Constrained and Heterogeneous Dynamics in the Mobile and the Rigid Amorphous Fractions of Poly(dimethylsiloxane): A Multifrequency High-Field Electron Paramagnetic Resonance Study

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 ≃213 K, above the onset of the melting at ≃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 (MAF) and the rigid (RAF) amorphous fractions of PDMS, respectively. The results suggest that MAF 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 8% and softens above 213 K, well below the melting transition (≃230 K). Similarities between the disordered phase of semicrystalline PDMS and the PDMS layers in poly(styrene)–PDMS diblock are discussed.

Glassforming Liquids, Amorphous and Semicrystalline Polymers: Exploring their Energy Landscape and Dynamical Heterogeneity by Multi-frequency High-Field EPR

“…The dynamics of the amorphous fraction in semicrystalline PDMS from the glassy region (below 147 K) up to the melt (above about 230 K) was investigated by HF-EPR [23,24]. Two different thermal protocols, slow and quench cooling, were applied to PDMS obtaining samples, PDMSsc [23] and PDMSq [24], with different amounts of RAF.…”

“…The dynamics of the amorphous fraction in semicrystalline PDMS from the glassy region (below 147 K) up to the melt (above about 230 K) was investigated by HF-EPR [23,24]. Two different thermal protocols, slow and quench cooling, were applied to PDMS obtaining samples, PDMSsc [23] and PDMSq [24], with different amounts of RAF. In fact, slow cooling from above the melting point ( T m ) down to the temperature of interest T ( T g < T < T m ) leads to less polycrystallinity than quench cooling in the glass region and subsequent reheating to reach the temperature T [76][77][78].…”

“…The present paper reviews in a concise way the experimental efforts carried out in Pisa using the HF-EPR spectroscopy to provide novel insight into a wide class of disordered systems both in the solid state, i.e., amorphous polymers [14][15][16][17][18][19][20], and in the liquid state, i.e., glassforming viscous liquids [21,22], polymer melts [21] and semicrystalline polymers [23,24]. For completeness, in addition to the previous studies, we also mention other investigations employing the same equipment [25][26][27][28][29][30][31].…”

“…Two main issues were addressed, namely the distribution of energy barriers which must be overcome by the spin probe during the reorientation process [14][15][16][17][18][19][20] and the spatial distribution of microscopic mobility [23,24]. The former aspect is strictly related to the features of the so-called "energy landscape" of glasses, whereas the latter, dubbed "dynamical heterogeneity" [36,37] is a distinctive feature of viscous liquids approaching the solidification process, known as glass transition, and is also present in semicrystalline polymers due to coexistence of liquid and solid fractions.…”

Probing the confining effect of clay particles on an amorphous intercalated dendritic polyester

“…In simplest terms, RAF represents the fraction of the amorphous phase that does not contribute to the change in ΔC p at either the T g or T m (melting). It is well-established [32][33][34][35][36][37][38][39][40][41][42][43][44] that the RAF is due to an immobilization of the disordered polymer chains that connect the crystalline lamellae. These chains are unable to undergo long range translational motions when crystalline constraints are imposed during crystallization of the polymer melt, implying that the RAF vitrifies in the vicinity of the crystallization temperature, T c .…”