Initiation of surface wrinkling during photopolymerization

It has long been observed when photo-curable resin is exposed to light, surface wrinkle pattern arises before the resin is fully photopolymerized. However, the critical condition for the onset of surface instability during photopolymerization process and how the wrinkling initiation is affected by photopolymerizing condition have not been theoretically resolved yet. In this paper, the onset of wrinkling initiation of a photopolymerization system is experimentally and theoretically investigated. The morphological patterns of surficial wrinkles are captured and characterized by in-situ monitoring of wrinkling initiation. Instability analysis of the transient and inhomogeneous photopolymerization system is implemented by state space method, combing the photochemical reaction kinetics with the coupled diffusion and large deformation theory of polymeric materials. Both the critical time and critical wavelength for wrinkling initiation can be predicted by our model, which is verified by experimental results. The critical time is significantly affected by the curing conditions that impact on the photochemical reaction kinetics, including irradiation intensity and photoinitiator concentration. Higher irradiation intensity and photoinitiator concentration can promote transformation of monomer and accelerate self-swelling behavior, generating a shorter time to surface instability. The critical wavelength is mainly affected by the mechanical properties of the material. Curing a thicker film with lower irradiation intensity and lower photoinitiator concentration results in a higher non-uniformity of material property, and generating a larger wavelength of wrinkle in the polymer surface. The theoretical and experimental analysis in this work provide guiding principles for controlling of surface morphology in photopolymerization-based designing and manufacturing.