The reorientation of TEMPO spin probe in semicrystalline poly(dimethylsiloxane) (PDMS) is investigated in the temperature range from the glassy region (below 147 K) up to the melt (above about 230 K) by high-field electron paramagnetic resonance (HF-EPR) spectroscopy at two different Larmor frequencies (190 and 285 GHz). The spin probe is confined in the disordered phase. Accurate numerical simulations evidence that the spin probe undergoes activated jump reorientation overcoming an exponential distribution of barrier heights characteristic of highly constrained systems and resulting in a power-law distribution of the reorientation times. Below 180 K the spin probe is coupled to local relaxations and does not sense the glass transition. A strong narrowing of the distribution of the reorientation times and a sudden drop of the mean value are observed at similar or equal to 213 K, above the onset of the melting at similar or equal to 209 K. Strikingly, it is found that the faster fraction of the spin probes does not sense the melting and couples to the segmental motion of the bulk amorphous PDMS from about 200 K onward. Our findings support the conclusion that the faster and the slower TEMPO molecules are located in (or very close to) the mobile (MAP) and the rigid (RAF) amorphous fractions of PDMS, respectively. The results suggest that MAP is negligible close to the glass transition but it is present above about 200 K, whereas RAF at about 211 K is reduced to about 896 and softens above 213 K, well below the melting transition (similar or equal to 230 K). Similarities between the disordered phase of semicrystalline PDMS and the PDMS layers in poly(styrene) PDMS diblock are discussed.
Constrained and Heterogeneous Dynamics in the Mobile and the Rigid Amorphous Fractions of Poly(dimethylsiloxane): A Multifrequency High-Field Electron Paramagnetic Resonance Study
Massa Carlo Andrea;Pizzanelli Silvia;Pardi Luca;Leporini Dino
2014
Abstract
The reorientation of TEMPO spin probe in semicrystalline poly(dimethylsiloxane) (PDMS) is investigated in the temperature range from the glassy region (below 147 K) up to the melt (above about 230 K) by high-field electron paramagnetic resonance (HF-EPR) spectroscopy at two different Larmor frequencies (190 and 285 GHz). The spin probe is confined in the disordered phase. Accurate numerical simulations evidence that the spin probe undergoes activated jump reorientation overcoming an exponential distribution of barrier heights characteristic of highly constrained systems and resulting in a power-law distribution of the reorientation times. Below 180 K the spin probe is coupled to local relaxations and does not sense the glass transition. A strong narrowing of the distribution of the reorientation times and a sudden drop of the mean value are observed at similar or equal to 213 K, above the onset of the melting at similar or equal to 209 K. Strikingly, it is found that the faster fraction of the spin probes does not sense the melting and couples to the segmental motion of the bulk amorphous PDMS from about 200 K onward. Our findings support the conclusion that the faster and the slower TEMPO molecules are located in (or very close to) the mobile (MAP) and the rigid (RAF) amorphous fractions of PDMS, respectively. The results suggest that MAP is negligible close to the glass transition but it is present above about 200 K, whereas RAF at about 211 K is reduced to about 896 and softens above 213 K, well below the melting transition (similar or equal to 230 K). Similarities between the disordered phase of semicrystalline PDMS and the PDMS layers in poly(styrene) PDMS diblock are discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
