The drying of industrially relevant latex dispersions designed for use as pressure-sensitive adhesives (PSAs) was followed using Forster resonance energy transfer (FRET) and scattering intensity as indicators for the progress of film formation. FRET and scattering intensity report the state of polymer interdiffusion and of particle deformation, respectively. Because the exciting UV-radiation only penetrated a few micrometers deep into the film, FRET measurements undertaken from the top and the bottom yielded different results. The combination of the two evidenced skin formation. Particle deformation occurs in two steps. There is a significant, but incomplete, decrease in turbidity because of skin formation. Only after the drying front has propagated to the substrate, the top layer turns fully clear. This second step is interpreted as coalescence, meaning the breakup of lamellae separating particles. Coalescence is followed by a sharp increase in interdiffusion. Further aspects studied included crosslinking, hydroplasticization, and tackifying resins.

The impact of ionic polymer cross-linking on the film formation kinetics of waterborne dispersions designed for use as acrylic pressure-sensitive adhesives (PSAs) was studied with Forster resonance energy transfer (FRET). Aluminum acetylacetonate (Al(acac) 3 ) was added as an ionic cross-linker to a latex with un-cross-linked chains. The polymer interdiffusion (studied with FRET) suggests that the degree of polymer cross-linking in the wet dispersion is small, as the interdiffusion kinetics is similar to that of a latex with un-crosslinked chains only. Cross-linking mainly takes place when the film dries, and it is slower than interdiffusion; thus, a homogeneous film can be formed. Ionic cross-linking in the final film through Al 3+ was proven by a large gel content and an increased cohesion, the latter evaluated with tensile and tack tests. Further aspects studied include the influence of serum pH on interdiffusion and cross-linking reaction and how ionic crosslinking before film formation affects interdiffusion.

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Interfacial separation of soft, often viscoelastic, materials typically cause the onset of instabilities, such as cavitation and fingering. These instabilities complicate the pathways for interfacial separation, and hence hinder the...

Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability – the critical pressure – is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multi-material composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface) we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.