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(417e) Crosslinking Gradients of a Photopolymerized Multifunctional Acrylate Film Control Mechanical Properties

Authors: 
Hancock, M. L., University of Kentucky
Yang, F., University of Kentucky
Hawes, E., Hummingbird Nano, Inc.
Grulke, E. A., University of Kentucky
The mechanical properties of a microfluidic device by rapid photopolymerization is dependent on the crosslinking gradient observed throughout the depth of the film. Quantitative information regarding the degree of polymerization of thin film polymers polymerized by free radical polymerization through the application of ultraviolet (UV) light is crucial to estimate material properties. In general, less cure leads to more flexibility, and more cure leads to brittleness. The objective is to quantify the degree of polymerization to approximate the carbon-carbon double bond (C=C) concentration and directly relate it to the mechanical properties of the polymer. SR-399 (dipentaerythritol pentaacrylate) is a fast cure, low skin irritant monomer that contains five C=C. It is a hard, flexible polymer, and also resistant to abrasion. It can be used as a sealant, binder, coating, and as a paint additive. When polymerized, a crosslinking gradient is observed throughout the depth of the coating.

Polymerization of C=C groups is conducted using a photoinitiator and an UV light source from one surface of a thin film of a multifunctional monomer. The C=C fraction in the film is found to vary with film depth and UV light intensity. The degree of crosslinking is estimated and linked directly to mechanical properties. Raman microspectroscopy is utilized to measure the C=C concentration and thus give an approximation of the degree of crosslinking of the resulting polymer. Mechanical properties measured from nanoindentation along the film depth reveal gradients in the local Young’s modulus and hardness. An UV polymerization model of SR-399, based on measured absorptivity of the components, is constructed utilizing the Beer-Lambert Law. The extents of conversion and crosslinking estimates is compared to local mechanical moduli and optical properties. A mathematical model linking the mechanical properties to the degree of polymerization, C=C composition, as a function of film depth and light intensity is then developed. Trends observed by Raman microspectroscopy and nanoindentation, along with polymerization kinetics, are confirmed by the model. The model is appropriate for optimizing mechanical properties of thin films within the manufacturing constraints for the microfluidic device. For a given amount of light energy, one can predict the hardness and modulus of elasticity. This information is highly valuable because it means that the manufacturing process can be altered to provide a range of mechanical properties in the product.