Figure 9
Part A: TSDC spectra obtained on a PET films with different drawn ratio (l). The relaxation time is obtained according relationship (10) and it dependence with the temperature is shown on the part B.
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Many works focused on glassy polymers determine values of glass transition temperature (Tg) and an overview of the literature shows that depending on the method used, values of Tg are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility charact...
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Context 1
... temperature below T g , the variations of the relaxation time with the temperature are obtained by TSDC. Figure 9 A shows some complex spectra obtained for the different samples studied in this work. A peak of current is observed attributed to the a transition (i.e. the dielectric manifestation of the glass transition) when the temperature reaches the glass transition. ...
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... The temperature dependence of the loss moduli (E ′′ ) of the PLA/PHBHHx blend, PLA/PHBHHx/HNT, and the PLA/PHBHHx/HNTs/PLA-g-MA nanocomposites is displayed in Figure 9b. The relaxation α associated with the glass transition temperature (Tg) of the samples coincides with the maximum temperature of the loss modulus shown [70,71]. All curves show two peaks at approximately Tg related to the α relaxation of PHBHHx and PLA. ...
Given the global challenge of plastic pollution, the development of new bioplastics to replace conventional polymers has become a priority. It is therefore essential to achieve a balance in the performances of biopolymers in order to improve their commercial availability. In this topic, this study aims to investigate the morphology and properties of poly(lactic acid) (PLA)/ poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) (at a ratio of 75/25 (w/w)) blends reinforced with halloysite nanotubes (HNTs) and compatibilized with poly(lactic acid)-grafted maleic anhydride (PLA-g-MA). HNTs and PLA-g-MA were added to the polymer blend at 5 and 10 wt.%, respectively, and everything was processed via melt compounding. A scanning electron microscopy (SEM) analysis shows that HNTs are preferentially localized in PHBHHx nodules rather than in the PLA matrix due to its higher wettability. When HNTs are combined with PLA-g-MA, a finer and a more homogeneous morphology is observed, resulting in a reduction in the size of PHBHHx nodules. The presence of HNTs in the polymer blend improves the impact strength from 12.7 to 20.9 kJ/mm2. Further, with the addition of PLA-g-MA to PLA/PHBHHX/HNT nanocomposites, the tensile strength, elongation at break, and impact strength all improve significantly, rising from roughly 42 MPa, 14.5%, and 20.9 kJ/mm2 to nearly 46 MPa, 18.2%, and 31.2 kJ/mm2, respectively. This is consistent with the data obtained via dynamic mechanical analysis (DMA). The thermal stability of the compatibilized blend reinforced with HNTs is also improved compared to the non-compatibilized one. Overall, this study highlights the effectiveness of combining HNTs and PLA-g-AM for the properties enhancement of PLA/PHBHHx blends.
... Article established; namely, the relaxation time measured at the calorimetric T g (taken as the inflection point) is on the order of 10 seconds. 45 Taking this into consideration, the starting point of our approach is to first do a relatively simple full characterization of the homopolymers' DSC behavior, which would encode the unsolved intricate connection between the segmental dynamics and the glass formation process. After that, we will use this simple picture to establish the connection between the segmental dynamics and the DSC data of the blends following a scheme mirroring that used before for the BDS data description. ...
We have disentangled the contributions to the glass transition as observed by differential scanning calorimetry (DSC) on simplified systems of industrial interest consisting of blends of styrene-butadiene rubber (SBR) and polystyrene (PS) oligomer. To do this, we have started from a model previously proposed to describe the effects of blending on the equilibrium dynamics of the α-relaxation as monitored by broadband dielectric spectroscopy (BDS). This model is based on the combination of self-concentration and thermally driven concentration fluctuations (TCFs). Considering the direct insight of small-angle neutron scattering on TCFs, blending effects on the α-relaxation can be fully accounted for by using only three free parameters: the self-concentration of the components φself SBR and φself PS) and the relevant length scale of segmental relaxation, 2R c. Their values were determined from the analysis of the BDS results on these samples, being that obtained for 2R c ≈ 25Å in the range usually reported for this magnitude in glass-forming systems. Using a similar approach, the distinct contributions to the DSC experiments were evaluated by imposing the dynamical information deduced from BDS and connecting the component segmental dynamics in the blend above the glass-transition temperature T g (at equilibrium) and the way the equilibrium is lost when cooling toward the glassy state. This connection was made through the α-relaxation characteristic time of each component at T g, τg. The agreement of such constructed curves with the experimental DSC results is excellent just assuming that τg is not affected by blending.
... As indicated in a previous section, Tg is used to describe the glass transition [1] as a very important property of glass-forming materials determining the industrial application and processing of these materials. There are various techniques to determine the value of Tg, such as thermodilatometry (TD), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC), dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization currents (TSDC), viscosity measurements, electrical conductivity measurements, and optical methods [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. It should be emphasized that different values of Tg for the same material can be observed depending on the experimental methods and the measurement conditions, e.g., different measurement frequencies or cooling rates. ...
... Therefore, these differences can be attributed to the existence of various relaxation times of different motions of the macromolecular chains [10]. It has been also found [49] that, by using three independent experimental procedures (dielectric, thermally depolarized current, and calorimetric), the value of the glass transition and the value of the relaxation time at Tg can be correctly determined only if the thermal history is the same for all these experiments. ...
This review addresses the impact of different nanoadditives on the glass transition temperature (Tg) of polyvinyl chloride (PVC), which is a widely used industrial polymer. The relatively high Tg limits its temperature-dependent applications. The objective of the review is to present the state-of-the-art knowledge on the influence of nanofillers of various origins and dimensions on the Tg of the PVC. The Tg variations induced by added nanofillers can be probed mostly by such experimental techniques as thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and dielectric thermal analysis (DETA). The increase in Tg is commonly associated with the use of mineral and carbonaceous nanofillers. In this case, a rise in the concentration of nanoadditives leads to an increase in the Tg due to a restraint of the PVC macromolecular chain’s mobility. The lowering of Tg may be attributed to the well-known plasticizing effect, which is a consequence of the incorporation of oligomeric silsesquioxanes to the polymeric matrix. It has been well established that the variation in the Tg value depends also on the chemical modification of nanofillers and their incorporation into the PVC matrix. This review may be an inspiration for further investigation of nanofillers’ effect on the PVC glass transition temperature.
... Using the VTF parameters, a value of dielectric glass transition temperature can be calculated for a relaxation time equal to 100s and 10s (it has been proven the latter better corresponds to the experimental conditions used for MT-DSC experiments) [70] ( Table 3) ) by TSDC [57,75]. The Angell's plot [72] obtained with DRS data, and J o u r n a l P r e -p r o o f then extended to a temperature range below the glass transition thanks to TSDC, is reported in [29], as well as an increased asymmetry due to position isomerism (2,5-PEF vs. 2,4-PEF), are expected to affect not only the regularity of the macromolecular conformations in the solid state [64] and the aptitude of the aromatic rings to flip [19], but also the relaxation dynamics. ...
Poly(ethylene 2,5-furandicarboxylate) (2,5-PEF) is one of the most credible biobased alternative to poly (ethylene terephthalate) (PET). The Henkel disproportionation reaction that leads to furandicarboxylic acid (FDCA) provides three position isomers: 2,5-FDCA (obtained with the highest yield), 2,4-FDCA (so far considered as a by-product), and 3,4-FDCA (traces). The copolymerization of the two main isomers of FDCA with a diol, e.g. ethylene glycol (EG), is an interesting approach to obtain a family of furan-based biopolymers with adjusted physical properties. This work investigates the molecular mobility of three copolymers obtained with EG and ratios of 2,5-FDCA and 2,4-FDCA ensuring the complete disruption of crystallization (90:10, 85:15 and 50:50 mol % of 2,5:2,4 FDCA), as compared to the homopolymers 2,5-PEF and 2,4-PEF. The molecular mobility was investigated by cross-comparing the results obtained by Modulated-Temperature Differential Scanning Calorimetry (MT-DSC), Dielectric Relaxation Spectroscopy (DRS) and Thermo-Stimulated Depolarization Currents (TSDC), with the aim of evaluating the local and segmental molecular mobilities, their activation energies, as well as the temperature dependence of the relaxation time and of the cooperatively rearranging regions at the glass transition. The furan ring in 2,5-FDCA (2,5-PEF) has a rotation axis that is less linear compared to the benzene ring in terephthalic acid (PET), with consequences on the ring-flipping mechanisms. 2,5-FDCA and 2,4-FDCA differ by the position of the carbonyl groups on the furan ring, which adds asymmetry to non-linearity. The incorporation of 2,4-FDCA-based units into a polymer backbone mainly constituted of 2,5-FDCA-based repeating units is responsible for longer relaxation times associated with the local β relaxation processes, no striking effects on the kinetic fragility index m, no obvious effects on cooperativity (a slightly increase in the cooperativity length is observed in the liquid state), no effects on the activation energy for the segmental α relaxation in the liquid state, and a decrease in the activation energy in the glassy state.
... An important aspect of the crystallization at quiescent conditions is the crystallization with a reduced diffusion at temperatures in the vicinity of the glass transition. Structural relaxation occurs in the glassy state if the cooling from the melt is performed sufficiently fast [30][31][32][33]. During this process, precursors for crystals can be formed. ...
Structural relaxation in polymers occurs at temperatures in the glass transition range and below. At these temperatures, crystallization is controlled by diffusion and nucleation. A sequential occurrence of structural relaxation, nucleation, and crystallization was observed for several homopolymers during annealing in the range of the glass transition. It is known from the literature that all of these processes are strongly influenced by geometrical confinements. The focus of our work is copolymers, in which the confinements are caused by the random sequence of monomer units in the polymer chain. We characterize the influence of these confinements on structure formation and relaxation in the vicinity of the glass transition. The measurements were performed with a hydrogenated nitrile-butadiene copolymer (HNBR). The kinetics of the structural relaxation and the crystallization was measured using fast differential scanning calorimetry (FDSC). This technique was selected because of the high sensitivity, the fast cooling rates, and the high time resolution. Crystallization in HNBR causes a segregation of non-crystallizable segments in the macromolecule. This yields a reduction in mobility in the vicinity of the formed crystals and as a consequence an increased amount of so-called “rigid amorphous fraction” (RAF). The RAF can be interpreted as self-assembled confinements, which limit and control the crystallization. An analysis of the crystallization and the relaxation shows that the kinetic of both is identical. This means that the Kohlrausch exponent of relaxation and the Avrami exponent of crystallization are identical. Therefore, the crystallization is not controlled by nucleation but by diffusion and is terminated by the formation of RAF.
... The dielectric value of the glass transition temperature is usually selected as a temperature of a relaxation time equal to 100 s. However, the temperatures obtained by DRS at the relaxation time equal to 10 s match much better with the dynamic glass transition temperatures obtained by MT-DSC with a period of 60 s [56]. Several studies of dielectric values of the glass transition temperature may be found in literature that reveal good agreement with the values observed by thermal techniques, such as DSC and MT-DSC [57,58]. ...
... In the racemic mixture of PLLA and PDLA, there is a competition between homocrystallization and stereocomplexation. Several parameters that could influence this competition are follows [25,33,37,[39][40][41][42][43][44][45][46][47]: 19 blending ratio of two homopolymers, PLLA and PDLA [25,[48][49][50] molecular weight of two homopolymers [49][50][51] optical purity of the two isomeric polymers [41,52] temperature and time after blending of two homopolymers in solutions [42,49,50,52] the solvents utilized for polymer casting [49,51] the structure of co-monomer units and length of the lactide unit sequences in copolymers [53][54][55][56][57]. ...
... Gas permeability was measured by the so called "time-lag" method [43] by using the experimental device reported by Joly et al. [44] (shown in Fig. 2. 20). The gases used are: ...
The originality of this work is based on analysis of physical and physicochemical properties of polylactide mixtures of different chirality (poly L-lactic acid and poly D-lactic acid) and on the influence of the chirality on the amorphous phase’s properties. The materials mixtures are elaborated from two homopolymers (PLLA and PDLA) according to two methods; solution casting or extrusion. Totally amorphous and isotropically crystallized materials with more or less confined amorphous phase were studied. It is shown that a stereocomplex crystalline phase can be obtained only under certain experimental conditions. The results of the thermal and permeation analyzes showed that the PLLA / PDLA mixture improved certain properties of the material, namely higher barrier properties towards liquid water and gases were obtained compared to parent homopolymers. In order to study the molecular mobility of amorphous phases, physical aging and structural relaxation (α and β relaxation), the Cooperative Rearrangement Region (CRR) concept has been applied. It has been shown that the amorphous phases of the homopolymers and the mixture have exactly the same properties at the glass transition and in the vitreous state when the materials are totally amorphous.
... The result usually agrees with the DSC measurement within a few degrees (Richert, 2014). However, the method is limited, because not all compounds become glass as τ = 100 s (Saiter et al., 2007;Bahous et al., 2014). Furthermore, this method does not take into account kinetic effects on glass transition, specifically the effect of cooling and heating rates (Elmatad et al., 2009(Elmatad et al., , 2010Keys et al., 2013;Limmer and Chandler, 2014;Hudson and Mandadapu, 2018), as the glass transition temperature changes with cooling and heating rates. ...
Glass transitions from liquid to semi-solid and solid phase states
have important implications for reactivity, growth, and cloud-forming (cloud
condensation nuclei and ice nucleation) capabilities of secondary organic
aerosols (SOAs). The small size and relatively low mass concentration of SOAs
in the atmosphere make it difficult to measure atmospheric SOA glass
transitions using conventional methods. To circumvent these difficulties, we
have adapted a new technique for measuring glass-forming properties of
atmospherically relevant organic aerosols. Aerosol particles to be studied
are deposited in the form of a thin film onto an interdigitated electrode
(IDE) using electrostatic precipitation. Dielectric spectroscopy provides
dipole relaxation rates for organic aerosols as a function of temperature
(373 to 233 K) that are used to calculate the glass transition temperatures
for several cooling or heating rates. IDE-enabled broadband dielectric
spectroscopy (BDS) was successfully used to measure the kinetically
controlled glass transition temperatures of aerosols consisting of glycerol
and four other compounds with selected cooling and heating rates. The glass
transition results agree well with available literature data for these five
compounds. The results indicate that the IDE-BDS method can provide accurate
glass transition data for organic aerosols under atmospheric conditions. The
BDS data obtained with the IDE-BDS technique can be used to characterize
glass transitions for both simulated and ambient organic aerosols and to
model their climate effects.
... Up to date, there have been many studies concerning the physical stability of ASDs since the concept was first introduced in 1961 (1). It has been reported that a number of factors, such as glass transition temperature (T g ) of amorphous drugs and polymers, molecular mobility of drugs, drug-polymer miscibility, solid solubility of drugs in the polymers, physical stability of amorphous drugs alone, fragility index of amorphous drugs, drug-polymer interaction, molecular weight of drugs, recrystallization temperature of amorphous drugs, storage environment (temperature and humidity) and preparation process, were related to and may significantly affect the physical stability of ASDs (16)(17)(18)(19)(20)(21)(22)(23)(24). Formulation design for ASDs are therefore guided under these factors. ...
... The super-cooling method has been one of the most widely used methods for the preparation of amorphous material from its crystalline form (19,21,99,100). The relationship between the thermodynamic properties, such as enthalpy (H) or specific volume (V), and the temperature during the super-cooling process is shown in Fig. 6. For crystalline material at temperature below the melting point (T m ), the enthalpy increases slightly with increasing temperature (100). ...
Purpose:
Amorphous solid dispersions (ASDs) have been widely used in the pharmaceutical industry for solubility enhancementof poorly water-soluble drugs. The physical stability, however, remainsone of the most challenging issues for the formulation development.Many factors can affect the physical stability via different mechanisms, and therefore an in-depth understanding on these factors isrequired.
Methods:
In this review, we intend to summarize the physical stability of ASDsfrom a physicochemical perspective whereby factors that can influence the physical stability areclassified into thermodynamic, kinetic and environmental aspects.
Results:
The drug-polymer miscibility and solubility are consideredas the main thermodynamicfactors which may determine the spontaneity of the occurrence of the physical instabilityof ASDs. Glass-transition temperature,molecular mobility, manufacturing process,physical stabilityof amorphous drugs, and drug-polymerinteractionsareconsideredas the kinetic factors which areassociated with the kinetic stability of ASDs on aging. Storage conditions including temperature and humidity could significantly affect the thermodynamicand kineticstabilityof ASDs.
Conclusion:
When designing amorphous solid dispersions, it isrecommended that these thermodynamic, kinetic and environmental aspects should be completely investigatedand compared to establish rationale formulations for amorphous solid dispersions with high physical stability.
... Thus, the relaxation time of the cooperative movement of polymer chains (α relaxation) at a certain temperature increases as the draw ratio increases (Fig. 5). This is consistent with observations of other semicrystalline polymers, such as drawn low-density polyethylene [31] and drawn polyethylene terephthalate [37] . ...
Dielectric relaxation spectroscopy (DRS) of poly(ε-caprolactone) with different draw ratios showed that the mobility of polymer chains in the amorphous part decreases as the draw ratio increases. The activation energy of the α process, which corresponds to the dynamic glass transition, increases upon drawing. The enlarged gap between the activation energies of the α process and the β process results in a change of continuity at the crossover between the high temperature a process and the α and β processes. At low drawing ratios the a process connects with the β process, while at the highest drawing ratio in our measurements, the a process is continuous with the α process. This is consistent with X-ray diffraction results that indicate that upon drawing the polymer chains in the amorphous part align and densify upon drawing. As the draw ratio increases, the α relaxation broadens and decreases its intensity, indicating an increasing heterogeneity. We observed slope changes in the α traces, when the temperature decreases below that at which τ
α
≈ 1 s. This may indicate the glass transition from the ‘rubbery’ state to the non-equilibrium glassy state.
