Structural Relaxation of Ultrathin Polymer Films Prepared by the Langmuir−Blodgett Technique:  Characteristics of the Two-Dimensional System

Abstract

Thermal relaxation of ultrathin polymer films prepared by the Langmuir−Blodgett (LB) technique was studied by a comparison with a three-dimesional bulk system. Poly(vinyl alkanal acetal)s (PVAA) with different side chain lengths were synthesized and the thermal stability of their LB films was investigated by the energy-transfer phenomenon between fluorescence probes labeled to the polymer chains. The multilayer structure was irreversibly disordered and mixed by the thermal treatment. A close relationship was found between the glass-transition temperature (T g) of the polymer bulk and the temperature at which the layered structure of LB films started to disorder. Theoretical calculations based on Förster kinetics were applied to measure the diffusivity of polymer segments, which was successfully evaluated by assuming time-dependent Gaussian distributions of chromophores in the direction normal to the layer plane. The apparent activation energy ΔE a of the diffusion constant for the LB films, however, was much smaller than that for polymer bulk obtained by the viscoelastic measurement. It was concluded that the relaxation process of the ultrathin polymer films occurs by conformational changes from the two-dimensional nonequilibrium state given by the LB deposition. The confined form could be released by the local motion of the polymer segments under the heating to temperatures higher than T g.

Two-Dimensional Polymers Investigated by Scanning Near-Field Optical Microscopy: Phase Separation of Polymer Blend Monolayer

“…(iv) The fluorescence decay with the SNOM probe located at position x , S ( t ; x ), was calculated as the sum of the decay curves for perylene molecules under the aperture. The detailed calculation procedure for steps iii and iv is described elsewhere. , (v) The calculations ii−iv were carried out with respect to the different probe positions. Finally, the fluorescence lifetime at each probe position was obtained according to eq 2.…”

Nanostructure and Thermal Stability of the Polyglutamate Langmuir−Blodgett Films Probed by Interlayer Energy Transfer Method

“…35 This process is mostly governed by the "entropy relaxation" from a 2-dimensional conformation to a 3-dimensional random coil state. 37 On the contrary, in the PG LB films, the R-helical structure of the polyglutamate chain which is stabilized by hydrogen bonds makes the macromolecules behave like a rigid rod. This rigid structure restrains the segmental motion of the polymer main chains; therefore, the layered structure is kept stable.…”

Conformational relaxation of single polymer chains confined in a two-dimensional plane studied by scanning near-field optical microscopy

“…In the last few decades, the thermal stability of Langmuir-Blodgett films has been examined by polarized infrared spectroscopy, 3,4 the optical second-harmonic generation method, 5 small-angle X-ray scattering, 6,7 X-ray reflectivity 8,9 and the energy transfer technique. 10,11 Several research groups reported the structural relaxation of polymer LangmuirBlodgett films in nanometer dimensions by thermal treatment and suggested that chain conformation was rearranged from a twodimensionally restricted form to a three-dimensional equilibrium conformation by the local motion of polymer segments. [8][9][10] In contrast to the methods used in previous studies, the direct observation of individual chains is expected to provide clear evidence on chain conformation and would be helpful in understanding the previous results obtained from ensemble-averaged measurements.…”

“…10,11 Several research groups reported the structural relaxation of polymer LangmuirBlodgett films in nanometer dimensions by thermal treatment and suggested that chain conformation was rearranged from a twodimensionally restricted form to a three-dimensional equilibrium conformation by the local motion of polymer segments. [8][9][10] In contrast to the methods used in previous studies, the direct observation of individual chains is expected to provide clear evidence on chain conformation and would be helpful in understanding the previous results obtained from ensemble-averaged measurements. Recently, several studies on the direct observation of a single polymer chain using atomic force microscopy have been reported.…”