Undulating the Lamellar Interface of Polymer ... - ACS Publications

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Undulating the Lamellar Interface of Polymer−Surfactant Complex by Dendrimer Chih-Ying Liu and Hsin-Lung Chen* Department of Chemical Engineering and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Self-assembly of the supramolecules formed by the complexation between poly(amidoamine) (PAMAM) generation four (G4) dendrimer and the surfactant, dodecylbenzenesulfonic acid (DBSA), generated a hexagonal columnar phase and a typical flat lamellar phase at low and high surfactant binding ratio, respectively, due to the dominance of the dendrimer to attain its natural curvature and the surfactant alkyl tails to reduce their hydrophobic interaction energy and packing frustration. Most strikingly, the delicate balance between these two free energy components at the intermediate binding ratios resulted in an undulated lamellar structure characterized by the centered rectangular unit cell. The finding demonstrates the power of dendrimer as a building block for expanding the morphological window of polymeric assemblies by modulating the interfacial curvature of microphase-separated structure.

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INTRODUCTION Self-assembly of supramolecules is an effective approach for creating new functional materials with well-defined and controllable structures. A supramolecule is built up by the constituent molecules linked by noncovalent attraction between complementary groups. The introduction of additional repulsive interaction in the supramolecules may drive microphase separation between the repulsive moieties, leading to the formation of a variety of structures with different interfacial curvatures.1−3 The commonly observed microphase-separated structures such as lamellar, cylinder, and sphere phases are characterized by homogeneous curvature; the bicontinuous cubic phases and the intermediate phases, such as perforated lamellae (PL), ribbon, and sponge (L3) phases, exhibit inhomogeneity in interfacial curvature.1−10 The self-assembled structures may be tuned by the system parameters, including temperature, pressure, constituent composition, and binding strength. The complexation of polymer with amphiphilic surfactant offers a facile way to produce self-assembled supramolecules. The noncovalent binding takes place between the headgroup of the surfactant and the complementary moiety in the polymer backbone. Because of the strong repulsion between the polar component (composed of the polymer and the surfactant headgroup bound to the polymer) and the nonpolar alkyl tails in the complex, microphase separation between the two components occurs spontaneously.11 Previous studies of polymer−surfactant complexes have centered on the systems with linear polymers; in this case, the binding of the amphiphilic molecules onto the polymer backbone yields a comblike supramolecule. Under moderate to high surfactant © 2017 American Chemical Society

binding density, the excluded volume interaction between the alkyl tails coupled with the strong polar−nonpolar repulsion cause high degrees of stretching of both the polymer backbone and the side chains.12−17 The self-assembly of such conformationally stiff comblike supramolecules yields predominantly a lamellar structure with essentially no interfacial curvature.12−17 The flat interface is entropically favored in that it minimizes the packing frustration arising from the nonuniform stretching of the alkyl tails in the nonpolar layer for attaining a uniform density. Here we introduce a concept of expanding the morphological window of polymeric assemblies by conferring interfacial curvature to the lamellar phase using dendrimer as the building block. It will be shown that a unique two-dimensionally (2-D) ordered undulated lamellar phase can be created by exploiting the molecular curvature of dendrimer in its complex with surfactant. Dendrimers are a unique type of treelike hyperbranched macromolecules with well-defined architecture.18−20 The branching starts from a central core with two or more reactive groups; each grafting cycle outward is called a “generation” (denoted as Gn with n being the generation number). The global conformation of dendrimers at low generations (e.g., G2 and G3) in the solution resembles that of star polymers.21 Increasing the generation number increases the compactness of dendrimer and hence transforms the conformational characteristic to sphere with internal monomer density fluctuations (∼G4 to G9).22 Dendrimers have been considered Received: May 31, 2017 Revised: August 16, 2017 Published: August 28, 2017 6501

DOI: 10.1021/acs.macromol.7b01122 Macromolecules 2017, 50, 6501−6508

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Macromolecules Scheme 1. Structures of PAMAM G4 Dendrimer and DBSAa

a

The repeating units and the surface groups of PAMAM dendrimer are omitted for clarity.

tional characteristics. It can be envisioned that an even broader spectrum of new structures can be generated based on the present concept, as the surface functional groups of dendrimer is varied for binding with various types of guest molecules.

as an attractive building block for supramolecular assemblies due to the precise number of functional groups and high structural uniformity.23−28 Considering the distinct curvature of dendrimers, their binding with surfactant molecules may lead to nanostructures with interesting interfacial curvatures not obtainable in the complexes with linear polymers. The incorporation of mesogen or long alkyl chains onto dendrimers by chemical functionalization or complexation had indeed been reported in a number of studies.29−40 It was proposed that the commonly observed lamellar or columnar mesophase was a consequence of the balance between the dendrimer struggling to maintain its isotropic shape and the attached mesogens or alkyl chains tending to arrange into their preferential ordering. However, the incorporation of dendrimer was not found to be particularly effective in generating new structures. This could be attributed the fact that most of the previous works had centered around the structures of the dendrimer-based systems in the bulk state,31−39 whereas the spherically symmetric conformation of dendrimer is attained in the solution state, in which the dendrimer is swollen by a significant amount of solvent molecules residing at its interior.41 Without the support of the solvent molecules, the dendrimer molecules collapse into a compact disordered conformation in the bulk state.34,35,37 Consequently, the special structures derived from the balance between the dendrimer to retain its swollen conformation and the guest molecules to attain their liquid crystalline order may be realized when the system is in the hydrated state. The present study investigates the self-assembled structures of the complexes of poly(amidoamine) (PAMAM) G4 dendrimer with a surfactant, dodecylbenzenesulfonic acid (DBSA) in the presence of excess water, where the sulfonic acid group of DBSA bound with the amine moiety of dendrimer via electrostatic interaction.42 We will demonstrate that the complex is mesomorphic in nature, showing interesting structural transition with respect to the change of the surfactant binding ratio. In particular, we will disclose a unique 2-D undulated lamellar phase with the interfacial curvature conferred by the dendrimer. The present work demonstrates the use of dendrimer as a supramolecular building block to construct curved lamellar structure via its unique conforma-

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EXPERIMENTAL SECTION

Materials and Complex Preparation. Ethylenediamine (EDA)cored primary amine-terminated poly(amidoamine) (PAMAM) dendrimer of generation four (G4) was acquired from Dendritech, Inc. (Midland, MI). Dodecylbenzenesulfonic acid (DBSA) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Scheme 1 shows the structures of PAMAM G4 dendrimer and DBSA. The dendrimer was stored in methanol which was removed before use. The bulk solutions of PAMAM G4 dendrimer were prepared by dissolving the dried dendrimer in deionized water. All the chemicals were used without further purification. Dendrimer−DBSA complexes were prepared by mixing the aqueous solutions of dendrimer and DBSA according to the prescribed binding ratios. The final dendrimer concentration of all the samples was kept at 0.5 wt %. The binding ratio x is given by the feed molar ratio of the sulfonic acid group of DBSA to the total amine group of dendrimer, where x = 1.0 signifies the stoichiometric binding. Upon mixing PAMAM dendrimer with DBSA solution, white precipitates formed immediately when x was sufficiently large (x ≥ 0.1). Complexes with x lower than 0.1 remained well dispersed in the solution and did not form any characteristic mesophase. Small-Angle X-ray Scattering (SAXS) Measurements. SAXS measurements of dendrimer−DBSA complexes with different DBSA binding ratios were conducted at beamline BL23A1 of the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The energy of the employed monochromatic radiation was 15 keV, corresponding to a wavelength of 0.83 Å. The 2D scattering patterns were collected with a Mar charge-coupled device (CCD) detector. The measured scattering wave vector q ranged from 0.007 to 0.4 Å−1 (q = 4π sin(θ)/λ, where 2θ is the scattering angle and λ is the wavelength). Samples in condensed state, i.e., the white precipitates dispersed in water, were held in the Kapton cells with the exposure time of 60 s. The collected scattering patterns were radially averaged to obtain onedimensional (1D) scattering intensity profiles. All the scattering profiles were corrected by for the scatterings from air and cell. Polarized Optical Microscopy (POM). The mesomorphic phase of the dendrimer−DBSA complexes was observed by an Olympus BX51 polarized optical microscope at room temperature. The samples 6502


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Figure 1. Representative polarized optical micrographs of PAMAM G4 dendrimer−DBSA complexes with the surfactant binding ratio x = (a) 0.25, (b) 0.6, and (c) 1.0. The appearance of birefringent patterns in the micrographs indicates the formation of mesophases in the complexes. were prepared by sandwiching the white precipitates between the microscope slides.

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RESULTS AND DISCUSSION In a previous study, we have investigated the complex of PAMAM G4 dendrimer with DBSA in aqueous medium. At low surfactant binding ratio x ≤ 0.08, the complex remained well dissolved in the aqueous solution without forming any mesophase. In this case, the hydrophobic tails of DBSA were encapsulated within the dendrimer, causing perturbation in the spatial distribution of the dendritic segments and the hydration level.42 As the binding ratio increases, we may envision that the internal cavity of the dendrimer is no longer sufficient to accommodate all the attached surfactant molecules; therefore, the unencapsulated alkyl tails have to face toward the water, which is energetically unfavorable. Once the hydrophobic interaction of the alkyl tails dominates, the complex will aggregate spontaneously, resulting in its precipitation out of the aqueous solution. The complex in this case is said to be in the “condensed state” to distinguish from the solution state attained at low binding ratio. Such a condensed state is attained at x ≥ 0.1 and is the focus of the present study. Despite the possible steric hindrance caused by the branch structure of the dendrimer, isothermal titration calorimetry (ITC) result shows that all the amine groups were able to complex with DBSA at x = 1.0 (see Supporting Information). Figure 1 shows the polarized optical micrographs of PAMAM G4 dendrimer−DBSA complexes with the binding ratios x = 0.25, 0.6, and 1.0. The micrographs revealed the presence of the birefringent patterns characteristic of mesomorphic structure. Since dendrimer is an amorphous material,41 the formation of the mesophase must result from its complexation with DBSA. SAXS was employed to explore the characteristic nanostructures associated with the mesomorphic order in the condensed state. Figure 2 shows the scattering profiles of the complexes with x ranging from 0.1 to 1.0. The presence of multiple peaks in the scattering profiles attests the formation of long-range ordered structures. For the complexes with x from 0.1 to 0.5, a series of peaks with the position ratio of 1:31/2:41/2 were observed, indicating the formation of a hexagonal columnar phase, as schematically illustrated in Figure 3. In this case, the dendrimer molecules were interconnected with each other along the axial direction, forming polar columns surrounded by the nonpolar alkyl tails of DBSA. Once the surface curvature of the dendrimer column does not deviate greatly from the its natural curvature, hexagonal packing is favored for alleviating the packing frustration of the alkyl tails for attaining uniform density of the matrix phase. It is noted that the lattice parameter (51.8 Å for x/0.1 complex) deduced from the primary peak position was about 8−10 Å smaller than the sum

Figure 2. SAXS profiles of PAMAM G4 dendrimer−DBSA complexes with the binding ratios x = 0.1, 0.25, 0.5, 0.6, 0.75, and 1.0. The scattering profiles were vertically shifted for clarity. The arrows mark the position ratios of the observed scattering peaks.

of the diameter of free G4 dendrimer (41.3−43.2 Å)44 and the interlamellar distance (ca. 28 Å) of the lamellar mesophase formed by neat DBSA,17 which implies that the dendrimer molecules were elongated into prolates that fused with each other at their ends to form the columns. Because of the prevalent segmental fluctuations within the hydrated dendrimer molecules,22,44,45 the columns built up by them should also be fluctuating. Nevertheless, the sharp diffraction peaks in the SAXS profiles attests that the centerlines of the columns in the hexagonal lattice were welldefined due to the mediation of the alkyl tails bound to them. The dynamic fluctuations of the dendrimer columns, on the other hand, gave rise to the broad halos superposing on the sharp diffraction peaks, as observed in Figure 2. Such a fluctuation-induced broad component in the scattering curve had been predicted for block copolymer mesophases46 and is expected to be evident for the supramolecular assemblies with hydrated dendrimer as the building block due to its generic nature in segmental fluctuations. The lattice parameter of the hexagonal columnar phase was found to decrease progressively from 51.8 to 47.9 Å as x increased from 0.1 to 0.5. The reduction of the nearestneighbor distance between the columns was attributed to the shrinkage of the dendrimer molecules arising from the decrease of their hydration level for reducing the interfacial energy of the microphase-separated structure. The hydrophilicity of the dendrimer is determined by the amount of water situating within it, where its hydrophilicity is greater at the higher hydration level. As the surfactant binding density on the dendrimer increased, the interfacial energy prescribed by the strength of the polar−nonpolar repulsion at the interface would also increase, if the hydration level of the dendrimer retained. 6503


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Figure 3. Schematic illustrations of the structural transformation of PAMAM G4 dendrimer−DBSA complexes with respect to the increase of surfactant binding ratio.

As shown in Figure 4b, five peaks can be identified for the x/ 0.75 complex; similar scattering patterns have been observed for several polymer, surfactant, and lipid systems, especially in the process of phase transformation to or from the lamellar structure.8,47−50 These scattering profiles were explained by the formation of the rhombohedral phase or a periodically ripplelike structure depending on the structural characteristics of the studied system.8,47−50 Since the complex with stoichiometric composition (i.e., x = 1.0) displayed the lamellar structure with flat interface (to be shown later), it was deduced that the x/0.75 complex formed an intermediate structure between hexagonal columnar and flat lamellar phase. Therefore, the SAXS pattern of this complex was assigned to a lamellar structure with a 2-D periodic ripple, as schematically illustrated in Figure 3. In this case, all the scattering peaks were indexed according to a 2-D centered rectangular unit cell with the lattice parameters of a = 68.7 Å and b = 42.8 Å, as listed in Table 1. Considering the intensity of the scattering peaks and the lattice parameters of the structures formed by the complexes with low and high binding ratios, the structural detail of the 2-D

To reduce the polar−nonpolar repulsion and hence the interfacial energy between the dendrimer domain and the nonpolar alkyl domain, a fraction of water molecules originally situating within the dendrimer were rejected to the bulk solution, leading to a reduction of the interdomain distance. It is noted that the complexation with DBSA tended to amplify the fluctuation-induced component in the scattering profile, as Figure 2 demonstrates that the broad halos in the SAXS profiles became stronger at larger x. The rearrangements of segments and water molecules were found to influence the local segmental motion of dendrimers,45 which might affect the dynamic fluctuations of dendrimer columns, leading to changes in the intensity of broad halos. It was deduced that as the surfactant binding density on the dendrimer increased, the combined effects of the DBSA and dendritic segmental rearrangement as well as the decrease in the hydration level may enhance the fluctuations, presumably because a decrease in the monomer−water interaction may soften the dendrimer columns. As the binding ratio x exceeded 0.5, the hexagonal columnar phase became unstable and new structures formed. As shown in Figure 2, despite the differences in peak position and peak intensity, the scattering profiles of the complexes with x = 0.6 and 0.75 both contained two sets of peaks with the position ratio of 1:2, as demonstrated more clearly in the enlarged SAXS profiles in Figure 4. Because the scattering peaks of the x/0.75 complex were more pronounced compared to those of the x/ 0.6 complex, the scattering profile of the x/0.75 complex was analyzed first to identify the corresponding structure formed.

Table 1. Miller Indices of the Observed Scattering Peaks of PAMAM G4 Dendrimer−DBSA Complexes with Different Binding Ratios x x 0.1

0.25

0.5

0.6

0.75

1.0

Figure 4. Enlarged SAXS profiles of PAMAM G4 dendrimer−DBSA complexes with (a) x = 0.6 and (b) x = 0.75 for clear identifications of the scattering peaks. The arrows mark the diffraction planes assigned by a 2-D rectangular unit cell.

peak position (Å−1)

position ratio

proposed index

0.140 0.242 0.278 0.143 0.247 0.287 0.152 0.262 0.304 0.159 0.167 0.309 0.331 0.173 0.183 0.240 0.346 0.364 0.184 0.366

1 1.73 1.99 1 1.73 2.01 1 1.72 2.00 1 1.05 1.94 2.08 1 1.06 1.39 2.00 2.10 1 1.99

(10) (11) (20) (10) (11) (20) (10) (11) (20) (11) (20) (22) (40) (11) (20) (21) (22) (40) (10) (20)

structurea

dhkl (Å)

Hex

44.9

Hex

44.0

Hex

41.5

UL

39.5 37.7

UL

36.3 34.3

FL

34.2

Hex = hexagonal columnar phase; UL = undulated lamellae; FL = flat lamellae. a

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Macromolecules undulated lamellae was deduced as follows. As shown in Figure 4b, the (20) peak had the highest intensity, indicating that the dendrimers were interconnected along the (20) plane with a well-defined interlayer distance d20 of 34.4 Å. The wavelength of the undulated layer, d01, which was given by the lattice parameter b of 42.8 Å, can be expressed in terms of d20 and d11 as d01 =

d11

⎡ 2 sin⎢cos−1 ⎣

( )⎤⎥⎦ d11 2d 20

The (20) peak later became the first-order peak of the flat lamellar structure in the scattering profile of the x/1.0 complex, as shown in Figure 2. On the other hand, the weaker (11) peak was associated with the periodically undulated layer, which was no longer observable when the binding ratio reached 1.0. The disappearance of the (11) peak revealed the structural transformation from a 2-D to a 1-D lamellae, during which the dendrimer layer lost its in-plane periodicity with further increase of the surfactant binding ratio. Comparing the calculated dimension of the unit cell and the diameter of a free G4 dendrimer, it was evident that the undulated lamellar structure consisted of a monolayer of dendrimers, incorporated between DBSA layers. Furthermore, the dendrimers in the polar domains were elongated and interconnected along the [01] direction while preserving some extent of their curvature, contributing to a set of atypical scattering peaks. The scattering peaks of the x/0.6 complex could also be assigned according to the diffraction indices of the x/0.75 complex, as listed in Table 1. It was however found that the (11) peak relating to the wavelength of the undulated layer was significantly broader than that of the (20) peak associated with the interlayer distance, suggesting that the correlation of interlayer distance was stronger than the in-plane undulating wavelength. The scattering peaks became sharper when x increased from 0.6 to 0.75, signaling that increasing the binding density of DBSA effectively enhanced the ordering of the lamellar structure by lowering the fluctuations of the undulating wavelength. As x increased from 0.75 to 1.0, two pronounced scattering peaks with the positions nearly identical to those of (10) and (20) peaks of x/0.75 complex were clearly observed, showing that the lamellar characteristic was retained at the stoichiometric binding. The disappearance of the scattering peaks relating to the in-plane undulating wavelength attests that the dendrimer layers lost their undulating feature and formed flat layers, as also supported by the TEM observation (see Figure S2 of the Supporting Information). By fitting the observed SAXS profile with the paracrystalline model of flat lamellar structure (see Supporting Information for detailed procedure),43 the average thickness of the polar layer was determined to be 20 Å (Figure 5), which proved the drastic flattening of dendrimers. The conformational perturbation of dendrimers has indeed been observed in liquid crystalline dendrimer systems in bulk state,33−36,39 whereas only a slight collapse of the dendrimer sandwiched between the layers of surfactants in the hydrated state has been reported by Li et al. for the complex of anionic PAMAM dendrimers possessing carboxylate terminal groups with the cationic didodecyldimethylammonium bromide (DDAB).51

Figure 5. SAXS profile of PAMAM G4 dendrimer−DBSA complex with x = 1.0. The solid curve represents the scattering curve calculated from the paracrystalline model of flat lamellar structure using the average thickness of the polar and nonpolar layer of 20 and 14 Å, respectively.

Our SAXS results have revealed the transition of the selfassembled structure of PAMAM G4 dendrimer−DBSA complex with respect to the increase of surfactant binding ratio, where the structure transformed from the hexagonal columnar phase to the 2D undulated lamellae and finally to the lamellar phase with flatten dendrimer layer. The use of dendrimer as the supramolecular building block was particularly effective in conferring interfacial curvature even to the lamellar phase. The observed structural transition may be explained by the compromise of the dendrimer curvature to accommodate the packing of DBSA alkyl tails with low packing frustration. The free energy of the complex in the microphase-separated state is divided into four components: (1) the free energy of the electrostatic interaction between dendrimer and DBSA, Felectro; (2) the interfacial free energy arising from the polar−nonpolar repulsion, Fint; (3) the free energy of dendrimer which includes its conformational free energy and hydration free energy, Fden; and (4) the free energy of the alkyl tails of DBSA which includes the hydrophobic interaction energy and the conformational free energy associated with tail stretching and packing frustration, Falk. The deformation of dendrimer induced by binding with DBSA increases Fden as a result of reduction of conformational entropy and increase of hydration energy (due to decrease of water content). The deformation of dendrimer may however decrease Falk, as a flatter interface reduces the hydrophobic interaction energy and packing frustration, since pure DBSA tends to form flat lamellar structure.17 The self-assembled structures formed by the complexes with different DBSA binding ratios can be roughly divided into two groups based on their geometrical features. At lower DBSA binding density (x ≤ 0.5), most of the DBSA molecules bound with the primary amine groups located at the dendrimer surface; the formation of the hexagonal columnar phase suggests that the dendrimers were able to maintain their natural spherical geometry to a large degree, indicating that Fden dominated the structure formation. On the other hand, when the DBSA binding ratio exceeded 0.5, the strong hydrophobic association between the alkyl tails of DBSA combined with the binding location at the interior of the dendrimer compromised the curvature of the dendrimers, causing the flattening of the dendrimer columns to reduce Falk. The columns further fused to form a layered structure of alternating hydrophobic and hydrophilic domains to decrease 6505


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layer thickness due to conformational flattening and dehydration might be offset by the increase of the nonpolar layer thickness as a result of the increase of surfactant binding density (or volume fraction).

the interfacial free energy and the packing frustration of the alkyl tails (and hence Falk). The SAXS results show that the dendrimer layers of x/0.6 and x/0.75 complexes were able to maintain a certain degree of curvature, resulting in the formation of the lamellar structure with a 2-D periodic ripple due to the balance between Falk and Fden. The structure consisting of the collapsed dendrimers sandwiched between the DBSA bilayer is analogue to the deformation of dendrimer when contacting with a substrate or membrane.52,53 The compressed state of dendrimer maximizes the number of functional groups that directly contact with the substrate or membrane.52 In the case of dendrimer−surfactant complexes with higher surfactant binding ratios, the flattening of dendrimer aligned the surface amine groups to decrease the distortion of the bound alkyl layers, which facilitated the ordering of the nonpolar domain. The packing of the DBSA tails was manifested by the broad halo located at 1.34 A−1 in the wide-angle X-ray scattering profiles, which corresponds to a typical liquidlike arrangement of alkyl chains (see Figure S3 in the Supporting Information).38 Furthermore, the alignment of the surface amine groups also prevented the tails of DBSA molecules from overstretching to maintain the density uniformity of the nonpolar domains. Moreover, the deformation might expose the tertiary amine groups within the dendrimer to provide additional binding sites for the DBSA molecules. Consequently, at the stoichiometric binding ratio (x = 1.0) Falk greatly outweighed Fden, leading to the sacrifice of the dendrimer curvature to accommodate the densely packed DBSA molecules. Figure 6 displays the spacing between dendrimer domains, L, associated with the characteristic structures as a function of

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CONCLUSION The present study has shown that the self-assembled structure of dendrimer−surfactant complex in the condensed state can be tailored conveniently by the binding ratio of the surfactant. In particular, dendrimer was demonstrated to be a unique polymeric building block for constructing an undulated lamellar structure. The structure of the complex was governed by the delicate balance between dendrimer free energy relevant to its curvature and the free energy of the surfactant alkyl tails prescribed by the hydrophobic interaction and packing frustration. As the binding ratio increased from 0.1 to 1.0, the structure transformed from a hexagonal columnar phase to a 2D undulated lamellar structure and eventually to the lamellar phase with flat layers. The domain spacing of the characteristic nanostructure decreased with the increase of surfactant binding ratio, due mainly to the reduction of water content in the dendrimer for decreasing the interfacial free energy associated with the polar−nonpolar repulsion. The phase transition and the accompanied deformation of the dendrimer were induced by the disturbance of the DBSA complexation to the internal tertiary amine groups of the dendrimer as well as the sacrifice of dendrimer curvature to accommodate the densely packed alkyl tails. The flattening of the dendrimers aligned the surface amine groups, which effectively decreased the distortion of alkyl layers and lowered the packing frustration. With the unique characteristics of molecular curvature and tunable functional groups for binding with different guest molecules, it can be envisioned that more new supramolecular structures can be designed for the development of functional materials by exploiting dendrimer as a building block.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01122. Isothermal titration calorimetry results, transmission electron microscopy and wide-angle scattering profiles of the PAMAM G4 dendrimer−DBSA complexes, details on the determination of the polar and nonpolar layer thickness for the x/1.0 complex in the hydrated state (PDF)

Figure 6. Spacing between dendrimer domains associated with various structures as a function of binding ratio.

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binding ratio. L was found to decrease monotonically with increasing x until x = 0.75, after which it reached a saturated value of ca. 34 Å. There was a change of slope in the variation of L with x across at the structural transition point, where the reduction of L was more drastic in the undulated lamellae regime than that associated with the hexagonal columnar phase. The decrease of L with increasing x was consistent with the gradual flattening and dehydration of the dendrimer molecules driven by the increasing dominance of the hydrophobic interaction of the alkyl tails. Interestingly, the interlayer distance remained virtually unperturbed as the structure transformed from the undulated lamellae at x = 0.75 to flat lamellae at x = 1.0. In this case, the decrease of the dendrimer

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.-L.C.). ORCID

Hsin-Lung Chen: 0000-0002-3572-723X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology (MOST) under Grant MOST 106-2221-E-007097. The authors appreciate the beamtime of BL23A provided by NSRRC in Taiwan and the helpful discussion with Professor 6506


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Macromolecules

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Takeji Hashimoto, the Emeritus Professor of Kyoto University, Japan.

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