Letter pubs.acs.org/Langmuir
The Importance of Proper Anchoring of an Amphiphilic Dispersant for Colloidal Stability Markus Andersson Trojer,* Krister Holmberg, and Magnus Nydén Department of Chemical and Biological Engineering, Applied Surface Chemistry, Chalmers University of Technology, SE-41296 Göteborg, Sweden S Supporting Information *
ABSTRACT: Highly stable poly(methyl methacrylate) (PMMA) based microcapsule suspensions without excess dispersant are obtained via the solvent evaporation route using poly(methyl methacrylate)-block-poly(sodium methacrylate) or poly(methyl methacrylate)-block-poly(sodium acrylate) diblock copolymers as dispersant. The stable suspension is characterized by a high ζpotential that does not change with time or after washing steps. It is indirectly proven on model PMMA surfaces using quartz crystal microbalance with dissipation monitoring that the PMMA block of the copolymer is embedded in the underlying PMMA microcapsule. Such anchoring of the dispersant is key for the good colloidal stability.
■
INTRODUCTION Microencapsulation of active substances is an efficient and robust method for controlling and thereby prolonging their release.1−3 Most applications of the so-called microcapsules are found within the pharmaceutical community.4 However, the coatings industry has recently emerged as another important sector for microcapsule applications.5 Within our group, encapsulation of different biocides in poly(methyl methacrylate) (PMMA) based microcapsules has been investigated for use in marine antifouling paints.1,2,5 The encapsulation relies on the solvent evaporation technique developed by Loxley and Vincent3 where the microcapsule dispersion is most often stabilized by conventional dispersants such as poly(vinyl alcohol) (PVA)1−4 or poly(methacrylic acid) (PMAA).3 However, in order to obtain proper colloidal stability, an excess of dispersant in the aqueous phase is required. Use of practice is approximately 2 wt % dispersant in the aqueous phase.5 This type of stabilization may be viewed as the reverse of depletion attraction, sometimes referred to as depletion stabilization (even though this label in a strict sense only applies to nonadsorbing polymers), and is kinetic in nature and of entropic origin.6,7 The requirement of excess dispersant has unwanted consequences since the microcapsules must be added to the coating as a concentrated aqueous dispersion.5 Obviously, it would be preferable to add the microcapsules as a solid powder without excess dispersant. © 2012 American Chemical Society
It is well-known that the combination of steric and electrostatic repulsion is a very effective avenue for obtaining colloidal stability.8 This combined stabilization may be obtained by polyelectrolyte brushes attached to the colloidal particle surface. Pure polyelectrolytes are generally too water-soluble to adsorb on a noncharged surface and therefore need to be anchored. A potential route to achieve a polyelectrolyte brush surface on the microcapsules is to use an amphiphilic block copolymer, consisting of a hydrophobic anchor block and a polyelectrolyte buoy block, as emulsifier/dispersant. It is important to recognize that during the encapsulation the dispersion changes from an emulsion to a suspension through the evaporation of the volatile solvent (often dichloromethane1,3 or ethyl acetate2,5). This offers the possibility to dissolve the anchor block in the nonaqueous phase. However, if the anchor block is intended to remain embedded during the encapsulation and in the final polymeric shell of the microcapsule, the design of the block copolymer is of paramount importance since most polymer−polymer mixtures are incompatible. A logical choice of anchor block is consequently the same polymer as the underlying surface. Therefore, a promising type of emulsifier/dispersant for PMMA microcapsules is poly(methyl methacrylate)-blockReceived: January 8, 2012 Revised: February 15, 2012 Published: February 15, 2012 4047
dx.doi.org/10.1021/la300112t | Langmuir 2012, 28, 4047−4050
Langmuir
Letter
Scheme 1. Microencapsulation Process and Potential Dispersants
poly(sodium methacrylate) (PMMA-b-PMANa) diblock polymers (see Scheme 1). This Letter presents indirect proof that the anchor block of the diblock copolymer is indeed embedded in the underlying surface. Through a combination of QCM-D measurements on model surfaces and analysis (light microscope and microelectrophoresis) of the microcapsule suspensions, we demonstrate that simple physisorption of the block copolymer is not sufficient to achieve proper anchoring of the polymeric dispersant. It is required that the anchor block is embedded in the underlying surface in order to prevent desorption, thus providing long-term colloidal stability.
■
MATERIALS AND METHODS
The emulsifier PMAA (Polysciences Europe, Eppelheim, D) and the amphiphilic block copolymers poly(methyl methacrylate)-block-poly(sodium methacrylate) (PMMA-b-PMANa) and poly(methyl methacrylate)-block-poly(sodium acrylate) (PMMA-b-PANa) (Polymersource, Montreal, CA) are presented in Table S1 in the Supporting Information. The microcapsules were prepared as previously described by Loxley and Vincent3 with some small modifications (see Scheme 1), as is also presented in the Supporting Information. The ζ-potential of the microcapsules was measured using a microelectrophoresis apparatus Mk II (Rank Brothers, Cambridge, U.K.). The adsorption and desorption of the dispersant on a PMMA model surface was measured using a QCM-D E4 instrument (Q-Sense, Göteborg, SE). The details of the sample preparations, including liposome preparation, are presented in the Supporting Information.
Figure 1. ζ-Potential of microcapsules (based on PMAA (green, ○), 600-b-4600 (white, ○), 4000-b-9300 (grey, ●) and 4300-b-17500 (black, ●)) before and after washing. The first figure in the diblock copolymers relates to the molecular weight of the PMMA block and the second figure relates to the molecular weight of the PMANa or PANa block. The inset displays the relative change of the ζ-potential.
■
RESULTS AND DISCUSSION By examining the change of the ζ-potential (see Figure 1) of microcapsules prepared with the conventional emulsifier PMAA and with a small series of block copolymers, it was apparent that PMAA desorbs during the washing step whereas the block copolymers remained on the surface. While the measured change of the ζ-potential when the block copolymers were used fell within the standard deviation, the change when PMAA was used as emulsifier was significant (see Figure 1). The washing of the PMAA based microcapsules requires an explanation. If the crude dispersion was treated with a base prior to the washing step, the microcapsules were redispersible without excess dispersant, in contrast to an untreated microcapsule dispersion or a PVA based microcapsule dispersion. However, the PMAA based microcapsule dispersion was only stable for approximately 2 weeks, after which aggregation started. This
phenomenon has been studied in some detail elsewhere.9 On the other hand, microcapsules prepared with the block copolymers were stable, at least for 6 months. No difference in dispersion stability between different block copolymers could be observed. The type of block copolymer does however effect the size distribution (see Supporting Information) and surface charge of the capsule. The detailed characterization of the block copolymer properties will be the topic of separate publication. However, when the adsorption and desorption behavior of the same block copolymers dissolved in water was investigated on a model PMMA surface using the QCM-D technique (see 4048
dx.doi.org/10.1021/la300112t | Langmuir 2012, 28, 4047−4050
Langmuir
Letter
Scheme 2. Model Surfaces for QCM-D
Scheme 2), it was obvious that the block copolymers slowly desorbed when the surface was rinsed with water (see Figure 2a). If, on the other hand, the block copolymer was adsorbed from an acetone containing aqueous solution (in order to swell the PMMA film) followed by slow evaporation of the acetone (in order to amalgamate the PMMA film and the PMMA block of the copolymer, as described in the Supporting Information), no desorption was visible. The tendency for desorption can be illustrated by the competition between the desorption promoting PMANa block and the adsorption promoting PMMA block of the copolymer dispersant. Thus, the electrostatic interaction between the polyelectrolyte block and a dissolved charged species, such as a dissolved polyelectrolyte or a charged liposome needs to be overcompensated by the hydrophobic interaction between the PMMA block and the surface. If the latter interaction is not strong enough, desorption will occur. A good example of the importance of having the block copolymer embedded in the PMMA film is provided by a study of the interaction between a liposome and a PMMA surface with either physisorbed or embedded block copolymer. The embedded block copolymers provided a good support for liposome adsorption, rupture, and subsequent lipid bilayer formation,10 whereas the physisorbed block copolymer was ripped off the PMMA surface (see Figure 2 and Supporting Information). Another example is polyelectrolyte bilayer adsorption. Embedded block copolymers efficiently facilitated successive polyelectrolyte adsorption in contrast to physisorbed block copolymers (Supporting Information). Both the polyelectrolyte bilayer buildup and the liposome adsorption have proven to be successful on the real microcapsule systems. This will be published in a separate paper. The difference between physisorbed (eq 1) and embedded (eq 2) block copolymer may be derived and explained from simplified thermodynamical arguments (a more detailed description is given in the Supporting Information). Note that while adsorption is purely energy driven, the entropy always favors desorption due to the loss of conformational entropy for the polymer coil in vicinity of the surface. ∂Gdesorb 2 A − SA = f kT χAS + T Sads liq ∂nA zb
(
)
Figure 2. Block copolymer 4000-b-9300 and cationic liposome adsorption/desorption in pure Milli-Q water represented by the frequency shift of the third harmonic as measured using QCM-D on (a) physisorbed block copolymer surfaces and (b) embedded block copolymer surfaces. The adsorption in (b) displays the typical features of bilayer formation,10,11 but given the frequency shift of −13 Hz approximately half of the surface appears to be covered.
∂Gdesorb A A = kT χAS + T Samorph − Sliq ∂nA
(
)
(2)
∂Gdesorb/∂nA represents the free energy change for the A-block (PMMA) per monomer unit upon moving one block from the surface to the solution at infinite dilution. χAS is the Flory− Huggins interaction parameter for the A-block and the solvent (water), k is the Boltzmann constant and T is the temperature. The entropy for the A-block in PMMA, in water, and adsorbed A A A on PMMA is given by Samorph , Sliq , and Sads , respectively. The fraction of monomers of the PMMA block in contact with the surface is given by f which is
