Mechanical hole burning spectroscopy in an SIS tri‐block copolymer

Abstract

We have previously developed a mechanical hole burning spectroscopy (MSHB) technique that promises to be a powerful tool for the investigation of dynamic heterogeneity of polymeric materials. This is because, unlike its dielectric analogue, MSHB can be used to characterize materials having a weak dielectric response, a particular feature of polymeric materials. However, while both mechanical and dielectric hole burning show behaviors that are consistent with dynamic heterogeneity in the materials, it is still unclear what the relationship between the hole properties, for example, frequency and amplitude, and the actual nature of the heterogeneity and particularly the length scale being probed. Here, we provide first evidence that a known length scale can be probed by the MSHB method by using a tri‐block copolymer as a “calibration” sample. The heterogeneity of a styrene‐isoprene‐styrene copolymer was investigated in the vicinity of the order‐disorder transition temperature (ODT). It was found that the amplitudes of the mechanical holes gradually decrease as the order‐disorder transition is traversed from the region with ordered structures. In the disordered state of the block copolymer, no apparent mechanical holes were detected. In contrast, mechanical holes were burned in the heterogeneous region and the hole amplitude increased as the depth into the ordered structure increased. Hence, the MSHB technique provides a qualitative correlation between the amplitude of the burned holes and the corresponding heterogeneity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3277–3284, 2007

Mechanical Spectral Hole Burning in polymer solutions

“…Upon closer examination of Figures 4(a) and 5(a) for the horizontal hole positions, it is found that the horizontal hole positions under the pump frequency of 0.02 Hz to 0.005 Hz are similar for both samples. As we presumed in the previous work,10 the hole position seems to be associated with the length scale of the heterogeneity. This finding would imply similar length scale heterogeneities for the two samples in the transition regime.…”

“…The results of the MSHB experiments over that regime are shown in Figure 4(a). As interpreted previously,1, 2, 10 the presence of mechanical holes signifies dynamic heterogeneity in the material response. The presence of both vertical and horizontal holes seen in Figure 4(a), although incomplete, implies dynamic heterogeneity in the transition from the rubbery plateau to the terminal regime for sample 30K_30% (It is worth remarking here that the dynamic response shows a plateau that is often attributed to the rubbery response or entangled response.…”

“…Although both NSHB and MSHB seem able to successfully probe dynamic heterogeneity, the length scale of the heterogeneity being probed has not yet been established. In our prior work, 10 we had found that a tri-block copolymer gave mechanical holes in the heterogeneous or ordered phase, but not above the order-disorder transition temperature (or microphase separation temperature) where homogeneous melt dynamics were anticipated. That work was suggestive of the hole burning being characterized by the heterogeneity at a nonchanging length scale.…”

“…The obtained holes were interpreted as being due to the microstructural heterogeneity arising from the branched chains. Shi and McKenna1, 2 were also able to show that hole burning occurred in a polystyrene solution, and Qian and McKenna10 have burned holes in a triblock copolymer below the order‐disorder transition temperature. Those works showed that MSHB can be used to probe the dynamics of low dielectric constant polymers; hence, it is potentially a viable method to probe dynamic heterogeneity of a different class of materials that can not be probed using the dielectric NSHB.…”

“…Hence, at present, it appears that the MSHB has to be further refined to make experimentation near the glass transition regime possible. However, the method remains of interest because the Mechanical Spectral Hole Burning (MSHB), unlike the Nonresonant Dielectric Spectral Hole Burning (NSHB), gives holes in systems that are far above their T g s. 1,2,10 For example, Shi and McKenna burned mechanical holes in the melt state of a long chain branched polyethylene 1 ; a result which was surprising as the hole burning event in such a system was unanticipated. The obtained holes were interpreted as being due to the microstructural heterogeneity arising from the branched chains.…”