Characterization of the Phase Transitions of Ethyl Substituted Polyhedral Oligomeric Silsesquioxane
Abstract
This study describes the synthesis and molecular mobility of both partially deuterated and fully protonated ethyl polyhedral oligomeric silsesquioxane (POSS) crystals. Phase transitions were identified with differential scanning calorimetry at ^258 K and ^253 K for partially deuterated and fully protonated ethyl POSS, respectively. A change in entropy of ^ 20. 8 K was observed for both transitions. The crystallographic phase transitions were identified as a high temperature rhombohedral unit cell with a contraction in volume and symmetry described by a low temperature triclinic unit cell past the transition temperature. Abrupt changes in the spin lattice relaxation and linewidth were detected with solid state proton nuclear magnetic resonance (NMR) spectroscopy, at the same temperatures detected with calorimetry. This NMR behavior suggests a transition in molecular motions of both ethyl derivatives. For deuterated ethyl POSS, the motions become increasingly anisotropic after the temperature is lowered past its transition point. Both derivatives exhibit an increase in the correlation time (^30 2 ns to ^530 15 ns ) and activation energy (^16 2 kJ/mol to ^ 20 2 kJ/mol ) for molecular tumbling at temperatures past their respective transitions.
Document Details
- Document Type
- Technical Report
- Publication Date
- Feb 01, 2005
- Accession Number
- ADA435983
Entities
People
- G. M. Poliskie
- K. K. Gleason
- R. L. Blanski
- T. S. Haddad
Organizations
- Massachusetts Institute of Technology