Enthalpy Recovery Research Articles

Glass transition and aging of the rigid amorphous fraction in polymorphic poly(butene-1)

The origin of “annealing peaks” in semicrystalline polymers, obtained with aging thermal protocols at temperatures between the glass transition and the melting temperature, and revealed by calorimetry as small endotherms preceding the main melting peak, has been recently debated in the literature. On the one side it is claimed that these small endotherms are related to the melting of defective/thinner crystals, on the other side it is proposed that they can be associated to the enthalpy relaxation of the rigid amorphous fraction (RAF), i.e., that part of the amorphous region directly coupled to the crystalline stems. With this work we aim to contribute to the debate, by exploiting the different thermal properties of the RAF coupled with different crystal modifications of polymorphic poly (butene-1) (PB-1). In this polymer the RAF mobilizes at largely different temperatures when it is coupled with crystals of trigonal Form I, or tetragonal Form II. By exploiting this peculiarity, we performed isochronous aging experiments with differential scanning calorimetry (DSC) on both crystalline phases, in a wide temperature range between the glass transition of the mobile (bulk) amorphous fraction and the onset of crystal melting. An endothermic peak above the aging temperature was typically observed. The trend of the enthalpy of this annealing peak with temperature can be described by a bell-shaped curve, approaching zero recovered enthalpy at temperatures of 100–110 °C, and 40–50 °C for Form I and Form II, respectively. These temperatures coincide with previous literature report about mobilization of PB-1 RAF and are thus identified as the upper limit of the RAF glass transition for the two polymorphs. Isothermal aging experiments as a function of time suggest that the enthalpy recovery of Form I RAF show the typical signature of glassy dynamics, similarly to what occurs for Form II RAF after short aging times. At longer times, the superposition of the polymorphic phase transition and RAF aging changes the kinetics of the recovered enthalpy for the latter polymorph, possibly due to the increased stress at the crystal-amorphous interface and creation of additional RAF in parallel with Form II to Form I transformation. Overall, our results demonstrate that for PB-1, at least within the investigated temperature range, the annealing peaks can be related to the RAF strive to attain thermodynamic equilibrium in its glassy state.

Effects of Annealing on Enthalpy Recovery and Nanomechanical Behaviors of a La-Based Bulk Metallic Glass

Structural relaxation and nanomechanical behaviors of La65Al14Ni5Co5Cu9.2Ag1.8 bulk metallic glass (BMG) with a low glass transition temperature during annealing have been investigated by calorimetry and nanoindentation measurement. The enthalpy release of this metallic glass is deduced by annealing near glass transition. When annealed below glass transition temperature for 5 min, the recovered enthalpy increases with annealing temperature and reaches the maximum value at 403 K. After annealed in supercooled liquid region, the recovered enthalpy obviously decreases. For a given annealing at 393 K, the relaxation behaviors of La-based BMG can be well described by the Kohlrausch-Williams-Watts (KWW) function. The hardness, Young’s modulus, and serrated flow are sensitive to structural relaxation of this metallic glass, which can be well explained by the theory of solid-like region and liquid-like region. The decrease of ductility and the enhancement of homogeneity can be ascribed to the transformation from liquid-like region into solid-like region and the reduction of the shear transition zone (STZ).

Physical aging of polystyrene blocks under three‐dimensional soft confinement in PS‐b‐poly(n‐butyl methacrylate) diblock copolymer: Two equilibrations on the way

AbstractPolystyrene‐block‐poly(n‐butyl methacrylate) (PS‐b‐PnBMA) was used to investigate three‐dimensional (3D) soft confinement effect on physical aging of the PS block therein. The soft confinement is constructed by phase‐separated PnBMA domains, as PnBMA is liquid on the aging temperatures of PS blocks due to its low glass transition temperature. In enthalpy recovery, aging response of PS blocks is represented by a low and broad heat capacity peak associated with an enhanced aging rate with respect to homo‐PS, when the aging temperature is relatively low. However, the aging response exhibits opposite characteristics at relatively high temperatures, compared with the results of homo‐PS. The phase‐separated morphology and thus the soft confinement on PS blocks was confirmed by atomic force microscope imaging using the Peak Force quantitative nanomechanical mapping (QNM) technique. Two local maximums of recovered enthalpy versus aging temperature indicate that two equilibration processes exist during aging of confined PS blocks, within a substantially shorter timescale to the bulk. The 3D soft confinement effect on aging of PS blocks is attributed to dual equilibration mechanisms: one dominates at higher aging temperatures, leading to a restrained aging rate, while the other plays a key role at lower aging temperatures, resulting in accelerated physical aging.

The origin of the faster mechanism of partial enthalpy recovery deep in the glassy state of polymers.

A novel finding made by Cangialosi and coworkers in the physical aging of several polymers way below the glass transition temperature Tg is that equilibrium recovery occurs by reaching a plateau in the enthalpy with partial enthalpy recovery. This observation points to the existence of a much faster mechanism capable of partial equilibrium recovery deep in the glassy state. A similar phenomenon was found in different glassy materials. The generality of the phenomenon indicates that the faster mechanism of equilibrium recovery is universal and fundamental. In this paper the faster mechanism is identified to be the universal JG β-relaxation having dynamic and thermodynamic properties analogous to the α-relaxation, and thus capable of effecting enthalpy and volume recovery far below Tg in several high-Tg polymers. The JG β-relaxation is also the mechanism responsible for the first step of two steps in the approach to equilibrium found in another polymer with much lower Tg. The Coupling Model is used to explain why the first step transpires far below Tg in some polymers but much closer to Tg in another polymer.

The Origin of Annealing Peaks in Semicrystalline Polymers: Enthalpy Recovery or Melting?

The present work provides unambiguous evidence that annealing peaks in differential scanning calorimetry heating experiments of semicrystalline polymers are caused by crystallization and not enthal...

Air conditioning system with enthalpy recovery for space heating and air humidification: An experimental and numerical investigation

This study presents the evaluation of an air conditioning system with enthalpy recovery for space heating and air humidification. Desiccant assisted air conditioning is potentially beneficial against other air conditioning processes in terms of air humidification, counteracting dry indoor air conditions during winter. Furthermore, the overall process can be operated efficiently relying on renewable energy sources. The objective of this work is to investigate a geothermal and desiccant assisted system for different system configurations experimentally and numerically during winter operation. The experimental investigation focuses on differences in system performance for two desiccant materials, lithium chloride and silica gel. Experimental results show that moisture recovery relying on a desiccant wheel with silica gel is beneficial against lithium chloride. An average moisture recovery efficiency of 0.83 was achieved for silica gel. Thermal comfort was maintained on a high level throughout the winter seasons investigated. Based on component and system simulation models, system performance is shown for different boundary conditions during winter requiring space heating and humidification to ensure comfortable indoor air conditions. The simulation results show a reduction in primary energy demand for heat supply by up to 54% by using a high efficient ground-coupled heat pump without backup system.

Analysis of the minimal model for the enthalpy relaxation and recovery in glass transition: application to constant-rate differential scanning calorimetry

The so-called minimal model is formulated for describing the enthalpy relaxation and recovery in glass transition. The model is based on the Arrhenius law for the enthalpy relaxation, which uses two-dimensional parameters, namely the activation energy and the so-called pre-factor (relaxation time at relatively high temperature). A numerically effective exact analytical solution is obtained for the case of constant-rate differential scanning calorimetry. The developed model is analyzed according to the logic of the model itself without introducing additional simplifying assumptions of thermodynamic nature. For typical range of the model parameters, the resulting differential equation contains a large parameter, which offers an opportunity for the application of asymptotic and approximate techniques. A number of simple approximations have been provided for some thermodynamic quantities of interest.

Dynamics of structural relaxation in bioactive 45S5 glass

Kinetics of physical aging in archetypic 45S5 bioactive silicate glass composition with different types of phase separation are studied in situ below the glass transition temperature (Tg). The qualitative nature of aging is found to be almost independent of the structural differences on the micrometer scale. A well-expressed step-like behavior in the enthalpy recovery kinetics is observed for aging temperatures Ta ∼ 0.90Tg and Ta ∼ 0.85Tg, which, however, disappears when the aging occurs at Ta ∼ 0.95Tg. The overall kinetics are described by a stretched-exponential function with stretching exponent close to 3/7 at Ta ∼ 0.95Tg, and 1/3 when the aging temperature drops to ∼0.90Tg and below. The values correlate well with the predictions of Phillips’ diffusion-to-traps and percolating fractals models. Appearance of step-like behavior at larger departure from Tg is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.

Performance Comparison of Enthalpy Recovery Devices Using Liquid Desiccant and Desiccant Wheel

Abstract Heat recovery between outdoor air and indoor exhaust air is an effective approach for energy-saving in the air-conditioning system. Performances of enthalpy recovery devices using liquid desiccant (LD) and desiccant wheel (DW) are compared in the present study. Effects of key factors including desiccant flow rate, number of transfer units (NTUm), and air inlet parameters on recovery performance are analyzed. There exists the same ideal recovery efficiency with a given NTUm for these two kinds of devices. However, an optimal solution flow rate could be achieved for enthalpy recovery device using LD, while a higher rotation speed leads to a higher recovery efficiency for that using DW. Recovery efficiencies of the two devices increase with the increase in NTUm, while they have different ranges of NTUm. NTUm of the DW is usually superior to that of the LD, due to the material difference. Then, the optimum methods to improve recovery efficiency of the two devices are clarified, i.e., increasing NTUm for an enthalpy recovery device using LD and choosing a reasonable rotation speed for that using DW, respectively. The present research will be beneficial to cast light on the relation between enthalpy recovery devices using LD and DW.

Sorbent based enthalpy recovery ventilator

A novel concept – disc type sorbent based enthalpy recovery ventilator (SERV) – is presented for the first time. The proposed SERV is a compact system that includes two separate sections for vapor and heat recovery, has low pressure drop, and eliminates frozen exhaust issue that affects the performance of conventional heat and enthalpy recovery ventilators in cold climates. A new theoretical model is developed for the heat and mass transfer in the air channels of the sorbent discs. A sensitivity analysis is performed to design the main parameters, e.g. channel diameter and spacing, to achieve optimum performance. A prototype SERV is built and tested in a custom-made experimental set-up designed based on ASHRAE 84 standards. The performance of SERV is evaluated for several air flow rates (3–18 CFM), cycle times, and outdoor air temperatures down to -15 °C. The SERV prototype – consisting of 2.1 kg of active sorbent material and 2 kg of heat storage materials –recovered up to 70% of heat and 80% of moisture from exhaust air with 60% less pressure drop relative to measured comparable systems in the literature.

Applications with semipermeable membranes for humidity recovery in HVAC systems

Both the sensible heat recovery and the combined heat and humidity recovery are state of the art. Of particular importance is the humidity recovery in winter season. The transfer of water vapor from the humidity-laden exhaust air to the very dry fresh air is very important for a good room air quality. However, all systems established on the market require that supply and exhaust air streams crosses within the device. Practical enthalpy recovery systems for plant concepts with local separation of supply and exhaust air are not available on the market until now. In order to close this gap, the ILK Dresden works on a R&D project. The objective is the development of a textile heat exchanger, which - integrated into circulatory composite systems - can transmit not only sensible heat but also humidity between spatially separated air streams. The functional principle is based on a liquid sorption process via semipermeable membranes. The current state of the development will be presented. Furthermore an outlook is given how it is intended to proceed in the project in further steps.

Solid-state analysis of amorphous solid dispersions: Why DSC and XRPD may not be regarded as stand-alone techniques

Amorphous solid dispersions (ASDs) are single-phase amorphous systems, where drug molecules are molecularly dispersed (dissolved) in a polymer matrix. The molecular dispersion of the drug molecules is responsible for their improved dissolution properties. Unambiguously establishing the phase behavior of the ASDs is of utmost importance. In this paper, we focused on the complementary nature of (modulated) differential scanning calorimetry ((m)DSC) and X-ray powder diffraction (XRPD) to elucidate the phase behavior of ASDs as demonstrated by a critical discussion of practical real-life examples observed in our research group. The ASDs were manufactured by either applying a solvent-based technique (spray drying), a heat-based technique (hot melt extrusion) or mechanochemical activation (cryo-milling). The encountered limiting factors of XRPD were the lack of sensitivity for small traces of crystallinity, the impossibility to differentiate between distinct amorphous phases and its impossibility to detect nanocrystals in a polymer matrix. In addition, the limiting factors of (m)DSC were defined as the well-described heat-induced sample alteration upon heating, the interfering of residual solvent evaporation with other thermal events and the coinciding of enthalpy recovery with melting events. In all of these cases, the application of a single analytical technique would have led to erroneous conclusions, whilst the combination of (m)DSC and XRPD elucidated the true phases of the ASD.

Experimental investigation of a ground-coupled air conditioning system with desiccant assisted enthalpy recovery during winter mode

In the framework of a research project at Hamburg University of Technology the combination of an open cycle desiccant assisted air conditioning system and a geothermal system is investigated experimentally by means of a test facility during summer and winter. The system is designed to use a large amount of renewable energies and to reduce electricity demand for air conditioning. Thus, the main objectives of this study are the investigation of the overall system performance and evaluation of geothermal assisted heat supply during winter mode for a temperate climate region, based on measured data. As an important comfort aspect, moisture recovery is considered in detail in terms of coupled heat and mass transfer within the enthalpy wheel; related energy demands are limited to auxiliary energies. Different reference systems are defined to evaluate the performance of the investigated system in terms of energy demand required for air treatment, especially air humidification. The geothermal system is an essential subsystem regarding seasonal storage of thermal energy. Thus, the influence of heat supply using a ground-coupled heat pump on soil temperature development is investigated. Besides system evaluation, measured data regarding thermal comfort are presented.

Accelerated Aging of PS Blocks in PS- b-PMMA Diblock Copolymer under Hard Confinement.

This letter presents an accelerated physical aging of polystyrene (PS) blocks in polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers under hard confinement. The three-dimensional hard nanoconfinement was provided by the PMMA component owing to its high elasticity and was formed via self-assembled microphase separation. Aging was observed by measuring enthalpy recovery of the PS blocks in the copolymers for which the degree of polymerization ( N) of PS blocks is fixed, whereas the N of PMMA blocks varies. Our results demonstrate that the aging speed of the PS blocks can increase by a factor of three to that of the neat PS as the N of PMMA blocks increases. Therefore, the hard confinement accelerates physical aging of the PS blocks, i.e., the relatively soft component in the copolymer.

Segmental α-Relaxation for the First Step and Sub-Rouse Modes for the Second Step in Enthalpy Recovery in the Glassy State of Polystyrene

Cangialosi and co-workers found two steps in the enthalpy recovery deep in the glassy state of high and low molecular weight bulk polystyrene (PS). We attribute the first step to the segmental α-relaxation and the second step to the sub-Rouse modes and explain the two-step enthalpy recovery by the dynamic properties of the two processes in bulk PS by the coupling model (CM). The two-step enthalpy recovery was found in nanometer thin films of polystyrene (PS). On decreasing film thickness h, the first step was shifted to much lower temperature than the second step, and the effect is explained by the segmental α-relaxation shifting to lower temperature much more than the sub-Rouse modes, as predicted by the CM. Furthermore, the dependences of the faster and slower processes of the two-step enthalpy relaxation on film thickness are analogues of two effects observed on decreasing h of freestanding polystyrene films. One effect is the observation of two transitions by ellipsometry. We associated the lower transition at Tgl(h) with the segmental α-relaxation and the upper transition at Tgu(h) with the sub-Rouse modes and explained the experimentally observation of Tgl(h) decreasing more rapidly than Tgu(h) on decreasing h also by the CM. The other effect is the rubbery stiffening observed by creep compliance measurements on decreasing h of freestanding films in polymers including PS. This effect was explained by the more rapid decrease of the segmental α-relaxation time τα(T,h) with decreasing h than the sub-Rouse relaxation time τsR(T,h), again using the CM. These two analogues of the two-step enthalpy recovery in bulk and nanometer PS thin films strengthen the common CM explanation of the first step by the segmental α-relaxation and the second step by the sub-Rouse modes in enthalpy recovery.

Thermal performance characterisation of a reverse-flow energy recovery ventilator for a residential building application

The European Union’s 2020 and 2030 sustainable energy policies seek significant reductions in both energy consumption and carbon emissions. These policies demand a greater use of energy efficient technologies and a transition away from fossil fuels. This paper studies one such technology, an indoor climate control system with a reverse-flow enthalpy recovery ventilator, capable of recovering both sensible and latent heat. The thermal performance characteristics are established using an experimental facility and calculation methods defined by European Standard EN 13141-7:2010. This involves measurement of temperature, humidity, pressure and volumetric air flow rates over a range of operating conditions. Total thermal energy recovery rates ranged from 0.63 kW to 2.2 kW, with energy recovery efficiency of 72.8 % to 88.6 %. The recovery efficiency ratio, which reflects the capacity of the indoor climate control system to recover thermal energy relative to its power consumption ranged between 6.87 to 19.97. Due to the unique reverse-flow defrost function, the system demonstrates operation down to -7 °C without frost formation. These results highlight the potential that this system can make towards the EU goals of reducing energy consumption, operating costs and carbon emissions associated with indoor climate control.

Entropic effect implication for change in polymer coils swelling state in the demixing enthalpy recovery of aqueous poly(vinyl methyl ether) solutions

ABSTRACTTime‐dependent demixing enthalpy recovery behavior of aqueous poly(vinyl methyl ether) (PVME) solutions exhibits distinct recovery characteristics in three concentration regions. The absence of recovery behavior below a water concentration of 38.3 wt % indicates that the PVME coil is in a globular state. The typically sigmoidal recovery behavior of demixing enthalpy above 38.3 wt % is ascribed to the reswelling of the collapsed polymer coils induced by the entropic effect. The increase in difference between the upper and lower limits indicates the continued swelling of the PVME coils. Above 65 wt %, a dominant diluting effect can be observed, and a much longer phase separation time is needed to reach the expected lower limit. In contrast, the recovery of demixing enthalpy in a wide range of water concentration (from 38.3 to 90 wt %) exhibits the same feature. The infrared spectroscopy results are in agreement with the above macroscopic differential scanning calorimetry results. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 142–151

Enhancement of the Physical and Chemical Stability of Amorphous Drug–Polymer Mixtures via Cryogenic Comilling

Amorphous dispersions of the Biclotymol antiseptic are obtained in polyvinylpyrrolidone (PVP), for different drug loadings, by comilling at temperatures below the glass transition of both components. Characterization of the dispersions by scanning differential calorimetry and temperature-dependent broadband dielectric spectroscopy shows them to be homogeneous amorphous molecular mixtures. A single glass transition with pronounced enthalpy recovery peak is observed, indicative of the formation of a homogeneous glass state characterized by substantial aging. The polymer has a considerable antiplasticizing effect on the drug, increasing its glass transition temperature (Tg) to well above room temperature. The Tg of the mixture increases linearly with the macromolecular mass fraction. Three (macro)molecular relaxations are identified in the isothermal dielectric spectra of the mixtures. The slowest process corresponds to the cooperative (α relaxation) dynamics of the polymer chains and drug molecules simultan...

Methanol partial oxidation accompanied by heat recirculation in a Swiss-roll reactor

Hydrogen production with high efficiency is a crucial issue for prospective hydrogen economy and carbon emission reduction. Methanol partial oxidation triggered over an h-BN-Pt/Al2O3 catalyst from a cold start in a Swiss-roll reactor with heat recirculation are investigated experimentally. The effects of methanol flow rate (0.5 and 0.6 mL min−1), O2 concentration (21–35 vol%), and O2-to-methanol (O2/M) molar ratio (1.0–3.0) on the performance of methanol partial oxidation are examined. Heat exchange by transferring the excess enthalpy in the product gas to the feed gas is achieved in the reactor where the temperature of the feed gas before entering the catalyst bed can be promoted to around 100 °C. The experimental results indicate that a methanol flow rate of 0.5 mL min−1 leads to more H2 production compared to those obtained with a flow rate of 0.6 mL min−1. In the conducted Swiss-roll reactor, oxygen supply plays an important role in accomplishing the partial oxidation, and the O2/M ratio should be controlled beyond 1.0. By increasing the O2 concentration, the H2 concentration at O2/M = 1.5 increases from 18.5 to 21.8%. However, the H2 yield decreases, resulting from progressively dominant combustion mechanism. At a fixed gas hourly space velocity of 10,000 h−1, the optimal O2/M ratio for H2 production is 2.5, for which the H2 concentration and H2 yield are 23.5% and 1.93 mol (mol methanol)−1, respectively. The highest H2 yield is close to the theoretical result. Overall, methanol partial oxidation along with heat recirculation in the Swiss-roll reactor can efficiently produce H2, and the excess enthalpy recovery in the reactor can improve energy utilization.

Moisture Diffusion Characteristics of a Paper Membrane Used for a Cross Flow Enthalpy Exchanger

An enthalpy exchanger is the key component of an enthalpy recovery ventilator, which is installed in apartments or office buildings to maintain proper indoor air quality. In this study, the performance data of the enthalpy exchanger made of paper membrane were obtained for wide range of temperature and humidity conditions. Using the data, diffusion coefficient correlations were developed through modeling of heat and moisture transfer. The heat transfer effectiveness was relatively constant independent of outdoor temperature condition. On the other hand, the moisture transfer effectiveness was highly dependent on outdoor humidity and temperature. In general, moisture transfer effectiveness increased as relative humidity increased. The moisture transfer effectiveness also increased as temperature difference between indoor and outdoor increased. Moisture diffusion coefficient correlations were developed from the data, which predicted the moisture transfer effectiveness was within [Formula: see text]%.