Emulsion Polymerization of Styrene-Butyl Acrylate Copolymers

Objectives

The primary objectives of this experiment were:

1. To prepare and characterize water-based emulsions of different compositions between n-butyl acrylate and styrene copolymers with the presence of acrylic acid in a batch process.

Introduction

Polymers are integral to various aspects of daily life due to their versatile properties and applications. Synthetic rubbers, a type of polymer, find extensive use in products such as tires, protective gloves, adhesives, and paints. Synthetic rubber can be produced through emulsion polymerization, a technique first patented in 1929 by R.

P. Dinsmore while working for The Goodyear Tire & Rubber Company.

Emulsion polymerization, initially developed to address the limited availability of natural rubber during World War II, has led to the synthesis of various copolymers with distinct properties. In this experiment, we focus on styrene-butyl acrylate (SBA) copolymer latex preparation via a batch process. The resulting emulsions were characterized for pH value, solubility in organic solvents, and film properties using spectroscopic techniques (FT-IR and 1H NMR) and Differential Scanning Calorimetry (DSC).

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Methods

Starting Materials

All starting materials were meticulously prepared for use:

• Butyl acrylate (BuA), acrylic acid (AA), and styrene (St) were purified by treating them with a 10% NaOH solution, followed by thorough washing with distilled water to remove inhibitors.
• Potassium persulphate (KPS) and sodium lauryl sulphate (SLS) were employed as the initiator and surfactant, respectively.
• Distilled water was used as the dispersion medium throughout the experiment.

Pre-emulsion

The following steps were taken for pre-emulsion preparation:

1. Purged distilled water with nitrogen gas for 3 minutes.
2. Mixed the purged distilled water, St, BuA, AA, surfactant, and initiator in a 500ml beaker.
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3. Mixed the components thoroughly using a glass rod and set aside in an ice bath.

Batch Emulsion Polymerization

The batch emulsion polymerization was carried out as follows:

1. Added 20% of the pre-emulsified mixture into a four-neck flask equipped with continuous stirring under reflux.
2. Conducted the polymerization reaction at 70.0˚C using a thermostated water bath for 10 minutes at a stirring rate of 350 rpm.
3. Added the remaining 80% pre-emulsified mixture step-wise into the reactor within 1 hour (2-3 mL per minute) via a dropping funnel.
4. Continued the reaction by adding 20 ml of distilled water at the same slow rate.
5. Reduced the stirring rate to 20 rpm and allowed the reaction to proceed at room temperature.

Recipe for Different Compositions of Polymer

Physical Characterization of Emulsions

The emulsions were characterized by the following methods:

• pH measurement using standard indicator paper.
• Solid content, wt. %, determined by weighing the P(BA/St) copolymers before and after drying at 60˚C for one week.

Characterization of Films

The films casted from copolymer emulsions were characterized as follows:

• Solubility in various common solvents as listed in Table 3.
• FT-IR analysis using Attenuated Total Reflectance accessory (FTIR-ATR) spectral measurements.
• 1H NMR analysis on a Bruker (400MHz) spectrometer using deuterated chloroform (CDCl3) as the solvent.
• Differential Scanning Calorimetry (DSC) analysis.

Results and Discussion

Properties of the P(BuA/St) Copolymer Emulsions

The characteristic properties of the synthesized copolymers are presented in Table (2). Both emulsions were found to be acidic with Set II displaying higher viscosity and solid content (44.2%) compared to Set I. This difference can be attributed to the molecular weight and crystallite size variations caused by the differing styrene content in the compositions.

Properties of the Films Casted from P(BA/St) Copolymer Emulsion

Both latex films were milky white, but Set I was ductile while Set II was brittle. The differences in mechanical properties can be attributed to variations in molecular weight and styrene content, with Set II displaying higher hardness due to its increased styrene content.

Solubility

FT-IR-ATR Analysis

Distinctive IR absorption bands for the copolymer samples are tabulated below:

1H NMR Analysis

The 1H NMR spectra of P(BuA/St) copolymer samples were analyzed to determine their composition:

Notably, in the case of poly(butyl acrylate) (PBA), the (-OCH2) proton appeared around δ 4.0 ppm. For polystyrene, the aromatic protons showed splitting into two broad bands. The P(Bu/St) copolymer spectrum displayed (-OCH2) peak splitting, indicating variations in BuA content.

Copolymer Composition Ratio Analysis

The copolymer composition was calculated from the relative intensities of phenyl (S1) and –OCH2 (S2) proton resonances, as described in Table (5).

Conclusion

In this experiment, we successfully prepared and characterized styrene-butyl acrylate (SBA) copolymer emulsions with varying compositions. These emulsions exhibited distinct properties, including differences in pH, solid content, and mechanical characteristics. The analysis of solubility in various solvents provided valuable insights into the materials' suitability for specific applications.

Furthermore, spectroscopic techniques, such as FT-IR and 1H NMR, were employed to elucidate the molecular structure and composition of the copolymers. The results demonstrated how variations in monomer composition affected the polymer's properties.

This study contributes to our understanding of emulsion polymerization processes and the ability to tailor polymer properties for specific applications in industries such as coatings, adhesives, and materials science.

Future Work

Future research could explore the optimization of copolymer compositions to achieve specific desired properties, such as enhanced mechanical strength or improved solubility in particular solvents. Additionally, investigating the application potential of these copolymers in various industries, including automotive, paints, and adhesives, would be valuable.

References

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