Chains entanglements between

The interconnection between the two different polymers can cause some essentially thermoplastic polymers to behave as thermosets when combined in an IPN. This arises because of chain entanglement between different polymers. The entanglements behave as crosslinks at lower temperatures but they can be broken by heat. 

Now, let us discuss why those chains with a high content of hydrophobic MACA form smaller aggregates. Picarra and Martinho [143] showed that in the phase separation of a thin-layer dilute homopolymer solution on the surface, the collision would not be effective as long as the collision (or contact) time (rc) is shorter than the time (re) needed to establish a permanent chain entanglement between two approaching aggregates. Quantitatively, Tanaka [144] showed that rc and re could be roughly characterized as... 

Figure 9.2 Main elements constituting the structure of a polymer network (1) crosslink point, (2) elastically active chain, (3) dangling chain, (4) loop or cycle, (5) multiple connection between two crosslink points, and (6) permanent chain entanglements between two adjacent crosshnks.

Considering the competition between intrachain contraction and interchain association, we have to discuss an overlooked viscoelastic effect in the formation of stable mesoglobules in dilute solutions. Otherwise, it would be difficult to understand why copolymer chains with a high content of hydrophobic comonomers could form smaller interchain aggregates. In the micro-phase separation, copolymer chains in solutions contract and associate. The collision between contracted and associated chains would not be effective if the collision (or contact) time (tc) is much shorter than the time (te) needed to establish a permanent chain entanglement between two ap-... 

The poled and cured film of the polyimide/ASD has NLO coefficient, d33, of 28 pmAf-This d33 value is comparable to that of lithium niobate. The temporal stability at 25 and 120 °C of the second-order nonlinearity after poling and curing of the polyimide/ASD is demonstrated in Figure 3. The sample shows excellent temporal stability at room temperature. In contrast to the stability of a poled/cured ASD sample which showed a rapid decay of the d33 value at 100 °C,20 the poled/cured polyimide/ASD sample shows a stable d33, after an initial reduction of 27%, as it is subjected to a thermal treatment at 120 °C for over 168 h. The existence of polyimide in the ASD network shows a tremendous improvement in the temporal stability. This synergistic property is a result of high Tg of polyimide and the chain entanglement between the two polymers in the composite. 

The major drawback of cellulose fibers in the present context resides in their highly polar and hydrophilic character, which make them both poorly compatible with commonly used non-polar matrices, such as polyolefins, and subject to loss of mechanical properties upon atmospheric moisture absorption. That is why they should be submitted to specific surface modifications in order to obtain an efficient hydrophobic barrier and to minimize their interfacial energy with the often nonpolar polymer matrix, and thus generate optimum adhesion. Further improvement of this interfacial strength, which is a basic requirement for the optimized mechanical performance of any composite, is attained by chain entanglement between the matrix macromolecules and the long chains appended to the fiber surface (brushes) or, better still, by the establishment of a continuity of covalent bonds at the interface between the two components of the composite. 

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

The interdiffusion of polymer chains occurs by two basic processes. When the joint is first made chain loops between entanglements cross the interface but this motion is restricted by the entanglements and independent of molecular weight. Whole chains also start to cross the interface by reptation, but this is a rather slower process and requires that the diffusion of the chain across the interface is led by a chain end. The initial rate of this process is thus strongly influenced by the distribution of the chain ends close to the interface. Although these diffusion processes are fairly well understood, it is clear from the discussion above on immiscible polymers that the relationships between the failure stress of the interface and the interface structure are less understood. The most common assumptions used have been that the interface can bear a stress that is either proportional to the length of chain that has reptated across the interface or proportional to some measure of the density of cross interface entanglements or loops. Each of these criteria can be used with the micro-mechanical models but it is unclear which, if either, assumption is correct. 

Recently, Tong et al. [195] have shown that in methyl methacrylate-b-alkyl acrylate-b-methyl methacrylate (MAM) and SIS-type triblock copolymers, the ultimate tensile strength is inversely proportional to the molecular weight between the chain entanglements in the middle soft block at comparable proportion of the outer block. 

Tong J.D. and Jerome R., Dependence of the ultimate tensile strength of thermoplastic elastomers of the triblock type on the molecular weight between chain entanglements of the central block. Macromolecules, 33, 1479, 2000. 

Gelation involves an extended structure and some type of linking between chains. The concept of salt-like crosslinks has already been described (Section 5.5). Other possibilities may be considered. Hill, Wilson Warrens (1989) examined the possibility that chain entanglements might account for the strength of polyelectrolyte cements. They used in particular... 

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