This research investigates complex phenomena during the polymer crystallisation and self-assembly of chiral liquid crystals through theoretical concepts in physics and molecular dynamics simulation.
Self-poisoning has been reported in three poly(ethylene brassylate) (PEB) polymers,
which also exhibit quantised lamellar thickening; where the crystalline layer increases by
multiples of the repeating unit’s length. The poisoning is characterised by two minima in
the crystallisation rate of the polymers. A one-dimensional quantitative model is
developed from the extension of the Higgs and Ungar model to study the self-poisoning.
The parameterised model agrees with the experimental curves, even capturing the
minima.
Another case of self-poisoning was identified in precision polyethylene derivatives, specifically bromine-substituted poly(ethylene bromine) (PEBr). The three PEBr polymers exhibiting self-poisoning showed a growth rate minimum near the melting point of a less stable form of the polymer. The one-dimensional model developed for PEB was found to better fit the experimental growth rate data of PEBr compared to models attributing poisoning to competition between more and less stable forms. A recent united-monomer model of polyethylene (PE) was extended by incorporating bromine atoms to mimic PEBr, enabling further study of its crystallisation. In this model, one bead represents a −C2H4− unit or −C2HBr2−. Two crystallisation protocols were employed: self-seeding and continuous cooling. The self-seeded systems displayed temperature-dependent quantised lamellar thickening, while the continuous cooling systems did not. Quenching the system revealed bromine alignment into layers, similar to experimental observations in PEBr.
The crystallisation of the high-molecular-weight poly(lactic acid) stereocomplex is
significantly hindered in a 1:1 mix. It has been suggested that this is due to the local
fluctuations in enantiomer concentration within the melt. A Monte Carlo simulation on a
two-dimensional lattice is employed to mimic the diffusion-driven motion of PLA enantiomers, capturing the effects of temperature and diffusion on stereocomplex
formation. Crystallisation and melting probabilities are parameterised to investigate
their interplay with diffusion. Metrics such as crystallinity of the stereocomplex and
homochiral clustering are tracked during the simulations. The results of the simulation
suggest that the stereocomplex can grow at lower temperatures.
A liquid crystal phase (LC) with chiral columns composed of achiral molecules with
straight cores and aliphatic long chains is also reported. The observed LC phases exhibit
an equal number of left- and right-handed columns, rendering the overall phase achiral.
The chirality arises from a balance between the parallel stacking of the cores to maximise
π − π interactions and the avoidance of clashes by the tails. Two interacting dimers of
the molecules were found to prefer perpendicular arrangements to optimise packing and
minimise head-on clashes. A model of rotating quadrupoles was developed to capture the interactions of dimers within the columns due to their similarity to linear electric
quadrupoles. The experimentally observed Fddd configuration was found to have the
lowest energy per dimer compared to three alternative configurations.
