Grain Structure in Block Copolymer Thin Films Studied by Guided Wave Depolarized Light Scattering

Abstract

A new optical technique for characterizing the grain structure of ordered block copolymer thin films has been developed. The technique, which we refer to as guided wave depolarized light scattering (GWDLS), is an adaptation of previous work wherein polarized light was used to characterize the grain structure in bulk block copolymer samples [Newstein et al. Macromolecules 1998, 31, 64]. Spin-casting and vacuum-annealing were used to prepare thin films of an ordered poly(α-methylstyrene-block-isoprene) copolymer on a flat, fused silica substrate. The ordered phase consisted of poly(α-methylstyrene) cylinders in a polyisoprene matrix. A plane-polarized laser beam was coupled into and out of a transverse magnetic (TM) mode of the film, which acts as a planar waveguide. The angle of incidence is adjusted to optimize the coupling efficiency into a particular mode of the waveguide. The path length traveled by the guided wave between the input and output coupling points was approximately 1 cm. The polarization of some of the incident light changes due to encounters with randomly oriented, optically anisotropic grains. This results in the coupling of light into propagating transverse electric (TE) modes in the sample. We show that the TE beam intensity of annealed samples with well-developed grain structure is significantly larger than that obtained from unannealed samples with poorly developed grain structure. The GWDLS results are consistent with atomic force microscopy results obtained from the annealed and unannealed films.

Measurement and Analysis of the Angular Guided-Wave Depolarized Light Scattering Patterns from Block Copolymer Thin Films

“…Experiments described in this paper were performed primarily on an anionically synthesized poly(styrene-block-isoprene) copolymer. The molecular weights of the polystyrene and polyisoprene blocks were 7.3 and 22.6 kg/mol, respectively, and this polymer is referred to as SI (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). The volume fraction of the polystyrene block in SI(7-23) was 0.217, and the morphology in the ordered state was previously determined, through SAXS and TEM, to be hexagonally packed cylinders of polystyrene in a polyisoprene matrix.…”

“…As the light propagated along the film it generated depolarized scattered light, which was then coupled out of the film and analyzed. To date, this method, called guided-wave depolarized light scattering (GWDLS), remains the only reciprocal space technique that quantifies the grain structures within ordered block copolymer thin films. − …”

“…GWDLS experiments have been conducted using two different coupling methods. Prism-coupled GWDLS was the first attempt at this technique, in which the absolute grain size was measured from the ratio of the integrated depolarized power to the integrated polarized power. , However, this coupling method is applicable only to hard films and cannot be used to study soft samples near the order−disorder transition temperature ( T ODT ). Grating-coupled GWDLS was developed to overcome these shortcomings.…”

“…To date, this method, called guided-wave depolarized light scattering (GWDLS), remains the only reciprocal space technique that quantifies the grain structures within ordered block copolymer thin films. [21][22][23] In a GWDLS measurement, laser light is coupled into and out of the thin-film waveguide through optical coupling devices. The incident and scattered light propagate in either discrete transverse electric (TE) or transverse magnetic (TM) modes, which are orthogonal in polarization and can be isolated using an analyzing polarizer.…”

Analysis of Order Formation in Block Copolymer Thin Films Using Resonant Soft X-ray Scattering

“…16 GWDLS results reported thus far are qualitative in nature and restricted to films that are 1 µm thick. 18 In classical scattering of light, X-rays or neutrons the relationship between the transmitted and incident beam fluxes J x and J 0 is given by Beer's law 19…”