PISA Block Copolymers prepared via RAFT Polymerisation for use as Friction Modifiers

Friction modifiers are chemical additives for motor oils, they are used to reduce frictional
losses within an engine in order to improve its efficiency. Poly(Stearylmethacrylate)-
Poly(Benzylmethacrylate) (PSMA-PBzMA) nanoparticles were synthesised via a one-pot
polymerisation-induced self assembly reaction using a RAFT control group and tested
for use as friction modifiers. 11 different sized nanoparticles were synthesised with and
without ethylene glycol dimethacrylate cross-linking. These were synthesised with good
chain length control, as measured by GPC, and formed stable spherical nanoparticles with
small variance in nanoparticle diameter, as measured by DLS, TEM and SAXS.
These nanoparticles were tested in a variety of different tribological contacts in order
to establish their effect on friction within a contact, these include across all sliding:rolling
ratios as well as in a linear and reciprocating contact. These tests show that small nanoparticles (PSMA30PBzMA40 primarily tested) show reduced friction across all contact environments and speeds, with particularly large friction reduction in the boundary regime,
with cross-linking of these nanoparticles appearing to improve their long term friction reduction. Larger nanoparticles (PSMA30PBzMA200 primarily tested) show larger friction
reduction in the boundary lubrication regime in some contact environments, in particular
the cross-linked equivalent. This effect though is not present across all types of contact, e.g.
high sliding:rolling ratios. This shows that small cross-linked PSMA-PBzMA nanoparticles (23-30nm) can be effective friction modifiers in an internal combustion engine, while
larger nanoparticles are likely only effective in niche environments. These nanoparticles
also have no effect on wear of the contact or viscosity of the oil and were shown to be stable long term upon completion of tribological testing, indicating these nanoparticles can
be effective long term friction modifiers with little effect on other tribological properties
within an ICE.
Mechanistic analysis of how these nanoparticles reduce friction within a contact was
undertaken by analysing the film generation and nanocompression. These showed that
these nanoparticles rapidly formed a stable tribofilm upon the contact, these nanoparticles
are then easily compressed within a contact while being soft and easily sheared, resulting
in a very low friction contact by preventing contact of the hard steel surfaces. Upon exiting
the contact they then display an elastic property to rebound back to their original shape,
allowing for long-term friction reduction.
Testing of PDMAC-PDAAM water-soluble nanoparticles also displayed friction reduction in the boundary lubrication regime, indicating that this friction-reducing effect is
common across a wide variety of polymerisation induced self assembled nanoparticles.