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

The lateral order of poly(styrene-block-isoprene) copolymer (PS-b-PI) thin films is characterized by the emerging technique of resonant soft X-ray scattering (RSOXS) at the carbon K edge and compared to ordering in bulk samples of the same materials measured using conventional small-angle X-ray scattering. We show resonance using theory and experiment that the loss of scattering intensity expected with a decrease in sample volume in the case of thin films can be overcome by tuning X-rays to the pi* resonance of PS or PI. Using RSOXS, we study the microphase ordering of cylinder- and phere-forming PS-b-PI thin films and compare these results to position space data obtained by atomic force microscopy. Our ability to examine large sample areas (~;9000 mu m2) by RSOXS enables unambiguous identification of the lateral lattice structure in the thin films. In the case of the sphere-forming copolymer thin film, where the spheres are hexagonally arranged, the average sphere-to-sphere spacing is between the bulk (body-centered cubic) nearest neighbor and bulk unit cell spacings. In the case of the cylinder-forming copolymer thin film, the cylinder-to-cylinder spacing is within experimental error of that obtained in the bulk