal control over polymer properties. Many click reactions can themselves be photoinitiated;14−17 they can also be combined with traditional chaingrowth polymerizations to produce materials with complex behavior. Nair et al.18,19 introduced the concept of “dual-cure” polymer networks by using a thiol-Michael click reaction to form the first stage step-growth polymer network and a lighttriggered, chain-growth homopolymerization of excess acrylates in the second stage. In the cited studies, a sharp increase in the rubbery modulus and glass transition temperature of the original polymer was achieved upon light exposure due to the increase in cross-linking density in the second stage. An interesting polymer network feature arises when considering the connectivity between the polymer chains Received: January 31, 2014 Revised: May 6, 2014 Published: May 12, 2014 3473
dx.doi.org/10.1021/ma500244r | Macromolecules 2014, 47, 3473−3482
Macromolecules
Article
resolution in creating such small objects, as it is not limited by nozzle diameter or powder size that characterize fused deposition modeling (FDM) and selective laser sintering (SLS), respectively.37,38 SL also confers the advantage of a large library of photo-cross-linkable monomers that may be incorporated into the photoresist, including the many acrylate, methacrylate, and epoxy monomers that are commercially available. Thus, network properties may be tuned to suit particular needs, as is particularly important in transitioning from prototyping technologies to manufacturing. In a typical SL process, a liquid monomer is selectively exposed to light to initiate a cross-linking reaction, converting the exposed portion into a permanent thermoset. Unexposed regions are later removed during development, which usually involves dissolving the unexposed material in solvent. Therefore, a significant cost and environmental consideration in stereolithographic processes is in consumption and proper disposal of development solvent. Layer-by-layer additive manufacturing comes with the inherent challenge of providing physical support for structures during their formation. Overhanging, high aspect ratio, or freely moving features must be adequately supported throughout fabrication; otherwise, feature collapse, sedimentation, or disfigurement may occur. Often, in SL processes, temporary support material must be cofabricated along with the desired object. Large quantities of support materialwhich are often required for complex objectsrepresent an additional expense, both to fabricate and to later remove. A clean removal of supports may also be difficult to achieve if the object’s features are very fragile or on the micro- or nanoscale. Freely moving structures have been formed without the use of support material using 3D single-photon34 and two-photon39 techniques. However, these methods rely on greatly increasing the viscosity of the photoresist to support the freely moving features. High-viscosity resins are more expensive to remove, which is an unfortunate trade-off in these particular selfsupporting fabrication processes. Previously, a thermoreversible Diels−Alder (DA) network was employed to avoid the viscosity/support trade-off entirely.31 A photoinitiated, radical-mediated thiol−ene reaction between the DA adduct itself and embedded thiols prevented the adduct from undergoing the retro-DA reaction and effectively rendered the entire network irreversible in exposed regions. This outcome was accomplished using a twophoton technique during the exposure step, referred to as photofixation. The unexposed region could be converted into a low-viscosity liquid by heating and removed simply by increasing the temperature of the system. Thus, the Diels− Alder network itself served as a temporary solid scaffold that could be made permanent where and when desired. However, the reactivity of thiols presents some limitations, particularly at high temperatures, where they are prone to react in unexposed regions during the development step. A network with a reverse gel temperature that is too high will almost inevitably lead to side reactions and polymerization in unwanted regions. Furthermore, two-photon lithography comes with several inherent drawbacks as compared with layer-by-layer techniques, such as limited focal lengths due to diffraction, slow write speeds, and high-energy requirements of the lasers.40,41 Here, we show the feasibility of using Diels−Alder networks in combination with multifunctional acrylate monomers to create a novel dual-cure system which may be useful as a stereolithographic resin. Acrylates have improved stability over
formed in each stage of these thiol−acrylate dual-cure polymer networks. The system can be compared to sequential interpenetrating networks (IPNs) where monomer is swelled into an initial network and then polymerized;20 however, the thiol−acrylate “dual-cure” network is not a true IPN. Both the first- and second-stage networks involve the same multifunctional acrylate monomers, indicating that the two networks are covalently linked throughout their structures. However, since the reactions happen at two different times, the resulting network is likely heterogeneous. Additionally, though the reactions occur in two separate stages, all reactive species are present initially, so the “swell-in” step (used in sequential IPNs and semi-IPNs) is not required. The advantage of the dual-cure approach is that the material properties of the polymer can be dramatically altered in a spatiotemporally controlled manner. Taking advantage of the network’s unique properties, thiol− acrylate dual-cure networks were explored for shape memory polymers, lithographic imprinting, and optical materials;21 however, in these previous networks, as in most conventional networks, the entire network structure was connected through irreversible covalent bonds. In this study, we have developed and analyzed a novel dualcure polymer network which is formed in two distinct steps; however, one of the network structures is composed of a thermoreversible Diels−Alder (DA) network which preserves its reversible character following the second stage reaction. DA cycloadditions, which occur between diene−dienophile pairs, are often included in the click chemistry family as they are orthogonal to many other reactions, proceed readily, do not require harsh conditions, and do not result in any reaction byproducts.22 In these reactions, diene−dienophile pairs and the resulting cyclic products exist in a dynamic equilibrium. Depending on the reactants chosen, the equilibrium will shift with temperature to favor either the cycloaddition product (via forward DA reaction at low temperature) or the reactants (via the retro-DA reaction at high temperature). Furan- and maleimide-containing reactants have received significant attention for thermoreversion, as the retro-DA reaction becomes significant at relatively low temperatures (
