High-Field Electron Paramagnetic Resonance Reveals a Stable Glassy Fraction up to Melting in Semicrystalline Poly(dimethylsiloxane)

Abstract

The reorientation of the 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. Accurate numerical simulations of the HF-EPR lineshapes evidence that the reorientation times of the spin probes are distributed between the melting temperature Tm and Tmâ\u80\u9430 K. The distribution exhibits, in addition to a broad component, a narrow component with low mobility up to the PDMS melting point. It is shown that the temperature dependence of the reorientation time of the spin probes with low mobility is the same of the spin probes in glassy PDMS. The result suggests that the low-mobility fraction is localized in the so-called rigid amorphous fraction

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

“…Figure 6 shows that the Arrhenius temperature dependence of the reorientation time of the trapped fraction τ trapped , observed up to T m , is in ideal continuation of the one of the single reorientation time τ SRT characteristic of the spin probe reorientation below ∼T cc . 42 These findings suggest that on increasing the temperature above 200 K, a part of the spin probes persists in the glassy dynamics up to T m , whereas an increasing fraction leaves the trapped fraction and accelerates its reorientation in a heterogeneous way with a power-law distribution of correlation times (see Figure 7, top).…”

“…The molecular size of the spin probe is quite comparable with the one of the PDMS monomer (see Figure ) and smaller than the RAF thickness, typically a few nanometers . An analysis of the interactions between the radical and PDMS, given in the Supporting Information, suggests that the spin probe has good coupling with PDMS (see also ref ). The sample was prepared by dissolving the spin probe and PDMS in chloroform according to the solution method .…”

“…In contrast, more insight is provided by techniques sensitive to mobility variations like NMR, , dielectric relaxation, or measurements of the solubility of a gas (for a review see ref ). Following the same approach, earlier , and novel ,, investigations of SCPs addressed the rotational mobility of suitable guest radicals (spin probes) in SCPs. They are carried out by electron paramagnetic resonance (EPR), in particular high-field EPR (HF-EPR), , and exploit the expertise gained on both ice–water mixtures − and amorphous polymers. − One major advantage in using guest molecules to investigate SCPs is their selectivity.…”

“…No distinctive spectral features associated with RAF were observed. In the present study, aiming at revealing signatures of reversible local melting, we adopt an improved strategy 42 to increase the amount of RAF as well as the coupling between the spin probe and PDMS. As first outcome, we provide evidence of RAF persisting up to T m .…”