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Epoxy-Based Nanocomposites—What Can Be Learned from Dielectric and Calorimetric Investigations?
(2022)
Epoxy-based nanocomposites are promisingmaterials for industrial applications (i.e., aerospace, marine, and automotive industries) due to their extraordinary mechanical and thermal properties. Regardless of the broad field of applications, there is still a considerable need to identify their structure–property relationships. Here, a detailed dielectric and calorimetric (DSC and fast scanning calorimetry) study on different epoxy-based nanocomposites was performed. Bisphenol A diglycidyl ether (DGEBA) cured with diethylenetriamine (DETA) was employed as the polymeric matrix, which was reinforced with three diverse nanofillers that exhibit different interaction strengths with the epoxy matrix (halloysite nanotubes, surface modified halloysite nanotubes, and taurine-modified layered double hydroxide). The structure, molecular mobility, and vitrification behavior are discussed in detail, focusing on the intrinsic structural and dynamic heterogeneity, as well as interfacial properties.
The chapter gives a brief introduction to (nano)compositecomposite materials having the focus on polymer-based nanocomposites. The different dimensionalities of nanoparticles are introduced, along with their distribution in the matrix.
Different application fields of polymer-based nanocomposites, like flame retardancy, filled rubbers, nanofilled thermosets and thermoplastics, separation membranes and nanodielectrics, are considered in greater detail.
The complex effect of nanoparticles on an epoxy-based and anhydride cured DGEBA/Boehmite nanocomposite with different particle concentrations is considered in this chapter. A combination of X-ray scattering, calorimetry (fast scanning and temperature modulated calorimetry) and dielectric spectroscopy was employed to characterize the structure, vitrification kinetics and the molecular dynamics of the nanocomposites. Firstly, the unfilled polymer was found to be intrinsically heterogeneous, showing regions with different crosslinking density, indicated by two separate dynamic glass transitions. Moreover, the glass transition temperature decreases with increasing nanoparticle concentration, as a result of changes in the crosslinking density. In addition, it was shown that the incorporation of nanoparticles can result in simultaneous increase in the number of mobile segments for low nanoparticle concentrations and on the other hand, for higher loading degrees the number of mobile segments decreases, due to the formation of an immobilized interphase.
A combination of different complementary methods is employed to investigate scaling of the molecular dynamics of two different liquid crystals. Each method is sensitive to different kind of fluctuations and provides therefore a different window to look at the molecular dynamics. In detail, broadband dielectric spectroscopy is combined with specific heat spectroscopy and neutron scattering. As systems the nematic liquid crystal E7 and a discotic liquid crystalline pyrene are considered. First of all it was proven that both systems show all peculiarities which are characteristic for glassy dynamics and the glassy state. Especially for the nematic liquid crystal E7 it could be unambiguously shown by a combination of dielectric and specific heat spectroscopy that the tumbling mode is the underlying motional process responsible for glassy dynamics. Dielectric investigations on the discotic liquid crystalline pyrene reveal that at the phase transition from the plastic crystalline to the hexagonal columnar liquid crystalline phase the molecular dynamics changes from a more strong to fragile temperature dependence of the relaxation rates. Moreover a combination of results obtained by specific heat spectroscopy with structural methods allows an estimation of the length scale relevant for the glass transition.
In this chapter broadband dielectric spectroscopy (BDS) is employed to polymeric blend systems. In its modern form BDS can cover an extraordinary broad frequency range from 10-4 to 1012 Hz. Therefore, molecular and collective dipolar fluctuations, charge transport, and polarization effects at inner phase boundaries can be investigated in detail including its temperature dependence. In the first part of the chapter, the theoretical basics of dielectric spectroscopy are briefly introduced covering both static and dynamic aspects. This section is followed by short description of the various experimental techniques to cover this broad frequency range. To provide the knowledge to understand the dielectric behavior of polymeric blend systems, the dielectric features of amorphous homopolymers are discussed in some detail. This concerns an introduction of the most important relaxation processes observed for these polymers (localized fluctuations, segmental dynamics related to the dynamic glass transition, chain relaxation), a brief introduction to the conductivity of disordered systems as well as polarization effects at phase boundaries. Theoretical models for each process are shortly discussed. In the last paragraph the dielectric behavior of polymer blends is reviewed where special attention is paid to binary systems for the sake of simplicity. In detail the dielectric behavior of binary miscible blends is described. The two most important experimental facts like the broadening of the dielectric relaxation spectra and the dynamic heterogeneity of the segmental dynamics are addressed in depth. Appropriate theoretical approaches like the temperature-driven concentration fluctuation model and the self-concentration idea are introduced.
Amorphous polymers
(2012)
The nature of the amorphous state in polymers is a classical topic in soft matter science with an immediate impact on modern technology. The chapter introduces the central concepts in this field and reviews a wealth of experimental information obtained by structural and dynamic techniques, such as neutron and X-ray scattering, broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and mechanical and specific heat spectroscopy, among others, covering dimensions between atomic (0.1 nm) to macroscopic (100 nm) length scales and timescales spanning more than 12 orders of magnitude, from picoseconds to seconds and longer. The theoretical models to describe the experimental findings are discussed in detail. Special attention is given to polymers having different architectures such as rings, stars, and block copolymers, and to the dynamics in nanometer thin layers.