Enthalpic Relaxation Research Articles

Multiple Glass Transitions of Microphase Separed Binary Liquids Confined in MCM-41

The confinement of fluids in channels of nanometer size presents unprecedented opportunities to reveal emergent physicochemical properties that have no equivalent in the corresponding bulk system. We present an experimental study of fully miscible binary low-molecular weight liquids confined in nanopores. Under these conditions, the mixtures form a microphase-separated state with two regions of different compositions forming the core and the shell of a concentric tubular nanostructure. We combine neutron diffraction and DSC measurements to assess the thermal behavior of this unusual phase. These results show that crystallization is suppressed, leading to glassy behaviors characterized by anomalies in the density and the heat capacity. We reveal the coexistence of two different glass transitions, providing the direct proof for two different enthalpic relaxation times. It leads to the conclusion that the subcomponents of the microphase-separated mixture each obey distinct dynamical behaviors. This phenomeno...

Spectroscopic evaluation of a freeze-dried vaccine during an accelerated stability study

This research evaluates a freeze-dried live, attenuated virus vaccine during an accelerated stability study using Near Infrared (NIR) and Fourier Transform Infrared (FTIR) spectroscopy in addition to the traditional quality tests (i.e., potency assay and residual moisture analysis) and Modulated Differential Scanning Calorimetry (MDSC). Therefore, freeze-dried live, attenuated virus vaccines were stored during four weeks at 4°C (i.e., recommended storage condition) and at 37°C (i.e., accelerated storage condition) and weekly analyzed using these techniques. The potency assay showed that the virus titer decreased in two phases when the samples were stored at 37°C. The highest titer loss occurred during the first week storage at 37°C after which the degradation rate decreased. Both the residual moisture content and the relaxation enthalpy also increased according to this two-phase pattern during storage at 37°C. In order to evaluate the virus and its interaction with the amorphous stabilizer in the formulation (trehalose), the NIR spectra were analyzed via principal component analysis (PCA) using the amide A/II band (5029–4690cm−1). The FTIR spectra were also analyzed via PCA using the amide III spectral range (1350–1200cm−1). Analysis of the amide A/II band in the NIR spectra revealed that the titer decrease during storage was probably linked to a change of the hydrogen bonds (i.e., interaction) between the virus proteins and the amorphous trehalose. Analyzing the amide III band (FTIR spectra) showed that the virus destabilization was coupled to a decrease of the coated proteins β turn and an increase of α helix. During storage at 4°C, the titer remained constant, no enthalpic relaxation was observed and neither the Amide A/II band (NIR spectra) nor the Amide III band (FTIR spectra) varied.

Thermal behaviour below and inside the glass transition region of a submicron P3HT layer studied by fast scanning chip calorimetry

Fast scanning chip calorimetry was utilized to investigate the thermal behaviour of a submicron layer of P3HT, an important polymer for organic electronics, when isothermally treated below and inside the glass transition region. A similar study was performed on the conventional polyester PBT, for comparison. It was observed that a heating and cooling rate of 30 000 K s−1 was sufficient to avoid all non-isothermal effects for P3HT, while the faster reordering kinetics of PBT still allowed reordering upon heating at these scanning rates. For both materials, treatments clearly below the glass transition region yield enthalpic relaxation of the glass phase. When treatments are performed inside the glass transition region, however, a combined process of both enthalpic relaxation and the melting of small (nano-sized), imperfect crystals is observed. Due to the influence of the glass phase on the crystallization process, it seems that large, more stable crystals can not be formed. Significant isothermal perfectioning can be observed for the imperfect crystals in P3HT, while very fast non-isothermal reordering upon heating is seen for PBT. An approach based on a 2-step nucleation mechanism of crystallization may be used to interpret the observations of polymer crystallization around and in the glass transition region.

Isothermal Crystallization of PC61BM in Thin Layers Far below the Glass Transition Temperature

An isothermal study was performed on 1.25 μm thin layers of PC61BM, a benchmark acceptor for organic photovoltaics. Using fast scanning chip calorimetry allowed for a scanning rate of 30 000 K·s–1, ensuring that all non-isothermal effects were avoided. The effect of isothermal treatments above and far below the glass transition was investigated. It was proven that PC61BM can crystallize below its glass transition, a behavior seen before for several organic glasses. Isothermal treatments as far as ca. 100 K below the glass transition lead to an initial enthalpic relaxation process around the glass transition, which over time gives rise to crystals melting around 300 °C. These observations show a remarkable similarity with the two-step nucleation mechanism documented in the literature. A standard crystallization process was observed for treatments above the glass transition. By combining chip calorimetry pretreatments with atomic force microscopy, different crystal morphologies were visualized for these two...

Investigation on Temperature-Dependent Electrical Conductivity of Carbon Nanotube/Epoxy Composites for Sustainable Energy Applications

Composites with carbon nanotubes are becoming increasingly used in energy storage and electronic devices, due to incorporated excellent properties from carbon nanotubes and polymers. Although their properties make them more attractive than conventional smart materials, their electrical properties have been found to be temperature-dependent which is important to consider for the design of devices. To study the effects of temperature in electrically conductive multi-wall carbon nanotube/epoxy composites, thin films were prepared and the effect of temperature on the resistivity, thermal properties and Raman spectral characteristics of the composite films was evaluated. Resistivity-temperature profiles showed three distinct regions in as-cured samples and only two regions in samples whose thermal histories had been erased. In the vicinity of the glass transition temperature, the as-cured composites exhibited pronounced resistivity and enthalpic relaxation peaks, which both disappeared after erasing the composites' thermal histories by temperature cycling. Combined DSC, Raman spectroscopy, and resistivity-temperature analyses indicated that this phenomenon can be attributed to the physical aging of the epoxy matrix and that, in the region of the observed thermal history-dependent resistivity peaks, structural rearrangement of the conductive carbon nanotube network occurs through a volume expansion/relaxation process. These results have led to an overall greater understanding of the temperature-dependent behaviour of conductive carbon nanotube/epoxy composites, including the positive temperature coefficient effect.

Evolution of structural (α) relaxation time anomalies in GexSe1−x chalcogenide glasses

We examine enthalpy relaxation across the chalcogenide glass series GexSe1−x, prepared over close-by compositions using conventional melt-quenching technique. We estimate the timescale τ(Tg) characterizing enthalpic relaxation near the kinetic glass transition temperature, using the non-reversing heat-flow data obtained from modulated differential scanning calorimetry measurements over a wide range of compositions (2.1 ≤ r ≤ 2.8, r = 2x + 2). Anomalies in the enthalpy-relaxation characteristic-times τg(r) are identified with three rigidity-transitions encountered in successive Ge-doping of polymeric selenium chains.

Formation of a Rigid Amorphous Fraction in Poly(3-(2'-ethyl)hexylthiophene).

Herein, we detail the formation of a rigid amorphous fraction in poly(3-(2'-ethyl)hexylthiophene) (P3EHT) at high relative crystallinity, yielding a more complete picture of the solid-state structure. In the differential scanning calorimetry (DSC) heating scans of isothermally crystallized P3EHT a distinct endothermic peak appears slightly above the crystallization temperature. This previously undescribed endothermic feature of P3EHT's thermal behavior is observed consistently ∼20 °C above the crystallization temperature-shifting to higher temperatures with increasing crystallization temperature-and increases in magnitude with both time and crystallization temperature. Here, we determine the origins of this endothermic peak with DSC and temperature-modulated DSC (TMDSC). TMDSC reveals that the annealing peak observed in the total heat flow (THF)-heat flow equivalent to that of conventional DSC-is a consequence of an enthalpic relaxation observable as an endothermic peak in the nonreversible heat flow (NHF) and a glass transition evident as a step increase in the reversible heat flow (RHF). In conjunction with conventional DSC observations, these results indicate that the observed annealing peak is a consequence of the formation of distinct amorphous regions-a mobile amorphous fraction (MAF) and a rigid amorphous fraction (RAF)-during the isothermal crystallization process and not the melting of a distinct crystallite population or melt recrystallization.

Supramolecular evolution over an initial period of biodegradation of lactide and caprolactone based medical (co)polyesters

Abstract Phase-structural changes and enthalpic relaxation behavior of poly( d , l -lactide) (PDLLA), poly( l -lactide) (PLLA) and poly( l -lactide-co-e-caprolactone) (PLCL) copolymers with a lactide molar content ∼70% and having different randomness character (PLCL r 1 : R = 0.69; PLCL r 2 : R = 0.92) were evaluated over 7 days in phosphate buffered saline (PBS) at 37 °C. Results obtained by Temperature Modulated Differential Scanning Calorimetry (TMDSC) and Dynamic Mechanical Analysis (DMA) showed an increase in the value of enthalpic relaxation ( δ ) and the narrowing of the tan δ peak, respectively, indicating the reduction in molecular mobility and a more uniform distribution of the entropic states as the aging time increased. The results obtained for PLCLs were clearly affected by chain microstructural magnitudes, l LA (PLCL r 1 = 4.35 and PLCL r 2 = 3.45). Both showed a crystallization process accompanied by amorphous phase separation during the early stage in PBS. DSC and DMA results also revealed faster structural changes as a result of new supramolecular arrangements as well as a higher tendency to crystallize for PLCL r 1 regarding PLCL r 2 .

Analysis of Structural Rearrangements of Poly(lactic acid) in the Presence of Water

The effects of hydration on two types of structural rearrangements, physical aging and crystallization, of poly(lactic acid) (PLA) have been characterized. Although the water absorbed is extremely small, on the order of 0.5% by weight, we found a significant increase in both the magnitude and kinetics of enthalpic relaxation near the glass transition temperature (Tg). The kinetics of aging is at least 2 times the speed as compared to dry samples. In addition, we also found that the crystallization kinetics can be increased significantly when PLA is hydrated. The initial rate of crystallization at 80 °C is extremely fast at 7 J/g/min for the hydrated state as compared to 2 J/g/min under dry conditions. However, the ultimate degree of crystallinity achieved is not different for the two types of samples. Both of these structural rearrangements in the hydrated state can be explained by an increase in segmental mobility. The fact that water has such a strong effect on PLA structure is attributed to the strong intermolecular interactions present and their changes in the hydrated state. Spectroscopic features associated with bound water or free water were found for PLA with different hydration levels. The increase in the segmental mobility was directly correlated to the presence of free liquid water disrupting the intermolecular interactions in PLA.

Effect of Spray Drying Conditions on the Physicochemical Properties and Enthalpy Relaxation of α-Lactose

The purpose of this work was to investigate the effect of different spray drying conditions on the physicochemical properties of dried lactose. Results indicated that the feed concentration and inlet temperature, as well as storage condition, influenced the thermal properties of the dried material, which were evaluated by the degree of molecular relaxation, presumably associated to different levels of molecular mobility in the sample. It was evident that higher drying temperatures decreased molecular mobility of the dried material while increasing its glass transition and crystallization temperatures. This is most likely due to the existence of states of lower mobility within the material as those with higher mobility were relaxed. The change in heat capacity at the glass transition temperature, , and the reduced crystallization temperature, , were used to compare the crystallization tendencies of the spray dried powders. It was observed that a decrease in and an increase in were associated with higher crystallization temperatures. On the other hand, the enthalpic relaxation did not change significantly with storage conditions. The findings illustrated that the initial enthalpy relaxation, which occurred during spray drying, was related to the crystallization tendencies and storage conditions.

Glass transitions and physical aging of cassava starch – Corn oil blends

Glass transition temperatures and physical aging of amorphous cassava starch and their blends with corn oil were assessed by differential scanning calorimetry (DSC). Two enthalpic relaxation endotherms, well separated in temperature values, were exhibited by neat amorphous cassava starch with 10.6% moisture content, evidencing two amorphous regions within the starch with different degrees of mobility. The phase segregation of these two amorphous regions was favored by added corn oil at low moisture contents during storage. The presence of amylose–lipid complexes in this matrix, may also affect the molecular dynamics of these two amorphous regions at low moisture contents. Increasing moisture content, leads to a homogeneous amorphous phase, with an aging process characterized by a single enthalpic relaxation peak. In all cases, after deleting the thermal history of the samples only one glass transition temperature was detected (during DSC second heating runs) indicating that a single homogeneous amorphous phase was attained after erasing the effects of physical aging. Trends of the enthalpic relaxation parameters were also different at the two moisture contents considered in this work.

Thermal characterization of poly-l-lactide by dielectric analysis and modulated DSC

Dielectric analysis (DEA) is a very sensitive technique, which allows for detection of small structural changes at the low scale. An advantage of DEA, with respect to other modulated techniques, is the possibility of using a wider frequency range. Molecular relaxations of the order of only a few nanometers are not observed by any other thermoanalytic method. Nevertheless, these small relaxations involve dipole changes that can be observed by DEA. Thus, this technique is used here, in combination with temperature-modulated differential scanning calorimetry (TMDSC) to obtain insightful information about the thermal transitions of poly-l-lactic acid (PLLA), one of the stereo-isomers of polylactide. Its complex thermal behavior is the subject of ongoing debate, with several overlapping crystallization and melting processes. The combined use of TMDSC and DEA provides a better insight of three important transitions of this polymer: the alpha relaxation, the enthalpic relaxation, and the cold crystallization. The dependences of the enthalpy relaxation on the dynamic glass transition relaxation and on the glass transition as a thermal event are evaluated. On the other hand, it will be shown how the cold crystallization can be identified by TMDSC, and DEA helps us understand the effect of crystallization on the dipole movements. The shape of the dielectric permittivity curve at low frequencies is compared to that of the reversing heat capacity to check whether both signals are sensitive or not to the same events. It is also verified how the experimental results of alpha relaxation of PLLA follow an Arrhenius or a Vogel trend.

Enthalpic Relaxation of Vital and Protease-Treated Wheat Gluten

Commercially available vital wheat gluten was purified with α-amylase and treated with protease in a one-step process. The supernatant of the protease-treated gluten was freeze dried, quench cooled, and aged for 2 weeks and analyzed using differential scanning calorimetry, sodium dodecyl sulfate poly-acrylamide gel electrophoresis, reverse phase high performance liquid chromatography, size exclusion high performance liquid chromatography, capillary zone electrophoresis, and dynamic mechanical analysis. The differential scanning calorimetry profile of the aged (un-purified or un-modified) gluten sample exhibited enthalpic relaxation beneath its glass transition (Tg) indicating molecular relaxation. Samples treated with 0.006 g protease/g gluten showed Tg at 49.11°C with ΔCp = 0.12 (J/°C g). Higher protease levels and longer aging time caused high Tg temperature, ΔCp, and ΔH of enthalpic relaxation. The size exclusion high performance liquid chromatography profile of the protease-treated gluten clearly showed differences in molecular size between the supernatant and the precipitate, while reverse phase high performance liquid chromatography profiles signified more hydrophilic gluten molecules than the control. The small difference in gluten molecular size after protease treatments (0.024 g protease versus 0.032 g) appeared to have a significant effect on the enthalpic relaxation, where the two treatments showed different degrees of enthalpic relation. The capillary zone electrophoresis data confirmed the insignificant difference between 0.024 and 0.032 g protease/g gluten regarding the size to charge ratio. The molecular interactions for protease-treated gluten were much weaker than vital gluten, as indicated by much weaker network and sensitivity to temperature, especially above Tg. The weaker network was evidenced by higher G″ versus G′, which explained the fluid like behavior of the protease-treated gluten.

Structural relaxation in Se-rich As–Se glasses

Enthalpic structural relaxation in the AsySe(100-y) glassy system was studied by means of differential scanning calorimetry (DSC). Glasses in the compositional range 0≤y≤15 were examined. Based on the curve-fitting results, the description of relaxation behavior was obtained in terms of the Tool–Narayanaswamy–Moynihan model. These results were further confirmed by a novel simulation-comparative non-fitting method. Evolution of TNM parameters with changing arsenic content was interpreted in terms of joint movements of occasionally linked Se polymeric chains continuously transiting towards rather independent positional changes within inter-linked segments or clusters. The interpretation represents a novel approach associating a phenomenological description with factual changes in the molecular structure of the material.

Enthalpy relaxation in Ge–Se glassy system

Differential scanning calorimetry measurements were used to study enthalpic structural relaxation in the GeySe(100 − y) glassy system. The examined compositional range was 0 ≤ y ≤ 15. Based on the results of both, fitting and non-fitting methods, the description of relaxation behavior was obtained in terms of the Tool–Narayanaswamy–Moynihan (TNM) model. Evolution of TNM parameters with increasing germanium content was interpreted within the conception of movements of occasionally linked Se polymeric chains continuously transiting toward rather independent positional changes within heavily inter-linked segments or clusters. As such, the interpretation represents a novel approach associating a phenomenological description with factual changes in molecular structure of the material. In addition, new non-fitting method is introduced, based on the examination of the overall shape of relaxation peak and its comparison to the simulated data.

A Study on Physical Aging of Semicrystalline Polyethylene Terephthalate below the Glass Transition Point

Physical aging of semicrystalline polyethylene terephthalate was studied using differential scanning calorimetry(DSC). PET samples with crystallinity content of 0.28 were aged at two different temperatures, 25 and 45°C. Thesamples were stored for several days and periodically tested using DSC method. The glass transition temperature forthe samples aged at 25°C was about 73-74°C, and the position and intensity of endothermic peaks wereapproximately constant. Higher glass transition of the samples aged at 45°C, 73-86°C, was attributed to the enthalpyrelaxation process of amorphous regions of semicrystalline PET. For the samples aged at 45°C, the endothermicpeaks shifted to higher temperatures with increasing aging time. The position of the endothermic peaks determined bythe temperature of the maximum, Tmax, tended to increase with aging time for samples aged at 45°C, and theintensity of the peaks continuously increased with time; however, the results showed that the aging of PET samples at45°C even after 120 days continued the enthalpic relaxation of semicrystalline PET and that the process could bestudied by DSC method. The results also showed that the aging process could affect the final degree of crystallinity ofc-PET samples and the samples stored at 45°C showed higher degree of crystallinity than the samples aged at 25°C.

Enthalpic relaxation in Ge2Sb2Se5 glass

Differential scanning calorimetry measurements were used to study enthalpic structural relaxation of Ge2Sb2Se5 glass. The Tool-Narayanaswamy-Moynihan (TNM) model was applied to describe the relaxation behavior. The curve-fitting results showed a significant dependence on experimental conditions, true values of the TNM parameters needed to be confirmed by the alternative non-fitting methods – apparent activation energy was confirmed by evaluations from classic and intrinsic cycles, the non-linearity parameter was determined on the basis of peak-shift method. Discussion on the origin of the trend in curve-fitting results was conducted in terms of influence of either the nonlinear Cp-T dependence or anomalous relaxation behavior of the material itself. For the latter case a suggestion regarding the proceeding molecular mechanisms was made.

Structure evolution of α′‐phase poly(lactic acid)

AbstractPoly(lactic acid) films consisting of α′‐forms were prepared and uniaxially drawn. The effects of the draw rate at temperatures above the glass transition temperature on chain conformation, degree of crystallinity, and crystalline phase transformation were investigated by a combination of vibrational spectroscopy (infrared and Raman), differential scanning calorimetry, and wide‐angle X‐ray diffraction (WAXD). It was established that the α′‐crystal's phase of poly(lactic acid) films does not transform into either an α or β crystals on uniaxial drawing at a fixed draw ratio of 4. However, the degree of crystallinity was significantly increased on deformation. The structural change as a function of deformation also promotes an increase in the strain‐induced enthalpic relaxation endothermic peak appearing near the glass transition region. While the overall changes in physical properties can be attributed to the changes in the degree of crystallinity as a function of strain rate, polarized Raman spectra, and WAXD clearly illustrated changes and the differences in the amorphous and crystalline orientation as a function of processing conditions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1446–1454, 2011

Structural Relaxation of Salmon Gelatin Films in the Glassy State

The unique property of gelatin to form networks and induce plasticity and elasticity is considered beneficial in the preparation of biopolymer-based packaging materials. However, its use in food products has been limited by some religious groups (Gudmundsson 2002). Gelatins from marine sources have emerged as an alternative, although they are different from mammalian collagen in terms of their biochemical constituents and therefore in their functional properties. It has been shown that cold-water fish collagen can have a lower content of amino acids (~16–18 %) (Gilsenan and Ross-Murphy 2000). Joly-Duhamel et al. (2002) established a positive correlation of the concentration of amino acids, proline (Pro) and hydroxyproline (Hyp), with the melting temperature of native collagen (helix to coil) and gelatin molecular weight with renaturation temperature (coil to helix), suggesting an important effect of biochemical composition on the structure stability of the gel network. An aspect that has not been explored in detail for marine gelatin is the structural stability of these systems at temperatures below their glass transition temperature (T g ). Early studies in glassy carbohydrates by Noel et al. (1999) described the ageing kinetics of maltose in the glassy state in terms of an overshoot in heat capacity on consecutive heating runs using a differential scanning calorimeter (DSC). Badii et al. (2005) evaluated the kinetics of enthalpic relaxation of bovine gelatin state as a function of the difference between ageing temperature and T g . In their later work, the same authors quantified the enthalpic relaxation of the same model system by DSC, correlating enthalpic values with an increase in the elastic modulus (E) obtained by mechanical spectroscopy. Work by Lourdin et al. (2002) described the mechanical relaxation of amorphous potato starch in the glassy state, obtaining characteristic relaxation parameters by the application of the Kohlrausch–Williams–Watts (KWW) model (Eq. 1). Anderssen et al. (2004) discussed the KWW equation in terms of a spectrum of relaxation times describing the ageing of a polymer. The KWW is an equation that can quantitatively describe the kinetics of the relaxation process toward an absolute relaxed state (Anderssen et al. 2004): $$ \phi (t)= \exp \left[-{\left(\frac{t}{\tau_0}\right)}^{\beta}\right] $$ where φ is the relaxation function, t is time, β (0 < β ≤ 1) is the width of the relaxation time distribution spectrum, and τ0 is the characteristic relaxation time, being dependent on temperature and material structure.

Effects of corn oil on glass transition temperatures of cassava starch

Glass transition temperatures of cassava starch–corn oil blends were determined by means of differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA), in a moisture content range of 4–35% (dry basis, d.b.). The samples were equilibrated at ambient temperature (25°C) during 4 weeks. A sub-Tg endothermic peak, occurring between 45 and 60°C, appeared in samples with relatively low moisture contents (less than: 14.4% (d.b.), 10.1% and 8.5% for added corn oil levels of 1.1, 4.5 and 6.6%, respectively). In correspondence with DSC measurements, secondary relaxation temperatures (Tβ) at around 55°C, measured by DMTA were also present at low moisture contents (less than: 18% (d.b.), 16% and 15% for added corn oil levels of 1.1, 4.5 and 6.6%, respectively). At intermediate moisture contents enthalpic relaxation events associated to ageing processes were evidenced by DSC first heating scans, while at higher moisture contents only the glass transition was exhibited by the samples. In all cases, increasing levels of added corn oil decreased the moisture content at which the samples exhibited the above described thermal events. The effect of water on glass transition temperatures (as determined during a DSC second heating scan and DMTA results) was well described by the Gordon–Taylor equation. It was found that cassava starch was better plasticized by water when the levels of added corn oil were decreased. The added corn oil was also found to be a plasticizer for cassava starch probably due to hydrophobic–hydrophilic type interactions with starch.