Network structure and properties of imidazole-cured epoxy novolac adhesives

The structural characteristics of four epoxy adhesives, obtained by crosslinking an epoxy novolac with various levels of a substituted imidazole curing agent, were investigated and correlated with thermal and mechanical properties. Variations in network structure were characterized by measuring crosslink densities and by qualitatively assessing glassy state free volume from densities and coefficients of thermal expansion. Differential scanning calorimetry was used to obtain glass transition temperatures, and dynamic mechanical thermal analysis was used to follow primary (alpha) and secondary (beta) transitions. Bulk behavior was characterized by tensile modulus, strength, and toughness, together with compressive modulus and yield strength. The effect of sub-T_g aging on compressive yield strength was investigated as well. As the level of imidazole increased, crosslink density, and hence network packing efficiency and free volume, decreased. For fully cured networks, both the glass and the alpha transition temperatures increased with crosslink density. Calculated activation enthalpies and entropies indicated significant degrees of network cooperativity in the alpha transitions, particularly for the more highly crosslinked systems. Beta transition temperatures, however, were found to be independent of crosslink density. Bulk properties generally showed a dependence both on crosslink density and free volume. Yield stress, for example, was highest for the network with lowest crosslink density and free volume. Volume relaxation associated with physical aging also caused yield stress to increase.     

Influence of vinyl ester/styrene network structure on thermal and mechanical behavior

The influence of vinyl ester/styrene network structure on thermal and mechanical properties was investigated. The crosslink density of the resins was altered by changing the molecular weight of the vinyl ester oligomer and by varying the amount of styrene used during the crosslinking reaction leading to variations in both the physical network structure and the chemical composition of the polymeric networks. The glass transition temperatures of the network polymers were found to increase systematically with increasing crosslink density without the additional influence of the chemical composition as determined from both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The breadth of the glass transition regions increased with crosslink density for the DSC data, but the breadth assessed from the DMA data did not vary significantly for the network materials. A secondary relaxation was observed for the materials using DMA, and this relaxation did not appear to be significantly affected by changes in either the crosslink density or the composition of the network. Cooperativity studies involving time–temperature scaling of dynamic mechanical data in the glass formation temperature region were also conducted. The degree of segmental cooperativity at T_g appeared to be primarily influenced by the chemical composition of the networks. These issues dealing with the structure of the networks provided insight into the associated fracture properties in the glassy state (ambient temperature). Specifically, an empirically based linear correlation was found between the fracture toughness of the networks and the cooperative domain size at the glass transition temperature normalized by the crosslink density. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 917–927, 2001     

Mechanical and gas transport properties of poly-ϵ-caprolactone model networks

The glass transition temperature increases with increasing crosslink densities in model networks formed by endlinking poly-?-caprolactone with a triisocyanate crosslinking agent. In the noncrystalline networks, the gas permeability decreases with increasing crosslink density. These results are consistent with an interpretation that the crosslinks reduce the main-chain molecular motions which are important to these processes. At the lowest crosslink density, where poly-?-caprolactone networks are crystalline, the gas permeability is lower than would be expected based on the volume fraction of amorphous polymer. The excess reduction in permeability is attributed to crystallization-induced enrichment of crosslink junction points in the amorphous fraction of the network. This reduces the permeability by creating an artificially high crosslink density in those regions of the network responsible for gas transport. Since crosslinking increases the stiffness and reduces the flexibility of the network polymer chains, it affects large penetrants more strongly than small ones. Therefore, increasing the crosslink density proves to be a useful method for increasing gas separation factors.     

Rheological behavior and thermomechanical properties of polyethylene—Glycidyl methacrylate vitrimers utilizing disulfide exchange

Four vitrimers were made using polyethylene – glycidyl methacrylate copolymer (PE-GMA) and 3,3′ – dithiodipropionic acid (DTDPA) using a two-step procedure. The amount of DTDPA added was varied, and therefore, the crosslink density was also varied. Rheological experiments were performed. The vitrimers exhibit different rheological behavior depending on the angular frequency applied, which most likely stems from the presence of a crosslinked network. Additionally, there appears to be an on-going reaction occurring during rheological tests, all of which occur above the melting temperature. Thermal and mechanical tests were also conducted. Thermomechanical testing determined that increasing crosslink density resulted in a decrease in the yield strength.     

Dynamic mechanical analysis of segmental relaxation in unsaturated polyester resin networks: Effect of styrene content

The effect of styrene content on non-exponential and non-Arrhenius behavior of the α-relaxation of cured unsaturated polyester resins (UPR) was investigated by dynamic mechanical analysis (DMA). To compare the temperature dependence of the relaxation times, the Angell fragility concept was applied to samples with different crosslink densities. Furthermore, the number of structural units per cooperatively rearranging region (CRR) was estimated using random walk model and the modified Adam-Gibbs theory. The results showed that rising styrene content enhanced the crosslink density of the networks, which altered the intensity and broadness of the α-relaxation. The fragility index, a measure of temperature dependence of relaxation time, and the average size of CRR at glass transition region was also increased by styrene content. Therefore, the segmental relaxation in networks with higher crosslink density could be associated with stronger intermolecular coupling. In addition, it was observed that the mean required energy for internal rearrangement of structural units within the CRR decreased as the fragility index increased, while the mean barrier height for repositioning of a CRR in cooperation to its local environment was nearly constant.     

Effect of chemical structure and crosslinking density on the thermo-mechanical properties and toughness of (meth)acrylate shape memory polymer networks

The objective of this work is to characterize and understand structure-mechanical property relationships in (meth)acrylate networks. The networks are synthesized from mono-functional (meth)acrylates with systematically varying sidegroup structure and multi-functional crosslinkers with varying mole fraction and functionality. Fundamental trends are established between the network chemical structure, crosslink density, glass transition temperature, rubbery modulus, failure strain, and toughness. The glass transition temperature of the networks ranged from −29 to 112 °C, and the rubbery modulus (E_r) ranged from 2.8 to 129.5 MPa. At low crosslink density (E_r < 10 MPa) network chemistry has a profound effect on network toughness. At high crosslink densities (E_r > 10 MPa), network chemistry has little influence on material toughness. The characteristic ratio of the mono-functional (meth)acrylates' components is unable to predict trends in network toughness as a function of chemical structure, as has been demonstrated in thermoplastics. The cohesive energy density is a better tool for relative prediction of network mechanical properties. Due to superior mechanical properties, networks with phenyl sidegroups are further investigated to understand the effect of phenyl sidegroup structure on toughness.     

Relationships of tensile strength with crosslink density for the high-carbon black-filled EPDM compounds with various softeners

Equilibrium swelling and rheological tests were adopted to systematically investigate the effects of softener type and dosage on the crosslink densities. The results turned out that the chemical crosslink density could be distinguished from the physical crosslink density by comparing the results of equilibrium swelling and rheological tests. The liquid butadiene (LB) as a softener leads to the greatest reduction in crosslink density, followed by polyethylene wax (PW) and paraffinic oil (PO). The tensile strength decreases with increasing PO content while shows peak values with increase of LB and PW contents. The dependencies of chemical crosslink density on the aging time under 150°C are quite different for the three softeners, which can be expected from the double crosslinking networks consisting of small softener and large main crosslinking networks. Further investigation has been performed to correlate the tensile strength with chemical crosslink density of ethylene propylene diene monomer elastomer vulcanizates. Three different linear relationships can be obtained for the softeners independent of the aging time. It can now be expected from this study that the role of some new softeners in rubber compounds is not only confined to plasticization but also forms crosslinking networks in the peroxide-cured rubbers.     

Effect of chain length distribution on thermal characteristics of model polytetrahydrofuran (PTHF) networks

A series of model polytetrahydrofuran (PTHF) networks were synthesized via end-linking reactions of α, ω-allyl PTHF oligomers with a stoichiometric tetrafunctional crosslinker. The telechelic PTHF oligomers were synthesized by living cationic ring-opening polymerization of tetrahydrofuran followed by a termination reaction with allyl alcohol. Networks thus prepared have well-controlled architecture in terms of the inter-crosslink chain length (M_c) and chain length distribution: resulting in unimodal, bimodal and clustered structures. Unimodal network was prepared by using polymer chains of same molecular weight, bimodal networks were synthesized by using two groups of polymer chains with different average molecular weights, and the clusters are prepared by incorporating clusters of networks with small molecular weight chains in a network matrix made of longer chains. Thermal characteristics of these model networks were investigated as a function of crosslink density, as well as inhomogeneities of crosslink distribution using DSC. We demonstrate that glass transition temperature (T_g) and crystallization behavior (melting temperature and crystallinity) of the networks are both strongly influenced by crosslink density (M_c). By comparing the unimodal, bimodal and clustered networks with similar average M_c, the effects of inhomogeneities in the crosslink distribution on the thermal properties were also investigated. Results show that inhomogeneities have trivial influence on T_g, but strongly affects the crystallization behavior. Moreover, the effects of the content ratio and length ratio between long and short chains, and the effects of cluster size and size distribution on the thermal characteristics were also studied.     

Molecular modeling of crosslinked epoxy polymers: The effect of crosslink density on thermomechanical properties

Molecular dynamics and molecular mechanics simulations are used to establish well-equilibrated, validated molecular models of the EPON 862-DETDA epoxy system with a range of crosslink densities using a united atom force field. Molecular dynamics simulations are subsequently used to predict the glass transition temperature, thermal expansion coefficients, and elastic properties of each of the crosslinked systems. The results indicate that glass transition temperature and elastic properties increase with increasing levels of crosslink density and the thermal expansion coefficient decreases with crosslink density, both above and below the glass transition temperature. The results demonstrate reasonable agreement with thermomechanical properties in the literature. The results also indicate that there may be a range of crosslink densities in epoxy systems beyond which there are limited changes in thermomechanical properties.     

Effects of crosslinking on thermal and mechanical properties of polyurethanes

The effects of chemical crosslinking on the thermal and dynamic mechanical properties of a polyurethane system were examined. The polyurethanes were prepared from poly(propylene glycol), a diol; trimethylolpropane propoxylate, a triol; and poly(propylene glycol), tolylene 2,4‐diisocyanate terminated, a diisocyanate monomer. The crosslink density was controlled by varying the triol concentration from 10 to 70 mol % and the isocyanate‐to‐hydroxyl (NCO/OH) ratio from 1.0 to 1.3. All the samples had one glass‐transition temperature and no crystalline regions. In addition, there were larger increases in glass‐transition temperature over the range of triol concentrations studied than over the range of NCO/OH ratios studied. For all samples, the Dibenedetto equation relating glass‐transition temperature to extent of crosslinking fit the data very well. Also, samples with higher crosslink densities had much larger elastic moduli for temperatures above the glass‐transition temperature. By assuming the system was a phantom network, approximate crosslink densities for stoichiometric samples were obtained from the dynamic mechanical data and these agreed fairly well with theoretical predictions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 212–223, 2002     

Controlled degradation of epoxy networks: analysis of crosslink density and glass transition temperature changes in thermally reworkable thermosets

The characteristics of networks formed in cured ‘reworkable’ epoxy thermosets capable of controlled thermal degradation were studied. Dynamic mechanical thermal analysis, swelling measurements, and glass transition temperature measurements were used to obtain information regarding the time and temperature dependence of the crosslink densities of these materials. By applying isothermal conditions, networks containing up to 36 mol% non-degradable components could be completely degraded, i.e. progress from a network of infinite molecular weight to a finite one with zero crosslink density. Percolation theory was used to facilitate the interpretation of these results. The degradation behavior of the reworkable thermosets were well-described by gel degradation theory, i.e. the reverse of the gelation process, and the experimental results were in good agreement with calculated values obtained by replacing the extent of reaction, p, in Macosko and Miller's branching theory with the extent of degradation, 1−p.     

A comparative study of the effect of some paraffinic oils on rheological and dynamic properties and behavior at low temperature in EPDM rubber compounds

The influence of some paraffinic oils on rheological properties, dynamic properties, and behavior at low temperature of various ethylene–propylene–diene monomer rubber (EPDM) compounds was studied. Three different types of EPDM, Dutral TER 4049, Dutral Ter 4038, and Nordel IP 4770 R, and five different paraffinic oils were used. The properties of the compounds were evaluated with reference to the oil characteristics: viscosity, composition, glass transition temperature, and solubility parameter. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1825–1834, 2005     

交联密度对氢化丁腈橡胶热性能的影响

用核磁共振法(NMR)测定了氢化丁腈橡胶(HNBR)的横向弛豫时间(T21),用以反映硫化胶的交联密度,并研究了交联密度对硫化胶热性能和力学性能的影响。结果表明:随着硫化剂DCP用量的增多,胶料交联密度增大,玻璃化转变温度升高,耐热性提高,拉伸强度先增大后减小,拉断伸长率、永久变形和撕裂强度明显减小。     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

Poly(ether urethane)/poly(ethyl methacrylate) IPNs with high damping characteristics: The influence of the crosslink density in both networks

Interpenetrating polymer networks (IPNs) with a controlled degree of microphase separation were synthesized from a poly(ether urethane) (PUR) and poly(ethyl methacrylate) (PEMA). The influence of the crosslink density of both networks was investigated in the 70:30 PUR/PEMA IPN. The extent of damping was evaluated by dynamic mechanical thermal analysis. Mechanical properties were studied using tensile testing and hardness measure-ments. Control of crosslinking was successful in tailoring the damping profile. Higher crosslinking in the first-formed network (polyurethane) seemed to increase slightly the area under the linear loss modulus curve, LA, whereas no influence was obvious when changing the crosslink density in the second network. TGA studies revealed improved thermal properties for the IPNs with a higher crosslink density in the PUR network. TEM micrographs confirmed a finer morphology for the materials with a higher crosslink density in the PUR, whereas increasing the crosslink density in the PEMA network resulted in a decrease of phase mixing. © 1996 John Wiley & Sons, Inc.     

Effects of crosslink density on mechanical properties of high glass transition temperature polycyanurate networks

Polycyanurate networks of different architecture were synthesized using different curing cycles. Networks with a variable extent of reaction were obtained; the small variation of the cyanate conversion (0.8 to 1) corresponds to a large variation of glass transition temperature (150–290°C) and crosslink density. The mechanical behavior at small and large deformations and the fracture toughness were examined at room temperature and related to the network structural parameters. To explain the puzzling variation of the yield stress and yield strain with the cyanate conversion, recovery experiments were conducted to discriminate anelastic deformation from plastic deformation. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2471–2479, 1997     

氢化羧基丁腈橡胶/环氧化POSS复合材料的硫化反应和性能研究

通过溶液共混和硫化成型制备了氢化羧基丁腈橡胶(HXNBR)/环氧环己基POSS复合材料,并进行了硫化性能、动态力学性能、FTIR谱图和交联密度测试。结果表明,环氧化环己基POSS和HXNBR发生交联反应并产生醇羟基;在实验温度范围内,200℃时复合材料硫化效率最佳,正硫化时间(t90)约为10min;复合材料高弹储能模量和交联密度随POSS用量的增大而明显增大,损耗因子降低,玻璃化转变温度提高;交联密度在20～40min区间增加幅度较大,当硫化时间为60min时交联密度有所降低。     

Dependence of the network structure of cured styrene butadiene rubber on the sulphur content

An investigation about the dependence of the physical properties of styrene-butadiene rubber copolymers (cured with different sulphur content, in order to obtain various networks) on the crosslink density was carried out by means of dynamical mechanical analysis, differential scanning calorimetry and positron annihilation lifetime spectroscopy. SBR specimens were cured with different sulphur content, in order to obtain various networks. On increasing the crosslink density, the glass transition temperature increases and the fractional free volume decreases. The thermal expansion coefficient of the free volume decreases in the rubbery phase by increasing the crosslink density, owing to the slower rate of expansion of nanoholes; furthermore, it seems influenced by the percentage of polysulfide density. The density of nanoholes is independent of the temperature, but decreases on adding sulphur. The results can be framed within the Simha-Somcynsky free volume theory.     

Dynamic mechanical response of model epoxy networks in the glassy state

The dynamic mechanical properties of model epoxy-amine networks are investigated in the glassy state over a wide range of frequencies, at temperatures between 123 K and 350 K. The effects of crosslink density and network chain flexibility on the β relaxation are examined. Motions responsible for the β process begin to develop at the same temperature, whatever the crosslink density. However, an increase in crosslink density is accompanied by an increase in amplitude and a broadening towards high temperatures of both damping tan δ and loss modulus E″. This effect is responsible for the decrease of elastic modulus E′ at room temperature with increasing crosslink density.     

Studies on double networks in natural rubber vulcanizates

The concept of double networks is a rather new idea, by which one imparts chain orientation in elastomers. Double networks were made in natural rubber vulcanizates cured with a single and a new binary accelerator system. Double networks with different extensions were prepared and their effects on tensile properties were analyzed. The influence of extent of initial cure on double‐network formation was examined. Thermal stability of the double network formed was analyzed by ageing of the double networks and was found to improve with residual extension. Crosslink density of the networks formed was determined by swelling methods and stress–strain analysis. It was found that crosslink density increased with double‐network formation and residual extension. The stress–strain behavior and moduli were analyzed to study the effect of these properties on double‐network formation. Double networks were hardly affected by the binary accelerator system. Based on the studies it was found that residual extension was the major factor determining the final properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1068–1076, 2004