Superior Mechanical Properties

​

Studies of molecular dynamics and conformation of polymers by solid state NMR for the superior mechanical properties.

 

In this work, the molecular dynamics and conformation of polymers was studied by solid state NMR and discovered the superior mechanical properties. We investigated molecular level structure and molecular dynamics inside of the a-form of isotactic polypropylene (iPP) by SS-NMR. Initially, we evaluated the order-disorder phenomenon for the stem orientations and their packing in the a-form of iPP with a high isotacticity sample under isothermal crystallization and annealing conditions. We applied a conventional high-resolution 13C SS-NMR incorporating a 1H spin-lattice relaxation time in the rotating frame (T1ρH) filter. We found that a significant amount of disorders in the stem orientations is included even at high isothermal crystallization temperature (Tc ) and annealing temperature (Ta). Moreover, we studied chain-level structure changes based on the stem orientations and chain-folding rules. We found that ordering of polymer chain arrangement occurs with increasing Tc. Secondly, we explore space heterogeneity between the ordered and disordered packing areas by 1H T1ρH. The obtained results indicated that ordered and disordered packing areas form domain structures on a scale >∼9 nm. Thirdly, we have characterized lamellar thickness, <l>, of the a-form by SAXS and treated the supercooling and annealing effects on <l> and forder. We found a unique relationship between forder and <l> that molecular ordering occurs prior to lamellar thickness enhancement under isothermal crystallization and annealing conditions. Fourthly, we investigated correlation time <τc> and activation energy of helical jump motions of stems in the a-form of iPP by applying center-bands only detection of exchange NMR. It was found that there are large variations in <τc> and activation energy of stem dynamics depending on supercooling. Through these experimental evidences, we concluded that not only stem dynamics but also molecular level ordering inside of the crystal play important roles for lamellar thickness enhancement at very high temperatures.

​

 

​

​

​

​

​

​

​

​

In the second part of this work, we explored the outstanding mechanical properties of isotactic Polybutene (iPB). We studied the molecular dynamics of the stable form I of iPB using centre band-only detection of exchange (CODEX) method and modified wide-line separation NMR. It was found that crystalline stems in form I do not undergo any overall or side-chain motions, which reorient the principle axes of chemical shift anisotropy (CSA). On the basis of dynamics results, we discussed the relationship between chain packing and the molecular dynamics of forms I and II. In addition, we investigated how chain mobility influences the lamellar thickness. Two types of iPB1 samples was used displayed different crystallization mechanisms (through or bypassing form II). We demonstrated that the lamellar thickness of iPB1 is highly dependent on the crystalline mobility. Through information about molecular dynamics in this work, we discovered that unique crystallization and irreversible solid–solid transitions dominate the outstanding mechanical properties of iPB1. We have also showed in this project, a large undercooling can increase of the nucleation density and results faster transition process.

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

[T Miyoshi, A Mamun, W Hu, Molecular ordering and molecular dynamics in isotactic-polypropylene characterized by solid state NMR, The Journal of Physical Chemistry B (ACS Publisher), 2009]

​

[T Miyoshi, A Mamun, D Reichert, Fast dynamics and conformations of polymer in a conformational disordered crystal characterized by 1H−13C wise NMR, Macromolecules (ACS Publisher), 2010]

​

[T Miyoshi, A Mamun, Critical roles of molecular dynamics in the superior mechanical properties of isotactic-poly(1-butene) elucidated by solid-state NMR, Polymer Journal (Nature Publishing Group), 2012]

​