Hyperbranched Polyesters Terminated with Alkyl Chains of Different

Jul 17, 2014 - Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka ... modes), Kelvin potential measurements, Brewster angle micros...
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Hyperbranched Polyesters Terminated with Alkyl Chains of Different Length at the Air/Water Interface and on Solid Substrates Anna Brzozowska,† Jan Paczesny,† Paweł Parzuchowski,‡ Monika Kusznerczuk,‡ Kostyantyn Nikiforov,† Gabriel Rokicki,‡ and Jacek Gregorowicz*,† †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: Hyperbranched polymers (HBPs) are considered less expensive substituents of dendrimers in many applications. It is still a matter of discussion to what extent properties of structurally defected HBPs are similar to those of dendrimers with almost perfect structures. To answer this question, thorough investigations of both classes of dendritic polymers are needed. In this work we present the studies of self-assembly of amphiphilic hyperbranched polyesters with end groups modified with long hydrophobic chains of different length. Despite the structural nonideality, hyperbranched polyesters form remarkably stable monolayers at the air/water interface even when the terminal groups are functionalized with comparatively short heptanoic chains. Depending on the chain length of the terminal functionalities and temperature, the polymers form monolayers of morphologies ranging from stiff solidlike to soft liquidlike. Langmuir films of the modified polyesters were studied using surface pressure (static and dynamic modes), Kelvin potential measurements, Brewster angle microscopy, relaxation measurements, and compression/decompression experiments. The monolayers were transferred onto silicon and gold substrates using the Langmuir−Blodgett technique. The transferred films were studied using AFM, X-ray reflectivity, and PM-IRRAS measurements. The investigated polymers uniformly covered the surface of the substrates forming compact monolayers.

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INTRODUCTION Macromolecular architecture is one of the main factors that determines properties and area of applicability of polymeric materials. Polymers with linear architecture gained ground as a useful material with a broad range of applications. New areas of applicability have been opened in recent years as a result of the design of new effective paths of synthesis of polymers with partly or highly branched architecture. Hyperbranched polymers (HBPs) have irregular chemical structure compared with that of linear block copolymers and dendrimers. Consisting of dendritic, linear, and terminal units, HBPs are highly branched macromolecules with three-dimensional dendritic globular architecture. HBPs have the advantages of facile one-pot or pseudo-one-pot preparation, a large number of terminal functional groups, lower viscosity, and better solubility than those of linear polymers of the same molecular weight. Because of the irregular architecture and relatively high molecular mass dispersity, little attention has been paid to the molecular self-assembly of HBPs. Only very recently, it has been found that HBPs are potentially good precursors for molecular self-assembly. Yan et al.1 showed that amphiphilic HBPs can self-assemble in solution despite of irregular chemical structure. The amphiphilic HBP obtained by the authors consisted of a hydrophobic hyperbranched poly(3-ethyl-3oxetanemethanol) interior and hydrophilic poly(ethylene © 2014 American Chemical Society

oxide) arms (HBPO-star-PEO). When added to acetone, it formed macroscopic tubes of 1 mm diameter and average length of 1.8 cm.2 Amphiphilic HBPs with smaller interior and longer PEO arms self-assembled into giant vesicles in water.3 These new types of polymer vesicles are called branched polymersomes. It was found that such vesicles exhibit controllable temperature-responsive phase transition with the lower critical solution temperatures.4 Self-assembly of HPBs on solid substrates was also investigated. Tsukruk et al. have shown that amphiphilic hyperbranched polyesters form uniform and stable onedimensional surface morphologies upon transfer onto silicon surface.5,6 Polyglycerol with terminal thiol groups was used for preparation of self-assembled monolayers on gold plates.7 Evaporation-induced self-assembly of HBPs on surfaces such as metal, silicon, quartz, and glass was studied by Stenzel et al.8 The authors investigated star polymers with hyperbranched polyester (Boltorn H20) interior and polystyrene arms. After solvent evaporation the HBP formed a honeycomb film on a cold substrate under a humid air flow. Water-soluble aliphatic hyperbranched poly(amido-amine)s (PAMAMs) also formed Received: May 7, 2014 Revised: June 24, 2014 Published: July 17, 2014 5256

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Figure 1. Idealized structure of Boltorn H30 and its derivatives studied in this work.

regular honeycomb porous films on silicon wafer, quartz, and mica.9 These films can be used for encapsulation of guest molecules.10 Recently, it has been proved that amphiphilic HBPs can be also used for layer-by-layer self-assembly technique. Ionizable hyperbranched polyesters terminated with carboxylic groups acted as polyanions, whereas linear poly(diallyldimethylammonium chloride) acted as polycation.11 Comprehensive reviews of surface behavior, microstructure, surface morphology, and properties of highly branched macromolecules are available in the literature.12,13 Polymers form stable films when their solutions in appropriate solvents are casted onto a surface. Thickness of the films depends mainly on the concentration of the solution. However, the mechanism of the film formation is still not fully understood. The basic processes of polymeric films formation can be revealed studying formation of monolayers of polymers at the air/water interface and their transfer on solid substrates. Formation of monolayers by amphiphilic dendritic polymers was already reported in the literature.5,14−21 Zhai et al.21 suggested that, in contrast to modified dendrimers, irregular branching and random attachments of terminal alkyl tails in HBPs prevent the formation of regular lateral ordering and crystallization of the alkyl tails within Langmuir monolayers. However, our previous investigations22 proved that the hyperbranched polymer Boltorn H30 modified with mixture of eicosanoic and docosanoinc acids formed welldefined monolayer at the air/water interface. The X-ray reflectivity (XRR) investigations performed for the monolayers transferred onto silicon substrates proved that the alkyl tails were arranged in an upright orientation with dense liquidcrystalline-like ordering. The difference between our view on the structure of the monolayer formed by the hyperbranched polymers end-capped with the alkyl chains and those presented by Zhai et al. arises probably from the difference of the length of the terminal groups. A functionalization of hyperbranched polymers is nowadays a preferred strategy for obtaining a variety of novel materials serving various scientific and technological applications.9 The research is just at very early stage, and much work needs to be done to understand the relationship between the structure and properties of amphiphilic HBPs. It is well-known that to design HB molecules capable of forming monolayers at interfaces, amphiphilic balance should be introduced by appropriate

modification of terminal groups with e.g. hydrophobic tails. There are two possible ways of controlling the amphiphilic balance of modified HB molecules. One method is based on a systematically changing number of the terminal alkyl groups attached to the polar interior. This method was explored by Zhai et al.,17 but it seems that preparation of a molecule with a desired amphiphilicity with this methodology may be difficult from the synthetic point of view. The other method is based on complete substitution of terminal hydroxyl groups with alkyl chains of systematically changing length, giving a more efficient way of controlling amphiphilicity. Our work aims at the understanding of the influence of the length of terminal functionalities in hyperbranched polyesters on the formation of the monolayers at the air/water interface and on solid substrates. The interplay of the length of terminal alkyl tails and temperature, as the main factors determining the morphologies of the monolayers, was also studied.

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EXPERIMENTAL SECTION

Materials, experimental details, and experimental techniques are described in detail in the Supporting Information.

RESULTS AND DISCUSSION

Synthesis. In this study we focused on the self-assembly behavior of the hyperbranched polyester Boltorn H30 with terminal OH groups functionalized with hexanoic, lauric, or stearic carboxylic acids. A set of polymers of the same core (Boltorn H30) and various lengths of the terminal aliphatic chains was synthesized. Appropriate carboxylic acids were converted into acid chlorides with the use of thionyl chloride and used for esterification of terminal hydroxyl groups of parent hyperbranched polyester (Boltorn H30). The reactions were conducted at −5 °C in dry THF in the presence of triethylamine as a HCl acceptor. The excess of acid chloride was removed by multiple washing of the product’s solution in dichloromethane with aqueous sodium bicarbonate and water. The details of the synthetic procedure are provided in the Supporting Information. Following the nomenclature proposed by Perstop AB the polymers consisting of H30 core substituted with stearic (C18), lauric (C12), and heptanoic (C7) acids are abbreviated throughout the paper as H3180, H3120, and H3070, respectively. The structures of the hyperbranched macromolecules are presented schematically in Figure 1. 5257

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Characterization of Polymer. The core polymer (Boltorn H30) used in this study was a third-generation polyester synthesized from 2,2-bis(hydroxymethyl)propionic acid with ethoxylated pentaerythriol as the core molecule. The structure of the core polyester with the terminal OH groups was characterized by 1H NMR spectroscopy. Analysis of the NMR data revealed that the polymer molecule consisted on average of 15.4 monomeric units and had 19.3 OH terminal groups. The number-average molecular weight of the unmodified polymer was estimated to be 2080 g mol−1. From the methylene signals of the bis-MPA the degree of branching (DB) was estimated to be 0.45. Molecular weights and degrees of substitution of hydroxyl groups were estimated assuming the macromolecules contained cores consisted on average of one pentaerythritol and five oxyethylene groups. The structures of polyesters with substituted terminal OH groups were also analyzed by 1H NMR spectroscopy, and the number of substituted terminal groups was determined. It was estimated that over 98% of OH groups reacted in all studied cases. Within the accuracy of the characterization method it was assumed that all the terminal groups of polyester H30 were esterified with the fatty acids. 1H NMR spectra for polymers H30, H3070, H3120, and H3180 are provided in the Supporting Information. The number-average molecular weights of polymers H3070, H3120, and H3180 estimated from NMR measurements were 4240, 5600, and 7220 g mol−1, respectively. For the polymers the number-average molecular weights (Mn), weight-average molecular weights (Mw), and molecular mass dispersity (DM) were estimated from GPC-RI measurements with polystyrene as a calibration standard. The results are presented in Table 1.

constant surface area. The results of the static mode measurements in the log−log scale are presented in Figure 2.

Figure 2. Surface pressure−surface concentration isotherms obtained according to the static method at 293 K.

The isotherms obtained for H3070 and H3120 coincide, and what follows the crossover from the dilute to the intermediate region occurs at the same surface concentration for both polymers. The isotherm for the polymer H3180 is shifted toward higher values of surface concentration in comparison to H3070 and H3120. It lies close to the curve for H3200, and thus both polymers have similar overlap concentration. The experiment shows that polymers terminated with shorter alkyl chains (heptanoic and lauric) occupy larger surface area than macromolecules functionalized with stearic and eicosanoic chains. Although this observation contradicts commons sense, it can be easily explained. Mutual interactions of shorter chains are weaker than those of the longer ones, and as a result macromolecules H3070 and H3120 have more loose structure than H3180 and H3200. Longer terminal chains of polymers H3180 and H3200 stick to each other due to van der Waals interactions reducing the area occupied by these molecules. Going one step forward, it can be concluded that at 293 K H3070 and H3120 form soft liquidlike monolayers, while H3180 and H3200 form stiff solidlike molecular film. Data for H3180 show large scatter at low concentrations (dilute region). The scatter was observed in all repeated runs of the experiment. It seems that it was caused by the structural changes within the monolayer during the successive deposition of H3180. This effect was not observed for other systems. The scaling theory predicts that within the intermediate (semidilute) regime the relation between surface pressure and surface polymer concentration is expressed by π ∝ cy, where y = 2ν/(2ν − 1) and ν is Flory’s exponent for the radius of gyration.23−26 Theoretical and experimental considerations, as indicated by Vilanove et al.,26 suggest that the Flory’s exponent at Θ conditions has the value of about 0.57. The ν exponents obtained by fitting the experimental data for the polyesters H3070, H3120, and H3180 to the scaling relation was 0.53, 0.53, and 0.54, respectively. This result indicated that the air/ water interface was a less than Θ solvent for the investigated polyesters. For all studied systems the values of ν exponents were very similar. It is a straightforward conclusion that the length of the terminal functionalities did not influence the interaction of the macromolecules with the subphase. At low surface concentrations the surface pressure can be expressed by a virial expansion truncated to the first-order

Table 1. Molecular Weights and Molecular Mass Dispersities of the Investigated Polymers GPC

NMRa

theoreticalb

polymer

Mn (g mol−1)

Mw (g mol−1)

DM

Mn (g mol−1)

Mn (g mol−1)

H3070 H3120 H3180 H3200

3800 5960 5120 8400

6280 8460 6970 17600

1.7 1.4 1.4 2.1

4240 5600 7220 8200

7190 9430 12120 13460

a

Both mass of the H30 interior and number of terminal alkyl chains were obtained from NMR measurements. bPolymer contain 28 bisMPA repeating units in the internal part and 32 terminal alkyl tails.

For H3070 and H3120 Mn estimated from GPC-RI and NMR agreed reasonably well, while for H3180 the discrepancy was significant. The obtained values of DM were at the expected level. In Table 1 theoretical molecular masses of modified polyesters are also provided. The theoretical mass assumes perfect dendrimer-like structure of the internal part of the polymer containing ethoxylated pentaerythriol as the core molecule and 28 bis-MPA units. Throughout this study the Mn obtained from NMR analysis was used to estimate the number-average surface area occupied by the macromolecules. Langmuir Monolayers at the Air/Water Interface: Influence of Temperature and Length of the Terminal Aliphatic Chains on the Structure of the Monolayers. Langmuir Monolayers at the Air/Water Interface: Static Experiments. Small portions of the investigated polyesters were deposited in successive steps onto the air/water interface of 5258

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Figure 3. Surface pressure−area isotherms (a) and static compressibility modulus (b) recorded for the polymer H3180.

term.23−26 This formula made it possible to estimate numberaveraged molecular weight of a hyperbranched polymer (Mn) and second surface virial coefficient (Ac22). The estimated number-averaged molecular weights from the surface measurements at very low concentrations for the polymers H3070, H3120, and H3180 were 6900, 7460, and 9880, respectively. Comparison with the molecular weights estimated from NMR (Table 1) made it clear that all polymers aggregated even at low surface concentration. The estimated surface second virial coefficients had negative sign, which indicated that the polyesters were in a less than Θ state at the air/water interface at 293 K. As expected, similar as for the polymer H3200, a limited penetration of water into the polyester interior of the polymer molecule occurred. Langmuir Monolayers at the Air/Water Interface: Dynamic Experiments. Langmuir isotherms were recorded with simultaneous surface potential measurements and Brewster angle microscopy (BAM) observations. The behavior of the polymers H3180, H3120, and H3070 at the air/water interface was studied and compared with the polymer H3200.22 The results of the surface pressure (π)−surface area per molecule (A) isotherms and respective static compressibility moduli (εs= −A(∂π/∂A)T) calculated along the π−A isotherms are presented in Figures 3−5. The polyester core of the investigated polymers is flexible, and the spatial arrangement of the terminal tails determines the surface area occupied by the whole molecule. The macromolecule occupies minimal surface area when alkyl terminal tails are straightened out, arranged perpendicular to the surface of the water, and aligned parallel to each other, forming liquidcrystalline-like or crystalline structure. The limiting crosssectional surface area per alkyl chain in the monolayer is 0.205 nm2.27 Taking the average number of the terminal groups in the macromolecule as 19.3 (as proven by means of NMR) gives the minimal surface area of the modified hyperbranched polyester molecule to be approximately 4 nm2 molecule−1. H3180. The shape of Langmuir isotherms for H3180 changed uniformly with temperature, but for the clarity of presentation we discuss the results obtained in three different temperature regimes, i.e., (1) from 283 to 304 K, (2) from 304 to 309 K, and (3) at very high temperature (at around 318 K). In the low temperature region (from 283 to 304 K) the π−A isotherms showed a steep surface pressure increase (Figure 3a). Further compression caused that eventually the Langmuir π−A

curves bent. Beyond the bend the isotherms continued with a smaller slope. In this temperature range, the value of surface pressure at which the bend appeared increased with increase of the temperature. The characteristic shape of the π−A isotherms was similar to those observed for the polymer H3200.22 The static compressibility modulus (Figure 3b) in the low temperature region showed one maximum. The curves started with a relatively low value of surface compressibility at higher surface areas per molecule and reached maxima at the surface area of about 4.3 nm2 molecule−1. The maximum corresponds to the inflection point on the π−A isotherm. At this point the molecules are in the most compact arrangement possible for the monolayer in this temperature region. A high value of the static compressibility modulus indicates that the structure of the monolayer resemble those of liquid condensed or solid surface phase. Compression of the monolayer beyond the inflection point resulted in a collapse. The collapse manifested itself as a bent on the isotherm and a steep decrease of the elasticity modulus. After the collapse the static compressibility modulus stabilized at a low level. At the collapse relatively rigid monolayer fractured along straight strips. The strips piled up and form multilayered structure as can be seen in the BAM image shown in Figure S6 of the Supporting Information. In the low temperature region the behavior of H3180 at the air/water interface closely resembled the behavior of the polyester H3200. In the low temperature region (283−304 K) the lift-off surface area, at which surface pressure started to increase, is about 6.5 nm2 molecule−1. In the medium temperature range (304−309 K) the lift-off surface area increases to about 8 nm2 molecule−1 (see Figure 3a). Apparently increase of temperature weakens interactions between the terminal alkyl tails, making the area occupied by the H3180 macromolecules bigger. This is manifested by an increase of surface pressure at lower surface concentration. During the compression the structural changes within the monolayer occur as revealed by the shape of the static compressibility modulus. In the medium temperature region the maxima at the static compressibility modulus curves are shifted to about 3.7 nm2 molecule−1, and at the same time additional maxima appear at higher surface area per molecule. The surface area per molecule at the inflection point lower than those of the most compact arrangements (i.e., 4 nm 2 molecule−1) is an indication that multilayer structures within 5259

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Figure 4. Surface pressure−area isotherms (a) and static compressibility modulus (b) for the polymer H3120.

Figure 5. Surface pressure−area isotherms (a) and static compressibility modulus (b) for the polymer H3070.

the film were formed. The fact that polymer was a multicomponent system might also influence the structure of the monolayer formed upon compression. The behavior of the polymers H3180 at the air/water interface changes dramatically upon increase of the temperature above 309 K. As presented in Figure 3a, the Langmuir isotherm at 318 K differs from the isotherms recorded at lower temperatures. The surface pressure lift-off appeared at about 11 nm2 molecule−1 and showed a wide flat region which is not encountered for the polymer H3200 even at high temperatures.22 The static compressibility modulus at 318 K shows two well-developed maxima: one at about 7.5 nm2 molecule−1 and the other at about 2 nm2 molecule−1. These maxima corresponded to the beginning and the end of the plateau at the isotherm. The ratio of the areas per molecule at both ends of the plateau is close to 4. It suggests quadruple layer formation which sets in at the beginning of the plateau.28 The area occupied by a H3180 molecule is determined by the interaction of the terminal alkyl chains. At lower temperatures the interactions between the stearic chains are strong, and as a result the chains adopt the upward orientation and stick to each other already at low surface concentration. The surface area of the flexible hyperbranched polyester interior adjusts to the surface area occupied by the bunch of the terminal alkyl chains, and the final surface area per H3180 molecule is only slightly higher than 4 nm2. Furthermore, as a

result of relatively strong interactions of stearic chains at lower temperatures, the polyester macromolecules stick to each other even at surface pressure of 0 mN m−1 to form stiff monolayer. Upon increase of temperature the interactions of the terminal chains are weakened. The effect is twofold. First, weaker interactions between the terminal chains result in an increase of the surface area occupied by H3180 molecules. Second, weaker interactions between molecules facilitate detachment of molecules from the subphase and formation of multilayer structures. The effect manifests itself also as a decrease of the surface pressure at the bend with the increase of temperature. These processes start at about 304 K to reach the final form at 318 K as can be deduced from the shapes of the π−A and static compressibility moduli curves. H3120. The shapes of the π−A isotherms of H3120 recorded at lower temperatures (279−293 K, Figure 4a) resemble the isotherms obtained for the polymer H3180 in the medium temperature region, i.e., from 304 to 309 K. Decrease of surface pressure at the bend with the increase of temperature also justified this conclusion. At least two bends were observed on the π−A curves of H3120 at lower temperatures. The bends indicated that compression imposed the structural changes within the monolayer. Upon compression the arrangements of the H3120 macromolecules in the monolayer changed in a few steps. The maxima at the elasticity modulus indicated more precisely the surface areas at which the changes occurred 5260

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Figure 6. Surface pressure−area isotherms (a) and static compressibility modulus (b) for the hyperbranched polyesters with terminal alkyl chains of different length.

which is very close to the value expected for the dense packing of alkyl chains. The same value of surface area at the maximum of the static elasticity modulus was also observed for H3180 in low and medium temperature ranges. At higher temperatures in case of H3180 and for the polymers with shorter chains (H3120 and H3070) an increase of the surface area per molecule at the maximum of εs was observed as a result of weakened interactions between terminal alkyl groups. The hyperbranched polyester H30 functionalized with fatty acids forms an amphiphilic compound with well-defined and separated polar and hydrophobic regions. The internal part of the polyester molecule consists of hydrophilic carbonyl and alkoxy groups, while the terminal aliphatic chains form hydrophobic external shell. When placed at the air/water interface, the molecules adopt orientation in which the interior is in contact with water. However, interface is a poor solvent for the polyesters as proven by Langmuir experiments at low surface concentrations. As a consequence, the internal part of the polyester is not extensively swelled by water. Thus, the polar interior of the polyester remain flexible, and the spatial arrangements and interactions of the terminal tails determine the surface area occupied by the modified polymer molecule. At low surface concentrations terminal alkyl tails within one molecule are separated and randomly oriented with respect to the interface. As the surface concentration increases or temperature is lowered, the interaction of the alkyl tails becomes predominant. As a result, the chains straighten out, align parallel to each other, and arrange perpendicular to the surface of water. The flexible interior of the polyester adjusts its volume to the compact arrangement of the tails. Stronger mutual interactions of the tails result in a decrease of the surface area occupied by the polyester molecules and at the same time promote formation of clusters by the polymers molecules. Simultaneously with the surface pressure the surface potential was measured using the Kelvin-electrode method. This technique provides additional information about the behavior of molecules at the air/water interface. The surface potential changes upon compression are proportional to changes of a vertical component of a dipole moments within the film. Such changes are usually connected with reorientation of alkyl chains from parallel to perpendicular to the air/water interface. Structural changes within hydrophilic headgroup (in our case H30 molecule) do not influence the measurements.

(Figure 4b). The structural changes of the H3120 monolayer are possible as a result of weaker interactions of the terminal alkyl groups in comparison to the polyester H3180. At 279.9 K the first maximum is at about 5.5 nm2 molecule−1. The relatively soft monolayer does not collapse but reorganizes its structure to form a more compact molecular film. Increase of temperature further weakens the interactions between both the aliphatic tails and molecules, and the monolayer changes into a fluid surface phase. Above 304.5 K the elasticity modulus shows one maximum at about 6 nm2 molecule−1. The monolayer compressed beyond this surface area collapses, forming bulk liquid phase. No second steep increase of surface pressure is observed at small values of surface area. Apparently interactions of lauric chains are too weak to support formation of welldefined multilayer structures as it was observed in the case of H3180. H3070. In Figure 5 Langmuir isotherms and static elasticity moduli for the polymer H3070 in a temperature range 285− 311 K are presented. The H3070 macromolecules containing terminal groups consisting of seven carbon atoms also form stable monolayers at the air/water interface. The shapes of the π−A curves are independent of temperature and closely resembled Langmuir isotherms obtained for the polyester H3120 at temperatures higher than 304 K. Similarly as for H3120, the maxima of the elasticity modulus were at 6 nm2/ molecule and the monolayers compressed beyond this surface area collapse (Figure 5b). As can be deduced from the shape of the Langmuir isotherms, the mechanism of the collapse is the same as for H3120 in higher temperatures range. Langmuir Monolayers: Effect of the Length of the Terminal Chains. From the above considerations it is clear that the length of the terminal alkyl chains and temperature are two interrelated factors influencing the formation of the monolayers by the alkyl modified hyperbranched polyesters. The systematic change of the shapes of the isotherms in Figure 6a (different chain lengths, high temperature region) closely resembles the evolution of the shape of the isotherms of H3180 as a function of temperature presented in Figure 3a (constant chain length, different temperatures). More information on the effect of the length of the alkyl terminal chains is given in the Supporting Information. The polyester H3200 upon compression forms monolayer with the compact arrangement of the molecules. The maximum of the elasticity modulus appears at about 4.3 nm2 molecule−1, 5261

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interface seemed to be characteristic for hyperbranched polyesters having end-groups functionalized with long enough aliphatic chains. From the BAM images presented in Figure S7c it can be concluded that the floes had uniform textures. Similar as for H3200, this observation justified a conclusion that in the condensed phases the aliphatic tails were perpendicular to the water surface. This is in line with the value of the surface area per molecule at the onset of the collapse. Images of monolayers formed by the polymers H3120 and H3070 did not show any characteristic structures. Relaxation Experiments. Additional information about the structure of the monolayer formed by the polymers H3180, H3120, and H3070 comes from step-compression experiments. The experiments were performed according to a method proposed by Hilles et al.29 Results are presented in Figure 8.

The hydrophilic interior is hydrated by water molecules and changes of dipole moments due to reorientation of C−O and CO bonds are in fact screened. Figure 7 shows the course of

Figure 7. Surface pressure isotherms and surface potential for the investigated polymers at 293 K.

the surface potential as a function of surface area per molecule at 293 K for the polymers H3180, H3120, and H3070. The dipole moment of terminal alkyl chains (especially the −CH3 groups, giving the main contribution to total dipole moment) makes it possible to follow the changes of the orientation of the terminal functionalities during the compression. Increase of the surface potential indicates rising up of the alkyl terminal groups. For all investigated polymers curves of surface potential vs surface area closely resemble the behavior of the surface potential for the polymer H3200.22 At larger surface areas per molecule the surface potential was small, indicating that the molecules were sparsely distributed at the surface of the water with the terminal alkyl chains lying flat at the water surface. Upon compression the terminal alkyl tails rise up and arrange perpendicular to the surface of water. At this moment a sudden increase of the surface potential was observed. Then the surface potential stabilized at a constant level. It is evident that the change of the orientation of the terminal functionalities is independent of the length of the alkyl chains. It is worth noting that at 293 K the polymer with the longest terminal tails, i.e. H3180, show the lowest surface area per molecule at a takeoff of the isotherm (see Figure 7). The takeoff of the isotherm coincides with the sharp increase of the surface potential. At this point the transition from the dilute to the semidilute region occurs. The observed phenomenon is in line with the view on the structures of the monolayers emerging from the static and dynamic Langmuir experiments. Relatively stronger interactions of terminal alkyl chains of the polymer H3180 at 293 K result in effectively smaller surface area per molecule at the same surface pressure and smaller overlap concentration than those observed for the polymers H3070 and H3120. The BAM images obtained for H3180 at three temperatures are shown in Figure S7 of the Supporting Information. The image taken at a surface pressure of 0 mN m−1 revealed that the polymer formed pieces of the condensed phase floating at the water surface even before the compression started. At the lowest temperature (278 K) the condensed phase formed sharp-edged panes resembling ice floes, randomly distributed in size and similar to those formed by the polymer H3200.22 The formation of the panes of the condensed phase at the air/water

Figure 8. Results of the step-compression experiments for monolayers of the polymers H3070, H3120, and H3180 at 293 K.

Measurements were performed just below the inflection points of the Langmuir isotherms, i.e., close to the most compact arrangements of the macromolecules within the monolayers. At this surface concentrations the systems were close to the crossover from the semidilute to the concentrated regimes. For all experiments the film area was rapidly reduced by about 10% of the initial surface area. The relaxation curves were recorded as a function of time. The curve for H3070 in Figure 8 clearly shows the existence of a nearly exponential relaxation with long relaxation time. This behavior is typical for a liquidlike monolayer. The curve for H3120 is also of the exponential type but with much shorter relaxation time. This is an indication of stronger interactions of the H3120 molecules within the film; however, it seems that the monolayer is still of the liquidlike type. Elongation of the terminal tails to 18 carbon atoms leads to the change of the character of the relaxation curve. The relaxation process for the polyester H3180 is rather slow and highly nonexponential, indicating that strong interactions of the H3180 molecules result in the formation of the liquid condensed or solid type monolayer. The relaxation experiments confirmed the structural changes in the monolayer observed in the static and dynamic Langmuir experiments. The experiments were also performed for the polymer H3200, but the scatter of the data made the interpretation of 5262

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Figure 9. Compression and decompression π−A isotherms for H3180 at two temperatures: (a) 292.4 K, isotherms with surface potential; (b) 292.4 K, two cycles of compression and decompression isotherms; (c) BAM image obtained on decompression at low surface area; (d) 318.0 K, two cycles of compression and decompression isotherms.

H3120 forms the monolayer at the air/water interface, which structure is softer than the structure of the monolayer formed by H3180. Already at about 280 K the isotherms show only small hysteresis during compression/decompression cycles (Figure 10a), whereas the compression runs show perfect reproducibility, i.e., identical compression curves in successive runs (Figure 10b). At this temperature the monolayer disintegrates during decompression at surface pressure of approximately 20 mN m−1. Also in this case disintegration is a slow process visible as inflection points on the decompression curve. This reversible aggregation manifests itself as reproducibility of the isotherms, i.e., repeatable subsequent cycles. Additional information about the self-assembly and aggregation of the polymer H3120 comes from the analysis of surface potential. As seen in Figure 10a, surface potential abruptly increases at surface area per molecule of about 10 nm2. From this moment the surface potential usually stabilizes at a constant level even after collapse (see Figure 7). However, in case of H3120 a sudden decrease of the surface potential is observed at the surface area per molecule of about 5 nm2, i.e., at the onset of the collapse. The decrease of the surface potential can be easily explained by the fact that the multilayer film is formed at this conditions. Molecules in the successive layers organize in the way that alkyl chains from one layer are always in contact with the alkyl chains of the molecules forming the next layer. As a result, dipole moments of the alkyl chains from the adjacent layers are arranged in opposite directions, giving rise to lowering of the total dipole moment, and therefore surface potential also decreases.

the results difficult. However, it was clear that the relaxation process for the polyester H3200 was also nonexponential. Reversibility and Reproducibility of the Langmuir Isotherms. The results of compression−decompression experiments for the polymers H3070, H3120, and H3180 are presented in Figures 9−11. The compression−decompression experiments provide additional information about the structure and the mechanism of a collapse of the monolayers. H3180 forms stiff monolayers as revealed by π−A, surface potential, BAM, and relaxation investigations. At 292.4 K the isotherm cycle of polymer H3180 exhibits strong hysteresis (see Figure 9a), indicating that the isotherm is not reversible. During decompression surface pressure and surface potential drop down to their initial values, but the successive compression runs are not reproducible (see Figure 9b). The obvious conclusion is that the aggregates of H3180 (BAM image in Figure 9c) formed during the initial compression run do not disintegrate during decompression even at π = 0 mN m−1. At the highest investigated temperature, i.e. 318 K, the isotherms show only small hysteresis during compression/decompression cycles (Figure 9d). At this temperature the compression runs show reproducibility: in the second and subsequent cycles almost identical compression curves were obtained. At 318 K the monolayer disintegrates during decompression at surface pressure of approximately 15 mN m−1. Disintegration is a slow process visible as inflection points on curve of decompression. This reversible aggregation manifest itself as reproducibility of the isotherms, i.e., repeatable subsequent cycles. 5263

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Figure 10. Compression and decompression π−A isotherms for H3120 at two temperatures: (a) 279.9 K, isotherms with surface potential; (b) 279.9 K, two cycles of compression and decompression isotherms; (c) 304.5 K, two cycles of compression and decompression isotherms.

At about 304 K the compression/decompression cycles of H3120 are perfectly reproducible (Figure 10c). For the polymer H3070, i.e., with heptanoic terminal chains, there is almost no hysteresis between compression and decompression runs already at low temperature (285 K, Figure 11a). The hysteresis becomes bigger in a second compression/ decompression cycle (Figure 11b), indicating that aggregates accumulate in the successive cycles. At this temperature the time between the cycles was not long enough for the aggregates to disintegrate. Increase of temperature to 311 K makes the compression/decompression isotherms perfectly reversible in the first and successive cycles (Figure 11c). Langmuir−Blodgett Molecular Films. In contrast to the result published previously,21 we have shown that hyperbranched polyesters (in our case Boltorn H30) modified with alkyl modified terminal groups (in our case with heptanoic, lauric, and stearic alkyl chains) form stable monolayers at the air/water interface. Our study has shown that the structures of the monolayers can be tuned by a proper adjustment of temperature and the length of the terminal functionalities. These observations prove that hyperbranched polymers with modified end groups have a high potential for application. The next important question is whether a properly formed monolayer at the air/water interface can be efficiently transferred onto a solid substrate. The transfers of the monolayers were performed at 313 K and the values of surface pressure corresponding to the most

compact arrangements of the macromolecules (i.e., maximum of the compressibility modulus). Figures 12a−14a show the surface morphologies of LB monolayers as viewed by AFM. In the Supporting Information corresponding 3D AFM images are provided. No significant differences in the surface morphologies of the monolayers formed by the polymers of different length of the terminal functionalities are detected. The observations confirm the conclusions from all the described above experiments that the modified hyperbranched polymers are forced to form close-packed crystalline-like arrangements with vertically oriented alkyl tails upon compression. In the AFM images some amount of aggregates is visible. The presence of the aggregates can be explained by the fact that the transfers of the monolayers were performed at the onset of the collapse. However, the most important is that despite dispersity of the material and defected structure of the polymers the transfer is effective and that the formed LB molecular films are stable at the surface of the substrate. The X-ray reflectivity (XRR) studies of all sample were performed. The experimental and simulated XRR profiles are presented in Figures 12b−14b. Well-developed intensity curves with well-defined minima were obtained. The CPK model was used for modeling of molecular structures and estimation of molecular dimensions.30 The reflectivity data were analyzed using the three-box model31,32 of the electron density profile. For the simulation purposes the polymer film was divided into three sublayers of different electron density with thickness 5264

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Figure 11. Compression and decompression π−A isotherms for H3070 at two temperatures: (a) 285.2 K, isotherms with surface potential; (b) 285.2 K, two cycles of compression and decompression isotherms; (c) 311.6 K, two cycles of compression and decompression isotherms.

Figure 12. LB monolayer of H3070 transferred onto the silicon substrate at about 15 mN m−1: (a) AFM image; (b) X-ray reflectivity curve.

macromolecules in the monolayer the terminal chains are oriented perpendicular to the surface. To evaluate changes in the coverage and packing of monolayers generated from the HBPs, the PM-IRRAS spectra were collected. Figure 16 shows the C−H stretching regions of the PM-IRRAS spectra of the monolayers generated from H3070, H3120, H3180, and H3200. The C−H stretching region of monolayers is strongly influenced by the conformation of the alkyl chains and their environment.33−38 Specifically, the frequency and width of the methylene antisymmetric (ν as (CH 2 )) and methylene symmetric

estimated from the geometry of the molecule. Final result of the fits gave the average total thickness of the polymer molecular films equal 1.62, 1.95, and 2.9 nm for H3070, H3120, and H3180, respectively. The results were compared with a simple geometrical model. The model assumes that the thickness of the waterlogged part of the modified polyester interior is about 0.7 nm, the length of the CH2 group is 0.2 nm, and the terminal alkyl chains are in extended conformation. Comparison of the calculated and measured thicknesses of the monolayers is presented in Figure 15. The excellent agreement justifies the conclusion that at the compact arrangements of the 5265

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Figure 13. LB monolayer of H3120 transferred onto the silicon substrate at about 15 mN m−1: (a) AFM image; (b) X-ray reflectivity.

Figure 14. LB monolayer of H3180 transferred onto the silicon substrate at about mN m−1: (a) AFM image; (b) X-ray reflectivity.

Figure 15. Thickness of H3070, H3120, H3180, and H3200 monolayers transferred onto the silicon substrates.

Figure 16. Surface infrared (PM-IRRAS) spectra of monolayers generated from H3070, H3120, H3180, and H3200 at 293 K.

(νs(CH2)) bands are sensitive to the conformational order of the alkyl chains. The significant increases in the CH2 stretching frequencies as a function of temperature are well-known from previous studies of alkyl chain systems.33−38 Qualitatively, it was suggested that when the frequency of the νas(CH2) peak is below 2918 cm−1 and νs(CH2) peak is below 2850 cm−1, alkyl chains are considered to be ordered, and when the frequencies

are above these values, it is considered that a large degree of disorder, in the form of gauche defects, exists.33 Figure 16 shows that νas(CH2)) and νs(CH2) appear at 2920 and 2850 cm−1 for H3200, what suggests monolayer having “crystalline” conformational order. In the case of H3180 the νas(CH2)) and νs(CH2) bands are shifted to higher wavenumber, appearing at 5266

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2925 and 2855 cm−1. These observations indicate more disordered conformations of the terminal alkyl chains; however, similarities in the structures of monolayers of H3180 and H3200 are apparent, which supports the conclusions drawn from the results of the investigations of the H3180 and H3200 monolayers at the air/water interface. The νas(CH2) and νs(CH2) bands are broaden and shifted to higher wavenumber for H3120 and H3070. The bands are so broad that it is difficult to assign the frequencies to the specific oscillation modes. These observations indicate a more liquidlike conformational odder in the case of these polymers. Figure 16 shows a high intensity of both methylene C−H stretching bands in case of H3120 and H3070 as compared to the intensities of these bands for H3120. The effect arises probably from the fact that PM-IRRAS on gold surface is sensitive mostly to vibrations perpendicular to the surface. Longer chains are mostly linear, and the difference in the intensity of the bands for H3180 and H3200 comes mostly from the differences in the chains lengths. The high intensity of the νas(CH2) and νs(CH2) bands for H3120 and H3070 is the result of the onset of considerable disorder in the terminal chains, leading eventually to a random orientation of the methylene groups relative to the surface, which increases the number methylene groups with large components perpendicular to the surface.

morphologies of monolayers formed by modified HB polyesters resemble those observed for modified dendrimer molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data on the synthesis, NMR spectra of the polymers, description of experimental procedures, BAM images of the monolayer formed at the air/water interface, and AFM images of molecular films transferred onto silicon substrate. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], tel +48 22 343 3172, fax +48 22 343 3172 (J.G.). Author Contributions

A.B. and J.P. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is based upon work supported by the Polish National Science Centre research grants N N209 028440 and 2012/07/B/ST5/02045. We gratefully acknowledge Prof. E. Górecka and Dr. D. Pociecha (Department of Chemistry, Warsaw University) for their help in X-ray reflectivity experiments and Dr. P. Bernatowicz (Institute of Physical Chemistry, PAS) for help in NMR measurements.

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SUMMARY AND CONCLUSIONS A series of alkyl-terminated, hyperbranched polyesters with variable amphiphilic core−shell balance were synthesized. Hyperbranched polyester Boltorn H30 was chemically modified by substituting terminal hydroxyl groups with alkyl chains of different length. In result amphiphilic molecules were obtained. The applied procedure of substitution of the hydroxyl groups with the alkyl tails was very efficient. The number of alkyl tails attached to the branches was close to the theoretical expectations. Despite defected structure and molecular mass dispersity, HBPs with terminal hydroxyl groups substituted with alkyl chains form well-organized monolayers at the air/water interface and on solid substrates. The monolayers were studied using surface pressure (static and dynamic modes) and Kelvin potential measurements, Brewster angle microscopy, relaxation measurements, and compression/decompression experiments. It was shown that the HBPs molecules arranged into monolayers have a core polyester part in contact with the water subphase, and the alkyl chains are all pointing toward the air, forming a parallel-packed hydrophobic layer. The structure of this hydrophobic layer changes from soft liquidlike to stiff solidlike depending on the length of the terminal functionalities and temperature. Nevertheless, even for HBP with the alkyl chains composed of seven carbon atoms, a well-organized monolayer is formed. The revealed interplay between temperature and the length of the alkyl terminal functionalities offers a new possibility for a control of the preparation of monolayers with a desired properties. The HBPs monolayers were transferred onto silicon and gold substrates using the Langmuir−Blodgett technique. Detailed structural analysis of monolayers deposited at solid substrates reveled that alkyl tails remained arranged in an upright orientation with dense liquid-crystalline ordering. It was shown that irregular branching of HBPs molecules does not prevent the formation of regular ordering and “crystallization” of alkyl tails within Langmuir−Blodgett molecular films. The

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REFERENCES

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