Mechanical Spectral Hole Burning in polymer solutions

Abstract

Mechanical Spectral Hole Burning (MSHB) was previously developed to investigate dynamic heterogeneity for polymeric materials, which exhibit relatively weak dielectric responses. In our previous work, MSHB was applied to a densely entangled block copolymer and successfully distinguishes the heterogeneous from the homogeneous states. Here, a series of polystyrene (PS) solutions was chosen to further investigate the effect of different types of heterogeneity on mechanical spectral hole burning. The three types of heterogeneity of interest include the entanglement spacing, the entanglement density (or number of entanglements per chain), and chain end density. The heterogeneity was varied by changing either solution concentration or molecular weight of the PS. Different types of dynamics from close to the Rouse regime into the terminal region were also examined. Our results are consistent with a heterogeneous dynamics over the time scales from close to Rouse regime into the rubbery plateau regime and the rubbery plateau-to-terminal flow transition regime. Terminal relaxation dynamics, on the other hand, were found to be homogeneous for the PS/diethyl phthalate solutions investigated. The results also indicate the hole properties are dominated by the type of dynamics rather than the length scale of heterogeneity.

Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

“…Solution samples were prepared in diethyl phthalate (DEP) with a purity of 99.5% obtained from Sigma-Aldrich. To make solution samples, toluene was used as a cosolvent to facilitate mixing, as described by Qin et al 47 After the solvent and solute were fully mixed, the toluene was removed by vacuum evaporation without the significant loss of DEP because of its low vapor pressure. The concentration of the solution was calculated based on the masses of polyDODT and DEP added before the toluene evaporation; the total mass of polyDODT and DEP was measured to confirm that there is no significant loss of DEP after the toluene evaporation.…”

Some open challenges in polymer physics*

“…(B) Vertical holes and (D) horizontal holes. Data courtesy of Qin et al [85] McKenna [81,82] did observe holes in the terminal flow regime. A point of importance in all hole burning experiments is that, as noted above, the magnitude of the holes is very small relative to the overall linear or modified response.…”

“…This is seen clearly in Figure 16B where one can see the modified response is <10% different from the linear response. Perhaps more importantly, though, is the fact that the holes in Figures 17A,C correspond to dielectric measurements on the glass‐forming liquid propylene carbonate near to the glass transition temperature [ 80,84 ] while those for Figures 17B,D correspond to a mechanical hole burning response of a polystyrene (PS) in diethylphthalate (DEP) solution [ 85 ] far above the glass transition temperature . Since it is frequently invoked that the hole burning response occurs due to heterogeneities that can be effectively attributed to local heating events [ 80,84,86–90 ] (either changing the local fictive temperature or due to a distribution of effective heat capacities that lead to a similar result) it is surprising that the response far above the can still exhibit holes because in this regime small changes in temperature or fictive temperature do not have large effects on the dynamics, unlike near to the glass transition.…”

Recovery rheology via rheo‐SANS: Application to step strains under out‐of‐equilibrium conditions

“…Subsequently, the two relaxation moduli that are calculated from the opposite directions are averaged and compared with the equilibrium G ( t ). Differences are referred to as “holes.” It is interpreted that holes exist when the LAOS step is carried out at frequencies in both the rubbery plateau and rubbery plateau‐to‐terminal regimes, but not in the terminal regime . In spite of the similarities in combining a step strain with LAOS, we present a fundamentally distinct viewpoint and analysis framework and emphasize the enhanced understanding in both the rheology and microstructure via recovery rheology.…”