Fracture of and adhesion between biological and synthetic macromolecular materials

A molecular theory was developed to predict the fracture energy of glassy polymers with molecular weight, M, smaller than a threshold value for the onset of chain entanglements, 2M$\sb{\rm e}$. A fracture mechanism is assumed that calls for sliding of the polymer chains in a microscopic craze at the crack tip. The plastic work is related to the chain interpenetration distance which is obtained from the solution of the Fokker-Planck equation. Theoretical predictions agree with experimental data of the fracture energy of polystyrene. A stochastic model is presented to predict the molecular weight dependence of the brittle fracture energy and strength of polymers with M $>$ 2M$\sb{\rm e}$. A chain scission criterion is invoked for the polymer chain segments crossing the fracture plane and being entangled about it. The presence of dangling ends is shown to be responsible for the change of the fracture properties with the molecular weight. The predictions of the model are in good agreement with experimental measurements of the fracture energy and strength of poly(methyl methacrylate) and polystyrene. Stochastic modeling was also employed to investigate the influence of chain entanglements on the autohesion of linear polymers. The fracture energy of a polymer interface is expressed in terms of the total number of effective crossings which is a function of the molecular weight distribution and the contact time. Experimental studies of autohesion of dried and polished poly(methyl methacrylate) surfaces at 117$\sp\circ$C validate the theoretical predictions. New scaling expressions were derived for the description of chain interpenetration and fracture at the gel-gel interface. Different scaling relationships are applicable depending on the fracture mechanism of the polymer chains crossing the interface. The fracture energy at contact time, t, smaller than the terminal relaxation time, $\tau\sb{\rm r}$, scales to t$\sp{1/2}$ for chain rupture and to t$\sp{5/4}$ for chain pull-out. The results of the scaling analysis are applied to the bioadhesion of polymers on soft tissues for the design of high-performance bioadhesive systems for drug delivery and targeting. A novel in vitro or ex vivo experimental method was proposed to study the interfacial interactions between polymer microparticles and a biological surface. The technique can be used to determine the adhesive forces between particles of diameter up to 200 $\mu$m in contact with a mucin gel or the mucosa of a tissue.