Formation Of New Cross-links Research Articles

Comparative structural, digestion and absorption characterization of three common extruded plant proteins

Extruded plant proteins, also known as textured vegetable proteins (TVPs), serve as vital components in plant-based meat analogue, yet their structural and nutritional characteristics remain elusive. In this study, we examined the impact of high-moisture (HM) and low-moisture (LM) extrusion on the structures, digestion and absorption of three types of plant proteins. Extrusion transformed plant proteins from spherical to fibrous forms, and formed larger aggregate particles. It also led to the disruption of original disulfide bonds and hydrophobic interactions within protein molecules, and the formation of new cross-links. Intriguingly, compared to native plant proteins, TVPs’ α-helix/β-sheet values decreased from 0.68 to 0.69 to 0.56–0.65. Extrusion increased the proportion of peptides shorter than 1 kD in digesta of TVPs by 1.44–23.63%. In comparison to unextruded plant proteins, TVPs exhibited lower content of free amino acids in cell transport products. Our findings demonstrated that extrusion can modify protein secondary structure by diminishing the α-helix/β-sheet value, and impact protein tertiary structure by reducing disulfide bonds and hydrophobic interactions, promoting the digestion and absorption of plant proteins. These insights offer valuable scientific backing for the utilization of extruded plant-based proteins, bolstering their role in enhancing the palatability and nutritional profile of plant-based meat substitutes.

Thermomechanical Stress Analysis of Hydrated Vital Gluten with Large Amplitude Oscillatory Shear Rheology.

Vital gluten is increasingly researched as a non-food product for biodegradable materials. During processing, the protein network is confronted with increased thermal and mechanical stress, altering the network characteristics. With the prospect of using the protein for materials beyond food, it is important to understand the mechanical properties at various processing temperatures. To achieve this, the study investigates hydrated vital gluten under thermomechanical stress based on large amplitude oscillatory shear (LAOS) rheology. LAOS rheology was conducted at increasing shear strains (0.01-100%), various frequencies (5-20 rad/s) and temperatures of 25, 45, 55, 65, 70 and 85 °C. With elevating temperatures up to 55 °C, the linear viscoelastic moduli decrease, indicating material softening. Then, protein polymerization and the formation of new cross-links due to thermal denaturation cause more network connectivity, resulting in significantly higher elastic moduli. Beyond the linear viscoelastic regime, the strain-stiffening ratio rises disproportionately. This effect becomes even more evident at higher temperatures. Lacking a viscous contribution, the highly elastic but also stiff network shows less mechanical resilience. Additionally, at these elevated temperatures, structural changes during the protein's denaturation and network shrinkage due to water evaporation could be visualized with confocal laser scanning microscopy (CLSM).

Analysis of riboflavin/ultraviolet a corneal cross-linking by molecular spectroscopy

Corneal cross-linking (CXL) with riboflavin and ultraviolet A light is a therapeutic procedure to restore the mechanical stability of corneal tissue. The treatment method is applied to pathological tissue, such as keratoconus and induces the formation of new cross-links. At present, the molecular mechanisms of induced cross-linking are still not known exactly. In this study, we investigated molecular alterations within porcine cornea tissue after treatment with riboflavin and ultraviolet A light by surface enhanced Raman spectroscopy (SERS). For that purpose, after CXL treatment a thin silver layer was vapor-deposited onto cornea flaps. To explore molecular alterations induced by the photochemical process hierarchical cluster analysis (HCA) was used. The detailed analysis of SERS spectra reveals that there is no general change in collagen secondary structure while modifications on amino acid side chains are the most dominant outcome. The formation of secondary and aromatic amine groups as well as methylene and carbonyl groups were observed. Even though successful cross-linking could not be registered in all treated samples, Raman signals of newly formed chemical groups are already present in riboflavin only treated corneas.

Keratin intrinsic fluorescence as a mechanism for non-invasive monitoring of its glycation

We have studied the evolution of keratin intrinsic fluorescence as an indicator of its glycation. Steady-state and time-resolved fluorescence of free keratin and keratin-glucose samples were detected in PBS solutions in vitro. The changes in the fluorescence response demonstrate that the effect of glucose is manifest in the accelerated formation of fluorescent cross-links with an emission peak at 460 nm and formation of new cross-links with emission peaks at 525 nm and 575 nm. The fluorescence kinetics of these structures is studied and their potential application for the detection of long-term complications of diabetes discussed.

Weatherability of different monomers in melamine-crosslinked polyester binders: Towards increasing renewable content in coil coatings

The stability of coil coatings towards exterior influences is one of their most important properties as it ensures substrate quality. The weatherability of eight melamine crosslinked polyester coatings was studied in accelerated aging in order to systematically examine the effect of individual monomers including phthalic anhydride and the bio-based monomers succinic acid, sebacic acid, 1,2-propanediol, 1,3-propanediol and glycerol. Coatings with and without hindered amine light stabilisers were characterised upon degradation through their gloss retention, infrared spectra, film thickness and μ-hardness.A greater susceptibility to radical degradation mechanisms forming new polar chain ends was observed for sebacic acid compared to succinic acid, and for 1,3-propanediol compared to 1,2-propanediol. On the other hand, μ-hardness results suggested that 1,2-propanediol and phthalic anhydride accelerate the formation of new crosslinks during degradation.As radical and hydrolytic bond scission compete with the formation of new crosslinks and each mechanism influences the others, both monomeric composition and overall structure have to be considered to understand the degradation behaviour. The presented findings can be used as a guide by those aiming to increase the bio-based content in coil coatings.

Influence of the Gamma Radiation on the Structure of PVDF/PANI Blend

In this study, the gamma radiation altering effects on the structure and optical properties of Polyvinylidene fluoride (PVDF)/polyaniline (PANI) blend films was investigated. The PVDF and PANI films were synthesized by casting method and gamma irradiated at 50, 100, 125, and 150 kGy, respectively. Following the irradiation, the films morphology and optical properties were characterized by X-ray diffraction technique (XRD), Fourier Transform Infrared spectroscopy (FT-IR) and ultraviolet–visible (UV–Vis) spectrophotometer. The optical constants, absorbance X (λ) and complex dielectric constant were calculated by using Swanepole method whereas the optical dispersion parameters Eo and Ed were determined by Wemple–DiDmenico and the high frequency dielectric constant (e∞) was analyzed. XRD patterns revealed the partial crystalline nature of the films before and after irradiation. FT-IR spectral analysis showed a slight shift and increase in the transmittance for all films post-irradiation. In conclusion, irradiating the PVDF and PANI films altered their optical properties suggesting degradation of the polymeric chains upon irradiation and the formation of new cross links.

Mechanisms of Plastic Deformation in Collagen Networks Induced by Cellular Forces

Contractile cells can reorganize fibrous extracellular matrices and form dense tracts of fibers between neighboring cells. These tracts guide the development of tubular tissue structures and provide paths for the invasion of cancer cells. Here, we studied the mechanisms of the mechanical plasticity of collagen tracts formed by contractile premalignant acinar cells and fibroblasts. Using fluorescence microscopy and second harmonic generation, we quantified the collagen densification, fiber alignment, and strains that remain within the tracts after cellular forces are abolished. We explained these observations using a theoretical fiber network model that accounts for the stretch-dependent formation of weak cross-links between nearby fibers. We tested the predictions of our model using shear rheology experiments. Both our model and rheological experiments demonstrated that increasing collagen concentration leads to substantial increases in plasticity. We also considered the effect of permanent elongation of fibers on network plasticity and derived a phase diagram that classifies the dominant mechanisms of plasticity based on the rate and magnitude of deformation and the mechanical properties of individual fibers. Plasticity is caused by the formation of new cross-links if moderate strains are applied at small rates or due to permanent fiber elongation if large strains are applied over short periods. Finally, we developed a coarse-grained model for plastic deformation of collagen networks that can be employed to simulate multicellular interactions in processes such as morphogenesis, cancer invasion, and fibrosis.

Combination of Cyclic Fatigue Testing and Materials Characterisation to Investigate Ageing of Elastomers

Cyclic fatigue testing and elastomer characterisation were combined to study changes in material properties and network structure of elastomers during thermal ageing. Natural rubber containing a typical additive package with carbon black was studied as a model material. The samples were aged at different temperatures in air or under a nitrogen atmosphere. The fatigue life in number of cycles to failure (S-N curves) was determined from force- and displacement-controlled fatigue tests on tensile bar specimens after different thermal ageing times. Changes in mechanical properties and crosslink density were studied by tensile tests, dynamic mechanical analysis, stress relaxation experiments, compression set measurements, swelling measurements and solid-state NMR. Changes in network density during thermal ageing are related to the interplay between the formation of new crosslinks and chain scission. The average molecular mass of the network chains was found to be a suitable parameter for comparing different characterisation methods. An initial decrease in the molecular mass between two crosslinking points due to post-curing is followed by an increase due to chain scission. A similar trend was found for fatigue life in number of cycles to failure (N) in force-controlled fatigue tests: an increase in N for short ageing times is followed by a decrease after longer ageing times.

Factors affecting the mechanical behavior of collagen hydrogels for skin tissue engineering

The effect of the production factors yielding a functional dermal substitute was investigated by means of monotonic and cyclic uniaxial tensile tests, as well as electron microscopy visualizations. The role of (i) plastic compression, (ii) product incubation, and (iii) cell permanence in the collagenous matrix in order to achieve a skin-like behavior were characterized in terms of material and structural stiffness, in-plane kinematics, and cyclic response, as well as pore size and network density. The plastic compression resulted in a denser and stiffer material, while no corresponding change was observed in the behavior of the entire structure. This was related to the progressive reduction in product thickness and amount of excess water, rather than to formation of new crosslinks between fibers. Contrary, irrespective of the presence of human fibroblasts, the product incubation induced both material and structural stiffening, indicating the formation of a denser network. These results were confirmed by similar evolutions in the construct in-plane kinematics and cyclic stress reduction. Finally, comparison of constructs incubated in different culture media indicated a determinant contribution of the biochemical environment, rather than of the seeded cells, to the achieved mechanical properties. The observed features are relevant in terms of mechanical biocompatibility of the implant and might direct future optimizations of the production process in order to rapidly attain the desired mechanical properties.

Transition‐Metal‐Catalyzed Chain‐Growth Polymerization of 1,4‐Diethynylbenzene into Microporous Crosslinked Poly(phenylacetylene)s: the Effect of Reaction Conditions

Chain‐growth polymerization of 1,4‐diethynylbenzene into conjugated crosslinked polyacetylene‐type poly(1,4‐diethynylbenzene)s (PDEBs) is reported. While metathesis catalysts (WCl6/Ph4Sn, MoCl5/Ph4Sn, Mo Schrock carbene) fail in this polymerization, insertion Rh catalysts ([Rh(nbd)acac], [Rh(nbd)Cl]2) provide microporous PDEBs in high yields. The Brunauer–Emmett–Teller (BET) surface, SBET, of PDEBs prepared with [Rh(nbd)acac] increases, in dependence on the polymerization solvent, in the order: THF << pentane < benzene < methanol < CH2Cl2. SBET further increases with both increasing monomer concentration and increasing polymerization temperature and reaction time, reaching a highest value of 1469 m2 g−1. In addition to micropores, PDEBs contain mesopores. The mesopore volume and average mesopore diameter increase with the time and the temperature of the polymerization up to 2.52 cm3 g−1 and 22 nm (72 h, 75 °C). The post‐polymerization thermal treatment of PDEB (280 °C) results in formation of new crosslinks and modification of PDEB texture and sorption behavior manifested mainly by enhancement of H2 adsorption capacity up to 4.55 mmol g−1 (77 K, 750 Torr). image

Rheology of Broken Rice-Foxtail Millet-Maize Flour Blends Related to Extrusion Cooking

Rheological properties of the various flours from the broken rice, foxtail millet and maize used during extrusion conditions were investigated for pasting properties and dynamic oscillation using Rheometer. Results pointed out that each of the different flour might be utilized for specific applications in food processing. In addition, the functional properties of individual flours were measured. The swelling power of broken rice flour showed the highest values, indicated that the viscoelastic properties of pastes were dependent on the swelled starch–protein complex granules and the formation of new cross-links in the network. All formulation flours exhibited variable pasting behavior, among different temperatures and formulations at 90°C and 20:60:20 had the highest peak viscosity 6741 MPa.s and final viscosity 6842 MPa.s and was found to be more effective than other temperature and formulation combinations. Viscosity decreased with the increase of temperature and formulation combinations. The storage modulus was invariably higher than loss modulus at all stages during heating, holding and cooling of the sample, showing that the paste has a tendency to behave like weak gel. G´ was the most primary dynamic rheological parameter that reflected rheological properties, G´ represented the elastic properties of rice starch paste.

디클로로아세트산 처리에 따른 무수말레산-그래프트 EPDM의 팽윤 거동

Swelling behaviors of raw (Specimen-R) and compressed (Specimen-C) samples of maleic anhydride-grafted EPDM (MAH-g-EPDM) depending on the treatment with dichloroacetic acid were investigated. Structural characteristics of the samples were analyzed by nuclear magnetic resonance spectroscopy (NMR) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). When the samples were not treated with dichloroacetic acid, the swelling ratio of Specimen-R was greater than that of Specimen-C by about twice and the swelling ratio change was negligible though the process of swelling and drying was repeated. When the samples were treated with dichloroacetic acid, the first swelling ratios were increased but the second ones were decreased. For the Specimen-C, the swelling ratio of the sample without the dichloroacetic acid treatment and the second swelling ratio of the sample treated with dichloroacetic acid were nearly the same. However, for the Specimen-R, the second swelling ratio of the sample treated with dichloroacetic acid was strikingly lower than that of the sample without the dichloroacetic acid treatment. The swelling ratio change according to the dichloroacetic acid treatment was explained by dissociation of the existing crosslinks and formation of new crosslinks.

Rheological properties of extruded dispersions of flaxseed-maize blend

Rheological properties of the extruded pastes of various extrusion conditions were investigated by dynamic oscillation and creep-recovery tests. Temperature sweep test showed that the starch gelatinization of non-extruded pastes took place at about 67 °C and no gelatinization was observed for the extruded sample. Frequency sweep tests could be represented by a Power law model and the samples showed gel-like behavior since storage modulus was much larger than loss modulus. The creep-recovery data were modeled by the Burger’s model. With the addition of flaxseed, the viscoelastic modulii of pastes were increased due to the formation of starch–protein–fat network, however, with further increasing the flaxseed content the viscoelastic modulii of pastes were decreased due to excessive fat which lead to network weakening. The water holding capacity results indicated that the viscoelastic properties of pastes were depended on the swelled starch–protein complex granules and the formation of new cross-links in the network.

Light emission on heating polymer networks formed by radical polymerization

Organic–inorganic hybrid monomers based on alkoxysilanes bearing methacrylate groups, or the pure organic dimethacrylate monomers were photo-polymerized to densely cross-linked networks. Storage of the polymer samples in nitrogen and at ambient laboratory temperature was associated with a slow decay of trapped free radicals and formation of new cross-links. In air, reactions with oxygen lead competitively to formation of peroxides due to easier diffusion in the polymer matrix. The formation and existence of peroxides in samples stored in air was shown by the chemiluminescence (CL) induced at elevated temperatures. The peroxide accumulation proceeds in three stages. At first, approximately up to 6 weeks, a high rate of accumulated peroxides activated by long living radicals is assumed. During the next 20 weeks a steady state oxidation stage is observed before another accumulation of peroxides occurs. The glass transition temperature ( T g) changes were used to estimate variation of cross-link density during ageing the networks. As far as the entirely organic network is concerned the main portion of precursors giving rise to cross-links in the post-curing period was deactivated within 9 weeks under the influence of oxygen. Contrary to this finding in networks based on organic–inorganic hybrid monomers and a copolymer of the inorganic and organic monomers the additional cross-linking is considerably less influenced by oxygen. It is anticipated that a pronounced network structure favours further cross-linking over oxygen addition.

Influence of the cure systems on long time thermal aging behaviors of NR composites

NR composites with different curing systems were aged thermally at 60, 70, 80, and 90 °C for 2–185 days in a convection oven, and the changes in the crosslink density were investigated as a function of the accelerated thermal aging. The overall crosslink densities increased with increasing aging time irrespective of the aging temperatures and curing systems. The changes in crosslink density were enhanced by increasing the aging temperature. The degree of the increased crosslink density was in the following order: “the conventional cure system > the semi-EV system > the EV system”. For short term thermal aging, the change in crosslink density with the aging time was complicated, particularly for low temperature aging. The activation energies of the change in crosslink density with thermal aging using the conventional and semi-EV cure systems increased and then remained relatively constant with increasing aging time, whereas that of the specimen with an EV cure system tended to increase linearly. The experimental results were explained by the dissociation of the existing polysulfidic linkages and the formation of new crosslinks through the crosslinking-related chemicals remaining in the sample.

Accelerated Thermal Aging Behaviors of EPDM and NBR Vulcanizates

The crosslink densities of rubber vulcanizates cured by a sulfur accelerator system are changed by thermal aging. Change of the crosslink density occurs by formation of new crosslinks and dissociation of existing crosslinks. Crosslink density change of a rubber vulcanizate with a high sulfur content after thermal aging was larger than that with a low sulfur content. Permanent deformation of a rubber vulcanizate results from the crosslink density change by thermal aging under the deformed state. Strain applied to a rubber vulcanizate affects the change of crosslink density. The crosslink density of the more strained vulcanizate was changed by thermal aging more than those of the less strained one. Ethylene-propylene diene rubber (EPDM) is a weatherresistant rubber and has slow cure characteristics of long induction time and slow cure rate. Acrylonitrile-butadiene rubber (NBR) is oil-resistant rubber and has fast cure characteristics of short induction time and fast cure rate. EPDM and NBR have different chemical properties each other. EPDM is nonpolar and has slow cure characteristics, whereas NBR is polar and has fast cure characteristics. In this work, we studied changes of crosslink densities of EPDM and NBR vulcanizates by thermal aging at high temperatures of 70-100 C for long time of 180 days. Levels of the crosslink density change and the activation energies of the vulcanizates were also compared. Thermal aging behaviors depending on the aging time were explained with the activation energy variation. Figures 1 and 2 show variations of the crosslink density changes of the EPDM and NBR vulcanizates with the aging time, respectively. The crosslink density change was obtained by dividing the difference in the crosslink densities after and before the thermal aging by the crosslink density of the initial sample. The crosslink density change increases as the aging time elapses. Level of the crosslink density change becomes larger by increasing the aging temperature. For the EPDM vulcanizate, the crosslink density change increases notably during the initial period and then the increment becomes smaller. The crosslink densities increase by 10.0, 17.6, and 22.6% after the thermal aging at 70, 85, and 100 C for 30 days, respectively, and increase by 17.3, 27.9, and 42.5% after the thermal aging for 180 days, respectively. The initial 30 day increments (0 to 30 days) are 10.0, 17.6, and 22.6% for 70, 85, and 100 C, respectively, while the final 30 day increments (150 to 180 days) are 0.2, 0.4, and 1.7%, respectively. For the NBR vulcanizate, the crosslink density change increases notably with increase of the aging time as shown in Figure 2. Especially, the crosslink density change at 100 Figure 1. Variation of the crosslink density change of the EPDM vulcanizate by the thermal aging as a function of the aging time. Squares, circles, and triangles indicate the aging temperatures of 70, 85, and 100 oC, respectively.

The Effect of Prolonged Thermo-oxidative Ageing on the Mechanical Properties of Dynamically Vulcanized Poly(Vinyl Chloride)/Nitrile Butadiene Rubber Thermoplastic Elastomers

ABSTRACT Plasticized poly(vinyl chloride)/nitrile butadiene rubber (PVC/NBR) thermoplastic elastomers (TPEs) were dynamically vulcanized in the melt stage with the incorporation of a semi-efficient vulcanizing system using a Brabender Plasticorder at 150°C and 50 rpm rotor speed. Curative concentration was progressively increased from 0 to 1 part per hundred (phr) NBR in order to study the effect of dynamic curing on the plasticized blend. The mechanical properties investigated include tensile strength, elongation at break (%EB), modulus at 100% elongation (M100), tear strength, and hardness. The effect of thermo-oxidative ageing (TOA) on the mechanical properties was investigated by exposing the PVC/NBR TPEs in an air oven at 100°C for 3, 7, 14, and 21 days. It was found that tensile and tear strength passed through a maximum value, whereas, hardness and M100 increased steadily with the sulfur content and ageing time. On the contrary, the elongation at break reduced gradually with ageing time until it reaches a minimum value. The increase in crosslink density as well as the steady reduction in swelling index with increasing concentration of curatives provided an excellent evidence for the significant increase in crosslink density with ageing time. The changes in physical and mechanical properties of the TPEs is believed to be closely related to some microstructural changes taking place as a result of the formation of new crosslinks due to prolonged thermo-oxidative environment.

Rheological and mechanical properties of dynamically cured poly(vinyl chloride)/nitrile butadiene rubber thermoplastic elastomers

AbstractPlasticized poly(vinyl chloride)/nitrile butadiene rubber (PVC/NBR) thermoplastic elastomers (TPEs) were dynamically cured in the melt stage with the incorporation of a semi‐efficient curing system using a Brabender Plasticorder at 150 °C and rotor speed of 50 rev min−1. Sulfur concentration was progressively increased from zero to 1 part per hundred NBR to study the effect of dynamic curing on mechanical and rheological behaviour of the TPEs. The compounds were characterized in respect of their rheological and mechanical properties. The effectiveness of dynamic curing was studied using Brabender plastograms, a moving disc rheometer and swelling index measurement. The mechanical properties investigated include tensile strength, elongation at break, modulus at 100% elongation, tear strength and hardness. The influence of thermooxidative ageing on the mechanical properties was investigated by incubating the PVC/NBR TPEs in an air oven at 80 °C for 168 h. The torque values obtained from both rheometers increased with increasing sulfur dosage, while the swelling index decreased. The significant increase in the degree of curing evidenced by the steady reduction in the swelling index provided excellent proof of the efficiency of the dynamic curing technique. Thermo‐oxidative ageing resulted in a pronounced enhancement in mechanical properties as a function of sulfur content. This observation seems to indicate that some microstructural changes, such as the formation of new crosslinks, occur in the thermo‐oxidatively aged TPEs. This trend was supported by further reduction in the swelling index of the aged TPEs.© 2003 Society of Chemical Industry

Thermo-oxidative aging and fatigue behavior of dynamically vulcanized PVC/ENR thermoplastic elastomers

Dynamically vulcanized poly(vinyl chloride)/epoxidized natural rubber (PVC/ENR) thermoplastic elastomers (TPEs) were prepared following a semi-efficient vulcanization system (semi-EV) with the incorporation of di-ethylhexyl phthalate (DOP) as a plasticizer. The PVC/ENR TPEs were melt mixed at 150°C and 50 rpm using a Brabender Plasticorder. The effect of dynamic curing on the fatigue behavior of the PVC/ENR TPEs was investigated. The properties investigated include fatigue life, stress - strain behavior, strain energy, morphology, and the effect of hysteresis. The unaged samples showed that as the sulfur concentration increases the fatigue life increases which could be related to the increase in cross-link density as indicated from the swelling index data. The increase in cross-link density increases the resistance of these materials against external dynamic stress. Hysteresis study revealed that samples with higher sulfur concentration are less sensitive towards changes in strain energy. As a result they exhibited lower heat build-up and consequently higher fatigue life. The effect of thermo-oxidative aging (TOA) on the fatigue behavior was investigated by exposing the PVC/ENR TPEs in an air oven at 80°C for 168 hours. It was found that all the properties studied decreased after thermo-oxidative aging process. This suggests that some micro-structural changes such as the formation of new cross-links due to post curing has taken place. This accounts for the reduced fatigue life and resilience while hysteresis, modulus, and hardness increased.

Physicochemical behaviour of chitin gels

Syneresis of chitin gels formed in the course of N-acetylation of chitosan in hydroalcoholic media has been studied. A critical cross-linking density related to a critical acetylation degree for which the gel undergoes weak syneresis and swells in water was shown (degree of acetylation (DA) 88%). Above this value, the weight loss during syneresis increases with DA. Conversely, syneresis decreases on increasing the polymer concentration, but disappears at a macroscopic level for a polymer concentration close to the critical concentration of entanglement in the initial solution. An increase in temperature favours the formation of hydrophobic interactions and new inter- and intramolecular hydrogen bondings. Due to the weak polyelectrolyte character of chitin, the weight of the gel depends on the pH and ionic strength of the media. Swelling–deswelling experiments show that the swelling of the gel is not fully reversible in relation with the formation of new cross-links during the depletion of the network. Our results reveals that the balance between segment–segment and segment–solvent interactions as well as the molecular mobility play the major role.