Aguilar / Román Smart Polymers and their Applications

1. Auflage 2014
ISBN: 978-0-85709-702-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 584 Seiten

Reihe: Woodhead Publishing in Materials

Smart polymers are polymers that respond to different stimuli or changes in the environment. Smart Polymers and their Applications reviews the types, synthesis, properties, and applications of smart polymers. Chapters in part one focus on types of polymers, including temperature-, pH-, photo-, and enzyme-responsive polymers. Shape memory polymers, smart polymer hydrogels, and self-healing polymer systems are also explored. Part two highlights applications of smart polymers, including smart instructive polymer substrates for tissue engineering; smart polymer nanocarriers for drug delivery; the use of smart polymers in medical devices for minimally invasive surgery, diagnosis, and other applications; and smart polymers for bioseparation and other biotechnology applications. Further chapters discuss the use of smart polymers for textile and packaging applications, and for optical data storage. Smart Polymers and their Applications is a technical resource for chemists, chemical engineers, mechanical engineers, and other professionals in the polymer industry; manufacturers in such sectors as medical, automotive, and aerospace engineering; and academic researchers in polymer science. - Reviews the different types of smart polymer, discussing their properties, structure, design, and characterization - Reviews applications of smart polymers in such areas as biomedical engineering, textiles, and food packaging

Autoren/Hrsg.

Aguilar, Maria Rosa

Román, Julio San

Leseproben

2 Temperature-responsive polymers: properties, synthesis and applications
R. Hoogenboom,    Ghent University, Belgium Abstract:
This chapter describes polymers that undergo a temperature-induced phase transition in aqueous solution providing an important basis for smart materials. Different types of temperature-responsive polymers, including shape-memory materials, liquid crystalline materials and responsive polymer solutions are briefly introduced. Subsequently this chapter will focus on thermoresponsive polymer solutions. At first, the basic principles of the upper and lower critical temperature polymer phase transitions will be discussed, followed by an overview and discussion of important aspects of various key types of such temperature-responsive polymers. Finally, selected potential applications of thermoresponsive polymer solutions will be described. Key words
smart material; responsive polymer; lower critical solution temperature (LCST); upper critical solution temperature (UCST); phase transition 2.1 Introduction
Smart materials that respond with a property change to a variation in the environmental conditions are an attractive class of materials for advanced applications. Responsive and adaptive materials are also omnipresent in natural systems. Examples include the focusing of the eye, opening and closing of pores, and wound healing (Stryer, 1999). The majority of such natural responsive and adaptive processes are driven by conformational changes and/or aggregation of proteins, which can be regarded as nature’s smart polymers. Similarly, responsive synthetic polymers are attractive building blocks for the development of artificial smart materials. A wide variety of responsive polymer materials have been reported that respond to various external parameters, such as temperature, pH, mechanical stress and even certain molecules, including CO2 and sugars (see Roy et al., 2010 for a recent review). The response of the polymer can also be manifold, such as a change in shape, color or solubility. Temperature-responsive polymers are especially interesting since variations in temperature can be applied externally in a non-invasive manner. Furthermore, spontaneous temperature fluctuations also occur in nature, for example, during day and night cycles as well as the increased temperature of inflamed tissue. Different working mechanisms can be exploited for the development of temperature-responsive polymers as will be briefly outlined in this introductory paragraph. The three main classes of temperature-responsive polymers are: 1. shape-memory materials; 2. liquid crystalline materials; 3. responsive polymer solutions. Shape-memory materials are thermoplastic elastomers consisting of a hard phase with a high glass transition temperature (Tg; Tg,1 in Fig. 2.1) and a second switching phase with intermediate Tg,2 or melting temperature that enables the temperature-responsive behavior (Lendlein and Kelch, 2002; Liu et al., 2007). Such shape-memory materials can be deformed in any shape when heating above the highest Tg resulting in the permanent shape. When these materials are subsequently deformed in between the two transition temperatures a temporary shape can be induced, which can be ‘frozen’ in by cooling the deformed state below the switching temperature. This shape-memory material will transform back to the permanent shape when heated above the switching temperature (Fig. 2.1). As such, these materials are thermoresponsive, but they have to be ‘reprogrammed’ after each switching cycle. By introducing multiple intermediate temperature transitions, the number of programmable shape changes can be increased and a recent example demonstrated four independent states in a shape-memory material having one broad Tg (Xie, 2010). 2.1 Schematic representation of the thermoresponsive behavior of a shape-memory polymer. Tg,1 represents the Tg of the hard phase and Tg,2 represents the Tg of the switching phase. Liquid crystalline polymers have a liquid crystalline phase in addition to the glassy state and the isotropic rubbery phase (Donald et al., 2005; Weiss and Ober, 1990). This liquid crystalline phase has a certain anisotropic order of the mesogens present in the polymer. It has been reported that polymers with main-chain nematic liquid crystalline blocks have an elongated main-chain in the liquid crystalline phase that contracts to a random coil state when heated to the isotropic phase, which is a fully reversible polymer phase transition that has been utilized as the main switching mechanism for developing artificial muscles (Fig. 2.2) (Li and Keller, 2006). Up to 40% contraction has been demonstrated for such materials upon heating. Polymeric networks with side-chain mesogens have also been developed having a chiral nematic liquid crystalline phase as is also used in liquid crystal display (LCD) screens. Such side-chain liquid crystalline polymer networks have been utilized in, for example, the development of thermochromic materials (Sage, 2011). 2.2 (a) Change in conformations of main-chain LC polymers from the extended nematic phase to a collapsed isotropic phase upon heating. (b) Corresponding macroscopic shape change during this nematic–isotropic phase transition. The third and most widely studied type of thermoresponsive polymers are polymers that undergo a solution liquid–liquid phase transition in response to variation of the temperature, that is, phase separation occurs from a homogeneous solution into a concentrated polymer phase and a diluted polymer phase. This phase transition is often accompanied by a transition from a clear solution to a cloudy solution, also known as the cloud point temperature (TCP), for low concentration polymer solutions. This clouding is due to the formation of droplets of the high concentration polymer phase in combination with the difference in refractive index between the two phases. When the phase separation occurs at an elevated temperature, this is referred to as lower critical solution temperature (LCST) transition while the reversed phase behavior is known as upper critical solution temperature (UCST) transition. Early examples of LCST and UCST behavior of polymers were reported in organic solvents, such as the UCST of poly(styrene) in cyclohexane (Schultz and Flory, 1952) and the LCST of poly(methyl methacrylate) (PMMA) in 2-propanone (Cowie and McEwen, 1976). Most interesting, however, are thermoresponsive polymer phase transitions in aqueous solutions since this phenomenon provides high potential for biomedical applications, such as drug delivery and switchable synthetic cell culture surfaces (de las Heras Alarcón et al., 2005; Schmaljohann, 2006; Ward and Theoni, 2011). The remainder of this chapter will focus on such temperature-responsive polymers in aqueous solution, by discussing basic principles (Section 2.2) and key types of temperature-responsive polymers (Section 2.3) as well as selected applications (Section 2.4). 2.2 Basic principles of temperature-responsive polymers in aqueous solution
The different types of polymer phase transitions that can occur in aqueous solutions of homopolymers are schematically depicted in Fig. 2.3, namely LCST transition, UCST transition and closed loop coexistence of LCST and UCST transitions. These schematically drawn binodal or coexistence curves represent the equilibrium concentration of the two phases in the phase separated state. The LCST is defined as the lowest temperature of this binodal curve (Fig. 2.3a) while the UCST is defined as the highest temperature of this binodal curve (Fig. 2.3b). Closed loop coexistence has also been reported for a small number of polymers that have coinciding LCST and UCST phase behavior (Fig. 2.3c). The most prominent example of a polymer with such closed loop coexistence in water is poly(ethylene glycol); albeit both LCST and UCST transitions only occur when heated beyond the boiling point of water in closed vessels (Saeki et al., 1976). Other polymers exhibiting closed loop coexistence phase behavior include partially acetylated poly(vinyl alcohol) (Nord et al., 1951) and poly(hydroxyethyl methacrylate) (Longenecker et al., 2011). Besides these three types of polymer phase diagrams, there are also a few examples of polymers that show a low temperature UCST transition and a high temperature LCST transition (not shown in Fig. 2.3), including poly(vinyl methyl ether) (van Assche et al., 2011) and mixtures of poly(dimethylaminoethyl methacrylate) with a trivalent [Co(CN)6]3- anion (Plamper et al., 2007). 2.3 Schematic representation of the polymer phase diagrams (binodal or coexistence curves) for polymers exhibiting LCST behavior (a), UCST behavior (b) and closed loop coexistence (c). The majority of recent reports on thermoresponsive polymers evaluate the phase transition temperature at a certain polymer concentration by turbidity measurements, that is, light scattering of a polymer solution at 500–700 nm as function of temperature. It is important to note that the transition temperature thus obtained is the...