Polymer Colloids - ACS Publications - American Chemical Society

Chapter 10

Kinetics of Emulsion Polymerization of Styrene in the Presence of Polyurethane Resins 1

2

Downloaded by CHINESE UNIV OF HONG KONG on March 10, 2016 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch010

I. W . Cheong , M . Nomura , and J. H. K i m

1,*

1

Nanosphere Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea Department of Material Science and Engineering, Faculty of Engineering, Fukui University, Fukui 910-8507, Japan 2

The role of reactive and non-reactive polyurethane resins in the emulsion polymerization of styrene was investigated. They exhibited low cmc values, high internal viscosities, and high solubilization ability for hydrophobic materials, i.e., pyrene. Continuous nucleation and broad particle size distribution were remarkable in the resin system, especially near and above the cmc. The average number of radicals per particle was below 0.5 and the parameter m in the resin system showed higher values than the estimated values from classical micellar nucleation theory.

In our recent work, non-reactive polyurethane resins were synthesized by polyaddition reactions of isophorone diisocyanate (IPDI), polypropylene glycol (1,2-PPG, M W : 750 and 2000 g/mol), and dimethylol propionic acid (DMPA). The polyurethane resins contained carboxylic groups, D M P A , in their backbones. Therefore, they were water-soluble or water-dispersible according to the degree of neutralization of the carboxylic groups. The polyurethane resins exhibited surface-active properties, such as micelle or aggregate formation, change of surface tension, and the ability to solubilize hydrophobic materials.

126

© 2002 American Chemical Society

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


127 Aggregate numbers of the resins were 7.2 and 6.6 for PUR-2000 and PUR-750, respectively. The solubilization ability of the resin for pyrene increased as the resin concentration increased, as is the case with electrolyte concentration. The solubilization ability was much higher than that of SDS (sodium dodecyl sulfate) at the same concentration (/). Several batch emulsion polymerizations of styrene were conducted in the presence of the polyurethane resin. The average particle sizes of the polystyrene latexes were very small (10 ~ 100 nm) and the size distributions were very broad. The emulsion polymerizations of styrene using the resins as emulsifiers showed somewhat different kinetic dependencies on the stabilizer and initiator concentrations compared to conventional anionic surfactants. It was found that the kinetic exponents describing the effect of the resin and initiator concentrations on the number of particles were 0.63-0.66 and 0.38-0.39, respectively (2). For the rate of polymerization, the exponents were lower than the values from Smith-Ewart theory. Moreover, there was no variation between the initial and final particle size especially in the small particle size range ( 5 - 1 5 nm). Broad particle size distributions and continuous nucleation were notable in comparison with a conventional surfactant system. This difference was mainly attributed to the hydrophobicity, relatively high molecular weight, and high internal viscosity of the resin molecules, which builds up a rather thick surface layer on the colloidal particle like protective colloids. In this work, a reactive polyurethane resin was synthesized by introducing 2hydroxyethyl methacrylate ( H E M A ) for a comparative study between the previous non-reactive and the reactive resins. The H E M A was reacted with NCO-terminated polyurethane prepolymer at room temperature by using dibutyltin dilaurate as a catalyst. The reactive polyurethane resin has vinyl groups at each end of the main chain. Thus, the resin can take part in the polymerization reaction with styrene monomer during the emulsion polymerization. The existence of H E M A monomer units in the resin was confirmed by C N M R . The chemical structures of PUR-750 and P U R 750HEMA are illustrated in Figure 1. 1 3

Experimental Materials Styrene monomer was obtained from Wako Pure Chemical Ind. (Japan) and purified by distillation under a nitrogen atmosphere. Commercial sodium dodecyl sulfate (SDS), potassium persulfate (KPS), and potassium bicarbonate ( K H C 0 ) were extra pure grade and were used without further purification. Distilled and deionized (DI) water was used in all experiments. Two kinds of 3


128 polyurethane resins were synthesized and used in emulsion polymerizations of styrene. Properties of the resins are given in Table I.

(a)


HO^O (b)

Figure 1. Simplified chemical structures of the polyurethane resins; (a) repeating unit of PUR-750; (b) PUR-750HEMA.

Table I. Physical Properties of the Polyurethane Resins Property M

n

M

w

PUR-750

Type of Resin PUR-750HEMA

(g/mol)

5600

5700

(g/mol)

11000

7000

2.05

1.23

47.4 0.36 7.2

48.7 0.28 Not analyzed

PDI ( M / M ) w

n

7

A V (mg KOH/g PUR) cmc (mM in water) N (-) 2

3

m

1. Acid value (AV) was measured by titration method. 2. Critical micelle concentration (cmc) was measured by surface tension. 3. Aggregation number was measured by pyrenefluorescencemethod (excitation and emission wavelengths were 345 and 380 nm).

Emulsion Polymerization Emulsion polymerizations were carried out in a double-jacketed 1L glass reactor, equipped with a mechanical stirrer, a thermometer, a reflux condenser, a temperature controller, an initiator funnel, and a nitrogen inlet. The basic recipe


129 for emulsion polymerization in the presence of the resin is given in Table II. The reactor was charged first with the desired amount of PUR solution (5 wt% solids content), styrene, KHC0 , and DI water. The calculated amount of KPS solution was added into the initiator funnel. A highly pure nitrogen gas was used to purge the reactor for 20 min to remove dissolved oxygen in the reaction mixture. The polymerization was initiated by dripping in the initiator solution, which had been deoxygenated with the nitrogen gas. The reaction temperature was adjusted at 60 ± 1 °C in a thermostated water bath. The stirring rate was kept at 400 rpm. Downloaded by CHINESE UNIV OF HONG KONG on March 10, 2016 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch010

3

Table II. Basic Recipe for Emulsion Polymerization of Styrene Using the Polyurethane Resins Components Styrene Polyurethane Resin KPS KHCO3 DI water

;

2

Amount (g)

wt% (base material)

20.000 0.100-5.000 0.020 ~ 0.200 0.007 - 0.070 328.000

5.7 (total wt.) 0.5 - 25.0 (monomer wt.) 0 . 1 - 1 . 0 (monomer wt.)

1. The resins were dissolved in water using appropriate sodium hydroxide before the polymerization and degree of neutralization was fixed at 100%. 2. Same molar concentration of buffer as KPS.

Latex Characterization The monomer conversion was determined by gravimetry using a 100 ppm hydroquinone in methanol solution as a short-stop agent. 6-8 g latex samples obtained from the reaction vessel were immediately quenched with 4-5 drops of the short-stop solution and cooled after a predetermined time of polymerization. Particle size and distribution of the latexes were analyzed by electron microscopy (H-600A, Hitachi, Japan) and dynamic light scattering (Photal D L S 7000, Otsuka Electronics, Japan) at 25 °C. Diluted samples were measured in the dynamic scattering mode. Average particle sizes were calculated by counting over 500 particles in the T E M photos. The number of particles was calculated from the monomer conversion and the average particle size obtained from the electron micrographs as follows: 6mX

n


130 where N is the number of particles in the system, D , the volume-average diameter, X , fractional conversion, m , amount of monomer with respect to the aqueous phase, and p , the density of polymer. The rate of polymerization can be expressed by the following equation: p

v

p

0

p

_k n[M] N


v

p

v

( 2 )

where, η is the average number of radicals per particle, [M] , concentration of styrene monomer in the polymer particles, and N Avogadro's constant. The propagation rate coefficient was obtained using the following equation (3): p

Ai

k =lxl0

7 6 3

p

1

L molds' exp(-32,510J

(3)

l

moY /RT)

The maximum rate of polymerization (/? , ) was calculated from the slopes of conversion as a function of time from 0.2 to 0.4 fractional monomer conversion. Subsequently, the number of particles was calculated in the period of constant polymerization rate. The monomer concentration in the polymer, [ M ] , is a function of monomer conversion, X . In Interval II, it was assumed that [ M ] was constant at 5.5 mol/L polymer. The η was obtained using eq. (2). Parameters related to η were calculated by the semi-empirical equation introduced by Nomura (4). In the calculation, aqueous phase termination of free radicals was assumed to be negligible. Thus, the following equation can be used: p

max

p

p

p

,0.5

, α'Ί + 2 (α*+— α'+m mJ

η =0.5

where, α ' = p v fk N w

p

tp

and m = k v ίk .

p

f

p

(4)

+ 0.25+^2

lp

p

w

is the rate of radical production

per unit volume of water and was approximately given by p

w

= 2 ^ : [ I ] . v is d

0

w

p

the volume of a particle, & , the rate constant of mutual termination of radicals in tp

the particles, and k the rate constant of desorption of radicals from a particle {l

(5). The decomposition rate coefficient (6) was expressed as follows: 1

fi

d

1

=96xl0 V exp(-143,250//?r)


(5)

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Results and Discussion In all experiments, the two polyurethane resins given in Table I, PUR-750 and PUR-750HEMA, were used. The existence of - C O O H groups from D M P A and unsaturated carbon bonds from H E M A were confirmed by C N M R spectra. No peaks indicating a side reaction between carboxyl and isocyanate groups (amide bond peak) were detected in the spectra. As seen in Table II, the appropriate amount of DI water was used for maintaining a constant initiator concentration, and total solid content. Subsequently, the total solid contents of all the emulsion latexes were about 5.7 wt%.


1 3

Particle Size and Particle Size Distribution Figure 2 shows T E M photos of polystyrene latexes prepared in the presence of the reactive polyurethane resins (PUR-750HEMA) after the completion of polymerization. Variation in the resin concentration ranged from below to above the cmc. The cmcs of PUR-750 and PUR-750HEMA were measured as 0.36 and 0.28 m M in water, respectively. Figures 2 (e) and (f) represent latexes prepared at the resin concentrations below the cmc, 0.133 and 0.053 m M in water. The volume-average particle sizes ranged from 30 to 120 nm depending on the concentration of the resin. Above the cmc, the particle size distribution was very broad; however, the distribution became narrower as the resin concentration decreased. In addition, a large number of small particles were freshly nucleated up to high fractional conversions (< 0.8) as were observed in the T E M photos. Below the cmc, the particle size distribution was narrow and the PDI was smaller than 1.1. In this case, few freshly-nucleated small particles were detected. In this paper the particle size distributions of the polystyrene latexes prepared using PUR-750 (non-reactive type resin) are not illustrated, however, the distributions showed similar trends. In both cases, the particle size distributions were very broad and continuous nucleation was observed especially near and above the cmc. Continuous nucleation was explained by the characteristics of the polyurethane resins. The polyurethane resin demonstrated a superior solubilization ability for hydrophobic materials and they have relatively high molecular weight and high internal viscosity in comparison with conventional short chain surfactants. Therefore, a few high molecular weight polyurethane resins can form an aggregate in the aqueous phase both above and below the cmc since the resins have relatively broad molecular weight distribution as well. The aggregate can be a locus of polymerization, as well as act as a polyelectrolyte. The molecular weight and acid number can significantly affect the formation of aggregates and the surface



132

Figure 2. TEM photos of final polystyrene latexes prepared using the PUR750HEMA with different concentrations; (a) 25 wt%; (b) 15 wt%; (c) 10 wt%; (d) 5 wt%; (e) 1.25 wt% ; (f) 0.5 wt% based on monomer; bar = 100 nm, magnification =3 OK.


133 active properties. It was difficult to make aggregates or micelles to produce new particles for the resins having high acid numbers and relatively low molecular weights and they showed somewhat different kinetic behavior (7, 8).


Rate of Polymerization and Final Number of Particle Figure 3 (a) shows the relationship between the rate of polymerization, and resin concentration. The dependencies, estimated from the slope of lst-order regressions, were 0.48 and 0.34 for PUR-750HEMA and PUR-750, respectively. According to the micellar nucleation theory, the exponent should be 0.6. In the case of the reactive resin, PUR-750HEMA, the exponent was higher than that of PUR-750. These results can be explained by the high polymerization rate of H E M A monomer and the irreversible adsorption behavior of the PUR750HEMA resin. It was found that all the polymerization data was located on the same linear line regardless of the existence of the cmc. This result suggested that the nucleation mechanisms below and above the cmc should be the same. From the slopes, it can be suggested that the aggregates formed by a few high molecular weight resins could be a polymerization locus in emulsion polymerization due to its high solubilization ability and high internal viscosity. However, it cannot be concluded that the particle nucleation follows a specific nucleation theory due to insufficiency of influential evidence concerning the particle nucleation. In the case of the effect of initiator concentration on /? , in Figure 3 (b), the exponents showed a similar trend considering the classical micellar nucleation theory. Figure 4 shows the final number of particles with the variation of resin and initiator concentrations. The exponents from the slopes were 0.78 and 0.69 for PUR-750HEMA and PUR-750, respectively. The final number of particles in PUR-750HEMA was relatively higher than that of PUR-750. These high exponent values have been observed often in homogeneous nucleation systems. In this work, however, these high values were due to the characteristics of the resin molecules as mentioned above. Moreover, from the slopes in Figure 4 (b), it was found that the exponent for the dependence of particle number on initiator concentration was higher in the case of PUR-750HEMA than that of PUR-750. This tendency can be explained by the role of unsaturated carbon bond in PUR750HEMA. A l l through the reaction, secondary particles were nucleated continuously; however, PUR-750HEMA reacted with styrene monomer and it could be chemically anchored onto a freshly nucleated particle. In comparison with PUR-750HEMA, PUR-750 resin molecules could adsorb to stabilize preexisting growing particles rather than adsorbing to the freshly nucleated precursor particle.

^ ,max> P

p


max


134

Figure 3. (a) Rate of polymerization (R ) versus initial resin concentration; round symbols: PUR750-HEMA; square symbols: PUR-750; (b) rate of polymerization versus initial initiator concentration; round symbols: PUR750HEMA; square symbols: PUR-750. pMax



135

Figure 4. (a) Final number of particles versus initial resin concentration; round symbosl: PUR750-HEMA; square symbols: PUR-750; (b) final number of particles versus initial initiator concentration; round symbols: PUR750-HEMA; square symbols: PUR-750.


136 Average Number of Radicals per Particle Figure 5 shows average number of radicals per particle, η , as a function of the parameter, a'. The η values for both resins were below 0.5. The parameter m showed higher values than estimated from conventional styrene/SDS emulsion systems, m has been reported as lxlO" in a polystyrene/SDS latex system with 62 nm volume-average particle size. For some water-soluble monomers, such as vinyl chloride and vinyl acetate, m has been reported as 2x10~ from the results of Ugelstad and Nomura (9). In this work, the high value of m was because of the unique properties of polyurethane resins. High m values implied that the radical loss process was dominant during the emulsion polymerization. The radical loss could occur by radical desorption due to the chain transfer reaction to resin molecules as well as styrene monomer. Especially, the transfer reaction to the resin leads to grafting reaction with the monomer or to termination by another radical entering from water phase. In fact, a grafting portion of the monomer was observed at 40% based on the total monomer according to our unpublished results. As mentioned above, chain-transferred resin molecules desorb from the growing particles and might be terminated in the aqueous phase. Radical desorption was significant especially in PUR-750, non-reactive resin system. According to the micellar nucleation theory, R , should be directly proportional to N . In this resin system, # , was proportional to N® and 4


3

p max

62

p

49

N°

p

p

max

for PUR-750HEMA and PUR-750, respectively.

Surface Tension Analysis Figure 6 shows the surface tensions for resin solutions only and the polystyrene latexes prepared using these resins. PUR-750HEMA showed better surface activity than that of PUR-750. In the case of the polystyrene latexes, the surface tension of PUR-750HEMA was much higher in comparison with that of PUR-750. From these results, it was concluded that PUR-750 (non-reactive resin) desorbed from the growing polymer particles during the polymerization; therefore, the surface tension value was low. At a higher concentration of the resin, a large number of the resin molecules existed in the aqueous phase. Consequently, the rate of continuous nucleation increased due to the free resin molecules or aggregates of the molecules.



137

Figure 5. The average number of radicals per particle as a function of the parameter a* (aqueous phase termination is negligible, Y = 0); round symbols: PUR750-HEMA; square symbols: PUR-750.

-7

-6

-5

-4

-3

-2

log[S] , Resin Cone. (mol/L water) o

Figure 6. Surface tensions of the polyurethane resins and final polystyrene latexes prepared using the resins; round symbols: PUR750-HEMA; square symbols: PUR-750.


138


Conclusions Polymeric or polymerizable surfactants are usually tailor-made and subsequently they exhibit unique properties as seen in many recent publications. In this work, the polyurethane resins showed a different behavior in comparison with those of conventional styrene/short-chain surfactant systems. Discrepancies in the power dependency of the polymerization rate and the final number of particles on the resin or initiator concentration was illustrated and explained by the typical properties of the resins, such as relatively high molecular weight and broad molecular weight distribution, high internal viscosity, and superior solubilization ability for hydrophobic materials. It was concluded that the low η and high value of m were due to radical loss process which was dominant all through the reaction.

Acknowledgments This work was supported by the Korea Institute of Science & Technology Evaluation and Planning (National Research Laboratory Program, 1999). The authors thank Dr. K . Suzuki and M r . S. Kato in Fukui University for helpful suggestions and instructive correspondence.

References 1. Cheong, I. W.; Nomura, M.; Kim, J. H . Macromol. Chem. Phys., in press. 2. Cheong, I. W.; Nomura, M.; Kim, J. H . Macromol. Chem. Phys., in press. 3. Buback, M.; Gilbert R. G.; Huchinson, R. Α.; Klumperman, B.; Kutcha, F. D . ; Manders, B . G.; O'Driscoll, K . F.; Russell, G. T.; Schweer, J. Macromol. Chem. Phys., 1995, 196, 3267. 4. Nomura, M.; Fujita, K . Makromol. Chem. Suppl., 1985, 10/11, 25. 5. Ugelstad, J.; Mørk, P. C.; Aassen, J. J. Polym Sci., 1967, A-1, 5, 2281. 6. Behrman, E . J.; Edward, J. O. Rev. Inorg. Chem., 1980, 2, 179. 7. Lee, D . Y.; Kim, J. H . J. Appl. Polym. Sci. 1998, 69, 543. 8. Kato, S; Sato, K ; Maeda, D ; Nomura, M. Colloids & Surfaces A, 1999, 153, 127. 9. Nomura, M; Satpathy, U . S.; Fujita, K ; Kouno, Y ; J. Polym. Sci. Part C, Polymer Letters, 1988, 26, 385.


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