Raft crystals of poly(isoprene)-block-poly(ferrocenyldimethylsilane) and their surface wetting behavior during melting as observed by AFM and NanoTA

Abstract

We report on the morphology evolution during heating and melting of lamellar poly(isoprene)-block-poly(ferrocenyldimethylsilane) (PI₇₆-b-PFDMS₇₆) raft crystals deposited at the native oxide surface of silicon (SiO₂) or at a highly ordered pyrolytic graphite (HOPG) surface, studied by in situ temperature controlled atomic force microscopy. Crystals deposited on hydrophilic SiO₂ surfaces revealed an irreversible decrease in length at temperatures of up to tens of degrees above their expected melting temperature, while maintaining their platelet-like structure. Crystals deposited on hydrophobic HOPG surfaces initially decreased in length below their expected melting temperature, while at 120 °C and above a typical molten morphology was observed. In addition, the irreversible formation of a PI₇₆-b-PFDMS₇₆ wetting layer around the crystals was observed upon increasing the temperature. These observations in the morphological behavior upon heating emphasize the role of interfacial energy between a surface deposited block copolymer based macromolecular nanostructure and its supporting substrate.

Introduction

The self-assembly of amphiphilic block copolymers (BCP) into well defined nanometer sized aggregates with a distinctive core–shell morphology in block-selective solvents has attracted extensive interest in recent years. In particular, the wide diversity available in architecture, shape, size, functionality and composition makes these self-organized structures attractive for utilization in e.g. drug delivery [1], [2], [3], [4], catalysis [5], [6], [7], [8] and nanolithography applications [9], [10], [11].

The functionality and properties of both the corona and core of the self-assembled BCP structures can readily be adjusted by modifying the composition of their constituent BCP chains. Poly(ferrocenyldimethylsilane) (PFDMS) based BCP assemblies prepared in a selective solvent for the non-PFDMS block have been reported in the literature and comprise an interesting class of architectures [12], [13], [14], [15]. Due to the presence of both Fe(II) and Si in the organometallic polymer main chain, PFDMS possesses appealing properties in terms of redox activity [16], [17], [18] and etch resistance [19], [20]. Furthermore, PFDMS containing BCPs have been reported to be suitable precursors for ferromagnetic ceramics [21], [22], [23]. Poly(dimethylsiloxane)-b-PFDMS, poly(methylvinylsiloxane)-b-PFDMS, poly(isoprene)-b-PFDMS, poly(styrene)-b-PFDMS and poly(methyl methacrylate)-b-PFDMS were reported to yield rod-like cylindrical micelles in solution [24], [25], [26]. Manners and coworkers [27], [28] provided evidence that crystallization of the PFDMS core forming block was the driving force for the formation of the cylindrical architectures. The semi-crystallinity of the PFDMS-rich core was detected by wide angle X-ray scattering (WAXS) measurements in which peaks with a d spacing of ∼6.4 Å were typically observed. The position of the peak was in good agreement with typical spacings reported for semi-crystalline PFDMS homopolymer films and PFDMS pentamer single crystals [29], [30]. The peak intensity observed was often weak and higher order peaks were typically not detected, suggesting a lower degree of crystallinity of PFDMS in micellar cores compared to PFDMS homopolymer in bulk. Interestingly, results reported by Vancso and coworkers [12] revealed a rather dynamic PFDMS core for PMMA-b-PFDMS micelles assembled in acetone (i.e. a selective solvent for PMMA), as was observed with temperature controlled ¹H NMR measurements. The absence of a crystalline phase in the PFDMS core for these micelles was independently confirmed by the absence of Bragg peaks in recorded WAXS spectra. These results pointed toward a solvophilicity driven micellar self-assembly process for PMMA-b-PFDMS block copolymers in acetone.

While a variety of self-assembled morphologies have been obtained for a range of PFDMS based block copolymers in selective solvents [31], [32], [33], [34], reports on their utilization and structural stability on surfaces remain scarce [21]. Wang and coworkers [35] reported on the microfluidic alignment of corona crosslinked poly(isoprene)-b-PFDMS (PI-b-PFDMS) cylindrical micelles on flat substrates and their subsequent use as precursors to 1D magnetic ceramic replicas. To our knowledge, there are no accounts of the behavior of non-crosslinked PFDMS-containing micelles on solid substrates in the literature. Such knowledge is of potential interest for utilization in etch resist patterns, guided assembly into ceramic nanolines and fabrication of other functional nanostructures.

This paper describes the melting behavior of platelet-like PI₇₆-b-PFDMS₇₆ micelles with a semi-crystalline core, deposited on either HOPG (hydrophobic) or SiO₂ (hydrophilic) substrates (Fig. 1), as studied by in situ temperature controlled AFM. In addition, NanoTA was used as an enabling nanotechnology tool for the thermal analysis of these nanometer sized architectures [36], [37] to determine the melting temperature of the crystalline PFDMS phase at the nanoscale. Interestingly, the irreversible formation of a wetting layer upon melting the HOPG supported crystals shed light on the role of interfacial energy between block copolymer based nanostructures and their supporting substrates. Hence, the observed substrate dependent, thermally induced morphology changes provide insight in the molecular organization within PI₇₆-b-PFDMS₇₆ platelet-like crystals and their thermal transitions.