Thermomechanical properties of glassy cereal foods

The main objective of this paper is to discuss the relationship between physical state, fracture mechanism, and texture for low moisture cereal-based foods. Experiments were also carried out to get a better understanding of the role of water. At room temperature, extruded bread and white bread (previously) dehydrated, then rehydrated in atmospheres with controlled humidities) exhibited a brittle behavior up to around 9% moisture. At 13.7% moisture, they were ductile. A significant loss in the crispness of extruded bread was observed between 8.5 and 10% moisture. The glass transition temperature (T _g) was measured, using dynamic mechanical thermal analysis (DMTA), for samples with up to 40% moisture. The resultingT _g curve showed that the important changes in fracture mechanisms and crispness occurred while the samples were still in the glassy state. The viscoelastic behavior of both extruded and white breads suggested that a secondary relaxation occurred around 10?C. Another event was observed around 70?C for low moisture sample, using DMTA. This event was attributed to disruption of low energy interactions.     

Moisture absorption and diffusion in an underfill encapsulant at T > T g and T < T g

Moisture absorption and diffusion behavior of an underfill material used for electronic packaging with a glass transition temperature (T _g) slightly above room temperature have been investigated by the sorption thermogravimetric analysis technique. It has been found that moisture diffusion in this material follows the Fick’s diffusion model, and moisture absorption–desorption is reversible and repeatable. Based on moisture-induced mass gain versus time curve, the diffusion constant can be determined. It was found that below T _g, moisture diffusivity exhibits an Arrhenius temperature dependence, which changes to a different Arrhenius temperature dependence as the temperature increases to T > T _g. The change in diffusivity from T < T _g to T > T _g is accompanied by a significant decrease in the energy barrier for moisture diffusion. Results shed light on the change in moisture diffusion in polymer-based materials in the glassy and the rubbery state.     

Glass transitions in starch,gluten and bread as measured

Dielectric Spectroscopy (DS) and Thermomechanical Analysis (TMA) were used to identity the glass transition temperature (T _g) of native wheat starch, vital wheat gluten and a commercial bread, in response to changes in moisture content. An open-ended coaxial probe technique was used to measure the permittivity or dielectric constant (?′) and the loss factor (?″) as functions of moisture, for 2.45 GHz frequency, at constant density and temperature. Plots of ?′ and ?″ as functions of moisture content showed dramatic changes in mobility-based dielectric properties, which occur upon transition from the glassy solid to the rubbery liquid state. The modified TMA method can measure the change in viscoelastic properties aroundT _g. This study further confirms that synthetic polymer science principles can be applied to food systems.     

Comparative curing kinetics of 1,4-bis (4-diaminobenzene-1-oxygen) n-butane and 4,4′-bis-(diaminodiphenyl) methane with tetraglycidyl methylene dianiline systems

Aromatic amine curing agent with flexible unit in backbone, 1,4-bis (4-diaminobenzene-1-oxygen) n-butane (DDBE), was synthesized, and the structure was confirmed by FT-IR and ¹H NMR. The curing kinetics of tetraglycidyl methylene dianiline (TGDDM, or AG80) using DDBE and 4,4′-bis-(diaminodiphenyl) methane (DDM) as curing agents, respectively, were comparatively studied by non-isothermal DSC with a model-fitting Málek approach and a model-free advanced isoconversional method of Vyazovkin. The dynamic mechanical properties and thermal stabilities of the cured materials were investigated by DMTA and TG, respectively. The results showed that the activation energy of AG80/DDBE system was slightly higher than that of AG80/DDM system. ?esták-Berggren model can generally simulate well the reaction rates of these two systems. DMTA measurements showed that the storage modulus of cured AG80/DDBE is similar to that of cured AG80/DDM at the temperature below glass transition temperature (T _g) and lower than that of cured AG80/DDM at the temperature above glass transition temperature, while T _g of cured AG80/DDBE is lower than that of cured AG80/DDM. TG showed that the thermal stabilities of these two cured systems are similar.     

Dynamic mechanical analysis of multiblock (segmental) polyesterimides

Thermal, deformation-strength, and dynamic mechanical properties of films of segmental polyesterimides prepared from pyromellitic anhydride, aromatic diamines, and poly(butylene adipate) (M _n = 1000) with hydroxy terminal groups were studied. The glass transition points T _g of the samples obtained are below 0°C. The dynamic elastic modulus curves at temperatures higher than T _g are characterized by a portion in which the modulus only weakly depends on temperature.     

Glass transition, fragility, and structural features of amorphous nickel–tellurate–vanadate samples

The experimental FTIR spectra and DSC curves of the ternary 40TeO₂–(60?x)V₂O₅–xNiO glasses with 0 ≤ x ≤ 30 (in mol%) have been investigated. The glass transition properties that have been measured and reported in this paper, include the glass transition temperature (T _g), glass transition width (ΔT _g), heat capacity change at glass transition (ΔC _P) and Fragility (F). Thermal stability, fragility, and glass-forming tendency of these glasses have been estimated. Also, Poisson’s ratio (μ) and IR spectra of the presented systems have been investigated, to determine relationship between chemical composition and the thermal stability or to interpret the structure of glass. In addition, Makishima and Makenzie’s theory was applied for determination of Young’s modulus, bulk modulus, and shear modulus, indicating a strong relation between elastic properties and structure of glass. In general, results of this work show that glasses with x = 0 and 30 have the highest shear and young’s modulus which make them as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 30 has higher handling temperature and super resistance against thermal shock.     

Dynamic mechanical analysis

Using dynamic mechanical analysis (DMA) we have studied thermal degradation for a system containing a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcylohexane (1,3-BAC). The changes of dynamic mechanical properties during thermal degradation indicated a shift of the glass transition temperature (T _g) to higher temperatures and a decrease in the peak value of the dynamic loss factor (tan δ) with an increasing of aging time. The value of dynamic storage modulus (E′) at the rubbery state showed an increase with aging time, whiteE′ at the glassy state only underwent a moderate change with increased thermal degradation. From these results it can be argued that thermal degradation during the stage prior to the onset of the severe degradation involves structural changes in the epoxy system, as further crosslinking and loss of dangling chains in the crosslinked network.     

Thermal characterizations of multifunctional epoxy/anhydride systems used for pultruded composite ropes

The viscoelastic characterization and thermal stability property of some multifunctional epoxy/anhydride systems cured at different schedules were investigated by dynamic mechanical thermal analysis (DMTA) in single cantilever mode at fixed frequency, and by non-isothermal thermogravimetric (TG) analysis, respectively. According to the DMTA results, three obviously different glass transition temperatures (T _g), were observed, among which TGDDM/MHHPA system exhibits the largest T _g. While from the TG curves, the results of the mass loss and thermal stability showed that, after cured for a prolonged duration, the TGDDM/MHHPA system possessed the most excellent performance in heat resistance.     

Thermomechanical investigation of a thick film of aniline-formaldehyde copolymer and poly(methyl methacrylate)

A thick film of aniline-formaldehyde copolymer and PMMA is synthesized via dispersion of aniline-formaldehyde copolymer powder as filler particles in PMMA with two different concentrations. Variation of the complex elastic modulus and mechanical loss factor (tanδ) with temperature is studied. It is observed that the complex elastic modulus decreases with temperature owing to thermal expansion of films. On the other hand, tanδ increases up to a characteristic temperature beyond which it shows a decreasing trend toward melting. Transition temperature T _g of sample S₁ (pure PMMA) is found to be 80°C. In sample S₂ (1 wt % aniline formaldehyde copolymer), the peak of tanδ at a lower temperature (66°C) corresponds to glass transition temperature T _g of the PMMA matrix, while the peak of tanδ at a higher temperature (107.8°C) corresponds to T _g of a polymer chain restricted by filler particles of aniline-formaldehyde copolymer. A further increase (10 wt % aniline-formaldehyde copolymer) in the concentration of filler particles of aniline-formaldehyde copolymer results in a more compact structure and a shift of T _g to a higher temperature, 122.2°C. This shift in the glass transition temperature of thick films of aniline-formaldehyde copolymer and PMMA is dependent upon the concentration of filler particles in the sample.     

Measurement of size‐dependent glass transition temperature in electrospun polymer fibers using AFM nanomechanical testing

The glass transition temperature (T_g) of individual electrospun polymer polyvinyl alcohol fibers of varying diameter was measured using atomic force microscopy (AFM) based nanomechanical thermal analysis. Indentation and bending of individual electrospun fibers using AFM allowed the calculation of the elastic modulus of the polyvinyl alcohol (PVA) fibers across a range of different temperatures. The elastic modulus of electrospun PVA fibers was observed to decrease significantly when passing through T_g, which allowed accurate determination of T_g. The T_g of electrospun PVA fibers was shown to decrease for smaller fiber diameters especially for fiber diameters below 250 nm. This size‐dependent glass transition behavior of electrospun PVA fibers is indicated as being due to polymer chain confinement. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Effect of the degree of cure on the viscoelastic properties of vinyl ester resins

Dynamic mechanical thermal analysis (DMTA) was performed on model vinyl ester resin (VER) samples prepared from bisGMA and styrene, partially cured using either benzoin ethyl ether (BEE) as a conventional photoinitiator or p-xylylene bis-(N,N-diethyldithiocarbamate) (XDT) as a photoiniferter, and on fully cured samples using benzoyl peroxide as a thermal initiator. Thermogravimetric studies showed that minimal styrene monomer was lost during the DMTA experiments. The DMTA experiments for the samples partially cured using BEE revealed two plateaus in the storage modulus and two peaks in the tan δ curves, attributed to trapped radicals gaining mobility and continuing polymerization after the samples were heated beyond their initial T_g. Only a single transition was observed for samples cured using XDT because this system did not contain trapped radicals. Performing photopolymerizations with XDT thus allowed us to probe the effect of cure conditions on the glass transition temperature, rubber modulus and relaxational behaviour of partially cured resins. The breadth of the glass transition was characterized in the temperature and frequency domains (obtained by time-temperature superposition). Partially cured bisGMA/styrene blends showed a narrowing of the transition in the frequency domain with increased cure but the bisGMA system showed no variation. When partially and fully cured bisGMA/styrene blends were studied, the breadths of the frequency domains showed opposite trends as the styrene concentration was varied. Thus, the breadth of the transition could not be interpreted as a simple measure of the network heterogeneity but is believed to be related to the distribution of mobilities in the network structure which determine the distribution of relaxation times. Time-temperature superposition yielded shift factors which, when fitted to the WLF equation, indicated that the free volume at the glass transition temperature (T_g) and the free volume expansion coefficient depended on both crosslink density and also the ability of the monomer units to pack into the network.     

Uniaxial tensile tests and dynamic mechanical analysis of satin weave reinforced epoxy shape memory polymer composite

The utilization of epoxy shape memory polymer composite (SMPCs) as engineering materials for deployable structures has attracted considerable attention in recent decades due to high strength and satisfactory stiffness in comparison with shape memory polymers (SMPs). Knowledge of static and dynamic mechanical properties is essential for analyzing structural behavior and recovery properties, especially for new epoxy SMPCs. In this paper, a new weave reinforced epoxy shape memory polymer composite was prepared with satin weave technique and resin transfer molding technique. Uniaxial tensile tests and dynamic mechanical analysis were carried out to obtain basic mechanical properties and glass transition temperatures, respectively.The tensile strength and breaking elongation of warp specimens were comparable with those of weft specimens. The increment of elastic modulus and hysteresis loop areas became smaller with loading cycles, meaning that cyclic tests could obtain approximate stable mechanical properties. For dynamic mechanical properties, glass transition temperature (T_g) obtained from storage modulus curves was lower than that determined from tan delta curves and T_gs in the warp and weft directions were similar (29.4 °C vs 29.7 °C). Moreover, the storage modulus in response to T_g was two orders of magnitude less than that with respect to low temperature, which demonstrated the easy processibility of epoxy SMPCs near glass transition temperature. In general, this study could provide useful observations and basic mechanical properties of new epoxy SMPCs.     

Water sorption and glass transition of freeze-dried camu-camu (myrciaria dubia (H.B.K.) Mc Vaugh) pulp

Differential scanning calorimetry (DSC) was used to determine phase transitions of freeze-dried camu-camu pulp in a wide range of moisture content. Samples were equilibrated at 25°C over saturated salt solutions in order to obtain water activities (a_w) between 0.11–0.90. Samples with a_w>0.90 were obtained by direct water addition. At the low and intermediate moisture content range, Gordon–Taylor model was able to predict the plasticizing effect of water. In samples, with a_w>0.90, the glass transition curve exhibited a discontinuity and T^’_g was practically constant (–58.8°C), representing the glass transition temperature of the maximally concentrated phase(T_g ).     

Effect of moisture on the bulk properties of salted poly(caproamide)

The glass-transition temperature (T_g) and the elastic modulus of nylon-6 and its mixtures with 4% w/w LiCl and with 2% LiCl+3% LiBr were investigated. In one set of experiments care was taken to avoid absorption of moisture during the measurements. In another set of experiments, samples were exposed to atmospheric humidity for different lengths of time. Both unoriented and oriented specimens were investigated. The presence of 4% LiCl almost doubles the amount of moisture absorbed at equilibrium by nylon-6. The T_g was found to be greatly affected by the moisture content and by salt type. When strict precautions to exclude moisture are taken, the addition of 4% LiCl raises the T_g of nylon-6 by about 25°C. The elastic modulus of oriented samples is increased by the addition of salts provided moisture is completely absent. This beneficial effect is, however, completely eliminated when samples are allowed to equilibrate with atmospheric moisture.     

Kinetics and network structure of thermally cured vinyl ester resins

Bisphenol-A diglycidyl ether dimethacrylate was blended with styrene at varying concentrations and this model vinyl ester resin (VER) was compared with two commercial VERs. The VERs were characterized using gravimetry, FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetry (DSC) and DMTA. NMR spectroscopy differentiated between a novolac epoxy-based multimethacrylate oligomer and the two bisphenol-A epoxy-based dimethacrylate oligomers. Reaction kinetics were studied using scanning and isothermal DSC and isothermal FTIR spectroscopy using benzoyl peroxide as the thermal initiator. The presence of oxygen was found to inhibit significantly the polymerization. Increased initiator concentration raised the rate of isothermal polymerization, but did not affect the final conversion while increased styrene concentration reduced the polymerization rate constant and increased the total conversion. This was interpreted in terms of the variations in the termination rate and the stability of the styryl radical on the cure rate and the effect of vitrification on the extent of cure. From measurements of the dynamic mechanical properties as a function of temperature, the breadth of the glass transition tan δ curve and the magnitude of the rubbery modulus was found to increase while the tan δ maximum decreased with increased crosslink density. The T_g, as measured by DSC, and the temperature of the tan δ maximum, as measured by DMTA, were not significantly affected by the styrene content in the resin per se, but were dependent on the combined effects of composition and crosslink density of the network.     

Temperature dependence of the dynamic mechanical properties of filled polymers

A theory has been developed to explain the jump in the relative modulus of filled polymers near the glass transition temperature T_g and the subsequent decrease in relative modulus at temperatures above the glass transition temperature. The theory is based upon the concept that there are some particle–particle contacts in doublets and in agglomerates containing a larger number of particles. Below T_g motion of particles at the contact points is possible because of the high modulus of the polymer. At T_g particle–particle motion mostly ceases because of the low modulus of the polymer. At higher temperatures, the mismatch in the coefficients of expansion allows some motion to occur at points of contact and slippage may occur at the polymer–particle interfaces, so the modulus decreases. It is shown theoretically and experimentally that both the elastic modulus and the mechanical damping depend upon the nature of the surface of the particles.     

On the role of TiO<Subscript>2</Subscript> nanoparticles on thermal behavior of flexible polyurethane foam sandwich panels

A series of flexible polyurethane foam (FPUF) and monolithic polyurethane (PU) sandwich panels reinforced with different contents of TiO₂ nanoparticles (0, 0.5 and 1 mass%) have been successfully prepared by compression molding process at room temperature. The influence of TiO₂ nanoparticles on the thermal properties of PU matrix has been investigated by thermogravimetric and dynamic mechanical thermal analysis (DMTA). The morphology of porous structure of FPUF sandwich panels has been characterized by scanning electron microscopy. The presence of TiO₂ nanoparticles as reinforcement has improved the thermal properties of the FPUF and PU sandwich panel samples. It has been observed that FPUF and PU sandwich panel reinforced with 1 mass% of TiO₂ nanoparticles possessed the highest enhancement in thermal properties in all accomplished thermal tests. The DMTA results for the FPUF and PU sandwich panel reinforced with 1 mass% of TiO₂ nanoparticles indicated that the storage modulus and loss modulus have increased about 1.22 and 1.25 times, 1.5 and 1.55 times, respectively, compared to pure samples. Furthermore, the glass transition (T _g) obtained from the damping factor (tanδ) curves has increased 2 and 1 °C for FPUF and PU sandwich panels, respectively.     

On the predictability of the high-frequency elastic modulus of semicrystalline polymers as function of crystallinity

The relation of the high-frequency elastic moduli of semicrystalline polymers to volume fraction crystallinity is correctly described by the Hashin-Shtrikman theory, without any disposable constants, as a function of the ratio of the modulus of the amorphous to that of the crystalline phase. Hence the (high-frequency) reduced modulus of semicrystalline polymers is largely a function of the temperature T/T_g. The importance of T/T_m for the modulus of the crystalline phase precludes the existence of a single universal reduced modulus versus temperature curve.     

The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T _g) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T _g due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T _g1, T _g2 and T _g3 in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E′), the peak of the loss modulus (E″) and the peak of the loss tangent (tan δ). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T _gDSC). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T _g1 and T _g2 have the same molecular mechanism as T _gDSC, i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T _g3 is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.     

The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H₂O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of theT _g of gelatin (T _g anhydrous, 200?C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H₂O (d.w.b.), but at higher water contents, the7 _g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with aT _g of 145?C for anhydrous gelatin Possibly, theT _g is elevated by crystallization — a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.