Surface segregation structure of polymer-grafted silica nano-particle in polymer thin films
Motoyasu Kobayashi（Kogakuin University）
1. Introduction
Polymer brushes, assemblies of surface-tethered polymers on solid substrates, are useful for surface
modification. The "grafting-from" synthetic procedure is suitable method to fabricate the densely grafted polymers
on the material surfaces, however, it is not easy to fabricate the polymer brushes on a large surface area at once bythe "grafting-from" method because of its complex procedure and inert chemical condition for surface-initiatedpolymerization reactions.
We propose the simple surface coating of polymer brush-immobilized nano-particle dispersed in polymer
matrices to prepare the brush-like surface. Silica nano-particles (SiNP) covered with high-density polymer brushes
are expected to segregate at the film surface, as follows. Theoretically, intermixing of polymer brush and theunbound (free) polymer chains in a matrix hardly occur even for chemically identical polymer brush and matrix
when the graft density of the brush is high, because intermixing of high-density brush and free polymer isentropically unfavorable. Therefore, the self-consistent field theory predicted phase separation of the high-density
polymer brush and matrix polymer to restrict a fine dispersion of polymer-grafted nano-particles or fillers in thebulk. In addition, free chain ends of grafted polymers tend to segregate at the near the surface due to its low freesurface energy. In this proposal, we prepare the spin coat film of the mixture of deuterated PMMA (d-PMMA)and hydrogenous PMMA (h-PMMA)-grafted SiNP (h-PMMA-g-SiNP) mixture to confirm whether the SiNPsegregate near the film surface to form a brush-like structure at the surface or not by neutron reflectivity (NR).

2. Experiment
Surface-initiated atom transfer radical polymerization of MMA from alkylbromide- immobilized silica
nano-particle (SiNP) (d = 12 nm) was carried out to fabricate PMMA with Mn = 100000 and Mw/Mn = 1.34 graftedon SiNP (PMMA-g-SiNP). A mixture of 2.0wt% d-PMMA (PMMA-d8, Mn = 68000, Mw/Mn = 1.12) and 0.03 ~0.06wt% PMMA-g-SiNP in toluene was spun casted on flat silicon wafer (20  50  0.5 mm3) to form a thin
PMMA film with 74 ~ 110 nm thickness, as shown in Figure 1(a). The film was annealed at 403 K for 1 h inthermal oven. NR measurements were carried out using a SOFIA reflectometer installed in BL-16 at MLF. The
neutron beam irradiated the PMMA spin coat film on the Si substrate, and the reflected neutrons were collected bya 2D detector. Three incident beam angle (0.3, 0.7 and 1.6 degree) were used to obtain the reflectivity in a q rangeof q = 0.10 ~ 1.5 nm-1 within a 5% resolution. A 12  40 mm of beam footprint with was maintained on thesample surface by using incident slits. The MOTOFIT program was used to fit the reflectivity profiles to modeled
scattering length density (SLD) layers, in which the thickness of each layer, SLD and Gaussian roughness wereoptimized to minimize the least-squares of the measured and calculated reflectivity curves. SLD of d-PMMA,
SiO2, and Si were 6.62  10-4, 3.47  10-4, 2.07  10-4 nm-2 were used. All the NR measurements were conducted
at 298 K.

3. Results
Reflectivity curve of a d-PMMA film containing h-PMMA-g-SiNP on silicon wafer after annealing at 403 K for
1 h represented sequential fringes attributable to thickness of the d-PMMA film (74 nm), as shown in Figure 1(b).
1The fringes gradually became unclear with increasing q, probably due to the SiNP. The SLD profile in Figure 1(C)showed segregation of h-PMMA-g-SiNP near the interface between d-PMMA and Si wafer. NR measurement of
the film before the annealing also revealed similar pattern. These results unfortunately indicated that the h-PMMAbrush layer did not exit at the outer most surface of d-PMMA film. The segregation of h-PMMA-g-SiNP at thebottom of the d-PMMA film occurred probably due to gravity of SiNP and phase separation of h-PMMA brushand unbound free d-PMMA.
(b) 10074 nm
Si waf er (20 x 50 x 0.5 mm3)
SiO2 nano particle

12 nm

(c)
80

Air-1

10Reflectivity

Air

Neutrond-PMMA
Mn = 68000
Mw /Mn = 1.1260-2Depth, nm

(a)

10-3

10d-PMMA

40
20PMMA-g-SiNP

10-4
h-PMMA brush
Mn = 100000
Mw /Mn =1.34010-5
0.10q, nm-1

1.0

substrate

01234567Scattering Length Density, 10-4 nm-2Figure 1. (a) Schematic view of d-PMMA film containing h-PMMA-g-SiNP on silicon wafer, (b) NR curve of thesample after annealing and the corresponding fit, and (c) SLD profile estimated by NR fitting curve.

4. Conclusions and perspective
NR measurement in this work revealed that PMMA-g-SiNP heterogeneously dispersed in the PMMA thin filmmatrix to form a concentrated region of SiNP near the interface between silicon substrate and PMMA film,
probably due to the gravity. The resulting PMMA film formed an undesired structure, however, the bottom
segregation of PMMA-g-SiNP is also interesting structure for a accumulation of polymer brush at the interface. In
future work, we are going to investigate the effect of chemical structure of surface-grafted polymers, graft density,and molecular weight distribution on the aggregation structure in the polymer thin film. In addition, removal of
the resulting film containing polymer-g-SiNP from the substrate surface will be also tried by using a sacrificial
film layer on the substrate, to isolate the film with brush-like surface structure.

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