Recent Advances in Structural Characterization of Elastomers

Abstract

Abstract One of the main targets of the structural characterization of elastomers so far has been the correlation of the polymerization conditions with the properties of the resulting polymers. The first step is the analysis of polymer structure, such as the chemical composition of copolymers, isomeric structure of diene polymers, degree of branching, extent of modification, functionality of end groups, amounts of abnormal groups, tacticity, and so on. Progress in nuclear magnetic resonance spectroscopy (NMR) makes possible the second step, which is the structural characterization of polymer chains, such as the sequence distribution of comonomer units in copolymer, isomeric units in diene polymers, configurational sequences in vinyl polymers, head and tail arrangement of monomer units. Recent development of FT-NMR spectroscopy, high-field spectroscopy from 300 MHz to 600 MHz at 1H-NMR, solid-state 13C-NMR, and two-dimensional NMR has facilitated a more detailed analysis of these structural features. The complexity of the structure of elastomers, which is derived from highly controlled copolymerization processes, leads to the widespread application of modern FT-NMR spectroscopy. It may reasonably be said that a fair number of structural problems in elastomers has been solved by NMR analysis. The high sensitivity of Fourier-transform infrared spectroscopy (FT-IR) has enabled one to determine trace structural changes in elastomers. Coupled on-line techniques such as gas-chromatography-mass-spectrometry combined with pyrolysis, liquid chromatography-NMR, and gel permeation chromatography-FT-IR will be powerful tools for the characterization of elastomers. Progress in analytical instruments has stimulated the development of high-performance elastomers, the synthesis of new speciality elastomers, and the elucidation of mechanisms for property enhancements. The use of modern instruments and a combination of ordinary methods of structural analysis have satisfied needs to some extent. However, a newer method of structural characterization is always desired in order to achieve higher orders of information. for example, the characterization of inhomogeneity along the polymer chain and between the polymer chains has become an important problem, especially in polymer blend systems. As to the former problem, spectroscopic methods provide only limited information. Although the NMR and FT-IR spectroscopies are very powerful tools for the analysis of short sequence distributions, it is difficult to characterize the distribution of long sequences and hybrid systems containing random and blocked sequences along the polymer chain. Gel permeation chromatography (GPC) is one of the most popular techniques for the analysis of molecular-weight distribution. However, it provides complicated information including molecular-weight distribution and chemical-composition distribution in the case of copolymers. Recent progress of high-performance liquid chromatography (HPLC) has provided a new powerful tool for the structural characterization of copolymers. It is appropriate to review the recent advances in structural characterization of elastomers, especially the characterization of microstructure of polymer chains, from the viewpoints of methodology and applicability of new methods. As to the NMR analysis of elastomers, reviews are available. Here, considerable attention is focused on the procedures for the assignment of signals, because the applicability of a NMR method is based on the reliability of the signal assignments. The other topics are selected to provide direct information for polymer synthesis or polymer properties.