MODEL 2010/M PRISM COUPLER APPLICATION NOTES

APPLICATION NOTE 107

MEASURING INDEX ANISOTROPY/BIREFRINGENCE OF FREE-STANDING POLYMER FILMS

The Model 2010/M measures refractive index in machine (x), transverse (y), and perpendicular (z) directions for free-standing polymer films permitting rapid and accurate determination of polymer density and crystallinity in approximately one minute. In-plane refractive index can also be measured along any arbitrary in-plane direction by a simple rotation of the sample about the coupling point. In some cases, information concerning index gradients in the material vs depth is obtainable, particularly in the case of high index surface skins. Measurement of coating thickness and index is also possible for coating thicknesses greater than approximately one micron. With a single prism, indices ranging from 1.3 to 2.0 are easily measured and measurements are rapid (typically 20 seconds), free of operator subjectivity, and do not require the use of index matching fluids. The spatial resolution of the measurement is approximately 1 mm, permitting high resolution mapping of index/crystallinity differences.

Index of free-standing polymer films thicker than 10 microns (0.4 mil) is easily analyzed by the 2010/M's bulk index measurement mode. In this mode, the 2010/M simply senses the critical angle at the interface between the prism and the material in contact with it:



If a material of index n is in contact with a prism of index np, as the sample and prism are rotated with respect to the stationary laser beam, light striking the base of the prism will be totally reflected to the system photodetector until the angle of incidence becomes less than the critical angle, where

= arcsin(n/np) (1)

The Model 2010/M determines the critical angle automatically and since np is well known, the film index is easily determined from equation (1).

The ability to measure refractive index in x, y, and z directions (see Fig .1) allows measurement of % crystallinity since the average of the x, y and z indices is directly proportional to density/crystallinity (Robert J. Samuels, J. Appl. Pol. Sci., Vol. 26 (1981), p. 1383). Fig. 2 is an example of average index vs crystallinity for polypropylene from this reference and similar data exists for PET. Measurement of x, y, and z indices has also been used to study crystallinity, orientation, and tear properties in polyethylene (Krishnaswamy. et al., Polymer, vol. 41 (2000), p. 9205 and Fruitwala et al., 1994 ANJEC Proceedings, p. 2252). In addition, since the 2010/M's index measurement probes primarily the first few microns at the surface of the material (in some cases, refractive index information can be obtained for deeper regions (see below) refractive index data can be obtained for both sides of the film and sample-side differences (e.g., preferential cooling rates) can be studied.

Thickness and index measurements of coatings or coextruded layers thicker than 1.0-1.5 microns is usually possible (see Fig. 3). The Model 2010/M also offers unique capabilities for characterizing index gradients vs depth in free standing films. If a high index region is present at the surface of a material, there may be two breaks in the intensity vs angle curve and it is often possible to measure the index at the surface as well as the bulk index. In some cases, individual waveguiding modes may exist in the high index skin, and it may even be possible to calculate an approximate thickness and index for the skin layer.

To provide compatibility with Abbe refractometer measurements made at 589 nm, a 594 nm HeNe laser can easily be substituted for the 633 nm HeNe which is normally provided with the 2010/M.

Fig. 1. Measurement of x. y, and z indices for 2-liter PET beverage bottle: Refractive indices along axial (long axis), circumferential, and perpendicular plane directions for outer bottle surface.


Fig. 2. Volume fraction crystallinity (Vc) vs. average index for polypropylene

Fig. 3. Measurement of coating thickness and index on polyester sheet

Press on chart for larger view

 

MONITORING OF PHOSPHORUS AND OTHER DOPANT CONCENTRATIONS IN SILICON DIOXIDE
HIGH ACCURACY MEASUREMENT OF RESISTS, POLYIMIDES, AND POLYMERS
DUAL FILM MEASUREMENTS
OPTICAL WAVEGUIDE CHARACTERIZATION
BULK MATERIAL OR THICK FILM INDEX/BIREFRINGENCE MEASUREMENT
MEASURING INDEX ANISOTROPY/BIREFRINGENCE OF FREE-STANDING POLYMER FILMS

 

 

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Last revised: 11/8/06