Different Degrees Of Crosslinking Research Articles

Physical, mechanical and antibacterial properties of alginate film: Effect of the crosslinking degree and oregano essential oil concentration

Alginate films with different degrees of crosslinking obtained by internal gelation, and alginate films incorporated with oregano essential oil (OEO) were prepared. The impact of the degree of crosslinking caused by the use of calcium carbonate as crosslinking agent and the incorporation of OEO into the alginate films on their antibacterial, optical, mechanical, microstructural and water vapour barrier properties was evaluated.An increase in the degree of crosslinking produced alginate films that were significantly thicker (0.031–0.038mm) and stronger (51.9–52.9MPa) but less elastic (2.3%) than those non-crosslinked films (0.029mm; 39.7MPa; 4.4%). The water vapour permeability (WVP) of the films decreased significantly only with the highest level of crosslinking.The incorporation of OEO in alginate films affected significantly their physical properties. Thickness and percent elongation at break of the films were increased by the addition of OEO (0.036–0.042mm and 2.7–3.7%), while the tensile strength and water vapour permeability decreased (31.1–55.5MPa and 2.7–3.0×10−9g/msPa).Films incorporated with OEO were more effective against Gram-positive bacteria (Staphylococcus aureus and Listeria monocytogenes) than Gram-negative bacteria (Escherichia coli and SalmonellaEnteritidis). A minimum concentration of 1.0% of OEO was necessary to ensure their antibacterial efficacy.

Photo- and thermally initiated polymerization of methacrylate monomer derivative of bis(4-hydroxyphenyl)sulfide with N-vinyl-2-pyrrolidone

The results of the studies concerning photo- and thermally initiated copolymerization of bis[4(2-hydroxy-3-methacryloyloxypropoxy)phenyl]sulfide (BES-DM) with N-vinyl-2-pyrrolidone (NVP) and thermo-mechanical properties of resulting compositions are presented. BES-DM was obtained in the two-step reaction. In the first step, the epoxy resin was synthesized. It was obtained in the reaction of bis(4-hydroxyphenyl)sulfide with 2-(chloromethyl)oxirane. In the second step, esterification of the obtained diglycidyl ether was carried out with the use of methacrylic acid. New copolymers with different degrees of crosslinking were obtained. The study describes the effect of crosslinking degree and the method of polymerization initiation on the properties of the new compositions. Density, viscosity, glass temperature, Young’s modulus, hardness, tensile strength were determined before and after curing for the compositions of BES-DM and NVP. Moreover, the dynamic-mechanical and thermal properties for the chosen samples of copolymers were studied.

Thermal properties of porous copolymers of BM-DVB and their carbonization products

Thermogravimetry and differential scanning calorimetry were used for characterization of the thermal properties of new 4,4′-bismaleimidodiphenylmethane (BM) and divinylbenzene (DVB) porous copolymers and their carbonization products. Bead-shaped porous copolymers BM-DVB with the following monomer ratios 1:4, 1:1, 4:1 were synthesized using suspension copolymerization under the same conditions. Differences in the monomer ratio caused a different degree of cross-linking of the starting polymers. Before carbonization, the BM-DVB copolymers were pretreated using two methods. In one method, the starting material was stabilized in hot air (product was labeled PO-C800). In the other method, the copolymer was soaked in H 3PO 4 (product was named P800). Then, materials obtained by both methods were carbonized at 800 °C in an argon atmosphere. To characterize the heat resistance of the BM-DVB copolymers and their carbonized derivatives, their thermostabilities were evaluated. The data suggest the existence of a relationship between the composition and thermal stability of the copolymers and their carbonized derivatives. The most thermally resistant copolymer was that obtained with a 4:1 molar ratio of BM to DVB. Its thermal stability is caused by the high concentration of nitrogen atoms in the polymeric structure. 1:4 BM-DVB copolymer with a high degree of cross-linking was the least thermally stable, which might be caused by its microporous nature and small fraction of nitrogen. The derived carbons have very similar thermal properties, and an insignificant influence of the nature of the polymer precursor was observed. More important factors affecting thermal stability were the porosity and surface chemistry, which were created in the thermal pretreatment processes.

Correlations for prediction of specific surface area and bulk and apparent densities of porous styrene‐divinylbenzene copolymers

AbstractMacroporous styrene‐divinylbenzene copolymers with different degree of crosslinking were prepared by suspension polymerization in presence of different binary mixtures of toluene and heptane, as diluent. Specific surface area, bulk and apparent densities, and pore volume of the resulting beads were determined experimentally. Applying the least square method to the experimental data, correlations for prediction of these properties were obtained. Effects of divinylbenzene concentration, diluent to comonomer volume ratio, and composition of the diluent mixture were considered in developing the aforementioned correlations. The influence of the reaction recipe on porous structure of the samples was also studied using scanning electron microscope (SEM). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Naturally crosslinked gelatin gels with modified material properties

Natural crosslinking of gelatin using a simple plant-derived phenolic compound, caffeic acid has been studied. For the first time, we are reporting a thermo-irreversible gelatin gel formation at 60 °C. Controlling the crosslinking reaction has resulted in obtaining gelatin with modified material properties. Reaction parameters such as reaction pH, reaction time and concentration of caffeic acid have been optimised to obtain different degrees of crosslinking. The modified material properties were studied using small and large deformation rheology. Gelatin crosslinked at 60 °C and pH 9 for 20 min using 1.5% concentration of caffeic acid showed higher melting and setting temperatures. The storage modulus and gel strength were also found to be higher for crosslinked gelatin at higher temperatures.

High Performance Polysodium Acrylate Superabsorbents Utilizing Microfibrillated Cellulose to Augment Gel Properties

Microfibrillated cellulose was utilized at low concentrations as a filler material, added prior to free radical polymerization, in cross-linked superabsorbent polysodium acrylate hydrogels. The effect of microfibrillated cellulose concentration on equilibrium swelling, shear modulus after synthesis, and shear modulus at equilibrium swelling was studied at different degree of cross-linking. For the characterization of the microfibrillated cellulose optical microscopy, atomic force microscopy, and transmittance analysis were used. The shear modulus of the samples was determined using uniaxial compression analysis. The swelling of the gels was determined using classical gravimetrical measurements. It was found that microfibrillated cellulose was highly efficient in increasing the shear modulus of the gels. Furthermore, the microfibrillated cellulose was found to have the same effect on the swelling and shear modulus at equilibrium swelling as the same mass of the conventional covalent cross-linker N,N'-methylenebisacrylamide (MBA), while in fact improving the fracture resistance of the gels. In conclusion, microfibrillated cellulose shows great potential as an additive to enhance the performance of soft materials.

ChemInform Abstract: High Catalytic Activity of Chiral Amino Alcohol Ligands Anchored to Polystyrene Resins.

Enantiomerically pure (2S,3S)-2,3-epoxy-3-phenylpropanol (3) has been anchored to Merrifield resins with different degrees of cross-linking and functionalization. The resulting epoxy-functionalized...

Chitosan-genipin microspheres for the controlled release of drugs: clarithromycin, tramadol and heparin.

The aim of this study was to first evaluate whether the chitosan hydrochloride-genipin crosslinking reaction is influenced by factors such as time, and polymer/genipin concentration, and second, to develop crosslinked drug loaded microspheres to improve the control over drug release. Once the crosslinking process was characterized as a function of the factors mentioned above, drug loaded hydrochloride chitosan microspheres with different degrees of crosslinking were obtained. Microspheres were characterized in terms of size, morphology, drug content, surface charge and capacity to control in vitro drug release. Clarithromycin, tramadol hydrochloride, and low molecular weight heparin (LMWH) were used as model drugs. The obtained particles were spherical, positively charged, with a diameter of 1–10 μm. X-Ray diffraction showed that there was an interaction of genipin and each drug with chitosan in the microspheres. In relation to the release profiles, a higher degree of crosslinking led to more control of drug release in the case of clarithromycin and tramadol. For these drugs, optimal release profiles were obtained for microspheres crosslinked with 1 mM genipin at 50 ºC for 5 h and with 5 mM genipin at 50 ºC for 5 h, respectively. In LMWH microspheres, the best release profile corresponded to 0.5 mM genipin, 50 ºC, 5 h. In conclusion, genipin showed to be eligible as a chemical-crosslinking agent delaying the outflow of drugs from the microspheres. However, more studies in vitro and in vivo must be carried out to determine adequate crosslinking conditions for different drugs.

Understanding the swelling of poly (N-isopropyl acrylamide) gels through the study of free volume hole size distributions using positron annihilation spectroscopy

Poly (N-isopropyl acrylamide) gels were prepared by UV polymerization with different degree of cross-linking in different solvents. Ethylene glycol dimethacrylate and penta-erythritol tetra-acrylate were used as cross-linkers, and methanol and dimethyl formamide (DMF) were used as solvents for gel preparation. The free volume fraction and hole size distribution in the dry gels were measured using positron annihilation lifetime spectroscopy. The equilibrium swelling of the samples in water was measured at room temperature. Both swelling properties of gels and the free volume distributions were seen to be sensitive to the amount, type, and functionality of cross-linkers as well as solvent medium used for synthesis. The gels prepared in DMF showed poor swelling properties than those prepared in methanol. The mean free volume hole size was higher while the variance of hole size distribution was smaller in the gels prepared in DMF medium compared to those prepared in methanol. The free volume fractions in the gels were found to be inversely correlated to the extent of equilibrium swelling for similar chemical compositions. The possible reasons are discussed.

Effect of crosslink density on the properties of monolithic polymer matrices: Reversed-phase gas-chromatography study

Monolithic polymer matrices of different natures and different degrees of crosslinking have been synthesized in capillaries with an inner diameter of 100 µm. The properties of the monolithic matrices are characterized by reversed-phase gas chromatography. Solubility coefficient S, Flory-Huggins parameter gC 12 ∞ , and reduced Flory-Huggins parameter gC 12 ∞ are evaluated. For all tested sorbates, the values of S depend on the degree of crosslinking of the polymer, which is characterized by parameter gC 12 ∞ . In the case of all monolithic polymer matrices under study, the logarithm of the solubility coefficient plotted as a function of the squared critical temperature of sorbate is described by a straight line, a circumstance that is likewise typical of linear polymers. Parameter D/d 2 , which characterizes the rate of diffusion of low-molecular-mass compounds in the monolithic matrix, is calculated. For both polar and nonpolar polymers, the dependence of D/d 2 on the degree of crosslinking follows an extremum pattern.

Physicochemical properties of cross-linked and acetylated starches and products of their hydrolysis in continuous recycle membrane reactor

The aim of the present work was to study the physicochemical properties of doubly modified, by cross-linking and acetylating, starches as well as the products of their enzymatic hydrolysis. A two step procedure of hydrolysis, including the batch and membrane reactors, were investigated. The second step of enzymatic processes were carried out in a continuous recycle membrane reactor (CRMR). Three kinds of commercial starches – two preparations of acetylated distarch adipate E1422 of different degrees of cross-linking, as well as one preparation of acetylated distarch phosphate E1414 were examined. It was found that the degree of substitution of acetyl groups in the macromolecules of starch did not influence the effectiveness of hydrolysis. However, the degree of cross-linking with adipate groups slightly decreased the efficiency of processing in the CRMR. Additionally, the relationship between the type of hydrocolloid and its adsorption activity in the air/water and oil/water systems was considered. All obtained derivatives revealed adsorption properties and reduced the surface/interface tension in the air/water and oil/water systems. The efficiency and effectiveness of adsorption of the investigated hydrocolloids were affected by the type of modification as well as the degree of substitution of acetyl groups in the macromolecules of starch. Particle size distributions formed in aqueous solutions for all investigated hydrolyses were determined and compared with results obtained for commercial products.

Characterization of monolithic capillary columns using inverse gas chromatography

Polymeric monoliths of different polymeric natures and different degrees of cross-linking were prepared in capillaries of inner diameter 100 microm. The properties of the monoliths prepared were investigated by means of inverse gas chromatography. Solubility coefficient S, Flory-Huggins parameter chi(12)(infinity), and reduced Flory-Huggins parameter chi(12)(infinity') were evaluated. The solubility parameter S of all samples depends on the degree of cross-linking of the monoliths, which is characterized by the reduced Flory-Huggins parameter chi(12)(infinity'). A linear dependence of the logarithm of the solubility parameter S on the solute critical temperature was also demonstrated, as it is well known for non-cross linked polymers. The diffusivity of low molecular mass solutes in monoliths was evaluated by calculation of the ratio D/d(f)(2), and a maximum was detected for the dependence of the ratio D/d(f)(2) on the degree of cross-linking of monoliths. Correlation of the measured monolith properties with the separation performance of capillary columns was carried out and revealed that the best performance in GC is provided by monolithic columns prepared from purely bifunctional monomer, i. e. having 100% cross-linking.

Calorimetric and X-ray Studies of Clathrate Hydrates of Tetraisoamylammonium Polyacrylates

The structure of clathrate hydrates with tetraisoamylammonium polyacrylate salt incorporated as guest has been studied in this work. Also, quantitative studies on the stability changes of the clathrate hydrates with different degrees of cross-linking of the guest polymer (varied from 0 to 3%) have been conducted. A single crystal X-ray diffraction study of a crystal of the hydrate with linear (uncross-linked) tetraisoamylammonium polyacrylate as guest reveals a hexagonal structure (space group P6m2, a = 12.15 A, c =12.58 A at 100 K) with 39 host framework water molecules per one guest monomeric unit. Powder X-ray diffraction analyses confirm the identity of the above crystal structure of the hydrate with linear guest polymer and the crystal structure of the hydrates with cross-linked guest (hexagonal, a = 12.25 A, c =12.72 A at 276 K). In order to quantitatively determine the stability differences of the hydrates with the included guests having various degrees of cross-linking of the anionic chain, a series of differential scanning calorimetry measurements of the fusion enthalpy of the hydrate samples has been carried out. On the basis of the results obtained, a structural model describing the decrease in the stability of the clathrate hydrates with tetraisoamylammonium polyacrylate guest as a function of the degree of cross-linking of the guest polymer has been suggested.

Vitamin B12 diffusion and binding in crosslinked poly(acrylic acid)s and poly(acrylic acid-co- N-vinyl pyrrolidinone)s

The diffusion mechanism of vitamin B12 in two types of crosslinked hydrogels, poly(acrylic acid) (cPAA) and copolymers of acrylic acid and N-vinyl pyrrolidinone (cP(AA-NVP)) was studied. The PAA and P(AA-NVP) synthesized by three different degrees of crosslinking have limited water absorption capabilities ranging from 3% to 18%. In the copolymers permeability of B12 is controlled by both intramolecular and intermolecular hydrogen-bonding between the pyrrolidinone and carboxylic acid side chains. The diffusion kinetic data in two types of polymers were best described by Peppas models instead of Higuchi models. Permeation from both crosslinked PAA and P(AA-co-NVP) copolymers followed a Super Case II transport mechanism, most likely driven by macromolecular chain relaxation and swelling of hydrophilic polymers. A special FTIR spectroscopic method for drug binding study, FTIR difference spectroscopy, is used to probe the strong interactions between vitamin B12 and the side chains of the hydrogels. The FTIR differential spectra of B12 in PAA hydrogels revealed dramatic changes of the spectral marker bands of B12 after binding in the crosslinked gels, indicating significant interactions occurring in the amide and phosphate moieties of B12. Such interactions retard the diffusion of vitamin B12.

Frontal polymerization as a new method for developing drug controlled release systems (DCRS) based on polyacrylamide

Stimuli-responsive polymers are macromolecular materials that undergo changes in response to small external stimuli in the environmental conditions. Among stimuli-responsive hydrogels are several polyacrylamides. Frontal polymerization is a fast, easy and inexpensive polymerization technique used for the synthesis of macromolecules. Aim of this work was the evaluation of the Frontal polymerization technique as new method for the preparation of controlled release dosage forms in which drug loading and polymer preparation occur together, as well as the possibility of obtaining more dosage units by a unique preparation. Hydrogels based on polyacrylamide containing diclofenac sodium salt were prepared using the Frontal polymerization and compared with similar systems obtained by the classic batch method. Polymers characterized by three different degree of cross-linking were prepared. The stability of the drug during the sample preparation was evaluated by IR analysis. The obtained samples were characterized in terms of drug content, morphology, in vitro drug release and swelling properties. Samples were studied also divided into disks. The results show that hydrogels based on polyacrylamide can be prepared by Frontal polymerization; these samples show similar properties to those obtained by batch polymerization. The drug is stable in the polymerization reaction conditions. Samples characterized by the lowest degree of cross-linking show drug loading values always higher than samples with the highest one regardless of the preparation method employed. The swelling ratio decreases as the degree of cross-linking increases. Loaded samples swell more than drug free ones. From a single preparation of hydrogel, three disks showing same drug content and in vitro release behaviour can be obtained and thus they can be used as three single dosage units.

Roughness of mesoporous silica surfaces deduced from adsorption measurements

The complete understanding of the properties of a material goes in close relationship with a correct description of their structure. This is particularly true for mesoporous silica that exhibits different shapes and sizes of the porous framework as well as different degrees of crosslinking of the silica network itself. These variations can affect drastically the specific properties of these materials as well as their hydrothermal resistance. Hydrothermal resistance and adsorption properties are governed by the surface structure, but the later parameter is not frequently studied because it can be difficult to characterize. Therefore a simple parameter such as surface roughness could be helpful in relating the properties of materials with their surface structure. The direct determination of surface roughness is impossible, but it has been demonstrated that a single mathematical parameter, the surface fractal dimension Ds, can be related within a certain limit with surface roughness. Here we demonstrate using nitrogen adsorption data that a correlation can be drawn between the values of the surface fractal dimension Ds used as a gauge of the surface roughness and the structural evolution of a series of mesoporous silicas. Among the different adsorption models for obtaining Ds that are available nowadays, we selected the one proposed by Avnir and Jaroniec. We demonstrate in this report that there exists a real correlation between the evolution of Ds, when applied to a series of samples prepared under different synthesis conditions, with the evolution of their structure and properties. By example, this study allowed us to link the range of hydrothermal resistance of different mesoporous silicas (e.g. the HMS, MSU, MCM, SBA families of silicas) with their surface roughness. Moreover, we found that HMS silica, a very resistant material toward hydrothermal treatment, exhibits actually the lowest surface roughness. We could demonstrate too how different synthesis parameters such as temperature, can modify this surface roughness, hence the final properties of the mesoporous silica. Finally, we show that this method can even be applied to water ellipsoporometry and help to follow the crystallization of thin layers of metal oxide. Bearing in mind that Ds is a parameter that can readily be determined using the existing software – based on different models – associated with most commercial adsorption equipment, we urge the broader use of the Ds parameter for the structural characterization of surface roughness in mesoporous materials and metal oxide thin layers.

Pattern Formation during Deformation of a Confined Viscoelastic Layer: From a Viscous Liquid to a Soft Elastic Solid

We study pattern formation during tensile deformation of confined viscoelastic layers. The use of a model system [poly(dimethylsiloxane) with different degrees of cross-linking] allows us to go continuously from a viscous liquid to an elastic solid. We observe two distinct regimes of fingering instabilities: a regime called "elastic" with interfacial crack propagation, where the fingering wavelength scales only with the film thickness, and a bulk regime called "viscoelastic," where the fingering instability shows a Saffman-Taylor-like behavior. We find good quantitative agreement with theory in both cases and present a reduced parameter describing the transition between the two regimes and allowing us to predict the observed patterns over the whole range of viscoelastic properties.

The influence of periodate oxidation on the moisture sorptivity and dimensional stability of paper

The hygroexpansion of paper was significantly reduced, up to 28% lower amplitude of change when the paper was subjected to a change in relative humidity from 20 to 85% RH, by oxidation of the fibre wall. Never-dried bleached kraft fibres were oxidised with sodium periodate, which specifically oxidises the C2–C3 bond of 1,4-glucans so that the cellulose is partly converted into dialdehyde cellulose. Since both the dry and wet strength of laboratory sheets were significantly improved, the dry tensile strength increased from 24 kNm/kg up to 66 kNm/kg and the relative wet tensile strength increased from 1.5% up to 40%, it is suggested that the aldehydes form hemiacetal linkages within the fibre wall during the consolidation and drying of the sheets. The mechanical, hygroexpansive and moisture sorptive properties of the sheets made from the oxidised fibres were studied. The results showed that the main reason for the reduced hygroexpansion was a decrease in moisture sorptivity, i.e. when the sheets made of fibres with different degrees of cross-linking were subjected to the same change in relative humidity, the more cross-linked fibres showed a smaller change in moisture content. It was also shown that the hygroexpansion coefficient, i.e. the moisture-normalised dimensional change, was not significantly changed by the periodate oxidation, i.e. indicating that there are no improvement in dimensional stability if the paper is subjected to a specific amount of water.

Buttressing Staples with Cholecyst-derived Extracellular Matrix (CEM) Reinforces Staple Lines in an Ex Vivo Peristaltic Inflation Model

Staple line leakage and bleeding are the most common problems associated with the use of surgical staplers for gastrointestinal resection and anastomotic procedures. These complications can be reduced by reinforcing the staple lines with buttressing materials. The current study reports the potential use of cholecyst-derived extracellular matrix (CEM) in non-crosslinked (NCEM) and crosslinked (XCEM) forms, and compares their mechanical performance with clinically available buttress materials [small intestinal submucosa (SIS) and bovine pericardium (BP)] in an ex vivo small intestine model. Three crosslinked CEM variants (XCEM0005, XCEM001, and XCEM0033) with different degree of crosslinking were produced. An ex vivo peristaltic inflation model was established. Porcine small intestine segments were stapled on one end, using buttressed or non-buttressed surgical staplers. The opened, non-stapled ends were connected to a peristaltic pump and pressure transducer and sealed. The staple lines were then exposed to increased intraluminal pressure in a peristaltic manner. Both the leak and burst pressures of the test specimens were recorded. The leak pressures observed for non-crosslinked NCEM (137.8 +/- 22.3 mmHg), crosslinked XCEM0005 (109.1 +/- 14.1 mmHg), XCEM001 (150.1 +/- 16.0 mmHg), XCEM0033 (98.8 +/- 10.5 mmHg) reinforced staple lines were significantly higher when compared to non-buttressed control (28.3 +/- 10.8 mmHg) and SIS (one and four layers) (62.6 +/- 11.8 and 57.6 +/- 12.3 mmHg, respectively) buttressed staple lines. NCEM and XCEM were comparable to that observed for BP buttressed staple lines (138.8 +/- 3.6 mmHg). Only specimens with reinforced staple lines were able to achieve high intraluminal pressures (ruptured at the intestinal mesentery), indicating that buttress reinforcements were able to withstand pressure higher than that of natural tissue (physiological failure). These findings suggest that the use of CEM and XCEM as buttressing materials is associated with reinforced staple lines and increased leak pressures when compared to non-buttressed staple lines. CEM and XCEM were found to perform comparably with clinically available buttress materials in this ex vivo model.

Long time stress relaxation of amorphous networks under uniaxial tension: The Dynamic Constrained Junction Model

The constrained junction model that represents the stress–strain relations of amorphous networks in equilibrium is modified to analyze stress relaxation. Deviation of stress from equilibrium when a network is stretched suddenly is represented by a time dependent constraint contribution that is of the same form as that of the equilibrium theory. The time dependent motions of the junctions are assumed to obey the Langevin equation. The only new term in the model is a time dependent κ parameter that vanishes at long times. Results of the model are compared with uniaxial stress relaxation experiments on polyisoprene networks with different degrees of cross-linking. Experiments show that the time dependent κ parameter obeys a stretched exponential form, κ(t)=κ0exp[−(t/τ)β] with β=0.4 and τ=40s, both of which are the same for all extensions and cross-link densities studied. The front factor κ0 depends on cross-link density in the same way as in the equilibrium case. Comparison with stress relaxation experiments shows satisfactory agreement at a wide range of extensions and for different degrees of cross-linking. The relatively low value of the stretched exponent parameter, β=0.4, is interpreted in terms of a molecular picture where entanglements contribute to relaxation at a wide spectrum of time scales.