Local Reversible Melting in Semicrystalline Poly(dimethylsiloxane): A High-Field Electron Paramagnetic Resonance Study

Abstract

The reorientation of the paramagnetic guest 4-methoxy-TEMPO (spin probe) in the disordered fraction of semicrystalline poly(dimethylsiloxane) (PDMS) is investigated by high-field electron paramagnetic resonance (HF-EPR) at 190 and 285 GHz. The distribution of reorientation times is evidenced by accurate numerical simulations of the HF-EPR line shapes above 200 K. The distribution exhibits a bimodal structure with (i) a broad component corresponding to spin probes with fast and intermediate mobility located in the disordered fraction far from the crystallites and (ii) a narrow component corresponding to spin probes with extremely low mobility trapped close to the crystallites in a glassy environment persisting up to the PDMS melting. The spin probe undergoes an exchange process between the trapped and the more mobile fractions which is accounted for by an equilibrium reversible process with standard Gibbs free energy of reaction per spin probe mole Î\u94Gr0 â\u89\u83 4(Î\u94Hm - TÎ\u94Sm), where Î\u94Sm is the equilibrium melting entropy per monomer mole following the absorption of the heat Î\u94Hm. The process is interpreted as signature of reversible tertiary nucleation, occurring at the intersection of crystalline surfaces, thus suggesting surface roughness of the crystal-amorphous interface. It becomes thermodynamically favored at temperatures higher than T â\u88¼ 209 K where the onset of PDMS melting is located according to differential scanning calorimetry

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

“…The failure of the SRT model is anticipated in that it misses any detail on the heterogeneous dynamics occurring in the disordered region between the crystallites. Differently, in PDMSq, the SRT model satisfactorily predicts the line shape up to 200 K and becomes inadequate in the range 200-230 K, where the analysis gives clear indications that the distribution of the rotational mobility of the spin probes has a bimodal structure with (1) a broad component corresponding to spin probes with fast and intermediate mobility, as detected in PDMSsc, and (2) a narrow component corresponding to spin probes with extremely low mobility, characterized by the reorientation time trapped [24]. The two fractions of the spin probes are expected to be located in the disordered fraction far from the crystallites and trapped close to the crystallites, respectively.…”

“…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.…”