Interfacial chain entanglements

Fig.4.a A perfectly flat wall in presence of sufficient polymer adsorption and interfacial chain entanglements, b An entangling melt under high stresses (o>oc) in contact with a molecularly smooth wall, where the adsorbed (thick) chains undergo a coil-stretch transition and the unbound chains are no longer in entanglement with the tethered chains. Here the first layer of adsorbed chains is stagnant, as the unbound chains flow by. 

The molecular meaning of b is best seen from the second or third equality of Eq. (3). In other words, b is explicitly related to the steady shear melt viscosity q and depends on the chain-chain interactions near the melt/wall interface as quantified by the friction coefficient p. In the limit of no polymer adsorption or in absence of interfacial chain entanglements due to the coil-stretch transition, P involves an interfacial viscosity q , which is as small as the viscosity of a monomeric liquid and independent of the molecular weight Mw p=qj/a, where a is a molecular length. Thus at the stick-slip transition, the molecular weight dependence of b arises entirely from q in Eq. (3). 

The second problem is that the experimental data indicating a presence of interfacial regions should be carefully considered, avoiding a misinterpretation. In addition to manifestation of a real interface, which is formed due to chain entanglements of the components or a presence of special additives, one should be aware about effects related to tip geometry and a complex local... 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Interfacial adhesion is the adhesion in which interfaces between phases or components are maintained by intermolecular forces, chain entanglements, or both, across the interfaces. Interfacial adhesion between rubber and PMMA must be sufficient to permit the effective transfer of stress to the rubber particles and also to provide multiple sites for crazing and localized shear yielding for effective impact energy dissipation. 

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. 

The interfacial thickness. Lb, for crystallites formed in dilute solution is about 10 A, independently of the molecular weight [226]. Under these crystallization conditions, chain entanglements are minimal and there is no significant chain-mobility restraint on the process of crystallization. In addition to the interface, there is also a substantial disordered overlayer associated with crystals formed in solution [227]. Thus it is not surprising that results obtained using many different experimental methods indicate that crystals formed in dilute solution are only 85 %-90% crystalline [3]. They are not completely crystalline. 

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