Polymers became the center of research over the past few decades due to their good adaptability [1, 2] and various potential applications in the field of medical [3, 4], pharmaceutical [5], and other industrial applications [6, 7]. The prerequisite for complete control over the application of polymers is the “polymerization process”. The advantages and shortcomings of polymerization process are dependent on its low versatility or compatibility over different monomer, solvent system, provided conditions and suitable initiators [8]. One of the most prominent polymerizations include controlled/living radical polymerization (CLRP). Among all other CLRP, the most advance type of polymerization which uses thiocarbonylthio functionality, provides relatively high versatility over a provided condition (solvent, temperature, pH and initiator), functional and nonfunctional monomers to yield desire material with complex architectures, narrow molecular weight distributions, and pre-determined molecular weights [9]. This thiocarbonylthio functionality is provided by reversible addition-fragmentation chain-transfer (RAFT) agents, which leads to RAFT polymerization. Since the invention of RAFT polymerization, numerous RAFT agents have been synthesized and reported [10].
RAFT polymerizations have been used in the synthesis of various polymeric architectures, such as star [11], brush [11], linear [12], dendrimer [13], core-shell [14], and graft [15], along with different conditions, namely, solution, suspension, emulsion, and miniemulsion polymerizations [16]. Dispersity is one of the prominent parameters that affects the properties of polymers. Control over dispersity can be attained by mixing two RAFT agents with adequately dissimilar chain-transfer behaviors in different ratios, affording polymers with monomodal molecular weight distributions over a broad dispersity range [17]. Henkel and Vana [18] studied the effect of RAFT agents on the mechanical and the microstructure behavior of poly(butyl acrylate). For this purpose, a photoinitiated polymerization of 1,4-butanediol diacrylate and butyl acrylate was conducted in the absence and presence of RAFT agents. It was found that RAFT-based polymers have lower Young's moduli and high swelling degree. Moreover, kinetic differential scanning calorimetry studies illustrated that the gel point was retarded with enlarging the content of RAFT agent [18]. Masuda and Takai [19] studied the effect of RAFT agent content on the microstructure and properties of poly(N-isopropylacrylamide) (PNIPAAm) gels. A millimeter-sized cylinder was synthesized from PNIPAAm gels. Swelling and deswelling behaviors were studied, and we found that a cylinder with high “RAFT agent content” showed fast deswelling properties [19]. The various multi-arm RAFT agents have been used in stereolithographic 3D printing. Further, it was widely found that changing the functionality and content of RAFT agents result in obtaining control over material mechanical behavior in a broad span [20]. Application of RAFT polymerization instead of free radical polymerization produced major variations in the mechanical, uptake behaviors and thermal properties, which seem to reflect the improvement in polymer uniformity and mobility frequently related with controlled polymerization [21].
The application and properties of core-shell nanomaterials can be promisingly controlled by the right selection of shell or core materials according to the environment/condition and applications. A vast study of core-shell material “as sensing” device have been reported, i.e., as optical sensors, gas adsorptive sensors, electrochemical sensors, and wearable sensing devices. These devices have various potential uses in food analysis and biological, industrial, environmental, and clinical applications. Moreover, numerous synthetic approaches with various prominent properties of core-shell materials, such as high ion transport properties, high conductivities, and high surface area have been studied. [22]
Although the synthesis of RAFT agents and their use in the preparation has already been reported [23, 24], to our knowledge, their effect on the final characteristics of the prepared water-soluble polymer (2-acrylamido-2-methylpropane sulfonic acid) (polyAMPS) and their core-shell particles has not been reported. Hence, considering various advantages of RAFT polymerization, this effort was made to determine their role in controlling the thermal stability, particle size distribution, crystallinity, and average particle size of the resultant core-shell particles with polyAMPS as a shell and butyl methacrylate (BMA) as a core.
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 K
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.
Structures of the as-prepared RAFT agents. RAFT, reversible addition-fragmentation chain-transfer.
For the preparation of water-soluble polymer poly(2-acrylamido-2-methylpropane sulfonic acid) (polyAMPS
For the preparation of CS
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 polyAMPS
FTIR spectra of the as-prepared polyAMPS
The CS
Identifying characteristic peaks of CS
S. No. | Wave number (cm−1) | Functional group | Belonging molecule | References |
---|---|---|---|---|
1. | 1,633 | C=O | ||
2. | 1,583 | N–H (bending) | ||
3. | 1,364 | C–N | polyAMPS | [25] |
4. | 1,190 and 1,039 | O=S=O (asymmetric and symmetric stretching) | ||
5. | 603 | C–S stretching | ||
6. | 3,333 | Vibration of bonded and non-bonded OH groups | H2O | Disperse medium |
7. | 2,957 | Stretching CH | ||
8. | 1,726 | C=O stretching | BMA | [27] |
9. | 1,240 | C–O–C stretching |
BMA, butyl methacrylate; polyAMPS, poly(2-acrylamido-2-methylpropane sulfonic acid).
FTIR spectra of the CS particles.
The wave numbers axis in the FTIR spectrum of synthesized CS
For studying the influence of RAFT agents on thermal degradation of their respective synthesized polymer, both polyAMPS
Comparing TGA results of polyAMPS. polyAMPS, poly(2-acrylamido-2-methylpropane sulfonic acid).
Comparative analysis of thermal degradation with respect to temperature.
Polymer | Temperature (°C) with weight reduction | ||
---|---|---|---|
20% | 50% | 70% | |
polyAMPS |
177 | 242 | 308 |
polyAMPS |
131 | 232 | 243 |
polyAMPS, poly(2-acrylamido-2-methylpropane sulfonic acid).
Due to relatively high thermal stability of polyAMPS
Comparative TGA results of CS particles.
Comparative analysis of thermal degradation with respect to temperature.
Core-shell particles | Temperature (°C) with weight reduction | |||
---|---|---|---|---|
25% | 50% | 75% | 92% | |
CS |
340 | 368 | 392 | 446 |
CS |
302 | 333 | 352 | 374 |
Comparing XRD results of polymeric CS particles. XRD, X-ray diffraction.
The XRD study revealed three major peaks: the first one was a more pronounced peak at 2
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 CS
EDX result of polymeric core-shell particles.
SEM images (highlighting spherical particles and crystals) of CS
SEM images (highlighting spherical particles and crystals) of CS
The microscopic analysis of the resultant polyAMPS@BMA core-shell particles was nonuniform. Through SEM study a small portion of crystallinity and large amorphous nature with spherical particles shapes have been observed. Three different particle shapes have been obtained: spherical, spheroidal, and agglomerated. The shapes of the crystals are needle-like and block-like, with a diameter range from ~10 nm to 300 nm, which is due to crosslinking [39]. This irregular shape of resultant polyAMPS@BMA core-shell particles provided the advantage of high surface area for various purposes. The relative analysis concluded a high agglomeration in CS
ImageJ (Java-based image processing program developed at the NIH, USA) software was used to analyze PSD and average particles size. From Figure 10 (left), it can be observed that the sizes of particles for CS
Histograms of PSD of polymeric CS particles, where the histogram for CS
Figure 11 shows the scanning electron micrograph images of both core-shell particles. Since SEM analysis was done in bulk form without treatment with any chemical species, a prominent number of particles were found in agglomerated foam. Non-agglomerated and spherical particles are highlighted with white arrows. Core-shell morphologies have also been confirmed for SEM images. The white blurred portion of each spherical particle illustrates the shell, while the gray inner portion is the core of particles.
Core-shell morphologies of obtained particles.
The basic purpose of this study was to evaluate the effect of RAFT agents on the net characteristics of the polyAMPS and their resultant core-shell particles. FTIR, XRD, EDX, and SEM studies confirmed the successful synthesis of the RAFT agents, polyAMPS, and polyAMPS@BMA particles. FTIR, XRD, and EDX studies authenticate that with the changing of RAFT agents, the structural properties of the resultant core-shell particles are less affected. The thermal properties, particle sizes, and yields are prominently influenced. Overall, it has been concluded that hyper-branched ability and crystallinity are the major factors that make the RAFT agents to influence the particle size, shape, size distribution, and thermal behavior of obtained polymers.