Polymer Optics: Amorphous polyimides are thermally stable, promising for optics

Amorphous polyimides have thermo-optic coefficients and volume coefficients of thermal expansion much lower than the widely used PMMA and polycarbonate polymers used in optics.

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Visible and near-infrared (near-IR) optics made of polymers are generally much lighter in weight than glass optics and can be easily molded into asphere and freeform shapes, as well as shapes that include integral mounting tabs and other useful additions. Two disadvantages of polymer optics, however, are that they typically have a high thermo-optic coefficient (change in refractive index as temperature changes, or dn/dT), as well as a high volume coefficient of thermal expansion (VCTE) that causes mechanical movements and deformations of optical components.

Researchers at the Naval Research Laboratory (NRL; Washington, DC) and KeyW (Hanover, MD) have been studying a group of amorphous polyimide polymers that have dn/dT and VCTE about 50% smaller than those for standard crystalline polymers, and have released a report showing that these polyimides can lead to polymer optics that are thermally more stable than current polymer optics.1

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Refractive index vs. temperature was measured for the various amorphous polyimides; for example, the BTDT polyimide (a) and linear least-squares lines fit to the data. The volume coefficient of thermal expansion (VCTE) vs. 1/m* (where m* is the molecular weight) was calculated from the data for the various polyimides (b) and the results (colored dots) compared to data (open black dots) from a 2017 study of on crystalline polyimides done by Ishige et al.2 The VCTE for the amorphous polyimides is significantly lower than those for the crystalline polyimides.

Fabrication and measurement

Polyimides prepared from various monomers including 4,4’- (hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3’,4,4’- biphenyltetracarboxylic dianhydride (BPDA), bicyclo [2,2,2]oct-7-ene- 2,3,5,6-tetracarboxylic dianhydride (BTDA), and 4,4’-oxydiphthalic anhydride (OPDA) were studied. To make the polyimides, the monomers, in solution, were polycondensed at room temperature, then subjected to thermal imidization in which the solutions were laid down in a film on a glass substrate, then dried at 80°C and imidized at higher temperatures. The result was polyimide films 50 to 100 µm thick, which were them peeled from the glass substrates.

The refractive indices of the films were measured using a prism coupler in a dry nitrogen atmosphere, with a piston pressing the films against the prism. The researchers added a heater to the setup to vary the temperature, with calibrated thermocouples monitoring the temperature. Measurements were taken at a wavelength of 632.8 nm. Measurement of dn/dT, which is essentially the measurement of relative changes in refractive index rather than absolute measurements, can be done to an accuracy of ±5 × 10-5, the researchers say.

The films showed no anisotropy between crossed polarizers and were thus assumed to have no in-plane anisotropy. The researchers measured the birefringence between the in-plane and out-of-plane directions using s- and p-polarized light and found the resulting birefringence to be significantly larger than the uncertainty in measuring refractive index. This birefringence was taken into account for the thermo-optic measurements by using an average of the indices measured with s- and p-polarizations.

After drying the film samples in a dry nitrogen atmosphere, refractive-index measurements were taken at temperatures between 25° and 105°C, with about 20 minutes of temperature equilibration between measurements.

Measurements were taken for the polyimides created from the various monomers (see figure), yielding both the dn/dT and the VCTE data. The material with the smallest VCTE was the BPDA, with a VCTE of 97 ppm/°C between 25° and 105°C. This is in comparison to the widely used conventional polymers PMMA and polycarbonate, which have VCTE at near 25°C of 215 ppm/°C and 184 ppm/°C, respectively. In other words, the VCTE of BPDA is 45% and 60% of PMMA and polycarbonate, respectively.

The researchers say that the amorphous polyimides with the lowest VCTE values had in nonplanar structures with “sterically hindered, kinked linkages between phenyl rings.” This is in contrast to the typical VCTE-lowering mechanism in crystalline polyimides, which is a rigid linear backbone molecular structure.

The next step for the researchers will be to take the most-promising polyimides and optimize the processing and annealing procedures in their fabrication.

REFERENCES

1. A. Rosenberg et al., Opt. Mater. Express, (2018); https://doi.org/10.1364/ome.8.002159.

2. R. Ishige et al., Macromolecules, 50, 5, 2112–2123 (2017); doi:10.1021/acs.macromol.7b00095.

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