DSC Protocol for Polymer Analysis

Found 10 papers. Key excerpts:

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A Phenomenological Model for Enthalpy Recovery in Polystyrene Using Dynamic Mechanical Spectra. — Koh-Hei Nitta et al., 2023

1. IntroductionWhen an amorphous polymer is cooled from well above glass transition temperature Tg to some lower temperature, rotational mobility around the main-chain bonds is frozen, and the polymer has no time to attain conformational equilibrium within the time scale of the given experiment. As a result, the glassy polymer solidified below Tg usually shows time-dependent physical properties such as heat capacity Cp, modulus, and density. This is a feature known as “physical aging” [1,2,3]. Then, a series of atomic rearrangements of the main chain towards the new equilibrium state proceeds in order to lose the excess configurational energy, and the subsequent time-dependent change is referred to as “structural relaxation” [4,5,6,7].Figure 1 illustrates schematical plots of enthalpy H and heat capacity Cp of a glassy polymer under cooling and subsequent reheating. The glass transition temperature Tg has been conventionally defined as a temperature that is the intersection of the slopes of the liquid state and the glassy one. The maintenance at temperature Ta below Tg after cooling from a liquid state induces enthalpy relaxation toward the equilibrium value. This phenomenon is called enthalpy relaxation or physical aging. The equilibrium state is represented by the extension of the slope of the liquid state (the dashed line in Figure 1). A cross point of the dashed line and the H–T curve gives a limiting fictive temperature

T

f

0

T f 0

. The enthalpy H overshoots and deviates downward from the equilibrium line during heating after enthalpy relaxation. The enthalpy recovery behavior can be detected as a peak around the glass transition during reheating on the Cp–T curve.

Figure 1.
Schematic plots of enthalpy and heat capacity during cooling and subsequent heating at a fixed rate for a glassy polymer.

The experimental technique most frequently

used for the characterization of structural relaxation is the measurement of enthalpy changes by differential scanning calorimetry (DSC). The internal rotation of the main chain toward reducing configurational energy, i.e., structural relaxation, emerges as an overshoot in the Cp curve under a constant heating rate after annealing at a temperature below Tg. The overshoot peak area resulting from the difference in both corresponding Cp–temperature lines is in accordance with the enthalpy difference

∆

H

DSC

∆ H DSC

between the extrapolated enthalpy of the liquid and that of the glass. The overshoot in the Cp curve shows a positive dependence on the annealing temperature and time. This thermodynamic feature is known as “enthalpy relaxation” [8,9,10]. Consequently, the changes in the enthalpy difference

∆
H

∆ H

with annealing time at a fixed annealing temperature can be expressed using a decay function

ϕ

t

ϕ t

of the annealing time t as follows:

∆

H

DSC

=
∆

H

e

1
−
ϕ

t

∆ H DSC = ∆ H e 1 − ϕ t

(1)

and here we introduce the normalized recovery function

Λ

T

t

Λ T t

as:

Λ

T

t

=
1
−
ϕ

t

Λ T t = 1 −</ --- **[Enthalpy Relaxation of Polyamide 11 of Different Morphology Far Below the Glass Transition Temperature](https://doi.org/10.3390/e21100984)** — René Androsch et al., 2019 1. IntroductionPolyamide 11 (PA 11) is an important thermoplastic material produced from short-term renewable castor oil, gaining increasing attention since it does not harm the environment like consumption of non-renewable crude oil. Due to its balanced property profile such as good chemical resistance, low oxygen- and hydrocarbon permeability, excellent low-temperature impact strength, or high thermal stability, it has found many industrial applications. These include off- and onshore oil and gas pipes, hydraulic and pneumatic hoses, electrical cable sheathing, sporting goods, or, related to its piezoelectricity, electronic-device applications [1,2,3,4]. PA 11 products are typically semi-crystalline, containing up to about 30% crystals, owing to its rather high rate of melt-crystallization. The critical cooling rate to suppress melt-crystallization and fully vitrify the melt at its glass transition temperature (Tg) of around 40 °C is between 500 and 1000 K/s. Slower cooling allows crystallization, however, with the final semi-crystalline morphology strongly depending on the exact crystallization conditions. It was found that crystallization at low supercooling of the melt proceeds via heterogeneous crystal nucleation, leading to formation of lamellar α-crystals and spherulites while crystallization at high supercooling of the melt, at temperatures below about 110 °C, proceeds via homogenous nucleation and non-spherulitic growth of a nodular mesophase [4,5,6,7]. Such temperature-controlled change of the pre-dominant nucleation mechanism is observed for many polymers, [8,9,10] including further representatives of the polyamide family such as PA 6 [11,12], PA 66 [13], or PA 12 [14].PA 11 typically is melt-processed by extrusion, blow-molding, injection-molding, rotomolding, but also 3D printing, and laser sintering, involving rather fast solidification during cooling and the generation of a large variety of unstable or metastable non-equilibrium structures [15,16,17,18]. Structural changes towards equilibrium may involve both the crystalline and amorphous phases, and often lead to a change of properties, requiring research for its quantification and understanding. Such irreversible changes of structure include enthal py relaxation of the amorphous phase, crystallization of the amorphous phase, and reorganization of crystals, with these processes briefly described below.A thermodynamically non-equilibrium amorphous structure is obtained on cooling the equilibrium liquid phase to below the equilibrium melting temperature Tm, 0 of the inherently crystallizable system, being in case of PA 11 203 °C [19] or 220 °C [20]. However, the structure of the supercooled non-equilibrium liquid below Tm, 0 apparently adjusts instantaneously on variation the temperature due to the short relaxation time of the order of magnitude of picoseconds [21]. As such, supercooled liquids are considered metastable, that is time-independent, unless crystal nucleation and growth occurs. Metastability, at least within a certain timeframe defined by the relaxation kinetics, is lost on vitrification of the supercooled liquid phase on further cooling the system to below Tg, leading to the formation of an initially thermodynamically unstable glass [22,23]. Due to constraints imposed by the reduced free volume between molecular segments, structural relaxation of the system by changes of conformations of covalent bonds distinctly slows down, allowing its recognition at experimentally assessable time scales well above milliseconds, even millions of years [23,24,25,26]. Relaxation processes in unstable polymer glasses, as well as glasses of other classes of materials, include its densification towards a final state defined by the density/free volume and enthalpy of the corresponding liquid at identical temperature. Such relaxation occurs by both cooperative rearrangement of molecular segments at the nanometer-length scale but also non-cooperative changes of local chain conformations at the sub-nanometer scale, e.g., depending on temperature [27,28]. Importantly, though connected with decreases of the enthalpy and entropy of the system, these relaxation processes do not involve the formation of a new phase, as would be the case upon crystallization. The decrease of the free volume during glass relaxation has enormous impact on properties of polymeric materials as it may cause detrimental changes of, e.g., mechanical or transport properties, often denoted as physical aging [29,30,31,32,33].Further processes occurring in non-equilibrium amorphous phases, in both the supercooled liquid and the glass, leading to a decrease of Gibb’s enthalpy towards equilibrium, are crystal nucleation and growth. Focusing on the glassy state, being in foreground in this manuscript, quantitative analysis of the kinetics of glass-crystallization in polymers recently became possible with the opportunity to prepare glasses of well-defined cooling history and to analyze efficiently the progress of structural changes on annealing the glass using fast scanning chip calorimetry [34]. A main conclusion derived from recent, tailored glass-relaxation- and -crystallization-experiments in polymers is the rather strict sequence of enthalpy relaxation, homogeneous crystal nucleation, and crystal growth [35,36,37,38,39,40]. It is explained such that the cooperative rearrangements of highly mobile short molecule segments at the length scale of few nanometers during enthalpy relaxation suppress growth of stochastically appearing nuclei to supercritical size, rather than lead to their disappearance. The interplay between enthalpy relaxation and crystal nucleation and growth in glasses has been confirmed for several polymers [35,36,37,38,39,40] but also for small inorganic molecules [41]. Worth noting, analysis of the temperature dependence of the kinetics of homogeneous crystal nucleation revealed that nucleation is fastest slightly above Tg and that nucleation is not affected by the main glass transition, that is, at temperatures around Tg nuclei formation requires segment mobility at a length scale shorter than freezing at the glass transition [8,9,37,42,43].Besides enthalpy relaxation of the amorphous glass, crystallization of the supercooled liquid or glass in absence or presence of already existing crystals, further structural changes driving a decrease of Gibbs enthalpy of the system involve an increase of the stability of crystals, commonly described as crystal reorganization. Crystal reorganization typically occurs at temperatures close to their stability limit, that is, their melting point, and includes processes like lamellar thickening to decrease the surface-to-volume ratio, or healing of lattice defects. In this work, crystal reorganization is out of the scope, with further information available in the literature [44,45,46,47,48,49,50].The present study focusses on changes of structure of PA 11 at temperatures far below Tg, being also important for practical reasons. The temperature range of application of this particular material includes ambient and sub-ambient temperatures, with superior --- **[Enthalpy Relaxation, Crystal Nucleation and Crystal Growth of Biobased Poly(butylene Isophthalate)](https://doi.org/10.3390/polym12010235)** — Silvia Quattrosoldi et al., 2020 1. IntroductionPlastic production continues to grow around the world, mainly using fossil resources. The production of plastic reached 64.4 Mt in 2017 in Europe [1]. Among the various fields of the application of plastics, the packaging sector is, by far, the most important for the volumes involved. Due to the short life of plastic packaging, a huge amount of waste is accumulating in the environment, creating dramatic marine as well as terrestrial pollution problems. Decreasing finite fossil resources, and mitigating the environmental impact of plastics, are both becoming very urgent needs. In this view, bio-based materials are one of the solutions for the development of a sustainable society. In the last few years, much research had focused on the development of bioplastics, showing comparable performances in terms of cost and properties to traditional, petroleum-based plastics. Promising materials are the derivates of isophthalic acid (IPA). IPA is a bio-based building block, obtainable by the cycloaddition of bio-acrylic acid and bio-isoprene units, or directly via the fermentation of biomasses [2,3]. Solvent free polycondensation with a bio-based glycol, such as 1,4-butanediol [4,5,6], can lead to a completely bio-based homopolymer, poly(butylene isophthalate) (PBI). This polyester belongs to the class of poly(alkylene phthalate)s, with a chemical structure similar to the well-known isomer poly(butylene terephthalate) (PBT) [7], only differing in the position of the linkage of the phenylene group with the neighbored ester groups. While in case of PBT, the ester groups are in the para-position, in PBI they are in the meta-position.PBI was patented in 1952 in the United States [8], and shortly afterwards, a first report about the crystallization and melting behavior of meta-phenylene groups containing polyesters including PBI was published [9]. With the glass transition temperature Tg being at about room temperature [10,11], any commercial use of PBI crucially depends upon its crystallization capability. Though PBI is able to crystallize [9], the maximum crystallization rate is much lower than in case of PBT. For PBT, the minimum crystallization halftimes of the order of magnitude of 0 .1 and 1 s are observed at around 70 and 145 °C [12,13,14], related to crystallization via homogenous and heterogeneous crystal nucleation, respectively [14,15,16]. For PBI, in contrast, reports suggest that crystallization is fastest between around 80 and 100 °C, with the minimum crystallization halftime being of the order of magnitude of several minutes [17,18]. Regarding the structure of crystals, to date detailed information about the unit cell or conformation of chain segments are not available, despite early observation of the X-ray fiber pattern [7]. The fiber identity period was determined being 2.6 nm, pointing to a slightly distorted planar zigzag conformation of the butylene sequence with two chemical repeat units per unit cell [19]. The equilibrium melting temperature of PBI crystals is reported being between 143 °C and 165 °C, while a larger discrepancy exists regarding the bulk enthalpy of melting [9,11,17,18,20]; in a more recent study a value of 125 J/g is suggested [18]. Little is known about the morphology of PBI crystals. It may be assumed that melt-crystallization leads to formation of lamellae, since growth proceeds spherulitically; however, at an extremely low rate [19]. Further characterization of PBI concerns the segmental dynamics [21] and the rheological behavior in the molten state [22] and in solution [23,24].Nowadays, PBI is not commercially available, though several patents report suitable industrial uses [25,26]. Research efforts often regard the derivates of PBI, such as end-capped materials, block copolymers and random copolymers [27,28,29,30,31,32,33,34], with potential application, e.g., as a hot-melt adhesive or coating. The main drawback for an industrial application of the PBI homopolymer is likely the rather low crystallization rate, which complicates obtaining semicrystalline products via melt processing, despite excellent mechanical behavior [27], good barrier properties and easy melt-processability [27]. In order to explore the possible potential of fully bio-sourced PBI for industrial uses, as well as to further understand its particularly slow crystallization with respect to its terephthalic counterpart, the present study attempts to provide a thorough analysis of the formation of semicry stalline morphologies from the melt and the glassy state. The crystallization rate and semicrystalline structure of polymers are largely dependent on the crystal nucleation. Therefore, the great part of this work is devoted to investigate the rather fast formation of crystal nuclei at high supercooling of the melt and even in the glassy state, being an alternative to the more traditional route of the acceleration of crystallization by using heterogeneous nucleators [35,36]. These nuclei then may enhance cold-crystallization, and possibly yield a non- or fine-spherulitic structure in shorter time than in the case of direct melt-crystallization [37,38]. This approach has already proven successful for other slow crystallizing polymers including poly(ethylene terephthalate) (PET) [39] or poly(l-lactic acid) (PLLA) [40,41], for which it has been shown that slow cooling allows the formation of nuclei, not yet crystals, which then grow to crystals on heating.Fast crystal nucleation at high supercooling of the melt has been attributed to the formation of homogenous nuclei [15,16,42], however, with their detection being complicated due to their small size as well as the low enthalpy of formation. In order to overcome this problem, a defined nucleation experiment inspired by Tammann can be applied, based upon the observation that the temperatures of the maximum rate of homogenous crystal nucleation and of crystal growth often are largely different [43,44]. Tammann’s two-stage crystal nuclei development method implies the formation of nuclei at high supercooling of the melt or even in the glassy state, and their subsequent growth at higher temperature, in order to detect them. The method was initially applied to investigate the nucleation and crystallization of glycerol [43,44] and of organic liquids [45], and later on, of silicate glasses [46,47]. Recently Tammann´s method was proven advantageous to gain information about homogeneous nucleation in polymers, including poly (ε-caprolactone) (PCL) [48,49,50], PLLA [51,52,53], isotactic poly(butene-1) (iPB-1) [54], polyamide 6 (PA 6) [55] or PET [56]; recently, a modified --- **[Long-Term Physical Aging Tracked by Advanced Thermal Analysis of Poly(](https://doi.org/10.3390/molecules25173810)** — Anna Czerniecka-Kubicka et al., 2020 1. IntroductionPoly(N-isopropylacrylamide) (PNIPA) belongs to the smart polymer group (Figure 1). It is an ideal candidate for use in drug delivery systems because it is a biocompatible, a low-toxic, and has a Lower Critical Solution Temperature (LCST) around human body temperature. PNIPA is a thermoresponsive polymer having LCST = 32 °C. Above the LCST, the PNIPA is hydrophobic and interacts with the components of the cells, while, below that, it is hydrophilic and does not interact with them [1,2,3,4,5]. The extensive research has been carried out on utilizing this property of PNIPA for the delivery of drugs in stimuli responsive drug delivery systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. These properties of PNIPA have caused this thermosensitive polymer to be utilized in many drug delivery systems, including for cancer therapeutics. In response to changes in temperature, solutions of PNIPA exhibit rapid, reversible phase transition/phase separation phenomena.PNIPA is an amorphous polymer which easily ages. This process, commonly so-called a structural recovery, is linked with storage of material. If the storage temperature is below a temperature of glass transition (Tg), the structural recovery (physical aging) of an amorphous or a semicrystalline substance is observed. This process applies to natural and synthetic materials. The structural recovery can be monitored by the volume recovery or the enthalpy recovery [18,19,20,21,22,23,24,25,26]. In literature, the physical aging term is also used, but it applies to changes in mechanical properties. The result of the physical aging process is the change of material properties, such as in length, hardness, and brittleness [23,24,25,26,27,28,29,30]. On the other hand, the physical properties of amorphous and semicrystalline material at the non-equilibrium state are changed versus time and temperature. In the case of the amorphous material, the values of volume, enthalpy, and entropy are higher than the value for material at the equilibrium state. Thermodynamic properties at the storage temperature below Tg are changed in the direction of equilibrium value. It is linked with the ordering of molecules under process conditions [31,32,33,34]. The characteristic thermogram of the aging process in the differential scanning calorimetry (DSC) measurement shows overlapping the enthalpy relaxation on the change of heat capacity in the glass transition region, as is presented in Figure 2 [34,35,36,37,38].Enthalpy relaxation (ΔHa) and the fictive temperature (Tf) are parameters which describe the progress of structural recovery in DSC experiments [38,39,40,41,42,43]. The enthalpy relaxation can be estimated as a difference between the A and B areas (see Figure 2). These areas are bounded of aged and unaged curves of heat flow rate or heat capacity for these full amorphous or semicrystalline samples [43,44,45]. The formation of ΔHa results from the aim for the amorphous material to state at better thermodynamic stability, and Tf is the second parameter related to the structural recovery, which characterizes the structure of the amorphous phase. The fictive temperature can be determined directly based on the DSC measurement during the heating process by analyzing the raw data of aged and unaged heat flow rate (or heat capacity) or based on analysis of total enthalpy function [46,47,48,49,50,51,52,53].The scheme presented in Figure 3 shows the quantitative analysis of the enthalpy relaxation estimation. This approach established and used the equilibrium solid, Cpsolid, and liquid, Cpliquid, heat capacities as reference lines.The baselines at solid (Cpsolid) and liquid (Cpliquid) state of PNIPA were established early in Ref. [54]. The insert of Figure 3 shows an example of the advanced thermal analysis for experimental heat capacities, Cp(exp)DSC obtained from the standard DSC, using Cpsolid and Cpliquid heat capacities. The advanced thermal analysis of glass transition using the equilibrium solid and liquid heat capacity allowed to establish the glass transition temperature (Tg) with estimated value of 415 K and the change of heat capacity (ΔCp) of PNIPA at Tg as 50.36 J·mol−1·K−1 [54].In this paper, for a first time, the structural recovery of PNIPA is monitored based on the experimental thermodynamic functions enthalpy, and transition parameters of PNIPA referenced to equilibrium enthalpy of solid (Hsolid) and liquid (Hliquid) [45,54]. Figure 3 shows the Hsolid and Hliquid of PNIPA as the temperature function. At equilibrium melting temperature of 622 K, the value of Hsolid increased by the value of the equilibrium heat of fusion, ∆Hf°, of 26.591 kJ·mol−1 to reach the level of Hliquid [45,54].Figure 4 shows a scheme of the results from advanced thermal analysis of physical aging of full amorphous polymeric material in details at around glass transition temperature (Tg) similar as in Ref [45]. First, the insert of Figure 4 illustrates the experimental apparent heat capacities of both unaged and aged samples in frame of equilibrium solid Cp(solid) and liquid Cp(liquid) heat capacities. The unaged sample presents a step in heat capacity at Tg and aged sample shows an endothermic peak. Figure 4 primarily shows the integral enthalpy function of aged (curve b) and unaged (curve a) polymer versus temperature compared with equilibrium liquid Hliquid and solid Hsolid enthalpies for data Cp presented in the insert this figure. The physical aging process described in Figure 4 was carried out isothermally at the aging temperature (Ta).The Hliquid line has been elongated towards the low temperatures (dash line). Under cooling the polymeric material at liquid state at a given rate, the enthalpy of aged polymer diffracted at the glass transition temperature (Tg). The unaged material reached the Ho value at Ta and then, subjected to the isothermal aging process, lowered its enthalpy to the Ha value at the aging temperature (Ta). The curve ‘a’ indicates the enthalpy of unaged polymer obtained during the cooling, while the curve ‘b’ points at the enthalpy of the aged polymer obtained under the heating. The intersection of the experimental enthalpy (the curve ‘b’) of the aged polymer with the extension of Hliquid determines the so-called fictive temperature (Tf) [47,48]. If the extrapolated liquid line is not reached at the completion of structural recovery, the fictive temperature should cease --- **[Electrospun Fibrous Membrane with Confined Chain Configuration: Dynamic Relaxation and Glass Transition.](https://doi.org/10.3390/polym14050939)** — Nuozi Zhang et al., 2022 Physical aging of glassy polymer materials, which is below their transition temperature [ 1 ] during storage and application, leads to changes in the electrical, thermal and mechanical properties [ 2 , 3 , 4 ]. Shifts in material properties lead to further changes in material functionality, especially those designed for long service life [ 1 , 2 , 3 , 4 , 5 ]. A popular example is polyurethane, which is used to insulate pacemakers [ 6 , 7 ]. Aging and degradation of the polyurethane coating lead to insulation failure and leakage in pacemakers, which can cause muscle irritation [ 8 ]. Another example is poly(lactide-glycolide) (PLGA), a biodegradable polymer used to make absorbable sutures for wound healing [ 9 , 10 , 11 , 12 ]. Physical aging and degradation are critical for application performance and storage processes of such polymers [ 13 , 14 ].It is also crucial for shape memory polymers, which are intelligent materials, rely on the different stress relaxation models. Polymers can be easily deformed into a temporary shape in one state and then quenched into a designed permanent shape by receiving heat or other stimuli [ 15 , 16 , 17 ]. Most simulations experiments based on accelerated aging are unreliable because accelerated aging does not always follow a linear equation with time and temperature as variables. Aging experiments typically require extended periods of time at temperatures far below the T g measured by differential scanning calorimetry (DSC). Otherwise, the amount of the endothermic change or time scale during this change would be difficult to be observed for analysis, given the sensitivity of the DSC instrument [ 18 ]. Mode-coupling theory (MCT) [ 19 ], is one of the most popular theories to study glass transition of structural materials [ 3 , 20 , 21 , 22 ]. N relaxation times constantly appear for an N-particle system in MCT. Götze MCT [ 23 ] has truncated all high-order correlation terms after the second term (fourth-order correlation in the reciprocal space) because of a complicated operation. Although Götze MCT [ 24 ] is a truncated theory, it only provides two modes (α and β) for glass transformation and demonstrates an accelerated slowdown of the α branch (or mode) relative to the β relaxation time in a simple system. However, the importance of the dynamic modes and the slowdown of long wavelength modes [ 25 ] are the most important characteristics of any glassy materials in a temperature below the DSC-measured T g and in the release of the shape memory function in applications for a glassy material. Although any MCT theory for understanding the glass transition process includes Götze MCT [ 24 ] failed to provide detailed quantitative guidance for data analysis, multiple relaxation times are essential. according to the general principle of MCT, extensive relaxation times slow down non proportionally at temperatures lower than the DSC T g . This DSC T g is a convenient temperature point, which marks the emergence of an observable endothermic change rate of the testing specimen at the DSC scanning rate. This work selected PLGA-regenerated film and electrospun membrane to study dynamics relaxation during the glass transition near the DSC-defined T g . The packing density of the regenerated film is higher than the electrospun membrane, but the electrospun membrane has a large surface-to-volume ratio base on a uniform nanofiber morphology. This electrospun membrane’s relaxation process (spectrum or modes) was vastly extended as PLGA chains are stretched, constrained, and confined in these fibers. The polymer chains are physically constrained and confined in the electrospun fibers of the membrane to a certain scale. Thus, the polymer chains are not only important in a membrane application but also a favorable model for studying the glass transformation and aging processes in general. Moreover, PLGA electrospun membranes with different molecular weights are studied to investigate the relaxation process in various timescales given their different chain mobilities, which are coupled into the long-distance relaxation modes. The relaxation process is influenced by the local structural environment, which varies at different annealing times and temperatures. The process can be accelerated by increasing the annealing temperature, thereby obtaining the DSC curves and reflecting the chain length and segmental mobility. Therefore, through this study, instantaneous structures during the DSC measurements over different annealing times and temperatures can be obtained to understand better the dynamic nature of glass transition and a favorable control of its structure/morphology during manufacturing, storage, and shape memory function delivery in the application. PLGA (Mw = 20,000 g/mol [20 k], 60,000 g/mol [60 k], 100,000 g/mol [100 k]; LA/GA = 75/25, mol/mol) was purchased from Jinan Daigang Biology Engineer Co., Ltd., Jinan, China. N, N-dimethyl formamide (DMF), acetone, chloroform, and deuterated chloroform were obtained from the China National Medicines Corporation, Ltd., Beijing, China. All other reagents used were of commercial analytical grade. PLGA solutions were prepared in a mixed solvent system of acetone and DMF ( v / v = 5/5). The 20 k, 60 k, 100 k membrane electrospinning process was performed in a sterile environment at 16, 20, 24 kV and a steady flow rate of 20, 15, 10 μL/min (spinneret with an inner diameter of 0.3 mm). The electrospun fibers were collected on a metal drum (as an electrode; diameter of 9 cm; tip-to-collector distance of 18, 23, 28 cm), rotating at approximately 120 rpm. The thickness of the fibrous scaffolds was set to 100 ± 10 µm by controlling the spinning time. The “regenerated film” was prepared by melting the electrospun membrane (a PLGA fibrous membrane with an Mw of 60 k was placed between two flat templates and was heated up to 65 °C for 6 h) and was used for further studies for easy comparison with a fibrous membrane in aging and other experiments. The DSC measurement was conducted to ensure that the treatment had completely melted the fibrous structure and provided a regenerated film. To remove the residual solvents, all the samples were further dried under 8–10 °C in the vacuum for 1 month to remove the residual solvents. All characterizations were performed after removing the residual solvents. The morphologies of the electrospun fiber were observed using a scanning electron microscope (JEOL JSM-6700F, Tokyo, Japan) at an accelerating voltage of 5 kV. Samples were sputter-coated with platinum before analysis. The water contact angles of the PLGA electrospun membrane were measured with a sessile drop method using a digital contact angle measurement system with a CCD camera (Powereach JC2000A, Shanghai, China). A water droplet of 5 µL was used and a snapshot of the image was taken to measure the static contact angle. Molecular weight and molecular weight dispersity were measured through gel permeation chromatography (GPC). --- **[Glass transitions and physical aging of cassava starch - corn oil blends.](https://doi.org/10.1016/j.carbpol.2014.01.032)** — Adriana Pérez et al., 2014 Introduction Glassy polymers can experience physical aging as a consequence of their desire to approach thermodynamic equilibrium after undergoing their glass transition temperature (Struik, 1978). This enthalpy reduction process simultaneously decreases both, free volume and segmental mobility, as the system approaches a pseudo-equilibrium state. Similar to synthetic polymers, physical aging is an important phenomenon in amorphous starch and starchy materials in general. Many starchy products are processed under high temperature and relative low moisture conditions; and later consumed in the glassy state, as for example snacks and breakfast cereals. Changes occurring during physical aging are often responsible for storage-induced changes and for quality deterioration. These effects should be controlled for the sake of long term stability of starchy products ( Champion et al., 2000;Chung & Lim, 2006). Physical aging leads to significant changes in the material properties such as densification, increased brittleness and decreased permeability; which make it a phenomenon of particular interest from both, scientific and technological points of view ( Badii et al., 2005;Kim et al., 2003;Lourdin et al., 2002). The physical aging of different glassy polymers has been investigated by means of dilatometry and differential scanning calorimetry (DSC). DSC is probably one of the most popular techniques to study physical aging. During aging, which is usually induced by annealing at temperatures close but lower than the glass transition temperature (T g ), the sample loses enthalpy as it approaches equilibrium. When the sample is heated during a DSC scan, the enthalpy is regained by the superheated glass. As the enthalpy must rapidly increase in the superheated glass in order to attain the value of the rubbery state, an endothermic process develops and a peak appears just above or around the glass transition temperature range. The area under this endothermic peak is equal to the enthalpy value that was regained by the sample and should be approximately similar to that lost during aging. Apart from allowing reliable temperature control of heating and cooling rates, the DSC experiments can also be performed at constant moisture content, as the sample can be encapsulated in hermetic pans. Although enthalpic relaxation has been widely studied in starch ( Borde et al., 2002a;Borde et al., 2002b;Chung & Lim, 2003;Kalichevsky et al., 1992;Kim et al ., 2003;Lourdin et al., 2002), only limited work has been carried out in more complex systems (e.g., Gonzalez et al., 2010), where interactions with other added food components such as other biopolymers, water and oil, might have an effect on the associated endothermic event. The heterogeneity of a system (i.e., the presence of two coexisting phases for instance) might play an important role in determining the relaxation behavior of starchy samples. Madrigal, Sandoval, & Muller (2011) reported two endothermic events from first DSC heating scans of cassava starch blended with different levels of corn oil, and containing low moisture content, after being kept at room temperature for 4 weeks for moisture equilibration purposes. These double peaks corresponded to two different enthalpic relaxation events correlated with two different T g in the material. They were also evidenced by two peaks exhibited by the same samples in tan i curves obtained by dynamic mechanical thermal analysis. It was shown, by these authors, that the greater the corn oil content added to cassava starch, the lower the moisture content at which the enthalpic relaxations appeared, which indicated that this oily component favored phase separation and molecular mobility during storage. Consequently, the aim of this study was to carry out a more systematic study on the appearance of such enthalpy relaxation endotherms, and to determine the aging parameters of cassava starch when corn oil was added. Materials and methods Raw material Native cassava starch (CS) donated by Inveyuca (PDVSA Agricola), located in San Tome (Anzoategui state, Venezuela) was used. Its initial moisture content was 14.4% (dry basis, d.b.). Commercial corn oil (CO) Mazeite (r) was bought from a local supplier. Sample preparation Transformation of native samples into amorphous materials by thermo-moulding required their moisture content to be around 25%. Humidified CS was prepared by slowly adding the required amount of distilled water to the CS. The humidified CS-CO blends, on the other hand, were prepared by simultaneously adding CO by slow dripping, in a proportion of 1, 5 and 10 g/100 g of CS, and distilled water to the CS. The required levels of water and corn oil were added separately, but at the same time using two different plastic containers from which the liquids were squeezed drop by drop onto the starch compound. Accordingly, samples with 25% moisture content (wet basis), and the required amount of CO, were obtained in each case. The amount of water added to reach 25% moisture content was calculated considering the initial moisture content of CS. Continuous mixing of samples was applied during addition of water and CO, by using a laboratory mixer at medium speed. Lumps were eliminated by sieving the samples, before being stored in plastic bags at 10 * C overnight to equilibrate moisture content within the sample. Transformed cassava starch (TCS) and TCS-CO blends were obtained by compression moulding in a ADQ11 (model PP25T) hydraulic press. The samples were placed between kapton sheets and moulded at 3900 psi and 160 * C for 30 min. Pressure was maintained during cooling of the amorphous samples to a temperature of 30 * C, so as to avoid expansion. It has been demonstrated in a previous work by means of DSC in excess of water and wide angle scattering X-rays that these thermo-moulded conditions fully transformed cassava starch to the amorphous state ( Garcia et al., 2012;Luk et al., 2013;Madrigal et al., 2011;Perdomo et al., 2009). Samples were cut into small discs (diameter 4 mm and thickness of 1 mm) of approximately 10 mg in weight. Moisture equilibration at different levels was carried out by storing amorphous samples during four weeks in controlled environments generated by the following eleven different oversaturated salt solutions at 25 * C: KOH (8.2%), LiCL (11.3%), MgCl 2 (32.8%), K 2 CO 3 (43.2%), Mg(NO 3 ) 2 (52.9%), KI (68.9%), NaCl (75.3%), (NH 4 ) 2 SO 4 (80.9%), KCl (84.3%), KNO 3 (95.4%), and K 2 SO 4 (97.3%). The numbers in parenthesis indicate the relative humidity in each environment (Greenspan, 1977). In order to prevent microbial spoilage of samples, crystalline thymol was placed inside the environments with relative humidity greater than 80%. It has been proven that at ambient temperature, thymol does not affect the sample water sorption (Sand --- **[Physical Aging Behavior of a Glassy Polyether.](https://doi.org/10.3390/polym13060954)** — Xavier Monnier et al., 2021 A glass can be formed by several routes, all of them sharing the prerequisite of allowing circumventing crystallization. Among them, that based on cooling through the melting, T M , and the glass transition, T g , temperatures is by far the most common [ 1 ]. The kinetic nature of the glass transition, well exemplified by the cooling rate dependence of T g [ 2 , 3 ], implies that glasses are thermodynamically in non-equilibrium. The slow evolution of the thermodynamic state toward the metastable equilibrium state represented by the supercooled liquid is generally addressed as structural recovery [ 4 ] or physical aging [ 5 , 6 , 7 , 8 ]. This phenomenon induces a general time-dependent modification of the glass properties and, therefore, is of utmost importance from both fundamental and technological viewpoints. Indeed, volume shrinkage could be detrimental for the glass lifetime [ 9 ]; therefore, aging must be by glass manufacturers to avoid any undesired alteration of properties over the course of time. Notable examples in this sense are those on the effect of aging on gas transport properties [ 10 , 11 , 12 , 13 ] also in relation to mechanical properties [ 14 ]. The relation between gas transport properties and the thermodynamic state of the glass has also been discussed [ 15 ]. Furthermore, knowledge of physical aging can provide insights of utmost importance on dynamics and thermodynamics of glasses sitting at the bottom of the energy landscape [ 16 , 17 ]. The conventional belief, based on the archetypal volume recovery experiments of Kovacs [ 4 ] and later by a wealth of experiments [ 6 , 7 ], is that recovery of equilibrium takes place with a monotonous sigmoidal-shaped evolution of the glass thermodynamic state and that such evolution is triggered exclusively by the main relaxation process, generally addressed as ” α relaxation”, exhibiting super-Arrhneius behavior and diverging not far below T g . The common features of studies showing this behavior is that physical aging is carried out either in proximity of T g or, if aging temperatures considerably smaller than T g are considered, for aging times not long enough to allow attaining the final equilibrium. Recently, experiments by differential scanning calorimetry (DSC)—where the evolution of the enthalpy of the glass is monitored—showed that, if physical aging is conducted considerably below T g and for aging times as long as about one year, two steps in the approach to equilibrium of the enthalpy, each characterized by the attainment of a plateau, are observed for different polymers [ 18 ]. This event was shown independently in other glasses, including chalcogenides [ 19 ], a small molecule [ 20 ], metallic glasses [ 21 , 22 ], and polysulfone [ 23 ]; and variously modeled according to different approaches [ 24 , 25 , 26 ]. Furthermore, if aging is conducted far below T g , prolonged aging results in the attainment of partial recovery of equilibrium even though a plateau in the enthalpy is achieved [ 27 ]. The presence of multiple steps indicates that there exist different mechanisms of equilibrium recovery, whose existence is evidenced by the thermal response of glasses aged well below T g . Specifically, in these cases, specific heat scans show the presence of an excess endotherm of the aged sample with respect to the unaged one. This outcome appears to be general, as it was found in glasses of different nature [ 28 , 29 ], including polymers [ 27 , 30 , 31 ], metallic glasses [ 29 , 32 ], and a plastic crystal [ 28 ]. Importantly, this behavior is magnified in polymer glasses exhibiting large free interfacial area [ 33 , 34 ], as a result of the acceleration of physical aging in these systems [ 35 , 36 , 37 , 38 , 39 ], which amplifies the separation among different mechanisms of equilibrium recovery [ 36 ]. Despite the variety of experiments showing these features, results showing several steps in isothermal conditions are relatively scarce. The main reason is likely that, to observe this behavior, a wide interval of aging times is required. The lower aging time bound attainable by standard calorimetric techniques is typically of the order of minutes. Hence, to cover an aging time interval of several decades (for instance, 5), time scales of months to years are required [ 18 , 19 , 27 , 29 ]. In the present work, we employ the capabilities of fast scanning calorimetry (FSC) [ 40 , 41 ] permitting to access heating/cooling rates of the order of several thousands kelvin per second to study the physical aging behavior in an amorphous polyether. By reducing the time scale of the experiments over orders of magnitude with respect to standard calorimetry, FSC allows accessing sub-second evolution of physical aging. Furthermore, samples heated at high rates are less amenable to chemical degradation, since the time spent at high temperatures is very short. This allows investigating phenomena, including glass transition [ 42 , 43 ], melting [ 44 ], and polymer adsorption [ 45 ], otherwise impossible to study by standard calorimetry. The choice of an amorphous polyether rests on his chemical difference with previously employed polymers [ 18 ], which were either vinyl polymers or presenting an aromatic ring in the backbone. We find that, similarly to other amorphous polymers with substantially different molecular structure [ 18 , 23 ], physical aging exhibits two mechanisms of equilibration, which, in the case of isothermal experiments, is indicated by the presence of two decays towards equilibrium. Poly(1,4-cyclohexanedimethanol) (PCDM), whose chemical structure is reported in Figure 1 , was synthesized by polycondensation of 1,4-cyclohexanedimethanol (CHDM) using the previously reported non-eutectic acid-base organocatalysts based on MSA and TBD (3:1) [ 46 , 47 ]. It is worth noting that the employed CHDM contains a cis:trans isomer mixture equal to 70:30, that strongly inhibits the crystallization of the material. In a typical reaction, CHDM was polymerized in the presence of the catalyst (7.5 and 2.5 mol % of MSA and TBD, respectively, with respect to the monomer) for 72 h with a gradual increase of temperature from 130 to 200 ∘ C under vacuum. The resulting product was purified by precipitation in cold MeOH from a CHCl3 solution. This procedure was repeated three times to yield the pure homopolymer that was characterized by 1H NMR (300 MHz, CDCl3 δ ): = 3.58–3.49 (1H), 3.31–3.10 (16H), 1.84 (16H), 1.5–1.3 (15H) 0.95 (13H). 13C NMR (300 MHz, CDCl3 δ ): = 77.9, 39.7, 30.7. GPC. (THF, 25 ∘ C) Mn: 10 kDa, Đ = 1.8. The kinetics of physical aging was studied following the evolution of the enthalpy by fast scanning calorimetry. To this aim, the Flash DSC-1 by Mettler-Toledo (Nänikon, Switzerland), based on chip calorimetry technology, was --- **[Enthalpy relaxation behavior of dry wood detected by temperature-modulated differential scanning calorimetry](https://doi.org/10.1007/s10086-012-1264-8)** — Tsunehisa Miki et al., 2012 Polymeric materials with glassy structures (glassy polymers) undergo enthalpy relaxation below their glass transition temperatures. Enthalpy relaxation is usually accompanied by a gradual reduction in volume (known as volume relaxation), which affects the physical properties of the material. The glass transition temperatures and enthalpy relaxation behaviors of industrial amorphous polymers (including composites) have been investigated by differential scanning calorimetry (DSC) with the aim of controlling their physical properties [ Wood is a natural polymer composite consisting of crystalline cellulose and amorphous matrices of cellulose, hemicellulose, and lignin. The wood matrix exhibits glass transitions. Since these glass transitions are affected by the presence of water in the matrix, the glass transition temperature of wood depends on the humidity and temperature of the environment. Many studies based on static and dynamic mechanical measurements have reported softening temperatures associated with a glass transition at about 80 °C for wet wood [ In this study, temperature-modulated DSC was used to investigate enthalpy relaxation of dry wood. The effects of the cooling rate and the holding time at a given temperature (which are referred to as the annealing time and temperature) on the thermal behavior [e.g., the temperature dependence of the total heat flow (THF)], which is equivalent to that measured by conventional DSC, as well as the dynamic heat capacity and NRHF are discussed. Based on the observed thermal behaviors, microstructure changes associated with unstable states of wood [ Sample preparation An air-dried hinoki ( For the DSC measurements, about 5 mg of these solid wood samples was placed in an aluminum pan with a pinhole; contact between the sample and the pan was maintained during heating/cooling runs and drying occurred through the pinhole. The same solid samples in pinhole hermetic pans were used in all the measurements for investigating the effects of the cooling rate and annealing. The same procedure was applied for the measurements of the cellulose sample. DSC measurements A heat flux differential scanning calorimeter (DSC Q100, TA Instruments) with modulation control was used. Modulated-temperature DSC has been described by Reading and Schawe [ In this study, three temperature programs were used to investigate the effects of the cooling rate and the temperature and annealing (holding) time at a given annealing temperature on the thermal behaviors of the samples between −80 and 170 °C, as shown in Fig.  Temperature-modulated DSC measures the RHF by modulating the temperature and the THF, which is equivalent to the heat flow obtained by conventional DSC. The RHF is obtained from the heat capacity change of a material, which responds rapidly to the oscillating temperature. The NRHF is defined as the difference between the THF and the RHF and it is originated from slow relaxation processes such as enthalpy relaxation. The temperature was modulated by applying a sinusoidal wave with a period of 100 s to the underlying heating/cooling programs. To maintain heating/cooling-only conditions for constant underlying cooling rates of 0.5, 1, 3, 5, and 10 °C/min, the amplitude was set to 0.13, 0.26, 0.79, 1.32, and 2.65 °C, respectively, in program (a), whereas the amplitude was fixed to 1.0 °C in programs (b) and (c). When analyzing the results obtained by these programs, this study focuses on the heat flow during heating because different heat histories (e.g., different cooling rates and annealing conditions) drastically affect the thermal behavior during subsequent heating processes with the same heating rate. Total heat flow Figure  Since the glass transition and enthalpy relaxation generate non-equilibrium states in glassy materials, it is important to consider the previous heat history (e.g., the cooling rate and time at the environmental temperature) before performing measurements on industrial polymers. Figure  Figure  Reversing heat flow and non-reversing heat flow derived by temperature modulation Figure  The glass transition and relaxation kinetics of glassy polymers have been characterized by conventional and temperature-modulated DSC [ Figure  In the present DSC study of dry wood containing cellulose, hemicellulose and lignin, moisture adsorption/desorption (which is detected by weight gain/loss) was hardly observed during dry nitrogen purging in cyclic heating/cooling runs, even for annealing at 0 °C. Moreover, wood that has been dried for longer annealing times exhibited greater endothermic behavior, even in later heating runs. Therefore, the endothermic behavior observed in the present study cannot be explained by water loss alone; it must also involve structural relaxation . Dry wood has a fine structure, which exhibits relaxation without typical glass transitions. Enthalpy relaxation at various annealing temperatures The enthalpy loss was determined from the area enclosed by heat flow curves after annealing at a given temperature and the base curve of the second heat flow. The effect of the annealing time at a given temperature on enthalpy relaxation was investigated (see Fig.  At the annealing temperature of 0 °C, the enthalpy loss behavior of the cellulose was smaller than that of wood over the measured time due to much crystalline state. Based on the annealing temperature at which enthalpy relaxation occurs, the observed relaxation does not appear to mainly originate from lignin because isolated lignin has a glass transition temperature of over 150 °C [ Temperature-modulated DSC clearly detected enthalpy relaxation of dry wood from the NRHF, although no clear glass transitions were observed in the RHF. The enthalpy relaxation is greatly affected by the annealing temperature and holding time. It was maximized at an annealing temperature of about 0 °C. Very little enthalpy relaxation was observed at temperatures above 40 °C or below −60 °C for holding times of up to 250 min. The endothermic peak temperature in the relaxation decreased when the annealing time was increased. These results suggest that temperature changes alter the microstructure of wood (even dry wood) and that lignin exists in different structures in wood because their enthalpy relaxation behaviors are quite different from those of isolated lignin. In particular, the observed enthalpy relaxation could be related to the other components besides lignin so that the time-dependent physical properties due to unstable states of wood originate from not only lignin but also from other components such as cellulose and hemicellulose and their interactions. This work was partially supported by a Grant-in-Aid for Young Scientists (A) (No. 19688010) and a Grant-in-Aid for Scientific Research (A) (No. 23246129) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. --- **[Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven-based experiments*](https://doi.org/10.1002/pen.25911)** — Shuang Jin et al., 2022 1 INTRODUCTION When polymeric materials are cooled from the equilibrium liquid state, their glassy state is reached when crystallization (if the polymer is semi-crystalline) is avoided, viz., through rapid quenching. The glass transition is a kinetic process and the temperature associated with the transition is called the glass transition temperature, T g . The relationship between thermodynamic properties such as volume or enthalpy and temperature at the glass transition is shown in Figure 1.[1, 2] FIGURE 1Open in figure viewerPowerPoint Schematic diagram for thermodynamic properties such as volume or enthalpy as a function of temperature. Adapted with permission from Simon and McKenna.[2] Copyright 2017 by Taylor & Francis Group, LLC With reference to Figure 1, for amorphous polymers, the volume or enthalpy depart from the equilibrium liquid line at T g and follow the glass line when the rate of cooling becomes comparable to the molecular mobility and the material cannot relax into the equilibrium state in the time frame of the experiment, thereby becoming hindered kinetically in a nonequilibrium state.[3] When held isothermally below T g at an aging temperature T a , the structure of the glassy polymer (volume or enthalpy) evolves spontaneously towards the equilibrium state, as shown by the arrow in Figure 1.[4] This phenomenon is called physical aging when it involves the changing mechanical responses as a result of the changing glassy structure.[5] Two quantities, departure from equilibrium, δ, and fictive temperature, T f , are used to describe the structural recovery process. In the volume recovery experiment, δ is defined by Kovacs as[1]: δ t = v t − v ∞ v --- **[Physical Ageing of Amorphous Poly(lactic acid)-Indapamide System Studied by Differential Scanning Calorimetry.](https://doi.org/10.3390/pharmaceutics15092341)** — Marcin Skotnicki et al., 2023 1. IntroductionActive pharmaceutical ingredients (APIs) may exist in a crystalline or amorphous form. The crystalline state is characterised by a regularly ordered lattice structure. In practical terms, the structures of these systems are generally thermodynamically stable and are relatively simple to study using techniques such as differential scanning calorimetry (DSC) or X-ray diffraction methods. On the other hand, there is no long-range order to amorphous forms and their “structures” are not easy to characterise by standard X-ray diffraction methods. A valuable technique for studying amorphous pharmaceuticals is DSC [1,2,3]. Amorphous drugs usually dissolve more readily and are more bioavailable than their crystalline counterparts [2]; however, amorphous APIs may recrystallise during the shelf-life of the formulation [4]. A common approach to stabilise amorphous drugs is polymeric amorphous solid dispersions (PASDs) [5]. In PASDs, the improved stability of an amorphous API is achieved by entrapping the drug in a high-energy glassy state between the polymer chains [6,7]. Although the excipients are often considered inert, it is known that they can interact with APIs, changing their stability, absorption and bioavailability [8,9]. Therefore, APIs must be investigated during preformulation studies at the early phase of the drug development process, in order to provide the necessary information to develop a stable formulation with increased bioavailability [10].The amorphous forms of API may be desirable due to their improved apparent solubility and, as a consequence, their bioavailability in comparison with its crystalline counterparts. However, in contrast with crystals, glasses are not thermodynamically stable [11,12,13,14,15]. Thus, the stability of amorphous APIs is a primary issue associated with their use in the formulation. The amorphous forms of APIs, during storage below or above the glass transition temperature Tg, may revert to the crystalline form [6,11,16], losing their superior properties. Below the glass transition temperature and above a Kauzman temperature, glasses undergo a physical ageing process, i.e., structural relaxation towards thermodynamic equilibrium as a function of time and temperature [17]. In contrast with chemical or biological ageing, physical ageing is a reversible phenomenon involving the ordering of the amorphous phase, during which no breaking or forming of chemical bonding occurs. More significant structuring may cause, among other things, the deterioration of solubility [18], diffusivity and permeability [19], a decrease in physical stability [20], and a change of mechanical properties [21,22]. For instance, a decrease in solubility has been observed for physically aged cinnarizine-Soluplus solid dispersions [18]. Annealing may also positively affect amorphous APIs, for instance, it can increase chemical stability [23].In this work, a polymer–drug system was obtained using amorphous poly(lactic acid) and an amorphous API—indapamide.Poly(lactide) (PLA) belongs to the group of aliphatic polyesters. Depending on D-, L-isomers content, it can exist in a semi-crystalline or an amorphous state exhibiting different physicochemical properties [24,25,26]. It is widely used in the food, pharmaceutical and medical industries. In pharmaceutical applications, PLA is used as a drug carrier matrix in drug delivery systems as well as the bulk component of medical devices due to its biocompatibility and bioresorbability [27,28,29]. PLA is widely used in formulations in order to modify the dissolution profile of an API or to improve its stability [30,31]. The drug can be released from the polymer matrix in a controlled and prolonged manner. For example, Leroueil-Le Verger et al. successfully used solid dispersion with polylactide in an oral controlled release system for isradipine [30].Indapamide (IND) is a thiazide-like diuretic drug used to treat hypertension [32]. IND is practically insoluble in water [33] and belongs to the biopharmaceutics classification system class II (low solubility, high permeability). Indapamide available on the market is formulated in a crystalline form, e.g., [34], however, it can also be obtained in an amorphous form [33,35,36].The ability of a material to transform into its amorphous state is called its glass-forming ability (GFA) [37]. APIs are categorised into three classes, I, II and III, based on their GFA [37,38]. The GFA of indapamide belongs to class III, where the material, after melting the crystalline form, does not recrystallise during the DSC cooling/heating cycle, and the sample remains amorphous.As previously mentioned, amorphous materials, unlike crystals, are in a thermodynamic, non-equilibrium state, and, therefore, such materials undergo the physical ageing process. The process is a physical phenomenon related only to the amorphous solid phase. At a temperature lower than the glass transition temperature, the amorphous materials gradually evolve toward the thermodynamic equilibrium of the glassy state [17]. The aged amorphous material tends to be in a more equilibrium state, with less energy [39,40,41]. The physical ageing process can be studied, among other things, using differential scanning calorimetry [36,42]. On the scan obtained from the standard DSC measurement of an unaged amorphous material, a glass transition with a change in specific heat is observed in the heat flow or heat capacity versus temperature plot (Figure 1). When the amorphous sample is aged isothermally at a temperature lower than the glass transition temperature (called the ageing temperature) and is reheated through the glass transition region, an endothermic peak in the DSC curve is observed in addition to the change of heat capacity (Figure 1). Figure 1. Scheme for determination of enthalpy relaxation from DSC plot of heat capacity vs. temperature. The physical ageing process can be described by measuring the enthalpy relaxation or enthalpy recovery, Δhr, which manifests an order in the amorphous structure and molecular mobility [17,43,44]. Enthalpy relaxation depends on the sample’s thermal history and is the additional energy burden required to reverse the ageing process—to destroy/reverse the ordering resulting from the evolution of the glass towards equilibrium. It can be calculated from the area difference between the heat flow or heat capacity curves of aged (A) and unaged (B) samples (ΔHr = A − B) (see Figure 1). The relaxation kinetic of the non-equilibrium glassy state can be determined using various models [43]. One of these models is the Kohlrausch–Williams–Watts model (KWW) [45,46].One of the parameters describing the molecular mobility of the amorphous material is the fragility