Polymer Latexes - American Chemical Society

Chapter 15

Control of Core—Shell Latex Morphology

Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch015

S. Lee and Alfred Rudin Institute for Polymer Research, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Composite latex particles with core-and-shell structures made via staged emulsion polymerization have gained interest in recent years. Control of particle structures has been a challenge, particularly when the desired core polymer is more hydrophillic than the shell polymer. The structure of a PMMA/PS system can be controlled by altering the thermodynamic and kinetic variables. These variables include particle surface polarity, stage ratio, core particle size, the mode of monomer addition, and the degree of crosslinking. Latexes made via two consecutive emulsion polymerization stages are commonly referred to as "core-shell" latexes, implying a particle structure with the initiallypolymerized material at the center and the later-formed polymer as the outer layer. By polymerizing different polymer types in each separate stage, composite polymer particles can be obtained which are neither like random copolymers nor like polymer blends. Such two-stage latexes have a wide range of potential applications in a variety of technologies, from paints and organic opacifiers, to impact modifiers for plastics, to carriers for biomolecules. For examples, two-stage latexes can be prepared for use as binders in latex paints. In such an application, the paint properties can be modified by varying the balance of properties of the first and second-stage polymers, e.g. the glass transition temperature of each stage (J). In addition, microvoid particles have been made by preparing particles with a base-sensitive hydrophillic core polymer surrounded by a hydrophobic shell polymer. These particles become hollow upon treatment under conditions of elevated temperature and pH, so that they contain mainly water at the center. Such hollow particles have been used as organic opacifiers in paints (2). Coreshell particles with elastomeric centers and rigid shells have also been made, which can be useful as impact modifiers in high impact plastics. Early workers largely assumed that two-stage polymerizations led to coreshell structures. However, upon more thorough investigation, and with the

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15. LEE AND RUDIN

Control of Core-Shell Latex Morphology

235

advent of electron microscopy techniques, it was discovered that the morphology of two-stage latexes could vary greatly. Okubo et al. have reported a number of morphologies other than the core-shell type exhibited by two-stage latex particles, including structures which they have designated "raspberry-like", "confetti-like", "snowman-like", "octopus ocellatus-like", and "mushroom-like" (3\ Cho and Lee, in addition, reported two-stage latex particles with "sandwich-like and "halfmoonlike" arrangements (4). Lee and Ishikawa obtained "inverted" particles, with the second-stage polymer at the particle center and the first-stage polymer on the outside (5). A few of these configurations are illustrated in Figure 1. It is notable that in the majority of cases where these unexpected morphologies occur, the first-stage polymer is more hydrophillic in nature than the second-stage polymer. To a large degree, the physical properties of two-stage latexes depend on the morphology of the particles. Since such wide variation in morphology is possible, it is necessary to have an understanding of how particle moiphology is controlled in order to control product properties. The control of two-stage latex particle morphology can be understood in terms of two major types of influences in the system, the thermodynamic forces and the kinetics of the morphological development. These two types of influences combine to determine the final particle structure. The effects of specific thermodynamic and kinetic factors will be discussed in this paper to illustrate this point. An analysis of the thermodynamics of two-stage particle formation has been developed by Sundberg et al. (6) in which the system was considered simply in terms of the free energy changes at the interfaces of a three phase system (i.e. polymer 1, polymer 2, and water) based on the following equation: G

=E , ^

(1)

where: G is the Gibbs' free energy of the system; 7y is the interfacial tension between phases i and j, and Ay is the interfacial area between phases i and j . According to this analysis, each particular morphological configuration will have a different value for G, and the arrangement with minimal free energy will be the one which is more thermodynamically favoured. For example, the free energy of the two extreme morphologies, the core-shell and the inverted configurations, can be calculated using equation 1 as: core-sheU

Z

G

4

Vl2 W

+

1*,*™!

(2) ^inverted =

+

where x and r^ are the appropriate radii shown in Figure 2. Using these equations, Sundberg et al. demonstrated the importance of the interfacial thermodynamics, particularly the interfacial tensions in predicting the morphology of the particles. Y-C. Chen and coworkers have developed a thermodynamically-based mathematical model to describefreeenergy differences between different possible particle structures (7). Additionally, another process for achieving composite polymer latex particles of controlled morphology prepared by the combination of preformed polymer particles has been reported recently by Waters (8). In this process, particles of polymer 1 are combined with particles of polymer 2 dispersed in a continuous medium, phase 3, at temperatures above the functional glass x

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

236

POLYMER LATEXES

core-shell


raspberry-like

sandwich-like

acorn-like

octopus ocellatus-like

•

first stage polymer

• second stage polymer

Figure 1. Examples of possible composite latex particle morphologies.

core-shell

inverted

Figure 2. Radii of core-shell and inverted morphologies.


15. LEE AND RUDIN


237

transition temperature of polymer 2. Starting from the same fundamental equations as those of Sundberg et al. as cited above, further equations expressed in terms of phase volume instead of interfacial area were derived. It was reported that the final particle morphology could be controlled by adjusting the relative phase volumes of polymers 1 and 2. For example, core-shell morphology would be expected to be more favorable than inverted morphology if G ™ . ^ < Gfo^^j, which when expressed in terms of interfacial tensions and polymer phase volumes becomes:


Y 2 w

> * f - 4f

(4)

Y12 x and 4>2 represent the volumes of polymers 1 and 2. All the models cited are mutually consistent. Waters' theory is particularly convenient for present purposes and is considered further below. Although the thermodynamics of the two-stage latex system determines which configuration will be ultimately the most stable, the kinetics of the morphological development determine the degree to which such a low energy arrangement is realized. Since the polymeric phases are subject to serious diffusional limitations, a kinetic barrier may exist between the initial state and the thermodynamically favoured state. Therefore, kinetic factors also play a role in determining two-stage particle morphology. In this paper, a number of poly(methyl methacrylate)/potystyrene twostage latexes (PMMA/PS) are described which demonstrate the effects of both thermodynamic forces and kinetic considerations on particle morphology. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition and the use of crosstinking agents.

where

Experimental Materials. Styrene (Aldrich) was purified by reduced pressure distillation, and was stored in a refrigerator. Water was obtained from Milli-Q purification system. All other materials were used as received without further purification. Reagent grade methyl methacrylate, allyl methacrylate, ammonium persulfate, potassium persulfate, and technical grade divinylbenzene were also obtained from Aldrich; 2,2'-azobisisobutyronitrile (AIBN) from Fluka; ethylene glycol dimethacrylate from Polysciences, and the surfactants Igepal CO-890 onylphenol ethoxylate with an average 40 ethylene oxidants) from Domtar and S-10 (sodiumdodecylbenzene sulfonate) from Alcolac.

S

Polymerizations. All polymerizations were performed in a 1L glass kettle reactor suspended in a thermostated water bath and equipped with a condenser, mechanical stirrer, and, for semi-batch polymerizations, a monomer pump. The temperature was maintained at 80°C for methyl methacrylate polymerizations with semi-batch addition of monomer to the reactor, and at 60°(J for styrene polymerizations with either batch or semi-batch addition of the monomer. Typical recipes are listed in Tables I to III.


238

POLYMER LATEXES Table I. Recipe for PMMA Core Latex using Ionic Initiator Ingredient

Amt.

water

375

methyl methacrylate

150


ammonium persulfate

(R)

1

Table II. Recipe for PMMA Core Latex using Nonionic Initiator Ingredient

Amt. (g)

water

225

DS-10 surfactant

1.3

methyl methacrylate

150

AIBN

1.3

Table III. Typical Recipe for Second Stage Polymerization Ingredient

Amt. (g)

PMMA core latex (dry weight)

10

water

90

Pre-emulsified feed: water

100

Igepal CO-890

0.2

styrene

10

AIBN

0.2

PMMA seed latexes were used as prepared, without purification. Characterization. Dried PMMA/PS latex pieces were embedded in a mixture of Poly/Bed 812 and Araldite 502 epoxy resins (Polysciences) and cured at 60°C for 48 h. The samples were ultramicrotomed to 900 Angstrom thin sections using a Reichert ultramicrotome. The polystyrene domains were preferentially stained by exposure to R u 0 solution vapours (Polysciences). The stained sections were examined and photographed using a Phillips 300 T E M at an accelerating voltage of 60 keV. 4

Results and Discussion Alteration of Particle Surface Polarity. It is important to note that since the PMMA/PS system is a hydrophillic/hydrophobic system, one can predict a priori


15. LEE AND RUDIN


239

that generally the most thermodynamically favorable arrangement will be an inverted morphology rather than a true core-shell structure. This is because 7 ^ is greater than 7 . Therefore, one would expect the following relationships to apply: l w


and so:

The formation of inverted morphology eliminates the most unfavorable hydrophobe-water surface and replaces it with a hydrophile-water surface. This situation may be represented by the diagram in Figure 3. This type of system is the one which is most complex in terms of morphological control and in which it is most difficult to achieve core-shell morphology. Cho and Lee (4) reported that the use of an ionic initiator in the second stage of a PMMA/PS latex system resulted in anchoring of the polystyrene phase by the ionic surface groups generated from decomposition of the initiator. The ionic groups at the polystyrene chain ends rendered the polystyrene surface more polar, and thus the polystyrene phase could remain on the outside of the particle. PMMA/PS latex particles made using an ionic initiator in the first stage and a nonionic initiator in the second stage have a very strong tendency to invert due to the inherent difference in the polarity of the two polymers coupled with the presence of ionic groups from the initiator on the PMMA surface. Particles containing many polystyrene domains within the PMMA core were obtained, as shown in Figure 4. A core-shell arrangement for the PMMA/PS system can be made more favorable by using a nonionic initiator such as AIBN in the first-stage polymerization and an ionic initiator in the second stage. Figure Sa shows that mdeed such particles do exhibit a core-shell morphology. In addition to the thermodynamic effects, the use of a nonionic initiator led to increased PMMA molecular weight ( M = 517,000; M . = 227,000) compared to the PMMA made with an ionic initiator (M^ = 81,500; M = 38,900). Higher molecular weights should result in reduced phase mobility. Thus the formation of core-shell morphology can be promoted by using a nonionic initiator. Furthermore, PMMA/PS particles were made using AIBN initiator in both stages. Under semi-batch conditions, a true core-shell structure was obtained with no polystyrene domains within the PMMA core, as shown in Figure 5b. Nevertheless, when nonionic initiators were used in both stages, and the shell polymerization carried out under batch conditions, a core-shell structure with many polystyrene inclusions was again obtained. The significance of batch versus semi-batch polymerizations will be discussed later. Apparently, eliminating covalently bound ionic groups from the PMMA surface can greatly reduce the core particle surface polarity and make a PMMA/PS core-shell structure more favorable thermodynamically. w

n

The Effect of Stage Ratio. To evaluate the applicability of Waters' thermodynamic equations to a real two-stage latex system, and to investigate the possibility of using the relative amounts of each polymer, or the stage ratio, to counterbalance the influence of the polymer polarities, which tend to favour inversion, the following latexes were prepared. PMMA/PS two-stage particles were made with polymer 1/potymer 2 weight ratios of 3:1 and 1:3 prepared via


240

POLYMER LATEXES

hydrophilic polymer B

hydrophobic polymer

O Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch015

core-shell

• inverted

Increasing phase mobility Figure 3. Free energy (G) of core-shell and inverted morphologies.

Figure 4. Microtomed section of 1:1 PMMA/PS latex (PS phase stained black with Ru0 ). Initiators: first stage ionic; second stage nonionic; semi-batch second-stage polymerization. 4




LEE AND RUDIN

Figure 5. 1:1 PMMS/PS. First-stage initiator: nonionic. Semi-batch second-stage polymerization (a) second-stage initiator: ionic, (b) secondstage initiator: nonionic.


241

242

POLYMER LATEXES


semi-batch polymerizations. These particles were made under the same conditions as those shown in Figure 4, in which a 1:1 stage ratio was used, where thefirst-stageinitiator was ionic and the second-stage initiator was nonionic. It can be seen from Figures 4, 6a and 6b that as the relative amount of polymer 2 increases, core-shell morphology becomes more favorable. This corresponds to the results predicted by thermodynamics, from equation 4 above. That is, as the volume of polymer 2 increases, the value of the right hand side of equation 4 decreases, implying that a core-shell should become more stable. The Influence of First-Stage Particle Size on Two-Stage Particle Morphology. In the preceding section, Waters' equation regarding the influence of relative phase volumes was discussed and experimental support for the applicability of that equation to real two-stage latexes was presented. This section will demonstrate the implications of the same equation to the role of core particle size on two-stage particle morphology. In Waters' patent, the relative phase volumes were controlled by one of two methods: (i) By varying the number of polymer 1 and polymer 2 particles with both particle types having approximately equal diameters (Figure 7), or (ii) by varying particle size for a given ratio (1:1) of polymer 1 and polymer 2 particles (Figure 8). Furthermore, as was shown in the preceding section, equation 4 can apply to two-stage latex formation as well. The two-stage particle morphology changes with stage ratio (Figure 9). Interestingly, the thermodynamic equations also appear to hold true for the situation where 4> l is held constant, but the particle size is varied, as demonstrated by the following experiments. A series of twostage PMMA/PS latexes were made with persulfate-initiated PMMA first-stage particles and AIBN initiation of the second stage, under semi-batch conditions. The monodispersefirststage latex diameter was different in each polymerization, varying from 100 to 900 nm. A 1:1 stage ratio was used in all the polymerizations. Figure 10 shows a schematic diagram of the morphologies obtained. Usingfirst-stageparticles of small diameter, the two-stage particles obtained had an inverted morphology (Figure 11a). The same morphology was obtained using 200 nmfirst-stageparticles. Again with a 1:1 PMMA/PSstage ratio, usingfirst-stageparticles of intermediate diameter (400 or 600 nm), the final particles had 'Taspbeny-like" structures with polystyrene inclusions, as in Figure 4. Finally, with larger PMMAfirst-stageparticles of 900 nm diameter, a core-shell morphology was formed with no inclusions of PS inside the PMMA cores, as shown in Figure lib. Thus, under the polymerization conditions used, and for core particles in the range 100 to 900 nm, the larger the core particle diameters, the more favorable is formation of core-shell morphology. Such behaviour may be explained using the equation derived by Waters. From equation 4, a core-shell morphology becomes more thermodynamically favoured as 2 increases, or conversely as x decreases. This equation, however, was derived to describe the phase behaviour of the combination of preformed particles. Thus, the phase volume was a convenient parameter to consider, since it is a parameter directly related to the surface area of each phase. The fundamental equation from which equation 4 was derived, however, was expressed in terms of interfacial areas. Let us now consider Waters' experiments in terms of phase surface areas rather than phase volumes. Firstly, particles of polymers 1 and 2, of similar diameters were combined. As the relative number of polymer 2 particles was increased, core-shell morphology became more thermodynamically favorable. Therefore, as Ai^ the total surface area of polymer 2 was increased, relative to x

2




LEE AND RUDIN

Figure 6. Semi-batch second-stage polymerizations with ionic first-stage initiator and nonionic second-stage initiator, (a) 3:1 PMMA/PS (b) 1:3 PMMA/PS.


243

POLYMER LATEXES


244

Figure 7. Varying phase volume () by altering particle numbers.



245


15. LEE AND RUDIN

Figure 8. Varying phase volume () by altering particle sizes.



246

POLYMER LATEXES

,/=3:l 2

polymer 1

o o

m^^mer

oo

© ® @©

ty4>2" 1:3 polymer 1

o

• So o

oo

monomer 2

oo

Figure 9. The influence of stage ratio on two-stage latex particle morphology.


15. LEE AND RUDIN

247



! / ) 2= 1:1 ; 100 nm core particles

• ® ® @

® ®® ® ® ® ® ®

®

l / 2= 1:1 ; 900 nm core panicles

Figure 10. The influence of core latex particle size on two-stage particle morphology.

American Chemical Society library 1155 16th St., N.W.

In Polymer Latexes; Daniels, E., et al.; O.C.Society: 20036Washington, DC, 1992. ACS Symposium Series;Washington, American Chemical

POLYMER LATEXES


248

Figure 11. Microtomed morphology of 1:1 PMMA/PS particles: (a) 100 nm core (b) 900 nm core.



15. LEE AND RUDIN

249


the total surface area of polymer 1, core-shell morphology became more favorable. Secondly, particles of polymers 1 and 2 of dissimilar diameter were combined in a 1:1 ratio (i.e. equal numbers of each particle type). As the diameter of polymer 2 particles increased, core-shell morphology became more favorable. Here again, as the total surface area of polymer 2 was increased relative to the total surface area of polymer 1, core-shell morphology became more favorable. From equation 1, it is evident that the thermodynamics of morphological development are actually dependent on the minimization of the interfacial free energy, as determined by the interfacial tensions and interfacial areas in the system. In short, the morphologies obtained using a constant stage ratio, but different first-particle sizes can be explained by replacing 4> and ^ with kAj and in equation 4, where A is the total surface area of the polymer 1 particles and k is a constant equal to (36w)" . The physical meaning of A2 is ill-defined for a two-stage polymerization, but this will be discussed later. Equation 4 can be re-expressed as follows: x

2

x

1/3

Y l

- W ^ m n + *

A

"

>«1

G

th

< inverted 8™»

Polymer Latexes - American Chemical Society

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