Chapter 2

Exploitation of Self-Assembly Phenomena in Liquid-Crystalline Polymer Phases for Obtaining Multifunctional Materials

M. Giamberini1 and G. Malucelli2*

1Universitat Rovira i Virgili, Departament de Enginyeria Química, Tarragona, Spain

2Politecnico di Torino, Dipartimento di Scienza Applicata e Tecnologia, Alessandria, Italy

*Corresponding author: giulio.malucelli@polito.it

Abstract

This chapter aims to provide a recent overview on the self-assembly phenomena that take place within specific liquid-crystalline polymer (LCP) phases and can be driven by different strategies (such as surface effects, amphiphilic interactions and weak intermolecular forces like hydrogen bonding, p-p stacking, etc.). Indeed, these phenomena are able to induce the formation of liquid-crystalline domains that may show peculiar thermal, mechanical, barrier, optical, and/or dielectric properties, which can be exploited for the build-up of multifunctional materials for a wide range of applications. Some examples of self-assembled LCP systems are thoroughly discussed, showing the correlations between their structure, the final properties, and some potential applications.

Keywords: Self-assembly, liquid-crystalline polymers, surface effects, multifunctional materials

2.1 Introduction

Self-assembly phenomena are dynamic processes responsible for the formation of highly ordered/structured domains at nanometric scale or beyond in different materials systems, including polymer and copolymers: the research on this topic has grown significantly over the past two decades [1-6]. The term self-assembly is usually exploited for describing spontaneous processes occurring as nanoscaled entities pack into regular arrangements; as a consequence, a minimum free energy is achieved through minimization of repulsive and maximization of attractive molecular interactions [7, 8]. The constituents of a system tend to organize themselves in a spontaneous way, exploiting specific physical interactions: as a result, a larger functional structured unit is obtained.

In nature itself, several spontaneous self-assembly processes take place, leading to the production of living systems having different levels of complexity. Some examples can be summarized as follows: naturally occurring phospholipids tend to aggregate to vesicular forms known as cell membranes; nucleic acids, i.e. DNA and RNA and the related biomolecules, arrange into a supramolecular information storage system, while chlorosomal chromophores result into self-assembled structures that allow collecting and transferring photonic energy. Furthermore, several biochemical systems exploit processes related to supramolecular reactivity: as an example, hemoglobin links and releases oxygen through this type of interaction.

The obtainment of these self-assembled systems can be interpreted from a thermodynamic point of view and exploits the decrease of the free energy of the assembled system, with respect to the one of the random counterpart. Usually, the decrease of free energy is a consequence of feebler intermolecular forces taking place within the assembling structures and is essentially enthalpy driven.

In order to describe self-assembly phenomena from a thermodynamic point of view, a simple Gibbs free energy equation can be utilized:

where ?Hsa and T?Ssa are the enthalpy variation of the process (mainly governed by the potential energy/intermolecular forces between the assembling components) and the entropy change related to the formation of the ordered or hierarchical arrangement, respectively.

Usually, self-assembly phenomena are accompanied by an entropy decrease: this means that they will be spontaneous only when ?Hsa is negative and exceeds the entropy contribution T?Ssa. In addition, the Gibbs free energy equation clearly indicates that self-assembly processes will progressively decrease as the magnitude of T?Ssa approaches that of ?Hsa: more specifically, spontaneous self-assembly will not occur above a critical temperature.

Despite that in most self-assembly processes the building up of regular arrangements is enthalpy driven, in certain conditions, entropy-driven processes can give rise to the formation of ordered arrangements: the formation of more organized structures as entropy increases is strictly related to the fact that these structures allow more degrees of freedom within the system [9]. Some examples of entropy-driven processes refer to definite micelles [10], liquid-crystal molecules and colloidal particles [11], and to several biological systems like viruses [12].

The existence and formation of self-assembled arrangements can be experimentally assessed by microscopy, spectroscopy, and structural analytical techniques, despite that they may undergo changes during the observation, as they are dynamic systems.

Furthermore, it is worthy to note that not all self-assembly processes can be described through simple thermodynamics: indeed, self-assembly can give rise to metastable states that are strictly affected by an external energy source (e.g. temperature, radiation, magnetic fields, chemical reactions, and so on), which is able to differentiate the metastable states being formed [13].

Within the nanotechnology field, the expression "self-assembly" has been used interchangeably with "self-organization", although they should be differentiated: first of all, unlike the latter, self-assembly represents a true equilibrium process and does not need an external (to the system) energy source. The self-organization is kept as long as the energy source is maintained. In addition, self-assembly gives rise to well-defined structures, the stability of which depends on the constituents and the types of physical interactions taking place between them; conversely, self-organization processes lead to the formation of less stable systems [14].

Finally, unlike self-assembly that needs a limited number of components, self-organization takes place only when a high number of components are involved [15].

A great potential importance for industrial applications is attributed to the patterns obtained in self-assembly processes: indeed, they can help in designing suitable methods for producing nanostructured surfaces, avoiding the use of such expensive techniques as lithography.

Among different engineered materials, liquid-crystalline polymers (LCPs) represent a very interesting class of materials that significantly exploit self-assembly phenomena: indeed, these latter are able to induce the formation of liquid-crystalline domains that provide the polymeric matrices with multifunctional features, like enhanced thermal, mechanical, barrier, optical, and/or dielectric properties. This justifies the great interest raised by these materials in the past 30 years, among a wide range of researchers, working in the fields of chemistry, solid, and soft matter physics, biology, medicine, macromolecular science, and nanotechnology. Indeed, a wide range of applications in different fields can be accomplished by exploiting the designed self-assembled polymeric structures, hence opening new pathways toward the development of "smart" polymeric materials.

Self-assembly phenomena taking place in LCPs can involve both lyotropic and thermotropic systems. Thermotropic LCPs have become important in the field of advanced materials, such as electro-optic devices and high-strength fibers. Lyotropic LCPs involving amphiphilic biomacromolecules are mainly related to living systems. Self-assembly is very important for both these classes and can be driven by exploiting different strategies, involving surface effects, amphiphilic interactions, and weak intermolecular forces like hydrogen bonding, p-p stacking, etc. (Figure 2.1). The resulting polymer structure may depend on shape/stiffness, surface-surface interactions, electrostatics, as well as on the homogeneity of the assembling units.

Figure 2.1 Possible strategies for promoting self-assembly in LCPs.

This chapter is aimed at describing the self-assembly phenomena that occur in LCPs and allow designing enhanced polymer systems, suitable for different functional and structural applications. Furthermore, some recent examples of self-assembled LCP systems will be presented and thoroughly commented, highlighting the structure-property relationships and discussing some of their potential uses.

2.2 Amphiphilic Self-Assembled LCPs

The self-assembly of amphiphiles has been already widely exploited for different application fields, including food, pharmaceutical, and cosmetic formulations. Generally speaking, amphiphilic molecules consist of at least two parts having an "opposite" behavior, i.e. usually hydrophilic and hydrophobic moieties: this feature makes surfactants to be considered as the most typical amphiphiles.

Amphiphilicity is one of the main driving forces for self-assembling surfactants: indeed, the thermodynamic features of amphiphiles in solution are determined by the strong tendency of hydrophobic tails to avoid direct contact with the aqueous medium (i.e. hydrophobicity). The easiest way for minimizing this unfavorable interaction implies the aggregation of amphiphilic molecules into micelles (i.e. the so-called micellization, through which self-assembly occurs): unlike the hydrophobic counterparts, the hydrophilic domains become exposed to water. This process can also be...