Changes In Crosslink Density Research Articles

Evaluation of Interfacial and Mechanical Properties of Glass Fiber/Poly-Dicyclopentadiene Composites with Different Post Curing at Ambient and Low Temperatures

Recently new poly-dicyclopentadiene (p-DCPD) has increasing been used for composites in military, aerospace, and transportation applications due to its favorable impact properties. In this work, the cross-linking density of p-DCPD with different post curing conditions was indirectly measured by thermal analysis using differential scanning calorimetry (DSC), weight variation accompanying swelling and density. The effects of the post curing conditions on mechanical properties of p-DCPD and glass fiber (GF)/p-DCPD were determined via tensile, Izod impact, and flexural tests at ambient and low temperatures (-20 °C). Interfacial properties were evaluated by fragmentation and cyclic loading tests. The work of adhesion was determined via Static contact angle using four solvents measurements, to provide the information on the differences in the interfacial properties between GF and p-DCPD. The change of cross-linking density by post curing of p-DCPD affected the mechanical and interfacial properties of GF/p-DCPD composites.

Influence of hydraulic oil on degradation behavior of nitrile rubber O-rings at elevated temperature

Abstract The degradation of nitrile rubber O-rings exposed to hydraulic oil and air were investigated by adopting accelerated aging test. The chemical structures and mechanical properties of the aged samples were evaluated by measuring attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results indicate that chemical structure changed significantly due to loss of additives and oxidation reactions. Hydraulic oil and temperature played a critical role in surface chemical changes. The crosslinking density changes were attributed to the competition between crosslinking and chain scission. The mechanical test results show that there existed large differences in changes of mechanical properties for samples under different conditions, which correlated to loss of fillers, the changes in crosslinking density and the formation of defects. Additionally, the fracture morphologies demonstrate that the formation of hardened brittle outer layer, voids and agglomerates also promoted the decrease in mechanical properties.

Improvement of elastomer swelling measurement for sulfur cross-link density determination

Due to growing demand from both environmental and industrial sectors the extent of rubber recycling is constantly increasing. Recycled rubber ranges from slabs of thermochemical reclaim, resembling virgin rubber, to fine rubber powders of diverse surface morphologies. Cross-Link Density (CLD) changes may characterize the effectiveness of rubber processing, but their measurements are often problematic. Most difficulties arise during determination of the volume fraction of polymer in the swollen rubber gel, also known as the Vr parameter (comparable to swell ratio), which closely relates to CLD. Vacuum, blotting and centrifugation were used to remove a swelling solvent (benzene) from the surface of the swollen rubber matrix. Centrifugation was more accurate for evaluating Vr of morphologically different types of rubber. Mechanical shear was used to produce a porous rubber sheet, while addition of acetamide and NaCl generated powder of devulcanized rubber. These samples were tested along with tire buffings and SBR sheets to show that extraction duration and time, needed to establish the swelling equilibrium between rubber and benzene, could be reduced from 72 h to less than 24 h in the case of porous or powdered rubbers. While compressed rubber might need longer durations, the overall time consumption for the CLD analysis could be shortened significantly, which helps in evaluating recycled rubber more thoroughly.

Effect of titanium dioxide on the UV‐C ageing behavior of silicone rubber

ABSTRACTThe ultraviolet (UV)‐C ageing behavior of silicone rubber (SiR) incorporated with titanium dioxide nanoparticles (nano‐TiO2) was studied under UV‐C radiation. The SiR incorporated with nano‐TiO2 displayed excellent physical properties when exposed to UV‐C radiation. With the increase of the ageing time, the SiR with nano‐TiO2 showed no significant change in crosslinking density and Shore A hardness. Moreover, compared with the SiR without nano‐TiO2, the SiR incorporated with nano‐TiO2 also exhibited high retention ratio in tensile properties, especially elongation at break. It was found that nano‐TiO2 was a good ultraviolet light stabilizer during the UV ageing process of SiR and the optimum content of nano‐TiO2 was 2 phr. The results of Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy showed the appearance of OH and CO and the decrease of the intensity of SiC2O2 in SiR samples during the process of UV‐C ageing. Based on these results, a possible UV ageing mechanism of SiR could be proposed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46099.

A Model for Calculating Hyperelastic Material Properties Under Thermal Aging

Understanding the degradation of material properties and stress–strain behavior of rubberlike materials that have been exposed to elevated temperature is essential for rubber components design and life time prediction. The complexity of the relationship between hyperelastic materials, crosslinking density (CLD), and chemical composition presents a difficult problem for the accurate prediction of mechanical properties under thermal aging. In this paper, a new and relatively simple mathematical formulation is presented to expresses the change in material properties of hyperelastic materials under thermal aging. The proposed formulation has been applied to a natural rubber (NR). Testing was performed on more than 130 specimens that were thermally aged then subjected uniaxial tension and hardness tests. The aging temperatures ranged from 76.7 °C to 115.5 °C, and the aging times ranged from 0 to 600 h. Based on the recorded experimental data, the NR mechanical properties under thermal aging showed a similar behavior to the rate of change of the CLD with aging time and temperature. Three mechanical properties have been chosen to be studied in this paper: the ultimate tensile strength, the fracture stretch value, and the secant modulus at 11.0% strain. The proposed mathematical formulation is a phenomenological equation that relates the material properties with the change in CLD based on a form of Arrhenius equation. The proposed equation showed promising results compared to the experimental data with an acceptable error margin of less than 10% in most of the cases studied.

Switchable release nano-reservoirs for co-delivery of drugs via a facile micelle-hydrogel composite.

Precise and controlled drug delivery systems are required to facilitate effective therapeutics. To address this need, we devised a micelle-hydrogel composite based on amphiphilic polypeptides as a general carrier model for the switchable and controlled release of dual drugs. Two different di-block polypeptides, poly(l-lysine-b-l-phenylalanine) and poly(l-glutamic acid-b-l-phenylalanine) (PGA-PPA), were synthesized to form distinct self-assembling micellar systems that were loaded with curcumin and amphotericin B, respectively, as model drugs. The drug-loaded micellar mixture was crosslinked utilizing the pendant amino groups of the l-lysine side chains via genipin to yield a micelle-hydrogel composite with PGA-PPA micelles trapped in an interlinked hydrogel system. This composite allowed for controlled multiphasic drug release and could be effectively tuned to moderate the pace and amount of drug release and be easily regulated to switch the drug release kinetics over a range of simple factors such as change in pH, cross-linking density, and composition.

The influence of pore size and stiffness on tenocyte bioactivity and transcriptomic stability in collagen-GAG scaffolds

Orthopedic injuries, particularly those involving tendons and ligaments, are some of the most commonly treated musculoskeletal ailments, but are associated with high costs and poor outcomes. A significant barrier in the design of biomaterials for tendon tissue engineering is the rapid de-differentiation observed for primary tenocytes once removed from the tendon body. Herein, we evaluate the use of an anisotropic collagen-glycosaminoglycan (CG) scaffold as a tendon regeneration platform. We report the effects of structural properties of the scaffold (pore size, collagen fiber crosslinking density) on resultant tenocyte bioactivity, viability, and gene expression. In doing so we address a standing hypothesis that scaffold anisotropy and strut flexural rigidity (stiffness) co-regulate long-term maintenance of a tenocyte phenotype. We report changes in equine tenocyte specific gene expression profiles and bioactivity across a homologous series of anisotropic collagen scaffolds with defined changes in pore size and crosslinking density. Anisotropic scaffolds with higher crosslinking densities and smaller pore sizes were more able to resist cell-mediated contraction forces, promote increased tenocyte metabolic activity, and maintain and increase expression of tenogenic gene expression profiles. These results suggest that control over scaffold strut flexural rigidity via crosslinking and porosity provides an ideal framework to resolve structure-function maps relating the influence of scaffold anisotropy, stiffness, and nutrient biotransport on tenocyte-mediated scaffold remodeling and long-term phenotype maintenance.

Low-temperature air plasma modification of chitosan-coated PEEK biomaterials

Novel chitosan-coated PEEK biomaterials were prepared by air plasma modification. Low-temperature plasma effect on changes of specific thermal, mechanical and adhesive properties of polyetheretherketone (PEEK) was investigated. The topography and surface roughness of the prepared materials were determined using an optical profilometer. The wetting and energetic properties of biomaterials were studied by means of advancing and receding contact angles measurements and then apparent surface free energy (and its components) were evaluated applying the LWAB (Lifshitz–van der Waals Acid Base) theory and contact angle hysteresis model. After air plasma treatment a fairly hydrophobic character of PEEK was changed to strongly hydrophilic one. Significant differences in the wettability and thermal stability of samples were observed. However, hardly any differences in excellent mechanical properties were noticed. The profilometer images showed an increase in the surface roughness of PEEK modified surface due to the change of cross-link density, elasticity and formation of additional polar groups on the surface. Plasma treated polyetheretherketone surfaces had better adhesive features and stable chitosan coating was created. Modification by chitosan improved antibacterial properties, inherent haemostatics and polymer biocompatibility. These advantages allowed to obtain new attractive biomaterials from the same polymer differing in properties for a wide spectrum of applications, mainly regenerative medicine and orthopedic surgery.

COMPATIBILITY STUDY OF LOW-TEMPERATURE–CAPABLE FLUOROELASTOMERS IN JET FUELS

ABSTRACT A series of four peroxide-cured fluoroelastomer (FKM) samples, varying in fluorine level and low-temperature resistance, were assessed in jet fuels JP-8 and JP-8+100 at 125 °C from 1 to 5 weeks. The key factors were fluorine level, FKM chemical structure, and the jet fuel +100 additive package. FKM stiffened upon aging with a corresponding loss in both tensile and elongation at break properties. Volume swell increased with immersion time, whereas any changes in chemical cross-linking density by equilibrium swelling were minor. The +100 additive package had a mild effect in causing property deterioration through stiffening and volume swell increases. Loss of fluorine from attack at the vinylidene fluoride group by basic additives is likely taking place. Lower fluorine containing FKM with flexible side chains was less affected by the additive package and provided the lowest glass-transition temperature (Tg; approaching −30 °C). The volume swells (<15%) are within the realm of an acceptable material to be in contact with jet fuels.

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration.

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.

PEROXIDE CROSS-LINKING OF EPDMs HAVING HIGH FRACTIONS OF ETHYLENIC UNITS

ABSTRACT The considerable amount of research in the literature has practically allowed the elucidation of the mechanism of peroxide cross-linking of ethylene–propylene–diene–monomer rubber (EPDM), which occurs through a radical chain reaction initiated by the thermal decomposition of the peroxide molecule. According to this radical chain reaction, all types of labile hydrocarbon bonds (i.e., allylic, methynic, and methylenic CH bonds) would be exposed to alkoxy radicals and involved in the formation of the elastomeric network. However, for high fractions of ethylenic units (typically ≥60 mol.%), simple chemical kinetics and thermochemical analyses have shown that the radical attack would essentially occur on the methylenic CH bonds. Starting from this assertion, a simplified mechanistic scheme has been proposed for the three commercial EPDMs under study. The corresponding kinetic model, derived from this new scheme by using the basic concepts of the chemical kinetics, provides access to the changes in concentration of the main reactive chemical functions (against exposure time), among which are double bonds and changes in cross-linking density. The validity of these predictions has been eventually successfully verified by five distinct analytical techniques frequently used for studying the cross-linking of rubbers.

Effects of Increasing Tungsten on Structural, Elastic and Optical Properties of xWO3–(40−x)Ag2O–60Te2O Glass System

Ternary tellurite glasses with composition of xWO3(40−x)Ag2O–60TeO2 (x = 0–40 mol%) have been prepared by melt-quenching method. Elastic and optical properties of the glass system were obtained by ultrasonic velocity measurements and UV–vis spectroscopy, respectively, while structural investigation was carried out by using Raman spectroscopy. The longitudinal and shear velocities, vL and vs showed large increase at x from 0 to 20 mol% before decrease with further addition of WO3. Independent longitudinal modulus CL and shear modulus μ bulk modulus Ke Young's modulus Y and Debye temperature θD showed similar behaviors to both velocities. The large increase of the elastic moduli at x from 0 to 20 mol% is suggested to be due to the increase in WO6 octahedral unit structure indicating the increase of bridging oxygen (BO) and formation of stronger Te–O–W bonds compared to Te–O–Te bonds. On the other hand, for x > 20 mol%, the decrease in the elastic moduli was due to the increase of non-bridging oxygen (NBO) as a result of formation of WO4 tetrahedral via breaking Te–O–W network. Further analysis by using bulk compression and ring deformation models showed a slight decrease in the ratio of ideal bulk modulus Kbc to the experimental bulk modulus Ke and average atomic ring size, l for x 20 mol%. Our analysis also indicates that limited ring deformation took place and the main compression mechanism in this glass system was mainly ideal isotropic compression. On the other hand, optical band gap Eopt showed small variation for x = 0–20 mol% but decreased upon higher WO3 content while refractive index n showed the opposite trend. This optical behavior is suggested to be related to the changes in cross link density and NBO concentration in the glass system.

The effects of varying poly(ethylene glycol) hydrogel crosslinking density and the crosslinking mechanism on protein accumulation in three-dimensional hydrogels

Matrix stiffness has been shown to play an important role in modulating various cell fate processes such as differentiation and cell cycle. Given that the stiffness can be easily tuned by varying the crosslinking density, poly(ethylene glycol) (PEG) hydrogels have been widely used as an artificial cell niche. However, little is known about how changes in the hydrogel crosslinking density may affect the accumulation of exogenous growth factors within 3-D hydrogel scaffolds formed by different crosslinking mechanisms. To address such shortcomings, we measured protein diffusivity and accumulation within PEG hydrogels with varying PEG molecular weight, concentration and crosslinking mechanism. We found that protein accumulation increased substantially above a critical mesh size, which was distinct from the protein diffusivity trend, highlighting the importance of using protein accumulation as a parameter to better predict the cell fates in addition to protein diffusivity, a parameter commonly reported by researchers studying protein diffusion in hydrogels. Furthermore, we found that chain-growth-polymerized gels allowed more protein accumulation than step-growth-polymerized gels, which may be the result of network heterogeneity. The strategy used here can help quantify the effects of varying the hydrogel crosslinking density and crosslinking mechanism on protein diffusion in different types of hydrogel. Such tools could be broadly useful for interpreting cellular responses in hydrogels of varying stiffness for various tissue engineering applications.

Aging behavior and mechanism of bio‐based engineering polyester elastomer nanocomposites

ABSTRACTBio‐based elastomers used in industry have attracted much attention. We prepared bio‐based engineering polyester elastomer (BEE) nanocomposites by mixing bio‐based engineering polyester elastomers with carbon (CB). The CB/BEE nanocomposites were exposed to an artificial weathering environment for different time periods. Both its aging behavior changes and aging mechanism were investigated in this article. The tensile strength retention rates were each above 90% after aging at 100°C and 125°C for 72 h. CB/BEE nanocomposites exhibited good anti‐aging properties. Furthermore, the chemical changes were detected by Fourier transform infrared spectroscopy and differential scanning calorimetry. The crosslink density changes during aging of BEE were determined as well. A plausible aging mechanism of BEE was proposed. It can be concluded that the thermal oxidation process gives priority to further crosslinking in the initial period of aging. As the aging time increases, chain scission becomes the dominant element in the subsequent thermal oxidation process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40862.

Effect of crosslinker structure and crosslinker/monomer ratio on network parameters and thermodynamic properties of Poly (N-isopropylacrylamide) hydrogels

The effect of polymer composition and polymerization parameters such as the type of crosslinker and comonomer, crosslinker/monomer ratio and polymerization temperature on the polymer-solvent interactions and mechanical properties of tetraallylammonium bromide (TAB)-crosslinked N-isopropylacrylamide (NIPAAm), N,N’-methylenebisacrylamide (BIS)-crosslinked NIPAAm and TAB-crosslinked NIPAAm - itaconic acid (IA) hydrogels were studied. It was observed that the hydrogels prepared by 0.7 mole / L concentration of NIPAAm and crosslinked with BIS exhibited mechanical weakness and, they were broken even under a pressure, being less than 5.0 N at a swelling temperature of 25 °C while the PNIPAAm hydrogels crosslinked with ionic-octafunctional crosslinker, TAB, had better compression properties. A decrease in the concentration of NIPAAm, increasing TAB content, addition of a hydrophilic/weakly acidic comonomer, IA and an increase in the functionality of crosslinker resulted in the decreasing values of the poymer-solvent interaction parameter, χ at 25 °C. The values of χ , effective crosslinking density (νe) and enthalpy changes during the shrinkage process in the ranges of 33o - 45 °C and 37o- 45 °C indicated the exothermal nature of phase transition and the importance of intermolecular interactions between PNIPAAm chains. The partial molar dilution-enthalpies (ΔH1) and -entropies (ΔS1) that were calculated from the values of enthalpic and entropic components of χ also indicated a similar trend, referring to a decrease in ordered-structuring of water molecules around hydrophobic isopropyl groups with an increase in the swelling temperature.

Crosslink Density Changes during the Hydrolysis of Tridimensional Polyesters

The hydrolysis of almost ideal networks based on macrodiols of average molar mass about 2 kg mol−1, with L = 18 ester groups per chain is studied. Tensile testing is used to evaluate the crosslink density through the statistical theory of rubber elasticity at two temperatures and three values of relative humidity. A kinetic model for ester consumption including an autocatalysis term is proposed and combined with two original approaches for modeling the crosslink density changes. This allows kinetic parameters of hydrolysis to be determined, and very good predictions are obtained for the variations of crosslink density (or elastic modulus) in the three aging conditions considered. The initial curvature of elastic modulus versus time is predicted positive for weak autocatalysis and negative for strong autocatalysis. The obtained conversion ratio at degelation is found to decrease sharply with the number of esters per elastically active chain.

Morin hydrate as pro-ecological antioxidant and pigment for polyolefin polymers

Abstract In our studies, we propose the use of natural, pro-ecological substances, such as flavonoids to protect elastomers against ageing. One of these substances, namely morin hydrate, was incorporated into ethylene–octene rubbers (Engage); then the vulcanisates of Engage containing the antioxidant under investigation were subjected to ageing processes. The changes in deformation energy, colour and cross-linking density and OIT index of the vulcanisates of Engage were measured before and after each ageing process. The test results obtained show that morin hydrate has a very positive influence on the stability of the vulcanisates of Engage, protecting them against the negative effects of ageing. In addition to its anti-oxidative function, morin hydrate changes the colour of the final polymeric product, fulfilling the role of a natural pigment. Thus, morin hydrate is not only a natural antioxidant but also a pigment in the polymers, imparting to them the features of pro-ecological materials.

Ultrasonic-assisted synthesis of nano-BaSO4 and its effect on thermal and cross-linking density of epoxy nanocomposites

Thermal and morphological behavior of epoxy nanocomposite system was investigated under the influence of barium sulfate nanoparticles. Initially, epoxy resin and nano-BaSO4 were synthesized under controlled ultrasound cavitation technique. At the time of epoxy resin synthesis, nano-BaSO4 (2–10 mass% loading) was added under controlled ultrasound. The intense collapse effect was arising due to cavitation, which enhances the polymerization process with uniform mixing of nano-BaSO4. The effect of nano-BaSO4 on thermal and functional group linkage was studied using differential scanning calorimetry, thermogravimetric analysis, and Fourier transform infrared spectroscopy, respectively. Addition of nBaSO4 accelerates the rate of epoxy resin curing during composite formation with uniform dispersion of nano-BaSO4 under controlled ultrasonic waves (50 kHz), which shows remarkable change in cross-linking density. The thermal stability of epoxy resin nanocomposite was found to be improved as compared to neat epoxy resin. The increasing amount of loading of nano-BaSO4 shows enhancement in thermal properties of epoxy nanocomposite. It was demonstrated that ultrasonic synthesis is a unique and ultimate way for preparation of epoxy resin with nanoaddition. Due to controlled ultrasonic waves, formation of long-term stable polymerization occurs, which consists of polymer/inorganic nanoparticles composite.

Mechanics of intermediate filament networks assembled from keratins K8 and K18

We have investigated intermediate filament networks assembled from the recombinant keratins K8 and K18 in vitro at various protein and MgCl2 concentrations using mechanical rheometry. Experimental parameters were chosen such that artifacts from sample surface elasticity or wall slip were avoided, and the gap width did not affect network formation. The modulus G0 depends weakly on the protein concentration (G0 ∼ c0.5) and the critical deformation γcrit at which non-linear response sets in is concentration independent. These findings can be rationalized assuming that the cross-link density decreases with decreasing protein concentration, while the filament contour length between cross-links remains unchanged. Thus, filaments are more stretched at lower protein concentrations and this increase in conformational energy partly compensates the free energy decrease related to the change in cross-link density. G0 is independent of the MgCl2 concentration indicating that the contribution of stretched filaments decreases when the cross-link density increases. Networks rupture when a critical strain is exceeded, but fully recover within 30 minutes. The non-linear network response is characterized by pronounced strain stiffening with increasing shear stress σ. Reduced differential modulus K′ data obtained at different protein or MgCl2 concentrations collapse onto a master curve. Two scaling regimes K′ ∼ σα are observed with α = 1 at intermediate and α = 0.6 at high stresses. These exponents may be rationalized in terms of the glassy wormlike chain model assuming sticky contacts with finite, constant bond strength. Two distinct scaling regimes could also result from the existence of two types of filament contacts with different bond energies or by the compliance of individual filaments.

Influence of rubber and fabric cord on deformation of a fabric cord-inserted rubber composite by thermal aging

NR composite and PET cord-inserted NR composite were prepared to investigate the contribution level for permanent deformation of fabric cord-inserted rubber composite by thermal aging. The degree of deformation of the PET cord-inserted NR composite was much larger than that of the NR composite without cord. The level of the contribution of the PET cord part on the permanent deformation was much larger than that of the rubber part for the thermal aging below 70°C. The experimental results were explained by the increased crosslink density change of the rubber part and increased crystallinity of the PET cord part.