ma9b01678_si_001.pdf (458.11 kB)
Thermal Metamorphosis in (Meth)acrylate Photopolymers: Stress Relaxation, Reshaping, and Second-Stage Reaction
journal contribution
posted on 2019-10-18, 17:48 authored by Maciej Podgórski, Brady T. Worrell, Jasmine Sinha, Matthew K. McBride, Christopher N. BowmanThermally transformable/responsive
(meth)acrylate photopolymer
networks were constructed from commercial (meth)acrylate esters and
synthetic di- and mono(meth)acylate monomers bearing thioester functionalities.
The thermal responsiveness, here self-limited exchange, relied on
the catalytic metamorphosis of thioesters into esters with the concomitant
depletion of hydroxyls and subsequent generation of free thiols. The
thioester–hydroxyl cross-exchange was demonstrated in network
systems with interchain thioesters as well as in networks with side-chain
pendant thioacetyls. The interchain metamorphosis resulted in close
to 80% conversion of thioesters into esters when 2 equiv of hydroxyl
groups was initially present. In practical terms, such an outcome
enabled efficient stress relaxation (60%) and good shape adaptation
(90% shape fixity) in 1 h at 105 °C. On the other hand, side-chain
S → O acyl transfer reactions were found to vary in efficiency
depending on the vicinity of thioesters and hydroxyls. When in close
vicinity, efficient noncatalytic exchange was observed in materials
with 1-to-1 thioester-to-hydroxyl ratios nearing 60%. The benefits
of generating free thiols postpolymerization were further explored
in enhanced two-stage curing systems where a subsequent thiol–ene
photopolymerization was demonstrated at ambient as well as at elevated
temperatures.