Radical Crosslinking Copolymerization Research Articles

Randomly cross-linked amphiphilic copolymer networks of n-butyl acrylate and N,N-dimethylacrylamide: synthesis and characterization

Abstract Five randomly cross-linked amphiphilic copolymer networks (ACPN) were prepared via the free radical cross-linking copolymerization of the hydrophobic n-butyl acrylate (BuA) and the hydrophilic N,N-dimethylacrylamide (DMAAm), in the presence of a small amount (5 mol% with respect to the sum of BuA plus DMAAm monomers) of the hydrophobic 1,6-hexanediol diacrylate (HexDA) cross-linker, initiated by 4,4ʹ-azobis(4-cyanovaleric acid) in 1,4-dioxane at a 10 % w/v total monomer concentration. The five ACPNs differed in their BuA content, fixed at 10, 30, 50, 70 and 90 mol%. The two homopolymer networks, BuA and DMAAm, were also prepared using the same polymerization method. Thus, this study involved a total of seven polymer networks, forming a homologous series with BuA contents ranging from 0 to 100 mol%. These networks were characterized in terms of their degrees of swelling in tetrahydrofuran (THF) and water, their mechanical properties in water, and their adhesion to human skin. The degrees of swelling (DS) of the networks in THF were higher than those in water because THF is a non-selective solvent, whereas water is selective for the hydrophilic DMAAm units. The DSs in THF increased with the network content in the less polar BuA units, while the opposite was true for the DSs in water which decreased with the content in the hydrophobic BuA units from 11 (0 mol% BuA) down to 1.1 (100 mol% BuA). A maximum in the elastic modulus was observed for the hydrogel with 50 mol% BuA, reflecting the opposing effects of polymer composition in soft polymer (polyBuA) content and hydrogel water content. In contrast, the tensile strain at break increased monotonically with the hydrogel BuA content, reaching a remarkable ∼900 % for the hydrogel with 90 mol% BuA. Finally, the adhesion of the ACPNs, both in their dried and hydrated states, onto human skin was explored. The adhesion of the hydrated samples onto skin was stronger than that of the dried ones. The hydrated ACPN with 30 mol% BuA exhibited the strongest adhesion onto skin, attributable to the best combination of a rather high content in polar DMAAm units (70 mol%), and a rather low aqueous DS (∼2.5), with the low DS value causing only a small dilution in the DMAAm units participating in the polar interactions with skin. The present work demonstrates that, even in this synthetically simple ACPN system, the multiple effects of ACPN composition on a certain property, in some cases opposing and in some other cooperating, lead to a rather complicated behavior. In particular, as the BuA content increases, some properties display maxima (elastic modulus, stress at break and fracture energy of hydrated ACPNs, and adhesion of hydrated ACPNs onto skin) while some other properties exhibit monotonic increases (strain at break of hydrated ACPNs, and adhesion of dried ACPNs onto skin). Thus, the optimal ACPN for a particular application will highly depend on the property best-serving the particular application, e.g., the ACPNs with 30, 50 and 90 mol% BuA for strongest wet adhesion to skin, stiffest hydrogel response, and highest hydrogel extensibility and toughness, respectively.

The Effect of Pyranine and MoS2 Content on the Optical Properties of Polyacrylamide-MoS2 Composite Gels

The most favorable members of the class of two-dimensional (2D) materials, molybdenum sulfide (MoS2), graphene, and tungsten disulfide (WS2) have become a thriving research frontier for many applications, ranging from catalysis to optoelectronic devices, due to their superior electronic behavior and mechanical properties. Making a new, smart, hydrogel when incorporating MoS2 into basic hydrogels can change their optical, electronic, and mechanical properties. In this work, MoS2-doped polyacrylamide (PAAm) gels were prepared via the free radical crosslinking copolymerization technique at room temperature without pyranine which is often used for crosslinking PAAm. The optical properties of PAAm-MoS2 composite gels were investigated, and they were analyzed using ultraviolet (UV) and fluorescence spectroscopy. The absorbance was measured before and after diffusion of MoS2-doped PAAm, and after the release of pyranine in and out of the composite gel, respectively. The shift of the absorption edge toward shorter wavelengths was seen when the quantity of MoS2 in the PAAm composite gels was decreased. The constituents included in the polymer composites exhibited notable photon energy absorption within the visible and ultraviolet spectra. This absorption phenomenon induced electron transitions to states of elevated energy. Parameters, such as refractive index and extinction coefficient, were analyzed with respect to the content of MoS2. When the MoS2 amount was increased in the gels, the percentage of reflectance increased. The extinction coefficient increased at longer wavelength and it decreased in the UV range, the shorter wavelength region. The results suggested that MoS2-doped PAAm composite gels show promising optoelectronic properties for developing optoelectronic devices.

Highly efficient removal of Sr2+ from aqueous solutions using a polyacrylic acid/crown-ether/graphene oxide hydrogel composite.

With the development of nuclear power, efficiently treating nuclear wastes generated during operation has attracted extensive attention. Hydrogels are common adsorbent materials in the treatment of wastewater due to their high swelling rate and easy post-treatment. In this work, a novel polyacrylic acid/crown-ether/graphene oxide (PAA/DB18C6/GO) hydrogel composite was synthesized by a radical cross-linking copolymerization method and characterized using various analytical tools such as SEM, FT-IR, TGA and XPS. The effects of time, pH, initial Sr2+ concentration, and temperature on Sr2+ adsorption onto the PAA/DB18C6/GO were studied. The PAA/DB18C6/GO shows a high adsorption capacity of 379.35 mg g-1 at an initial Sr2+ concentration of 772 mg L-1 due to the unique structure of dibenzo-18-crown-ether-6 and high swelling. The composite has a high selectivity for Sr2+ with a removal rate of 82.4% when concentrations of Na+ and K+ were 10 times higher than that of Sr2+. The pH and temperature have no apparent impact on adsorption performance of the PAA/DB18C6/GO under the experimental conditions. The composite shows excellent reusability with more than 92% removal rate for Sr2+ after five continuous cycles. In addition, the mechanism of Sr2+ adsorption by PAA/DB18C6/GO was analyzed by fitting the adsorption data to the theoretical models and XPS data.

Optical and mechanical properties of PNIPAm‐MWCNTs and PNIPAm‐GO composites

AbstractThe aim of this study is to investigate drying, swelling, elastic properties, and to produce optical band gap energies on the performance of Poly (N‐Isopropyl acrylamide) ‐ Multi walled carbon nanotubes (PNIPAm – MWCNTs) and Poly (N‐Isopropyl acrylamide)‐ Graphene Oxide (PNIPAm‐GO) composites, respectively. The composites were prepared by free radical crosslinking copolymerization. Drying and swelling of PNIPAm‐MWCNTs and PNIPAm‐ GO composites were performed by using steady state fluorescence technique. Firstly, the desorption coefficients of the composites were modeled by using Fick's Law. Cooperative diffusion coefficients were calculated by employing Li‐Tanaka Model. When composite doped with 10 wt% of MWCNTs has a higher desorption coefficient than composite doped with the same amount of GO, 40 wt% of GO content in the PNIPAm has a higher diffusion coefficient than the composite that has 40 wt% of MWCNTs content in the swelling process. Secondly, the composite that has 40 wt% of MWCNTs content has a higher modulus before swelling than the composite that has the same amount of GO contents. Lastly, optical band gap energies were calculated by using Tauc's approach, and Urbach's relation by using the data performed UV Spectroscopy. We found that PNIPAm ‐GO has a much narrower band gap value.

2D Materials (WS2, MoS2, MoSe2) Enhanced Polyacrylamide Gels for Multifunctional Applications.

Multifunctional polymer composite gels have attracted attention because of their high thermal stability, conductivity, mechanical properties, and fast optical response. To enable the simultaneous incorporation of all these different functions into composite gels, the best doping material alternatives are two-dimensional (2D) materials, especially transition metal dichalcogenides (TMD), which have been used in so many applications recently, such as energy storage units, opto-electronic devices and catalysis. They have the capacity to regulate optical, electronic and mechanical properties of basic molecular hydrogels when incorporated into them. In this study, 2D materials (WS2, MoS2 and MoSe2)-doped polyacrylamide (PAAm) gels were prepared via the free radical crosslinking copolymerization technique at room temperature. The gelation process and amount of the gels were investigated depending on the optical properties and band gap energies. Band gap energies of composite gels containing different amounts of TMD were calculated and found to be in the range of 2.48–2.84 eV, which is the characteristic band gap energy range of promising semiconductors. Our results revealed that the microgel growth mechanism and gel point of PAAm composite incorporated with 2D materials can be significantly tailored by the amount of 2D materials. Furthermore, tunable band gap energies of these composite gels are crucial for many applications such as biosensors, cartilage repair, drug delivery, tissue regeneration, wound dressing. Therefore, our study will contribute to the understanding of the correlation between the optical and electronic properties of such composite gels and will help to increase the usage areas so as to obtain multifunctional composite gels.

Synthesis and Characterization of Stretchable Ionic Cable

A transparent and flexible polymer with unique electric and sound signal transmission capability under highly stretchable conditions has been synthesized. In this paper the milestone achieved in the polymerization of intrinsic ionic Ω–conjugated polymer, hereinafter known as Ionic Cables (ICs), through free radical crosslinking co-polymerization process/technique and structural modifications encountered so far in our experiments are discussed. EIS and stress-strain assessments had showed that ionic conductivity and flexibility are dependent on the amount of accelerator Tetramethylethylenediamine (TEMED) and curing time (CT). By varying the amount of accelerator, in our case TEMED, and CT the ICs with ionic conductivity of 0.021 S/cm to 0.111S/cmand 350% elongation to 448% elongation conditions was synthesized with highly water absorption capability up to 78% of its total volume. The synthesized ICs with ultra-stretchability, transparency and conductivity can be used in versatile applications, including stretchable ionic cables, tissue engineering, ionic mass transport systems and bio sensors.

In Situ RheoNMR Correlation of Polymer Segmental Mobility with Mechanical Properties during Hydrogel Synthesis.

Understanding polymer gelation over multiple length‐scales is crucial to develop advanced materials. An experimental setup is developed that combines rheological measurements with simultaneous time‐domain 1H NMR relaxometry (TD‐NMR) techniques, which are used to study molecular motion (<10 nm) in soft matter. This so‐called low‐field RheoNMR setup is used to study the impact of varying degrees of crosslinking (DC) on the gelation kinetics of acrylic acid (AAc) and N,N′‐methylene bisacrylamide (MBA) free radical crosslinking copolymerization. A stretched exponential function describes the T 2 relaxation curves throughout the gelation process. The stretching exponent β decreases from 0.90 to 0.67 as a function of increasing DC, suggesting an increase in network heterogeneity with a broad T 2 distribution at higher DC. The inverse correlation of the elastic modulus G′ with T 2 relaxation times reveals a pronounced molecular rigidity for higher DC at early gelation times, indicating the formation of inelastic, rigid domains such as crosslinking clusters. The authors further correlate G′ with the polymer concentration during gelation using a T 1 filter for solvent suppression. A characteristic scaling exponent of 2.3 is found, which is in agreement with theoretical predictions of G′ based on the confining tube model in semi‐dilute entangled polymer solutions.

Evaluation of the fractal dimension of polyacrylamide during gelation and swelling

The photon scattering method was performed to monitor the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) applied with N,N′-methylenebis (acrylamide) (Bis) in time. FCC experiments were conducted using different Bis contents to create fractal-like network structures in polyacrylamide (PAAm) gels. Higher intensity values of the scattered photons, Isc were observed with increasing Bis content, BC, at a given time, which was attributed to fractal-like macrogel formation from interconnected microgels. The same gels were used in swelling experiments after drying, and increased Isc intensities from the PAAm gels were observed during the swelling process. Fractal dimensions, d during the gelation and swelling processes were measured and found to be increased as gelation and swelling times increased. According to the results obtained from the gelation measurements, it was seen that the fractal size, d, increased from very small values to 3.00. A similar increase in the range 1.00–1.66 was observed during swelling experiments.

Cross-linking of double oil-in-water-in-oil emulsions: A new way for fragrance encapsulation with tunable sustained release

This paper describes a new route to synthesize polymeric capsules with interesting and diverse diffusive and mechanical properties. Such capsules were obtained by free radical cross-linking copolymerization of the intermediate water phase of a double oil-in-water-in-oil (O/W/O) emulsion. Such a strategy allows separating the locus of polymerization from the fragrance- containing compartment. Indeed fragrance molecules are known to be reactive. The encapsulated species, also corresponding to the innermost oil, was a model fragrance composed of more than 10 various molecules usually used in perfumery. In a first step, the double emulsion containing a hydrophilic monomer, either Methacrylamide or Oligo(ethyleneglycol) methacrylate, and a hydrophilic cross-linker, either Tetra(ethyleneglycol) diacrylate or N,N-Methylene-bis-Acrylamide, was formulated. The influence of the cross-linker agent on the feasibility of the double emulsion was examined. Then, in a second part, the polymerization was performed. Four different systems of capsules were developed and compared in terms of diffusive and mechanical properties. We show that changing the monomer /cross-linker couple allowed modulating the capsule properties.

Stable cross-linked gel terpolymer electrolyte containing methyl phosphonate for sodium ion batteries

The novel bifunctional methyl phosphonate which combines the good polymerization ability of acrylate, the adjustable flexibility of ethylenedioxy group and the flame retardant properties of phosphonate is designed and synthesized. The corresponding ternary gel polymer electrolyte is prepared by radical cross-linking copolymerization. The gel terpolymer electrolyte has good thermal stability with the onset decomposition temperature of 250 °C, excellent electrochemical stability of up to 5.0 V (vs. Na+/Na) and high ionic conductivity of 5.13 mS cm−1. Moreover, the gel polymer electrolyte can enhance interfacial adhesion between the membrane and the electrodes. The Na3V2(PO4)3/Na cell using phosphorus containing gel terpolymer electrolyte shows ultra-long cycling performance and better cycle stability and rate performance than the corresponding conventional liquid electrolyte, phosphorus containing gel bipolymer electrolyte and other crosslinker based cells. Meanwhile, the glass fiber separator filled with gel polymer electrolyte maintains better integrity than liquid electrolyte after 1000 cycles, which means the gel polymer electrolyte has good mechanical property and compatibility. The gel polymer electrolyte based sodium-ion battery also exhibits superior cycle stability compared to the battery with liquid electrolyte when running at high temperature of 60 °C. The strong interaction between methyl phosphonate and sodium ion is studied by FT-IR, 31P NMR and HRMS, and demonstrated by theoretical calculation.

Temperature Effect on the Elasticity of Acrylamide-n-Isopropylacrylamide Copolymers

ABSTRACTAcrylamide (AAm) – N-isopropylacrylamide (NIPA) copolymers were prepared via free radical crosslinking copolymerization with various weight percentages (wt%) of AAm and NIPA. The temperature dependence of the compressive elastic modulus, G, and toughness, UT, of the PAAm- NIPA copolymers due to a volume phase transition was found using a compressive testing technique. It was observed that the compressive elastic modulus increased comprehensively when the temperature was increased between 30°C and 60°C. The PAAm- NIPA copolymers presented higher values of the compressive elastic modulus than pure NIPA above the lower critical solution temperature (LCST) (NIPA exhibits a volume phase transition from hydrophilic to hydrophobic in water at 31°C) and their compressive elastic modulus and toughness had a strong temperature dependence.

Dynamical modeling and experimental aspects of multi-responsive hydroxy-functional methacrylate-based gels with tunable swelling induced by multivalent ions

Novel materials displaying multi-responsive property were developed by forming crosslinked copolymer systems that exhibit distinct temperature, pH and salt-sensitivity independently. An investigation of the mechanical properties of a novel multi-responsive poly(hydroxypropyl methacrylate) (PHPMA)-based hydrogel system was carried out with two major objectives. First was to study the effect of various preparation conditions; reaction temperature, comonomer content on the elasticity as well as on the absorbency of resulting hydroxy-functional methacrylate-based gels and the second was to interpret their water uptake data by various kinetic models. Experiments were conducted to characterize the equilibrium swelling and temperature/pH-dependent phase transitions of PHPMA-based copolymeric gels prepared by radical crosslinking copolymerization in aqueous solution with tetraethyleneglycol dimethacrylate (TEGDMA) as crosslinker. The aqueous equilibrium swelling properties of PHPMA hydrogels and cryogels containing methacrylate-based comonomer were described using N,N-dimethylaminopropyl methacrylate (DMAEMA) having weakly basic cationic groups. For PHPMA-based copolymeric hydrogels, the scaling laws relating the optimum preparation conditions with the crosslinking density N and the swelling degree qv were derived. Dynamic kinetics profiles were evaluated to outline the swelling/deswelling response of the resulting hydrogels and cryogels which presents useful information when designing specific applications that pursue or require the absorption of pH-sensitive or ionic-strength-sensitive molecules.

Mechanical properties of graphene oxide–polyacrylamide composites before and after swelling in water

Polyacrylamide (PAAm) hydrogels have been proposed for use as promising biomaterials in biomedical and tissue engineering but their poor mechanical and water-retention properties have hindered their development. Graphene oxide (GO), an excellent nanofiller, was added to PAAm to produce mechanically strong GO–PAAm composites. The free radical crosslinking copolymerization method was used with GO content varying in the range between 5 and 50 µl of GO. The mechanical properties of the GO–PAAm composites are measured by compressive test, revealing a decrease in the shear modulus and toughness of the composites above 8 µl of GO by increasing GO content.

Elastic properties of a swollen polyacrylamide (PAAm) gel doped with various multiwalled carbon nanotube (MWNT) contents

Abstract The toughness UT and shear modulus S of polyacrylamide (PAAm) – multiwalled carbon nanotube (MWNTs) composites were studied with various MWNT contents. These composites were prepared via free radical crosslinking copolymerization with different amounts of MWNT varying in the range between 0.1 and 50 wt.-%. The MWNT content dependency of the shear modulus of the composites due to volume phase transition was measured using tensile testing at 30 °C. It was observed that the composite preserves the ability to undergo the volume phase transition and its elastic modulus is strongly dependent on the MWNT content. The observed shear modulus first increased and then decreased above 1 wt.-% MWNT content during such loading tests. The toughness showed exactly the reverse behavior dependent on the MWNT content, as compared to the elasticity and as expected.

Optical energy band gap of PAAm-GO composites

Graphene oxide (GO) can be used as an electron acceptor for polymers. The generation of the exciton in such materials is intimately related to the optical band gap. Polyacrylamide (PAAm) – graphene oxide (GO) composites were prepared via free radical crosslinking copolymerization with GO content varying in the range between 5 and 40µl of GO. The influence of the GO content on the optical properties of the composites is characterized by ultraviolet visible spectrophotometer. The optical band gap of the composite has been calculated by using Tauc’s model. The tail width of the localized states has been calculated by using Urbach’s relation, and the values of Urbach’s energy with the variation of GO have been found from 2,89 to 2,96eV. The results demonstrated that GO can be well dispersed in the PAAm-GO composites and consequently the optical band gap has been correlated with the various amounts of GO.

Role of ion conducting polymeric electrolyte in suppressing deterioration of cathode electrodes in dye-sensitized solar cells

In this study, an ion conducting polymeric hydrogel (ICPH) is synthesized, characterized and used as a matrix for trapping the electrolyte solution to suppress the deterioration of cathode electrodes for dye-sensitized solar cells (DSSCs). Polymeric hydrogels are synthesized as free radical cross-linking co-polymerization of acrylamide (AAm) and methylenebisacrylamide (BIS) as crosslink agent in slab forms with the same thickness. The polymer host is characterized by scanning electron microscopy (SEM). Impedance spectroscopy studies are carried out in the frequency range of 5 Hz–13 MHz at room temperature. It is observed that while the dielectric constant decreased, the loss tangents increased with increasing electrolyte content which is trapped in ICPH. The as-prepared ICPH/graphene electrodes, assembled with this material as cathode, are confirmed as substitution for platinum (Pt) counter electrode in DSSCs. The photovoltaic tests of the ICPH-DSSCs are carried out by the current–voltage (I–V) measurements and it is observed that ICPH-DSSC exhibited significantly enhanced electrical properties such as increased photovoltaic cell efficiency (PCE) which reached to 5.41% after using the ICPH. We predict that this work will open a way to fabricate the DSSC devices with a cathode electrode based on ion conductive polymeric hydrogels.

Rheological behavior and reversible swelling of pH sensitive poly(acrylamide-co-itaconic acid) hydrogels

The study involved preparation of poly(acrylamide-co-itaconic acid) hydrogels by radical cross-linking copolymerization. The copolymer hydrogels were characterized through infrared spectroscopy, thermal analysis, swelling measurements and in oscillatory and steady shear rheology. Results showed that more stable copolymers were formed due to the strong interaction in the hydrogels. These hydrogels have shown substantial percent swelling in water and shrinking in saline solution and acidic buffers. The rheological properties were described by the Herschel-Bulkley and power-law models to explore their non-Newtonian behavior. The results showed that higher itaconic acid content raised the polymer viscosity; the degree of shear-thinning and polymer elasticity (G′) were also increased. The transition from the viscous (G′ G″) occurs at a crossover frequency ranged from 17.8 rad/s for polyacrylamide to 15.7, 12.8 and 12.5 rad/s for copolymers.

Ion-stimuli responsive dimethylaminoethyl methacrylate/hydroxyethyl methacrylate copolymeric hydrogels: mutual influence of reaction parameters on the swelling and mechanical strength

Novel ion-stimulus-responsive copolymeric hydrogels were synthesized by free radical crosslinking copolymerization from the monomers N,N-dimethylaminoethyl methacrylate (DMAEMA) and 2-Hydroxyethyl methacrylate (HEMA) in the presence of a crosslinker, diethyleneglycol dimethacrylate (DEGDMA). The influences of the reaction parameters, the comonomer composition and the ionic strength of salt solutions on the swelling behavior of P(DMAEMA-co-HEMA) hydrogels were examined. The ion-stimulus-responsive swelling behavior of the prepared copolymers was studied in water as well as in aqueous solutions of NaCl, KCl, KBr, KI, CaCl2, BaCl2 and MgCl2. It was found that, starting from some characteristic concentration of a salt, a further increase of the salt concentration results in the shrinking of copolymeric P(DMAEMA-co-HEMA) hydrogels. The Flory-Rehner theory correctly predicts the swelling behavior of the hydrogels in salt solutions if the variation of the comonomer HEMA content is taken into account. The calculation of the interaction parameter χ between P(DMAEMA-co-HEMA) network and water showed that the specific interactions between cations and side groups of polymeric network affect the mixing term of the free energy. The extent and kinetics of water absorption were studied to determine their relationship with the reaction parameters. The kinetics of the hydrogel collapse is strongly dependent on the kind of salt used. The swelling results will be useful in designing and developing novel controlled delivery systems.

Direct one-pot synthesis of poly(ionic liquid) nanogels by cobalt-mediated radical cross-linking copolymerization in organic or aqueous media

Nanogels of controlled kinetic chain length were synthesized by cobalt-mediated radical cross-linking copolymerization (CMRccP) involving a vinyl monomer and a divinyl cross-linker.

Swelling behavior and network characterization of hydrogels from linear polyacrylamide crosslinked with glutaraldehyde

Polyacrylamide hydrogels have been prepared by crosslinking reaction between linear polyacrylamide (PAAm) and glutaraldehyde (GA) in acidic medium. The equilibrium swelling degree, modulus of elasticity and structure parameters of the network have been investigated as functions of the initial polymer concentration and the crosslinker content. Stability of GA crosslinks between PAAm chains has been demonstrated under the acid and basic conditions. It has been suggested that the crosslinking reaction can be presented as two stage process, which distinguishes different accessibility of aldehyde and amide groups for cross-linking agent in conditions of macromolecule distancing. The method provides formation of PAAm hydrogels with the values of the swelling degree and mechanical characteristics, which are close to the hydrogels prepared by traditional method of free radical crosslinking copolymerization.