Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

Abstract

Pressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Pressure sensitive adhesives based on interpolymer complexes

“…By default, until recently this reasonable question was considered to be irrelevant and unanswerable because, as is seen from Table 1, existing PSAs exhibit markedly dissimilar chemical compositions and structures. As has been quite recently shown [38], the [39].…”

“…Equation (1) is not useful for quantitative calculations of peel force due to a range of crude assumptions made in its derivation [38]. Nevertheless, Equation (1) predicts qualitatively the significance of diffusion and relaxation processes (both of which require molecular mobility) for the adhesive properties of polymers.…”

“…As the molecular theory of pressure sensitive adhesion predicts [17,38,41] and solids are viscoelastic [18] and that is why all PSAs are viscoelastic materials. Hence, the molecular theory of pressure sensitive adhesion predicts that adhesion strength goes through a maximum with the change of PSA molecular mobility as an adhesive material composition or temperature is varied [17,38,41].…”

“…Hence, the molecular theory of pressure sensitive adhesion predicts that adhesion strength goes through a maximum with the change of PSA molecular mobility as an adhesive material composition or temperature is varied [17,38,41].…”

“…The derivation of Equation (1) [38] and its forerunners [17,41] was based on consideration of a model PSA behavior, i.e. the blends of high MW poly(N-vinyl pyrrolidone), PVP, with oligomers of poly(ethylene glycol), PEG, ranging in molecular weight from 200 to 600 g/mol.…”