Article pubs.acs.org/Macromolecules
Self-Assembly of Poly(methacrylic acid)‑b‑poly(butyl acrylate) Amphiphilic Block Copolymers in Methanol via RAFT Polymerization and during Film Formation for Wrinkly Surface Pattern Longhai Guo, Yongjing Jiang, Siyu Chen, Teng Qiu,* and Xiaoyu Li* College of Materials Science and Engineering, State Key Laboratory of Organic−Inorganic Composites, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing Engineering Research Center of Synthesis and Application of Waterborne Polymer, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *
ABSTRACT: Reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization was utilized to synthesize poly(methacrylic acid)-b-poly(butyl acrylate) (PMAA-b-PBA) amphiphilic block copolymer dispersions in methanol by using the PMAA homopolymer with dithiobenzoate end-group as macro-RAFT agent. And the PMAA macroRAFT agent was synthesized first by using 4-cyanopentanoic acid dithiobenzoate (CADB) as RAFT agent, where the intermolecular hydrogen bondings can be formed between the MAA units. With the formation of solvophobic PBA block by propagating BA monomer on the PMAA homopolymer chain, the PMAA-b-PBA block copolymer in situ self-assembled into core/shell sphere with PBA and PMAA blocks as core and shell matrix, respectively. The repulsive steric interaction within the PMAA block on shell matrix stabilized the copolymer particles in methanol dispersion, which further resulted in the formation of wrinkly surface pattern on the PMAA-b-PBA copolymer film. During the film formation process, the core/shell copolymer particles were concentrated and then anisotropically aggregated with the evaporation of methanol. The aggregated copolymer particles further assembled into the fibrous structure, so that the film with wrinkly surface pattern was obtained. And the fibrous width and the amplitude of fluctuant film surface can be controlled by copolymer molecular structure and film casting temperature, which are synergetically governed by both the self-assembly of core/shell copolymer particles and the hydrogenbonding network within the PMAA blocks. self-assembly (PISA).16,17 PISA can be started from the RAFT polymerization of first lyophilic block, which is carried out in a good solvent for both the monomer and the polymer chains with the existence of necessary initiator and chain transfer agent. Then, the lyophobic monomer was used for the subsequent molecular chain extension so as to in situ form the amphiphilic block copolymer, which can self-assemble into self-stabilized spheres, fiber, and/or vesicles, depending on the polymerization condition and molecular structure.5,6 Although the great efforts have been devoted to the morphological study on nano-objects and the structural prediction on them in polymer dispersions, we still cannot image what kind of surface patterns would be obtained from the film prepared by casting the copolymer dispersion. It has been suggested by a large amount of studies on polymer latex that the film formation of polymer colloidal dispersion is complicated.18,19 The dispersed particles would first accumulate in a concentrating process to a dense packed lattice and then coalesce among the particles driven mainly by the surface tension and capillary forces.
I. INTRODUCTION Particular interest has been focused on the synthesis and selfassembly behavior of block copolymers, especially for the amphiphilic block copolymers, which have been widely applied in bio-, optical, and electrical fields with nanostructures.1−8 The microphase separation between the different blocks in a selective solvent can induce the diverse array of micelles.9 And in solid film, the intricate microscope lithography can also be observed or manipulated as patterned surfaces. The explorations on novel and convenient synthesis method of block copolymers have approached for the new morphologies. The mechanisms for the achieving of supramolecular structures and the applications of them were extensively studied and reviewed.10−12 The investigation on block copolymer was accelerated by the rapid development of controlled living radical polymerization (CPR) technique based on reversible deactivation of the propagating radical, such as reversible addition−fragmentation chain transfer (RAFT) polymerization.13−16 Based on the recent reports, RAFT polymerization can be carried out not only in bulk and solution but also in heterophase conditions, which makes it possible to directly prepare the polymer colloidal dispersion via the so-called polymerization-induced © 2013 American Chemical Society
Received: October 21, 2013 Revised: December 15, 2013 Published: December 27, 2013 165
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
during film formation process, so as to obtain the film with special surface pattern. In our work, we systematically investigated the self-assembly of PMAA-b-PBA copolymer during film casting process, which provided a novel method to prepare the polymer film with the wrinkly surface pattern. The PMAA-b-PBA copolymers with different degree of polymerization (DP) were synthesized via one-pot, two-step RAFT polymerization in methanol. The copolymer particles with core/shell sphere structure (PBA block as core and PMAA block as shell) were obtained in methanol dispersion due to the PISA process. Subsequently, the directional association of these particles was performed during the film formation process. The wrinkle-like surface morphology with the periodical wavelength was finally obtained. The geometrical size and morphology of PMAA-bPBA film surface pattern are controllable by adjusting the molecular structure of synthesized amphiphilic block copolymer and the evaporation rate of methanol solvent.
Generally, fcc packed lattice or homogeneous flat surface would be obtained from a dispersion of random polymerized polymer. However, the wrinkly surface morphology due to the anisotropic aggregation can be occasionally observed in some practices. The surface wrinkle of soft material is another academic discipline with growing interest.20−22 Based on the recent review on the surface wrinkling, periodic or chaotic wrinkly surfaces were generally observed on the human skin and elastomer surfaces.10 Except the theoretical approaches for understanding the formation of wrinkly surface in different materials and geometries, there has been a resurgence of interest in the biomimetic or periodic nature of wrinkles. Theoretical and experimental works have been done to fabricate the tunable wrinkly patterns, which showed the potential applications as functional coatings on lithography, metrology, electronics, and biomedical engineering. Recently, the studies on high-order self-assembly of nanoparticles or micelles proved that the anisotropic long fibrous aggregation of particles could happen in dispersions.9 Meanwhile, hierarchical assembly would also occur in the block copolymer dispersion, which is induced by the solvent condition. And the wrinkly surface can be obtained, if the anisotropic aggregation or assembly can be accompanied during the film formation process, which will give out a texture-like surface with the controllable wrinkle period (λ) by the diameter of fibrous assemblies. For this purpose, the structure of the dispersed particles should satisfy the following requirements: (i) particles possess enough bond sites for coordination; (ii) there is specific repulsive force between monomers. It has been proved by theoretic and experimental studies that polypeptidegraft nanoparticle could satisfy the above requirements and selfassemble into the fibrous supramolecular structure, when adopted a relatively high energy stage to allow for thermodynamically favored incorporation of nanoparticles into the supramolecular structures.12 The synthesis of the amphiphilic block copolymers, such as poly(acrylic acid)-b-poly(butyl acrylate) (PAA-b-PBA), has been reported via RAFT polymerization.23,24 Jacquin et al.23 reported the synthesis of PAA-b-PBA block copolymers by MADIX and focused on their self-assembly and micellization in aqueous solution, even through the cylinder morphology was found via cast film route in tetrahydrofuran solution. Besides, the block length of lyophilic PAA was noticeably larger than that of lyophobic PBA. Chenal et al.24 reported the synthesis of PBA-b-PAA block copolymer particles via RAFT emulsion polymerization in water, in which the PAA macro-RAFT agent was synthesized first in 1,4-dioxane. By adjustment the pH of reactor in the second step, the weight percentage of hydrophilic PAA block (with respect to PBA block) could be reduced to 1.2 wt %, and the high molar mass of copolymer was possible to reach up to 100 kg mol−1. However, most of the previous literatures just focused on the RAFT polymerization control and self-assembly behavior in dispersion at nanoscale. Actually, for poly(methacrylic acid)-b-poly(butyl acrylate) (PMAA-bPBA) block copolymer, the PMAA block is subject to hydrogen bonding for cohesive interactions, which can meet the first requirement for hierarchical assemblies. Moreover, the well dissolved and slightly ionized PMAA block can provide the specific repulsive forces between the in situ formed assemblies, and hence it can meet the second requirement. Therefore, based on the nanoscaled self-assembly structure of block copolymer in dispersion, the further assembling into hierarchical structure at submicrometer scale might be achieved
II. EXPERIMENTAL SECTION II-1. Materials. Methacrylic acid (MAA) and n-butyl acrylate (BA) were purified by distillation under reduced pressure prior to use, and the initiator of 4,4-azobis(4-cyanopentanoic acid) (ACPA, >98%, Adamas-Beta) was directly used as received. The RAFT agent of 4cyanopentanoic acid dithiobenzoate (CADB) was obtained by the reaction of ACPA with di(thiobenzoyl) disulfide synthesized in a first step, according to the literature.25 The 1H NMR results of the assynthesized CADB in this work is shown in Supporting Information Figure S1. Methanol and tetrahydrofuran (THF) were all analysis grade and purchased from Beijing Chemical Works. II-2. One-Pot, Two-Step Synthesis of PMAA-b-PBA Amphiphilic Diblock Copolymers in Methanol. In the first step, the PMAA homopolymer with the dithiobenzoate end-group was synthesized via RAFT polymerization in methanol by using CADB as RAFT agent. And then, the solution of as-synthesized PMAA homopolymer was directly employed as macro-RAFT agent for the second step RAFT polymerization of BA monomer. The experimental protocol is sketched in Scheme 1, and the experiment conditions for
Scheme 1. Schematic Representation of the Synthesis of Poly(methacrylic acid)-b-poly(butyl acrylate) (PMAA-bPBA) Amphiphilic Diblock Copolymers via RAFT Dispersion Polymerization by Using 4-Cyanopentanoic Acid Dithiobenzoate (CADB) as RAFT Agent in Methanol
the synthesis of PMAA and PBA blocks are summarized in Table 1. For the sake of convenience, the PMAA homopolymers and the PMAA-b-PBA block copolymers with different block lengths of PMAA and PBA are designed hereafter as MX and MX-BY, where M stands for PMAA block, B stands for PBA block, X stands for the theoretical DP of PMAA block, and Y stands for the theoretical DP of PBA block. 166
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
b-PBA copolymers was injected at a concentration of 3 mg mL−1 after filtration trough a 0.45 μm of microporous membrane. Columns and detectors were maintained at 30 °C. The molecular weight (MW) of copolymer was calculated with a calibration curve based on narrow polystyrene standards. The PMAA-b-PBA copolymer dispersion in methanol was dropped on the copper grids and then treated by phosphotungstic acid solution (2 wt %, aq) for TEM characterization. The TEM image of the copolymer was recorded at an acceleration voltage of 200 kV (Hitachi-800 Japan). The surface morphology and roughness of the as-prepared film were investigated by using SEM and AFM techniques. The SEM measurement was performed at an acceleration voltage of 30 kV (TESCAN VEGA3 SBH, Czechic). The AFM image was recorded by using a Multimode Nanoscope IIIa, Digital Instruments, operated in tapping mode under ambient conditions. The transmission FTIR spectrum was measured at rt for the PMAA-b-PBA amphiphilic copolymers by using a Bruker Tensor 37 Fourier transform IR spectrometer via KBr pellet method. A total of 64 scans were accumulated for signal-averaging of each IR spectral measurement to ensure a high signal-to-noise ratio with a 2 cm−1 resolution.
Table 1. Experimental Conditions and Results of the RAFT Polymerization of PMAA and PMAA-b-PBA in Methanol entrya
[M]0/[CADB]0
conv (%)
Mn (g mol−1)
Mw (g mol−1)
PDI (Mw/Mn)
M50 M100 M50-B50 M50-B100 M50-B150 M100-B100 M100-B300
50 100 50 100 150 100 300
99 99 59 41 40 36 39
4 317 9 670 8 105 9 381 11 971 13 539 19 926
4 878 10 562 9 969 12 101 15 442 16 788 26 700
1.13 1.09 1.23 1.29 1.29 1.24 1.34
a
M stands for PMAA block, B stands for the PBA block, and the numbers after M and B correspond to the degree of polymerization of PMAA and PBA block, respectively.
First Step: Synthesis of PMAA Macro-RAFT Agent. A typical example (Table 1, entry M100) for the synthesis procedure is as follows: MAA (4.30 g, 50.0 mmol), CADB (140.0 mg, 0.5 mmol), ACPA (35.0 mg, 0.125 mmol), and methanol (25.37 g) were added into a 100 mL round-bottom septum-sealed flask equipped with a magnetic bar, where the RAFT agent over initiator molar ratio was kept at 4/1 and the solid content was controlled at 15 wt %. The mixed solution in a sealed reaction vessel was degassed for 30 min with nitrogen and subsequently placed in a preheated thermostated water bath at 60 °C to carry out the raft polymerization. The reaction was stopped after 24 h by immersion of the flask in an iced bath. Finally, the methanol solution of PMAA macro-RAFT agent was obtained, which was directly employed for the further RAFT polymerization of BA monomer without any further purification. The conversion of MAA monomer was determined by gravimetric analysis. After dying and subsequent methylation of the COOH groups,26 the polymer was analyzed by gel permeation chromatography (GPC) in THF. Second Step: RAFT Dispersion Polymerization of BA in Methanol with PMAA Macro-RAFT Agent. The present description corresponds to the typical example of M100-B100 as listed in Table 1. BA (6.40 g, 50 mmol), ACPA (35.0 mg, 0.125 mmol), and methanol (36.47 g) were directly added into a 100 mL round-bottom septum-sealed flask equipped with a magnetic bar and purged for 30 min with nitrogen in an ice bath. Then the as-prepared solution was injected into the reaction flask with PMAA macro-RAFT agent obtained in the first step (Table 1, entry M100) under the continuous degassing by nitrogen, in which the macro-RAFT agent over initiator molar ratio was kept at 4/ 1 and the total solid content was controlled at 15 wt %. After the degassing by nitrogen for another 30 min in an iced bath, the RAFT polymerization was carried out at 60 °C in a thermostated water bath for 24 h. Finally, the reaction flask was quenched by immersion in iced water. The conversion of BA monomer was determined by gravimetric analysis. After drying and subsequent methylation, the polymer was analyzed by GPC in THF. II-3. Film Preparation of PMAA-b-PBA Amphiphilic Diblock Copolymers. Two drops of the synthesized PMAA-b-PBA amphiphilic diblock copolymer dispersion in methanol were casted on the center of glass substrate in a closed environment. The glass substrate was preheated at the different temperatures, such as room temperature (rt) and 50 °C. And then, the films were kept in a vacuum oven at rt for another 24 h in order to evaporate the methanol completely. The sample thus prepared is designated hereafter as “asprepared” sample and held in the vacuum oven until measurement. II-4. Characterization Techniques. The DP of PMAA synthesized in first step of RAFT polymerization was determined by 1 H NMR spectroscopy (Bruker AV600) in d6-DMSO solution at rt in 5 mm diameter tubes. The chemical shift peak (δ, in ppm) of the carboxylic and benzene ring protons of PMAA-CAT was used to determine the DP by calculation of their integral areas. GPC measurement was performed by using a GPC515-2410 from Waters Corporation (Milford, MA) that consists of a HPLC pump and refractive index detector. THF was employed as the mobile phase at a flow rate of 1 mL min−1. The THF solution of the methylated PMAA-
III. RESULTS AND DISCUSSION III-1. Control on MW and Molecular Weight Distribution (MWD). As described in the Experimental Section, a series of PMAA-b-PBA amphiphilic block copolymer were synthesized by using the PMAA homopolymer with the dithiobenzoate end-group as macro-RAFT agent at 15 wt % of solid concentration. In order to obtain the block copolymer having a constant block length of PMAA, the MAA monomer was polymerized first to yield the solvophilic macro-RAFT agent with the DP at 100 and 50 (M100 and M50), respectively. The as-synthesized PMAA macro-RAFT agents of M100 and M50 were analyzed by 1H NMR as shown in Supporting Information Figure S2, and the DP of them was calculated at 102 and 50, respectively. Besides, almost a full monomer conversion (>99%) was achieved as judged by gravimetric analysis. The slight higher experimental DP then the theoretical one might be due to the partial inactivation of the RAFT agent during the polymerization process. In this work, the PMAA block was synthesized in the first step via RAFT solution polymerization in methanol, which behaved as both the solvophilic and hydrogen bonding providing segment for PMAA-b-PBA amphiphilic block copolymer. It has been reported by Pelet et al.27 that methanol is the suitable solvent for the polymerization of MAA monomer via RAFT technique, which was suggested by the comparison between the results of experiments carried out in different solvents. In our work, we performed the RAFT solution polymerization of MAA monomer in methanol to synthesize PMAA macro-RAFT agent with the relatively narrow PDI (∼1.13). The target block copolymers of PMAA-b-PBA were subsequently prepared by using the as-synthesized PMAA macro-RAFT agent in methanol. The GPC technique was employed to trace the RAFT polymerization process after the quantitative addition of the BA monomer and the initiator into the PMAA macro-RAFT agent solution obtained in the first step. The well-controlled polymerization of BA monomer and the molecular chain extension of the PMAA homopolymer were clearly proved by the typical evolution of number-average molecular weight (Mn) (M50-B100) as a function of monomer conversion (red square plot) in Figure 1a, in which the evolution of theoretical Mn (black straight line) is also displayed. A good agreement between the experimental Mn and the theoretical one as a function on monomer conversion can be obtained. And the GPC chromatograms in Figure 1b 167
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
PMAA-b-PBA block copolymer in the methanol dispersion was viewed by TEM. In all experiments, the spherical particles with core/shell structure can be observed. It is a quite reasonable morphology for the PMAA-b-PBA amphiphilic block copolymers in methanol via PISA process. Based on a kinetic stability mechanism, the solvophobic PBA block would in situ associate to form the particle “core”, and the well-dissolved PMAA block would form the particle “shell”.17 As observed by Chenal et al.,24 the particle diameter of PAA-b-PAA copolymer was bellow 100 nm with broad particle size distribution, which was synthesized via RAFT emulsion polymerization in water. And the smaller particle diameter of PAA-b-PBA copolymer in ethanol (bellow 50 nm) was reported by Jacquin et al.,23 which was due to the different block length of PAA and PBA. In this work, we obtained the PMAA-b-PBA copolymer particles with larger diameter, which was contributing for the formation of special film surface pattern at submicrometer scale as discussed in section III-3. The typical TEM image of M100-B100 is displayed in Figure 2a, and that of single copolymer particle
Figure 1. Typical synthesis of PMAA-b-PBA amphiphilic diblock copolymers of M50-B100 via RAFT dispersion polymerization of BA by using PMAA-CTA as macro-RAFT agent: (a) evolution of the number-average molecular weight (Mn) and polydispersity indexes (PDI, Mw/Mn) as a function of monomer conversion in the second step of RAFT polymerization; (b) evolution of the GPC chromatograms.
clearly show the increasing MW of the copolymer with reaction time and monomer conversion. Besides, the PDIs of them were all below 1.3. Thus, it is illustrated that the BA monomer was successfully reinitiated and propagated on the PMAA macroRAFT agent, based on RAFT polymerization mechanism. Eventually, different from what Jacquin et al.23 observed when preparing PAA-b-PBA block copolymer by MADIX in ethanol, the block length of lyophobic PBA was remarkably increased (with respect to PMAA block). This would result in the morphological variation of our synthesized copolymer in solution and on casted film, which will be further discussed in sections III-2 and III-3. It has to be noted that the PBA block cannot be dissolved in methanol, even though BA monomer can be dissolved in the condition. However, the synthesized PMAA-b-PBA block copolymers with different block lengths of PBA can all be achieved with the conversion of PBA monomer in the range from 36 to 59%. The propagating of solvophobic PBA block under RAFT mechanism should behave as a dispersion polymerization, which was stabilized by the solvophilic PMAA blocks.28 It was expected to cause that the reaction system transformed from transparent state to the translucent state after the initiation of the RAFT polymerization of BA monomer. In this work, the stable dispersions of PMAA-b-PBA block copolymers were obtained in all the reactions with the various block lengths of PMAA and PBA, which were directly employed for the further investigation on their self-assembly during film formation process and the film surface structure of them. III-2. Self-Assembly of PMAA-b-PBA Amphiphilic Diblock Copolymer in Methanol. The morphology of
Figure 2. TEM images of the final PMAA-b-PBA core/shell particles obtained after one-pot, two-step RAFT dispersion polymerization in methanol of M100-B100 (a) and the enlarged single copolymer particle (b). The SEM image of the final films of PMAA-b-PBA (M50-B50) copolymers casted by their diluted dispersions in methanol (0.5 wt %) (c).
with clear core/shell structure is displayed in Figure 2b. It is obviously shown in Figure 2a,b that the diameter of the particles was located in the range from 100 to 300 nm and behaved as a broad particle size distribution, which should be attributed to the coalescent of these particles. The kinetically stable smaller particles would naturally coalesce to the larger but more stable ones during the RAFT dispersion polymerization of BA monomer. Besides, the sign of the higher order structure can also be distinguished in Figure 2a, where most of the particles were arranged and linked in a way to form the topological network, which is sketched as red dotted line. The more positive evidence for the formation of network in PMAA-b-PBA copolymer film was observed, when the 168
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
by the variations of the film casting temperature and the block length of PBA. It is noteworthy that the increase of film casting temperature from rt to 50 °C resulted in the denser intertexture-like morphology for M100-B100 and M100-B300. For the as-prepared films of M100-B100 and M100-B300 as casted at rt in Figures 3a and 3c, only the peanut and random worm-like morphologies can be observed, where the fibrous width was around 800 nm and the length was about 2 μm. For those as casted at 50 °C in Figure 3b and 3d, however, the fibrous structure was twisted and weaved together, so as to form the complete surface. The typical periodical wrinkly surface pattern was obtained with the fibrous width around 500 nm and the distance between the crossing points about 1.5 μm. It is reasonable that the surface structure formation of the asprepared PMAA-b-PBA copolymer film is attributed to the balance between the isotropic coalescence and the anisotropic self-assembly of the core/shell particles. With the evaporation of methanol, the copolymer particles in methanol were gradually concentrated. And hence, the distance between these particles became closer and closer, but the dispersion was still stable by the lyophilic PMAA block due to the steric effect. With the further increase of concentration in the dispersion, the aggregation of the core/shell particles occurred, and then the shell layer of PMAA block from different particles would collapse and coalesce with each other. Based on the steric stabilization effect, the approach between well-separated particles during film formation process would lead to the entropy reduction due to the increasing overlap volume between the molecular chains on particle shell layer.30,31 The random aggregation or the linear aggregation of copolymer particles would result in the different overlap volume of the PMAA shell layer, and the linear aggregation should bring out the smallest overlap volume. Thus, in order to depress the contribution of entropic reduction due to the collapse and coalescence of PMAA shell, the core/shell particles tended to anisotropically assemble and formed the fibrous structure. Moreover, Choueiri et al.9 have proved that for the nanoparticle dispersion the poor solvency condition in conjunction with strong electrostatic repulsion can yield the anisotropic selfassembly of these nanoparticles and result in the formation of nanoparticle chains. In our works, with the evaporation of methanol and the increase of copolymer concentration during the film formation process, the solvency condition became poorer and poorer. Moreover, the dissolved and slightly ionized PMAA block provided the specific repulsive force between the particles. Therefore, the anisotropic self-assembly of PMAA-bPBA core/shell particles can also be supported, and the fibrous structure was in turn obtained as shown in Figure 3. Meanwhile, it cannot be ignored that the film casting temperature (rt and 50 °C) was always higher than the glass transition temperature of PBA block. Thus, the PBA block was still able to move and adjust its molecular conformation, so as to bury some PMAA segments inside and generate the large homogeneous coalesced object, if the sufficient time was provided. On the other hand, however, with the rapid evaporation of methanol, the mobility of PBA block was also restricted by the PMAA block (Tg of PMAA is above 100 °C) and also frozen by the hydrogen-bonding network due to inter between the MAA units on PMAA block, which was useful for the copolymer film to keep the fibrous structure due to the anisotropic self-assembly of PMAA-b-PBA core/shell particles. The competition between these two contrary effects can be controlled by the film casting temperature, which determined
copolymer film was prepared by casting their diluted dispersion in methanol with the solid content at 0.5 wt %. And the typical SEM image of M50-B50 film as prepared by this method is displayed in Figure 2c. It can be distinctly seen that the block copolymer formed the end-capped fibrous structure with the diameter around 300−400 nm and the aspect ratio around 5. Actually, the network structure in Figure 2c is the intertexture of the microfibers. The texture structure convinced our molecular design in the Introduction: the PMAA block functions in stabilizing the particle dispersion and associating hierarchical assembly for the anisotropic structure in diluted condition. It has been reported that copolymers with MAA units are strongly self-associated in the condensed state at ambient temperatures through the formation of intermolecular hydrogen bondings within the carboxylic acid dimers of MAA unit (designated hereafter as inter).29 And the self-assembly of PMAA-b-PBA core/shell particles into the network structure (Figure 2c) is also due to the formation of inter. The variation of inter as a function of molecular structure will be further discussed in section III-4. III-3. Film Surface Morphology of PMAA-b-PBA Amphiphilic Diblock Copolymer. Instead of the irregular porous intertexture structure as obtained in the drying film of the diluted copolymer dispersions in methanol, the cast of the synthesized copolymer dispersions would result in the formation of dense copolymer coating on the substrate. The typical SEM images of M100-B100 and M100-B300 films prepared by casting their dispersions at rt are summarized in Figures 3a
Figure 3. SEM images of the final films of PMAA-b-PBA copolymers as prepared by casting their dispersions on glass substrate: (a) M100B100 at rt; (b) M100-B100 at 50 °C; (c) M100-B300 at rt; and (d) M100B300 at 50 °C.
and 3c, respectively, and those of M100-B100 and M100-B300 films prepared by casting their dispersions at 50 °C are shown in Figures 3b and 3d, respectively. In these copolymers, the DP of PMAA block was controlled at 100. The wrinkle surface pattern of as-prepared films can all be found in these SEM images with the obvious difference in appearance, which should be caused 169
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
the final wrinkly surface structure and morphological size. For the as-prepared film with the casting temperature at rt, the width and the distance between crossing points of the wrinkly surface morphology were obviously larger than those of film with casting temperature at 50 °C, as shown in Figure 3. In this work, we also investigated the influence of block length of PMAA-b-PBA copolymers on the surface morphology of as-prepared film. Thus, we synthesized the PMAA-b-PBA block copolymers with the various block lengths, such as M50B50, M50-B100, and M50-B150, in which the DP of PMAA block was controlled at 50 and the block length ratio of PBA over PMAA was controlled from 1 to 3. The typical SEM images of the M50-B50, M50-B100, and M50-B150 as-prepared films with casting temperature at rt are summarized in Figures 4a, 4c, and
however, the wrinkly surface morphology became hazy with the fibrous width around 1.5 μm, which was remarkably larger than that in case (i), and the distance between crossing points was around 5 μm. Compared with the film surface structure of copolymers with various block lengths as shown in Figures 3 and 4, the difference in appearance can also be found. Let us first discuss one group of M100-B100 (Figure 3a and 3b) and M50-B50 (Figure 4a and 4b), where the PBA over the PMAA block length ratio equaled to 1. When the casting temperature was at rt, M100-B100 copolymer film only behaved as the peanut morphology with fibrous length around 2 μm, but M50-B50 copolymer film showed the longer fibrous structure with the distance between crossing points around 5 μm. It can be imaged that the restriction effect on the mobility of PBA block by the PMAA block with high Tg and the hydrogen-bonding network between MAA units can be improved, if the molecular chain length of PMAA-b-PBA copolymers is shortened. Therefore, the PBA block in M50-B50 film is more effectively restricted than that in M100-B100 film, so that the more integral wrinkly pattern can be observed on M50-B50 film surface. Moreover, the same results were also obtained in the other group of M100-B300 (Figure 3c and 3d) and M50-B150 (Figure 4e and 4f). Thus, it is further proved that the mobility of PBA block is an important factor to affect the wrinkly structural size of the final film surface pattern. Besides, by adjusting the DP of PMAA and PBA blocks in the PMAA-b-PBA block copolymers, the film surface morphology can be controlled. In this work the surface morphologies of the as-prepared copolymer films were also investigated by AFM. The height image and its corresponding cross section of the M50-B50 copolymer film with casting temperature at 50 °C are shown in Figures 5a and 5b, respectively. The topographic image (Figure 5a) indicates the formation of wrinkly structure on the film surface with the fibrous width around 500 nm and distance between crossing points about 3 μm, which are consistent with the SEM results as shown in Figure 4b. Besides, the amplitude of fluctuant film surface was around 270 nm as suggested by the red points in the cross-sectional image (Figure 5b), which almost equaled to half of the fibrous width (500/2 nm). In this work, the casting temperature induced surface morphological variation of the as-prepared films was also investigated, and one composition of the synthesized PMAA-b-PBA copolymer (M50B100) was selected for this comparison. The AFM height image and the corresponding cross section of the M50-B100 film with casting temperature at 50 °C are shown in Figures 5c and 5d, respectively, and those of M50-B100 film with casting temperature at rt are displayed in Figures 5e and 5f, respectively. It can be found in Figure 5c that the periodic wrinkle structure with the fibrous width around 500 nm was clearly observed on the as-prepared film surface, when the film casting temperature was 50 °C. However, when the film casting temperature was rt as shown in Figure 5e, only the hazy wrinkle structure with the width around 1.5 μm could be observed, which was wider than that as shown in Figure 5c (film casting temperature was 50 °C). From the detailed AFM analysis of the film surface patterns in Figure 5c and 5e, it is concluded that the change of film casting temperature can induce the variation of wrinkly surface structure on the PMAA-b-PBA as-prepared film, which is consistent with the results of SEM as discussed in conjunction with Figures 3 and 4. Besides, when the M50-B100 film was casted at 50 °C, the cross-sectional image in Figure 5d shows that the amplitude of fluctuant film surface was around 250 nm, which was half of the fibrous width (500/2 nm) (same
Figure 4. SEM images of the final films of PMAA-b-PBA copolymers as prepared by casting their dispersions on glass substrate: (a) M50-B50 at rt; (b) M50-B50 at 50 °C; (c) M50-B100 at rt; (d) M50-B100 at 50 °C; (e) M50-B150 at rt; and (f) M50-B150 at 50 °C.
4e, respectively. Besides, those of the as-prepared films with casting temperature at 50 °C are shown in Figures 4b, 4d, and 4f, respectively. The differences in appearance between these SEM images of these two groups (casting temperature at rt and 50 °C) can also be clearly observed, which are similar as those of M100-B100 and M100-B300 as shown in Figure 3: (i) when the casting temperature was at 50 °C, the film surface pattern with periodic wrinkles was clearly obtained with the fibrous width around 500 nm and the distance between the crossing points around 3 μm; (ii) when the casting temperature was at rt, 170
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
Figure 5. AFM images of the final films of PMAA-b-PBA copolymers (M50-B50) as prepared by casting their dispersions on glass substrate at 50 °C: (a) height image, (b) corresponding cross section in (a), and those of the final films of PMAA-b-PBA copolymers (M50-B100) with the casting temperature at 50 °C: (c) height image, (d) corresponding cross section in (c) and at rt, (e) height image, and (f) corresponding cross section in (e).
and formation of inter on PMAA block, which is consistent with the conclusion from SEM results. In this work, we also studied the film structure at edge region of the as-prepared films with film casting temperature at 50 °C, and the typical AFM and SEM images of M100-B100 film are displayed in Figure 6. It is quite remarkable in Figures 6a and 6c that the wrinkles were regularly arranged with the width and the length of orientated fibrous structure around 500 nm and 5 μm, respectively. During the film formation process, the fast evaporation of methanol solvent first occurred at edge region of the droplet of copolymer dispersion, and the droplet gradually shrunk with the evaporation of methanol, which resulted in the internal stress at the edge region of the as-prepared films. So the PMAA-b-PBA copolymer particles tended to anisotropic aggregation along the shrinking direction of the dispersion droplet due to the internal stress, and the formation of inter network preserved the self-assembly structure, which is
as the results of M50-B50 in Figure 5a). It is suggested that the aggregation of the fibrous structure behaved as the rigid packing process without the discriminable diffusion of PBA block to bury the PMAA segment and form the homogeneous phase, which was due to that the mobility of PBA block was effectively restricted by PMAA block and the formation of inter between MAA units. Therefore, the amplitude of fluctuant film surface was around half of the fibrous width. However, when the M50B100 film was casted at rt, the cross-sectional image in Figure 5f shows that the amplitude of fluctuant film surface was around 210 nm, which was remarkably smaller than half of the fibrous width (1.5/2 μm). It should be attributed to the diffusion of PBA block so as to bury some PMAA segments inside and form the homogeneous phase during the film formation process. These results further prove that the variation of wrinkly film surface structure as a function of film casting temperature can be controlled by the molecular chain diffusion of PBA block 171
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
Figure 6. AFM height image (a), corresponding cross section (b), and the SEM image (c) of the edge region of PMAA-b-PBA copolymer (M100B100) film as prepared by casting their dispersions on glass substrate at 50 °C.
expected to cause the formation of orientated wrinkly surface structure at the edge region of the as-prepared film as shown in Figure 6. Especially for the films as-prepared under faster evaporation of methanol with film casting temperature at 50 °C, it was more notable to assemble into the orientated structure. Meanwhile, the cross-sectional image in Figure 6b shows that the amplitude of fluctuant film surface with orientated wrinkly structure was around 240 nm, which also equaled to about half of the fibrous width. III-4. FITR Spectra in the CO Stretching Band Region of PMAA-b-PBA Copolymers. Based on the discussion above, it is illustrated that the hydrogen-bonding interaction between the MAA units on PMAA block is an important factor to affect the film surface morphology. In this section, we aimed to further confirm the effect of inter on surface morphology, and FTIR spectroscopy was employed to characterize the amount of inter CO groups of PMAA block. Figure 7 shows the normalized FTIR spectra of the PMAA-bPBA copolymer films in the CO stretching vibration region, in which two strong bands centered at 1733 and 1703 cm−1 can be obviously found due to the free CO groups (designed hereafter as f ree) and inter CO groups of MAA units, respectively.15 The peak intensities of these two bands and the fraction of inter CO groups of MAA units were calculated by the ratio of peak intensity as listed in Table 2. Let us first compare the FTIR spectra of M50-B50, M50-B100, and M50-B150, where the DP of PMAA block was constant at 50. With the increasing block length of PBA, the fraction of inter CO groups obviously decreased and that of f ree CO groups increased, suggesting that the increasing block length of PBA could depress the formation of inter on PMAA block. Thus, for the PMAA-b-PBA copolymers with constant block length of PMAA, the mobility of PBA block with short block length can be more effectively restricted by PMAA block. Moreover, for
Figure 7. FTIR spectra in the CO stretching band region of M50B50, M50-B100, M50-B150, M100-B100, and M100-B300 with two peaks around 1733 and 1703 cm−1 due to free and intermolecular hydrogenbonded carbonyl groups of MAA units, respectively.
the spectrum of M50-B100, the free and inter CO stretching bonds shows the almost equivalent peak height, indicating that both the fractions of f ree and inter were around 0.5. It means that about one-half of the CO groups of MAA units formed inter and the other half remained at f ree state for M50-B100 at rt. Now, let us next compare the FTIR spectra of M100-B100 and M100-B300, where the DP of PMAA block was controlled at 100. With the increasing block length of PBA, the fraction of inter notably decreased, which is same as the results of M50-B50, M50B100, and M50-B150. However, compared with M50-M50, the fraction of inter in M100-B100 was remarkably lower, even though 172
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
(M100-B100 and M100-B300), even though the films were casted at rt.
Table 2. List of CO Stretching Vibration Bands Intensity Due to Free and Intermolecular Hydrogen-Bonded Carbonyl Groups of MAA Units, Respectively, and the Fraction of the Hydrogen-Bonded CO Groups of MAA Units M50-B50 M50-B100 M50-B150 M100-B100 M100-B300
I1733
I1703
fraction of intera (%)
0.66 0.98 1 1 1
1 1 0.95 0.71 0.52
60.2 50.5 48.7 41.5 34.2
IV. CONCLUSIONS In this work, we synthesized the PMAA-b-PBA amphiphilic diblock copolymers via RAFT polymerization in methanol and obtained the copolymer films with wrinkly surface pattern by casting the copolymer dispersions. The structural development of the self-assembled PMAA-b-PBA copolymer film was investigated. The film formation process is partially presented by schematic illustration shown in Figure 8. The polymerization-induced self-assembly of PMAA-b-PBA copolymer with core/shell spheral structure was first built up in methanol dispersion, where the core matrix and shell layer were consisted of PBA and PMAA, respectively, as shown in Figure 8a. When the copolymer dispersion was casted on glass substrate, the copolymer particles anisotropically aggregated first with the increasing concentration and then assembled into the fibrous structure with the collapse of PMAA shell matrix. Meanwhile, the intermolecular hydrogen-bonding interactions between MAA units on PMAA block were formed, so as to build up the network between PMAA blocks, which was able to restrict the mobility of PBA block with low Tg. So the self-assembled fibrous structure was preserved and the wrinkly surface pattern was obtained as shown in Figure 8b and 8c. It was found that the surface structure of the copolymer film can be adjusted by
a
Fraction of inter = I1703/(I1703 + I1733), where I1703 and I1733 are the peak intensities of IR band due to inter and f ree, respectively.
the ratio between the block lengths of PMAA and PBA was almost same. And the same results were also obtained from the comparison between M50-B150 and M100-B300. Therefore, when the ratio between the block length of PMAA and PBA is constant, the mobility of PBA block can be more effectively restricted by PMAA block, if the molecular chain length of PMAA-b-PBA copolymers is short. It is expected to cause the morphological results as shown in Figures 3 and 4 that the copolymers with short molecular chain length (M50-B50, M50B100, and M50-B150) showed the more perfect wrinkly surface morphology than those with long molecular chain length
Figure 8. Sketch of self-assembly of PMAA-b-PBA copolymers during film formation process. The copolymers in regions A1 and A2 in (b) schematically represent the continuous phase due to diffusion of PBA blocks and the hints of fibrillar morphology due to self-assembly of MAA units with intermolecular hydrogen-bonding dimers. 173
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
Macromolecules
Article
(14) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559−5562. (15) Moad, G.; Rizzardo, E.; Thang, S. H. Poymer 2008, 49, 1079− 1131. (16) Keddie, D. J.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 2012, 45, 5321−5342. (17) Charleux, B.; Guillaume, D.; Rieger, J.; D’Agosto, F. Macromolecules 2012, 45, 6753−6765. (18) Soleimani, M.; Haley, J. C.; Lau, W.; Winnik, M. A. Macromolecules 2010, 43, 975−985. (19) Jiang, B.; Tsavalas, J.; Sundberg, D. Langmuir 2010, 26, 9408− 9415. (20) Nehlig, E.; Schneider, R.; Vidal, L.; Clavie, G.; Balan, L. Langmuir 2012, 28, 17795−17802. (21) Samyn, P.; Laborie, M. P.; Mathew, A. P. Langmuir 2012, 28, 1427−1438. (22) Basu, S. K.; Scriven, L. E.; Francis, L. F.; MaCormick, A. V. Prog. Org. Coat. 2005, 53, 1−16. (23) Jacquin, M.; Muller, P.; Talingting-Pabalan, R.; Cottet, H.; Berret, J. F.; Futterer, T.; The odoly, O. J. Colloid Interface Sci. 2007, 316, 897−911. (24) Chenal, M.; Bouteiller, L.; Rieger, J. Polym. Chem. 2013, 4, 752− 762. (25) Chong, Y. K.; Krstina, J.; Le, T. P. T.; Moad, G.; Postma, A.; Rizzardo, E.; Thang, S. H. Macromolecules 2003, 36, 2256−2272. (26) Couvreur, L.; Lefay, C.; Belleney, J.; Charleux, B.; Guerret, O.; Magnet, S. Macromolecules 2003, 36, 8260−8267. (27) Pelet, J. M.; Putnam, D. Macromolecules 2009, 42, 1494−1499. (28) Wan, W. M.; Pan, C. Y. Polym. Chem. 2010, 1, 1475−1484. (29) Lee, J. Y.; Painter, P.; Coleman, M. M. Macromolecules 1988, 21, 346−354. (30) Mackor, E. L. J. Colloid Interface Sci. 1951, 6, 492−495. (31) Chern, C. S. Principles and Application of Emulsion Polymerization; John Wiley & Sons, Inc.: Hoboken, NJ, 2008.
controlling the formation of inter on PMAA blocks and the mobility of PBA blocks during film formation process. In one case (Figure 8, process 2) that the copolymer dispersion was casted at 50 °C with the fast evaporation rate of methanol, the copolymer particles instantaneously contacted with each other and formed inter within PMAA block, which resulted in the effective restriction on the mobility of PBA block. The wrinkly structure with the fibrous width around 500 nm was obtained. In the other case (Figure 8, process 1) that the copolymer dispersion was casted at rt with slow evaporation rate of methanol, the formation of inter within PMAA block was depressed as sketched in region A2 of process 1. Meanwhile, there was enough time for the PBA blocks to diffuse from core matrix and form the continuous phase to bury some PMAA segments inside as show in region A1 of process 1, which resulted in the obviously larger fibrous width of film wrinkly structure than those of films casted at 50 °C.
■
ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra of the RAFT agent. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected] (X.L.). *E-mail
[email protected] (T.Q.). Author Contributions
L.G. and Y.J. contributed equally to this work. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grant No. 21304007), the Scientific and Technological Project of Beijing (Grand No. Z131103002813071), and Fundamental Research Funds for the Central University at Beijing University of Chemical Technology.
■
REFERENCES
(1) Hilmyer, M. A.; Baes, F. S.; Almdal, K.; Mortensen, K.; Ryan, A. J.; Fairclough, J. P. Science 1996, 271, 976−978. (2) Forster, S.; Abetz, V.; Muller, A. H. E. Adv. Polym. Sci. 2004, 166, 173−210. (3) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200−1205. (4) Zhulina, E. B.; Borisov, O. V. Macromolecules 2005, 38, 6726− 6741. (5) Sugihara, S.; Blanazs, A.; Armes, S. P.; Ryan, A. J.; Lewis, A. L. J. Am. Chem. Soc. 2011, 133, 15707−15713. (6) Blanazs, A.; Madsen, J.; Battaglia, G.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2011, 133, 16581−16587. (7) Chambon, P.; Blanazs, A.; Battaglia, G.; Armes, S. P. Langmuir 2012, 28, 1196−1205. (8) Jones, E. R.; Semsarilar, M.; Blanazs, A.; Armes, S. P. Macromolecules 2012, 45, 5091−5098. (9) Choueiri, R. M.; Klinkova, A.; Therien-Aubin, H.; Rubinstein, M.; Kumacheva, E. J. Am. Chem. Soc. 2013, 135, 10262−10265. (10) Genzer, J.; Groenewold, J. Soft Matter 2006, 2, 310−332. (11) Li, B.; Cao, Y. P.; Feng, X. Q.; Gao, H. Soft Matter 2012, 8, 5728−5745. (12) Wang, J.; Xia, H.; Zhang, Y.; Lu, H.; Kamat, R.; Dobrynin, A. V.; Cheng, J.; Lin, Y. J. Am. Chem. Soc. 2013, 135, 11417−11420. (13) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728−1731. 174
dx.doi.org/10.1021/ma402167d | Macromolecules 2014, 47, 165−174
