Gradient refractive index materials

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

Nadareishvili Levan and Lekishvili Nodar / Fabrication of Gradient Refractive Index Materials with Controlled Gradient Formation // POLYCHAR WORLD FORUM ON POLYMER APPLICATIONS AND THEORY, POLYCHAR-9. University of North Texas, Denton, Program and Book of Abstracts, 2001, January 9-12 (P.62). 

The three modes of FP have proven to offer advantages for different applications. Photofrontal polymerization is driven by a continuous flux of energy and has been applied to the preparation of microfluidic chips. It can be applied to any photopolymerization. IFP relies on the gel effect to create a slowly moving localized polymerization through monomers like methyl methacrylate. This method can be used to prepare gradient refractive index materials. 

Figure adapted from X. Li, L. Xue, Y. Han, Broadband antireflection of block copolymer/ homopolymer blend films with gradient refractive index structures. Journal of Materials Chemistry 21 (2011) 5817-5826. 

Ellipsometry is a method of measuring the film thickness, refractive index, and extinction coefficient of single films, layer stacks, and substrate materials with very high sensitivity. Rough surfaces, interfaces, material gradients and mixtures of different materials can be analyzed. 

Fig. 2.14 The scheme of the cylindrical lens method for diffusion coefficient measurement (1) the source with the horizontal slit (2) the condenser supplying a handle of parallel beams (3) the cuvette with a refraction index gradient where the beams are deflected (4) the objective lens focusing the parallel beams to a single point (5) the optical member with an oblique slit and a cylindrical lens (6) the photosensitive material...

Spin the samples with 200 000 x g at 4 °C for 15 -17 h. After the run, displace the gradient using a more dense solution, e.g., 40% sucrose in Soln. A, colored by a droplet Amido Black 10 B solution. The principle of a displacement apparatus is shown in Fig. 5.4. The RNA content of the fractions is measured either by reading the UV absorption at 260 nm or, if labeled material was used, by counting the radioactivity. To monitor the sucrose gradient, estimate the refractive index of the obtained fractions (concentration, density and refractive index of sucrose solutions are given in Table 8.17). 

However, one anomalous feature of this technique is that there occurs an apparent rapid removal of material from the concentration gradient at the boundary, as evidenced by a reduction in the area under the Schlieren curve. For the standard PVP/dextran T10 system, we observed a reduction of 20 % during the 10 min after the formation of the initial boundary no further changes in the area occurred after this initial event. This reduction in area is not accompanied by the appearance of refractive index gradients elsewhere in the cell47 . The redistribution of material within the cell has been shown to occur by monitoring PVP 360 directly using absorption optics at 237 nm. 

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