Transparent, Ultrahigh-Refractive Index Polymer Film (n ∼1.97) with Minimal Birefringence (Δn <0.0010)

Abstract

High refractive index (RI) thin films are of critical importance for advanced optical devices, and the high refractive index polymers (HRIPs) constitute an interesting class of materials for high RI thin films due to low cost, good processability, light weight, and high flexibility. However, HRIPs have yet to realize their full potential in high RI thin film applications due to their relatively low RI, strong absorption in the blue light region, and limited film formation methods such as rapid vitrification. Herein, we report a development of a new HRIP thin film generated through a one-step vapor-phase process, termed sulfur chemical vapor deposition (sCVD), using elemental sulfur and divinyl benzene. The developed poly­(sulfur-co-divinyl benzene) (pSDVB-sCVD) film exhibited RI (measured at 632.8 nm) exceeding 1.97, one of the highest RIs among polymers without metallic elements reported to date. Because the sCVD utilized vaporized sulfur with a unique sulfur-cracking step, formation of long polysulfide chains was suppressed efficiently, while high sulfur content as high as 85 wt % could be achieved with no apparent phase separation. Unlike most of inorganic high RI materials, pSDVB-sCVD was highly transparent in the entire visible range and showed extremely low birefringence of 10 × 10–4. The HRIP thin film with unprecedentedly high RI, together with outstanding transparency and low birefringence, will serve as a key component in a wide range of high-end optical device applications.

Systematic Control of Sulfur Chain Length of High Refractive Index, Transparent Sulfur-Containing Polymers with Enhanced Thermal Stability

“…While different properties of polymer films from sCVD and bulk polymers from IV should be considered while taking into account these fundamental differences, the significantly enhanced thermal properties and low crystallinity from polymers with similar sulfur contents and identical comonomers are notable. With the T g values of polymers from sCVD measured through variable temperature ellipsometry on substrate-bound samples being identical to those measured through DSC of samples peeled from their original substrates, the effect of the substrate–polymer interactions on the film’s thermal properties is negligible. We hope that further investigations into various methods for the direct polymerization of elemental sulfur would give rise to various benchmark materials against which a more direct property comparison could be made without the need for adjustments for differences in processing methods.…”

“…23,24 Recently, we reported a one-step vapor-phase copolymerization, termed sulfur chemical vapor deposition (sCVD), between elemental sulfur and a wide range of multifunctional olefins. 25,26 The thickness and optical properties of polymer films from sCVD could be adjusted systematically by controlling variables such as process temperature, pressure, feed ratio, and substrate temperature, allowing for the achievement of transparent thin films with high and controllable refractive index.…”

Tracking side reactions of the inverse vulcanization process and developing monomer selection guidelines

“…The absorption wavelengths of simple poly-thioethers should not be in the visible region and polymers with no absorption in the visible region have been synthesized using the inverse vulcanization method. 28 However, there have been some reports on polymers colored red, orange, yellow, or black. 3,20,22 Therefore, by-products are expected to contribute to the coloration of the polymer.…”

Transcending the Trade-off in Refractive Index and Abbe Number for Highly Refractive Polymers: Synergistic Effect of Polarizable Skeletons and Robust Hydrogen Bonds

“…As a solution to this problem, polymers with large numbers of short polysulfide chains synthesized by sulfur chemical vapor deposition of elemental sulfur using comonomers containing multiple vinyl groups were recently reported. , These polymers exhibited superior transparency and much higher RI values (up to n D = 1.982, where D denotes the sodium D line at 589.3 nm) compared to those of previously reported HRIPs, although the versatility of the synthetic route was limited by the need for vapor deposition. Therefore, the development of facile and versatile preparation methods for HRIPs with ultrahigh RI values, adequate Abbe numbers, and film formability has become a critical issue in the field.…”