Polymer Science Faculty Research
Slow Chain Dynamics in Isotactic-poly (4-methyl-1-pentene) Crystallites near the Glass Transition Temperature Characterized by Solid-State 13C MAS Exchange NMR
Abstract
The chain dynamics for isotactic-poly(4-methyl-1-pentene) (iP4M1P) crystallites near the glass transition temperature (Tg = 304 K) is characterized by solid-state 13C MAS NMR methods at natural abundance. The 13C line width under high-power proton decoupling and the 13C spin−lattice relaxation time in the rotating frame (T1ρc) detect the segmental motions in the amorphous and crystalline regions with correlation times of about 0.2 × 10-5 s at 360−382 K and about 448 K, respectively. Centerband-only detection of exchange (CODEX) with an additional T1ρc filter is applied to investigate the motional geometry and kinetic parameters for the main- and side-chain dynamics in iP4M1P crystallites in a sample. The CODEX evolution-time dependence of the resolved signals indicates a large-angle reorientational process: the simulation of the experimental data of the main-chain CH2 signal reveals that iP4M1P crystallite performs the helical jump motions with jump angles of 72−145° in the disordered 72 helix. The CODEX mixing-time dependence permits the determination of kinetic parameters for the main- and side-chain motions over about 4 orders of magnitude. The determined correlation times for the main-chain carbons match these of the side-chain signals over the investigated temperature range, indicating that the side chain does not perform an independent slow dynamic process in the crystallites. The temperature dependence of the correlation time does not obey an Arrhenius behavior but must be analyzed in terms of WLF behavior with a reference temperature of Ts = 294 K. This exceptional behavior of a crystalline material is explained in terms of the amorphous and/or interfacial constraints around Tg. Furthermore, 2-D exchange NMR shows that helical jump motions accompany conformational transition. We also discuss our NMR results in relation to previously reported bulk mechanical relaxation and other data.