Structure and Properties Relationships of Epoxy Resins Part 1: Crosslink Density of Cured Resin: (II) Model Networks Properties

Carefully designed resin precursors of high purity, viz. N,N-bis-(2,3-epoxypropyl)-N',N-dimethyl-4,4′-diaminodiphenylenemethane (G2A) and N,N-bis-(2,3-epoxypropyl)-N,N-dimethyl-4,4′-diamino-diphenylenemethane (G2S) were used in combination with N,N,N',N-tetrakis-(2,3-epoxypropyl)-4,4′-diaminodiphenylene methane, TGDDM, and cured with stoichiometric amounts of 4,4′-diamino-diphenylene methane (DDM) to produce networks with a range of controlled crosslink density. The tensile moduli E of the networks in the rubbery state, at T_g+30°C, T_g+45°C and T_g+60°C, were measured using a thermal mechanical analyser. Using the statistical theory of rubber elasticity and the observed values of E, the number average molecular weights between crosslink points M_c for the cured resins were deduced. The experimental M_c values were then compared with those derived by calculations based on a probabilistic model of the network proposed by Chu and Seferis.¹ The experimental M_c values were 2.5 to 5.5 times larger than the calculated ones. The differences were attributable to a consumption of only 40% of the available secondary amino hydrogen via epoxy-amine reaction. A direct relationship was established between the glass transition temperature and the crosslink density 1/M_c for the resins, and the dynamic mechanical properties were studied. The thermal stability of cured resins studied by thermo-gravimetric analysis indicated an enhancement of stability as 1/M_c was reduced. The amount of water absorbed by cured resin was directly proportional to 1/M_c.     

Dynamic mechanical properties of UV-curable polyurethane acrylate with various reactive diluents

UV-curable polyurethane acrylate (PUA) based on polycaprolactone and m-tetramethylxylene diisocyanate were considered with different acrylate monomers as reactive diluents: ethylhexylacrylate, hexanediol diacrylate, and isobornyl acrylate. The effect of the chemical structure and functionality of the reactive diluent (33 wt %) on their thermal and mechanical properties were investigated. The synthetized PUA networks are homogeneous from a thermodynamical point of view. The initial glass transition temperature (T_g) and the functionality of the reactive diluent do not affect the onset value of the glass transition temperature of the network. Nevertheless, the main mechanical relaxation, denoted α, associated with the glass transition temperature becomes broader as the T_g of the homopolymer of the considered reactive diluent becomes higher than the T_g of the PU soft segments. The increase of the amount of the diacrylate monomer leads to an increase in the equilibrium and storage moduli in the rubbery state and to a decrease in the amplitude of the α relaxation. © 1996 John Wiley & Sons, Inc.     

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.     

A relationship between the glass transition temperature (Tg) and fractional conversion for thermosetting systems

An equation, based on thermodynamic considerations to relate the glass transition temperature, T_g, to compositional variation of a polymer system, is adapted in this article for modeling the T_g vs. fractional conversion (x) relationship of reactive thermosetting systems. Agreement between the adapted equation and experimental T_g vs. x data is found for several thermosetting crosslinking systems (i.e., epoxies and cyanate ester/polycyanurate) as well as for reactive thermosetting linear polymer systems (i.e., polyamic acid and esters to polyimides). The equation models the experimentally obtained T_g vs. x behavior of thermosetting systems which include competing reactions. Agreement for widely varying molecular structures demonstrates the generality of the equation. The entire T_g vs. x relationship can be predicted for a thermosetting material by using the T_g vs. x equation and the values of the initial glass transition temperature, T_g0, the fully reacted system glass transition temperature, T_g∞, and the ratio of the change in specific heat from the liquid or rubbery state to the glassy state (Δc_p) at T_g0 and T_g∞, Δc_p∞/Δc_p0. The values of T_g0, T_g∞, and Δc_p∞/Δc_p0 can be measured generally from two differential scanning calorimetric experiments. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 3–14, 1997     

Synthesis and Thermomechanical Behavior of (Qua)ternary Thiol-ene(/acrylate) Copolymers

The objective of this work is to characterize and understand the structure-to-thermomechanical property relationship in thiol-ene and thiol-ene/acrylate copolymers in order to complement the existing studies on the kinetics of this polymerization reaction. Forty-one distinct three- and four-part mixtures were created with systematically varied functionality, chemical structure, type and concentration of crosslinker. The resulting polymers were subjected to dynamic mechanical analysis and tensile testing at their respective glass transition temperature, T_g, to quantify and understand their thermomechanical properties. The copolymer systems exhibited a broad range of T_g, rubbery modulus – E_r and failure strain. The addition of a difunctional high-T_g acrylate to several three-part systems increased the resultant T_g and E_r. Higher crosslink densities generally resulted in higher stress and lower strain at failure. The tunability of the thermomechanical properties of these copolymer systems is discussed in terms of inherent advantages and limitations in light of pure acrylate systems.     

Thermal and thermomechanical properties of poly(ethylene oxide) networks produced with silica and organic crosslinking agents

The thermal and thermomechanical properties of two series of poly(ethylene oxide) networks (NPEOs) were investigated as a function of the chain length between crosslink sites (M_c) and the concentration of LiClO₄ (C_L) in the NPEOs. The two series of networks were produced with silica and organic crosslinking agents and, therefore, had crosslink sites of different natures: one was an inorganic silicate network (silica NPEO), and the other was an organic polar group (organic NPEO). The crosslink sites in both series of networks were commonly covalently bonded to the poly(ethylene oxide) (PEO) phase through a urethane group in the NPEOs. The glass‐transition temperatures (T_g's) of the PEO phases in the NPEOs, according to differential scanning calorimetry, increased with a decrease in M_c and were higher in the silica NPEOs than in the organic NPEOs under the same M_c conditions. The difference in T_g between the two series of networks with the same M_c values increased with decreasing M_c. These results suggested that the interaction of crosslink sites with the PEO phase was stronger in the silica NPEOs than in the organic NPEOs. The addition of LiClO₄ to the NPEOs resulted in T_g of the PEO phase in the NPEOs being elevated and increased according to the increase in C_L. The increase of T_g of the PEO phase according to the increase of C_L in the NPEOs was retarded or saturated at high values of C_L, and this indicated that the limit of solubility of the salt in the polymer was attained. The retardation or saturation of the increase of T_g was also observed in dynamic mechanical analyses. The curves of the loss factor tan δ and temperatures from the dynamic mechanical analyses for the NPEOs with high values of C_L showed shoulders or double peaks indicating the existence of the second phase in the polymer networks. In the curves of tan δ for salt‐complexed NPEOs with high values of C_L, silica NPEOs showed a shoulder of low intensity, but organic NPEOs showed a distinguished second peak becoming stronger with increasing C_L. The results of the T_g behavior and tan δ curves suggested that the salt solubility in the NPEOs was limited and that the salt solubility of PEO in the silica NPEOs was higher than that in the organic NPEOs. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 270–277, 2003     

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.     

Structure–property relationships in photopolymerizable polymer networks: Effect of composition on the crosslinked structure and resulting thermomechanical properties of a (meth)acrylate‐based system

Photoinitiated polymer networks were formed by copolymerization of tert‐butyl acrylate with di(ethylene glycol) dimethacrylate (DEGDMA) or poly(ethylene glycol) dimethacrylate (PEGDMA). The degree of crosslinking was systematically varied by modifying the weight fraction and molecular weight of the dimethacrylate crosslinking agent. An increase in effective crosslink density with increasing crosslinking agent concentrations was confirmed by decreasing equilibrium swelling ratios (q) and increasing rubbery moduli (E_R). Glass transition temperatures (T_g) varied from ?22 to 124°C, increasing with increasing DEGDMA content and decreasing with increasing PEGDMA content. Tensile deformation behavior (at T_g) ranged from an elastomeric‐like large‐strain response for lightly crosslinked materials to a small‐strain brittle response for highly crosslinked networks. At low crosslinking levels, the strain‐to‐failure of the network polymers decreased quickly with increasing crosslinking agent concentration. The stress at failure demonstrated a more complex relationship with crosslinking agent concentration. The effect of composition on network structure and resulting properties (q, E_R, strain‐to‐failure) decreased as the crosslinking agent concentration increased. The results reveal trade‐offs in T_g, E_R, strain‐to‐failure, and failure stress with composition and network structure, and are discussed in light of the wide range of potential applications suggested in the literature for (meth)acrylate‐based photopolymerizable polymer networks including biomaterials and shape‐memory polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

A structure–property rationalization of some poly(carboxyl–epoxy) thermosets

The glass transition temperatures, T_g, and strength properties have been determined for an acrylic-based prepolymer containing 25 mole-% carboxyl functionality crosslinked with five different diepoxides. The T_g values vary from 100° to 200°C, and ultimate elongations of 2% to 7% at 25°C are observed with the different crosslinking agents. These variations are rationalized in terms of the structural elements present in the diepoxides. Thermosets possessing rigid structural units at the crosslink points connected by flexible segments have the best all-around combination of T_g and tensile properties. A decline in these properties was noted when the epoxide/carboxyl ratio exceeded unity owing to the formation of mixed networks and free chain ends.     

On the toughness of photopolymerizable (meth)acrylate networks for biomedical applications

Photopolymerizable networks are being explored for a variety of biomedical applications because they can be formed in situ, rendering them useful in minimally invasive procedures. The purpose of this study was to establish fundamental relationships between toughness, network chemical structure, and testing temperature of photopolymerizable (meth)acrylate networks deformed in air and under hydrated conditions. Networks were formed by combining at least one monofunctional (meth)acrylate with a difunctional methacrylate, and weight ratios were adjusted to vary the degree of crosslinking, elastic modulus, and glass transition temperature (T_g). Stress–strain behavior and toughness were determined by performing tensile strain to failure tests at temperatures spanning the glassy and rubbery regimes of each network both in air and phosphate‐buffered saline. In air, all of the networks demonstrated a peak in toughness below the network's T_g. At an “equivalent” test temperature relative to T_g, crosslinking concentration and monomer chemistry influenced the toughness of each network. Apparent toughness is significantly altered in an aqueous environment, an effect driven by water absorption into the network causing the T_g to decrease. The results from this study provide the fundamental knowledge required to guide the development of tougher photopolymerizable networks for mechanically strenuous biomedical applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

A hybrid model for mechanical spectra of filled and unfilled elastomers

The dynamic mechanical properties of elastomers are of vital importance in determining the product design/performance relationship. Unfortunately, the statistical theory of Gaussian networks, commonly used for the ideal rubbery state, cannot adequately model the moduli of elastomers in engineering applications. The WLF equation, although not originally designed to predict moduli, has a functional form that predicts moduli for the range from T_g to 100°K plus T_g. A hybrid equation which incorporates elements of the WLF equation and the statistical theory of Gaussian networks in an ideal rubbery state has been developed for explaining the mechanical spectrum of elastomeric materials. The new equation satisfactorily models the mechanical properties for both filled and unfilled elastomers. This model shows that filler loading tends to broaden the relaxation spectrum. This finding agrees with a previous study on the viscosity of uncured elastomer-filler systems.     

Glass transition temperature of ideal polymeric networks

Measurements of the glass transition temperature (T_g) have been carried out on polystyrene networks prepared by the anionic copolymerization of styrene and divinylbenzene, and star-shaped polystyrene of varying functionality. The results show a linear variation of T_g versus $\text{M}\text{?}{}^{\mathrm{?1}}{}_{\mathrm{n}}$ in all cases. The value of the slope interpreted in terms of the free volume theory shows that the glass transition temperature depends closely on the average functionality of the crosslinks. In order to study the influence of free chains on the glass transition of crosslinked polymers a series of networks were contaminated with increasing ratios of linear polystyrene chains, slightly polydisperse.     

Single-phase bicomponent network by random crosslinking of hydroxyl-terminated polyisobutylene/polytetrahydrofuran mixtures

Summary Random bicomponent networks were prepared from 50/50 wt % mixtures of hydroxyl-terminated polyisobutylene (HO-PIB-OH) and polytetrahydrofuran (HO-PTHF-OH) by the use of triphenylmethane triisocyanate (TTI) endlinking agent. Differential scanning calorimetry (DSC) and dynamic mechanical techniques were employed to study the networks. The glass transition temperature (T_g) of HO-PIB-OH was found to increase upon network formation. This T_g increment was used to estimate the molecular weight between crosslinking points (M_c). HO-PIB-OH/HO-PTHF-OH mixtures are immiscible and exhibit an upper critical solution temperature, however, the 50/50 network in a two-phase regime for the precursors gives a single T_g situated between the T_gs of the component networks. The single-phase network may be due to rapid nondiscriminatory endlinking and to an increase of the single-phase region of the phase diagram because of crosslinking.     

Viscoelastic properties and film morphology of heterogeneous styrene–butadiene latexes

Heterogeneous carboxylated styrene–butadiene (S/Bu) latexes were prepared by a twostage emulsion polymerization process, using three PS seeds with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu ratio of 1 : 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. Morphological studies by transmission electron microscopy (TEM) as well as studies of the viscoelastic properties by mechanical spectroscopy have been performed on films prepared from the latexes. The studies showed that the glass transition temperature, T_g, of the second-stage polymer was considerably affected by copolymerization with MAA. An increase in the MAA content in the second-stage polymer increased the T_g of this phase significantly. Addition of DVB as a crosslinking agent in the preparation of the PS seed phase substantially increased the rubbery moduli of the films, whereas the glass transition temperature of the second-stage polymer was unaffected. On the other hand, the presence of a chain transfer agent reduced the glass transition of the second-stage copolymer containing 1 wt % MAA dramatically, whereas the rubbery modulus was unaffected. When the MAA content was increased to 10 wt % the influence of the MAA monomer had a dominating effect on T_g. Latexes containing 10 wt % MAA had T_g values close to each other, regardless of chain transfer agent present in the second-stage polymerization. It was found that the morphology of the latex particles influenced the rubbery modulus of the films. The presence of irregularly shaped seed particles in samples prepared from a crosslinked PS seed had a considerable reinforcing effect on the films, whereas spherical seed particles originating from core–shell particles had a less reinforcing effect. © 1996 John Wiley & Sons, Inc.     

Viscoelastic response of model epoxy networks in the glass transition region

Six epoxy networks with various structures built up from a diepoxy prepolymer, DGEBA, and three different diamines or mixtures of a monoamine and a diamine were studied by dynamic mechanical analysis in the glass transition region. The systems were designed in order to investigate the dependence of glass transition T_g on both crosslink density and network chain flexibility. The time (frequency)—temperature superposition principle (WLF equation) was used to determine the viscoelastic coefficients C^g₁ and C which are related to some free volume characteristics on the molecular scale. C^g₁, related to the free volume fraction available at T_g depends mainly on crosslink density, even though the product C^g₁C, related to the free volume expansion coefficient, is dependent on both chain flexibility and crosslink density. Thus, viscoelastic properties determined over large temperature and frequency ranges are shown to yield more precise information on epoxy network structure than the simple analysis of glass transition temperature.     

Relationships between thermally induced residual stresses and architecture of epoxy‐amine model networks

The capability of epoxy‐amine resins to develop residual stresses was studied as a function of temperature and network architecture. These residual stresses were induced while cooling epoxy‐glass bilayers from temperatures higher than the network glass transition temperature, T_g. This behavior was the result of the marked differences (α_r − α_g), in linear thermal expansion coefficient of the two components, as evidenced by the measurement of α_r for the epoxy networks under study. Various network architectures were selected, resulting from variation of (1) the chemical nature of both epoxide and curing agent, (2) the nature and relative amount of the chain‐extensor agent, and (3) the stoichiometric ratio. Three ranges of cooling temperature were observed systematically: first, the range of temperatures above T_g, where no stress has been detected, then an intermediate temperature range (from T_g to T*), where stresses develop quite slowly, and finally, the low temperature range (T < T*), where a linear increase in stress accompanies the decrease of temperature. The two latter regimes were quantitatively characterized by the extent, T_g − T*, of the first one and by the slope, SDR, of the second one. T_g − T* values were shown to be governed by the T_g of the network: the higher the T_g, the larger the gap between T_g and T*. This result was interpreted by accounting for the variation of relaxation rate at T_g from one network to the other. It was also shown that a semiempirical relationship holds between SDR and T_g: SDR decreases monotonically as T_g increases. By inspecting the effects of network architecture in more details, it turned out that SDR is governed by the Young's moduli, E_r(T − T_g), of the epoxy resins in the glassy state: the lower E_r(T − T_g), the lower SDR in a series of homologous networks. As E_r(T − T_g) values are known to be related to the characteristics of the secondary relaxation β, which depends, in turn, on crosslink density, SDR values were finally connected to the amplitude of the β relaxation processes. This finding was corroborated by the measurements on an antiplasticized dense network. Finally, data relative to thermoplastic‐filled networks showed that the addition of thermoplastic reduces the development of residual stresses, whatever the system, is homogeneous or biphasic. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 638–650, 2000     

Degradable epoxy resins based on bisphenol A diglycidyl ether and silyl ether amine curing agents

A series of silyl ether amine curing agents were synthesized by selective substitution reactions of chloroalkylsilanes or the transetherification of alkoxysilanes. Crosslinked networks were prepared by mixing a stoichiometric ratio of bisphenol A diglycidyl ether (D.E.R 331) with the amine curing agents. The networks were characterized by ATR‐FTIR spectroscopy, TGA, DSC, and DMA. The onset of thermal degradation, glass transition temperatures, and storage moduli for the networks were 350 °C, 70–108 °C, and 5–25 MPa, respectively. The degradation behavior of the cured samples was monitored for 30 days in PBS, NaOH 5% (w/v), and HCl 5% (v/v) solutions and the degradation products were characterized by spectroscopic methods. The thermal, mechanical, and degradation studies indicated that crosslink density, T_g, storage modulus, and the rate of degradation were affected by the functionality of the amine curing agents and the number of hydrolyzable silyl ether bonds present per mole of curing agent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44620.     

Synthesis and characterization of polyfunctional naphthoxazines and related polymers

In pursuing the new polymers that could provide high mechanical properties and good thermal stabilities, a series of naphthoxazines are synthesized from different hydroxynaphthalenes with aniline and formaldehyde. The molecular structures are confirmed by NMR spectroscopy. After being polymerized in an autoclave, the naphthoxazine derived from 1,5-dihydroxynaphthalene is successfully cured to form the void-free resin. The density and tensile properties of these polynaphthoxazines are measured. Dynamic mechanical tests are performed to determine the T_g, crosslink density, and the activation enthalpy of the glass transition process for the polynaphthoxazines postcured in air at different temperatures. The effect of postcure temperature on the T_gs of the polynaphthzoxazines is investigated and discussed in terms of crosslink density. The polynaphthoxazine shows a T_g higher than the cure temperature. Fourier transform IR spectroscopy is applied for the molecular characterization of the curing systems. Thermal properties of these polynaphthzoxazines are studied in terms of the weight loss after isothermal aging in static air, the decomposition temperature from thermogravimetric analysis, and the change of dynamic storage moduli at high temperatures. © 1996 John Wiley & Sons, Inc.