Temperature-modulated Differential Scanning Calorimetry Research Articles

Modification of poly(lactate) via polymer blending with microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers

This study investigated the effect of polymer blending of microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers (LAHB) with poly(lactate) (PLA) on their mechanical, thermal, and biodegradable properties. Blending of high lactate (LA) content and high molecular weight LAHB significantly improved the tensile elongation of PLA up to more than 250 % at optimal LAHB composition of 20–30 wt%. Temperature-modulated differential scanning calorimetry and dynamic mechanical analysis revealed that PLA and LAHB were immiscible but interacted with each other, as indicated by the mutual plasticization effect. Detailed morphological characterization using scanning probe microscopy, small-angle X-ray scattering, and solid-state NMR confirmed that PLA and LAHB formed a two-phase structure with a characteristic length scale as small as 20 nm. Because of mixing in this order, the polymer blends were optically transparent. The biological oxygen demand test of the polymer blends in seawater indicated an enhancement of PLA biodegradation during biodegradation of the polymer blends.

Fragility and Tendency to Crystallization for Structurally Related Compounds.

The present study was designed to investigate the physical stability of three organic materials with similar chemical structures. The examined compounds revealed completely different crystallization tendencies in their supercooled liquid states and were classified into three distinct classes based on their tendency to crystallize. (S)-4-Benzyl-2-oxazolidinone easily crystallizes during cooling from the melt; (S)-4-Benzylthiazolidine-2-thione does not crystallize during cooling from the melt, but crystallizes easily during subsequent reheating above Tg; and (S)-4-Benzyloxazolidine-2-thione does not crystallize either during cooling from the melt or during reheating. Such different tendencies to crystallize are observed despite the very similar chemical structures of the compounds, which only differ in oxide or sulfur atoms in one of their rings. We also studied the isothermal crystallization kinetics of the materials that were shown to transform into a crystalline state. Molecular dynamics and thermal properties were thoroughly investigated using broadband dielectric spectroscopy, as well as conventional and temperature-modulated differential scanning calorimetry in the wide temperature range. It was found that all three glass formers have the same dynamic fragility (m = 93), calculated directly from dielectric structural relaxation times. This result verifies that dynamic fragility is not related to the tendency to crystallize. In addition, thermodynamic fragility predictions were also made using calorimetric data. It was found that the thermodynamic fragility evaluated based on the width of the glass transition, observed in the temperature dependence of heat capacity, correlates best with the tendency to crystallize.

Ion transport, mechanical properties and relaxation dynamics in structural battery electrolytes consisting of an imidazolium protic ionic liquid confined into a methacrylate polymer

The effect of confining a liquid electrolyte into a polymer matrix was studied by means of Raman spectroscopy, differential scanning calorimetry, temperature-modulated differential scanning calorimetry, dielectric spectroscopy, and rheology. The polymer matrix was obtained from thermal curing ethoxylated bisphenol A dimethacrylate while the liquid electrolyte consisted of a protic ionic liquid based on the ethyl-imidazolium cation [C2HIm] and the bis(trifluoromethanesulfonyl)imide [TFSI] anion, doped with LiTFSI salt. We report that the confined liquid phase exhibits the following characteristics: (i) a distinctly reduced degree of crystallinity; (ii) a broader distribution of relaxation times; (iii) reduced dielectric strength; (iv) a reduced cooperativity length scale at the liquid-to-glass transition temperature (T g); and (v) up-speeded local T g-related ion dynamics. The latter is indicative of weak interfacial interactions between the two nanophases and a strong geometrical confinement effect, which dictates both the ion dynamics and the coupled structural relaxation, hence lowering Tg by about 4 K. We also find that at room temperature, the ionic conductivity of the structural electrolyte achieves a value of 0.13 mS/cm, one decade lower than the corresponding bulk electrolyte. Three mobile ions (Im+, TFSI-, and Li+) contribute to the measured ionic conductivity, implicitly reducing the Li+ transference number. In addition, we report that the investigated solid polymer electrolytes exhibit the shear modulus needed for transferring the mechanical load to the carbon fibers in a structural battery. Based on these findings, we conclude that optimized microphase-separated polymer electrolytes, including a protic ionic liquid, are promising for the development of novel multifunctional electrolytes for use in future structural batteries.

Carbon Dots-Biomembrane Interactions and Their Implications for Cellular Drug Delivery.

The effect of carbon dots (CDs) on a model blayer membrane was studied as a means of comprehending their ability to affect cell membranes. Initially, the interaction of N-doped carbon dots with a biophysical liposomal cell membrane model was investigated by dynamic light scattering, z-potential, temperature-modulated differential scanning calorimetry, and membrane permeability. CDs with a slightly positive charge interacted with the surface of the negative-charged liposomes and evidence indicated that the association of CDs with the membrane affects the structural and thermodynamic properties of the bilayer; most importantly, it enhances the bilayer's permeability against doxorubicin, a well-known anticancer drug. The results, like those of similar studies that surveyed the interaction of proteins with lipid membranes, suggest that carbon dots are partially embedded in the bilayer. In vitro experiments employing breast cancer cell lines and human healthy dermal cells corroborated the findings, as it was shown that the presence of CDs in the culture medium selectively enhanced cell internalization of doxorubicin and, subsequently, increased its cytotoxicity, acting as a drug sensitizer.

Rationalizing the interfacial layer in polymer nanocomposites: Correlation between enthalpy and dielectric relaxation

The accurate and convenient characterization of the interfacial layer is a crucial prerequisite for understanding the relationship between structure and macroscopic properties in polymer nanocomposites (PNCs). Herein, enthalpy relaxation and interfacial layer properties estimated from temperature-modulated differential scanning calorimetry (TMDSC) and broadband dielectric spectroscopy (BDS) of model PNCs with attractive interactions are investigated. We demonstrate that enthalpy relaxation enables the accurate quantification of the volume fraction of the interfacial layer (φint) as TMDSC and BDS. The parameter (χint) corresponding to the decreased degree of enthalpy hysteresis (ΔHR) related to the contribution of the interfacial layer, is identical with φint estimated from BDS, while presents only similar dependence on nanoparticle loading with φint estimated from TMDSC. This interesting phenomenon can be ascribed to the consistency between enthalpy and dielectric relaxation, which both characterize the dynamic change of polymer mainly originating from segmental relaxation. Such an unexpected correlation between enthalpy and dielectric relaxation provides a new approach to characterize the interfacial layer properties of PNCs and understand its impact on the glass transition behavior.

Tranilast-matrine co-amorphous system: Strong intermolecular interactions, improved solubility, and physiochemical stability

There is a great interest to develop co-amorphous drug delivery systems to enhance the solubility of biopharmaceutics classification system (BCS) class II and IV drugs. However, most reported systems only resulted in severalfold solubility improvement. Tranilast (TRA) is an anti-allergic drug used to treat bronchial asthma and allergic rhinitis. It is a BCS class II drug and its poor aqueous solubility affects its absorption in vivo. To address this issue, a natural alkaloid matrine (MAR) with interesting biological activities was chosen to form a co-amorphous system with TRA, based on the solubility parameter and phase solubility experiment. The TRA-MAR drug-drug co-amorphous system was prepared by the solvent evaporation method, and further characterized by powder X-ray diffraction and modulated temperature differential scanning calorimetry. Fourier transform infrared spectroscopy, FT-Raman, and X-ray photoelectron spectroscopy revealed the formation of salt and the presence of strong intermolecular interactions in the TRA-MAR co-amorphous system, which are also supported by molecular dynamics simulations, showing ionic and hydrogen bonding interactions. This co-amorphous system exhibited excellent physical stability at both 25 °C and 40 °C under anhydrous silica gel condition. Finally, co-amorphous TRA-MAR showed greatly enhanced solubility (greater than 100-fold) and rapid release behavior in the vitro release experiments. NMR spectroscopy revealed the strong intermolecular interactions between TRA and MAR in both DMSO‑d6 and D2O. Our study resulted in a TRA-MAR co-amorphous drug system with significant solubility improvement and showcased the great potential to improve the dissolution behaviors of BCS class II and IV drugs through the co-amorphization approach.

A skin pharmacokinetics study of permeation enhancers: The root cause of dynamic enhancement effect on in vivo drug permeation

Skin pharmacokinetics (SPK) of permeation enhancers can answer the question of why enhancement effects different at the kinetic level. Herein, SPK of permeation enhancers were classified into two categories, namely, lateral elimination (elimination to surrounding stratum corneum (SC)) and longitudinal elimination (elimination to deep epidermal (EP)). They were evaluated with a specific parameter for permeation enhancers, diffusion ratio (DRSC-EP), according to results of tissue-distribution test, molecular dynamic (MD) simulation, and confocal laser scanning microscopy (CLSM). The linear relationship between ke-enahcer and Δ Cmax-drug (R2 = 0.92), MRTenhancer and Δ Tmax-drug (R2 = 0.97), AUCt-enhancer and Δ AUCt-drug (R2 = 0.90) suggesting that SPK of permeation enhancers precisely controlled dynamic process of drug permeation in vivo. The molecular mechanisms of the dynamic effect of SPK process on drug transdermal behaviors were characterized by modulated-temperature differential scanning calorimetry (MTDSC), dielectric spectroscopy, small-angle X-ray scattering (SAXS), solid-state NMR. Permeation enhancers with high molecular weight (M.W.) and high polar surface area (P.S.A.) had good compatibility and strong interaction strength with SC, leading their lateral-elimination behavior, causing their low DRSC-EP and resulting in low ke-enhancer, long MRTenhancer, and large AUCt-enhancer. Consequently, skin barrier can be rapidly opened fast and to a great extent. In summary, compared with SPK of permeation enhancers with longitudinal elimination, SPK of permeation enhancers with lateral elimination can enable more sustainable and greater drug permeation. The information about SPK of permeation enhancers offered a criterion to estimate its permeation-enhancement effect on the drug and its subsequent application in transdermal formulations.

Cross-Linking Mechanisms of a Rigid Plant Oil-Based Thermoset from Furfural-Derived Cyclobutane

cis-Cyclobutane-1,2-dicarbocylic acid (CBDA-2) was synthesized and used as a biobased hardener to cross-link epoxidized linseed oil to produce a fully biobased thermoset. The polymerization mechanism was studied by means of real-time attenuated total reflection-Fourier transform infrared (ATR-FTIR), differential scanning calorimetry (DSC), stochastically modulated temperature DSC, and rheometry. The study highlights the importance of hydrogen bonding of CBDA-2 and catalytic effect of hydroxyl groups created during the reaction. Evolution of reactive functions with temperature and time, variation of activation energy, and pre-exponential factor was correlated to macroscopic changes, such as heat capacity variations and chemorheology. A macrokinetic model was proposed to describe the complete polymerization mechanism. It is shown that the obtained parameters accurately describe the polymerization using parameters with a real physical meaning.

Calculation of the degree of crystallinity of HDPE/GNPs nanocomposites by using various experimental techniques: a comparative study

This work is a comparative study of four different techniques to determine the crystallinity of high-density polyethylene (HDPE) nanocomposites filled with different diameter sizes (5, 15 and 25 μm) of graphene nanoplatelets (GNPs) at various amounts (0.5–5 wt%). The structure of HDPE/GNPs nanocomposites was extensively studied by using different experimental methods, such as X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. To further provide a complete comparison, differential scanning calorimetry measurements were utilized to calculate the crystallinity values, while temperature-modulated DSC was employed to investigate the possible mechanism of the different crystalline structures. It was found that these methods can be used to estimate the crystallinity, but the sample parameters and experimental conditions must be taken into consideration. All the techniques showed that the crystallinity depends on GNPs size and content. The distance between dispersed platelets was substantial at low concentrations of GNPs, making it comparatively easy for additional nucleation sites to incorporate the polymer matrix, and the crystal nucleus was simply formed. However, at high concentrations of GNPs, the diffusion of polymer chains to the growing crystallites was hindered by large GNPs particles, despite the formation of additional nucleation sites.

Mechanistic insights of different release behaviors dominated by drug physicochemical properties in polyisobutylene pressure sensitive adhesive

The purpose of this study was to investigate the effect of the physicochemical parameters of drugs on their own release behaviors in polyisobutylene pressure sensitive adhesive (PIB PSA), which provided a theoretical guidance for the application of PIB in transdermal drug delivery system (TDDS). Seven drugs with different physicochemical parameters including clonidine (CLO), flurbiprofen (FLU), diclofenac (DIC), ibuprofen (IBU), zolmitriptan (ZOL), lidocaine (LID), tulobuterol (TUL) and the mixed adhesive (7:3, w/w) of Oppanol® B 15 N (M.W. = 108,000 Da) and Oppanol® N 50 (M.W. = 565,000 Da) were selected for in vitro drug release and skin permeation studies. Regression analysis was used to study the relationship between physicochemical parameters and release behaviors. The release behaviors of drugs were a negative correlation with polarizability and dipole moment per molecular volume (μ/V), which represented van der Waals and dipole–dipole interaction, respectively. Fourier transform infrared spectroscopy (FT-IR), modulated temperature differential scanning calorimetry (MDSC) and molecular dynamics simulation were used to provide molecular details of the interaction between the drug and PIB. The free volume and molecular mobility of PIB were characterized using mechanical property tests, rheology study, MDSC and molecular dynamics simulation. Based on the above results, drugs with high polarizability and μ/V had stronger van der Waals and dipole–dipole interaction with PIB, reducing the free volume and molecular mobility of PIB, so that the drug struggled to release from PIB. In addition, the diffusion activation energy of the drug was calculated by using the variable temperature release study to characterize the ease of drug release from the kinetic aspect. And the trends of in vitro drug release and skin penetration profiles were basically similar. Thus, it was thought that the physicochemical parameters of the drug played a vital role in the drug release behavior of PIB PSAs and would affect the skin penetration process, which provided a reference for the design and application of patches based on PIB PSAs in TDDS.

Co-amorphous Systems of Sinomenine with Platensimycin or Sulfasalazine: Physical Stability and Excipient-Adjusted Release Behavior.

There is strong interest to develop affordable treatments for the infection-associated rheumatoid arthritis (RA). Here, we present a drug-drug co-amorphous strategy against RA and the associated bacterial infection by the preparation and characterization of two co-amorphous systems of sinomenine (SIN) with platensimycin (PTM) or sulfasalazine (SULF), two potent antibiotics. Both of them were comprehensively characterized using powder X-ray diffraction, temperature-modulated differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The co-amorphous forms of SIN-PTM and SIN-SULF exhibited high Tgs at 139.10 ± 1.0 and 153.3 ± 0.2 °C, respectively. After 6 months of accelerated tests and 1 month of drug-excipient compatibility experiments, two co-amorphous systems displayed satisfactory physical stability. The formation of salt and strong intermolecular interactions between SIN and PTM or SULF, as well as the decreased molecular mobility in co-amorphous systems, may be the intrinsic mechanisms underlying the excellent physical stability of both co-amorphous systems. In dissolution tests, two co-amorphous systems displayed distinct reduced SIN-accumulative releases (below 20% after 6 h of release experiments), which may lead to its poor therapeutic effect. Hence, we demonstrated a controlled release strategy for SIN by the addition of a small percentage of polymers and a small-molecule surfactant to these two co-amorphous samples as convenient drug excipients, which may also be used to improve the unsatisfactory dissolution behaviors of the previously reported SIN co-amorphous systems. Several hydrogen bonding interactions between SIN and PTM or SULF could be identified in NMR experiments in DMSO-d6, which may be underlying reasons of decreased dissolution behaviors of both co-amorphous forms. These drug-drug co-amorphous systems could be a potential strategy for the treatment of infection-associated RA.

Physical aging of the 62.5GeS2-12.5Sb2S3-25CsCl chalcogenide glass: Assessing the mechanisms of equilibration and crystallization

The goal of this work is to provide an answer to the important scientific question: will a glass-forming liquid reach its equilibrium state or not before to crystallize during physical aging process, i.e. when it is maintained below its glass transition temperature? For this goal the physical aging process in a 62.5GeS2-12.5Sb2S3-25CsCl glass has been investigated for a long period (5 months) at 30°C below the glass transition temperature Tg. A crystallization was observed during this process, and was characterized through X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Temperature Modulated Differential Scanning Calorimetry (MT-DSC) analyses. By evaluating the enthalpy loss related to the physical aging and by comparing this experimental quantity to the theoretical one, we have proved that this glass has reached its thermodynamic equilibrium state before crystallizing. Thus, this work evidences experimentally the ability for a glass former to crystallize under Tg after reaching its equilibrium state.

Efficacy and safety of permeation enhancers: A kinetic evaluation approach and molecular mechanism study in the skin

This study sought to provide approach for evaluating and predicting the efficacy and safety of permeation enhancers on the basis of their kinetic distribution behavior in the skin dictated by physicochemical properties. Herein, the efficacy-safety regularity of eight permeation enhancers were studied with ex vivo skin permeation study, small-angle X-ray scattering, MTT assay, H&E staining, and in vivo skin erythema analysis, classifying into the following three categories: high enhancement–low irritation, medium enhancement–high irritation, and low enhancement–low irritation. These three modes were positively correlated with the distribution amount of permeation enhancers in the skin layers and verified by the in vitro tape-stripping study. The kinetic parameter, effective-safety index (IES), was proposed to describe the regularity of enhancement effect tendency and irritation risk, and the relationship between IES and physicochemical properties of permeation enhancers was analyzed with multiple regression analysis. According to the results of modulated temperature differential scanning calorimetry and dielectric spectrum, permeation enhancers with high lipophilicity and low polarity had IES > 1, suggesting high enhancement effect and low irritation due to their higher affinity with the stratum corneum (SC) than with epidermis (EP). Permeation enhancers with medium lipophilicity and medium polarity exhibited 0 < IES ≤ 1, showing medium enhancement effect and high irritation, as determined by their comparable affinity with the SC and epidermis (EP). However, permeation enhancers with low lipophilicity and high polarity had IES → 0, demonstrating low enhancement effect and irritation, as indicated by their poor affinity with the SC. In summary, different physicochemical properties of permeation enhancers influenced their affinities with skin layers, resulting in their different enhancement effect and irritation potential. This study will provide a theoretical basis and criteria for evaluating and predicting the safety and efficacy of permeation enhancers, which will enable a more rational selection of permeation enhancers in the optimization of transdermal patches.

Kinetics of enthalpy recovery studied by temperature-modulated differential scanning calorimetry

An analyzing method using periodic temperature modulation is applied to the kinetics of enthalpy recovery that occurs when an amorphous material in the glassy state devitrifies during heating. In particular, this study demonstrates that an analytical expression of the kinetic response can be formulated by assuming temperature modulation with a periodic temperature jump and isothermal holding. This analysis provides a reasonable interpretation for the well-known fact that the magnitude of the dynamic heat capacity of complex quantity obtained by temperature modulation does not exhibit a large peak corresponding to the enthalpy recovery peak. Furthermore, it is demonstrated that the phase angle peak of the dynamic heat capacity was affected by the kinetics. Its quantitative evaluation provides useful information on the relaxation time during glass transition. Numerical simulations and experimental results of atactic polystyrene examined by conventional differential scanning calorimetry and fast scanning calorimetry confirm these findings.

Phase change characterization of eco-friendly isopropyl palmitate-based graphene nanoplatelet nanofluid for thermal energy applications

• Design of stable isopropyl palmitate-based nanoenhanced phase change materials. • Analysis of solid–liquid transition and enthalpy-temperature curves. • Reductions in sub-cooling higher than 2 K, increases in the latent heat up to 5.9%. • The IPP polymorphism was not significantly altered due to the GnPs dispersions. • Reductions up to 12% in the isobaric heat capacities compared to the base material. Thermal energy storage (TES) facilitates the integration of renewable energy by decoupling production and consumption, mitigating intermittence issues. However, current TES systems use fossil fuel-based products as storage media, so the development of sustainable and efficient materials should be investigated. This work aims to the design and characterization of new eco-friendly nanoenhanced phase change materials (NePCMs) based on dispersions of graphene nanoplatelets of two different lateral sizes (7.2 and 40 μm) in isopropyl palmitate for cold storage applications. The stability of the NePCMs was analysed by dynamic light scattering, selecting Span® 80 as surfactant to improve the stabilization of both nanomaterials within the base material. The influence of the nanoadditive concentration on the temperature transitions and on the solid–liquid phase change were comprehensively studied by differential scanning calorimetry, finding out that the dispersed nanoadditives do not alter the polymorphism of the isopropyl palmitate. Additionally, reductions in the sub-cooling effect higher than 2 K for both nanoplatelets were found, with increases in the latent heat up to 5.9 and 3.9% for the shorter and the longer nanoplatelets, respectively. Isobaric heat capacities were also determined by temperature-modulated differential scanning calorimetry, reporting maximum reductions of 12% compared to the base material.

Heat capacity and index of refraction of polyzwitterions

Specific heat capacities and indices of refraction were measured for a series of six sulfobetaine zwitterionic polymers and compared to topological and group contribution models. Poly(sulfobetaine methacrylate) (PSBMA), poly(sulfobetaine acrylate) (PSBA), poly(ethyl sulfobetaine methacrylate) (PESBMA), poly(sulfobetaine methacrylamide) (PSBMAm), poly(sulfobetaine 2-vinylpyridine) (PSB2VP) and poly(sulfobetaine 4-vinylpyridine) (PSB4VP) were specially synthesized with changes of chemical structure in order to study effects of either the polymer backbone or the side group on their properties. Temperature modulated differential scanning calorimetry (TMDSC) as well as quasi-isothermal TMDSC were used to determine the temperature dependent specific heat capacity, cp(T) from 30 to 200 °C. The solid state specific heat capacities were measured for all the polyzwitterions, while liquid state heat capacity measurements were limited to PSBA and PESBMA. Ellipsometry at wavelengths from 300 nm to 3000 nm was used to measure the index of refraction for polyzwitterion thin films at room temperature. Theoretical room temperature values of cp and n were calculated for the polyzwitterions using topological and group contribution methods and compared to the measured values. The measured index of refraction values agree well with predictions for the polyzwitterions. However, for the solid state heat capacity, the model produces accurate prediction only for PSB2VP and PSB4VP. The discrepancy between the measured and predicted cp values is most likely due to the reduction of vibrational degrees of motion in the solid state for some of the polyzwitterions due to physical crosslinking.

Multiscale Dynamics of Lipid Vesicles in Polymeric Microenvironment.

Understanding dynamic and complex interaction of biological membranes with extracellular matrices plays a crucial role in controlling a variety of cell behavior and functions, from cell adhesion and growth to signaling and differentiation. Tremendous interest in tissue engineering has made it possible to design polymeric scaffolds mimicking the topology and mechanical properties of the native extracellular microenvironment; however, a fundamental question remains unanswered: that is, how the viscoelastic extracellular environment modifies the hierarchical dynamics of lipid membranes. In this work, we used aqueous solutions of poly(ethylene glycol) (PEG) with different molecular weights to mimic the viscous medium of cells and nearly monodisperse unilamellar DMPC/DMPG liposomes as a membrane model. Using small-angle X-ray scattering (SAXS), dynamic light scattering, temperature-modulated differential scanning calorimetry, bulk rheology, and fluorescence lifetime spectroscopy, we investigated the structural phase map and multiscale dynamics of the liposome–polymer mixtures. The results suggest an unprecedented dynamic coupling between polymer chains and phospholipid bilayers at different length/time scales. The microviscosity of the lipid bilayers is directly influenced by the relaxation of the whole chain, resulting in accelerated dynamics of lipids within the bilayers in the case of short chains compared to the polymer-free liposome case. At the macroscopic level, the gel-to-fluid transition of the bilayers results in a remarkable thermal-stiffening behavior of polymer–liposome solutions that can be modified by the concentration of the liposomes and the polymer chain length.

Differential scanning calorimetry for authentication of edible fats and oils - What can we learn from the past to face the current challenges?

Fats and oils authentication has become an important issue recently, due to the growing interest in consumption of cold-pressed oils. Therefore, it is becoming more and more difficult to maintain official control over the growing assortment of new cold-pressed oils. Authenticity of plant oils is also an important issue for religious or cultural reasons.This review article focuses on the application of differential scanning calorimetry (DSC) in the field of assessing authenticity of various fats and oils (e.g. olive oil, palm oil, confectionery fats, butter). Extra virgin olive oil (EVOO) is the most comprehensively tested oil by means of the DSC technique in terms of the authenticity of origin as well as the adulteration with foreign oils. In most of the studies on DSC applicability for authentication, crystallization and melting curves were analyzed by the conventional DSC, although other modified DSC methods were also applied, such as isothermal freezing, modulated temperature DSC (MT-DSC) and fast DSC. However, the most promising are the melting profiles, which, due to the complexity of transitions, need advanced chemometric tools as well as tools for peaks deconvolution. The future prospect of using DSC in the authenticity assessment lies also in the use of DSC techniques along with other complementary chromatographic or spectroscopic techniques.

Temperature modulated DSC for composition analysis of recycled polyolefin blends

Post-consumer plastic waste contains blends of numerous types of polyethylene (PE) and isotactic polypropylene (i-PP), whose recycling is challenging due to the complexity of this waste stream. A comprehensive knowledge of the composition of these recyclates is essential to understand the structure-property relationship of these systems and therefore upcycle them for high-value applications. To this aim, we used Temperature Modulated DSC (TM-DSC) to develop a quantitative method to evaluate PE and Low-density PE (LDPE) content in recycled polyolefin blends. TM-DSC was carried out on 29 virgin PE materials, spanning densities between 960 and 862 kg/m3, characterizing a wide range of PE microstructures. Moreover, several PE/i-PP model blends were prepared by selecting LDPE, High-density PE (HDPE) and Linear Low-density PE (LLDPE) materials to blend them with i-PP of three types: homopolymer (PP–H), block copolymer (PP–B) and random copolymer (PP-R), mimicking the composition of real recyclates. Results from the TM-DSC analysis of these blends allowed us to establish methods for quantifying the amount of overall PE content and also the LDPE fraction within recyclates. The developed methods were applied to real post-consumer recycled grades, and results were compared with the ones obtained from Cross-Fractionation Chromatography (CFC) analysis and Nuclear Magnetic Resonance (NMR) spectroscopy, displaying good agreement between the latter and the TM-DSC method.

Structural evolutions of the amorphous fraction of polyethylene terephthalate during the secondary crystallization

We have studied the structural evolution of polyethylene terephthalate (PET) during the isothermal crystallization through the temperature-modulated differential scanning calorimetry (TMDSC) and the positron annihilation lifetime spectroscopy (PALS). By tracing the changes in the amorphous free volume size, and the content of the crystal, the mobile amorphous fraction (MAF), and the rigid amorphous fraction (RAF), we clarified how the RAF participates in the crystallization. It was found that the limiting temperature of the RAF, T* determines the behaviors of the RAF in the crystallization. For isothermal crystallization at temperatures below T*, the RAF is strictly constrained by the lamella such that it cannot participate to the crystallization. For isothermal crystallization at temperatures above T*, chain segments inside the lamella are able to move, and the restrictions exerted on the RAF by the lamella are eased. Then, the activated RAF participates in the crystallization mainly through the lamellar perfection. As a result, instead of a monotonic increase in the RAF content, a non-monotonic change and even a monotonic decrease as a function of the isothermal crystallization time were observed. Moreover, an anomalous increase in the MAF content at the late stage of isothermal crystallization was found. Our results demonstrate that the activated RAF can indeed participate to the crystallization, and reveal how the structural evolutions of the activated RAF and the MAF proceed.