Chapter 22
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Ethylene-Modified Latexes: Preparation, Miscibility Enhancement, and Barrier Properties Jin-Sup Shin, Doug-Youn Lee, and Jung-Hyun Kim* Nanosphere Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea
Ethylene-modified latexes were prepared by the emulsion polymerization of styrene using poly(ethylene-co-acrylic acid) (EAA), [ M : 18,800 g mol , acid number: 140], as a polymeric emulsifier. E A A containing 20% portion of acrylic acid could form aggregates like micelles. The rate of polymerization of styrene using E A A is found to increase with increasing E A A concentration. This result is similar to that obtained in emulsion polymerization using conventional surfactant. The polystyrene (PS) latexes prepared using E A A exhibited small particle size and monodisperse particle size distribution in the presence of excess neutralizing agent which reduces the repulsion between charges along the E A A chain. The EAA¬ -1
n
-grafted-PS formed in situ during emulsion polymerization led to the improvement of miscibility between E A A and PS. The dynamic mechanical properties of the ethylene-modified PS films exhibit single glass transition temperature values, thus conforming to the law of miscibility. The grafted or anchoring of EAA into the particles improved the barrier properties, such as chemical resistance, of ethylene-modified PS films.
© 2002 American Chemical Society
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
323
324 It is well known that amphiphilic polymers, which consist of both hydrophobic and hydrophilic groups, can stabilize polymer particles and can form aggregates, like micelles, as a result of intermolecular and/or intramolecular hydrophobic interactions (1,2). Aqueous solutions of such polymers are characterized by unusually low viscosities, and by high solubilization capacities. These properties are attributed to the aggregation of the surfactant side chains providing hydrophobic microdomains in isotropic aqueous solution. Such a behavior resembles the micelle formation of low-molecularweight surfactants in water. Hence, the micellar polymers were coined (3), and the hydrophobic aggregates formed in polysoaps are referred to as polymeric micelles. Recently, alkali-soluble resin (ASR) containing carboxylated random copolymers were used as a polymeric surfactant in the emulsion polymerization of polystyrene (PS) and poly(methyl methacylate) ( P M M A ) latex particles (4). Very small latex particles were formed compared with a conventional surfactant system. The A S R was found to become adsorbed as well as grafted on the surface of the latex particles. Thus, this would appear to offer a new route of changing the nature of the polymeric material of the latex particles. Poly(ethylene-co-acrylic acid) ( E A A ) containing an ethylene group are flexible thermoplastics having water resistance and barrier properties similar to low-density polyethylene (LDPE). Due to a high degree of inertness to chemicals, polyethylene is an appropriate material for containers and automobile tanks. The role of adsorbed surfactants in the process of film formation has received some interest and has been summarized by Bindschaedler et al. (5), and the barrier properties caused by them have been discussed (6). At high carboxyl levels, there are enough acid groups to be neutralized with alkali in hot water to form a surfactant-free dispersion. This dispersion can be applied as a polymeric surfactant in emulsion polymerization. However, few publications can be found that are concerned with E A A as a polymeric emulsifier in emulsion polymerization (7). The properties of E A A are expected to be similar to those of other polymeric surfactants, but their behavior and contribution in emulsion polymerization would be different from those of conventional polymeric surfactants. The importance of the interface in multiphase polymer systems has been long recognized. Physical and chemical interactions across the phase boundaries are known to control the overall performance of immiscible polymer blends. Searches have shown that the blending of P E T and P A in the melt produces polyester-polyamide block copolymers, which could improve the compatibility of the blends (8). Unfortunately, very few investigations on the improvement of compatibility of the blends without compatibilizer or functional groups present seems to be available in the literature. The present study was undertaken to investigate the contributions of E A A in the emulsion polymerization of styrene with a water-soluble initiator, potassium
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
325 persulfate. The aqueous solution behavior of E A A , such as aggregate formation, and the effects of the concentration of E A A on the latex particle size and size distribution were investigated. Also, the improvement of miscibility between PS and E A A during emulsion polymerization and the barrier properties of ethylenemodified latex films were investigated.
Experimental Materials 1
Poly(ethylene-co-acrylic acid) (EAA) [ M : 18,800 g mol" , M : 111,000 g mol" , acid number: 140], obtained from Dow Chemical Co. was used as received. Styrene and butyl methacrylate monomers were obtained from Junsei Chemical and were distilled under the reduced pressure of nitrogen. Pyrene was purchased from Fluka. Potassium persulfate, sodium hydroxide, triethylamine, sodium chloride, toluene, and methyl ethyl ketone were all analytical grade materials and were used without further purification. Distilled and deionized water was used throughout. n
w
1
Emulsion Polymerization Using EAA Batch polymerizations were carried out in a 1L double-wall round-bottom reactor equipped with a temperature controller, nitrogen inlet, and stirrer. In the reactor, E A A and NaOH were dissolved in distilled-deionized water, and monomer was added. The reactor was maintained for about 30 min for the contents to attain the reaction temperature, 70 °C. During this time, the contents of the reactor were stirred and flushed with nitrogen. The initiator was dissolved in the remaining water and added to the reactor. The addition of the initiator marked the start of the reaction. The polymerization was carried out under a nitrogen atmosphere until the reaction was substantially completed. The basic recipe of the emulsion polymerization in the presence of E A A is given in Table I. The sample I D . for prepared latex samples is listed in Table II. The conversion of styrene was determined by gravimetry.
Dynamic Mechanical Analysis (DMA) The simple extension mode was employed for the dynamic mechanical analysis. The dynamic mechanical behavior of the films was investigated using a dynamic mechanical thermal analysis instrument (DMTA) (Polymer
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
326 Labotatories, Model MK-III, U K ) equipped with an attachment for temperature control. This apparatus enabled us to measure the extension storage modulus (Ε'), the loss modulus (E") and the loss tangent (tan δ) over a wide range of temperature (-50 °C - 200 °C). The isochronal temperature dependence of moduli and tan δ was obtained for a frequency of 10 Hz at a constant heating rate of 2 °C min" under a nitrogen atmosphere. 1
Table I. Basic Recipe of Emulsion Polymerization Using Poly(ethylene-coacrylic acid) as a Polymeric Emulsifier Component D.D.I. Water Poly(ethylene-co-acrylic acid) (EAA) Sodium Hydroxide (NaOH) Styrene or Butyl Methacrylate Potassium Persulfate a
1
Amount 600 20, 40, 50, 60 variable 100 0.5
a
b
1
EAA: M = 18,800 gmol , M = 111,000 gmol" , Acid No. = 140 n
w
b
NaOH or triethylamine was added in all systems in order to change the degree of neutralization of EAA.
Table II. Sample I.D. for Prepared Latex Samples Composition
Sample I.D.
60 wt% EAA-Ethylene-Modified PS
EMPS-E60
40 wt% EAA-Ethylene-Modified PS
EMPS-E40
20 wt% EAA-Ethylene-Modified PS
EMPS-E20
60 wt% EAA-Blended PS
SBPS-E60
40 wt% EAA-Blended PS
SBPS-E40
20 wt% EAA-Blended PS
SBPS-E20
Chemical Resistance Test The latexes for the chemical resistance study were dried in a freeze dryer unit at -40 °C. Sheets in the form of disks were obtained by compression molding in a heated press (Carver hot press) at 130 °C and 3000 psi. The preheating time was 10 min and molding time was 60 min. From the prepared sheets, specimens were cut into disks of 50.80 mm in diameter and 3.175 mm in
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
327 thickness for the chemical resistance tests. This chemical resistance test was based on A S T M D543. The weight loss of the samples was calculated after their immersion in methyl ethyl ketone for 5 hours to compare the ethylene-modified PS with the simple blends at the same E A A concentration in the film.
Results and Discussion Aggregate Formation of E A A in Aqueous Solution Amphiphilic polymers, having both hydrophobic and hydrophilic groups, can form aggregates in aqueous solution. In order to find out whether the random copolymer forms aggregates in aqueous solution, pyrene solubility in E A A solution was studied. Pyrene is an aromatic hydrocarbon exhibiting a very low solubility in pure water ([Py] , ter ~ 7xl0" M ) and is largely used as a probe for the study of micelles and other hydrophobic aggregates in water (2). A n increase in pyrene absorbance with E A A concentration was observed (Figure 1). This indicated the formation of polymer aggregates like micelles in aqueous solution. The increase in solubility of pyrene could be attributed to an increase in the number of the aggregates. This trend is similar to that observed for conventional surfactants. 7
s wa
Figure 1. UV absorbance of pyrene at 360 nm and surface tension of EAA solution as a function of EAA concentration (wt% based on total). Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
328 In the surface tension versus concentration measurement, an equivalence to the critical micelle concentration (cmc) of low-molecular-weight surfactants is missing. These findings are in general agreement with recent work by Anton and Laschewsky (9) on the aggregation behavior of polymeric emulsifiers. As shown in the pyrene solubility results, however, E A A molecules in the aqueous phase seem to form aggregates at low concentrations before they start to transfer to the air-water interface.
Kinetics of Emulsion Polymerization of Styrene Investigation of the kinetics of emulsion polymerization typically involve the determination of the polymerization rate and its dependence on polymerization parameters such as the monomer, emulsifier, and initiator concentrations, and the temperature (10). It is suggested that both the solubility of the monomer in water and its polarity affect the mechanism of particle formation (11,12). For the conventional emulsion polymerization of styrene, examined by Harkins (71), the most important sites of particle generation are the monomer-swollen emulsifier micelles; with the increase of monomer solubility in water, the locus of particle nucleation may move to the aqueous phase (12). In the case of water-insoluble monomer such as styrene, micellar nucleation is the predominant mechanism in the polymerization process. Because of this fact, styrene was used as the monomer for this study on the kinetics of emulsion polymerization. Figure 2 depicts the conversion of monomer versus time for the emulsion polymerization of styrene at different E A A concentrations. Determination of the fractional conversion was accomplished by a gravimetric method. As shown in Figure 2, with increasing concentration of E A A as polymeric emulsifier, the rate of polymerization increased. It is generally expected that the increase of E A A concentration results in an increase in the number of aggregates providing polymerization loci, and thus, the rate of emulsion polymerization increases. This behavior of E A A in emulsion polymerization is similar to that observed with conventional emulsifier (13).
Particle Size and Particle Size Distribution of Latex Particle formation in emulsion polymerization using E A A would be affected by the properties of the E A A aggregates, which depend on the neutralization of E A A , as well as the content of electrolytes, and the water solubility of the monomer. The PS latexes prepared using excess neutralizing agent, NaOH, exhibits a small particle size (ca. 70 nm) compared to that observed for
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
329
Ο
20
40
60
80
100
120
Time (min) Figure 2. Conversion of monomer versus reaction time for the emulsion polymerization of styrene with different concentrations of EAA (wt% based on monomer). conventional emulsion polymers. In this system, very stable submicron size latexes are formed with a monodisperse size distribution. Figure 3 shows the PS latex particle size and size distribution obtained from this system using different ratios of E A A to styrene. The particle sizes of latexes increased from 69.1 to 82.4 nm in diameter with decreasing weight ratio of EAA/styrene and the particle size distribution became monodisperse with increasing weight ratio of EAA/styrene. This result was similar to that obtained in the emulsion polymerization using conventional surfactants, namely, an increase in E A A concentration results in more aggregate formation providing polymerization loci, and therefore, the particle size is smaller. Grafting The quantitative separation and characterization of PS homopolymer, alkali water-soluble E A A , and EAA-grafted-PS were carried out by using two consecutive selective solubilization steps. First, the EMPS-E40 powder was stirred in toluene. The toluene-soluble PS homopolymer is then separated from the sample by centrifugation. Second, hot alkaline water was used for selective solubilization of E A A to separate the alkaline water-soluble E A A from E A A -
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
330 grafted-PS. PS homopolymer was extracted in the toluene and the amount obtained was less than the amount calculated based on the theoretical PS weights. Thus, it was assumed that the remaining PS, 20.2% based on total PS, was grafted onto the E A A . After the second selective treatment in a hot alkaline water for 48h, 39.9% of the precipitated E A A was dissolved in the alkaline water, while the remaining 60.1% of the total E A A was alkali-water-insoluble ( E A A grafted-PS). Due to grafting of E A A molecules on the latex particles, the hindered desorption of the emulsifier from the latex would improve the latex stability and facilitate a core/shell-like structure. Also, it would prevent the migration and accumulation of E A A molecules (e.g., near the surface) after the film fomation of the latex, which deteriorates the performance of the final polymer products. That is, the EAA-grafted-PS formed in situ will then act as a compatibilizer for the main polymer components. The behavior and contribution of E A A in emulsion polymerization were found to be different from those of conventional polymeric surfactants. The E A A molecules not only act like
120
0
40
80
120
160
200
Particle size (nm) Figure 3. Particle size and size distribution of ethylene-modified PS with different EAA concentrations at 140% degree of neutralization of EAA.
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
331 surfactants but also provide their own characteristics to the latex for the E A A modified emulsion system (4).
Dynamic Mechanical Properties The miscibility of different polymer blends through various specific interactions can be predicted theoretically using the solubility parameter. The solubility parameter components may be predicted from group contribution theory, using the method of Hoftyzer and Van Krevelen (14). The solubility parameter for a random copolymer can be calculated by using the Hildebrand equation, as follows:
where 5, and
