Welding of polymers of nonuniform molecular weight

Healing of polymer interfaces: Interfacial dynamics, entanglements, and strength

Self-healing of polymer films often takes place as the molecules diffuse across a damaged region, above their melting temperature. Using molecular dynamics simulations we probe the healing of polymer films and compare the results with those obtained for thermal welding of homopolymer slabs. These two processes differ from each other in their interfacial structure since damage leads to increased polydispersity and more short chains. A polymer sample was cut into two separate films that were then held together in the melt state. The recovery of the damaged film was followed as time elapsed and polymer molecules diffused across the interface. The mass uptake and formation of entanglements, as obtained from primitive path analysis, are extracted and correlated with the interfacial strength obtained from shear simulations. We find that the diffusion across the interface is significantly faster in the damaged film compared to welding because of the presence of short chains. Though interfacial entanglements increase more rapidly for the damaged films, a large fraction of these entanglements are near chain ends. As a result, the interfacial strength of the healing film increases more slowly than for welding. For both healing and welding, the interfacial strength saturates as the bulk entanglement density is recovered across the interface. However, the saturation strength of the damaged film is below the bulk strength for the polymer sample. At saturation, cut chains remain near the healing interface. They are less entangled and as a result they mechanically weaken the interface. Chain stiffness increases the density of entanglements, which increases the strength of the interface. Our results show that a few entanglements across the interface are sufficient to resist interfacial chain pullout and enhance the mechanical strength.

An overview of polymer latex film formation and properties

The literature on polymer latex film formation has grown enormously in recent times--driven by the need to find alternatives for solvent-based systems with their adverse environmental impacts. Although greater insight has been shown by the use of modern instrumental techniques such as small angle neutron scattering, direct non-radiative energy transfer and atomic force microscopy, the actual mechanisms involved in deforming spherical particles into void-free films are still the subject of controversy and debate. Surfactant-free homopolymer model colloid latices, favoured in academic studies, together with latices containing surfactants whose redistribution can influence film properties, and the more complex copolymer, blended, core-shell and pigmented systems needed to satisfy a full range of film properties are all considered.