Materials for High and Low Refractive Index Coatings

Introduction

Refractive index (RI or n) is of critical importance for photonics applications such as optical waveguides and ophthalmic devices. Based on their unique refractive index characteristics and good optical clarity, polymeric materials are also used as anti-reflective coatings for solar cells, displays and contact lenses.1,2 With the need for mechanical strength, environmental stability and ease of processing, polymeric-based materials have the potential to be excellent alternatives to traditional inorganic optical films.3 Other advantages over their inorganic counterparts include light weight, insensitivity to vibrational stress and low cost.

Schematic of a display (e.g. LCD) with protective and RI-control polymer overlayer films image

Figure 1: Schematic of a display (e.g. LCD) with protective and RI-control polymer overlayer films image

 

Anti-reflection coatings for LCD displays are one example of an application that uses polymeric RI-control technology (Figure 1). Films of PET (polyethylene terephthalate) or TAC (triacetyl cellulose) are typically applied to protect outer layers of modern multi-layer LCD stacks. PET and TAC are inexpensive strong polymers with n ~ 1.7 much larger than for air (n = 1). A large fraction of light beam incident on an abrupt interface with ?n ~ 0.7 would bounce off (reflect) from the interface. For an LCD, this would result in a significant fraction of ambient light reflected from the display surface, causing glare and wash-out of displayed images. To solve this problem, a light management film stack with gradually varying RI can be applied to the LCD surface. The stack consists of multiple layers with gradually diminishing RI from high adjacent to the PET/TAC surface to low facing air. By matching the refractive index of the display surface to that of air, surface reflection is reduced, leading to higher effective image contrast and reduced washout by ambient reflections.

Zirconium and Hafnium Acrylate Monomers

RI engineering of light management stacks depends on availability of additives to adjust (raise or lower) RI of polymer films. Possible RI-raising additives include metal-oxide nanoparticles and monomers/polymers containing atoms with high (compared to carbon) atomic numbers. Zirconium and hafnium based acrylate monomers newly available from Sigma-Aldrich (Table 1) are particularly well suited for controlling RI of acrylate films, for example urethane acrylates commonly used to formulate protecting films of LCD displays. Cured acrylate films incorporating these products are UV-stable and combine:

  • Excellent optical clarity (>95%)
  • Good hardness and scratch resistance (>2H Pencil Hardness)
  • >Tuning of RI by choosing loading of functional monomer (Figure 2)

Sample experimental protocols for using Zr and Hf acrylate products are described in Aldrich Technical Bulletin AL-238.


Table 1: Polyfunctional zirconium and hafnium acrylate monomers

Product No. Product Name Structure Package Size Add to Cart
686239 Zirconium acrylate 686239 Structure Image 25 gm Shopping Cart
686247 Zirconium carboxyethyl acrylate, 60% in n-propanol 686247 Structure Image 100 gm Shopping Cart
686220 Hafnium carboxyethyl acrylate, 60% in 1-butanol 686220 Structure Image 25 gm Shopping Cart
686204 Zirconium bromonorbornanelactone carboxylate triacrylate (PRM30) 686204 Structure Image 25 gm Shopping Cart


 

Physical properties of fully cured acrylate films

Figure 2: Physical properties of fully cured acrylate films

High and Low RI Polymers

In addition to the polyfunctional zirconium and hafnium acrylates, Sigma-Aldrich offers a range of high RI polymers based on aromatic and brominated aromatic monomers (Table 2), as well as low RI materials based on fluorinated monomers (Table 3). The associated monomers are also available and can be used to synthesize polymers whose refractive index can be tailored. Also listed are UV/thermal crosslinkable polymers for patterning or heterolayered device fabrication. The tables list products sorted in order of refractive index and can be used as material selection tools for your photonics research.

Table 2: High Refractive Index Materials

Name n4 Tg (°C)4 Homopolymer Photo Crosslinkable Polymer Monomer
Poly(pentabromophenyl methacrylate) 1.710 592064 591831,
591610
592439
Poly(pentabromophenyl acrylate) 155 591521 591505,
591408
592552
Poly(pentabromobenzyl methacrylate) 1.710 640301 640298 640336
Poly(pentabromobenzyl acrylate) 1.670 180 640328 640263
Poly(2,4,6-tribromophenyl methacrylate) 1.660 640247 640255 640239
Poly(vinylphenylsulfide) 1.657 113 640212 640220
Poly(1-napthyl methacrylate) 1.641 205 640190 640204
Poly(2-vinylthiophene) 1.638 640182
Poly(2,6-dichlorostyrene) 1.625 167 639974 640174 D74509
Poly(N-vinylphthalimide) 1.620 201 639982 349542
Poly(2-chlorostyrene) 1.610 103 640018 160679
Poly(pentachlorophenyl methacrylate) 1.608 640034

 

Table 3: Low Refractive Index Materials

Name n4,5 Tg (°C)4,5 Homopolymer Photo Crosslinkable Polymer Monomer
Poly(1,1,1,3,3,3-hexafluoroisopropyl acrylate) 1.375 -23 630152 367656
Poly(2,2,3,3,4,4,4-heptafluorobutyl acrylate) 1.377 -30 630179 443751
Poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate) 1.383 65 591971 592102,
592099
444006
Poly(2,2,3,3,3-pentafluoropropyl acrylate) 1.389 -26 630136 470961
Poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate) 1.390 56 591327 591548,
591432
367664
Poly(2,2,3,4,4,4-hexafluorobutyl acrylate) 1.394 -22 630160 474452
Poly(2,2,3,4,4,4-hexafluorobutyl methacrylate) 591645 591874,
591769
371971
Poly(2,2,3,3,3-pentafluoropropyl methacrylate) 1.395 70 592080 590894,
591106
474193
Poly(2,2,2-trifluoroethyl acrylate) 1.411 -10 630098 297720
Poly(2,2,3,3-tetrafluoropropyl acrylate) 1.415 -22 630144 371920
Poly(2,2,3,3-tetrafluoropropyl methacrylate) 1.417 68 591637 591211,
590991
371998
Poly(2,2,2-trifluoroethyl methacrylate) 1.418 69 591963 591750,
591866
373761

 References

  1. Beercroft, L. L.; Ober, C. K. J.M.S.-Pure Appl. Chem. 1997, A34(4), 573.
  2. Nalwa, H. S., Ed. Polymer Optic Fibers, American Scientific Publishers, 2003. (Product No. Z552151)
  3. Prasad, P.; Williams, D. Introduction to Nonlinear Optical Effects in Molecules & Polymers, John Wiley & Sons, 1991.
  4. Brandrup, J. et al. Polymer Handbook, 4th ed.; John Wiley & Sons, 1999. (Product No. Z412473)
  5. Gaynor, J. et al. J. Appl. Polym. Sci. 1993, 50, 1645.

 

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