Journal of Undergraduate Research
Volume 4, Issue 1 - September 2002

The Role of Solvent Polarity: In the Production of Poly (styrene-co-diethyl (aminoethyl) methacrylate MICROSPHERES

Vasana Maneeratana

ABSTRACT

The purpose of this experiment is to be able to clearly evaluate the role of the polarity of the solution in a dispersion polymerization with poly (Styrene-co-Diethyl (aminoethyl) methacrylate). The polarity of the solution affects not only the particle (microspheres) size but also the particle size distribution (the polydispersity index). The aim of this research experiment is to be able to confirm from previous research that as the polarity increases the microsphere size decreases. Therefore, the ability to calculate the data points from the trend, will allow assessment of the curves to delineate the trends during the polymerization of the microspheres within the polar solutions.

INTRODUCTION

Dispersion Polymerization is an important technique in attaining micron sized polymeric particles. The size of the particles and the distribution are important in the determination of the application. There have been many studies about the differing factors involved in the polymerization, such as the monomer(s) and its concentration, initiator concentration, temperature of the reaction, the presence and concentration of stabilizer, and solubility of the solvent. Dispersion Polymerization is ideal because the reaction takes place in a homogenous solution. The solubility of the components of the reaction is an important issue, because the polarity has to be able to keep the initial components soluble but allowing the polymeric spheres to precipitate out.

This experiment is a subset of a larger project in the evaluation of secondary caries. In our group, it is thought that microspheres, more specifically, magnetically labeled microspheres can be used to improve its detection. Wherein, secondary caries is defined as "carious conditions located at the margins of existing restorations." (Mjor and Toffnetti, 2000). It is noted that secondary caries is the most commonly cited reason for failure of directly placed restorations. Secondary caries have an existence in the micro-leakage channel between the tooth structure and the restoration, which provides bacterial and nutrient access. The general etiology of a carious lesion involves the formation of a plaque layer, which provides a good attachment and nutrient base for bacterial colonization; the primary strain of initial colonizing bacteria is Streptococcus mutans (S. mutans), which are eventually followed by lactobacilli. These are acidogenic species that cause the local pH to drop below the threshold at which demineralization of the tooth tissue occurs. Since the hydroxyapatite (the mineral component of tooth and bone) remodeling is a function of the environment concentration of its structural components. A local pH causes an undersaturation of these structural species in the fluid, and demineralization occurs. The pH at which the latter occurs is between 5.5 and 6.0, with the rate of demineralization increasing at lower pH values. (Liesenfeld, 2000) Our group and others have investigated means of incorporating dyes into such polymers, which may be released at specific pH values. (Leckey, 1997). Such microspheres can be prepared which will release dyes at various pH ranges thus indicating the pH necessary for carious activity. These microspheres may contain inclusions of magnetite present in roughly nanometer size particles. These particles have excellent magnetic properties and hence the whole microsphere can be moved easily with permanent magnets. Very strong permanent magnets based on the iron boron neodymium ceramic materials can exert extremely strong magnetic forces and have become available at low cost. Hence strong magnetic forces can now be applied conveniently at low cost to move magnetic objects. The amount of vibrational energy necessary to stimulate migration under the effect of some external force can be characterized, although it depends on the viscosity of the medium through which migration is desired. Therefore, the combination of pH sensitive monodisperse microspheres can be combined with magnetic material to produce a system, which will improve evaluation of micro-leakage channels beneath restorations, and demonstrate carious activity, if present. (Liesenfeld, 2000).

The need to explore each factor of dispersion polymerization is important. The most important purpose is to be able to synthesize the polymeric microspheres and be able to apply it to specific applications. Therefore the purpose of studying the role of solvent specifically is a crux in understanding more about the parameters surrounding dispersion polymerization. The importance of the particle size and its size distribution is therefore one of the deciding factors. The ability to deduce the role and the path of the polarity of the solvent as the polymerization progresses is therefore necessary to not only to observe the trend but also to conclude an ultimate polarity in which success can be defined.

TECHNICAL APPROACH

Dispersion Polymerization is an appealing single step process for synthesizing micron-size polymer particles. The mechanism of these processes is of considerable interest, particularly with respect to the formation of monodisperse vs. polydisperse particles, and in the control of particle size (Tseng, C.m., Lu, Y.Y., El-Asser, M.S., Vanderhoff, J.W., 1986). Within our group, previous work was demonstrating that pH-responsive polymer swelling, with attendant release of entrapped dye or drug would be feasible for many polymer systems (Leckey, 1997). Expanding on Leckey's experiment, the monodispersity will be ideal for the measurement of swelling, because polydispersity presents a problem with consistency of swelling data. The size will be needed for determination of its application. The level of success in each of these applications is ultimately dependent upon the particle size and its distribution. In order to achieve the objective, the microspheres will be produced from a copolymer of (St-co-DEA) with DVB as stabilizer. Poly (vinylpyrrolidone) (PVP) will be used as a co-stabilizer in controlling the narrow polydispersity of microspheres. The solvent will be that of an ethanol and water mixture. Measuring the sizes and polydispersity indexes by varying the volume percentage of the ethanol and water mixture will test the hypothesis.

Unlike other processes of polymerization, such as emulsion and suspension, the single step technique of dispersion polymerization is initially a homogenous solution in a continuous phase with the monomer. The latter, therefore puts certain constraints on the types of solvents that can be used in combination with monomers and stabilizers. Tseng, et al. (1986) and Lok and Ober (1985 and 1986) discussed the mechanism for particle formation and growth in dispersion polymerization. Upon heating, the initiator decomposes and the free radicals react with solute monomer to form oligomeric radicals. It is at a critical chain length that the oligomers will eventually precipitate from the homogenous solution and adsorb stabilizer to form an unstable nuclei. It is noted that a key characteristic of dispersion polymerization is that the nuclei or particle will eventually become immiscible with the homogenous solution and precipitate, thus forming a new phase. When sufficient mature microspheres are formed that can capture all the radicals and nuclei in the continuous phase, no more particles will be formed, the nuclei or microsphere formation stage is complete (Horak, D., Shapoval, P. 2000). Therefore the relative amount in each phase is determined by choice of reaction conditions.

In accordance with Cao,K., Li, B., Huang, Y., Li, B., Pan, Z. (2000) and Lu, Y.Y., El-Assser, M.S., Vanderhoff, J.W. (1990) one of the key features of dispersion polymerization, is the copolymer stabilizer, poly (vinylpyrrolidone) (PVP) that forms in the continuous phase in the solution. The costabilizer functions as a steric stabilizer, which can prevent flocculation and aggregation of the particles being formed. Hence, this method is dependent on incipient aggregation of the polymerizing species at the early stages of the polymerization; the numbers of particles are determined by these growing nuclei. In order to achieve monodispersity, the role of the costabilizer is to not allow additional growth of the particle, because the growth of the nuclei or particles results in the particles of differing diameters in the system. The stable microsphere formation stage becomes shorter, thus reducing the nucleation time and resulting in narrow size distribution samples (Horak, Svec, Frechet 1995).

The initial issue with the solvent of the system is that it has to be of a certain polarity to allow miscibility of the initiator, monomers, and stabilizers. The solvent's medium polarity, according to Horak and Shavopal (2000), indicates that it is a key factor in determining the particle size and distribution because it controls the critical molecular weight above that the polymer precipitates.

The procedures are detailed in Leckey (1997). Dispersion polymerizations will be conducted in 25mL glass vials with screw-on caps placed in a constant temperature shaker bath set at 55ºC and roughly 100 cycles per minute. The standard components of the reaction consist of the monomers of Styrene and Diethyl (aminoethyl) methacrylate, stabilizers of Divinyl Benzene and poly (vinylpyrrilidone) (PVP 40, 40,000 MW), AIBN (2,2' &endash; azobisisobutryonitrile) (recrystallized in methanol), and the varying volume percent of ethanol/water mixtures varying from 100% ethanol to 55%. All monomers will be distilled prior to use. In the case of DVB, it will be washed with a 10% sodium hydroxide aqueous solution.

The following recipe will be used for the dispersion polymerization of the microspheres: 5% of monomers, 3% PVP, 0.3% AIBN, and the balance of 91.7% solvent. Initially the PVP is made soluble in ethanol and hence filtered directly into the reaction vessel through 0.45 mm pore size syringe filter. The AIBN is then dissolved into the monomer while mixed into the solution by the vortex for approximately a minute. Nitrogen gas is the bubbled through the reaction vessel for 2-3 minutes prior to the capping of the vessel and placing it horizontal into the shaker bath for 24 hours at 55ºC. Once the reaction is complete, the solution is hence poured into the 100mL water and stirred for 1 hour. After the washing of the microspheres, the microspheres are hence collected by centrifugation, and lastly dried in a paper filter.

Tasks

1. Produce Microspheres

2. Organize and Produce Variable Solvent Mixtures.

3. Test the Size and Distribution of Microspheres.

4. Graph Data.

5. Apply data to further applications.

EXPERIMENTAL PROGRESS

Microspheres were produced using the solvent systems as prescribed earlier. Only "success" was defined for the solvent systems of 80% Ethanol and 20% Water. Success has been defined as a product of microspheres with a polydispersity index of 1.0 &endash; 1.1. The polydispersity index was calculated from the data obtained in the Coulter Laser Particle Size Analyzer. See Appendix A.

The results of the successful microspheres follow the expected patterns of previous experiments. The size of the microspheres inversely follows the increase in the polarity of the solvent.

IMPLICATIONS OF RESEARCH

The solvent system of 80%-Ethanol will be utilized to assess its compatibility in further experimentation of the application of these microspheres. Currently the microspheres are being tested for the polymerization of the microspheres incorporating magnetite particles. The existing difficulties arise with the adherence of the microsphere to the Fe3O4 surface. Previous research has shown that attaching a coupling agent leads to the incorporation of the magnetic particle into the polymer but has is still being worked on in our group. However, research is simultaneously being investigated in utilizing the nitrogen groups within the co-polymer system to induce magnetite precipitation within the microsphere realm. Swelling experiments, also are being conducted for the microspheres, however the fusion of the data obtained from the solvent system, magnetic incorporation, and swelling experiments are still being compiled.

REFERENCES

Cao K, Li BF, Huang Y, Li BG and Pan ZR, Mechanism and Model of Dispersion Polymerization Using Homopolymer as Dispersant in Polar Media, Macromolecular Symposia, 2000; 150:187-194

Horak D et al., J Polym Sci A, Polym Chem ed., 2000; 38:3855-63

Horák D, _vec F and Fréchet JMJ, Preparation and Control of Surface Properties of Monodisperse Micrometer Size Beads by Dispersion Copolymerization of Styrene and Butyl Methacrylate in Polar Media, Journal of Polymer Science A: Polymer Chemistry, 1995; 33:2329-38

Leckey A, Active Microspheres for use in the Treatment of Hepatic Tumors, [Masters Thesis] University of Florida; 1997

Liesenfeld Bernd, Detection of Secondary Caries with Polymeric Magnetic Microspheres, [NSF Grant Proposal] University of Florida; 2000.

Lok PK and Ober CK, Particle size control in dispersion polymerization of polystyrene, Canadian Journal of Chemistry, 1985; 63:209-216

Ober CK and Lok KP, Formation of Large Monodisperse Copolymer Particles by Dispersion Polymerization, Macromolecules, 1987; 20:268-73

Mjor IA and Toffenetti F, Secondary Caries: A Literature review with case resports, Restorative Dentistry, 2000 31(3): 165-179.

Tseng CM, Lu YY, El-Aasser MS and Vanderhoff JW, Uniform Polymer Particles by Dispersion Polymerization in Alcohol, Journal of Polymer Science: Part A: Polymer Chemistry Edition, 1986; 24:2995-3007

APPENDIX A

Graph of "Successful" Dispersions

This is one of many data plots that delineate the trend that our experiments conveyed. Note:

Solubility parameter varied: Ethanol:H²O from 80:20 to 90:10
D030-D032 indicate sample numbers
90:10 produced bi-modal (D030-31) and tri-modal (DO32) samples: not desirable!

The above graph indicates the decreasing trend in the size of the micron sized co-polymeric particles as the solubility parameter increases. This is the specific region we would like to further concentrate our research. In each of the samples names: such as DO30, there were 5 samples that had a different amount of percent ethanol to water. (Ranging from 80.0, 82.5, 85.0, 87.5, and 90.0 percentages). Note: some of the data were not accountable due to experimental error in measurements from the Coulter Laser Particle Size Analyzer.

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