Enthalpy Relaxation Rate Research Articles

A simple approach to understand the physical aging in polymers

Glassy-state structural relaxation is a physical phenomenon that occurs in most amorphous and semicrystalline polymers after their melt-processing. As such, after these industrial conformation processes, polymers are usually found in a thermodynamic non-equilibrium state. Therefore, polymer chains undergo thorough conformational changes on their way towards a thermodynamically stable state. These molecular-level events involve macroscopic-level modifications, which are manifested as changes in physical properties. Here we propose a simple approach to monitor the physical aging in polymers by differential scanning calorimetry (DSC). Accordingly, the βH enthalpy relaxation rate is directly extracted from the DSC curves taking into account the enthalpy loss at different aging times. This method allows a simple quantification of the bulk aging dynamics in polymers, and can be extrapolated to systems containing reinforcing particles; i.e. composites and nanocomposites. This safe, low cost and simple experiment designed for undergraduate students of physical, chemical and engineering specialities serves as a guide to easily determine the physical aging suffered by polymeric materials during their storage and to understand its implications at macroscopic level, which is a relevant field in condensed-matter physics. Given the paramount relevance of physical aging in glassy materials, this experiment may also serve to raise awareness of the importance of the thermal history of materials on their resulting properties, which is overlooked too often.

Co-amorphous Formation Induced by Combination of Tranilast and Diphenhydramine Hydrochloride

In this study, we investigated the formation of a co-amorphous system of tranilast (TRL) and diphenhydramine hydrochloride (DPH), which are drugs used for treating allergies and inflammation. The crystallization from undercooled melts of the drugs and drug mixtures was evaluated by thermal analysis. Both drugs in the amorphous state underwent crystallization on heating, although the mixture remained in the amorphous state, indicating the formation of a co-amorphous system. The physicochemical properties of co-amorphous TRL-DPH prepared by the melting-cooling process were studied. The glass transition temperature of co-amorphous TRL-DPH deviated from the theoretical value. The enthalpy relaxation rate of the amorphous drugs, which reflected the molecular mobility, was reduced by the formation of a co-amorphous system. The intermolecular interactions between TRL and DPH in the co-amorphous system were measured by the change in the IR spectra. These results were consistent with the high physical stability. The co-amorphous sample remained in the amorphous state for over 30 days at 40°C, whereas the amorphous drugs showed rapid crystallization. Our findings demonstrate that TRL and DPH form a co-amorphous system, which dramatically decreases their crystallization without an excipient.

Physical aging and mechanical performance of poly(l‐lactide)/ZnO nanocomposites

ABSTRACTIn this work nanocomposites based on poly (l‐lactide) (PLLA) and zinc oxide (ZnO) nanoparticles with a concentration up to 5 wt % have been prepared by solvent‐precipitation followed by compression moulding at 200 °C. Structural evolution of nanocomposites as a function of time and nanoparticle concentration has been monitored by differential scanning calorimetry (DSC). Results reveal a marked reduction of the enthalpy relaxation rate βH from 3.273 J/g for neat polymer to 0.912 J/g for its 0.25 wt % reinforced counterpart, revealing slower aging dynamics induced by zinc oxide. It is shown by field emission scanning electron microscopy (FE‐SEM) that concentrations larger than 1 wt % yield nanoparticle agglomeration. These large aggregates decrease the amount of nanoparticle surfaces exposed to PLLA chains, notably reducing the efficiency of ZnO nanoparticles to delay the physical aging of its hosting matrix. Mechanical tests show an increased stiffness upon ZnO loading as denoted by the increase in modulus from 2310 MPa to 2780 MPa for the 1 wt % nanocomposite. Obtained findings through this work lead the way for the development of nanocomposites based on renewable polymers and natural fillers to be used in packaging applications, where the use of nonbiodegradable materials for short‐term applications is extended. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43619.

The impact of elastic and plastic strain on relaxation and crystallization of Pd–Ni–P-based bulk metallic glasses

The effects of deformation and subsequent heat treatment on the low-temperature heat capacity, enthalpy relaxation rate and mechanical losses of two Pd–Ni–P-based bulk metallic glasses of slightly different compositions and different thermal stabilities have been investigated. It was found that the crystallization temperatures decreased significantly with imposed strain and the effect was more pronounced for the alloy with a higher thermal stability. The boson heat capacity peak increases with increasing strain in both alloys. However, after annealing treatments above room temperature, it relaxes to a lower enthalpy state as compared to that of the as-quenched state for the alloy with a lower thermal stability. The existence of two counteracting processes that might be related to different shear band structures within one homogeneously deformed sample is suggested. These results agree with the internal friction data, which indicate different regimes of mechanical damping as a function of the strain amplitude, while the critical amplitude of a transition between the regimes depends on the imposed strain. The results are interpreted within the energy landscape approach and advocate that the composition-dependent local atomic configurations affect significantly the response of the glass to an applied strain.

Physical Aging in Poly(L‐lactide) and its Multi‐Wall Carbon Nanotube Nanocomposites

AbstractSummary: In this work the effects of the structure evolution of neat PLLA and its multi‐wall carbon nanotube nanocomposites at an aging temperature of 45 °C (Tg‐12 °C) have been investigated. The analysis of enthalpy relaxation rate (βH) of composites by means of differential scanning calorimetry (DSC) reveals the existence of a critical concentration of 1.25–2.5 wt. % in which aging rate reaches a mi–imum value of 1.351 J/g. Mechanical tests showed that neat polymer is much more sensitive to the aging process than the reinforced one. As shown by transmission electron microscopy (TEM) all the samples present randomly dispersed nanotubes, yielding an agglomerated structure at MWCNT contents exceeding 1.25–2.5 wt.%. The obtained results are explained from the viewpoint of polymer/filler interfacial effects that perturb the structural relaxation of glassy polymer due to the drastic reduction on the molecular mobility of polymer chains in the vicinity of carbon nanotube surfaces. In this way, nanocomposite approach represents an efficient strategy to successfully reduce the physical aging process of polymers.

On the nature of enthalpy relaxation below and above the glass transition of metallic glasses

Within the framework of the interstitialcy theory, we derived a simple and testable expression for the heat flow (enthalpy relaxation rate) occurring upon warming of glass at a constant rate. Formally, this heat flow is mostly determined by the shear moduli of glass and parent crystal. The underlying physical reason is relaxation of an intrinsic interstitialcy-like defect system. An extensive check of the derived heat flow law for temperatures near the glass transition showed good agreement with the experiment specially performed on two Pd-based metallic glasses.

Molecular weight dependence in a relaxation phenomenon at glassy state: Physical aging of polycarbonate

At the glassy state of polymers, the reptation-type molecular motion should be frozen-in and the relaxation should be only by local motions, such as rotation, vibration, and torsion. Such local motions would not depend on molecular weight. This may be current understanding of the polymer glass. By contrast, we found that the rate of enthalpy relaxation in polycarbonate at 100°C (50°C below Tg) clearly depends on molecular weight. Deterioration of impact strength was also found to depend on molecular weight. The surprising results are presented in this letter.

Enthalpy Relaxation during Non-Isothermal Crystallization from the Melt of Polymer: For <i>I</i>PP as Example

The enthalpy relaxation rate (rh) is defined by enthalpy change in unit time to describe non-isothermal crystallization behavior of iPP from its melt. Dates of rhare transformed from those of heat flow rate which are collected by means of DSC directly. The curve by plotting with rhto time indicates that the process of non-isothermal crystallization can be separated into two stages by the time point of inflection at rhcurve. The two stages are called stage Ⅰand stage Ⅱ. The curve of rhis differentiated and the curve of Ahproduced. There are two time inflection point in curve of Ah. Combined different trends of rhand Ahduring the whole process, the non-isothermal crystallization are further separated into four sub-stages, which are called stage Ⅰ1, stage Ⅰ2, stage Ⅱ1and stage Ⅱ2. The critical values of time and relative crystallization degree which divide the process of non-isothermal crystallization of iPP are present. It provides one new method to uncover the details of non-isothermal crystallization process.

Desiccation-Induced Structuralization and Glass Formation of Group 3 Late Embryogenesis Abundant Protein Model Peptides

Anhydrobiotic (i.e., life without water) organisms are known to produce group 3 late embryogenesis abundant (G3LEA) proteins during adaptation to severely water-deficient conditions. Their primary amino acid sequences are composed largely of loosely conserved 11-mer repeat units. However, little information has been obtained for the structural and functional roles of these repeat units. In this study, we first explore the consensus sequences of the 11-mer repeat units for several native G3LEA proteins originating from anhydrobiotic organisms among insects (Polypedilum vanderplanki), nematodes, and plants. Next, we synthesize four kinds of model peptides (LEA models), each of which consists of four or two repeats of the 11-mer consensus sequences for each of the three organisms. The structural and thermodynamic properties of the LEA models were examined in solution, in dehydrated and rehydrated states, and furthermore in the presence of trehalose, since a great quantity of this sugar is known to be produced in the dried cells of most anhydrobiotic organisms. The results of Fourier transform infrared (FTIR) spectroscopic measurements indicate that all of the LEA models transform from random coils to alpha-helical coiled coils on dehydration and return to random coils again on rehydration, both with and without trehalose. In contrast, such structural changes were never observed for a control peptide with a randomized amino acid sequence. Furthermore, our differential scanning calorimetry (DSC) measurements provide the first evidence that the above 11-mer motif-containing peptides themselves vitrify with a high glass transition temperature (>100 degrees C) and a low enthalpy relaxation rate. In addition, they play a role in reinforcing the glassy matrix of the coexisting trehalose. On the basis of these results, we discuss the underlying mechanism of G3LEA proteins as desiccation stress protectants.

Glass Transitions in Aqueous Solutions of Protein (Bovine Serum Albumin)

Measurements by adiabatic calorimetry of heat capacities and enthalpy relaxation rates of a 20% (w/w) aqueous solution of bovine serum albumin (BSA) by Kawai, Suzuki, and Oguni [Biophys. J. 2006, 90, 3732] have found several enthalpy relaxations at long times indicating different processes undergoing glass transitions. In a quenched sample, one enthalpy relaxation at around 110 K and another over a wide temperature range (120-190 K) were observed. In a sample annealed at 200-240 K after quenching, three separated enthalpy relaxations at 110, 135, and above 180 K were observed. Dynamics of processes probed by adiabatic calorimetric data are limited to long times on the order of 10(3) s. A fuller understanding of the processes can be gained by probing the dynamics over a wider time/frequency range. Toward this goal, we performed broadband dielectric measurements of BSA-water mixtures at various BSA concentrations over a wide frequency range of thirteen decades from 2 mHz to 1.8 GHz at temperatures from 80 to 270 K. Three relevant relaxation processes were detected. For relaxation times equal to 100 s, the three processes are centered approximately at 110, 135, and 200 K, in good agreement with those observed by adiabatic calorimetry. We have made the following interpretation of the molecular origins of the three processes. The fastest relaxation process having relaxation time of 100 or 1000 s at ca. 110 K is due to the secondary relaxation of uncrystallized water (UCW) in the hydration shell. The intermediate relaxation process with 100 s relaxation time at ca. 135 K is due to ice. The slowest relaxation process having relaxation time of 100 s at ca. 200 K is interpreted to originate from local chain conformation fluctuations of protein slaved by water. Experimental evidence supporting these interpretations include the change of temperature dependence of the relaxation time of the UCW at approximately T(gBSA) approximately = 200 K, the glass transition temperature of protein in the hydration shell, similar to that found for the secondary relaxation of water in a mixture of myoglobin in glycerol and water [Swenson et al. J. Phys.: Condens. Matter 2007, 19, 205109; Ngai et al. J. Phys. Chem. B 2008, 112, 3826]. The data all indicate in hydrated BSA or other proteins that the secondary relaxation of water and the conformation fluctuations of the protein in the hydration shell are inseparable or symbiotic processes.

Effects of Chemical Modification of Thiol−Ene Networks on Enthalpy Relaxation

The highly uniform and dense network structure of photopolymerized thiol−enes was chemically modified, and the enthalpy relaxation of the networks was measured. n-Alkyl acrylate and hydroxyl acrylate groups were incorporated into thiol−ene networks using a phosphine-catalyzed Michael addition reaction. The effect of flexible alkyl side chains and hydrogen bonding on sub-Tg relaxation was evaluated without sacrificing network uniformity. Overall both the rate and extent of enthalpy relaxation decreased as a function of the flexible n-alkyl chain length, while hydrogen bonding resulted in enhanced enthalpy relaxation. A trithiol−triene−triacrylate ternary system was investigated by correlating enthalpy relaxation and network uniformity. A multifunctional acrylate (TMPTA), being capable of homopolymerization as well as thiol−acrylate copolymerization, was incorporated into a thiol−ene network structure, thereby decreasing the network uniformity and significantly affecting the enthalpy relaxation behavior. In...

Thermal relaxation of water due to interfacial processes and phase equilibria in 1.8 nm pores of MCM-41

The available data on the enthalpy relaxation rate, d H m/d t, and specific heat, C p,m , of water in 1.8 nm pores of MCM-41 are interpreted by considering surface interactions and the number of H 2O molecules available to crystallize or vitrify. Out of a maximum of five H 2O molecules in a close-packing along the 1.8 nm diameter pore, at least two would form an uncrystallizable shell bonded to the silica wall and three would remain as a 1.1 nm diameter nanocore, which is too small for nucleation of the usual ice crystals. The d H m/d t features observed in the temperature range of 90–130 K and 125–175 K show kinetic unfreezing or glass-softening characteristics. The first is attributed to the reorientation of H 2O in the nanoshell with little change in their center of mass position and the second to the change in the population of bonds between water and silica surface. The third d H m/d t feature observed in the 180–230 K range is inconsistent with kinetic unfreezing or glass-softening and is attributed to the formation and melting of distorted ice-like unit cells with or without the growth- and stacking-faults that remain at equilibrium with the melt. The large increase in C p,m at T near 210 K is attributed to the latent heat of their “melting”, as occurs on premelting of fine-grain ice and other solids. Data on the pore size dependence qualitatively support this interpretation. Structure of the water in nanopores depends on the pore size, and its properties differ from those of bulk water.

Observations of physical aging in a polycarbonate and acrylonitrile–butadiene–styrene blend

AbstractThe effects of physical aging of a 75 : 25 PC/ABS blend have been studied using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). From DSC, two distinct peak endotherms at about 90°C and 110°C, which are associated with the glass transition of ABS (Tg,ABS) and PC (Tg,PC) components, respectively, were observed. When progressive aging was monitored at 80°C for over 1000 h, the changes in enthalpic relaxation, glass and fictive temperatures for the blend followed similar trends to those already seen in the literature for PC aged between 125 and 130°C. The rate of enthalpy relaxation was also comparable. The plot of peak endotherm against logarithmic aging time for the PC blend constituent, however, behaved quite differently from the linear relationship known for highly aged PC. The ABS peak component also appeared to be insensitive to aging. Both observations were confirmed to be statistically significant using analysis of variance methods. Using temperature modulated‐DSC, there is evidence that aging increases the blend miscibility as the Tg,PC shifts toward the stationary Tg,ABS during aging. Parallel FTIR investigations found oxidation of butadiene during aging to be even at this relatively low temperature, forming hydroxyl and carbonyl degradation products. The presence of ABS in the blend also appeared to have prevented the shifting from the trans‐cis to trans‐trans arrangement of the carbonate linkage, which is a well‐known phenomenon during elevated temperature aging of PC alone. Moreover, the carbonate linkage appears to have been at the lower energy, trans‐trans, arrangement prior to the aging process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Correlation between Chemical Reactivity and the Hammett Acidity Function in Amorphous Solids Using Inversion of Sucrose as a Model Reaction

The goal was to evaluate the effects of acidity, expressed as the Hammett acidity function, on chemical reactivity in freeze-dried materials (lyophiles). Dextran-sucrose-citrate and polyvinyl pyrrolidone (PVP)-sucrose-citrate aqueous solutions, adjusted to pH values of 2.6, 2.8, and 3.0 were freeze dried, and characterized by X-ray powder diffractometry, DSC, isothermal microcalorimetry, and Karl Fischer titrimetry. Lyophiles were also prepared from identical solutions but containing bromophenol blue (BB). Diffuse reflectance-visible spectroscopy was used to measure the extent of BB protonation from which the Hammett acidity functions were determined. The stability studies were performed at 60 degrees C. All the freeze-dried samples were observed to be X-ray amorphous with <0.15% w/w water content. The T(g) of dextran lyophiles were approximately 20 degrees C higher than that of PVP lyophiles whereas enthalpy relaxation rates at 60 degrees C were similar. The Hammett acidity functions were significantly lower (i.e., higher acidity) for dextran systems (<2.2-2.6) when compared with PVP systems (3.3-3.9). The rate of sucrose inversion was significantly (an order of magnitude) higher in dextran lyophiles. This study showed that in amorphous matrices with comparable water content and structural relaxation times, chemical reactivity could be significantly different depending on the matrix "acidity".

Structural ordering and ice-like glass transition on cooling the nano-channel water formedwithin a crystalline framework

Heat capacities and enthalpy relaxation rates were measured for crystalline[Ni(cyclam)(H2O)2]3(TMA)2·24H2O,where cyclam is 1,4,8,11-tetraazacyclotetradecane, TMA is 1,3,5-benzene tricarboxylic acid, and24H2O represents the water forming a nano-channel. A phase transition was found to occur at196.9 K, and a glass transition at 87 K. A potential picture is given for the progress of theordering of water molecules and hydrogen atoms in the channel. At room temperature,interfacial water molecules form hydrogen bonds with the oxygen atoms of the channelwall, and the aggregation of water molecules is recognized, as the average structure,to be in a crystalline state with a long-range order. The bond formation playsimportant role in the stabilization of the crystalline framework with a channelstructure. The aggregate transforms to a more stable crystalline state at 196.9 K.All the water molecules should be fixed completely there. The positions of thehydrogen atoms on the network are, however, essentially in the disordered statewhile keeping an ice rule, and freeze at 87 K only with a short-range order in thearrangement. The progress of such ordering of channel water reveals a strikingcontrast to the behaviour of the water within meso-porous silicas, in which themolecules in the interface are always in the non-crystalline state and those in the porecentre tend to crystallize only when the pore diameter is greater than 2 or 3 nm.

Low-Temperature Glass Transitions of Quenched and Annealed Bovine Serum Albumin Aqueous Solutions

To investigate the glass transition behaviors of a 20% (w/w) aqueous solution of bovine serum albumin, heat capacities and enthalpy relaxation rates were measured by adiabatic calorimetry at temperatures ranging from 80 to 300K. One series of measurements was carried out after quenching from 300 down to 80K and another after annealing in 200–240K. The quenched sample showed a heat capacity jump indicating a glass transition temperature Tg=170K, and the annealed sample showed a smaller jump with the Tg shifted toward the higher temperature side. The temperature dependence of the enthalpy relaxation rates for the quenched sample indicated the presence of two enthalpy relaxation effects: one at around 110K and the other over a wide temperature range (120–190K). The annealed sample showed three separate relaxation effects giving 1) Tg=110K, 2) 135K, and 3) temperature higher than 180K, whereas nothing around 170K. These effects were thought to originate, respectively, from the rearrangement motions of 1) primary hydrate water forming a direct hydrogen bond with the protein, 2) part of the internal water localized in the opening of a protein structure, and 3) the disordered region in the protein.

Finding of an unexpected thermal anomaly at very low temperatures due to water confined within a globular protein, bovine serum albumin

Heat capacities and enthalpy relaxation rates of completely anhydrous bovine serum albumin (BSA) and hydrous BSA with 1.85% (w/w) water were measured by using an adiabatic calorimeter to examine the thermal behavior at low temperatures of a small amount of water left within the globular protein molecule. The heat capacities of the hydrous BSA were larger a little on account of the presence of water molecules. In addition, two new phenomena appeared for the hydrous BSA while not for the anhydrous one: spontaneous exothermic and endothermic effects depending on the pre-cooling rates were observed and interpreted as due to a glass transition. Anomalous behavior of heat capacities was found in 60–140 K and recognized as potentially originating from a kind of phase transition. It is suggested that the water left a little within the BSA is responsible for both the phenomena occurring at low temperatures.

Properties of poly(lactic acid) blends with various starches as affected by physical aging

AbstractPoly(lactic acid) (PLA) and starch are both biodegradable and renewable polymers derived from agricultural feedstock. A previous study showed that a small amount (0.5%) of methylenediphenyl diisocyanate (MDI) could enhance the mechanical properties of starch and PLA blends by improving the interfacial interaction. In this study, blends of PLA (1/1, w/w) and starch with or without MDI were evaluated for thermal and mechanical properties as well as morphology, as affected by physical aging when stored up to 12 months at 25 °C and 50% relative humidity. The blends were prepared by thermally blending PLA with wheat starch, corn starch, and/or high amylose corn starch, with or without MDI. All samples exhibited phenomena of physical aging. The samples with MDI aged more slowly, showing a slower reduction rate of excess enthalpy relaxation, than those without MDI. The mechanical properties decreased slowly as aging proceeded. Microstructure showed a reduced interaction between starch and PLA around the interface with aging. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3683–3689, 2003

Influence of physical aging and side group on the free volume of epoxy resins probed by positron

The effect of physical aging and side group on the free volume has been studied by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimeter (DSC) for a series of epoxy resins cured separately with novolac resin and esterified novolac resins, that is, the hydroxyl group of phenol novolac resin was placed by a CH 3COO-group, a CH 3CH 2 CH 2COO- and a C 6H 5 CH 2COO-group, which were named EP, EPA, EPB and EPP, respectively. Positron experimental results show that the side group plays an important role in determining free volume properties of epoxy resins, i.e. the stronger the cross-linking of curing agent, the smaller the free volume. This is confirmed by DSC measurement. On the other hand, the free volume relaxation properties were investigated by the Kolrausch–Williams–Watt (KWW), the single exponential and the double exponential functions in aging course for all samples, respectively. We found that ortho-positronium ( o-Ps) intensity decreased with increasing aging time, which meant that the free volume diminished resulting from the formation of the local order structure due to the chain segmental rearrangement. The relaxation time constants of both τ and the parameter β have been calculated, which indicated that the order of relaxation rate of free volume characterized by τ −1 measured by PALS was the same as the enthalpy relaxation rate characterized by ∂ΔH/ ∂( log t) measured by DSC.

Contributions of side groups to the water diffusion in cured epoxy resins (2) study on physical aging

AbstractNovolac epoxy resins cured with novolac resin, novolac acetate resin, novolac butyrate resin, and novolac phenylacetate resin named as EP, EPA, EPB, and EPP, respectively, were prepared. Their physical aging behavior at a Tg‐30 °C (30 °C below glass‐transition temperature) was examined by positron annihilation lifetime spectroscopy and differential scanning calorimetry. The ortho‐positronium annihilation lifetime τ3 variation extent of EP is less apparent than that of the other three esterified samples during physical aging. The time dependence of ops intensity I3 agreed with the Kohlrausch‐Williams‐Watts (KWW) equation. The relaxation time (τ0) and nonexponential parameter were calculated. The free volume and enthalpy relaxation rate characterized by the reciprocal of τ0 and ∂ΔH/∂logt, respectively, exhibit the same order—EPP &gt; EPB &gt; EPA &gt; EP. These results suggest that the extend and rate of relaxation are not only related to the frozen free volume produced by quenching but also significantly influenced by segmental mobility of the network that attributed to the side‐group flexibility and their interaction with networks. This work also supports the fact that side‐group flexibility and the free‐volume fraction and distribution act in concert to control the water‐diffusion behavior in epoxy networks. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1135–1142, 2003