Enthalpic Relaxation Research Articles

Synthesis and Characterization of Well-Defined High-Molecular-Weight PDLA-b-PLLA and PDLA-b-PLLA-b-PDLA Stereo-Block Copolymers

A facile method is developed to synthesize poly(lactide) (PLA) stereo-diblock and stereo-triblock copolymers using a bismuth catalyst. The judicious choice of the catalytic system allows us to synthesize the PLA stereocomplex having a molar mass exceeding 450 kg/mol and a polydispersity of below 2. The synthesis of such a high molar mass became feasible because in the chosen catalytic system, unlike in the reported synthesis routes, the presence of water in traces does not promote chain transfer. The observations are that during polymerization, single crystals of poly(l-lactide)-b-poly(d-lactide) (PLLA-b-PDLA) and poly(d-lactide)-b-poly(l-lactide)-b-poly(d-lactide) (PDLA-b-PLLA-b-PDLA) stereo-block copolymers are formed. These platelet-like single crystals tend to aggregate, forming a globular spherical morphology equivalent to that perceived on the crystallization of the stereocomplex from solution. The adopted synthesis route provides molecular mixing of the stereo-regular PLA chains in the absence of any detectable traces of homo-crystals, which are unavoidable when the mixing is performed using a homopolymer dissolution route. The molecularly mixed stereo-regular PLAs in a polymer melt retain their molecular interaction, even after isothermal crystallization or rheological studies that follow the anticipated viscoelastic response of a polymer melt. The increasing molar masses of the PLLA/PDLA diblock and triblock copolymers show an increase in the tensile modulus and elongation to break, retrospectively indicating the brittle to ductile transformation of the polymer with increasing molar mass. Thermal and imaging experimental methods such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, scanning electron microscopy, electron diffraction, wide-angle X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy are employed to follow the enthalpic relaxation, conformational, and structural changes.

Highly stable petroleum pitches provide access to the deep glassy state.

Differential scanning calorimetry (DSC) was used to study the fast aging behavior of two petroleum pitch materials despite being only three to five years old. We observe that these highly aromatic pitches with broad distributions of both molecular weight and aromaticity exhibit large enthalpic relaxation endotherms in initial DSC heating scans, and 20-32 °C reductions in the fictive temperature and 0.35-0.87 of θK, which are indicative of aged glasses similar to ultrastable glasses and 20 MA aged amber. Quantifying the degree of thermodynamic stability relative to the Kauzmann temperature vs. the aging time demonstrates that these materials age just as quickly as low fragility metallic glasses. Additionally, we observe that pitches age faster than polymers reported in the literature when compared using down-jump experiments. We hypothesize that the fraction of higher aromaticity of pitch molecules plays a crucial role in faster dynamics. The unique aging behavior and the ability to produce pitches in bulk quantities using pilot-scale equipment, while being possible to tailor their molecular composition, make them a useful material for studying complex aging dynamics in the deep glassy state.

Effects of Accelerating the Ageing of 1D PLA Filaments after Fused Filament Fabrication.

The effects of post-treatment temperature-based methods for accelerating the ageing of PLA were studied on 1D single-PLA filaments after fused filament fabrication (FFF). The goal was to answer the questions whether the PLA can be safely aged-i.e., without degrading-at higher temperatures; at which temperatures, if any; how long it takes for the PLA to fully age at the chosen temperature; and which are the main differences between the material aged at room temperature and the material aged at higher temperatures. We also share other helpful information found. The use of 1D filaments allows for decoupling the variables related to the 3D structure (layer height, raster angle, infill density, and layers adhesion) from the variables solely related to the material (here, we analysed the molecular weight, the molecular orientation, and the crystallinity). 1D PLA filaments were aged at 20, 39, 42, 51, 65, 75, and 80 °C in a water-bath-inspired process in which the hydrolytic degradation of the PLA was minimised for the ageing temperatures of interest. Those temperatures were selected based on a differential scanning calorimetry (DSC) scan of the PLA right after it was printed in order to study the most effective ageing temperature, 39 °C, and highlight possible degradation mechanisms during ageing. The evolution of the thermal and mechanical properties of the PLA filaments at different temperatures was recorded and compared with those of the material aged at room temperature. A DSC scan was used to evaluate the thermal and physical properties, in which the glass transition, enthalpic relaxation, crystallisation, and melting reactions were analysed. A double glass transition was found, and its potential implications for the scientific community are discussed. Tensile tests were performed to evaluate the tensile strength and elastic modulus. The flow-induced molecular orientation, the degradation, the logistic fitting, and the so-called summer effect-the stabilisation of properties at higher values when aged at higher temperatures-are discussed to assess the safety of accelerating the ageing rate and the differences between the materials aged at different temperatures. It was found that the PLA aged at 39 °C (1) reached almost stable properties with just one day of ageing, i.e., the ageing rate accelerated by 875% for the elastic modulus and by 1635% for the yield strength; (2) the stable properties were higher than those from the PLA aged at room temperature; and (3) no signs of degradation were identified for the ageing temperature of interest.

Understanding the difference in the stretched structural relaxations probed by dielectric and enthalpic studies of glass forming substances.

We investigated the stretched dynamics of the structural relaxation in molecular glass formers by using dielectric and thermal (or enthalpic) relaxations. The dielectric stretching exponents βdie are determined by the Havriliak-Negami function, while the enthalpic βTNMH is quantified by using the Tool-Narayanaswamy-Moynihan-Hodge formalism. We found βTNMH is anticorrelated with the degree of freedom, a molecule addressed by the concept of beads. Referring to the reported relation of βdie to the dipole moment μ, we proposed a combined parameter of μ2*beads, which can rationalize the difference in stretching exponents obtained by dielectric and enthalpic relaxations. For the majority of glass-forming molecules, the difference is trivial, but for those molecules with both unusually high dipole moments and flexibility, a large difference is obvious. The interplay of the degree of freedom and dielectric dipole-dipole interaction in molecular dynamics is addressed.

Characterisation and Modelling of PLA Filaments and Evolution with Time.

The properties of polylactic acid (PLA) filaments have not yet been analysed in detail, and they are strongly affected by the extrusion process used in some additive manufacturing systems. Here we present the mechanical, thermal, physical, and fractographical properties of an extruded filament (not the bulk material or scaffolds), the basic building block of any PLA structure printed via material extrusion. This research aims to create a reference point for the modelisation of additively manufactured structures via extrusion processes, as the main building block is characterised in detail for a deep understanding. Furthermore, we investigated the natural ageing (up to one year), the effect of the printing (extruding) temperature (180 and 190 °C), and the effect of the crosshead speed during the tensile tests (10−1 to 102 mm/min) to provide a deeper analysis of the material. The results showed that the material extruded at 190 °C performed better than the material extruded at 180 °C. However, after one hundred days of natural ageing, both materials behaved similarly. This was related to the flow-induced molecular orientation during the extrusion. The crosshead rate produced a logarithmic increase of the mechanical properties, consistent with the Eyring model. Additionally, the ageing produced significant changes in both the elastic modulus and the yield strength: from 2.4 GPa and 40 MPa, in one-day-aged samples, up to 4 GPa and 62 MPa once entirely aged. Finally, it was observed that the glass transition and the enthalpic relaxation increased with ageing, agreeing with the Kohlraushch–William–Watts model.

Dynamic Mechanical Analysis Investigations of PLA-Based Renewable Materials: How Are They Useful?

Interest in renewable polymers increased exponentially in the last decade and in this context poly(lactic acid) (PLA) became the leader mainly for practical reasons. Nevertheless, it is outstanding also from a scientific point of view, because its thermal and morphological properties are offering challenging new insights. With regard to dynamic mechanical analysis (DMA), PLA does not have the classical behavior of a thermoplastic polymer. Often, overlapping events (enthalpic relaxation, glass transition and crystallization) that occur as the temperature increases make the DMA result of a PLA look inexplicable even for polymer scientists. This review offers a perspective of the main phenomena that can be revealed in a DMA experiment and systematizes the information that can be obtained for every region (glassy, glass transition, rubbery, cold-crystallization and melting). Also, some unusual patterns registered in some cases will be commented upon. The review intends to offer indices that one should pay attention to in the interpretation of a DMA experiment, even if the investigator has only basic skills with DMA investigations.

Dielectric and thermal characteristics of Beidellite nanoclay-reinforced poly(butylene succinate)

Polymer nanocomposites based on organo-modified Beidellite nanoclays (1–3 wt%) and poly(butylene succinate) (PBS) were elaborated and investigated using Scanning electron microscope (SEM), X-Ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Broadband Dielectric Spectroscopy (BDS). SEM analysis and XRD diffractograms demonstrated a good dispersion of added nanoclays in the poly(butylene succinate) matrix. A possible correlation between the crystallinity rate provided by the DSC results and the dielectric response of the samples was evaluated in order to establish a morphology-property relation. Additionally, a step-like variation related to an enthalpic relaxation within the rigid amorphous fraction (RAF) indicated a delayed contribution of this region to the overall dynamics of the polymer. The BDS results have shown various relaxation processes for which the activation energies were calculated after fitting the dielectric data by means of the Havriliak–Negami model. No significant effects of the added nanofiller on the glass transition temperature were observed. The extracted parameters confirm that a 2 wt% of Beidellite exhibits a better matrix/filler adhesion and hence improved interfacial properties. Thus, such filler load is recommended for a higher performing nanocomposite with potential applications.

Crystallization behavior of zinc oxide/poly(lactic acid) nanocomposites

In this work, films of commercial PLA Ingeo 4043D with nano-zinc oxide particles (nano-ZnO) with average diameter around 100 nm and ZnO concentrations of 0.1, 0.25, 0.3 and 0.5 mass% were prepared by flat die extrusion. The addition of nano-ZnO had an objective to promote the crystallization of PLA, but it decreased the thermal stability of the films. Both neat PLA and PLA/nano-ZnO composites films did not crystallize during the cooling, but crystallize during a first heating, showing a cold crystallization process, which was significantly influenced by the heating rate. A low chain mobility was evidenced by the values of relaxation time (T1H). This low chain mobility avoided the high degree of crystallization of the films which were predominantly amorphous. For all films, an enthalpic relaxation phenomenon was detected during the first heating. The glass transition temperature (Tg) was shifted to higher values as the heating rate and nanofiller content are increased. Addition of nano-ZnO in PLA films did not increase their crystallinity, indicating that it is not an effective nucleating agent for this low-crystallinity PLA grade.

Dynamic and stress signatures of the rigid intermediate phase in glass-forming liquids.

We study the evolution of enthalpic changes across the glass transition of model sodium silicate glasses (Na2O)x(SiO2)100-x, focusing on the detection of a flexible-rigid transition and a possible reversibility window in relationship with dynamic properties. We show that the hysteresis resulting from enthalpic relaxation during a numerical cooling-heating cycle is minimized for 12% ≤ x ≤ 20% Na2O, which echoes with the experimental observation. The key result is the identification of the physical features driving this anomalous behavior. The intermediate-flexible boundary is associated with a dynamic onset with increasing depolymerization that enhances the growing atomic motion with a reduced internal stress, whereas the intermediate-stressed rigid boundary exhibits a substantial increase in the temperature at which the relaxation is maximum. These results suggest an essentially dynamic origin for the intermediate phase observed in network glass-forming liquids.

Mechanical and Thermal Properties of Mixed-Tacticity Polyhydroxybutyrates and Their Association with Iso- and Atactic Chain Segment Length Distributions

We investigated the effect of including S enantiomers on the mechanical and thermal properties of predominantly (R)-β-polyhydroxybutyrate (PHB). From tensile testing, we determined resulting ratios of meso to racemo diads, for which elastic modulus, strength, and fracture strain combine to provide maximized fracture energies. We found that these coincide with an inversion of the respective elastic moduli of the amorphous and crystalline phases. From thermocalorimetric analyses, we determined the glass-transition temperatures and enthalpic relaxations, the heat capacities of the materials and their constituent phases, the directional crystallization rates and melting points, as well as the melting enthalpies for the α-PHB phase as functions of tacticity. We present a unifying characteristic, accounting for tacticity mismatches, based on the previously determined random polymerization action of the catalyst ethylzinc β-diketiminate and 4-methoxybenzyl alcohol. This characteristic provides a qualitative indication of the transition points in nonlinear correlations encountered between the ratios of meso to racemo diads and mixed-tacticity polyhydroxybutyrates’ fracture energies, amorphous and crystalline phase elastic moduli, melting enthalpies, and lattice vibrational frequencies.

About the endothermal transitions of galactomannans: A multi-analytical DSC, LF-1H NMR and DMA study

The thermal transitions of biopolymers were subject of great discussion in the 90s due to their relevance in structure development during processing and stability on storage. In the present work two galactomannans, vinal gum and guar gum, were evaluated by DSC, DMA and LF-1H NMR in order to compare them, establishing their potential operational application range and promoting the use of the non-conventional VG in foods or other products. Three endothermal transitions appeared when heating the samples in the DSC: one at temperatures −90 to −10 °C (LTT), other around 50 °C (MTT) and a third one between 50 and 100 °C (HTT). Both LTT and HTT showed water content dependence and low ΔCp values, which difficulted the assignment of a glass transition. MTT appeared as an enthalpic relaxation independent on frequency or on water content. This transition was related to changes in mechanical properties and with the stabilization of proton mobility.

Accelerated physical ageing of poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate).

Successfully evaluating plastic lifetime requires understanding of the relationships between polymer dynamics and mechanical performance as a function of thermal ageing. The relatively high Tg (Tg = 110 °C) of poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PCTT) renders it useful as a substituent for PET in higher temperature applications. This work links thermal ageing and mechanical performance of a commercial PCTT plastic after exposure to 40–80 °C for up to 2950 h. No chemical or conformational changes were found while pronounced physical ageing, measured as enthalpic relaxation, caused yield hardening (28% increase in yield strength) and embrittlement (80% decrease in toughness). Enthalpic relaxation increased with temperature and time to 3.8 J g−1 and correlated to the determined toughness and yield strength. Finally, a 9% increase in Young's modulus was observed independent of temperature and with no correlation to enthalpic relaxation. Enthalpic relaxation followed Vogel–Fulcher–Tammann behaviour, while yield strength and charpy v-notch toughness followed Arrhenius behaviour enabling prediction of the different properties with time and temperature.

Deviations of dynamic parameters characterizing enthalpic and dielectric relaxations in glass forming alkyl phosphates.

In our recent study [T. Wu et al., J. Chem. Phys. 147, 134501 (2017)], an alkyl phosphate glass former was studied and it suggested that the enthalpy relaxation involving the motions of all parts of the molecule is global, while the dielectric relaxation detects the local rotation of the polar core. In this work, we study a series of trialkyl phosphates using calorimetric and dielectric measurements over a wide temperature range. The results indicate a departure of the dielectric fragility indexes from the enthalpic ones as the length of the branch chain increases in the trialkyl phosphates. The Kirkwood correlation factor (g k ) is found to coincide at ∼0.6 at glass transition temperature (T g ) from triethyl phosphate to tributyl phosphate, indicating a similar structural alignment. The enthalpic relaxation serving as the more fundamental relaxation relevant to the structural relaxation is confirmed. Strikingly, we observed the relation of T g to the chain length in alkyl phosphates, revealing a minimum T g behavior, and its explanation assists in the understanding of the glass transition in relation to the structure of the glass-formers.

Thermal transitions and enthalpic relaxations as related to the stability of microencapsulated paprika powders

Present work investigates the effect of thermal transitions and enthalpy relaxation phenomenon on the physico-chemical stability of paprika microencapsulated powders. Wall materials were combinations of soy protein isolates, maltodextrin and gum arabic. Powders stability was evaluated through the solid-water interactions, determined by sorption kinetics, calorimetry and color changes. The water sorption isotherms were well described by the GAB model. Below the critical condition, aw 0.6 and water content 11% (d. b.) the systems were in amorphous state at 25 °C, and enthalpy relaxations were detectable as endothermal events in the stored samples. The magnitude of the enthalpy relaxations increased with storage temperature below Tg and with time of storage. In the amorphous microencapsulates, luminosity changes measured as (Lo*-L*) values decreased as a function of time and aw, indicating pigment decoloration. The kinetics of the enthalpy relaxations changes, instead of the punctual values of the variables under study, were related to the dynamics of deteriorative changes. It can be concluded that enthalpy relaxation phenomenon can be established as a criterion for defining storage conditions or predict changes in microencapsulated and other systems that have to be stored in the glassy state.

Properties of injection-molded poly (l-co-d,l-lactic acid) using different melt temperatures and stress concentrator in the specimen geometry

This paper investigates the morphology and mechanical properties of poly (l-co-d,l-lactic acid) (PLDLA) specimens injection-molded using different melt temperatures and stress concentrator in the specimen geometry. The values for the tensile strength and ultimate strain increased with a reduction in the melt injection temperature for unnotched specimens. The notched specimens molded using low and high melt injection temperatures showed similar values for the tensile strength compared with the unnotched specimens. The fracture surfaces of the notched and unnotched specimens molded at the low melt temperature showed the characteristic of ductile failure presenting fibrillation and displacement in the oriented skin layer. A restriction in the chain rotation and conformation due to the oriented skin layer explains the less viscous behavior observed in the dynamic mechanical analysis for low injection temperature specimens. The photoelastic analysis indicated a birefringence along the specimens manufactured using the low melt injection temperature, suggesting the existence of residual stress due to filling phase and rapid solidification. On the other hand, specimens injected using the high temperature showed residual stress concentration near the gate due to the packing effect of holding pressure. The enthalpic relaxation peak at the glass transition (Tg) observed in the differential scanning calorimetry analysis confirmed the existence of significant residual stress in all PLDLA specimens, especially when injected using the low temperature.

Interplay of intermolecular interactions and flexibility to mediate glass forming ability and fragility: A study of chemical analogs.

We have investigated the enthalpic and dielectric relaxations of four groups of quinoline analogs having similar structural properties (i.e., rigidity, stiffness, and bulkiness) but a different steric character and the nature of intermolecular interactions and flexibility. The dielectric fragility index (md) and the enthalpic one (mH), determined by the Tool-Narayanaswamy-Moynihan-Hodge formalism, are comparable. Generally, for the four sets of molecules of similar structures, both the interactions and flexibility are found to be critical in making the large span of fragility (i.e., from 59 to 131) and glass forming ability. By contrast, individual impacts of the interaction and flexibility can only explain fragility partly among each group of isomers. We found that the molecules with high fragility are of relatively low liquid density, reflecting the joint impact of the interactions and flexibility. An interesting result is observed among the isomers that the molecules which are fragile have enhanced glass forming ability. The results are unveiling the joint impacts of molecular structure (flexibility) and intermolecular interaction on the molecular dynamics.

Specific Mechanical Energy and Thermal Degradation of Poly(lactic acid) and Poly(caprolactone)/Date Pits Composites

The compatibility of date pits (DP) with polylactic acid (PLA) or polycaprolactone (PCL) is investigated. Composites were prepared by compounding PLA or PCL with date pits at 10, 20, 30, and 40% wt/wt and extruded. Wheat vital gluten (VG) was also used as a filler and in combination with DP. The specific mechanical energy (SME) was calculated and the composites thermal properties were tested using DSC (peak temperature, enthalpic relaxation, and glass transition) and TGA (degradation temperature and mechanism and degradation kinetics). Because DP is hard filler, the SME of PCL-DP composites increased as the amount of filler increased. At 40% fill, the SME decreased due to the lubricating effect of oil found naturally in DP. As illustrated by lower SME, PLA composites exhibited softer texture because PLA is harder than DP. The DSC melting peak temperature of both polymers has increased at higher DP; however, PLA exhibited enthalpic relation between 66 and 68°C. The TGA profile of the composites displayed two distinct peaks versus one peak for the pure polymer. The degradation kinetics showed multistep process for the composites and one-step process for the pure polymer. The utilization of date pits as a hard filler in developing biodegradable plastics is good for the environment and a value added for the date industry.

Presence of global and local α-relaxations in an alkyl phosphate glass former.

The dynamics of a molecular glass former, tributyl phosphate (TBP), with an alkyl phosphate structure (three alkyl branches emanating from a polar core of PO4) is studied in the supercooled regime by dielectric and thermal (or enthalpic) relaxations. The dielectric fragility index md and the stretching exponent βd of the Kohlrausch-Williams-Watts correlation function are determined. Analyses of the enthalpic relaxation data by the Tool-Narayanaswamy-Moynihan-Hodge formalism yield the enthalpic fragility index mH and stretching exponent βH. The large difference between the dielectric md and the enthalpic mH, as well as between βd and βH, is a remarkable finding. The differences are interpreted by the formation of molecular self-assemblies. The interpretation is supported by the quite comparable fragility determined by viscosity and the enthalpic relaxation. The Kirkwood factor calculated at low temperatures is also consistent with the interpretation. The results suggest that the enthalpic relaxation involving the motions of all parts of TBP is global, while the dielectric relaxation detects the local rotation, which might originate from the rotation of the dipole moment of the core. The presence of two structural α-relaxations, one global and one local, with a large difference in dynamics is revealed for the first time in a molecular glass former.

Structural Transitions in Solution-Cast Films of a New AABB Type Thiophene Copolymer

The structural transitions in solution-cast film of poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT) copolymer have been systematically studied by a combination of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), in situ Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy, and in situ wide-angle X-ray scattering (WAXS). The glass transition temperature of the main chain as well as the melting and crystallization temperatures of the polymer were determined as 10, 238, and 221 °C, respectively. The out-of-plane deformation vibration of PDCBT Cβ–H groups in FTIR has been manifestly assigned for the first time. A broad endothermic peak between 30 and 120 °C was observed during DSC heating process and was attributed to the enthalpic relaxation of the twist glass transition, which resulted in a small negative effect on the power conversion efficiency (PCE) of solar cells. A reorganization process in the crystalline re...

Thermal and Mechanical Properties of Compression-Moulded Poly(Lactic Acid)/Gluten/Clays Bio(Nano)Composites

Bio(nano)composites comprising agricultural-based polymers blended with biodegradable plant-based fillers and clays were produced to develop novel hydrophobic, yet biodegradable materials that have properties comparable to those of petroleum-based plastics. Poly(lactic acid) (PLA), vital wheat gluten (VG), and 3, 6, and 9% clays (NC1 or NC2) were mixed in a Haake Rheocord torque rheometer at 170 °C for 10 min. Blends were compression-moulded and tested using modulated DSC (MDSC), thermogravimetric analysis (TGA), Instron, FTIR and HPLC. The DSC profile of neat PLA exhibited a glass transition (Tg) and exothermic (Cry) followed by endothermic (Mel) transitions. The profile showed a Tg of 0.46 J/g/°C, Cry with 29.6 J/g, whereas Mel exhibited 28.3 J/g. Vital wheat gluten displayed a single Tg (0.45 J/g/°C). Enthalpic relaxation of PLA was noted in the presence of clays, where the overall DSC profiles of PLA were different, especially during melting. The hydrophobic nature of NC1 clay created shoulders during PLA melting. The degradation kinetics of the blends followed a multistep mechanism, as shown by the TGA data. The tensile strength dropped because of processing and due to the addition of clay, whereas elongation was reduced by the presence of NC1. Higher Young's modulus value due to NC1 or NC2 indicated stiffer bio(nano)composites. Better biodegradability was indicated after proteinase-K enzyme treatment in the presence of NC1. Due to the hydrophobic nature of NC1, the amount of the acetic acid-extractable protein was reduced. FTIR analysis showed no evidence of chemical reaction between PLA, gluten and the clays, but suggested the occurrence of significant non-covalent intermolecular interactions.