Hybrid sol-gel materials have been a subject of intensive research during the past decades because these nanocomposites combine the versatility of organic polymers with the superior physical properties of glass. Here, we report the synthesis, by spin coating, of hybrid interpenetrating networks in the form of free-standing nanomembrane (around 35-nm thick) with unprecedented macroscopic size and characteristics. The quasi-2D interpenetration of the organic and inorganic networks brings to these materials a unique combination of properties that are not usually compatible within the same film: macroscopic robustness and homogeneity, nanoscale thickness, mechanical strength, high flexibility and optical transparency. Interestingly, such free-standing nanofilms of macroscopic size can seal large openings, are strong enough to hold amounts of liquid 70,000 times heavier than their own weight, and are flexible enough to reversibly pass through holes 30,000 times smaller than their own size.

Remarks on the Peel TestsConceptually, a peel test determines the force necessary to tear a 20 mm-wide strip of tape from a solid substrate at a constant speed, and is expressed herein in cN/20mm. Other authors have performed similar tests with different tape widths (for instance 10 mm or 25mm) and report adhesion values accordingly normalized (for instance N/10mm or N/25mm). All adhesion results reported in this study can be critically compared with the literature values by normalizing the reported peel forces with the specific geometries employed (for instance 500 cN/20mm = 2.5 N/cm = 6.25 N/25mm).

This Article describes the synthesis and characterization of renewable self-adhesive coatings with tunable viscoelastic properties and equipped with well-defined amounts of carboxylic acid "sticker" groups with adhesion promoting characteristics. Hydroxyl-ended polyesters with various architectures (linear, branched) were synthesized by melt polycondensation of dimerized fatty acids and fatty diols and then cured with maleic anhydride-modified triglycerides (such as maleinized soybean oil) in the presence of the amidine catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene. The curing reaction of alcoholysis has the dual effect of chain extending/cross-linking the base polymers via creation of polymeric half-esters linkages while introducing carboxylic acid functions within the gel structure. We demonstrated how the adhesion properties can be finely tuned from molecular design and formulation of the network precursors and how the rheology and functionality of the coatings influence the adhesive bond formation and development. These renewable polyester adhesives proved to be suitable materials for pressure-sensitive adhesives applications with respect to adhesion strength, viscoelasticity, and functionality. In addition, the environmental benefits of such materials are briefly discussed.

In this review, we provide a bird's eye view of recent developments in the field of pressure-sensitive adhesives (PSAs) derived from renewable monomeric building blocks. This emerging research field has been driven by increasing sustainability requirements in the adhesive industry and bridges the gap existing between highly optimized petroleum-based synthetic PSA systems, which display superior performance but lack biobased content, and historical PSAs derived from naturally occurring biopolymers (e.g., starch and natural rubber), which provide more environmentally friendly bonding solutions but have inherent technical limitations that prevent their more widespread implementation in today's technically demanding applications. We critically reviewed a representative (and exhaustive) survey of recent synthetic approaches to the development of biobased PSAs from the academic (articles) and industrial (patents) literature categorized in two families: chain-growth and step-growth polymerization routes. Finally, we draw a parallel between renewable synthetic PSAs and nature's selfadhesive glues, highlighting how the synergy between green chemistry and biomimetic concepts could inspire the emergence of a new generation of smart, synthetic, biobased PSAs with differentiated properties that approach the ones that are found in the natural world and with a wide spectrum of potential applications in the industrial and medical sectors.

CA adhesives are the strongest and most homogenous group in terms of mechanical strength. Hydrogels (FG, AB) are heterogeneous, with lower mechanical strength than CA. FG are mechanically the weakest adhesives. Rheological profiles correlate to mechanical strength and may be useful for predicting mechanical performance.