10/11/2011

www.gupta-verlag.de/thermoplastic-elastomers

Polyurethane Elastomers. From Morphology to Mechanical Aspects

This book is not only a review of the state of the art but also a collection of new contributions in some specialized areas resulting from an international collaboration between scientists at the Institute of Macromolecular Chemistry "Petru Poni” in Iasi, Rumania and at the Department of Engineering Science of the University of Oxford in the United Kingdom.[image_0]As indicated by its title, the book covers a variety of aspects from the morphology to mechanical characteristics. It focuses on the elasticity and inelasticity of amorphous to crystalline polyurethane elastomers, as relating to their sensitivity to chemical and physical structure. In such polymers, resilience of the material is an important attribute. In many applications, they are in commercial competition with other, relatively soft elastomeric materials. The choice of material for any given application then hinges on a spectrum of key properties offered by relatively soft polymers - stiffness and strain recovery characterizing their elasticity, but also inelastic effects such as hysteresis and stress relaxation. In these respects the mechanical properties of polyurethane elastomers are similar to those of other elastomers. The main structural feature that is explored in this book is the relationship between the nature of the hard segment (crystallizing or not) and that of soft segment. A study was made to determine how aspects of the constitutive responses of polyurethane elastomers vary with composition. The polyaddition procedure, the hard segment, the soft segment and chain extenders (mostly diols) were varied systematically in a large number of systems of model and novel materials. Results were related to microstructural changes, on the basis of evidence from a variety of analytical methods. The book consists of six chapters: Chapter 1 (Chemistry of polyurethane elastomers) describes general aspects of the chemistry of polyurethane elastomers: their origins and development, the mechanisms involved in their synthesis, characteristics of the individual raw materials used as well as general considerations regarding the main chemical parameters that define these materials. In chapter 2 (Structural studies on polyurethane elastomers), aspects of the morphology and phase separation, and the phase separation kinetics of two large categories of polyurethane elastomers: materials based on single diisocyanates, and materials derived from mixtures of diisocyanates are discussed. The characterization methods used here are: wide and small angle X-ray scattering (WAXS) and (SAXS), and transmission and scanning electron microscopy (TEM and SEM respectively). The results are revealing particularities of the polyurethane elastomers morphology. Conventional materials derived from rigid diisocyanates (mainly MDI) are compared with the relatively recent polyurethane elastomers obtained by the current research of researchers at the Institute of Macromolecular Chemistry "Petru Poni” in Iasi, Romania.  The latter materials contain crystallizing hard segments generated by 4,4'-dibenzyl diisocyanate (DBDI), which in the presence of suitable chain extenders (diols or diamines) gives rise to significant degrees of crystallinity. Thermal analytical techniques such as differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) used to describe the thermal behavior of the materials are also discussed in relatively great detail in this chapter. In chapter 3 (Thermal behaviour of polyurethane elastomers), a comparison between the thermal behavior of conventional materials and those derived from dibenzyl structures is made. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) and thermogravimetry are the methods used in these studies. Particular attention is given to the thermal behavior of a series of novel polyurethane blends obtained with DBDI. Inelastic features in the polyurethane elastomers constitutive response are investigated in chapter 4 (Mechanical aspects of polyurethane elastomers). A comparison has been made between the mechanical response of elastomers achieved without and with excess of isocyanate-NCO groups. A study of the kinetics and postcure reaction mechanisms has been included. Particular consideration is given to the investigation of deformation induced morphological developments and crystallization of polyurethane elastomers. In chapter 5 (Deformation induced morphological developments), a brief review has been made of selected research in this area, by including materials with hard segments crystallizing or non-crystallizing, based on diisocyanates of variable geometries. The evaluations of these phenomena were done by SEM and infrared (IR) dichroic measurements. Chapter 6 (Perspectives. Novel crosslinked polyurethanes as shape -memory materials) is dedicated to a detailed description of a new family of cross-linked polyurethanes, with regard to their thermorheological characterization as shape memory materials and the prediction of their shape-memory performance. The individual sections of this chapter are: Thermorheological characterizationLinear theory of shape recoveryRecovery temperatureWidth of recovery windowRelaxed modulus The results of the studies discussed in this chapter illustrate the potential of this family of polyurethanes to act as shape-memory polymers, and to be tailored chemically to suit particular practical applications. One of the promising field worth of additional research is according to the author the area of biomedicine where candidate shape memory polyurethane materials could enable technology for future applications. The main goal stated by the author in the preface is to advance this understanding, by means of a systematic study of the effects of varying chemical composition of model polyurethane elastomers on: (a) their physical structure at the important nanometre length-scale, and (b) the resulting mechanical properties of interest. The reaction pathways involved in resolving the subtle morphological evolution at this level, and capturing mathematically the complex, large-deformation nonlinear viscoelastic mechanical behavior are assumed to bring new important insights in the basic research in the field of polyurethanes and towards applied industrial research in this area. In general, the author is able to explain a variety of complex concepts by leading the reader through them without getting lost. One interesting feature of her writing style is the use of fitting thoughts or quotations by well known or unknown scientists, authors or other thinkers (or her own) to start a chapter or a section. This in a way catches the reader's attention and prepares him/her for the journey through the often demanding text and/or illustrations. The size of the publication is only 255 pages but it is amazing how much it covers: chemistry, structural studies, thermal behavior, mechanical aspects, morphology, new developments, and perspectives. The treatment of the subject is thorough and systematic, yet the text is clearly written and the style is highly readable. Given that, the book will be a very useful resource for a wide range of readers, from advanced students to university educators and scientists working in the field of polymer science and technology, in materials science and technology in general, and in the field of polyurethanes and polyurethane elastomers in particular.

www.gupta-verlag.de/thermoplastic-elastomers