Decrease In Crosslink Density Research Articles

Investigating the Stress–Strain Behavior in Ring-Opening Metathesis Polymerization-Based Brush Elastomers

Brush networks are intriguing materials that are able to replicate the stress–strain behavior of soft tissue, but the effect of the backbone chemical composition on the network mechanics is largely unknown. Here, we show that brush elastomers made by ring-opening metathesis polymerization (ROMP) of norbornene-terminated poly(dimethylsiloxane) macromonomers are less extensible than brush elastomers with the methacrylate backbone yet not as extensible (λmax) as predicted by the strain-stiffening parameter (β) derived from fitting the experimental stress–strain curves. The softness (E0) and firmness (β) of the norbornene-based networks decrease with decreasing cross-link density as expected, but the λmax significantly drops when the grafting density becomes low enough, which has not been previously observed in this class of materials.

A self-healing smart photonic crystal hydrogel sensor for glucose and related saccharides.

A self-healing smart PhC hydrogel sensor that combines the optical property of photonic crystal and the dynamic regeneration property of boronate ester bond has been prepared for determination ofglucose and related saccharidesusing Debye diffraction ring detection. The boronate ester bond formed through phenylboronic acid and dopamine endows the hydrogel network self-healing ability, and the tensile stress of the healing hydrogel can recover to 94.4%; this excellent self-healing property can effectively improve the reliability and lifetime of thehydrogel. Dueto the high bonding capacity between 1,2- and 1,3-diol and phenylboronic acid, the hydrogel sensor has a good recognition ability for glucose and related saccharides. The reaction between the monosaccharides and the phenylboronic acid group makes the sensor swell and the diameter of the Debye diffraction ring decrease. The sensor shows good reuse and responsive ability for saccharides; the RSD of the recoverability assays is 4.3%. Thedetermination range of the sensor to glucose is 0.5 to 12mM. The sensor also has good response to glucose in urine, exhibiting potential application value in thepreliminary screening of diabetes. Although the sensor has poor selectivity for specific monosaccharides, the process of measuring the Debye ring makes the determination no longer rely on expensive and complicated equipment and greatly simplifies the determining process and reduces the cost of determination, which shows a broad application prospect. The boronate ester bond formed through phenylboronic acid and dopamine results in the self-healing property of hydrogel network, which can effectively improve the reliability and lifetime of hydrogel. And due to the high bonding capacity between 1,2- and 1,3-diol and phenylboronic acid, the smart hydrogel sensor has a good recognition ability for glucose and related saccharides. The reaction between the monosaccharides and the phenylboronic acid group breaks the original boronate ester bond; this will lead to a decrease in cross-linking density of the PhC hydrogel sensor and further makes the sensor swell and the diameter of the Debye diffraction ring decrease.

Thermomechanical Devulcanisation of Ethylene Propylene Diene Monomer (EPDM) Rubber and Its Subsequent Reintegration into Virgin Rubber.

Rubber waste remains a challenge for material science because its covalently cross-linked structure hinders the establishment of the circular economy of rubber. Devulcanisation may provide a solution, as it converts rubber vulcanisates back into their original, uncured state. Devulcanised rubber may be revulcanised or incorporated into virgin rubber, thus waste is utilized and the use of primary resources is reduced at the same time. In this paper, we treated sulphur-cured EPDM (ethylene propylene diene monomer) rubber on a two-roll mill at various temperatures and frictions. We determined the effectiveness of devulcanisation via Horikx’s analysis, which suggested that low devulcanisation temperatures would result in a 50% decrease in cross-link density with minimal polymer degradation. The devulcanisate was recycled via two methods: (a) revulcanisation with extra curing agents, and (b) mixing it with various amounts of the original rubber mixture, preparing rubber samples with 25, 50, 75, and 100 wt% recycled content. Tensile tests revealed that the samples’ elastic properties were severely compromised at 75 and 100 wt% devulcanisate contents. However, tensile strength decreased only by 15% and 20% for revulcanisates containing 25% and 50% recycled rubber, respectively.

Chemo-thermomechanical behaviors of Enzyme-degradable shape memory composite and its heat-enzyme triggered shape memory properties

Designed with elaborated chemistry of the structure, shape memory composite (SMC) exhibits excellent biocompatibility and biodegradability, and therefore find a wide potential biomedical application. Recently, a two-constituent enzyme degradable shape memory composite (EDSMC) was reported utilizing a polyether-based polyurethane thermoplastic called Pellethane (PE) and a semicrystalline Poly (ε-caprolactone) (PCL). The heat-enzyme triggered melt-crystal transition of PCL was utilized to realize the invertible “switch” of the fixing and recovery configuration. A chemo-thermomechanical 3D viscoplastic model is developed for the EDSMC aiming to provide theoretical foundation for its structure design and engineering applications. The enzyme-triggered decrystallization of crystals is captured by an energy-dependent kinetical model, and the further dissolution under enzyme activated chain-broken chemical reaction is modeled as decrease of crosslinking density. The chemo-thermomechanical constitutive model for EDSMC is established by combining the viscoplastic behaviors of the PE and that of PCL. The model is calibrated and validated by the associated experiments in the published work, and then applied to simulations of the diffusion-controlled enzyme degradation of the PCL as well as the chemomechanical behavior of the EDSMC. Some parametric examples are conducted for the heat-crystal activated shape memory properties of EDSMC. From the simulations and experiments, the enzyme-triggered dissolution of the crystals activates effectively recovery of the PCL assisted fixed configuration. The researches in this work provide the theoretical understandings for the EDSMC, and also the bio-composites characterized with the chemically triggered melt-crystal transitions.

SANS and NMR study on nanostructure of thermoresponsive double network hydrogels

Temperature induced changes of nanostructure of double network (DN) hydrogels were investigated by a combination of small-angle neutron (SANS) and 1H nuclear magnetic resonance (NMR) spectroscopy. The DN hydrogels consist of the first densely crosslinked thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) network swollen in water in which a second loosely crosslinked hydrophilic polyacrylamide (PAAm) network was prepared. In the series, the crosslinking density of the first network is varied while the crosslinking density of the second network is kept constant. At high-q range (q being the magnitude of scattering vector), SANS profiles are governed by a term originating from thermal movement of polymer segments in water. For q > 0.1 Å−1, this term scales as q−α and the scaling exponent α obtains values lower than 5/3 (at T = 25 °C) to ca 2 (at T ≥ 35 °C) in agreement to the change of quality of PNIPAAm-water interaction. At low-q range, an excess scattering due to formation of large loose domains with non-uniform concentration of crosslinks is observed. At temperatures below the deswelling temperature, Tdsw, the excess scattering is relatively week since the domains differ little in the neutron scattering length density. However, NMR results show that mobility of a great part of NIPAAm units in DN hydrogels is significantly reduced even below Tdsw. At temperatures above Tdsw the excess scattering becomes much stronger showing q−4 scaling in Porod’s region corresponding to formation of water-poor and water-rich domains separated by well-defined interface. The size of domains grows with increasing temperature and decreasing crosslinking density of the first network. The SANS intensities from DN hydrogels are reduced with respect to SN hydrogels which is attributed to lower scattering contrast due to an increased water content in water-poor phase caused by the presence of the second hydrophilic PAAm network.

Influence of PCL on the epoxy workability, insights from thermal and spectroscopic analyses

Epoxy compound based on diglycidyl ether of bisphenol A (DGEBA), methyl tetrahydrophthalic anhydride (MTHPA) as hardener and 2,4,6-tris (dimethylaminomethyl) phenol (DEH 35) as catalyzer, at 100/87/5 as fixed concentration was successfully processed. Aiming workable thermoset, thermoplastic Polycaprolactone (PCL) was added at 10, 20 and 30 phr contents to the epoxy compound. Analyses of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and optical microscopy (OM) were conducted in order to obtain evidences from chemical interactions, curing kinetics and phase separation. From FTIR spectra partially miscibility between Epoxy and PCL can be assumed, mainly due to the hydrogen bonding between PCL carbonyl and epoxy hydroxyl, whereas after reaching epoxy's solubility limit PCL segregates and it is suggested separated phases take place through spinodal decomposition (Kinloch et al., 1994) [1] and nucleation and growth (NG) mechanisms which depend on temperature and raw material concentrations, as illustrated in OM images. Curing kinetics followed through released heat during DSC scans, indicated decreased crosslinking density upon PCL addition, as also lower Tg related to neat epoxy. Overcoming epoxy brittle fracture, rough surface fracture images with toughening character were captured from epoxy/PCL. Evidences of workable epoxy/PCL compounds are provided and related results presented in this work offer reliable tools to determine the exact aimed degree of crosslinking widening processing window to desired application.

Influence of dicumyl peroxide and Type I and II co-agents on cross-linking and physical–mechanical properties of rubber compounds based on NBR

ABSTRACT The aim of the work was the investigation of the effect of dicumyl peroxide and different types of co-agents on cross-linking and physical–mechanical properties of vulcanisates based on acrylonitrile–butadiene rubber. First, dicumyl peroxide was applied for cross-linking at different curing temperatures. Based on the achieved results, the optimum temperature and peroxide concentration was chosen for the next experiments, in which organic peroxide was combined with various Type I and Type II co-agents. The results revealed that Type I co-agents take active part in vulcanisation process as they contribute to the increase in cross-link density of vulcanisates. This was reflected in the increase in modulus and hardness and the decrease in elongation at break of vulcanisates. The tensile strength was also improved by the application of Type I co-agents. On the other hand, the decrease in cross-link density and deterioration of physical–mechanical properties was recorded by application of Type II co-agents.

Silicone dielectric elastomer with improved actuated strain at low electric field and high self-healing efficiency by constructing supramolecular network

Dielectric elastomers (DE) suffer from cracks and breaks during repeated cycles, thus a DE material with both high actuated strain (SA) and self-healing ability is highly desired. Herein, for the first time we report a self-healable silicone DE (SiR-SN) with large SA under low electric field by constructing supramolecular network assembled by hydrogen bonds and ionic bonds from two components involving carboxyl modified polymethylvinylsiloxane (PMS-g-COOH) and amino terminated polydimethylsiloxane (PDMS-NH2). The results show that as PMS-g-COOH content increases, the dielectric constant (ε′) of SiR-SN increases owing to an increase in dipole content whereas the elastic modulus (Y) decreases because of a decrease in crosslinking density, leading to the significantly increased SA at a given electric field. SiR-SN with 0.2/1 of the mass ratio of PMS-g-COOH to PDMS-NH2 shows much higher ε′ and SA than the commercial silicone DE. Interestingly, self-healing of SiR-SN at 80 °C leads to the re-formation of hydrogen bonds, and thus the recovery of network structure, whereas self-healing at 100 °C can lead to the conversion from hydrogen bonds into ionic ones, and thus the change of network structure. As a result, a self-healing efficiency of 115% in tensile strength and almost 100% in SA at a given electric field can be achieved after self-healing at 80 °C for 5 h, whereas an increase in Y and higher breakdown strength can be achieved after self-healing at 100 °C, which may sustain the repeated stress and prolong the service life. It is promising that this new self-healable silicone dielectric elastomer with large actuated strain under low electric field could be used especially in biological and medical fields.

Polyurethane networks based on polycaprolactone and hyperbranched polyester: Structural, thermal and mechanical investigation

A series of environmentally friendly polyurethane (PU) networks based on polycaprolactone as soft segment (SS) and Boltorn® aliphatic hyperbranched polyester of the second pseudo generation as crosslinking agent was prepared by a two-step polymerization in solution. The chemical structure of the PU networks was analyzed by Fourier transform infrared spectroscopy, while XRD analysis revealed that these samples are amorphous. Swelling measurements, water absorption study, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, mechanical tests and scanning electron microscope revealed that, swelling behavior, hydrophobicity, thermal and mechanical features and morphology of PU networks show clear dependence on polycaprolactone content. The increase of the SS content induced appearance of the microphase separated morphology, increase of thermal stability, elongation at break, hydrophobicity and swelling ability in tetrahydrofuran and toluene, but at the same time it is responsible for the decrease of crosslinking density, glass transition temperatures, rigidity, tensile strength and Young’s modulus of PU networks. Scanning electron microscope images of the fracture surfaces of PU networks show that their surface is rough, irregular, with flake-like structure, where size of flakes increases with increasing SS content. These results indicate that by choosing adequate SS content, PU coatings with diverse features based on polycaprolactone and Boltorn® aliphatic hyperbranched polyester can be designed for appropriate and desired application.

Structure-property correlation of poly(ethylene glycol) based form stable phase change materials with different crosslinking structure

Chemical crosslinking is an important strategy for preparation of form stable phase change materials (FSPCMs). However, the influence of different crosslinking structure on the properties of FSPCMs is not very clear currently. In this work, PEG-based FSPCMs are synthesized by free-radical polymerization. They possess different crosslinking densities, and the location of PEG moieties include two different sites (backbone or side chain). The chemical structure, phase change property, crystal structure, mechanical performance, and thermal reliability of the FSPCMs are studied and compared with respect to their different crosslinking structure. It is found that the phase change enthalpy and degree of crystallinity increase with decreasing crosslinking density, and PEG moieties locating in side chain is preferable. With decreasing crosslinking density, the spherulites of the FSPCMs become larger, but always smaller than that of pristine PEG. The results can be explained by the restriction on movement/assembly of PEG moieties by the crosslinking network. The mechanical performance of FSPCMs with PEG moieties located in backbone is preferable than that in side chain. Tensile strength is maximized with poly(ethylene glycol) acrylate (Mn = 12000) as monomer, and elongation at break increases monotonically with decreasing crosslinking density. The findings of this work will contribute to the future design of FSPCMs with optimized overall thermal energy storage and mechanical properties.

Ultrafine calcium carbonate-filled natural rubber latex film: mechanical and post-processing properties

This study aims to improve the dispersion of treated calcium carbonate (CaCO3) in natural rubber latex (NRL) films. Ball mill and ultrasonic methods were used to break down the particle size of CaCO3, and the combined effect of ball mill/ultrasonic and ultrasonic/ball mill methods with their optimum processing times was applied to re-agglomerate and further break down the particle size of CaCO3. The particle size and dispersibility of treated CaCO3 were characterized by particle-size analysis, transmission electron microscopy (TEM), and Zeta-potential measurement. The particle-size analysis and TEM results revealed that the particle size of CaCO3 reduced down to 1.4 µm with less agglomeration using ultrasonic for 120 min and ball mill for 72 h. Zeta--potential measurement also indicated that the ultrafine-treated CaCO3 with a broad particle-size distribution was compatible with NRL particles. The treated CaCO3 loadings from 5 to 20 phr were filled in the NRL compounds. NRL/CaCO3compounds showed decrease in viscosity and cross-link density, but increase in swelling index. It was found that tensile strength, modulus at 100% elongation, elongation-at-break, swelling index, and cross-link density of the films were improved with addition of 10 phr of CaCO3. At 20 phr of filler loading, the increment rate of tensile strength of NRL/CaCO3 films was 44% after leaching, and their retention percentage was 62% after heat aging. Therefore, ultrafine CaCO3-incorporated NRL can withstand under accelerated heat-aging condition.

Effect of the combination of a crosslinking agent and a thiourethane additive on the properties of acrylic denture bases processed with microwave energy

Thiourethane (TU) additives and difunctional, polymerizable crosslinking agents have been demonstrated to increase toughness in methacrylate-based materials. The aim of this study was to evaluate the potential reinforcement of acrylic denture bases by combining thiourethane additives and 1,6 hexanediol dimethacrylate (HDDMA) as an additional crosslinking agent. One commercial acrylic resin (Nature-Cryl MC; GC America) was tested by adding 0 (control) or 10 wt% TU, each of them combined with 0 (control), 10, 20 and 30 wt% HDDMA, for a total of 8 experimental groups. Materials were processed using microwave energy (500 W for 3 min) using microwave-safe molds and flasks. Flexural strength, modulus and toughness were obtained in 3-point bending (ISO 4049) using bars measuring 2 × 2x25 mm (n = 6). Dynamic mechanical analysis was used to determine glass transition temperature (Tg), breadth of tan delta (as a measure of polymer heterogeneity) and crosslinking density in 1 × 3x15 mm bars (n = 6) tested in tension, using a 3 °C/min heating rate (−30 to 180 °C). Viscosity samples were evaluated in a parallel plate reometer. Data were analyzed by two-way ANOVA and Tukey's test (α = 0.05). Results showed that on the samples not containing TU, HDDMA up to 20 wt% increased the flexural strength and thoughness (and up to 30 wt% HDDMA increased the modulus). The addition of TU did not affect those properties (except for the increase in elastic modulus), but the combination TU + HDDMA led to decreased properties overall. The addition of HDDMA decreased the viscosity for all materials, and the presence of TU did not affect viscosity. The Tg increased linearly with the concentration of HDDMA, except in the groups containing TU – in general, the addition of TU reduced Tg. The crosslinking density increased with the addition of HDDMA for all materials, regardless of the presence of TU. The addition of TU significantly decreased crosslinking density. The breadth of tan delta was not affected by the addition of HDDMA, but significantly increased with the addition of TU. In conclusion, the chain-breaking effect of TU on polymerizing methacrylates was deleterious in the case of methyl methacrylate, since it forms a linear polymer. The addition of HDDMA up to 20 wt% and not combined with TU significantly improved the tested properties.

Reactive Molecular Dynamics Simulations of the Atomic Oxygen Impact on Epoxies with Different Chemistries

Atomic oxygen (AO) is one of the most abundant species present in the lower earth orbit and is responsible for the aggressive degradation of polymers used in spacecraft structures. In this investigation, we use ReaxFF reactive force field molecular dynamics simulations to evaluate the disintegration of several different thermosetting epoxy polymers subjected to hypervelocity AO impact. Our simulations indicate that epoxy with aromatic curative displays higher resistance to the AO impact because of its stable benzene functionality. Decreased cross-linking density and increased simulation temperature both lead to faster disintegration of the polymer. Our simulation results indicate that ReaxFF force field simulations can be a useful tool to evaluate the response of various thermosetting epoxies to AO impact and identify promising candidate materials for spacecraft applications.

Development of a Self-Adhesive Cellulosic Hydrogel Wound Dressing

In this study, a self-adhesive hydrogel wound dressing was developed by combining hydroxyethyl cellulose (HEC) hydrogel with a tannic acid-polyethylene glycol (TAPE) adhesive bioadhesive with gelatin. Test samples of the cellulosic wound dressing were prepared with three (3) different mixing ratios of the crosslinking solution, three (3) different adhesive formulation, and two different hydrogel/adhesive contact area (flat, ridged). Adhesion performances of these samples on porcine skin were evaluated by performing a T-peel test. Analysis of the HEC/TAPE-gelatin interface showed that the HEC cross-linking agent formulation, adhesive thickness, and presence of surface ridges showed significant three-way interaction effects, and these parameters were modeled using orthogonal polynomials and optimized via response surface methodology (RSM). The adhesion on the HEC-TAPE-gelatin interface was also investigated further using scanning electron microscopy (SEM), where it had been observed that greater adhesion occurred with a decrease in cross-linking density, thinner adhesive layer, and the presence of ridges. Lastly, disk diffusion testing indicated greater antimicrobial activity (mean inhibition zone = 12 mm) against S. aureus and P. aeruginosa in contrast to commercial hydrogel dressings (mean inhibition zone = 7.5 mm), while MTT assay on human lymphocytes resulted to a 98% cell survival rate. Based on these results, it was concluded that it is feasible to use HEC hydrogel with TAPE-gelatin adhesive for manufacturing self-adhesive wound dressing products.

Study on the Correlation of Crosslink Network Structure and Tensile Properties of HTPB Binder

To study the crosslink network structure and the correlation with tensile mechanical properties of hydroxyl-terminated polybutadiene (HTPB) binder, tensile test and nuclear magnetic resonance test were introduced, the crosslink data and tensile stress-strain curves were obtained at various temperatures. The results show that the temperature has a significant effect on the crosslink density. When heating the samples from 30°C to 90°C, the crosslink density decreases and the transverse relaxation time increases. From 90°C to 130°C, the crosslink density raises at first and then becomes lower, and the transverse relaxation time keeps increasing. When the temperature rises from 30°C to 90°C, the elastic modulus and tensile strength of the binder are reduced, the elongation at break gets greater, and the tensile strength and the crosslink density has a linear relationship, which can predict the tensile strength of the binder at different crosslink densities.

Fabrication of Cellulose Nanocrystal/Chitosan Hydrogel for Controlled Drug Release.

In this work, a novel nanocomposite hydrogel based on cellulose nanocrystal (CNC) and chitosan (CS) was fabricated and applied as a carrier for the controlled delivery of theophylline. CNC was firstly periodate-oxidized to obtain dialdehyde nanocellulose (DACNC). Then, chitosan was crosslinked using DACNC as both the matrix and crosslinker in different weight ratios, to fabricate CNC/CS composites. The prepared composites were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), zeta potential measurement and swelling ratio tests. FT-IR results confirmed the successful reaction between the free amino groups on chitosan and the aldehyde groups on DACNC. With the increase of chitosan percentage in the hydrogel, the isoelectric point was shifted towards an alkaline pH, which was probably caused by the higher content of free amino groups. The swelling ratio of the composite also increased, which may have been due to the decrease of crosslinking density. Because the swelling ratio of the drug-loaded hydrogels differed under varied pH values, the cumulative drug release percentage of the composite hydrogel was achieved to approximately 85% and 23% in the gastric (pH 1.5) and intestinal (pH 7.4) fluids, respectively. Therefore, CNC/CS hydrogel has application potential as a theophylline carrier.

Tailoring PNIPAM hydrogels for large temperature-triggered changes in mechanical properties

N-isopropylacrylamide (NIPAM)-based hydrogel films are used for touch-controlled applications, where the temperature-induced change in the mechanical properties is utilized to create tactile feedback. N,N′-methylenebisacrylamide (BIS) and poly(ethylene glycol)diacrylate (PEGDA) are used as cross-linkers to study the influence of their size and concentration on the viscoelastic properties in a temperature-controlled rheology setup. The changes in water content between swollen and collapsed state of the hydrogel samples increase with decreasing cross-linking density and increasing size of the cross-linker resulting in bigger meshes in the network. The difference in the viscoelastic properties of the hydrogels increases with increasing deswelling ratio and is highest for the P(NIPAM-PEGDA) hydrogels with low cross-linking density with a 50-fold increase in the storage modulus. The deswelling ratio of these P(NIPAM-PEGDA) hydrogels is up to five times higher compared to the P(NIPAM-BIS) hydrogels of the same cross-linking density. The mesh sizes are estimated from the mechanical properties.

An antioxidant stabilized, chemically cross-linked UHMWPE with superior toughness.

Chemical cross-linking of ultrahigh molecular weight polyethylene (UHMWPE) using an organic peroxide followed by high temperature melting results in a large increase in toughness accompanied by a decrease in cross-link density, which, surprisingly does not compromise the wear resistance. We compared the mechanical properties and wear behavior of a vitamin E blended, chemically cross-linked and high temperature melted UHMWPE produced by ram extrusion (PRX HTM) to those measured with the clinically available 100-kGy irradiated and melted UHMWPE (CISM 100). We also assessed the local biocompatibility of PRX-HTM in rabbit subcutaneous pouch and osteochondral defect models. The ultimate tensile strength and pin-on-disc wear rate were similar to CISM 100; whereas the elongation-at-break and impact toughness were much higher with PRX-HTM. The stress intensity factor range at crack inception was also higher with PRX-HTM. Accelerated aging did not result in any measurable oxidation or changes in mechanical properties. Hip simulator wear rate of acetabular liners made with PRX-HTM was 0.3 ± 0.4 mg/million-cycle, similar to that reported for CISM 100 liners. The wear particles were largely spherical with a number-averaged particle size of 0.95 μm with ~75% of particles below 1 μm. The subcutaneous and osteochondral rabbit implantations showed no histological differences between PRX-HTM and the control CISM 100. Pre-clinical wear, mechanical, and biocompatibility testing of PRX HTM showed feasibility for the use of this material as a total joint arthroplasty implant bearing surface. This process has the potential of eliminating the additional step of radiation cross-linking by combining consolidation and cross-linking while improving toughness. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1945-1952, 2019.

Construction of sustainable polyurethane-based gel-coats containing poly(ε-caprolactone)-grafted lignin and their coating performance

In order to use a lignin as a polyol in polyurethane-based gel-coated films, poly(ε-caprolactone) grafted lignin (PCL-g-lignin) was synthesized by a ring-opening polymerization of ε-caprolactone in presence of a pristine lignin. This reaction was confirmed by FT-IR spectroscopy. By usage of the PCL-g-lignin, polyurethane-based gel-coated films was prepared through a simple urethane reaction following which the effects of the PCL-g-lignin content on the physical properties and surface characteristics of the polyurethane-based gel-coat films was examined. With increased PCL-g-lignin content, the decreased cross-linking density resulted in the reduction of physical properties such as the tensile strength and surface hardness of the gel-coated films. Their surface characteristics (e.g., adhesion strength, abrasion, and gloss) were constant, regardless of the PCL-g-lignin content increase in the gel-coated films.

Self-healing 8-armed star-shaped ɛ-caprolactone oligomers dually crosslinked by the Diels-Alder and urethanization reactions

The reactions of a hydroxy-terminated 8-armed star-shaped ɛ-caprolactone oligomer (H8CLO), which was prepared by the ring opening polymerization of ɛ-caprolactone initiated by tripentaerythritol, with furfuryl isocyanate and (4-maleimidophenyl)isocyanate at the OH/NCO ratios of 1/1.5 and 1/1.2 produced furan- and maleimide-functionalized star-shaped oligomers (F8CLO and M8CLO), respectively. Furthermore, the same reactions except that the OH/NCO ratios are 1/0.75 and 1/0.6 produced half furan- and maleimide-functionalized star-shaped oligomers (H4F4CLO and H4M4CLO). By the furan/maleimide Diels-Alder (DA) reaction and/or OH/NCO urethanization reaction with 1,6-hexamethylene diisocyanate, a DA network (DA8CLO), polyurethane/DA network (PU4DA4CLO) and polyurethane network (PU8CLO) were synthesized. The differential scanning calorimetric analysis revealed that the endothermic peak temperature (113 and 143 °C) due to retro DA reaction of PU4DA4CLO was lower than that (157 °C) of DA8CLO. The conversion (87%) of DA reaction for PU4DA4CLO was a little higher than that (77%) for DA8CLO, and their DA conversions decreased with increasing annealing temperature over the range of 90–150 °C by the progression of retro DA reaction. The decrease of crosslinking density by the retro DA reaction was confirmed by the swelling test. When partially overlapped cut-films of DA8CLO, PU4DA4CLO and PU8CLO were stood at 70 °C for 24 h, the PU8CLO film was not self-repaired, but the DA8CLO and PU4DA4CLO films were self-healed. Especially, the self-repaired PU4DA4CLO film maintained the original shape during the treatment at 70 °C, while the self-repaired DA8CLO film considerably deformed by softening.