Role Transformation of Poly(N-isopropylacrylamide) Microgels from

Nov 10, 2017 - However, they turned into seeds when water was added into the methanol solution, with the formation of octopus-like nanoparticles. Furt...
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Role Transformation of Poly(N‑isopropylacrylamide) Microgels from Stabilizer to Seed in Dispersion Polymerization by Controlling the Water Content in Methanol−Water Mixture Rui Chen, Ning Ren, Xin Jin,* and Xinyuan Zhu School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

ABSTRACT: Poly(N-isopropylacrylamide) (PNIPAM)-based ionic microgels with different diameters were first prepared and then used as particulate stabilizer or seed in dispersion polymerization of styrene. The role of PNIPAM-based ionic microgels could be transformed from particulate stabilizer to seed by controlling the water content in methanol−water mixture. Generally, PNIPAM-based ionic microgels served as particulate stabilizer in methanol in the absence of water, leading to the formation of spherical polystyrene nanoparticles. However, they turned into seeds when water was added into the methanol solution, with the formation of octopus-like nanoparticles. Further study demonstrated that the mechanism for this role transition was related to the special thermosensitivity of PNIPAM microgels in methanol−water mixture. They lost their thermosensitivity in pure methanol solution but restored their thermosensitivity when increasing the water content in methanol−water mixture.

1. INTRODUCTION Polymer particles, especially nonspherical ones, play very important roles in many areas such as drug delivery system,1 analytical chemistry,2 and model system.3 Various methods have been developed to design nonspherical polymer particles. Spherical polymer particles could be stretched into ellipsoid ones by controlling the glass transition temperature,4 squeezed into rod-like ones by microfluidic chips,5,6 or assembled into raspberry-like ones by hydrogen interaction.7 However, these methods could not be applied to the preparation of nonspherical nanoparticles in a large scale because of the sophisticated procedures. The classical method for producing nonspherical polymer nanoparticles is seeded emulsion/ dispersion polymerization.8,9 Nanoparticles bearing core− shell,10 hollow,11 raspberry-like,12 Janus,13 yolk−shell,14 and snowman-like15 morphologies are prepared by this method. Among these examples, different types of seeds such as polymer nanoparticles,16 silica nanoparticles,17 gold nanoparticles,18 and hydrogel particles19 are introduced. Poly(N-isopropylacrylamide) (PNIPAM) microgels were first reported by Pelton in 1986. 20 Owing to their thermosensitivity, they are widely applied in many areas including biotechnology21 and emulsion polymerization.22 It © XXXX American Chemical Society

is worth mentioning that they can be used either as seeds in emulsion polymerization23 or as particulate stabilizer in dispersion polymerization.24−26 However, the reason why PNIPAM microgels serve as seeds in emulsion polymerization while they serve as particulate stabilizer in dispersion polymerization is still unclear. In Suzuki’s work, PNIPAM microgels were used in pure water or in pure alcoholic system.12,24 The property of the PNIPAM microgels in the water−alcohol mixture for dispersion polymerization has not been reported yet. Considering the special thermosensitivity of PNIPAM,27 a detailed study to reveal the above phenomenon of role transformation in different solvents was performed. In this work, we report for the first time the dispersion polymerization of styrene by using PNIPAM-based ionic microgels28 as either particulate stabilizer or seed in the methanol−water system. PNIPAM-based ionic microgels with different diameters were first prepared by surfactant-free emulsion polymerization and then used in dispersion polymerization of styrene. An interesting phenomenon was observed Received: September 27, 2017 Revised: November 8, 2017 Published: November 10, 2017 A

DOI: 10.1021/acs.langmuir.7b03381 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Table 1. Recipe, Hydrodynamic Diameter, and Yield of PNIPAM-Based Ionic Microgels Dh (nm)b code

a

NIPAM (mg)

VIM (μL)

DBB (μL)

V50 (mg)

water (mL)

25 °C

70 °C

yields (%)

678 904 1808

108 108 216

144 144 288

100 100 150

400 400 400

330 430 510

180 240 270

83.2 86.5 89.1

PNIM330 PNIM430 PNIM510

a PNIM refers to PNIPAM-based ionic microgels, and the following number refers to the diameter of PNI microgels at 25 °C. bThe diameter of PNI microgels is characterized in water solution by DLS.

Table 2. Recipe, Characterization, and Yield of Polystyrene Nanoparticles

a

codea

PNIM type

PNIM content (wt %)

styrene volume (mL)

water content (vol %)

property

morphology

yield (%)

DP1 DP2 DP3 DP4 DP5 DP6 DP7 DP8 DP9 DP10 DP11 DP12 DP13

PNIM330 PNIM330 PNIM330 PNIM430 PNIM510 PNIM330 PNIM330 PNIM330 PNIM330 PNIM330 PNIM330 PNIM430 PNIM510

0.5 0.75 1 0.75 0.75 0.75 0.75 0.75 0.75 0.75 1 1 1

2.3 2.3 2.3 2.3 2.3 4.6 3.45 2.3 2.3 1.15 2.3 2.3 2.3

0 0 0 0 0 5−10 5−10 5−10 >15 7.5 5−10 5−10 5−10

dispersion dispersion dispersion dispersion sediment dispersion dispersion dispersion sediment dispersion dispersion dispersion sediment

spherical spherical spherical spherical

93.4 96.3 97.1 51.2

octopus octopus octopus

97.1 95.4 96.4

raspberry octopus octopus

90.4 93.2 92.6

DP refers to dispersion polymerization. and linear polymers. Then, the centrifuged PNIPAM-based ionic microgels were dispersed into methanol by ultrasonic treatment. The methanol dispersion containing PNIPAM-based ionic microgels was centrifuged at 4000 rpm for 30 min. This dispersion/centrifugation process was repeated three times. The final purified PNIPAM-based ionic microgels were dried in a vacuum oven or dispersed in absolute methanol. 2.5. Preparation of Spherical and Octopus-Like Polystyrene Nanoparticles. The recipe for the preparation of spherical nanoparticles was given as follows: 2.3 mL of styrene, 25 mg of AIBN, and 0.4 g of dried PNIPAM-based ionic microgels (0.5 wt % compared to the weight of methanol) were added into a four-necked round-bottom flask containing 100 mL of absolute methanol (the weight of methanol was about 80 g). The flask was equipped with a reflux condensing tube, a nitrogen purging system, and a tetrafluoroethylene impeller. This solution was bubbled by nitrogen for about 20 min. Then, the temperature of this solution was increased to 70 °C. The stirring speed was maintained at 80 rpm during the whole process. After 24 h, this reaction was stopped by cooling down to room temperature. The obtained solution was centrifuged at 2000 rpm for 30 min to clear off PNIPAM-based ionic microgels. Then, the centrifuged spherical polystyrene nanoparticles were dispersed in methanol. This centrifugation/dispersion process was repeated three times. Other recipes for the preparation of spherical polystyrene nanoparticles were shown in Table 2. The procedure for the preparation of octopus-like nanoparticles was similar to that of the spherical ones, except that absolute methanol was changed to methanol−water mixture, such as 95 mL of methanol and 5 mL of water (compared to the total addition volume of methanol and water, the water content was 5 vol %). Other recipes for the preparation of octopus-like nanoparticles were shown in Table 2.

that the PNIPAM-based ionic microgels served as particulate stabilizer in pure methanol system. However, with the addition of a little amount of water, PNIPAM-based ionic microgels turned into seeds instead. We found that water played an important role in the transformation of PNIPAM-based ionic microgels from particulate stabilizer to seed. Hence, the mechanism for this transformation was strictly proposed.

2. EXPERIMENTS 2.1. Materials. 2,2-azobisisobutyronitrile (AIBN), 1,4-dibromobutane (DBB), vinylimidazole (VIM), and N-isopropylacrylamide (NIPAM) were purchased from Adamas Co.; 2,2′-azodiisobutyramidine dihydrochloride (V50) was purchased from Accela Co.; styrene (distilled before use) was purchased from Aladdin Co.; and absolute methanol was purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Instruments. Scanning electron microscopy (SEM) was used to characterize the morphology of nanoparticles (JEOL JSM-7800F Prime). Dynamic light scattering (DLS) was used to detect the particle size of nanoparticles (Malvern Instruments Ltd, ZS90). X-ray photoelectron spectroscopy (XPS) was used to characterize the elements of nanoparticles (AXIS UltraDLD). 2.3. Preparation of PNIPAM-Based Ionic Microgels. The recipe for the preparation of PNIPAM-based ionic microgels was given as follows: 904 mg of NIPAM, 108 μL of VIM, and 144 μL of DBB were added into a four-necked round-bottom flask containing 400 mL of water. The flask was equipped with a reflux condensing tube, a nitrogen purging system, and a tetrafluoroethylene impeller. This solution was bubbled with nitrogen for about 30 min. Then, the temperature of this solution was increased to 70 °C. After the temperature became stable, 100 mg of V50 was dissolved in 10 mL of water and added into this solution. The stirring speed was maintained at 350 rpm during the reaction process. The reaction was stopped 6 h later by cooling down to room temperature. By changing the monomer concentration, the diameter of PNIPAM-based ionic microgels varied according to the DLS, as shown in Table 1. 2.4. Purification of PNIPAM-Based Ionic Microgels. After obtaining the emulsion of PNIPAM-based ionic microgels, this emulsion was centrifuged at 9000 rpm for 15 min to clear off water

3. RESULTS AND DISCUSSION 3.1. Preparation of PNIPAM-Based Ionic Microgels. PNIPAM-based ionic microgels (encoded as PNI microgels) with different diameters were prepared, as shown in Table 1. There was no sedimentation during the preparation process, and the final PNI microgel solution was homogeneous. With B

DOI: 10.1021/acs.langmuir.7b03381 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. SEM of PNI microgels. (a) Sample of PNIM330 in Table 1; (b) sample of PNIM430 in Table 1; and (c) sample of PNIM510 in Table 1.

Figure 2. Polystyrene nanoparticles bearing different morphologies prepared at different conditions. (a) Spherical nanoparticles (sample of DP3 in Table 2); (b) octopus-like nanoparticles (sample of DP7 in Table 2); and (c) raspberry-like nanoparticles (sample of DP10 in Table 2).

Figure 3. Diameter of polystyrene nanoparticles decreases when increasing the content of PNI microgels in dispersion polymerization. (a) Sample of DP1 in Table 2; (b) sample of DP2 in Table 2; and (c) sample of DP3 in Table 2.

Therefore, it was quite straightforward that PNI microgels served as particulate stabilizer in this case. They provided steric hindrance in prohibiting the agglomeration of polystyrene nanoparticles. However, their stabilization capacity was affected by the diameter of PNI microgels, which was in accordance with the previous report.24 PNI microgels with a large diameter presented poor capacity in stabilizing polystyrene nanoparticles, which led to the sedimentation when used as particulate stabilizer. In addition, the yield of spherical polystyrene nanoparticles decreased when increasing the particle size of PNI microgels. Polystyrene nanoparticles with decreased diameters were obtained by increasing the concentration of PNI microgels, as shown in Figure 3. 3.3. Preparation of Octopus-Like Nanoparticles. For the preparation of spherical polystyrene nanoparticles, methanol was used as solvent in the absence of water. However, octopus-like nanoparticles were formed instead of spherical ones when a trace amount of water was added into this methanol solution, as shown in Table 2 and Figure 2b. The octopus-like nanoparticles consisted of two parts: on the one side of octopus-like nanoparticles, they were spherical with smooth surface similar to the spherical polystyrene nanoparticles; on the other side, they were rough similar to the raspberry-like nanoparticles which were prepared by reducing the monomer concentration (DP10), as shown in Figure 2c. In addition, both the spherical part and the raspberry-like part

increasing monomer concentration, the particle size of PNI microgels increased. Because of the existence of PNIPAM chains in the backbone, PNI microgels were thermosensitive. The yield of PNI microgels was high and increased with increasing monomer concentration. The observed weight loss came from the uncross-linked linear polymer which was removed during the centrifugal process. The morphology of PNI microgels was characterized by SEM, as shown in Figure 1. The PNI microgels were narrowly distributed spherical particles, thus providing a robust seed (or stabilizer) to perform dispersion polymerization. In accordance with their hydrodynamic diameters (Table 1), the particle size of dried PNI microgels increased with increasing monomer concentration. 3.2. Preparation of Spherical Nanoparticles. Dispersion polymerization of styrene by using PNI microgels as particulate stabilizer was performed in pure methanol solution. The preparation conditions were listed in Table 2. We could see that spherical polystyrene nanoparticles were prepared in this methanol solution, as shown in Figure 2a. Compared to PNI microgels, they were larger in size and led to the formation of a smoother surface of the polystyrene nanoparticles. In dispersion polymerization of styrene, polystyrene chains aggregated into nanoparticles because they were insoluble in methanol. In addition, they would further aggregate into huge sedimentation without the participation of PNI microgels. C

DOI: 10.1021/acs.langmuir.7b03381 Langmuir XXXX, XXX, XXX−XXX

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Figure 4. Elemental analysis of nanoparticles by XPS. (a) Spherical polystyrene nanoparticles: the sample of DP3 in Table 2; (b) octopus-like nanoparticles: the sample of DP7 in Table 2; and (c) raspberry-like nanoparticles: the sample of DP10 in Table 2.

Scheme 1. Illustration of the Mechanism for Forming Spherical and Octopus-Like Nanoparticles

combined with each other. The elemental analysis by XPS demonstrated that O, N, and Br elements existed on the surface of both octopus-like and raspberry-like nanoparticles rather than spherical nanoparticles, as shown in Figure 4. This result indicated that PNI microgels existed on the surface of both octopus-like and raspberry-like nanoparticles. In addition, compared to the raspberry-like nanoparticles, the content of O, N, and Br elements in octopus-like nanoparticles was relatively low. Therefore, for octopus-like nanoparticles, the PNI microgels only existed on the surface of the raspberry-like part rather than the spherical part. We could further conclude that the spherical part of octopus-like nanoparticles was polystyrene and the raspberry-like part was PNI microgels within which polystyrene was formed. According to the above analysis, polystyrene nanoparticles were formed both inside and outside of PNI microgels. Those polystyrene nanoparticles formed outside of PNI microgels adsorbed onto the surface of PNI microgels. In this case, we could see that PNI microgels were used as seeds. 3.4. Mechanism. The mechanism for forming spherical and octopus-like nanoparticles was illustrated in Scheme 1. Before illustrating the mechanism, some reported studies needed to be noted first: (a) PNIPAM lost their thermosensitivity and was in the swollen state in the methanol−water mixture when the molar fraction of methanol was higher than 0.45;27 (b) competitive hydrogen bonds between PNIPAM and water/ methanol molecules existed;29 and (c) cooperative hydration of water molecules to PNIPAM existed.30 Depending on the above hypothesis, we could conclude that (i) in pure methanol solution, PNI microgels were in the swollen state and lost their thermosensitivity; (ii) upon the addition of water, water molecules competed with methanol molecules for forming hydrogen bonds with PNI microgels; (iii) because of the cooperative hydrogen effects, the water content in the PNI microgel local area was very high; and (iv) at a relatively high water content, PNI microgels restored their thermosensitivity.

The above conclusions were verified by characterizing the thermosensitivity of PNI microgels by controlling the water content in the methanol−water mixture, as shown in Figure 5.

Figure 5. Thermosensitivity of PNI microgels in the methanol−water mixture.

PNI microgels did not shrink when increasing the temperature in pure methanol. This indicated that they lost their thermosensitivity in pure methanol. However, they began to shrink with increasing temperature when water was added into the methanol solution. This indicated that they began to restore their thermosensitivity in the methanol−water system. Water (5−10 vol %) in methanol was enough to help restore the thermosensitivity of the PNI microgels. In addition, with increasing water content, the thermosensitivity became more apparent. Considering that the reaction temperature was 70 °C, PNI microgels were still in a swollen state which could be considered hydrophilic in pure methanol. In this situation, D

DOI: 10.1021/acs.langmuir.7b03381 Langmuir XXXX, XXX, XXX−XXX

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Figure 6. Variation of the particle size of octopus-like nanoparticles by changing the amount of styrene. (a) Sample of DP6 in Table 2; (b) sample of DP7 in Table 2; and (c) sample of DP8 in Table 2.

styrene could not penetrate into the PNI microgels and the content of styrene within PNI microgels was relatively low, whereas most of the styrene was dissolved in methanol. Similarly, polystyrene as the hydrophobic material formed in the methanol solution could not adsorb onto the surface of the hydrophilic PNI microgels. In this case, PNI microgels served as particulate stabilizer to stabilize the spherical polystyrene nanoparticles. However, PNI microgels became hydrophobic in the methanol-water mixture at the reaction temperature because they restored their thermosensitivity. On one hand, styrene could penetrate into the PNI microgels and the content of styrene in PNI microgels was relatively high. Thus, polystyrene could be formed and separated into nanoparticles within the PNI microgels because of the incompatibility between PNI microgels and polystyrene. These polystyrene nanoparticles bulged on the surface of PNI microgels forming raspberry-like nanoparticles. This was the reason why raspberry-like nanoparticles were formed. On the other hand, styrene was still soluble in the methanol−water mixture when the water content is low. Therefore, polystyrene could also be formed outside the PNI microgels. These polystyrene chains aggregated into nanoparticles because of their poor solubility in the methanol−water mixture and adsorbed onto the surface of the hydrophobic PNI microgels via the hydrophobic interaction. In addition, the latter formed polystyrene chains inclined to adsorb onto the area in which polystyrene chains were enriched on the surface of PNI microgels. According to this process, octopus-like nanoparticles were produced. According to this proposed mechanism, parameters that might affect the final morphology of octopus-like nanoparticles were studied. With increasing the amount of styrene, the size of the spherical part increased but no change occurred in the raspberry-like part, as shown in Figure 6. Interestingly, raspberry-like nanoparticles were obtained when the styrene content was low enough, as shown in Figure 2c. This was because almost all of the styrene penetrated into the hydrophobic PNI microgels. Variation of water in a narrow range of 5 to 10 vol % content could not affect the morphology of octopus-like nanoparticles dramatically. However, sedimentation occurred when the water content was more than 15 vol %. This was due to the reduced dispersion capacity of the methanol−water mixture for octopus-like nanoparticles when the water content increased. Generally, increasing the content of PNI microgels had the same effect on the morphology of the octopus-like nanoparticles with decreasing amount of monomer. As for the effect of the particle size of PNI microgels on the final morphology of octopus-like nanoparticles (DP11, DP12, and DP13 in Table 2), we found that with increasing diameter of PNI microgels, their stabilization capacity for octopus-like nanoparticles decreased. For other parameters,

methanol could be replaced by ethanol and any oil-soluble initiator could be used in this system. In terms of application, the octopus-like nanoparticles had excellent film-forming property on different substrates such as glass, plastic, and metal. This was mostly due to their special structure that had a hydrophilic raspberry-like part and a hydrophobic spherical part.

4. CONCLUSIONS In this work, the special thermosensitivity of PNIPAM-based ionic microgels was illustrated and utilized to design the morphology of nanoparticles. In pure methanol solution, they lost their thermosensitivity and spherical nanoparticles were prepared. However, they restored their thermosensitivity in the methanol−water mixture and octopus-like nanoparticles were obtained. Further study demonstrated that PNIPAM-based ionic microgels served as particulate stabilizer when spherical nanoparticles were prepared. However, they turned into seeds when octopus-like nanoparticles were prepared. The role transformation of PNIPAM-based ionic microgels was strictly controlled by the water content. This was due to the water content that could affect the thermosensitivity of PNIPAMbased ionic microgels in the methanol−water mixture. Finally, this study answered the question why the PNIPAM microgel served as the seed in emulsion polymerization but as the particulate stabilizer in dispersion polymerization.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xin Jin: 0000-0003-1779-6407 Xinyuan Zhu: 0000-0002-2891-837X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Basic Research Program of China (2015CB931801), National Natural Science Foundation of China (51503122 and 51690151), and Shanghai Rising-Star Program (17QC1401100).

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DOI: 10.1021/acs.langmuir.7b03381 Langmuir XXXX, XXX, XXX−XXX