Fabrication and Structural Characterization of ... - ACS Publications

Article pubs.acs.org/Macromolecules

Fabrication and Structural Characterization of Module-Assembled Amphiphilic Conetwork Gels Takashi Hiroi,*,† Shinji Kondo,‡ Takamasa Sakai,‡ Elliot Paul Gilbert,§ Young-Soo Han,∥ Tae-Hwan Kim,∥ and Mitsuhiro Shibayama*,⊥ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Bioengineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Bragg Institute, Australian Nuclear Science and Technology Organization, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ∥ Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Korea ⊥ Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Tokyo, Chiba 277-8581, Japan ‡

S Supporting Information *

ABSTRACT: Structural analysis of inhomogeneity-free poly(ethylene glycol)− poly(dimethylsiloxane) (PEG−PDMS) amphiphilic conetwork gels has been performed by the complementary use of small-angle X-ray and neutron scattering. Because of the hydrophobicity of PDMS units, the PEG−PDMS gels exhibit a microphase-separated structure in water. Depending on the volume fraction of PDMS, the microphase-separated structure varies from core−shell to lamellar. The obtained X-ray and neutron scattering profiles are reproduced well using a core− shell model together with a Percus−Yevick structure factor when the volume fraction of PDMS is small. The domain size is much larger than the size of individual PEG and PDMS unit, and this is explained using the theory of block copolymers. Reflecting the homogeneous dispersion conditions in the as-prepared state, scattering peaks are observed even at a very low PDMS volume fraction (0.2%). When the volume fraction of PDMS is large, the microphase-separated structure is lamellar and is demonstrated to be kinetically controlled by nonequilibrium and topological effects.

■

INTRODUCTION Amphiphilic conetworks are composed of hydrophilic and hydrophobic polymers.1,2 Their amphiphilicity is promising for many applications; for example, amphiphilic conetworks can absorb both polar and nonpolar solutes. This is useful for drug delivery applications, which is currently limited to the delivery of hydrophilic materials,3−5 drug release systems,6 antifouling coatings,7 gas and biosensors,8 chiral separation membranes,9 activating carriers for biocatalysts,10 and soft contact lenses which demand contradictory characters: a hydrophilic nature for biocompatibility and a hydrophobic nature for oxygen permeability.11 Amphiphilic conetworks meet these criteria. Starting from de Gennes’ theoretical investigation,12,13 structural analyses for many conetwork systems have been reported.14−27 For example, Yamamoto et al. observed structure development of polydimethylsiloxane-α,ω-diacrylate and N,Ndimethylacrylamide (PDMS-DA/DMAA) amphiphilic conetworks during a radical polymerization by using small-angle Xray scattering (SAXS).14 The results showed the appearance of a scattering peak which was interpreted as a bicontinuous structure on the nanometer scale.28 Domján et al. reported higher order peaks in their SAXS studies of poly(2hydroxyethyl methacrylate) and polyisobutylene (PHEMA/ PIB) amphiphilic conetworks which was reported to be due to a weakly ordered local structure.15 When copolymerization reactions are performed in solution, the resultant amphiphilic conetworks are gels. Vamvakaski et al. reported novel methods © XXXX American Chemical Society

to prepare amphiphilic gels using group transfer polymerization.29 Small-angle neutron scattering (SANS) studies of 2(dimethylamino)ethyl methacrylate and methyl methacrylate (DMAEMA/MMA) amphiphilic gels revealed the existence of hydrophobic domains. However, it is difficult to control the reaction of cross-linking since radical or γ-ray radiation reactions are often used for copolymerization. For a better understanding for amphiphilic conetworks, preparation of inhomogeneity-free networks is desirable. For this purpose, we reported the preparation of homogeneous amphiphilic gels by the cross-linking of hydrophilic tetra-arm poly(ethylene glycol) and hydrophobic linear poly(dimethylsiloxane) to form PEG−PDMS gels.30 In contrast to the conventional way of copolymerization,31 this reaction does not require cross-linkers.32 Therefore, the resultant gels are homogeneous similar to the inhomogeneity-free gels we have reported using only tetra-arm PEG units.33 For the structural analysis of gels, observations in real space from techniques such as TEM, SEM, and AFM are usually unavailable because of the existence of solvents and the difficulty in the preparation of thin samples with a network structure. Scattering techniques, on the other hand, enable structural investigations to be conducted in the presence of Received: April 22, 2016 Revised: June 16, 2016

A

DOI: 10.1021/acs.macromol.6b00842 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Schematics of sample preparation. (a) Conventional Tetra-PEG gels. (b) PEG−PDMS gels, r = 1. (c) PEG−PDMS gels, r = 0.5.

Table 1. Experimental Conditions of SANS and SAXS 40 m SANS source average wavelength sample-to-detector distance (SDD) q rangea temperature a

QUOKKA

SPring-8

neutron 6.0 Å 2.0 m/17.5 m

neutron 5.0 Å 3.1 m/16.1 m

X-ray 1.0 Å 1.2 m/4.4 m

0.02 Å−1 < q < 0.3 Å−1 (SDD: 2.0 m) 0.003 Å−1 < q < 0.04 Å−1 (SDD: 17.5 m) room temperature

0.02 Å−1 < q < 0.3 Å−1 (SDD: 3.1 m) 0.004 Å−1 < q < 0.05 Å−1 (SDD: 16.1 m) 25 °C

0.02 Å−1 < q < 1.0 Å−1 (SDD: 1.2 m) 0.003 Å−1 < q < 0.2 Å−1 (SDD: 4.2 m) room temperature

q = 4π sin θ/λ where λ and 2θ represent the wavelength and the scattering angle, respectively. were both 10 kg/mol, and that of −NH2 terminated linear-PDMS was 2.5 kg/mol unless noted otherwise. A 27 kg/mol sample of −NH2 terminated linear-PDMS was also prepared for comparison (Figure 6). Typical gelation time of the PEG−PDMS gels in the present condition was of the order of several minutes, which was evaluated from rheological measurements (see Supporting Information for details, Figure S1).34 From elastic modulus measurements, the degree of conversion of end groups was estimated to be more than 80% (Figure S2). After gelation was completed (typically waiting for 12 h), each gel was immersed into toluene. After equilibrium swelling was accomplished, the solvent of the swollen gels was substituted from toluene to methanol. The substitution was done gradually by using solvent mixtures whose compositions of toluene and methanol were 4:0, 3:1, 2:2, 1:3, and 0:4. Each gel was immersed into an adequate amount of each mixture solvent for approximately 12 h with stirring. Some of them were further substituted from methanol to water in a similar manner. For the SANS measurements, the solvent of each gel was substituted by corresponding deuterated solvents. Photographs of the gels are shown in the Supporting Information, Figure S3. Small-Angle Neutron Scattering (SANS). The SANS experiments were performed on the 40 m SANS at High-flux Advanced Neutron Application Reactor (HANARO)35 at Korea Atomic Energy Research Institute (KAERI), Korea, and QUOKKA36 at the OPAL Reactor at the Australian Nuclear Science and Technology Organization (ANSTO), Australia. The experimental conditions are shown in Table 1. The sample gels were cut to a disc shape with radius and thickness of ca. 10 mm and ca. 2 mm, respectively; they were subsequently installed into demountable cells and immersed in the solvents. As for the measurements of as-prepared gels, the gels were prepared in the cells by filling up the cells with the pregel solutions. The thickness of gels was precisely measured by a CCD laser

solvent. The purpose of this study is to investigate the influence of solvent, the volume fraction of PDMS, and the molecular weight of the polymers on the resultant PEG−PDMS gels’ structure. Through the combined use of SANS and SAXS, the influence of changing solvent on the gel structure is highlighted, by virtue of differences in scattering contrast; in addition, the process of phase separation during solvent substitution is clarified.

■

EXPERIMENTAL SECTION

Sample Preparation. Schematics of the sample preparation are shown in Figure 1.30 The preparation scheme of PEG−PDMS gels is similar to that of tetra-PEG gels.33 In our previous works, tetra-PEG gels were prepared from two types of tetra-arm PEGs; −COO-NHS terminated PEG (NHS: N-hydroxysuccinimide) and −NH2 terminated PEG (Figure 1a). Similarly, PEG−PDMS gels were prepared by mixing −COO-NHS terminated tetra-arm PEG and −NH2 terminated linear-PDMS in toluene as a solvent (Figure 1b). In addition, the weight fraction of PDMS in the conetworks was tuned by using −NH2 terminated linear-PEG. By using equimolar prepolymers carrying −COO-NHS and −NH2, a precise tuning of PDMS content rate was achieved (Figure 1c). In this paper, the molar ratio of linear-PDMS to the total linear polymers is called r. In other words, r ≡ (linearPDMS)/(linear-PDMS + linear-PEG). All PEG species were purchased from NOF Corporation, and the −NH2 terminated linear-PDMS was purchased from Sigma-Aldrich. The total concentration of the as-prepared gels was set to 75 mg/mL except for SANS measurements of the as-prepared gels (100 mg/mL), which was the same condition as per our previous paper.30 The molecular weights of −COO-NHS terminated tetra-PEG and −NH2 terminated linear-PEG B


Article

Macromolecules displacement sensor (LK-G30A, KEYENCE). The scattered intensities were converted onto an absolute intensity scale by using direct beam attenuation method. Small-Angle X-ray Scattering (SAXS). The SAXS experiment was performed on BL03XU at SPring-8, Japan. The experimental conditions are shown in Table 1. Immediately on cutting, the sample gels were attached to the sample holder to enable SAXS experiments to be conducted before dehydration. The thickness of the gels was ca. 2 mm, which was measured by the CCD laser displacement sensor. The scattered intensity was converted into the absolute intensity scale by using glassy carbon as a secondary standard.

the case of r = 1, a slow reaction may result in the homogeneous gel shown in Figure 2a. To study the structure of these as-prepared gels, a model fitting was performed. The SANS profiles are reproduced well by the Ornstein−Zernike function (fitting results are shown in Figure 2a with solid lines) I(q) =

■

I(0) + IBKG 1 + (qξ)2

(1)

where ξ and IBKG correspond to the correlation length and the background (including incoherent) scattering, respectively. In the case of r = 0, the fitting was done for q > 0.01 Å−1 to neglect the effect of the upturn at low-q region. Note the Ornstein− Zernike function is commonly used for semidilute polymer solutions;38 the fact that the SANS profiles are reproduced by the Ornstein−Zernike function is consistent with the lack of static inhomogeneities arising from cross-linking. The resultant values for ξ as a function of the average molecular weight between cross-link points (cf. Figure S4) are shown in Figure 2b. It is clearly shown that there is a positive correlation between ξ and the average molecular weight between cross-link points; this result is reasonable since ξ is a measure of the mesh size of the gels. Figure 3a shows the SANS profiles of swollen PEG−PDMS gels of r = 1 (Figure 1b) with various solvents. The profile of

RESULTS AND DISCUSSION Microphase Separation Induced by Solvent Substitution. Figure 2a shows the r dependence of SANS profiles of the

Figure 2. (a) r dependence of SANS profiles of as-prepared PEG− PDMS gels. Solid lines are the fitting results by using eq 1. Each profile except for r = 0 is vertically shifted for clarity. (b) Relationship between correlation length and average molecular weight between cross-link points; the correlation length was evaluated from the fitting.

as-prepared PEG−PDMS gels in toluene. Interestingly, there is no upturn in the low-q region except for r = 0 (no PDMS component). This is different from conventional gels which show a noticeable upturn in the low-q region.37 This upturn is assigned to inherent inhomogeneities and indicates that longrange inhomogeneities are largely suppressed by the introduction of PDMS components. One of the possible reasons for this is the difference in reaction rate; indeed, the reaction between −COO-NHS terminated tetra-PEG and −NH2 terminated linear-PEG is faster than the reaction between −COO-NHS terminated tetra-PEG and −NH2 terminated linear-PDMS (Figure S1). Consequently, in the case of r > 0, the reaction may proceed in stepwise manner. First, a reaction between tetra-PEG and linear-PEG proceeds, followed by a coupling reaction between tetra-PEG and linearPDMS. In the case of r = 0, on the other hand, the reaction is terminated by the first reaction, and inhomogeneities result. In

Figure 3. (a) SANS profiles and (b) swelling ratios of PEG−PDMS gels (r = 1) with different solvents. The ratios of toluene/MeOH and MeOH/water are both 1/1 (volume fraction).

the sample swollen in toluene shows a slight upturn in low-q region as is consistent with inhomogeneities induced by swelling.39 After solvent substitution from toluene to a mixture of toluene and methanol, the low-q upturn is suppressed and indicates that the mixture is a better solvent for the PEG− C


Article

Macromolecules PDMS gel; this consideration is also supported by the macroscopic gel size. Figure 3b shows the linear-swelling ratio of swollen gels where the linear-swelling ratio is defined by the ratio of the gel sizes (m/m) of swollen to the as-prepared gels. The linear-swelling ratio of the toluene/methanol (the ratio is 1/1) gel is larger than that of the toluene gel. This expansion shows the high affinity of PEG and PDMS for the mixture solvent. In addition to the low-q region behavior, the scattered intensity from this toluene/methanol gel is lower than that from the toluene gel over the entire q region investigated; the latter is simply due to a decrease of polymer concentration due to swelling. Note that the scattering length densities of dtoluene and d-methanol are almost the same. Hence, the solvent substitution from d-toluene to d-methanol does not produce any additional effect in the absolute scattering intensity. When the solvent is completely substituted by methanol, a scattering peak is observed. Scattering peaks like this are often observed in amphiphilic block copolymers and in amphiphilic conetworks.14,15,29 In our case, this peak is due to phase separation between PEG-rich and PDMS-rich domains. This phase separation is induced by collapsing of PDMS units since PDMS units are hydrophobic. This collapse also induces a macroscopic shrinkage of the gel size as shown in Figure 3b. This shrinkage proceeds further on substituting the solvent with water. Correspondingly, the peak becomes sharper and shifts to higher-q consistent with a decrease of the interdomain distance; this agrees well with the shrinkage of the macroscopic gel size. From the peak position q*, the interdomain distance d* may be estimated from d* ≡ 2π/q*. The estimated value of d* is around 200 Å for the gels with water solvent. Since the radius of gyration for the tetra-arm PEG (molecular weight: 10 kg/mol) is around 15 Å and is much smaller than the interdomain distance,40 the emergence of the 200 Å order interdomain spacing indicates that several PDMS chains are collapsed to form a PDMS domain. Because of the chain connectivity, such a collapsing (microphase separation) must be accompanied by contraction of PEG chains. The morphology of the resultant gels may be changed therefore by varying the compositions of PEG and PDMS. To explore this issue, the SANS profiles of PEG−PDMS gels with different compositions were measured. r Dependence of Microphase Separation. Figure 4a shows the SANS profiles of swollen PEG−PDMS gels for various r values. Table 2 is the volume fractions of PEG, PDMS, and water estimated from the preparation conditions and the observed swelling ratios. Except for r = 0 sample, clear scattering peaks are observed for all gels even when the volume fraction of PDMS is around 0.2% (r = 0.25, Table 2). This result indicates a uniform distribution and strong aggregation of PDMS units. An increase of r also induced a systematic shrinkage of the macroscopic gel size as shown in Figure 4b. The relationship between the interdomain distance d* and the swelling ratio is shown in Figure 4c. This figure shows a positive correlation between d* and the swelling ratio. However, the variation of SANS profiles at values of q > q* with r does not exhibit such systematic behavior as shown in Figure 4a. For example, r = 0.75 and 1 shows an additional peak around q ∼ 0.12 Å−1. From this result, the morphology of the microphaseseparated structure may depend on r. From the viewpoint of scattering, a structure factor does not depend on r while the form factor does. To carry out this structural analysis one step

Figure 4. r dependence of (a) SANS profiles and (b) swelling ratios in water (deuterated). (c) Relationship between the interdomain distance and the swelling ratio. The interdomain distance was evaluated from the peak positions of (a).

Table 2. r Dependence of the Estimated Volume Fractions of Swollen PEG−PDMS Gels in Water r

PEG

PDMS

water

0 0.25 0.5 0.75 1

0.021(3) 0.035(3) 0.044(4) 0.058(4) 0.098(12)

0 0.0021(2) 0.006(1) 0.017(1) 0.058(7)

0.979(3) 0.963(3) 0.950(4) 0.925(5) 0.844(18)

further, an analysis by through the complementary application of SANS and SAXS was performed. Analysis of Microphase-Separated Structure. Figure 5a shows the scattering length densities (SLD) of water, PEG, and PDMS for SANS (left) and SAXS (right). In the case of SANS, the difference of SLD between PEG and PDMS is smaller than that between D2O and both polymers (PEG, PDMS); consequently, it is difficult to distinguish between PEG and PDMS in D2O. In contrast for SAXS, the SLD of PEG is larger D


Article

Macromolecules

Note that the difference in absolute intensities between the SANS and SAXS data is modeled only by the difference of SLD. In this modeling, PEG chains are assumed to form a core while PDMS chains to form a shell since it is reasonable to assume the longer component (PEG) to be a core. In this modeling, a clear boundary between PDMS shell and the solvent is assumed for simplicity although there should be some hydrophilic PEG thin layers. The profiles constructed from the model are shown as the solid lines in Figure 5b. The parameters used for the model are determined by trial and error, and the result is n = 2.8 × 1016 cm−3, ϕ′ = 0.32, R = 36 Å, L = 45 Å, and dR = 7.5 Å. This size is significantly larger than the radius of gyration of tetra-PEG (15 Å),40 which indicates the aggregation of up to several tens of polymer units. Although this model might be too simple, qualitative aspects for both profiles are reproduced. For example, the peak of SANS and the dip of SAXS around q ∼ 0.04 Å−1 are reproduced simultaneously. The intensity difference between SANS and SAXS is also reproduced only by the difference of SLDs. Further to that, the number density of this model agrees well with the value calculated from the preparation condition of PDMS (3.3 × 1016 cm−3; see the explanation of Figure S5 for details). The small difference may be attributed to the single PDMS units dispersed in the gel matrix. From this number density, it is clarified that the most of PEG organizes the gel matrix and only around 5% of PEG is embedded as a core. In the case of a PDMS-core PEG-shell model, this intensity difference is not reproduced (Figure S5). In addition, the additional peak appearing at around q ∼ 0.12 Å−1 in the SANS profile is attributed to the fringe of the form factor. As mentioned above, the experimental data suggest a PEGcore PDMS-shell structure. This result is counterintuitive; each PDMS unit is likely to shrink independently and disperse homogeneously. To address this point, energy variation induced by microphase separation should be considered.44 From the viewpoint of entropy loss due to the localization of joints and the restriction of conformation, each domain should disperse homogeneously. In contrast to this, from the viewpoint of interfacial enthalpy, each domain should aggregate. The latter enthalpy term induces microphase separation, and this is the reason why the observed aggregates are much larger than the size of the prepolymers. Since the PEG here is a tetrapod shape, microphase separation occurs throughout three dimensions. In addition, the PDMS-core PEG-shell cannot be large compared to the PEG-core PDMS-shell since the PDMS (2.5K) is short compared to the PEG (10K) in this system. The reason why the PDMS is assigned to the shell, therefore, is to reduce the interfacial enthalpy at the expense of the entropy loss. The formation of the ordered structure shown here is similar to that of block copolymers.45,46 The free energy calculation for the block copolymer of PEG and PDMS is shown in Figure S6. The major difference between block copolymers and the PEG−PDMS gels is the existence of crosslinking. The cross-linking introduces additional effects such as elasticity and osmotic pressure. These effects make the situation complicated. Although quantitative analysis has not been accomplished yet, the experimental results show the possibility of the manipulation of a microphase-separated structure created in the amphiphilic gels. PDMS Size Dependence of Microphase Separation. The morphology of the microphase-separated structure, in principle, is determined by the volume fraction of the two components.47,48 In other words, the microphase-separated

Figure 5. (a) Scattering length densities of water, PEG, and PDMS for (left) SANS and (right) SAXS. (b) SANS and SAXS profiles of PEG− PDMS gels with r = 1. The molecular weights of tetra-PEG and PDMS are 10 and 2.5 kg/mol, respectively. The solid lines are the simulated profiles obtained from the model described (see main text).

than that of H2O while that of PDMS is smaller than that of H2O. Figure 5b shows the SANS and SAXS profiles of PEG− PDMS gels of r = 1. The SAXS profile shows a dip while the SANS profile shows a peak around the same q region. In the toluene solvent, PEG and PDMS chains are both miscible, and the samples are regarded as a two-component system: polymer and the solvent. In contrast to this, in the water solvent, aggregation of PDMS components leads to phase separation of PEG-rich and PDMS-rich domains. In this case, the system is regarded as a three-component system: PEG, PDMS, and solvent. The simplest model to explain the dip shown in the SAXS profile is such a three-component system such as core− shell−solvent structure.41,42 In addition, a modified Percus− Yevick structure factor is included to explain the peak shown in the SANS profile; this structure factor has already been used for the analysis of amphiphilic gels.29 The experimental profiles are compared to the model curve as follows: I(q) = nSmPY(q; ϕ′, R + L)Pcs(q; R , dR , L) + IBKG

(2)

where n, SmPY(q; ϕ′, R + L), and Pcs(q; R, dR, L) correspond to the number density of core−shell particles, the modified Percus−Yevick structure factor, and the core−shell form factor,43 respectively. Here, ϕ′, R, L, and dR correspond to the effective volume fraction, the radius of core, the thickness of the shell, and the polydispersity of the core, respectively. A Schulz distribution for the radius is assumed for the analysis.43 E


Article

Macromolecules

(Figure 7). In the case of the as-prepared PDMS-rich gel, both PEG and PDMS are mixed with toluene. Therefore, the (PEG

structure is manipulated by changing the volume fraction of the unit. To see the similarity and the difference between the PEG−PDMS gels and block copolymers, PDMS-rich PEG− PDMS gels were prepared by using long linear-PDMS. The SANS and SAXS profiles of the PEG−PDMS gels with long PDMS units are shown in Figure 6. Interestingly, the SANS and

Figure 7. Schematics of the composition variation of the PEG−PDMS gels. Figure 6. SANS and SAXS profiles of PEG−PDMS gels with r = 1. The molecular weights of tetra-PEG and PDMS are 10 and 27 kg/mol, respectively. The dashed lines indicate the peak positions.

+ toluene) phase (ϕ = 0.14) is smaller than (PDMS + toluene) phase (ϕ = 0.86). The fact that PEG and PDMS are mixed homogeneously is also observed by SANS of the as-prepared PDMS-rich gel (Figure S9). However, because PDMS is insoluble to water, only PEG is swollen in water. In this case, the (PEG + water) phase (ϕ = 0.69) is larger than PDMS phase (ϕ = 0.31) although the volume fraction of PEG (ϕ = 0.05) is smaller than that of PDMS (ϕ = 0.31). As a result, there is an inversion of the volume fraction between the PEG and PDMS phases during the solvent substitution. To explain the reason for the formation of lamellar domain further, let us consider a nonequilibrium aspect of microdomain formation. In the case where the adjustment of the chemical junctions of diblock copolymers at the interface is very slow in response to solvent substitution, the morphology of diblock copolymers may differ from a thermodynamically favorable morphology and be kinetically controlled (nonequilibrium effect).50 In the case of solutions, the typical reason for the slowing down of the adjustment is the high concentration of the polymers. This consideration can be applied to the PEG−PDMS gels. In this case, the reason for the slowing down of the adjustment is attributed to the three-dimensional cross-linking. In the case of PDMS-rich gel, microphase separation occurs during the substitution from toluene to water due to the shrinkage of PDMS. A decrease of the composition of PDMS phase induces a formation of lamellar domains. Once these are created, further morphology changes do not occur since a decomposition of the lamellae into cylinders or spherical domains is kinetically unfavorable. The SANS and SAXS measurements of r-tuned PDMS-rich gels did not show lamellar domains except for r = 1, which also supports this proposition (Figure S8 and Table S2). In contrast to the PDMS-rich gel, the PEG-rich gel shows a large composition of PEG phase in all of the solvent (Table 3) which results in the formation of the core−shell structure. The microphase separation in the PEG−PDMS gels is illustrated schematically in Figure 8. It is clarified that the microphase-separated structure is tunable by changing the molecular weight of PEG and PDMS and their proportion. This may open the door for the precise design of the mesoscopic structure of amphiphilic gels.

SAXS profiles obtained from PDMS-rich gels (Figure 6) are totally different from those from PEG-rich gels (Figure 5b). In the case of PDMS-rich gels, both SANS and SAXS profiles show clear peaks. The SAXS profile shows more clear peaks compared to the SANS profiles; this may be due to the difference in the instrumental broadening. In addition, higher order peaks are observed up to fourth order in SAXS. The relative peak positions are approximately 1:2:3:4 with respect to the first-order peak, which strongly suggests a presence of a lamellar domain structure composed of PEG and PDMS layers. Peak assignment using other structure factors did not work well (Figure S7). An additional peak was also observed at q ∼ 0.8 Å−1 (Figure S8),49 indicating strong aggregation of PDMS chains. In the case of block copolymers, a lamellar system appears when the volume fraction of two components is almost the same. Since the building block of the PEG−PDMS gels is a triblock copolymer of PEG−PDMS−PEG segments, the theory of block copolymers may also work well for the PEG−PDMS gels. However, this is not the case for the PDMS-rich gel studied here. The volume fraction of the each component of the gels shown in Figures 5b and 6 is summarized in Table 3, which shows that the volume fraction of PDMS for the PDMSrich gel is much smaller than 0.5 (volume fraction, ϕ = 0.31), although it is significantly larger than that of PEG (ϕ = 0.05). To consider this contradiction, let us also consider the solvent Table 3. Comparison of the Estimated Volume Fractions of Each Component of PEG−PDMS Gels Composed of Short and Long PDMS PDMS: 2.5 kg/mol

PEG

PDMS

solvent

as-prepared (toluene) swollen (water) PDMS: 27 kg/mol

0.041(1) 0.098(12) PEG

0.024(1) 0.058(7) PDMS

0.935(1) 0.844(18) solvent

as-prepared (toluene) swollen (water)

0.010(1) 0.050(3)

0.061(1) 0.313(19)

0.929(1) 0.637(22) F


Macromolecules

■

ACKNOWLEDGMENTS This work was supported by a Grant-In-Aid from the Japanese Ministry of Education, Science, Culture and Sports, Japan. This work has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 25248027). The SANS experiment at 40 m SANS at HANARO was transferred from SANS-U at the Research Reactor JRR-3 with the approval of Institute for Solid State Physics (ISSP), The University of Tokyo (Proposal No. 13592). The SANS experiment at QUOKKA at OPAL was also transferred from SANS-U with the approval of ISSP (Proposal No. 14587). The SAXS experiment was performed at the second hutch of the Frontier Soft Matter Beamline (FSBL; BL03XU), SPring-8, Hyogo, Japan, with the assistance of Atsushi Izumi, Sumitomo Bakelite, Co., Ltd. (Proposal No. 2014B7260).

■

Figure 8. Proposed phase separation mechanism during solvent substitution.

CONCLUSION The structural analysis of inhomogeneity-free amphiphilic PEG−PDMS gels was performed by SANS and SAXS. The as-prepared PEG−PDMS gels in toluene did not show a noticeable upturn in the low-q region, which indicates the suppression of inhomogeneities. Solvent substitution from toluene to methanol gradually induces microphase separation together with a decrease of macroscopic gel size. This microphase separation proceeds further by solvent substitution from methanol to water. The reason for this microphase separation is attributed to the shrinkage of the PDMS units by solvent substitution. This microphase separation occurs even when the volume fraction of PDMS is 0.2%. Further analysis of the microphase-separated structure was performed by varying the composition of PDMS components. The structural analysis by complementary use of SANS and SAXS indicates a PEGcore and PDMS-shell structure for the PEG-rich gels. The domain size of the PEG−PDMS gels is much larger than that of the size of individual tetra-PEG and PDMS units. This is explained by considering the balance of the gain of interfacial enthalpy and the entropy loss due to the localization of joints and the restriction of conformation which is similar to that of block copolymers. As for the PDMS-rich gels, a lamellar structure was observed though the volume fractions of the (PEG + water) phase and PDMS phase were not the same. From this fact, it is revealed that the microphase separation is kinetically controlled by nonequilibrium and topological effects. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00842. Figures S1−S9; Tables S1 and S2 (PDF)

■

REFERENCES

(1) Patrickios, C. S.; Georgiou, T. K. Covalent amphiphilic polymer networks. Curr. Opin. Colloid Interface Sci. 2003, 8 (1), 76−85. (2) Erdodi, G.; Kennedy, J. P. Amphiphilic conetworks: Definition, synthesis, applications. Prog. Polym. Sci. 2006, 31 (1), 1−18. (3) Barakat, I.; Dubois, P.; Grandfils, C.; Jérôme, R. Macromolecular engineering of polylactones and polylactides. XXV. Synthesis and characterization of bioerodible amphiphilic networks and their use as controlled drug delivery systems. J. Polym. Sci., Part A: Polym. Chem. 1999, 37 (14), 2401−2411. (4) Iván, B.; Kennedy, J. P.; Mackey, P. W. Synthesis and Characterization of and Drug Release from Poly(N,N-dimethylacrylamide)-1-polyisobutylene. In Polymeric Drugs and Drug Delivery Systems; Dunn, R. L., Ottenbrite, R. M., Eds.; American Chemical Society: 1991; Vol. 469, pp 194−202. (5) Iván, B.; Kennedy, J. P.; Mackey, P. W. Synthesis and Characterization of and Drug Release from Poly(2-hydroxyethyl methacrylate)-1-polyisobutylene. In Polymeric Drugs and Drug Delivery Systems; Dunn, R. L., Ottenbrite, R. M., Eds.; American Chemical Society: 1991; Vol. 469, pp 203−212. (6) Lin, C.; Gitsov, I. Preparation and Characterization of Novel Amphiphilic Hydrogels with Covalently Attached Drugs and Fluorescent Markers. Macromolecules 2010, 43 (23), 10017−10030. (7) Gudipati, C. S.; Greenlief, C. M.; Johnson, J. A.; Prayongpan, P.; Wooley, K. L. Hyperbranched Fluoropolymer and Linear Poly(ethylene glycol) Based Amphiphilic Crosslinked Networks as Efficient Antifouling Coatings: An Insight into the Surface Compositions, Topographies, and Morphologies. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (24), 6193−6208. (8) Hanko, M.; Bruns, N.; Rentmeister, S.; Tiller, J. C.; Heinze, J. Nanophase-Separated Amphiphilic Conetworks as Versatile Matrixes for Optical Chemical and Biochemical Sensors. Anal. Chem. 2006, 78 (18), 6376−6383. (9) Tobis, J.; Boch, L.; Thomann, Y.; Tiller, J. C. Amphiphilic polymer conetworks as chiral separation membranes. J. Membr. Sci. 2011, 372 (1−2), 219−227. (10) Savin, G.; Bruns, N.; Thomann, Y.; Tiller, J. C. Nanophase Separated Amphiphilic Microbeads. Macromolecules 2005, 38 (18), 7536−7539. (11) Nicolson, P. C.; Vogt, J. Soft contact lens polymers: an evolution. Biomaterials 2001, 22 (24), 3273−3283. (12) de Gennes, P. G. Effect of cross-links on a mixture of polymers. J. Phys., Lett. 1979, 40, 69−72. (13) Briber, R. M.; Bauer, B. J. Effect of crosslinks on the phase separation behavior of a miscible polymer blend. Macromolecules 1988, 21 (11), 3296−3303. (14) Yamamoto, K.; Ito, E.; Fukaya, S.; Takagi, H. Phase-Separated Conetwork Structure Induced by Radical Copolymerization of Poly(dimethylsiloxane)-α,ω-diacrylate and N,N-Dimethylacrylamide. Macromolecules 2009, 42 (24), 9561−9567.

■

■

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.H.). *E-mail [email protected] (M.S.). Notes

The authors declare no competing financial interest. G


Article

Macromolecules (15) Domján, A.; Erdö di, G.; Wilhelm, M.; Neidhö fer, M.; Landfester, K.; Iván, B.; Spiess, H. W. Structural Studies of Nanophase-Separated Poly(2-hydroxyethyl methacrylate)-l-polyisobutylene Amphiphilic Conetworks by Solid-State NMR and Small-Angle X-ray Scattering. Macromolecules 2003, 36 (24), 9107−9114. (16) Iván, B.; Almdal, K.; Mortensen, K.; Johannsen, I.; Kops, J. Synthesis, Characterization, and Structural Investigations of Poly(ethyl acrylate)-l-polyisobutylene Bicomponent Conetwork. Macromolecules 2001, 34 (6), 1579−1585. (17) Erdodi, G.; Kennedy, J. P. Ideal tetrafunctional amphiphilic PEG/PDMS conetworks by a dual-purpose extender/crosslinker. II. Characterization and properties of water-swollen membranes. J. Polym. Sci., Part A: Polym. Chem. 2005, 43 (20), 4965−4971. (18) Gomes, S. R.; Margaça, F. M. A.; Salvado, I. M. M.; Falcão, A. N.; Almásy, L.; Teixeira, J. SANS investigation of PDMS hybrid materials prepared by gamma-irradiation. Nucl. Instrum. Methods Phys. Res., Sect. B 2008, 266 (24), 5166−5170. (19) Fodor, C.; Kali, G.; Iván, B. Poly(N-vinylimidazole)-l-Poly(tetrahydrofuran) Amphiphilic Conetworks and Gels: Synthesis, Characterization, Thermal and Swelling Behavior. Macromolecules 2011, 44 (11), 4496−4502. (20) Kali, G.; Vavra, S.; László, K.; Iván, B. Thermally Responsive Amphiphilic Conetworks and Gels Based on Poly(N-isopropylacrylamide) and Polyisobutylene. Macromolecules 2013, 46 (13), 5337− 5344. (21) Bruns, N.; Scherble, J.; Hartmann, L.; Thomann, R.; Iván, B.; Mülhaupt, R.; Tiller, J. C. Nanophase Separated Amphiphilic Conetwork Coatings and Membranes. Macromolecules 2005, 38 (6), 2431−2438. (22) Haraszti, M.; Tóth, E.; Iván, B. Poly(methacrylic acid)-lPolyisobutylene: A Novel Polyelectrolyte Amphiphilic Conetwork. Chem. Mater. 2006, 18 (20), 4952−4958. (23) Kali, G.; Georgiou, T. K.; Iván, B.; Patrickios, C. S.; Loizou, E.; Thomann, Y.; Tiller, J. C. Synthesis and Characterization of Anionic Amphiphilic Model Conetworks Based on Methacrylic Acid and Methyl Methacrylate: Effects of Composition and Architecture. Macromolecules 2007, 40 (6), 2192−2200. (24) Georgiou, T. K.; Patrickios, C. S.; Groh, P. W.; Iván, B. Amphiphilic Model Conetworks of Polyisobutylene Methacrylate and 2-(Dimethylamino)ethyl Methacrylate Prepared by the Combination of Quasiliving Carbocationic and Group Transfer Polymerizations. Macromolecules 2007, 40 (7), 2335−2343. (25) Kali, G.; Georgiou, T. K.; Iván, B.; Patrickios, C. S.; Loizou, E.; Thomann, Y.; Tiller, J. C. Synthesis and Characterization of Anionic Amphiphilic Model Conetworks of 2-Butyl-1-Octyl-Methacrylate and Methacrylic Acid: Effects of Polymer Composition and Architecture. Langmuir 2007, 23 (21), 10746−10755. (26) Scherble, J.; Thomann, R.; Iván, B.; Mülhaupt, R. Formation of CdS Nanoclusters in Phase-Separated Poly(2- hydroxyethyl methacrylate)-l-polyisobutylene Amphiphilic Conetworks. J. Polym. Sci., Part B: Polym. Phys. 2001, 39 (12), 1429−1436. (27) Binder, W. H.; Petraru, L.; Roth, T.; Groh, P. W.; Pálfi, V.; Keki, S.; Iván, B. Magnetic and Temperature-Sensitive Release Gels from Supramolecular Polymers. Adv. Funct. Mater. 2007, 17 (8), 1317− 1326. (28) Teubner, M.; Strey, R. Origin of the scattering peak in microemulsions. J. Chem. Phys. 1987, 87 (5), 3195−3200. (29) Vamvakaki, M.; Patrickios, C. S.; Lindner, P.; Gradzielski, M. Amphiphilic Networks Based on Cross-Linked Star Polymers: A Small-Angle Neutron Scattering Study. Langmuir 2007, 23 (21), 10433−10437. (30) Kondo, S.; Hiroi, T.; Han, Y. S.; Kim, T. H.; Shibayama, M.; Chung, U. I.; Sakai, T. Reliable Hydrogel with Mechanical “Fuse Link” in an Aqueous Environment. Adv. Mater. 2015, 27, 7407−7411. (31) Cui, J.; Lackey, M. A.; Tew, G. N.; Crosby, A. J. Mechanical Properties of End-Linked PEG/PDMS Hydrogels. Macromolecules 2012, 45 (15), 6104−6110.

(32) Erdödi, G.; Iván, B. Novel Amphiphilic Conetworks Composed of Telechelic Poly(ethylene oxide) and Three-Arm Star Polyisobutylene. Chem. Mater. 2004, 16 (6), 959−962. (33) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules 2008, 41 (14), 5379−5384. (34) Winter, H. H.; Mours, M. Rheology of Polymers Near LiquidSolid Transitions. Adv. Polym. Sci. 1997, 134, 165−234. (35) Han, Y. S.; Choi, S. M.; Kim, T. H.; Lee, C. H.; Kim, H. R. Design of 40M SANS instrument at HANARO, Korea. Phys. B 2006, 385−386, 1177−1179. (36) Gilbert, E. P.; Schulz, J. C.; Noakes, T. J. ‘Quokka’the smallangle neutron scattering instrument at OPAL. Phys. B 2006, 385−386, 1180−1182. (37) Norisuye, T.; Masui, N.; Kida, Y.; Shibayama, M.; Ikuta, D.; Kokufuta, E.; Ito, S.; Panyukov, S. Small Angle Neutron Scattering Studies on Structural Inhomogeneities in Polymer Gels:Radiation Cross-linked Gels vs Chemically Cross-linked Gels. Polymer 2002, 43, 5289−5297. (38) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University: Ithaca, NY, 1979. (39) Shibayama, M. Spatial Inhomogeneity and Dynamic Fluctuations of Polymer Gels. Macromol. Chem. Phys. 1998, 199, 1−30. (40) Matsunaga, T.; Sakai, T.; Akagi, Y.; Chung, U.; Shibayama, M. SANS and SLS Studies on Tetra-Arm PEG Gels in As-Prepared and Swollen States. Macromolecules 2009, 42 (16), 6245−6252. (41) Manet, S.; Lecchi, A.; Impéror-Clerc, M.; Zholobenko, V.; Durand, D.; Oliveria, C. L. P.; Pedersen, J. S.; Grillo, I.; Meneau, F.; Rochas, C. Structure of Micelles of a Nonionic Block Copolymer Determined by SANS and SAXS. J. Phys. Chem. B 2011, 115 (39), 11318−11329. (42) Groenewolt, M.; Brezesinski, T.; Schlaad, H.; Antonietti, M.; Groh, P. W.; Iván, B. Polyisobutylene-block-Poly(ethylene oxide) for Robust Templating of Highly Ordered Mesoporous Materials. Adv. Mater. 2005, 17 (9), 1158−1162. (43) Bartlett, P.; Ottewill, R. H. A neutron scattering study of the structure of a bimodal colloidal crystal. J. Chem. Phys. 1992, 96, 3306. (44) Helfand, E.; Wasserman, Z. R. Block Copolymer Theory. 4. Narrow Interphase Approximation. Macromolecules 1976, 9, 879−888. (45) Bates, F. S.; Schulz, M. F.; Khandpur, A. K.; Förster, S.; Rosedale, J. H.; Almdal, K.; Mortensen, K. Fluctuations, conformational asymmetry and block copolymer phase behaviour. Faraday Discuss. 1994, 98, 7−18. (46) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics: Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (47) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: Oxford, 2003. (48) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (49) Andrianov, K. A.; Slonimskiĭ, G. L.; Zhdanov, A. A.; Levin, V. Y.; Godovskiĭ, Y. K.; Moskalenko, V. A. Some physical properties of polyorganosiloxanes. I. Linear polyorganosiloxanes. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10 (1), 1−22. (50) Shibayama, M.; Hashimoto, H.; Kawai, H. Ordered Structure in Block Polymer Solutions. 5. Equilibrium and Nonequilibrium Aspects of Microdomain Formation. Macromolecules 1983, 16, 1434−1443.

H


Fabrication and Structural Characterization of ... - ACS Publications

Recommend Documents