Search for: All records

Decades of advances in understanding and simulating the polymerization kinetics and structural evolution that arises in free‐radical photopolymerizations of multifunctional monomers are combined into a single, first‐principles 3D model. The model explicitly accounts for polymerization features including diffusion‐controlled kinetics, oxygen inhibition, light attenuation, chain‐length dependent termination, reaction‐diffusion termination, heat transfer, composition and conversion‐dependent material properties, crosslinking effects, and species diffusion. Using the homopolymerization of 1,6‐hexanediol diacrylate as a model system, a minimum of two kinetics experiments performed at different initiation rates are required to fit model parameters. The model accurately predicts known relationships regarding oxygen inhibition, light intensity, and curing temperature for samples of different geometries and boundary conditions. The emphasis of the results herein is placed on the interactions between polymerization features, motivating the importance of a model that accommodates these features all in one simulation. The model is shown to be robust in its handling of thermal boundary conditions, alternative polymerization techniques or mechanisms, and characteristics of 3D voxel formation. The model in this work provides a useful tool for property prediction in a wide variety of applications, most notably coatings, dental materials, industrial photocuring processes, additive manufacturing, and holography, where complex interactions of the various features of polymerization play a substantial role.

 

Liquid crystal elastomers leap via thermally induced snap-through transitions facilitated by spatial programming of properties. 

Radical-disulfide exchange reactions in thiol–ene–disulfide networks were evaluated for several structurally distinct thiol and disulfide containing monomers. A new dimercaptopropionate disulfide monomer was introduced to assess how different disulfide moieties affect the exchange process and how the dynamic exchange impacts polymerization. The stress relaxation rate for the disulfides studied herein was highly tunable over a narrow range of network compositions, ranging from 50% relaxation over 10 minutes to complete relaxation over a few seconds, by changing the thiol–disulfide stoichiometry or the disulfide type in the monomer. The thiol/disulfide monomer pair was shown to have significant influence on how radical-disulfide exchange impacts the polymerization rate, where pairing a more stable radical forming thiol ( e.g. an alkyl thiol) with a less stable radical-forming disulfide ( e.g. a dithioglycolate disulfide) reduces the rate of the thiol–ene reaction by over an order of magnitude compared to the case where those two radicals are of the same type. The variations in rates of radical-disulfide exchange with dithioglycolate and dimercaptopropionate disulfides had a significant impact on stress relaxation and polymerization stress, where the stress due to polymerization for the final dimercaptopropionate network was about 20% of the stress in the equivalent dithiogylcolate network under the same conditions. These studies provide a fundamental understanding of this polymerization scheme and enable its implementation in materials design.