Molecular Dynamics and Structure of the Crystalline Region of Isotactic-Polyolefins Characterized by Solid-State NMR

Abstract

A review is given of recent solid–state NMR work on structure and molecular dynamics of polyolefins such as isotactic poly(1–butene) (iPB1) and isotactic polypropylene (iPP). In iPB1, melt–crystallization leads to metastable form II, and subsequent solid–solid transition results in stable form I. 1H–13C wide line separation (WISE) and center bands only detection of exchange (CODEX) NMR techniques provided geometric and time–kinetic parameters of molecular dynamics for overall and side chains in both forms at natural abundance. The 1H line–shape analysis using a two–spin approximation and 13C T 1 results indicate that form II chains perform uniaxially rotational diffusions accompanying side–chain conformational transitions in the fast motional limit (correlation time < 10-7 s). After irreversible solid transition, 13C CODEX results indicate that crystalline stems and side–chain conformations are completely fixed, up to the melting points (correlation time > 10 s). The effects of the unusual molecular dynamics on the mechanical properties are discussed. The second part of this article deals with a re–investigation (by high resolution 13C NMR) of packing structures of the α form of iPP. Two–Pulse Phase–Modulation decoupling during a rather long acquisition time results in significantly narrowed signals corresponding to ordered packing. The enhanced spectral resolution allows us to properly evaluate order–disorder phenomenon in polymer crystals. From CODEX experiments, lamellar thickness and packing order apparently suppress helical jumps in the crystalline regions. These experiments provide valuable information on chain–level structures and dynamics, particularly on the variations in lamellae thickness as a function of supercooling.