Relaxation modes of metaphosphate glasses with entropic shrinkage

Takatoshi Yaoita,Seiji Inaba

doi:10.1063/5.0053213

Abstract

We have reconfirmed the anomalistic reversible strain (shrinkage) after tensile creep in the specific metaphosphate glass melts with entropic elasticity in a wide range of applied stresses. We attempted to elucidate the origin of the anomalistic shrinkage by focusing on the relaxation modes. The fast mode was identified for all glasses by two different measurements, i.e., the dynamic viscoelasticity in the linear range and the creep recovery in the non-linear range. In contrast, the slow mode in addition to the fast mode was confirmed only for the glasses showing the anomalistic shrinkage. The slow mode induced the essential shrinkage at whole tensile stresses, whereas the fast mode, especially at higher tensile stresses, contributed to the moderate shrinkage. As the most likely mechanism in the linear range, the local segmental motion with mobile cation motion and the large-scale relaxation of the connected molecules through scission and renewal of the bond between alkali cations and the oxygen anion were assigned to the fast and the slow modes, respectively. In the non-linear range, the dimensionless strain-rate dependence of the reversible strains for both modes had a similar characteristic to that of the viscosity.

Highlights

Glass-forming liquids show the relaxation behavior originated from the glassy mode
We investigate the strain-rate-dependence of ε−slow and ε−fast with comparing their relaxation times
We focused on the relaxation modes in order to clarify the origin of the anomalistic reversible stain in the alkali metaphosphate glass melts

Summary

INTRODUCTION

Glass-forming liquids show the relaxation behavior originated from the glassy mode. The glassy relaxation irrespective of organic or inorganic glass-forming liquids in the vicinity of glass transition temperature (Tg) first takes place mainly through the local segmental motion including cooperative rearrangements of a cluster of atoms (α relaxation) followed by slower motions if exist, whereas the relaxation at significantly higher temperature than Tg is accompanied by the local bond interchange in the inorganic liquids. A ubiquitous character of glassy materials around Tg is non-exponential viscoelastic relaxation of the α relaxation, which is often called fast mode. We have recently found some mixed alkali metaphosphate glasses that show the character originated from entropic elasticity.25–29 These glasses with the specific cations having a large ionic radius exhibit the significant birefringence and lengthwise shrinkage of several tens of percent after tensile creep above Tg. It is notable that the degree of the shrinkage observed in these glasses is far superior to those reported in numerous inorganic oxide glasses.. We attempt to elucidate the origin of the anomalistic reversible strains (shrinkage) from the aspect of relaxation mode(s) and time(s) For this purpose, we first measured the linear viscoelasticity, which probes the dynamics reflecting internal structure under small deformation, for several alkali metaphosphate glasses with and without the distinct shrinkage. We analyzed the recovery data to evaluate the number of relaxation mode(s) and time(s) comparing with those obtained in the linear range

EXPERIMENTAL

RESULTS AND DISCUSSION

Relaxation of reversible strain in non-linear range

CONCLUSIONS