1 Glass-Transition Phenomena in Polymer Blends Ioannis M

1 1 Glass-Transition Phenomena in Polymer Blends Ioannis M. Kalogeras University of Athens, Faculty of Physics, Department of Solid State Physics, Zografos 15784, Greece 1.1 Introduction The ever-increasing demand for polymeric materials with designed multi- functional properties has led to a multiplicity of manufacturing approaches and characterization studies, seeking proportional as well as synergistic properties of novel composites. Fully integrated in this pursuit, blending and copolymerization have provided a pair of versatile and cost-effective procedures by which materials with complex amorphous or partially crystalline structures are fabricated from combinations of existing chemicals [1–3]. Through variations in material’scom- position and processing, a subtle adaptation of numerous chemical (corrosion resistance, resistance to chemicals, etc.), thermophysical (e.g., thermal stability, melting point, degree of crystallinity, and crystallization rate), electrical or dielectric (e.g., conductivity and permittivity), and manufacturing or mechanical properties (dimensional stability, abrasion resistance, impact strength, fracture toughness, gas permeability, recyclability, etc.) can be accomplished effortlessly. It is therefore not surprising the fact that related composites have been widely studied with respect to their microstructure (e.g., length scale of phase homoge- neity in miscible systems, or type of the segregation of phases in multiphasic materials) and the evolution of their behavior and complex relaxation dynamics as the material traverses the glass transformation range [4–7]. The reversible transformation of amorphous materials (including amorphous regions within semicrystalline polymers) from a molten or rubber-like state into a stiff and relatively brittle glassy state is denoted as “glass transition” (or “liquid–glass transition”). Originally, this term was introduced to describe the striking changes in thermodynamic derivative properties (e.g., heat capacity, compressibility, and thermal expansivity) that normally accompany the solidification of a viscous liquid, such as a polymer melt, during cooling or even compression. In time course, however, the term “glass transition” acquired a broader meaning and is now frequently Encyclopedia of Polymer Blends: Volume 3: Structure, First Edition. Edited by Avraam I. Isayev. 2016 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2016 by Wiley-VCH Verlag GmbH & Co. KGaA. 2 1 Glass-Transition Phenomena in Polymer Blends employed for describing “any phenomenon that is caused by a timescale (on which some interesting degree of freedom equilibrates) becoming longer than the timescale on which the system is being observed” [6]. The conventional route to the glassy state of matter is the (rapid) cooling of a melt, provided that crystalization is bypassed. Interestingly, melt mixing provides one of the most common techniques for the large-scale preparation of compression or injection molded polymer blends. The freezing-in of a structural state during cooling, commonly referred to as glass- ification or vitrification, corresponds to a loss of the state of internal equilibrium possessed by the initial liquid. The vitrification process occurs over a narrow temperature interval, the so-called glass transformation range, over which the characteristic molecular relaxation time of the system changes by some 2–2.5 orders of magnitude, reaching the order of 100 s (the laboratory timescale). In macro- molecular substances, this relaxation time is connected with the time response of cooperative (long-range) segmental motions. For convenience, the glass transformation region is traditionally represented by a single value, denoted as the “glass-transition temperature” (Tg) [7]. Because of the range of temperature involved and also because of its history, path, and cooling (or heating) rate dependences, assigning a characteristic Tg to a system becomes frequently a problematic task. Nonetheless, when appropriately measured, the glass-transition temperature is very reproducible and has become recognized as one of the most important material properties, directly relating to several other thermophysical and rheological properties, processing parameters, and fields of potential application [8]. Nowadays, polymer engineering largely relies on chemical and compositional manipulation of Tg, in an attempt to target particular technological or industrial requirements. A notable paradigm provides meticulous studies on the physical stabilization of active pharmaceutical ingredients (typically, poorly soluble drugs, but potentially also of proteins or other compounds) in the form of binary or ternary solid dispersions/solutions with biologically inert glassy polymers, with the aim of increasing their solubility, dissolution rate, bioavailability, and therapeutic effective- ness [9]. Significant improvements in the performance of related systems are frequently accredited to a combination of factors, including the effects of hydrogen- bonding networks or ion–dipole intercomponent interactions (i.e., stabilizing enthalpic contributions), and strong crystallization-inhibitory steric effects owing to the high viscosity of the polymeric excipient [10]. The implementation of solid-state glassy formulations as a means to preserve the native state of proteins (biopreserva- tion) entails a higher level of complexity in behavior, since chemical and physical stabilization heavily relies on manipulating the local anharmonic motions of the individual protein molecules (fast dynamics) in addition to their slow (glass-transition) dynamics [11]. The established practice in the preservation of proteins primar- ily focuses on hydrated solid-state mixtures of proteins with glass-forming disaccharides (e.g., trehalose or sucrose) or polyols (e.g., glycerol), serving as lyopro- tectants. Still, processing problems such as surface denaturation, mixture separation, and pH changes that lead to physical and chemical degradations, in addition to degradations occurring on storage, make clear that manufacturing of alternative solid-state formulations remains a challenging issue. In this pursuit, however, one 1.1 Introduction 3 has to take into account the complex internal protein dynamics and the fact that in order to successfully maintain protein’s structural integrity the selected glass- forming polymer will have to sustain a strong and complicated hydrogen-bonding network (around and to the protein) that will effectively couple matrix dynamics to the internal dynamics of the protein molecules [11]; the adaptability in local structures and chemical environments, offered by polymers’ blending, might provide a viable solution to the problem. While general consensus exists as to the usability of glass transitions in exploring molecular mobilities, molecular environments, and structural heterogeneities in segmental length scales, conflicting arguments still appear regarding the interpretation of fundamental phenomenological aspects of the transition itself. The nature of the glass transition remains one of the most controversial problems in the disciplines of polymer physics and materials science, and that in spite of the in-depth experimental and theoretical research conducted hitherto [12–14]. The difficulty in treating glass transitions even in relatively simple linear-chain amorphous polymers is caused by the almost undetectable changes in static structure, regardless of the qualitative changes in characteristics and the extremely large change in the timescale. Given the significance of this subject, this chapter begins with an overview of important aspects of the phenomenology of glass formation. To address the per- plexing behavior encountered when a system passes through its glass transformation range, a number of theoretical models approach this phenomenon using arguments pointing to a thermodynamic or a purely dynamic transition. Although we have not arrived at a comprehensive theory of supercooled liquids and glasses, it is frequently recognized that the observed glass transitions are not bona fide phase transitions, but rather a dynamical crossover through which a viscous liquid falls out of equilibrium and displays solid-like behavior on the experimental timescale. Basic notions and derivations of common theoretical models of the glass transition are presented in Section 1.2, with particular reference to early free volume and con- figurational entropy approaches, in view of their impact on the development of “predictive” relations for the compositional dependence of the Tg in binary polymer systems. Note that regardless of the multitude of treatments we clearly lack a widely accepted model that would allow ab initio calculation of the glass-transition temperature. Most theoretical approaches simply allow a prediction of changes in Tg with – among other variables – applied pressure, degree of polymerization (molar mass) or curing (cross-linking), and composition. Important chemical factors that influence the affinity of the components and the magnitude of Tg in polymer blends, in addition to manufacturing processes or treatments that are typically used for manipulating the glass-transition temperature of polymer composites, are briefly reviewed in Section 1.3. The largely experimentally driven scientific interest on glass transitions in composite materials is equipped with a collection of experimental methods and measuring techniques, with the ability to probe molecular motions at distinctly

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