On the supramacromolecular structure of core–shell amphiphilic macromolecules derived from hyperbranched polyethyleneimine

https://doi.org/10.1016/j.jcis.2014.06.034Get rights and content

Highlights

  • The supramacromolecular structure of core–shell amphiphilic macromolecules was studied.

  • The influence of the assembly conditions on their structure was addressed.

  • Thin films were formed at air water interface and their structure was described.

Abstract

The supramacromolecular structure of core–shell amphiphilic macromolecules (CAMs) with hyperbranched polyethyleneimine (HPEI) cores and fatty acid chain shells (HPEI-Cn) for different chain lengths was investigated both, in colloidal suspension, solid phase and at the air–water interface using Small Angle X-ray Scattering (SAXS), Wide Angle X-ray Scattering (WAXS), X-ray Reflectometry (XRR) and Langmuir isotherms.

At low temperatures colloidal toluene suspensions of the HPEI-Cn polymers form, as evidenced by peaks arising in the structure factor of the system showing mean particle-to-particle distances correlated with the length of the aliphatic chains forming the shells of HPEI-Cn unimicelles. The CAM sizes as found from the SAXS experiments also display a clear dependence on shell thickness suggesting that the aliphatic chains adopt a brush-like configuration.

After solvent extraction, HPEI-Cn adopts ordered structures with hexagonal packing of the aliphatic chains.

Submitted to lateral pressure Π at the air–water interface, HPEI-Cn undergoes a disorder–order transition with increasing transition pressure for increasing chain lengths. The CAMs show different behaviors in-plane and out-of-plane. While out-of-plane the aliphatic chains behave as a brush remaining almost fully unfolded, whereas parallel to the air–water interface the chains fold down in a mushroom way with increasing lateral pressure Π.

Introduction

The search for smart building blocks has attracted the attention of the soft-matter community for a while [1], [2], [3]. Among others, amphiphilic building blocks have the ability to adopt several structural conformations by tuning their thermodynamic variables [4].

Along these lines, dendrimers and dendrimeric structures with amphiphilic core–shell architectures displaying micelle-like properties, proved to be attractive building blocks to form supramolecular materials with designed properties [5], [6]. Seminal works of Percec [7], [8], [9] and Meijer [10], and their co-workers, provided the framework for designing an unprecedented number of superstructural assemblies from precisely engineered dendrons and dendrimers.

In particular, the modification of peripheral groups of hydrophilic dendrimers with hydrophobic tails led to the creation of molecularly defined systems with inverse micelle properties [10], [11]. However, dendrimer synthesis is time consuming and limits their application almost to laboratory scale. Alternatively, core–shell amphiphilic macromolecules (CAMs) [12], [13] with hyperbranched cores [14], a less perfect variant of the dendrimers have gained popularity, among other properties because of the possibility of easy synthesis of unimolecular micelles [15], [16], [17].

CAMs with unimolecular micelar (unimicelar) structures can behave in different ways allowing a tuning of their aggregation state depending on solvent, temperature conditions [18] external fields, pressure, etc. Unimicelles derived from hyperbranched polyethyleneimine (HPEI) are very well known, and they probe their versatility as phase transfer agents among other properties [19], [20], [21], [22], [23]. Despite the number of papers reporting results in solution/suspension using HPEI as polar core, to date, there is a lack of information on the structure of HPEI CAMs in solid state.

However, molecular and supramolecular properties are far from being the ultimate hierarchical level and interesting properties arise from supramacromolecular assembly [24]. Recently, we used supramacromolecular assemblies of HPEI-C16 to organize gold nanoparticles into lamellar structures [21]. Among other techniques, Langmuir–Blodgett film assembly has proven to be one of the most versatile methods for controlled assembly of interfacial dendritic structures [25], [26], [27], [28], [29], [30], [31], [32].

In this paper we investigate the supramacromolecular structure and properties of the amphiphilic structure (HPEI-Cn) formed by the hyperbranched polar polymer polyethyleneimine (HPEI) and long chain saturated fatty acids (Cn, n = 8, 10, 12, 14, 16 and 18. From now on, Cn will indicate the whole set of chain lengths) in toluene suspension, solid state and Langmuir films at air–water interface.

Section snippets

Materials and methods

Hyperbranched polyethyleneimine (HPEI, Mn = 10,000 Da) and fatty acid chlorides (capryl, capric, lauryl, myristoyl, palmitoyl and stearoyl chlorides) were purchased from Sigma–Aldrich. Triethylamine (TEA) was purchased from Sintorgan. All chemicals and solvents used were of the maximum purity available in market. Prior to use, HPEI was kept in vacuum for 2 days, chloroform and TEA were purified and dried following standard protocols [33].

The synthesis of HPEI capped with carboxylic acids via amide

HPEI-Cn CAMs suspended in toluene

The amphiphilic nature of HPEI-Cn with a hydrophilic core partially shielded by an aliphatic shell (the degree of capping in our CAMs was kept fixed at 52%) opens the possibility to tune the supramacromolecular structure of the micelles controlling solvent properties (temperature, electric susceptibility, etc.). In fact, Picco et al. [18] showed that HPEI-C16 displays a liquid crystal-to-colloid thermally activated transition near room temperature. Moreover, HPEI-C16 displays also negative

Conclusions

Supramacromolecular assembly of functional soft-matter structures proves to be a flexible strategy for controlling material properties [1], [2], [3] and designing novel collective behavior.

CAMs with hyperbranched polyethyleneimine core building blocks are especially useful acting as unimicelles, based on the ability to transfer polar substances to non-polar media due to the hydrophilic nature of the cores. Previous results on HPEI-C16 unimicelles demonstrated that tight control over

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments

A.S.P. is recipient of a CONICET doctoral fellowship. O.A., G.S. and M.C. are staff members of CONICET (Argentina). O.A. gratefully acknowledges financial support from the Max Planck Society (Germany), ANPCyT (Argentina, Projects: PICT/PRH 163/08 and PICT-2010-2554). O.A. and M.C. acknowledges Laboratório Nacional de Luz Síncrotron (Campinas – Brazil, proposals SAXS1-10737, SAXS1-13502 and SAXS1-14537) for partial financial support. The authors acknowledge the contribution of Lic. Agustín

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