Fracture and adhesion of soft materials: a review

Abstract

Soft materials are materials with a low shear modulus relative to their bulk modulus and where elastic restoring forces are mainly of entropic origin. A sparse population of strong bonds connects molecules together and prevents macroscopic flow. In this review we discuss the current state of the art on how these soft materials break and detach from solid surfaces. We focus on how stresses and strains are localized near the fracture plane and how elastic energy can flow from the bulk of the material to the crack tip. Adhesion of pressure-sensitive-adhesives, fracture of gels and rubbers are specifically addressed and the key concepts are pointed out. We define the important length scales in the problem and in particular the elasto-adhesive length Γ/E where Γ is the fracture energy and E is the elastic modulus, and how the ratio between sample size and Γ/E controls the fracture mechanisms. Theoretical concepts bridging solid mechanics and polymer physics are rationalized and illustrated by micromechanical experiments and mechanisms of fracture are described in detail. Open questions and emerging concepts are discussed at the end of the review.

Fatigue fracture of tough hydrogels

“…The stress-stretch curve of a hydrogel or an elastomer often depends on loading history due to various inelastic processes such as the Mullins effect [37,38], viscoelasticity [39][40][41][42][43], poroelasticity [42][43][44][45][46] and viscoplasticity [47]. In particular, the Mullins effect has been considered as the major factor to explain the softening of double network hydrogels over consecutive loading cycles [48][49][50][51][52][53][54], and has been attributed to progressive damage of the polymer networks [50,55].…”

Flaw sensitivity of highly stretchable materials

“…Stretchable, transparent, ionic conductors (e.g., hydrogels and ionogels) enable devices of unusual functions, such as transparent loudspeakers [12], artificial skins [13], artificial axons [14,15], and electroluminescence of giant stretchability [16][17][18]. The interest in the mechanics of stretchable materials has surged [19][20][21][22][23][24][25][26][27][28][29][30][31][32].…”

“…Another frequently discussed material length is the ratio of fracture energy and elastic modulus, Γ / E . [32] [59] This length overestimates the length of flaw sensitivity by orders of magnitude. For a highly stretchable material, the stretchability λ * is on the order of ten, so that W * ≫ E .…”

Adhesion between oppositely charged polyelectrolytes

“…the edge of the circular contact. Contact stresses are a longstanding problem in contact mechanics [34] and although their measurement has been of longstanding interest [2] new techniques for the in situ measurement of the stress are continually being developed [35]. The stress is given by…”

“…Adhesion between polymer interfaces has attracted considerable interest, partly due to its industrial importance [1,2]. The use of 'smart' materials for adhesive applications has become an area of some significant effort [3].…”