Dispersion polymerization has been widely applied to the synthesis of monodisperse micron-sized polymer colloidal spheres. Many efforts have been devoted to studying the influence of initial conditions on the size and uniformity of the resultant microspheres, aiming to synthesize micron-size monodisperse colloidal spheres. However, the inner contradiction between the size and the size distribution of colloidal spheres hinders the realization of this goal. In this work, we drew our attention from the initial conditions to the growth stage of dispersion polymerization. We tracked the size evolution of colloidal sphere during the dispersion polymerization, through which we established a kinetic model that described the relationship between the monomer concentration and the reaction time. The model may provide a guideline to prepare large polymer colloidal spheres with good monodispersity by continuous monomer feeding during the growth stage to maintain the concentration of monomer at a constant value in a dispersion polymerization process.
As a new functional macromolecule material, monodisperse polymer microspheres have many applications in environmental conservation, biomedicine, colloid science, electronic information material, and many other areas [
The first report on monodisperse polymer colloidal spheres was the polystyrene spheres prepared by Vanderhoff and Brandford [
To attain larger microspheres with excellent monodispersity, many efforts have been made to explore the influence of the initial condition of the reaction system on the resultant microspheres, such as the dosage of monomer, initiator, and dispersing agent. Generally, with the increase in the monomer’s initial concentration, the sphere diameter increases and the distribution of sphere diameter broadens. Thus, there is a suitable range for monomer’s initial concentration to achieve uniform and large microspheres [
Recently, Song et al. [
In this work, we focused on the whole process in one-stage dispersion polymerization and tracked the size evolution of colloidal sphere at different reaction time. Based on the experimental results, we established a theoretical model to describe the relationship between the monomer concentration and the reaction time during dispersion polymerization. The model was expected to provide guidance to the two-stage or the multi-stage dispersion polymerization, aiming to synthesize micron-sized large and monodisperse polymer colloidal spheres.
All chemicals, including styrene (St, 99%, Aldrich), anhydrous ethanol (99.95%, Aldrich), polyvinylpyrrolidone (PVP, MW 40000, TCI), and azobis(isobutyronitrile) (AIBN, Sinopharm Chemical Reagent Co., Ltd), were used as received without further purification. Deionized (DI) water (resistivity greater than 18.2 MΩ·cm, Ultra Pure UV, China) was used in all experiments.
Briefly, the synthesis was conducted in a three-neck round-bottom flask equipped with a condenser and immersed in a water bath with a preset temperature. Firstly, the monomer of 17 mL styrene and the stabilizer of 1.5 g PVP were dissolved in 98 mL anhydrous ethanol and then added to the flask. The solution was stirred with a rate of 200 rpm and the temperature was kept at 70°C while nitrogen gas was continuously led into the flask to avoid the oxidation of the initiator AIBN in the whole process. After 30 minutes, an initiator solution of 0.15 g AIBN dissolved in 28 mL ethanol was introduced into the system. The reaction continued for 24 h and then was stopped by cooling at ambient conditions. To track the evolution of sphere size during dispersion polymerization, the mixture in the reaction flask was sampled at specific intervals and dried on a glass substrate. The sphere size was measured using an optical microscope.
The optical microphotographs of the PS colloidal microspheres were taken using an optical microscope (Olympus, BX51TRF), which was connected to a CCD camera (Pixelink-B742) and a computer for real-time image recording.
As can be seen from the optical microscope photograph shown in Figure
Optical microphotograph of PS colloidal spheres prepared by dispersion polymerization.
The process of dispersion polymerization can be basically divided into several stages. At the beginning of the reaction, monomer, dispersing agent, and initiator are all dissolved in the solvent. When the temperature reaches the decomposition temperature of the initiator, the initiator decomposes and produces radicals. The radicals trigger the polymerization process, which results in formation of the oligomers. When the chain length of the oligomers exceeds a critical value, nucleation occurs and the oligomers precipitate from the solvent. At the same time, dispersing agents are absorbed by the nuclei and form graft copolymers with the polymer chains, which makes microspheres stable in the solvent. Then the synthesis enters the stage of growth. Monomer and copolymer in the solvent are persistently captured by polymer nuclei to provide the motive power of the growth, which keeps on until termination of the reaction. In the process of dispersion polymerization, the nucleation stage is a transition period for the system from homogenous phase to heterogeneous phase. This period determines the number of microspheres in the system which should be constant during the polymerization to guarantee the uniformity of final microspheres [
Knowing the evolution of monomer concentration during dispersion polymerization is of importance especially for synthesis of large and uniform colloidal spheres. Considering the reaction mechanism involved in dispersion polymerization, however, a large proportion of the monomers exist in the solvent in the form of oligomers whose chain length does not exceed the critical value for nucleation. Thus, direct measurement of the monomer concentration is not feasible. Herein, we establish a theoretical model to describe monomer concentration at different reaction time based on the diameter of microspheres measured at different time intervals.
The following are the major assumptions used in formulating the model. Firstly, in view of the good monodispersity of the final microspheres, the reaction is considered as an ideal dispersion polymerization, in which nucleation number is just determined by the nucleation stage, and secondary nucleation does not occur in the later growth stage. It is also assumed that the number of microspheres remains during growth stage equal to the nucleation number at the beginning of reaction. Secondly, although the microspheres in the growth stage may mostly seize the oligomer chains rather than the monomers, it is considered that the increasing rate of the microspheres’ weight or volume is proportional to the equivalent concentration of the monomers, some of which are in the form of free oligomers. Thirdly, considering the actual slight volume change during the mixture process of styrene and ethanol, which can hardly be observed in the experiment, the deviation of volume is neglected in our model to reduce the complexity of the calculation. Thus, a simple approximation was adopted by the replacement of the total volume of the mixture of several liquids with the sum of their own volumes.
According to the conservation of mass, the relationship between the product and the reactant at any time
where
where
The concentration of styrene in the system at time
where
By putting together the three equations above, out comes the relationship between the concentration of monomer and the average diameter of microspheres at time
Based on our experimental data, we input relevant parameters. The initial parameters were the same as the data described in the experimental section, while the density of microspheres was taken as that of polystyrene. According to the former assumption, the number of microspheres was a constant value which is equal to the nucleation number. Thus, we set the time
Thus, with the input of the initial parameters and the estimated number of microspheres
To introduce the time dimension, we measured the diameter of the PS spheres sampled from the reaction system at different time intervals. According to the relationship expressed in (
The sphere diameters and monomer concentration at different time.
Reaction time (h) | Sphere diameter ( |
Monomer concentration (g/mL) |
---|---|---|
0 | 0 | 0.105 |
1 | 1.170 | 0.098 |
1.5 | 1.364 | 0.094 |
2 | 1.360 | 0.094 |
3 | 1.760 | 0.080 |
4 | 1.960 | 0.069 |
5 | 2.114 | 0.060 |
7 | 2.376 | 0.039 |
9 | 2.410 | 0.036 |
11 | 2.652 | 0.010 |
23 | 2.720 | 0.001 |
In order to further deduce the relationship between the monomer concentration and the reaction time, we calculated different function forms of monomer concentration versus reaction time and processed the data with linear fitting. The results are shown in Figure
Linear fitting results of different function forms of concentration
As can be seen in Figure
where the initial monomer concentration
Combining (
The evolution of spheres diameter with extended reaction time. The squares stand for the experimental points.
Through the deformation of (
With the relationship between the monomer concentration and the reaction time described in (
Thus, assuming that the initiator does not lose efficacy during the reaction, the monomer concentration will be held at a constant value if one continuously feeds extra monomer into the system according to the consumption rate described in (
According to the analysis above, we calculated the relationship between the beginning time of the monomer feeding and the monomer feeding rate within the present system of our experiment, which is shown in Table
Different monomer feeding rates required at various beginning time of continuous monomer feeding.
|
|
|
|
|
---|---|---|---|---|
1 | 0.0950 |
|
140.9 | 1.48 |
2 | 0.0860 |
|
139.4 | 1.32 |
3 | 0.0778 |
|
138.0 | 1.18 |
4 | 0.0704 |
|
136.8 | 1.06 |
5 | 0.0637 |
|
135.7 | 0.95 |
6 | 0.0576 |
|
134.7 | 0.86 |
7 | 0.0521 |
|
133.9 | 0.77 |
8 | 0.0472 |
|
133.1 | 0.69 |
9 | 0.0427 |
|
132.4 | 0.62 |
10 | 0.0386 |
|
131.8 | 0.56 |
In summary, a kinetic model has been established to describe the exponential relationship between the monomer concentration and the reaction time during synthesis of colloidal spheres by using dispersion polymerization. Accordingly, one may gain the quantitative relationship between the monomer consumption rate and the reaction time. Taking advantage of this model, it is possible to attain larger colloidal spheres by continuously feeding monomer into the reaction system during the growth stage according to the monomer consumption rate. In this way, the reaction rate is kept at a high constant all the time until the reaction is artificially ended. Furthermore, secondary nucleation can be effectively avoided because both the average concentration and the local concentration of monomer in the reaction system are less than the critical value. Ultimately, large and uniform colloidal spheres can be prepared using the dispersion polymerization technique.
The authors declare no competing financial interests.
This work was supported by the Natural Science Foundation of China (Project nos. 50903046 and 51173097) and the National Key Basic Research Program of China (no. 2013CB632902). The Tsinghua University Initiative Scientific Research Program and the Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation are also acknowledged for partial financial support.