Synthesis and Characterization of Thermosensitive Nanosupports with Core–Shell Structure (PSt-PNIPAM) and Their Application with Silver Nanoparticles

Abstract

The present study synthesized silver nanoparticles supported on a thermosensitive polymer with a core–shell structure, formed by a polystyrene (PS) core and a poly(N-isopropylacrylamide) (PNIPAM)/Poly(N, N-methylenebisacrylamide) (MBA) shell. The PS core was synthesized via semicontinuous heterophase polymerization at a flow of 0.073 g/min, enabling polystyrene nanoparticles with an average size (Dz) of 35.2 nm to be obtained. In the next stage, the conditions required for polymerization synthesis were established in seeded microemulsion using PS nanoparticles as seed and semicontinuously adding the thermosensitive shell monomer (PNIPAM/MBA) under monomer-flooded conditions to favor shell formation. The non-homopolymerization of PNIPAM/MBA was demonstrated by obtaining nanoparticles with a core–shell structure, with average particle sizes of 41 nm and extremely low and narrow polydispersity index (PDI) values (1.1). The thermosensitive behavior was analyzed by QLS, revealing an average shrinkage of 4.03 nm and a percentage of shrinkage of 23.7%. Finally, silver nanoparticles were synthesized on the core–shell heat-sensitive nanoparticles in a colloidal solution containing the latices, while silver nanoparticles were anchored onto the cross-linked heat-sensitive network via the formation of complexes between the Ag+ ions and the nitrogen contained in the PNIPAM/MBA network, favoring anchorage around the network and maintaining a size of 5 nm.

1. Introduction

Since size and the creation of micro- and nanocomposites affect their optical and electrical characteristics, metallic nanoparticles have been the subject of extensive research in recent years. This research has mostly concentrated on the synthesis and characterization of stable dispersions of silver, gold and other noble metal nanoparticles [1]. The use of silver nanoparticles as bactericides has garnered a lot of interest [2]. The increasing application of Ag nanoparticles as virucides, that is, to kill viruses like COVID-19 on surfaces, is being investigated in other fairly recent publications [3]. Surface plasmon resonance (SPR), which is a collective excitation of charge density that takes place in systems with high electronic densities, such as metallic nanostructures and nanoparticles, has become more significant since the 1990s. We can cite research on structures made of noble metals like silver (Ag) and gold (Au) as an example [4]. Numerous intriguing technical applications have been made possible by the investigation and comprehension of LSPR’s characteristics in these nanostructures, including the production of ultrasensitive sensors. Additionally, silver nanoparticles are excellent candidates for catalysis because of their high surface-to-volume ratio [5,6,7]. According to the specialized literature in this area [5], great progress has been made in the research on silver nanoparticles due to their properties, which are substantially different to those that present when they are constituted by larger particles. However, by their nature, nanocatalysts tend to aggregate, leading to supports being developed to avoid this phenomenon and enable full advantage to be taken of the catalytic properties of these nanoparticles [8,9,10].

The synthesis and characterization of thermosensitive polymer particles, which provide special qualities for a variety of applications, have been the subject of more research in recent years [11,12,13,14]. Three main types of thermosensitive particles with the following structures have been reported: (i) cross-linked PNIPAM micro- and nanogels; (ii) core–shell particles in which PNIPAM forms the two structures; and (iii) core–shell particles with a rigid polymer core and a cross-linked PNIPAM shell [15,16,17]. One interesting application of the latter is as supports for metal nanoparticle immobilization [8,9,10]. The literature reports the preparation of a large number of thermosensitive particles, with average particle sizes greater than 100 nm [10]. Some of these studies have reported microspheres with diameters up to 1 µm and composed of a PS core and a PNIPAM/PMBA cross-linked shell with thermosensitive properties [9,14,18]. The properties of core-shell polymers are determined by the properties of the polymers that comprise the core and the shell., such as the use of a rigid polymer to form the core and the use of a polymer with thermosensitive properties to form the shell, thus creating an intelligent system that responds to an external stimulus [15]. The core–shell particles are ordinarily obtained via emulsion polymerization in two stages, wherein the core (seed) is formed in the first stage and then coated with another polymer to form the shell [19,20,21].

Anion-shielded PNIPAM microspheres shrink discontinuously when approaching their critical solution temperature (CST) of PNIPAM, whereas they shrink gradually on being heated from 20 to 30 °C. Because the PNIPAM shell is thermosensitive and exhibits LCST as temperatures reach 32 °C, this slow shrinkage is ascribed to the development of agglomerates in the PNIPAM chains [22,23]. The hydrogen bridges formed between the amino groups and water cause PNIPAM to seem enlarged at room temperature, but this interaction stops at 32 °C [24].

In an effort to exploit the core–shell structure and its thermosensitive properties, these nanostructured particles have been synthesized for use as supports for metal nanoparticles and to facilitate their catalyst function. The 140 nm spherical Dpn support provided modulates nanoparticle activity on the thermosensitive PNIPAM/MBA shell. While the Ag, Au, Rh and Pt particles embedded in the shell are reactive at temperatures below the transition temperature, above this temperature, the shrinkage of the polymeric network decreases the reaction rate of the catalyst due to the low diffusion of the reactants throughout the shrunken network. In this way, the polymeric network is able to act as a nanoreactor, which can be closed or opened as appropriate [8,9,10].

The literature reports a two-step emulsion synthesis of thermosensitive core–shell nanoparticles with average diameters of 150 nm [10]. Another option for the preparation of this type of particle is the microemulsion polymerization technique, which allows for the preparation of colloidal polymeric particles with diameters usually less than 50 nm dispersed in a continuous aqueous phase. The reduction in particle size would considerably increase the surface area of these systems, significantly enhancing their efficiency. However, the high amount of surfactant needed and the low polymer content in the final lattices are two significant drawbacks of microemulsion polymerization that limit its potential applications. Semicontinuous polymerization is one of the suggested solutions to remove these limitations. As a way of raising the polymer/surfactant ratio in the finished latex, this method was first published in 1997 by Rabelero et al. [25] and Roy and Devi [26]. This operation method is based on the idea that by dosing the monomer into the reaction mixture, new particles can be created, utilizing the high surfactant content that is usually seen in microemulsions. The usage of this type of procedure has since been documented in a number of papers [27,28,29,30,31]. High polymer content is made possible by this novel microemulsion process with semi-continuous monomer dosing. According to studies, this mode of operation keeps the particle size within the usual range for batch microemulsion polymerization while increasing the polymer surfactant ratio [25,29,30,31,32]. However, in order to decrease the particle size and surfactant content in this type of heterogeneous polymerization, it is necessary to operate under conditions known as monomer avidity during the addition period [33]. Krackeler and Naidus introduced this concept [33]; they relied on the correlation developed for emulsion polymerization by Smith and Ewart to predict the particle number (N_p) for the kinetics, in which NP is inversely proportional to the volumetric growth rate of the particles during nucleation to explain the reduced particle growth and the formation of a large number of particles in styrene emulsion polymerization carried out in semicontinuous mode compared to the batch process [34]. When the particles are saturated with monomer, they grow at their maximum speed; as a consequence, particle nucleation is minimal. In semicontinuous emulsion (or microemulsion) polymerization, the maximum volumetric particle growth rate occurs at so-called monomer-flooded conditions [30]. In contrast, operation under monomer avidity conditions decreases the particle growth rate and leads to a higher number of small particles. Despite the importance of operation under monomer avidity conditions, in semicontinuous microemulsion polymerization, only four reports [25,29,31,35] refer to it. Only Ledezma et al. study the effect of monomer dosing rate and show that polymerization is carried out under monomer avidity conditions [31].

The present study reports the synthesis of silver nanoparticles on a support formed by a polystyrene (PS) core with a poly(N-isopropylacrylamide) (PNIPAM)/poly(N,N-methylenebisacrylamide) MBA thermosensitive shell. The reaction conditions were established for three stages that constituted the totality of the project. In the first stage, the PS core was synthesized via microemulsion in semicontinuous monomer starved conditions, and in the next stage, the polymerization synthesis conditions were established in seeded microemulsion using PS nanoparticles as seeds and adding the thermosensitive shell monomer (PNIPAM/MBA) semicontinuously under monomer-flooded conditions to favor the formation of the shell. The silver nanoparticles were finally synthesized on the thermosensitive core–shell nanoparticles in the colloidal solution that contained the latices. The Ag+ ions and the nitrogen in the PNIPAM/MBA network formed complexes that anchored the silver nanoparticles in the cross-linked thermosensitive network.

2. Materials and Methods

2.1. Materials

The reagents used were of reagent grade. Styrene (S) (99%), potassium persulfate (KPS) (99.99%), sodium dodecyl sulfate (SDS) (98%), N-isopropylacrylamide (NIPAM) (97%), N,N Methylenebisacrylamide (MBA) (99%), silver nitrate (AgNO₃) (99.99%) and sodium boron hydride (NaBH₄) (99%) were supplied by Sigma Aldrich, Mexico. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was supplied by Fluka, México. Distilled water was used.

2.2. Method of Experimentation

The synthesis process was carried out in three stages with the aim of establishing the necessary conditions in each one to obtain the desired characteristics.

In the first stage, the conditions for the addition of monomer (S) were established at a sufficiently slow flow rate (0.071 g/min) to promote the use of the added monomer in the formation of a large number of particles and to prevent the already existing particles from continuing to grow. In this way, a large number of particles with sizes smaller than 50 nm will be obtained. The nanoparticles produced in the first stage were utilized as seeds in the second stage. In order to prevent homopolymerization of the thermosensitive monomer, the reaction conditions were set, so that polymerization would take place on the surface of the PS seeds and not encourage the production of additional PNIPAM/MBA particles adjusting the NIPAM/MBA monomer adding flow to 0.11 g/min. In the third stage, the core–shell nanoparticles were purified via dialysis to eliminate the surfactant and reaction residues. Subsequently, the aqueous silver nanoparticles will form in the presence of thermosensitive supports.

2.3. Synthesis of the PS Core

During the first stage of the present research, the PS core was synthesized via semicontinuous heterophase polymerization at a flow rate of 0.073 g/min for 8 h, using the SDS/AOT surfactant system and KPS as the initiator.

A micellar solution was prepared, which contained 3.35 g of SDS, 1.13 g of AOT, 96.03 g of water and 0.062 g of the KPS initiator. Subsequently, the micellar solution and the initiator were placed in a jacketed 100 mL reactor. The system was sealed to prevent the entry of oxygen. Afterward, degassing of the reaction mixture was initiated. To achieve this, the solution was bubbled with argon for 1 h while stirring at 450 rpm. Ten minutes before starting the reaction, water at 60 °C was passed through the reactor jacket. After this time, the reaction was initiated by adding the monomer at a flow rate of 0.073 g/min. The process of adding the monomer took eight hours to complete. One hour was allotted for post-addition time at the conclusion of the addition. The monomer addition flows were established by calibrating the dosing pump.

2.4. Synthesis of Core–Shell Nanoparticles

The conditions for polymerization synthesis were established in seeded microemulsion using PS nanoparticles as seeds and semicontinuously adding the thermosensitive shell monomer (PNIPAM/MBA) under monomer-flooded conditions (0.11 g/min for 4 h) at 70 °C. For this reaction, 22.5 g of PS seed latex with a content of 5.35 g of PS was placed in a jacketed 100 mL reactor. The system was sealed to prevent the entry of oxygen. Then, degassing of the seed was initiated, making the micellar solution bubble with argon for 1 h while simultaneously stirring. The reaction was initiated by simultaneously adding 2.0 g of an aqueous solution with 2% KPS in relation to the monomers (NIPAM/MBA) and the aqueous solution of the shell monomers (NIPAM/MBA) at a flow rate of 0.11 g/min for 4 h. An amount of 1.75 g of NIPAM and 3.0% of the cross-linking agent (MBA) in relation to NIPAM were added to 28 g of deionized water to create the monomer solution. The core–shell nanoparticle was purified via dialysis using an MC18 cellulose membrane purchased from Aldrich. The goal of purifying core–shell nanoparticles is to eliminate the surfactants and residues from unreacted reagents. Dialysis using an Aldrich MC18 cellulose membrane was used to complete it. The membrane was filled with the colloidal solution containing core–shell nanoparticles, and the ends were sealed. Twenty liters of distilled water was placed inside the barrier. The conductivity levels were checked every day to ensure that the membrane had been cleared of all surfactant and residue leftovers. The procedure was then repeated with a different water source until conductivity remained constant.

2.5. Synthesis of Ag Nanoparticles

Finally, silver nanoparticles were synthesized in the core-shell thermosensitive nanoparticle solution.

The synthesis of silver nanoparticles in an aqueous solution was carried out by adding silver nitrate in an aqueous solution of core–shell nanoparticles of PS-NIPAM (MBA) and followed by reduction with 0.1 M sodium borohydride. After adding 2 g of the thermosensitive solution diluted in 98 g of deionized water and 0.5 g of 0.1 M AgNO₃ to a 100 mL jacketed reactor, the colloidal mixture began to degas, and the argon bubbled for 30 min while being agitated at 450 rpm. Sodium borohydride (0.043 g) in 5 g of water was rapidly added to the mixture and swirled for one hour. The reaction was carried out at 25 °C.

2.6. Characterization

Particle size was measured at 25 °C using light scattering with a Nano z-sizer S90 light disperser by Malvern, México. The final samples obtained in the three stages of the reaction were measured at various temperatures between 25 and 50 °C. Prior to testing, latex samples were diluted with water up to 200 times.

The average decay rate is calculated using the cumulant approach to examine the intensity correlation data (Γ2 = q2D), where Γ is the correlation function; q = (4πη/λ)sin(θ/2) is the scattering vector; η is the refractive index; and D is the diffusion coefficient. Assuming that the solvent has the viscosity of water, Stokes’ law is used to express the measured diffusion coefficients in terms of the apparent diameters. The intensity diameter, or D_z, is the diameter that is obtained using this method.

Transmission electron microscopy was used to determine particle size distributions and polydispersity index (PDI) values, acquiring micrographs in the scanning mode (STEM) at various magnifications. A JEM2200Fs+STEMCs Transmission Electron Microscope by JOEL, México; was used with an acceleration voltage of 200 k. A drop of this solution was placed onto a copper grid and allowed to dry after 0.01 g of latex was diluted in 10 g of water. The PDI values (D_w/D_n), which are the weight and number average diameters, respectively, were determined by measuring at least 300 particles from the micrographs using the following equations:

$$D_n={\displaystyle \frac{{\sum }_i{n}_i{D}_i}{{\sum }_i{n}_i}}={\displaystyle \frac{\sum n_i{D}_i}{n}}$$

(1)

$$D_w={\displaystyle \frac{{\sum }_i{n}_i{D}_i^4}{{\sum }_i{n}_i{D}_i^3}}$$

(2)

where n_i is the number of particles with size D_i, and n is the total number of measured particles.

3. Results

3.1. Morphology and Size

The final sample of the polystyrene seed was analyzed by means of STEM, with the corresponding micrographs shown in Figure 1. The particles presented a spheroidal morphology and formed agglomerates of different sizes, while their size distribution appeared to be very uniform, with the majority of spheres falling within an average size of around 35 nm, although spheres measuring between 20 and 40 nm in diameter were also found.

Micrographs of the final polystyrene seed sample were used to obtain a particle size distribution histogram. In

Table 1. Comparison of particle sizes determined by QLS and STEM for styrene polymerization at an addition rate of 0.073 g/min.

QLS	STEM
Dz (nm)	Dn (nm)	Dw (nm)	Dv (nm)	Dz (nm)	Dw/Dn
42.4	30.7	34.3	31.9	35.2	1.12

, the average diameters in number (D_n), weight (D_w), volumetric (D_v) and z (D_z), calculated from the STEM micrographs, as well as the D_z determined by QLS, are reported. It can be seen that the sample has a polydispersity index (D_w/D_n) of 1.12, which indicates that the sample has a narrow population of particle sizes, which is characteristic of semicontinuous microemulsion polymerization.

Figure 2 shows the micrographs obtained via STEM and the histogram for the core–shell nanoparticles, which present a spheroidal morphology and whose size ranges from 20 to 50 nm in diameter. In seed nanoparticles (PS), the limited distribution of nanoparticle sizes is preserved, while the transition to larger particle sizes is notable in core–shell nanoparticles.

Table 2. Particle sizes of the seeds and core–shell nanoparticles, with average diameters reported in number (D_n) and weight (D_w), as calculated from the STEM micrographs, and D_z, as determined via QLS.

Sample	METB			QLS
	Dn (nm)	Dw (nm)	PDI	Dz 25 °C (nm)
Seed	30.7	34.3	1.12	42.4
Core–shell	41.4	45.7	1.1	51.9

shows the particle sizes of the seeds and the core–shell nanoparticles prepared, with average diameters reported in number (D_n) and weight (D_w), as calculated from the STEM micrographs, and D_z, as determined via QLS. The table shows that the particle size increases in line with the size increase from seed to core–shell nanoparticles. The PDIs in all cases are relatively narrow (1.10–1.12), with even the core–shell particles presenting slightly lower values than the PDI of the seed.

Figure 3 compares the histograms of the seed and core–shell nanoparticles. As can be observed, the narrow distribution of nanoparticle sizes is preserved, and a shift toward larger particle sizes is noticeable in the core-shell nanoparticles. The PDIs in all cases are relatively narrow (1.10–1.12), with core–shell nanoparticles presenting slightly smaller values than the PDIs of the seed.

3.2. Thermosensitive Properties

Particle diameter was determined via QLS at different temperatures for core–shell nanoparticles, and the particle size behavior of the PS seed was also included. Figure 4 shows the results of the comparison between the core–shell nanoparticles and their PS seed. As expected, the particle diameter of the seed is very insensitive to temperature. The size of the PS seed at room temperature (25 °C) is smaller than that of the core–shell nanoparticles at the same temperature as a result of the formation of the shell.

Table 3. Size of core–shell nanoparticles at temperatures below and above the LCST of NIPAM and volume percentage loss. * Values acquired from a prior study.

PS/PNIPAM-MBA	%MBA	Dz (25 °C) (nm)	Dz (55 °C) (nm)	Volume Percentage Loss	Shrinkage (nm)
(75.0/25.0)	3.0	53.19	48.59	23.70	4.6
* (78.1/21.9)	2.0	51.8	44.5	33.0	7.3
* (74.1/25.9)	2.0	57.8	50.3	36.0	7.5

includes the values of D_z for core–shell nanoparticles, determined at 25 and 55 °C, and the volume percentage loss within this temperature interval as a measure of particle shrinkage due to the expelled water from the particles.

Volume percentage loss was calculated using the following equation:

$$Volume\%loss=(1-{\displaystyle \frac{V_{shrunk}}{V_{swollen}}}) \times 100$$

(3)

where V_shrunk and V_swollen are the particle volumes calculated from D_z values at 25 and 55 °C, respectively.

The evolution of particle diameter with temperature indicates that the core–shell particles shrink markedly up to about (32 °C) and then decrease in a less noticeable manner. In the case of core–shell nanoparticles with diameters in the order of 1 μm, this behavior has also been observed [24]. The explanation is that when an aqueous solution of PNIPAM is heated above its critical solution temperature (LCST), which is 32 °C, a very drastic change occurs around its hydrophobic (propyl) groups. The water molecules that hydrate the propyl groups through hydrogen bridges are released and move away from the propyl groups, causing these hydrophobic groups to bind, resulting in chain shrinkage [5,6,7].

3.3. Silver Nanoparticles

Figure 5 corresponds to the micrograph, obtained via STEM, of the final colloidal solution obtained after the reduction of silver nitrate (AgNO₃) with boron sodium hydride (NaBH₄). It is observed that all particles are immobilized within the PNIPAM/PMBA thermosensitive network, and no particles are located outside of it. This figure shows dark field images acquired at different magnifications. In the dark field images, core–shell nanoparticles are observed in dark tone and silver (Ag) nanoparticles in bright tone. The silver particles are found with smaller sizes between 5 and 3 nm. All particles are located on the thermosensitive PNIPA lattice, and the high-resolution images indicate that the silver nanoparticles are crystalline.

In Figure 6, the hydrodynamic ratios of the core-shell nanoparticles containing Ag (squares) and those without Ag (circles) are displayed. Both sets of data demonstrate that the thermosensitive shell experiences a volume transition at 32 °C. The Au and Pt nanocomposite particles exhibit similar behavior, which is consistent with earlier findings that the embedding of nanoparticles does not disrupt the volume transition within the network.

4. Discussion

4.1. Morphology and Size

The particle size obtained via QLS of the latex sample at the end of polymerization of styrene shows a Dz value of 42.4 nm, and the corresponding measurement via STEM gives Dn values of 30.7 nm (Table 1 and Figure 1). The particle sizes and the PDI values obtained match the typical results obtained in a semicontinuous heterophase polymerization carried out under monomer-starved conditions [18,36]. Krackeler and Naidus [33] established that if the monomer dosing flow rate is low enough, this favors the formation of a large number of small particles. They observed that under these conditions, particle growth was reduced, and the formation of a large number of particles was favored in the emulsion polymerization of styrene. They predicted the number of particles using the correlation for emulsion polymerization proposed by Smith and Ewart [34] to calculate the number of particles (Np) for case-II kinetics, in which Np is inversely proportional to the volumetric growth rate of the polymer particles during nucleation. The monomer dosing rate was sufficiently low to result in a low monomer concentration in the particles and, consequently, to facilitate the creation of a large number of small particles, in accordance with Krackeler and Naidus [33]. These results indicate that the monomer addition flow rate at which the work was carried out was adequate to obtain polystyrene latices with the indicated characteristics to be used as seed in the seeded polymerization of PNIPAM/PMBA.

The seeded polymerization of PNIPAM/MBA was carried out with a higher monomer addition flow rate than the flow rate used in the polymerization of the PS seed. These addition conditions will favor the saturation of the monomer and the formation of the PNIPAM/MBA shell over the PS seed, and they will prevent the formation of new PNIPA/MBA particles.

The results shown in Table 2 and the micrograph presented in Figure 2 correspond to the final sample obtained from seeded polymerization for the formation of the thermosensitive shell. This suggests that the shell was indeed formed on the surface of the polystyrene core. The analysis conducted on different micrographs indicates that it was not possible to identify the PNIPAM/MBA particles, which, together with the PDI values obtained for the core–shell particles, suggests that the monomer, as added semicontinuously, will grow the already existing PS particles, and homopolymerization of NIPAM/MBA was not observed. In other words, it was found that the majority of the monomer added swelled the particles, while the other part of it dissolved in the aqueous medium in the presence of water-soluble initiator radicals (KPS), forming oligomeric radicals, which could be easily captured by the monomer-swollen seed to form a cross-linked PNIPAM-PMBA shell on its surface. When the particles are saturated with monomer during the monomer addition period, in what is known as monomer-flooded conditions [32], they develop at their fastest rate, which results in minimal particle nucleation in semicontinuous heterophase polymerization.

4.2. Thermosensitive Properties

The manner in which the particle diameter evolves in line with temperature indicates that the core–shell particles shrink markedly, by up to nearly 32 °C, and then decrease less notably. This behavior has also been observed for core–shell particles with diameters in the order of 1 μm [14] and can be explained by the fact that when an aqueous solution of PNIPAM is heated to over its CST of 32 °C, a very drastic change occurs in its hydrophobic groups (propyl). The water molecules that hydrate the propyl groups via hydrogen bonds are released and then move away from the propyl groups, causing the hydrophobic groups to bond, in turn causing chain shrinkage.

Table 3 presents the average particle diameters of core–shell nanoparticles at 25 °C and 55 °C. The contraction percentage (23.70%) and particle size (4.6 nm) were lower than those found in earlier research, which indicated values of 36% and 7.5 nm [20]. The temperature-sensitive nature of the particles is demonstrated by the inverse relationship between size and temperature, as observed in Figure 5 and Table 3. Figure 4 clearly shows a declining trend in particle size with temperature, whereas PS particle size essentially remains constant as the temperature rises. The three points in the 20–30 °C period may be fixed to a straight line (shown in Figure 4), with a slope three to five times that of the line for the five points in the 35–55 °C interval (shown in Figure 4). This indicates that the core–shell nanoparticles behave differently. As indicated by the dotted lines in Figure 3, the crossing of these two straight lines actually occurs between 31 and 33 °C.

Given the stated LCST value of 31–32 °C for PNIPAM [21,37] this behavior is consistent with that expected for particles having a PNIPAM shell. Given that both curves show a size change with temperature of the samples generated at various core–shell weight ratios using PNIPAM as the shell, this cannot be a coincidence.

A volume loss of 23.70% was observed in these thermosensitive core–shell nanoparticles, which was less than the normal values (60–95%) reported in the literature for PS-PNIPAM core–shell particles [30,34] and somewhat less than the results of earlier research conducted by this working group (Table 3) [22]. For the core–shell curve, a somewhat consistent trend of a decreasing particle size with temperature was also noted. According to the literature, a low volume percentage loss and a somewhat continuous decay trend of the particle size with temperature in thermosensitive core–shell particles could be ascribed to a low PNIPAM proportion in the shell and/or a highly cross-linked PNIPAM shell [38]. It can be observed that the values of the percentage volume loss are not very high, but there is a downward trend in the contraction percentage with the decrease in the PNIPAM ratio [22]. Comparatively, core–shell particles with a PS/PNIPAM weight ratio of 25.5/74.5 and 1.5 weight percentage of MBA in relation to NIPAM were synthesized by Yi and Xu [21]. Between 20 and 50 °C, these authors recorded a volume percentage loss of about 96.5%. In this study, core–shell particles were prepared with a weight ratio of PS/PNIPAM of 75/25 and were compared with particles reported in a previous work [22] obtained using similar proportions of PS/PNIPAM but containing 3.0% by weight of MBA relative to NIPAM as a cross-linking agent. The increase in the % of MBA was carried out in order to improve the cross-linking in the shell and promote the retention of silver nanoparticles in the shell of the thermosensitive support. It can be observed that the values of percentage volume loss are not very high in both cases, but there is a downward trend in the percentage of shrinkage with the decrease in the proportion of PNIPAM, and it can be observed that the values of percentage volume loss are not very high in both cases, but there is a downward trend in the percentage of shrinkage with the decrease in the proportion of PNIPAM and the increase in the cross-linking agent (MBA). This finding could serve as a basis for the development of very small polymer nanoparticles with thermosensitive properties and core–shell structures with a quick response time to an appropriate temperature change, with the aim of increasing the thickness of the thermosensitive shell.

4.3. Silver Nanoparticles

The high-resolution images in Figure 5 indicate that the silver nanoparticles are crystalline. The Ag+ ions were shown to be strongly localized within the lattice, most likely as a result of their complex formation with PNIPAM’s nitrogen atom [8,10]—an immobilization, which gives the system great stability, namely that once the silver is formed in the network, it remains fixed. Recent research has shown that Au, Ag and Pd nanoparticles may be uniformly incorporated into negatively charged microgel particles. Other metal nanoparticles, including Au, Rh and Pt, were immobilized within these microgels [38]. This immobilization gives the system great stability, namely that once the silver is formed in the network, it remains fixed.

It is also noteworthy that there is greater cross-linking in the NIPAM/MBA shell, the Ag nanoparticles are mostly found around the edge of the corona rather than close to the dark PS core in the majority of core–shell particles. Smaller Ag particles are produced in the system at greater cross-link densities [8]. The particle sizes of various systems were compared by Yan Lu and colleagues: Ag nanoparticles with diameters of more than 100 nm had sizes of 8.5, 7.3 and 6.5 nm, respectively, when cross-linked with 3.4, 7.3 and 13.7% BIS in relation to the NIPAM. This could be because the creation of Ag nanoparticles may be restricted by the heavily cross-linked network.

The reason for the achievement of particle sizes of approximately 4 nm in this work, despite using a lower quantity of the cross-linker (3.5% of MBA relative to NIPAM), is that the surface area of the core–shell particles significantly increased as their size shrank (<50 nm) in comparison to the findings of the previously mentioned study (100 nm) [8]. As a result, a greater area is covered by the Ag+ ions in the thermosensitive shell, which increases the quantity of silver particles and decreases their size.

Making silver nanoparticles bind to the thermosensitive PNIPAM shell ensures that the natural aggregation that occurs in nano-sized particles is avoided. This “smart system” will allow modulating the catalytic activity of silver nanoparticles through thermodynamic transition, which takes place in their shell. Previous research [18] has shown that this transition is perfectly reversible, and the shrinking and swelling process can be repeated without degradation or coagulation of the nanoparticles.

Analysis using dynamic light scattering of composite particles at different temperatures showed that the PNIPA network’s thermosensitivity is unaffected by the inclusion of metal nanoparticles. Comparable to the carrier particle, the Ag-composite particles (Figure 6) exhibit a volume transition temperature at 32 °C. This implies that the volume transition inside the network is not interfered with by metal nanoparticles [8]. The addition of metal nanoparticles to the PNIPAM network does not impair its thermosensitivity or its ability to shrink and reswell based on composite particle dynamic light scattering measurements at different temperatures. This implies that the volume transition inside the network is not disturbed by metal nanoparticles.

5. Conclusions

PS–PNIPAM core–shell nanoparticles were produced via semicontinuous heterophase polymerization and seeded polymerization. They had narrow particle size distributions (1.1) and average diameters of about 41 nm. The increased seed size and comparable PDI values for the core and core–shell nanoparticles at the end of the seeded polymerizations indicate that secondary nucleation was not present, even though some coagulation might have taken place. The NIPAM polymerized nearly exclusively onto the PS cores, according to this theory.

The core–shell nanoparticles prepared in the present study showed thermosensitive behavior in the 20–55 °C temperature interval.

The present study found that it is possible to deposit silver nanoparticles with diameters of between 3 and 5 nm inside the cross-linked shell of P (NIPAM-MBA), thus obtaining a system in which the catalytic activity of silver could be controlled via changes in temperature.

The addition of metal nanoparticles to the PNIPAM network does not impair its thermosensitivity or its ability to shrink and reswell, according to dynamic light scattering tests. This suggests that metal nanoparticles do not disrupt the volume transition inside the network.