An autonomous self‐healing hydrogel based on surfactant‐free hydrophobic association

Self‐healing hydrogels are attractive for a variety of applications including wound dressings and coatings. This paper describes the facile preparation and characterization of an autonomous self‐healing hydrogel system comprising surfactant‐free hydrophobic associations. The hydrogel comprised a copolymer of benzyl methacrylate (B), octadecyl methacrylate (O), and methacrylic acid (MA). The hydrogels were prepared via a controlled dehydration procedure to achieve the formation of strong intermolecular hydrophobic associations of the octadecyl groups above a critical polymer concentration. Fractured hydrogels healed within 30 min without any external intervention. Increasing hydrogel polymer content from 31 wt % to 39 wt % resulted in a threefold increase in the shear modulus and 50% reduction of the relaxation time. Addition of 4 mM NaCl to a hydrogel of 31 wt % polymer content resulted in 2.5 times longer relaxation time and 24% decrease in shear modulus. The hydrogels swelled up water by up to four times its weight, which corroborates the robustness of the hydrophobic association crosslinks. The bulk properties of the hydrogels are discussed in terms of the hydrophobic associations of the O‐groups and the electrostatic interaction of the MA‐groups in the polymer chains. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44800.     

Cyclopentadiene‐functionalized polyketone as self‐cross‐linking thermo‐reversible thermoset with increased softening temperature

Self‐cross‐linkable thermo‐reversible thermosets were obtained by a two‐steps post‐functionalization of aliphatic alternating polyketones yielding two different cyclopentadiene functionalization degree of 9 and 22% (with the respect of initial 1,4‐dicarbonyl units). Thermo‐reversibility was verified by gelation experiments and differential scanning calorimetry (DSC) scans displayed a broad transition varying from 75–100°C till 160°C that can be related to retro‐Diels Alder de‐bonding of the dicyclopentadienyl moieties. The dynamic mechanical thermal (DMTA) analysis showed the complete thermo‐mechanical recovery of the material up to six thermal cycles with a softening temperature around 210°C, thereby ensuring a suitable application window for high‐temperature resistant thermosets. Independently of the exact mechanism at the molecular level and in addition to previous studies which used the same Diels‐Alder diene‐dienophile system, it must be noticed that all prepared materials retained their mechanical behavior during at least six consecutive thermal cycles, thus indicating the re‐workability of the system. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42924.     

An injectable and self‐healing novel chitosan hydrogel with low adamantane substitution degree

Chitosan (CS) is a semi‐natural polymer with supreme biological function, while the strong interchain hydrogen bonds cause poor water solubility and limit its broader use. To break the semi‐rigid structure of CS, a kind of CS modified by adamantane (AD‐CS) was successfully synthesized by amidation reaction with 1‐(3‐dimethylaminopropyl)‐3‐ethylcarbodiimide hydrochloride as catalyst. The chemical structure of AD‐CS was characterized by Fourier transform infrared spectroscopy and ¹H NMR. The AD substitution degree of CS is around 2%, calculated by ¹H NMR. A soft and transparent hydrogel composed of hydrogen bonds was obtained directly by simply adding a certain amount of water under mild conditions. Rheological measurements were carried out to research the mechanism of hydrogel formation by measuring the influence of different additives and conditions on the AD‐CS hydrogel. Reinforced hydrogels were prepared by freezing and thawing. The mechanical strength and self‐healing property of reinforced and pristine hydrogels were assessed with an oscillatory rheometer. The modulus of the reinforced hydrogel was obviously enhanced without much loss of self‐healing property. Other properties such as adhesion, injectability and temperature response were also studied. These injectable and self‐healing hydrogels show potential value in medical care. Additionally, this is a new method to design CS hydrogels with their original interchain hydrogen bonds. © 2019 Society of Chemical Industry     

Multi‐point monitoring of cross‐linking reactions

The feasibility of deploying optical fiber sensors to obtain qualitative and quantitative information on the cross‐linking characteristics of thermosetting resin systems has been demonstrated by a number of researchers. This article is concerned with the demonstration of a low‐cost fiber‐optic Fresnel sensor system for monitoring the cross‐linking reactions at multiple locations. Cross‐linking reactions can be influenced by a number of parameters including the stoichiometry of the resin and hardener, and the heat‐transfer rates in and out of the preform as a function of the cross‐section of the preform. In situations where there is a variation in the thickness of the preform or when large components are processed, a facility to monitor the cross‐linking reactions at multiple locations will be valuable because the rate and extent of the cross‐linking can be inferred. In this article, six Fresnel sensors were immersed in individual vials containing an epoxy/amine resin system and processed (cross‐linked) in an air‐circulating oven. One additional vial with a Fresnel sensor immersed in the neat epoxy resin (no hardener) was co‐located in the oven to enable the effect of temperature to be monitored. The feasibility of using the multiplexed Fresnel sensors for cure monitoring was demonstrated successfully. The sensors in the post‐cross‐linked samples were used to study the effects of heating and cooling cycles. The feasibility of detecting the glass transition temperature using the Fresnel sensor is discussed along with the factors that give rise to variability in the output Fresnel signals. © 2014 The Authors. Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41088.     

Environment‐friendly silicone sealants by self‐catalytic cross‐linking reaction of α‐aminomethyl triethoxysilanes

On the basis of the self‐catalytic characteristic of α‐aminomethyl triethoxysilanes, (diethyl) aminomethyl triethoxysilane, cyclohexylaminomethyl triethoxysilane, and anilinomethyl triethoxysilane were used as self‐catalytic cross‐linkers to prepare a series of environmental friendly, nano‐CaCO₃ reinforced silicone sealants based on poly(dimethylsiloxane) (PDMS). The tensile properties, hardness, adhesive properties, hot‐air aging resistance, and oil‐resistance properties of these materials were studied by universal electronic tensile machine, scanning electron microscope, and atomic force microscopy, etc. Results show that the comprehensive properties of the self‐catalytic cross‐linking PDMS sealants are much better than that of the traditional ones. The chemical structure and reactivity of the α‐aminomethyl triethoxysilanes have a great effect on the properties of PDMS sealants. The tensile strength of the reinforced sealants with 100 phr (100 parts per hundreds of PDMS) nano‐CaCO₃ is about 4.4 times than that of the unreinforced ones, and the elongation at break is about three times. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Silica‐based self‐healing microcapsules for self‐repair in concrete

New self‐healing material for concrete repair has been fabricated through microencapsulation of silica sol via interfacial polymerization of poly(urea‐urethane). Smooth, uniform, and spherical capsules of 60–120 µm sizes were synthesized and optimized by studying the emulsification and polymerization steps of its synthesis. A hydrophile–lyophile balance (HLB) value of 3.0–5.0 and a rotational speed of 500 rpm were necessary to produce a stable emulsion system of silica sol in surfactants. The polymerization speed of 900 rpm and the methylene diisocyanate (MDI) monomer content of 15 wt % were both significant in getting the right size and smoothness of the microcapsules. With this method and cheap healing agents, industrial scale‐up is highly possible. The viscoelastic properties of the microcapsules were examined, and further characterizations were carried out through scanning electron microscopy, energy dispersive X‐ray spectroscopy, and Fourier transform infrared spectroscopy. After healing the microcracks in concrete, the results showed that the significant increase of compressive and bending strengths manifested the self‐healing ability of the microcapsules in concrete. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43090.     

Functionalization and cross‐linking of high‐density polyethylene in the presence of dicumyl peroxide—An FTIR study

High‐density polyethylene was thermolysed with short‐chain organic compounds (additives) with an ester functional group in the presence of free radical initiator such as dicumyl peroxide at 160°C. The experimental results inferred that an additive with a hydrogen donor containing a thiol group showed better efficiency toward functionalization and an additive with a methylene bridged group showed better efficiency toward cross‐linking. The FTIR spectral results indicated that the area of the carbonyl peak at 1730 cm^?1 increased with the increase in percentage grafting of additives with simultaneous increase of cross‐linking. The ester grafting was further confirmed by saponification reaction. The possibility of acid functionalization via intramolecular hydrogen transfer was also checked through acid hydrolysis of the saponified products. A suitable reaction mechanism was proposed in order to explain the experimental and spectral data obtained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 766–774, 2005     

Rate‐dependent self‐healing behavior of an ethylene‐co‐methacrylic acid ionomer under high‐energy impact conditions

In this work, the mechanical and the self‐healing behaviors of an ethylene‐co‐methacrylic acid ionomer were investigated in different testing conditions. The self‐healing capability was explored by ballistic impact tests at low‐velocity, midvelocity, and hypervelocity bullet speed; different experimental conditions such as sample thickness and bullet diameter were examined; in all impact tests, spherical projectiles were used. These experiments, in particular those at low and midspeed, allowed to define a critical ratio between sample thickness and bullet diameter below which full repair was not observed. After ballistic damage, the healing efficiency was evaluated by applying a pressure gradient through tested samples. Subsequently, morphology analysis of the affected areas was made observing all tested samples by scanning electron microscope. This analysis revealed different characteristic features of the damaged zones affected at different projectile speed. Stress–strain curves in uniaxial tension performed at different temperatures and strain rates revealed yield strength and postyield behavior significantly affected by these two parameters. A rise of temperature during high strain rate tests in the viscoplastic deformation region was also detected. This behavior has a strong influence on the self‐repairing mechanism exhibited by the studied material during high‐energy impact tests. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1949–1958, 2013     

Transparent thermoplastic resin with electron beam cross‐linking

The mechanical, thermal and optical properties of a transparent thermoplastic resin which is based on non‐crystalline nylon with cross‐linking agents were improved by electron beam (EB) irradiation cross‐linking. When the EB irradiation power was increased, the yield stress improved by 34% and the glass transformation temperature increased from 116 to 165°C. Transformation from the brittle to the ductile state was observed by the dependence of a strange elastic module on the temperature and EB irradiation power. Above the glass transformation temperature, an optical element produced using this resin could maintain its shape. The birefringence decreased when the EB irradiation power was increased. The decrease of the birefringence was dependent on the initial birefringence value before EB cross‐linking. The molecular orientation was relaxed by EB cross‐linking. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Reversibly crosslinked self‐healing PCL‐based networks

Poly(ε‐caprolactone) (PCL)‐based thermoreversible networks with self‐healing properties were prepared through Diels–Alder (DA) and retro‐DA reactions. Bis‐ or Tris‐maleimide compounds and a series of copolymer(caprolactone‐diene) PCL_XF_Y (X: degree of polymerization and Y: furan‐average functionality) with Y between 2.4 and 4.9 were used. The successive sequences of formation and dissociation of polycaprolactone networks via DA and retro‐DA reactions were observed repeatedly by dynamic mechanical analyses (DMA) and their gel‐temperatures determined. The cross‐linking densities, thermal properties, and thermal reversibility of the PCL_XF_Y/multimaleimide polymers have been modulated by the structure and functionalities of the used diene and dienophile moieties. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermoplastic acrylic resin with self‐healing properties

This article presents a novel processing method of a self‐healing acrylic thermoplastic material starting from a healing agent in solution form. The self‐healing system consisted of a solution of the healing agent dicyclopentadiene (DCPD) in dimethylformamide (DMF) and a solution of the catalyst bis(tricyclohexylphosphine) benzylidene ruthenium (IV) dichloride (called Grubbs' catalyst) in dichloromethane (DCM). Hollow glass tubes filled with the self‐healing components were incorporated into autopolymerizing acrylic resins. The one set of tubes was filled with a solution of DCPD (containing the dye Rhodamine B as a marker) and the other set with a solution of Grubbs' catalyst in dichloromethane. FTIR and DSC analyses revealed that a poly(DCPD) film formed at the healed interface. The low energy impact tests of the samples showed a recovery of 83% after 4 days. The benefits of the Grubb's catalyst solution are twofold; besides the repair of the cracks, which is common for such a system, the reaction could decrease the content of residual monomer in the acrylic resin, which could reduce diffusion of residual monomer out of the resins. POLYM. ENG. SCI., 56:251–257, 2016. © 2015 Society of Plastics Engineers     

UV‐activated silicone oligomer cross‐linking through photoacid and photobase organocatalysts

Diphenyl iodonium hexafluorophosphate salt and N‐alkyl morpholino acetophenone were shown to be effective photocatalyst generators for the cross‐linking of α,ω‐silanol terminated silicone oligomers. These two photoacid and photobase‐induced polycondensation pathways provided an attractive and efficient alternative to toxic and expensive organometallic catalysts. The utility of this novel UV‐curing process was demonstrated with a combination of time‐resolved infrared spectroscopy to follow the fast reaction kinetics and solid‐state ²⁹Si nuclear magnetic resonance to investigate the polysiloxane network. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39875.     

Carbon nanotubes inhibit the free‐radical cross‐linking of siloxane polymers

Carbon nanotubes (CNTs) are found to inhibit the free‐radical cross‐linking of vinyl‐terminated polydimethylsiloxane (PDMS) by trapping the free radicals in a CNT/PDMS composite. The cross‐linking density values measured by swelling test, equilibrium stress‐strain test, glass‐transition temperature and Raman spectroscopy all decrease with the addition of CNTs. The inhibition effect is shown to be more significant with increased functionality and curvature in the CNTs used. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40355.     

Evaluation of cross‐linking after accelerated photo‐ageing of silicone rubber

Silicone formulations based on polydimethylsiloxane (PDMS) were investigated by using a number of analytical techniques. Horizontal attenuated total reflectance Fourier‐transform infrared (HATR‐FTIR) spectroscopy studies have confirmed the stability of these materials towards photo‐oxidation. No significant oxygenated products were detected after 5000 h of accelerated photo‐ageing. However, by using thermoporosimetry, hardness measurements and densitometry, an important vulnerability of these formulations towards the cross‐linking reactions provoked by UV visible radiation was identified. Thermoporosimetry, using the solid‐solid cyclohexane transition as a textural probe, was also used to visualize the profile of cross‐linking inside the materials by calculation of the mesh size distribution in successive slices of 40 µm in thickness. Copyright © 2003 Society of Chemical Industry     

Tung oil‐based thermosetting polymers for self‐healing applications

Several bio‐renewable thermosetting polymers were successfully prepared from tung oil through cationic polymerization for the use as the healing agent in self‐healing microencapsulated applications. The tung oil triglyceride was blended with its methyl ester, which was produced by saponification followed by esterification. The changes in storage modulus, loss modulus, and glass transition temperature as functions of the methyl ester content were measured using dynamic mechanical analysis. In addition, the fraction of cross‐linked material in the polymer was calculated by Soxhlet extraction, while proton nuclear magnetic resonance, Fourier transform infrared spectroscopy and TEM were used to investigate the structure of the copolymer networks. The thermal stability of the thermosets as a function of their methyl ester blend contents was determined by thermogravimetric analysis. Finally, the adhesive properties of the thermosets were studied using compressive lap shear and the fracture surfaces were analyzed using SEM. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40406.     

Constitutive equation of peroxide cross‐linking of thermoplastic polyolefin rubbers

A nonlinear constitutive equation has been proposed to describe the dependence of the cross‐link density vs. peroxide and coagent concentrations, following the dynamic vulcanization of ethene‐propene copolymers (EPM) rubbers. This was obtained by integrating the rate equations of the fundamental reactions of the cross‐linking processes (β‐scission, chemical coupling, coagent grafting). The compression set test ASTM D 395 was then modeled and put in correlation with the cross‐linking density. The model was then validated with compression set experimental data, obtained from a series of EPM rubbers dynamically cross‐linked under various conditions of peroxide/coagent content. The two most meaningful model parameters (N₁ and N₂, which are linked to the order of the rate equations) have been correlated with the elastomer microstructure, particularly with the ethene intramolecular chain distribution and content. POLYM. ENG. SCI., 45:333–342, 2005. © 2005 Society of Plastics Engineers     

Recyclable bio‐based crosslinked polyurethanes with self‐healing ability

We report the synthesis of a linear bio‐based polyurethane (bio‐PU) containing furan ring by using renewable polylactide copolymer diol and 2,5‐furandimethanol as a soft segment and chain extender, respectively, in which the reversible crosslinked covalent bonds between hard segments were incorporated via Diels–Alder (D‐A) reaction between the furan ring of the chain extender and bismaleimide (BM) crosslinker. By simply controlling the amount of BM, mechanical properties of the obtained crosslinked bio‐PUs (CBPUs) were varied widely. In particular, the CBPU100 sample shows the highest tensile strength of 10.8 MPa, Young's modulus of 193 MPa, and an elongation of 155%. The differential scanning calorimetry experiments verify the recycle property of the CBPUs by the D‐A/retro‐D‐A reaction at the proper temperature. The thermal recyclability and remolding ability of these materials are demonstrated by two kinds of polymer processing methods, i.e., solution casting and hot‐compression molding. The recycled CBPUs display almost identical elongation and slightly decreased tensile strength compared to the as‐synthesized samples. Furthermore, the CBPUs also exhibit excellent self‐healing ability. Therefore, the resulting CBPUs possess tunable mechanical properties, good thermal recyclability, re‐mending, and self‐healing ability, which makes the bio‐based materials more eco‐friendly. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46272.     

Thermally assisted self‐healing polyurethane containing carboxyl groups

Self‐healing polymer is a kind of intelligent material with the capability to repair damage automatically. In this paper, a type of polyurethane containing carboxyl groups is reported that demonstrates thermally assisted healing effects. This polymer can be healed even 72 h after cutting and also has a repeatable healing property. The self‐healing efficiency can reach 90% when comparing the tensile strength of the healed sample to the original sample. Carboxyl content plays an important role in the self‐healing property; polyurethane with no carboxyl groups cannot be healed. The mechanism of self‐healing showed that hydrogen bonding interactions between carboxyl groups and diffusion of poly(ethylene glycol) chains contribute to the self‐healing behavior. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45929.     

Electrospinning of highly aligned and covalently cross‐linked hydrogel microfibers

Smart polyelectrolyte hydrogels are increasingly studied toward the realization of soft microactuators. This study focuses on the fabrication of highly aligned and covalently cross‐linked polyacrylamide hydrogel microfibers by electrospinning technique following two‐stage polymerization. The engineering of the reaction timescale of the precursor, such that the material gels shortly after spinning, is described and the design of the electrospinning setup, to generate highly aligned fibers, is presented. In addition the effect of the operating parameters on the fibers average diameter is investigated. The generated fibers are 15 cm long with average diameter ranging between 100 nm and 1.10 μm. The fibers diameter is controlled by adjusting the thickener type and concentration in the precursor and the electrospinning processing parameters. Thinner fibers are generated at lower thickener molecular weights and concentrations as well at lower flow rate and higher voltage. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41092.