Crosslinking,filler, or transition constraint of polymer networks. I

The basic theory of modulus/swelling is developed to allow for limited extensibility, filler reinforcement or transition effects, and steric hindrance of aligned segments by extended chains or filler particles. Filler forms an effective hard fraction C_h per cubic centimeter of compound with v_c a new (compound) index of swelling. For 1/M_c + σ fix points having ratio φ to gum values 1/F₀(v_r) and with F(v_c) replacing the Flory function F(v_r): where σ denotes entanglement. Linkage reinforcement φ does not vary with sulfur crosslinking of SBR. Vacuoles invalidate φ from mass-increment F₀(v_r)/F(v_r) for inert fillers. Then, or for Graphon, with negligible φ ≈ 1: The effective C_h includes rubber stretched hard on Graphon by swelling or trapped inside hard aggregates. Only the right-hand equation fits normal blacks. In theory, C_h can always be obtained from swollen moduli G by linear slopes (1 + 1.4C_h) relating F(v_c) and (1 ? C)ρRT/G. For filler fractions C ≥ 0 cm^?3 and low strains α = 1.5?2.0 below prestretch the modulus G is given a new basic definition: Here C₂* ≈ 0.7 corresponds to Mooney-Rivlin C₂ and the effective crosslinking 1/[M_c] = (ρRT)^?1G is equal to (1 ? C)(1/M_c + σ) for unswollen prestretched rubber (v_r = 1). For higher strains a hypothesis of strain hardening is proposed. This is distinct and opposite in character to the initial prestretch softening (Mullins effect). Nonlinear effects of crosslinks are expressed by a fractional stress-upturn Ω (1/M_c + σ), effective mesh wieght (1/M_c + σ)^?1 ? Ω, and hard fraction Ω(1/M_c + σ). For μ_h characterizing strain hardening up to the prestretch (α_h ? 1) their contribution is: The sixth-power refinement has J = j(α_b ? 1)^1/2 with j ≈ 0.4. The hard phase is augmented by filler and grows with increasing strain up to the prestretch.     

Hyperelasticity and stress softening of filler reinforced polymer networks

Starting from an analysis of filler networking in bulk polymers, a constitutive micro-mechanical model of stress softening and hysteresis of filler reinforced polymer networks is developed. It refers to a non-affine tube model of rubber elasticity, including hydrodynamic amplification of the rubber matrix by a fraction of hard, rigid filler clusters with filler-filler bonds in the unbroken, virgin state. The filler-induced hysteresis is described by an anisotropic free energy density, considering the cyclic breakdown and re-aggregation of the residual fraction of soft filler clusters with already broken, damaged filler-filler bonds. Experimental investigations of the quasi-static stress-strain behaviour of silica and carbon black filled rubbers up to large strain agree well with adaptations found by the developed model. The microscopic material parameters obtained appear reasonable, providing information on the mean size and distribution width of filler clusters, the tensile strength of filler-filler bonds and the polymer network chain density.     

Crystallization in stretched polymer networks. II. Trans-polyisoprene

Measurements are described of the rates of crystallization and crystal melting temperatures in stretched test pieces of trans-polyisoprene, crosslinked to various degrees. The increase in rate with extension are attributed to the corresponding increases in melting temperature and hence degree of supercooling. The rise in the melting temperature of both the α- and β-crystal forms is found to be in satisfactory agreement with Flory's treatment of oriented crystallization. The changes in tensile stress are also generally in accord with the formation of oriented crystallites. For the more lightly crosslinked materials, a pronounced rise in tensile stress occurred during the later stages of crystallization, at extensions below about 100%. Reasons are given for attributing this phenomenon primarily to the relatively large contraction in volume on crystallization, rather than to the formation of folded-chain crystallites.     

Interpenetrating polymer networks from polyurethanes and methacrylate polymers. II. Interpenetrating polymer networks with opposite charge groups

Interpenetrating polymer networks (IPNs) with opposite charge groups (tertiary amine and carboxyl groups) made from polyurethanes and methacrylate polymers have been synthesized and their properties and morphology, studied. With increasing carboxyl group concentration the mechanical properties and compatibility between the component networks were significantly improved, possibly because of the negative (or zero) free energy produced by the interaction contribution between the tertiary amine groups in the polyurethanes and the carboxyl groups in the methacrylate polymers determined by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The improved molecular mixing in these IPNs was thought to be due to the influence of the opposite charge groups in these systems.     

Effect of filler on kinetic characteristics of glass transition in polymer composite materials

Dynamic mechanical spectroscopy and differential scanning calorimetry were used to study the effect of various fillers (carbon fiber, glass fiber, and aramid fiber) on the kinetic characteristics of glass transition in polymer composite materials based on epoxy resin. It is shown that the composite based on carbon fiber is the most fragile among the materials studied, whereas the polymer composite material based on aramid fiber exhibits the lowest rate of variation of the relaxation time above the glass-transition temperature. A relationship is determined between the heat conductivity and fragility of polymer composite materials. The effect of various fillers on the curing kinetics of the epoxy matrix upon glass transition is prognosticated, with the difference in the degree of curing reaching a value of 4–5%. The strongest filler effect on the curing kinetics is observed in the chemically controlled region, which may be due to the catalytic effect of functional groups on the fiber surface.     

Equilibrium conformations of polymer chains Part II. Conformons in networks

With regard to the ideal network it is shown that the concept ofN non-interacting polymer chains can be transformed in a problem of non interacting excitations (called conformons) for rubber elasticity. Modelling the interaction on permanent crosslinks as a scattering problem and taking the finite chain length into account, an interpretation of the second Mooney coefficient can be given. There is some evidence that the junctions move by constrained self diffusion.Dedicated to Prof. Dr. W. Ruland on the occasion of his 60th birthday.     

Crosslinking a palladium(II) polymer gives a laminated sheet structure

Chains of a zigzag coordination polymer containing bis(pyridyl) bridging groups between palladium(II) centres can be arranged to give a laminated sheet structure by a biomimetic approach in which hydrogen bonding involving amide groups is the key feature.     

Graft interpenetrating polymer networks of polyurethane and epoxy. II. Toughening mechanism

Graft interpenetrating polymer networks (graft-IPNs) of polyurethane (PU) and the diglycidyl ether of bisphenol A (epoxy) were prepared by first grafting excess PU prepolymer to the epoxy and then simultaneously polymerizing the PU prepolymer and epoxy. The fracture properties, at high shear rate (e.g., impact) and low shear rate (e.g., pseudostatic tensile fracture energy measurement) of these graft-IPNs exhibit opposite behavior. Although dispersed rubber particles can enhance the Izod impact strength, toughening of the matrix of graft-IPNs was found to be the main contribution. In contrast, it was found that a heterogeneous morphology with suitably dispersed rubber domains of appropriate size as well as the toughness of the matrix are requirements for effectively increasing the fracture energy at low shear rate. A reinitiating crack in the plastic matrix is proposed as the main toughening mechanism and can be invoked to interpret the fracture behavior at high and low shear rates of the graft-IPNs.     

Cyclolinear polysiloxanes. II. Crosslinking and characterization

The synthesis and characterization of two groups of novel networks prepared from cyclolinear polysiloxanes are described. The first group of networks from cyclolinear polysiloxanes (N‐CLPSs) was synthesized by the hydrosilation of vinyl‐terminated cyclolinear polyorganosiloxanes [prepared from diacetoxydiethyltetramethylcyclotetrasiloxane (D₄Et₂OAc₂) or diacetoxytriethylpentamethylcyclopentasiloxane (D₅Et₃OAc₂)] with a copolymer of dimethylsiloxane and methylhydrosiloxane as the crosslinking agent. Hydrosilation was effected with a platinum carbonyl catalyst with a cyclovinylsiloxane moderator. The second group of networks (N‐eCLPSs) was prepared similarly with extended cyclolinear polysiloxanes. The mechanical properties of the novel networks were comparable to those of polydimethylsiloxane networks (N‐PDMS). The oxygen permeabilities were similar to or slightly higher than that of N‐PDMS. The glass‐transition temperatures of D₄Et₂OAc₂‐ and D₅Et₃OAc₂‐based N‐CLPSs were ?67.8 and ?90.8 °C, respectively, whereas the incorporation of polydimethylsiloxane spacers into similar N‐eCLPSs lowered their glass‐transition temperatures to ?109.7 and ?115.0 °C. Upon heating to 800 °C in air, N‐CLPSs yielded more residue than N‐eCLPSs, which in turn yielded more residue than N‐PDMS. These results may have been due to the presence of T units in the cyclic siloxane units, which may have inhibited chain degradation or the formation of volatile products. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4053–4062, 2006     

Crosslinking of starch xanthate. II. Reaction with polyacrolein

The reaction between polyacrolein bisulfite adduct and sodium starch xanthate in aqueous alkali gives a rapid viscosity increase and subsequent gelation. The magnitude and pattern of the viscosity development depend on the degree of substitution of the starch xanthates in the range of 0.12 to 0.45, varying from around 5000 cP centipoises (nongelling) for the lowest to more than 100 000 cP for the highest. The viscosity patterns developed with the more highly substituted xanthates show a minimum valley after a 20-30-min reaction. Analytical and spectral analyses of solid materials obtained from the ethanol precipitation of the gels indicate that the product is a monothiocarbonate stemming from the elimination of sodium hydrogen sulfide between a xanthate group of the starch and a hydroxyl group of the polyacrolein adduct. A hydrated form of polyacrolein does not react with starch xanthate under the same aqueous alkaline conditions.     

Determination of filler content for natural filler polymer composite by thermogravimetric analysis

Determination of filler content by thermogravimetric (TG) analysis is commonly utilized to investigate the effectiveness of processing methods for composite materials and to quantify the dispersion of filler within the matrix. However, the existing analysis method is not capable of accurately predicting the filler content for natural fiber composites for the case where thermal degradation of the filler and matrix occurs within similar temperature ranges. In the present study, the authors have proposed a generic equation for the determination of filler content which can be utilized for any given range of thermal degradation temperatures in natural filler polymer composites. Oil palm shell unsaturated polyester composites were selected to verify the proposed equation using the TG test with the results indicating good agreement between the estimated and experimental filler contents with a maximum error on the order of 10 %. The suggested technique provides a simple, yet generic, approach to determining the filler content of green or lignocellulose-based polymer composites by TG analysis.

     

A study on the glass transition behavior and morphology of semi-interpenetrating polymer networks

The epoxy resin/polyurethane semi-IPN was prepared and the glass transition behavior of the semi-IPN was discussed with DSC and DMA methods. The results show that the two glass transition temperatures (T_g) referring to epoxy resin and polyurethane respectively get closer. Between the two T_g there exists another T_g related to the interface between the two polymers. SEM indicates that this semi-IPN has a two-phase continuous structure which changes with different weight compositions. This is also proved by testing the Young's modulus. It is found that the IPN system has a cellular structure. Comparatively, the compatibility between the two polymers is the best when the weight ratio of EP/PU is 70/30. © 1996 John Wiley & Sons, Inc.     

Deformation behavior of polymer networks containing or interacting with a solvent

General formalism to describe both equilibrium and nonequilibrium states of polymer networks containing a solvent or interacting with the solvent medium is proposed. Two classes of problems have been formulated. It is necessary to determine the stress-strain state of an inhomogeneously swollen material in one case and that of a statically loaded material occurring in thermodynamic equilibrium with the solvent in the other case. The state of the swollen material is characterized in terms of the global mechanical stress tensor and the solvent chemical potential. In the case of incompressible material and liquid, an osmotic stress tensor is introduced. A method for deriving physical expressions for the mechanical stress tensor, the chemical potential, and the osmotic stress tensor is proposed on the basis of the known free energy relations that follow from different theories of rubber elasticity. The efficacy of the general formalism is demonstrated using particular examples in which the deformation behavior and the equilibrium swelling of mechanically loaded polymer networks are considered.     

Crosslinking of polyimides via spirodilactone unit in polymer backbone

A new approach for the crosslinking of polyimides via the lactamization of spirodilactone unit in polyimide backbone was studied by two means: model reaction and the comparison of the properties of the polyimide precursors to those of the crosslinking polymers. Polyimides 4 and 5 were soluble in N,N′dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), N′-methylpyrrolidone (NMP), and other common organic solvents, whereas their corresponding crosslinking polymers were insoluble in these solvents. The glass transition temperatures for polyimide 5 and its crosslinking polymer were 262°C and 291°C, whereas those for polyimide 4 and its crosslinking polymer were 265°C and 360°C. The weight-loss rate of the crosslinking polymers was apparently slower than that of the precursors when the temperature was > 400°C. The 10% weight-loss temperature for the polyimides 4 and 5 was < 500°C, whereas that for the crosslinking polymers was close to or above 600°C. The results indicate that this type of crosslinking polymer has good thermal properties. The temperature for the formation of lactam was above 180°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3680–3686, 1999     

Physicochemical properties of ocher used as polymer filler

Principal physicochemical properties of ocher were studied in order to use it as a mineral filler for polymers.     

Polyethylene networks crosslinked in solution: Preparation,elastic behavior,and oriented crystallization. I. Crosslinking in solution

A study has been made of the crosslinking of linear polyethylene in solution. Networks containing a low number of trapped entanglements and elastically ineffective chain ends were prepared by crosslinking high molecular weight linear polyethylene in 1,2,4-trichlorobenzene solutions with dicumyl peroxide at 120°C. No syneresis was observed during crosslinking except at high peroxide concentrations. The networks were characterized by swelling measurements, infrared spectroscopy, and differential scanning calorimetry. The crosslinking efficiency, calculated from swelling, was found to be proportional to the square of the polymer volume fraction. The proportionality constant was 0.8, indicating close to unit efficiency for undiluted polymer. Chemical modification of the polyethylene chains by attachment of peroxide and solvent fragments was of the order of one foreign unit per elastically active network chain, depending on peroxide and polymer concentration. Sol–gel analysis indicated that no chain scission occurred. These results are shown to be consistent with a “cage” mechanism for crosslinking. The possible topological consequence of this mechanism, preferential crosslinking of entanglements, is discussed. The concentration of trapped entanglements was also found to be proportional to the square of the polymer volume fraction. The proportionality constant corresponds to a molecular weight between entanglements of 4000 for the undiluted polymer, which is close to the value of 4200 found for networks prepared from the undiluted polymer. Since the results obtained are based mainly on the use of the swelling equation, different aspects of the applicability of this equation for the evaluation of the crosslinking process are discussed. As regards the reference dimensions, which should be known for a quantitative application of the elastic theory, the results strongly support the use of the dimensions of the network chains after completion of crosslinking.     

Reactivity of polymer substituents. II. Aminolysis of p-nitrophenyl acrylate copolymers with acrylamide or N,N-dimethylacrylamide in aqueous solution

Rates of the benzylaminolysis of acrylamide and dimethylacrylamide copolymers with a small amount of p-nitrophenyl acrylate in water solution were compared with the analogous reaction of p-nitrophenyl isobutyrate. Rates of the dimethylacrylamide copolymer and analog were also measured in methanol. The copolymer reactions were much slower than those of the analog, in contrast with results obtained in dioxane. It was found that the reaction of the analog is much more sensitive to the medium than the reaction of the polymer, because the “effective solvent medium” in the polymer domain is largely determined by properties of the polymer chain backbone.     

Formation,structure and properties of polymer networks

The systematic study of intramolecular reaction and gel points together with measurements of moduli of trifunctional and tetrafunctional end-linked networks show that gel points are always delayed and network imperfections always result from the intramolecular reaction which must occur when such networks are formed. The quantitative interpretation and prediction of gel points and moduli directly from reactant structures and reaction conditions are discussed.     

Swelling kinetics of anisotropic filler loaded PDMS networks

The kinetics of swelling of spherical poly(dimethyl siloxane) (PDMS) composite networks in cyclohexane has been studied. The PDMS composite gels contain magnetite and iron particles built in the network either in randomly distributed form or in chain-like structure. The composite gel becomes anisotropic in the terms of both, mechanical and swelling properties. As a consequence the swelling kinetics can be characterized by direction dependent relaxation time. The swelling kinetics of these samples has been compared to that of pure PDMS gels. It was found that Li-Tanaka theory can be applied to describe the swelling kinetics of PDMS network.