Hu Shape Memory Polymers and Textiles

1. Auflage 2007
ISBN: 978-1-84569-306-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 360 Seiten

Reihe: Woodhead Publishing Series in Textiles

Shape memory polymers (SMPs) are smart materials that, as a result of an external stimulus such as temperature, can change from a temporary deformed shape back to an original shape. SMPs are finding an increasing use in such areas as clothing where they respond dynamically to changes in heat and moisture levels, ensuring greater comfort for the wearer. Shape memory polymers and textiles provides an authoritative and comprehensive review of these important new materials and their applications.After an introductory chapter on the concept and definition of shape memory materials, the book reviews methods for synthesising, characterising and modelling SMPs. It goes on to consider the properties of particular materials such as shape memory polyurethane and environmentally-sensitive polymer gels. The book concludes by assessing potential applications such as wrinkle-free fabrics and smart fabrics providing improved protection and comfort for the wearer.Shape memory polymers and textiles is a valuable guide to R&D staff in such areas as textile apparel in developing a new generation of smart textiles and other products. - Reviews the structure, synthesis and preparation of shape memory polymers - Assesses methods for analysing and modelling shape memory properties - An authoritative overview of particular fibres such as shape memory polyurethane (SMPU)

Jinlian Hu is a Professor at the Institute of Textiles and Clothing, Hong Kong Polytechnic University. A Fellow of the Textile Institute, she was also the recipient of the 2001 Award for Distinguished Achievement from the US Fiber Society. Professor Hu has published over 300 articles and several books on textile materials. She is currently the Editor-in-Chief of the Research Journal of Textiles and Apparel.

Autoren/Hrsg.

Hu, Jinlian

Leseproben

2 Preparation of shape memory polymers
Publisher Summary
This chapter provides a detailed description of the structure of Shape Memory Polymers (SMPs) with an emphasis on the mechanism of the shape memory effect and the unique molecular structure behind it. It also describes the thermal-mechanical properties of SMPs, reviews key issues in the preparation of SMPs and their preparation for use in the medical field. Because of their unique performance, shape memory materials are, at the moment, on the verge of commercialization for medical applications, such as catheters, bone casts, prosthetics, self-dilating introducers, and stents. In recent years, there has been an increasing interest in shape memory thermoplastic polymers because it is easier to adjust their transition temperature and because of their superior maximum deformation. The range of temperatures for shape transition covers the human body temperature range. These attributes, together with their proven biocompatibility, competitive price, and the ease with which they can be synthesized, make SMPs exploitable for medical applications. This chapter will provide readers with a detailed description of the structure of shape memory polymers (SMPs) with an emphasis on the mechanism of the shape memory effect and the unique molecular structure behind it. The thermal-mechanical properties of SMPs are also described. Key issues in the preparation of SMPs are reviewed and, finally, their preparation for use in the medical field. 2.1 Structures of shape memory polymers
2.1.1 Hard and soft segments
An elastomer can be said to have shape memory functionality if it can stay in the deformed state in a temperature range that is relevant for a particular application. This unique property can be acquired by using the network chains as a kind of molecular switch. If temperature is designed to trigger the transformation, the flexibility of some segments should be a function of temperature. However, the shape memory effect is not related to a specific material property of a single polymer, but is realized from a combination of the polymer structure and its morphology. Shape memory behavior is a result of the appropriate structure/morphology combination. Chapter 1 briefly described SMPs as composed of incompatible hard and soft segments distributed at a molecular level to form a micro-phase separated structure and netpoints, which form either a soft or a hard micro-phase domain. The hard micro-phase is a hard segment-rich micro-domain, which is generally semi-crystalline or physically cross-linked and imparts stiffness and reinforces the material (controlling the mechanical properties of the SMP). The soft segment-rich micro-phase, which is responsible for the thermo-elastic behavior of the polymer, is usually amorphous with a glass transition temperature, or semi-crystalline with a melting temperature. A large, reversible change in elastic modulus across the transition temperature (Ttrans) makes shape change and shape retention possible.1 At temperatures above Ttrans, the chain segments are flexible, but below this thermal transition the flexibility of the chains is at least partly limited. In the case of a transition from the rubber-elastic or viscous state to the glassy state, the flexibility of the entire segment is limited. If the thermal transition chosen for fixing the temporary shape is the melting point, strain-induced crystallization of the switching segment can be initiated by cooling the material that has been stretched above the Ttrans value.2 Crystallization is a complex phenomenon characterized by many interrelated factors. The intrinsic nature of the crystallizing substance is critical for crystallization. The speed at which nuclei grow into crystals is defined by the rate of crystallization. This rate depends on the concentration of the solute in the solution. In general, a more concentrated (more supersaturated) syrup will crystallize faster than a less concentrated syrup. The rate of crystallization is slow at a higher temperature and becomes more rapid at a lower temperature. If the crystal-forming nuclei are distributed by agitation, the crystallization rate is also increased. The crystallization achieved in an SMP such as polyurethane (PU) is always incomplete, which means that a certain number of the chains remain amorphous. The crystallites formed prevent the segments from immediately reforming the entangled structure and from spontaneously recovering the permanent shape defined by the netpoints. The permanent shape of shape memory networks is stabilized by covalent netpoints, whereas the permanent shape of the shape memory thermoplastic is fixed by the phase with the highest thermal transition at Tperm. Polymers such as polynorbornene, trans-polyisoprene, styrene–butadiene copolymer, crystalline polyethylene, some block copolymers, ethylene–vinyl acetate copolymer, and segmented polyurethane have been characterized as having a shape memory effect. Among them, shape memory polyurethane (SMPU) has been particularly widely studied and used.1–16 2.1.2 Phase separation
In terms of thermal properties, the incompatibility between the soft and hard segments can be related to the difference in their glass transition temperatures. However, the shift to a higher Ta (melting of crystalline polyol) indicates that regions of considerable phase mixing exist. Although the hard and soft segment domains in a two-phase system can behave independently, each displaying characteristic thermal and mechanical behavior, a mixed hard segment–soft segment region exists in the transition zone between the two types of domain. It will be shown in Sections 3.1 and 3.3, using dynamic mechanical analysis (DMA), that the soft segment has little effect on the Tg of the hard segment. However, increasing hard segment content HSC depresses the phase separation of the soft and hard segments and enhances their compatibility. This leads to an increase in Ta and storage modulus. The degree of phase separation in the copolymer can be related to the crystallization of the soft segment (see Section 3.1). It implies that the chemical bonding between the polyol and the urethane block restricts both the phase separation and crystallization of the polyol segments. In Section 3.5, the degree of separation can further be visualized using Raman spectroscopy (RS). The degree of depolarization shown by RS highlights the change in molecular orientation of the hard segment with various HSC. The data from RS can be compared with that from the DSC measurement to show the efficacy of using Raman scattering in this respect. 2.1.3 Crystallinity
The properties of textile fibers are determined by their chemical structure, degree of polymerization, orientation of chain molecules, crystallinity, package density and cross-linking between individual molecules. Polymer crystallinity is one of the important properties of all crystalline or semi-crystalline polymers. Semi-crystalline polymers have both a crystalline and an amorphous form such that they exhibit short-range as well as long-range ordering of macromolecular chains. Both types of ordering contribute towards the degree of crystallinity of the polymer. An understanding of the order and structure in semi-crystalline SMPs, using techniques such as differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and IR spectroscopy, is essential for designing these materials for various applications and predicting their properties. The presence of crystalline and amorphous regions together in a semi-crystalline polymer is illustrated in Fig. 2.1. The schematic representation presumes that the ordered and disordered domains are intermingled with no demarcation between the two regions. The ordered and disordered portions shown in the figure can be related to the crystalline and amorphous regions observed at molecular level. The ordered regions in a polymer’s physical structure are called crystalline regions.
2.1 Schematic representation of the superstructure of a semi-crystalline polymer. Crystals and their crystalline states can be characterized in terms of many physical parameters, such as density, enthalpy, and free energy change on heating. A crystal is defined as having an ordered 3D structure. In a semi-crystalline polymer, 100% crystallinity is never obtained since, in a real polymer piece, there are large interfacial regions where a disordered structure is present. A perfect 3D order requires that the structure repeats itself in all directions so that, by describing the structure locally (in a repeating 3D unit cell), the entire structure can be uniquely described. Based on the findings of thermal analytical techniques such as DSC and WAXD, the differences between polyester and polyether-type PUs mainly depend on the chemical structure of polyol, the interaction between the soft and hard segments, and the degree of phase separation. These differences are the cause of different crystalline structures. Crystalline samples have an anisotropic structure. The size and concentration of crystallites are different for different PUs. The differences in polarizing microscopy (POM) images for different samples of PUs are...