Mechanical Property Enhancement of ABA Block Copolymer-Based

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Mechanical Property Enhancement of ABA Block Copolymer-Based Elastomers by Incorporating Transient Cross-Links into Soft Middle Block Mikihiro Hayashi, Satoru Matsushima, Atsushi Noro,* and Yushu Matsushita* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We propose a new strategy to enhance mechanical properties of ABA triblock copolymer-based elastomers by incorporating transient cross-links into the soft middle block. An ABA triblock-type copolymer, poly(4vinylpyridine)-b-[(poly(butyl acrylate)-co-polyacrylamide]-bpoly(4-vinylpyridine) (P−Ba−P), was synthesized via RAFT polymerization. In the molecular design, the poly(4-vinylpyridine) (P) end blocks with a high Tg formed pseudo-crosslink domains due to segregation against the soft Ba middle block, while acrylamide units on the middle block formed selfcomplementary hydrogen bonding, serving as transient crosslinks. According to tensile tests, the Young’s modulus, elongation at break, maximum stress, and material toughness were 1.9 MPa, 200%, 2.6 MPa, and 2.8 MJ/m3, respectively. Comparison between mechanical properties of P−Ba−P and those of another triblock copolymer, poly(4-vinylpyridine)-bpoly(butyl acrylate)-b-poly(4-vinylpyridine) (P−B−P), revealed that P−Ba−P showed larger Young’s modulus, longer elongation at break, and larger maximum tensile stress than P−B−P. Particularly, the material toughness of P−Ba−P (2.8 MJ/ m3) was more than 100 times larger than that of P−B−P (0.02 MJ/m3). Rheological analysis on the basis of sticky Rouse relaxation of Ba middle block of P−Ba−P suggested that the hydrogen bonds on the middle block serve as dynamic stickers in elastic strands of elastomers under stress. Such dynamic behavior of the hydrogen bonds could prevent local concentration of applied stress for activating break/failure of the materials during elongation, leading to mechanical property enhancement of the materials. In addition, zinc chloride was blended with P−Ba−P to form metal−ligand coordination in the P end block domains, which also affected the mechanical properties of the elastomers.

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of the hard segment and that of the soft segment while they flow at higher temperatures than the Tg of the hard segment. As typical TPEs, ABA triblock copolymers,27−30 such as polystyrene-b-polybutadiene-b-polystyrene (SBS) and polystyrene-bpolyisoprene-b-polystyrene (SIS), have been studied extensively.31−34 This is because distinct polymeric “networks” can be spontaneously constructed at around the 10 nm scale due to selfassembly of ABA triblock copolymers, where the B middle blocks serve as melt strand matrix bridging the pseudo-cross-link domains of A blocks.35−38 The use of precisely synthesized ABA triblock copolymers has an advantage in preparing elastomers because the bridge-type molecular structure can directly affect the macroscopic physical properties of the materials. Physical properties of the elastomers associated with self-assembled ABA triblock copolymer morphologies have also been investigated.39−42

INTRODUCTION Elastomers are one of the representative polymeric network materials.1−5 Since the materials show unique mechanical features, such as stretchability and damping function,6−8 they have been widely used in our daily lives as automotive tires or shock absorbers. Rubber is a typical elastomer,9,10 which is produced through a chemical reaction of vulcanization. Because of the chemical cross-links located inhomogeneously in the polymer networks, there are some drawbacks, such as lack of recyclability and difficulty of reshape after processing the materials. For overcoming the drawbacks, thermoplastic elastomers (TPEs) have gained attention due to their processability and recyclability.11−20 It is well-known that TPEs are composed of two essential components: One is a hard segment with a high glass transition temperature (Tg) serving as a pseudo-cross-link domain to prevent macroscopic flow, where the term “pseudocross-link domains” refers to the assembly of glassy polymer chains via van der Waals interaction.21−23 The other is a soft segment with a low Tg which serves as an elastic strand in the network.24−26 TPEs can be elastic at temperatures between a Tg © XXXX American Chemical Society

Received: November 4, 2014 Revised: December 19, 2014

A

DOI: 10.1021/ma502239w Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In recent years, the studies of network materials composed of an ABA triblock copolymer and a cross-linker polymer keeping a noncovalent interaction ability to the A end blocks have been developing. Noro et al. have reported the preparation of thermoreversible supramolecular polymer gels (supramacromolecular gels) by blending an ABA triblock copolymer, poly(2vinylpyridine)-b-poly(ethyl acrylate)-b-poly(2-vinylpyridine), and a cross-linker, poly(4-hydroxystyrene), in an ionic liquid, where hydrogen bonds were formed between the poly(2vinylpyridine) end blocks and poly(4-hydroxystyrene). It has been revealed that the physical properties can be tuned by various parameters, such as the blend ratio, the cross-linker molecular weight, and the molecular weight of end blocks of the triblock copolymers.43−46 More recently, preparation of supramolecular polymer gels via metal−ligand coordination by blending an ABA triblock copolymer, poly(4-vinylpyridine)-bpoly(ethyl acrylate)-b-poly(4-vinylpyridine), and metal salts in an ionic liquid has been reported.47 Metal−ligand coordination was confirmed between the end block and the metal salts added, demonstrating that the thermoresponsive viscoelasticity can be controlled by varying the blend ratio, the end block chain length, and kinds of metal salts. Although there are a lot of other reports on the physical property control of elastomers bearing noncovalent cross-links of the end block of ABA triblock copolymers,48−53 there are few reports regarding the mechanical property control in terms of noncovalently cross-linking of the soft middle block. If transient weak cross-links of noncovalent bonding54 are incorporated into the soft middle blocks without a loss of the softness, such cross-links would improve the stiffness due to the increase in the cross-link density. Furthermore, the transient cross-links incorporated on the middle block can be repeatedly dissociated and reassociated during stress application, which could prevent local stress concentration in the network. In other words, incorporating transient cross-links into the soft middle block enables enhancement of the mechanical properties of elastomers. In this report, we propose a new molecular design of ABA triblock copolymer-based elastomers with transient cross-links on the soft B middle block for mechanical property enhancement of elastomers. An ABA triblock copolymer with a random copolymer in the B middle block, that is, poly(4-vinylpyridine)b-[poly(butyl acrylate)-co-polyacrylamide]-b-poly(4-vinylpyridine) (P−Ba−P, Figure 1), was synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization. Poly(4-vinylpyridine) (P) end blocks with a much higher Tg than room temperature periodically segregate from the middle block, which should serve as pseudo-cross-link domains. To attain elastomeric properties, butyl acrylate was chosen as a major component of the middle block due to its low Tg, while the randomly incorporated acrylamide units as a minor component on the soft middle block form self-complementary hydrogen bonding, which serves as transient cross-links in the matrix. For control experiments, a random copolymer, poly(butyl acrylate)co-polyacrylamide (Ba), and another triblock copolymer, poly(4vinylpyridine)-b-poly(butyl acrylate)-b-poly(4-vinylpyridine) (P−B−P), were also synthesized. Mechanical properties of these materials were investigated by dynamic mechanical measurements and tensile tests. In addition, zinc chloride was blended with P−Ba−P to form rigid metal−ligand coordinated domains of the end blocks, which can affect the mechanical properties.55,56

Figure 1. Chemical structure and schematic illustration of a P−Ba−P polymer. The P block assemblies are illustrated as blue spheres while the Ba middle block and acrylamide units are drawn as pink lines and pink “L-type” blocks, respectively.

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EXPERIMENTAL SECTION

Synthesis of Triblock Copolymers. For preparation of a P−Ba−P triblock copolymer, P blocks were first synthesized via RAFT polymerization with a bifunctional chain transfer agent S,S′-bis(α,α′dimethyl-α″-acetic acid)trithiocarbonate and azobis(isobutyronitrile) (AIBN).57 After purification of P with reprecipitation procedures, a mixture of butyl acrylate and acrylamide was randomly copolymerized from the center of P blocks, where the RAFT agent residue was located (Scheme 1). The unreacted precursor P was removed with a selective solvent. To evaluate effects of the transient cross-links on the mechanical properties, poly(4-vinylpyridine)-b-poly(butyl acrylate)-b-poly(4-vinylpyridine) (P−B−P) was synthesized in a similar manner to the P−Ba−P synthesis (Scheme 1). In addition, a random copolymer poly(butyl acrylate)-co-polyacrylamide (Ba) and a linear poly(butyl acrylate) (B) were also synthesized via RAFT polymerization as control samples to confirm effects of the P end block on the mechanical properties (Schemes S1 and S2). Note that the polymerization of butyl acrylate and acrylamide should proceed in a nearly random sequence, judging from their reactivity ratios, where the reactivity ratio of acrylamide to butyl acrylate and that of butyl acrylate to acrylamide are 0.72 and 1.26, respectively.58,59 Random sequence of butyl acrylate and acrylamide was also evaluated by performing the differential scanning calorimetry (DSC) measurements for various poly(butyl acylate)-co-polyacrylamide with varying the fraction of each monomeric unit (see Figure S7 and Table S1 in Supporting Information). A single endothermic peak was observed in the DSC thermograms and that was shifted to a higher temperature with an increase in the fraction of acrylamide units, suggesting the copolymerization of butyl acrylate and acrylamide via RAFT polymerization proceeds in a random fashion. Detailed synthesis procedures and conditions were described in the Supporting Information. Polymer Characterization. Polydispersity indices (PDIs) of the polymers synthesized were determined by size exclusion chromatography (SEC), using three TSK gel columns (two G3000HHR and one B

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Macromolecules Scheme 1. Synthesis of P−Ba−P and P−B−P (*Note RAFT agent residue was omitted in their chemical structures for simplification)

Table 1. Codes and Molecular Characteristics of Polymers sample code B Ba P−B−P P−Ba−P

NPa

NBa

48 48

194 195 196 178

Naa

fab

30

0.13

30

0.15

Mmiddlec

Mtotald

PDIe

Tg (°C)

25 000 25 000

25 000 27 000 30 000 30 000

1.05 1.10 1.22 1.17

−50f −26f −45,f 115g −23,f 106g

a

Average number of monomeric units of 4-vinylpyridine (NP), butyl acrylate (NB), and acrylamide (Na) in a polymer chain determined by 1H NMR. Mole fraction of acrylamide units in a random copolymer chain, which was calculated from Na/(NB + Na). cNumber-average molecular weight of the middle block (Mmiddle). dTotal number-average molecular weight of the polymers (Mtotal), which was calculated from the number of monomeric units and the mass of each monomeric unit. ePolydispersity indices measured by SEC. fGlass transition temperatures determined by DSC. gGlass transition temperatures determined by rheological measurements. b

units 0.5. Since ZnCl2 possesses two available coordination sites,47 the mole ratio between available coordination sites and ligands should attain the stoichiometric equivalence when the mole ratio of ZnCl2/pyridine units is 0.5. The total mass of the polymers and ZnCl2 was fixed at 250 mg to prepare the films with the same thickness. Blends of P−Ba−P (or P−B−P) and ZnCl2 were coded as P−Ba−P/ZnCl2 (or P−B−P/ ZnCl2). Spectroscopy. Fourier transform infrared spectroscopy (FT-IR) measurements were carried out to confirm hydrogen bonding and metal−ligand coordination. The instrument used was an IR Prestige-21 spectrometer combined with an AIM 8800 attachment (Shimadzu) for microscopic sample positioning. Each measurement was conducted at room temperature. Calorimetry. Differential scanning calorimetry (DSC) measurements were conducted for all samples with a Q2000 calorimeter (TA Instruments) to investigate Tgs. The temperature range for the measurements was from −60 to 200 °C, and the temperature increase rate was 10 °C/min. Morphological Investigation. Small-angle X-ray scattering (SAXS) measurements were carried out for all samples at room temperature to acquire the structural information at the nanometer scale by Rigaku Nano Viewer, where an X-ray generator was operated at the voltage of 45 kV and the current of 60 mA. The wavelengths of the incident X-rays and the camera length were 0.154 and 735 mm, respectively, while imaging plates were used as detectors. Mechanical Properties. Dynamic mechanical measurements for the triblock copolymers and the blends were performed with an uniaxial rheometer RSA-G2 (TA Instruments) by using strip-shaped samples. The measurements for B and Ba were performed with a shear rheometer of ARES-G2 (TA Instruments) with 25 mm parallel plates. Dynamic temperature ramp and dynamic frequency sweep tests were conducted

G4000HHR), combined with an HPLC pump and a refractive index detector (Tosoh Corp.). Dimethylformamide (DMF) was used as an eluent, and a flow rate was 1 mL/min. The elution volume was calibrated with polystyrene standards. SEC chromatograms of the polymers are shown in Figure S1, which reveals that polymers with relatively low PDIs were prepared and that the precursor P for P−Ba−P (or P−B−P) was completely removed by purification. The degree of polymerization and the number-average molecular weight (Mn) of polymers were estimated by 1H NMR (see Figures S2−S6). Molecular characteristics of B, Ba, P− B−P, and P−Ba−P are summarized in Table 1. Since the entanglement molecular weight (Me) of poly(butyl acrylate) is approximately 30 000 g/mol according to the literature,6 the molecular weights of B, Ba, and the middle block of P−B−P and P−Ba−P are all lower than Me of poly(butyl acrylate). It should be also noted that the molecular weight of the middle block (Mmiddle) and that of the triblock copolymer (Mtotal) for P−Ba−P and P−B−P are almost the same. Preparation of the Bulk Films. Preparation of bulk films of neat polymers was carried out as follows. P−Ba−P (or P−B−P) was dissolved in a mixed solvent of pyridine/water (9/1 by volume), which was poured into a Teflon made mold (W × 25 mm, D × 10 mm, H × 10 mm). The solvent was evaporated slowly during solvent casting at 40 °C and dried in vacuo at 40 °C. The mass of the polymers used was fixed at 250 mg to prepare films with almost the same thickness. Preparation of the Blends. Preparation of blends was carried out as follows. First, P−Ba−P (or P−B−P) and zinc(II) chloride ZnCl2 were dissolved in a mixed solvent of pyridine/water (9/1 by volume) separately. The use of the solvent mixture was to prevent rapid aggregation between P−Ba−P (or P−B−P) and ZnCl2.60,61 Then, both solutions were mixed in a Teflon made mold, and the solvent was evaporated in the same way as explained above. P−Ba−P (or P−B−P) and ZnCl2 were mixed with keeping the mole ratio of ZnCl2/pyridine C

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Macromolecules for the samples. All dynamic tests were performed in a linear regime in a N2 gas atmosphere. It should be noted that degradation by heating of the polymers synthesized via RAFT polymerization can be negligible in an inert gas atmosphere under 130 °C.62 Detailed experimental conditions for the rheological measurements are described in the Supporting Information. Tensile tests for the triblock copolymers and the blends were performed with an ARES-G2 rheometer (TA Instruments) by using dogbone-shaped samples prepared with a cutting die (detailed sample shape is shown in Figure S8). The tests were conducted with an elongation rate of 10 mm/min at room temperature.

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RESULTS Molecular Characterization of Polymers. Figure 2 represents the macroscopic appearance of bulk polymers. B

Figure 3. (a) FT-IR spectra of Ba and P−Ba−P within a wavenumber range from 3300 to 3600 cm−1, where a dashed line at 3350 cm−1 and a dotted line at 3450 cm−1 correspond to the absorption bands of hydrogen bonded N−H and free N−H stretching vibration modes, respectively. (b) FT-IR spectra of P−B−P and P−Ba−P within a wavenumber range from 970 to 1010 cm−1, where a dotted and a dashed line represent the wave numbers corresponding to the absorption bands of free and hydrogen bonded pyridine groups, respectively. (c) DSC thermograms for polymer samples, where the arrows represent Tgs. (d) Small-angle X-ray scattering profiles for the four polymers. Arrows with numbers in P−B−P profiles represent relative q positions at peaks.

Figure 2. Photos of macroscopic appearance of (a) B, (b) Ba, (c) P−B− P, and (d) P−Ba−P.

and Ba were sticky materials at room temperature,63 while selfstanding and elastic films were prepared from P−B−P and P− Ba−P. These indicate that the P end blocks play an important role in attaining elasticity of these materials, and the P end blocks probably form pseudo cross-link domains. Note that the yellow color is derived from the RAFT agent residue in the polymers. Figure 3a represents FT-IR spectra of Ba and P−Ba−P in a wavenumber range from 3300 to 3600 cm−1, where a dashed line at 3350 cm−1 and a dotted line at 3450 cm−1 correspond to hydrogen bonded N−H and free N−H stretching vibration modes, respectively.64,65 It was found by the FT-IR spectra that not all but a part of acrylamide units formed self-complementary hydrogen bonding66,67 in P−Ba−P and Ba. In addition, the FTIR spectra of P−B−P and P−Ba−P within a wavenumber range between 970 and 1010 cm−1 are shown in Figure 3b, where the absorption at 993 cm−1 corresponds to free pyridine groups.68,69 According to the literature,68,69 the peak originating from the hydrogen bonded pyridine groups should appear at approximately 1005 cm−1. However, there are no distinct peaks at this wavenumber in a P−Ba−P spectrum as well as a P−B−P spectrum. These results suggest that hydrogen bonding was not formed between pyridine and acrylamide units in either P−Ba−P or P−B−P. DSC thermograms obtained are shown in Figure 3c. One endothermic peak was observed in thermograms of B and Ba at −50 and −26 °C, respectively, which can be assigned to their Tgs. The reason for the higher Tg of Ba than that of B can be attributed to acrylamide units in Ba. Similarly, an endotherm appeared in the thermograms of triblock copolymers at −45 °C for P−B−P and −23 °C for P−Ba−P, which could also correspond to the Tgs

of P−B−P and P−Ba−P. Since the Tgs of P−B−P and P−Ba−P are similar values to the Tgs of B and Ba, respectively, the observed Tg in P−B−P or P−Ba−P originates from segmental motions of the middle block. The Tg of P block was not observed clearly in the DSC thermograms or the derivative-type thermograms (Figure S9) because of small fraction of the P block (∼10%) in each chain end and the amount of endothermic heat from glass transition of P is relatively small.68 In contrast, a peak was evidently observed at approximately 100 °C in the tan δ spectra acquired by rheological measurements (Figure S10), which should correspond to an α relaxation of the P block; therefore, the peak position on the tan δ spectra was assigned to a Tg of the P block (Table 1). Figure 3d shows SAXS profiles of the four polymers. The vertical axis represents the intensities in logarithmic scale, while the horizontal axis expresses the scattering vector q (= 4π(sin θ)/ λ), where λ and 2θ are the wavelength of X-rays and the scattering angle, respectively. There were no scattering peaks in the profiles of B and Ba, meaning that there is no or little electron density fluctuation at the nanometer scale in B and Ba. This also indicates that acrylamide units did not form their own domain, probably because acrylamide units should be randomly distributed in Ba. In contrast, distinct peaks can be seen in the profiles of P−Ba−P and P−B−P. These are attributed to the electron density fluctuation or phase separation at a nanometer scale between the component of the P end block and that of the D

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Macromolecules

were observed in P−B−P and P−Ba−P at the temperature range from −20 to 90 °C for P−B−P and from 20 to 90 °C for P−Ba− P. The E′ in the rubbery region of the P−B−P spectra remained almost constant whereas that of P−Ba−P spectra decreased gradually with an increase in temperatures, which could be attributed to the fact that the hydrogen-bonded cross-links in the Ba middle block of P−Ba−P are sensitive to the temperatures as evidenced in Figure S13 of FT-IR spectra at various temperatures. The plateau modulus, EPs, determined at the temperature with a minimum tan δ value was 0.62 MPa for P−B−P and 1.6 MPa for P−Ba−P. Since EP is known to be proportional to the average cross-link density ν (EP ∼ νkBT, where kB and T are the Boltzmann constant and the absolute temperature, respectively),70−73 these results indicate that P−Ba−P has higher crosslink density. This resulted from formation of the selfcomplementary hydrogen bonds between acrylamide units on the middle blocks (more detailed discussion of the EP will be provided in the Discussion section). The E′ decreased at the temperature between 90 and 100 °C in both P−B−P and P−Ba− P, suggesting the appearance of rubbery flow region, where the temperature was approaching the Tg of P block. This decrease in E′ of P−B−P was more gradual than that of P−Ba−P. The rheological behavior of the polymers against frequencies was also investigated. Figure 5a represents the master curves of B and Ba created on the basis of the time−temperature superposition (TTS) principle with a reference temperature

middle block, where spherical structures could presumably be formed as judged from the volume fraction of the P block. Unfortunately, however, structure specification was not clear for both P−Ba−P and P−B−P by TEM observation (see Figure S11). Note that scattering intensities of the peak on the P−Ba−P profile are apparently lower than those on the P−B−P profile, and higher order peaks were not observed in P−Ba−P unlike in P−B−P, suggesting segregation power difference between them. To test this difference, P/B and P/Ba blends were prepared. It was found that macrophase separation was easier to occur in P/B than in P/Ba (see Figure S12), confirming that the segregation power between P and B was stronger than that between P and Ba. This may be coupled with higher degree of long-range ordering in the structure of P−B−P than in P−Ba−P, which is indeed realized as the sharper peaks on the SAXS profile of P−B−P. Therefore, the incorporation of acrylamide units into the middle block lowered the effective interaction parameter between the middle block and the end blocks. Viscoelastic Properties of Polymers. First, we compare the results of dynamic temperature ramp tests for B and Ba obtained by using a shear rheometer in Figure 4a. The loss

Figure 4. Rheological curves of storage moduli and loss moduli acquired by dynamic temperature ramp tests for B and Ba (a) and for P−B−P and P−Ba−P (b).

modulus G″ was larger than the storage modulus G′ for B in the measurement temperature range. Although G′ of Ba were slightly larger than the G″ at lower temperatures than 10 °C, a distinct rubbery plateau was not observed. In addition, both B and Ba represent flow regions at higher temperatures than −20 and 20 °C, respectively. Figure 4b shows the storage Young moduli E′ and the loss moduli E″ spectra acquired by dynamic temperature ramp tests with an uniaxial rheometer. Both triblock copolymers are in the glassy state having constant E′ with the order of 109 Pa at the temperature region from −60 to −40 °C for P−B−P and from −60 to −20 °C for P−Ba−P. At higher temperatures than the glassy regions, the transition from the glassy state to the rubbery state was present. It should be noted that the transition region (−20 to 20 °C) in P−Ba−P spectra was wider than that in P−B−P spectra (−40 to −20 °C) probably due to the influence of the hydrogen bond formation in the middle block of P−Ba−P. In contrast to the rheological spectra of B and Ba, rubbery regions

Figure 5. (a) Master curves of B and Ba with the reference temperature of 30 °C. The dashed lines represent the fitting curves with the Rouse model for B and the sticky Rouse model for Ba, respectively. The frequency dependence of E′ for P−B−P and P−Ba−P at (b) 60 °C and (c) 90 °C was also shown, where the unfilled and filled symbols represent E′ spectra of P−B−P and P−Ba−P, respectively. E

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Macromolecules (Tref) of 30 °C. It is apparent that the TTS worked well for the whole measurement temperature range in both B and Ba, suggesting that a single relaxation mechanism governs the modulus in each system at temperatures examined.74 Obviously, Ba showed much slower relaxation than B because of incorporation of acrylamide units. Since molecular weight of B is lower than Me of poly(butyl acrylate),6 the viscoelastic behavior of B can be described by the Rouse relaxation model as expressed in eqs 1 and 275 G′(ω) =

G″(ω) =

N

ρRT M

∑

ρRT M

∑

p=1 N p=1

The dynamic frequency sweep tests were also performed for the triblock copolymers, and the spectra of E′ and E″ for P−B−P and P−Ba−P against frequencies at each temperature are presented in Figures S15 and S16, respectively. Figures 5b and 5c show the E′ spectra at 60 and 90 °C, where the temperatures correspond to the rubbery plateau region and the rubbery flow region of dynamic temperature ramp spectra, respectively. The E′ of P−Ba−P was larger than that of P−B−P in the whole frequency range (0.1−100 Hz) at 60 °C because of the higher network density in P−Ba−P than in P−B−P, originating from incorporation of the transient cross-links of acrylamide hydrogen bonds. It should be also noted that E′ of P−B−P remains nearly constant while that of P−Ba−P decreased gradually with a lowering of frequencies. This is probably because some part of the hydrogen bonded cross-links in the Ba middle block of P− Ba−P gradually dissociated with a lowering of frequencies, which decreased the average cross-link density. In contrast to the spectra at 60 °C, E′ was larger for P−B−P than for P−Ba−P at the low frequencies (