Study on Structure and Dynamics of Polymers at VariousInterfaces by Neutrons
Keiji Tanaka
Department of Applied Chemistry, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
In the last two decades, it has been accepted that aggregation structures and physical properties ofpolymer materials at the air facing surface are different from those in the corresponding bulk. However,the surface will contact with a different phase if the polymer is used in applications such as biomaterials,
nanocomposites, etc. Thus, the interfaces contacted with different phases should be studied structurally
and dynamically as a next step. In this talk, we focus on the aggregation structure of perdeuteratedpoly(methyl methacrylate) (dPMMA) films in the presence of alcohol, which are all non-solvents for
dPMMA, obtained by neutron reflectivity (NR).1-3) Monodisperse dPMMA with a number-averagemolecular weight (Mn) of 296k was used. Methanol, ethanol, 1-propanol, and 1-butanol were used as
typical non-solvents for PMMA. Films of dPMMA were prepared from a toluene solution onto quartz
blocks by spin-coating. The thickness of the films measured by ellipsometry was in the range of 63 to68 nm. The films were annealed for 24 hr at 423 K in vacuum to relax the film preparation history.
Density profiles of the dPMMA films along the direction normal to the surface under the alcohols wereexamined by NR measurements. The neutron beam was guided into the dPMMA film from the quartz
side and the reflected beam was detected under the specular condition. Reflectivity was acquired as afunction of scattering vector (q), q = (4π/λ)sinθ, where λ and θ are the wavelength and theincident angle of the neutrons, respectively. The model to express a film structure was composedof three layers; alcohol, dPMMA and quartz. Two interfaces between alcohol and dPMMA and between
dPMMA and the quartz substrate were, respectively, expressed by error and exponential decay functions.
Thus, three parameters, the interfacial roughness between alcohol and dPMMA (σ), the alcohol content
in the internal part of the dPMMA film (φ) and the decay length of the excess amount of alcohol at thesubstrate interface (ξ), were used to fit the experimental data. Since the alcohol molecules were notdeuterated, the (b/V) value at a depth in the film decreased if they penetrated into the films. Also, at thesame time, the film became thicker to conserve the mass of PMMA. The extent was in the order ofmethanol > ethanol > 1-propanol > 1-butanol. This order seems to be quite reasonable because
methanol possesses the smallest size among them and also forms the highest number density of hydrogenbonds with side chains of dPMMA. The interfacial width of dPMMA with alcohol and the excess
amount of alcohol at the substrate interface also varied in the same order as the extent of sorption.

Figure 1 (a) Neutron reflectivity for dPMMA films in contact with air and non-solvents. Open symbols denote experimental
data, and solid lines are reflectivity calculated on the basis of the scattering length density profiles shown in (b). (b) Scattering
length density profiles for dPMMA films in contact with alcohols. The abscissa is normalized by the original thickness in air. (c)A model for a dPMMA film on quartz in an alcohol to fit the experimental reflectivity in panel (a). The model involves three fitting
parameters: the width of the alcohol-dPMMA interface (φ), the alcohol content in the internal region of the film (σ) and the decay
length at the substrate interface (ξ).

References
1. Langmuir 24(1), 296-301 (2008).
2. Chem. Lett. 39(8), 810-811 (2010).
3. J. Phys. Chem. Lett. 1(5), 881-885 (2010).