Due to the unique combination of amphiphilicity combined with the particle character, Janus particles are expected to strongly adsorb and orient at the interface. The adsorption energy at the interface is significantly enhanced as compared to standard block copolymers, which are commonly used as stabilizers in polymer alloys. To the best of our knowledge, none of the theoretical studies has included Janus particles into their considerations. Note that Janus particles impose another significant enthalpic contribution to the free energy of the system. The interfacial tension between the sides and the respective polymers is negligibly small, whereas the interfacial tensions of the two sides with the unlike blend component is in the region of the interfacial tension of the mixture.
In order to apply this new concept and to study the effect of the Janus particles on the compatibilization of polymer blends, we prepared PS/PMMA blends at weight ratios of 6/4 and 8/2 with various amounts of Janus particles. To allow a meaningful estimation of the performance of the bead-like stabilizer, several polymer blends using linear block copolymers were prepared for comparison.
In a first step, the weight fraction of Janus particles was changed from one to 20 wt% to allow a monitoring of the morphological changes. The mixing parameters like temperature, shear rate and mixing period were kept constant for all mixtures to minimize the experimental errors during the blending and allowing a reliable comparison. Note that the statistical analysis of polymer blends with 8/2 blending ratio leads to more accurate values, as a thermal relaxation of the droplets after blending can be done in order to reduce the shear-induced asymmetry of the dispersed droplets. In case of a blend ratio of 6/4 this is not possible, because the morphology may relax into a co-continuous one after prolonged annealing37, for which a straightforward droplet analysis cannot be applied anymore. The symmetric blend ratio was additionally chosen though for its significance in terms of tailoring mechanical properties or e.g. in polymer solar cells, which would in the best sense require co-continuous morphologies. Furthermore, it was theoretically shown that for different mixing ratios of polymers, the compatibilization efficiency of a given particle can be different.14
The SEM and TEM images shown in Figure 8 - 2 give a first overview of the development of the domain sizes of the dispersed phase. The SEM images (PS/PMMA = 6/4) are shown at the same magnification to allow a straightforward overview of the changes occurring with increasing Janus particle content. The magnification of the TEM images (PS/PMMA = 8/2) is changed progressively to allow a good display of the domain sizes. Clearly, the PMMA domains, appearing as dips or white domains in the PS matrix in the SEM and TEM images, respectively, show a distinct and rapid decrease in size. Initially, the number-average domain diameter, Dn, of the reference sample without addition of any stabilizer is in the micrometer range (Dn = 1.8 m for 6/4 and Dn = 1.1 m for 8/2). Upon addition of only one percent of Janus particles, a rapid decrease of the domain diameter to an average PMMA domain diameter of Dn = 1.2 m (6/4) and Dn = 710 nm (8/2) can be observed. This decay steadily continues with increasing amount of compatibilizer and truly nanoscopically sized domains can be obtained. With a further increase of the Janus bead content, a continuous decreases of the domain sizes even below Dn = 140 nm (6/4) and Dn = 80 nm (8/2) for 20 wt% Janus particle content can be observed. Importantly, this
VIII-8 Engineering Nanostructured Polymer Blends with Janus Particles nanostructuring of the polymer blend can be achieved without special high-shear processing units or complex chemical reactions within the blend mixture. Moreover, no optimization of the blending conditions was done, demonstrating furthermore the efficiency of the stabilizer used.
Figure 8 - 2. SEM images obtained for blends at a PS/PMMA ratio of 6/4 (top row). Non-stained TEM images obtained for blends at a PS/PMMA ratio of 8/2 (centre and lower row). The amount of compatibilizer is indicated in each image.
The decay of the number-average domain diameters of the PMMA droplets (Dn,PMMA), is shown in Figure 8 - 3.
A strong and strikingly rapid decay of the PMMA domain sizes can be observed. The decrease is yet ongoing for high concentrations, indicating an only insignificant loss of stabilizer in one of the blend components. The rapid decay itself is an indication for the presence of a compatibilizer of very high interfacial activity.
Engineering Nanostructured Polymer Blends with Janus Particles VIII-9
Figure 8 - 3. Decrease of the number-average PMMA domain diameter as a function of the content of Janus particles at PS/PMMA ratios of 6/4 (a) and 8/2 (b). The vertical bars show the standard deviation of the size distribution of the PMMA domains.
In conclusion of the observed decrease of the domain diameter with the increased addition of Janus particles, it is evident that the Janus particles act as stabilizers at the interface of the PS/PMMA blends. Figure 8 - 4 shows a representative overview of two selected TEM images at higher magnification for two different Janus particle contents and two different blending ratios of PS and PMMA. In both images, small black dots can be observed at the interface of the PMMA and PS phases. These small black particles can be identified, in terms of their size, as the cores of the Janus particles. Those cores consist of polybutadiene, which was crosslinked with S2Cl2, thus explaining the contrast in TEM. Furthermore, both domains, PS and PMMA, appear homogeneous and hardly any single or aggregated Janus particles within one phase can be found. Note that Janus particles can be clearly identified within the PS or PMMA phase (see e.g.
Figure 8 - 1).
Figure 8 - 4. TEM images obtained for (a) 10 wt% JP in a 8-2 PS/PMMA blend and for (b) 20 wt% JP in a 6/4 PS/PMMA blend.
VIII-10 Engineering Nanostructured Polymer Blends with Janus Particles These observations give a first explanation of the excellent performance of Janus particles as stabilizers in polymer blends. Obviously, the Janus particles are almost exclusively located at the interface and only a negligibly small fraction is “lost” as unimers or micellar aggregates in one of the components, also representing a significant improvement as compared to block copolymer compatibilizers.3-6 This is a direct consequence of the high interfacial activity and increased adsorption energy at interfaces of Janus particles.
The strong adsorption of Janus particles can be elucidated with an energetic consideration of the system. The surface activity of a particle can be expressed in terms of the desorption energy of the particle from an interface. Binks et al. derived an expression for the calculation of the desorption energy of an amphiphilic particle, adsorbed at an interface of two immiscible liquids, based on its surface free energy, E().7
for ≤ (I)
The surface free energy (equation (I)) depends on the immersion depth of the particle at the interface, , the asymmetry of the biphasic particle, , and the radius, R, of the particle. A particle is completely immersed into one of the phases for = 0o and = 180oand adsorbed along its equator for = 90o. For completely symmetric hemispheres, approaches 90o. Furthermore, the equation depends on the various interfacial tensions between the PMMA (MJP) and PS (SJP) side of the Janus particle with the two bulk phases PS (S) and PMMA (M) ((SJPS),(MJPM),(MJPS)), as well as on the interfacial tension of the blend components ((MS)).
Note that this equation is valid under the condition that the Janus particles are sufficiently smaller than the stabilized domains, allowing the assumption of quasi-planar interfaces. The desorption energy, which is a measure for the interfacial activity of a particle, can be calculated from the energy which is necessary to transfer a particle from the interface to one of the bulk phases (EPS - Einterface orEPMMA - Einterface). Assuming negligible interfacial tensions ((SJPS),(MJPM)) for the sides of the Janus particle with their respective blend phases and an equatorial adsorption ( = 90o) of the indeed completely symmetric Janus particle ( = 90o), the desorption energy for a Janus particle (EJP,des) reads as:
(II)
Similarly, Pieranski derived an expression for the desorption energy of a homogeneous particle, P,7, 38 ( = 0o, Ehom,des). Herein (PS), (PM) and (SM) are the interfacial tensions of the particle with PS and PMMA, and between the two polymers (PS & PMMA), respectively.
(III)
The maximum of the desorption energy, Ehom,des, can be reached if │(PS)-(PM)│<<(SM) and reads as πR2(SM).
Clearly, these considerations show the stronger adsorption of the Janus particles as compared to homogeneous particles of similar size. The desorption energy of an ideal Janus particle is three
Engineering Nanostructured Polymer Blends with Janus Particles VIII-11 times higher than for a standard particle. This gap widens for asymmetrically (non-equatorially) adsorbed homogeneous particles, which are preferentially wetted by one blend component. In particular for small nanoparticles and higher temperatures, the energetic difference between biphasic and normal particles matters most. Note that the probability of the displacement of a particle from the interface scales with p ̴ exp(-E/kT). Therefore, the probability of a Janus particle to desorb from the interface is lower by a factor of e3 ≈ 20. Consequently, the biphasic nature of the Janus particles is the origin of a crucial additional energetic contribution and is the reason for a complete adsorption in combination with a firm attachment of the particles at the interface.
In terms of a general applicability of this approach for other polymer blend systems, the Flory-Huggins interaction parameter, , can be used rationalize the effect of changing the polymer blend components. This interaction parameter is related to the interfacial tension, , of a binary blend system via ̴ 1/2.39, 40 Thus, the Flory-Huggins parameter can be directly related to the desorption energies in equations (II) and (III). Following these considerations, the adsorption is favored and simplified for polymer systems with larger unfavorable interaction, . Herein, we used a PS/PMMA polymer blend model system with a relatively low incompatibility ((SM) ≈ 0.036 at 210 oC(SM) ≈ 1.12 mN/m at 210 oC42). Note that a complete particle adsorption at the interface could even be achieved for such a system with relatively low incompatibility for which the location of the particles at the interface is less favored. Most polymer blend systems show a higher incompatibility and would thus lead to a strengthened adsorption at the interface.
In this context, a comparison with previous results in literature is appropriate. Kramer and co-workers29-35 could show a control of the location of homogeneous particles and mixed (possibly biphasic) particles in block copolymers in one of the phases or predominantly at the interface, respectively. However, they required extremely mild film casting processes in their preparation of the composite materials, which is thus not representative of the conditions used here.
Concerning high shear blending, this is the first time that Janus particles are used and so any comparison with literature can be made. For homogeneous particles, it is known that the exclusive location of particles at the interface remains challenging. Often only a fraction of the particles can be located at the interface or there is an incomplete coverage of the interface.43, 44 Therefore, the order at the interface is low as well and a significant fraction of particles is “lost”
in one of the phases and may contaminate the properties of a nanocomposite material. For instance, Vermont et al.44 demonstrated that they could obtain some compatibilization of polyisobutylene /polydimethylsiloxane (PIB/PDMS) blends with silica particles. However, the particles were also located in both phases, particularly in the PDMS phase, thus altering the properties of all phases and resulting in ill-defined nanocomposites. Additionally, the particles did not exhibit any order at the interface. An inversion of the weight fractions of the polymer blend resulted in an unsatisfying compatibilization, demonstrating the need of tailoring the surface chemistry of the particles. Some further success was recently reported by Chung et al.45, who showed that nanoparticles with specifically tailored surface chemistry assembled to a large extent at a polymer blend interface after a soft annealing step in a thin film. After casting of the film, the particles were located in all components. Whether the interfacial adsorption would persist during high shear blending could not be shown, but is at least uncertain.
VIII-12 Engineering Nanostructured Polymer Blends with Janus Particles Aside the strong adsorption of the Janus particles, even an ordering of the particles could be observed. Due to the yet finite thickness of 70 – 90 nm of the ultrathin sections used for the TEM investigations, several circular endcaps of the PMMA droplets can be identified at closer observation of the TEM images. Two of those caps are shown in Figure 8 - 5.
The section analysis in Figure 8 - a shows a core diameter of around 8 – 10 nm, which confirms the black points as the diameter of individual Janus particles. The particles adsorb strongly at the interfaces, but are yet separated from each other by the repellence of their polymer brush-like corona. Both TEM images exhibit an ordered arrangement of the Janus particles (dark dots) at the interface. All Janus particles are separated from each other by a similar centre-to-centre distance of around 30 – 35 nm and are closely packed. The efficient repellence of the Janus particles along the PS/PMMA interface also goes along with a strong reduction of the coalescence of Janus particle stabilized PMMA droplets during the blending process. The exhibited order at the interface is a very interesting observation that would allow a lateral two-dimensional nanoscopic structuring of the interface
These results additionally demonstrate that desired functionalities based on modified Janus particles (e.g. bimetallic Janus particles46) could be precisely and exclusively located at the blend interface even under the strong shear and high temperature conditions of the twin-screw mini-mixer. In a next step, this implies that particles can be selectively located in the two phases or at the interface, just depending on the fact whether they are homogeneous in their corona or phase-segregated into two hemispheres. This offers a high control for engineering nanocomposite polymer blends of desired compositions and properties.
Figure 8 - 5. Endcaps of PMMA droplets with Janus particles located at the interface. (a) and (c) are representative examples at 10 wt% and 6 wt% of JP content in a 6/4 PS/PMMA blend. (b) Section analysis of the red bar shown in image (a). One side of a Janus particle is schematically sketched to show its brush-like character and the location of the polybutadiene core.
Engineering Nanostructured Polymer Blends with Janus Particles VIII-13