ISSN 1560-0904, Polymer Science, Series B, 2017, Vol. 59, No. 6, pp. 708–717. © Pleiades Publishing, Ltd., 2017. Original Russian Text © O.G. Zamyshlyayeva, E.A. Smirnov, N.S. Zakharycheva, 2017, published in Vysokomolekulyarnye Soedineniya, Seriya B, 2017, Vol. 59, No. 6, pp. 450–460.
FUNCTIONAL POLYMERS
Linear-Dendritic Block Copolymers Based on N-Isopropylacrylamide and Perfluorinated Polyphenylenegermane: Synthesis and Properties at Various Interfacial Boundaries O. G. Zamyshlyayevaa,*, E. A. Smirnova, and N. S. Zakharychevab a National b
Research Nizhny Novgorod State University, pr. Gagarina 23, Nizhny Novgorod, 603950 Russia Research Institute of Chemistry, National Research Nizhny Novgorod State University, pr. Gagarina 23, Nizhny Novgorod, 603950 Russia *е-mail:
[email protected] Received January 9, 2017; Revised Manuscript Received July 20, 2017
Abstract—It is shown that linear-dendritic block copolymers poly(N-isopropylacrylamide)–block–polyphenylenegermane can be prepared by the polymerization of N-isopropylacrylamide in the presence of bis(pentafluorophenyl)germane followed by activated polycondensation with tris(pentafluorophenyl)germane. The properties of the dilute solutions and Langmuir monolayers of the functional polymers and the linear-dendritic block copolymers of N-isopropylacrylamide are studied. DOI: 10.1134/S1560090417060112
INTRODUCTION The aqueous solutions of poly(N-isopropylacrylamide) (PIPAA) are characterized by a lower critical solution temperature ranging from 30 to 35°С [1]. This motivated a wide application of PIPAA and polymers of other N-substituted acrylamides and hydrogels on their basis in bioengineering and for manufacturing membranes with the temperature-dependent permeability, materials for biotechnological applications, and matrices for the directed transport of drugs [2–5]. The key approach to governing the solubility and properties of the polymers based on N-isopropylacrylamide is its chemical modification via a change in the ratio of hydrophilic and hydrophobic groups, for example, the synthesis of copolymers with meth(acrylamides) [6, 7] and N-vinylpyrrolidone [8]. The modification of PIPAA does not always preserve its capability for the LCST, but it may lead to the appearance of new properties at various interfacial boundaries. In particular, this tendency is typical of copolymers with fluorinated monomers. For example, the graft copolymers of N-isopropylacrylamide with vinylidenefluoride were synthesized [9] and used as porous membranes. In this context, fluorinated linear-dendritic PIPAA-based block copolymers, in which perfluorinated polyphenylenegermane (PPG) is used as a fluorinated component, may be of interest to researchers.
As was shown in [10, 11], the combination of radical polymerization and activated polycondensation makes it possible to modify linear PMMA and PS with hyperbranched PPG with a yield of up to 80% [10, 11]. The efficiency of these reactions is ensured by high constants of chain transfer to the organogermanium compound (in the polymerization of styrene and MMA, the constants of chain transfer to (С6F5)2GeH2 are 3.0 and 0.87 and the constants of chain transfer to (C6F5)3GeH are 3.4 and 0.3). The properties of solutions and films of these linear-dendritic block copolymers differ from the properties of their linear analogs—PMMA and PS. The goals of this study are to ascertain whether the reaction of chain transfer to bis(pentafluorophenyl)germane in the radical polymerization of N-isopropylacrylamide and the subsequent activated polycondensation of the isolated functional polymers with tris(pentafluorophenyl)germane may be used for the synthesis of linear-dendritic block copolymers with various molecular masses of blocks and to study the processes of self-assembly of hybrid macromolecules in solutions and Langmuir monolayers. EXPERIMENTAL Solvents used in this study were purified according to common procedures [12]. Monomer IPAA (Aldrich) was recrystallized two times from hexane
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LINEAR-DENDRITIC BLOCK COPOLYMERS BASED Table 1. Conditions of synthesis and characteristics of functional polymers PIPAA PIPAA
[η], [BPPG] d(w), nm d(n), nm cm3/g × 102 mol/L
Мη × 10–3
1
4.0
1.6
1.3
4.2
11.5
2
2.0
3.8
3.6
6.7
23.7
3
1.0
2.0
1.9
6.4
22.0
4
0.8
5.1
5.0
6.4
22.1
5
0.5
5.6
4.9
8.3
32.7
6
0
13.9
10.8
53.6
580.0
K = 9.59 × 10–3, α = 0.65, T = 27°С, THF as a solvent [13].
and dried in vacuum at room temperature. AIBN used as an initiator of radical polymerization was recrystallized two times from isopropyl alcohol.
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resulting polymers were dried in vacuum at room temperature to a constant mass. The yield of reaction products was 42.8% for PIPAA–PPG-1 and 57.1% for PIPAA–PPG-2. For the comparative analysis, PPG was prepared from TPPG under the same conditions. The solution turned yellow during activated polycondensation; this fact indicated consumption of the activator and formation of the side product Et3N · HF [16]. After the synthesis, the monomer mixture was a turbid white solution, and upon evaporation of the solvent, the reaction product was insoluble in THF. Therefore, the block copolymers PIPAA–PPG-1 and PIPAA–PPG-2 were isolated by the method of hot extraction with THF using a Soxhlet apparatus (the time of extraction was 4 h). The polymer dissolved in THF was reprecipitated into hexane, and the product insoluble in THF was reprecipitated into hexane from dioxane solution. The yields of fractions soluble in THF and insoluble in THF were 96.5 and 3.5% for PIPAA–PPG-1 and 87.9 and 12.1% for PIPAA– PPG-2, respectively.
Synthesis of Functional and Linear-Dendritic Polymers The polymerization of IPAA was performed in the presence of bis(pentafluorophenyl)germane (C6F5)2GeH2 (BPPG) as a chain-transfer agent (at concentrations from 0 to 4 × 10–2 mol/L) and using AIBN as an initiator (at concentration of 7.7 × 10‒3 mol/L) (Table 1). Polymerization was conducted in ampoules at 60°С, and the time of synthesis was 24 h [13]. The reaction mixtures were preliminarily purified via three freeze–pump–thaw cycles. After completion of polymerization, the ampoules were cooled with liquid nitrogen to stop the reaction. The polymers were reprecipitated three times from acetone in hexane and dried in vacuum at 25°С. The relative constant of chain transfer to BPPG was determined in the mixed solvent acetone–benzene (1 : 1, vol/vol) ([IPAA] = 1.1 mol/L) by the Mayo method (Table 1) [14]. Linear-dendritic block copolymers were subsequently synthesized using two functional polymers: PIPAA-1 and PIPAA-2, which were prepared in the presence of BPPG (concentrations of 4 × 10–2 and 2 × 10–2 mol/L, respectively). The block copolymers were synthesized by the activated polycondensation of polymers PIPAA-1 and PIPAA-2 with tris(pentafluorophenyl)germane (C6F5)3GeH (TPPG) in THF solution in an argon atmosphere at 25°С (the synthesis time was 1 h) under continuous stirring. Triethylamine was used as an activator (molar ratio of TPPG : Et3N = 1 : 8, m(TPPG) : m(PIPAA) = 1 : 2, and m(TPPG) : m(THF) = 1 : 10) [15]. After the synthesis, the reaction mixture was reprecipitated from dioxane solution into hexane. The POLYMER SCIENCE, SERIES B
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Characterization of the Functional and Linear-Dendritic Polymers The intrinsic viscosity of polymer solutions was measured in THF at 27°С on an Ubbelohde viscometer (the efflux time of the solvent was t0 = 77.3 s). The value of Mη for the homopolymer PIPAA and the functional polymers PIPAA-1 and PIPAA-2 were calculated by the Kuhn–Mark–Houwink equation [η] = K(Mη)α (the values of constants K and α are listed in Table 1). The structure of the polymers was investigated by IR and 1Н NMR spectroscopy. The IR spectra of the samples were measured on an InfralumFT-801 IR spectrometer using KBr pellets. NMR spectra were taken on an Agilent DD2 400 instrument (400 MHz, T = 25°С) using CDCl3 as a solvent and tetramethylsilane as a reference. The spectra were treated with the aid of MestReNova software. The fraction of the hydrophilic block in the lineardendritic block copolymers was determined using a vario EL cube elemental analyzer for simultaneous CHN(S) determination. The average hydrodynamic radius (d(w)) of macromolecules of the studied polymers dissolved in THF, water, or chloroform was estimated by dynamic light scattering via correlation function multimodal analysis. Measurements were performed at 25 ± 0.1°С in chloroform and water and at 27 ± 0.1°С for THF at angles of 90° and 173° in 1-cm-quartz cuvettes using a NanoBrook Omni spectrometer (Brookhaven Instruments, United States). The time of correlation function accumulation was 180–300 s, and the number of photons falling on the detector per
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second was in the range of 50–450 kcps. The average hydrodynamic radius was calculated from the results of 5 to 10 parallel measurements. Before analysis, the quartz cuvette was dedusted by 2- to 3-fold rinsing with the solvent filtered through a 0.2-μm Nylon filter (Chromafil Xtra). A number of PIPAA solutions containing 0.01, 0.1, 0.25, 0.5, 1.0, and 2.0 wt % of the polymer were preliminarily analyzed at various angles in order to obtain concentration dependences and to register only diffusion modes. It was found that the number and position of modes do not change beginning from 0.5 wt %. The solutions of all the studied polymers with a concentration of 0.5– 1.0 wt % which were prepared in chloroform or THF were filtered through the 0.2-μm Nylon filter (Chromafil Xtra) before measurements. Surface pressure isotherms were recorded in air on a KSV Mini system (Finland) using a Tef lon bath (surface area of 273 cm2) equipped with polyacetal membranes. The π–A isotherms of monolayer compression were registered at 21 ± 1°С by a Langmuir automated balance with a Wilhelmy platinum plate. The solutions of polymers (1 g/L) in chloroform were deposited on the surface of the subphase (deionized water with a specific conductivity 0.07 μS/cm or 0.1 N HCl solution), allowed to stand for 30 min to evaporate the solvent from the subphase surface, and compressed at a rate of 10 mm/min. The surface area per milligram of the copolymers in a monolayer A0 was determined graphically by extrapolating the descending branch of the isotherm π = f(A) to π = 0. RESULTS AND DISCUSSION In order to synthesize linear-dendritic block copolymers under activated polycondensation conditions, the bis(pentafluorophenylgermanium) group should occur at chain end of the linear polymer. In this case, addition of the activator and the comonomer tris(pentafluorophenyl)germane ensures formation of the linear-dendritic block copolymer. The end group of the polymer may be functionalized via the reaction of chain transfer to BPPG in radical polymerization [10, 11, 15, 17]. The value of the relative constant of chain transfer to BPPG, as calculated by the Mayo method for the polymerization of IPAA, was found to be 2.2. This finding provides evidence that this organogermanium compound is an efficient chain-transfer agent (Table 1).
Figure 1 shows the fragmented IR spectra of PIPAA B
D H C C n H2 C O C NH C H3C H CH3 A A
the functional polymer PIPAA-1 containing bis(pentafluorophenylgermanium) groups F D H C C Ge n H2 C OH
B
F
C NH C H3C H CH3 A A
and the block copolymer PIPAA–PPG-1 B
D H C C Ge PPG n H2 C O C NH C H3C H CH3 A A
It is seen that, in the IR spectrum of the functional polymer, absorption bands corresponding to groups ‒С6F5 (969.1 cm–1) and –CF (1080.9 cm–1) appear. This fact confirms the presence of perfluorophenylgermanium groups in macromolecules (Fig. 1, spectra а, b). In the case of the block copolymer (spectrum c), the absorption band due to the group –C6F4 (1236.8 cm–1) emerges in the IR spectrum. This observation suggests that the hyperbranched block is formed in activated polycondensation. The block copolymers were studied by 1H NMR spectroscopy (Table 2). For the functional polymer and block copolymer, the values of chemical shifts due to protons of the group –СН2– shift (1.88 and 1.68 ppm). This may be associated with the presence of the – GeH(C6F5)2 group at the end of a macromolecule and the appearance of the branched PPG block at chain
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а
C(O)NH
CH(CH3)2 CH2, CH3, CN
NH
b
CF
CH(CH3)2
C6 F 5
C(O)NH CH2, CH3, CN
NH
c
C6 F 4 CH(CH3)2 C(O)NH 1800
1700
NH 1600
CF
C6 F 5
CH2, CH3, CN
1500
1400
1300
1200
1100
1000
900
800 ν, сm−1
Fig. 1. Fragmented IR spectra of (a) the homopolymer PIPAA, (b) the functional polymer PIPAA-1, and (c) the linear-dendritic block copolymer PIPAA–PPG.
end. According to the elemental analysis data, the linear-dendritic block copolymers contain 5.5 (PIPAA– PPG-1) and 5.0 wt % nitrogen (PIPAA–PPG-2). The mechanism behind formation of a hyperbranched macromolecule at the end of PIPAA chains
involves two stages [16, 18]. At the first stage, the activation of the monomer TPPG proceeds via formation of ion pairs, and at the second stage, the stepgrowth polymerization occurs to afford hyperbranched macromolecules.
Table 2. Assignment of 1H NMR signals of PIPAA, PIPAA-1, and PIPAA–PPG-1 Chemical shifts of protons, ppm Polymer –CH3 (A)
–CH2– (B)
–NH (C)
–CH– (D)
PIPAA
1.14
1.68
7.26
2.61
PIPAA-1
1.13
1.88
7.26
–
PIPAA–PPG-1
1.13
0.86
7.26
–
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ZAMYSHLYAYEVA et al. F
F
CH2
H C
Ge
CO
H
F
+ HGe
Et3N, THF, Ar −Et3NHF
F
CH2
Ge
CO
F
NH H3C
H C
NH
CH
H3 C
CH3 n
CH CH3 n
F F
Ge
F F
Ge
F
F F
Ge
=
F
F
F
F
Ge
F
F
Ge H
F
Ge
Ge
F
F F
Ge
F F
F
F F
Ge
Ge
F
F F
F
Solution Behavior of Functional Polymers and Block Copolymers The synthesized functional and linear-dendritic polymers are of diphilic nature; therefore, it was of interest to explore their behavior in solutions and at the water/air interface (in Langmuir monolayers). As is known, the aqueous solutions of PIPAA are characterized by the LCST. Below the LCST, hydroη, сm3/g y = −0.1274x + 4.1785 4
2
0
6
8
10
12 с, g/L
Fig. 2. Dependence of reduced viscosity on the concentration of functional polymer PIPAA-1 in THF solution.
gen bonds of water with N–H and C=O groups of macromolecules prevail, and this circumstance is favorable for dissolution of the polymer. Above the LCST, hydrogen bonds weaken, while interactions between hydrophobic side groups of the polymer grow and become dominant. As a consequence, phase separation occurs. At the LCST, there is competition of dispersion interactions between the hydrophobic groups and the hydrogen bonds formed between water molecules and polymer macromolecules [19]. The viscosity-average molecular mass of PIPAA, as estimated in THF, was 5.8 × 105, in agreement with the conditions of polymer synthesis described in [13]. The viscosity-average molecular masses of PIPAA-1 and PIPAA-2 were determined in THF; these values were 11.5 × 103 and 23.7 × 103, respectively (Table 1, samples 1, 2). For all of the studied polymers, including the homopolymer, an anomaly of viscosity was observed (the reduced viscosity increased somewhat with a decrease in the concentration of polymer solutions). As an example, Fig. 2 shows the concentration dependence of reduced viscosity for the solution of PIAA-1 in THF. This phenomenon, which was described in [20], is typical of certain linear polymers, whose macrocoils unfold and become looser at high dilutions, which leads to a rise in the viscosity of solutions, and macromolecular compounds comprising
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branched blocks, for which the abnormal viscosity manifests itself to a higher extent. An analysis of the dynamic light scattering data obtained for PIPAA solutions (Table 1, sample 6) revealed the existence of two modes with the following maxima (d(w)): 14.1 and 34.5 nm in water, 13.9 and 37.9 nm in THF, and 9.9 and 23.8 nm in chloroform. A difference in the values of the first mode on passage from one solvent to another may be attributed to its thermodynamic quality for this polymer and, accordingly, to the size of the coil in this solvent. For example, for PIPAA, THF is a good solvent, water is a θ-solvent [13], and chloroform is the worst solvent from this series in terms of quality. The second mode may be attributed to the high-molecular-mass fraction of the sample owing to a wide molecular-mass distribution of the polymer [13]. It should be noted that, in the case of water, THF, and chloroform used as solvents, the numerical distribution shows only one mode with number-average diameters d(n) of 10.6, 10.8, and 7.2 nm. Because the functional polymers PIPAA-1 and PIPAA-2, as opposed to the homopolymer, are insoluble in water, their solutions in THF and chloroform were analyzed. On passage from the functional polymer PIPAA-1 to the block copolymer PIPAA–PPG-1, the value of d(w) increases from 1.6 to 2.2 nm in THF and from 1.1 to 1.6 nm in chloroform, while for PIPAA-2 and PIPAA–PPG-2 parameter d(w) decreases from 3.8 to 3.6 nm in THF and from 2.9 to 2.2 nm in chloroform. It is possible that reduction in the hydrodynamic diameters of functional and linear-dendritic polymers is associated with the incorporation of hydrophobic fragments into PIPAA macromolecules and the additional worsening of thermodynamic quality of the solvent. For example, chloroform is a poorer solvent than THF for both PIPAA and PPG. This manifests itself as a strong “compression” of blocks in hybrid macromolecules in chloroform solutions compared with THF solutions. The value of d(w) = 5.4 nm, which was obtained for PPG using the same method, is slightly above the respective value of d(w) for the block copolymer PIAA–PPG-2. This may apparently be explained by the fact that the activated polycondensation of TPPG mostly affords the third-generation PPG with М = (1.7–2.2) × 10 4 as the reaction product [16]. The polycondensation of the functional polymer PIAA-1 with ТPPG yields the second-generation PPG block (М = 7.2 × 103 and Reff = 1.5 nm [16]). The reason behind this phenomenon is unclear, but it may be assumed that it is associated with the presence of the PIPAA hydrophilic block that affects the preceding polycondensation reaction giving rise to the second functional group in a monomer molecule—the anionic center on the germanium atom. The first POLYMER SCIENCE, SERIES B
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functional group, namely, para fluorine atoms in ‒C6F5 groups, is contained in the initial monomer. In order to gain insight into the behavior of amphiphilic macromolecules at the water/air interface, it was important to evaluate the state of macromolecules in spreading solutions, that is, in chloroform solutions. Upon deposition of polymer solutions in chloroform on the water/air interface, it is of importance to ascertain the state of macromolecules in solution (individual macromolecules, aggregates, or micelles). As was shown in [21] for amphiphilic block copolymers based on N-vinylpyrrolidone and 2,2,3,3-tetrafluoropropyl methacrylate, the behavior of macromolecules in a layer is determined by the properties of spreading solutions from which the monolayer is deposited. For all of the studied polymers (except the homopolymer) in chloroform and THF solutions, a single mode is seen (for both the weight and the numerical distribution of particles). Note that the values of d(w) and d(n) do not change with the solution concentration; therefore, the concentration dependences of d(w)/d(n) reflect the motion of individual molecules. Note that the values of d(w) and d(n) are close. This is evidence for a narrow size distribution of particles. Properties of Functional Polymers and Block Copolymers at the Water/Air Interface Surface pressure isotherms were measured for the mentioned polymers at various concentrations of spreading solution under compression on aqueous (pH 7.0) and acidic subphases (pH 1.3); the data are generalized in Fig. 3 and Table 3. The values of area per 1 mg of the polymer in a monolayer А0 with a change in the volume of the spreading solution (10–50 μL) on the aqueous subphase are different. This implies that the spreading of polymer solution does not provide a true monomolecular layer (each macromolecular unit is in contact with the water/air interface). For polymers PIAA-1 and PIAA-2, the values of А0 in a spreading solution volume of 10 μL are 0.88 and 1.30 m2/mg, respectively. Taking into account that the Mη of PIAA-1 is lower than that of PIAA-2, this difference may be explained by the fact that the length of the hydrophilic block in the functional polymers affects the behavior of macromolecules at the water/air interface. At a spreading solution volume of 50 μL, the state of macromolecules in the monolayer and solution is controlled by dispersion (intramolecular hydrophobic) and polar interactions (intermolecular contacts between IPAA units and between IPAA units and water) [22]. For the polymer with Mη = 11.5 × 103, the pressure of monolayer destruction πdestr is 17.2 mN/m (Table 3). This estimate is higher than the value of 12.3 mN/m obtained for the polymer with Mη = 23.7 × 103, but the values of A0 for this volume of spreading solution are the same for these polymers (0.46 m2/mg). It appears that, at a
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ZAMYSHLYAYEVA et al.
(а)
π, mN/m
F
F
F
1
F
GeH
GeH
20 А0 = 16.72 nm2/molecule 15
2
2
1 Ge
Ge
10 А0 = 21.48 nm2/molecule
5
0
0.5
1.0
1.5
(b)
F
π, mN/m
A, m2/mg
2.0
F
F
F GeH
GeH
1
16 1 Ge
Ge
2
2
12 А0 = 8.82 nm2/molecule 8
4
0
0.1
0.2
0.3
0.4
A, m2/mg
Fig. 3. Surface pressure isotherms of (1) the polymer PIPAA-1 and (2) block copolymer PIPAA–PPG-1 at spreading solution volumes of (a, c) 10 and (b) 50 μL on the subphase at pH (a, b) 7.0 and (c) 1.3. The insets show the orientation of macromolecules at the interfacial boundary.
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715
(c) 1 π, mN/m 25
F
F
F
F GeH
GeH
А0 = 33.88 nm2/molecule
20
2 Ge
Ge 15 А0 = 19.09 nm2/molecule 10 2
1
5
0
0.5
1.0
1.5
2.0
A, m2/mg
Fig. 3. (Contd.)
small length of PIPAA chains in the monolayer, intermolecular interactions prevail (πdestr(PIPAA-1) > πdestr(PIPAA-2), while for the high-molecular-mass block intramolecular interactions in the hydrophilic fragment of a macromolecule predominate. A comparison of the behavior of the functional polymer and the block copolymer PIPAA–PPG-1 shows that the presence of the branched block at the end of the PIPAA hydrophilic unit is responsible for the increase in the value of А0 (e.g., for a spreading solution volume of 10 μL А0 = 1.13 m2/mg), but for a spreading solution volume of 50 μL, the values of А0 for PIPAA-1 and PIPAA–PPG-1 are equal and amount to 0.46 m2/mg. For the second series of the polymers, the values of А0 for PIPAA-2 and PIPAA– PPG-2 at Vspr = 10 μL are almost the same (1.30 and 1.25 m2/mg). It may be suggested that in this case the linear block occupies the same area at the water/air interface both under the group H(C6F5)2Ge– at the chain end of the functional polymer and under the branched block in the linear-dendritic macromolecule. At a spreading solution volume of 50 μL, the value of А0 for both PIPAA-2 and PIPAA–PPG-2 and the first series of the polymers is 0.46 m2/mg. Note POLYMER SCIENCE, SERIES B
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that the values of surface pressure for the films of the studied polymers are fairly high (π = 12–24 mN/m). In addition, surface pressure isotherms were obtained using a 0.1 N solution of HCl (pH 1.3) as a subphase. Figure 3 and Table 3 demonstrate that the values of А0 on the acidic subphase for PIPAA-1 and PIPAA-2 are 1.78 and 1.59 m2/mg, respectively (Vspr = 10 μL). These values are higher than the analogous parameters measured at pH 7.0. This finding may be explained by unfolding of the hydrophilic blocks of poly(N-isopropylacrylamide) at the water/air interface. If all PIPAA units could fully escape to the acidic subphase, then the value of А0 would correspond to the analogous value of А0 for PPG in the densely packed condensed monolayer, which at the water/air interface was of billiard-ball-type packing [23] with А0 = 0.2 m2/mg at πdestr = 39.78 mN/m (pH 1.3, Table 3). In the case of the block copolymers at respective concentrations of the spreading solution, the values of А0 decrease (e.g., for PIPAA–PPG-1 at Vspr = 10 μL, А0 = 1.00 m2/mg). In our opinion, for the studied polymers at a spreading solution volume of 30–50 μL on the acidic subphase, exactly this concentration of macromole-
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Table 3. Characteristics of monomolecular polymer films on aqueous (pH 7.0) and acidic subphases (pH 1.3) obtained from surface pressure isotherms π = f(A) Polymer
Vcalcd, μL
πdestr, mN/m
А0, m2/mg pH 7.0
PIPAA-1
PIPAA-2
PIPAA–PPG-1
PIPAA–PPG-2
PPG
πdestr, mN/m
А0, m2/mg pH 1.3
10
0.88
19.27
1.78
24.22
20
0.83
23.41
1.20
20.35
30
0.80
22.37
0.79
17.58
40
0.60
17.87
0.54
15.88
50
0.46
17.20
0.41
13.86
10
1.30
23.99
1.59
25.27
20
1.03
24.00
1.20
20.99
30
0.79
19.46
0.80
13.62
40
0.60
14.93
0.58
12.70
50
0.46
12.30
0.40
10.59
10
1.13
21.26
1.00
21.54
20
1.04
23.55
0.89
25.12
30
0.80
19.49
0.71
21.72
40
0.59
17.58
0.55
16.68
50
0.46
15.27
0.41
14.69
10
1.25
24.61
1.17
22.86
20
1.13
24.24
0.88
24.59
30
0.80
16.39
0.71
24.99
40
0.60
12.26
0.58
17.02
50
0.46
9.67
0.41
15.34
40
0.17
30.44
0.20
39.78
50
0.14
36.70
0.19
49.80
cules is achieved in the monolayer, at which intramolecular polar interactions between PIPAA units predominate over intermolecular interactions in the monolayer and the main fraction of PIPAA units appears to be within macrocoils. This suggestion is confirmed by low surface pressure values, at which the destruction of monolayers occurs (Table 3), and low values of A0 (0.4–0.8 m2/mg) for all of the studied polymers at a given acidity of the subphase (pH 1.3). Exactly at this concentration of the spreading solution, macrocoils experience compression at the water/air interface. This is due to the fact that the bis(pentafluorophenylhermanium) group occurs at the chain end and that, at a high content of macromolecules per unit area in the monolayer, it is oriented
toward air. At a spreading solution volume of 10– 20 μL, the monolayer films of the examined polymers feature higher destruction pressures of monolayers, namely, 20–25 mN/m. This is indicative of considerable intermolecular interactions both between PIPAA units and between PIPAA units and water. As a result, the hydrophilic blocks are unfolded and this process is accompanied by an increase in A0. The values of А0 obtained at a spreading solution volume of 50 μL on the subphase at pH 7.0 almost coincide with the analogous values measured at pH 1.3 for all the studied polymers. The hyperbranched perfluorinated PPG is characterized by high surface pressures equal to 30– 49 mN/m and low values of A0 = 0.14–0.2 m2/mg
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(Table 3) regardless of the acidity of the subphase. Therefore, it may be proposed that, in the monolayer of the linear-dendritic block copolymers, the hydrophobic fluorinated block is uninvolved in intermolecular interactions at the water/air interface and, as in the case of functional polymers, is expelled in air.
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The behavior of PPG obtained by activated polycondensation under the same conditions as lineardendritic block copolymers was investigated (Table 3). The values of А0 for PPG at spreading solution volumes of 40 and 50 μL on the aqueous subphase were 0.17 and 0.14 m2/mg, respectively (the pressures of film destruction were 30.44 and 36.70 mN/m). Comparing these values with the analogous data obtained for the block copolymers makes it possible to infer that, when block copolymer macromolecules are oriented toward the water/air interface, the hydrophilic blocks of IPAA do not dissolve in the subphase but are retained on the surface of water through the abovementioned interactions and create a so-called “cushion” for the hyperbranched block (which is expelled in air), thereby screening the aqueous subphase from the hydrophobic block. The values of А0 obtained for PPG in Langmuir monolayers were used to recalculate the sizes of hyperbranched polymer macromolecules with allowance made for the density of PPG (2.39 g/cm3) [23]. It was found that the effective radius of PPG molecules in the monolayer is 1.5 nm (for the water subphase, А0 = 0.14 m2/mg). Using the surface pressure isotherms and taking into account the Mη of the hydrophilic block in the linear-dendritic block copolymer, the area per macromolecule in the dense monolayer А0 was calculated. The value of А0 for the functional and linear-dendritic polymers in the dense monolayer was 8.82 nm2/molecule (insets in Fig. 3) at a spreading solution volume of 50 μL (Fig. 3b, А0 = 0.46 m2/mg). Thus, an efficient method has been developed for preparing linear-dendritic block copolymers based on N-isopropylacrylamide and perfluorinated polyphenylenegermane containing 5.5 and 5.0% of the hydrophilic block. It has been shown that these macromolecules are able to form monomolecular films at the water/air interface regardless of destruction acidity.
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ACKNOWLEDGMENTS
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This work was supported by the Ministry of Education and Science of the Russian Federation, 4.5510.2017/BCh. POLYMER SCIENCE, SERIES B
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23. S. I. Gusev, S. D. Zaitsev, Yu. D. Semchikov, and O. G. Zakharova, Russ. J. Appl. Chem. 79, 1338 (2006).
