Dynamics around the liquid-glass transition in poly(propylene-glycol) investigated by wide-frequency-range light-scattering techniques

Abstract

The structural dynamics of poly͑propylene-glycol͒ of molecular weight M W ϭ4000 have been investigated over a large temperature range 10-375 K and in a wide dynamical window, corresponding to 10 Ϫ3 -10 14 Hz, using various light-scattering techniques. The slow dynamics were investigated using a wide time-range photon-correlation spectroscopy; for the faster dynamics a combination of interferometric and grating spectrometer techniques were used. We observe four distinguishable kinds of dynamics; ͑i͒ slow normal-mode dynamics, ͑ii͒ the main (␣) relaxation which is related to the viscosity, ͑iii͒ a faster ͑␤͒ relaxation, and ͑iv͒ a low-frequency vibrational peak. The data are discussed in relation to the mode-coupling theory ͑MCT͒ for the liquid-glass transition. Surprisingly, the slow dynamics observed using the PCS technique close to the glass transition temperature T g are found to be in good agreement with predictions of MCT and a T c ϭ236 K can be extracted. In contrast, the high-frequency data taken above T c are not consistent with MCT. In this range a strong vibrational peak, the so-called boson peak, seriously affects the relaxational spectrum and simple MCT analysis cannot be applied. This finding is in agreement with recent light-and neutron-scattering investigations of other hydrogen-bonded intermediate glass formers and also strong covalently bonded systems.

Breakdown of time–temperature superposition: From experiment to the coupling model and beyond

“…The dynamic crossover temperature has been estimated for a few polymers [15][16][17], but not for PIP. The easiest way to estimate the crossover temperature is the use of the so-called Stickel plot [18].…”

Positron annihilation response and broadband dielectric spectroscopy: Poly(propylene glycol)

“…The positron lifetime spectra were analyzed using the well-known PATFIT-88 software package [22] in terms of: a short-term component from para-positronium p-Ps, s 1 , an intermediate one attributed to ''free" positron s 2 , and a long-term one, related to free volume: ortho-positronium o-Ps, s 3 . The PPG sample pre-treated with the drying procedure from [27] was measured during heating cycle. The acquisition time per each temperature point was at least 2 h.…”

“…¼ ð2:11 AE 0:23Þ Â 10 À2 T À ð2:87 AE 0:61Þ r ¼ 0the first characteristic PALS temperature is close to the glass transition temperature from calorimetry T DSC g = 200 K[27], so that it defines the glass transition temperature as obtained by the PALS method. Below T PALS g , s 3 $ 1.3-1.5 ns and falls into the typical range for many amorphous systems studied so far.…”

Harmonic Vibrational Excitations in Disordered Solids and the “Boson Peak”

“…The boson peak seems also to persist at elevated temperatures. Here its relation to the liquid-glass transition and the corresponding relaxation dynamics is a matter of discussion [9][10][11][12][13]. Due to the development of new experimental techniques allowing to perform Brillouin scattering measurements in the THz range [14][15][16][17], as well as pertinent molecular dynamics simulations [18][19][20][21][22] the question concerning the nature of the modes in the boson peak region has gained much additional interest recently.…”