Surface Forces Characterization of Concentrated PMMA Brush Layers under Applied Load and Shear

Area of Science:

• Surface Science and Tribology
• Polymer Physics
• Materials Science

Background:

• Concentrated polymer brushes (CPBs) offer excellent lubrication but exhibit variable frictional behaviors.
• Understanding the relationship between CPB structure and friction under shear is crucial for controlling their properties.
• Previous studies highlight the importance of preparation and operating conditions on CPB friction.

Purpose of the Study:

• To investigate the structure of poly(methyl methacrylate) (PMMA) brushes in toluene under shear conditions.
• To correlate the structural properties of CPBs with their measured frictional behavior.
• To elucidate the mechanisms governing friction in CPBs, enabling tailored applications.

Main Methods:

• Employed surface forces, resonance shear measurement (RSM), and refractive index measurement (Fringes of Equal Chromatic Order - FECO).
• Analyzed elastic (k_s) and viscous (b_s) parameters derived from RSM.
• Determined the density distribution of PMMA brushes using FECO measurements.

Main Results:

• PMMA-PMMA interfaces showed higher elastic and viscous parameters than PMMA-silica interfaces.
• Under increasing shear amplitude, PMMA-PMMA friction parameters (b_s) varied, while elastic parameters (k_s) decreased monotonically.
• Brush dynamics were independent of thickness, and lower outer brush density facilitated interpenetration, with chain pull-out observed under shear.

Conclusions:

• The study reveals that the dynamics of CPBs under load and shear are not dependent on brush thickness.
• The interpenetration of swollen PMMA brushes, facilitated by lower outer density, plays a key role in their frictional response.
• Regulating friction in CPBs by controlling their structure and interpenetration is possible, broadening their application potential.