Molecular dynamics study of the structure of functionalised copolymers
View/ Open
Apóstolo2020.pdf (57.32Mb)
Date
10/02/2020Item status
Restricted AccessEmbargo end date
07/02/2025Author
Apóstolo, Rui Filipe Gonçalves
Metadata
Abstract
Polymers can be decorated with functional groups (FGs) to confer specific and
tuneable properties to the molecules. By varying the chemical architecture,
concentration, and distribution of the FGs on the polymer, it is possible to control
the chemical and physical properties of the polymers. Such functionalised polymers
are applied broadly, in chemical synthesis, biological science, chemical engineering,
medicine, agriculture, food science, optical science, and the energy sector.
This thesis is devoted to the study of functionalised polyethylene/polypropylene
(PE/PP) copolymers used to control friction and wear in combustion engines. The
aim is to investigate how the distribution of FGs on linear PE/PP copolymers controls
the structural, dynamical, and tribological properties of the molecules when dissolved
in n-heptane (representing an oil), either in bulk, or confined and sheared between solid
iron oxide surfaces (representing the moving parts of engines). Large-scale atomistic
molecular dynamics simulations are used to gain detailed and unique insights on the
links between molecular-scale and tribological properties. Simulations of isolated
copolymers in solution are carried out to explore how the radius of gyration, end-toend
distance, and scattering form factor are dictated by the distribution of a proprietary
FG.
It is shown that the size of a copolymer can vary by as much as 38% by varying
the FG distribution, due to the association of the FGs in solution. It is found
that solvophobic interactions are responsible for this association, and the results are
supported by simulations of different FGs without the opportunity to stack. Predicted
form factors will enable comparisons to be made with future X-ray and neutron
scattering experiments. The shear viscosity is found to correlate well with the FG
distribution and copolymer size at low concentration, while at high concentration the
viscosity does not seem to depend with FG distribution, in agreement with the lack of
variation in static structure. Next, the properties of copolymers solutions confined
between iron oxide surfaces are explored, in static conditions and under shear. It
is found that the FG distribution strongly affects the nature of the adsorption of the
copolymers, and the response of the adsorbed film to shear, as measured by mass-density
and velocity profiles across the liquid layer.