ARTICLE pubs.acs.org/Langmuir
Electrostatic Contributions in the Increased Compatibility of Polymer Blends Elis^angela M. Linares,† Sergio A. V. Jannuzzi,† and Fernando Galembeck*,†,‡ † ‡
Institute of Chemistry, University of Campinas, P.O. Box 6154, 13083-970 Campinas, S~ao Paulo, Brazil National Nanotechnology Laboratory/CNPEM, P.O. Box 9192, 13083-970 Campinas, S~ao Paulo, Brazil
bS Supporting Information ABSTRACT: Successful blending of different polymers to make a structural or functional material requires overcoming limitations due to immiscibility and/or incompatibility that arise from large polymerpolymer interfacial tensions. In the case of latex blends, the combination of capillary adhesion during the blended dispersion drying stage with electrostatic adhesion in the final product is an effective strategy to avoid these limitations, which has been extended to a number of polymer blends and composites. This work shows that adhesion of polymer domains in blends made with natural rubber and synthetic latexes is enhanced by electrostatic adhesion that is in turn enhanced by ion migration, according to the results from scanning electric potential microscopy. The additional attractive force between domains improves blend stability and mechanical properties, broadening the possibilities and scope of latex blends, in consonance with the “green chemistry” paradigm. This novel approach based on electrostatic adhesion can be easily extended to multicomponent systems, including nonpolymers.
’ INTRODUCTION Miscibility and compatibility are essential issues in the development of polymer blends since they determine the blend final properties. Polymer pairs are generally immiscible due to the positive mixing enthalpy together with negligible entropy contribution to the mixing free energy. Moreover, high interfacial tension leads to low interfacial area, low domain adhesion, and finally poor mechanical properties.1 Many efforts have been done to overcome compatibility problems in polymer blends since these are very attractive for making new materials. 2 Several blending strategies were developed based on the control of chemical and physical interactions between blend components.37 Latex blends are especially interesting because the interfacial properties can be controlled in many different ways, depending on the dispersant and stabilizers used in the blended latex dispersions. Blends prepared from latex have some peculiar advantages often allowing fine dispersion and adhesion between the phases. The presence of surfactants in immiscible blend interfaces may decrease the interfacial tension between polymers, thus reducing the coarsening rate of the domain structures.8,9 Electrostatic adhesion is another factor which has not yet been exploited in polymer blends, but it makes an important contribution to joining different phases in polymer nanocomposites.10,11 Latex particles generally contain charges originated from water-soluble initiator residues and acidic or basic comonomers, as well as those acquired by adsorption of surfactants and polyelectrolytes.12,13 Ion distribution in particles and its effects on latex film morphology have been previously explored1418 r 2011 American Chemical Society
using scanning electric potential microscopy (SEPM)19,20 and electron spectroscopy imaging in transmission electron microscopy (ESI-TEM),21,22 which evidenced the location of buried charges within latex particles. These results showed that even if the dry latex particles are electroneutral overall, ions of the same charge signal are clustered in domains extending for tens or hundreds of nanometers. In addition, Velegol et al. described the charge heterogeneity of surfaces on polystyrene latex spheres, using electrophoresis of spherical particles to measure the random distribution of the ζ potential.23,24 Nonuniformity and its effect on surface forces of bicompositional (zwitterionic, bipolar) particles were also investigated by Drelich et al., analyzing diffuse-layer charge mapping by the atomic force microscopy (AFM) technique.25 The assumption that polymer dielectrics are electrically neutral has been challenged on the basis of electret formation.2628 McCarty and Whitesides29,30 showed that materials with covalently bound ions and mobile counterions are charged up upon contact with other materials, probably due to ion transfer at the interface. Very recently, Gouveia and Galembeck31 showed that water adsorption and desorption change the state of electrostatic charging of particulate solids. Pinchuk et al. showed the formation of a nonequilibrium structure in blend compositions induced by generation of free charge carriers and their entrapment during extrusion of polyamidepolyethylene blends.32 Received: August 1, 2011 Revised: October 17, 2011 Published: October 18, 2011 15199
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Figure 1. Electric potential on the cut sample surfaces. Topography (a, c, e) and electric potential images (b, d, f) of the cast surface of the natural rubber film (a, b) and poly(styrenebutyl acrylate) film (c, d). Topography and electric potential images (e, f) of the interface between NR and P(SBA) films that were cast separately. Histograms (g) are shown for all electric potential images. Line profile (h) of the cut surface of the interface of NR and P(SBA) films, which were dried separately and juxtaposed.
The present work shows that ion transfer can also occur between polymers containing ionic species during latex blend formation, and electric potential gradients thus created contribute to the overall latex film properties. A model is presented to explain ion partition within blend domains.
’ EXPERIMENTAL DETAILS Poly(styrenebutyl acrylate) latex Acronal 295 D (P(SBA); Tg = 23 ( 1 °C, ζ = 80 ( 2 mV) and poly(vinyl chloride) latex Norvil L66GA (PVC; Tg = 85 ( 1 °C, ζ = 24 ( 3 mV) are commercial resins supplied by BASF (S~ao Paulo, Brazil) and Braskem (Maceio, Brazil), respectively. Natural rubber latex (NR; Tg = 62 ( 1 °C, ζ = 80 ( 4 mV) was supplied by Talism~a (Mirassol, Brazil). Latexes were mixed in
different proportions [NR/P(SBA) 8:2, 7:3, 6:4, and 5:5 (wt %) and NR/PVC 7:3 (wt %)] at room temperature, diluted to 30 wt % solids content, stirred for 30 min, cast on flat polyethylene molds, and air-dried at 60 °C for 24 h. Specimens for tensile testing were cut from the samples prepared by casting, following the DIN 52504 standard. Prior to testing, specimens were kept at 23 ( 2 °C and 50 ( 5% relative humidity for 48 h. Measurements were performed using an EMIC DL 2000 universal testing machine under a strain rate of 200 mm min1. Results are averages for eight test specimens. SEPM analyses were performed using a Discoverer TMX 2010 microscope from Topometrix, operating in noncontact mode and using a Pt-coated silicon tip. The microscope setup yields topographical and electric potential images from the same area and simultaneously. 15200
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Figure 2. Electric potential images of cut surfaces of the NR/P(SBA) blends. Topography (a, c) and electric potential (b, d) images of the cut surface of natural rubber/poly(styrenebutyl acrylate) blends prepared by casting in different proportions: 7:3 (wt %) (a, b) and 5:5 (wt %) (c, d). Electric potential histograms (e) of images b and d and line profiles (f) of images a and b. SEPM provides information on the electrostatic field present on the sample surface. The electrostatic force, Fel, between the probe and sample surface, without external potential, is given by Fel ðdÞ ¼ ð1=4πεo Þ
N
N
∑ ∑ qi qj =jri þ rj j2 j¼1 i¼1
SEPM provides information on the electrostatic field adjacent to the sample surface. The principle of measuring the potential difference between the probe and the sample is analogous to that of the vibrating capacitor method, or Kelvin method. It is based on the distance of oscillation between two parallel plates, at frequency ω, resulting in the current i(t) given by iðtÞ ¼ Vpc ωΔC cosðωtÞ where Vpc is the potential difference at the contact between two plates and ΔC is the capacitance variation. To determine the Vpc value, a potential is applied between the plates to cancel the electric field when the current i(t) becomes zero. In SEPM, the electrostatic force is analyzed instead of the current: for each pixel, an additional voltage is applied between the probe and the sample until the electric field and the electrostatic force are nil. The applied voltage equals the surface potential.33 Image acquisition is done as follows: a standard AFM system is used to acquire topography images in noncontact mode, while a second oscillator feeds an ac signal to the probe, a few kilohertz below the frequency of the first oscillator, which is adjusted to the natural frequency of the probe (between 40 and 70 kHz). The metal-coated probe vibrates mechanically during a free oscillation at the frequency of the first oscillator, and it is fed an ac at the frequency of the second
oscillator. The photodetector signal, containing all this information, is separated into the original frequencies by using two lock-in amplifiers. Feedback to the second oscillator changes the potential on the probe surface and shifts the component of the mechanical vibration at the electrical signal frequency, ac. Then a dc signal is added to the ac signal to cancel the potential difference between the probe and the surface, keeping the phase constant at the ac frequency. Plotting the dc signal at each pixel provides the electric potential micrograph. Images were also acquired using a Carl Zeiss CEM-902 transmission electron microscope equipped with a CastaingHenryOttensmeyer filter spectrometer. Energy-filtered transmission electron microscopy (EFTEM) was used to obtain elemental maps imaging inelastic scattered electrons. Energy losses were 532 eV (O), 165 eV (S), and 132 eV (P). The images were recorded using a Proscan high-speed slow scan digitized (10241024 pixels, 8 bits) with a charge-coupled device (CCD) camera and an iTEM universal TEM imaging platform. Samples for microscopy were prepared using a Leica EM FC6 cryoultramicrotome. Ultrathin (ca. 60 nm) sections for TEM analysis were cut at 150 °C with a diamond knife (Drukker). A drop of supersaturated sucrose solution was used to collect the thin cuts from the cooled microtome and to transfer them to microscope grids. After that, the grids were left floating in deionized water in a beaker for 510 min to wash the sucrose off. They were then removed and dried at room temperature. For the cut surface analysis by SEPM, pure polymer and blend films were cut at 150 °C with a glass knife and the trimmed surfaces were imaged. Blends were cut normal to the casting plane. Droplets of PVC latex and NR/PVC blend dispersion were deposited on freshly cleaved mica sheets and allowed to dry at room temperature. The dry surfaces were analyzed by SEPM. 15201
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A computational model was developed to simulate the effects of charge migration between two different material domains on the electric potential energy of the system, as described elsewhere.10 MATLAB 6.5 was used to create a virtual object and execute the calculation. The detailed procedure is explained in the Supporting Information.
’ RESULTS AND DISCUSSION Electric Charge Distribution in Isolated Latex Polymer Films and Their Blends. AFM and SEPM images from the cut
surface of NR and P(SBA) cast films are presented in Figure 1. Surfaces were cut and trimmed in a cryo-ultramicrotome to minimize roughness. Natural rubber forms partially coalesced films containing a few small holes which likely derive from microbubbles trapped during film casting, while the P(SBA) cut surface is smoother than the NR surface. SEPM images from the same areas show domains with different electrostatic potentials, evidencing charge segregation within the films. The electric potential ranges from 1.0 to +4.5 V and from 0.7 to +5.1 V in NR and P(SBA), respectively, but following different patterns: natural rubber shows prevalence of positive domains, while the styreneacrylic copolymer is mostly negative (between 0.7 and 0 V) with a few positive spikes. Charge segregation was previously reported for latex films and many types of particles,18,19,21,34 and it has been assigned to chemical heterogeneity, immiscible catalyst residues, oxidized chains, and contaminants introduced during fabrication.35,36 In addition, dielectric materials composed of two or more phases with different dielectric constants present interfacial polarization according to the Maxwell WagnerSillars theory.37,38 Parts e and f of Figure 1 show a cut surface normal to the interface of superimposed films of natural rubber and P(SBA) cast and dried separately, where both films are clearly distinguished. The natural rubber film on the right-hand side of the image is rougher than the P(SBA) film, the same as observed in the images of the separate polymers (Figure 1a,c). The SEPM image shows an electric potential difference between the juxtaposed polymers due to charge imbalance intrinsic to each polymer. The line profile and histograms in Figure 1g,h show a marked electric potential contrast only at the interface. Thus, SEPM images show unmistakably that the films are not electrically neutral, challenging the conventional assumption that bulk materials always have equal numbers of opposite charges. Moreover, charge imbalance is different in each material, and the charge mobility is low. Blend films of natural rubber and P(SBA) latexes were prepared by casting and analyzed by SEPM (Figure 2). Two phases are clearly distinguished with a large contact surface that is strongly interconnected, forming extended patches. The higher domains in the topography images are from natural rubber, and their height is due to the larger thermal dilation coefficient,39 which causes greater expansion of rubber as compared to P(SBA) when the sample is heated from the cutting temperature (123 K) to room temperature. The size of the P(SBA) domains increases as the content of this polymer is increased, as expected. SEPM images show a significant electric potential difference between domains across the blend bulk, in excess of 6 V, displaying positive domains dispersed in the negative matrix. This potential difference between the nanometric polymer domains is in turn due to a small charge imbalance. The line profiles (Figure 2f) show that the natural rubber phase corresponds to the negatively charged domains in the
Figure 3. Bright-field image and elemental maps of a thin cut of the NR/P(SBA) blend. Bright-field (BF) image of a thin cut of NR/ P(S-BA) 7:3 (wt %) and oxygen, sulfur, and phosphorus elemental maps of the same area. The higher concentrations of S and P allow the identification of natural rubber as the matrix, while the dispersed domains are made of P(SBA).
SEPM image. This is according to the electric potential histogram (black line), which reveals that negative domains are more frequent than positive domains in the NR/P(SBA) 7:3 blend, as expected due to the higher proportion of natural rubber in this blend. It is remarkable that electric potential contrast is opposite that shown by polymer films cast separately and shown in Figure 1. The natural rubber film is predominantly positive, while its domains in the blend film are slightly negatively charged. To confirm the assignment of the blend domains, a TEM bright-field image and elemental maps of a thin cut of the NR/ P(SBA) 7:3 blend were also acquired and are shown in Figure 3. The bright-field micrograph shows a brighter phase dispersed in the gray matrix that is assigned to P(SBA) dispersed in natural rubber, considering the sulfur, phosphorus, and oxygen EFTEM maps. S and P are found in protein and phospholipids from natural rubber, while oxygen is rather abundant in the dispersed domains, which is expected for the P(SBA) phase due to the acrylic monomer. Thus, the elemental maps confirm that dispersed domains are from the P(SBA) phase while the matrix is made out of natural rubber. Electric contrast inversion between juxtaposed films (Figure 1f) and the blend film (Figure 2b,d) can be understood considering the occurrence of ion partition between the phases during blend film formation. P and S shown in elemental maps from natural rubber derive from ionic groups, such as sulfates and deprotonated phospholipids.39 Cations (Ca2+, Na+, K+, and others) are mobile, while most anions are attached to the polymer chains and their diffusion is thus hindered.4042 Cation migration from the rubber toward the P(SBA) domains is thus the suggested mechanism responsible for the change in the electric potential contrast between bare (Figure 1) and blended (Figure 2) polymers. The charge migration is pronounced during blend casting from aqueous dispersion, but not in the dry films, due to the large difference in ion mobility. 15202
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Figure 4. Electric potential images of the PVC and NR/PVC blend surfaces. Topography (a, c) and electric potential (b, d) images of the surfaces of PVC (a, b) and NR/PVC 7:3 (wt %) blend (c, d) films. PVC particles are more positive in the natural rubber blend than in the PVC bare film, evidencing positive charge migration toward PVC particles. The 3-D image (e) and line profile (f) of the indicated area on the SEPM image (d) shows charge depletion at the interface of natural rubber and PVC particles.
Poly(vinyl chloride) and natural rubber/poly(vinyl chloride) 7:3 (wt %) blend films were also analyzed using SEPM, and the images are shown in Figure 4. PVC particles are quasi-spherical with a lower electric potential than the interstitial domains. Topography and SEPM images from the NR/PVC blend show PVC particles well dispersed in the natural rubber matrix, forming elongated aggregates together with isolated particles. The electric potential map indicates a large difference between the two phases, in excess of 18 V. Furthermore, the 3-D electric potential map and line profile under higher magnification show pronounced positive charge depletion on rubber, close to the interface, suggesting that mobile cations migrated toward PVC particles. Effect of Ion Partition on the Electric Potential Energy of the Polymer Blends. Cation migration from natural rubber to P(SBA) during blend formation is expected for two reasons. First, cations in these polymers are much more mobile than anions that are bound to the polymer chains. Second, the styreneacrylate polymer has a higher dielectric constant (2.75) than natural rubber (2.37), which contributes to lowering the overall electrostatic energy for a given excess cation concentration. This is shown by the results of electric potential energy calculation in the model depicted in Figure 5. In this calculation, monovalent cations from natural rubber were transferred stepwise to the P(SBA) phase and the electric potential energy was determined before and after migration. The detailed calculation procedure is described in the Supporting Information.
The calculated electrostatic energy lowering may reach 100 kJ (mol of ions1) considering two effects: (i) dilution of charge excess throughout a larger volume and (ii) migration toward a phase with higher dielectric constant, according to Figure 5. However, calculation results do not show the experimentally observed charge inversion that probably arises from specific interactions between cations released from natural rubber and anionic groups in the synthetic latex particles. This requires further refinement of the present model that is beyond the scope of the present work. Mechanical Properties of the Blends. Mechanical properties for both latex blends were measured and are given in Table 1. Even though the blends are immiscible (see the glass transition temperatures in the Supporting Information), their Young modulus and the tensile strength increase relative to those of the natural rubber matrix for the NR/PVC blend containing 20% PVC. NR/P(SBA) blends also show a prominent increase of the Young modulus and tensile strength when compared to the NR matrix. This behavior is not often observed in immiscible polymer blends, and it suggests high compatibility between the two polymer phases. In general, domains of immiscible polymer blends are round-shaped to minimize the interfacial area. This is not the case for NR/P(SBA) blends shown in Figure 2, where elongated domains with meandering interfaces prevail. The electric potential differences between the phases allow us to conclude that this unexpected phase compatibility and the 15203
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Figure 5. Electrical potential energy variation as a function of the cation migration fraction from NR to P(SBA). Energy stabilization caused by cation migration to the phase with higher dielectric constant. Schematic representation of the virtual object used to calculate the electrical potential energy variation from the system after cation migration, showing clearly the electric charge dilution between the phases.
Table 1. Mechanical Properties of Natural Rubber and NR/ P(SBA) Blends Prepared by Casting sample
tensile strength
elongation
modulus
(MPa)
(%)
(MPa)
830 ( 168
1.0 ( 0.5
NR
1.2 ( 0.3
P(SBA)a
NA
NA
NA
NR/P(SBA) 8:2 (wt %) NR/P(SBA) 7:3 (wt %)
0.73 ( 0.01 2.0 ( 0.1
390 ( 31 573 ( 43
6.2 ( 0.6 18 ( 2
NR/P(SBA) 6:4 (wt %)
3.0 ( 0.3
656 ( 46
20 ( 3
NR/P(SBA) 5:5 (wt %)
4.3 ( 0.1
515 ( 25
64 ( 7
NR/PVC 9:1 (wt %)
1.3 ( 0.3
714 ( 87
1.3 ( 0.1
NR/PVC 8:2 (wt %)
2.7 ( 0.5
880 ( 78
1.9 ( 0.5
a
Mechanical properties for P(SBA) were not obtained due to the difficulty of obtaining a uniform P(SBA) film using this latex.
associated low interfacial tension are due to attractive interactions between opposite charges in the NR and P(SBA) domains. Interfacial tension in immiscible polymer blends is related to the intermolecular forces existent across the interfaces. If electrostatic interactions are included in the definition of surface (and interfacial) tension of electrified phases,43 they contribute to lower tension. As a consequence, adhesion44 between oppositely charged phases in the immiscible polymer
blend is enhanced, enabling mechanical stress transmission, spreading, and dissipation within the material. The final result is the improved mechanical performance of the polymer blends.
’ CONCLUSION Latex blends have large nonelectroneutral domains heterogeneously dispersed in the matrix, and the charge density of each domain depends on the actual blend. The difference in electric charge excess between coexisting phases in a polymer material provides an additional attractive force between domains, which reduces the interfacial tension, increases compatibility, and improves the bulk mechanical properties. ’ ASSOCIATED CONTENT
bS
Supporting Information. Glass transition temperatures of the polymers and blends and a description of the calculation procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +55-19-3521-3080. Fax: +55-19-3521-2906. E-mail:
[email protected]. 15204
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’ ACKNOWLEDGMENT E.M.L. and S.A.V.J. acknowledge fellowships from Fundac-~ao de Amparo a Pesquisa do Estado de S~ao Paulo (Fapesp) and Conselho Nacional de Desenvolvimento Científico e Tecnologico (CNPq), respectively. This is a contribution from the Millennium Institute for Complex Materials, Programa de Apoio ao Desenvolvimento Científico a Tecnologico (PADCT)/ CNPq, and Instituto Nacional de Ci^encia e Tecnologia em Materiais Complexos Funcionais (INOMAT), Instituto Nacional de Ci^encia e Tecnologia (INCT) for Complex Functional Materials, supported by the Programa dos Institutos Nacionais de Ci^encia e Tecnologia (Ministerio da Ci^encia e Tecnologia (MCT)/CNPq and Fapesp, Brazil). ’ REFERENCES (1) Paul, D. R.; Newman, S. Polymer Blends, 1st ed.; Academic Press: London, 1978. (2) Ryan, J. R. Nat. Mater. 2002, 1, 8. (3) Coleman, M. M.; Painter, P. C.; Graf, J. F. Specific Interactions and the Miscibility of Polymer Blends; Technomic Publishing Co.: Lancaster, PA, 1991. (4) Zhu, S.; Rafailovich, M. H.; Sokolov, J.; Gersappe, D.; Winesett, D. A.; Ade, H. Nature 1999, 400, 49. (5) Pernot, H.; Baumert, M.; Court, F.; Leibler, L. Nat. Mater. 2002, 1, 54. (6) Alice, N. C. W.; Lindway, M. J.; MacKnight, W. J. Macromolecules 1994, 27, 3027. (7) Pham, H. H.; Winnik, M. A. Macromolecules 2006, 39, 1425. (8) Laradji, M.; Guo, H.; Grant, M.; Zuckermann, M. J. Phys.: Condens. Matter 1992, 4, 6715. (9) Kawakatsu, T.; Kawasaki, K.; Furusaka, M.; Okabayashi, H.; Kanaya, T. J. Chem. Phys. 1993, 99, 8200. (10) Valadares, L. F.; Linares, E. M.; Braganc-a, F. C.; Galembeck, F. J. Phys. Chem. C 2008, 112, 8534. (11) Braganc-a, F. C.; Valadares, L. F.; Leite, C. A. P.; Galembeck, F. Chem. Mater. 2007, 19, 3334. (12) Amalvy, J. I.; Asua, J. M.; Leite, C. A. P.; Galembeck, F. Polymer 2001, 42, 2479. (13) Schumacher, H. C.; Alves, M.; Leite, C. A. P.; Santos, J. P.; Teixeira-Neto, E.; Murakami, M. M.; Galembeck, F.; Amaral, M. do. J. Colloid Interface Sci. 2007, 305, 256. (14) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1998, 14, 3187. (15) Keslarek, A. J.; Costa, C. A. R.; Galembeck, F. J. J. Braz. Chem. Soc. 2004, 15, 66. (16) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447. (17) Tang, J.; Dimonie, V. L.; Daniels, E. S.; Klein, A.; El-Aasser, M. S. J. Appl. Polym. Sci. 2000, 77, 644. (18) Kientz, E; Holl, Y. Colloids Surf., A 1993, 78, 255. (19) Teixeira-Neto, E.; Galembeck, F. Colloids Surf., A. 2002, 207, 147. (20) Santos, J. P.; Corpart, P.; Wong, K.; Galembeck, F. Langmuir 2004, 20, 10576. (21) Linares, E. M.; Valadares, L. F.; Silva, C. A.; Rezende, C.; Leite, C. A. P.; Galembeck, F. Anal. Chem. 2009, 81, 2317. (22) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Colloids Surf., A 2001, 181, 49. (23) Jones, J. F.; Holtzer, G. L.; Snyder, C.; Yake, A. M.; Velegol, D. Colloids Surf., A 2005, 267, 79. (24) Velegol, D.; Feick, J. D; Collins, L. C. J. Colloid Interface Sci. 2000, 230, 114. (25) Drelich, J.; Wang, Y. U. Adv. Colloid Interface Sci. 2011, 165, 91. (26) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763. (27) Partington, J. R.; Planer, G. V.; Boswell, I. I. Nature 1946, 158, 835.
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