Understanding the molecular origin of aging in the three topological phases of network glasses

ABSTRACT

Network glasses composed of chalcogenide and modified oxides display three topological phases: the flexible, intermediate, and stressed-rigid. Modulated-DSC experiments, Raman scattering, and molar volume measurements along with topological constraint theory have shown that glass compositions in the flexible and stressed-rigid phases display glass transitions that age with time. The change of the enthalpy of relaxation (ΔH_nr) at the glass transition temperature (T_g) steadily evolves with waiting time which can be traced to the presence of Open Degrees of Freedom (ODF). In the isostatically constrained intermediate phase (IP), glasses possess an enthalpy of relaxation which is substantially reduced in aging due to a lack of ODF. IP glasses are composed of self-organised Isostatically Rigid Local Structures (ISRLSs) which have no ODF. The melt fragility index (m) across the three topological phases displays a Gaussian-like global minimum of m = 15 near the IP centre composition, illustrating a super-strong melt behaviour leading to the realisation of the ideal Phillips glass condition where glass forming tendency is globally optimised as demonstrated in the present work. For melt compositions away from the IP centre and into the flexible and stressed-rigid phases, the fragility index steadily increases due to the increased presence of non-ISRLSs. The non-ISRLSs admix with the ISRLSs formed near the mid IP composition, thus promoting the melt networks to relax, age, and diffuse as one goes away from IP centre composition.

KEYWORDS:

• Topological phases
• topological constraint theory
• isostatically rigid local structures
• modulated DSC
• Raman scattering
• fragility index

Acknowledgements

PB acknowledges with pleasure discussions with Jim Phillips, Alan Bishop, Matthieu Micoulaut, Matthieu Bauchy, the late Richard Zallen, John deNeufville and Len Thomas on glass science issues over the past four decades. Many former students have played a significant role in developing a reliable database by synthesising dry and homogenous glasses and understanding their topological phases as part of their thesis work and these include Deepak Selvanathan, Siddhesh Bhosle, Kapila Gunasekera, Shibalik Chakraborty, Ralph Chbeir, Avik Mandal, Vamshi Gogi, Badriah Almutairi, Charles Skipper, Matthew Burger and Chandi Mohanty.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by NSF [grant number DMR 08-53957].