Formation,structure, and elasticity of loosely crosslinked epoxy-amine networks. II. Mechanical and optical properties

The mechanical and optical behavior in the dry and swollen states of loosely crosslinked epoxy networks prepared from the diglycidyl ether of bisphenol A, phenylglycidyl ether, and 4,4′-diaminodiphenylmethane was investigated, and the weight fraction of sol in the networks was determined. The crosslinking density was controlled by an excess of diamine and by the fraction of monoepoxide. The reaction proceeded to almost full conversion of epoxy groups. With increasing content of monoepoxide or with increasing excess of diamine, the main transition region is shifted to lower temperatures. The dependence of the viscoelastic modulus on temperature and the optical behavior indicate that the networks are homogeneous. In all cases, the sol fraction is adequately described by the theory of branching processes (cf. Part I). The equilibrium modulus related to the dry state is the same irrespective of whether it is obtained by measurements in the dry or swollen state. The mechanical behavior in the rubbery state can be described by the theory of phantom networks with fully suppressed fluctuations of crosslinking (front factor A = 1) or by the theory of phantom networks with fully released fluctuations of crosslinks (front factor) A = f_e?2/f_e] and contribution of trapped entanglements of the Langley-Graessley type (cf. Part I). In the analysis of the equilibrium behavior, it is advantegeous to use the plot of superimposed dependences of G_e on the gel fraction, which considerably reduces the effect of experimental inaccuracy in determination of composition and degree of conversion.     

Thermoelasticity of weak polyelectrolyte networks

The thermoelastic behavior of poly(vinyl alcohol)–poly(acrylic acid) networks was evaluated in pure water and CaCl₂ solution. The ratio f_e/f of the energy component of the force to the total force, evaluated without taking into account polymer–diluent specific interactions, ranged from ?0.75 for networks swollen in pure water to ?5.7 in 0.1M CaCl₂. However, an analysis based on Flory's theory of polyelectrolyte gels yields f_e/f constant at ?1.32 when specific interactions are accounted for. In addition, the variation of In (r²)₀ with CaCl₂ concentration is 2000 times that with water. In neither pure water nor CaCl₂ solution can specific interactions be neglected.     

Thermoelasticity of networks in swelling equilibrium with mixed diluents: Specific polymer-diluent interactions

Thermoelasticity studies of poly(vinyl alcohol) networks swollen in aqueous glycol indicate that the effects of specific polymer-diluent interactions diminish at high swelling, much as in the situation of poly(vinyl alcohol) in pure water reported earlier. In 20% glycol, the ratio of the energy component f_e of the force to the total force f was found to be ?0.59 at high swelling. Swelling was controlled by the degree of network crosslinking. Evidence that f_e/f could be determined under the condition of a null thermal expansion coefficient was not obtained; in fact, the contrary was indicated.     

Thermoelasticity of networks in swelling equilibrium with single component diluents: Specific polymer-diluent interactions

Using thermoelastic measurements, specific polymer-diluent interactions have been demonstrated for atactic poly(vinyl alcohol) networks swollen in water. This was done by following a thermodynamic parameter as a function of network swelling, which was controlled by varying the extent of network crosslinking. At low crosslinking (high swelling) the ratio of the energy component f_e of the force to the total force f was found to be perhaps a little less than ?0.38 for poly(vinyl alcohol) swollen in water to less than 0.36 volume fraction of polymer. The method of evaluation is new and should be applicable to other polymer single component diluent systems.     

Statistical thermodynamics of networks at large deformations

A general theory of non-Gaussian elasticity is presented for real polymeric chains having fixed bond angles and restricted internal rotations. The theory contains the displacement-vector distribution given by Nagai, and the Flory-Wall-Hermans procedure is used for the calculation of network properties. Whereas the treatment is valid for all types of polymer chains, it is not totally satisfactory from a practical standpoint because of a slow series convergence if the chains are stiff. It is best utilized for flexible polymers under conditions of light crosslinking. Detailed network behavior is investigated only for polyethylene type chains having uncorrelated internal rotations. In this instance the fractional contribution f_e/f of the internal energy of the total force f is found to be a function of elongation at high degress of stretching. It may decrease, or increase, depending upon the sign of f_e/f at low elongations. Furthermore, the variation of f_e/f with elongation is independent of the fixed bond angle of the chain backbone. Stress–strain behavior and energy–strain behavior are in opposition, i.e., when the non-Gaussian contribution to the stress is greatest, it is the least for the ratio f_e/f, and vice versa. The presence of correlated internal rotations would not be expected to greatly alter these general conclusions.     

Thermoelastic properties of networks in swelling equilibrium: Poly(vinyl alcohol)

The energy component of the stress has been determined for poly(vinyl alcohol) networks in swelling equilibrium with a series of water–ethylene glycol compositions. The data are analyzed by using the equations describing the thermoelasticity of networks in swelling equilibrium. The ratio f_e/f of the energy component to the total force, as calculated from these equations, varies systematically with diluent composition but is independent of elongation in a given diluent. For a network crosslinked by terephthalaldehyde, f_e/f varied from ?0.33 to ?0.42 as the diluent composition was changed from pure water to 20% ethylene glycol. Similar effects were found in a network crosslinked by formaldehyde. It is not yet certain whether this effect represents a real solvent dependence of f_e/f or a failure of the equation of state to account for the effect of composition changes on the force.     

CONFORMATIONAL PROPERTIES OF STRETCHED POLYETHYLENE CHAIN*

 When polyethylene chains are stretched, the chains are regarded as being confined in an infinite cylinder withdecreasing diameter. The conformational properties of polyethylene chains confined in an infinite cylinder are investigatedby using rotational isomeric state model. Using the average conformational energy and entropy and the average length, wecan determine the elastic force f or the fraction of the energy term to the total forcef_e/f,where f_e=?<∪>/?and f=?/?.Comparisons with experimental data are also made.The results of these microscopic calculations are discussed in therms of the macroscopic phenomena of rubber elasticity.     

Thermoelastic properties of crosslinked polyurethanes

Thermoelastic (stress–temperature) measurements in elongation have been carried out on polyurethane elastomers obtained from poly(ethylene adipate), poly(ethylene maleate), poly(ethylene glycol), and 4,4′-diphenylmethane diisocyanate. The elastomers were crosslinked by an excess of diisocynate and some of them were additionally crosslinked by dicumyl peroxide. Values of the temperature coefficient d ln r /dT of the unperturbed dimensions of the network chains and the fraction f_e/f of the stress, which is due to energetic effects, were calculated from measurements of unswollen samples and samples swollen in benzene to a constant volume fraction ν₂ of about 0.6. The coefficient d ln r /dT and the ratio f_e/f for unswollen samples were positive but dependent on the extension ratio α and crosslinking density. The f_e/f ratio displayed a tendency to decrease with increasing α and this decrease was smaller for elastomers of higher crosslinking density. Additional crosslinking due to dicumyl peroxide reduced the dependence of f_e/f on α. Swollen samples had lower positive values of f_e/f; which were somewhat independent of α and crosslinking density. Departures from the molecular theories of rubberlike elasticity are explained by intermolecular effects, which, in general, are influenced by crosslinking.     

Epoxide networks as model networks

Summary The equilibrium mechanical behaviour of weak diepoxide-monoepoxide-diamine networks, prepared with an excess of diamine and measured in the rubbery state, was compared with theoretical predictions obtained by using the theory of branching processes. The experimental equilibrium moduli fit well the shape of theoretical curves over a broad range of crosslinking density regardless of whether the contribution by trapped entanglements is considered or not. The data fit equally well the theoretical dependence for the front factor A = 1 without entanglement contribution and forA = (f _e - 2)/f _e , (f _e is the average effective functionality of a junction) with an entanglement contribution based on the contact probability between any two units within elastically active network chains (Langley).Dedicated to Prof. Dr.G. Rehage on the occasion of his 60th birthday.With 5 figures     

Dynamic - mechanical properties of poly(oxypropylene)di-amine-diepoxide and poly(oxypropylene)triamine-diepoxide networks and their relationship to the structure of elastically active network chains

The effect of the initial mole ratio of reactive components on the shape and position of dynamic mechanical functions in the main transition and rubbery region was investigated for two series of networks made from poly(oxypropylene)diamine (D-400)-diglycidyl ether of Bisphenol A (DGEBA) and poly(oxypropylene)-triamine (T-403)-DGEBA. The networks were prepared with an excess of amine groups up to the highest conversion of epoxy groups; the ratio ^rH = 2 [ NH₂ ]₀/ [E]₀ ranged from unity to 2,1 for networks from D-400 and from unity to 3,5 for networks from T-403. By using the theory of branching processes, structural parameters of these networks were calculated, in particular, the molecular weights of elastically active network chains (EANC's) including dangling chains, of backbone EANC's and of dangling chains. A comparison between theory and experiment led to the following conclusions: (a) the mechanical behaviour in the rubberlike region can be described either by using an affine deformation model (front factor A = 1), or by means of a phantom model (A = (f_e-2)/f_e, f_e being functionality of the crosslink) with the contribution of permanent interchain interactions; (b) the temperature and frequency position of viscoelastic functions in the main transition region is conclusively affected by the concentration of EANC's; (c) the shape of visco-elastic functions, especially of retardation spectra in the main transition and rubbery region, depends on the detailed structure of EANC's, but it cannot be decided from the result which structural parameter has the strongest effect on the shape of the functions.     

Elastic and thermoelastic properties of “inorganic rubber”

Polymerization of cyclic phosphonitrilic chloride trimer was carried out at sufficiently high temperatures and for sufficiently long periods to cause gelation of the poly(dichlorophosphazene) formed. The resulting crosslinked material, called “inorganic rubber”, was found to be stable enough in the absence of moisture to permit reliable stress-strain and stress-temperature (thermoelastic) measurements in elongation. The stress-strain isotherms at low and moderate values of the elongation, α, showed that the networks had relatively low degrees of crosslinking; at high α and low temperature, they underwent strain-induced crystallization. The thermoelastic data confirmed the occurrence of crystallization at high α and low T. Thermoelastic values of the fraction f_e/f of the force that is of energetic origin were negative and generally large, with a magnitude that increased with increase in α, presumably because of the crystallization. In any case, the experimental values of f_e/f are in at least qualitative agreement with rotational isomeric state theory, which predicts this ratio to be large and negative.     

Dynamics of supramolecular associative polymer networks at the interplay of chain entanglement,transient chain association,and chain‐sticker clustering

The dynamic mechanical properties of supramolecular associative polymer networks depend on the average number of entanglements along the network‐forming chains, N_e, and on their content of associative groups, f . In addition, there may be further influence by aggregation of the associative groups into clusters, which, in turn, is influenced by the chemical structure of these groups, and again by N_e and f of the polymer. Therefore, the effects of these parameters are interdependent. To conceptually understand this interdependency, we study model networks in which (a) N_e, (b) f , and (c) the chemical structure of the associative groups are varied systematically. Each network is probed by rheology. The clustering of the associative groups is assessed by analyzing the rheological data at the end range of frequency covered and by comparison of the number of supramolecular network junctions with the maximum possible number of binary transient bonds. We find that if the total number of the network junctions, which can be formed either by interchain entanglement or by interchain transient associations, is greater than a threshold of 13, then the likelihood of cluster formation is high and the dynamics of supramolecular associative polymer networks is mainly controlled by this phenomenon. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1209–1223     

Preparation and properties of poly-(N-butylmethacrylamide) networks

The gel-effect influences the copolymerization of N-butylmethacrylamide with methylene-bis-acrylamide similarly to other vinyl-divinyl copolymerizations. The fraction of crosslinks wasted in cycles is very high at the gel point, reaching 0.84–0.95; it decreases with increasing content of the crosslinking agent and with decreasing content of dimethylsulphoxide (DMSO) in the polymerization mixture. The effect of residual time processes on the mechanical characteristics in the rubberlike region decreases with increasing concentration of elastically active network chains, ν_e. The equilibrium deformational behaviour obeys the kinetic theory of rubber elasticity. With increasing dilution of the system, the efficiency of the cross-linking reaction decreases and so does the effect of trapped entanglements. A comparison of the gel points and the mechanical characteristics for poly(N-butylmethacrylamide) and poly(N-ethylmethacrylamide) networks swollen in DMSO indicates that the substituent at the amide nitrogen atom has a dilution effect depending on its size. The interaction parameter of the system poly(N-butylmethacrylamide)-DMSO increases linearly with the volume fraction of the polymer in the swollen gel, ν₂; χ = 0.498 + 0.362 ν₂ for ν₂? (0.1, 0.33) and 25°.     

Polarization of fluorescent light from a uniaxially oriented rubber

The polarization of fluorescence from a polymer characterizes two segment orientation functions, f_s⁽²⁾ and f_s⁽⁴⁾. These may be calculated as a function of elongation using the kinetic theory of rubber elasticity. Three cases are considered: (a) the transition moment direction lies parallel to the segment axis and does not change its orientation during the lifetime of the excited state, (b) the transition moment direction lies at an angle γ to the segment axis, and (c) the transition moment changes its orientation during the lifetime of the excited state.     

The effect of network architecture on the thermal and mechanical behavior of epoxy resins

The effects of crosslink functionality (f_c), molecular weight between crosslinks (M_c), and chain stiffness display on the thermal and mechanical behavior of epoxy networks are determined. Both f_c and M_c are controlled by blending different functionality amines with a difunctional epoxy resin. Chain stiffness is controlled by changing the chemical structure of the various amines. In agreement with rubber elasticity theory, the rubbery moduli are dependent on f_c and M_c, but independent of chain stiffness. The glassy moduli and secondary relaxations of these networks are relatively independent of f_c, M_c, and chain stiffness. However, the glass transition temperatures (T_g) of these networks are dependent on all three structural variables. This trend is consistent with free volume theory and entropic theories of T_g. f_c, M_c, and chain stiffness control the yield strength of these networks in a manner similar to that of T_g and is the result that both properties involve flow or relaxation processes. Fracture toughness, as measured by the critical stress intensity factor (K_Ic), revealed that f_c and M_c are both critical parameters. The fracture behavior is the result of the fracture toughness being controlled by the ability of the network to yield in front of the crack tip. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1371–1382, 1998     

Elastic and thermoelastic behavior of cis-polybutadiene crosslinked in the swollen state

Samples of cis-polybutadiene were cross-linked in the presence of decalin at values of ?_r (the volume fraction of rubber) ranging from 0.25 to 0.50. After the diluent had been removed, a study was made of the force-extension and thermoelastic behavior of the networks. Within experimental error, the ratio of the Mooney-Rivlin constants 2C₂/2C₁ was found to be zero for networks prepared at low ?_r (0.25 and 0.30) but started to increase towards the conventional value at higher ?_r. The quantity f_e/f was found to be independent of the ratio 2C₂/2C₁ and of the extension ratio α; the average value of f_e/f was found to be 0.11.     

Thermodynamic behavior of D-sphingosine/cholesterol monolayers and the topography observed by AFM

Lipid rafts are of a dynamic microdomain structure found in recent years,enriched in sphingolipids,cholesterol and particular proteins.The change of structure and function of lipid rafts could result in many diseases.In this work,the monolayer behavior of mixed systems of D-sphingosine with choles-terol was investigated in terms of the mean surface area per molecule(Am),excess molecular area(Aex),surface excess Gibbs energy(Gex),interaction parameter(ω),activity coefficients(1 and 2) as well as elasticity(C...     

Energetic Contribution to the Elastic Force in Polyphenylmethylsiloxane Networks

Abstract

Energetic contribution to the total elastic force in polyphenylmethylsiloxane networks has been measured in order to elucidate the influence of the phenyl group on chain con -formational mobility. A value of 0.20 ± 0.03 for f_e /f was found.     

Glycerol‐lactic acid star‐shaped oligomers as efficient biobased surface modifiers for improving superabsorbent polymer hydrogels

The glycerol‐lactic acid‐based star‐shaped modifier (SM) were synthesized and employed for surface modification of hygienic superabsorbent polymer (SAP) hydrogels for the first time. Surface crosslinking was carried out to increase the saline‐absorbency under load (AUL) and the swollen gel strength. The surface treatment process was analyzed employing free absorbency and AUL tests, salt sensitivity, attenuated total reflectance‐Fourier‐transform infrared spectroscopy (ATR‐FTIR), oscillatory rheometry, as well as scanning electron microscopy analysis. The effect of the branched architecture and the contents of SM on the properties of the modified SAPs were studied. The implementation of surface treatment leads to increase up to a 28% in the AUL of the modified SAP. Moreover, the loss modulus was surprisingly increased, while the storage modulus was enhanced (possibly due to the star architecture of oligomers). M_c and crosslink density values have been estimated based on modified rubber elasticity theory. Salt sensitivity factor (f) was calculated; the modified samples showed lower salt sensitivity in NaCl (f = 0.7) and CaCl₂ (f = 0.93–0.95) compared with the intact SAP (f = 0.84 for NaCl, and f = 0.95–0.97 for CaCl₂).     

A Monte Carlo Study of the Elastic Behavior of Polymethylene Chains

The rubber elastic behaviors of long polymethylene (PM) chains are investigated using Monte Carlo simulations and considering the rotational‐isomeric‐state model. Through the Monte Carlo method we can generate many PM chains in the equilibrium states, and obtain the average Helmholtz free energy 〈A〉 and average energy 〈U〉. Chain dimensions and thermodynamic statistical properties of long PM chains under various elongation ratios λ are also calculated. We find that the elastic force f increases with elongation ratio λ, and that energy contribution f_u to the elastic force is negative and significant. The ratio f_u /f ranges from –0.37 to –0.32 at T = 300 K and from –0.53 to –0.40 at T = 413 K, and decreases with increasing temperature, which agrees with the experimental data. Our calculations may provide some insights into the macroscopic phenomena of rubber elasticity.