Unraveling the Formation of Two-Photon Polymerized Materials at the Nanoscale

University Of California-Irvine, Irvine CA

Investigators

Abstract

This project supports fundamental research on an emerging method for the fabrication of microstructures, called two-photon polymerization (TPP). In TPP, a focused laser beam writes three-dimensional polymer structures of any configuration, enabling the fabrication of intricate objects and devices at a length scale many times smaller than the thickness of a human hair. Developments in TPP have been hampered by the inability to improve the repeatability, structural integrity, mechanical properties and further downsizing of the written structures. These limitations are chemical in nature, but deciphering the chemical composition at such small length scales has remained a significant challenge. In this project, this hurdle is overcome by using a new approach capable of generating detailed chemical maps of TTP microstructures. Using a light-based analytical method called tip-enhanced Raman scattering, a close-up view of evolving chemical changes in TTP microstructures is obtained with nanometer precision. By unraveling the hitherto hidden chemical and structural changes at the nanoscale, TTP procedures can be much improved, making a large-scale implementation of TTP technologies possible. This project thus accelerates the translation of TPP from academic labs towards large-scale industrial applications. The project also provides training to a specialist and two undergraduate students who have the opportunity to acquire sought-after skills in high-end instrumentation. In addition, this project includes an outreach effort to middle school students in the community, in which students have the chance to produce micrometer-sized versions of 3D objects of their choosing. Two-photon polymerization (TPP) is a popular technology for the fabrication of micro-structures, yet the industrial use of TPP has been hindered by poor consistency between written structures limiting a large-scale implementation of this additive manufacturing technique. Variations in the degree of conversion, solvent permeation, voxel overlap, and material inhomogeneities are not well understood, giving rise to a low overall repeatability. The root of these problems is insufficient knowledge of the polymerization process at the nanoscale. This project addresses several longstanding questions regarding the photo- physics and photo-chemistry of TPP that have limited a broader implementation of the technique. The project focuses on outstanding questions about the time-sequenced process of photo- initiation followed by polymerization as well as the resulting nanoscale morphology and chemistry. The excitation dynamics, radical formation and resin interactions of photoinitiators are studied by ultrafast spectroscopy experiments, and the degree of cross-linking, material inhomogeneities and solvent permeation at the nanoscale are studied by tip-enhanced Raman scattering. The insights acquired in this work enable an optimization of key TPP parameters, producing new guidelines for photoinitiator development, directions for controlling the degree of polymerization, new laser illumination strategies of improving resolution and management of the pyrolysis process at the nanoscale. Consequently, the deliverables in this project accelerate the translation of TPP from academic labs towards large-scale industrial applications. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.