Photon correlation spectroscopy of bulk poly(n‐hexyl methacrylate) near the glass transition

Abstract

Slowly relaxing longitudinal density fluctuations in an optically perfect sample of bulk poly(n‐hexyl methacrylate) (PHMA) have been studied by photon correlation spectroscopy in the temperature range 10–36°C. The glass transition temperature for this sample was measured to be Tg = −3°C by differential scanning calorimetry. The optical purity of the sample was verified by Rayleigh‐Brillouin spectroscopy and the Landau‐Placzek ratio was observed to be 2.3 at 25°C. Light‐scattering relaxation functions were obtained over the time range 10−6‐1 s. The shape of the relaxation functions broadened as the temperature was lowered towards the glass transition. Quantitative analysis of the results was carried out using the Kohlrausch‐Williams‐Watts (KWW) function to obtain average relaxation times, 〈τ〉, and width parameters, β. The width parameter decreased from 0.43 to 0.21 over the temperature interval, as suggested by visual inspection. Average relaxation times shifted with temperature in a manner consistent with previous mechanical studies of the primary glass‐rubber relaxation in PHMA. The relaxation functions were also analyzed in terms of a distribution of relaxation rates, G(Γ). The calculated distributions were unimodal at all temperatures. The average relaxation times obtained from G(Γ) were in agreement with the KWW analysis, and the shape of the distribution broadened as the sample was cooled. The rate at which G(Γ) displayed a maximum correlated well with the corresponding frequency of maximum dielectric loss for PHMA. The temperature dependence of these two quantities could be reproduced with an Arrhenius activation energy of 21 Kcal/mol. A consistent picture of the molecular dynamics of PHMA near the glass transition requires a strong secondary relaxation process with a different temperature dependence from the primary glass‐rubber relaxation. The present results suggest that the behavior of PHMA is similar to the other poly(alkyl methacrylates). © 1996 John Wiley & Sons, Inc.

Evidence for the γ‐relaxation in the light scattering spectra of poly (n‐hexyl methacrylate) near the glass transition

“…A study by Geibel, looking at the PCS of PHMA over a temperature range from −2 to 25 °C reported a constant value for β of 0.27 ± 0.2 7. Hwang found that the β parameter varied between 0.21 and 0.43 over the temperature range from 10 to 36 °C 3. In the current studies, the β parameter is weakly temperature dependent over the experimental temperature range.…”

“…The LPR reported for many Brillouin scattering studies readily exceeds the intrinsic value. Meier reports an LPR of 8 at 50 °C,18 and Hwang reports an LPR of 2.3 at 25 °C for PHMA 3. This increase relative to the intrinsic value is a result of impurities in the sample that give rise to light scattering that does not come solely from longitudinal density fluctuations.…”

“…The relaxation functions for poly (alkylmethacrylates) are highly nonexponential 3, 5, 38. Initial analysis represented the relaxation function as a stretched exponential function (eq 18).…”

“…The random motion of molecules leads to local density fluctuations within the medium, and these fluctuations can be represented as a distribution of relaxation times 2. Light scattering has been employed to study the time dependence of density fluctuations for many poly(alkyl methacrylates) (PAMAs) near the glass transition 3–8. Two different experiments allow for the study of different time scales.…”

“…Three previous studies of the light scattering from PHMA have been published, but they exhibit conflicting results 3, 7, 18. To study the full range of relaxation behavior and improve the analysis of light scattering relaxation functions in PHMA, an optically perfect sample is necessary and the experimental data must be more refined.…”