Adhesion and friction of polymer surfaces: The effect of chain ends

Using a surface forces apparatus we studied the adhesion and friction of glassy polystyrene (PS) and poly(vinylbenzyl chloride) (PVBC) surfaces having various molecular weights (MW) from 2 000 000 to 1240 Da. Cross-linking of high MW polymers leads to lower adhesion and friction relative to the untreated polymers, whereas scission (bond-breaking) leads to higher adhesion and friction. "Asymmetric" bond-broken against cross-linked surfaces behave intermediately between the two "symmetric" systems. Both the friction and adhesion of untreated polymers increase with decreasing MW, often resulting in irreversible damage during sliding or separation. We conclude that the population of the chain "ends" at the surfaces is the most important factor that determines the adhesion, adhesion hysteresis, friction, and wear between two polymer surfaces. Segment polarity and layer thickness play secondary roles.