Drug-Biopolymer Dispersions: Morphology- and Temperature- Dependent (Anti)Plasticizer Effect of the Drug and Component-Specific Johari-Goldstein Relaxations

Amorphous molecule-macromolecule mixtures are ubiquitous in polymer technology and are one of the most studied routes for the development of amorphous drug formulations. For these applications it is crucial to understand how the preparation method affects the properties of the mixtures. Here, we employ differential scanning calorimetry and broadband dielectric spectroscopy to investigate dispersions of a small-molecule drug (the Nordazepam anxiolytic) in biodegradable polylactide, both in the form of solvent-cast films and electrospun microfibres. We show that the dispersion of the same small-molecule compound can have opposite (plasticizing or antiplasticizing) effects on the segmental mobility of a biopolymer depending on preparation method, temperature, and polymer enantiomerism. We compare two different chiral forms of the polymer, namely, the enantiomeric pure, semicrystalline L-polymer (PLLA), and a random, fully amorphous copolymer containing both L and D monomers (PDLLA), both of which have lower glass transition temperature (T_g) than the drug. While the drug has a weak antiplasticizing effect on the films, consistent with its higher T_g, we find that it actually acts as a plasticizer for the PLLA microfibres, reducing their T_g by as much as 14 K at 30%-weight drug loading, namely, to a value that is lower than the T_g of fully amorphous films. The structural relaxation time of the samples similarly depends on chemical composition and morphology. Most mixtures displayed a single structural relaxation, as expected for homogeneous samples. In the PLLA microfibres, the presence of crystalline domains increases the structural relaxation time of the amorphous fraction, while the presence of the drug lowers the structural relaxation time of the (partially stretched) chains in the microfibres, increasing chain mobility well above that of the fully amorphous polymer matrix. Even fully amorphous homogeneous mixtures exhibit two distinct Johari-Goldstein relaxation processes, one for each chemical component. Our findings have important implications for the interpretation of the Johari-Goldstein process as well as for the physical stability and mechanical properties of microfibres with small-molecule additives.

Microscopic understanding of the Johari-Goldstein β relaxation gained from nuclear γ-resonance time-domain-interferometry experiments

Traditionally the study of dynamics of glass-forming materials has been focused on the structural α relaxation. However, in recent years experimental evidence has revealed that a secondary β relaxation belonging to a special class, called the Johari-Goldstein (JG) β relaxation, has properties strongly linked to the primary α relaxation. By invoking the principle of causality, the relation implies the JG β relaxation is fundamental and indispensable for generating the α relaxation, and the properties of the latter are inherited from the former. The JG β relaxation is observed together with the α relaxation mostly by dielectric spectroscopy. The macroscopic nature of the data allows the use of arbitrary or unproven procedures to analyze the data. Thus the results characterizing the JG β relaxation and the relation of its relaxation time τ_{β} to the α-relaxation time τ_{α} obtained can be equivocal and controversial. Coming to the rescue is the nuclear resonance time-domain-interferometry (TDI) technique covering a wide time range (10^{-9}-10^{-5}s) and a scattering vector q range (9.6-40nm^{-1}). TDI experiments have been carried out on four glass formers, ortho-terphenyl [M. Saito et al., Phys. Rev. Lett. 109, 115705 (2012)10.1103/PhysRevLett.109.115705], polybutadiene [T. Kanaya et al., J. Chem. Phys. 140, 144906 (2014)10.1063/1.4869541], 5-methyl-2-hexanol [F. Caporaletti et al., Sci. Rep. 9, 14319 (2019)10.1038/s41598-019-50824-7], and 1-propanol [F. Caporaletti et al., Nat. Commun. 12, 1867 (2021)10.1038/s41467-021-22154-8]. In this paper the TDI data are reexamined in conjunction with dielectric and neutron scattering data. The results show the JG β relaxation observed by dielectric spectroscopy is heterogeneous and comprises processes with different length scales. A process with a longer length scale has a longer relaxation time. TDI data also prove the primitive relaxation time τ_{0} of the coupling model falls within the distribution of the TDI q-dependent JG β-relaxation times. This important finding explains why the experimental dielectric JG β-relaxation times τ_{β}(T,P) is approximately equal to τ_{0}(T,P) as found in many glass formers at various temperature T and pressure P. The result, τ_{β}(T,P)≈τ_{0}(T,P), in turn explains why the ratio τ_{α}(T,P)/τ_{β}(T,P) is invariant to changes of T and pressure P at constant τ_{α}(T,P), the α-relaxation time.

New insight into relaxation dynamics of an epoxy/hydroxy functionalized polybutadiene from dielectric and mechanical spectroscopy studies

Dielectric and mechanical spectroscopy methods have been employed to describe the temperature dependencies of the segmental and macromolecular relaxation rates in epoxy/hydroxy functionalized polybutadiene. Dielectric studies on the dynamics of segments of the polymer as well as the mobility of small ions trapped in the system have been carried out both as a function of temperature and pressure under isobaric and isothermal conditions, respectively.

Identifying the origins of two secondary relaxations in polysaccharides

The main goal of this paper is to identify the molecular origins of two secondary relaxations observed in mechanical as well as in dielectric spectra in polysaccharides, including cellulose, and starches, such as pullulan and dextran. This issue has been actively pursued by many research groups, but consensus has not been reached. By comparing experimental data of monosaccharides, disaccharides, and polysaccharides, we are able to make conclusions on the origins of two secondary relaxations in polysaccharides. The faster secondary relaxations of polysaccharides are similar to the faster secondary relaxations of mono-, di-, and oligosaccharides. These include comparable relaxation times and activation energies in the glassy states, and also all the faster secondary relaxations have larger dielectric strengths than the slower secondary relaxation. The similarities indicate that the faster secondary relaxations in the polysaccharides have the same origin as that in mono-, di-, and oligosaccharides. Furthermore, since the relaxation time of the faster secondary relaxation in several mono- and disaccharides was found to be insensitive to applied pressure, the faster secondary relaxations of the polysaccharides are identified as internal motions within their monomeric units. The slower secondary relaxations in polysaccharides also have similar characteristics to those of the slower secondary relaxations of the disaccharides (maltose, cellobiose, sucrose, and trehalose), which indicates the analogous motions govern the slower process in these two groups of carbohydrates. Earlier we have shown in disaccharides that the rotation of the monomeric units around the glycosidic bond is responsible for this process. The same motion can occur in polysaccharides in the form of a local chain rotation. These motions involve the whole molecule in disaccharides and a local segment in polysaccharides. It is intermolecular in nature (with relaxation time pressure dependent, as found before in a disaccharide), and hence, it is the precursor of the structural alpha-relaxation. These results lead us to identify the slower secondary relaxation of the polysaccharides as the Johari-Goldstein beta-relaxation, which is supposedly a universal and fundamental process in all glass-forming substances.