ma300228z_si_001.pdf (301.58 kB)
Stress Relaxation via Addition–Fragmentation Chain Transfer in High Tg, High Conversion Methacrylate-Based Systems
journal contribution
posted on 2012-07-24, 00:00 authored by Hee Young Park, Christopher
J. Kloxin, Ahmed S. Abuelyaman, Joe D. Oxman, Christopher N. BowmanTo reduce shrinkage stress which arises during the polymerization
of cross-linked polymers, allyl sulfide functional groups were incorporated
into methacrylate polymerizations to determine their effect on stress
relaxation via addition–fragmentation chain transfer (AFCT).
Additionally, stoichiometrically balanced thiol and allyl sulfide-containing
norbornene monomers were incorporated into the methacrylate resin
to maximize the overall functional group conversion and promote AFCT
while also enhancing the polymer’s mechanical properties. Shrinkage
stress and reaction kinetics for each of the various functional groups
were measured by tensometry and Fourier-transform infrared (FTIR)
spectroscopy, respectively. The glass transition temperature (Tg) and elastic moduli (E′)
were measured using dynamic mechanical analysis. When the allyl sulfide
functional group was incorporated into dimethacrylates, the polymerization-induced
shrinkage stress was not relieved as compared with analogous propyl
sulfide-containing resins. These analogous propyl sulfide-containing
monomers are incapable of undergoing AFCT while having similar chemical
structure and cross-link density to the allyl sulfide-containing methacrylates.
Here, a monomethacrylate monomer that also contains a cyclic allyl
sulfide (PAS) was found to increase the cross-linking density nearly
20 times as compared to an analogous monomethacrylate in which the
allyl sulfide was replaced with an ethyl sulfide. Despite the much
higher cross-link density, the PAS formulation exhibited no concomitant
increase in stress. Thiol–norbornene resins were copolymerized
in PAS to promote AFCT as well as to synergistically combine the ring-opening
benefits associated with the thiol–ene reaction. AFCT resulted
in a 63% reduction of polymerization stress and a 45 °C enhancement
of the glass transition temperature in the allyl sulfide-containing
thiol–norbornene–methacrylate system compared with rubbery
dimethacrylates. When compared with conventional glassy dimethacrylates,
this combined system has less than 10% of the typical shrinkage stress
level while having similarly excellent mechanical properties.