EXOTIC TRANSITIONS OF RANDOM HETEROPOLYMERS INTERACTING WITH SOLID-SURFACES

In many applications, it is desirable to control interfacial propertie s by employing adsorbed polymer layers, In this work, we study the ads orption of random heteropolymers and find a rich surface phase diagram that suggest ways in which the properties of the adsorbed layers can be controlled rather precisely by manipulating the physical conditions . Specifically, we present a comprehensive field-theoretic analysis of the surface phase diagram of a solution of random heteropolymers inte racting with a chemically homogeneous solid surface, and find many sur face transitions that may be exploited in applications. The different types of polymer segments interact with the solid surface in arbitrari ly different ways. Our analysis, wherein a replica method is employed to average over the quenched sequence fluctuations, allows us to obtai n the surface free energy functionals that show that our problem parti ally resembles a semi-infinite Ising spin system. Thus, akin to the Is ing system, the phase diagram exhibits exotic surface transitions. In the infinitely dilute limit we find four ''massless'' transition lines : the ordinary (OT), the surface (ST), the extraordinary (ET), and the special (SPT) transition. At finite bulk solution concentration, we f ind two transitions: viz. the OT and the adsorption-depletion (ADT) tr ansitions. The nature of the critical points that reside on the transi tion lines are analyzed, and the physical meaning of each of the surfa ce transitions is elucidated, Our results are related to experiments a nd it is shown that the interesting behavior that random heteropolymer s exhibit near surfaces is due to the quenched sequence fluctuations. (C) 1996 American Institute of Physics.