Highly Stretchable Free-Standing Poly(acrylic acid)-block-poly(vinyl

Article pubs.acs.org/Macromolecules

Highly Stretchable Free-Standing Poly(acrylic acid)-block-poly(vinyl alcohol) Films Obtained from Cobalt-Mediated Radical Polymerization Fu-Sheng Wang,† Tzu-Fang Wang,‡ Hung-Hsun Lu,† Wai-Sam Ao-Ieong,‡ Jane Wang,‡ Hsin-Lung Chen,*,‡ and Chi-How Peng*,† †

Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters and ‡Department of Chemical Engineering, National Tsing Hua University 101, Sec 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: The block copolymers of poly(acrylic acid)-bpoly(vinyl alcohol) (PAA-b-PVA) were obtained from the hydrolysis of poly(methyl acrylate)-b-poly(vinyl acetate) (PMA-b-PVAc), which was synthesized by cobalt-mediated radical polymerization (CMRP) using the cobalt(II) porphyrin complex (CoII(TMP)) as the mediator. The mechanical properties of the PAA-b-PVA free-standing films could be tuned by the pH of the aqueous solution used to cast the films. The block copolymer films showed a much higher tensile strain and fractural tensile strength than the films prepared from the blends of PAA and PVA homopolymers. FTIR and morphological characterizations suggested that the tensile properties of the films were governed by both the hydrogen bonding between PVA and PAA that led to interpolymer complexation and the phase-separated morphology. For a given type of material, the greater extent of interpolymer complexation attained at lower solution pH led to the film with better tensile properties. The difference in the length scale of phase separation was responsible for the large difference in the tensile properties between block copolymer and blend films, where the characteristic nanostructure formed in the block copolymer prescribed a considerably larger amount of interface which enhanced the tensile properties significantly.

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INTRODUCTION Given by the rapid development of controlled/living radical polymerization methods such as NMP,1,2 ATRP,3,4 RAFT,5,6 MADIX,7,8 TERP,9,10 ITRP,11,12 and CMRP,13,14 various block copolymers have been synthesized and showed unique properties that are potentially useful to the fields of drug delivery,15,16 photolithography,17,18 and nanopatterning.19,20 The block copolymers with poly(vinyl alcohol) (PVA) segment yet have seldom been reported. PVA, which is obtained from the hydrolysis of poly(vinyl acetate) (PVAc), shows the properties of good solvent resistance,21 strong mechanical properties,22,23 high water-containing capacity,24 and good biocompatibility25,26 that render PVA as one of the most widely used hydrophilic materials adaptable for textile industry,27 food packaging,28 and medical devices.29,30 However, the difficulty to approach the controlled/living radical polymerization of vinyl acetate31 has limited the development of PVA-based block copolymers. Cobalt-mediated radical polymerization (CMRP) is to date one of the most efficient ways to synthesize the controlled polymers of vinyl acetate and thus controlled poly(vinyl alcohol).31−33 Cobalt complexes such as CoII(acac)2,31,34 CoII(TMP),32,35 and CoII(salen*)33 (Figure 1) have already been demonstrated as the good mediators to control the radical © XXXX American Chemical Society

Figure 1. Cobalt complexes used in controlled/living radical polymerization of vinyl acetate: (a) cobalt(II) acetylacetonate, CoII(acac)2; (b) cobalt(II) tetramesitylporphyrin, CoII(TMP); (c) cobalt(II) [N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine], CoII(salen*).

polymerization of vinyl acetate and to obtain the well-defined PVAc-based block copolymers. The block copolymer of poly(vinyl alcohol) and poly(acrylic acid) was first reported in 2008 as a pH-responsive material that formed the nanoparticles through self-assembly in water.36 Received: April 3, 2017 Revised: June 26, 2017

A

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electron microscope. The turbidity test was performed by mixing 2 mL of water solution of block copolymer, or polymer blend (10 mg/mL) was 2 mL buffer solutions with different pH value. The transmittance of the solution was recorded by a SHIMADZU UV-1800 instrument. DLS measurements were performed by a goniometer obtained from Brookhaven Instruments Corp. (Holtsville, NY), equipped with a diode-pumped laser (Coherent, Santa Clara, CA) with a wavelength (λ) of 532.15 nm and a power of 10 mW. The scattered light was collected at 90°. The chamber temperature was set as 20 °C with a water circulator. The autocorrelation function was computed using a digital correlator (BI 9000, Brookhaven Instruments Corp) and then analyzed using the non-negative least-squares (NNLS) method. SAXS measurements of the polymer films 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 0.07 to 4 nm−1 (q = 4π sin(θ)/λ, where 2θ is the scattering angle and λ is the wavelength). The collected scattering patterns were radially averaged to obtain one-dimensional (1D) scattering profiles. All the scattering profiles were corrected by removing the scattering from air and cell. The swelling test was performed by weighing dry polymer films and swollen films which were immersed in water for 24 h at room temperature. Free water on swollen film was removed by using filter paper. The swelling ratio was defined as follows:

Herein, the block copolymers of PMA-b-PVAc were synthesized by cobalt-mediated radical polymerization using CoII(TMP) and then converted to PAA-b-PVA via the hydrolysis. The thermal properties of the PAA-b-PVA freestanding films evaluated by TGA and DSC are similar to those of PVA/PAA hompolymer blend films. However, the tensile properties of the block copolymer films could be varied by the pH value of the solution used to cast the film and could reach as high as 623% in tensile strain, which is significantly higher than those of the corresponding blend films. According to the previous reports, the tensile strain of PVA was found to vary from 50% to 350%37 but mostly lay below 100%.38−41 Even the doped PVA films exhibited the tensile strain below 200%.38 The mechanical properties of PAA films were rarely studied, but the tensile strain of PAA fibers has been found to fall in the range 100%−150%.42,43 Obviously, the PAA-b-PVA block copolymers prepared here displayed a record high tensile strain over other PVA- or PAA-based materials. The small-angle X-ray scattering (SAXS) results suggested that the enhanced tensile properties of the block copolymer films arose mainly from the formation of the nanostructure due to microphase separation between the constituent blocks.

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EXPERIMENTAL SECTION

Materials. Solvents such as dimethylformamide, methanol, chloroform, and benzene were refluxed for 24 h over CaH2 to remove water. 2,2′-Azobis(isobutyronitrile) (AIBN, Showa), cobalt acetate (Alfa Aesar), mesitaldehyde (Sigma-Aldrich), pyrrole (Acros), boron trifluoride diethyl etherate (Sigma-Aldrich), 2,3-dichloro-5,6-dicyano1,4-benzoquinone (Alfa Aesar), N,N′-bis(3,5-di-tert-butylsalicylidene)1,2-cyclohexanediaminocobalt (CoII(salen*), Sigma-Aldrich), potassium diphosphate (Sigma-Aldrich), sodium citrate (Sigma-Aldrich), sodium carbonate (Sigma-Aldrich), sodium hydroxide (Shimakyu’s pure chemicals), poly(vinyl alcohol) (MW = 130K, 99% hydrolyzed, Sigma-Aldrich), and poly(acrylic acid) (MW = 240K, 25 wt % solution in water, Sigma-Aldrich) were used without any further purification. Deuterated solvents (Aldrich) were dried over molecular sieves. Vinyl acetate (Acros) was distilled under reduced pressure for inhibitor removing and degassed by three freeze−pump−thaw cycles before use. Methyl acrylate were purchased form Acros and were passed through basic Al2O3 to remove the inhibitor. OrDial D14b (nominal MWCO: 12 000−14 000, Orange Scientific) was used for dialysis. Tetramesitylporphyrin ((TMP)H2) was synthesized according to previous literatures44 and will be briefly described below. All reactions were carried out under an inert atmosphere. Characterization. NMR spectroscopy was used for characterization of chemical structure and calculation of monomer conversion. The spectrum was recorded by a Mercury-400 spectrometer at 298 K. The chemical shifts in 1H NMR were shown in ppm refer to residual solvent in CDCl3 as δ 7.24 ppm. GPC equipped with an Ultimate 3000 liquid chromatograph, a 101 refractive index detector, and Shodex columns (Shodex KF-802, Shodex KF-803, and Shodex KF-805) was used to analyze the polymeric products using THF as the eluent at 30 °C with flow rate equal to 1 mL min−1. The calibration was based on a linear poly(styrene) Shodex standard (SM-105) ranging in molecular weight from 1.20 × 102 to 2.61 × 106 g mol−1 with low PDI value. The Mn, Mw, and PDI of the polymeric products were given by DIONEX chromeleon software. A Seiko TG/DTA 300 was applied to give DSC and TGA curve from 30 to 600 °C under nitrogen. Tensile test was performed by universal test machine (AGS-2000G, Shimazu) with a 10 kN load cell. The rate was 8 mm/min. The samples were prepared as a 3 mm × 15 mm slide. Fourier transform infrared spectra (FT-IR) of the polymer films coated on silica wafer were performed with a Bruker Vertex 80v FTIR spectrometer. The spectra were recorded within the range of 4000−400 cm−1. The microstructure of the polymer membrane cross section was investigated by a JEOL JSM-6700F field emission scanning

swelling ratio =

wswollen − wdry wdry

where wdry and wswollen represent weight of the dry and swollen film, respectively. Synthesis of Tetramesitylporphyrin ((TMP)H2). Boron trifluoride diethyl etherate (110 μL, 0.088 mmol) was added dropwise into chloroform solution (250 mL) of 2,4,6-trimethylaldehyde (1.83 mL, 12.5 mmol) and pyrrole (0.89 mL, 12.5 mmol) with vigorous stirring. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (2.13 g, 9.4 mmol) was then slowly added after 2 h reflux and reacted for another 1 h. Triethylamine (120 μL, 0.088 mmol) was used to stop the reaction. After filtration, the mixture was purified by column chromatography and recrystallized with methanol to give the dark purple solid (25%). Synthesis of Cobalt(II) Tetramesitylporphyrin (CoII(TMP)). Dimethylformamide solution (20 mL) of TMP (188 mg, 0.24 mmol) and cobalt acetate (89.6 mg, 0.36 mmol) was refluxed under nitrogen for 16 h. The solvent was then removed by vacuum, and the crude dry product was washed with methanol. The solid was further purified by column chromatography using silica gel and eluants of dichloromethane and hexane (2:1). The final product of purple red powders was obtained from the recrystallization in dichloromethane and methanol (93%). Synthesis of Macroinitiators. The macroinitiators were synthesized under the condition of [CoII]0/[AIBN]0/[monomer]0 = 1:5:x at 55 °C. To keep the macroinitiators living, the reaction was stopped when the conversion of monomer ranging was about 10% to 30%. The x was set ranging from 4000 to 6000 depends on desired molecular weight of the macroinitiator. The polymerization of VAc was held under bulk. On the other hand, the polymerization of MA was in benzene solution to make [MA] = 2.47 M. For example, the synthesis of the macroinitiator PMA-CoIII(TMP) for chain extension of VAc with AIBN was held under [CoII]0/[AIBN]0/[monomer]0 = 1:5:4000 at 55 °C in benzene with [MA] = 2.47 and stopped at 380 min, and the monomer conversion was 24.5% with Mn = 88 000 and PDI = 1.25. Solvent and unreacted monomers were removed by evaporation under vacuum. Typical Process for Chain Extension and Block Copolymer Synthesis. Macroinitiator and monomer were mixed in a 250 mL Schlenk flask under inert atmosphere. The mixture was heated to 50 °C. The reaction was stopped when the molecular weight of block B

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Figure 2. Time-dependent GPC traces of chain extension of (a) MA from PVAc-Co(salen*), (b) VAc from PMA-Co(salen*), (c) MA from PVAcCo(TMP), and (d) VAc from PMA-Co(TMP). The molecular weights of the macroinitiators were (a) 43.5K, (b) 16.3K, (c) 22.5K, and (d) 19.3K.

Figure 3. Reanalyzed time-dependent GPC traces of chain extension of (a) MA from PVAc-Co(salen*), (b) VAc from PMA-Co(salen*), (c) MA from PVAc-Co(TMP), and (d) VAc from PMA-Co(TMP). The molecular weights of the macroinitiators were (a) 43.5K, (b) 16.3K, (c) 22.5K, and (d) 19.3K.

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copolymer reached the desired value via removing solvent and unreacted monomers by evaporation under vacuum with ice bath. Preparation and Purification of PAA-b-PVA Block Copolymers.45 A 20 mL methanol solution of sodium hydroxide (2 g) was slowly added into a 250 mL methanol solution of block copolymer (20 g) with vigorous stirring. The solution was refluxed for 24 h. The precipitate (yield = 95%) was collected by filtration, washed by methanol, and dried under high vacuum. The solid was then dissolved in water and dialyzed against RO water at 60 °C for five times; each took at least 12 h. Water was removed under high vacuum to give green or brown solids. Fabrication of Polymer Films. 70 mL aqueous solutions of the block copolymer or the homopolymer blends (1 g) at desired pH value were poured into a 7 cm diameter glass beaker. Water was evaporated slowly at 50 °C for 3 days to obtain the polymer films.

RESULTS AND DISCUSSION Synthesis of Well-Defined PAA-b-PVA Block Copolymer. The block copolymers of poly(vinyl alcohol) and poly(acrylic acid) could be obtained from the hydrolysis of various vinyl acetate (VAc)-based block copolymers such as poly(vinyl acetate)-b-poly(acrylonitrile)36 and poly(vinyl acetate)-b-poly(methyl acrylate).32,33 In this work, PMA-b-PVAc was selected as the precursor to prepare the PAA-b-PVA block copolymers. Since both CoII(TMP) and CoII(salen*) were capable of mediating the CMRP to synthesize the PMA-bPVAc,32,33 chain extension of MA or VAc from different macroinitiators of PVAc-CoIII(TMP), PVAc-CoIII(salen*), PMA-CoIII(TMP), and PMA-CoIII(salen*) was evaluated to determine the synthetic route. The chain extension reaction C

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Macromolecules was performed by following the reported method32,33 with the macroinitiators having Mn around 10K−100K. The GPC traces moved smoothly in MA polymerization initiated by PVAcCoIII(salen*) and VAc polymerization initiated by PMACoIII(TMP) (Figure 2a,d). However, the residue of the macroinitiators was observed in the chain extension of VAc from PMA-CoIII(salen*) and MA from PVAc-CoIII(TMP) (Figure 2b,c), indicating an inefficient initiation or the formation of terminated chains in these two systems. The GPC analysis is based on the correlation between the refractive index (RI) signal and elution time. However, the RI signal is proportional to the concentration of monomer but not the number of molecule so that the influence of low molecular weight polymers could be underestimated. To better evaluate the efficiency of chain extension for these systems, the GPC traces were thus reanalyzed to give the plots of polymer concentration versus molecular weight (Figure 3) obtained from the value of (RI signal)/Mn (eq 1) because the refractive index (dn/dc) is RI =

dn dn ×C= × M × MW dc dc

PVA. The macroinitiators were obtained by following the reported CMRP method41 in benzene at 55 °C, and the chain extension was performed at 50 °C in bulk (Table 1). The Table 1. PMA-Co(TMP) and PMA-b-PVAc Used To Prepare PAA-b-PVA PMA sample PMA976-bPVAc1825 PMA1034-bPVAc1147 PMA1348-bPVAc202

PMA-b-PVAc

Mn,GPC

Mn,NMR

PDI

Mn,NMR

PDI

abbrev after hydrolysis

88K

84K

1.25

241K

1.41

block 1,2a

92K

90K

1.23

188K

1.35

block 1,1a

110K

116K

1.40

133K

1.51

block 10,1a

hydrolysis of block copolymers was performed by using the procedure in the literature,45 and the conversion from PMA-bPVAc to PAA-b-PVA was evaluated by 1H NMR spectrum (Figures S2−S7). The homopolymers of PVA and PAA with DPn equal to 2790 and 2816, respectively, were purchased from Sigma-Aldrich and were mixed with the molar ratios of 1/2, 1/ 1, and 10/1 to form the corresponding homopolymer blends for comparison with the block copolymers (Table 2).

(1)

where dn/dc is specific refraction index increment, C is mass concentration of polymer solution, M is molar concentration of polymer solution, and MW is the molecular weight of polymer. The polymer concentration was steady when the size of polymer is large enough.46 The elution time was converted to molecular weight by using the calibration curve built with polystyrene standards. Given by Figure 3, the residue of macroinitiator can also be clearly observed in the chain extension of MA from PVAc-CoIII(salen*) (Figure 3a). The inefficiency of chain extension from PVAc macroinitiator to MA (Figure 3a,c) could be attributed to the fast propagation of MA (kp = 13 00033) and the stronger Co−C bond in PVAc− CoIII complexes32,33,47 that lead a much faster propagation than the initiation. The poor control of VAc polymerization initiated by PMA-CoIII(salen*) (Figure 3b) was proposed to be due to the relatively small equilibrium constant between CoII(salen*) and PMA-CoIII(salen*) (2.4 × 107 M−1).33 At the beginning of chain extension, part of PMA-CoIII(salen*) would dissociate to CoII(salen*) and polymeric radicals in order to reach the CoIII/ CoII equilibrium, but at the same time, most of the polymeric radicals terminated. Once the residual PMA radicals propagate to VAc, the CoIII/CoII equilibrium constant raised to the level of 1011 M−1,48,49 implying that the CoII(salen*) formed at the beginning would now suppress the radical concentration and thus retarded the polymerization. On the other hand, the molecular weight still increased smoothly when the polymerization of VAc was initiated by PMA-CoIII(TMP) (Figure 3d). Although the difference in molar masses of the macroinitiators could be a concern to the efficiency of chain extension, the molecular weights of the macroinitiators in Figure 2b−d were comparable (16.3K, 22.5K, and 19.3K, respectively). Although the macroinitiator in Figure 2a was relatively large (43.5K), it still showed a better chain extension efficiency than the cases in Figure 2b,c, indicating that the difference in molecular weight at this level did not have a significant impact on the chain extension efficiency. Therefore, the system of chain extension from PMA-CoIII(TMP) to PVAc was applied to synthesize the PMA-b-PVAc block copolymers with different MA/VAc ratio. Three block copolymers of PMA-b-PVAc with Mn ranging from 260K to 129K and MA/VAc ratio of 1/2, 1/1, and 10/1 have been prepared and subsequently hydrolyzed to PAA-b-

Table 2. Polymer Samples Used To Form the Films in This Study composition abbrev

PAA

PVA

description

block 1,2a block 1,1a block 10,1a PVAb PAAb blend 1,2 blend 1,1 blend 10,1

72K 77K 100K

86K 50K 7K 130K

hydrolyzed PMA976-b-PVAc1825 hydrolyzed PMA1034-b-PVAc1147 hydrolyzed PMA1348-b-PVAc202 homopolymer (DP = 2790) homopolymer (DP = 2815) polymer blend (AA:VA = 1:2) polymer blend (AA:VA = 1:1) polymer blend (AA:VA = 10:1)

240K 240K 240K 240K

130K 130K 130K

a

All the block copolymers were synthesized via CMRP mediated by CoII(TMP). bThe commercial PVA and PAA were chosen to have similar degree of polymerization to PMA976-b-PVAc1825.

Thermal Properties of the Block Copolymer Films. The thermal properties of the block copolymer and polymer blend films were studied by DSC and TGA. Figure 4 displays the DSC heating scans of the as-cast films of block 1,2, block 1,1, block 10,1, and blend 1,2. All the film samples exhibited a melting endotherm and a glass transition (marked by the arrow). The melting transition, located at the temperature near the melting endotherm of neat PVA, was associated with the melting of the PVA crystallites formed in the films, indicating that the as-cast films were semicrystalline. The melting temperature of the block 10,1 was much lower than that of other samples due to the low volume fraction of PVA in the copolymer. It is noted that all samples were found to display a Tg around 78 °C. The observed Tg was very close to that of neat PVA, implying that a significant fraction of PVA and PAA components in all the block copolymer and blend films were phase separated. The TGA and dTGA thermograms of the block copolymers and polymer blend shown in Figure 5 demonstrated that all polymer samples decomposed in three steps. The first one occurred at 150 °C with around 10% weight loss was associated D

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Figure 6. Stress−strain curves obtained from the tensile test of the films of PVA-b-PAA and PVA/PAA blends prepared from the aqueous solutions of different pH values: (a) block 1,2, pH = 2.7; (b) block 1,2, pH = 6.5; (c) block 1,2, pH = 11.4; (d) blend 1,2, pH = 3.1; and (e) blend 1,2, pH = 6.5.

Figure 4. DSC heating scans of the as-cast films of PVA, block 1,1, block 1,2, block 10,1, and blend 1,2. The inset at the right displays the enlarged thermograms in the temperature region near the glass transitions. The glass transition temperatures are indicated by the arrows.

in stress, when the pH of the casting solution increased to 6.5 and 11.4, respectively (Figure 6b,c). The highest strain at break and tensile stress was only 271% and 23.3 MPa for the blend film prepared from low pH environment (Figure 6d); the mechanical properties of the blend deteriorated to 152% and 0.8 MPa as the pH of the casting solution was raised to 6.5 (Figure 6e). The high solution pH of 11.1 even led to the blend film that was too soft for tensile test. Although block 1,2 and blend 1,2 were significantly different in strain at break, the effect of the pH of the casting solution on their mechanical properties was identical; i.e., decreasing the solution pH created the film with higher fracture strength and strain. Since the change of solution pH may affect the dissociation of the proton in the carboxylic acid group of PAA and thus the formation of hydrogen bonding between PAA and PVA, FTIR spectroscopy was employed to evaluate the hydrogen bonding in the films of block 1,2 and blend 1,2 prepared under a different pH environment (Figure 7). According to the literature,54 the formation of hydrogen bonds with the carboxylic acid group would broaden its CO stretching peak at around 1700 cm−1. It can be seen in Figure 7a−c that the CO peak of block 1,2 film prepared from pH 2.7 solution was clearly broader than that of the films prepared from pH 6.5

with the evaporation of residual water. The main weight loss took place from 300 to 400 °C and then from 400 to 500 °C, which could be attributed to the elimination of side chains and the decomposition of polymer backbone, respectively. Similar two-stage process had also been observed in the TGA and dTGA thermograms for the degradation of PVA50,51 and PAA52,53 homopolymers. Tensile Properties and Hydrogen Bonding of PAA-bPVA and PAA/PVA Blend Films. All polymer films were prepared via solvent casting at 50 °C using the aqueous solutions of different pH values. The tensile tests of these films with the dimension of 3 mm × 15 mm were then performed using a Universal Testing Machine with a 10 kN load cell and 8 mm/min extension rate. We first compared the tensile properties of PAA-b-PVA and PAA/PVA blend films prepared under different pH using block 1,2 and blend 1,2 as the representatives (Figure 6). Interestingly, the block copolymer film was found to show a much higher strain at break than that of the blend. The highest value approached 641% for block 1,2 prepared from pH 2.7 solution with the corresponding tensile stress of 27.3 MPa (Figure 6a). The strain at break gradually decreased to 490% and then to 409% with 20.3 and 19.2 MPa

Figure 5. TGA and dTGA thermal diagrams of (a) block 1,2, (b) block 1,1, (c) block 10,1, and (d) blend 1,2. E

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Figure 8. Stress−strain curves obtained from the tensile test of the films of PVA-b-PAA and PVA/PAA blends prepared from the aqueous solutions of pH 6.5. (a) block 1,2, (b) block 1,1, (c) block 10,1, and (d) blend 1,2.

Although the film of block 10,1 showed a high strain at break of 672%, the corresponding stress was only 0.9 MPa (Figure 8c), which was attributable to the low overall crystallinity present in the film due to low PVA composition. The IR spectra of these films in Figure 9a showed that the extent of hydrogen bonding

Figure 7. FTIR spectra in the wavenumber range of 1300−1900 cm−1 of the films of PVA-b-PAA and PVA/PAA blends prepared from the aqueous solutions of different pH values: (a) block 1,2, pH = 2.7; (b) block 1,2, pH = 6.5; (c) block 1,2, pH = 11.4; (d) blend 1,2, pH = 3.1; (e) blend 1,2, pH = 6.5; and (f) blend 1,2, pH = 11.1.

and 11.4 solutions, indicating a higher extent of hydrogen bonding between the −OH group of VA and the −COOH moiety of AA monomers in the former. In the aqueous solution with higher pH, more protons were dissociated from the carboxylic acid groups of PAA, thereby lowering the extent of hydrogen bonding. It appears that the pH dependence of the extent of hydrogen bonding in the solution (to be presented later) was largely retained the film, as similar pH dependence was observed for the film in Figure 7a−c. The effect of solution pH on the extent of hydrogen bonding in the film of blend 1,2 was analogous to that observed for the corresponding block copolymer film, as demonstrated in Figure 7d−f. For a given type of material (e.g., block 1,2), the FTIR results demonstrated that the film showing the best tensile properties (e.g., the film prepared from pH 2.0 solution) also displayed the highest extent of hydrogen bonding, suggesting that the hydrogen bonding played a role controlling the mechanical properties. However, considering that there was no large difference in the extent of hydrogen bonding between block 1,2 and blend 1,2, the consideration of the hydrogen bonding interaction alone cannot explain the large difference in mechanical properties between the blend and the copolymer films. The effect of composition on the tensile properties of the copolymer films prepared from the pH 6.5 solution was revealed in Figure 8. Block 1,1 showed the strain at break and stress of 319% and 14.6 MPa, respectively (Figure 8b), which were smaller than those of block 1,2 (490% and 20.3 MPa; see Figure 8a) but were still significantly larger than the values of 152% and 0.8 MPa displayed by blend 1,2 (Figure 8d).

Figure 9. FTIR spectra in the wavenumber range of 1300−1900 cm−1 of the films of PVA-b-PAA and PVA/PAA blends prepared from the aqueous solutions of pH 6.5: (a) block 10,1, (b) blend 1,2, (c) block 1,1, and (d) block 1,2.

should be highest in the film of block 10,1, followed by blend 1,2 (Figure 9b) and then block 1,1 and block 1,2 (Figure 9c,d). The extent of hydrogen bonding did not match the order of the strain at break (block 10, 1 > block 1,2 > block 1,1 > blend 1,2) and thus supported the assertion that the tensile property was not solely governed by the extent of hydrogen bonding. F

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was decreased to 5.0; the solution turbidity reached a maximum at pH 3.0 (Figure 10, upper panel). The change of the solution turbidity with respect to the change of pH was significant enough to be observed by the naked eye (Figure 10, bottom panel). It is interesting to note that the critical pH below which the interpolymer complexation took place was higher for the block copolymer solution than that for the blend solution; as can be seen in Figure 10, the turbidity of the blend solution did not increase obviously until pH decreased to 2.0. The result attested that the PVA and PAA in the copolymers formed complex much more easily than the corresponding homopolymers, which was attributable to the close proximity of the two block chains in the copolymer due to their chemical connectivity. The formation of interpolymer complex in PVAb-PAA has also been pointed out in a previous study.36 The particle size measurement by dynamic light scattering was performed for block 1,2 solution under different pH environment to provide the quantitative evidence for aggregation of the interpolymer complex. As shown in Figure 11, the average particle size at pH= 4 was 45.9 nm, and it

Table 3. Thickness, Maximum Stress, and Maximum Strain under Different pH Environment for the Films of Block Copolymers and Polymer Blends sample

pH

block 1,2 block 1,2 block 1,2 block 1,1 block 10,1a blend 1,2 blend 1,2 blend 1,2

2.7 6.5 11.4 6.5 6.5 3.1 6.5 11.1

max stress (MPa) 25.31 22.23 15.95 17.70 0.93 22.11 1.12

± ± ± ±

3.07 2.25 2.32 4.66

± 3.03 ± 0.51

max strain (%) 595.56 464.68 423.34 282.38 660.09 276.29 128.04

± ± ± ±

45.40 16.68 26.60 73.20

± 16.27 ± 23.95

thickness (mm) 0.31 0.28 0.45 0.31 0.18 0.25 0.50

± ± ± ± ± ± ±

0.02 0.03 0.11 0.09 0.01 0.05 0.08

The film of block 10,1 showed a poor physical property and was broken apart easily during the film fabrication so that only limited data could be collected.

a

Interpolymer Complexation of PAA and PVA in the Aqueous Solutions. The FTIR spectra in Figure 7 suggested that the extent of hydrogen bonding in the film increased with the decrease of the pH of the solution used to prepare the film, implying that although the interpolymer complex structure in the solution might undergo reorganization during the solvent evaporation, the state of interaction between PAA and PVA in the solution was largely retained in the film. The occurrence of the complexation between the two polymers was manifested by the increase of the turbidity of the solution due to the aggregation of the poorly soluble complex to form particles.48 Figure 10 shows the turbidity of the solutions of the block copolymers and blends as a function of pH. The block copolymer solutions started to become turbid when pH value

Figure 11. DLS for block 1,2 (10 μg/mL) in water at different pH values. The particle size reached maximum value at pH = 3.

reached the maximum value of 120.0 nm at pH = 3, consistent with the occurrence of the highest turbidity. The average particle size then decreased to 63.3 nm when the solution pH was further reduced to 2.0. The interpolymer complexes formed in the solution may remain intact throughout the solvent removal process in film casting, such that they were transferred into the films. The polymer films obtained thus composed of the matrix phase comprising uncomplexed PVA-b-PAA or PVA/PAA homopolymers and the domains containing the interpolymer complex. The hydrogen bonding revealed by the IR spectra of the films existed in the complex domains, as the PAA and PVA chains constituting the remaining matrix phase were phase separated as evidenced by the DSC results in Figure 4. The hydrogen bonding interaction between the two polymers led to high Tg for the interpolymer complex domains, such that they could act as the fillers for promoting the mechanical strength. Phase-Separated Morphology of the Films. Since the mechanical properties of the polymer films were not governed solely by the extent of hydrogen bonding relevant to the interpolymer complexation, the phase-separated morphology of the remaining matrix phase could be the key factor. Figure 12a shows the SAXS profiles of the as-cast films of block 1,2 and blend 1,1 prepared from pH 3.0 solutions. The scattering curve of neat PVA was also displayed for comparison. The SAXS

Figure 10. (top) Turbidity of the polymer solutions under different pH values. (bottom) Photographs of block 1,2 aqueous solution under different pH values. The concentration of the solution was 0.01 g/mL (in water). G

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Figure 12. (a) SAXS profiles of the as-cast films of block 1,2 and blend 1,1 prepared from pH 3.0 solutions and neat PVA. (b) SAXS profile presented in terms of I(q)q2 of block 1,2 prepared from pH 3.0 solution to show the scattering peaks (marked by the solid arrows) associated with the hexagonally packed cylinder structure more clearly.

profile of neat PVA exhibited a broad hump at ca. 0.7 nm−1 associated with the semicrystalline lamellar structure in the film. Blend 1,2 also showed a similar broad peak but was located at higher q (ca. 1.0 nm−1), implying that thinner PVA crystallites formed in the blend. Therefore, the morphology of the blend film was characterized by the presence of the domains of the interpolymer complex and the macrophase-separated domains consisting of nearly pure PAA and PVA. The PVA phase was semicrystalline, containing both crystallites and amorphous regions. Block 1,2 was found to display a totally different SAXS pattern, where a strong peak at ca. 0.1 nm−1 was identified along with a broad hump at ca. 0.75 nm−1. The presence of the low-q peak attested that copolymer molecules forming the matrix phase underwent a microphase separation, leading to a characteristic nanostructure with the domain spacing of ca. 63 nm. The SAXS profile of block 1,2 was further presented in the I(q)q2 plot in Figure 12b for revealing the higher-order peak more clearly. It can be seen that position of the higher-order peak relative to that of the primary peak was 31/2, suggesting that the block 1,2 cast from pH 3 solution self-assembled to form hexagonally packed cylinder morphology. The broad peak located at ca. 0.75 nm−1 was again attributed to the interlamellar distance of the crystallites formed in the PVA microdomains in the copolymer. On basis of the SAXS results, the block copolymer film was proposed to compose of the interpolymer complex domains distributed over the matrix phase in which PAA and PVA blocks underwent a microphase separation yielding a hexagonally packed cylinder nanostructure. The PVA microdomain thus generated was again semicrystalline. In light of the FTIR and morphological characterizations, we proposed that the tensile properties of the polymer films were governed by both the hydrogen bonding interaction between PVA and PAA that led to interpolymer complexation and the phase-separated morphology of the films. For a given type of the material, the extent of hydrogen bonding or interpolymer complexation, which could be tuned by the pH of the solution used to cast the film, played an important role controlling the mechanical properties, as the domains composed of the complex could act as the fillers for enhancing the tensile strength. The difference in the characteristic length scale of the

phase-separated morphology was responsible for the large difference in the tensile properties between block copolymer and blend films. The domains generated from the phase separation in the blends were normally of micrometers in length scales, whereas those formed in the block copolymers were of nanometers in length scale due to chemical connectivity between the block chains. Moreover, the interfacial adhesion between the two phases in the homopolymer blends is normally poor without compatibilizer; under this condition, the stress cannot be transmitted effectively across the interface, leading to poor mechanical properties. In the case of block copolymers, the characteristic nanostructure formed via microphase separation bears a considerable amount of interface, and the chemical connectivity between the block chains would allow the stress to transmit across the interface effectively, so as to enhance the tensile properties significantly. Water Absorbability of PAA-b-PVA. In addition to the tensile properties, the water absorbability of PAA-b-PVA was evaluated here, since both PAA and PVA have been used as hydrogel materials. The dry block copolymer films were immersed in RO water for 24 h for the measurement of water absorption capacity. Although both block 1,1 and block 10,1 were dissolved in water during the test, most possibly due to the deprotonation of PAA segment to increase the solubility of block copolymers, block 1,2 film demonstrated a significant water absorbability as shown in Figure 13. The film formed

Figure 13. Photographs showing the water uptake of the films of block 1,2 prepared from the aqueous solution with different pH values. H

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from basic solution (pH 11.4) exhibited the highest swelling ratio of 5000% (Figure 13). The insolubility of this film could be attributed to the longer PVA segment and the hydrogen bonding between PVA and PAA segments, which was proposed to suppress the deprotonation of PAA. The swelling ratio then gradually decreased to 2710% and 120% as the solution for film casting changed to neutral (pH 6.5) and acidic (pH 2.7) environment, respectively. The much better water absorbability displayed by the film formed under basic condition may be attributed to proton dissociation from the carboxylic acid moiety of PAA block, which rendered the polyelectrolyte nature to the PAA block.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan (MOST 104-2113-M-007-012-MY3), for support of this research. We also thank the assistants of Professor C.-C. Chang for the dynamic light scattering (DLS) measurement. The instrument of DLS is supported by Center for Biomedical and Biological Engineering, “Aim for the Top University Plan” of the National Chiao Tung University and Ministry of Education, Taiwan, R.O.C.

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CONCLUSIONS The free-standing films of PAA-b-PVA obtained from the hydrolysis of PMA-b-PVAc synthesized by using the CMRP technique with CoII(TMP) as the mediator have been prepared. Although the thermal properties characterized by DSC and TGA were similar between the block copolymer and homopolymer blend, the tensile properties of the block copolymer films were in general much better than those of the corresponding blend, where the maximum fractural tensile stress and strain attained were 27.3 MPa and 641%. The FTIR study indicated that the extent of hydrogen bonding between PAA and PVA transferred from the solution state could explain the variation of the mechanical properties of a given type of films prepared from the solutions with different pH values, but it was unable to rationalize the large difference between the block copolymer and the blend films prepared under a given pH environment. Combining the results of FTIR spectra, DSC, solution turbidity, and SAXS, we proposed that the films composed of the interpolymer complex domains distributed over a matrix phase in which PVA and PAA were phase separated. In the blend films, the phase separation occurred at the macroscopic length scale, generating the PAA and PVA domains of micrometers in size. On the other hand, PVA and PAA blocks in the block copolymer underwent the microphase separation, generating a characteristic nanostructure as evidenced by the SAXS profile. The nanostructure thus formed may prescribe a large amount of interface that led to the considerable enhancement of the tensile properties. Finally, the block copolymer film was found to display a high water absorptivity with the highest swelling ratio of 5000%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00700. Procedure of chain extension experiment with different macroinitiators in detail, representative GPC trace and NMR spectrum of block copolymers, and titration curves of block 1,2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

Hsin-Lung Chen: 0000-0002-3572-723X Chi-How Peng: 0000-0002-8305-8624 I

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