NSRRC Activity Report 2022

084 NSRRC ACTIVITY REPORT 2022 Effect of Homopolymer Chain Length on Phase Behavior of Polystyrene-Block-Poly(methyl Methacrylate) Films The morphology and phase interactions of polystyrene-block-poly(methyl methacrylate) blended with homopolystyrenes of three molecular weights were investigated. S elf-assembling block copolymers (BCPs) offer an extremely versatile method for forming various types of nanodomains with diverse morphologies and orientations. The complexity of BCP thin films can be increased by blending a homopolymer with them. A team led by Ya-Sen Sun (National Central University) systematically studied the effects of chain length on the phase behavior of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA): homopolystyrene (hPS) (B wt% H wt% ) blend films. Figure 1 shows the scheme for the phase behavior of B 75 H 25 x (x = 2.8, 6, and 17) blend films. This scheme is based on the morphological observations and grazing-incidence small-angle X-ray scattering (GISAXS) characterizations at TLS 23A1 . The perforated layers (PLs) in the PS-b-PMMA-rich region of the B 75 H 25 2.8 films form with a parallel orientation only, regardless of film thickness. Because of their short length, hPS 2.8 chains tend to be distributed uniformly in polystyrene (PS) perforations and layers. Furthermore, the uniform distribution of hPS 2.8 chains in PS perforations causes a swell in the chemical junctions at the interface between PS and poly(methyl methacrylate) (PMMA) (case i). A uniform distribution may not exist for hPS 6 and hPS 17 . hPS 6 and hPS 17 exhibit different local segregation behaviors (cases ii and iii). Almost all hPS 17 chains locally segregate at the middle of the perforation, whereas hPS 6 chains tend to be distributed in an intermediate state in which uniform distribution and local segregation coexist. The local segregation behavior affects the phase behavior of B 75 H 25 6 and B 75 H 25 17 films. To further investigate the morphology, the team of Sun used neutron reflectometry (NR) instrument ( PLATYPUS ) at the Australia's Nuclear Science and Technology Organisation (ANSTO) to trace the vertical distribution of hPS x chains within self-assembled nanodomains. Two deuterated homopolymers (dPS) were used to enhance the contrast in scattering length density (SLD) between PS-b-PMMA and dPS. The fitting model contained (1) a top layer of mixed PS and dPS near the free surface of a film, (2) alternating layers with a certain periodicity as the middle region of the film, and (3) a bottom layer of PMMA chains richly anchored onto the substrate interface. Comparisons of the SLD profiles displayed in Figs. 2(c) and 2(d) indicate one similarity and two discrepancies between the prepared B 75 D 25 4 and B 75 D 25 17 films. The similarity is that the top layer always consists of mixed PS and dPS chains. Therefore, the SLDs of the top half-layer were 3.57 × 10 −6 and 4.80 × 10 −6 Å −2 for B 75 D 25 4 and B 75 D 25 17 , respectively. The top layer containing mixed PS and dPS was formed because of their lower surface energies. The first discrepancy is that the bottom half-layer near the substrate interface may consist of either neat PMMA chains or a mixture of dPS and PMMA chains. The SLDs of the bottom Fig. 1 : Scheme for the spatial distributions of hPS x chains in the PS-b-PMMA-rich region of B 75 H 25 x thin films (thickness = 80 nm) annealed at 270 °C: x = 2.8 (i), 6 (ii), and 17 (iii). [Reproduced from Ref. 1]