Self-assembling of Thermo-Responsive Block Copolymers: Structural, Thermal and Dielectric Investigations

Abstract

Plethora of amphiphilic polymers and copolymers have been synthesized that form self-assembled structures in aqueous media, resembling the assemblies of biopolymers invented by nature. Such polymeric systems serve as stimuli-responsive materials, i.e. they respond to small external changes in the environmental conditions, which is a common process for biopolymers in living organisms. Temperature is the most widely used stimulus in environmentally responsive polymer systems. Thermoresponsive polymers have attracted much research interest because of their potential applications, which include rheological control additives, thermal affinity separation, controlled drug release, gene therapy and regenerative medicine. On the other hand, they represent model systems for many biological systems, for example for the investigation of the interaction between peptide-like groups and solvents and, thus, for the study of protein stability in aqueous solutions. In this chapter, we provide a comprehensive view on recent investigations on the micellar aggregation and the thermoresponsive behavior of amphiphilic model polymers. Firstly, we will present general characteristics of the thermoresponsive behavior of macromolecules and discuss in more detail their applications with biomedical interest. Next, we will focus on the experimental investigation of thermoresponsive polymers and present, briefly, research outcomes concerning the properties of the well-studied poly(N-isopropylacrylamide) (PNIPAM) polymer. Then, we will present results with respect to the thermoresponsive behavior of a rather new class of polymers based on the nonionic poly(methoxy diethylene glycol acrylate) (PMDEGA) polymer. Copolymers with various architectures, namely diblock, triblock and star block copolymers are studied, as well as a PMDEGA homopolymer as reference. To that aim, complementary methods were applied, such as small-angle X-ray (SAXS), differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). Seeking for understanding of fundamental aspects of the macromolecular thermoresponsive behavior, we present, in a comparative way, results obtained on PNIPAM- and PMDEGA-based systems. Characteristic differences between the two series of polymeric solutions are worked out, concerning the self-organization, the width and hysteresis of the transition, and the chain conformations during the demixing phase transition.