The Nature of IPNs

In this review we deal only with phase-separated IPNs. There are some data about compatible IPNs, but a more typical and much more interesting situation arises when we consider superposition of chemical and physical processes, both having a mutual influence on one another. 

After analyzing over many years the development of science, we have come to the conclusion that many up-to-date results often repeat what was already known, although accounting for the modernized or new concepts. We think we have no right to forget our predecessors, and hence in this review we give references to some rather old works, where the main principles were formulated. 

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Characterisation of Glass Transition Behaviour in Interpenetrating Polymer Networks The multi-phase nature of IPNs results in complicated glass transition behaviour [101]. Figure 3.46 shows that heat capacity changes with temperature for a series 60 40 polyurethane (PU)/ polystyrene (PS) IPNs (see Table 3.5 for the compositional details) [131,132]. It is, however, not possible to obtain much detailed information from these heat capacity signals. 

While most of the work described in this monograph emphasizes the two-phased nature of IPNs and related materials, it is interesting to explore more deeply the characteristics of phase separation in polymer/polymer systems. Of key importance, McMaste/ " showed that most polymer/polymer phase diagrams are expected to exhibit a lower critical solution temperature (LOST). This means that as the temperature is raised the polymer pair becomes less mutually soluble, and phase separates. This effect is not immediately predicted by equations (2.1) and (2.2), which suggest the usual phenomenon of an upper critical solution temperature (UCST). 

While stress-strain and impact tests are destructive in nature, electrical insulation and permeability measurements usually are not. This section will discuss the behavior of IPNs in these key engineering applications. 

Some applications of IPNs of note include sound and vibration damping, biomedical applications, natural products and renewable resources, tough and impact-resistant materials, etc. IPN review articles include those by Sperling and Hu (2), Lipatov (3), and Athawale and co-workers (4). There are also papers of note by Srivastava and co-workers (5), and edited books by Klempner and coworkers (6), and Kim and Sperling (7). Two older books review early references, and develop the concepts of IPNs (1,8). 

The nature of the co-continuous phases in the polypropylene S-EB-S blends leads to a generalization of the specific function of the block copolymer in systems like this. The now-classical model of this characteristic phase structure is that of the chemical IPNs where each network is actually a phase which is constrained or condensed by a mechanism which has infinite relaxation time and where three dimensional spatial continuity of each phase is achieved. This is topologically identical to those IPNs which are achieved by sequential polymerization and crosslinking within a pre-existing network structure or the... 

A novel interpenetrating polymer network (IPN) of polyethylene glycol, polyvinyl alcohol and polyacrylamide is prepared and its potential for sorption and delivery of diatase, a digestive enzyme, is evaluated. The effects of experimental parameters such as varying chemical composition of the IPN, percent loading of diatase, pH and temperature of the release medium and molecular weight of PEG are investigated on the release dynamics of the diatase. On the basis of Pick s equation, the diffusional exponent (n) and diffusion (D) are evaluated for different IPN compositions. From the kinetic parameters data, an attempt is made to explore the nature of the mechanism of the release process of diastase. The IPNs are characterised by IR and examined for zero-order release behaviour of loaded enzyme. 39 refs. 

Therefore, nowadays researchers pay more attention to reclaimed interpenetrating rubber. The main reason for this is the stable nature of IPNs in crosslinked rubbers and the strict specifications regarding the quality of products. The disposal problem of crosslinking rubber rejects and possible solutions are available. Another method to reuse large volumes of scrap IPN rejects is to use it as an impact plastic such as PS. 

Natural rubber blends have not been substantially limited to only reactive blends, such as those produced by grafting, epoxidation and halogenation studies. IPNs are one choice for modification of the natural rubber to increase the compatibility between natural rubber and acrylate polymers. Full IPNs of natural rubber and acrylate polymers are defined as a polymer blend having a crosslinked polymerization of each polymer type in their networks. Semi-IPNs, in contrast, are those in which only one type of polymeric component, mostly acrylate polymer, is crosslinked. The polymerization of IPNs would be a potential method for the modification of natural rubber. ... 

Reports in the literature on the transport of penetrant molecules through NR based blends and IPN s are few when compared to the existing literature on most common/commercial polymers/blends. The sorption and diffusion of aromatic penetrants into different NR blends such as NR/BIIR, NR/CIIR, NR/neoprene, NR/EPDM, NR/polybutadiene, and NR/SBR were studied by Siddaramaiah et al. The diffusion coefficient (D) of the penetrants was found to range from 6.8 to 84.3 x 10 cm /s at a temperature range of 25-60 °C. Results indicated that the transport data were affected by the nature of the interacting solvent molecule rather their sizes, and also by the structural variations of the elastomers blended with NR. The activation parameters for the diffusion of the penetrants ranged from 4.16 to 30.48 kJ/mol. 

Now, if we apply the model approach proposed above, we shall see that the main effect on the effective cross-linking density in semi-IPNs is produced by the kinetic conditions and the nature of the components. Changing reaction conditions leads to the formation of the more defective structures of networks as compared with the pme network. Thus, the effective network density is determined not only by the theoretical topology of the network but also by the reaction conditions. It is worth noting that the formation of IPNs imder phase-separated conditions enhances the formation of the defective network because phase separation leads to the formation of two phases and of a transitional region, which may be considered as an independent IPN with its own composition and cross-linking density. 

Minssen, M., Walgenbach, W. (1985) Naturstoffe, Kunststoffe und das Makro-molekulkonzept [Natural and Synthetic Polymers and the conception of Macromolecules]. Kiel IPN. 

As can be seen from all of these works, sequential IPNs have been extensively used with silicone with various crosslinking reactions and polymers. As a result, attention must be paid to the influence of processing conditions, of the chemical nature of components (silicone and the other polymer), of the evolution of the material at each step of the IPN synthesis etc. Depending on the mastering of these parameters, reactivity, morphology and entanglement can be controlled and consequently targeted properties can be reached. 

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