A split before yield
Two different ways of measuring mobility in a polymer glass gave sharply different results as the material approached its yield point. In a laboratory test of a lightly crosslinked PMMA film, the relaxation time measured with a molecular probe fell by a factor of 130, while the mechanical relaxation time changed by a factor of 3. The contrast was specific to the pre-yield part of the deformation: after yield, the two measures tracked much more closely. The paper reports these as approximate factors and gives differing magnitudes elsewhere without reconciling them, so the exact size of the contrast is uncertain.
The film’s reported mechanical response put yield at 27 MPa and a strain of 0.028 ± 0.002; its modulus was 1,700 ± 100 MPa.
How the two measures were built
The experiment asked whether a mechanical relaxation time, τmech, could track segmental mobility during deformation as well as a probe-based measure, τprobe. The researchers deformed the glass at a constant global strain rate, initiated stress relaxation at different strain values, and used optical measurements of a dilute DPPC probe’s reorientation. The work was carried out at 19 kelvins below the material’s glass-transition temperature.
Those measurements were run at three global strain rates: 3.1×10−5, 3.1×10−6 and 1.5×10−6 s−1. The mechanical time was operationalized from the slope of approximately the first 3% of the stress-relaxation decay. The probe time came from fitting optical anisotropy decay with a stretched exponential; the same fit supplied β, an indirect indicator of the width of the relaxation-time distribution.
Closer after yield
Once deformation moved into the post-yield regime, the two measures nearly converged. At a given strain rate, their absolute relaxation times differed by no more than about a factor of 2. Both also followed the same power-law relationship with strain rate, with an exponent of −0.8 ± 0.1. In plain terms, higher strain rates were associated with shorter relaxation times after yield.
The rate comparison carries an important qualification. A linear actuator controlled global strain rate, local strain was tracked optically, and a seventh-order polynomial produced a smooth strain-time function. The paper notes that the comparison depends on whether local or global strain rate is used.
The meaning of the mismatch
Alongside the two relaxation times, the study tracked dynamic heterogeneity through β. The indicator initially decreased during deformation and then remained constant in the post-yield regime, which the authors report as a narrowing of the relaxation-time distribution. Because β is indirect, this result is an indicator of changing relaxation behavior, not a direct map of dynamics at each location in the film.
The authors interpret τprobe as a representation of average segmental relaxation time during deformation. They speculate that τmech reflects two things at once: changes in segmental mobility and changes in the relaxation-time distribution. On that reading, the pre-yield mismatch and post-yield agreement are not contradictory; they may show that the mechanical measure is responding to more than average molecular motion.
A result from one sample
The study’s scope is limited by its sample and protocol. All experiments used the same thermal history, and the presented data came from one lightly crosslinked PMMA sample with a dilute DPPC probe; the authors state that other samples were consistent. The evidence therefore describes a specific tensile-deformation experiment in this PMMA glass at the tested temperature and rates.
The mechanical relaxation time is an operational initial-slope measure, and treating it as a direct readout of segmental dynamics is an assumption rather than a result established by this comparison. The probe reorientation and β measures are likewise indirect indicators of segmental dynamics and heterogeneity.
Paper data and sources
Original title: Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass
Authors: Benjamin Bending, M. D. Ediger
Journal/Repository: JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2016, 54, 1957-1967
Status: Peer-reviewed
First online: 2026-08-24
DOI: 10.1002/polb.24110
Original paper · Full text