Effect Of Crosslink Density Research Articles

Effect of cross-link density on modification of epoxy resins by N-phenylmaleimide-styrene copolymers

The effect of cross-link density on the toughening of modified resins was investigated for the modification of epoxy resins with N-phenylmaleimide-styrene alternating copolymers (PMS). The cross-link density of the epoxy matrix was controlled by a combination of two kinds of epoxy resins [diglycidyl ether of bisphenol-A (DGEBA) or triglycidyl aminocresol (TGAC)] and hybrid hardeners composed of p, p′-diaminodiphenyl sulphone (DDS) and p, p′-( N, N′-dimethyl)-diaminodiphenyl sulphone (MDS). The addition of 10 wt% of PMS ( M w 214,000 ) led to 120% increase in the fracture toughness ( K 1C) of the DGEBA resin cured with the hybrid hardener (DDS: MDS, 67:33 mol ratio). On addition of 15 wt% of PMS ( M w 214,000 ), K 1C for the modified resin increased 110% in the TGAC/hybrid hardener (DDS: MDS, 67:33 mol ratio) system. Morphologies of the modified resins depended on PMS molecular weight and concentration, and the cross-link density of the matrix. The toughening of epoxies could be explained by the cocontinuous phase structure in every case.

Synthesis of polymer electrolytes based on poly[2‐(2‐methoxyethoxy)ethyl glycidyl ether] and their high ionic conductivity

AbstractNetwork polymer electrolytes based on poly[2‐(2‐methoxyethoxy)ethyl glycidyl ether] (PME2GE) were synthesized and characterized. The effects of crosslinking density, the introduction of ethylene oxide (EO) unit into the main chain by copolymerization, and the electrolyte (LiClO4) concentration on the ionic conductivity were explored. The random copolymer system improved the conductivity considerably compared with the polymer electrolytes previously obtained for network polymer systems and exhibited a high ionic conductivity of 10−4 S/cm at 40°C and 10−5 S/cm at 10°C, and even 3 X 10−7 S/cm at −20°C with optimization of the copolymer composition, the crosslinking density and the amount of LiClO4. The optimum composition was EO/ME2GE = 70/30 in molar ratio. The most preferable ratio of LiClO4 to etheric oxygen was in the range of 0.02–0.03.

The Effect of Crosslink Density and Complex Fields of Stresses on the Network Chain Mobility of Model Epoxy Resins

AbstractThe viscoelastic properties of model epoxy resins were investigated using a tensile stress relaxation technique. Simultaneously with the applied tensile strain, the specimens were subjected to torsional oscillations in a specially designed instrument. The crosslinking density of the resins was controlled by the length of the aliphatic chain of the diamine hardener. Smooth master curves were obtained by Computed Aided Superposition (CAS) using a software written especially for this work. The shape of the master curves changed significantly as a function of the frequency of torsional oscillations. Based on previously published data, as well as on pertinent information from literature, it was concluded that the variations in the shape of the master curves reflect corresponding changes in the spectrum of relaxation times of the investigated resins. It is suggested that the observed phenomena are caused by physical processes, such as short-range orientation and physical aging, which lead to densification of the network and a corresponding increase in its stiffness.

Structure and mobility within anisotropic networks obtained by photopolymerization of liquid crystal molecules

Liquid crystalline mixtures of mono- and diacrylates were polymerized after macroscopic orientation of the molecules. Refractive indices of the mixtures were investigated both in the LC phase and after polymerization in the macroscopically oriented state. In this way the effect of cross-link density on the optical anisotropy was measured and correlated with order parameter measurements using infrared dichroism. Dielectric measurements were also carried out to investigate the effect of cross-linking on the movement of the side groups around the main chain

Effect of cross-link density on modification of epoxy resins with reactive acrylic elastomers

The effect of cross-link density on the toughness of modified resins was investigated for modification of epoxy resins with acrylic elastomers with pendant epoxy groups. The elastomers were prepared by terpolymerization of butyl acrylate (BA), glycidyl methacrylate (GMA) and acrylonitrile (AN). The cross-link density of the epoxy matrix was controlled using hybrid hardeners composed of p, p′-diaminodiphenyl sulphone (DDS) as a primary diamine and p, p′-( N, N′-dimethyl)-diaminodiphenyl sulphone (MDS) as a secondary diamine. The terpolymers were effective as modifiers for toughening of difunctional epoxy resins (bisphenol A diglycidyl ether). The addition of 20 wt% of the terpolymer (62 mol% BA, 25 mol% GMA and 13 mol% AN, M n 7900 ) led to a 100% increase in the fractural toughness of the resin cured with the hybrid hardener (DDS/MDS, 48:52 mol ratio, 65:35 NH ratio). The lower the cross-link density, the larger was the fracture toughness and the lower was the glass transition temperature. On the other hand, the terpolymers were less effective in the modification of trifunctional epoxy resins of higher cross-link density (triglycidyl aminocresol) even when using the hybrid hardener (DDS/MDS, 48:52 mol ratio). Both modified systems afforded cured resins with two-phase morphologies. The toughening mechanism is discussed in terms of the morphological and dynamic mechanical behaviours of the modified epoxy resin systems. It is concluded that the ductility of the epoxy matrix contributes greatly to toughening of epoxy resins with acrylic elastomers.

Dynamic mechanical properties of hydrogenated nitrile rubber: effect of cross-link density, curing system, filler and resin

The effect of cross-link density, curing system, filler and resin on dynamic mechanical properties of hydrogenated nitrile rubber (HNBR) is reported. The storage modulus,G′, increases and the loss angle, tan δ, decreases in the plateau zone with cross-link density. At equal cross-link density, the tan δ peak value at the transition zone of sulphur cured system, is less than that of peroxide cured system. Tan δ decreases from 90 to 180 °C and increases beyond 180 °C due to post vulcanization reaction and sulphidic linkage cleavage, respectively. The ZSC 2295 resin increasesG′ and decreases tan δ, and is found to be compatible with HNBR. The plateau modulus,G′, increases with carbon black loading. The tan δ is lower for high structure carbon black in the transition zone and higher in the plateau zone when the surface area is high.

Effect of cross-linking density on the toughening mechanisms of rubber-modified thermosets

Effect of cross-linking density on the toughening mechanisms of rubber-modified thermosets

Physical adhesion of rubber to glass: cross-link density effects near equilibrium

Pre-cross-linked rubber adheres to glass mainly by physical interactions (van der Waals' bonds). Nevertheless, the measured adhesion is often greater than expected even at low separation rates. By adopting the Johnson, Kendall and Roberts (JKR) technique for assessing adhesion between a rubber hemisphere and a glass slide, it is shown that the apparently stable equilibrium state depends on the manner in which initial contact is obtained. Although the work of adhesion due to osculating contact is close to the Dupré value, that obtained following the application of direct pressure to the system increases with the average inter-cross-link molecular weight, M c , of the elastomer. The kinetics of evolution towards equilibrium are considered using a simple dynamic energy balance argument.

Effect of cross-linking density on the toughening mechanisms of rubber-modified thermosets

Solid rubbers have been incorporated into thermosets using the compositional quenching process. Electron micrographs reveal that rubber particles a few micrometres or less in size have been dispersed in the matrix. Two model systems were studied: an epoxy resin cured with primary amines which represents a tight network structure, and a phenoxy resin cross-linked with methylene diisocyanate which represents a loose network. The present study indicates that a small amount of a cross-linking agent can reduce the degree of matrix yielding with a resultant drop in impact strength. SEM fractography provides complementary information on the transition from ductile fracture to brittle fracture.

Deuterium NMR of Strained Swollen Crossunked Elastomers: Effect of Crosslink Density on the Orientational Order of Chain Segments

Abstract The 2H NMR of uniaxially strained swollen chloroprene rubbers and butadiene-styrene copolymers with different crosslink densities is studied. the relative number of chain segments between cross-links of various samples is obtained. the results are in approximate agreement with those obtained from the Flory equilibrium-swelling method. It is found that the enhancement factor G depends not only on the lattice-walk model and swelling of rubber, such as Tanaka etc. expected, but also on the crosslink density and the structure of the rubber. the formula relating G to the number of chain segments between cross links is derived from theory. Moreover, we can also estimate the relation between the enhancement factor Gr of real rubber and G calculated from the theory of the lattice model. the 2H NMR results of these samples indicate that the interaction between chain segments decreases with increasing crosslink density.

Deuterium Nmr of Strained Swollen Crosslinked Elastomers: Effect of Crosslink Density On the Orientational Order of Chain Segments

Deuterium Nmr of Strained Swollen Crosslinked Elastomers: Effect of Crosslink Density On the Orientational Order of Chain Segments

Study of Equilibrium Properties of Cr(III)/Polyacrylamide Gels by Swelling Measurement and Equilibrium Dialysis

Summary In observations of the long-term properties of a series of Cr(III)/polyacrylamide (PAAM) gels, the gels either underwent syneresis upon aging or swelled in contact with excess brine. Both syneresis and swelling can substantially change the volume and properties of a gel placed in a formation and therefore influence the effectiveness of a crosslinked-polymer treatment. Efforts were made to develop an understanding of these phenomena and to describe them in terms of the gel's physical and chemical states. A long-term gel's physical and chemical states are characterized by two parameters, effective crosslinking density and chromium density, determined by equilibrium swelling and equilibrium dialysis, respectively. Swelling and syneresis properties were correlated to the effective cross-linking density described in polymer network theory. A model based on Flory and Hermans' swelling equations was developed to calculate the effective crosslinking densities of gels prepared from solution. Attempts were made to relate the swelling and syneresis properties to the compositions of gel systems to allow prediction of long-term stability of a gel based on its composition. An analysis of the amount of chromium that reacted with the PAAM was made by successive equilibrium dialyses of the gel followed by chromium analyses of dialysates by atomic and visible absorptions. The overall fractional conversion of chromium in the gelation was between 0.39 and 0.70. The equilibrium conversion of Cr(VI) to Cr(IH) ranged from 0.65 to 0.85. The effective crosslinking density obtained from the swelling experiment was <5 % of the chromium density for all samples studied. A substantial fraction of reacted chromium is suspected to be present in the gel matrix either in the form of intramolecular crosslinks or attached to only one polymer segment.

Properties of poly( N-vinyl-2-pyrrolidone) hydrogels crosslinked with ethyleneglycol dimethacrylate

N-Vinyl-2-pyrrolidone (VP) in the presence of ethyleneglycol dimethacrylate (EDMA) has been polymerized to high conversion by γ-irradiation. Poly( N-vinyl-2-pyrrolidone) (PVP) hydrogels were obtained by swelling the resultant solid xerogels to equilibrium in water. The hydrogels were characterized by differential scanning calorimetry, compression-strain measurements and determinations of sol fraction, water content and dimensions. For EDMA contents within the range 0.5–5.0 wt%, Young's moduli and the polymer-water interaction parameter χ lay within the intervals 0.019–0.504 MN m −2 and 0.491–0.572, respectively, at 294 K. At a fixed EDMA concentration, an increase from 275 to 342 K resulted in a small increase and a small decrease in χ and in water content, respectively, but Young's modulus and the effective crosslinking density ( v e) remained practically independent of temperature. Inefficient crosslinking and inhomogeneity of the products are indicated by (i) the presence of large sol fractions and (ii) small values of v e relative to the theoretical crosslinking densities. The findings are supported by calculation of the compositional drift of unreacted monomer at all stages of the copolymerization (crosslinking) reaction. The water contents of the hydrogels were very high (∼ 95 wt%) and were reduced significantly only at a high content of EDMA. The ratio of the contents of non-freezing water to total water within the hydrogels increased with increasing extent of crosslinking.

Permeabilities of radiation cured materials

The permeability of radiation cured materials to oxygen and water molecules was investigated. Several classes of radiation curable materials were studied, including: epoxy, urethane, polyether, polyester, and aliphatic acrylates. Permeabilities were measured as a function of irradiation dose for Ultra-Violet and Electron Beam sources. The effect of cross-link density on permeability was examined by varying the functionality of the molecule and the degree of cure. The results were interpreted in terms of structure/property relationships.

Mechanical and thermodynamic properties of butyl acrylate-N-vinylpyrrolidone hydrogels

Copolymerization to almost 100% conversion has been effected by the γ-irradiation of mixtures of n-butyl acrylate (BA) and N-vinyl-2-pyrrolidone (VP) in the presence of a crosslinking agent, 1,1,1-trimethylolpropane trimethacrylate (TPT). Swelling in water at 294 K yielded hydrogels having equilibrium water contents in the range 1–89%, the corresponding extension ratios and volume fractions of water being obtained from dimensional changes on swelling. Stress(compression)-strain measurements yielded the compression and Young's moduli, the effective crosslinking density ( v e) and the copolymer-water interaction parameters (χ). From the linear dependence of v e on the theoretical value based on the concentration of TPT, the efficiency of crosslinking was found to increase with increasing BA content; a finite v e was also interpolated to exist even in the absence of added TPT. Increasing the VP content of xerogels produced a corresponding increase and decrease in the fractional contents of water and BA respectively within the hydrogels, whereas the fractional content of VP displayed an increase followed by a decrease. This latter behaviour mirrors that exhibited by v e and the elastic moduli. Possible explanations are proposed.

Structurizing action of aerosil on the formation of the network structure of polyurethanes

The formation of network polyurethanes based on polyoxypropylene glycol, toluylene-2,4- or diphenylmethane-4,4'-diisocyanates, and trimethylolpropane in the presence of various amounts of Aerosil, which does not affect the kinetic parameter of this reaction, is studied. The quantities indicating the beginning of gel formation and the effective crosslinking density as a function of temperature are of the same type for the polymer series based on each of the diisocyanates. In the case of systems cured with participation of diisocyanates of unsymmetrical and symmetrical structure these relations differ in character. This difference is a result of formation of secondary structures of coagulation type at the initial stages of the reaction with participation of the filler particles in the case of asymmetrical diisocyanates, these structures being impossible in the case of symmetrical diisocyanates,

High-temperature viscoelasticity and heat-setting of poly(ethylene terephthalate)

Viscoelasticity of semicrystalline drawn filaments of poly(ethylene terephthalate) was studied in the temperature range critical in forming processes: between the glass transition and melting regions. Measurements were made of strain recovery following heat-setting in torsion. Microstructural features were also monitored, by density measurement and X-ray scattering. Heat-treated specimens were found to exhibit two distinct heat-setting processes. One corresponded to the glass transition. The other was situated between the glass transition and melting regions. This second process has been compared with predictions for heat-setting due to (a) melting/recrystallization, (b) viscous flow. The mechanism is deduced as being viscous flow, corresponding to mechanical relaxation — an α′-relaxation — separate from the glass transition, and probably arising from entanglement slippage. On heat-treatment, the dominant α′-relaxation time at the temperature of treatment was found consistently to increase to approximately equal the time allowed for heat-treatment. This indicates a link between the molecular diffusion processes responsible for α′ stress relaxation and those involved in evolution of microstructure and properties during heat-treatment. The magnitude of the α′-relaxation showed increases with increasing time, but decreases with increasing temperature of pre-treatment above 200°C. These are explicable in terms of microstructural changes by using a simple quantitative model for calculating the effective cross-link density of the non-crystalline matrix provided by crystals.

Effect of crosslinking density on the orientational order generated in strained networks: A deuterium magnetic resonance study

The orientational order generated in uniaxially strained rubbers is investigated as a function of crosslinking density. This study was carried out on two sets of rubbers; on randomly crosslinked diene networks and on model end-linked silicone rubbers. The degree of orientational order was measured by means of the deuterium n.m.r. technique; the experiments were carried out either on dissolved deuterobenzene as an n.m.r. probe (dienes) or directly on labelled polymeric chains (silicones). The induced order increases with the crosslinking density as predicted by the kinetic description of rubber elasticity. Data are related to the thermodynamics of the networks rather than to their chemical mesh size which does not accurately describe the topological structure of interest. For both sets of samples, the degree of order varies linearly with the corresponding volume fraction of polymer at swelling equilibrium.

Engineering Plastics from Lignin - V. Effect of Crosslink Density on Polyurethane Film Properties — Variation in Polyol Hydroxy Content

Article Engineering Plastics from Lignin - V. Effect of Crosslink Density on Polyurethane Film Properties — Variation in Polyol Hydroxy Content was published on January 1, 1984 in the journal Holzforschung (volume 38, issue 5).

Engineering Plastics from Lignin - IV. Effect of Crosslink Density on Polyurethane Film Properties — Variation in NCO:OH Ratio

Article Engineering Plastics from Lignin - IV. Effect of Crosslink Density on Polyurethane Film Properties — Variation in NCO:OH Ratio was published on January 1, 1984 in the journal Holzforschung (volume 38, issue 4).