Crosslinking Temperature Research Articles

Chemically cross-linked UHMWPE with improved toughness

Event Abstract Back to Event Chemically cross-linked UHMWPE with improved toughness Brinda N. Doshi1, Colin Ng1, David Bichara1, 2, Ebru Oral1, 2 and Orhun K. Muratoglu1, 2 1 Massachusetts General Hospital, Harris Orthopaedic Laboratory, United States 2 Harvard Medical School, Orthopaedic Surgery, United States Introduction: Cross-linked, anti-oxidant stabilized UHMWPE is preferred in total hip replacements due to its wear and oxidation resistance [1]. Cross-linking can be achieved by radiation or using chemical cross-linkers like peroxides. High temperature melting (HTM) of radiation cross-linked UHMWPEs around 300°C was shown to improve toughness by inducing controlled chain scission and enhanced intergranular diffusion of chains simultaneously [2]. In this study we combined vitamin E blending, peroxide cross-linking and high temperature melting to obtain a highly wear and oxidation resistant cross-linked UHMWPE with superior toughness. We also optimized the large-scale manufacturing process and tested the biocompatibility. Materials and Methods: Medical grade GUR1050 UHMWPE was blended with vitamin E (0.2 wt%) and with 2,5-Di(tert-butylperoxy)-2,5-dimethyl-3-hexyne or ‘P130’ (0.8 wt%) and consolidated by ram extrusion into 4” diameter and 5ft. long cylinders. These blocks (4” dia, 8cm thick) underwent an optimum high temperature melting (HTM) cycle (PRX-HTM; Figure 1a). The blocks were tested for wear rate, impact strength, tensile properties (UTS and EAB) and oxidation induction time (OIT) 'as is' or after 25 kGy gamma sterilization. In vivo biocompatibility tests were done by implanting the PRX-HTM (1cm dia x 3.75 cm length implant) subcutaneously in the dorsum of rabbits for 105 days. Tissue in subcutaneous pockets was evaluated histologically. Results: The strength, toughness and oxidation resistance of the PRX-HTM was superior compared to the clinically relevant control (Virgin UHMWPE irradiated to 100 kGy and subsequently melted), without sacrificing the wear rate. The material showed no significant changes after gamma sterilization (Figure 1b). No adverse reactions were encountered in vivo subcutaneously after a 105 day implantation of the PRX-HTM (Figure 2). Discussion: We were able to successfully scale up the PRX-HTM material from laboratory to large scale manufacturing without sacrificing any properties and demonstrating local implant biocompatibility. This was achieved by processing the peroxide cross-linked UHMWPE to an optimum HTM cycle (Figure 1a) specially designed to improve the properties and reduce the residuals formed during peroxide decomposition. This material showed promise by providing the ability to consolidate and cross-link the UHMWPE in a single step which would lead to substantial cost reduction by eliminating high dose irradiation. This is a breakthrough implant material showing tremendous improvement over currently available alternatives. Conclusions: Large scale peroxide cross-linking of UHMWPE followed by high temperature melting yields a feasible implant material with improved toughness without sacrificing the wear resistance.

Understanding the in situ crosslinked gelatin hydrogel

AbstractThe present study focuses on an in‐depth investigation at the molecular level of in situ crosslinked gelatin hydrogel by oxidized carboxymethylcellulose (OCMC), intended for biomedical purposes. The crosslinking parameters, namely the crosslinking temperature, crosslinking time and the amount of crosslinker, were varied. The surface and bulk properties were investigated to construct a representative model postulating the interaction of gelatin and OCMC encompassing their physical and covalent linking which determines the properties of the hydrogel. It was inferred that covalent crosslinking facilitates the interfacial interaction between gelatin and OCMC which beyond 30% OCMC composition is impeded due to depletion: microdomain formation leading to phase separation. This study might enable the fabrication of tunable hydrogel with crosslinking parameters in accordance with its purpose. The preliminary biocompatibility studies of the hydrogel conclude that the crosslinked hydrogel generates favourable conditions for cells to adhere and proliferate showing potential for biomedical purposes. © 2015 Society of Chemical Industry

Catalysis of Thermal Cure and Ceramization of Phenylethynyl-Terminated Oligosilane

Phenylethynyloligosilane (PEPSI), prepared by lithium condensation, was stable in liquid state after a bench life over one year, and PEPSI can be used as precursor of SiC–C ceramic. The thermal cure occurred readily through polymerization reaction of ethynyl groups and hydrosilylation of Si–H with ethynyl at 230–330 °C. Thermal cure was dramatically accelerated by catalyst addition; among the investigated catalysts of Fe(acac)3, Co(acac)3 and Ni(acac)2, the highest catalysis performance was achieved for Ni(acac)2. The effect of catalyst amount [1, 2 and 3 % Ni(acac)2, respectively] on thermal cure and ceramization was studied using DSC, TGA, XRD, and SEM–EDS, etc. The results show: 1 % weight of catalyst Ni(acac)2 was enough for catalytic thermal cure and pyrolysis; the thermal cure process showed a decrease in the initiation crosslinking temperature from 230 to 120 °C and the peak temperature from 280 to 155 °C; the ceramic yield was improved from 58 to 71 %; the ceramic crystallization was accelerated by catalytic sintering of Ni(acac)2.

Capillary condensation-induced anomalous water sorption in urethane-based coatings exposed to high humidity conditions

In this work, the water uptake of crosslinked urethane-based coatings in high humidity conditions as a function of crosslinking temperature was investigated. The water uptake values were measured using an electrochemical impedance spectroscopy technique by applying the Brasher–Kingsbury procedure. Water uptake values as high as 200% were observed in the current research. These anomalous results were explained based on capillary condensation of diffused water molecules in the nano-metrically (or nano-scale) rough hydrophilic surfaces of the pores formed by thermal degradation of the polyurethane network. The water uptake value is at its minimum when an effectively pore-free tight network formed at moderate crosslinking conditions.

Effective genetic modification and differentiation of hMSCs upon controlled release of rAAV vectors using alginate/poloxamer composite systems

Viral vectors are common tools in gene therapy to deliver foreign therapeutic sequences in a specific target population via their natural cellular entry mechanisms. Incorporating such vectors in implantable systems may provide strong alternatives to conventional gene transfer procedures. The goal of the present study was to generate different hydrogel structures based on alginate (AlgPH155) and poloxamer PF127 as new systems to encapsulate and release recombinant adeno-associated viral (rAAV) vectors. Inclusion of rAAV in such polymeric capsules revealed an influence of the hydrogel composition and crosslinking temperature upon the vector release profiles, with alginate (AlgPH155) structures showing the fastest release profiles early on while over time vector release was more effective from AlgPH155+PF127 [H] capsules crosslinked at a high temperature (50°C). Systems prepared at room temperature (AlgPH155+PF127 [C]) allowed instead to achieve a more controlled release profile. When tested for their ability to target human mesenchymal stem cells, the different systems led to high transduction efficiencies over time and to gene expression levels in the range of those achieved upon direct vector application, especially when using AlgPH155+PF127 [H]. No detrimental effects were reported on either cell viability or on the potential for chondrogenic differentiation. Inclusion of PF127 in the capsules was also capable of delaying undesirable hypertrophic cell differentiation. These findings are of promising value for the further development of viral vector controlled release strategies.

Influence of crosslinking functionality, temperature and conversion on heterogeneities in polymer networks

Crosslinked polymer formation commonly occurs when two or more multi-functional precursors react to form a three-dimensional network. The resulting networks may contain a significant number of topological imperfections such as loops or dangling ends when formed using crosslinkers with high functionality or when crosslinking at high temperatures. We employ molecular dynamics simulations to analyze these topological imperfections in coarse-grained networks generated from precursors consisting of ‘chain extenders’ composed of two beads (dimers) and a crosslinker of functionality f = 3 or 6 for a wide range of crosslinking temperatures and final conversions. It is shown that these imperfections result in networks in which the number of elastically active chains, the cycle rank and the number of elastically active junctions is smaller than predicted by the Miller–Macosko theory. Such defects must adversely affect the mechanical properties, resistance to solvent swelling and, possibly, the long-term protective properties of polymer networks.

Effects of crosslinking temperature and time on microstructure and stability of cassava starch microspheres

Starch microspheres (SMs) were prepared by a water-in-water emulsion–crosslinking technique at 4°C and 30°C for 1, 6, 12 and 24h; the SMs obtained were analyzed for crosslinking density, morphology, crystalline structure, and stability against temperature, pH, and α-amylase hydrolysis. The crosslinking degree at 30°C was considerably higher than that at 4°C. SMs prepared at 4°C for less than 12h incubation had larger size and more porous structure as compared with those prepared at 30°C, but the morphology became comparable (spherical shape with smooth surface and dense structure) after 24h incubation. All SMs samples displayed amorphous structure. Stability tests revealed that the SMs were very stable under acidic and mild basic pH; however, stability against α-amylase hydrolysis varied depending on incubation temperature and time.

Nanocomposite SPEEK-based membranes for Direct Methanol Fuel Cells at intermediate temperatures

Novel nanocomposite membranes were prepared by infiltration of a blend of sulfonated PEEK (SPEEK) with polyvinyl alcohol (PVA), using water as solvent, into electrospun nanofibers of SPEEK blended with polyvinyl butyral (PVB). The membranes were characterized for their application on Direct Methanol Fuel Cells (DMFCs) operating at moderate temperatures (>80°C). An important role of the solvent on the crosslinking temperature for the SPEEK-PVA system was observed. A mat of hydrated SPEEK-30%PVB nanofibers revealed higher proton conductivity in comparison with a dense membrane of similar composition. Incorporation of the nanofiber mats to the SPEEK-35%PVA matrix provided mechanical stability, methanol barrier properties and certain proton conductivity up to a crosslinking temperature of 120°C. Not remarkable effect of the nanofibers was found above that crosslinking temperature. The combined effect of the nanofibers and crosslinking temperature on the properties of the membranes is discussed. DMFC performance experiments concluded promising results for this new low-cost type of membranes, although further optimization steps are still required.

Fabrication of size-controlled SiC/Si3N4 ceramic microspheres with enhanced extinction property

SiC/Si3N4 ceramic microspheres with good spherical structure were fabricated via a simple emulsion technique combined with the polymer-derived ceramics (PDCs) method. After pyrolyzed at 1400°C, the SiC/Si3N4 ceramic microspheres exhibited controllable size distribution in the range of 0.60–0.66µm and stable spherical morphology. The size and surface morphology of the ceramic microspheres were tuned by adjusting the operation conditions, such as O/W mass ratio, emulsifier amount, cross-linking time, cross-linking temperature and pyrolysis temperature. The effective specific extinction, e*, was used to evaluate the extinction performance of ceramic microspheres. e* of the ceramic microspheres pyrolyzed at 1400°C reached maximum value, 40.7m2/kg, and it indicated that the SiC/Si3N4 ceramic microspheres were promising high-temperature thermal insulating materials.

Tubular thermal crosslinked-PEBA/ceramic membrane for aromatic/aliphatic pervaporation

Pervaporation could be a cost-competitive process for separating aromatic/aliphatic hydrocarbon mixtures in the chemical industry. In this study, commercial poly(ether-block-amide) (PEBA) and ceramic tubular substrates were used to prepare pervaporation membrane for separating aromatic/aliphatic mixtures through a facile thermal crosslinking method. The adsorption and diffusion properties of toluene and n-heptane in PEBA were determined by swelling experiment and inverse gas chromatography technique. FTIR and XRD were used to investigate the hydrogen bonding and crystallinity of PEBA molecule through thermal crosslinking. The morphology and structure of PEBA/ceramic composite membrane were characterized by SEM and EDX. The separation performance of the composite membrane can be adjusted by controlling PEBA concentration, thermal crosslinking temperature and time. Ceramic tubular substrate could suppress the excessive swelling and thereby enhance the stability of the membrane. Furthermore, different types of fillers were incorporated into PEBA matrix to improve the separation performance. The results demonstrate that PEBA/graphite hybrid membrane has the highest separation factor for separating 50wt% toluene/n-heptane mixtures. This study is expected to extend the membrane material and simplify the membrane preparation procedures in aromatic/aliphatic pervaporation fields.

High-Performance Flexible Tandem Polymer Solar Cell Employing a Novel Cross-Linked Conductive Fullerene as an Electron Transport Layer

A novel thermally cross-linkable, n-doped conductive fullerene material is developed by incorporating tetrabutylammonium iodide (TBAI) as the dopant into an azidofullerene derivative PCBN3. The application of TBAI-doped cross-linked PCBN3 film as the electron transport layer (ETL) in polymer solar cells (PSCs) delivers several remarkable features, including easy solution-processability, reasonable electrical conductivity (2.8 × 10–3 S cm–1), good ambient and chemical stability, fine-tunability of the work function of the electrode, wide applicability in a variety of efficient polymers, relative weak thickness-dependent performance property, and moderate cross-linking temperature (∼140 °C). With this ETL, a single-junction solar cell based on the blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) delivers a power conversion efficiency (PCE) up to 8.8...

Novel Alkaline Anion-exchange Membranes Based on Chitosan/Ethenylmethylimidazoliumchloride Polymer with Ethenylpyrrolidone Composites for Low Temperature Polymer Electrolyte Fuel Cells

Novel alkaline anion-exchange membranes composed of chitosan (CS) and 1-Ethenyl-3-methyl-1H-imidazoliumchloride polymer with 1-ethenyl-2-pyrrolidone (abbreviated as EMImC-Co-EP) are prepared by a combined thermal and chemical cross-linking technique. The hydroxide conductivity (σOH-), water uptakes, ion exchange capacity (IEC), thermal stability, mechanical property, oxidative stability and alkaline stabilities of CS/EMImC-Co-EP membranes are measured to evaluate their applicability in H2/O2 alkaline fuel cells (FC). The effects of thermal cross-linking temperature and membrane composition on membrane OH− conductivity are studied using AC impedance technique. FTIR, SEM and TG analysis are used for structural characterization of these membranes. It is found that the OH− conductivity of the membranes increases with temperature and exceeds 10−2Scm−1 at 80°C. The CS/EMImC-Co-EP membranes show excellent thermal stability with onset degradation temperature high above 200°C. In particular, the high alkaline stability is achieved for the CS/EMImC-Co-EP membranes in hot 8.0M KOH at 85°C without losing their integrity and OH− conductivity during 300hours of evaluation, and also a relatively high oxidative stability. The membrane electrode assembly (MEA) fabricated with CS/EMImC-Co-EP (1:0.5 by mass) gives an initial power density of 21.7mWcm−2 using H2 as the fuel and O2 as oxidant at room temperature, on a low metal loading on both the anode and the cathode of 0.5mg (Pt)cm−2 at ambient temperature.

Preparation and thermal properties of polyethylene-based carbonized fibers

In this study, carbonized fibers were prepared by using acidically cross-linked LDPE fibers. The surface morphologies of the carbonized fibers were observed by SEM. The effects of cross-linking process temperatures were studied using thermal analyses such as DSC and TGA. The melting and heating enthalpy of the fibers decreased as the cross-linking temperature increased. The cross-linked fibers had a carbonization yield of over 50%. From SEM results the highest yield of carbonized LDPE-based fibers was obtained by cross-linking at a sulfate temperature (170oC). As a result, carbonation yield of the carbonized fibers was found to depend on the functions of the cross-linking ratio of the LDPE precursors.

Graphene Oxide–Polymer Nanocomposite Anion-Exchange Membranes

Functionalized graphene oxide (FGO) was incorporated into polyvinyl alcohol/ poly(diallyldimethylammonium chloride) semi-interpenetrating polymer networks (PVA/PDA SIPNs) in order to create novel nanocomposite anion-exchange membranes (AEMs) with improved OH− conductivity and good thermo-mechanical stability at high relative humidity. The nanocomposites were fabricated by a solvent-casting method and subsequently thermally crosslinked to improve their mechanical stability in the hydrated state. The effects of PVA/PDDA ratio, cross-linking method, cross-linking temperature, and FGO content were studied in order to determine the optimal conditions to fabricate AEMs with improved material properties. The membranes were characterized by XRD, FTIR, TGA, FE-SEM, and impedance spectroscopy to assess the material properties of FGO and the nanocomposite membranes. The membranes were also evaluated as polymer electrolytes in anion exchange membrane fuel cells. The results reveal that the membrane fabricated at a PVA/PDDA weight ratio of 70/30 and 20 wt% FGO possesses the highest value of OH− conductivity (12.1 mS cm−1 @ 30°C and 21 mS cm−1 @ 80°C), as well as improved thermo-mechanical stability at 100% R.H. However, the fuel cell performance reaches a maximum using the membrane fabricated at 10 wt% FGO.

Polymers Based on Norbornene Derivatives

This study presents the influence of exo,exo-N,N’- hexylene-di(5-norbornene-2,3-dicarboximide) on the degree of cross-linking and glass transition temperature of the polymers obtained from a mixture of dimethyl esters of exo,exo- and endo,endo-5-norbornene-2,3-dicarboxylic acid. It was found that exo,exo-N,N’- hexylene-di(5-norbornene-2,3-dicarboximide) can be used as a cross-linking agent in ROMP. Addition of the cross-linking agent leads to an increase in the degree of cross-linking polymers. The glass transition temperature of polymers increases with concentration increasing of the crosslinking agent.

Thermally Crosslinked Chitosan-EDTA/PVA Electrospun Nanofiber Mats: Crosslinking Conditions

This work was aimed to develop thermally crosslinked chitosan/PVA nanofiber mats. The CS/PVA nanofibers were fabricated using elctrospinning process. The appropriate crosslinking temperatures and crosslinking times in order to thermally crosslinked CS/PVA nanofiber and physicochemical properties of the thermally crosslinked nanofibers were investigated. The morphology of the nanofiber mats was investigated by scanning electron microscopy (SEM). The water insolubilization and FTIR were employed to evaluate the success of crosslinking. Increasing the crosslinking time and temperature, the crosslink was increased, thus causing a decrease of water solubilization. The temperature and time for successful crosslinking were 180-200 °C and 0.5-5 h, respectively.

Immobilization of pectinase from Bacillus licheniformis KIBGE-IB21 on chitosan beads for continuous degradation of pectin polymers

Abstract Pectinase catalyzes the degradation of pectin substances and has been used in various food and textile industries. In this study, pectinase from Bacillus licheniformis KIBGE-IB21 was immobilized on chitosan beads using formaldehyde as a crosslinking agent. Various concentration of formaldehyde was tried during immobilization procedure and 10% was found to immobilize maximum pectinase. The optimum crosslinking time and temperature for the activation of chitosan beads were observed to be 3.0 h and 30 °C, respectively. The immobilization did not influence the optimum pH and temperature of pectinase for maximum activity and both the native and immobilized pectinase showed maximum activity at same pH and temperature. However, the stability of pectinase against different pH and temperatures was increased after immobilization. The chitosan immobilized pectinase showed excellent recycling efficiency and retained more than 90% of its initial activity during 4th cycle.

Novel bioactive polyester scaffolds prepared from unsaturated resins based on isosorbide and succinic acid

In this study new biodegradable materials obtained by crosslinking poly(3-allyloxy-1,2-propylene succinate) (PSAGE) with oligo(isosorbide maleate) (OMIS) and small amount of methyl methacrylate were investigated. The porous scaffolds were obtained in the presence of a foaming system consisted of calcium carbonate/carboxylic acid mixture, creating in situ porous structure during crosslinking of liquid formulations. The maximum crosslinking temperature and setting time, the cured porous materials morphology as well as the effect of their porosity on mechanical properties and hydrolytic degradation process were evaluated. It was found that the kind of carboxylic acid used in the foaming system influenced compressive strength and compressive modulus of porous scaffolds. The MTS cytotoxicity assay was carried out for OMIS using hFOB1.19 cell line. OMIS resin was found to be non-toxic in wide range of concentrations. On the ground of scanning electron microscopy (SEM) observations and energy X-ray dispersive analysis (EDX) it was found that hydroxyapatite (HA) formation at the scaffolds surfaces within short period of soaking in phosphate buffer solution occurs. After 3h immersion a compact layer of HA was observed at the surface of the samples. The obtained results suggest potential applicability of resulted new porous crosslinked polymeric materials as temporary bone void fillers.

Enhancing stabilities of lipase by enzyme aggregate coating immobilized onto ionic liquid modified mesoporous materials

Abstract Mesoporous material SBA-15 as the matrix and hydrophilic methyl imidazolium ionic liquids [MSiIM] + BF 4 − as modifier were involved in preparing ionic liquid modified materials as enzyme carriers through after-grafting silane coupling reaction. The method of enzyme aggregates coating was firstly used to immobilize porcine pancreatic lipase (PPL) onto ionic liquid modified SBA-15. Characterization before and after modification and immobilization were conducted using infrared spectroscopy (FT-IR), differential thermal-thermal analysis (DTA-TG) and N 2 adsorption–desorption method (BET). The results indicated that the ordering degree of SBA-15 declined after ionic liquid modification, but mesoporous structure remained. After enzyme immobilization, pore size and specific surface area of carrier became smaller. The cross-linking agent amount, reaction temperature and pH were optimized in this paper. The result demonstrated that the initial activity of enzyme was raised from 35% to 53% after five times recycle by enzyme aggregate coating. 74% of the original activity remained after 25 days storage.

Characterization of Thermal Reversible Cross-Linking Agents for Flexible Poly(vinyl chloride)

In order to improve thermal stability of Poly (Vinyl Chloride), a kind of thermal reversible cross-linking agents, the end cyclopentadienyl ethanethiol salts, sodium cyclopentadienyl ethyl mercaptan (CPD-C2H4SNa) was synthesized in this paper. The DSC results proved that the prepared CPD-C2H4SNa has obvious thermal reversible characteristics and the thermal reversible temperature was at 177 °C. The effects of cross-linking agent amount, cross-linking reactive time and temperature on flexible PVC compounds were evaluated through gel content testing. It was showed that the cross-linking agent with 3 Phr is optimum for PVC compounds. Moreover, both the cross-linking reactive time and temperature has effects on gel content of PVC compounds. The gel content of CPD-C2H4SNa cross-linked PVC compounds reaches maximum at 76% for 20 min at 160 °C.The static thermal stability of flexible PVC was measured according to congo red method. The results showed that the thermal stability of PVC compounds was enhanced with increasing cross-linking agent amount.