Cross-linked polymers chain entanglement

In contrast to network copolymers, interpenetrating polymer networks (IPNs) (2, 38) are generally composed of two or more cross-linked polymers with no covalent bonds between them. They are often Incompatible polymers in a network form, and at least one is polymerized and/or cross-linked in the immediate presence of the other with the result of a complete entanglement of the chains. 

Moreover, such a hnear relationship tends to demonstrate that the actual cross-linking degree of the APCNs is only dependent on the initial molar fraction in PCLDMA crosshnkers introduced in the polymerization medium and therefore attests for the absence of additional (physical) cross-links that could be due to polymer chain entanglements. Therefore and owing to the control over the molecular parameters of the APCNs, a correlation between gel stracture and swelling behavior could be predicted from the Flory-Rehner relationship in the particular case of this study with a constant composition in PCL of about 30%. 

A variety of polymers matrices and gel types have been used—ranging from cross-linked polymer (polyacrylamide-bisacrylamide) to linear polymers such as polyacrylamide, hydroxycelluloses, polyvinyl alcohol and dextrans. This later class can be regarded as physical gel formed by the entanglement of the polymer chains. With cross-linked polymers the polymerisation is commonly carried out in sites while for linear polymers it is possible to change the capillary with a solution of polymer. 

Hydrogels are physically or chemically cross-linked polymer networks swollen with large amounts of water. Due to their crosslinked nature, these gels do not dissolve in aqueous media but contain an enormous amount of solvated water molecules within the entangled polymer chain matrix. Hydrogel properties are reviewed elsewhere in this book. This chapter is dedicated to a unique hydrogel family that responds to... 

Bond distributions can be analyzed for any molecular architecture. The notion of bonds can be taken in its widest meaning to comprise weakly bound solvation networks, molecular meshes, multiple entangled chains (including double-stranded DNA), and cross-linked polymers. Hydration clusters... 

With regard to the mechanical reactirai of a polymer network to a stress applied, it is important that loose ends of macromolecules in a network structure are as shmrt as possible and/or their concentration is low. As these ends mostly extend out of the lamellas of crystallites then, while crossUnking is taking place in an amorphous phase and with the simultaneous presence of crystallites, a network with small loose ends should be formed. The crosslink junctions stabilize the natural molecular network (entanglements and crystallites), and every chain in the system is potentially elastically operative and can contribute to the stress in a tensile experiment [33]. The stabilization effect of chemical crosslinks on entanglements and crystallites may be the direct cause of observed differences in the determination of the amount of chemical crosslinks from mechanical property measurements and sol-gel analysis of the cross-linked polymer. 

If there are two or more entanglement points per molecule, a type of network is set up. In order to flow, a molecule needs to pull neighboring molecules with which it is entangled. These molecules, in turn, may be coupled with others and so on throughout the system. When a stress is imposed on such a system, there is a resistance to flow so that the system resembles a cross-linked polymer. However, it is different from a cross-linked system in that, if the stress is maintained, slippage of the chains... 

It is well known that thermoset polymers such as epoxy, vinyl ester, polyester, phenolic, etc., are chemically or physically cross-linked polymers (chemical bonds between polymer chains, intermolecular van der Waals bonds, dipole-dipole interactions, and molecular entanglement). These cross-links serve as molecular anchorages, which prevent molecular motion of the... 

Figure 2.10 Six basic modes of linking two or more polymers are identified (20). (a) A polymer blend, constituted by a mixture or mutual solution of two or more polymers, not chemically bonded together, (b) A graft copolymer, constituted by a backbone of polymer I with covalently bonded side chains of polymer II. (c) A block copolymer, constituted by linking two polymers end on end by covalent bonds, (d) A semi-interpenetrating polymer network constituted by an entangled combination of two polymers, one of which is cross-linked, that are not bonded to each other, (e) An interpenetrating polymer network, abbreviated IPN, is an entangled combination of two cross-linked polymers that are not bonded to each other, (f) AS-cross-linked copolymer, constituted by having the polymer II species linked, at both ends, onto polymer I. The ends may be grafted to different chains or the same chain. The total product is a network composed of two different polymers.

In practise, aqueous systems containing synthetic polymers of this type exhibit complex behaviour that shows elements of both these effects. Consequently, the term viscoelastic is commonly used when referring to such systems. These effects arise in the entangled mass of chains present in a true solution of polymer, as well as in the interlinked polymer chain network present within the water swollen micro-particles of cross-linked polymers. 

A discussion on polymer blends cannot be considered complete without a reference to inteipmietrating polymer networks (EPNs). These form a separate class of polymer blends of a novel type composed of cross-linked polymers. They are more or less intimate mixtures of two or more distinct cross-linked polymer netwoiks with no covalent bonds between the polymers. Hius, polymer A cross-links only with other molecules of polymer A, and polymer B does likewise. In other words, IPNs may be described as combinations of chemically dissimilar polymers in which chains of one are completely and permanently entangled with those of the other. 

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