Mechanical Properties Of Gels Research Articles

Combined effect of denatured whey protein concentrate level and fat level in milk on rennet gel properties

A factorial experimental design was used to evaluate the effect of denatured whey protein and fat concentrations in milk on rennet gel properties. The coagulation properties were characterized by dynamic rheometry. Decreased coagulation rate and storage modulus (G′) were observed when denatured whey protein concentrate (DWPC) was added to milk. Increasing the fat content barely affected milk coagulation, with a slight decrease in G′ at 60 min. The contraction kinetics of the gels during cooking was also characterized. Only the increase in DWPC reduced the contraction capacity of the gel. The use of DWPC in miniature cheese production linearly increased cheese yield and moisture. Introducing DWPC did not reduce fat retention in the curd, except at 0.75% when combined with the highest fat level (3.4%). Fat globules and DWPC aggregates are both considered to be fillers in rennet gels, but our results clearly show different impacts on gel mechanical properties.

A novel mono‐methacryloyloxy terminated fluorinated macromonomer used for the modification of UV curable acrylic copolymers

ABSTRACTA novel macromonomer containing fluorinated units (PHFBMA‐GMA) was synthesized through a two‐step procedure: firstly, hexafluoro‐butyl methacrylate (HFBMA) was polymerized in the presence of functional chain transfer agent 3‐mercaptopropionic acid (MPA) and then the carboxyl acid group terminated polymer was end‐capped with glycidyl methacrylate (GMA). Chemical structures of PHFBMA‐GMA were characterized by gel permeation chromatography, fourier transform infrared spectroscopy (FTIR), and 1H nuclear magnetic resonance (NMR). Subsequently, PHFBMA‐GMA was employed as reactive surface additives added into UV‐cured polyacrylate to modify UV‐curable coatings. It is convenient to control the tail length of the fluorinated segments in this study by adjusting the ratio of initiator and chain transfer agent. The influence of both the concentration and the molecular weight of PHFBMA‐GMA on the surface properties of UV‐cured films was investigated. With increasing both the concentration and the molecular weight of PHFBMA‐GMA, the surface energy of the UV‐cured films decreased. X‐ray photoelectron spectroscopy was employed to characterize and quantify the surface composition and the results confirm the enrichment of fluorinate atoms on the surface. Moreover, the physical properties of UV‐cured films, such as gel content, water absorption, pencil hardness, adhesion, chemical resistance, mechanical properties, optical transmittance, and thermal properties, were also investigated in detail. The novel macromonomer was economical but effective to modify the properties of the UV‐curable coatings. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43116.

Rheological, mucoadhesive and textural properties of thermoresponsive polymer blends for biomedical applications

The development of binary polymeric mixtures (polymer blends) containing bioadhesive and thermoresponsive polymers can provide new materials for biomedical applications, with higher contact, increased adhesion, prolonged residence time, protection, and in determined cases, secured absorption of an active agent from the site of application. Mixtures were prepared using a wide range of poloxamer 407 and Carbopol 971P® amounts. The rheological (flow and oscillatory), sol–gel transition temperature, mechanical (hardness, compressibility, adhesiveness, cohesiveness and elasticity), softness, and mucoadhesive properties of formulations were investigated. Moreover, the interaction between the different proportions of polymers was also analyzed. Continuous shear and oscillatory rheometry identified the plastic flow with various degrees of thixotropy, besides the viscoelastic behavior of formulations. The determination of gelation temperature displayed values ranged from 27.17 to 41.09°C. It was also found that low carbomer concentrations were enough to provide positive interaction parameter. However, the highest values were obtained for the polymeric blends with higher concentration of poloxamer 407. The mucoadhesion and softness index were greater in preparations containing 20% (w/w) poloxamer 407. The rheological, mechanical and mucoadhesive properties of the polymeric blends can be manipulated by changing the concentrations of the polymers and they suggest the blends are worthy of biomedical applications.

Effects of electron beam irradiation on the gel fraction, thermal and mechanical properties of poly(butylene succinate) crosslinked by multi-functional monomer

Abstract In the present work, a multi-functional monomer triallyl isocyanurate (TAIC) of 4 wt.% is incorporated into neat poly(butylene succinate) (PBS) by twin screw extrusion technique and the PBS sheets are irradiated at various electron beam absorption doses. It has been found that electron beam irradiation at an appropriate absorption dose plays an effective role in crosslinking the PBS molecules in the presence of TAIC. As a result, the irradiation influences the gel fraction, glass transition temperature, thermal stability, thermo-dimensional stability, dynamic storage modulus, and tensile properties of PBS, depending on the electron beam absorption dose applied. The improvement of the thermal and mechanical properties of PBS is most significant in the range of 50 to 70 kGy. The results are consistent with each other, supporting the electron beam irradiation effect on the property improvement of PBS.

Topological structure influences on the gel formation process and mechanical properties of l-lysine based supramolecular gels

The influence of a minor modification of the topological structure of a gelator's core on the mechanism of the self-assembly processes and the resultant gel properties were researched.

Synthesis of amino acid based covalently cross-linked polymeric gels using tetrakis(hydroxymethyl) phosphonium chloride as a cross-linker

This work reports the design and synthesis of side-chain amino acid containing cross-linked polymer gels with high mechanical strength. The Boc-leucine methacryloyloxyethyl ester (Boc-Leu-HEMA) has been polymerized by reversible addition-fragmentation chain transfer (RAFT) polymerization to obtain the corresponding polymer, P(Boc-Leu-HEMA). After Boc group deprotection under acidic condition and pH neutralization amino groups remain in the polymer side chain, which were reacted with tetrakis(hydroxymethyl) phosphonium chloride (THPC) to obtain chemically cross-linked gels. The mechanical properties of gels have been studied by rheological measurements, which reveal that mechanical strength increases as weight% of polymer in solution, cross-linking density and polarity of solvents increases, and decreases as molecular weight of polymers increases. A plateau of storage modulus (G′) over a wide range of angular frequency demonstrates that viscoelastic property of gels is independent of angular frequency. Field emission scanning electron microscopy (FE-SEM) studies of cross-sectioned gels validate rheological observation that as the G′ values of gel increases, porosity of gel decreases.

Impact of Molecular Architectures on the Thermal and Mechanical Properties of Multi‐Phase Polymer Networks

SummaryMultiphase copolymer networks (CLEG) composed of crystallizable poly(ε‐caprolactone) (PCL) and crystallizable poly(ethylene glycol) (PEG) segments can exhibit a pronounced triple‐shape effect in the dry state or a dual‐shape effect in the water swollen, hydrogel state. We hypothesize that by adjusting the network architecture of CLEG copolymers networks the swelling behavior, the thermal and mechanical properties, as well as the crystal structure of PEG and PCL domains can be tailored. Here, we studied CLEG materials with a fixed PCL/PEG weight ratio but different polymer network architectures, whereby the PEG segments were incorporated as grafted side chains or as network chains connecting two netpoints. The prepared CLEGs were analyzed regarding their gel contents, swelling behavior, thermal and mechanical properties and finally the crystallinity of the polymer networks were determined by differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS) measurements. High gel content values of G ≥ 98% were achieved for all copolymer network samples, indicating an almost complete conversion of the reaction precursors. The degree of swelling (Q) determined with water increased from Q ≈ 150% to 223% with increasing poly(ethylene glycol) monomethyl monomethacrylate (PEGMMA) content to create more PEG grafted side chains. Only when PEGMMA moieties were present in the CLEG networks two separated melting transitions related to the PCL and PEG segments were obtained, whereby Tm,PEG was found to increase with increasing the PEGMMA weight fraction. Based on this observation it can be assumed that the incorporated poly(ε‐caprolactone) diisocyanoethyl methacrylate PEGDIMA segments forming the main network were not able to form crystallites in CLEG networks. An almost two‐fold decrease in the Younǵs modulus was observed with increasing amounts of grafted PEG side chains, while the elongation at break increased significantly. Based on Mooney‐Rivlin Equations, the crosslinking density increased from 0.10% to 0.21% when PEG segments switched from grafted chains to the network. WAXS and DSC investigations revealed an increase in the degree of crystallinity (DOC) of PEG segments with increasing PEGMMA moieties, while the DOC related to the crystalline PCL domains remained almost constant. The obtained results clearly demonstrated the importance of the molecular architecture in designing polymer networks.

Mechanochemical Reaction Cascade for Sensitive Detection of Covalent Bond Breakage in Hydrogels

A novel strategy is reported for sensing chemical bond breakage in hydrogels at low levels of mechanical stress using a mechanochemical reaction cascade to generate fluorescence. Hydrogels are promising substrates and frameworks for cell growth and tissue engineering, particularly for cardiovascular repair and cartilage replacement. For these applications, it is important to maintain careful control over gel mechanical properties so that these hydrogels not only match the properties of the desired tissue for replacement but also retain their integrity for extended periods. Since the failure of hydrogels begins with the breakage of cross-links within the structure, methods are needed to sense these initial events for monitoring the performance of implants. In this work, it was hypothesized that nonspecific covalent bond breakage would produce radicals that would react with water to produce reactive oxygen species, which in turn could activate fluorophores sensitive to these. A series of multiarm poly(ethyl...

Fibril formation pH controls intrafibrillar collagen biomineralization in vitro and in vivo

We demonstrate that intrafibrillar, homogenous collagen biomineralization can be achieved by controlling self-assembly under mildly alkaline conditions. Using dense collagen (DC) gels as an osteoid model, we modulated their fibrillogenesis environment to evaluate the effects of fibrillogenesis pH on the protein charge distribution and ultimately on biomineralization. Cationic and anionic dye staining and electron cryomicroscopy analyses established that fibrillogenesis under mildly alkaline conditions promotes the formation of electronegative charges within the protein (anionic DC gels). These charges are stable upon titration of the gel pH to physiological values. Subsequent exposure of anionic DC gels to simulated body fluid induced the intrafibrillar biomineralization of the gels, promoting a rapid, extensive formation of carbonated hydroxyapatite, and strongly impacting gel mechanical properties. The generality and significance of this approach has been addressed by implanting freshly made anionic DC gels in vivo, in a rat subcutaneous model. Subcutaneous implants showed an extensive, homogenous biomineralization as early as at day 7, indicating that anionic collagen gels rapidly self-mineralize upon contact with body fluids in a non-osseous implantation site. The control of collagen fibrillogenesis pH provides not only new interpretations to what has been called the collagen mineralization enigma by demonstrating that neat collagen can intrafibrilarly self-mineralize, but it will also set a new starting point for the use of DC gels in bone regenerative medicine, in addition as potential applications as mineralized tissue model or as slow-release delivery carriers.

The Effect of Polymerization Mode on the POSS Hybrid PVA Hydrogels

A novel POSS hybrid nanocomposite hydrogels was synthesized by introducing water-soluble Oa-POSS into the PVA hydrogels by freeze-draw mode. The freezing temperature and freeze-thaw cycle times affected the mechanical properties of gels. When polymerizing at lower temperature, the tensile strength and modulus increases and the elongation at break decreases obviously. Meantime, with the increase of freeze-thaw cycle times, the strength, modulus and elongation at break all improved gradually.

Radiation-chemical preparation of poly(vinyl alcohol) hydrogels

This work reports the usage of method of radiation-chemical synthesis to prepare cross-linked hydrogels from poly(vinyl alcohol) modified with glycidyl methacrylate. Synthesis kinetics of modified poly(vinyl alcohol) and properties of hydrogels were studied. The gel fraction, swelling, mechanical properties, and water content of the hydrogels were measured. It was found that gel fraction increases with increasing radiation dose, concentration of modified poly(vinyl alcohol), and reaches 60%. It was established by differential scanning calorimetry that a fraction of the “bound” water in hydrogels is 50–70% and independent of gel fraction content. In addition to “bound” and “free” states, water in hydrogels is also present in the intermediate state.

Effect of γ‐Irradiation on the Physicochemical Properties of LDPE/HCB and LDPE/iPP/HCB Composites

ABSTRACTThe influence of γ‐irradiation dose and the concentration of hexachlorobutadiene (HCB) on the thermal and mechanical properties of low‐density polyethylene (LDPE) and LDPE/isotactic polypropylene (iPP) blends have been investigated. Gel fraction, mechanical properties, and morphological evolution of gamma‐irradiated LDPE/HCB and LDPE/iPP/HCB composites were studied. The gel content of LDPE/HCB samples increased with increasing irradiation dose from 150 to 350 kGy and then it slightly decreased at higher irradiation dose (600 kGy). On the other hand, the gel content of LDPE/iPP/HCB (100/0/3, 80/20/3, and 50/50/3 wt%) composites increased gradually with increasing irradiation dose. For LDPE/HCB composites, the tensile strength increases gradually with increasing either the irradiation dose or HCB content. The tensile strength of LDPE/iPP/HCB blend formulations increased with increasing either the irradiation dose or iPP content. The addition of iPP to the LDPE matrix increased the thermal temperature parameters of composites as well as the incorporation of the HCB agent. The thermal stability of LDPE/iPP/HCB composites is higher than that of LDPE/HCB.

Biomaterials based on a natural polysaccharide: alginate

In this short review, our objective was to describe few new applications of a natural polysaccharide extracted from algae, the alginates. Firstly, the chemical composition is recalled and the main techniques used for characterization are cited. The role of guluronic acid blocks (GG blocks) is particularly important for gel mechanical properties in presence of calcium counterions. Then, the main applications in food and biomedicine are briefly mentioned followed by tentative applications in packaging, paper, textil and wound dressing which are described. Especially, fine fibers are produced now industrially under mixed sodium/calcium ionic form.

Biomimetic Fibrin–Hyaluronan Hydrogels for Nucleus Pulposus Regeneration

To develop a biomimetic polymeric injectable hydrogel that can support nucleus pulposus (NP) regeneration. Natural polymer-based hydrogels were synthesized using fibrinogen (FBG) and hyaluronic acid (HA), conjugated by a novel two-step procedure. Bovine NP cells were cultured in FBG-HA conjugate-based 3D beads in vitro and in a nucleotomized organ culture model. FBG-HA conjugate-based hydrogels prepared with 235 KDa HA at a FBG/HA w/w ratio of 17:1 showed superior gel stability and mechanical properties and markedly increased glycosaminoglycan synthesis compared with a FBG/HA mixture-based hydrogels or fibrin gels. Gene-expression levels of NP markers were maintained in vitro. In organ culture, NP cells seeded in FBG-HA conjugate-based hydrogels showed better integration with native NP tissue compared with fibrin gels. Moreover, FBG-HA conjugate-based hydrogels restored compressive stiffness and disc height after nucleotomy under dynamic load. Specific FBG-HA conjugate-based hydrogels may be suitable as injectable materials for minimally invasive, biological NP regeneration.

Mechanical Properties of Self-Assembled Fmoc-Diphenylalanine Molecular Gels

We explore the phase diagram and mechanical properties of molecular gels produced from mixing water with a dimethyl sulfoxide (DMSO) solution of the aromatic dipeptide derivative fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF). Highly soluble in DMSO, Fmoc-FF assembles into fibrous networks that form gels upon addition of water. At high water concentrations, rigid gels can be formed at Fmoc-FF concentrations as low as 0.01 wt %. The conditions are established defining the Fmoc-FF and water concentrations at which gels are formed. Below the gel boundary, the solutions are clear and colorless and have long-term stability. Above the gel boundary, gels are formed with increasing rapidity with increasing water or Fmoc-FF concentrations. A systematic characterization of the effect of Fmoc-FF and water concentrations on the mechanical properties of the gels is presented, demonstrating that the elastic behavior of the gels follows a specific, robust scaling with Fmoc-FF volume fraction. Furthermore, we characterize the kinetics of gelation and demonstrate that these gels are reversible in the sense that they can be disrupted mechanically and rebuild strength over time.

SYNTHESIS AND CHARACTERIZATION OF POLY(VINYL ALCOHOL)-HYALURONIC ACID BLENDED HYDROGEL MEMBRANES

Poly(vinyl alcohol)PVA is a hydrophilic polymer and water soluble . It is used in many biomedical and pharmaceutical applications, due to its advantages such as: non-toxic, non-carcinogenic, and biodegradable characteristics with the ease of processing. Physically cross-linked hydrogel membranes composed of different amounts of hyaluronic acid (HA) blend with (PVA) were prepared by freeze–thawing method. This freezing–thawing cycle was repeated for three consecutive cycles. Properties of (PVA–HA) hydrogel membrane such as gel fraction, swelling, mechanical properties(tensile strength, elongation to break),degradation and protein adsorption were investigated. With the increasing of HA content, the gel fraction, the maximum tensile strength and elongation at break(%) of (PVA-HA) hydrogel membranes were decreased. Furthermore, with the increase of HA content, the swelling, the protein adsorption and the hydrolytic degradation of PVA-HA hydrogel membrane were increased.After soaking of hydrogel membrane for three days in phosphate buffer saline (PBS), the maximum weight loss of PVA–HA hydrogel membranes ranged between 18% and 70% according to HA content, this indicates that they are biodegradable.

Doubly crosslinked poly(vinyl amine) microgels: hydrogels of covalently inter-linked cationic microgel particles.

Doubly crosslinked (DX) microgels are macroscopic hydrogels comprised of covalently inter-linked singly crosslinked (colloidal) microgel particles. In this study we demonstrate for the first time that DX microgels can be prepared from concentrated dispersions of singly crosslinked (SX) poly(vinyl amine) (PVAM) microgel particles. The latter were of micrometer size, cationic and contained high primary amine contents. The DX PVAM morphologies contained extensive inter-connected porosity as determined by optical microscopy and SEM. The effective porosity ranged from 76 to 93 vol% and was tuneable through microgel particle concentration. The mechanical properties of the DX PVAM microgels were investigated using dynamic rheology. The best DX PVAM microgel had a storage modulus (G') of 41 kPa and yield strain of 46%, which are a good combination of elasticity and ductility. This gel had an internal porosity of 76 vol%. The dependence of G' on the effective volume fraction (ϕeff) for the DX PVAM microgels was tuneable and followed the equation: G'∼ exp(bϕeff), with b = 16.4. The latter value indicated low particle softness. The DX PVAM gels were also injectable and could be prepared at 37 °C. Furthermore, the gel mechanical properties after swelling for 3 days at physiological pH and ionic strength were similar to those before swelling. Because these injectable DX PVAM microgels have high primary amine contents they are well suited to functionalisation and should have potential applications in areas including catalysis, composite hydrogels and biomaterials.

Supramolecular hydrogels assembled from nonionic poly(ethylene glycol)-b-polypeptide diblocks containing OEGylated poly-l-glutamate

A poly(ethylene glycol)-b-poly(γ-(2-methoxyethoxy)esteryl-L-glutamate) (PEG-b-poly-L-EG2Glu) diblock was prepared by the ring-opening polymerization (ROP) of L-EG2Glu N-carboxyanhydride (NCA) using PEG–NH2 as the macroinitiator. When the average degree of polymerization (DP) for poly-L-EG2Glu was below 40, the PEG-b-poly-L-EG2Glu diblock could spontaneously form hydrogels in water. For the different diblocks, a thermally-induced increase in mechanical strength was observed for the PEG-b-poly-L-EG2Glu diblock between 20–45 °C. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed that continuous networks of nanoribbons were formed in these hydrogel samples. Circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) characterization suggested that the formation of sufficient β-sheet content accounted for the nanoribbon assemblies and hydrogel networks. The minimal gelation concentration and gel mechanical properties were found to strongly depend on the composition of the PEG-b-poly-L-EG2Glu diblocks.

Decrosslinking of crosslinked high‐density polyethylene via ultrasonically aided single‐screw extrusion

A novel environmentally friendly process for decrosslinking of the peroxide crosslinked HDPE (XHDPE) via ultrasonic assisted single (SSE) is developed and its process characteristics are established. The specific ultrasonic energy decreases with the flow rate and increases with the ultrasonic amplitude, while die pressure increases with the flow rate and decreases with the ultrasonic amplitude. Application of ultrasonic treatment during extrusion enables an increase of productivity. Gel fraction, crosslink density, dynamic and mechanical properties, and thermal behavior of the virgin HDPE, XHDPE, and decrosslinked XHDPE are measured. Gel fraction and crosslink density of the decrosslinked XHDPE are decreased with increasing flow rate and ultrasonic amplitude. A unique linear relation between the normalized gel fraction and the normalized crosslink density is found, regardless of the type of extruders and processing conditions. SEM images reveals that the decrosslinked XHDPE is a composite of submicron size gel particles embedded in its sol matrix. The sol extracted from the decrosslinked XHDPE exhibits a higher complex viscosity and higher level of branching than the virgin HDPE. An increase of the ultrasonic amplitude leads to a decrease of the complex viscosity, storage and loss moduli, and an increase of the loss tangent of the decrosslinked XHDPE. The thermal behavior and mechanical properties of the decrosslinked XHDPE show a weak dependency on processing conditions. At some processing conditions, mechanical properties of the decrosslinked XHDPE are close or higher to those of XHDPE. POLYM. ENG. SCI., 54:2715–2730, 2014. © 2013 Society of Plastics Engineers

Correlation between Local and Global Inhomogeneities of Chemical Gels

It is well-known that mechanical properties of gels are strongly related to inhomogeneities of gels such as spatial and connectivity inhomogeneities (defects and variation of cycles). Here, a cycle is defined as the type of polygonal shape formed by several prepolymers. However, these inhomogeneities, especially connectivity inhomogeneities, have hardly been quantified due to the difficulty in directly observing them. To directly observe and analyze gel structures, we used a molecular dynamics simulation to reproduce gelation process and quantified defects and cycles of the obtained gels. Furthermore, we investigated the relation between defects or cycles and spatial inhomogeneities. Our investigation indicated that defects caused spatial inhomogeneities in dilute and semidilute systems. On the other hand, in concentrated systems, self-biting links, in which a cross-link connects with itself, caused spatial inhomogeneities. This could be observed in the swollen state, which is revealed in the difference in degree of swelling of cycles during the swelling process.