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 (T g ) 304 K) is characterized by solid-state 13 C MAS NMR methods at natural abundance. The 13 C line width under high-power proton decoupling and the 13 C spin-lattice relaxation time in the rotating frame (T1Fc) 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. Centerbandonly detection of exchange (CODEX) with an additional T1Fc 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 CH 2 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 mainchain 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 T s ) 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.

Critical roles of molecular dynamics in the superior mechanical properties of isotactic-poly(1-butene) elucidated by solid-state NMR

“…This value is a typical condition to characterize large amplitude motions of the crystalline segments. In former works, helical jump motions of iPP and iP4M1P were analyzed using Nt r ¼2 ms. 18,19 Thus, the current CODEX result denies the presence of large amplitude motions in iPB1 form I in the slow dynamic range even at very high temperatures just before T m . If iPB1 may have small amplitude motions, the decay curve requires a longer Nt r to detect the molecular dynamics.…”

“…Several works evaluated helical jump motions using the CODEX method. 18,19,29 Figure 3b shows the Nt r dependence of the CODEX (S/S 0 ) for all of the functional signals of the a 2 -rich form 18 of iPP with a t mix of 200 ms at 112 1C. The (S/S 0 ) intensity ratios of all of the signals decay with increasing Nt r .…”

“…13 C-13 C spin diffusion also leads to an (S/S 0 ) decay curve at a longer t mix of more than 1 s even in natural abundance. Thus, spin diffusion correction is necessary to investigate slow dynamics at any t mix longer than 1 s. 19 Figures 4b and c show the t mix dependence of the (S/S 0 )* intensity ratios of m-CH 2 , CH and CH 3 at ambient temperature and at 118 1C, respectively, where asterisk (*) represents the pure CODEX (S/S 0 ) decay after spin-diffusion corrections. The spin-diffusion effect, (S/S 0 ) SD , was obtained at À10 1C, where no motion is expected.…”

“…PE (orthorhombic), 16 iPP (a2 form) 17,18 and iP4M1P (form I) 19 commonly show helical jump motions in the crystalline regions. The helical jump motion accompanies jump rotations of overall stems to neighboring sites and translation of overall stems by one monomer unit.…”

“…Solid-state nuclear magnetic resonance (NMR) has been successfully applied to reveal the molecular dynamics of solid polymers in the amorphous and crystalline regions. [15][16][17][18][19][20] In the crystalline regions, the molecular dynamics of typical polyolefins such as PE, 16 iPP 17,18 and iP4M1P 19 were investigated by two-and one-dimensional exchange NMR, which can directly detect reorientations of magnetic anisotropic interactions that are due to molecular dynamics. 15 These crystalline polymers commonly display slow, helical jump motions in the crystalline regions, which include discrete jump motions around the chain axes and translations of overall stems by one monomer unit.…”