Polymers network

An interpenetrating polymer network, IPN, is defined as a blend of two or more polymers in a network form, at least one of which is synthesized and/or crosslinked in the immediate presence of the other(s). An IPN can be distinguished from polymer blends, blocks, or grafts in two ways (1) an IPN swells, but does not dissolve in solvents, and (2) creep and flow are suppressed. 

Latex IPN. The polymers are made in the form of latexes, each particle constimting a micro-IPN. Depending on the rates of monomer addition relative to the rates of polymerization, various degrees of interpenetration and/or core/shell morphologies may develop. There are several kinds of latex IPNs, see Section 6.4. 

Gradient IPN. In this case, the overall composition or crosslink density of the material varies from location to location on the macroscopic level. One way of preparing these materials involves partial swelling of polymer network I by the monomer II mix, followed by rapid polymerization before diffusional equilibrium takes place. Films can be made with polymer network I predominantly on one surface, and polymer network II predominantly on the other surface with a gradient composition existing throughout the interior. 

Thermoplastic IPN. When physical crosslinks rather than chemical crosslinks are utilized, the materials may flow at elevated temperatures. As such, they are hybrids between polymer blends and IPNs. Such crosslinks commonly involve block copolymers, ionomers, and/or semicrystallinity. 

Semi-IPN. These are compositions in which one or more polymers are crosslinked, and one or more polymers are linear or branched. 

So far we have discussed the striking effects of the noncrossability of polymers for uncrosslinked polymer melts and for solutions in the free draining 

Here we will discuss three groups of simulation. In all cases we consider only systems in which the role of the conserved topology is explicitly taken into account. First we discuss some work on short chain networks, where the crosslinks are fixed in space. These simulations were used to investigate to what extent the entropic spring concept, which is the basis of the theory of elasticity of networks, is valid in systems in which the excluded volume interactions are present. We then review some and 

This chapter will discuss the synthesis, morphology, and mechanical behavior of IPN s. Related materials, in particular the interpenetrating elastomeric networks (lEN s) of Frisch (Frisch and Klempner, 1970 Klempner et a/., 1970 Matsuo et a/., 1970h) will be included for comparison. 

The synthesis of an IPN first requires a crosslinked polymer I, which may be either the elastomer or plastic component. In the first papers on IPN s, the elastomer was the first component. When the plastic was component I, the term inverse IPN was employed on an arbitrary basis. 

With both the PEA/P(S-co-MMA) and PB/PS IPN s, an important variable is the ratio of elastomer to plastic in the final material. When the plastic component predominates, a type of impact-resistant plastic results. In this manner the PB/PS IPN s are analogous to the impact-resistant graft copolymers. When the elastomer component predominates, a self-reinforced elastomer results, the behavior resembling that of the ABA-type block copolymers (thermoplastic elastomers) described in Section 4.4. When the overall compositions of both the PB/PS and the PEA/P(S-co-MMA) series are close to 50/50, the materials behave like leathers. 

Although both polymeric networks in IPN s may be visualized as continuous, their milky appearance hints at a more complex morphology. In order to characterize the morphology, the elastomeric portion of the IPN s was stained selectively using osmium tetroxide (see Section 2.4) to facilitate staining, about 1% of butadiene was incorporated as a comonomer in the PEA. This special product was designated PEAB. 

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A different class, in between polymer lattices and polymer solutions, is tliat of microgels, consisting of weakly crosslinked polymer networks. Just as for polymer solutions, small changes in tire solvency conditions may have large... 

CHEOPS is based on the method of atomic constants, which uses atom contributions and an anharmonic oscillator model. Unlike other similar programs, this allows the prediction of polymer network and copolymer properties. A list of 39 properties could be computed. These include permeability, solubility, thermodynamic, microscopic, physical and optical properties. It also predicts the temperature dependence of some of the properties. The program supports common organic functionality as well as halides. As, B, P, Pb, S, Si, and Sn. Files can be saved with individual structures or a database of structures. 

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

Microreticular Resins. Microreticular resins, by contrast, are elastic gels that, in the dry state, avidly absorb water and other polar solvents in which they are immersed. While taking up solvent, the gel structure expands until the retractile stresses of the distended polymer network balance the osmotic effect. In nonpolar solvents, little or no swelling occurs and diffusion is impaired. 

Polymers with the mechanical and chemical properties we have discussed in this section are called elastomers. In the next couple of sections we shall examine the thermodynamic basis for elasticity and then apply these ideas to cross-linked polymer networks. 

Equation (5.47) is of considerable practical utility in view of the commercial importance of three-dimensional polymer networks. Some reactions of the sort we have considered are carried out on a very large scale Imagine the consequences of having a polymer preparation solidify in a large and expensive reaction vessel because the polymerization reaction went a little too far Considering this kind of application, we might actually be relieved to know that Eq. (5.47) errs in the direction of underestimating the extent of reaction at... 

S. netropsis Nettle extract Network analysis Network formation Network polymer Networks... 

Peroxides or other additives, eg, chlorinated paraffin, may also cause the thermoplastic resin to cross-link with the siloxanols. In this case, a tme interpenetrating polymer network forms, in which both phases are cross-linked. 

Gels are viscoelastic bodies that have intercoimected pores of submicrometric dimensions. A gel typically consists of at least two phases, a soHd network that entraps a Hquid phase. The term gel embraces numerous combinations of substances, which can be classified into the following categories (2) (/) weU-ordered lamellar stmctures (2) covalent polymeric networks that are completely disordered (2) polymer networks formed through physical aggregation that are predominantly disordered and (4) particular disordered stmctures. 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

Where the polyurethane comprises <30% of the blend, the polyurethane remains in discrete droplets within the polyacetal matrix. In this range the particle size and particle size distribution of the elastomer particles are of importance. Where the elastomer component is in excess of 30%, interpenetrating polymer networks exist in the sense that there are two interpenetrating continuous phases (as opposed to two cross-linked interpenetrating polymer systems). 

Over the years many blends of polyurethanes with other polymers have been prepared. One recent example is the blending of polyurethane intermediates with methyl methacrylate monomer and some unsaturated polyester resin. With a suitable balance of catalysts and initiators, addition and rearrangement reactions occur simultaneously but independently to give interpenetrating polymer networks. The use of the acrylic monomer lowers cost and viscosity whilst blends with 20% (MMA + polyester) have a superior impact strength. 

Deruelle, M., Tirrell, M., Marciano, Y., Hervet, H. and Leger, L, Adhesion energy between polymer networks and solid surfaces modified by polymer attachment. Faraday Discuss., 98, 55-65 (1995). 

To understand the global mechanical and statistical properties of polymeric systems as well as studying the conformational relaxation of melts and amorphous systems, it is important to go beyond the atomistic level. One of the central questions of the physics of polymer melts and networks throughout the last 20 years or so dealt with the role of chain topology for melt dynamics and the elastic modulus of polymer networks. The fact that the different polymer strands cannot cut through each other in the... 

If this is true, this should hold not only for polymer melts but, in the limit of long chains, also for polymer networks. In the simplest case the elastic properties of polymer networks are entirely governed by the entropic... 

H. E. Mark, B. Erman, eds. Elastomeric Polymer Networks, Englewood Cliffs, NJ Prentice Hall, 1992. 

Sheu and coworkers [111] produced polysty-rene-polydivinylbenzene latex interpenetrating polymer networks by the seeded emulsion polymerization of styrene-divinylbenzene in the crosslinked uniform polystyrene particles. In this study, a series of uniform polystyrene latexes with different sizes between 0.6 and 8.1... 

A wide range of polymer networks are constructed in this manner. Poly(vinyltrichloacetate) was used as the coinitiator with styrene, MMA and chloroprene as cross-linking units. Polycarbonates, polystyrene, N-haloge-nated polyamide, polypeptides, and cellulose acetate, suitably functionalized, have been used as a coinitiator... 

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