Properties and structure of elastomeric two-stage emulsion interpenetrating networks

Interpenetrating polymer networks (IPNs) consist of two crosslinked polymers which form a network within a network. Bulk-prepared IPNs are thermosetting and cannot be processed due to the formation of a macroscopic network. Emulsion IPNs, although thermosetting, can be processed as thermoplastic materials. This is due to a special particle-slippage flow mechanism which is practically insensitive to molecular weight. The morphology of the reported latex particles is unique in the sense that very small polystyrene domains are formed by phase-separation of polymerizing styrene added to a seeded flexible polyacrylate latex. Compression or injection moulded specimens of these crosslinked elastomeric materials show significant mechanical properties. Some properties and structural observations using a special electron microscopy technique are described in this article. This method is based on the differential radiation damage to various polymers embedded in ice and can be used as an analytical tool to determine the microstructure of certain multiphase systems.     

High internal phase emulsion foams: Copolymers and interpenetrating polymer networks

High internal phase emulsion (HIPE) copolymer and interpenetrating network foams were prepared from 2‐ethylhexyl acrylate (EHA), styrene (S) and divinylbenzene (DVB) using a unique process. The morphologies, thermal properties and dynamic and static mechanical properties of these foams were investigated. The glass transition temperatures and damping properties of the EHA/S copolymer foams vary with its composition. IPN foams with very broad tan 5 peaks were obtained. The damping properties of IPN foams were tailored through changing copolymer composition and monomer composition. The IPN foams based on a copolymer foam and styrene had a broader tan δ peak, a higher glass transition temperature and a higher modulus than tne copolymer foams of similar overall styrene contents. It is therefore possible to prepare novel damping foams based on polyHIPE foams through the synthesis of interpenetrating polymer networks.     

Porous elastomeric beads from crosslinked emulsions

The production of porous polymeric particles is attractive for a large number of applications and can be achieved by various techniques. Although numerous production schemes exist for glassy polymers, difficulties arise for soft, rubbery materials that need a chemical crosslinking step, such as elastomers. This is particularly true for poly(dimethylsiloxane) (PDMS), which shows the lowest glass‐transition temperature among the polymers. Recent studies suggest in situ hydrogen bubble formation or vacuum drying of water droplets dispersed in the polymer matrix in order to generate porous PDMS structures. In this work we report early results based on the chemical crosslinking of water in PDMS emulsion droplets in a mechanically stirred thermostated water vessel. This approach is shown to lead to high porosity PDMS beads (ca. 10^?3 m particle diameter) with an open structure whose properties (diameter and porosity) are strongly influenced by the starting composition (solvent, surfactant, and polymer types and ratios), as well as the operating parameters (agitation and temperature). The possible uses of these derived beads are discussed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 967–971, 2002     

Mechanical properties of interpenetrating polymer network hydrogels based on hybrid ionically and covalently crosslinked networks

Hydrogels are polymer networks swollen in water. Because of their soft and wet nature, and their ability to show large volume changes, hydrogels can be useful in many biomedical and actuator applications. In these applications, it is crucial to tune the mechanical and physical properties of a hydrogel in a controllable manner. Here, interpenetrating polymer networks (IPNs) made of a covalently crosslinked network and an ionically crosslinked network were produced to investigate the effective parameters that control the physical and mechanical properties of an IPN hydrogel. Covalently crosslinked polyacrylamide (PAAm) or poly(acrylic acid) (PAA) networks were produced in the presence of alginate (Alg) that was then ionically crosslinked to produce the IPN hydrogels. The effect of ionic crosslinking, degree of covalent crosslinking, AAm : Alg and AA : Alg ratio on the swelling ratio, tensile properties, indentation modulus, and fracture energy of IPN hydrogels was studied. A hollow cylindrical hydrogel with gradient mechanical properties along its length was developed based on the obtained results. The middle section of this hydrogel was designed as a pH triggered artificial muscle, while each end was formulated to be harder, tougher, and insensitive to pH so as to function as a tendon‐like material securing the gel muscle to its mechanical supports. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2504–2513, 2013     

Semi-1 thermoplastic interpenetrating polymer networks of physically crosslinked poly(n-butyl acrylate) and polystyrene

Poly(n-butyl acrylate) (PnBA) chemically crosslinked with tetraethylene glycol dimethacrylate (TEGDM) and physically crosslinked PnBAs produced by neutralization of poly(n-butyl acrylate-stat-acrylic acid) with NaOH or Ca(OH)₂ were prepared as a polymer I network. Each polymer I was swollen with styrene and cured in situ into semi-IPN-TEGDM, semi-IPN-Na, or semi-IPN-Ca, respectively. Both physically crosslinked polymers maintained their shapes during the swelling procedure. Dynamic mechanical spectroscopy indicated that good mixing of the two polymers took place in the semi-IPN-Ca as well as in semi-IPN-TEGDM, but a distinct phase separation occurred in the semi-IPN-Na. These results were supported by their transparent or optical opaque appearances, respectively. Annealing at 180°C developed further phase separation in the semi-IPN-Na, but very little in the semi-IPN-Ca. Analyses by the incompatibility number (based on the modulus–temperature curve) and the calculation of individual phase compositions (from the glass transition temperature shifts) were used in estimating the extent of molecular mixing.     

Castor oil based interpenetrating polymer networks. II. Synthesis and properties of emulsion polymerized products

Polystyrene Latexes were synthesized using sodium ricinoleate (the ehief saponification product of castor oil) as the surfactant. Later sulfur, more sodium ricinoleate, and sometimes castor oil were added, and the emulsion heated to a temperature where the sulfur vulcanized the castor oil products, making a semi-interpenetrating polymer network. Stress-strain studies showed the presence of a well developed yield point and high elongation for some samples, indicating considerable toughening for slow rates of strain. Electron microscopy revealed a complex two-phased morphology. Usually polystyrene was the continuous phase. The rubbery phase domain size depended upon the amount of castor oil products added lzod impact strengths showed only modest improvements; probably because of the high glass transition temperature of the castor oil vulcanizate.     

Capillary extrusion of composite materials

Investigations on the extrusion characteristics of composite systems were performed on the Sieglaff-McKelvey capillary rheometer, with particular emphasis on the characterization of flow instability and “melt fracture” phenomena. The mechanisms of melt fracture appear to be identical for both the filled and unfilled polymers (1. Polyethylene with glass beads; 2. Ethylene-propylene copolymer with graphitized carbon black). In all cases, the flow curves exhibit a plateau at some value of the shear stress. Above this shear stress plateau, melt fracture occurs. Although slip flow is the dominant mode of transport during melt fracture, the slippage in the tube may not be a necessary condition for the subsequent severe melt fracture.     

Acronal-polymethyl acrylate interpenetrating polymer networks

Four interpenetrating polymer networks were prepared by swelling crosslinked Acronal (a copolymer of styrene and butyl acrylate) with methyl acrylate plus crosslinking agent and then polymerizing the methyl acrylate in situ. Certain properties of the constituent network materials, plus the interpenetrating polymer networks which contained 70, 50, 35 and 25% by weight of polymethyl acrylate, were investigated. Electron microscopy showed the interpenetrating polymer networks to be two-phase materials with the polymethyl acrylate domain size increasing with increasing polymethyl acrylate content. Longitudinal sonic velocity measurements indicate that at around 50% by weight of polymethyl acrylate both phases become continuous while dynamic mechanical spectroscopy leads to the view that the constituent networks were not extensively mixed.     

Elastomeric domain-type interpenetrating polymer networks

Elastomeric latex interpenetrating polymer networks (IPNs) can result from a two-stage emulsion polymerization procedure in which styrene is polymerized and cross-linked on a lightly cross-linked polyacrylate (PA) seed latex in a ratio of 75 : 25 PA-PS. The multiphase nature of these IPNs is indicated by two distinct T_gs and is confirmed by cold-stage transmission electron microscopy and by the unique mechanical and rheological properties that are intimately related to the material's structure. PS microdomains reinforce the elastomeric PA, yielding a significant modulus, and interparticle PS physical ties yield a significant ultimate tensile strength. The elastomeric latex IPN's dual thermoset-thermoplastic nature is revealed in a stick, slip, roll flow mechanism of the cross-linked submicrometer particles, which can be injection molded as a thermoplastic. The relationships among the polymerization procedure, the structure, and the physical properties are characterized by the examination of several different materials using a variety of analytic techniques.     

Design and synthesis of conducting secondary crosslinked interpenetrating polymer network

A new conducting composite of a secondary crosslinked interpenetrating polymer network (IPN) was first designed and synthesized by chemically incorporating a rigid conducting polymer within the flexible crosslinked network and forming the link between them. The new conducting composite produced with a low content of polypyrrole exhibited unusually good conductivity, processability, and mechanical properties, and the conductivity was not influenced by the formation of an IPN. © 1997 John Wiley & Sons, Inc.     

Thermodynamic characterization of interpenetrating polymer networks

Crosslinked poly(methyl methacrylate) (PMMA-c), poly(carbonate-urethane) (PCU-c), poly(vinyl pyridine) (PVP-c), and full, simultaneous interpenetrating polymer networks (IPNs) based on the above polymers were characterized by precise heat capacity (C_p) measurements in the temperature interval 4.2–450 K. The raw values of C_p scaled with temperature (T) as C_p ∼ T^d with d = 2 and 5/3, as expected for a fracton-like vibration regime, in the temperature intervals 8–10 and 10–30 K, respectively. A single glass transition temperature (T_g) and two T_g's were observed for apparently homogeneous and microphase-separated IPNs, respectively. Judged by the positive sign of the excess Gibbs free energy, the apparently single-phase state of homogeneous IPNs is thermodynamically unstable; however, its kinetic stability is ensured by permanent topological constraints (network junctions) prohibiting the incipient phase separation.     

Preparation of macrocellular PU-PS interpenetrating networks

Macromolecular monoliths were synthesised from concentrated emulsions. Matrixes with only a polyurethane network were too soft and it was necessary to add to the formulation a rigid network such as polystyrene to obtain a material with a good dimensional stability. Either unconnected or interconnected interpenetrating networks were prepared, the later by using hydroxybutyl methacrylate as a comonomer that chemically links both networks. The modifications of the mechanical properties were evaluated by estimating the Young's modulus from compression tests.     

High-quality conductive polypyrrole within a secondary crosslinked interpenetrating polymer network

A high-quality conductive polypyrrole was prepared within a secondary crosslinked interpenetrating polymer network by chemical oxidative polymerization, whose conductivity is high up to 0.25 S/cm only with 0.9% polypyrrole. The “microfiber” structure of polypyrrole was characterized by scanning electron microscopy, polarizing microscopy, and small angle X-ray diffraction. The formation of the microfiber was strongly dependent on the structure of the matrix. This might provide a new method for the preparation of conductive microfibers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1–4, 1997     

Recent studies on interpenetrating polymer networks

Recent investigations on interpenetrating polymer networks (IPNs) have included two component IPNs from polyurethanes and poly(methacrylates) and two component IPNs from polyurethanes and epoxies. All the IPNs were prepared by the simultaneous polymerization technique (SIN-IPNs). Two types of IPNs, polyurethane-poly(methyl methacrylate) (PU/PMMA) and polyurethane-poly(methyl methacrylate-methacrylic acid) (PU/PMMA-MAA) were prepared. Improved phase miscibility and decreasing extent of phase separation was observed in both types of IPNs with increasing the NCO/OH ratio, decreasing molecular weight of the polyol in the PU and introduction of charge groups. A comparison was made between full-IPNs, pseudo-IPNs, graft copolymers and related homopolymers from polyurethanes and epoxies. Increased compatibility in full-IPNs and graft copolymers was observed by means of DSC, SEM and was also further substantiated by a shift toward single T_gs as determined by dynamic mechanical spectroscopy. The introduction of opposite charge groups in two-component IPNs from polyurethanes and epoxies led to improved compatibility (no phase separation) and enhanced mechanical properties.     

Recent advances in interpenetrating polymer networks

Interpenetrating polymer networks (IPN's) can be defined as a combination of two polymers in network form, at least one of which was synthesized and/or crosslinked in the immediate presence of the other. Historically, the science of IPN's began with the papers of J. R. Millar in 1960 on homo-IPN's made from polystyrene, but the first recorded publication is a patent by J. W. Aylsworth in 1914. This latter system was based on phenol-formaldehyde for one network, and sulfur cured natural rubber for the other network. Early academic laboratories interested in IPN's include the Frisch team at Detroit and SUNY, who soon added their former student, Danny Klempner, and Yuri Lipatov's team at the Ukranian SSR Academy of Sciences in the USSR, as well as the author's laboratory. More recent academic teams interested in IPN's include Douglas Hourston at the University of Lancaster, England; Robert Cohen at MIT; S. C. Kim at the Korea Advanced Institute of Science and Technology, Seoul, Korea; G. Meyer and J. M. Widmaier in Strasbourg, France; and many others. Numerous industrial laboratories are interested, noting that about 90 U.S. patients have been granted, most of them in the past ten years. Systems of special interest include the new thermoplastic IPN's, which are really hybrid materials between polymer blends and IPN's, and the IPN-based RIM (reaction injection molding) materials. Other materials include the sequential IPN's and the SIN's, which have both polymers simultaneously polymerized, and the latex IPN's, which often exhibit core-shell characteristics.     

Recent advances in interpenetrating polymer networks

We review the synthesis, morphology, and physical and mechanical properties of IFNs as well as the related pseudo-IPNs, in which only one of the polymers is crossliriked. Recent studies have shown that the degree of phase separation achieved in these materials is strongly dependent on the compatibility of blends of the linear polymer constituents of the IPN components as well as the kinetics of chain extension and the presence of grafting between component polymers. We illustrate this by a series of IPNs consisting of a polyurethane and an acrylic copolymer. The acrylic is a typical automotive enamel. An enhancement in properties results, which is dependent on the amount of grafting and the kinetics of polymerization. Also discussed are IPNs of a polyurethane and an epoxy, which exhibit a synergism in adhesive properties, and IPNs of a RIM polyurethane with several epoxies and unsaturated polyesters. In addition, also reported are the preliminary studies on the first successful preparation of a three-component IPN, consisting of a polyurethane, an epoxy, and an acrylic.     

Polybutadiene/polystyrene interpenetrating polymer networks

Interpenetrating polymer networks (IPN's) of polybutadiene (PB) and polystyrene (PS) were prepared using both random (containing 36% cis, 55% trans, and 9% 1,2 vinyl) PB and high-cis PB. For both series, a wide range of PB/PS compositions were synthesized. Using samples stained with osmium tetroxide, electron microscope studies revealed an irregular cellular structure of a few hundred Ångstrom diameter with the first component, PB, making up the cell walls. The size of the cells was found to depend on the PB crosslink density for the random materials. Modulus-temperature data revealed two distinct glass transitions, confirming the microscopy finding of two phases. However, the transition temperature and transition slope varied with composition, and with the microstructure of the polybutadiene, giving evidence of significant molecular mixing. Stress-strain data on the IPN's showed that materials rich in PB behave like self-reinforced elastomers. Charpy impact resistance experiments on materials rich in PS indicated values of 5 ft-lb/in. of notch, which compares well with graft-type polyblends of similar PB/PS composition. The results were interpreted in the light of the recent theoretical work of Bragaw, who considered the importance of the distances between domain boundaries with respect to crack acceleration mechanics. Although the IPN's considered herein exhibited somewhat less than the predicted optimum phase dimensions, the arrangement of the domains is different from ordinary impact resistant plastics.     

Structure-Property-Processing relationships of polyurethane-polyester interpenetrating polymer networks

Interpenetrating polymer networks of polyurethane and unsaturated polyester were prepared by reaction injection molding (RIM) and transfer molding. The structures of the molded samples were analyzed by electron microscopy and dynamic mechanical analysis. It was found that polymer morphology and dynamic mechanical properties depend strongly on the molding temperature, reaction rate and reaction sequence. Simplified structure models based on Takayanagi's model and sample morphology can predict the storage modulus reasonably well but not the tanδ.     

Thermodynamic state of reinforced interpenetrating polymer networks

The free energies of mixing of two networks in the interpenetrating polymer network based on crosslinked polyurethane and poly(ester acrylate) have been determined by the vapour sorption method. It was established that the constituent networks in the IPN are not miscible. The introduction of fillers of different chemical nature increases the compatibility. The thermodynamic affinity of the fillers to the individual networks and IPN was estimated. It was established that when the free energy of interaction of one or both components of the IPN with the filler is negative, reinforcement leads to the formation of a compatible and equilibrium system. For fillers having no affinity to the polymers, compatibilization is observed, which is connected with slowing down of phase separation in the system in the presence of filler.