Low & Ultra-low Refractive Index Polymers

Plastics are becoming increasingly popular among the formulators for their variety of optical applications. They are extremely useful for optical applications such as:

• Fiber optic core and cladding
• Optical lens coating
• Anti-reflective coatings
• Optical adhesives
• Encapsulation of various optical components (Light-guides, patterning or hetero-layered device fabrication)

Refractive Index (RI) is one of the major factors deciding the optical properties of a plastic.

Let's take a detailed look at the most popularly known optical plastics, their relevance with the refractive index and the applications...

Understanding Refractive Index

The refractive index (RI) of a polymer is the ratio of the speed of light in a vacuum to the speed of light through the polymer. It varies with frequency (and thus wavelength) of light. Typically, it is measured at well-defined spectral wavelengths; for example, the yellow sodium double emission at 589nm wavelength. As other properties, refraction indices are temperature dependent. RI of air and water are 1.0 and 1.31 respectively.

Three properties of polymers are related to their refractive index:

• Light rays change direction when they cross the interface from air to the polymer.
• Light reflects partially from surfaces that have a refractive index different from that of their surroundings.
• The dispersive effect due to the diversity of the wavelengths of the light, the bending effect being frequency dependent.

The lower the refractive index, the less the material bends the light, decreasing the focusing power, the reflective effect and the light dispersion. Therefore, the polymer of an optical plastic must possess lower value of refractive index.

Optimum Refractive Index Range for Polymers

For the plastics to be optically sound, their polymers must possess the RI in the range of 1.3-1.4. For this speciality polymers are used, which have access to the required range. “Usual” low refractive index plastics are specialty fluorinated polymers possibly bearing chemical function such as: Acrylate Propionate Acetate Or siloxane Several polymer producers and some adhesive or coating manufacturers commercialize raw and compounded fluorinated polymers aiming optical applications.

One strategy to obtain RIs inferior to 1.3, consists of using polymers as precursors to generate, in a first step, polymer nanoparticles dispersed in a silica matrix. Then, in a second step, polymer nanoparticles are destroyed by a suitable heat treatment forming a film of mesoporous nanosilica.

So, a maximum free space is obtained, leading to refractive indices as low as 1.1 or even less. One of the issues of this promising strategy is that it leads to materials sometimes suffering from poor storage stability and poor processability.

Optical Plastics

It is because of versatility, processability, mechanical and chemical properties of Optical Plastics that they are being used in:

• Lenses
• Optical circuits
• Optical fibers
• Anti-reflective films and coatings
• Optical adhesives
• LCD displays
• Waveguides
• UV-reactive inks
• Varnishes

The most popularly known optical plastics include polymers such as:

• Polymethacrylates
• Polyurethanes
• Polycarbonates
• Polystyrenes
• Urethane-acrylates

All these have refractive indices in a medium or high range. In addition, heat or radiation (UV, EB, laser...) treatments such as annealing, crosslinking and high polymerization contribute to increase refractive indices of the basic materials.

Analyzing Refractive Index of Polymers

Polymers versus Other Materials

Shown below is the table representing RI of polymers and glass. This could be clearly seen from the table that silicon has the highest refractive index of the examined materials but compared to common glass, polymers are in the same refractive index range.

Material	Refractive Indices
Silicon	4.0
Diamond	2.4
Sapphire	1.76
Flint glass	1.52 - 1.92
Polymers	1.3 - 1.8
Crown glass	1.48 - 1.75
Pyrex	1.47
Fused Quartz	1.46
Fused silica	1.46
Water ice	1.31
Air	1.0

 Refractive Indices of Polymers and Glass

Statistical Analysis of Neat Polymers

If we enlarge the scope of investigation to 300 marketed or development grade as shown in the table and figure below. We can see that the lower limit of refractive indices is between 1.31 and 1.4 and more than 96% of the data are superior to 1.36.

Property	Value
Mean	1.519
Median	1.51
Mode	1.5
Standard deviation	0.080
Variance	0.00647
Kurstosis	0.193
Degree of asymmetry	0.0178
Range	0.469
Minimum	1.31
Maximum	1.77
Samples	300
96% data range	1.36-1.68

Refractive Index Statistics Analysis of 295 Resins

Polymer Refractive Index

Refractive indices of conventional polymers are in a limited range as we can see in Table below, displaying refractive indices of 16 thermoplastic and thermoset polymers ranked by types (columns 2 and 3) or by RI ascending order (columns 4 and 5). We can remark that fluorinated polymers form the only family with substantially lower refractive indices than the mean value (1.519).

Fluoropolymers and Non-Fluorinated Polymers

Taking into account the mean value and the standard deviation resulting from the statistical analysis, we consider that "low refractive index polymers" have RI between 1.3 and 1.4. However, it is obvious that many polymer producers, adhesive or coating manufacturers commercialize common polymers with refractive index superior to 1.4.

Classified by polymer type			Ascending ranking
Commodity	PE	1.51-1.54	Fluorinated	1.31-1.4
	PP	1.5-1.51	Epoxy	1.45-1.6
	PVC	1.53-1.55	POM	1.48-1.51
	PS	1.55-1.59	Acrylics	1.49-1.57
	UP	1.52-1.54	PP	1.5-1.51
	PUR	1.5-1.6	PUR	1.5-1.6
Engineering	PA	1.53-1.54	PE	1.51-1.54
	PET	1.55-1.64	UP	1.52-1.54
	Acrylics	1.49-1.57	PA	1.53-1.54
	PC	1.58-1.6	PVC	1.53-1.55
	POM	1.48-1.51	PS	1.55-1.59
	PEI	1.66-1.67	PET	1.55-1.64
	Sulfone	1.63-1.65	PC	1.58-1.6
	Epoxy	1.45-1.6	Sulfone	1.63-1.65
Specialty	Fluorinated	1.34-1.4	PEEK	1.65-1.77
	PEEK	1.65-1.77	PEI	1.66-1.67

Refractive Indices of Some Conventional Polymers

Table below shows the predominance of fluorinated polymers in the low refractive index (1.3-1.4) polymers also including a few non-halogenated polymers.

Fluoropolymers	Refractive indices
Teflon™ AF (DuPont Spin-off - Now Chemours)	1.31+
Cytop (Asahi)	1.31+
Poly(hexafluoropropylene oxide)	1.31+
Fluorinated Ethylene Propylene	1.338
Poly(tetrafluoroethylene-co-hexafluoropropylene)	1.338
Poly(pentadecafluorooctyl acrylate)	1.339
Perfluoroalkoxy	1.340
Poly(tetrafluoro-3-(heptafluoropropoxy)propyl acrylate)	1.346
Poly(tetrafluoro-3-(pentafluoroethoxy)propyl acrylate)	1.348
Poly(tetrafluoroethylene)	1.350
Tetrafluoroethylene hexafluoropropylene vinylidene fluoride	1.350
Poly(undecafluorohexyl acrylate)	1.356
Poly(nonafluoropentyl acrylate)	1.360
Poly(tetrafluoro-3-(trifluoromethoxy)propyl acrylate)	1.360
Poly(pentafluorovinyl propionate)	1.364
Poly(heptafluorobutyl acrylate)	1.371
Poly(trifluorovinyl acetate)	1.375
Poly(1,1,1,3,3,3-hexafluoroisopropyl acrylate)	1.375
Poly(octafluoropentyl acrylate)	1.380
Poly(methyl 3,3,3-trifluoropropyl siloxane)	1.383
Poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate)	1.383
Poly(pentafluoropropyl acrylate)	1.385
Poly(2,2,3,3,3-pentafluoropropyl acrylate)	1.389
Poly(2-heptafluorobutoxy)ethyl acrylate)	1.390
Poly(chlorotrifluoroethylene)	1.390
Poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate)	1.390
Poly(2,2,3,4,4-hexafluorobutyl acrylate)	1.392
Poly(2,2,3,4,4,4-hexafluorobutyl acrylate)	1.394
Poly(2,2,3,3,3-pentafluoropropyl methacrylate)	1.395
Non-fluorinated polymers	Refractive indices
Poly(methyl hydro siloxane)	1.397
Hydroxypropyl cellulose	1.337

Low Refractive Indices of Numerous Fluoropolymers and a Few Non-fluorinated Polymers

Ready-to-Use Low Refractive Index Polymers

Several polymer producers, adhesive or coating manufacturers commercialize polymers with refractive index in the range 1.31-1.39. That too, without claiming to be exhaustive, some examples:

Chemours' (DuPont Spin-off) Teflon™ AF Amorphous Fluoropolymers

Teflon™ AF amorphous fluoropolymers by Chemours (DuPont Spin-off) are distinct from other fluoropolymers in that they have the lowest index of refraction of any known polymer. In addition they are:

• Soluble in selected solvents
• Have high gas permeability
• High compressibility
• Creep resistance
• Low thermal conductivity
• Have the lowest dielectric constant of any known solid polymer
• Aimed optical applications include fiber optic core and cladding, optical lenses, anti-reflective coatings, molded- or solution-processed parts

Asahi Glass & Chemicals's Cytop Amorphous Fluoropolymer

Cytop Amorphous Fluoropolymer by Asahi Glass & Chemicals possessfollowing properties:

• High optical transparency
• Excellent chemical, thermal, electrical and surface properties
• Low refractive index, low coefficient of optical dispersion and good lamination properties
• Good solubility in certain fluorinated solvents due to its amorphous nature This, coupled with its thermoplastic characteristics, make it a popular choice as a coating for optic materials
• Low refractive index, low coefficient of optical dispersion and good lamination properties
• Aimed optical applications include anti-reflective coatings

Polymer Range by MY Polymers Ltd.

MY Polymers Ltd., a manufacturer of low refractive index optical adhesives and coating materials, offers a line of polymers with refractive indices in the range 1.32-1.33. Most of MY products are:

• < 100% solids UV curable resins but some are also made as single component moisture curable compositions
• The UV resins are more adapted for quick and deep curing
• The moisture-cured resins are more suitable for thin coating applications with good adhesion

MY-133 used in industrial applications since 2003, was built especially to answer the need for a low RI encapsulating material. It cures to a clear low modulus solid to a depth of a few millimeters by the action of radiation sources at 300-400nm. It is useful as a coating material and for the encapsulation of various optical components such as optical fibers, light-guides, lenses and other components where the enhancement of total internal reflection is the issue.

As a cladding material for fibers, it has the potential for the best available numerical aperture. A non-UV curable version, MY-133MC, with the same low refractive index of 1.33 is available as a moisture curable material. It is characterized by better adhesion and better mechanical strength than that of the UV curable version. In addition it does not need inserting and requires no radiation equipment.

MY-132 is the last addition having the lowest refractive index of 1.320 in the cured state at the near IR (0.9-1.0 micron).

UV-Opti->Clad™ by Ovation Polymers

Ovation Polymers markets a series of UV curable materials, UV-Opti->Clad™, with:

• Low refractive indices
• 100% solid after curing
• Suitable for spin-coating
• Dip-coating
• Brush-coating applications
• Used as cladding solutions in fiber optics industry and optical adhesive applications

Refractive Index can be as low as 1.33. Table below, displays some property examples.

Solution Properties
Viscosity (cps) @ 25°C	500
Solution Specific Gravity	1.634 g/ml
Cured Polymer Properties
Cure Time (min.)	1-2 µWatt/cm2
Cured Refractive Index 589nm @ 25°C	1.336
Hardness (shore D)	20
Maximum Shrinkage	3.8 %
Specific Gravity	1.699 g/ml
Elongation at Break	18 %
Tensile Strength	1.7 MPa
Modulus	23 MPa
Other Properties
Adhesion to glass and plastics	Good
Thermal Stability (TGA @ 5%)	260°C
Shelf Life (months)	12

Property Examples: UV-OPTI CLAD™ 1.33CM-500

Fluorinated Acrylate Monomers-based Materials by Sigma-Aldrich

Sigma-Aldrich offers a range of low RI (1.375 up to 1.395) materials based on fluorinated acrylate monomers that can be used to synthesize crosslinkable polymers for patterning or hetero-layered device fabrication.

Other Strategies to Obtain Ultra-Low RI

1. Asep Bayu Dani Nandiyanto, Nobuhiro Hagura, Ferry Iskandar, Kikuo Okuyama (Acta Materialia, Volume 58, Issue 1, Pages 282-289, January 2010) prepared a highly ordered arrangement of pores with multiple sizes in film and particle forms using dip-coating and spray-drying methods, respectively.
   
   The template-driven self-assembly technique was effective when a combination of 5nm silica and two different sizes of monodispersed polystyrene spheres were self-assembled to produce a composite silica/PS template. Heat treatment was then used to remove the PS, which produced the porous particles. The size of the pores (large pores: 100-1000nm; small pores: 30-200nm) could be controlled simply by adjusting the PS size. Sufficient numbers of large/small pores made it possible to produce a material of ultra-low refractive index.


2. Masato Yamaguchi, Hiroyuki Nakayama, Kazuhiro Yamada, and Hiroaki Imai (Optics Letters, Vol. 34, Issue 13, pp. 2060-2062, 2009) obtained ultra-low refractive index, n=1.07, transparent coatings consisting of mesoporous silica nanoparticles.
   
   Excellent anti-reflection performance was achieved with six anti-reflection layers including the mesoporous film as the top layer. The average reflectance in the entire visible wavelength region (380-780 nm) was 0.04-0.22 % at incident angle of 0°-40°.


3. Paolo Falcaro, Luca Malfatti, Tongjit Kidchob, Giacomo Giannini, Andrea Falqui, Maria F. Casula, Heinz Amenitsch, Benedetta Marmiroli, Gianluca Grenci and Plinio Innocenzi (Chem. Mater., 2009, 21 (10), pp 2055-2061) study bimodal hierarchical hybrid organic-inorganic thin films with pores in the meso and macro range. The first order of porosity is obtained through a self-assembly process using organic block copolymers as templates for mesopores; Fluorinated organic nanoparticles of around 70 nm are introduced in the precursor solution to act as template of the second order of porosity.
   
   After the film deposition, the nanoparticles become randomly dispersed within the mesostructured matrix. Hierarchical porous thin films with a bimodal distribution of the pores are obtained upon post-synthesis calcination at 350°C. The hierarchical porous thin films have a very low refractive index, 1.14 at 633 nm.


4. Venumadhav Korampally, Minseong Yun, Thiruvengadathan Rajagopalan, Purnendu K Dasgupta, Keshab Gangopadhyay and Shubhra Gangopadhyay (Journal Nanotechnology, Volume 20, Number 42, 2009) describe a method for the fabrication of nanoporous materials from colloidal dispersions of polymethylsilsesquioxane nanoparticles. Nanoparticle-polymer composites above the decomposition temperature form hydrophobic films with refractive indices as low as 1.048.


5. Venumadhav Korampally, Maruf Hossain, Minseong Yun, Keshab Gangopadhyay, Luis Polo-Parada and Shubhra Gangopadhyay (Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences, October 12 - 16, 2008, San Diego, California, USA) have developed novel ultra low refractive index (as low as 1.04), crack-free thick coatings (as thick as 3.6 microns) based on organosilicate nanoparticulate (NPO) networks.


6. Frank Marlow, Denan Konjhodzic, Helmut Bretinger and Hongliang Li (Advances in Solid State Physics, 2006, Volume 45/2006, 149-161) describe mesoporous silica films with a refractive index of 1.14 for use in 2D photonic crystal waveguide systems.


7. Lorenz Zimmermann, Martin Weibel, Walter Caseri, Ulrich W. Suter, Paul Walther (Polymers for Advanced Technologies, Volume 4, Issue 1, pages 1-7, January 1993) prepare nanocomposites of gelatine and gold by reduction of AuCl4H in a gelatine solution, followed by a spin coating process and a brief washing. The thickness of the films thus obtained was typically 200-400 nm.
   
   The refractive index depends linearly on the volume fraction of gold, at least at 1300 nm. The lowest measured refractive indices are 1.008±0.060 at 1300 nm and 0.963±0.062 at 632.8 nm.