Surface Ordering of Block Copolymers

A linear relationship between fractional surface excess of PDMS (Ψ) and PDMS block length (N_A, Ψ = α^′ -βN_A- ^1/2, is found for the PS-PDMS-PS type triblock copolymers from ESCA results over three different sampling depths, ranging from 27 to 103 Å. The effect of PDMS surface segregation Ψ is compared within the PS-PDMS diblock, PS-PDMS-PS and PDMS-PS-PDMS triblock copolymer series as N_A changes. At similar bulk composition, the PDMS segments in the PS-PDMS diblock copolymers are the most apt to segregate in the free surface region, while the PDMS segments of the PS-PDMS-PS triblock copolymers are the least probable to segregate in the free surface region. The surface morphologies of the PS-PDMS diblock and PS-PDMS-PS triblock copolymers are lamellar with the periods of 600 and 450 Å respectively, which are revealed by TEM micrographs of the cross sections of the films both near the free surface and in the bulk. A morphology of PS spheres in PDMS matrix is found for the PDMS-PS-PDMS triblock copolymer. The period of the lined PS sphere arrays separated by the PDMS domains is measured to be 130 Å.