Viscoelastic behavior of highly crosslinked poly(acrylic acid)

The effects of crosslinking upon the dynamic mechanical properties and swelling behavior of poly(acrylic acid) (PAA) have been studied. Materials were prepared by the free radical copolymerization of acrylic acid with varying amounts of the tetrafunctional monomer allyl acrylate (ALA). The results indicated a linear dependence of the glass-transition temperature (T_g) on composition, T_g increasing by ～43°C over the mole fraction range X = 0?0.37 ALA. Room temperature (25°C) modulus values, as determined by both dynamic and compression methods, were inversely proportional to the initial concentration of ALA. The degree of network formation has been characterized in terms of the molecular weight between crosslinks M_c, and the influence of this parameter on the swelling ratio was discussed.     

Network characteristics of homopolymer and some copolymers of poly(2-hydroxyethyl methacrylate)

Soluble poly(2-hydroxyethyl methacrylate) and copolymers of 2-hydroxyethyl methacrylate with n-butyl acrylate and 2-hydroxypropyl acrylate of varying compositions were synthesised by solution polymerisation. The polymers were characterised by dilute solution viscometry and shear viscosity. Crosslinked membranes were prepared from these polymers by introducing crosslinks in solution state through the reaction with 1,6-hexamethylene diisocyanate at a number of different crosslinker concentrations. The membranes swollen in dimethylformamide and water were characterised by performing equilibirium swelling measurements and calculating polymer volume fraction, the molecular mass between crosslinks (M?^c) and equilibrium water content (EWC) at 25°C. The calculation of M?^c used an expression developed by Peppas & Lucht for equilibrium swelling. This is a modified equation of the original Flory–Rehner expression, that allows non-Gaussian distribution of chain lengths. The variation of M?^c with crosslinker concentration and copolymer composition is discussed in terms of structural and molecular characteristics of these highly crosslinked networks.     

Network formation and swelling behavior of photosensitive poly(vinyl alcohol) gels prepared by photogenerated crosslinking

Poly (vinyl alcohol) with pendent styrylpyridinium groups (SbQ) is insolubilized by photoirradiation. An association takes place in SbQ groups. The association of polymer chains becomes marked with increasing the number of SbQ groups. Mainly intermolecular crosslinks were formed. Transparent and homogeneous macrogels consisting of several intermolecular crosslinks are obtained. The proportion of the free water to the bound water in PVA-SbQ gels was 3.3?2.9 despite of the large change in conversion of photodimerization of SbQ groups, x=0.27?0.58. The water uptake after swelling of the gels in water increased 6–27 times compared to the original weight at pH=7. The higher the degree of photocrosslinking, the lower was the degree of swelling. The water diffusion coefficients, D, were (2.2?5.8) × 10^?5 cm² S^?1 for a 88% saponified PVA with 1 . 3 mol% SbQ groups. The volume of the gel increased discontinuously about 10-fold for the 99% saponified PVA with 0 . 096 mol% SbQ and 51% water (49% acetone). The acetone concentration at the transition decreased with increasing the degree of saponification of the PVA.     

A comparative study of swelling properties of hydrogels based on poly(acrylamide‐co‐methyl methacrylate) containing physical and chemical crosslinks

Poly (acrylamide‐co‐methyl methacrylate) hydrogels of different ratios were prepared by using chemical and physical crosslinks to study the effect of nature of crosslinks on swelling behavior of hydrogels. The chemically crosslinked gels were prepared by using NN′‐methylene bis acrylamide, while physically crosslinked hydrogels were prepared by precipitation polymerization method, using dioxane as solvent. Detailed swelling kinetics such as swelling ratio, transport exponent n, diffusion coefficient D and the effect of pH on equilibrium swelling studies. The study revealed that the nature of crosslinks alter the swelling characteristics of the hydrogel. In chemically crosslinked hydrogels the water transport is Fickian in nature, while in the case of the physically crosslinked hydrogels the water transport mechanism is anomalous indicating major change in relaxation mechanism due to nature of crosslinks. The results also indicate that with increasing acrylamide content the swelling ratio of the hydrogels were also increased, but the transport exponent n remains nearly constant. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 779–786, 2003     

Novel physical hydrogels composed of opened‐ring poly(vinyl pyrrolidone) and chitosan derivatives: Preparation and characterization

A novel, physically stabilized hydrogel system composed of chitosan (Chi) or its derivatives [e.g., carboxymethyl chitosan (CMC), sodium carboxymethyl chitosan, or trimethyl carboxymethyl chitosan (TMCMC)] with poly(vinyl pyrrolidone) (PVP) or opened‐ring poly(vinyl pyrrolidone) (OR–PVP) were prepared and characterized. TMCMC was synthesized by a novel method with dimethylsulfate as the methylation agent. The synthesized materials were characterized by Fourier transform infrared spectroscopy, ¹H‐NMR, ¹³C‐NMR, and size exclusion chromatography. The mechanical properties, gel fraction, swelling behavior, and water state of the prepared hydrogels were investigated. Gelation occurred when the OR–PVP and Chi solutions were blended within a few seconds. However, the gelation of the OR–PVP and CMC solutions needed pH adjustment. No gelation occurred when the solutions of TMCMC and PVP or OR–PVP were blended. The quaternization or protonization of  NH₂ groups may have prevented the gelation of the solutions. The amino groups of Chi derivatives should have been free to take part in hydrophilic bonds between the two polymers. The physical entanglement of polymeric chains and strong hydrogen bonds between the polymers were considered as mechanisms for the formation of the physical hydrogels. The physical hydrogels showed ionic and pH‐sensitive swelling properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A study on distension index and distribution of crosslinks in formaldehyde-crosslinked cotton

A study on distension index (DI) and distribution of crosslinks is reported for cotton cellulose treated with selected swelling reagents and crosslinked by different processes with formaldehyde. Details of estimation of the distension index are also described. Distribution of crosslinks under different conditions of crosslinking has also been shown by electron microscopy. Marked differences in DI values and distribution of crosslinks in cellulose samples crosslinked under different conditions of treatments are discussed. The results of these analysis provide quantitative information on the gross uniformity of distribution of crosslinkages in the fiber structure of cotton celluloses.     

Ionizable interpenetrating polymer networks of carboxymethyl cellulose and polyacrylic acid: Evaluation of water uptake

A semi interpenetrating polymer network (IPN) of carboxymethyl cellulose (CMC) and crosslinked polyacrylic acid (PAA) has been prepared and its water‐sorption capacity has been evaluated as a function of chemical architecture of the IPN, pH, and temperature of the swelling medium. The water uptake potential of the IPNs has also been investigated in inorganic salt containing aqueous solutions and simulated biological fluids. The IPN was characterized by IR spectral analysis, and the network parameters such as average molecular weight between crosslinks (M_c), crosslink density (q), and number of elastically effective chains (V_e) have been evaluated by water‐sorption measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2054–2065, 2004     

High strength poly(acrylamide)-clay hydrogels

A series of polyacrylamide nanocomposite hydrogels were synthesized by in situ free radical polymerization of acrylamide (AAm) with ethylene glycol dimethacrylate (EGDMA) as a crosslinker in the presence of sodium montmorillonite (NaMMT) and organically modified montmorillonite (OrgMMT) clays. Modification of MMT was carried out with a quaternary salt of coco amine as intercalant having a styryl group whose contribution to both polymerization and crosslinking reactions via its reactive double bond was confirmed by solid state NMR. Exfoliation success was checked with X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques whereas mechanical performance was followed with uniaxial compression experiment. It has been found that exfoliated PAAm nanocomposites having 0.5% OrgMMT had both the maximum equilibrium swelling in water and compression strength as well as improved thermal stability due to the special and beneficial morphology observed via scanning electron microscopy (SEM).     

An eco-friendly approach for toughening of polylactic acid from itaconic acid based elastomer

Sustainable and biocompatible novel lactic acid based bioelastomer (LBPE) was synthesized by polycondensation process which has been confirmed by FTIR and ¹H NMR. Owing to the resemblances in the lactate structures of polylactic acid (PLA) and LBPE, the synthesized LBPE bioelastomers can act as an excellent PLA toughener in presence of free radical initiator dicumyl peroxide (DCP). The mechanical, morphological and thermal were investigated. Chemical crosslinks endow the LBPE with relatively high elasticity and environmental stability which ultimately enhances the mechanical properties of PLA matrix.     

Characterization and determination of swelling and diffusion characteristics of poly(n‐vinyl‐2‐pyrrolidone) hydrogels in water

Hydrogels in the form of rods with varying crosslink densities and three‐dimensional network structures were prepared from Poly(N‐vinyl‐2‐pyrrolidone) (PVP)/water and PVP/water/persulfate systems by irradiation with γ rays at ambient temperature. Average molecular weights between crosslinks, percent swelling, swelling equilibrium values, diffusion/swelling characteristics (i.e., the structure of network constant, the type of diffusion, the initial swelling rate, swelling rate constant), and equilibrium water content were evaluated for both hydrogel systems. Water diffusion to the hydrogel is a non‐Fickian type diffusion and diffusion coefficients vary from 6.56 × 10⁻⁷ to 2.51 × 10⁻⁷cm²min⁻¹ for PVP and 6.09 × 10⁻⁷ to 2.14 × 10⁻⁷ cm²min⁻¹ for PVP/persulfate hydrogel systems. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 994–1000, 2000     

Evaluation of compounding techniques for optimising sealing performance of nitrile rubber vulcanisates

Abstract

The minimisation of viscoelastic effects, which give rise to creep, stress– relaxation, hysteresis, and set in elastomers, is highly desirable in engineering applications, particularly in sealing applications. Natural rubber is an important engineering material and as such much work has been carried out to minimise the above properties in an attempt to enhance service life. Consequently, a diverse range of compounding techniques is available for natural rubber. These techniques have been utilised in nitrile rubber mixes primarily in an attempt to minimise compression stress–relaxation and compression set along with other properties, which are considered pertinent to elastomeric materials used as seals and gaskets exposed to hydrocarbon liquids and vapours. The way in which these compounding techniques affect network structure and thus various sealing properties in nitrile rubber vulcanisates is discussed in terms of crosslink concentration, crosslinking efficiency, and mean apparent sulphur rank of crosslinks.     

Polymer electrolytes based on lithium sulfonate derived from perfluorovinyl ethers; single ion conductors

A new lithium perfluoroakylsulfonate has been synthesized from a commercial perfluorovinylether fitted with a fluorosulfonyl group. By making use of the vinylether group, this species was incorporated into polyurethane networks based either on a poly(ethylene oxide)glycol or on a tri‐arm star‐poly(ethylene oxide)triol. The results, in terms of ionic conductivities, thermal behaviour and different domains of rigidity (solid‐state ¹H NMR), are reported for these single cation conductor polymer electrolytes. When compared to the polymer electrolytes where the same but untethered lithium perfluoroakylsulfonate is dissolved in the same initial networks, a loss of conductivity is observed despite the higher mobility of the ionomers. This loss corresponds to the conductivity attributed to the anionic species. The higher mobility of the ionomeric electrolytes is to be related to the quasi absence of physical crosslinks. © 2000 Society of Chemical Industry     

Observation of the redistribution of nanoscale water filled porosity in cement based materials during wetting

Spatially resolved GARField NMR has been used to follow the ingress of water into previously dried Portland cement concrete and mortar samples. It is shown that the amount of capillary water in the surface layers of different samples after 1 day of capillary absorption exceeds the amount found before drying but that over the subsequent 7 days the amount of capillary water decreases once more, even though the external source of water is maintained. The hydrate gel pore water was additionally tracked in the mortars. It shows complementary behaviour. The data is discussed in terms of C–S–H swelling.     

Effects of polymeric curing agent modified with silazanes on the mechanical properties of silicone rubber

Polymeric curing agent modified with hexamethyldisilazane (PCA‐D), or with hexamethylcyclotrisilazane (PCA‐T), was used to improve the mechanical properties of hydroxyl‐teminated polydimethylsiloxane (PDMS) rubber. The structure and the gel time of PCA were characterized by ²⁹Si NMR and shear viscosity measurement, respectively. The PCA modified with silazanes was more stable in storage than that without treatment (PCA‐0). Chemical bonds were formed during the reaction of silazanes and PCA according to ²⁹Si NMR results. The crosslink density (γ_e) and the mechanical properties of PCA/PDMS rubber were determined by swelling equilibrium and stress–strain tests. It was found that PCA treated with both silazanes could better enhance the mechanical properties of PCA/PDMS rubber compared with PCA‐0. PCA‐T/PDMS rubber, with additional crosslinks, was the best among the three types of PCA/PDMS rubber on the mechanical properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Swelling and mechanical properties of cellulose hydrogels. III. Temperature effects on the swelling and compliance levels studied by dilatometry and 1H-NMR spectroscopy

The temperature dependence of the swelling and the creep compliance has been investigated for swollen isotropic cellulose hydrogels. The measurements were performed in a high precision type of dilatometer between 5 and 65°C. The thermal expansion of the gels in silicone oil (closed system) and the temperature dependence of the equilibrium swelling in water (open system) were studied. The influence of compressive stress in these experiments was also evaluated. The swelling level in equilibrium with water diminishes slightly with increasing temperature due to migration of water from the gel phase to the surrounding water phase. A secondary transition was found at 35°C where the temperature dependence of the swelling level is changed. When measured at constant gel composition the creep compliance of a highly swollen gel decreases with increasing temperature. The decrease is not, however, large enough for entropy elasticity to dominate over energetic elasticity. The energetic contribution f_U/f was determined to be 0.61 for a gel swollen to 3.9 g water/g dry gel (g/g) and 1.24 for a gel swollen to 1.05 g/g. The swelling and compliance data have also been analyzed in terms of a model where the gels are assumed to behave as a filler-reinforced rubbery network. The amorphous parts of the hydrogels are thus assumed to be described by the statistical theory for polymeric networks. In proton magnetic resonance studies of a gel swollen to 4.4 g/g the spin-lattice relaxation time T₁ was determined to be considerably longer than the spin-spin relaxation time T₂. T₂ has a maximum at 30°C. This maximum marks the onset (on the NMR time scale) of an exchange process between two types of proton species. These species are suggested to be specific hydration water and free gel water, respectively.     

Effect of ethylamine pretreatment on the tensile strength of periodate oxycelluloses

A study has been made of the effect of ethylamine pretreatment on the changes in strength suffered by cotton fibres and viscose rayon filaments on oxidation with periodate and on subsequent reduction with borohydride. The extent of chain scission during periodate oxidation has been estimated roughly from intrinsic viscosity measurements in cadoxen on the borohydride-reduced materials and has proved to be small. The variation of strength with the degree of oxidation of the oxycelluloses themselves has been interpreted in terms of Bueche's model for rubbery polymers. It appears to be a combination of the effects of chain scission, chain flexibility, swelling and the formation of hemi-acetal crosslinks. By assuming that the first two are the same in both the original and the reduced oxycelluloses, the effects on the strength of the oxycelluloses of the destruction of intermolecular hydrogen bonds by swelling and the formation of intermolecular hemi-acetals have been estimated. In the early stages of the oxidation of cotton, hemi-acetal formation causes a fall in strength, which is much smaller in ethylamine-treated than in untreated cotton. This is because of the more uniform distribution of reactive sites in the former material. As oxidation proceeds the destruction of hydrogen bonds through swelling more than offsets the number of hemi-acetals formed, and an increase in strength occurs. In viscose rayon, no initial fall of strength occurs because this material is far less effectively crosslinked than cotton.     

Effects of free volume in thin‐film composite membranes on osmotic power generation

For the first time, the effects of free volume in thin‐film composite (TFC) membranes on membrane performance for forward osmosis and pressure retarded osmosis (PRO) processes were studied in this work. To manipulate the free volume in the TFC layer, a bulky monomer (i.e., p‐xylylenediamine) was blended into the interfacial polymerization and methanol immersion was conducted to swell up the TFC layer. Results from positron annihilation lifetime spectroscopy (PALS) show that p‐xylylenediamine blending and methanol induced swelling enlarge and broaden the free volume cavity. In addition, the performance of TFC membranes consisting of different free volumes were examined in terms of water flux, reverse salt flux, and power density under high pressure PRO operations. The TFC‐B‐5 membrane (i.e., a TFC membrane made of blending monomers) with a moderate free volume shows the highest power density of 6.0 W/m² at 9 bar in comparison of TFC membranes with other free volumes. After PRO operations, it is found that the free volume of TFC layers decreases due to high pressure compression, but membrane transport properties in terms of water and salt permeability increase. Interestingly, the membrane performance in terms of resistance against high pressures and power density stay the same. A slow positron beam was used to investigate the microstructure changes of the TFC layer after PRO operations. Compaction in free volume occurs and the TFC layer becomes thinner under PRO tests but no visible defects can be observed by both scanning electronic microscopy and PALS. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4749–4761, 2013     

Engineering hydrophobically associated hydrogels with rapid self-recovery and tunable mechanical properties using metal-ligand interactions

In this contribution, hydrophobic association and metal-ligand coordination have been employed in a dual physical crosslinking strategy to access hydrogels based on micellar copolymerization of acrylamide and a hydrophobic acrylic monomer (containing terpyridine (terpy) for metal-ligand interaction). The mechanical properties of these hydrogels are strongly influenced by the thermodynamic stability and kinetic lability of the metal-terpy crosslinks present in these materials. While the hydrogel tensile strength and stability on water exposure are enhanced by choosing stronger Fe²⁺-terpy crosslinks, the weaker and more kinetically labile Zn²⁺-terpy coordination bonds enable significantly higher energy dissipation under tensile loading and self-healing in the resultant hydrogels.     

The hydration of reactive cement-in-polymer dispersions studied by nuclear magnetic resonance

The behaviour of two novel cement-in-polymer (c/p) dispersions, namely cement-in-poly(vinyl acetate) and cement-in-poly(vinyl alcohol) upon exposure to water at room temperature was investigated by a combination of various NMR methods. The swelling, cracking, and the water ingress were monitored non-destructively using ¹H single point imaging. The hydration of the cement matrix was investigated using ²⁹Si NMR whilst ¹³C CPMAS NMR spectra allowed the quantification of the kinetics of the hydrolysis reaction of poly(vinyl acetate) into poly(vinyl alcohol). The polymer controls the rate of water ingress and swelling which in turn determines the behaviour of the c/p dispersions upon exposure to water. For the cement-in-poly(vinyl alcohol), the rates of water ingress and swelling are much faster than the hydration of the clinker whilst for the cement-in-poly(vinyl acetate) the slow rates of the two processes allow the formation of a cementious matrix which assures the stability of the sample.     

Biocompatible,stimuli‐responsive hydrogel of chemically crosslinked β‐cyclodextrin as amoxicillin carrier

A novel pH‐responsive, chemically crosslinked hydrogelator (cl‐β‐CD/pVP) has been fabricated using β‐cyclodextrin (β‐CD) and N‐vinyl pyrollidone (N‐VP) in presence of diurethane dimethacrylate (DUDMA) crosslinker/azobisisobutyronitrile initiator through free radical polymerization. Various grades of cl‐β‐CD/pVP have been synthesized and the best grade has been considered with higher crosslinking density, higher gel strength, and lower % swelling ratio. The hydrogelator has been characterized by FTIR, ¹H and ¹³C NMR spectroscopy, TGA, and FESEM analyses. The hydrogelator demonstrates pH‐responsive behavior, which has been confirmed by swelling behavior and gel characteristics at various pH (at 37 °C). Using hen egg lysozyme, degradation experiment has been performed, which confirms the biodegradable nature of the hydrogel. The in vitro cytotoxicity study and live–dead assay suggest that the hydrogelator is cytocompatible toward MG‐63 cells. Finally, the hydrogelator shows excellent efficacy as an antibiotic (amoxicillin) carrier. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45939.