(637f) Responsive, Flexible Optics and Optoelectronics with Precision Polymer Thin Films and Multilayers

As interest in flexible, wearable electrical, optical and optoelectronic devices intensifies, mechanically compliant optical and encapsulating thin film materials must be developed. Traditional coating technology based on inorganic compounds such as TiO₂, Al₂O₃, and MgF₂ are incompatible with soft, flexible polymer substrates commonly employed in next-generation devices. Polymeric thin films are a promising alternative class of materials for these applications. Using initiated chemical vapor deposition (iCVD), uniform multilayered thin film coatings can be synthesized conformally on flexible optical devices with precise compositional and thickness control. This presentation will describe two studies where iCVD has been exploited to design flexible, compliant optical coatings.

In the first study, a model two-layer coating design consisting of relatively low index poly(1H,1H,6H,6H-perfluorohexyl diacrylate) (pPFHDA) was synthesized directly on top of poly(4-vinylpyridine) (p4VP). Broadband antireflection over the visible wavelength range (400–750 nm) was conferred to an optical-grade thermoplastic polyurethane (TPU) substrate, reducing its surface reflectance to less than 2%. The superior mechanical compliance of polymer ARCs over conventional inorganic coatings on the TPU substrate was demonstrated through biaxial tensile testing. The in situ spectroscopic ellipsometry capability integrated into the iCVD chamber, which was essential to realizing the optical coating design specifications with nm-level resolution and real-time index monitoring, will be described.

The second study describes a facile approach to synthesizing high refractive index polymers. Halogen molecules are incorporated into poly(4-vinylpyridine) thin films through exposure to halogen solutions or vapor. The halogen compounds are immobilized in the polymer matrix due to the formation of charge transfer complex (CTC): P4VP-IX (X = I, Br, and Cl). The maximum refractive indexes measured at 587 nm for P4VP-I₂, P4VP-IBr, and P4VP-ICl were 2.08, 1.849, 1.774. The index for P4VP-I₂ is among the highest values reported for polymers. It is shown gravimetrically that one halogen molecule absorbs for every pendent pyridine. The high refractive index is due to the high molar refractivity coupled to the compact nature of the complex; the densities of the complexed films were shown to be greater than 2.0 g cm^-3. The spatial uniformity of the P4VP-I₂ film was also characterized, showing standard deviations in refractive index and thickness of 0.0769 and 1.91%, respectively, over a 10mm diameter. Future opportunities for polymer thin films in optical and optoelectronic applications will be described.