- Email: [email protected]
alumina nanoparticle composites
European Polymer Journal 47 (2011) 1240–1249
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Surfactant addition effects on dispersion and microdomain orientation in SBS triblock copolymer/alumina nanoparticle composites C. Ocando, A. Tercjak, I. Mondragon ⇑ ‘Materials + Technologies’ Group, Escuela Politécnica, Dpto Ingeniería Química y M Ambiente, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Pza, Europa 1, 20018 Donostia-San Sebastian, Spain
MACROMOLECULAR NANOTECHNOLOGY
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 28 February 2011 Accepted 6 March 2011 Available online 12 March 2011 Keywords: Alumina nanoparticles Surfactant Dispersion Triblock copolymer Self-assembling
a b s t r a c t The aim of this study was to investigate the effects of surfactant addition on the dispersion of 1–3 wt.% alumina nanoparticles on the self-assembled morphology of poly(styrene-bbutadiene-b-styrene) (SBS) linear triblock copolymer. The neat triblock copolymer microphase separated into PS cylinders self-assembled on a hexagonal array in the PB matrix, being the orientation of domains dependent on the annealing conditions. UV–vis and AFM analyses showed an improvement on dispersion of Al2O3 nanoparticles into SBS matrix by adding dodecanethiol as organic surfactant, due to its miscibility with PS block. Interactions between surfactant-coated Al2O3 nanoparticles with PS block were demonstrated by the variation on the glass transition temperature of this block. AFM analysis showed that the incorporation of surfactant-coated Al2O3 nanoparticles in the SBS matrix have great influence on the orientation of microphase separated domains in SBS nanocomposites. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Block copolymers are materials with a great potential for applications in nanotechnology, due to their ability for self-assembling into a long range of ordered morphologies at nanoscale. The segregation of blocks in the copolymer, resulting from thermodynamic incompatibility of their two polymer chains, and to the fact that they are covalently connected, leads to a great variety of microphase separated morphologies, with dimensions of segregated nanodomains ranging from 10 to 100 nm. Depending on the volume ratio between blocks, these microphase separated domains can be spheres, hexagonally packed cylinders, gyroidal and lamellae for diblock copolymers [1–11]. In addition to composition, architecture of chains and molecular weight of block copolymers, the microphase separated morphology and spatial orientation in the polymer–air interface are also dependent on the ⇑ Corresponding author. Tel.: +34 943017271; fax: +34 943017200. E-mail address: [email protected] (I. Mondragon). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.03.008
annealing conditions, among other possible variables [12– 16]. On the other hand, the microphase separated morphology can also develop upon temperature and under shear [17–21]. These typical features of block copolymers allow their use as nanotemplates for spatial organization of inorganic nanoparticles in thin films or in bulk samples [22–40]. For the preparation of hybrid inorganic/organic materials, different chemical and physical methods have been proposed to modify the surface of inorganic nanoparticles in order to promote particle–particle repulsive forces because the nanoparticles have the tendency to form aggregates by van der Waals attractions. Small molecules as amphiphilic surfactants can facilitate the dispersion of nanoparticles in a polymer matrix [41–45]. In addition, the use of a selective surfactant can allow the segregation of inorganic nanoparticles into one of the microphase separated domains in block copolymers [41–45]. On the other hand, the presence of a good interfacial interaction between the nanoparticles and the polymeric matrix is a key factor to obtain nanocomposites with enhanced thermal properties [46–48].
In this work, the viscoelastic, thermal and morphological behavior of neat poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer (30 wt.% PS) and its alumina (1– 3 wt.%) containing nanocomposites have been investigated. In order to promote the compatibility between alumina nanoparticles and the polymeric matrix, an organic surfactant miscible with PS was used [44,45]. Therefore, the effect of modification of alumina nanoparticles with a surfactant excess on dispersion in SBS matrix nanocomposites is discussed. The variations on the orientation of microphase separated domains in SBS matrix are also analyzed. Morphological changes were followed by atomic force microscopy (AFM) and viscoelastic analysis. 2. Experimental section 2.1. Materials Poly(styrene-b-butadiene-b-styrene) (Dynasol C500) linear triblock copolymer, with 70 wt.% PB, was kindly supplied by Repsol-YPF. The weight-average molecular mass was 136000 g/mol, being the glass transition temperatures (Tg) of PS and PB blocks 71 and 80 °C, respectively. Alumina (Al2O3) nanoparticles with an average diameter size of 39 nm and specific surface area 44 m2/g were purchased from Nanophase Technologies Corp. (NTC). As can be seen in Fig. 1 from AFM analysis, as-received Al2O3 nanoparticles formed aggregates. The particles were dried in a vacuum oven at 125 °C for a period of 24 h before used. Dodecanethiol and toluene, from Sigma Aldrich, were used as surfactant and solvent, respectively. 2.2. SBS nanocomposites preparation Pellets of SBS triblock copolymer were dissolved in toluene at a concentration of 5 wt.%. Films of the resultant solutions were prepared by solvent-casting. A 20 lL pipet was used to cast equal-sized droplets of the BC solutions onto a glass support and solvent was evaporated for 24 h
1241
in air at room temperature before analysis. Then, the films were annealed at two temperatures above the Tg of PS block, 80 and 110 °C under vacuum. It should be mentioned that for AFM analyses several measurements along the films were realised for each sample, and we do not observe any significant changes of the morphologies even though there can be some thickness variation in the films. Therefore, the phase behavior of samples formed under different conditions is representative and reliable. SBS matrix nanocomposites were prepared in solution by the sonication technique. A microprocessor sonicator 750 W; Vibracell 75043 from Bioblock Scientific with an amplitude of 30% was used. Alumina nanoparticles were dispersed in toluene at a concentration of 0.05 wt.%. Alumina nanoparticles, surfactant and SBS were sonicated during 4 h. Nanocomposites containing 1–3 wt.% Al2O3 nanoparticles with respect to the block copolymer and an excess of surfactant (1:1, 2:1, 4:1 weight ratio of dodecanethiol with respect to Al2O3) were developed. The amount of dodecanethiol (mS) adsorbed on the nanoparticles (mNP) can be calculated with the following equation:
qS 4=3 p ðrNP þ 2wSÞ3 ðrNPÞ3 mS ¼ mNP qNP 4=3 p ðrNPÞ3 where qNP is the nanoparticle density (3.6 g/cm3), qS is the surfactant density (0.845 g/cm3), rNP is the nanoparticle ratio and wS is the covering thickness of the surfactant surrounding the nanoparticle surface (0.2 nm). This last value was obtained assuming a monolayer coverage of surfactant on the nanoparticle and taking into account that the area for a linear dodecanethiol surfactant has been reported to be approximately 20 Å2 (0.2 nm2) per molecule [49]. From the previous calculations a 1.5 wt.% of surfactant is adsorbed on the nanoparticles. Consequently, the 1 wt.% Al2O3/SBS nanocomposites with 1:1, 2:1 and 4:1 weight ratio of dodecanethiol with respect to Al2O3 contain
Fig. 1. (a) TM-AFM height image of Al2O3 nanoparticles and (b) height profile.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1242
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
around 1, 2 and 4 wt.% of surfactant with respect to the block copolymer. The 3 wt.% Al2O3/SBS nanocomposites with 1:1, 2:1 and 4:1 weight ratio of dodecanethiol with respect to Al2O3 contain around 3, 6 and 12 wt.% of surfactant with respect to the block copolymer. The samples obtained are designed throughout the manuscript in the following form: I wt.% Al2O3/II/SBS (I being the amount of Al2O3 (wt.%) and II the weight ratio of dodecanethiol with respect to Al2O3). Additionally, SBS-matrix samples containing 4 and 12 wt.% of dodecanethiol were also prepared in order to compare the surfactant effects in 1 wt.% Al2O3/4:1/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites, respectively. Films of nanocomposites were prepared in the same way than the neat block copolymer.
MACROMOLECULAR NANOTECHNOLOGY
2.3. Characterization 2.3.1. Differential scanning calorimetry (DSC) Thermal transition temperatures of neat SBS and their nanocomposites were determined by using a differential scanning calorimeter Mettler Toledo DSC-822, under a nitrogen flow of 20 mL/min, working with 5–7 mg samples in aluminum pans. Dynamic scans were performed from 60 to 160 °C at a heating rate of 20 °C/min. The values of Tg were determined at the inflection point of the change in heat capacity. 2.3.2. UV–vis spectroscopy UV–vis spectra of the coating films with the same thickness, in the range 200–800 nm, were recorded with a UV– vis spectrophotometer (JASCO V-570). 2.3.3. Viscoelastic measurements To study the viscoelastic behavior of neat SBS and its nanocomposites, dynamic oscillatory shear measurements were performed using a Rheometrics Ares rheometer equipped with 15 mm diameter parallel plates and two transducers with a couple operating range of 0.02– 2.000 g cm. A mini-injection machine, Haake, was employed using a disks shape mold to make samples of around 2 mm thickness by injection at 160 °C and 780 bar for few seconds to avoid possible PB-block degradation. Dynamic temperature ramp tests were conducted to record the evolution of storage, G0 , and loss, G00 , moduli under isochronal conditions at a heating rate of 3 °C/min and at angular frequency of 0.1 Hz. The strain amplitude was varied to insure a linear viscoelastic response. Additionally, frequency sweep tests were carried out at 110 °C over a frequency range of 0.006–100 rad/s for the neat block copolymer. 2.3.4. Morphological analysis Morphologies of the neat SBS and its nanocomposites were studied by atomic force microscopy. AFM images were obtained with a Nanoscope IIIa scanning probe microscope (Multimode™, Digital Instruments). Tapping mode (TM) was employed in air using an integrated tip/ cantilever (125 lm in length with ca. 300 kHz resonant frequency). Typical scan rates during recording were 0.7– 1 line/s using a scan head with a maximum range of
16 16 lm. The brighter domains in height images correspond to PS phase [13]. The height profiles of the films were measured along the cross-section line in the middle of each TM-AFM image. It is important to emphasize that TM-AFM images of 10 and 3 lm were used to evaluate the dispersion and morphology in the nanocomposites, respectively.
3. Results and discussion 3.1. Self-assembled morphology of neat SBS The morphological behavior of neat triblock copolymer was analyzed by microscopy and rheology techniques for comparison with the behavior of the hybrid nanocomposites. Representative TM-AFM images for the air-surface morphologies of SBS thin films prepared by solvent-casting and then submitted to different annealing treatments are shown in Fig. 2ai and bi. It can be noticed in Fig. 2ai that the film without annealing clearly showed nanostructuring by solvent-induced microphase separation between dissimilar blocks, where ordered PS cylinders of around 20 nm on a hexagonal self-assembly array were oriented perpendicularly to the polymer–air interface in a continuous PB layer. The perpendicular ordering occurred probably due to the rate of solvent evaporation [15,16]. In order to investigate the morphology evolution upon temperature, the solvent-cast thin films of SBS were annealed at different temperatures and times. Although not shown here, the perpendicular cylinders morphology also remained when the block copolymer was annealed at 80 °C for 15–20 h and 110 °C for 6 h. On the other hand, the film annealed at 110 °C for 15 h showed a mixed morphology (Fig. 2bi) that consisted in PS cylinders of around 20 nm perpendicularly oriented and parallel to the polymer–air interface, suggesting that the microstructure formed by solvent evaporation in SBS films developed a transformation to more stable in-plane cylinder morphology with annealing temperature [16]. Totally parallel orientation of the cylinders in the polymer–air interface was achieved in the film annealed 110 °C for 24 h (not shown here). The corresponding height profiles of SBS thin films without annealing and annealed at 110 °C for 15 h are shown in Fig. 2aii and bii, respectively. From the cross-section profiles it can be seen that the morphology in SBS thin film with and without annealing is regular and periodic. The specific value of the periodicity distance in the images obtained is around 40 nm and the height differences between hills (PS phase) and valleys (PB phase) are around 4 and 6 nm. Since the linear viscoelastic behavior in low-frequency isochronal rheologic measurements is sensitive to the state of order, the temperature dependence of storage, G0 , and loss moduli, G00 , of bulk SBS was also studied. Dynamic temperature scans conducted to samples annealed at 110 °C for 15 h and without annealing are shown in Fig. 3, where the loss factor, tan d is also included. All samples revealed the Tg of PS block around 80 °C and a second transition around 120 °C. An increment in both G0 and G00 above 180 °C due to degradation of material was also
1243
Fig. 2. (i) TM-AFM height images and (ii) height profiles of SBS films: (a) without annealing and (b) annealed at 110 °C for 15 h.
Fig. 3. Evolution of storage (filled symbols) and loss (open symbols) shear moduli and loss factor versus temperature at 1 Hz for SBS: (j) unannealed, (N) annealed at 110 °C for 15 h. Frequency dependency of storage and loss moduli for SBS after annealing (inset on the right).
observed. The order–disorder transition, TODT, probably is over the degradation temperature [50]. The second transition observed around 120 °C possibly is linked to thermally induced rearrangements of the nanodomains, from perpendicular to parallel orientation of the PS cylinders embedded in the PB matrix with respect to the shear direction. It has to be emphasized that this morphological transition produced a drop in G0 and G00 due to the parallel alignment minimizes resistance to flow [17–21]. On the other hand, in the sample subjected to pre-annealing this second transition was less pronounced, suggesting that the entire macroscopic rearrangement of the cylinders probably was reached more quickly in this sample, as shown above by AFM, the sample annealed at 110 °C for 15 h showed a mixed morphology. In addition, dynamic frequency sweep test at 110 °C is also shown in the inset of Fig. 3. For the sample annealed for 15 h, in the low-frequency range G0 and G00 presented the power-law behavior (G a xa) characteristic of the viscoelastic response of a
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1244
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
self-assembled hexagonal close-packed cylinders structure, being a around 0.2 and 0.3 for G0 and G00 , respectively [51,52].
MACROMOLECULAR NANOTECHNOLOGY
3.2. SBS nanocomposites At first, a comparative evaluation on the dispersion of unmodified Al2O3 nanoparticles and modified ones with a surfactant excess (4:1 weight ratio of dodecanethiol with respect to Al2O3) to insure good dispersion in 3 wt.% Al2O3/SBS nanocomposites is presented in Fig. 4ai and bi. As can be seen, a better dispersion of nanoparticles in the triblock copolymer was obtained in the nanocomposite prepared with surfactant. The corresponding height profiles of 3 wt.% Al2O3/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites are shown in Fig. 4aii and bii, respectively. As can be seen, the height difference between the phases and distance precocity are higher for 3 wt.% Al2O3/SBS than for 3 wt.% Al2O3/4:1/SBS, which is probably due to the presence of Al2O3 aggregates in the nanocomposite without surfactant. Additionally, UV–vis transmittance spectra
Fig. 5. UV–vis transmittance spectra of: (d) 12 wt.% surfactant-SBS mixture, (j) 3 wt.% Al2O3/SBS, (.) 3 wt.% Al2O3/2:1/SBS and (J) 3 wt.% Al2O3/4:1/SBS nanocomposites films.
(Fig. 5) of obtained 3 wt.% Al2O3/2:1/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites films showed higher trans-
Fig. 4. (i) TM-AFM height images and (ii) height profiles of: (a) 3 wt.% Al2O3/SBS and (b) 3 wt.% Al2O3/4:1/SBS nanocomposites.
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
tant due to the improvement on nanoparticle dispersion. Similar results were obtained for 1 wt.% Al2O3/SBS nanocomposites.
MACROMOLECULAR NANOTECHNOLOGY
parency in the visible range than for 3 wt.% Al2O3/SBS film, demonstrating that the use of surfactant permits to retain optical transparency of the SBS matrix with 12 wt.% surfac-
1245
Fig. 6. (i) TM-AFM height images and (ii) height profiles of: (a) 4 wt.% surfactant-SBS mixture, (b) 1 wt.% Al2O3/4:1/SBS without annealing, and (c) 1 wt.% Al2O3/4:1/SBS annealed at 110 °C for 15 h.
1246
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
tant. Moreover, this plasticized effect was lower compared to the 4 wt.% surfactant-SBS mixture, demonstrating that the presence of nanoparticles created a reduction of polymer chains mobility. Similar results were obtained for 1 wt.% Al2O3/2:1/SBS (not shown here). On the other hand, the corresponding height profiles of 4 wt.% surfactant-SBS mixture, 1 wt.% Al2O3/4:1/SBS without annealing and 1 wt.% Al2O3/4:1/SBS annealed at 110 °C for 15 h are shown in Fig. 6aii–cii. From the cross-section profile it can be seen that the height difference between phases is irregular and increases from around 6 nm for neat SBS film (Fig. 2aii and bii) to a maximum of 20 nm for 1 wt.% Al2O3/4:1/SBS. In addition, the periodicity distance in the zone of higher height difference is around 80 nm for 1 wt.% Al2O3/4:1/ SBS. These facts confirm the presence of well-dispersed Al2O3 nanoparticles in 1 wt.% Al2O3/4:1/SBS. Additionally, the morphology for nanocomposites with higher nanoparticle content and different amount of surfactant was also analyzed. Fig. 7ai and bi shows AFM images for 3 wt.% Al2O3/2:1/SBS and 3 wt.% Al2O3/4:1/SBS
MACROMOLECULAR NANOTECHNOLOGY
The influence of surfactant excess in 1 wt.% Al2O3/SBS nanocomposites on self-assembled morphology was studied by AFM. Firstly, the self-assembled morphology of 4 wt.% surfactant-SBS mixture was analyzed. As can be seen in Fig. 6ai, the morphology remained hexagonal as in neat SBS, but the cylinders were placed parallel to the polymer–air interface, indicating that the addition of the surfactant got influence on the orientation of the microdomains possibly due to plasticization of PS block domains [53]. The morphological behavior of nanocomposites is shown in Fig. 6bi and ci. AFM images show that the morphology switched from cylinders totally perpendicular to the polymer–air interface for the unannealed 1 wt.% Al2O3/4:1/SBS nanocomposite (Fig. 6bi) to totally parallel cylinders, when the nanocomposites were annealed at 110 °C during 15 h (Fig. 6ci). This fact seems to indicate that the presence of surfactant-coated nanoparticles in SBS matrix could have an accelerating effect in the process of rearrangement of PS domains with respect to neat SBS (Fig. 2bi) produced by the plasticization effect of surfac-
Fig. 7. (i) TM-AFM height images and (ii) height profile of: (a) 3 wt.% Al2O3/2:1/SBS and (b) 3 wt.% Al2O3/4:1/SBS without annealing.
without annealing, respectively. It is worth noting that morphologies were defined by perpendicular and parallel cylinders to the polymer–air interface for 3 wt.% Al2O3/ 2:1/SBS and 3 wt.% Al2O3/4:1/SBS, respectively. This parallel orientation in 3 wt.% Al2O3/4:1/SBS could be a result of the high content of surfactant (12 wt.% with respect to SBS). Apparently, most of the surfactant amount added to the system modified the SBS morphology. Therefore, the effect of alumina nanoparticles on the block copolymer morphology in the investigated systems was possibly minimal. The corresponding height profiles of 3 wt.% Al2O3/ 2:1/SBS and 3 wt.% Al2O3/4:1/SBS are shown in Fig. 7aii and bii. As can be seen, the height difference between phases and periodicity distance are irregular and higher than for neat SBS film (Fig. 2aii), due to the presence of well-dispersed Al2O3 nanoparticles. On the other hand, the presence of interactions between surfactant-coated alumina nanoparticles and the PS phase was demonstrated by DSC. Table 1 summarizes the Tg values of neat SBS and its nanocomposites. The Tg of PS block in the neat SBS was 71 °C. In the case of 1 wt.% Al2O3/SBS nanocomposites with surfactant, the Tg corresponding to PS domains was higher, being 75, 74 and 73 °C for 1 wt.% Al2O3/1:1/SBS, 1 wt.% Al2O3/2:1/SBS and 1 wt.% Al2O3/4:1/ SBS nanocomposites, respectively. The increase was lower at higher surfactant concentration as a consequence of plasticization effect, since the Tg for the 4 wt.% surfactant-SBS mixture was 64 °C. Therefore, the Tg increase of 9 °C in 1 wt.% Al2O3/4:1/SBS nanocomposite compared with the surfactant-SBS mixture seems to be due to a good nanoparticle dispersion and the presence of interfacial interactions between the nanoparticles and the microphase separated PS domains. Similar results were obtained for SBS nanocomposites containing 3 wt.% Al2O3 nanoparticles. The Tg of PS block in the SBS dropped to 56 °C when the content of surfactant was 12 wt.% in the surfactantSBS mixture. Nevertheless, the introduction of 3 wt.% Al2O3 nanoparticles produced an increment in Tg of 21 °C for 3 wt.% Al2O3/4:1/SBS, being its value 77 °C. This fact indicates a higher hindering in chains mobility by increasing the amount of nanoparticles. The Tg of PS block in SBS for 3 wt.% Al2O3/2:1/SBS was 79 °C. On the opposite, the 1 wt.% Al2O3/SBS and 3 wt.% Al2O3/SBS nanocomposites without surfactant presented a slight decrease in the Tg of PS block with respect to the neat SBS, being its value similar for both nanocomposites (around 68 °C), thus indi-
1247
cating the absence of interactions between nanoparticles and PS phase, and also the poor dispersion of them in SBS matrix, as shown above by AFM and UV–vis. Rheologic analysis of SBS nanocomposites was also carried out. The temperature dependency of both storage and loss moduli for nanocomposites containing 1 wt.% Al2O3 nanoparticles and different amounts of surfactant is shown in Fig. 8. It can be noticed that the second transition at 120 °C, attributed to thermal induced rearrangement of cylindrical domains observed in neat SBS, was also distinguished in all nanocomposites. Moreover, this transition dropped to lower temperatures as the content of surfactant increased; appearing at 105 °C in 4 wt.% surfactant-SBS mixture, at 109 °C in 1 wt.% Al2O3/2:1/SBS nanocomposite and at 106 °C in 1 wt.% Al2O3/4:1/SBS nanocomposite, thus confirming the acceleration effect on rearrangement of domains caused by the surfactant, as observed by AFM. On the contrary, at low surfactant content as for 1 wt.% Al2O3/1:1/SBS nanocomposite, the thermal induced rearrangement of domains occurred at 125 °C, probably because the morphological effect provoked by the surfactant in this nanocomposite could be suppressed through the hindering effect of Al2O3 nanoparticles, even producing a delay in this morphological transition with respect to the neat SBS. Furthermore, for 1 wt.% Al2O3/SBS nanocomposite, without surfactant, the thermal induced rearrangement of domains also dropped to lower temperatures but less than with surfactant, appearing at 115 °C, that could be a consequence of the slight decrease on the Tg of PS block in this nanocomposite. On the other hand, as can be seen from Fig. 8, G0 and G00 values also diminished upon surfactant content in SBS nanocomposite. However at high temperatures, 1 wt.% Al2O3/1:1/SBS nanocomposite showed major values of G0 and G00 than 1 wt.% Al2O3/SBS and neat SBS, because the low plasticization effect at this surfactant concentration. Similar results were obtained when the content of nanoparticles was increased up to 3 wt.% in SBS nanocomposites. As can be seen in Fig. 9, the macroscopic rearrangement of PS cylinders was achieved at lower
Table 1 Glass transition temperatures of PS block in the neat SBS and its nanocomposites which were determined by DSC. System
Tg PS (°C)
SBS 1 wt.% Al2O3/SBS 4 wt.% S-SBS mixture 1 wt.% Al2O3/4:1/SBS 1 wt.% Al2O3/2:1/SBS 1 wt.% Al2O3/1:1/SBS 3 wt.% Al2O3/SBS 12 wt.% S-SBS mixture 3 wt.% Al2O3/2:1/SBS 3 wt.% Al2O3/4:1/SBS
71 68 64 73 74 75 68 56 79 77
Fig. 8. Evolution of storage (filled symbols) and loss (open symbols) moduli versus temperature for: (j) neat SBS and their nanocomposites containing (N) 1 wt.% Al2O3/SBS, (.) 1 wt.% Al2O3/1:1/SBS, () 1 wt.% Al2O3/ 2:1/SBS, (J) 1 wt.% Al2O3/4:1/SBS and (d) 4 wt.% surfactant/SBS mixture.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1248
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
mains and on the temperature where this morphological rearrangement takes place. Acknowledgments This work was carried out with the support of Basque Country Government in the frame of Grupos Consolidados (IT-365-07), inanoGUNE and SAIOTEK (S-PE07UN39) projects and also of Spanish Ministry of Education and Science for MAT2006-06331 (FUNAN).
References
MACROMOLECULAR NANOTECHNOLOGY
Fig. 9. Evolution of storage (filled symbols) and loss (open symbols) moduli versus temperature for: (j) neat SBS triblock copolymer and their nanocomposites containing (d) 3 wt.% Al2O3/4:1/SBS, (.) 3 wt.% Al2O3/ 2:1/SBS and (N) 3 wt.% Al2O3/SBS.
temperatures as the content of nanoparticles was higher, being at 98 for 3 wt.% Al2O3/4:1/SBS, at 111 °C for 3 wt.% Al2O3/2:1/SBS and at 116 for 3 wt.% Al2O3/2:1/SBS. This fact is due to the greater amount of surfactant excess necessary to disperse the nanoparticles has more influence in the block copolymer morphology. Furthermore, a shift of both G0 and G00 to minor values upon surfactant content was also observed for these nanocomposites. 4. Conclusions In this work, we have demonstrated that hybrid inorganic/organic materials with excellent nanoparticle dispersion can be achieved by surfactant addition in SBS triblock copolymer matrices. Morphological analysis of neat SBS revealed PS cylinders self-assembled in a PB matrix, being the spatial orientation of domains strongly dependent on temperature of annealing. In addition, the viscoelastic behavior of SBS showed a morphological transition above the Tg of PS block, attributed to thermally induced rearrangement of the nanodomains. On the other hand, when surfactant-coated nanoparticles were introduced in the SBS matrix, DSC analysis showed a variation on the Tg of PS block, indicating the existence of interactions between alumina nanoparticles and these domains in the nanocomposites. Moreover, the Tg increase in the PS was suppressed when the content of surfactant-coated nanoparticles was increased, that can be attributed to the high plasticization effect of surfactant in PS block. The highest increase in glass transition temperature was observed for 1 wt.% Al2O3/1:1/SBS and 3 wt.% Al2O3/2:1/SBS nanocomposite, while a better dispersion of nanoparticles was obtained for the nanocomposite with 4:1 surfactant:Al2O3 weight ratio, without loosing its glass transition temperature. On the contrary, for nanocomposites without surfactant, a decrease in the Tg of PS phase was always observed. The viscoelastic and morphological behavior of the nanocomposites showed that the addition of surfactant in the SBS had a great influence on the spatial orientation of microdo-
[1] Leibler L. Theory of microphase separation in block copolymers. Macromolecules 1980;13(6):1602–17. [2] Bates FS, Fredickson GH. Block copolymer thermodynamics: theory and experiment. Ann Rev Phys Chem 1990;41:525–57. [3] Bates FS. Polymer–polymer phase behavior. Science 1991;251(4996): 898–905. [4] Matsen MW, Bates FS. Unifying weak- and strong-segregation block copolymer theories. Macromolecules 1996;29(5):1091–8. [5] Milner ST. Chain architecture and asymmetry in copolymer microphases. Macromolecules 1994;27(8):2333–5. [6] Han CD, Baek DM, Kim JK, Ogawa T, Hashimoto T. Effect of volume fraction on the order–disorder transition in low molecular weight polystyrene-block-polyisoprene copolymers. 1. Order–disorder transition temperature determine by rheological measurement. Macromolecules 1995;28(14):5043–62. [7] Matsen MW. Equilibrium behaviour of asymmetric ABA triblock copolymer melts. J Chem Phys 2000;113(13):5539–44. [8] Lodge TP. Block copolymers: past successes and future challenges. Macrom Chem Phys 2003;204(2):265–73. [9] Funaki Y, Kumano K, Jinngi H, Yoshida H, Kimishima K, Tsutsumi K. Influence of casting solvents on microphase-separated structures of poly(2-vinylpyridine)-block-polyisoprene. Polymer 1999;40(25): 7147–56. [10] Sakurai S, Momii T, Taie K, Shibayama M, Nomura S, Hashimoto T. Morphology transition from cylindrical to lamellar microdomains of block copolymers. Macromolecules 1993;26(3):485–91. [11] Quintana JR, Jañez MD, Hernaez E, Garcia A, Katime I. Associative behavior if a triblock copolímero in mixed selective solvents. Macromolecules 1998;31(20):6865–70. [12] van den Berg R, de Groot H, van Dijk MA, Denley DR. Atomic force microscopy of thin triblock copolímero films. Polymer 1994;35(26): 5778–81. [13] Motomatsu M, Mizutani W, Tokumoto H. Microphase domains of poly(styrene-block-ethylene/butylene-block-styrene) triblock copolymers studied by atomic force microscopy. Polymer 1997;38(8):1779–85. [14] Bodycomb J, Funaki Y, Kimishima K, Hashimoto T. Control of microdomain orientation of block copolymers induced by temperature gradient and surface ordering and characterization of their orientation. Polym Mat Sci Eng 1998;79:378–9. [15] Kim G, Libera M. Morphological development in solvent-cast polystyrene-polybutadiene-polystyrene (SBS) triblock copolymer thin films. Macromolecules 1998;31(8):2569–77. [16] Kim G, Libera M. Kinetic Constrains on the development of surface microstructure in SBS thin films. Macromolecules 1998;31(8): 2670–2. [17] Polushkin E, Bondzic S, de Wit J, van Ekenstein GA, Dolbnya I, Brás W, et al. In-situ SAXS study on the alignment of ordered systems of comb-shaped supramolecules: a shear-induced cylinder-to-cylinder transition. Macromolecules 2005;38(5):1804–13. [18] Bondzic S, Polushkin E, Shouten AJ, Ikkala O, ten Brinke G. The influence of grain size on the alignment of hexagonally ordered cylinders of self-assembled diblock copolymer-based supramolecules. Polymer 2007;48(16):4723–32. [19] Angelescu DE, Waller JH, Register RA, Chaikin PM. Shear-induced alignment in thin films of spherical nanodomains. Adv Mater 2005;17(15):1878–81. [20] Fredrickson GH. Steady shear alignment of block copolymers near the isotropic-lamellar transition. J Rheol 1994;38(4):1045–67. [21] Lee HH, Register RA, Hajduk DA, Gruner SM. Orientation of triblock copolymers in planar extension. Polym Eng and Sci 1996;36(10): 1414–24.
[22] Lin Y, Boker A, He J, Sill K, Xiang H, Abetz C, et al. Self-direct selfassembly of nanoparticles/block copolymer mixtures. Nature 2005;434(7029):55–9. [23] Bockstaller MR, Thomas EL. Proximity effects in self-organized binary particle-block copolymer blends. Phys Rev Lett 2004;93(16):166106/1–6/4. [24] Thompson RB, Ginzburg VV, Matsen MW, Balazs AC. Predicting the mesophases of copolymer-nanoparticles composites. Science 2001;292(5526):2469–72. [25] Thompson RB, Ginzburg VV, Matsen MW, Balazs AC. Block copolymer-directed assembly of nanoparticles: forming mesoscopically ordered hybrid materials. Macromolecules 2002;35(3): 1060–71. [26] Balazs AC, Emrick T, Russell TP. Nanoparticles polymer composites: where two small worlds meet. Science 2006;314(5802):1107–10. [27] Lin BH, Morkved TL, Meron M, Huang ZQ, Viccaro PJ, Jaeger HM, et al. X-ray studies of polymer/gold nanocomposites. J Appl Phys 1999;85(6):3180–4. [28] Huh J, Ginzburg VV, Balazs AC. Thermodynamic behavior of particle/diblock copolymer mixtures: simulation and theory. Macromolecules 2000;33(21):8085–96. [29] Sohn BH, Choi JM, Yoo SII, Yun SH, Zin WC, Jung JC, et al. Direct selfassembly of two kinds of nanoparticles utilizing monolayer films of diblock copolymer micelles. J Am Chem Soc 2003;125(21):6368–9. [30] Bockstaller MR, Lapetnikov Y, Margel S, Thomas EL. Size-selective organization of enthalpic compatibilized nanocrystals in ternary block copolymer/particle mixtures. J Am Chem Soc 2003;125(18): 5276–7. [31] Yeh SW, Wei KH, Sun YS, Jeng US, Liang KS. Morphological transformation of PS-b-PEO diblock copolymer by selectively dispersed colloidal CdS quantum dots. Macromolecules 2003;36(21): 7903–7. [32] Chiu JJ, Kim BJ, Yi GR, Bang J, Kramer EJ, Pine DJ. Distribution of nanoparticles in lamellar domains of block copolymers. Macromolecules 2007;40(9):3361–5. [33] Kim BJ, Bang J, Haeker CJ, Kramer EJ. Effect of areal chain density on the location of polymer-modified gold nanoparticles in a block copolymer template. Macromolecules 2006;39(12):4108–14. [34] Chiu JJ, Kim BJ, Kramer EJ, Pine DJ. Control of nanoparticle location in block copolymers. J Am Chem Soc 2005;127(14):5036–7. [35] Lopes WA, Jaeger HM. Hierarchical self-assembly of metal nanostrutures on diblock scaffolds. Nature 2001;414(6865): 735–8. [36] Jeng US, Sun YS, Lee HY, Hsu CH, Liang KS, Yen SW, et al. Binding effect of surface-modified cadmium sulphide on the microstructure of PS-b-PEO block copolymers. Macromolecules 2004;37(12): 4617–22. [37] Hamley IW. Nanostructured fabrication using block copolymers. Nanotechnology 2003;14(10):R39–54.
1249
[38] Li Q, He J, Glogowski E, Li X, Wang J, Emrick T, et al. Responsive assemblies: gold nanoparticles with mixed ligands in microphase separated block copolymers. Adv Mat 2008;20(8):1462–6. [39] Gutierrez J, Tercjak A, Garcia I, Peponi L, Mondragon I. Hybrid titanium dioxide/PS-b-PEO block copolymer nanocomposites based on sol–gel synthesis. Nanotechnology 2008;19(15):155607/1–7/8. [40] Lan Q, Francis LF, Bates FS. Silica nanoparticle dispersions in homopolymer versus block copolymer. J Polym Sci: Part B: Polym Phys 2007;45(16):2284–99. [41] Weng CC, Wei KH. Selective distribution of surface-modified TiO2 nanoparticles in polystyrene-b-poly(methyl methacrylate) diblock copolymer. Chem Mater 2003;15(15):2936–41. [42] Kim BJ, Chiu JJ, Yi GR, Pine DJ, Kramer EJ. Nanoparticle-induced phase transitions in diblock-copolymer films. Adv Mat 2005;17(21): 2618–22. [43] Balazs AC. Nanocomposites: economy at the nanoscale. Nature Materials 2007;6(2):94–5. [44] Hu S, Brittain WJ, Jacobson S, Balazs AC. Selective ordering of surfactant modified gold nanoparticles in a diblock copolymer. Eur Polym J 2006;42(9):2045–52. [45] Peponi L, Tercjack A, Torre L, Kenny JM, Mondragon I. Morphological analysis of self-assembled SIS block copolymer matrices containing silver nanoparticles. Comp Sci Tech 2008;68(7–8):1631–6. [46] Rittigstein P, Torkelson JM. Polymer-nanoparticle interfacial interactions in polymer nanocomposites: confinement effect on glass transition temperature and suppression of physical aging. J Polym Sci: Part B: Polym Phys 2006;44(20):2935–43. [47] Ash BJ, Siegel RW, Schadler LS. Glass-transition temperature behavior of alumina/PMMA nanocomposites. J Polym Sci: Part B: Polym Phys 2004;42(23):4371–83. [48] Harton SE, Yang H, Koga T, Kumar SK. Glass transition behavior of polymer nanocomposites. PMSE Prepints 2007;97:791–2. [49] Yong-Kyun P, Sang-Hoon Y, Sungho P. Assembly of highly ordered nanoparticle monolayers at a water/hexane interface. Langmuir 2007;23(21):10505–10. [50] Zhang Q, Tsui OKC, Du B, Zhang F, Tang T, He T. Observation of inverted phase in poly(styrene-b-butadiene-styrene) triblock copolymer by solvent-induced order–disorder phase transition. Macromolecules 2000;33(26):9561–7. [51] Adams JL, Graessley WW, Register RA. Rheology and the microphase separation transition in styrene-isoprene block copolymers. Macromolecules 1994;27(21):6026–32. [52] Modi MA, Krishnamoorti R, Tse MF, Wang HF. Viscoelastic characterization of an order–order transition in a mixture of diand triblock copolymers. Macromolecules 1999;32(12):4088–97. [53] Yang H, Sa U, Kang M, Ryu HS, Ryu CY, Cho K. Near-surface morphology effect on tack behavior of poly(styrene-b-butadienestyrene) triblock copolymer/rosin films. Polymer 2006;47(11): 3889–95.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Surfactant addition effects on dispersion and microdomain orientation in SBS triblock copolymer/alumina nanoparticle composites C. Ocando, A. Tercjak, I. Mondragon ⇑ ‘Materials + Technologies’ Group, Escuela Politécnica, Dpto Ingeniería Química y M Ambiente, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Pza, Europa 1, 20018 Donostia-San Sebastian, Spain
MACROMOLECULAR NANOTECHNOLOGY
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 28 February 2011 Accepted 6 March 2011 Available online 12 March 2011 Keywords: Alumina nanoparticles Surfactant Dispersion Triblock copolymer Self-assembling
a b s t r a c t The aim of this study was to investigate the effects of surfactant addition on the dispersion of 1–3 wt.% alumina nanoparticles on the self-assembled morphology of poly(styrene-bbutadiene-b-styrene) (SBS) linear triblock copolymer. The neat triblock copolymer microphase separated into PS cylinders self-assembled on a hexagonal array in the PB matrix, being the orientation of domains dependent on the annealing conditions. UV–vis and AFM analyses showed an improvement on dispersion of Al2O3 nanoparticles into SBS matrix by adding dodecanethiol as organic surfactant, due to its miscibility with PS block. Interactions between surfactant-coated Al2O3 nanoparticles with PS block were demonstrated by the variation on the glass transition temperature of this block. AFM analysis showed that the incorporation of surfactant-coated Al2O3 nanoparticles in the SBS matrix have great influence on the orientation of microphase separated domains in SBS nanocomposites. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Block copolymers are materials with a great potential for applications in nanotechnology, due to their ability for self-assembling into a long range of ordered morphologies at nanoscale. The segregation of blocks in the copolymer, resulting from thermodynamic incompatibility of their two polymer chains, and to the fact that they are covalently connected, leads to a great variety of microphase separated morphologies, with dimensions of segregated nanodomains ranging from 10 to 100 nm. Depending on the volume ratio between blocks, these microphase separated domains can be spheres, hexagonally packed cylinders, gyroidal and lamellae for diblock copolymers [1–11]. In addition to composition, architecture of chains and molecular weight of block copolymers, the microphase separated morphology and spatial orientation in the polymer–air interface are also dependent on the ⇑ Corresponding author. Tel.: +34 943017271; fax: +34 943017200. E-mail address: [email protected] (I. Mondragon). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.03.008
annealing conditions, among other possible variables [12– 16]. On the other hand, the microphase separated morphology can also develop upon temperature and under shear [17–21]. These typical features of block copolymers allow their use as nanotemplates for spatial organization of inorganic nanoparticles in thin films or in bulk samples [22–40]. For the preparation of hybrid inorganic/organic materials, different chemical and physical methods have been proposed to modify the surface of inorganic nanoparticles in order to promote particle–particle repulsive forces because the nanoparticles have the tendency to form aggregates by van der Waals attractions. Small molecules as amphiphilic surfactants can facilitate the dispersion of nanoparticles in a polymer matrix [41–45]. In addition, the use of a selective surfactant can allow the segregation of inorganic nanoparticles into one of the microphase separated domains in block copolymers [41–45]. On the other hand, the presence of a good interfacial interaction between the nanoparticles and the polymeric matrix is a key factor to obtain nanocomposites with enhanced thermal properties [46–48].
In this work, the viscoelastic, thermal and morphological behavior of neat poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer (30 wt.% PS) and its alumina (1– 3 wt.%) containing nanocomposites have been investigated. In order to promote the compatibility between alumina nanoparticles and the polymeric matrix, an organic surfactant miscible with PS was used [44,45]. Therefore, the effect of modification of alumina nanoparticles with a surfactant excess on dispersion in SBS matrix nanocomposites is discussed. The variations on the orientation of microphase separated domains in SBS matrix are also analyzed. Morphological changes were followed by atomic force microscopy (AFM) and viscoelastic analysis. 2. Experimental section 2.1. Materials Poly(styrene-b-butadiene-b-styrene) (Dynasol C500) linear triblock copolymer, with 70 wt.% PB, was kindly supplied by Repsol-YPF. The weight-average molecular mass was 136000 g/mol, being the glass transition temperatures (Tg) of PS and PB blocks 71 and 80 °C, respectively. Alumina (Al2O3) nanoparticles with an average diameter size of 39 nm and specific surface area 44 m2/g were purchased from Nanophase Technologies Corp. (NTC). As can be seen in Fig. 1 from AFM analysis, as-received Al2O3 nanoparticles formed aggregates. The particles were dried in a vacuum oven at 125 °C for a period of 24 h before used. Dodecanethiol and toluene, from Sigma Aldrich, were used as surfactant and solvent, respectively. 2.2. SBS nanocomposites preparation Pellets of SBS triblock copolymer were dissolved in toluene at a concentration of 5 wt.%. Films of the resultant solutions were prepared by solvent-casting. A 20 lL pipet was used to cast equal-sized droplets of the BC solutions onto a glass support and solvent was evaporated for 24 h
1241
in air at room temperature before analysis. Then, the films were annealed at two temperatures above the Tg of PS block, 80 and 110 °C under vacuum. It should be mentioned that for AFM analyses several measurements along the films were realised for each sample, and we do not observe any significant changes of the morphologies even though there can be some thickness variation in the films. Therefore, the phase behavior of samples formed under different conditions is representative and reliable. SBS matrix nanocomposites were prepared in solution by the sonication technique. A microprocessor sonicator 750 W; Vibracell 75043 from Bioblock Scientific with an amplitude of 30% was used. Alumina nanoparticles were dispersed in toluene at a concentration of 0.05 wt.%. Alumina nanoparticles, surfactant and SBS were sonicated during 4 h. Nanocomposites containing 1–3 wt.% Al2O3 nanoparticles with respect to the block copolymer and an excess of surfactant (1:1, 2:1, 4:1 weight ratio of dodecanethiol with respect to Al2O3) were developed. The amount of dodecanethiol (mS) adsorbed on the nanoparticles (mNP) can be calculated with the following equation:
qS 4=3 p ðrNP þ 2wSÞ3 ðrNPÞ3 mS ¼ mNP qNP 4=3 p ðrNPÞ3 where qNP is the nanoparticle density (3.6 g/cm3), qS is the surfactant density (0.845 g/cm3), rNP is the nanoparticle ratio and wS is the covering thickness of the surfactant surrounding the nanoparticle surface (0.2 nm). This last value was obtained assuming a monolayer coverage of surfactant on the nanoparticle and taking into account that the area for a linear dodecanethiol surfactant has been reported to be approximately 20 Å2 (0.2 nm2) per molecule [49]. From the previous calculations a 1.5 wt.% of surfactant is adsorbed on the nanoparticles. Consequently, the 1 wt.% Al2O3/SBS nanocomposites with 1:1, 2:1 and 4:1 weight ratio of dodecanethiol with respect to Al2O3 contain
Fig. 1. (a) TM-AFM height image of Al2O3 nanoparticles and (b) height profile.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1242
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
around 1, 2 and 4 wt.% of surfactant with respect to the block copolymer. The 3 wt.% Al2O3/SBS nanocomposites with 1:1, 2:1 and 4:1 weight ratio of dodecanethiol with respect to Al2O3 contain around 3, 6 and 12 wt.% of surfactant with respect to the block copolymer. The samples obtained are designed throughout the manuscript in the following form: I wt.% Al2O3/II/SBS (I being the amount of Al2O3 (wt.%) and II the weight ratio of dodecanethiol with respect to Al2O3). Additionally, SBS-matrix samples containing 4 and 12 wt.% of dodecanethiol were also prepared in order to compare the surfactant effects in 1 wt.% Al2O3/4:1/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites, respectively. Films of nanocomposites were prepared in the same way than the neat block copolymer.
MACROMOLECULAR NANOTECHNOLOGY
2.3. Characterization 2.3.1. Differential scanning calorimetry (DSC) Thermal transition temperatures of neat SBS and their nanocomposites were determined by using a differential scanning calorimeter Mettler Toledo DSC-822, under a nitrogen flow of 20 mL/min, working with 5–7 mg samples in aluminum pans. Dynamic scans were performed from 60 to 160 °C at a heating rate of 20 °C/min. The values of Tg were determined at the inflection point of the change in heat capacity. 2.3.2. UV–vis spectroscopy UV–vis spectra of the coating films with the same thickness, in the range 200–800 nm, were recorded with a UV– vis spectrophotometer (JASCO V-570). 2.3.3. Viscoelastic measurements To study the viscoelastic behavior of neat SBS and its nanocomposites, dynamic oscillatory shear measurements were performed using a Rheometrics Ares rheometer equipped with 15 mm diameter parallel plates and two transducers with a couple operating range of 0.02– 2.000 g cm. A mini-injection machine, Haake, was employed using a disks shape mold to make samples of around 2 mm thickness by injection at 160 °C and 780 bar for few seconds to avoid possible PB-block degradation. Dynamic temperature ramp tests were conducted to record the evolution of storage, G0 , and loss, G00 , moduli under isochronal conditions at a heating rate of 3 °C/min and at angular frequency of 0.1 Hz. The strain amplitude was varied to insure a linear viscoelastic response. Additionally, frequency sweep tests were carried out at 110 °C over a frequency range of 0.006–100 rad/s for the neat block copolymer. 2.3.4. Morphological analysis Morphologies of the neat SBS and its nanocomposites were studied by atomic force microscopy. AFM images were obtained with a Nanoscope IIIa scanning probe microscope (Multimode™, Digital Instruments). Tapping mode (TM) was employed in air using an integrated tip/ cantilever (125 lm in length with ca. 300 kHz resonant frequency). Typical scan rates during recording were 0.7– 1 line/s using a scan head with a maximum range of
16 16 lm. The brighter domains in height images correspond to PS phase [13]. The height profiles of the films were measured along the cross-section line in the middle of each TM-AFM image. It is important to emphasize that TM-AFM images of 10 and 3 lm were used to evaluate the dispersion and morphology in the nanocomposites, respectively.
3. Results and discussion 3.1. Self-assembled morphology of neat SBS The morphological behavior of neat triblock copolymer was analyzed by microscopy and rheology techniques for comparison with the behavior of the hybrid nanocomposites. Representative TM-AFM images for the air-surface morphologies of SBS thin films prepared by solvent-casting and then submitted to different annealing treatments are shown in Fig. 2ai and bi. It can be noticed in Fig. 2ai that the film without annealing clearly showed nanostructuring by solvent-induced microphase separation between dissimilar blocks, where ordered PS cylinders of around 20 nm on a hexagonal self-assembly array were oriented perpendicularly to the polymer–air interface in a continuous PB layer. The perpendicular ordering occurred probably due to the rate of solvent evaporation [15,16]. In order to investigate the morphology evolution upon temperature, the solvent-cast thin films of SBS were annealed at different temperatures and times. Although not shown here, the perpendicular cylinders morphology also remained when the block copolymer was annealed at 80 °C for 15–20 h and 110 °C for 6 h. On the other hand, the film annealed at 110 °C for 15 h showed a mixed morphology (Fig. 2bi) that consisted in PS cylinders of around 20 nm perpendicularly oriented and parallel to the polymer–air interface, suggesting that the microstructure formed by solvent evaporation in SBS films developed a transformation to more stable in-plane cylinder morphology with annealing temperature [16]. Totally parallel orientation of the cylinders in the polymer–air interface was achieved in the film annealed 110 °C for 24 h (not shown here). The corresponding height profiles of SBS thin films without annealing and annealed at 110 °C for 15 h are shown in Fig. 2aii and bii, respectively. From the cross-section profiles it can be seen that the morphology in SBS thin film with and without annealing is regular and periodic. The specific value of the periodicity distance in the images obtained is around 40 nm and the height differences between hills (PS phase) and valleys (PB phase) are around 4 and 6 nm. Since the linear viscoelastic behavior in low-frequency isochronal rheologic measurements is sensitive to the state of order, the temperature dependence of storage, G0 , and loss moduli, G00 , of bulk SBS was also studied. Dynamic temperature scans conducted to samples annealed at 110 °C for 15 h and without annealing are shown in Fig. 3, where the loss factor, tan d is also included. All samples revealed the Tg of PS block around 80 °C and a second transition around 120 °C. An increment in both G0 and G00 above 180 °C due to degradation of material was also
1243
Fig. 2. (i) TM-AFM height images and (ii) height profiles of SBS films: (a) without annealing and (b) annealed at 110 °C for 15 h.
Fig. 3. Evolution of storage (filled symbols) and loss (open symbols) shear moduli and loss factor versus temperature at 1 Hz for SBS: (j) unannealed, (N) annealed at 110 °C for 15 h. Frequency dependency of storage and loss moduli for SBS after annealing (inset on the right).
observed. The order–disorder transition, TODT, probably is over the degradation temperature [50]. The second transition observed around 120 °C possibly is linked to thermally induced rearrangements of the nanodomains, from perpendicular to parallel orientation of the PS cylinders embedded in the PB matrix with respect to the shear direction. It has to be emphasized that this morphological transition produced a drop in G0 and G00 due to the parallel alignment minimizes resistance to flow [17–21]. On the other hand, in the sample subjected to pre-annealing this second transition was less pronounced, suggesting that the entire macroscopic rearrangement of the cylinders probably was reached more quickly in this sample, as shown above by AFM, the sample annealed at 110 °C for 15 h showed a mixed morphology. In addition, dynamic frequency sweep test at 110 °C is also shown in the inset of Fig. 3. For the sample annealed for 15 h, in the low-frequency range G0 and G00 presented the power-law behavior (G a xa) characteristic of the viscoelastic response of a
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1244
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
self-assembled hexagonal close-packed cylinders structure, being a around 0.2 and 0.3 for G0 and G00 , respectively [51,52].
MACROMOLECULAR NANOTECHNOLOGY
3.2. SBS nanocomposites At first, a comparative evaluation on the dispersion of unmodified Al2O3 nanoparticles and modified ones with a surfactant excess (4:1 weight ratio of dodecanethiol with respect to Al2O3) to insure good dispersion in 3 wt.% Al2O3/SBS nanocomposites is presented in Fig. 4ai and bi. As can be seen, a better dispersion of nanoparticles in the triblock copolymer was obtained in the nanocomposite prepared with surfactant. The corresponding height profiles of 3 wt.% Al2O3/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites are shown in Fig. 4aii and bii, respectively. As can be seen, the height difference between the phases and distance precocity are higher for 3 wt.% Al2O3/SBS than for 3 wt.% Al2O3/4:1/SBS, which is probably due to the presence of Al2O3 aggregates in the nanocomposite without surfactant. Additionally, UV–vis transmittance spectra
Fig. 5. UV–vis transmittance spectra of: (d) 12 wt.% surfactant-SBS mixture, (j) 3 wt.% Al2O3/SBS, (.) 3 wt.% Al2O3/2:1/SBS and (J) 3 wt.% Al2O3/4:1/SBS nanocomposites films.
(Fig. 5) of obtained 3 wt.% Al2O3/2:1/SBS and 3 wt.% Al2O3/4:1/SBS nanocomposites films showed higher trans-
Fig. 4. (i) TM-AFM height images and (ii) height profiles of: (a) 3 wt.% Al2O3/SBS and (b) 3 wt.% Al2O3/4:1/SBS nanocomposites.
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
tant due to the improvement on nanoparticle dispersion. Similar results were obtained for 1 wt.% Al2O3/SBS nanocomposites.
MACROMOLECULAR NANOTECHNOLOGY
parency in the visible range than for 3 wt.% Al2O3/SBS film, demonstrating that the use of surfactant permits to retain optical transparency of the SBS matrix with 12 wt.% surfac-
1245
Fig. 6. (i) TM-AFM height images and (ii) height profiles of: (a) 4 wt.% surfactant-SBS mixture, (b) 1 wt.% Al2O3/4:1/SBS without annealing, and (c) 1 wt.% Al2O3/4:1/SBS annealed at 110 °C for 15 h.
1246
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
tant. Moreover, this plasticized effect was lower compared to the 4 wt.% surfactant-SBS mixture, demonstrating that the presence of nanoparticles created a reduction of polymer chains mobility. Similar results were obtained for 1 wt.% Al2O3/2:1/SBS (not shown here). On the other hand, the corresponding height profiles of 4 wt.% surfactant-SBS mixture, 1 wt.% Al2O3/4:1/SBS without annealing and 1 wt.% Al2O3/4:1/SBS annealed at 110 °C for 15 h are shown in Fig. 6aii–cii. From the cross-section profile it can be seen that the height difference between phases is irregular and increases from around 6 nm for neat SBS film (Fig. 2aii and bii) to a maximum of 20 nm for 1 wt.% Al2O3/4:1/SBS. In addition, the periodicity distance in the zone of higher height difference is around 80 nm for 1 wt.% Al2O3/4:1/ SBS. These facts confirm the presence of well-dispersed Al2O3 nanoparticles in 1 wt.% Al2O3/4:1/SBS. Additionally, the morphology for nanocomposites with higher nanoparticle content and different amount of surfactant was also analyzed. Fig. 7ai and bi shows AFM images for 3 wt.% Al2O3/2:1/SBS and 3 wt.% Al2O3/4:1/SBS
MACROMOLECULAR NANOTECHNOLOGY
The influence of surfactant excess in 1 wt.% Al2O3/SBS nanocomposites on self-assembled morphology was studied by AFM. Firstly, the self-assembled morphology of 4 wt.% surfactant-SBS mixture was analyzed. As can be seen in Fig. 6ai, the morphology remained hexagonal as in neat SBS, but the cylinders were placed parallel to the polymer–air interface, indicating that the addition of the surfactant got influence on the orientation of the microdomains possibly due to plasticization of PS block domains [53]. The morphological behavior of nanocomposites is shown in Fig. 6bi and ci. AFM images show that the morphology switched from cylinders totally perpendicular to the polymer–air interface for the unannealed 1 wt.% Al2O3/4:1/SBS nanocomposite (Fig. 6bi) to totally parallel cylinders, when the nanocomposites were annealed at 110 °C during 15 h (Fig. 6ci). This fact seems to indicate that the presence of surfactant-coated nanoparticles in SBS matrix could have an accelerating effect in the process of rearrangement of PS domains with respect to neat SBS (Fig. 2bi) produced by the plasticization effect of surfac-
Fig. 7. (i) TM-AFM height images and (ii) height profile of: (a) 3 wt.% Al2O3/2:1/SBS and (b) 3 wt.% Al2O3/4:1/SBS without annealing.
without annealing, respectively. It is worth noting that morphologies were defined by perpendicular and parallel cylinders to the polymer–air interface for 3 wt.% Al2O3/ 2:1/SBS and 3 wt.% Al2O3/4:1/SBS, respectively. This parallel orientation in 3 wt.% Al2O3/4:1/SBS could be a result of the high content of surfactant (12 wt.% with respect to SBS). Apparently, most of the surfactant amount added to the system modified the SBS morphology. Therefore, the effect of alumina nanoparticles on the block copolymer morphology in the investigated systems was possibly minimal. The corresponding height profiles of 3 wt.% Al2O3/ 2:1/SBS and 3 wt.% Al2O3/4:1/SBS are shown in Fig. 7aii and bii. As can be seen, the height difference between phases and periodicity distance are irregular and higher than for neat SBS film (Fig. 2aii), due to the presence of well-dispersed Al2O3 nanoparticles. On the other hand, the presence of interactions between surfactant-coated alumina nanoparticles and the PS phase was demonstrated by DSC. Table 1 summarizes the Tg values of neat SBS and its nanocomposites. The Tg of PS block in the neat SBS was 71 °C. In the case of 1 wt.% Al2O3/SBS nanocomposites with surfactant, the Tg corresponding to PS domains was higher, being 75, 74 and 73 °C for 1 wt.% Al2O3/1:1/SBS, 1 wt.% Al2O3/2:1/SBS and 1 wt.% Al2O3/4:1/ SBS nanocomposites, respectively. The increase was lower at higher surfactant concentration as a consequence of plasticization effect, since the Tg for the 4 wt.% surfactant-SBS mixture was 64 °C. Therefore, the Tg increase of 9 °C in 1 wt.% Al2O3/4:1/SBS nanocomposite compared with the surfactant-SBS mixture seems to be due to a good nanoparticle dispersion and the presence of interfacial interactions between the nanoparticles and the microphase separated PS domains. Similar results were obtained for SBS nanocomposites containing 3 wt.% Al2O3 nanoparticles. The Tg of PS block in the SBS dropped to 56 °C when the content of surfactant was 12 wt.% in the surfactantSBS mixture. Nevertheless, the introduction of 3 wt.% Al2O3 nanoparticles produced an increment in Tg of 21 °C for 3 wt.% Al2O3/4:1/SBS, being its value 77 °C. This fact indicates a higher hindering in chains mobility by increasing the amount of nanoparticles. The Tg of PS block in SBS for 3 wt.% Al2O3/2:1/SBS was 79 °C. On the opposite, the 1 wt.% Al2O3/SBS and 3 wt.% Al2O3/SBS nanocomposites without surfactant presented a slight decrease in the Tg of PS block with respect to the neat SBS, being its value similar for both nanocomposites (around 68 °C), thus indi-
1247
cating the absence of interactions between nanoparticles and PS phase, and also the poor dispersion of them in SBS matrix, as shown above by AFM and UV–vis. Rheologic analysis of SBS nanocomposites was also carried out. The temperature dependency of both storage and loss moduli for nanocomposites containing 1 wt.% Al2O3 nanoparticles and different amounts of surfactant is shown in Fig. 8. It can be noticed that the second transition at 120 °C, attributed to thermal induced rearrangement of cylindrical domains observed in neat SBS, was also distinguished in all nanocomposites. Moreover, this transition dropped to lower temperatures as the content of surfactant increased; appearing at 105 °C in 4 wt.% surfactant-SBS mixture, at 109 °C in 1 wt.% Al2O3/2:1/SBS nanocomposite and at 106 °C in 1 wt.% Al2O3/4:1/SBS nanocomposite, thus confirming the acceleration effect on rearrangement of domains caused by the surfactant, as observed by AFM. On the contrary, at low surfactant content as for 1 wt.% Al2O3/1:1/SBS nanocomposite, the thermal induced rearrangement of domains occurred at 125 °C, probably because the morphological effect provoked by the surfactant in this nanocomposite could be suppressed through the hindering effect of Al2O3 nanoparticles, even producing a delay in this morphological transition with respect to the neat SBS. Furthermore, for 1 wt.% Al2O3/SBS nanocomposite, without surfactant, the thermal induced rearrangement of domains also dropped to lower temperatures but less than with surfactant, appearing at 115 °C, that could be a consequence of the slight decrease on the Tg of PS block in this nanocomposite. On the other hand, as can be seen from Fig. 8, G0 and G00 values also diminished upon surfactant content in SBS nanocomposite. However at high temperatures, 1 wt.% Al2O3/1:1/SBS nanocomposite showed major values of G0 and G00 than 1 wt.% Al2O3/SBS and neat SBS, because the low plasticization effect at this surfactant concentration. Similar results were obtained when the content of nanoparticles was increased up to 3 wt.% in SBS nanocomposites. As can be seen in Fig. 9, the macroscopic rearrangement of PS cylinders was achieved at lower
Table 1 Glass transition temperatures of PS block in the neat SBS and its nanocomposites which were determined by DSC. System
Tg PS (°C)
SBS 1 wt.% Al2O3/SBS 4 wt.% S-SBS mixture 1 wt.% Al2O3/4:1/SBS 1 wt.% Al2O3/2:1/SBS 1 wt.% Al2O3/1:1/SBS 3 wt.% Al2O3/SBS 12 wt.% S-SBS mixture 3 wt.% Al2O3/2:1/SBS 3 wt.% Al2O3/4:1/SBS
71 68 64 73 74 75 68 56 79 77
Fig. 8. Evolution of storage (filled symbols) and loss (open symbols) moduli versus temperature for: (j) neat SBS and their nanocomposites containing (N) 1 wt.% Al2O3/SBS, (.) 1 wt.% Al2O3/1:1/SBS, () 1 wt.% Al2O3/ 2:1/SBS, (J) 1 wt.% Al2O3/4:1/SBS and (d) 4 wt.% surfactant/SBS mixture.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
1248
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249
mains and on the temperature where this morphological rearrangement takes place. Acknowledgments This work was carried out with the support of Basque Country Government in the frame of Grupos Consolidados (IT-365-07), inanoGUNE and SAIOTEK (S-PE07UN39) projects and also of Spanish Ministry of Education and Science for MAT2006-06331 (FUNAN).
References
MACROMOLECULAR NANOTECHNOLOGY
Fig. 9. Evolution of storage (filled symbols) and loss (open symbols) moduli versus temperature for: (j) neat SBS triblock copolymer and their nanocomposites containing (d) 3 wt.% Al2O3/4:1/SBS, (.) 3 wt.% Al2O3/ 2:1/SBS and (N) 3 wt.% Al2O3/SBS.
temperatures as the content of nanoparticles was higher, being at 98 for 3 wt.% Al2O3/4:1/SBS, at 111 °C for 3 wt.% Al2O3/2:1/SBS and at 116 for 3 wt.% Al2O3/2:1/SBS. This fact is due to the greater amount of surfactant excess necessary to disperse the nanoparticles has more influence in the block copolymer morphology. Furthermore, a shift of both G0 and G00 to minor values upon surfactant content was also observed for these nanocomposites. 4. Conclusions In this work, we have demonstrated that hybrid inorganic/organic materials with excellent nanoparticle dispersion can be achieved by surfactant addition in SBS triblock copolymer matrices. Morphological analysis of neat SBS revealed PS cylinders self-assembled in a PB matrix, being the spatial orientation of domains strongly dependent on temperature of annealing. In addition, the viscoelastic behavior of SBS showed a morphological transition above the Tg of PS block, attributed to thermally induced rearrangement of the nanodomains. On the other hand, when surfactant-coated nanoparticles were introduced in the SBS matrix, DSC analysis showed a variation on the Tg of PS block, indicating the existence of interactions between alumina nanoparticles and these domains in the nanocomposites. Moreover, the Tg increase in the PS was suppressed when the content of surfactant-coated nanoparticles was increased, that can be attributed to the high plasticization effect of surfactant in PS block. The highest increase in glass transition temperature was observed for 1 wt.% Al2O3/1:1/SBS and 3 wt.% Al2O3/2:1/SBS nanocomposite, while a better dispersion of nanoparticles was obtained for the nanocomposite with 4:1 surfactant:Al2O3 weight ratio, without loosing its glass transition temperature. On the contrary, for nanocomposites without surfactant, a decrease in the Tg of PS phase was always observed. The viscoelastic and morphological behavior of the nanocomposites showed that the addition of surfactant in the SBS had a great influence on the spatial orientation of microdo-
[1] Leibler L. Theory of microphase separation in block copolymers. Macromolecules 1980;13(6):1602–17. [2] Bates FS, Fredickson GH. Block copolymer thermodynamics: theory and experiment. Ann Rev Phys Chem 1990;41:525–57. [3] Bates FS. Polymer–polymer phase behavior. Science 1991;251(4996): 898–905. [4] Matsen MW, Bates FS. Unifying weak- and strong-segregation block copolymer theories. Macromolecules 1996;29(5):1091–8. [5] Milner ST. Chain architecture and asymmetry in copolymer microphases. Macromolecules 1994;27(8):2333–5. [6] Han CD, Baek DM, Kim JK, Ogawa T, Hashimoto T. Effect of volume fraction on the order–disorder transition in low molecular weight polystyrene-block-polyisoprene copolymers. 1. Order–disorder transition temperature determine by rheological measurement. Macromolecules 1995;28(14):5043–62. [7] Matsen MW. Equilibrium behaviour of asymmetric ABA triblock copolymer melts. J Chem Phys 2000;113(13):5539–44. [8] Lodge TP. Block copolymers: past successes and future challenges. Macrom Chem Phys 2003;204(2):265–73. [9] Funaki Y, Kumano K, Jinngi H, Yoshida H, Kimishima K, Tsutsumi K. Influence of casting solvents on microphase-separated structures of poly(2-vinylpyridine)-block-polyisoprene. Polymer 1999;40(25): 7147–56. [10] Sakurai S, Momii T, Taie K, Shibayama M, Nomura S, Hashimoto T. Morphology transition from cylindrical to lamellar microdomains of block copolymers. Macromolecules 1993;26(3):485–91. [11] Quintana JR, Jañez MD, Hernaez E, Garcia A, Katime I. Associative behavior if a triblock copolímero in mixed selective solvents. Macromolecules 1998;31(20):6865–70. [12] van den Berg R, de Groot H, van Dijk MA, Denley DR. Atomic force microscopy of thin triblock copolímero films. Polymer 1994;35(26): 5778–81. [13] Motomatsu M, Mizutani W, Tokumoto H. Microphase domains of poly(styrene-block-ethylene/butylene-block-styrene) triblock copolymers studied by atomic force microscopy. Polymer 1997;38(8):1779–85. [14] Bodycomb J, Funaki Y, Kimishima K, Hashimoto T. Control of microdomain orientation of block copolymers induced by temperature gradient and surface ordering and characterization of their orientation. Polym Mat Sci Eng 1998;79:378–9. [15] Kim G, Libera M. Morphological development in solvent-cast polystyrene-polybutadiene-polystyrene (SBS) triblock copolymer thin films. Macromolecules 1998;31(8):2569–77. [16] Kim G, Libera M. Kinetic Constrains on the development of surface microstructure in SBS thin films. Macromolecules 1998;31(8): 2670–2. [17] Polushkin E, Bondzic S, de Wit J, van Ekenstein GA, Dolbnya I, Brás W, et al. In-situ SAXS study on the alignment of ordered systems of comb-shaped supramolecules: a shear-induced cylinder-to-cylinder transition. Macromolecules 2005;38(5):1804–13. [18] Bondzic S, Polushkin E, Shouten AJ, Ikkala O, ten Brinke G. The influence of grain size on the alignment of hexagonally ordered cylinders of self-assembled diblock copolymer-based supramolecules. Polymer 2007;48(16):4723–32. [19] Angelescu DE, Waller JH, Register RA, Chaikin PM. Shear-induced alignment in thin films of spherical nanodomains. Adv Mater 2005;17(15):1878–81. [20] Fredrickson GH. Steady shear alignment of block copolymers near the isotropic-lamellar transition. J Rheol 1994;38(4):1045–67. [21] Lee HH, Register RA, Hajduk DA, Gruner SM. Orientation of triblock copolymers in planar extension. Polym Eng and Sci 1996;36(10): 1414–24.
[22] Lin Y, Boker A, He J, Sill K, Xiang H, Abetz C, et al. Self-direct selfassembly of nanoparticles/block copolymer mixtures. Nature 2005;434(7029):55–9. [23] Bockstaller MR, Thomas EL. Proximity effects in self-organized binary particle-block copolymer blends. Phys Rev Lett 2004;93(16):166106/1–6/4. [24] Thompson RB, Ginzburg VV, Matsen MW, Balazs AC. Predicting the mesophases of copolymer-nanoparticles composites. Science 2001;292(5526):2469–72. [25] Thompson RB, Ginzburg VV, Matsen MW, Balazs AC. Block copolymer-directed assembly of nanoparticles: forming mesoscopically ordered hybrid materials. Macromolecules 2002;35(3): 1060–71. [26] Balazs AC, Emrick T, Russell TP. Nanoparticles polymer composites: where two small worlds meet. Science 2006;314(5802):1107–10. [27] Lin BH, Morkved TL, Meron M, Huang ZQ, Viccaro PJ, Jaeger HM, et al. X-ray studies of polymer/gold nanocomposites. J Appl Phys 1999;85(6):3180–4. [28] Huh J, Ginzburg VV, Balazs AC. Thermodynamic behavior of particle/diblock copolymer mixtures: simulation and theory. Macromolecules 2000;33(21):8085–96. [29] Sohn BH, Choi JM, Yoo SII, Yun SH, Zin WC, Jung JC, et al. Direct selfassembly of two kinds of nanoparticles utilizing monolayer films of diblock copolymer micelles. J Am Chem Soc 2003;125(21):6368–9. [30] Bockstaller MR, Lapetnikov Y, Margel S, Thomas EL. Size-selective organization of enthalpic compatibilized nanocrystals in ternary block copolymer/particle mixtures. J Am Chem Soc 2003;125(18): 5276–7. [31] Yeh SW, Wei KH, Sun YS, Jeng US, Liang KS. Morphological transformation of PS-b-PEO diblock copolymer by selectively dispersed colloidal CdS quantum dots. Macromolecules 2003;36(21): 7903–7. [32] Chiu JJ, Kim BJ, Yi GR, Bang J, Kramer EJ, Pine DJ. Distribution of nanoparticles in lamellar domains of block copolymers. Macromolecules 2007;40(9):3361–5. [33] Kim BJ, Bang J, Haeker CJ, Kramer EJ. Effect of areal chain density on the location of polymer-modified gold nanoparticles in a block copolymer template. Macromolecules 2006;39(12):4108–14. [34] Chiu JJ, Kim BJ, Kramer EJ, Pine DJ. Control of nanoparticle location in block copolymers. J Am Chem Soc 2005;127(14):5036–7. [35] Lopes WA, Jaeger HM. Hierarchical self-assembly of metal nanostrutures on diblock scaffolds. Nature 2001;414(6865): 735–8. [36] Jeng US, Sun YS, Lee HY, Hsu CH, Liang KS, Yen SW, et al. Binding effect of surface-modified cadmium sulphide on the microstructure of PS-b-PEO block copolymers. Macromolecules 2004;37(12): 4617–22. [37] Hamley IW. Nanostructured fabrication using block copolymers. Nanotechnology 2003;14(10):R39–54.
1249
[38] Li Q, He J, Glogowski E, Li X, Wang J, Emrick T, et al. Responsive assemblies: gold nanoparticles with mixed ligands in microphase separated block copolymers. Adv Mat 2008;20(8):1462–6. [39] Gutierrez J, Tercjak A, Garcia I, Peponi L, Mondragon I. Hybrid titanium dioxide/PS-b-PEO block copolymer nanocomposites based on sol–gel synthesis. Nanotechnology 2008;19(15):155607/1–7/8. [40] Lan Q, Francis LF, Bates FS. Silica nanoparticle dispersions in homopolymer versus block copolymer. J Polym Sci: Part B: Polym Phys 2007;45(16):2284–99. [41] Weng CC, Wei KH. Selective distribution of surface-modified TiO2 nanoparticles in polystyrene-b-poly(methyl methacrylate) diblock copolymer. Chem Mater 2003;15(15):2936–41. [42] Kim BJ, Chiu JJ, Yi GR, Pine DJ, Kramer EJ. Nanoparticle-induced phase transitions in diblock-copolymer films. Adv Mat 2005;17(21): 2618–22. [43] Balazs AC. Nanocomposites: economy at the nanoscale. Nature Materials 2007;6(2):94–5. [44] Hu S, Brittain WJ, Jacobson S, Balazs AC. Selective ordering of surfactant modified gold nanoparticles in a diblock copolymer. Eur Polym J 2006;42(9):2045–52. [45] Peponi L, Tercjack A, Torre L, Kenny JM, Mondragon I. Morphological analysis of self-assembled SIS block copolymer matrices containing silver nanoparticles. Comp Sci Tech 2008;68(7–8):1631–6. [46] Rittigstein P, Torkelson JM. Polymer-nanoparticle interfacial interactions in polymer nanocomposites: confinement effect on glass transition temperature and suppression of physical aging. J Polym Sci: Part B: Polym Phys 2006;44(20):2935–43. [47] Ash BJ, Siegel RW, Schadler LS. Glass-transition temperature behavior of alumina/PMMA nanocomposites. J Polym Sci: Part B: Polym Phys 2004;42(23):4371–83. [48] Harton SE, Yang H, Koga T, Kumar SK. Glass transition behavior of polymer nanocomposites. PMSE Prepints 2007;97:791–2. [49] Yong-Kyun P, Sang-Hoon Y, Sungho P. Assembly of highly ordered nanoparticle monolayers at a water/hexane interface. Langmuir 2007;23(21):10505–10. [50] Zhang Q, Tsui OKC, Du B, Zhang F, Tang T, He T. Observation of inverted phase in poly(styrene-b-butadiene-styrene) triblock copolymer by solvent-induced order–disorder phase transition. Macromolecules 2000;33(26):9561–7. [51] Adams JL, Graessley WW, Register RA. Rheology and the microphase separation transition in styrene-isoprene block copolymers. Macromolecules 1994;27(21):6026–32. [52] Modi MA, Krishnamoorti R, Tse MF, Wang HF. Viscoelastic characterization of an order–order transition in a mixture of diand triblock copolymers. Macromolecules 1999;32(12):4088–97. [53] Yang H, Sa U, Kang M, Ryu HS, Ryu CY, Cho K. Near-surface morphology effect on tack behavior of poly(styrene-b-butadienestyrene) triblock copolymer/rosin films. Polymer 2006;47(11): 3889–95.
MACROMOLECULAR NANOTECHNOLOGY
C. Ocando et al. / European Polymer Journal 47 (2011) 1240–1249