Characterization study of polyAMPS@BMA core-shell particles using two types of RAFT agents

Potassium persulfate, sodium hydride, magnesium turnings, and carbon disulfide were purchased from Daejung, Korea. 4-vinylbenzyl chloride, 4,4′-azobis(4-cyanovaleric acid) (ABCA), dimethylformamide (DMF), BMA, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), iodine, magnesium sulfate, potassium persulfate (K₂S₂O₈), n-hexane, diethyl ether, bromobenzene, dimethyl sulfoxide, petroleum ether, and tetra hydrofuran were purchased from Sigma-Aldrich. Pyrroles and silica gel were the products of Unichem, USA.

The FTIR analysis was done via Thermo Fisher Scientific model NICOLET iS5. Scanning electron microscopic and energy dispersive X-ray analyses were done via field-emission scanning electron microscopy (SEM), JEOL Japan, model JSM5910, with an acceleration voltage of 30 KV. X-ray diffraction (XRD) pattern was recorded via an X-ray diffractometer (model JDX-3532), JEOL Japan, by using Ni-filtered Cu Kα radiation and a wavelength of 1.5418 Å. Thermogravimetric analysis was done with TGA instrument from PerkinElmer (USA), model TGA 4000.

In the study, two different RAFT agents 4-vinylbenzyl pyrrolecarbodithioate and 4-vinylbenzyl dithiobenzoate (4VP and 4VD, respectively) were separately prepared using the reported protocols with some modifications [24]. Briefly, for the preparation of 4VP, 6.02 g of NaH was mixed in 160.00 ml of DMF, followed by stepwise addition of 10.02 g/20 ml of pyrrole, 9.01 ml/20 ml CS₂, and 22.2/20 ml of 4-vinylbenzyl chloride, which were all dissolved in DMF. The mixture was stirred for 12 h. For the isolation of 4VP, the resultant product was washed with diethyl ether and distilled water (1:1), followed by extraction via column chromatography, in which petroleum ether was used as a mobile phase. The petroleum ether was separated through vacuum distillation, and the final product, 4VP, was stored at −18 °C in an inert environment. For the synthesis of 4VD, 3.301 g of magnesium turning was dissolved in 14 ml/40 ml bromobenzene (in tetrahydrofuran), and to initiate this reaction, 0.1 g of iodine was added. CS₂ [7.91 ml/5 mL (in tetrahydrofuran)] and 4-vinylbenzyl chloride [7.19 mL/5 ml (in tetrahydrofuran)] were added in a stepwise manner after stirring for 12 h. The same isolation process was followed for 4VD. Structures of these two RAFT agents are given in Figure 1.

Fig. 1

For the preparation of water-soluble polymer poly(2-acrylamido-2-methylpropane sulfonic acid) (polyAMPSi), the previously reported method was adopted with certain minor modifications [23]. Briefly, for the synthesis of polyAMPSi, a known amount of AMPS monomer, 4VP (25:1), and ABCA were added to 25 ml of dimethyl sulfoxide under an oxygen-free environment. The reacting flask was deoxygenated with nitrogen gas purging and vacuum pump followed by fitting it to water bath for 12 h at 333 K. After 12 h at 333 K, a brownish color polyAMPSi was prepared which was stored at 268 K in an inert environment. The same protocol was used for polyAMPSii but 4VP was replaced with 4VD.

For the preparation of CSi, 25 ml of BMA was slowly added to 1 g/300 ml of polyAMPSi solution in distilled water, followed by addition of potassium persulfate (K₂S₂O₈) solution (1 g/15 ml). The whole setup was oxygen-free; this was ensured through the usage of an overhead condenser and magnetic stirrer, which was later fitted in a water bath for 12 h at 333 K. The same protocol was used for preparation of CSii but polyAMPSi was replaced with polyAMPSii.

FTIR analysis was carried out to confirm the presence of various functional groups. TGA analysis was used to study the control of RAFT agents over thermal properties (stability). XRD was done to determine the effect on the semicrystalline nature of the obtained CS particles. The SEM study was used to study the morphology and influence of RAFT agents on the particle size of the obtained CS particles.

The FTIR analysis given in Figure 2 reveals the successful preparation of polyAMPSi and polyAMPSii, which is confirmed by the presence of various characteristic peaks, such as asymmetric and symmetric vibrations of −SO₂ at 1,031 cm⁻¹ and 1,210 cm⁻¹, N-H and C=O vibrations at 1,538 cm⁻¹ and 1,665 cm⁻¹, and −CH₃ and −CH₂ asymmetric and symmetric stretching at 2,996 and 2,913 cm⁻¹, respectively. These peaks are found in the structure of polyAMPS, and the data are in agreement with the literature [25, 26].

Fig. 2

The CSi and CSii were synthesized with the application of as-prepared polyAMPSi and polyAMPSii. FTIR was carried out to evaluate their structure, and different characteristic peaks have been obtained, which are listed in Table 1 along with their specific regions and corresponding species. Figure 3 represents the two spectra of CSi and CSii. No major differences have been found in these graphs, which confirms that RAFT agents have no affect or influence on the core-shell structure.

Fig. 3

The wave numbers axis in the FTIR spectrum of synthesized CSi and CSii is given in Figure 3, which indicates the presence of various functional groups due to the presence of their corresponding characteristic peaks. A peak present at 1,586 cm⁻¹ illustrates the bending vibration of N-H, while peaks at 1,190 cm⁻¹ and 1,039 cm⁻¹ correspond to symmetric and asymmetric stretching of −SO₂; moreover, other peaks at 1,633 cm⁻¹, 1,364 cm⁻¹, and 603 cm⁻¹ illustrate the existence of C=O, C−N, and C−S, respectively. All these functional groups confirmed the existence of polyAMPS in CSi and CSii. Other peaks at 2,957 cm⁻¹, 1,726 cm⁻¹, and 1,240 cm⁻¹ correspond to stretching vibration of functional groups of BMA, which are C−H, C−O−C, and C=O, respectively. Functional groups with their respective peak regions are listed in Table 1.

For studying the influence of RAFT agents on thermal degradation of their respective synthesized polymer, both polyAMPSi and polyAMPSii were analyzed via thermogravimetry, and a temperature increase rate of 30 °C/min was adjusted. The obtained thermogram gave two major degradations: (a) the one between 125 °C and 200 °C illustrates thermal degradation of sulfonic acid moiety [26, 28] (b) and the second one at 200–300 °C corresponds to polyAMPS chain burning. Initially, there is not much difference in the degradation rates of the two RAFT agents, but it appears at higher temperatures in later stages of thermogravimetric analysis. The degradation rate and extent in polyAMPSii are higher than those in polyAMPSi. A clear difference in thermal stability appears in the prepared polyAMPS after 230 °C, and it was observed that polyAMPSii lost its 70% weight at 243 °C compared to at 308 °C for polyAMPSi. After 300 °C, the thermal degradation is very small and the weight of the polyAMPSi and polyAMPSii remains stable. The thermogram given in Figure 4 and Table 2 shows relative high stability of polyAMPSi, which is surely due to the application of pyrrole-based RAFT agents, as reported by Stace et al. [29], as the dithiocarbamate-based RAFT agents are thermally more stable than other RAFT agents, which is basically due to presence of pyrrole and bidentate legends (SC(=S)N) [30]. These factors increase crystallinity along with thermal stability [29, 30].

Fig. 4

Due to relatively high thermal stability of polyAMPSi, the corresponding CSi also showed high thermal stability, which is also elaborated by Figure 5 and Table 3. The thermogram showed three key weight decreases: the first one at 40–190 °C corresponds to complete dehydration [28]; the second degradation occurs in the range of 190–290 °C, which is specific for sulfonic acid groups in polyAMPS which produce sulfur oxide gases (SO₂ and SO₃) [26, 31]; and the third degradation was between 300 °C and 450 °C, which shows increased weight degradation of 76.757% for CSi and 76.269% for CSii. This major weight lost was due to deesterification [32], depolymerization, and denaturing of BMA [33], in which blazing of a hydro-carbon chain occurred and turned out into carbon monoxide, carbon dioxide, and traces of water vapor [34, 35]. Another major factor that enhances thermal stability of CSi was its relative high degree of crystallinity [36] since SC(=S)N (present in 4VP) has the capacity of being a bidentate legend [30]. A relative more pronounced XRD peak (Figure 6) supports this evidence.

Fig. 5

Fig. 6

The XRD study revealed three major peaks: the first one was a more pronounced peak at 2θ = 18.82°, corresponding to a high degree of amorphous nature of both core-shell polymeric particles [37, 38], and the second and third peaks at 2θ = 37° and 45° a very small amount of crystallinity. The relative study revealed high crystallinity in CSi with respect to CSii, which is due to the presence of bidentate legends (SC(=S)N) in CSii [30].

In the EDX study, both core-shell polymeric particles confirmed the presence of all the expected elements with the abundance trend of C>O>S>K. The atomic and weight percentages with their corresponding species are illustrated in Figure 7. The comparative study revealed the resultant relative high carbon content in CSi, which is due to three main reasons: high degree of crystallinity, branching, and monomer conversion capacity in CSi [24].

Fig. 7

Fig. 8

Fig. 9