Weight Swelling Ratio Research Articles

Mucoadhesive and pH-responsive behavior of gelatin containing hydrogels for protein drug delivery applications

Novel gelatin-containing polymer hydrogels that can be used as mucoadhesive delivery systems were developed. Poly(acrylic acid) hydrogels were modified by copolymerizing gelatin as adhesion promoter, to improve the adhesion to the mucus layer and the synthesized copolymer of acrylic acid (AA) and methacrylated gelatin (GelMA) were designated as P(AA-co-GelMA). The pH-sensitivity and the mucoadhesive property of the P(AA-co-GelMA) hydrogel were investigated as carries of an oral protein delivery system activated by pH changes of the human GI tract. There was a drastic change in the weight swelling ratio of P(AA-co-GelMA) hydrogels at a pH of around 5, that is, low swelling ratios at a pH below 5, while high swelling ratios at a pH greater than 5. In addition, the swelling ratio increased at a pH above 5, when the AA content in the hydrogel increased. In mucoadhesive experiments using the rheometer, when the GelMA concentration in the P(AA-co-GelMA) hydrogel increased, the maximum force of detachment increased, indicating that the mucoadhesion of the hydrogel was improved. The P(AA-co-GelMA) hydrogels also showed a pH-responsive release behavior. The ratio of the cumulative amounts of Rh-B released from P(AA-co-GelMA) hydrogels at pH 2.6 to pH 7.0 increased, when the AA content in the hydrogel decreased.

Physico-mechanical properties and automotive fuel resistance of EPDM/ENR blends containing hybrid fillers

This research aimed to investigate the effect of blend ratios on cure characteristics, mechanical and dynamic properties, morphology and automotive fuel resistance of ethylene-propylene diene rubber (EPDM) and epoxidized natural rubber (ENR) blends containing carbon black and calcium carbonate hybrid filler. The composition of EPDM/ENR blends varied were 100/0, 70/30, 50/50, 30/70 and 0/100 %wt/wt. All ingredients used for preparing each blended compound, except for the curatives, were mixed in a kneader. Thereafter, the compound was further mixed with curatives on a two-roll mill and then were vulcanized together with shaped by compression molding before determining cure characteristics, mechanical properties, morphology and weight swelling ratio in three automotive fuels; gasohol-91, diesel and engine oils. The results indicated that Mooney viscosity and cure time of EPDM/ENR blends tended to decrease with increasing ENR content, while cure rate index and crosslink density increased. Tensile strength of all EPDM/ENR blends is lower than that of the individual EPDM and ENR. This is attributed to the incompatibility between nonpolar and polar nature of EPDM and ENR, respectively, supporting by the glass transition temperature form dynamic mechanical thermal analysis (DMTA) and scanning electron micrographs (SEM). Owing to the differences in polarity of automotive fuels and rubbers, weight swelling of EPDM/ENR vulcanizates decreased in diesel and engine oils, but increased in gasohol-91 with increasing in ENR content.

Baclofen novel gastroretentive extended release gellan gum superporous hydrogel hybrid system: in vitro and in vivo evaluation

Baclofen is a centrally acting skeletal muscle relaxant with a short elimination half-life, which results in frequent daily dosing and subsequent poor patient compliance. The narrow absorption window of baclofen in the upper gastrointestinal tract limits its formulation as extended release dosage forms. In this study, baclofen extended release superporous hydrogel (SPH) systems, including conventional SPH, SPH composite and SPH hybrid (SPHH), were prepared aiming to increase the residence of baclofen at its absorption window. The applicability of different polymers, namely, gellan gum, guar gum, polyvinyl alcohol and gelatin, was investigated in preparation of SPHH systems. The prepared SPH systems were evaluated regarding weight and volume swelling ratio, porosity, mechanical properties, incorporation efficiency, degree of erosion and drug release. In vivo assessment was performed in dogs to evaluate gastric residence time by X-ray studies. In addition, the oral bioavailability of baclofen relative to commercially available Lioresal® immediate release tablets was also investigated. The novel baclofen gellan SPHH cross linked with calcium chloride was characterized by optimum mechanical properties, acceptable swelling properties as well as extended drug release. It also exhibited a prolonged plasma profile when compared to twice daily administered Lioresal®.

Studies of crosslinked biopolymer structure for environmental tools in terms of the rate of weight swelling ratio, viscosity, and biodegradability: part A

The crosslinking reaction caused by blending different biopolymers and/or adding a crosslinking agent to biopolymers was studied. The rate of crosslinking reaction was estimated with weight swelling ratio (WSR), viscosity, and biodegradability. The crosslinked material from blending two biopolymers or mixing a crosslinking agent with a mixture of two biopolymers showed a polymeric volume change, a high microbial resistance even after 80 days, and a viscosity increased because of chemical functional groups to form extensive intra- and interpenetrating polymer networks (IPNs). The organized IPN is a base block in building a three-dimensional (3-D) crosslinked biopolymer structure. This 3-D structure can be utilized in the environmental and geotechnical applications such as an environmental barrier or containment to encapsulate and treat hazardous materials against toxic flow in a subsurface engineering system.

Natural rubber/organoclay nanocomposites: Effect of filler dosage on the physicomechanical properties of vulcanizates

Natural rubber/organoclay nanocomposites of varying filler loading [2 to 10 per hundred rubber (phr)] using derivative of tea (Camellia sinensis) seed oil were prepared by melt intercalation. Effects of filler dosage on the physicomechanical properties of the natural rubber (NR) vulcanizates were examined. Results of the mechanical properties indicates that tensile strength and tear properties of the modified organoclay/NR nanocomposites increases with increasing filler loading compared with the unmodified filled NR vulcanizates. Furthermore, rheological measurement showed that modified filled NR vulcanizate exhibited higher storage modulus (GI) than the unmodified filled NR. The values of the weight-swelling ratio (Qt) of the modified filled nanocomposites decreased remarkably and are lower than the unmodified filled NR vulcanizate. The higher value of the chemical crosslink density of 0.629 at 6 phr for the organoclay/NR composite indicate better reinforcement of the filler-rubber matrix over the unmodified. The scanning electron microscopy revealed that incorporation of modified organoclay up to 6 phr has transformed the failure mechanism of the resulting NR vulcanizate compared to the unmodified. There is an indication that the optimum level of incorporation of sodium salt of tea seed oil is 6 phr. Key words: Vulcanizate, physicomechanical, nanocomposite, Camellia sinensis, natural rubber, organoclay.

Behaviors of Chemically Crosslinked CAAMPS Hydrogels in Uptake of Uranyl Ions from Aqueous Solutions

Uranyl ion adsorption from aqueous solutions has been investigated by chemically crosslinked (C) polyelectrolyte acrylamide/2-acrylamido-2-methyl-1-propanesulfonic acid (CAAMPS) hydrogels. CAAMPS hydrogels with various compositions were prepared from ternary mixtures of acrylamide (A), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and water by free radical polymerization in an aqueous solution using multifunctional crosslinkers such as ethylene glycol dimethacrylate (EGDMA) and 1,4 butanediol dimethacrylate (BDMA). The swelling equilibrium of polyelectrolyte copolymer gels containing of CAAMPS hydrogels has been studied as a function of copolymer composition. Swelling experiments were performed in water at 25°C, gravimetrically. The influence of AMPS content in hydrogels was examined. The weight-swelling ratio of CAAMPS hydrogels was increased up to 127.03 (for 300 mg AMPS and crosslinked by EGDMA) and 93.32 (for 300 mg AMPS and crosslinked by BDMA), while acrylamide hydrogels swelled up to 10.27 (crosslinked by EGDMA) and 10.06 (crosslinked by BDMA). Uranyl ion adsorption from aqueous solutions was studied by batch sorption technique at 25°C. The effect of uranyl ion concentration and mass of AMPS on the uranyl ion adsorption were examined. In the experiments of the sorption, L type sorption in the Giles classification system was found. Finally, the amount of sorbed uranyl ion per gram of dry hydrogel (q) was calculated to be 0.67 × 10−3–2.11 × 10−3 mol uranyl ion per gram for CAAMPS hydrogels. Removal effiency of uranyl ions (RE%) was changed range 9.05–29.92%. The values of partition ratio, (K d ) of uranyl ions was calculated to be 0.10–0.43 for CAAMPS hydrogels.

Effects of triethoxy and triphenylvinylsilanes on the porous structure of cross-linked poly(methyl methacrylate) beads

AbstractNovel microprous beads with the particle size of about 0.6-1.1 mm were prepared for the first time, from methylmethacrylate (MMA), ethyleneglycol dimethacrylate (EGDM), triethoxyvinylsilane (TEVS) and triphenylvinylsilane (TPVS) by suspension polymerization technique. In this study toluene was used as solvent. The swelling measurement results indicate that increasing silicone concentration increases the amount of pores and porosity percent, the weight swelling ratio, and particle sizes, but it has been observed that the apparent density decreases in the case of TEVS, and not in the case of TPVS. Also it has been demonstrated that the volume swelling ratio is independent of TEVS and increases by increasing TPVS concentration. The resulting copolymer beads were characterized by using swelling studies and Fourier transform infrared spectroscopy (FTIR). The morphology of copolymer beads were also investigated by scanning electron microscopy (SEM) and optical microscopy (OM).

Biocatalytic hydrogels by template polymerization

AbstractNovel ionizable hydrogels were prepared from poly(acrylic acid) and dimethylaminoethyl methacrylate monomer employing template polymerization technique as an alternative to traditional physical and chemical crosslinking. The mode of interaction, as proved by Fourier Transform Infrared Spectroscopy (FTIR), was multiple H‐bonding between the tertiary amino group of the monomer and the carboxylic groups of the polymer. The hydrogels represented suitable matrices for enzyme immobilization. The effect of varying the polymer–monomer molar ratio on the swelling kinetics and parameters was investigated. The dynamic swelling isotherm exhibited a Fickian mode of penetrant sorption and a plateau that increases with the amino group content. A polymer complex of molar ratio (polymer:monomer) 0.5:0.8 had a weight swelling ratio of 10 and 7 at pHs 3 and 8, respectively. The proven pH sensitivity together with the amphoteric character of these hydrogels make them good candidates for another bioapplication such as oral delivery systems of therapeutic peptides and proteins. The structural integrity of the hydrogels was proved by their swelling reversibility. β‐Galactosidase, as an acidic model enzyme, was immobilized covalently on the synthesized hydrogels. The maximum enzyme velocity (Vmax) was enhanced to 19 µmol/min/mg, for polycomplex of molar ratio 0.5:0.8, compared with 3.2 µmol/min/mg for the free enzyme. Copyright © 2008 John Wiley & Sons, Ltd.

A New Sorbent Chemically Cross-linked Highly Swollen Copolymeric Hydrogels for Dye Uptake

Water uptake and the sorption properties of polyelectrolyte hydrogels made by the polymerization of acrylamide (AAm) with maleic acid (MA) were investigated as a function of composition to find materials with swelling and dye sorption properties. Highly swollen acrylamide/maleic acid (AAm/MA) hydrogels were prepared by free radical solution polymerization in aqueous solutions of AAm with MA as comonomer and two multifunctional cross-linkers such as trimethylolpropane triacrylate (TMPTA) and 1,4-butanediol dimethacrylate (BDMA). Swelling experiments were performed in water at 25°C, gravimetrically. Chemically cross-linked AAm/MA hydrogels were used in experiments on sorption of water-soluble monovalent cationic dyes such as “Nil blue” (NB) and “Methyl violet (MV)”. Weight-swelling ratio values of AAm/MA hydrogels were calculated range 8.88–61.46. Some swelling kinetic parameters were found. Diffusion behavior of water and water sorption rate constant were investigated. Water diffusion into hydrogels was found to be non-Fickian in character. For sorption of cationic dyes, NB and MV into AAm/MA hydrogels were studied by batch sorption technique at 25°C. AAm/MA hydrogels in the dye solutions showed coloration. However, the AAm hydrogel did not show sorption of any dye from solution. The amount of the dye sorbed per unit mass of AAm/MA hydrogels was investigated.

Analysis of poly(ethylene glycol)-diacrylate macromer polymerization within a multicomponent semi-interpenetrating polymer network system.

Semi-interpenetrating polymer networks (semi-IPNs) containing poly(ethylene glycol)-diacrylate (PEGdA) and modified gelatin were prepared with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator. The effect of (i) initiator and PEGdA concentration, and (ii) weight ratio and type of modified gelatin on the conversion of PEGdA functional end groups was monitored in situ using attenuated total reflectance-Fourier transform infrared (ATR-FTIR). Reaction induction time was dependent on DMPA concentration and increased with decreasing DMPA concentration. Relative reaction rate was strongly dependent on both DMPA and PEGdA concentrations. Gelatin weight ratio and modification did not significantly affect reaction induction time, relative reaction rate, or reaction end time. Swelling/degradation kinetics at various aqueous conditions sought to establish relationships between diacrylate conversion and the resulting semi-IPN physical properties. Semi-IPN swelling weight ratio was strongly dependent on solvent conditions and semi-IPN exposure to gamma-irradiation. Gelatin backbone modification and UV exposure time exhibited no effect on semi-IPN swelling weight ratio. In conclusion, ATR-FTIR presents a viable means of monitoring the conversion of PEGdA functional end groups within a complex mixture. UV exposure >10 s did not significantly affect the weight swelling ratio, and supports our ATR-FTIR results that network formation reached completion before 3 min of UV exposure.

Water absorbency studies of γ-radiation crosslinked poly(acrylamide-co-2,3-dihydroxybutanedioic acid) hydrogels

Water absorbency behavior of acrylamide (AAm)/2,3-dihydroxybutanedioic acid (DBA) hydrogels synthesized by γ-radiation crosslinking of AAm and DBA in an aqueous solution was investigated. Different amounts of DBA containing AAm/DBA hydrogels were obtained in the form of rods via a radiation technique. Swelling experiments were performed in water at 25 °C, gravimetrically. The influence of absorbed dose and DBA content of the hydrogels on swelling properties were examined. The hydrogels showed enormous swelling in an aqueous medium and displayed swelling characteristics which were highly dependent on the chemical composition of the hydrogels and irradiation dose. Diffusion behavior and some swelling kinetic parameters were investigated. The values of the weight swelling ratio of AAm/DBA hydrogels were between 8.34 and 15.16, while the values of the weight swelling ratio of pure AAm hydrogels were between 7.58 and 8.28. Water diffusion into hydrogels was found to be non-Fickian in character. Equilibrium water contents of the hydrogel systems were changed between 0.8681 and 0.9340.

Degradation of phospholipid polymer hydrogel by hydrogen peroxide aiming at insulin release device

The purpose of this study is to ascertain the applicable possibility of H 2O 2 degradable hydrogel for fabrication of insulin release system synchronized with the change in the glucose concentration in the medium. The hydrogel was prepared by using 2-methacryloyloxyethyl phosphorylcholine (MPC) and crosslinker. The favorable characteristic of the hydrogel was H 2O 2 concentration responsive degradation. The H 2O 2 was utilized and produced by enzymatic reaction between glucose oxidase and glucose. Poly(MPC) (PMPC) was easily degraded in H 2O 2 aqueous solution, and the PMPC hydrogel was also degraded in H 2O 2 aqueous solution. The degradation mechanism was considered to be main chain scission of PMPC. The degradation profile was evaluated by using weight swelling ratio and volume swelling ratio. The weight swelling ratio of PMPC hydrogel firstly increased due to the reduction of crosslink density, then the ratio decreased to zero (complete degradation). The degradation profile was proportional to the H 2O 2 concentration. Furthermore, volume swelling ratio also increased, and complex elastic modulus decreased with degradation in H 2O 2 aqueous solution. These results indicated that the hydrogel was degraded by hydroxy and/or hydroperoxy radicals which was produced by H 2O 2, the crosslink density and mechanical property decreased. The release profile from the hydrogel was estimated by using lipid microsphere (LM) as an insulin model. The LM was released with the degradation of PMPC hydrogel. Taking these results into account, the PMPC hydrogel was available for H 2O 2 degradable hydrogel for synchronization with glucose concentration by using enzymatic reaction.

Synthesis and Characterization of Biodegradable Cationic Poly(propylene fumarate-co-ethylene glycol) Copolymer Hydrogels Modified with Agmatine for Enhanced Cell Adhesion

We synthesized positively charged biodegradable hydrogels by cross-linking of agmatine-modified poly(ethylene glycol)-tethered fumarate (Agm-PEGF) and poly(propylene fumarate-co-ethylene glycol) (P(PF-co-EG)) to investigate the effect of the guanidino groups of the agmatine on hydrogel swelling behavior and smooth muscle cell adhesion to the hydrogels. The weight swelling ratio of these hydrogels at pH 7.0 increased from 279 +/- 4 to 306 +/- 7% as the initial Agm-PEGF content increased from 0 to 200 mg/g of P(PF-co-EG), respectively. The diffusional exponents, n, during the initial phase of water uptake were independent of the initial Agm-PEGF content and were determined to be 0.66 +/- 0.08, 0.71 +/- 0.07, and 0.60 +/- 0.05 for respective initial Agm-PEGF contents of 0, 100, and 200 mg/g. The heat of fusion of water present in the hydrogels increased from 214 +/- 11 to 254 +/- 4 J/g as the initial Agm-PEGF content increased from 0 to 200 mg/g. The number of adherent smooth muscle cells increased dose-dependently from 15 +/- 6 to 75 +/- 7% of the initial seeding density as the initial Agm-PEGF content increased from 0 to 200 mg/g. These results suggest that the incorporation of the guanidino groups of agmatine into P(PF-co-EG) hydrogels increases the hydrogel free water content and the total water content of the hydrogels and also enhances cell adhesion to the hydrogels.

Poly(N-vinyl pyrrolidone)−Calcium Alginate (PVP−Ca-alg) Composite Hydrogels: Physical Properties and Activated Sludge Immobilization for Wastewater Treatment

An alternative composite hydrogel material for cell immobilization is presented, and some of its important physical properties, such as weight swelling ratio (WSR) and cell viability, are evaluated. The hydrogel is prepared with cross-linked poly(vinyl pyrrolidone), PVP, and calcium alginate as the encapsulating rigid polymer. Composite hydrogels prepared with different proportions of sodium alginate lead to PVP/calcium alginate hydrogels with weight swelling ratios close to 11 (water content ≈ 90 wt %), higher than the WSR of PVP by a factor of 2. Low-vacuum SEM images reveal that the composite is made of PVP randomly distributed in a solid continuous cage formed by the rigid calcium alginate polymer. When cell viability of these composite hydrogels is evaluated from observations carried out using low-vacuum SEM, the images show that a concentrated bacterial population is attached to and growing on the surface of calcium alginate and entrapped PVP particles. In addition, when the composite hydrogels are used to entrap activated sludge and their efficiency to remove glucose from synthetic wastewater is evaluated and compared to the efficiency of a suspended activated sludge through kinetics studies, it is observed that the glucose depletion rate is practically the same for both systems, indicating that the composite materials have the right structure to allow the diffusion of substrate, byproducts, and oxygen for cell metabolism and may compete favorably with suspended activated sludge.

Synthesis and characterization of poly(ethylene glycol)/poly( l-lactic acid) alternating multiblock copolymers

Thermoplastic hydrogels of alternating multiblock copolymers, consisting of poly(ethylene glycol) (PEG) and poly( l-lactic acid) (PLLA), were synthesized. Dicarboxylated oligomeric PLLAs were synthesized by the condensation reaction of l-lactic acid in the presence of succinic acid. Changing the feed ratio of l-lactic acid to succinic acid controlled PLLA molecular weights. Alternating multiblock copolymers with different block lengths of PEG and PLLA were obtained from the polycondensation reaction between PEG and dicarboxylated-PLLA in the presence of dicyclohexyl carbodiimide and N-dimethyl aminopyridine as catalysts. The chemical compositions of dicarboxylated PLLAs and multiblock copolymers were verified by 1H-NMR and FT-IR, and the molecular weight and distribution were measured by gel permeation chromatography. DSC thermograms showed that there existed a high degree of phase mixing, as well as microphase separation in the multiblock copolymer, demonstrated by a large T g peak from the amorphous phase-mixing domain and a small T m peak from crystalline microdomains of the PLLA component. The block copolymers with low molecular weights were water-soluble and clouded at a temperature which is associated with a lower critical solution temperature (LCST) phenomonon. However, the high molecular weight polymers could swell in water and their optical transparency was influenced by temperature. This observation can be attributed to an enhanced hydrophobic interaction between hydrophobic moieties in the polymer chains, caused by an increase in temperature. The weight swelling ratio (absorbed water/polymer) of the polymers was controlled by polymer composition and molecular weight. These block copolymers may offer potential for applications in drug delivery and various other biomedical projects.

Effect of solvent on aggregation structure and responsive properties of copolymeric gel with side chain crystals

The copolymeric gel films were prepared by a radical copolymerization of stearyl acrylate(SA), acrylic acid(AA) and N,N′-methylenebisacrylamide (MBAA) as a cross-linking agent. The copolymeric (molar ratio SA/AA/MBAA: 24.7/74.3/1.0) gel films with crystalline side chains were swollen in three different kinds of solvent and their aggregation structure and responsive properties have been investigated. In order to control the SA side chains-solvent interaction and to dissociate carboxyl groups in AA, solvents such as dimethyl sulfoxide (DMSO), (1-hexanol/DMSO) mixed solvents and water with different pH values were used. The responsive behaviors of the (SA/AA/MBAA) gel film swollen in different solvents were discussed on the basis of the weight swelling ratio. The X-ray diffraction study of the gel film revealed that the long period corresponding to a layer distance for the (SA/AA/MBAA) gel film swollen in DMSO increased with temperature or the swelling ratio, and the interchain distance of alkyl side chains remained constant with the variation of temperature up to the melting temperature of SA side chain crystals. The swelling ratio of the gel film in (1-hexanol/DMSO) mixed solvent exhibited an abrupt increase in the case of about 40wt% of 1-hexanol, because the SA side chain crystals of the gel film in the mixed solvents were dissolved due to an increase in SA-solvent interaction. On the other hand, the gel film in the water with different pH values showed a sharp increase in the swelling ratio above pH = 11, because the ionic repulsive force among the AA groups became greater than the aggregation one among alkyl side chains. These results indicate that the swelling ratio of the (SA/AA/MBAA) gel film can be controlled by temperature, pH and the magnitude of solubility parameter of swelling solvent.

Preparation and characterization of pH‐sensitive, interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid)

AbstractHydrogels with varying cross‐linking ratio and ionic content were prepared from interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid). Equilibrium swelling studies were conducted and the average molecular weight between cross‐links, Mc, and mesh size were determined. Hydrogels with large Mc, values were found to swell to a greater extent than those with small Mc values. It was also observed that an increase in Mc yielded faster swelling and deswelling rates, as the rates for membranes with Mc = 18,000 were about twice as fast as were the rates for membranes with Mc = 34,000. Oscillatory swelling behavior was investigated in response to changes in the pH and ionic strength of the swelling medium. A change in pH from 3 to 6 caused an ionization of the hydrogels and an increase in the weight swelling ratio, with a greater increase exhibited by IPNs with a higher ionic content. Increase in pH also caused an increase in the average mesh size. © 1995 John Wiley & Sons, Inc.

Formation and structural characteristics of porous ethylene glycol dimethacrylate networks

AbstractThe relationships between the synthesis conditions and the morphology of ethylene glycol dimethacrylate (EGDM) networks were investigated. EGDM networks were prepared by free‐radical polymerization in toluene as a diluent. The networks were characterized by swelling, porosity, and pendant vinyl group content measurements. The accessibility of pendant vinyls to low molecular weight compounds was tested using pyrrolidine as a model reactant. The results show that the polymerization of EGDM in toluene is accompanied by a phase separation in the system. The networks thus obtained are heterogeneous and exhibit an inhomogeneity in the porosity and swelling distributions. Both the weight swelling ratio and the pore volume of the networks increase with increasing monomer dilution. It was also found that the accessibility of pendant vinyls increases as the cross‐linking density of the network decreases or as its porosity increases. The results can be explained with the formation of highly cross‐linked microgels as the intermediates in the network formation processes. Two equations were derived to predict the equilibrium weight swelling ratio and the pore volume of heterogeneous networks. Satisfactory agreement of the results of measurements with the predictions of the equations was observed.

The correlation between the morphology and radiation resistance of polypropylene solid materials

The influence of morphology on the radiation-induced oxidative degradation of polypropylene solid materials has been studied. γ-Irradiation in air caused oxidation of polypropylene films to produce hydroperoxide and carbonyl groups, thus giving rise to a remarkable decrease in elongation at break (γ B) in the tensile test. Although the radiation-induced degradation of the mechanical properties was attributable to oxidation, it was not always determined by the extent of oxidation. The critical dose ( D C), which was defined by the irradiation dose leading to 50% residual elongation, increased as the yield stress (σ Y) before irradiation decreased: both quantities varied significantly depending upon the polymer processing conditions. The σ Y before irradiation increased with increase in the mean diameter of spherulites, up to ca 10 μm, which were formed in the polypropylene films. Films with spherulite diameter larger than ca 10 μm were broken before reaching their yield points and the corresponding tensile strength (σ B) decreased with increasing spherulite diameter. A universal curve was obtained such that σ Y before irradiation decreases with increase in weight-swelling ratio ( S W) measured by immersion in p-xylene at room temperature. The radiation resistance of polypropylene materials, as measured by the D C value, became greater as spherulite size decreased and S W at room temperature increased.

Flexible methylene chain length in crosslinked networks as a measure of the brittleness of γ-irradiated low density polyethylene sheet

Low density polyethylene (LDPE) sheets, untreated and pre-crosslinked by thermal reaction with dicumyl peroxide (DCP) in the molten state, have been γ-irradiated with doses up to 200 Mrad at room temperature in air. The elongation at break ( γ B ) of both materials was decreased by the irradiation. The radiation resistance of pre-crosslinked LDPE, as measured by the characteristic dose which leads to a 50% decrease in γ B , ( D 1 2 = 110 Mrad ) was 2·6 times greater than that of untreated LDPE ( D 1 2 = 42 Mrad ). The gel fraction (f g ) of untreated LDPE was increased by the irradiation, while that of pre-crosslinked LDPE with an initial f g value of 0·803 showed negligible change. The density (d p ) of both LDPEs increased and the weight swelling ratio (S w ) of the polymer network decreased with increasing radiation dose. The changes in these quantities for untreated LDPE were more rapid than for pre-crosslinked LDPE. The γ B value was universally correlated with both the d p and S w values, whether the samples were of untreated or pre-crosslinked LDPE. Characteristic values of d p and S w associated with a 50% decrease of residual γ B value for both LDPEs were obtained as 0·924 g/cm 3 and 4·3, respectively. The molar concentration of crosslinks (Φ) in the irradiated LDPE sheets was evaluated from the S w value by the use of Flory's theory. The flexible methylene chain length between two crosslink points, derived from the Φ value, was shown to be a universal molecular measure of the brittleness of irradiated LDPE sheets.