Low Temperature Modulus Research Articles

Rheological investigation of the network structure in mixed gels of Kappa and Iota Carrageenan

Carrageenans comprise linear sulfated high molecular weight polysaccharides obtained from seaweeds and are routinely used in food and home/personal care industries. Various kinds of carrageenans differ from others based on the ester sulphate group location on the polysaccharide chains. Pure and mixed systems of Kappa Carrageenan and Iota Carrageenan undergo a three-dimensional gel network structure formation or dissociation with a change in temperature. During the sol-gel and gel-sol transitions, the Carrageenan systems pass through a unique critical gel state, where dynamic moduli are scale-invariant owing to the self-similar structure of the three-dimensional network. In this work, we obtain the critical gel state associated with pure and mixed systems of Kappa and Iota Carrageenan during cooling and heating by exploring the material behavior for a range of frequencies. Interestingly, on the one hand, the mixed gels show a higher critical sol-gel transition temperature compared to the pure systems at equal individual concentrations. On the other hand, the low temperature moduli of mixed gels are closer to that of Kappa Carrageenan when the concentration of the same is more than half in the mixture. The rheological measurements demonstrate that the Kappa Carrageenan strongly affects the nature of aggregation of double helices of Iota Carrageenan, but Iota Carrageenan does not have a significant influence on that of Kappa Carrageenan. These results suggest an associative, interactive network formation between Kappa and Iota Carrageenan in the mixture, such that the gel behavior is predominantly influenced by Kappa Carrageenan.

Influence of epoxy resin polymer on recycled asphalt binder properties

Utilization of high-performance pavement materials with a high reclaimed asphalt pavement (RAP) content is gradually gaining importance given the impetus of green environmental policies. Epoxy resin polymer (EP) can greatly enhance the reuse of RAP materials in asphalt mixture. The influence of EP on the properties of aged asphalt binder was investigated wherein mechanical and chemical properties, and the micro-morphology of binders composed of aged asphalt binder and EP were also evaluated. EP significantly improved tensile, high-temperature, low-temperature, and fatigue properties of aged asphalt binder, and simultaneously reduced concentration of sulfoxide and carbonyl of aged asphalt binder. Increasing EP content reduced the impact of asphalt ageing degree on mechanical properties, micro-morphology, and chemical composition. Furthermore, the construction allowance time of the binder was increased with increasing EP content. At 40% EP content, an effective network structure was formed in the aged asphalt binder, leading to higher tensile elongation, high-temperature rheological properties, and fatigue life as compared to virgin asphalt binder. Meanwhile, low-temperature modulus and sulfoxide and carbonyl concentrations were lower than those of virgin asphalt binder. For application, a recommended EP content of 40% is suggested.

A triaxial linear viscoelastic characterization framework for asphalt concrete based on the 2S2P1D model

Asphalt concrete materials are known to exhibit pressure dependence in response to mechanical loading. However, for conventional dense-graded asphalt mixtures, only weak pressure sensitivity was noted in the relaxation spectrum, time-temperature shift factors, and low-temperature moduli. Given these facts, a triaxial linear viscoelastic characterization framework was developed, which consisted of a convenient experimental protocol for the confined dynamic modulus test and a modeling approach based on the 2S2P1D model. In this framework, the same relaxation spectrum and time-temperature equivalence were applied across all different confining conditions, while the pressure dependence was fully assigned to the equilibrium modulus. The resultant prediction errors in dynamic modulus were in general below or comparable to typical test variability. Numerical implementation of the modeling method in the time domain was accomplished via Prony discretization of the continuous relaxation spectrum, and was demonstrated via application to triaxial relaxation and creep data. The model's satisfactory performance suggested that the confining condition had an insignificant impact on the molecular processes occurring in the asphalt phase during relaxation, owing to the low aggregate mobility and thus minimal asphalt flow as restrained by the dense structure. Limitations of the proposed modeling scheme as well as methods available from the literature were revealed lastly upon asphalt concrete materials with unconventional gap and open gradations.

Structure and Mechanical Stability of Epoxy Modified Polyurethane Foam Under Heat and Stress

A series of phenolic epoxy resin (PEP) modified polyurethane foams (PUF) were prepared via an in-situ polymerization, one step process. It was found that the epoxy modified PUF foam exhibited a perforated network structure with larger cell size, higher open cell porosity and enhanced ovality compared with pure PUF. With increasing content of PEP, the tensile strength, elongation at break and low temperature modulus of PUF decreased. A single Tg was observed for PEP modified PUF, indicating that the two component phases of the polyurethane-epoxy were miscible. With increasing PEP content, the Tg of PUF shifted slightly to higher temperature, tan δmax dropped to lower values, and the retention value of the storage modulus at −20 and −10 °C increased. For pure PUF, the cell walls degraded and the structure became disordered after aging under heat and stress, while for PUF/20wt%PEP, the degradation degree was obviously reduced, and an orientation of the cells along the stress direction and a density increase was observed. During aging at 200 °C, the retention of the mechanical properties of PUF/20wt% PEP was much higher than that of pure PUF, and it showed superior stability under heat and stress, attributed to incorporation of the thermally resistant oxazolidone rings and benzene rings in the PU backbones, the highly cross-linked networks of the polyurethane-epoxy systems and the obvious orientation of the cells under stress.

Effect of milk fat content on the viscoelasticity of mozzarella-type cheese curds

The effect of fat content in cheese curds on their rheological properties was examined using dynamic shear measurements. Surplus fat addition to milk samples caused two distinct types of changes in the temperature dependence of the viscoelastic moduli of resultant curds. The first was a significant reduction in the moduli over a wide temperature range, which is attributed to the presence of liquefied fat globules within the milk protein network. The second was the excess contribution to the low-temperature moduli owing to the reinforcing effect of solidified fat globules. An upward shift in the sol–gel phase transition temperature driven by an increased fat content was also observed.

Structure and Mechanical Response of Protein Hydrogels Reinforced by Block Copolymer Self-Assembly.

A strategy for responsively toughening an injectable protein hydrogel has been implemented by incorporating an associative protein as the midblock in triblock copolymers with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) endblocks, producing materials with a low yield stress necessary for injectability and durability required for load-bearing applications post-injection. Responsive reinforcement triggered by PNIPAM association leads to significant increases in the gel's elastic modulus as well as its resistance to creep. The performance of these materials is a strong function of molecular design, with certain formulations reaching elastic moduli of up to 130 kPa, effectively reinforced by a factor of 14 over their low temperature moduli, and having stress relaxation times increased by up to a factor of 50. The nanostructural origins of these thermoresponsive enhancements were explored, demonstrating that large micellar cores, high PNIPAM volume fractions, and high densities of associating groups in the protein corona lead to the greatest reinforcement of the gel's elastic modulus. Gels with the largest micelles and the highest packing fractions also had the longest relaxation times in the reinforced state. These combined structure and mechanics studies reveal that control of both the micellar and protein networks is critical for making high performance gels relevant for biomedical applications.

Defects and supersolidity: effects of annealing and stress on elastic behavior of solid He4

Recent measurements have shown that solid He4 can decouple from a torsional oscillator below 200mK, and defects appear to be crucial to this behavior. Helium’s shear modulus increases in the same range, which can be understood in terms of dislocations pinned by He3 impurities at the lowest temperatures, but mobile above 100mK. We measure the pressure and shear modulus of helium to study the effects of annealing and stresses applied at low temperatures. Pressure gradients produced during crystal growth or plastic deformation are greatly reduced by annealing, but only at temperatures close to melting. Annealing does not change the low-temperature modulus but usually raises it at high temperature, as expected if annealing eliminates some dislocations. Large stresses also affect the modulus, but these changes are reversed by heating above 0.5K, suggesting that defects introduced by stress are easier to anneal than those produced during growth.

Structure–property relationships and melt rheology of segmented, non-chain extended polyureas: Effect of soft segment molecular weight

Novel, segmented non-chain extended polyureas were synthesized. Soft segments (SS) were based on poly(tetramethylene glycol) (PTMO) (average molecular weight 1000 or 2000g/mol) and hard segments (HS) were based on a single molecule of a diisocyanate, which was either 1,6-hexamethylene diisocyanate (HDI), 1,4-phenylene diisocyanate (pPDI) or 1,4-trans-cyclohexyl diisocyanate (CHDI). An increase in the SS molecular weight was found to lead to an increased formation of SS crystallites below 0°C, which increased the low temperature modulus. Both 1K and 2K PTMO-based polyureas showed a microphase separated morphology, where the HS formed thread-like, crystalline structures that were dispersed in the continuous SS matrix. Upon deformation, the HS were found to breakdown into distinctly smaller threads, which oriented along the direction of the strain; this effect was found to be partially reversible and time dependent. Both the 1K and 2K polyureas based on HDI HS were found to be thermally stable and potentially melt-processible.

Epoxy resin based nanocomposites: 1. Diglycidylether of bisphenol A (DGEBA) with triethylenetetramine (TETA)

AbstractThe preparation and properties of a series of nanocomposite materials obtained using different organically modified montmorillonite clays with a simple epoxy resin are reported. Dynamic mechanical thermal analysis is used to assess the effect of the incorporation of the clay platelets into the matrix of the polymer. In this system, it is observed that with the well‐dispersed clay system the low temperature modulus increases as would be predicted for a filled polymer system. The high temperature modulus increase is consistent with the premise that the polymer is interacting directly with the clay platelets. The glass transition temperature increases with the loading of the clay in the polymer resins. However, the extent to which enhancement of the physical properties of the composite occurs depends on the nature of the organic modifier. Copyright © 2004 Society of Chemical Industry

Synthesis and properties of poly(butylene terephthalate)‐poly(ethylene oxide)‐poly(dimethylsiloxane) block copolymers

Abstractchemical structure image Poly(butylene terephthalate)‐poly(ethylene oxide)‐poly(dimethyl siloxane)‐poly(ethylene oxide) block copolymers, (PBT‐PEO‐PDMS‐PEO)m, are synthesized by polycondensation (PC) of dimethylterephtalate (DMT), 1,4‐butanediol (BDO) and PEO‐PDMS‐PEO. The soft block has been incorporated from 10 to 70 wt‐%; the total molecular weight (MW) of the block‐copolymers amounts to 16000 ‐ 20000 g/mol.One major problem of polyether‐PBT thermoplastic elastomers is their poor thermo‐oxidative stability. Due to the excellent heat stability of PDMS, the resistance of this new thermoplastic elastomer against thermo‐oxidative degradation has been increased 80 %!From differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) in the PEO‐PDMS‐PEO based COPEs, three phases can be distinguished. Besides the crystalline PBT phase, an amorphous mixed phase of PBT and PEO and an almost pure PDMS phase have been found. Due to the high concentration of the mixed PBT‐PEO phase, the low temperature modulus and the glass transition temperature, Tg, are not dominated by the pure PDMS phase (Tg = −114°C). Depending on the amount of PBT and PEO present, the main glass transition lies in the range of −50°C to 50°C.

Low-temperature mechanical loss measurements of 5N and 6N lead

Measurements of the dynamic modulus and internal friction of 5N and 6N lead were performed after low-temperature plastic deformation. Low-temperature deformation induces an important decrease in the low-temperature modulus. This result is associated with lubrication dislocation motion due to intrinsic point defects. Finally, the evolution of the relaxation peak observed when the annealing temperature is increased demonstrates that Bordoni relaxation due to kink pair formation occurs in lead at a very low temperature compared with other f.c.c. metals.

Polymer–leather composites. IV. Mechanical properties of selected acrylic polymer–leather composites

An extensive study was made of the mechanical properties of the polymer–leather composite materials reported in previous articles. Polymer was deposited into leather by both emulsion and bulk (or solution) polymerization. Either methyl methacrylate, n-butyl acrylate, or a fixed comonomer mixture of n-butyl acrylate and methyl methacrylate were used over the widest feasible range of composition. Tensile strengths, in analogy with many polymer-treated fibers, were generally smaller than untreated controls, but entensions to break remained fairly constant as composition changed. Polymer–leather composites prepared by both methods were rheologically similar when correlated against the volume fraction of the polymer used. Relative tensile and torsional moduli were greater than unity at small volume fractions of polymer, but higher compositions assumed more of the viscoelastic characteristics of the modifying polymer. The constancy of the glass transition temperature of the polymeric component as composition changed indicated poor domain interactions. However, residual porosity reduced low-temperature moduli anomalously. A modified Halpin–Tsai equation was proposed that qualitatively predicted moduli increase by incremental space filling as either fiber aggregation (from simple air drying of untreated controls) or polymer content increased. The simultaneous rheological dependence of polymer–fiber interactions in composites was also accounted for by the equation.

Mechanism for Dislocation Pinning in the Alkali Halides

Changes in the elastic modulus and internal friction of plastically deformed rocksalt, caused by changes in the mean free length of oscillating dislocation segments, have been observed during x-irradiation at temperatures ranging from 10° to 300°K. The rates at which the modulus and damping changes occur during irradiation are found to be independent of temperature; but during irradiation at helium temperature, the magnitude of the effects are smaller. This decrease in magnitude has been shown to be due to the absence of thermal unpinning. Explanations of the pinning process based on the diffusion of point defects to dislocations appear to be ruled out by the rapidity of the low temperature modulus change. The dislocation pinning produced by irradiation of NaCl, KCl, KBr, and KI at low temperatures is found to be reversible when the crystals are subjected to illumination in, or near, the visible. The wavelength of the illumination which produces the maximum unpinning increases as the lattice constant increases; the effect disappears when the crystals are warmed to room temperature. However, at all temperatures, bleaching a deformed and partially irradiated crystal with F light causes a further modulus increase. The pinning point produced by low temperature irradiation is identified as a dislocation jog in close association with an F center whose absorption band is displaced towards the red by the dislocation strain field. Ionization of the F center results in recombination of its vacancy with the jog; warmup allows the F center to diffuse beyond recombination range.

A Study of Stiffness Testing of Elastomers at Low Temperatures

Abstract In light of the above discussion, it does not appear logical to choose a “best” test for determining low-temperature moduli. A study of the data as a whole indicates that the correlation between the two sets of values from the simple beam-method (laboratories C-1 and C-2) are closest, but there is nothing to show that they are the most valid. It should be emphasized that, in the temperature region of greatest technical importance, that is, where the rubber is changing from its ordinary flexible state to a hard stiff material, the different tests show good agreement. A low-temperature limit of serviceability would be close to −80° F for the Hevea gum and −60° F for the GR-S by each of the methods studied. Another conclusion that might well be reached by a study of the test methods and data presented is that the term “modulus”, as applied to rubberlike materials, is rather vague unless the method of obtaining that modulus is also given.