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Monitoring of glass transition at a polymer surface by localized surface plasmon resonance

A. Kaan Kalkan, R.K. Putla
Oklahoma State University, US

Keywords: localized surface plasmon resonance, surface glass transition temperature


The temperature below which a polymer is in glassy state and above which is in viscous state is called the glass transition temperature (Tg). Investigations on polymer surfaces by various groups revealed that the surface Tg (Tgs) is not equal to the bulk Tg (Tgb). The present work investigates Tg on a polymer surface by embedding gold nanoparticles (AuNP) and exploiting the localized surface plasmon resonance (LSPR) of the AuNP as the potential probe. The impregnation of AuNP into the polymer causes a spectral shift in the LSPR due to the changing dielectric environment surrounding nanoparticles. LSPR is sensitive to the close vicinity of the nanoparticles within a few nm from the nanoparticle surface. Therefore, the diffraction limit of light can be beaten by orders of magnitude and impregnation of AuNP into the polymer can be monitored to a spatial resolution down to a nm. AuNP, at an average size of 8 nm, were synthesized on poly isobutyl methacrylate (PiBMA) films on glass by physical vapor deposition. Subsequently, time series LSPR spectra of AuNP on PiBMA were acquired in transmission mode in a temperature controlled optical cell using a CCD spectrophotometer. The forced convective heating of the sample by Ar gas through the optical cell enables rapid stabilization of the temperature in few seconds. The temperature was measured at the sample surface by a K-type thermocouple. The effective dielectric constant surrounding the AuNP was calculated from LSPR optical extinction peak. Depth and velocity of penetration of AuNP were derived from the measured dielectric constant on the basis of dielectric mixing. Two types of measurements were conducted. First, the temperature was gradually increased from 25 to 70 ºC at 5 ºC increments maintaining the sample at each temperature for 10 min. The onset of AuNP penetration was found to be at 45 ºC, which is 10 ºC below the reported glass transition of PiBMA. Second, time series spectra were recorded at temperatures of 45, 55, 60, and 65 ºC until saturation of the LSPR signal (i.e., end of penetration of AuNP). The penetration velocity was found to be fairly constant in time while temperature activated at an energy of 0.65 eV.
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