(180au) Interfacial Interaction Induced Relaxation and Its Impact On the Long-Term Tribology Properties of Nanometer-Thick Polymer Melts

Interfacial Interaction Induced Relaxation and its Impact on the Long-term Tribology Properties of Nanometer-thick Polymer Melts

Yongjin Wang¹, Jianing Sun² and Lei Li^1*

1. Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261
2. J.A. Woollam Co., Inc., Lincoln, NE 68508

The relaxation of polymer chains in bulk melt is fast, e.g., on the order of milliseconds to seconds. However the relaxation of polymer melts in nanofilms at the polymer-solid interface could be orders of magnitude slower than in bulk. It is critical to understand the relaxation mechanism because it impacts the long-term performance of nanoscale polymer, which have found many applications in recent years such as photo-resistant nanofilms for nanoimprint lithography (NIL), nanocomposites for biomaterials, sensors, batteries, and lubrication nanofilms for MEMS/NEMS and hard disk drives (HDD). In this study, we report the experimental results indicating that the polymer-solid interfacial interaction, which does not have a bulk counterpart, plays a key role in both thermodynamic driving force and kinetics of the relaxation. Moreover, the slow relaxation results in a time-dependent tribology property of the polymer nanofilms. 

Two nanometer-thick perfluoropolyethers (PFPEs) with the same backbone and different end-groups, one polar and the other non-polar, were used in our experiment. As shown here, ZDOL has polar hydroxyl end-groups while Z-03 has nonpolar CF₃ end groups and their backbones are identical:

HOCH₂CF­₂O - (CF­₂CF­₂­O)_m - (CF­₂O)_n - CF­₂CH₂OH  ZDOL

CF₃CF­₂O - (CF­₂ CF­₂O)_m - (CF­₂O)_n - CF­₂CF­₃       Z-03

(m/n = 1/1)

ZDOL can form hydrogen bonds with the polar surface via hydroxyl end groups, and its backbone has no attractive interaction with the polar surface. Z-03 cannot form hydrogen bonds with the polar surface at all because their end groups are nonpolar. The ZDOL polymers that form hydrogen bonds with the substrate are defined as “bonded” ZDOLs while the ones that does NOT are defined as “mobile”. The nanometer-thick films were fabricated by dip-coating and the relaxation of the thin films was investigated by water contact angle testing. The friction and wear performance of the polymer nanofilms were also studied with nano-tribometer, optical microscopy and AFM. 

The experimental result¹ indicates that the surface energy of the PFPE with polar end-groups “relaxes” with time and the relaxation time constant, obtained from Kohlrausch-Williams-Watts (KWW) model, is ten orders of magnitude higher than that of bulk polymer. However, the surface energy relaxation of the PFPEs with non-polar end-groups and 100% bonded molecules were not observed. Thermodynamically, the relaxation of ZDOL is driven by the attractive interaction between the hydroxyl end-groups of mobile polymers and the polar sites on the silica wafer. Kinetically, the very slow relaxation can be attributed to the low mobility of the bonded ZDOLs and the motional cooperatively between bonded and mobile ZDOLs. The time-dependent tribology properties of ZDOL and Z-03s have been investigated and related to the relaxation behavior at molecular-level.

Reference

[1] Wang, Y.; Sun, J.; Li, L. What is the role of the interfacial interaction in the slow relaxation of nanometer-thick polymer melts on a solid surface? Langmuir 2012, 28, 6151−6156