Interpenetrating polymer network thermodynamics

This is a theoretical study on the entanglement architecture and mechanical properties of an ideal two-component interpenetrating polymer network (IPN) composed of flexible chains (Fig. la). In this system molecular interaction between different polymer species is accomplished by the simultaneous or sequential polymerization of the polymeric precursors [1 ]. Chains which are thermodynamically incompatible are permanently interlocked in a composite network due to the presence of chemical crosslinks. The network structure is thus reinforced by chain entanglements trapped between permanent junctions [2,3]. It is evident that, entanglements between identical chains lie further apart in an IPN than in a one-component network (Fig. lb) and entanglements associating heterogeneous polymers are formed in between homopolymer junctions. In the present study the density of the various interchain associations in the composite network is evaluated as a function of the properties of the pure network components. This information is used to estimate the equilibrium rubber elasticity modulus of the IPN. 

Within the thermodynamic framework proposed, only one reaction has been taken into account. But, generally, the modifier can also react with the matrix. Besides, the introduction of a second reaction quite different from the first one, for example chain and step reactions, is essential for modelling more complex systems like interpenetrating polymer networks. 

Utracki, L.A. (1994) Thermodynamics and kinetics of phase separation, in Interpenetrating Polymer Networks, (eds D. Klempner, L.H. Sperling, and L.A. Utracki), American Chemical Society, Washington, DC, pp. 77-123. 

Yu. S. Lipatov, L. M. Sergeeva, L. V. Karabanova, A. E. Nesterov, and T. D. Ignatova, Thermodynamic and Sorption Properties of Interpenetrating Polymer Networks Based on Polyurethane and a Styrene-Divinylbenzene Copolymer, Vysokomol. Soedin. Ser. A. 18(5), 1025 (1976). Transition layer studies of PU/PS IPNs. IPN heat of mixing. 

The data obtained were used to calculate enthalpy, entropy and the Gibbs functions for the seq-IPNs synthesis. It was shown that the isotherms of diverse thermodynamic properties of interpenetrating polymer networks plotted versus their composition, in particular the molar fraction of the CPU per conditional mole, can be described by straight lines. This made it possible to estimate the thermodynamic behavior of the seq-IPNs of any compositions at standard pressure within a wide temperature range. It was determined [50] that at molar content > 0.50 of PCN in seq-IPNs studied AG°p (AG° of process) < 0 and this has allowed authors to conclude about thermodynamical miscibility of the components for seq-IPNs of these composition... 

Eleonora G. Privalko - Senior researcher. Department of Polymer Thermophysics (DePTh), Institute of Macromolecular Chemistry, National Academy of Sciences of Ukraine. She graduated in Polymer Chemistry from the Shevchenko State University (Kyiv) and got her PhD (Polymer Chemistry) in the Institute of Macro-molecular Chemistry, National Academy of Sciences of Ukraine (1989). Areas of research thermodynamic properties and mechanical performance of heterogeneous polymer materials (filled polymers, polymer blends, interpenetrating polymer networks, polymer nanocomposites). Visiting positions NATO Research Fellow, National Technical University of Athens, Greece. Publications over 50 papers in refereed journals. 

Lipatov Y, Karabanova L and Sergeeva L (1994) Thermodynamic state of reinforced interpenetrating polymer networks, Polym Inf 34 7-13. 

The information available on aqueous polymer blends is qualitative in nature because of the lack of a suitable theory to interpret the experimental observations. Mixed gels can be comprised of an interpenetrating network, a coupled network (as discussed above), or a phase-separated network [2]. The latter is the most common as the blends have a tendency to form two phases during gelation. In such cases the miscibility and thermodynamic stability have to be empirically investigated and proper conditions for miscible blends identified. This involves a phase diagram study as is described in [3]. 

An interpenetrating network (IPN) can be defined as a mixture of two crosslinked polymers when at least one of them is synthesized and (or) crosslinked with another [114]. The components that make up an IPN are thermodynamically incompatible and a transition region of two phases is formed in such a system. The whole complex of IPN properties is determined by the availability and features of this region. 

Obtaining interpenetrating networks (IPNs) is in principle a new method of blending nomnelting and insoluble three-dimensional polymers. IPNs are known to be characterized by a number of thermodynamic and physical-mechanical peculiarities. Application of IPNs based on crosslinked polymers of various chemicsd compositions should produce adhesives with a wide range of properties. 

The advantages of full-interpenetrate networks preparation are related to IPN having the properties of both polymers and producing synergistic effect the thermodynamic incompatibility of components can be overcome due to the tight interaction of both polymeric chains, improved mechanical strength and biocompatibility acquired from collagen [63]. 

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