No 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.

Thiol-isocyanate-methacrylate two-stage reactive network polymers were developed and used for fabrication of well-defined surface patterns as well as functional geometric shapes to demonstrate a new methodology for processing of "smart materials". The dynamic stage I networks were synthesized in base-catalyzed thiol-isocyanate cross-linking reactions to yield tough, glassy materials at ambient conditions. Methacrylate-rich stage I networks, incorporating photoinitiator and photoabsorber, were irradiated with UV light to generate stage II networks with intricate property gradients. Upon directional straining and subsequent temperature-dependent stress relief of the predefined gradient regions, the desired surface or bulk geometric transformations were achieved. Depending on the gradient extent in conjunction with photoorthogonal initiators, the introduced deformations were shown to be easily erasable by heat or permanently fixable by bulk polymerization.