Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Room-Temperature Synthesis of Size-Uniform Polystyrene Latex and Characterization of Its Properties: Third-Year Undergraduate Teaching Lab Nimer Murshid,† Nicole Cathcart, and Vladimir Kitaev* Department of Chemistry and Biochemistry, Wilfrid Laurier University, 75 University Avenue W, Waterloo, Ontario N2L 3C5, Canada
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S Supporting Information *
ABSTRACT: Teaching emulsion polymerization is an important benchmark in undergraduate polymer courses. However, the introduction of hands-on experiments in undergraduate polymer laboratories is challenging: experimental time, reagents, and equipment involved are the primary restraints. We report a practical emulsion polymerization laboratory experiment for undergraduate students that includes the synthesis of polystyrene (PS) latex and exposure to multiple methods of characterization. Using a room-temperature redox initiation system of ascorbic acid (vitamin C) and hydrogen peroxide, and scaling the reaction down to ca. 10 mL, simplifies stirring and degassing and alleviates the aforementioned challenges. To prepare latex samples with different sizes and low size dispersity, students explore an effect of a steric stabilizer (polyvinylpyrrolidone, PVP). For effective time management, students perform characterization of previously prepared latex, while qualitatively and quantitatively monitoring particle growth during polymerization stages. Latex characterization performed by students includes measurements of average particle size and zeta potential, estimating conversion and preparation of latex films with determination of glass transition temperature (Tg). Other characterization methods that can be incorporated in the extended experiments include electron microscopy, atomic force microscopy, and other techniques of colloidal and polymer characterization. Additionally, the prepared latex is able to form colloidal photonic crystal films, enabling size evaluation by color or UV−vis measurements, where selective reflection of blue, green, and red corresponds to latex with particle diameters of 185, 240, and 300 nm, respectively. On the basis of the lab evaluation survey, students performing the experiment have an overall positive learning experience with 94% satisfaction rate. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Polymer Chemistry, Hands-On Learning/Manipulatives, Polymerization, Free Radicals, Colloids
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INTRODUCTION Polymers are omnipresent materials in our everyday lives and are especially important with respect to environmental sustainability. Not duly reflecting this importance, polymers are historically underrepresented in the undergraduate learning experience.1,2 This point is emphasized by Flores-Morales et al., who bring the arguments for a push to teach polymer science to undergraduate and graduate students,1 and is echoed by Moore and Stanitski, who present a convincing case for the importance of including polymers in general chemistry courses.2 Along with integrating polymers into the chemistry curriculum, polymer courses need laboratory experiments that are crucial for students to acquire strong applied knowledge, through hands-on experience and mastering practical skills. That is especially true for teaching emulsion polymerization, as one of the integral components of courses in polymers and polymerization.3,4 There is a limited number of published lab © XXXX American Chemical Society and Division of Chemical Education, Inc.
experiments that give an opportunity for students to have hands-on experience preparing and characterizing polymers. Recent lab experiments published in this Journal detail the preparation of colloidal polymer (poly(lactic-co-glycolic acid)) nanoparticles and their characterization using dynamic light scattering (DLS) in two 180 min sessions;5 related colloid and interface concepts were investigated using SiO2 particles modified with a surfactant for phase transfer in an emulsion.6 Previously reported polymerization experiments include the preparation of polymeric microparticles, encapsulating annatto seed extract,7 and a lab exercise preparing poly(vinyl alcohol) (PVA) glue through free-radical emulsion polymerization.8 One common theme of polymer experiments is adapting Received: July 31, 2018 Revised: April 16, 2019
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DOI: 10.1021/acs.jchemed.8b00618 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
ing, as well as analyzing and communicating their experimental results.
industrial processes to the microscale, as exemplified by Lewis et al.9 in their modification of the polymerization procedure reported by Taguchi et al.10 Polymer characterization experiments reported in this Journal include reaction monitoring with IR spectroscopy,11 integration of structure−property relationships, and mechanical property analysis.12 Additionally, strategies to include light-scattering characterization in the undergraduate curriculum have been presented.13 Building upon this experience with room-temperature initiation, the developed experiment presents a procedure for undergraduates to prepare colloidal polystyrene (PS) latex, provides hands-on experience with polymerization, and can be adapted to include several methods of polymer and colloid characterization depending on available instrumentation. Emulsion polymerization produces aqueous colloidally stable polymers with versatile environmentally friendly applications in latex paints, water-born adhesives, and synthetic rubbers.14−16 Polymers prepared using this method form colloidal systems, named latex based on their similarity in appearance to milk.15 Active species are commonly generated during the initiation step via an endothermic process involving homolytic scission of a chemical bond; this step is important for choosing suitable reaction conditions.17,18 Light-active initiators (where bonds are cleaved by UV or violet light) are commonly used for polymerization in films and coatings. In larger volumes, emulsion polymerization is usually performed at elevated temperatures (70−90 °C) to generate radicals by thermal cleavage of labile peroxo or diazo bonds.16 Using redox initiator systems enables initiation at lower temperatures.19 Common redox initiator systems used in emulsion polymerization are persulfates and tetramethylethylenediamine (TMEDA),20−23 which are quite toxic and hazardous. Mediating heat uniformity (thermal initiation) or hazards is a challenge for implementing teaching lab experiments. Additionally, reaction vessel and setups are complicated.24 There is a strong need for the development of an undergraduate lab to mitigate these factors and to introduce students to latex synthesis and related polymer and colloid characterization. Herein, we report an emulsion polymerization experiment for upper-division undergraduate students. In this experiment, third-year undergraduate students enrolled in an advanced chemistry course named Polymers and Soft Matter at Wilfrid Laurier University were able to synthesize PS latex at room temperature, using ascorbic acid and hydrogen peroxide as a green and safe redox initiation system. The reaction volume is scaled down to 11−12 mL, using affordable 20 mL scintillation vials as reactors. The main experimental parameter explored by students is the effect of a steric stabilizer (polyvinylpyrrolidone, PVP) on the latex particle size. Three groups of students work with different PVP concentrations to prepare latex with different sizes. The variation of particle size enables preparation of latex films of different colors with size-uniform latex particles. The growth of latex particles is monitored during the reaction process with the naked eye, laser beam (laser pointer), and dynamic light-scattering (DLS) measurements. To manage the lab time efficiently, students characterize previously prepared latex samples determining solid content, particle size, and zeta potential. Performing this experiment, students gain a wide range of science process skills,25 e.g., observing, measuring, predicting and experiment-
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PEDAGOGICAL VALUE Performing emulsion polymerization to prepare PS latex and characterize its diverse properties provides numerous pedagogical values. Most important is the translation of students’ theoretical knowledge of emulsion polymerization into real life practical skills. Taking inspiration from Kolb’s theory of stimulation, reflection, abstraction, and experimentation,26 in a more applied approach, such as science process skills, students developed and enhanced their experimental, measurement, and observation skills. The described hands-on approach enabled students to set up and monitor emulsion polymerization, following nucleation and particle growth, as well as to learn about the properties of latex as a polymer material and colloidal system. These concepts are introduced in lectures prior to the experiment and are reflected upon through postlab reporting, where trends in size and PVP concentration are considered. Students are introduced to redox reactions and their functioning in the initiation of free-radical polymerization. Students practice the inquiry approach by investigating the effect of the concentration of a steric stabilizer on the particle size and size uniformity. The described laboratory experiment provides ample opportunities to gain hands-on experience in interdisciplinary characterization methods, e.g., DLS, to determine particle size and zeta potential, and to learn about colloidal surface charge and its importance for the colloidal stability. Employing imaging techniques, such as atomic force microscopy (AFM) and electron microscopy (EM), offers further dimensions to the characterization relevant for research and industries. Overall, the described experiment provides important practical skills that should be valuable for students in a wide range of disciplines including chemistry, material science, and engineering.
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EXPERIMENTAL SECTION
Overview
Third-year chemistry students are introduced to emulsion polymerization in lectures and then prepare PS latex in this lab experiment. Free radicals are generated by an initiator system of ascorbic acid and hydrogen peroxide that are only soluble in the aqueous phase and not in the monomer (styrene). Initiation takes place in the aqueous phase, as well as initial chain propagation, as is characteristic of emulsion polymerization. Growing chains become increasingly insoluble and begin to phase segregate, forming colloidal particles that enlarge until the monomer is depleted.14,15,17 The resulting product of emulsion polymerization is a colloidally stable polymer dispersion in an aqueous medium, known as latex.14,15 Experimental Organization
The specific outcomes of this experiment for students are to (1) synthesize a sample of PS latex; (2) estimate polymer conversion; (3) investigate the effect of PVP concentration on the particle size and dispersity; (4) characterize different samples of PS latex using DLS and electron microscopy images; and (5) fabricate latex films and estimate particle sizes from the film colors based on diffraction of colloidal photonic crystals.27,28 This experiment has been performed for three years in a 3 h laboratory period with 8−12 third-year chemistry students that worked in groups of two (32 students in total). All groups B
DOI: 10.1021/acs.jchemed.8b00618 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 1. Schematic illustration of polymerization stages in preparation of PS latex.
Figure 2. Representative student results showing (a) DLS data and (b) optical photographs taken after the initiation and during the polymerization process (1.29 mM PVP). (c) DLS and zeta-potential measurements and (d) AFM image of the final product of PS polymerization (0.85 mM PVP). Data presented in parts c and d were measured 1 day after PS preparation.
nitrogen flow. (Detailed synthesis procedure and lab tips are provided in Supporting Information (SI).)
started their experiment with the synthesis of PS latex. After the students set up their polymerization reactions, they monitored the particle growth using DLS measurements. During downtime, students also performed the characterization of previously prepared latex samples that were provided to them.
Characterization
DLS measurements were performed to determine the hydrodynamic diameter (Dh) and polydispersity index (PDI) as the main characterization method in the described experiment, given the availability of a Malvern Zetasizer (Nano ZS). During the reaction time, a 10 μL sample was taken every 20 min and diluted to 2.0 mL with deionized water prior to particle size measurements in a 4 mL poly(methyl methacrylate) (PMMA) cuvette. To measure zeta potentials, the previously diluted latex sample was transferred to a disposable folded capillary cell (DTS1070, Malvern) in an electrophoretic light-scattering (ELS) setup of Malvern Zetasizer at 25 °C. For further characterization, both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed using Hitachi S-5200 to evaluate particle size and morphology. AFM experiments were conducted using Bruker Innova in noncontact mode. The EM and AFM measurements have not been performed by students in our experiments; instead, students analyzed the images provided. Imaging can be performed given the availability of instruments and additional lab time and can include optical microscopy
Synthesis of Polystyrene Latex
To a 20 mL scintillation vial (attached to a retort stand with a clamp and capped with a 19 mm siloxane rubber septum) containing a cross-shaped Teflon-coated magnetic stir bar was injected 10 mL of degassed water. Water was then stirred uniformly with high stirring speed of 800−1200 rpm. Using a 1 mL syringe, 150 μL of 0.025 M 4-vinylbenzenesulfonic acid sodium salt (St(−)) was added, followed by varying amounts of 0.05 M PVP (200 μL in a typical experiment) and 370 μL of 0.2 M hydrogen peroxide. During steady and fast stirring, 560 μL of purified styrene was added. Nitrogen gas was purged through the system using a series of tubing and needles (see Figures S4 and S8 in the Supporting Information for the details of the experimental setup), at ca. 30 bubbles per minute. After 10 min, while preserving nitrogen flow, 370 μL of 0.2 M ascorbic acid was injected to the vial through the septum to initiate the reaction. The stirring speed was kept high (1200 rpm) and uniform during the reaction under the steady C
DOI: 10.1021/acs.jchemed.8b00618 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 3. Representative student results showing optical microscopy and electron microscopy (EM) images of the PS latex samples with an average diameter of (a, b) 200 ± 5 nm, (c, d) 245 ± 10 nm, and (e, f) 295 ± 12 nm, prepared using 1.3, 0.85, and 0.60 mM of PVP, respectively. The insets in top panel images are optical photographs of the self-assembled latex films obtained by drying of a small droplet of the latex dispersion on a glass slide.
(Figure 1). Figures S3 and S4 in Supporting Information represent three intervals of the propagation stage in the emulsion polymerization. After 15−30 min of the reaction, students are able to observe the nucleation stage apparent by bluish light scattering; 40−60 min later, the reaction mixture becomes milky white, which is indicative of the formation of larger particles (“monomer-swollen” entities depicted in Figure 1). The growth of the latex particles monitored by DLS and optical photographs is shown in Figure 2a,b, respectively. DLS measurements are a convenient and readily accessible method to monitor particle growth during emulsion polymerization by measuring the hydrodynamic diameter (Dh) of the growing particles. Representative DLS data obtained by students, who monitored the reaction over time, are shown in Figure 2a. More detailed information for two different latex samples is given in Figure S9 in the Supporting Information. The final product of the developed emulsion polymerization process is size-uniform PS latex. As shown in Figure 2c, DLS analysis of a representative latex sample with prominently small PDI (
