Polymer Colloids - ACS Publications - American Chemical Society

Chapter 19

New Core-Shell Dispersions with Reactive Groups 1

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Hans-Jürgen P. Adler , Andrij Pich , Axel Henke , Carsten Puschke , and Stanislav Voronov 2

Downloaded by UNIV QUEENSLAND on June 15, 2014 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch019

1

Dresden University of Technology, Institute for Macromolecular Chemistry and Textile Chemistry, Mommsenstr. 4, D-01062 Dresden, Germany State University Lvivska Polytechnika, Bandera Strasse 12, 290646 Lviv, Ukraine

2

The aim of this work is a new approach to creating polymeric core-shell particles. By means of emulsion polymerization with a reactive peroxidic surfactant ('inisurf ) we obtain polymeric nanoparticles with peroxide groups on the particle surface. The polymer chains of shell monomer start growing from the initiator functionality and are finally covalently grafted to the core. The peroxide groups are useful for crosslinking with a matrix polymer. Reactive microgels with good properties for corrosion inhibition and adhesion promotion on aluminum surfaces were obtained via copolymerization with phosphate¬ -containing acrylic monomers.

Polymeric core microspheres and core-shell particles have a large and still growing importance in the past decade for a number of different applications, especially for coatings and adhesives. They are also used in medicine as substrates for biochemical functions. Core-shell particles are combinations of different, mostly incompatible, polymers. The use of these particles offers the opportunity for introducing

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© 2002 American Chemical Society

In Polymer Colloids; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


277 functional groups on the surfaces of the particles. In this way, useful systems for applications as reactive fillers and impact modifiers are available (1-3). Generally these particles are synthesised in a two-stage process by means of the sequential emulsion polymerization of polymers with various natures. However, it was discovered that the morphology of latex particles could vary greatly. A number of morphologies other than core-shell have been reported (4-6). The control of the two-stage latex particle morphology can be considered in terms of two major types of influences in the system which determine the final morphology: the thermodynamic forces and the kinetics of the morphological development. The core-shell morphology can be built up in a batch or semibatch procedure (by adding two monomers at the same time or one after another) or by seeded polymerization. In this latter case, the shell monomer is mixed with a preformed latex and polymerization of the second monomer is initiated by a free radical initiator (7). In the first case, the particle morphology is determined mainly by the copolymerization parameters of the two monomers; in the second case, the polarity and the compatibility of the shell and core polymer, type of monomer addition, type of initiator, and temperature could strongly influence particle morphology (8). We have studied particles with reactive groups located on the particle surface which are able to react with shell-monomers to form a covalently bonded shell layer or to react with a matrix of other polymers as well as with metal surfaces. According to previous publications of Funke (9) concerning functionalized microgels and our own results with self-assembling molecules based on phosphonate- and phosphate groups as reactive groups toward aluminum surfaces (10) polymeric nano particles with phosphate groups were synthesized. These particles can be used for corrosion inhibition of aluminum.

Peroxidic "Inisurfs" In this work we have used a novel method in which the above polymerization process was carried out by using a polymeric "inisurf'. Such a polymer combines the initiator and surfactant functions in one molecule. Several studies have been performed in recent years, where initiators have been attached to the surface of solid substrates or polymer particles (7, 10). This special system has a number of advantages with respect to the resulting particles and their properties. The synthesis of the peroxidic monomers was carried out at the Institute of Organic Chemistry State University-Lvivska Polytechnika, Lviv, Ukraine. The polymeric peroxide was synthesised (77) by means of the radical



278 copolymerization of maleic acid anhydride ( M A A ) and vinyl peroxide (VP) in acetone media at 333 Κ with benzoyl peroxide as initiator (Figure 1). The resulting copolymer has a molecular weight of 5000 g/mol. The polymeric peroxide becomes soluble in water after the reaction of maleic anhydride units with sodium hydroxide as is shown in Figure 1. The presence of groups with strongly differing polarities in the inisurf chains leads to certain surface-active properties. These properties enable the sorption of these polymers on phase boundaries of various colloidal systems. The inisurf exibits temperature-time dependent decomposition of the peroxide groups and therefore the emulsion polymerization process can be easily controlled. By means of thermogravimetric measurements the peroxide decomposition was studied in the interval from 90 to 110 °C. The results are presented in Figure 2. According to this result, after the core polymer synthesis by means of emulsion polymerization was complete, a certain amount of peroxide groups remain on the core particle surface and can be used for the next reaction step, i.e., the creation of a polymer shell. Finally, the shell is covalently bonded to the core particle. After polymerization of the shell monomers the inisurf becomes buried between the core and shell polymer phases which leads to destabilization of such a colloidal system. Therefore, the addition of another surfactant is necessary during the shell formation.

Synthesis of Peroxide Dispersions By using an inisurf in emulsion polymerization, the polymer nanoparticles are formed where the inisurf remains covalently bonded to the particle surface (Figure 3). Due to the presence of numerous peroxide groups in the inisurf chain, the resulting particles are crosslinked and, therefore, insoluble. The kinetics of emulsion polymerization of styrene (St) and butyl acrylate (BuA) in the presence of the reactive surfactant was studied at temperature intervals from 65 to 85 °C. The concentration of inisurf was varied from 1.6 to 3.14 mmol/L. Amounts of styrene and B u A were constant in all experiments; i.e. 1.00 mol/L. Conversion of monomer was determined by dilatometric measurements (checked by gravimetry). The conversion-time data are presented in Figure 4.


279


MAA

Vinyl peroxide (VP)

Co(MAA-VPO)

Co(MA-Na-VP) = Inisurf

Figure 1. Synthesis of the copolymer ofmaleic acid anhydride and 5-tert butyl peroxy-5-methyl-l-hexen-3-in with neutralization to peroxidic inisurf.

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Time [h] Figure 2. Thermogravimetric decomposition curves of the inisurf at different temperatures.



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Figure 3. Schematic representation of emulsion polymerization in the presence of the inisurf and the synthesis of the shell (Redox-system: FeS0 Na-hydrogen sulfoxylate*CH 0 (Rongalit), EDTA). 4i

2

Figure 4. Conversion-time curves of the emulsion polymerization of styrene and butyl aery late with different inisurf concentrations.



281 One can see that the polymerization rate for BuA is significantly higher if it is compared with styrene. This effect is probably due to the higher resonance stabilization of styrene radicals. In the case of BuA, the polymerization rate decreased rapidly at ca. 80% monomer conversion and for the samples where low inisurf concentrations were used, limited conversion was detected. On the contrary, polymerization of styrene is nearly quantitative for all inisurf concentrations. The kinetic data were used to calculate the polymerization rate and the overall activation energy (61 kJ/mol and 40.6 kJ/mol for styrene and butyl acrylate, respectively). Particle diameters of the obtained polymeric dispersions were investigated with dynamic light scattering (DLS) and Flow-Field-Flow-Fractionation (FFFF). The size of the particles decreased with increasing inisurf concentration and reaction temperature (Figure 5) and this indicates that the inisurf shows similar behavior to low molecular mass initiators and surfactants in emulsion polymerization.

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Polymer Colloids - ACS Publications - American Chemical Society

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