Article pubs.acs.org/Langmuir
Understanding and Controlling Morphology Formation in Langmuir−Blodgett Block Copolymer Films Using PS-P4VP and PSP4VP/PDP Iryna I. Perepichka, Qing Lu, Antonella Badia,* and C. Geraldine Bazuin* Département de Chimie, Centre de Recherche sur les Matériaux Auto-Assemblés (CRMAA/CSACS), Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal (QC), Canada H3C 3J7 S Supporting Information *
ABSTRACT: This contribution offers a comprehensive understanding of the factors that govern the morphologies of Langmuir−Blodgett (LB) monolayers of amphiphilic diblock copolymers (BCs). This is achieved by a detailed investigation of a wide range of polystyrenepoly(4-vinyl pyridine) (PS-P4VP) block copolymers, in contrast to much more limited ranges in previous studies. Parameters that are varied include the block ratios (mainly for similar total molecular weights, occasionally other total molecular weights), the presence or not of 3-npentadecylphenol (PDP, usually equimolar with VP, with which it hydrogen bonds), the spreading solution concentration (“low” and “high”), and the LB technique (standard vs “solvent-assisted”). Our observations are compared with previously published results on other amphiphilic diblock copolymers, which had given rise to contradictory interpretations of morphology formation. Based on the accumulated results, we re-establish early literature conclusions that three main categories of LB block copolymer morphologies are obtained depending on the block ratio, termed planar, strand, and dot regimes. The block composition boundaries in terms of mol % block content are shown to be similar for all BCs having alkyl chain substituents on the hydrophilic block (such as PS-P4VP/PDP) and are shifted to higher values for BCs with no alkyl chain substituents (such as PS-P4VP). This is attributed to the higher surface area per repeat unit of the hydrophilic block monolayer on the water surface for the former, as supported by the onset and limiting areas of the Langmuir isotherms for the BCs in the dot regime. 2D phase diagrams are discussed in terms of relative effective surface areas of the two blocks. We identify and discuss how kinetic effects on morphology formation, which have been highlighted in more recent literature, are superposed on the compositional effects. The kinetic effects are shown to depend on the morphology regime, most strongly influencing the strand and, especially, planar regimes, where they give rise to a diversity of specific structures. Besides film dewetting mechanisms, which are different when occurring in structured versus unstructured films (the latter previously discussed in the literature), kinetic influences are discussed in terms of chain association dynamics leading to depletion effects that impact on growing aggregates. These depletion effects particularly manifest themselves in more dilute spreading solutions, with higher molecular weight polymers, and in composition regimes characterized by equilibrium degrees of aggregation that are effectively infinite. It is by understanding these various kinetic influences that the diversity of structures can be classified by the three main composition-dependent regimes.
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INTRODUCTION It is well-known that block copolymers in the bulk self-organize into various morphologies (most commonly cubic, cylindrical, gyroid, and lamellar for diblock copolymers), depending on the block copolymer composition (relative block lengths), the χ parameter between the blocks, and the total molecular weight of the polymer.1−3 It is equally well-known that thin film morphologies are determined by interfacial energies (substrate and air) and film thickness in addition to the above parameters.4−7 In the latter, the relationships between the different parameters and the resulting morphologies are quite well understood for lamellar-forming block copolymers4 and partially understood for cylinder-forming and sphere-forming block copolymers.7 Equilibrium or pseudoequilibrium morphologies in these materials are achieved by thermal annealing in the bulk (limited mainly by the degradation temperature) and by thermal or solvent vapor annealing in thin films © XXXX American Chemical Society
(although the solvent itself adds additional factors affecting equilibrium conditions, and thus it is more accurate to speak of changes in order induced by solvent annealing8). In ultrathin films of (generally amphiphilic) block copolymers prepared by the Langmuir−Blodgett (LB) technique, considered to be monomolecular films, various morphologies are also observed, but the processes leading to these morphologies are particularly complex, due to the influence of kinetic effects during film formation, to the soft aqueous interface on which they form, and to the difficulty if not impossibility of ensuring the attainment of equilibrium conditions throughout. In early work on LB block copolymer films, it was shown that the block composition (relative block Received: October 15, 2012 Revised: February 1, 2013
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dx.doi.org/10.1021/la3040962 | Langmuir XXXX, XXX, XXX−XXX
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morphology is a coherent nanopatterned film held together by the hydrophilic block monolayer (obtained also by spincasting an ultradilute chloroform solution of PS-P2VP onto a mica substrate27). As will be detailed in the present contribution, the role of chain association in growing domains that may be frozen in at various stages should also be considered. Overall, at the present time, understanding of the underlying factors governing LB film morphology of block copolymers has become clouded. In this paper, we seek to clarify the issues by investigating a wide composition range of easily available copolymers in various relevant experimental conditions and by comparing with and reevaluating literature results, which are all based on much more limited ranges of compositions and conditions compared to the present paper. We thereby aim to arrive at a comprehensive and coherent understanding of morphology formation in LB diblock copolymer films that reconciles compositional and kinetic contributions to the final morphology. Ideal copolymers for this investigation are polystyrene-poly(4-vinyl pyridine) (PS-P4VP), which are commercially available in a wide variety of block compositions, and their blends with 3-n-pentadecylphenol (PDP), which hydrogen bonds via the phenol moiety to the pyridine in P4VP, as shown in Figure 2. This supramolecular diblock system has
lengths) is a determining factor in the morphology obtained.9−12 Three main types of morphologies were observed: (a) variably sized, including very large, planar-type aggregates (also called continents, pancakes, islands, etc.) at low hydrophilic block content; (b) nanostrands or strands (also called spaghetti, rods, ribbons, stripes, worms, elongated micelles, etc.) of relatively constant width but often highly variable lengths, sometimes well interconnected in the form of a nanostrand network,13−15 at somewhat higher hydrophilic block content; and (c) nanodots or dots (also called circular micelles) that are much more uniform in size and tend to pack in a two-dimensional array (typically hexagonal, but also square16) at still higher hydrophilic block content. The picture that is now held of these structures, including from the results of the present work, is illustrated in Figure 1. The nanodots
Figure 1. Schematic representation of the morphologies referred to in the text, red and green designating the hydrophobic and hydrophilic blocks, respectively.
(i.e., elevated core of the circular surface micelles) are composed of the condensed hydrophobic block that avoids the aqueous surface and are surrounded laterally and underneath by the relatively long hydrophilic block adsorbed as a monolayer on the water surface. The strand-like aggregates are composed of the elevated hydrophobic block in elongated form, with the significantly shorter hydrophilic block in monolayer form lying on both sides and below. Planar aggregates consist of an upper layer of the hydrophobic block that is protected from the water surface by the very short hydrophilic block acting as a wetting monolayer. It was later observed with diblock copolymers (generally of relatively low hydrophilic block content)17−21 and even polystyrene homopolymers22 that these kinds of morphologies, often mixed together and with variable shapes and sizes, can also be observed for one and the same composition, and that they are influenced by spreading solvent concentration and total block copolymer molecular weight. Other morphologies, such as planar aggregates with holes (“nanofoams”),19,23 linearly interconnected nanodots (“necklaces” and “chains”),18,24 and nanorings18 (or “nanodonuts”25), have also been observed under different experimental conditions. The dependence of the morphologies on experimental conditions and the observation of mixed morphologies in the same film were attributed to kinetic effects operating during the film forming process, leading to frozen-in morphologies.17−19,21,22,26 Moffitt and colleagues have interpreted the various morphologies as resulting from different frozen-in stages of a dewetting process of an initially uniform film of spreading polymer solution, where planar structures can dewet, among other forms, into strands and strands into dots.19,22 This has put into question the role of block composition in determining the morphologies. On the other hand, we recently proposed that the spreading film (before dewetting) may itself develop a particular structure at some stage that influences the final morphology, as explained in ref 15 to rationalize the highly uniform nanostrand morphology. Besides, the nanodot
Figure 2. Schematic structure of PS-P4VP/PDP.
been widely studied in the bulk28−34 and, more recently, in the form of thin films (usually obtained by spin-coating techniques),8,35−37 which, from a materials point of view, are thus complementary to the present work. Among the few block copolymer systems whose LB film morphologies have been investigated as a function of relative block length, the first one studied involves a diblock polyelectrolyte series based on PS-P4VP where the P4VP block is quaternized by an n-decyl chain (to be referred to hereafter as PS-P4VPQ10, or PS-P4VPQn for an n-alkyl chain of unspecified length).9,38,39 This series constitutes a particularly relevant comparison for the present PS-P4VP/PDP system, since both systems have the same molecular architecture that combines a linear block with a comb-like block. They differ by the type of attachment of the alkyl chain to P4VP, which is via noncovalent hydrogen bonding in PSP4VP/PDP and by covalent bonding in PS-P4VPQ10. They also differ in that P4VPQ10 is an ionic block, whereas PSP4VP/PDP is not. Other copolymer systems whose LB behavior was investigated at various block compositions are PS-PEO [(PEO: poly(ethylene oxide)]21 including a linear triblock40 and star diblocks,41 and some PS-poly[alkyl(meth)acrylates],10,11,42 all nonionic block copolymers. In general, they tend to show the same three basic LB morphologies as PSB
dx.doi.org/10.1021/la3040962 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Table 1. Characteristics of the PS-P4VP Diblock Copolymers Used nomenclature
Mn(PS)a
Mn(P4VP)a
Mw/Mn
n(PS)b
n(P4VP)b
mol % P4VP
4% 8% 9% 12% 14H%c 16% 19% 292%c 294%c 33H%c 46% 49%
41 400 34 000 35 500 40 000 252 000 42 100 32 900 31 900 41 500 72 000 20 000 20 000
1900 2900 3600 5600 43 000 8100 8000 13 200 17 500 35 000 17 000 19 000
1.07 1.07 1.06 1.09 1.09 1.08 1.06 1.08 1.07 1.09 1.08 1.09
398 326 341 384 2420 404 316 306 398 691 192 192
18 28 34 53 409 77 76 126 166 333 162 181
4.3 7.9 9.1 12.1 14.4 16.0 19.4 29.2 29.4 32.5 45.8 48.5
a
As indicated by the supplier. bn(PS), n(P4VP): number of PS and P4VP repeat units. cSubscript H indicates polymers with significantly higher molecular weight than the others; subscripts 2 and 4 indicate the decimal to distinguish two very similar compositions. isotherms (surface pressure vs mean molecular area isotherms) were obtained by symmetrical compression of the barriers at a speed of 10 mm/min (15 cm2/min). Isotherms were repeated at least three times for each composition. Under the same conditions as for the isotherms and following a 15− 25 min wait at the desired surface pressure (3−15 mN/m) for barrier stabilization (“standard” procedure), Langmuir−Blodgett (LB) monolayers were transferred onto mica substrates during vertical withdrawal from the subphase at a rate of 5−10 mm/min (no differences were observed between these two speeds). In the so-called “solventassisted” procedure,14 used with dilute solutions only, the waiting step for chloroform evaporation after solution deposition was omitted; instead, barrier compression was implemented as soon as practically possible after solution deposition. To minimize the time required, the Langmuir bath area was first decreased by about half (to approximately 150 × 230 mm2) and the volume of solution spread was adjusted to be near the onset of surface pressure. Atomic Force Microscopy. The deposited films were dried in a clean box overnight at room temperature and then imaged in air by atomic force microscopy (AFM) in tapping mode using a Multimode AFM with a Nanoscope IIIa controller (Digital Instruments/Veeco, Santa Barbara, CA) and silicon probes (MikroMasch U.S.A.: rectangular, no aluminum coating on tip and backside, resonance frequency 265−400 kHz, tip curvature radius
