Surface Segregation of Low Surface Energy Polymeric Dangling

Article pubs.acs.org/Macromolecules

Surface Segregation of Low Surface Energy Polymeric Dangling Chains in a Cross-Linked Polymer Network Investigated by a Combined Experimental−Simulation Approach A. C. C. Esteves,*,† K. Lyakhova,† L. G. J. van der Ven,†,‡ R. A. T. M. van Benthem,†,§ and G. de With† †

Laboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands ‡ AkzoNobel, Automotive & Aerospace Coatings, Sassenheim, The Netherlands § DSM Ahead, Geleen, The Netherlands ABSTRACT: The surface properties of polymeric materials are generally determined by the chemical groups present at their surface. For low surface energy polymeric films the preferential location of the low surface energy chemical groups at the surface is crucial for the low-adhesion properties which are of high interest for various engineering fields. Controlling the surface segregation of such chemical groups would allow maintaining the materials properties at high performance level all through its service lifetime. In this work we used a combined experimental−simulation approach to study the surface segregation and the bulk distribution of low surface energy polymeric dangling chains, chemically bonded to a cross-linked poly(urethane) network. The surface properties of the cross-linked polymeric films, prepared with different experimental parameters, were investigated by contact angle (CA) measurements and X-ray photoelectron spectroscopy (XPS). A dissipative particle dynamics (DPD) method was used to model the distribution of the low surface energy dangling chains, at the surface and in the bulk of the cross-linked systems, equivalent to the ones prepared experimentally. The combined results show with excellent agreement the segregation of the low surface energy polymeric dangling chains toward the air−polymer interface. The influence of different experimental parameters, such as fluorine concentration and dangling chains molecular mobility, on the surface segregation is discussed. DPD simulations revealed further details of the polymeric films structure: the formation of a depletion zone beneath the top surface and the presence of highly dynamic clusters of the polymeric dangling chains in the bulk of the network, but not at its surface.

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INTRODUCTION For many of the current technological applications the performance of the materials relies on their surface properties, e.g., adhesion, wettability, friction, lubricity, reflectivity, and biocompatibility. These special properties are intimately related to the surface chemical composition which determines the materials behavior at interfaces, allowing for advanced applications such as low-adhesion or low-friction, selfcleaning,1,2 antibacteria,3,4 drag-reducing, or anti(bio)fouling5 purposes. Hence, for the majority of these materials, the surface composition needs to differ significantly from the bulk. This fact is particularly relevant for polymeric materials since their macromolecular nature allows a large degree of freedom for the segregation of specific chemical groups/components at the surface, e.g., species with the low surface tension, low molecular weight, or poor compatibility with the bulk material. Because of the relevance of the surface segregation phenomena, this feature has been investigated for a wide variety of polymeric systems.6−8 Since many years, a common way to manipulate the surface properties of polymeric films is © 2013 American Chemical Society

through the introduction of a small amount of components, e.g., additives or surfactants, which will locate preferentially at the air−polymer interface,9,10 driven by low compatibility with the polymer matrix. However, this approach has become less attractive due to possible leaching of the added components. Furthermore, this segregation generally ends with all the additives located at the interfaces, where they are easily worn out by the routine handling of the materials, lowering rapidly its performance. One way to overcome this problem is to use low surface energy components chemically bonded to the polymer matrix. For this purpose polymers with controlled architectures and specific end-functional groups which trigger the segregation behavior have been widely investigated. Some well-studied examples are (co)polymers with per-fluorinated blocks or low Received: October 29, 2012 Revised: January 15, 2013 Published: February 6, 2013 1993

dx.doi.org/10.1021/ma302236w | Macromolecules 2013, 46, 1993−2002

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Scheme 1. Low Surface Energy Films Components (EXP = Experimental and SIM = Simulations): (1) Polyester Precursor (TMP-PCLx, x = 3n, with n Number of Caprolactone Units per Arm), (2) Tri-isocyanate, and (3) Fluorinated Dangling Chains (F17C8-PCLy, y = Total Number of Caprolactone Units)

information on how the segregation occurs and what are the changes at the molecular level (e.g., dynamics and kinetics of the polymer system) at the air−polymer interface and/or in the bulk. For this purpose, computational methods are an excellent tool to access the details of the segregation dynamics as well as the spatial distribution of components. Moreover, the search for optimum parameters to create certain behavior becomes significantly more efficient when guided by simulations. Hence, the aim of this work was to use a combined experimental−simulation approach to study in depth the surface segregation behavior of a series of model cross-linked polymeric systems, containing fluorinated dangling chains chemically bonded to the polymer matrix, varying the concentration and spacer length of the fluorinated dangling chains as well as the network density. The use of simulation techniques in combination with experiments is a very promising approach.23 A wide range of problems in polymer solutions have been successfully studied by coarse-grained simulation methods addressing polymers− surfactant interactions,24 viscoelastic properties,25 and polymer melt mixtures.26 One of the most widely used methods is dissipative particle dynamics (DPD), initially proposed by Hoogebrugge et al.27 DPD is a coarse-grained off-lattice simulation technique which involves the movement of coarsegrained particles (beads) in a box with discrete time steps. The stochastic DPD simulation method was later modified by Español28 to guarantee the proper thermal equation of state. The beads represent fluid regions as well as parts of molecules. The internal degrees of freedom of the particles are integrated out, and the beads interact with simplified pair potentials. Hence, the main advantage of the DPD method is that it allows studying phenomena for relatively long time and length scales. In fact, this method has been used to study many soft systems such as polymers and colloids29,30 and has been applied to more complex situations, e.g., the dynamics of a drop at a liquid−solid interface,31 flow, and rheology in the presence of polymers grafted to surfaces,32 vesicle formation of amphiphilic molecules,33 and polyelectrolytes.34 More recently, DPD was also successfully used to study self-healing systems.35,36

surface energy components chemically bonded to the main polymer chain.11,12 Another possible strategy is to use modified polymer networks with low surface energy components, mainly fluorinated10,13,14 or silicon based,15,16 which have enough molecular mobility to self-orient toward the air−polymer interface. Ming et al.,17 for example, reported low surface energy cross-linked polymer films obtained through the reaction of fluorinated-oligoester precursors with a poly(isocyanate) cross-linker. These films showed a decrease of about 20 mN/m of the surface tension as the fluorine (F) content in the initial formulation increased up to 1.4 wt %. Furthermore, the F % at about 5 nm depth from the top surface was found to be 20−80-fold above the stoichiometric level expected from the initial formulation.17 For such systems, it was also reported that the cross-linking conditions have a relevant effect on the self-segregation behavior;18 e.g., it was found that films cured at different temperatures have different cross-linking rates leading to dissimilar surface chemical compositions and wettability. A particularly successful system was reported by Dikić et al.19 consisting of a cross-linked polymer network with chemically bonded hydroxyl-terminated fluorinated dangling chains. A polymeric spacer was included in the dangling chains to control (1) the miscibility with the bulk network, avoiding extreme surface segregation, and (2) the molecular mobility of the fluorinated dangling chains. Hence, the low surface energy components orient easily toward the air−polymer interface, but a reasonable amount remains homogeneously distributed in the bulk. Consequently, these cross-linked films show a selfreplenishing behavior19 by the reorientation of the dangling chains upon wear or damage of the films. Up to the present, such surface segregated systems have only been studied by a few experimental techniques, such as contact angle (CA) measurements, which only provide an indirect probe of the chemical groups at the surface and little insight into the behavior of interfaces at the molecular level. The surface composition can be obtained by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) can be used to probe the different phases at the surface20,21 and the topography.22 Nevertheless, these methods still give limited 1994


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Table 1. Trifunctional Polyester Precursors (TMP-PCLx) and Respective Characterization TMP-PCLx

x (3n)

n (DP)

Mn 1H NMR (g/mol)

Mn GPC (g/mol)

PDI GPC Mw/Mn

Mn MALDI (g/mol)

DSC Tg (°C)

DSC Tm (°C)

TMP-PCL18 TMP-PCL24 TMP-PCL36 TMP-PCL48

18 24 36 48

6 8 12 16

1590 2230 2892 3890

2468 2480 4533 6428

1.18 1.19 1.14 1.11

1650 2455 2683 3826

−74 −72 −72 −71

21 35 36 41

Table 2. Polymeric Dangling Chains (F17C8-PCLy) and Respective Characterization F17C8-PCLy

y (DP)

theor Mn

Mn 1H NMR (g/mol)

Mn GPC (g/mol)

PDI GPC Mw/Mn

Mn MALDI (g/mol)

DSC Tg (°C)

DSC Tm (°C)

F17C8-PCL8 F17C8-PCL12 F17C8-PCL16 F17C8-PCL22

8 12 16 22

1400 1800 2300 2800

1648 1924 2042 2192

2818 3191 3804 3781

1.13 1.17 1.06 1.12

1415 2101 2329 2443

−65 −66 −65 −64

47 50 50 50

With our dual experimental−simulation approach we aim to understand the effect of using different experimental parameters on the surface segregation, rather than learning by trial and error which variables have to be tuned to achieve the desired segregation profile. We prepared and characterized experimentally (by CA measurements and XPS) several polymeric precursors and cross-linked films which were simultaneously represented by a DPD simulation method. The surface segregation of the polymeric dangling chains is described in a parallel between experimental characterization results and simulations. The influence of different experimental parameters, such as the concentration of the low surface energy component and the molecular mobility of the dangling chains, on the surface segregation is discussed. Further details of the polymeric network structure formed are revealed by the simulations and will be also described.

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Table 3. Parameters of the Cross-Linked Films Studied

RESULTS AND DISCUSSION

Several cross-linked polymeric films were prepared according to methods described in the literature.19,37 Well-defined polycaprolactone-based oligomer precursors were used to build the cross-linked bulk matrix (Scheme 1 (1EXP)). These precursors consisted of three-armed hydroxyl-functionalized polyesters which were synthesized by ring-opening polymerization (ROP) with controlled degree of polymerization (DP) (Scheme 1 (1EXP) and TMP-PClx in Table 1). The low surface energy polymeric dangling chains consisted of perfluoroalkyl-endcapped linear polyesters also prepared by ROP with controlled functionality and DP (Scheme 1 (3EXP) and F17C8PCLy in Table 2). Several low surface energy polyurethane films were prepared via reaction of the polyester precursors (TMP-PCLx) with the tri-isocyanate (Scheme 1 (2EXP) and Table 3), using fixed crosslinking conditions, such as temperature, solvents, and curing time. The cross-linked films were prepared with precursors with different DP (Table 1) and different fluorine concentrations (via adjusting the dangling chains concentration in the initial formulation) (Table 2). The characterization of the crosslinked films was carried out by differential scanning calorimetry (DSC) and differential mechanical analyses (DMA) while their surface was characterized by X-ray photolelectron spectroscopy (XPS) and dynamic water contact angle (CA) measurements. In the simulation study all the experimental components were modeled in a coarse-grained manner (Scheme 1). For the polymer precursor (TMP-PCLx), the caprolactone (CL) repeating units and the TMP initiating group were represented by “CL” and “TMP” beads, respectively (Scheme 1 (1SIM)).

a b

TMP-PCLx x=

F17C8PCLy y=

wt % dangling chainsa

wt % Fb

Tg (°C)

18 24 36 48 24 24 24 24 24 24 24 24 24 18 24 36 48

0 0 0 0 16 16 16 16 16 8 12 16 22 8 8 8 8

0.0 0.0 0.0 0.0 4.6 9.6 15.0 21.0 34.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0

0.0 0.0 0.0 0.0 0.5 1.0 1.5 2.0 3.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

−58 −59 −59 −60 −46 −46 −46 −44 −47 −48 −47 −44 −48 −47 −56 −57 −62

(Grams of dangling chains/grams of polymer precursor) × 100. (Grams of fluorine/total grams of the system) × 100.

The end-caprolactone reactive units were represented by “CL− H” beads. The fluorinated precursors were also modeled with “CL” beads and using two “F” beads for the fluorinated part (Scheme 1 (3SIM)). The cross-linker was represented by four beads (“CR” and “B” type) in correspondence with equal volume considerations (Scheme 1 (2SIM)). The simulation box was built up as a 3-dimentional cube with periodic boundary conditions. First, the polymer and cross-linker molecules were placed randomly in the box,26,34 keeping the overall reduced density, ρ = 3. Second, a layer of “void” beads was placed at the top of the polymer film, mimicking air and creating the polymer−air interface (Figure 1a, air beads are not shown). Finally, the polymer film was sufficiently cross-linked and equilibrated. The profiles of the fluorine-containing beads were extracted from the simulation results. To estimate the fluorine/carbon (F/C) ratio, the simulation boxes were divided in layers and the number of fluorine beads was calculated per layer integrated in the x−y direction. Figure 1b shows a typical example where the surface segregation at the air−polymer interface is clearly observed by the presence of a strong peak for concentration of fluorine beads at the air−polymer interface. The F/C ratio was obtained averaging the fluorine concentration over time to minimize fluctuations. 1995


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Figure 1. (a) Simulation box: (left) complete system and (right) details of the air-interface (up) and bulk (down) with the “F” beads highlighted and (b) fluorine (beads) profile (average concentration per layer) before (closed squares) and after (open circles) surface segregation.

Fluorine Concentration: Driving Force Effect. For the low surface energy polymeric films prepared, the fluorinated part of the dangling chains provides the necessary driving force to surface segregation due to its low surface energy. Hence, the overall fluorine (F) concentration is expected to have an effect on how and to what extent the dangling chains distribute in the bulk and/or orient toward the air−polymer interface. For this reason, we investigated the effect of the fluorine concentration on the surface segregation of the dangling chains upon film formation. Cross-linked polymer films with different fluorine concentration were prepared and characterized (Table 3). F17C8PCL16 was used as dangling chain, TMP-PCL24 as bulk precursor, and the cross-linking conditions/procedures were kept constant. Figure 2 shows the variation of (a) the advancing water contact angle (CAadv) and (b) the F/C atomic ratio of films made with different fluorine concentration. The CAadv increases with the F % in the initial formulation up to the 2 wt % F coatings. At

higher concentrations no further increase was observed (Figure 2a). In contrast with this observation, the fluorine concentration at the surface (as given by XPS) increases continuously with the increase of the F % in the initial formulation (Figure 2b). The corresponding simulations show very similar results (Figure 2b). The surface concentration of fluorine beads (“F”) increases with the F % in the initial formulation, in the same range and trend as the experimental values. Moreover, the F/C atomic ratios at the coatings top-surface, by both experimental and simulations were in general much larger than those of the initial formulations. For example, for a cross-linked film containing 2 wt % of F in the initial formulation, the expected theoretical F/C value for the “bulk” and the surface is 0.02, assuming that the fluorine is equally and homogeneously distributed all through the coating. The experimental F/C ratio at the surface (as determined by XPS) was 0.24 (Figure 2a). These results clearly indicate the preferential surface segregation of the fluorinated components toward the air− 1996


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Figure 2. (a) Advancing water contact angle (CA, deg) and (b) fluorine/carbon atomic ratio (F/C) as a function of the concentration of fluorine (F, wt %) in the initial formulation. EXP: determined experimentally by XPS; SIM: DPD simulation. TMP-PCL24 was used as bulk precursor and F17C8PCL16 as dangling chains.

Figure 3. (a) Dynamic water contact angles (CA, deg) and (b) fluorine/carbon atomic ratio (F/C) as a function of the length (DP) of the dangling chains (F17C8PCLy, y = total CL units), with an overall 2 wt % fluorine in the initial formulation. EXP: determined experimentally by XPS; SIM: DPD simulation. TMP-PCL24 was used as bulk precursor and F17C8PCL16 as dangling chains (dashed lines: F/C theoretical value according to the initial 2 wt % of F in the formulation).

the Teflon sample shows a F/C atomic ratio of 1.66 ± 0.18 and water CAadv of 112 ± 2°. Considering that Teflon has a fully fluorinated surface, this result confirms that for the coatings studied the fluorine concentration at the surface is far from being saturated with fluorinated compounds, even for the highest F concentrations studied. Nevertheless, the water CAadv of these coatings reaches values that are very close to the Teflon coating, indicating a high hydrophobic character even for considerably low F concentration at the coatings air−interface.

polymer interface. Similar results were previously reported by Dikic et al.19,38 Furthermore, it suggests that the leveling of the CAadv values at a certain F % in the initial formulation is related not only with the fluorine concentration but also with the orientation/distribution of the fluorinated dangling chains at the coatings air−interface, since from XPS results no saturation level is reached. For comparison purposes, a Teflon-coated substrate was analyzed by XPS and dynamic CA measurements. Interestingly, 1997


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The fluorine simulation profiles also showed further interesting details about the distribution of the fluorinated dangling chains through the coatings (e.g., in Figure 1a,b). The “F” beads concentration decreases considerably in the layers just beneath the air−polymer interface; i.e., a depletion zone is formed. This effect was observed for all the films prepared. The existence of this depletion zone was also proposed on previous experimental works,19,38 based on the F/C ratios obtained at different depths of the films, probed by XPS analyses of microtomed layers. Another interesting feature observed from the simulations was that the “F” beads which were unable to reach the air− polymer interface seem to form highly dynamic clusters in the bulk. These clusters fluctuate considerably in size. The large majority is composed by four “F” beads aggregated (i.e., ∼3 nm size), but larger aggregates of up to 18 “F” beads were also observed. We are currently investigating further the formation and behavior of such clusters by simulations. Furthermore, cryo-TEM analyses have been performed on cross-linked films with 2 wt % of fluorine to prove experimentally the existence of these clusters. Although some differences were observed as compared with cross-linked films without fluorinated dangling chains, the experiments carried out so far have not been conclusive. This is mainly due to low contrast of the images obtained, which is related to the low percentage of fluorine in the system and the similar chemical composition of the dangling chains and bulk polymer precursors. Further experiments are underway and will be reported in the forthcoming papers. While evaluating the wettability of films with varying F content, it was observed that their advancing water CA follows a similar trend as registered by the F/C ratio: it slowly increases with the F % in the initial formulation, reaching a maximum value of about 114° for 2 wt % F. For higher concentrations, the CA seems to reach a plateau, which is very close to the maximum CA that can be reached on a flat and fully fluorinated surface, according to Young’s equation.39 The advancing water CA experiments have also showed that, compared with the films with no dangling chains, the water CA increased considerably even with a small percentage of fluorine; e.g., for 0.5 wt % F the angle increased from 73° to 105°. This fact may be related to the long polymeric spacer used which provides enough molecular mobility for the dangling chains to reach more easily the air−polymer interface. In this case the polymeric spacer has ∼16 caprolactone units while each polymeric arm of the bulk-network precursor has an average of 8 units (TMP-PCL24). Molecular Mobility of the Dangling Chains. In order to study the influence of the molecular mobility of the dangling chains on the surface segregation, several films were prepared using TMP-PCL24 and fixed percentages of F17C8-PCLy dangling chains, where y is the degree of polymerization (DP) of the polymeric spacer varying from 8 to 22 (Table 2). Figure 3a shows the variation of the advancing water CA on films prepared with 2 wt % of F in the formulation. The water CA increases with the length of the polymeric spacer, up to DP = 16. These results seem to indicate that dangling chains with long polymeric spacers, therefore a more extended reach, segregate more easily to the air−polymer interface. Surprisingly, for dangling chains with a very long polymeric spacer (22 caprolactone units), a considerable decrease of the CA was registered. The F/C ratio obtained for these films, however, does not show a significant variance for dangling chains up to

DP = 16 and only a slight decrease for a DP = 22. The corresponding simulations of these systems showed a similar trend (see Figure 3b). To investigate this effect further, we used the simulations to study the depth of the depletion zone beneath the air−polymer interface as a function of the molecular mobility of the dangling chains (achieved by using polymeric spacers with different length) (Figure 4). We expected a deeper depletion zone for

Figure 4. Depletion zone depth (au) as a function of the length (degree of polymerization, DP) of the dangling chains (F17C8PCLy, y = total CL units), with an overall 2 wt % fluorine (SIM) and (b) simulation box with the “F” beads highlighted evidencing the existence of a depletion zone and the presence of clusters of the dangling chains.

systems containing dangling chains with more extended reach (higher length), as they would self-segregate more easily toward the surface. Unexpectedly, we did not find a clear dependence. These results seem to indicate that the difference observed on the wettability of the films (by the CA measurements) is not associated with surface concentration of the fluorinated dangling chains, as also suggested by the small differences in the F/C ratios obtained by XPS. A possible explanation could be different “surface arrangements” of the dangling chains at the air−polymer interface in the presence of water. For short polymeric spacers, e.g. DP from 8 to 16, the dangling chains that reach the air−polymer interface can arrange in an “extended” configuration which exposes the fluorinated parts toward the water phase (Scheme 2a). However, for DP = 22, the polymeric spacer is very long (as compared to 8 CL units per arm of the bulk precursor). Hence, once at the surface, the dangling chains may adopt a “coiled” configuration exposing more hydrophilic parts (from the caprolactone spacer) of the molecule (Scheme 2c) and reducing the hydrophobic character. Water-induced surface Scheme 2. Possible Scheme of the Configuration of the Dangling Chains at the Air−Polymer Interface with (a) Short Polymeric Spacer (DP = 8), (b) Long Polymeric Spacer (DP = 16), and (c) Very Long Polymeric Spacer (DP = 22)

1998


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Figure 5. (a) Advancing water contact angle (CA, deg) and (b) fluorine/carbon ratio (F/C) as a function of the length (DP) of the bulk precursor (TMP-PCLx=3n, n = CL units on each arm), with an overall 2 wt % F. EXP: determined experimentally by XPS; SIM: DPD simulation. TMP-PCL24 was used as bulk precursor and F17C8PCL8 as dangling chains (dashed line: F/C theoretical value according to the initial 2 wt % of F in the formulation).

dangling chains concentration at the air−polymer interface with the bulk precursor length (Figure 5b). Using polymer precursors with different length of polymeric chains allowed us to prepare polymer films with different network mobility (see Table 3). Nevertheless, these combined results indicate that at this concentration of dangling chains the change of mobility in the polymer network segments does not seem to have a strong influence on the surface segregation of the low surface energy groups. It should be noted however that all the Tg values obtained are still below room temperature, and this may be the explanation why no significant differences were observed. Similar results were observed for films with the same polymer bulk precursors and dangling chains with longer polymeric spacers, namely F17C8-PCL16. The possible effect on the size and distribution of the clusters of the dangling chains in these polymer films, with different mobility for the polymer bulk segments, is currently being investigated by the simulations and will be reported in the following papers.

rearrangements have been previously reported for several other polymeric films.11,40−42 These rearrangements occur very rapidly and have been attributed to the hydration and swelling, resulting in the emergence of the swollen part of the polymer at the surface40,41 or by reorientation of the polymer segments, backbone, or pendant groups.42 This surface rearrangements were further confirmed by the receding water contact angles measured on these coatings (CArec). The water CArec are more sensitive to hydrophilic groups at the surface. For all the coatings prepared the CArec were significantly lower than the CAadv (see example in Figure 3a) . This indicates that after a large water droplet is placed on the top of the coatings and sufficient time is given to “wet” the surface (as to measure CArec) it penetrates into the coatings and the hydrophilic parts of the system (from the polymeric spacer and the urethane groups of the polymer network) reorient toward the surface, rendering it less hydrophobic. A second explanation to be considered is the higher probability of formation of entanglements which can occur as the long polymeric dangling chains are mixed in the bulk network. This possibility would also explain the lower F/C ratios registered both by experiments and simulations, for very long polymeric spacers (Figure 3b). Network Parameters. Finally, we investigated polymeric films made from bulk precursors with different length on the polymeric segments between cross-links, which may provide different space ranges for the dangling chains to move. Figure 5a shows the water advancing water CA and Figure 5b the F/C ratio of films prepared with the TMP-PCLx precursors, where n is the DP of the polymeric chains per arm of bulk precursor, varying from 18 to 48 (Table 1). A fixed percentage of fluorine was used in the formulation (2 wt %) via F17C8-PCL8 dangling chains. For all the films prepared, no significant difference of the water CA or F/C ratio was registered. The corresponding simulation study did not show any significant dependence of

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CONCLUSIONS We have studied the surface segregation of low surface energy components (perfluorinated dangling chains) at the air− polymer interface of cross-linked poly(urethane) films. A number of important parameters of the system were varied systematically, and their influence on the hydrophobicity of the polymer film was investigated. It was found that the concentration of dangling chains influences significantly the surface segregation of the low surface energy species. However, after a certain threshold (2 wt % of F) the contact angle of the fluorinated components at the surface reaches a plateau, and the hydrophobicity cannot be further increased. Changing the length of the polymeric dangling chains does not seem to affect significantly the surface segregation. However, very long dangling chains are detrimental for the surface hydrophobicity, possibly due to spatial configuration 1999


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using the VGX900 data system collecting an average of 60 scans. All carbon C 1s peaks corresponding to hydrocarbons were calibrated at a binding energy of 284.5 eV, to correct for the energy shift caused by charging. The spectra were acquired at takeoff angle of 0° relative to the surface normal, corresponding to a probe depth of ∼10 nm. The fluorine/carbon (F/C) ratio was determined from curves fitted to the C 1s spectra, according to different carbon environment (C−C, C−H, C−O, C−N, CO, C−F2, and C−F3). The areas were corrected for the elements sensitivity, and the F/C ratio was estimated from the corresponding area ratios by the equation

rearrangements at the surface and in the bulk, in the presence of water.11,40−42 Hence, the combination of a significant fluorine concentration in the initial formulation (2 wt %) and a polymeric spacer with a specific length, allows reaching an optimum hydrophobicity for these cross-linked films. The methods used in this study, experiments−simulations, showed a very good correspondence. Experiments provided data to optimize and tune the surface-segregation behavior and to validate the simulation parameters. On the other hand, simulations gave additional insight into the system dynamics and network structure. Previously, the existence of a depletion zone beneath the top surface-segregated layer was speculated.19,38 The simulations confirmed the presence of this depletion zone for all the cross-linked films studied. This depletion zone does not seem to depend on the molecular mobility of the dangling chains. Finally, the presence of “clusters” of the dangling chains in the bulk was observed for all the polymeric films studied while no clusters were detected at the air−polymer interface. Further studies are underway with cryo-transmission electron microscopy (TEM) to prove experimentally the existence of these clusters. The results of our combined experimental−simulations study provide experimental data and a better understanding of the distribution of low surface energy polymeric dangling chains in the bulk and at the surface of model polymeric films, which may be applied to tune the surface segregation on many other surface segregated polymeric and composite materials.

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[A(F)] F = C [A(CF2) + A(CF3) + A(CO) + A(C−O) + A(C−C)] (1) Synthesis of Compound 1EXP (Scheme 1): Three-Arm Poly(caprolactone) Polyester Precursors (TMP-PCLx). The synthesis of the TMP-PCLx polyester precursors was made according to the method described by Dikic et al.37 A typical procedure is as follows: all the synthesis was performed under a dry nitrogen atmosphere. For the TMP-PCL24, a mixture of TMP (0.964 g, 0.0072 mol) and fumaric acid (2.005 g) was first heated to 90 °C, and then CL (20 mL, 0.18 mol) was added to the flask. The reaction mixture was stirred with a magnetic stirrer. After 24 h it was diluted with tetrahydrofuran (THF) (or CH2Cl2, 20 mL),43 and the polymer was precipitated in n-heptane (or n-hexane, 200−300 mL).43 The precipitate formed was filtered and washed with methanol (in the case of low-Mw PCL initiated by TMP, a mixture of water and methanol (1:2) was used).43 After washing a white, sticky powder was collected and dried in vacuum at 40 °C for 4 h. After 24 h of reaction, the yield determined by gravimetry was 87% and decreased to 65% after the second precipitation, carried out for extensive purification. 1H NMR (CDCl3): 1.39 (broad s, 2H), 1.65 (broad s, 4H), 2.31 (broad s, 2H), and 4.06 ppm (broad s, 2H).43 The polyester precursors synthesized with different degree of polymerization (DP), and the respective characterization is described in Table 1. To tune the DP of the polymer precursors, monomer and alcohol in different molar ratios were added to the reaction vessel. The molar ratio between monomer and fumaric acid ([CL]0/[fumaric acid]0) was kept at 10, and all the reactions were done at 90 °C. Compound 3EXP (Scheme 1): Fluorinated Poly(caprolactone) Dangling Chains (F17C8-PCLy). For the preparation of the F17C8PCLz dangling chains the same procedure described above was followed. Perfluorooctylethanol (F17C8-OH) was used as initiator, and typical recipes were as follows: for the preparation of F17C8-PCL8, 10.471 g, 0.011 mol of F17C8-OH and 2.005 g of fumaric acid were first heated to 90 °C, and then CL (20 mL, 0.18 mol) were used. All the dangling chains were synthesized with different degrees of polymerization (DP), and the respective characterizations are described in Table 2. Poly(urethane) Cross-Linked Films: with and without F17C8PCLy Dangling Chains. Polyurethane films were prepared from a mixture of the three-armed PCL precursors prepared (TMP-PCLx) and a polyisocyanate cross-linker in N-methylpyrrolidone (NMP) as the solvent (30 wt % solid content). The molar ratio of NCO/OH was kept at 1.1 to ensure the full conversion of OH groups. Films were applied on aluminum panels or glass slides (previously cleaned with ethanol and flushed with air) using a doctor blade driven by a 509 MC CoatMaster automatic film applicator and then cured at 125 °C in a vacuum oven (pressure was about 20 mbar) for 30 min. Typically, the thickness of the cured films was between 200 and 300 μm. The following fixed conditions were used: curing temperature of 125 °C; curing time of 45 min, NMP (N-methylpyrrolidone) as solvent; solid content of 30 wt %; NCO/OH ratio of 1.1, and glass slides as substrates. DPD Simulations Methodology. A coarse-grained simulation technique, dissipative particle dynamics (DPD), was used. The original DPD simulation method proposed by Hoogerbrugge et al.27 has been modified during the past 10 years. From the most important modification we can mention the introduction of the fluctuation− dissipation theorem by Groot et al.26 Another modification was

EXPERIMENTAL SECTION

Chemicals. ε-Caprolactone (CL) (Sigma-Aldrich, 99%) was vacuum-distilled to remove traces of water. 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol (TMP) (Merck, ≥99%) and fumaric acid (Fluka) were dried in vacuum oven at 40 °C for 3 h prior to use. Tolonate HDT-LV2, mainly consisting of hexamethylene diisocyanate trimer (compound 2EXP further noted as t-HDI), was used as cross-linker with no further purification (Perstorp, equivalent weight of NCO (EW)NCO = 183 g; NCO functionality = 2.8, molar mass = 504.6 g/ mol). Perfluorooctyl ethanol (F17C8-OH) (Clariant GmbH) was used as received. Solvents p.a. grade: THF, n-heptane, NMP (Nmethylpyrrolidone), and methanol were used as received without further purification. Characterization. 1H NMR spectra were recorded on a Varian 400 spectrometer at 25 °C, operating at 400 MHz. CDCl3 (with 0.01 wt % TMS) was used as solvent. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS) measurements were performed on a Voyager-DE Pro instrument (Perspective Biosystems, Framingham, MA). The polymers were dissolved in THF and NaTFA or KTFA (sodium or potassium trifluoroacetate) was used as the ionizing agent. The glass transition temperatures (Tg) were determined on a Q2000 TA differential scanning calorimeter (DSC) in a range from −90 to +100 °C. A first heating and cooling scans were performed at 10 °C/min to eliminate the thermal history of the samples. The Tg’s of polymers and cross-linked films were determined at the halfway heat capacity of a second heating run at heating rate of 10 °C/min. Dynamic water contact angle (CA) measurements were performed on a Dataphyscis OCA 30 at room temperature, using deionized water as probe liquid. Water droplets were measured in advancing and receding mode, and the reported results are the average of three separate drop measurements at different locations of the samples surface. The error bars provided correspond to the errors associated with the calculation of the advancing CA by the Dataphyscis software. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a K-Alpha, ThermoScientific spectrometer using an aluminum anode (Al Kα = 1486.3 eV) operating at 510 W with a background pressure of ∼2 × 10−8 mbar. The spectra were recorded 2000


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proposed by Pagonabarraga et al.,44 who introduced a densitydependent conservative force into the algorithm. To study the cross-linked low surface energy polymer systems, we have modified the traditional method described before45 in order to allow the formation of cross-links. The cross-links are established when the two reactive groups approach each at a distance smaller than a threshold. Since in the present study we were interested in cross-linked polymer coating and not in the cross-linking process itself, we did not assign a probability for the formation of the cross-links. In traditional DPD simulations, each bead represents the volume of several molecules or parts of molecules. DPD describes the temporal evolution of the system via integration of Newton’s equations of motion, m(dv⃗i/dt) = fi⃗ , where mi and v⃗i are the dimensionless mass and velocity of a bead i and fi⃗ is the force applied to the bead. The force which acts on bead is pairwise additive and consists of three parts: first, the conservative force, which is a sort repulsive force given by F⃗Cij = aij(1 − rij/ro)r ⃗ ̂ij, where aij is a maximum repulsion between beads i and j, r ⃗ ̂ij is a unit vector, and r0 is the characteristic length scale. Second, the random force is F⃗Rij = σωR (rij)ξijr ⃗ ̂ij. Here ξij is a zero mean Gaussian random variable of unit variance, σ2 = 2kBTγ, where kB is Boltzmann’s constant, T is absolute temperature, and γ is the viscosity related parameter. Third, the dissipative force given by F⃗ ijR = −γωD(rij)(r ⃗ ̂ij · v ⃗ ̂ij) r ⃗ ̂ij, where ωD is weight function which goes to zero at rij = r0 and v⃗ij = v⃗i − v⃗j. Weight functions ωR and ωD are related via ωD(rij) = ωR(rij)2. The dissipative and random forces are related via ωD = ωR(rij)2. The equations of motions are integrated with a modified velocity-Verlet algorithm. A characteristic time scale of the model used is defined as τ = (mr02/ kBT)1/2. The total dimensionless bead number density is ρ = 3. For any two beads of the same type we have taken the repulsion parameter to be aij = 25 (in units kBT).26 The excess repulsion between beads was calculated using Flory−Huggins interaction parameters χ. The relations between aij and aii is aij = aii + 3.49χ.26 The Flory−Huggins parameters χij were calculated via solubility parameters χij = V0(δi − δj)2/RT, where V0 is the molar volume of the solvent and δi and δj are self-solubility parameters. The interaction between the artificial “void” bead (V bead), and other beads was taken aVi = 60 in order to ensure the separation between polymer coating and “air” phase. The interaction between CL-H beads and C-beads was taken aCL‑H,C = 10 in order to increase probability of cross-linking. The driving force for the segregation of fluorine beads at the polymer−air interface was the incompatibility of F beads and the polymer matrix. Simulations were performed using periodical boundary conditions. The polymer beads were placed in the 3D simulation box. The layer of “void” beads mimicking air was placed at the top of the polymer film. The profile of the F beads (fluorine contacting beads) and F/C ratios was calculated after the simulation systems were significantly equilibrated. The fluorine profile was determined as the number density of fluorinated beads per layer in the simulation box. To calculate F/C ratio, the coarse-grained model of polymer chains was replaced by atomistic model after the sufficient equilibration of the coarse-grained model.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Daan Huisman for his help on the preparation and characterization of the polymeric films, Dr. A. J. Markvoort and Dr. E. A. J. F. Peters for discussions, and Agentschap NL for the financial support (IOP-Self-Healing materials; project # SHM08710). 2001


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NOTE ADDED AFTER ASAP PUBLICATION This article posted ASAP on February 6, 2013. In the Experimental Section, paragraph 8, sentences 1 and 3 have been revised. The correct version posted on February 21, 2013.

2002


Surface Segregation of Low Surface Energy Polymeric Dangling

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