Self-Adapting Peripherally Heterofunctionalized Hyperbranched

Article pubs.acs.org/Langmuir

Self-Adapting Peripherally Heterofunctionalized Hyperbranched Polymers: Formation of Janus and Tripodal Structures Ashok Zachariah Samuel and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: A peripherally clickable hyperbranched polyester carrying numerous propargyl terminal groups was prepared by a simple melt transesterification polycondensation of a suitably designed AB2 monomer; this clickable hyperscaf fold was then transformed into a variety of different derivatives by using the Cu-catalyzed azide−yne click reaction. Functionalization of the periphery with equimolar quantities of mutually immiscible segments, such as hydrocarbon, fluorocarbon, and PEG, yielded frustrated molecular systems that readapt and form structures wherein the immiscible segments appear to self-segregate to generate either Janus structures (when two immiscible segments are present) or tripodal structures (when three immiscible segments are present). Evidence for such self-segregation was obtained from a variety of studies, such as differential scanning calorimetry, Langmuir isotherms, AFM imaging, and small-angle X-ray scattering measurements. Crystallization of one or more of the peripheral segments reinforced this self-segregation; the weight-fraction-normalized enthalpies of melting associated with the different domains revealed a competition between the segments to optimize their crystalline organization. When one or more of the segments are amorphous, the remaining segments crystallize more effectively and consequently exhibit a higher melting enthalpy. AFM images of monolayers, transferred from the Langmuir trough, revealed that the thickness matches the expected values; furthermore, contact angle measurements clearly demonstrated that the monolayer films are fairly hydrophobic, and in the case of the tripodal hybramers, the presence of domains of hydrocarbon and fluorocarbon appears to impart nanoscale chemical heterogeneity that is reflected in the strong hysteresis in the advancing and receding contact angles.

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INTRODUCTION Although hyperbranched polymers (HBPs) are structurally imperfect analogues of the symmetrically branched dendrimers, several of the interesting functional properties of dendrimers have been emulated in suitably designed HBPs. For instance, core−shell HBPs, wherein the core is hydrophobic and the shell hydrophilic or vice versa, exhibit unimolecular micellar or reverse micellar properties, respectively; this is exemplified by their ability to occlude suitable guest molecules within their core region,1 similar to that seen with the dendritic box.2 Such core−shell domain separation within a single polymer molecule is realized in HBPs for two reasons: one is the branch-uponbranch topology, which causes the terminal segments to lie near the outer region of the molecule, and the other is the conformational flexibility of the highly branched polymer backbone to readapt and refine the core−shell domain segregation. Other examples that reaffirm this behavior of HBPs are (i) the substantially higher LCST (lower critical solution temperature) of peripherally PEGylated HBPs when compared to their linear analogues, which suggests a higher level of solvation of the PEG segments in the former,3 (ii) the formation of jellyfish-like structures by amphiphilic core−shell HBPs at the air−water interface, wherein there is a flattening of the hydrophilic core at the water surface,4 and (iii) the wide range of well-defined self-assembled aggregate morphologies revealed by amphiphilic core−shell HBPs.5 Recently, we © 2012 American Chemical Society

demonstrated that HBPs whose periphery was randomly decorated by hydrophilic PEG segments and hydrophobic docosyl (C-22 alkyl) units reconfigure to form Janus structures wherein the PEG and docosyl segments self-segregate; these Janus structures subsequently assemble to form bilayer disks, vesicles, lamellar structures, etc.6 Thus, despite the presence of a large number of branching defects and the broad molecular weight distribution, suitably derivatized HBPs do exhibit a number of very interesting dendrimer-like properties that could be exploited for useful applications. In the present study the periphery of the HBP has been modified to generate three types of structures: (i) with fluorocarbon (FC) and hydrocarbon (docosyl, DOCO), (ii) with FC and PEG, and (iii) with all three components, namely, FC, DOCO, and PEG (polyethylene glycol monomethyl ether, 350/1000 g/mol). The mutual incompatibility between these segments would be expected to drive their segregation at the periphery as in the case of Janus hybramers.6 Analogous Janus polymeric nanoparticles have been prepared by Müller and coworkers by exploiting the phase-separated morphologies formed by ABC-type triblock copolymers;7−9 however, these were formed by aggregation of numerous polymer chains. The Received: October 21, 2012 Revised: December 31, 2012 Published: December 31, 2012 1245

dx.doi.org/10.1021/la304146r | Langmuir 2013, 29, 1245−1257

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examined with respect to their wetting behavior in an effort to understand the role of chemical microheterogeneity due to the presence of both fluorous and hydrocarbon domains. In addition, using a number of other experimental techniques, such as differential scanning calorimetry (DSC), Langmuir− Blodgett measurements, small-angle X-ray scattering (SAXS), contact angle measurements, and AFM, we provide strong evidence for the reconfiguration of HBPs functionalized with two and three mutually incompatible segments to generate Janus and tripodal molecular architectures, respectively.

closest functional analogues of the three-component systems studied here are the miktoarm star polymers;10 these polymers carry three different types of polymer chains, with distinct solubility parameters, originating from a common molecular junction. These miktoarm polymers self-assemble in solution to generate a variety of interesting structures, such as hamburger micelles, segmented wormlike micelles, raspberry-like micelles, nanostructured bilayer vesicles, and other stimulus-responsive multicompartment nanostructures.11−16 HBPs whose periphery has been grafted with three different types of polymer chains have also been investigated, and they have been found to aggregate into nonuniform tubelike structures;17 since the arms (linear polymers) in this case are substantially larger than the size of the HB core, the behavior in solution and the solid state is mainly governed by the arms and is less limited by the reconfigurability of the HB core. Consequently, these HBP systems may be expected to behave similarly to the miktoarm star polymers. Unlike in these previous studies, in the present system, the periphery of the HBPs is decorated randomly using three relatively small mutually incompatible segments (Scheme 1); we have termed these HBPs as tripodal hybramers.

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION

Materials and Methods. The “clickable” HBP and docosyl and PEG azides were synthesized as described in our earlier paper.6 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Heptadecafluoro-10-iododecane (FC iodide) was purchased from Aldrich Chemical Co. The 1H NMR spectra were recorded in appropriate deuterated solvents using tetramethylsilane (TMS) as an internal reference. NMR spectra were recorded on a Bruker AV 400 MHz spectrometer. Gel permeation chromatography (GPC) analysis was performed using standard calibration based on polystyrene standards. Differential scanning calorimetric studies were performed using a Mettler-Toledo DSC1 instrument. All the samples were heated to melt and cooled to room temperature to eliminate the thermal history of the sample. The third heating−cooling cycle was always used for analysis, and all the measurements were repeated twice. The heating−cooling scans were performed at a rate of 10 °C/ min. All the measurements were performed using 2−3 mg of samples in aluminum pans with a lid supplied with the instrument. The selfassembly experiments were performed in the following way: to 6 mL of a solution of DOCO-PEG350-FC in THF (0.5 mg/mL) was added 600 μL of water dropwise over 20 min. After being stirred for 1 h, the resultant clear solution was spin coated onto a Si wafer (piranhatreated) and AFM images were recorded. Details of Langmuir− Blodgett (LB), AFM, SAXS, SEM, and TEM measurements are provided in the Supporting Information. Synthesis. 1-Azido-1H,1H,2H,2H-perfluorodecane. A 5 g (8.7 mM) sample of the FC iodide was mixed with 2.83 g of NaN3 (43.55 mM) in 25 mL of DMSO/H2O (75:1, v/v). The reaction mixture was stirred at room temperature for five days. After five days, 25 mL of water was added and the product was extracted into diethyl ether. The ether layer was washed with 10% Na2SO4 solution and subsequently with water and brine. The ether layer was removed in a rotary evaporator to obtain the product as a yellow liquid. Yield: 88%. 1H NMR (δ, ppm, CDCl3): 2.4 (m, 2H, CF2CH2CH2−); 3.62 (t, 2H, CF2CH2CH2−). Click Reaction. A 100 mg sample of the HB polyester (containing 0.36 mM peripheral propargyl units) was dissolved in THF and degassed using a N2 purge. Docosyl azide (64.13 mg, 0.18 mM) and FC-azide (88.2 mg, 0.18 mM) were added to it. CuSO4·5H2O (4.66 mg, 0.018 mM) and sodium ascorbate (7.17 mg, 0.0364 mM) were separately dissolved in a small amount of water and added to the reaction mixture. After degassing, the reaction mixture was stirred at 50 °C for three days. Later this was concentrated and reprecipitated in methanol to obtain a white polymer product (DOCO-FC). Similarly, by utilizing appropriate amounts of the other azides, namely, DOCO azide, PEG azide, and FC azide, the other polymers were also prepared. A detailed discussion of the proton NMR can be found in the main text. Polymers DOCO-PEG350-FC and DOCO-PEG1000FC were also reprecipitated in methanol; however, all the other polymers were purified by reprecipitating in diethyl ether. (typical yields were 75−85% after reprecipitation).

Scheme 1. Depiction of the Structural Reorganization of Tripodal Hybramers

Another aspect of these tripodal HBPs that we have examined is their thin-film properties, specifically in the context of the influence of chemical heterogeneity on the wetting behavior. It is well-known that the contact angle of water on a surface is extremely sensitive to the morphology and surface chemical constitution. Fluorocarbon-modified surfaces do not wet easily and therefore exhibit very high values of contact angles. It has been demonstrated that densely packed fluorocarbon chains with CF3 groups at the surface increase the contact angle very substantially.18−20 Several studies have investigated the correlation between surface tension and fluorocarbon content.21−23 Contact angle variation as a function of chemical heterogeneity and the size of the individual domains continues to be a matter of intense investigation, and many theoretical arguments have been proposed for predicting the contact angle values. Cassie theory24 correlates the observed value of the contact angle to the fractional area occupied by individual components and their corresponding contact angles. Many suggestions were put forward to include atomic-scale heterogeneities,25 surface roughness, and contact line curvature.26 Several experimental studies have attributed the positive deviation from Cassie’s equation to a variety of reasons, including the domain patch size and surface heterogeneities.27−29 Large contact angle hysteresis is also believed to be associated with a chemically heterogeneous surface with smaller domain sizes.49,50 In the present study, monolayer films of the tripodal hybramers were

Structural Characterization. The peripherally clickable hyperbranched polyester was prepared from dipropargyl 5-((4hydroxybutyl)oxy)isophthalate via a melt transesterification process (Scheme 2) that was described earlier.30 The polymer

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Scheme 2. Synthesis of Clickable Hyperbranched Polyesters and their Derivatization to Generate Different Types of Heterofunctionalized Hybramers

exhibited a very broad molecular weight distribution, Mw = 39 000 g/mol and Mn = 7800 g/mol, which is typical of hyperbranched polymers prepared via the self-condensation of AB2-type monomers.31 Control of the polydispersity of HBPs remains one of the important challenges in this area, though several interesting methods have been demonstrated to achieve reasonable control over the dispersity.32−35 The numerous propargyl groups on the periphery of the HBP core permit the direct single-step functionalization with a variety of segments using the versatile copper-catalyzed azide− yne click reaction;36 the functionalization was shown to be nearly quantitative. As mentioned earlier, the three segments chosen for this study were polyethylene glycol monomethyl ether (PEG350/1000), a C22 docosyl segment (DOCO), and a perflouroalkyl segment (FC). Although the primary objective of this study is to prepare tripodal structures carrying three mutually immiscible peripheral entities, a few control Janus

systems carrying just two of the segments were also prepared for comparison. To prepare the peripherally heterofunctionalized HBPs, the parent clickable polyester was treated with equimolar quantities of the two (or three) appropriate azides in the presence of the Cu catalyst; the Janus HBP carrying PEG350 and fluorocarbon is labeled PFG350-FC, the one carrying docosyl and fluorocarbon is termed DOCO-FC, and the tripodal HBP carrying equimolar quantities of all three components is labeled DOCO-PEG350-FC. In the case of the tripodal HBP, another sample, namely, DOCO-PEG1000FC, carrying PEG1000 instead of PEG350, was also prepared. All the click reactions were performed using the required azide components together instead of sequentially, since we believe this approach will provide a more random distribution of these segments at the periphery of HBPs. The 1H NMR spectra of the parent clickable polyester (HBP-PE) carrying propargyl ester groups at the periphery, 1247


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Figure 1. 1H NMR spectra of parent HBP (blue) and the heterofunctionalized HBPs. The peaks at 2.55 and 4.95 ppm due to the propargyl group disappear after clicking, indicating completion of the click reaction. Peaks are assigned as shown, and the relative intensities of the o, p, and q and a, e, and d sets of peaks were used to estimate the percentage incorporation of different segments.

Table 1. Structural Data Reflecting the Composition of Peripheral Segments on HBPs, their Molecular Weight (Mw) and Polydispersity Index (PDI), along with DSC Data of Janus and Tripodal HBPs incorporation from NMR (%) name DOCO-PEG1000FC

DOCO-PEG350FC

PEG350-FC DOCO-FC F100 parent HBP a

feed (%)

using o, p, and q peaks

using a, d, and e peaks

average

Mw

PDI

melting point (°C)

enthalpy of melting (J/g)

PEG

33

36

31

33

119 000

5.1

17 (26a)

0.33 (20a)

FC DOCO PEG

34 33 33

36 28 33

37 32 29

36 31 32

3.2

75 52 ncb

2.1 66 nc

FC DOCO PEG FC DOCO FC FC

34 33 50 50 50 50 100

37 30 50 50 45 55 100

35 36 49 51 46 54 100

36 33 50 50 45 55 100

67 51 nc 70 54 76 85

0.8 101 nc 7.9 78 5 7.6

component

59 600

53 400

3.7

39 600

3.3

18 300 39 000

2.4 5.0

Melting point and enthalpy values for PEG1000 domains obtained after thermal annealing experiment. bNot crystalline.

along with the various peripherally functionalized samples, DOCO-FC, PEG350-FC, DOCO-PEG350-FC, and DOCOPEG1000-FC, are shown in Figure 1, along with the essential peak assignments. We have confirmed the assignments by comparing with the NMR spectra of the HBPs that were fully functionalized with only one particular segment (FC, DOCO, or PEG) (Figure S1, Supporting Information); in all cases no residual propargyl unit peaks (at ∼2.55 and 4.95 ppm) are visible, which confirms the nearly quantitative transformation. The peaks due to the terminal methyl protons of the DOCO

and PEG units (a and e) as well as methylene peaks of the FC segment (d and l), are well-resolved; therefore, their protonnormalized intensities were directly employed to estimate the extent of incorporation. Interestingly, the single proton peak of the triazole ring, which is formed after click reaction, appears at three distinct positions for the three different segments (o, p, q), and remains well-resolved and, therefore, could also be used to estimate the relative composition; the compositions estimated using both these methods were in reasonable agreement (Table 1), thus reconfirming the peak assignments. 1248


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Figure 2. DSC thermograms of different heterofunctionalized HBPs. The left panel shows DSC thermograms of the Janus polymers, and the right panel shows those for tripodal hybramers. In panel E, the magenta curve shows the heating cycle after rapid quenching and annealing of DOCOPEG1000-FC at 5 °C; the annealing process induces effective crystallization of PEG1000 in the sample, and consequently, a more intense melting peak is observed.

PEG350 remains amorphous. The melting peak at 54 °C with a larger enthalpy is assigned to melting of the DOCO domain; this value matches the reported value for the DOCO segment in a similar Janus system reported earlier.6 It must be noted that the normalized enthalpy associated with the melting of the fluorocarbon phase is relatively small compared to that of the hydrocarbon, reflecting the weaker intermolecular interactions in the FC domains, which is consistent with earlier reports.37−46 The DSC curves for the HBPs containing three components at the periphery are also given in Figure 2; Figure 2D shows the DSC curve for DOCOPEG350-FC, where the melting associated with crystalline DOCO (51 °C) and FC (67 °C) domains is clearly observed but, as expected, no melting peak associated with PEG350 is observed. These melting temperatures compare reasonably with those exhibited by the homofunctionalized HBPs (63 °C for DOCO100 and 85 °C for FC100; see Figure S10, Supporting Information), thereby reaffirming that all three types of segments are self-segregated and do form separate domains rich in each of the segments; this configuration could be visualized as three pods linked to a central HB core, hence the term “tripodal hybramer”. When the length of the PEG segment is increased to PEG1000, the propensity for this segment to crystallize is significantly enhanced. Thus, the thermogram of the sample DOCO-PEG1000-FC exhibits three distinct melting peaks; the most dominant peak is due to the DOCO segment, while the other two segments exhibit only weak transitions. Since each of the peaks is believed to be associated with the melting of the three individual domains, the enthalpies associated with these peaks are best analyzed after normalization with respect to the weight fraction of the appropriate segment; such normalized enthalpies are listed in Table 1. The melting enthalpy associated with the PEG peak in DOCO-PEG1000-FC was found to be lower than that for the HBP fully functionalized with PEG1000 alone (HBP-PEG1000;

As evident from Table 1, the compositions of the different functionalized HBPs were in accordance with the feed ratios of the different azides used. The molecular weights of the parent HBP and the various amphiphilic derivatives were estimated by GPC, and the values obtained have also been listed in Table 1. The parent polymer, as stated earlier, had a broad distribution; however, upon derivatization the PDI decreased substantially. The reduction could be due either to fractionation that occurs during the precipitation of the polymer after derivatization because of the loss of some low molecular weight tail, and/or due to an apparent reduction in the PDI, which could be due to the fact that after peripheral functionalization the sensitivity of the hydrodynamic size to molecular weight becomes weaker; this causes an apparent decrease in the polydispersity. DSC Studies. It is well-known that long-chain alkyl segments exhibit a strong tendency to crystallize; this was earlier shown to reinforce the self-segregation and lead to the formation of lamellar morphologies in Janus DOCO-PEG hybramers.6 In the present tripodal systems, therefore, it would be interesting to explore the self-segregation process, especially in samples wherein more than one of the segments may crystallize, such as the ones that include FC and/or PEG1000 along with DOCO. For the sake of comparison, HBPs carrying only a single type of peripheral segment, DOCO, FC, or PEG350, were also prepared; the former two polymers exhibited melting peaks at 63 and 85 °C, respectively; while the latter was completely amorphous (Figure S10, Supporting Information). A stack plot of the DSC curves of FC100, DOCO-FC, and PEG350-FC polymers is shown in Figure 2; in all the cases a peak in the region between 70 and 85 °C clearly reveals the occurrence of phase separation and the consequent formation of a crystalline fluorocarbon phase. In the case of DOCO-FC two melting peaks are evident, one due to the DOCO phase and the other due to the FC phase, whereas in PEG350-FC only a single peak due to the FC phase is seen, as 1249


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In summary, the DSC studies clearly suggest that randomly functionalized HBPs carrying three mutually immiscible peripheral segments self-segregate to form tripodal structures that lead to microscopic phase separation; comparison of the normalized melting enthalpies indicates that the crystallization of one or more segments causes the formation of less ordered crystalline phases of the remaining segments due to reduced adaptability of the HBP backbone. Studies at the Air−Water Interface. Amphiphilic molecules are known to spread molecularly at the air−water interface.38,39 Isolated molecules can be compressed from a highly dispersed gaslike state to a solidlike state using movable barriers. The stability of the monolayers at the interface is most often decided by the polar segments which are solvated by water.40 The phase behaviors of many amphiphilic systems have been studied at the interface using this approach, for instance, surfactants,41 block copolymers,42 dendrimers,43 hyperbranched polymers,44,61,62 and a variety of mixed surfactants.45 In the context of highly branched polymers, conformational adaptability at the interface has been demonstrated in the case of core−shell dendrimers and hyperbranched polymers; in these systems peripheral hydrophobic alkyl chains were shown to project outward into air, while the hydrophilic core is solvated in the aqueous phase.43,44 In a previous study,6 we had examined the interfacial behavior of amphiphilic HBPs carrying DOCO and PEG segments at the periphery; these structures were clearly shown to reconfigure at the air−water interface to form Janus structures. In the present case only those systems that possessed the hydrophilic PEG segment were taken up for these investigations as the others are not expected to form stable monolayers. In the case of PEG350-FC, a completely reversible isotherm is seen during successive compression− expansion cycles (Figure S3, Supporting Information), and this suggests the formation of a highly stable monolayer. In addition, quasi-static compression experiments suggest the occurrence of crystallization of FC segments (Figure S4, Supporting Information). Such pseudocrystallization at the interface has been previously observed for similar FC segments in fluorocarbon amphiphiles.46 This reconfirms the Janus nature of PEG350-FC, where the PEG segments are selfsegregated and solvated in the water subphase; this is in complete accordance with our earlier observations using DOCO-PEG350.6 Langmuir isotherm studies for the tripodal HBPs containing three different segments at the periphery were also conducted; the π−A isotherms for samples PEG350-FC, DOCO-PEG350FC, and DOCO-PEG1000-FC are shown in Figure 3. Hyperbranched polymers, viz., DOCO-PEG1000-FC and DOCO-PEG350-FC, contain similar mole fractions of the three segments, except that the lengths of the PEG segments are different. All the isotherms are characterized by a gradual increase in surface pressure beyond a specific surface area; this onset represents the juncture at which molecules begin to make first contact with each other during compression.6 It is evident from the figure that the onset points are different for the different systems; since the surface pressure is normalized with respect to the amount of sample taken, the onset point is a reflection of the relative sizes of the peripheral segments in different polymers. In this case, the onset point appears to reflect the volume fraction of PEG in the HBPs; in other words, the first contact appears to occur in the aqueous subphase between the PEG segments. The volume fraction of PEG is smallest for DOCO-PEG350-FC, and hence, the surface area at

see Figure S2, Supporting Information); this lower enthalpy of melting suggests imperfect crystallization of PEG. Of the three components at the periphery, DOCO is expected to have the highest tendency for crystallization; thus, during the cooling of the melt, FC and DOCO crystallize before PEG and, therefore, appear to hinder effective packing and crystallization of the excluded PEG segments. This may be expected because the crystallization of two of the three segments would drastically reduce the adaptability of the HB core. To verify this hypothesis, the following control experiment was carried out: the completely molten polymer (at 90 °C) was rapidly cooled to 5 °C and maintained at this temperature for 10 min (isothermal). This temperature is considerably below the crystallization temperature of DOCO (37 °C) but close to the crystallization temperature of PEG; therefore, it may be expected that under these conditions PEG segments will have maximum mobility and hence a greater opportunity for crystallization. The heating cycle after this thermal annealing is given in Figure 2E (magenta curve). The melting peak corresponding to the PEG1000 domain shifts to a higher temperature (from 17 to 26 °C), one that is closer to the melting point of HBP-PEG1000 (36 °C) (Figure S2, Supporting Information); this is also accompanied by a considerable increase in the normalized melting enthalpy (Table 1). Taken together, these observations suggest the occurrence of more effective crystallization of PEG segments during annealing. It should also follow that the crystallization of PEG will affect the crystallization of the other two segments. Thus, the annealed sample exhibits a broad and weaker endotherm corresponding to the DOCO segment; a similar effect is also seen for the FC melting peak (Figure 2). Similar observations can be made by comparing the other normalized melting enthalpies, which are listed in Table 1; here again, it is evident that the enthalpy of melting varies depending on the architecture of the HBP, which includes both the nature and number of different segments at the periphery. It is seen that the enthalpy decreases with increasing number of crystallizable segments at the periphery; the ΔHm of the DOCO segment is largest (147 J/g)6 in the case of DOCOPEG350 and lowest (66 J/g) in the case of DOCO-PEG1000FC (where all three components tend to crystallize), while it takes an intermediate value (78 J/g) in case of DOCO-FC (where both the components tend to crystallize). Similarly, the enthalpy of melting of the perfluoroalkyl segment follows a similar trend: ΔHm(PEG350-FC) > ΔHm(DOCO-FC) > ΔHm(DOCO-PEG1000-FC) (Table 1). Furthermore, the presence of a noncrystallizable segment appears to cause an increase in the ΔHm of the crystallizable segment; the ΔHm value in DOCO-PEG350 (147 J/g) is higher than in the homofunctionalized sample HBP-DOCO (136 J/g). A similar trend is also seen in the case of FC; HBP-FC shows a lower enthalpy of melting compared to PEG350-FC, but here the difference is very small. Comparison of the ΔHm values of the FC peak of DOCO-PEG1000-FC and DOCO-PEG350-FC also reveals a very similar trend. Finally, incorporation of noncrystallizable PEG350 instead of PEG1000 increases the melting enthalpy of the DOCO segment from 66 J/g in DOCO-PEG1000-FC to 101 J/g in DOCO-PEG350-FC; however, the opposite trend was observed in the case of FC segments, where the ΔHm decreased from 2.1 to 0.8 J/g, respectively (Table 1). The exact reason for this is not clear at this time; further experiments are clearly required to fully understand this behavior. 1250


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interface.46,47 It would, therefore, be interesting to examine the surface characteristics of these transferred monolayers; this could be done at different stages during the compression, thereby permitting one to assess the evolution of the structure. AFM Imaging of the Interface. To visualize the morphology at the interface, the monolayers were transferred onto a solid substrate and dried; piranha-solution-treated Si wafers were used as the hydrophilic substrate. The AFM images of PEG350-FC, transferred at two different surface pressures, are shown in Figure S5 (Supporting Information); both images show a fairly uniform surface coverage (although the surface density appears a little lower in the low-pressure case) with an average roughness of about ∼2 nm, which reasonably matches the expected monolayer thickness. AFM images obtained for the tripodal DOCO-PEG350-FC are given in Figure 4. The transfer was performed at three different surface pressures, at 5 mN/m, which is near the onset point, at an intermediate surface pressure (20 mN/m), and after formation of a tight monolayer (40 mN/m). The morphology of the molecular aggregates at the interface at lower surface pressure is distinctly different from that seen for the Janus PEG350-FC. The chemical heterogeneity within the hydrophobic domain of tripodal HBP, due to the presence of FC and DOCO segments, could be one reason for this irregular lumpy morphology at surface pressures of 5 and 20 mN/m; at a higher surface pressure these structures fuse together to form a more compact monolayer (Figure 4). Direct evidence for the formation of distinct domains of fluorocarbon and hydrocarbon is difficult to obtain, although this might be expected to occur. An earlier investigation, using a mixture of fluorinated surfactant and fibrinogen, reported a similar morphology due to the presence of two different components, but there too the separate domains could not be resolved.47 Phase-segregated membrane

Figure 3. π−A isotherms for different heterofunctionalized hybramers. The surface pressure rise occurs at different normalized surface areas for different polymers because of the variation in the relative fractional volumes.

which the molecules make first contact with each other during compression is smallest (Figure 3, green curve). In DOCOPEG1000-FC, the longer PEG segment causes the surface pressure to rise at a significantly larger surface area (Figure 3, red curve), whereas in PEG350-FC the onset occurs at an intermediate value of the surface area because of the higher mole fraction of PEG in this two-component system (Figure 3, blue curve). In the tripodal HBPs, both the hydrocarbon (DOCO) and FC segments would remain above the air−water interface, leading to a chemically heterogeneous surface. Earlier experiments, using a mixture of hydrocarbon and fluorocarbon surfactants, have demonstrated the phase segregation of fluorocarbon and hydrocarbon segments into domains at the

Figure 4. AFM images of a transferred monolayer of DOCO-PEG350-FC at different surface pressures, before thermal annealing. (b), (e), and (g) show the AFM images of the monolayer transferred at surface pressures of 5, 20, and 40 mN/m, respectively; (a), (d), and (f) show the height variation along lines indicated in the respective images, and insets c and h show the water contact angle measured on these monolayer substrates at the respective surface pressures. 1251


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Figure 5. AFM images of a transferred monolayer of DOCO-PEG350-FC at different surface pressures, after thermal annealing. (b) and (e) show the AFM images of the monolayer transferred at surface pressures of 5 and 40 mN/m, respectively, after thermal annealing, (a) and (e) show the height variations along lines indicated in the respective images, and insets c and f show the water contact angle measured on these annealed monolayer substrates at the respective surface pressures.

morphology in the range of 10−200 nm, however, has been characterized using AFM.48 It is likely that the domain sizes formed here are smaller than 10 nm and therefore are not resolved. AFM images of DOCO-PEG1000-FC at different surface pressures also showed very similar morphology, suggesting that the length of the PEG segment does not appear to exert a significant influence (Figure S6, Supporting Information). Thermal annealing experiments were performed on the transferred monolayer films of DOCO-PEG350-FC; to achieve this, the films were maintained at 75 °C for 12 h prior to the AFM images being recorded. It is evident from Figure 5 that the morphology of the film transferred at 5 mN/m was not substantially affected; however, the one that was transferred at 40 mN/m appeared considerably smoother after annealing, although the height of the monolayer remained roughly the same (∼3 nm). It may be expected that the annealing process would also enhance the phase separation; however, discrete domain formation was still not discernible in the images. To examine the effect of chemical heterogeneity and thermal annealing, the wetting behavior of the surface was probed using contact angle measurements. Contact Angle (CA) Measurements. It is widely accepted that the contact angle is very sensitive to the chemistry and surface roughness of the topmost molecular layer.49 Hence, contact angle measurements would be an effective method to probe the surface of the transferred monolayer; here, it is important to recognize that the hydrophilic Si wafer would preferentially adsorb the PEG segments and the topmost layer is likely to be populated by the hydrocarbon and/or fluorocarbon segments. The contact angle measurements of the various transferred monolayers, whose AFM images were

recorded, reveal some interesting trends: the water contact angle on the PEG350-FC monolayer, transferred at 40 mN/m, is 94° (Figure S5, Supporting Information), which is close to the value obtained for poly(tetrafluoroethylene) (PTFE);50 this suggests that the fluorocarbon segments have preferentially segregated to the top and reconfirms the Janus nature of PEG350-FC. However, this value is smaller when compared to that of FC monolayers;19 the monolayer transferred at a lower surface pressure (20 mN/m) exhibited a slightly lower value as evident from Figure S5. This clearly suggests that the compactness of the FC chains plays a crucial role in governing the final contact angle. A similar effect of surface pressures is also seen in the case of the tripodal sample DOCO-PEG350FC: the values measured were 77° at 5 mN/m and 84° at 40 mN/m, as seen in Figure 4. Importantly, we have also performed CA measurements on spin-coated thin films of the different heterofunctionalized HBPs, which generally exhibited similar wetting characteristics, although the actual values of the contact angles were slightly higher (Figure S9). These thin-film studies suggest that such heterofunctionalized HBPs could be useful to tailor the wetting characteristics of thin-film coatings, given that the composition of the three peripheral segments can be readily adjusted. Furthermore, one may expect that thermal annealing, which was shown to improve the packing of the monolayer, may be expected to change the wetting behavior; thus, for instance, the annealed tripod monolayer films of DOCO-PEG350-FC exhibited an advancing contact angle of 104° (Figure 5), while it was 84° before annealing (Figure 4). We postulate that the annealing process has caused a more effective domain segregation and/or crystallization of FC and DOCO segments, which has led to the increase in the contact angle.51,52 As stated 1252


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Figure 6. Schematic depiction of the variation of the contact angle on the transferred monolayer of DOCO-PEG350-FC as a function of annealing and aging. Improved domain segregation and possible crystallization of the domains are being implicated in the contact angle variation.

Figure 7. SAXS patterns observed for the various functionalized HBPs, along with a schematic depiction of the lamellar organization. (a) SAXS pattern observed for FC100. The peak positions are in the ratio 1:2:3, confirming the lamellar morphology. Similarly, the lamellar patterns obtained for PEG350-FC (b) and DOCO-FC (c) are also shown. In the case of DOCO-FC the lamellar pattern is clearer in the melt (d), suggesting the possible formation of a liquid crystalline phase.

accordance with the very hydrocarbon-like nature of the film, as expected, for Janus DOCO-PEG350 films. According to Cassie theory,24 the contact angle of a surface containing a mixture of FC and hydrocarbon (DOCO) should follow the equation cos(θ) = f1 cos(θf) + f 2 cos(θh), where f1 is the fractional area covered by FC, f 2 is the fractional area covered by DOCO, and θf (=74°) and θh (=7°) are the contact angles observed on surfaces covered only by FC and DOCO, respectively. In the present studies, assuming that the fractional area is proportional to the relative molar composition of the two segments, the expected contact angle was estimated to be 50°, which matches remarkably well the observed value of 50°. Thermal annealing caused the hexadecane contact angle to increase to 67°; this positive deviation suggests a more effective phase segregation and the possible crystallization of FC and DOCO domains, which was earlier seen during studies of mixed alkyl−fluoroalkyl SAMs.59,46 Taken together, these observations clearly suggest the occurrence of self-segregation of the DOCO, FC, and PEG

earlier, the densely packed fluorocarbon chains with CF3 groups at the surface clearly cause an increase in the contact angle.19 The receding contact angle, on the other hand, was found to be substantially smaller (45°); one of the possible explanations that has been provided for this large contact angle hysteresis is the presence of chemical heterogeneity on the surface.53−56 Interestingly, when the film was aged under ambient conditions for 12 h, the contact angle dropped to the initial value of 84°; reannealing the film at 75 °C for 12 h again raised the contact angle back to 104°. This reversible annealing−aging behavior reveals the dynamic nature of the monolayer that permits gradual restructuring at the surface.57,58 To further understand the nature of the surface, hexadecane contact angles were measured on the same DOCO-PEG350-FC monolayer film; the value for the pristine film was 50°, whereas the values for the Janus PEG350-FC and DOCO-PEG350 films were found to be 74° and 7°, respectively. The very low value for the hexadecane contact angle for the latter case is clearly in 1253


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Figure 8. AFM images of aggregated structures obtained in a THF−water mixture for DOCO-PEG350-FC. The heights of these aggregated structures are near the bilayer dimension, but the lateral sizes are in the hundreds of nanometers regime. An AIDS ribbon kind of aggregated morphology is observed in AFM image and is highlighted, and a model is provided in the right panel.

Figure 9. Schematic depiction of the proposed mechanism for aggregate formation. Initially, when the water composition increases in the THF solution of the polymer, phase segregation of segments into domains takes place and the FC domains form dimers as the solubility of FC in the medium is expected to be the lowest among the three segments (step 1). When the composition of water increases further, the DOCO domains start to aggregate (step 2), leading to many different aggregated structures. A model (model 1) for the AIDS-ribbon-like aggregate observed in the AFM image is also provided.

morphology in the solid state.6 Both DOCO-FC and PEG350FC, which adopt a Janus configuration, also show similar bilayer lamellar morphology (Figure 7b,c), although the layering evidently does not appear as well-developed. Notably, in DOCO-FC the second-order SAXS peak becomes more prominent in the molten sample, suggesting that the phase separation is enhanced upon melting (Figure 7d), possibly because of the formation of a liquid-crystalline-type phase. However, the SAXS profiles of the tripodal HBPs containing three immiscible segments (Figure S7, Supporting Information) do not reveal any discernible morphology; expectedly, such tripodally segregated systems could lead to fairly complex morphologies that may require further detailed experimentation and modeling to fully fathom. Self-Assembly of Hybramers in Solution. Amphiphilic Janus hybramers with alkyl and PEG segments were shown earlier to assemble into flat disklike micelles or even vesicles when placed in suitable segment-selective solvents.6 Similar

segments, leading to the formation of the postulated tripod structures, which then could organize as depicted in Figure 6. SAXS Studies of Bulk Samples. Although the studies thus far appear to suggest the self-segregation of the peripheral segments in both Janus and tripodal systems, it would be instructive to examine the bulk morphology of these sample SAXS studies. Peripherally alkylated HBPs have been shown to adopt lamellar morphology in the solid state, where the lamellar spacing matches the single-molecule dimension.6,60 The SAXS pattern of HBPs containing peripheral FC segments alone (HBP-FC100) is shown in Figure 7a; these studies were done using short fibers formed by pulling from a molten polymer sample using a glass capillary. The SAXS pattern clearly reveals a lamellar morphology with a layer spacing of 4.48 nm; the appearance of the fourth-order peak reveals the effective core− shell phase segregation in these systems. Our earlier investigations on Janus hybramers carrying DOCO and PEG segments revealed that the Janus HBPs adopt a bilayer lamellar 1254


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segments that had the strongest propensity to crystallize cause the other segments to compromise in precision of their organization, as seen from the lower relative enthalpiesan apparent molecular tug-of-war. Furthermore, the wetting behavior of the transferred monolayers clearly reveals the effect of the presence of FC segments, especially when present along with the DOCO segments. Clear evidence for phase separation was noticed, the ramification of which was the large hysteresis between the advancing and receding contact angles. Furthermore, spin-coated thin films of the tripodal HBPs also exhibit similar wetting characteristics; this makes it particularly interesting from an application point of view as the surface characteristics can now be readily fine-tuned by varying the peripheral composition of the heterofunctionalized hybramers. In summary, these studies have clearly demonstrated that peripheral segments on a hyperbranched scaffold can indeed reconfigure to generate self-segregated molecular entities, such as Janus and tripodal structures; this is clearly possible because of the conformational adaptability of the hyperbranched backbone, which we believe may not have a parallel in the common linear polymers. It is this adaptability of these highly branched structures that needs to be targeted for novel and exciting potential applications.

structures may be expected from Janus systems containing any two of the three immiscible segments (viz., DOCO, FC, and PEG). However, the tripod hybramer DOCO-PEG350-FC shows unique self-assembly behavior in a THF−water (10:1) mixture. The morphology of the aggregated structures was dinstinctly different from that of the structures formed by Janus hybramers6 (see Figure 8). The height of these aggregated features was close to the bilayer (5−6 nm) dimension, but the lateral dimensions are in the range of hundreds of nanometers. In THF, all the segments would be reasonably well-solvated, but the addition of water causes both the FC and DOCO segments to self-segregate and form domains. It may be reasonable to assume that the FC domains aggregate first to form a dimer (step 1, Figure 9), while the DOCO domains aggregate at a later stage on the basis of their relative hydrophobicities, leading to a possible library of aggregated structures (step 2). The primary motivation for the formation of these aggregate structures is the strong tendency of the FC and DOCO segments to avoid contact with the water-rich medium, while at the same time their mutual immiscibility needs to be respected; this could lead to a variety of possible combinations of primary structures, d1, d2, n.c1, n.c2, n.t1, and n.t2 (Figure 9). Stretching our imagination a bit, a possible model for the “AIDS ribbon” kind of aggregated morphology observed in AFM can be explained, as depicted in Figure 9 (in the Supporting Information, see Figures S8 for the SEM image and S11 for the enlarged AFM image and line profiles). It is very clear that such a conjecture is just one of many possible explanations, but importantly, it should be recognized that the tripodal hybramers are intrinsically more complex and therefore will lead to a fairly convoluted range of aggregate morphologies. Preliminary TEM studies of these aggregates show the presence of small dark patches within the aggregates (Figure S12, Supporting Information), which could be interpreted as being due to the fluorocarbon domains surrounded by the other two domains that could not be discriminated. Further studies are clearly required to probe this further.

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ASSOCIATED CONTENT

S Supporting Information *

Details of LB, AFM, SAXS, SEM, and TEM measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank V. A. Raghunathan from the Raman Research Institute for the SAXS measurements and S. Sampath for the monolayer studies. Many valuable discussions with both of them are gratefully acknowledged. We also thank S. Umapathy for his valuable suggestions and discussions. S.R. thanks the Department of Atomic Energy for an ORI-Award (2006−2011) and the Department of Science and Technology for a J C Bose fellowship (2011−2016), and A.Z.S. thanks the Council of Scientific and Industrial Research for a fellowship.

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CONCLUSIONS In conclusion, we have demonstrated that clickable HBPs can serve as useful scaffolds to place distinctly different segments randomly on their periphery; important features of this approach are the use of the Cu-catalyzed azide−yne reaction to achieve this objective and the relatively simple chemistry to prepare the starting clickable HBPs carrying numerous propargyl groups on their periphery. In this study, three mutually immiscible segments, namely, a long-chain alkyl (DOCO), an FC, and PEG, were simultaneously clicked; it was shown that their mutual immiscibility leads to their selfsegregation, leading to the formation of tripodal hybramers. As was shown in our earlier paper,6 the incorporation of any two of the above three segments leads to the formation of Janus structures, which behave in a predictable manner; however, when all three segments are present, their bulk morphology and aggregates formed in solution are fairly complex and are not readily resolved. However, studies at the air−water interface and the thermal behavior of the tripodal systems could be readily rationalized by comparison with the suitable Janus analogues carrying any two of the appropriate segments. One interesting feature that emerged from the DSC studies was the competition between the three different segments to crystallize independently, as evident from the relative values of the segment-normalized melting enthalpies; it was clear that the

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REFERENCES

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