Chain Dynamics on Crossing the Glass Transition: Nonequilibrium Effects and Recovery of the Temperature Dependence of the Structural Relaxation

Abstract

In this paper we report thermally stimulated depolarization current results on the chain and segmental dynamics of two monodisperse polyisoprenes accessing both dynamics at ultralow frequency range and exploring the relationship between segmental and chain time scales when crossing the glass transition. In this range, we have recorded experimental evidence of nonequilibrium effects on the slowest chain mode dynamics. The nonequilibrium effects seem to occur simultaneously for both chain and α-relaxation. Moreover, detailed analysis strongly indicates the recovery of an even T-dependence for the chain and α-relaxation dynamics on crossing glass transition and in the glassy state. The obtained results can be understood taking into account the different temperature dependences of the length scales involved in the segmental and chain relaxations.A ccording to Rouse and reptation tube models 1−3 for polymer chain dynamics, the characteristic time of the chain motion is defined in terms of the so-called friction coefficient and its temperature variation. Traditionally this coefficient has been identified with that controlling the segmental motions of the polymer chain leading to the expectation that both chain dynamics and the structural α-relaxation have equal temperature (T)-dependences. However, systematic tests of this assumption have proven that this is not the case and that the α-relaxation shows a stronger Tdependence than chain modes especially as the temperature approaches the glass transition one, T g . 4−7 The thermally stimulated depolarization current (TSDC) is a dielectric technique that has proven recently to be a very powerful tool allowing the study of chain dynamics in detail at ultralow frequencies 8 and at relatively low temperatures close to T g a range where chain end-to-end fluctuations detected by dielectric experiments have scarcely been explored. In this paper we report TSDC results on the chain and segmental dynamics of two monodisperse polyisoprenes accessing both dynamics at the ultralow frequency range and in this way exploring the relationship between segmental and chain time scales when approaching and crossing the glass transition. For low molecular weight polyisoprene (PI), nonequilibrium effects on the slowest chain mode are observed for the first time. Moreover, the detailed analysis of the results strongly indicates the recovery of an even T-dependence for the chain and α-relaxation dynamics on crossing T g and deep in the glassy state.Two monodisperse PI samples with molecular weights M w = 10 500 Da, PI10, and M w = 2900 Da, PI3, were investigated. PI3 is below the entanglement regime, while PI10 is right at the transition between entangled and nonentangled systems. The PI chain contains dipolar components parallel to the chain backbone yielding a net "end-to-end" polarization vector. This allows to follow chain dynamics by dielectric relaxation techniques, and as a consequence PI is a very well-studied polymer by broadband dielectric spectroscopy (BDS) in particular. 6,7,10,11 A...

Dielectric relaxation of 2-ethyl-1-hexanol around the glass transition by thermally stimulated depolarization currents

“…It is interesting to note that the observed phenomenology is very much analogous to that found for the normal mode of a linear polymer, polyisoprene, 22 on crossing the glass transition. In that case, the constant τ N M /τα ratio observed below T g was rationalized appealing to the characteristic length scales of each process and their variation with temperature.…”

“…Similar results are well documented in the literature for other glass forming systems. 22,24,32 Although not as prominent as in the case of the α relaxation, the characteristic times obtained for the Debyelike relaxation by TSDC also show deviations with respect to the VFT fit of the isothermal data at low temperatures below the calorimetric glass transition (data at a reduced scale are included in Figure 5(c)). These small deviations have its reflect in the depolarization current.…”

“…As commented in the Introduction, TSDC technique is very well suited for the detailed and precise study of the possible dispersion of relaxation times in the Debye-like process, and provides a natural way to analyze the mode composition of a chain-like response. 21,22 For this purpose, a series of TSDC experiments were carried out where the polarizing field was switchedon at a sufficiently high temperature for the Debye-like process to be fully polarized, whereas T off was systematically varied to later eliminate the contribution of faster modes. Figure 1(a) shows depolarization currents for 2E1H obtained by varying the polarizing temperature window.…”

“…Finally, TSDC technique is very well suited for the detailed and precise study of the possible dispersion of relaxation times in the Debye-like process, and provides a natural way to analyze the mode composition of a chain-like response, as it has been recently demonstrated. 21,22 With these ideas in mind, we have investigated here the dielectric response of the extensively studied 2E1H as an archetype of MAs to illustrate the potential application of the experiments proposed for investigating the behavior of the Debye-like relaxation around the glass transition. Another goal of this work will be exploring to which extent the analogy of polymer chain dynamics is compatible with that of the chainlike aggregates formed by MAs.…”

A complete TSDC description of molecular mobilities in polylactide/starch blends from local to normal modes: Effect of composition, moisture, and crystallinity

“…Thermally stimulated depolarization current (TSDC) technique is probably the most adequate technique to study the normal mode as overlapping with neighboring relaxations can be resolved by the choice in the polarization conditions. Only in very recent literature, a few TSDC works on polyisoprene [9][10][11] or PLLA 12 with very low molecular weights report the observation of a normal mode.…”