Nano-structured soft matter: a synergy of approaches to amphiphilic and block copolymer systems
University of Lincoln, UK
An important class of soft materials is formed by molecules consisting of chemically distinct parts. In this workshop, we concentrate on two types of materials that posess this feature: lipid amphiphilic molecules and block copolymers. These molecules can self-assemble into nano-structures (mesophases) of similar symmetries, but on different scales [1-4].
Controlling the properties of the nano-structured soft materials is relevant for a range of industrial applications (energy materials, templates, pharmaceuticals, cosmetics, foodstuffs, detergents and advanced painting) . Their phase diagrams have marked differences, for instance in their stability regions ; the kinetics of their formation often plays an important role [2,6]. These systems span multiple scales both in space and time. Both short amphiphiles (lipids) and block copolymers can form bilayers (membranes), which can close into vesicles (e.g. spherical), which are important systems in biology and also in technological applications, for example as drug delivery vehicles.
Both in biological and synthetic systems lipid mesophases usually contain more than one lipid species, which have a tendency to phase separate. From a biological perspective, lateral lipid organizations into domains and membrane curvatures are ubiquitous features, and are known to play an important role for the membrane functionalities . From a materials perspective, this can lead to a systematic and rational functionalization of such mesophases. Most studies on lipid phase separation focus on simple membrane geometries, such as lipid vesicles and supported membranes [8-10]. Only very recently have lipids in more complex bicontinuous phases been theoretically investigated .
For block copolymers (BCP), due to the larger molecular sizes, mesoscale and multiscale techniques have to be involved. Much research has been done using soft, generic coarse-grained models of polymers that allow for accelerated dynamics . Self-consistent field theory (SCFT) and more recently field theoretical approaches [13-16], which work with polymer densities, can address much larger system sizes [17,18] compared to molecular dynamics simulations and were able to simulate experimentally relevant systems [19-22] and molecular weights . For even larger BCP system sizes advances have been made using a variety of phase field models offering the next scale level of description . For practical applications such as lithography there is an interest in defects and their removal [19,24].
Forming vesicles from amphiphilic molecules (both lipids and BCP) received much attention in recent experiments utilizing a wide range of techniques, including microfluidic templating , electroformation from membranes  and self-assembly in solution . A recently developed variation on the self-assembly process is polymerisation-induced self-assembly , during which polymerisation continues as self-assembly is taking place. In this process, spherical micelles can grow into vesicles as the polymers from which they are formed gradually lengthen. Simplified theoretical models were proposed for BCP vesicular and micellar formation using versions of SCFT [29,30]; however, most of the progress has been driven by experiment, and there is now a need for further theory and simulation to deepen our understanding and guide future investigations.
The proposed workshop aims at bringing together experts from two different subfields of soft matter physics, namely amphiphilic and block copolymer physics which, despite the differences in their objects of enquiry, share marked similarities in the thermodynamic phases observed both numerically and experimentally and also in the modelling these highly complex systems require to complement experimental studies. The workshop will strongly encourage simulation experts of these two subfields to interact with the aim of addressing challenges in the method development and multiscale modelling necessary to understand an ever-growing body of experimental results which will be summarised by leading experts in these fields.
Specific challenges to be addressed are listed in three overlapping sections. All these problems have been chosen because we believe that discussion among researchers from a range of scientific backgrounds will contribute strongly to their solution.
- Formulation of models for repartition of different species (other lipids or peptides) in lipid bilayers; discussion of raft hypothesis and its controversial aspects
- Formulation of models to describe accurately the thermodynamic and kinetic pathways to transition from one lipid phase to another (e.g. from minimal gyroid to primitive or diamond)
- Modelling and understanding the role of local phase instability in facilitating or hindering aggregation (e.g. protein crystallisation in the triply periodic primitive phase)
- Modelling of the specific experimental signatures (e.g. how to experimentally test species patterning on complex surfaces such as the P-surface)
- How to cope with defects/eliminate them/lower the annealing time in order to achieve perfect structures in applications? How to minimize the effect of box size on the defect dynamics?
- How to enhance the stability region of phases, such as the gyroid, to ease assembly of these structures? Can incorporation of additional components (e.g., nanoparticles) enhance the stability?
- Development of hybrid models with automatic numerical determination of the parameters of phase field models from self-consistent field theory
- Related to the accuracy of modelling, what is the effect of compositional fluctuations that are present in experiments, but neglected in some coarse grained models such as SCFT?
- Dynamics of vesicle formation in new experiments; e.g., polymerisation-induced self-assembly, which allows nano-objects to be formed at very high concentrations.
- Controlling the surface properties of vesicles to tune their interaction with their environment.
- The role of molecular architecture in controlling the properties and size of vesicles; for example, to allow fine control over the dosage of an encapsulated chemical.
And in general:
- What can the communities learn from each other: simulation techniques, preparation, inspiration from biology, external fields? Development of models for lipid/polymer mixtures.
Martin Greenall (University of Lincoln) - Organiser
Xiaohu Guo (Hartree Centre, Daresbury Laboratory) - Organiser
Fabien Paillusson (University of Lincoln) - Organiser
Bart Vorselaars (University of Lincoln) - Organiser
Andrei Zvelindovsky (University of Lincoln) - Organiser & speaker