Pressure-sensitive adhesives (PSA) represent special class of viscoelastic polymers forming strong adhesive joints with various substrates under application of slight contact pressure (1-10 Pa) over short contact time (1-5 s). Existing PSAs are mainly hydrophobic elastomers, common disadvantage of which is a lack of adhesion toward wet substrates. Meantime, many areas of industry and, especially, medicine need in development of hydrophilic PSAs, maintaining high adhesion with hydrated biological tissues.
Rational design of new PSAs requires an insight into molecular and supramolecular structures of polymers possessing PSA properties. As has been recently shown [1], at most fundamental, molecular level, pressure-sensitive adhesion results from specific balance between high energy of intermolecular cohesion and large free volume, arising owing to heat motion of macromolecules. Since these properties are usually incompatible, creation of new PSAs is difficult problem. Nevertheless, these conflicting properties can be realized in interpolymer complexes, wherein high cohesion strength results from noncovalent bonding of complementary functional groups of polymer chains (hydrogen or electrostatic bonding) and free volume is a result of loop formation [2]. Polymer complexes with oligomers bearing complementary reactive functional groups at both ends of their short chains, typically exemplified by polyvinyl pyrrolidone (PVP) – poly(ethylene glycol) (PEG) H-bonded complex, demonstrate excellent pressure sensitive adhesion [2]. Essential feature of innovative technology of PSA production is that the PSAs with tailored performance properties can be obtained by simple mixing of a wide range of hydrophilic functionalized polymers, which are not necessary to be tacky in unblended state.
As comparison of the structure of model PSAs with their adhesive properties has shown, the PSAs can be regarded as nanostructured polymer composites. The best adhesion is observed as the radius of free volume is between 2.95 and 3.08 Å, and the free volume content is 6.3 – 7.0 %. At the same time, the energy of interpolymer bonds should be in the range of 40.7 – 79.4 kJ/mol.
Adhesive, mechanical and water-absorbing properties of PSAs based on polyelectrolyte and polymer – oligomer complexes can be easily controlled by the change of their composition [2]. Polycomplex hydrophilic PSAs find currently increasing application in pharmacy, cosmetic products and industry.
