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

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 ΔGr0 ≃ 4(ΔHm - TΔSm), where ΔSm is the equilibrium melting entropy per monomer mole following the absorption of the heat ΔHm. 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 ∼ 209 K where the onset of PDMS melting is located according to differential scanning calorimetry.