Quantifying Changes in the Low-Frequency Dynamics of Amorphous Polymers by 2D Correlation Mechanical Spectroscopy

Abstract

The longer segmental dynamics of sub-Rouse modes in polystyrene with different molecular weights has been investigated by 2D correlation mechanical spectroscopy. The sub-Rouse modes were first separated from the α relaxation and Rouse modes, and their dynamics exhibits a similar change at the same temperature, T(B) ≈ 1.2T(g), as the α relaxation. The relaxation time of sub-Rouse modes at T(B) is independent of molecular weight and has a value of about 0.1 s, indicating that solely the time scale of the relaxation determines the change in dynamics of sub-Rouse modes. According to the coupling model, the change is caused by a strong increase in intermolecular cooperativity. The present work provides direct evidence for the intermolecular coupled nature of the sub-Rouse modes and demonstrates that the properties of the sub-Rouse modes resemble those of α relaxation, which could provide a new perspective for understanding the glass transition of polymers.

Segmental α-Relaxation for the First Step and Sub-Rouse Modes for the Second Step in Enthalpy Recovery in the Glassy State of Polystyrene

“…The sub-Rouse modes in polymers were first revealed by combined creep compliance and stress relaxation experiment, and by light scattering experiments on polyisobutylene in 1995. 21,22,23 Since then, the general existence of the sub-Rouse modes in other polymers has been confirmed and their properties brought out by various experimental studies 24,25,26,27,28,29 . In this section we invoke the properties of the sub-Rouse modes supported theoretically by the Coupling Model (CM) to justify that they are responsible for the slower process in the two-step enthalpy recovery observed by Cangialosi and coworkers 1,2,17,18,19 .…”

“…3 has the Vogel-Fulcher-Tammann (VFT) temperature dependence much stronger than the approximately Arrhenius temperature dependence of eq,1, but instead we know that sR has weaker temperature dependence than . 21,22,23,24,25,26,29,30,31,32,33,34,35,36,37,38,39,40,41,44 However, upon closer examination of the time evolution of the recovered enthalpy at different temperatures for the high molecular weight polystyrene with Mn=85 kg/mol (PS85k), the presence of second plateau in their 2 figure 2a at 363 K and 358 K used to obtain eq,2 and deduce its VFT dependence is not evident, and therefore may not be used to deduce that eq,2 has the strong VFT temperature dependence below Tg. To show this, we have reproduced the data at the two lowest temperatures 363 K and 358 K in Fig.…”

Durably Ductile, Transparent Polystyrene Based on Extensional Stress-Induced Rejuvenation Stabilized by Styrene–Butadiene Block Copolymer Nanofibrils

“…The shape of the loss modulus curve exhibits an asymmetric singlepeak structure, revealing the intensity of molecular motion at various temperatures. The position of the loss modulus peak represents the length of motion units, 34 while the area under the curve denotes the "relaxation strength" in the specimens. 35,36 Since the peak widths are approximately similar, we utilize the peak value to indicate relaxation strength.…”

Mechanical Spectroscopy: Some Applications On Structural Changes And Relaxation Dynamics In Soft Matter

“…Recently, a series of polymers such as polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc) have been measured by mechanical spectroscopy in torsion mode at a frequency range 10 -2 -10 2 Hz with a constant heating rate or at a given temperature versus frequency (isotherms) [9][10][11][12][13]. Fig.…”

“…Due to the overlap and coupling of different molecular modes, it is difficult to resolve the contribution of each mode and gain more information. With the help of two-dimensional correlation analysis, three peaks are well resolved in the synchronous 2D correlation spectra of tanδ with frequency, corresponding to the α-relaxation, the sub-Rouse mode and the Rouse mode, respectively [13]. For the sake of clarity, examples of the fits to the spectra at 402 K are given.…”