Langmuir 1999, 15, 1283-1290
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Spherical Polymer Containers with a Fluid Polymer Core: Synthesis and Characterization of Film Formation by AFM C. Schellenberg,† S. Akari,‡ M. Regenbrecht,‡ K. Tauer,† F. M. Petrat,§ and M. Antonietti*,† Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Kantstrasse 55, D-14513 Teltow-Seehof, Germany Received June 15, 1998. In Final Form: November 24, 1998 Two-stage emulsion polymerization controlled with a reaction calorimeter results in core-shell structured latexes with homogeneous size distribution and architecture. We describe the synthesis of so-called “container particles”, consisting of a low-viscosity core with a low glass transition temperature (poly(2-ethylhexyl methacrylate), (PEtHMA)), covered with a thin shell of a cross-linked rubber (poly(n-butyl acrylate), (PBA)). Drying of these dispersions results in nanostructured films. A controlled topography and a network superstructure are obtained, which may be adjusted by the size, composition, and architecture of the original particles. Atomic force microscopy (AFM) in the tapping mode is used to study the final latex films. In addition to topographic information, it is possible to display, with a nanometer resolution, the amplitude and phase of response of the cantilever in each pixel, which images the remainder of the former core and shell by their different mechanical loss behavior. The degree of cross-linking of the second stage polymer (PBA) is found to be the major factor influencing the morphology of the polymer films formed. At lower cross-linking densities, even and surface mechanically homogeneous films are obtained. For highly crosslinked shells, it is shown by a combination of AFM modes that the containers collapse and release the low molecular weight liquid core to form a continuous film containing the single, collapsed units.
Introduction Polymer dispersions with well-designed composite particle morphology are, due to their improved performance, promising for a large number of industrial applications, for example, paper coating.1 The synthesis of these structured latexes, such as core-shell particles, is simple by concept but quite difficult in practice, particularly if one is interested in well-defined and homogeneous dispersion particles of a distinct type. It has been demonstrated that both thermodynamic and kinetic factors dictate the particle morphology.2,3 The influence of many process parameters controlling the particle morphology, such as particle surface polarity,4,5 mode of monomer addition,6 effect of polymer crosslinking7,8 and chain transfer agents,9 role of surfactant,10 * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. ‡ Rudower Chaussee 5, D-12489 Berlin-Adlershof, Germany. § Hu ¨ ls Infracor GmbH, Paul-Baumann Str. 1, D-45764 Marl, Germany. (1) Lee, D. I. In Polymeric Dipersions: Principles and Applications; Asua, J. M., Ed.; NATO ASI Series; Kluwer Academic Publishers: Norwell, MA, 1997; Vol. 335, p 497-513. (2) Sundberg, D.; Casassa, A. P.; Pantazopoulos, J.; Muscato, M. R. J. Appl. Polym. Sci. 1990, 41, 1429. (3) Chen, Y. C.; Dimonie, V. L.; El-Aasser, M. S. Macromolecules 1991, 24, 3779. (4) Dimonie, V. L.; El-Aasser, M. S.; Vanderhoff, J. W. Polym. Mater. Sci., Eng. 1988, 58, 821. (5) Cho, I.; Lee, K.-W. J. Appl. Polym. Sci. 1985, 30, 1903. (6) Okubo, M.; Yamada, A.; Matsumoto, T. J. Polym. Sci., Polym. Chem. 1980, 18, 3219. (7) Lee, S.; Rudin, A. In Polymer Latexes; Daniels, E. S., Sudol, E. D., El-Aasser, M. S. Eds.; ACS Symposium Series 492; American Chemical Society: Washington, DC, 1992; p 234. (8) Merkel, M. P.; Dimonie, V. L.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polymer Chem. 1987, 25, 1755. (9) Jo¨nsson, J.-E.; Hassander, H.; Jansson, L. H.; To¨rnel, B. Macromolecules 1994, 27, 1932. (10) Sundberg, D. C.; Cassasa, A. P.; Pantazopoulos, J.; Muscato, M. R.; Kronberg, B.; Berg, J. J. Appl. Polym. Sci. 1990, 41, 1425.
and influence of initiator,9 have already been investigated and were recently summarized.11 The preparation of core-shell particles is usually a twostage emulsion polymerization via seed latexes with a physical or time separation between the two stages. In the second-stage polymerization, different addition modes, batch (swelling method) and the continuous addition process (dropwise method), are usual.6 The structured latexes in this work were synthesized by such a core-shell emulsion polymerization. We adopt this technique for the synthesis of so-called “container” particles in which a rather oily polymer core is encapsulated by a cross-linked rubbery shell. The oily core is polymerized via a regulated first-stage batch polymerization and a semibatch second-stage polymerization including controlled swelling of primary particles in the same reactor. To ensure a near-complete conversion at both stages, the rates of polymerization and conversion were determined online in each reaction by measurements of the heat flow (reaction calorimetry), which has been proven to be a powerful instrumental tool under emulsion polymerization conditions also.9,12,13 It is shown that under these controlled conditions, narrowly distributed latexes with well-defined architectures are obtained. The container particles not only produce coatings with profitable mechanical properties but also are excellent model particles to characterize some of the physical effects occurring during film formation.14 The morphologies of core-shell latexes and their resulting films are classically characterized by transmis(11) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley and Sons: New York, 1997; Chapter 9. (12) Varela De La Rosa, L.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 461. (13) Moritz, H. U. In Polymer Reaction Engineering; Reichert, K. H., Geiseler, W., Eds.; VCH Verlag: Weinheim, 1989; p 248. (14) (a) Keddie, J. L. Mater. Sci. Eng. Rep. 1997, 21, 101. (b) Visschers, M.; Laven, J.; German, A. L. Prog. Org. Coat. 1997, 30, 39.
10.1021/la980694d CCC: $18.00 © 1999 American Chemical Society Published on Web 01/28/1999
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Table 1. Standard Emulsion Polymerization Recipe for Core-Shell Latex Preparation First Step SLSa water KPSb EtHMAc CBr4 temp time BAd 1,3-DIPBe feed rate temperature time
0.2 g 80 g 0.1 g 20 g 0.33 g 70 °C 120-150 min Second Step 20 g 7, 9, 11, 15 mol % of BA 0.25 mL/min 70 °C 240-270 min
a SLS, sodium lauryl sulfate. b KPS, potassium persulfate. EtHMA, 2-ethylhexyl methacrylate. d BA, n-butyl acrylate. e 1,3DIPB: 1,3-diisopropenyl benzene.
c
sion electron microscopy (TEM) or scanning electron microscopy (SEM).15 Today, it is also possible to use atomic force microscopy (AFM)16 to qualify the morphology of single latex particles17,18 and to study latex dispersion films.19,20 The method offers the advantage of working in a nonvacuum system and in an aqueous environment, i.e., conditions close to those being used in common technological applications of latex films. This paper reports on a new approach of characterizing the structure of dispersion films formed by container latex particles using AFM in the tapping mode. Here, determination of spatial variations of height (topographic mode) and interaction (deformation amplitude and phase shift) allows simultaneous imaging of surface topography and fine morphological features. For model systems with sufficient mechanical contrast (as the container particles), the location of core and shell polymer can be determined using this technique with a nanometer resolution, and the setup of structured films can be easily followed. Experimental Section Preparation of Core-Shell Latex Particles. n-Butyl acrylate (Ro¨hm AG) and 2-ethylhexyl methacrylate (Ro¨hm AG) were purified from inhibitors by passing the monomer through a column filled with aluminum oxide (Aldrich, active base). The purified monomers were kept at 8 °C before use. All other chemicals were of analytical grade and were used as received from the suppliers. Deionized water (18 MΩ cm-1) was received from a REWA HQ 5 system. Latex particles were prepared using a two-stage emulsion polymerization technique carried out in a calorimetric reactor (ChemiSens RM2-S, Lund, Sweden).21 Sodium lauryl sulfate (SLS) was used as surfactant and potassium persulfate (KPS) as initiator. The polymerization was performed as a semi-batch process with continuous addition of the secondstage monomer. In the first stage, the monomer EtHMA was polymerized with addition of 1 mol % CBr4 as chain transfer agent. The recipe for this reaction is summarized in Table 1. The reaction rate profile of this reaction was determined in an independent experiment in the calorimetric reactor. The heat flow curve of this polymerization reaction and the resulting timeconversion curve are exemplarily shown in Figure 1. (15) Shen, S.; El-Aasser, M. S.; Dimonie, V. L.; Vanderhoff, J. W.; Sudol, E. D. J. Polym. Sci., Polym. Chem. 1991, 29, 857. (16) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (17) Sommer, F.; Duc, T. M.; Pirri, R.; Meunier, G.; Quet, Ch. Langmuir 1995, 11, 440. (18) Butt, H.-J.; Gerharz, B. Langmuir 1995, 11, 4735. (19) Juhue, D.; Lang, J. Langmuir 1993, 9, 792. (20) Goh, M. C.; Juhue, D.; Leung, O. M.; Wang, Y.; Winnik, M. A. Langmuir 1993, 9, 1319. (21) Nilsson, H.; Silvegren, C.; Tornell, B. Angew. Makromol. Chem. 1985, 112, 125.
Figure 1. Heat flow curve of the polymerization of the firststage monomer and determination of the conversion by integration of the curve. The arrows mark a conversion of 85% where the second monomer is added. Knowledge of this reaction rate profile and the reaction heat flow curve allows for the start of the addition of the second-stage monomer after 80-90% conversion of the first monomer for coreshell latex preparation. The second-stage monomer butylacrylate/ cross-linker was added continuously with a feed rate of 0.25 mL/ min. For simplicity, only symmetric systems were examined, i.e., the weight ration of first-stage monomer to second-stage monomer was always 1:1. The second stage monomer contained various amounts of m-diisopropenylbenzene (1,3-DIPB) in the range of 7 to 15 mol % to give a cross-linked shell with variable rigidity and permeability. Instrumental Techniques. The particle size of the final latices were analyzed by dynamic light scattering (DLS, NICOMP model 370, Santa Barbara, CA, particle sizer with a fixed scattering angle of 90°). The GPC analysis was performed with a P1000 pump and a UV1000 detector (λ ) 260 nm), both from Thermo Separation Products, equipped with 5 µm, 8 × 300 mm SDV columns with 106, 105, 103 Å, from Polymer Standard Service, in THF using a flow rate of 1 mL/min at 30 °C. The molecular weights were calculated with a calibration relative to PS standards. For cryo-transmission electron microscopy (cryo-TEM), a small amout of the diluted latex suspension was placed onto a perforated carbon film, supported on 200 mesh electron microscope copper grids. The grid was ultrarapidly cooled in liquid ethane, which results in an amorphous water film. A cryo transfer system was used to transport the sample to the cryo-transmission electron microscope (Zeiss EM 902). In the microscope, the sample was held at a temperature of about 100 K. For characterization of the size distribution, a series of photos were taken and evaluated with a manual particle size analyzer (Zeiss TGZ-3 with computer interface). Transmission electron microscopy (TEM) photos of latex films were made from the pure container latex dispersions as well as from a mix of the PEtHMA/PBA latex together with a pure PBA latex. The films were cut with an ultramicrotome at a temperature of 150 K. The samples were transferred on a copper grid and stained with RuO4. m-Diisopropenylbenzene, which is used for cross-linking of the shell, acts as a probe for ruthenium and causes the contrast in the TEM pictures. The samples were introduced into the transmission electron microscope (Philips TEM 402), and photos were taken at suitable magnifications. The atomic force measurements were performed with a Nanoscope IIIa MultiMode scanning probe microscope (Digital Instruments, Inc., Santa Barbara, CA) with a phase extender module. For the tapping mode experiments, Si cantilevers (nanosensors) with spring constants of 31-50 N/m and resonance frequencies of 270-310 kHz were used. After the measurements, the tip radius, r, was determined to be
