The Structure of Polymer Networks

Abstract

Network polymers comprise one of the most important classes of polymeric materials, from both a theoretical and a commercial perspective. The linking together of macromolecular chains usually through permanent covalent bonds confers unique properties to network polymers. These may be increased modulus and elasticity, lower rates of creep, solvent resistance, high temperature stability, to name just a few. The applications of network polymers are thus myriad. Thermosetting resins comprise the majority of polymers used in structural applications. Crosslinked polyolefins are ubiquitous as automotive tyres, as a component of asphalt, as o-rings, sheeting, in clothing and footware and so on. In more recent times, chemically crosslinked networks are becoming important in the field of biomaterials, as supports for tissue growth and for drug delivery. Many other applications can be listed. From the point of view of the NMR spectroscopist, the structural features that provide networks their unique physical properties give rise to a number of experimental challenges. Network polymers are by definition wholly or largely insoluble, and therefore conventional solutionstate NMR techniques cannot be routinely applied to their analysis. Resort must be made to solid-state NMR techniques, or to other methods for narrowing the dipolarbroadened line shapes. To compensate for this difficulty in extracting chemical information, there are a number of other NMR techniques for determining physical structure, for example, crosslink density, molecular weight between entanglements, and pore or mesh size. Thus, the aim of this review is to summarize the development of NMR methodologies for the study of network polymers, in particular the determination of chemical structure and network properties using high-resolution NMR and NMR relaxometry. We are not concerned in this review with other aspects of network behavior, for example, the motion of penetrants in polymer networks, or NMR imaging of networks, as these aspects are covered in depth elsewhere in this monograph. All of these issues have been addressed in detail on a number of reviews specifically within the field of NMR characterization of networks [1–6], as well as a number of reviews on general analytical methods, including NMR, for characterizing polymer networks [7–9].