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Surface Self-Assembly and Properties of Monolayers Formed by Reverse Poly(butylene Oxide)- Poly(ethylene Oxide)-Poly(butylene Oxide) Triblock Copolymers With Lengthy Hydrophilic Blocks Eva M Villar-Alvarez, Adriana Cambon, Mateo Blanco-Loimil, Alberto Pardo-Montero, Raquel MartinezGonzalez, Silvia Barbosa-Fernandez, Miguel A. Valdez, Josue Elias Juarez, and Pablo Taboada J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Surface

Self-Assembly

and

Properties

of

Monolayers Formed by Reverse Poly(butylene oxide)-Poly(ethylene oxide)-Poly(butylene oxide) Triblock Copolymers with Lengthy Hydrophilic Blocks.

Eva Villar-Alvarez,a,‡ Adriana Cambón,a,b,‡ Mateo Blanco,a Alberto Pardo,a Raquel Martínez,a Silvia Barbosa,a Miguel A. Váldez,c Josué Juárez,c* Pablo Taboadaa*

a

Grupo de Física de Coloides y Polímeros, Departamento de Física de Partículas,

Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain b

Centre for Nanomedicine and Theranostics, DTU Nanotech, Technical University of

Denmark, Building 423, 2800 Lyngby, Denmark c

Departamento de Física, Universidad de Sonora, Resales y Transversal, 83000

Hermosillo Sonora, Mexico

ACS Paragon Plus Environment

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ABSTRACT: The surface behavior and properties of several reverse poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) block copolymers, BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21, at the air/water interface have been analyzed by drop tensiometry, Langmuir film balance, and atomic force microscopy (AFM). The kinetic adsorption process of block copolymer chains at the air/water interface is a diffusion-controlled process at short times. Structural rearrangements of the copolymer backbones are progressively more important as the adsorption carries on. The adsorption layers formed at the interface display evident solid-like behavior in the whole range of frequencies analyzed even at the lowest frequencies used probably as a result of the interconnection between hydrophobic ends of polymeric chains. All the copolymer display adsorption isotherm profiles composed of four different regions in which the different characteristic regimes (“pancake”, mushroom”, “brush” and collapsed conformations) are observed. The differences observed between copolymers come from the different block lengths and, hence, hydrophilic to hydrophobic (EO/BO) block ratios. In this regard, it is observed that the shortest copolymer, BO8EO90BO8, having the lowest block ratio display the complete adsorption profile at much lower areas per molecule and within the narrowest range. Images of copolymer films transfer at solid substrates at determined transfer pressures enable to have direct information about the structure and size of formed structures. In this manner, relevant differences were observed between copolymers with the shortest blocks

(BO8EO90BO8,

BO12EO227BO12)

and

those

with

the

longest

ones

(BO20EO411BO20 and BO21EO385BO21). In this regard, surface circular micelles were observed for the former at low surface transfer pressures, evolving to continent-like structures, first, and then de-wetted structures as the transfer pressure increases. Conversely, for BO20EO411BO20 and BO21EO385BO21 copolymers micelle formation is


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noted at lower transfer pressures than the shortest counterparts, and the formed micelles appear to be elongated, interconnected and with larger thickness. As the transfer pressure increases, attractive micellar interactions enhanced and, then, led to form a dense network of interconnected micelles, first followed by an evolvement to continentlike and de-wetted structures, as also observed for BO8EO90BO8, BO12EO227BO12 copolymers.


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INTRODUCTION Due to to their composition and amphiphilic character, many block copolymers possess an important surface/interfacial activity at surfaces/interfaces compared to smaller surfactant molecules. As occurred in aqueous solution, chemical differences in monomer-forming blocks can result in phase separation that, in particular on a surface, can lead to the creation of polymeric nanostructured assemblies/monolayers in two dimensions with a broad domain of technological applications such as pharmaceuticals, ferrofluids, hybrid nanocomposites, sensors, nanopatterns, foaming and emulsification, amongst others.1-7The change of surface properties such as wettability, lubricity, adhesion, or protein repelling as well as the colloidal stability8,9 can be finely tuned by the modification of the properties of each copolymer block, i.e., their composition, structure and length.10 Diblock copolymers have been widely studied at the air-water interface by the Langmuir monolayer methodology as, for example, poly(styrene)-poly(ethylene oxide) (PS-PEO),11 poly(styrene)-poly(acrylic acid),12 PS-PAA, poly(methyl methacrylate)poly(ethylene

oxide),13

PMMA-PEO,

poly(ethylene

oxide)-poly(propylene

oxide),14PEO-PPO, poly(n-butyl acrylate)-poly(ethylene glycol),15 PnBA-PEG, or poly(ethylene oxide)-poly(lactic-co-glycolic acid),16 PEO-PLGA, amongst others; meanwhile, the behavior and surface properties of triblock or multiblock ones have been little less analyzed17-19 and specially focused on triblock copolymers of the Pluronic and Tetronic type composed of PEO and PPO blocks.20-22 This class of block copolymers is of special interest by their interfacial and associative properties in aqueous solution, commercial availability and biocompatibility of most varieties. In addition, some of them have been approved by regulatory agencies (i.e. FDA and EMA) to be used in pharmaceutical formulations and medical devices.23However, these copolymers present


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several drawbacks as, for example, their inherent polydispersity after oxyanionic polymerization as a consequence of the transfer reaction during the polymerization of the PO blocks,24 which results in an important diblock component in the synthesised polymeric material. This gives rise to a subsequent strong variation in their bulk and interfacial properties from batch to batch. To circumvent this problem and expand the amphiphilicity of the resulting copolymers, during last years a series of more hydrophobic block copolymer counterparts with similar architecture but with the PPO segment replaced by a more hydrophobic one, as poly(butylene oxide) (PBO), poly(styrene oxide) (PSO) or poly(phenyl glycidyl ether) (PG) have been proposed.25-28 In particular, great attention has been paid specially to copolymers bearing 1,2-butylene oxide (BO) end-capping a central PEO block, that is, BOnEOmBOn, where n and m would denote the lengths of butylene oxide and ethylene oxide blocks, respectively. With BO monomer transfer reaction is not a problem, and the broadening of the EO distribution can be avoided by polymerizing BO last.29 Also, the amphiphilicity of the resulting family of copolymers is largely enhanced provided that BO is six-fold more hydrophobic than the PO monomer unit.26 This fact involves that BOnEOmBOn copolymers self-assemble in the form of micelles at much lower concentrations than their POnEOmPOn counterparts, with the corresponding savings of material together with an enhanced ability to solubilize hydrophobic compounds in their micellar cores.30 For these reasons, BOnEOmBOn copolymers have recently been evaluated as suitable nanocarriers for the transport and delivery of chemotherapeutic drugs allowing sustained release patterns and reduced side toxicity.31-33In addition, as a result of the protrusion of the hydrophobic BO blocks out of the formed polymeric micelles upon association leading to their mutual bridging, these copolymers are also able to form in


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situ different types of physical gels, which have been shown to be excellent depots for the release of, for example, antifungal drugs.34 Despite the self-association in bulk and the micellar properties of this kind of copolymers have been extensively studied,30,35-37 an in-depth analysis of their behavior at the air-water interface and their surface properties is still lacking to the best of our knowledge. Hence, in the present work we analyze the surface behavior of five different BOnEOmBOn

copolymers,

BO8EO90BO8,

BO12EO227BO12,

BO14EO378BO14,

BO20EO411BO20 and BO21EO385BO21, which differs in their EO and BO block lengths. As a consequence of their chain architecture allowing the protrusion of the hydrophobic blocks out of one micelle and inclusion within a near one micelle, these block copolymers formed interconnected nanostructures acting as truly rheological modifiers in bulk solution. Hence, it would be also expected that not only the viscoelastic behavior at the air/water interface but also the structural integrity of the resulting architectures and the dynamic responses of these assemblies to their environment and formation conditions should be affected by the copolymer architecture and length. In this manner, we effectively observed that despite all the copolymers displayed complete adsorption isotherm profiles with the different conformational states of the copolymer chains (“pancake”, “mushroom”, “brush” and collapsed conformations); these are expanded over much shorter areas per molecule for copolymers bearing the shortest lengths, BO8EO90BO8, BO12EO227BO12. Nevertheless, normalizing the observed behavior by the number of EO and BO units, it is observed that EO influence is similar for all copolymers independent of their length, whereas the copolymer bearing the highest BO/EO ratio (BO8EO90BO8) displays an adsorption profile shifted to much lower areas. In addition, all copolymers display a characteristic solid-like behavior, which can be fitted to a single element Maxwell model and the formation of common


6

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patterns of assembled nanostructures depending on the transfer pressure and block copolymer architectures of the monolayers, as denoted by AFM analysis.

EXPERIMENTAL DETAILS Materials. Triblock copolymers were prepared by oxyanionic polymerization as reported elsewhere.30-34Table 1 summarizes the molecular characteristics of the resulting copolymers.

Table 1. Molecular Characteristics of the Selected Copolymers Copolymers BO8EO90BO8 BO12EO227BO12 BO14EO378BO14 BO20EO411BO20 BO21EO385BO21

Mn a (g/mol) 5100 11700

Mw/ Mnb 1.07 1.05

Mw (g/mol) 5460 12280

CMCc (mg/mL) 0.330 0.031

38d -

rh (nm) 13.0d 12.0f

18600 21000 20000

1.12 1.08 1.10

20830 22680 22000

0.058 0.012 0.025

18e 17d 9e

18.5e 18.9d 20.4e

N

a

Estimated by NMR; bEstimated by GPC; Mw calculated from Mn and Mw/Mn. Estimated uncertainty: Mn to ±3 %; Mw/Mn to ±0.01.Taken from DLS data in cref 30,dref. 36, and eref. 35.f From SANS data in ref. 37.

Dynamic Surface Tension and Dilatational Rheology Measurements. Pendant bubble tensiometry was used to determine the dynamic surface tension and the surface dilatational rheology of the present block copolymers adsorbed at the air/water interface. The bubble was formed at the tip of a U-shape stainless steel needle (0.5 mm inner diameter) immersed in an aqueous copolymer solution of desired concentration. Measurements were carried out in a Track Tensiometer equipment (I.T. Concept, France) adapted to determine surface tension values in real time with an accuracy of ± 0.1 mN/m. Surface tension and surface rheology estimations are based on the digital profile of a drop image and the resolution of the Gauss-Laplace equation. Win Drop software (I.T. Concept, France) was used to obtain the surface tension values by mean of

axisymetric

drop

shape

analysis.

Aqueous

solutions


of

BO8EO90BO8,

7


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BO12EO227BO12,

BO14EO378BO14,

BO20EO411BO20

and

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BO21EO385BO21block

copolymers were prepared at a constant concentration of 1·10-3 mg/mL, below their respective critical concentration values (cmc) values. At the air-water interface the analysis of the surface tension (γ) measurements was followed in a relative wide time (>10000 s) after the bubble was formed, and the dilatational elastic storage (E’) and loss (E’’) modulus were determined after such period using a frequency sweep of 3.14, 2.09, 1.04, 0.52, 0.31, 0.15, 0.078, and 0.039 rad/s. All measurements were carried out in a time interval of 400 s using a 10 % amplitude oscillation of the maximum volume drop. Adsorption Isotherms. Surface-pressure isotherms were recorded for monolayers spread from chloroform solutions (1 mg/mL) onto a nanopure-quality water subphase in a Langmuir-Blodgett (LB) Teflon trough (model 611 from Nima Technologies Ltc., Coventry, UK) equipped with a microbalance, Wilhelmy plate, dipper, and two moveable barriers. Volumes ranging from 5 to 120µl of either BO8EO90BO8, BO12EO227BO12,

BO14EO378BO14,

BO20EO411BO20

and

BO21EO385BO21block

copolymer solutions (1 mg/mL) were spread dropwise on a Millipore water subphase with a Hamilton microsyringe (the volume varied depending on the concentration and copolymer type in order to maintain an initial pressure of 0 mN/m). To ensure complete evaporation of the solvent, a time lag of 15 min was applied between the deposition of the copolymer and the beginning of compression at a rate of 5 mm min-1. Temperature was kept constant at 25 °C with a water bath and all experiments were carried out inside a dust-free glass box. Langmuir-Blodgett copolymer films were transferred onto freshly cleaved mica substrate for imaging. A piece of freshly cleaved mica was first dipped into the water and the polymer monolayer was applied to the water surface. A waiting time of at least 15 min was allowed for solvent evaporation. The monolayer was then compressed to the


8

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targeted surface pressures at a speed of 5 mm min-1. Once the targeted surface pressure was reached and stable, the cleaved mica was lifted up at a speed of 1 mm/min with the barriers maintaining the pressure during the transfer. Films were allowed to dry and stored in separate containers prior to imaging. Transfer ratios were between 0.9-1.0 depending on polymer and spreading solution concentration. Atomic Force Microscopy (AFM). The resulting LB polymeric films were imaged using a JEOL instrument (model JSPM 4210) in non-contact mode using nitride cantilevers NSC15 from MicroMasch, USA (typical working frequency and spring constant of 325 kHz and 40 N/m, respectively). The AFM samples were dried in air or under a nitrogen flow when required. Control samples (freshly cleaved mica and bare aqueous solution) were also investigated to exclude possible artifacts. Topography and phase-shift data were collected in trace and retrace directions of the raster, respectively. The offset point was adapted accordingly to the roughness of the sample. The scan size was usually 500 nm (aspect ratio, 1 x 1), with a sample line of 256 points and a step size of 1 µm. The scan rate was tuned proportionally to the area scanned and kept within the0.35-2 Hz range. Each sample was imaged several times at different locations on the substrate to ensure reproducibility. Diameters and heights of copolymer aggregates were determined by sectional analysis taken from the average of several sections through different aggregates.

RESULTS AND DISCUSSION Dynamic Surface Tension of Adsorbed Copolymers. Some of the most important molecular

characteristics

of

the

selected

block

copolymers

BO8EO90BO8,

BO12EO227BO12, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21 are shown in Table 1. As observed, the cmc of these block copolymers increases as the hydrophobic


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block length does, while almost a negligible effect is noted upon changes of the EO block lengths. The selection of the present five copolymers allowed us to analyze the polymer adsorption and surface rheological properties at the air/water interface in terms of the influence of the hydrophobic and/or hydrophilic block lengths and hydrophilic/hydrophobic block ratio. Previous papers have elucidated the influence of the relative poly(oxyethylene) block lengths of the present copolymers in their bulk properties,25-27,30 that is, in their micellization extent and rheological properties, with an special emphasis in their capability of forming flower-like micelles which can entangle thanks to the protrusion of the hydrophobic blocks out of the micellar core giving rise to micellar clusters and gels. This phenomenon allows regulating the viscosity and elastic behavior of these polymeric solutions, making them useful as associative thickeners and gelling systems to build up drug delivery depots.34-36 The temporal evolution of the surface tension of copolymers BO8EO90BO8, BO12EO227BO12,

BO14EO378BO14,

BO20EO411BO20

and

BO21EO385BO21

at

a

concentration of ca. 0.001 mg/mL is illustrated in Figure 1a. The adsorption of a polymer on an interface occurs in three stages. Firstly, the polymer must diffuse to the interface, secondly it must attach to the interface, and finally, it has to spread itself. Despite the low concentration used in the experiments, the true equilibrium state was not attained for any of the copolymers since surface tension is still decreasing after ca. 3 h of incubation, which points to a very slow surface polymer adsorption kinetics at the air water/interface.


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3.5

75

b) 3.0

70

2.5

-2

Γ (mg m )

65

γ (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

55

2.0

1.5

1.0 50

0.5

a)

45

0.0 1

10

100

1000

10000

0

10

20

30 1/2

time (s)

40

50

60

1/2

t (s )

Figure 1: a) Evolution of surface tension, γ, with time at the air-water interface for copolymers

()BO8EO90BO8,

()BO12EO227BO12,

()BO14EO378BO14,

()BO20EO411BO20, and () BO21EO385BO21. b) Variation of surface concentration, Γ, with time at the air-water interface for copolymers () BO8EO90BO8, () BO12EO227BO12, () BO14EO378BO14, () BO20EO411BO20, and () BO21EO385BO21.

For all copolymers, the dynamic surface tension plots displayed a quasi-plateau region at short times with γ values similar as those of water, which denotes the relatively slow migration of copolymer molecules from the bulk solution to the air/water interface if compared, for example, with more hydrophobic PS-PEO diblock and triblock copolymers, which possess faster diffusions.19 This area would correspond to the so-called lag phase. For copolymers bearing very long EO blocks such as BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21 this lag phase is little shorter, which can be attributed to the enhanced repulsion between EO chains giving rise to an increment of the surface pressure. Next, the post-lag phase is characterized by a rapid decrease of surface tension with time rather similar for all the studied copolymers. This reduction is associated with the progressively increased number of copolymer chains at the air/water interface by a diffusion controlled process, which is followed by


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conformational changes of polymer molecules upon adsorption through lateral interactions between the adsorbed chains stabilizing the formed monolayer. Strikingly, after a slowing down of the adsorption rate after the post-lag phase (between ca. 200 and 2000 s), which in classical dynamic surface isotherms would led to equilibrium surface tension values (the so-called final or last adsorption phase), surface tension values rapidly decrease again at longer times and for all the copolymers. In particular, faster γ decreases at this stage were observed for copolymer BO8EO90BO8 (followed by BO12EO227BO12) as a consequence of their shorter EO/BO ratios and, hence, of their larger relative hydrophobicities. For those copolymers bearing both longest BO and EO blocks the decrease rates are slower and take longer, in agreement with the need of longer diffusion times and/or additional configurational changes in the adsorbed polymeric monolayers. In addition, for BO8EO90BO8 and BO12EO227BO12 the smoothness of the decrease of γ at the largest incubation times may indicate the beginning of the expected quasi plateau region, which points to the existence of a dense layer exerting steric repulsions towards newly arrived macromolecules, which hinders their adsorption at the interface.38 Conversely, despite their hindrance the surface tension tends to continuously decrease during this stage for the other copolymers (BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21) because of their high surface activity. The existence of this second post-lag phase might be related to either polymer conformational rearrangements leading to polymeric cluster formation, allowing more monomer to be adsorbed, and/or the existence of polymer adsorption-desorption processes. However, further studies would be required to elucidate the predominant mechanism. To gain some insight into the kinetics of copolymers adsorption at the air/water interface, the time dependence of surface tension was analyzed. Fits of the experimental


12

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γ(t) curves in the whole time range to a single exponential function representing a pure diffusion-controlled adsorption were clearly inadequate. Instead, consideration of a mixed diffusion followed by a molecular reorientation process at the interface theoretically handled by Joos et al.39 allows the perfect fitting of the dynamic surface tension data through a simplified double exponential function of the form

= ⁄ + ⁄

(1)

where τ1 and τ2 correspond to the diffusive and reorientational relaxation times, respectively, A0 and B0 are adjustable parameters, and γ0 is the surface tension of pure water, γt is the surface tension at time t and, γ∞ is the extrapolated equilibrium surface tension at t→∞. Results obtained from the fitting procedure can be observed in Table 2. It is clearly seen that the first diffusive step is faster than the second one, and both appear separated. τ1 is rather similar for all the copolymers whereas τ2 increases with the block length (and hence, the molecular weight) of the copolymers. The presence of the observed smooth intermediate pseudoplateau in the γ(t) data, which is more visible as the copolymer block lengths increases, have been related to a transition from an initial diffusion stage to an internal reorganization of the copolymer chains which might be adopting a brush conformation, as also observed for Pluronic-type block copolymers.40 Nevertheless, this transition is here noted at lower concentrations (ca. two orders of magnitude) as a consequence of the much larger EO block lengths of the present BOnEOmBOn copolymers


13


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Table 2: Relaxation times and apparent diffusion coefficients from the bulk solution to the interface for copolymers BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20, and BO21EO385BO21 Block copolymer τ1 (s) τ2 (s) 108Da-w (m2/s) BO8EO90BO8 130 ± 10 4262 ± 40 0.3 ± 0.05 BO12EO227BO12 114 ± 2 5279 ± 30 3.7 ± 0.4 BO14EO378BO14 121 ± 2 9023 ± 44 1.6 ± 0.2 BO20EO411BO20 102 ± 2 6494 ± 82 3.9 ± 0.5 BO21EO385BO21 149 ± 3 9172 ± 66 8.7 ± 0.5 Moreover, considering that in the induction stage at short times and sufficiently low concentrations, the surface molecules interact quasi-ideally. In this case, the simplified form of the Ward-Tordai equation allows one to derive an apparent diffusion coefficient near the air – water interface as:

=

√

→

(2)

where Da-w is the apparent diffusion coefficient near the air – water interface (m2/s), R the universal gas constant (J mol-1 K-1), T the temperature in K, t is the time in seconds and C0 the bulk polymer concentration (mol/m3). A clear disagreement between Da-w (see Table 2) and bulk diffusion coefficients at infinite dilution (ranging between 1.21.0·10-11 m2s-1) for the present block copolymers is noted, and this deviation confirms the appearance of more complex adsorption processes as well as the diffusive pathway, in agreement with previous data. In addition, surface pressures values were derived from experimental dynamic surface tension data were converted to Γ values by using the scaling expression Π∼Γy, with y values calculated from adsorption isotherm experiments (see below) in order to get additional information about the underlying adsorption kinetic mechanisms of the present block copolymers at the air-water interface. The proportionality constant was set equal to 1 (m2·mg-2)y mN·m-1, which implies that the surface pressure is set to 1 mN·m-


14

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1

at a surface concentration of 1 mg·m-2. This value is typically found for many

polymers.41 Figure 1b shows that the short-time asymptotic behavior is clear diffusioncontrolled (below ca. 400 s) as denoted by the linear dependency of Γwith t1/2 as deduced from the Ward-Tordai equation, Γ(t) = 2c0(Dt/π)1/2,42 where Γ is the amount of adsorbed species at the air-water interface, c0 the polymer concentration, and D the bulk diffusion coefficient. Conversely, at longer incubation times > 400s configurational and reorientation processes of the polymer molecules at the surface seem to become predominant as observed for a severe change in the slope, which has been also found for other PEO-based block copolymers such as commercial Pluronics43 or those bearing poly(styrene) and poly(styrene oxide) hydrophobic blocks.19,44. Surface Elasticity. To obtain further information about the type of interactions between polymer chains at the air-water interface, the frequency dependence of dilational moduli was analyzed. Figure 2 shows the evolution of storage, E’, and loss, E’’ dilatational moduli with frequency in semi-logarithmic plots for the adsorbed layers of block copolymers BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21, respectively. Molar copolymer concentration differences only slightly affected the final dilatational modulus values (not shown). At the air/water interface the adsorption layers manifest obvious solid-like properties in the whole accessible frequency range (ω = 0.039-1.0 rad/s) with E´> E´´, thus, with the elastic contribution clearly dominating the surface dilatational behavior and with a very low significance of dissipative processes during dilational deformations of the block copolymer layers. The present values suggested a more fluid layer for copolymer BO8EO90BO8 followed by BO12EO227BO12,in agreement with their shorter blocks (especially EO ones) despite their smallest EO/BO ratios, which restricts the formation of interconnected polymeric chains, in particular for BO8EO90BO8,


15


resembling the observed behavior in bulk solution. In contrast, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21show the most important solid-like behavior, which involves the formation of interconnected copolymer chains at the interface to avoid contact between hydrophobic BO blocks and water; these bridging is also affected by the reduction of hydrophilicity of EO blocks as a result of their extraordinary length. 24

20

16 -1

E´, E´´ (mN m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

12

8

4

0 0.1

-1

1

ω (rad s )

Figure 2: Storage, E´(closed symbols), and loss, E´´ (open symbols), moduli as a function of frequency for copolymers () BO8EO90BO8, () BO12EO227BO12, () BO14EO378BO14, ()BO20EO411BO20, and () BO21EO385BO21.

The frequency dependence of both E’ and E’’ is generally explained as a result of the relaxation process undergone by block copolymer molecules at the air/water interface.45 In this regard, we observe that the storage modulus E´ is almost frequencyindependent for all the copolymers except at low frequencies, for which a certain increase is observed. Meanwhile, the loss moduli E´´ are close to zero. This might involve the existence of exchange processes between the surface layer and the bulk solution which are slightly important at low frequencies, with the system not reaching the equilibrium and returning most of the stored energy. Conversely, at larger frequencies the behavior is the opposite, with exchange processes being negligible thanks to the stability of the surface layer. In order to account for the observed slight


16

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observed frequency dependence of both E´ and E´, these were fitted to a single Maxwell element model in which the relaxation time τ is related to the adsorption-desorption exchange of polymer chains between the surface and the adjacent sub-interface layer during dilatational perturbation and slight molecular rearrangement of the adsorbed layer38 ! "#$

!! "#$

=

=

%'%&

( (

)%'%&

%'%&

(

)%'%&

(3)

(4)

where E’ is related to lateral interactions between polymer segments at the interface plane and is relevant for the rigidity of interfacial film, whereas E’’ values are influenced by different molecular reorganisation processes such as the expulsion of polymer chains from the interface upon compression and/or the interactions of polymer molecules with adjacent liquid ones.7 The obtained values are displayed in Table 3. From this Table, it can be observed a certain increase of both the relaxation time and the dilatational modulus of copolymers as the block lengths (molecular masses) rise, i.e. BO8EO90BO8 possesses the shortest relaxation time, τ, which indicates a slight faster adsorption-desorption exchange and molecular reorganization of the polymer chains adsorbed at the interface as a result of its lower block length and larger relative hydrophobicity compared to the other copolymers. Also, a comparison of copolymers BO14EO378BO14and BO21EO385BO21 also denotes an increase in the same quantities as the length of the BO block increases as a consequence of a probable larger entanglement of the polymeric chains to avoid contact with water.


17


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Page 18 of 42

Table 3: Relaxation Time,τ, and Dilatational Moduli, E0, of Copolymers BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20, and BO21EO385BO21 at the Air Water Interface Block copolymer E0 (mN m-1) τ/s BO8EO90BO8 98.6 ±5.7 17.2 ± 0.1 BO12EO227BO12 17.7 ± 0.2 100.4 ± 6.3 BO14EO378BO14 18.7 ± 0.1 112.9 ± 8.7 BO20EO411BO20 140.6 ± 19.8 20.7 ± 0.1 BO21EO385BO21 21.0 ± 0.1 139.2 ± 8.5 Adsorption

Isotherms.

Monolayers

of

BO8EO90BO8,

BO12EO227BO12,

BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21 block copolymers were spread on the air/water surface contained in a Langmuir-Blodgett trough and the Π-A isotherms were obtained (see Figure 3a). All five copolymers displayed a classical isotherm pattern divided in four well-differentiated regions. However, the surface pressures of BO8EO90BO8 monolayers were measurable at significantly smaller areas per molecule than the other copolymers. Several regions of the compression isotherms can be distinguished, which indicates that the block copolymers adopt different molecular conformations as the area decreases. When no pressure is exerted, polymeric chains should lie on the interface with a flat (“pancake”) conformation parallel to the surface plane,46,47 which agrees with the subsequent observed increment of surface pressure with decreasing area, with a slope that becomes larger as the monolayer is compressed, an indication of the transition from an expanded phase (the so-called pancake) to a more condensed phase. The area occupied is a function of the number of BO and EO units. Roughly, the maximum cross-sectional area occupied by an EO unit is 13-16.5 Å2 and that occupied by a BO unit 17.5-19.0 Å2.48 Once hydrated, the areas of EO and BO units increase by 8.5 Å2 (a water molecule) so that it is clear that the polymer chains remain expanded, and the monolayer can be considered as in a gas state. Once the compression of the monolayer began, the surface pressure gradually increased to different extents. For example, up to ca. 10700 and 2500 Å2 for


18

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BO21EO385BO21 and BO8EO90BO8, respectively, i.e. depending on the maximum transverse area occupied by all EO and BO units and, hence, by the total block lengths. At this respect, as the pressure is increased the hydrophobic BO blocks, initially on the air/water interface, might be lifted away. The surface pressure at which this phenomenon occurs is usually low owing to the weakness of the interaction between the aqueous medium and the hydrophobic groups (Figure 3a-ii). At larger pressures between ca. 10.0 to 13.0 mN/m, a region with lower slope appears as a result of a change in the copolymer conformation, which should enable the penetration of the hydrophilic EO chains into the subphase: the copolymer adopts the so-called “mushroom” conformation (Figure 3a-iii). As a consequence, this conformational transition denoting a larger condensation (higher slopes) is more evident for those copolymers bearing the longest EO blocks such as BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21than those with shorter ones (especially BO8EO90BO8). In this region, there is a slight increase in the surface pressure as the surface area is decreased pointing to a true first-order transition does not occur. This “pseudoplateau” (almost negligible for BO8EO90BO8)can be considered as a rearrangement of the BO and EO coils into “loops” within the monolayer regime (for those copolymers bearing very long EO blocks) and/or the immersion of more EO units (especially for those copolymers bearing the shortest EO blocks) in the aqueous subphase. Previously, adsorption isotherms of poly(ethylene oxide)-poly(styrene oxide), PEO-PSO, and poly(ethylene oxide)-poly(propylene oxide), PEO-PPO, copolymers with different EO lengths showed that the EO block length largely influences the character of this transition.49


19


35

BO8EO90BO8

a)

30

BO12EO227BO12

30 iv

25

BO12EO 227BO12

BO20EO411BO20

BO14EO 378BO14 BO20EO 411BO20

BO21EO385BO21

BO21EO 385BO21

20

20

Cs (mN/m)

15 iii

-1

Π (mN m-1)

b)

BO8EO90BO 8

BO14EO378BO14

10

10

5

ii

i

0 0

5000

10000

15000

20000

25000

30000

35000

0 0

Area per molecule (Å)

5

10

15

20

25

30

35

π (mN/m)

35

25

d)

c) 24

34

Surface pressure (mN/m)

23

Surface pressure (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

22 21 20 19 18

33

32

31

30

29 17 16 300

400

500

600

700

800

900

1000

28 400

600

800

2

Area/molecule (A )

1000

1200

1400

1600

2

Area/molecule (A )

Figure 3: a) Surface pressure, π, isotherms for spread monolayers of the different copolymers. b) Compression moduli of the copolymers. Compression-decompression cycles of c) BO8EO90BO8 and d) BO21EO385BO21 copolymers.

As seen from Figure 3a, at the transitions from pancake to mushroom and from mushroom to brush conformations the surface pressure mildly increases with decreasing area, which supports the view that the adsorbed EO segments are continuously squeezed out from the water surface as the available area becomes smaller and smaller during compression. Further compression of the interface causes a steep increase in surface pressure as a consequence of the block copolymer molecules becoming gradually much closer, as well as the mobility of the blocks being largely restricted due to both space limitations and stronger lateral interactions; as a consequence, the copolymer molecules


20

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reorganize into the so-called “brush conformation” (Figure 3a-iv).50 In the subphase, mainly EO chains entangle with neighboring copolymer molecules, whereas at the air/water interface BO blocks and some EO ones can form loops and, even, can be partially solubilized in the aqueous EO layer. If the area is further restricted, both hydrophilic and hydrophobic blocks become stretched (condensed state).48The extrapolation of the experimental data above this pressure indicates that the areas occupied per molecule at the condensed state (AL) are 635, 1035, 1070, 970 and 1280 Å2 for

BO8EO90BO8,

BO12EO227BO12

and

BO14EO378BO14,

BO20EO411BO20

and

BO21EO385BO21, respectively. The observed transitions are best observed by calculating the compressional modulus

of the monolayer (*+ = ,). As shown in Figure 3b, several phase transition with maxima at ca. ∼5.5, ∼8 and ∼25 were observed. The exact location of the maxima slightly varies depending on the copolymer. The first two maxima can be related to EOrelated phase transitions as a result of penetration of EO chains into the subphase in such a way that a swollen three-dimensional structure on the aqueous phase might be formed. The subsequent minimum and third maximum observed at larger surface pressures is a result of the progressive collapse of the interface as a result of repulsion between BO segments in the top layer and EO ones in this (thanks to the large copolymer block lengths) and in the subphase. Hence, this would facilitate chain desorption within the layer and the formation of loops and tails. Cycles of compression and expansion in π-A isotherms denote the presence of hysteresis for all the copolymers. Figure 3c-d shows the compression and decompression cycles for copolymers BO8EO90BO8 and BO21EO385BO21, respectively, as examples. For highly compressed layers, the surface pressure during expansion was lower at a given area per molecule than during compression. Also, compression results


21


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in substantial hysteresis for the first cycle, and this was more important as the molecular weight of the copolymer increases (for similar values of concentrations, the longest polymer adsorbs at lower surface pressures) in agreement with

previous

observations.51,52 Hysteresis between compression and decompression cycles is often observed for Langmuir layers due to dissolution into the subphase,53 slow rearrangements from orientation-dependent and short-range attractive forces within the layer,54 or monolayer collapse.55 In the present case, the compression/expansion hysteresis shows that this transition was either irreversible on the timescale of the expansion or that some copolymers was ejected from the interface into the subphase at elevated pressures; in fact, the progressive shift to lower surface pressure values as the number of cycles increases might also point to polymer desorption into the subphase as one of the causes of hysteresis.56 Additionally, conformational rearrangements are also present provided that the cycles were performed in the region of the mushroom to brush transition. In particular, the extremely lengthy EO blocks of copolymers BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21 would specially favor the formation of loops and the subsequent development of stronger entanglements than for copolymers bearing shorter hydrophilic blocks, which are expected to give rise to larger hysteresis upon expansion if relaxation and reptation times are slow, as observed in Figure 3c-d upon comparison of BO8EO90BO8 and BO21EO385BO21 copolymers. An additional confirmation of this point might be provided by the quasi-superimposition observed among the expansion step of one cycle and the compression of the following. The fact that higher molecular areas are obtained for the compression step of a next cycle in comparison to that of a previous expansion one indicates that compressed segments can return to the interface upon relaxation.14 The effect of the polymer structure can be easily visualized by plotting the surface


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pressure as a function of the average surface area per monomer. Isotherms plotted with the molecular areas normalized with respect to the number of EO and BO segments are shown in Figure 4a,b. Figure 4a demonstrates that the normalized isotherms of all the copolymers fall almost on a single curve for surface areas up to the so-called mushroom regime (the pseudoplateau region, area iii in Figure 4a). This indicates that the isotherms in this regime are mainly dependent on the EO block length, being hydrophobic forces between water molecules and hydrophobic blocks much less important. In particular, the limiting pancake area (Ap) was an average of ca. 47 ± 2 Å2/EO unit, similar to those reported for PEO homopolymers,57 which points to the negligible role of BO blocks in the pancake region. Of course, as the EO block length decreases the BO blocks might overlap upon compression diminishing the importance of the plateau transition, as observed for BO8EO90BO8 copolymer. From the pseudoplateau region, the isotherms slightly deviate one each other but still display rather similar behavior. This deviation can be originated from their different EO/BO ratios, suggesting higher amounts of BO leads to quicker compression of the EO chains within the plateau region. This is additionally corroborated when analysing the ratio of the limiting area in the condensed phase per BO unit, AL/BO, for which a decrease from values ca. 40-43 for the copolymers bearing the shortest EO blocks to ca. 25-30 for those with the longest ones takes place. This fact denotes the influence of BO blocks in film packing in the condensed state, and confirms the formation of brushes in which EO blocks going to the subphase. When isotherms normalization is performed in terms of BO units, the isotherms are rather

superimposed

in

the

whole

range

for

copolymers

BO12EO227BO12,

BO20EO411BO20 and BO21EO385BO21, with rather similar EO/BO ratios (between 1820), whereas deviated slightly for BO14EO378BO14 maybe as a consequence of its larger


23


polydispersity and also larger EO/BO ratio (ca. 27) (see Figure 4b). Conversely, the isotherm obtained for BO8EO90BO8 copolymer is rather different except at the concentrated regime. This denotes that the isotherm shape is now determined by the BO chain length and EO/BO molar ratio. The dependence of the EO/BO ratios can be directly related to the existence of more important rearrangement processes: under increasing compression, the polymers that possess shorter EO/BO ratios start first to lift the BO blocks off the interface (zone ii) and, then, reorder the EO blocks in the subphase (zone iii), in which different configurations can be adopted as the formation of loops or bridges. As a consequence, the behavior of the present block copolymers except BO8EO90BO8 is specially determined by the anchoring of EO chains at the air/water interface, with little influence of BO blocks on surface pressure or the localization of EO chains normal to the interface. Only for BO8EO90BO8 modification of repulsion interactions between BO and EO blocks can lead to slight changes in the expected concentration profile at the interface, as also observed for PS-PEO and PEOPS-PEO,47,49 Pluronics™51 and Tetronics™20 (four star-shaped ethylene oxidepropylene oxide block copolymers) block copolymers.The balance between these opposite interactions will depend on the copolymer surface density, the BO block length, and the solution temperature. 35

40

a)

BO8EO90BO8

b)

BO12EO227BO12

30

35

BO14EO378BO14 BO21EO385BO21

25

20

-1

Π (mN m )

-1

30

BO20EO411BO20

25

Π (mN m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

15

20 15

10 10

5

5

0 0

20

40

60 2

80

100

0 0

500

1000

-1

Area ( Å monomer )/NEO

2

1500

2000

2500

-1

Area (Å monomer )/NBO

Figure 4: Surface pressure isotherms considering a) the average area per EO unit and b)


24

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the average area per BO unit for the five copolymers analyzed. The behavior of the copolymer films may be examined regarding the degree of mixing within the adsorbed layer. In particular, in the semidilute regime the Flory coefficient of the system can be obtained from the des Cloizeaux equation,58 which describes the relationship between the surface pressure and the surface concentration (Γ) of a polymer by

Π = CA-y = KΓy with y = dν/(dν-1)

(5)

where C and K are proportionality constants, A is the molecular area, y is the scaling exponent, d is the geometrical dimension, and ν is the Flory coefficient used to express the radius of gyration in terms of molecular weight (Rg≈ Mν) being a measure of the solvent quality.59 For chains in good solvent conditions, ν is theoretically predicted to be 0.75 and 0.6 for a 2D and 3D self-avoiding walk, respectively. Consequently, the theoretical exponent for the scaling of the surface pressure with the area per molecule is y = 3 for the 2D semidilute regime and y = 2.25 for the 3D one. Plotting the variation of the surface pressure Π versus the inverse of the area per molecules 1/A on a doublelogarithmic scale for the different copolymers in the intermediate region between the dilute regime and the plateau we have obtained y exponents of 2.66, 2.68, 2.76, 2.92 and 2.98 for BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21, respectively. These values indicate that for the three first copolymers the layer is an intermediate between a two-dimensional and a three-dimensional array denoting certain intermixing between EO and BO blocks within the fully hydrated copolymer layer at the air-water interface; for BO20EO411BO20 and BO21EO385BO21it evolves toa 2D interpenetrated layer.


25


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Page 26 of 42

Langmuir-Blodgett Films. Finally, copolymers spread at the air/water interface were transferred as Langmuir-Blodgett (LB) films using cleaved mica as a substrate in order to gain further information on the film structure. Several types of aggregate structures were observed for all copolymers investigated. This surface assembly can take place (i) during the solvent evaporation step or (ii) during the LB transfer. Dynamic light scattering measurements (DLS, data not shown) of the copolymers in chloroform did not show any evidence of micelle formation at the concentration used, which implies that the formation of surface features in LB films is a result of spontaneous copolymer assembly at the air/water interface rather than the transfer of polymeric micelles formed in the spreading bulk solution.60 Figure 5 shows AFM images of block copolymer films of copolymers BO8EO90BO8, BO12EO227BO12 and BO20EO411BO20 on freshly cleaved mica obtained at different surface pressures as examples. ForBO8EO90BO8 at a surface pressure of 5 mN/m one few disperse spherical aggregates with diameters of ca. 30 ± 0.4 nm are observed (Figure 5a).


26

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a)#

b)#

c)#

d)#

e)#

f)#

g)#

h)#

i)#

j)#

k)#

l)#

Figure 5: AFM images of LB films of copolymer a-f) BO8EO90BO8, g) BO12EO227BO12, and h-l) BO20EO411BO20 at different surface pressures (see text for details). The inset denotes the 3-dimensional views of the corresponding structures. Scale bars denote in a) and g) 200 nm; in b) 1.5 µm; in c), d), e), f) and j) 1 µm; in h) 650 nm, and in i) 6 µm.

The average height of the aggregates obtained at this surface pressure is 0.8± 0.2 nm, pointing to a completely flattened micellar structure (a surface circular micelle) on the substrate. Also, it is worth mentioning that the roughness measured from Figure 5a was ca. 0.9 Å indicating only a fraction of the area is covered with the copolymer. When the transfer surface pressure is rise up to 10 mN/m, micelles of BO8EO90BO8


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evolve to larger aggregates of sizes ca. 1 µm and height ca. 1 nm ± 0.2 (Figure 5b,c) as a result of the enhancement of micelle-micelle interactions; these nanostructures somehow resemble the continent-like ones observed, for example, for PS-PEO block copolymers at rather similar surface pressures.11It is worth mentioning that we cannot rule out the possible influence of particle/micellar aggregation upon film drying in the present experiments. At larger surface pressures (ca. 15 mN/m), a dewetting process is observed, with the formation of several micron-sized structures with thickness of ca. 8 nm derived from the crystallization of EO units at large pressures and the incompatibility between blocks (Figure 5d,e). Finally, at very large surface pressures (>20 mN/m), a full layer with some cracks covering the full substrate can be observed with a thickness of ca. 10 nm, which confirms its multilayer nature (Figure 5f). On the other hand, for copolymers bearing longer EO and BO blocks micelle formation seems to appear also at relatively low surface pressures, as denoted by the larger aggregate sizes (ca. 240 ± 30 nm with a height of 0.7 ± 0.2 nm) observed at 5 mN/m for copolymer BO12EO227BO12 (Figure 5g). Additionally, enhancing the transfer pressure a rather similar behavior as that found for copolymer BO8EO90BO8is observed (not shown). Conversely, for copolymers bearing the longest EO and BO blocks, for example, BO20EO411BO20and BO21EO385BO21, some structural differences in the transfer LB films emerge. For example, for BO20EO411BO20at low surface pressures (5 mN/m), the presence of hairy-like aggregates with lengths of ca. 110 ± 20 nm and widths varying from ca. 20-25 nm for the thinnest elongated micelles to ca 80 nm for the more globular/denser observed aggregates in the sample is noted. This observation would agree with the formation of interconnected chains due to the protrusion of hydrophobic blocks out of one micelle to enter in the neighboring one (Figure 5h). As the surface pressure increases (ca. 8 mN/m) the density and number of interconnections


28

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between micellar clusters continues to grow, with the development of a denser network denote by height increments (from 2.8 to 4.3 nm) (Figure 5i). At a transfer pressure 12 mN/m this network evolves to larger, continent-like structures of micron size and ca. 4 nm in thickness (Figure 5j); for the present copolymer this type of structure appears at slightly larger pressures than for less lengthy copolymers asBO8EO90BO8. Finally, a similar dewetting process as for the shortest copolymers, followed by the formation of entire multilayer films can be distinguished at ca. 15 and 20 mN/m, respectively(Figure 5k-l).

CONCLUSIONS In summary, the spontaneous adsorption of the present BO8EO90BO8, BO12EO227BO12 and BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21 appears to be a relatively slow process slightly influence by the molecular weight and hydrophobicity of the copolymers, becoming the interface saturated earlier for the copolymer with the shortest blocks and lowest EO/BO ratio (BO8EO90BO8). At early stages, the adsorption process is diffusion-controlled and next, structural rearrangements of the copolymer chains become progressively more important with time. The adsorption layers at the air/water interface possess a clear elastic behavior (solid-like properties) for all the tested copolymer in the whole analyzed frequency range, with minimal contribution of dissipative processes. All the copolymers display adsorption isotherms composed of four different stages corresponding to the different conformations adopted by the polymeric chains at the interface, being these confined to smaller areas per molecule for the copolymers with the shortest blocks. Normalization of the adsorption isotherms per hydrophilic and hydrophobic monomeric units denote that the EO block length does not have almost influence on the surface behavior of the copolymers except at very large


29


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Page 30 of 42

surface pressures; conversely, a strong influence of the hydrophobic block takes places for the copolymer with the lowest molecular weight. Finally, surface circular micelles are observed for copolymers BO8EO90BO8 and BO12EO227BO12 at relatively low transfer surface pressures, which evolve to continent-like structures first and then to dewetted microstructures resulting from EO crystallization as the transfer pressure increases. In comparison, for copolymers bearing the longest blocks BO20EO411BO20 and BO21EO385BO21, elongated micelles seem to be formed at surface pressures much lower than surface micelles of shortest copolymers. Slight increments in surface pressure lead to more dense interconnected micellar layers, which progressively seem to evolve to continent-like first, and de-wetted and extended cracked films later as the transfer pressure increases and approaches to that of the collapsed state.

AUTHOR INFORMATION Corresponding Authors * (P.T.) Email: [email protected]. Phone: +34881814111 * (J.J.) Email:[email protected]. Phone: +52 16622592100, ext. 2212 Author Contributions ‡

These authors contributed equally

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Authors thank AEI and Xunta de Galicia for research projects MAT2013-40971-R, MAT2016-80555-R and EM2013-046, respectively. Authors also thank granted EDFR funds. S.B. is grateful to MINECO for her Ramón y Cajal fellowship.


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E´, E´´ (mN m-1)

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Sinusoidal oscilla,on

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poly(ethylene oxide) - American Chemical Society

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