Photochemical Synthesis of Polymer Nanoparticles in Aqueous Solutions Using Poly-N-Isopropyl-Acrylamide as a Template

Pokhrel, M. R., Janik, K., Bossmann, S. H.

Lehrstuhl für Umweltmesstechnik, Engler-Bunte-Institut, Universität Karlsruhe,

76128 Karlsruhe, Germany

ABSTRACT

The photoinitiated polymerization of methyl-methacrylate led to the formation of polymer latex particles using 2,3-diphenyl-butadiene as crosslinking agent and three photoinitiators of the benzoin-type in the presence of Poly-N-isopropyl-acrylamide as water soluble, hydrophobic template polymer in H2O/DMF. After recrystallization globular latex particles were found. By using benzoin-methylether as photoinitiator, "real nanoparticles" possessing a smaller diameter than 10 nm were formed among bigger particles (d > 20 nm). The elementary analysis of the synthesized polymers indicated that only a smaller fraction of the template polymer (4-10 percent) was incorporated into the latex particles.

INTRODUCTION

Compared to bigger particles, nanoscopic polymer particles feature substantial advantages. Their reduced size will permit a faster response to environmental changes. Furthermore, polymer nanoparticles will be used as building blocks for the nano-architecture of the future and for the tailoring of viable composite materials. Other potentially unusual or unique properties may be discovered because only little is known to date about the physical properties of these nanomaterials. In this report we describe a method for the preparation of polymer nanoparticles using Poly-N-isopropyl-acrylamide (PNIPAM) in mixtures of water and the consonsolvent DMF as reaction template. PNIPAM is known to undergo a coil to globe transition of individual polymer chains in water (T = 32°C) as well as many mixtures of water and water-mixable solvents. The later phenomenon has been called cononsolvency and ternary phase diagrams have been observed. Based on the solvent composition, characteristic LCST´s (lower critical solution temperatures) were observed. PNIPAM, as polymerisation template, methyl-methacrylate (MMA) and 2,3-diphenyl-butadiene (DPB) have been selected for the preparation of organic latex particles. MMA possesses a large chain propagation constant and therefore, a good monomer consumption can be predicted. Benzoin (BN), Benzoin-methylether (BME) and Irgacure (IGC) were selected as photoinitiators. Transmission Electron Micrography (TEM) was selected for the analysis of the particle´s size. Quantitative results are obtained when the TEM-images are analyzed with the aid of a suitable computer software (IMAGE/NIH).

EXPERIMENTAL

The TEM-images were recorded using an Transmission Electron Micrograph EM 912/Zeiss equipped with an Omega filter system. For the analysis of the TEM-images, the IMAGE-software, (NIH/USA), served as an excellent tool. Poly-N-isopropyl-acrylamide (PNIPAM) was prepared using AIBN in t-BuOH, purified and analyzed according to standard procedures. ([n] = 101± 4 [cm3 g-1] ([n] = viscosity at infinitesimally low polymer concentration); Mh = 1,540,000 g mol-1 (calculated according to h = 9.59 x 10-3 Mv0.65); Mn = 77,400 g mol-1; Mw = 110,000 g mol-1; LCST = 32.1 °C). Photopolymerization was performed in a photochemical batch reactor (V = 0.70 L, equipped with a Philips HKP 125 mercury medium pressure lamp). The gel permeation chromatography (GPC) experiments were carried out employing an HP-79911 GP-103 column (PL Gel 10 mm, 1000 A, 7,5 x 300 mm). THF was used as eluent (0.50 mL min-1).

TEM Sample Preparation

The standard sample preparation procedure consisted of the dispersion of 0.50 mL of polymer latex particles dissolved in THF/H2O (v/v: 1/1, c = 1 x 10-6 g L-1 ) using a sonication system (Siemens). The microdroplets were deposited on a special TEM-surface (available from Plano/Marburg) consisting of a copper grid(distance of the copper-wires: 85 µm), which are laminated with polyvinylformaldehyde (thickness: 20-40 nm). The PVF was sputtered using elementary carbon. This particular surface was exposed to the latex/solvent microdroplets for exactly 10 s (distance from the buffer surface: 5.0 cm). Finally, most of THF/H2O water was removed in high vacuum over 24h.

Gel Permeation Chromatography

The gel permeation chromatography (GPC) experiments were carried out employing an HP-79911 GP-103 column (PL Gel 10 mm, 1000 A, 7.5 x 300 mm). THF was used as eluent (0.50 mL min-1). The concentration of the injected samples (Vinj. = 1.0 mL) in THF was 1 x 10-3 g L-1. Calibration of the GPC has been performed using 5 monodisperse PMMA samples. (Mw = 2,000; 8.000; 30,000; 50,000; 75,000; 100,000; 150,000; 200,000). The detection of the polymers was achieved using l = 220 nm as detection wavelength.

RESULTS

Photoinitiated Polymerisation of Latex Particles

1.50 g of PNIPAM was dissolved in 0.50 L of H2O/DMF (70/30% per weight and warmed up to 60°C. The LCST of PNIPAM is transcended at the chosen H2O/co-nonsolvent composition. The mixture was agitated and purged by a constant nitrogen flow for 30 min. Then 1.0 g of liquid MMA (1.0 x 10-2 mol), 0.10 g of solid DPB (4.845 x 10-4 mol) and 0.40 g photoinitiator (BN: 1.885 x 10-3 mol; BME: 1.76 x 10-3 mol; IGC: 1.56 x 10-3 mol) was added. The N2-purging continued for another 30 min. Then the reaction mixture was irradiated for 15 min. at T = 60°C. After photolysis, the purging continued for an additional 15 min. During and after the photolysis, a pale yellow precipitate formed on two glass plates (2 x 5 cm2) inside the reactor. (A little precipitation occurred at the quartz socket of the lamp as well.) Typical primary yields were ranging from 0.48 to 0.38 g. The crude precipitate was recrystallized by dissolving it in 10 mL of anhydrous THF followed by the addition of 1 mL of H2O at T = 20°C. The resulting white precipitate was then filtered off and dried in high vacuum. The following yields were obtained (BN/MMA/DPB: 0.26 ± 0.05 g (23 %); BME/MMA/ DPB: 0.28 ± 0.05 g (25 %); IGC/MMA/DPB: 0.20 ± 0.05 g (18 %)).

GPC-Analysis

The progress of the photoinitiated polymerisation was monitored using GPC. A typical series of GPC-chromatograms, which were performed before polymerisation and at 5, 10 and 15 min. is reported in Fig. 1. From these experimental results we concluded that the formation of P(MMA/DPB) polymers, possessing a distinctly bigger macromolecular weight than the template polymer (PNIPAM), is proceeding upon irradiation. Furthermore, the occurrence of a steady state concentration of the formed P(MMA/DPB) polymers is observed. As reported previously in the experimental section, the created P(MMA/DPB) polymers deposit rapidly on the glass plates built in the photochemical reactor. From the quantitative analysis of the GPC peaks obtained from photolyses using BN, BME and IGC, we estimated that 25 to 30 percent of the PNIPAM was incorporated in the formed P(MMA/DPB) precipitate.

Figure 1. (Click on the picture to enlarge.) GPC chromatograms of the starting materials and the products formed during polymerisation employing MMA/DPB and BME as photoinitiator in the reaction system PNIPAM/H2O/DMF under continuous irradiation between 300 and 400 nm.

The next step in the synthesis of P(MMA/DPB) latex particles consisted in their purification by means of reprecipitation: The polymer precipitate on the glass plates was dissolved using anhydrous THF. Then, H2O was added until a white precipitate appeared (see experimental section), which was collected by filtration. The GPC chromatograms of the primary and the reprecipitated P(MMA/DPB) are shown in Fig. 2. It becomes apparent that the template PNIPAM as well as the residual MMA and DPB were removed by the employed reprecipitation method. Note that only the PNIPAM, which is codeposited beside the P(MMA/DPB) during the primary deposition at the glass plates could be removed by this process. Any PNIPAM, which was crosslinked to P(MMA/DPB) during photopolymerisation, remains within the formed latex particles.

Figure 2. (Click on the picture to enlarge.) GPC chromatograms of the precipitate, occurring during photopolymerisation of MMA/DPB /BME, on the glass scrubber plates before and after recrystallization in THF/H2O. The recrystallization procedure is described in the text.

Table 1: (Macro)molecular weights (Mn and Mw ) and polydispersities ( Pd) of the P(MMA/DPB) polymers formed by photoinitiated polymerisation in the system PNIPAM/H2O/DMF using benzoin (BN), benzoin-methylether (BME) and irgacure (IGC) as initiators.

Initiator		Mn	Mw	Pd
BN	crude	125,400	255,800	2.04
	recrystallized	130,800	247,500	1.89
BME	crude	170,500	245,000	1.44
	recrystallized	179,800	205,000	1.14
IGC	crude	105,750	251,800	2.38
	recrystallized	75,100	237,400	3.16

The data in Table 1 shows that the macro(molecular) weights of the P(MMA/DPB) polymers exceeded in all the experiments (except IGC) the weights of the employed template macromolecule (PNIPAM, Mn = 77,400 g mol-1; Mw = 110,000 g mol-1). The process of recrystallization led to lower values of Mw. When BN and BME were used as photoinitiators, the polydispersity measured by GPC decreased significantly during recrystallization.

TEM-Characterization

The characterization of the P(MMA/DPB) latex particles before and after reprecipitation revealed remarkable differences in their size distributions and their morphology. In Fig. 3, typical structures obtained from the photoinitiated polymerisation in the system MMA/DPB/PNIPAM/H2O/DMF are shown. Before the process of reprecipitation, PNIPAM is clearly present in all images (e.g. as hair-like structures, latex particles like pearls on a string or networks inter-connecting large latex particles).

Figure 3 (A):(Click on the picture to enlarge.) BN was used as photoinitiator. Large latex particles were present (diameter: 96 - 145 nm), interconnected by a network of very small structures, which resemble the morphology of micelles or microemulsions.

Figure 3 (B): BME was used as photoinitiator. Spherical latex particles were found (larger diameter 80 - 139 nm, smaller diameter 10 - 70 nm). Many particles were linked by linear polymer chain, which belong most likely to the PNIPAM template (pearls on a string). Furthermore, "hairy" surfaces of the latex particles can be identified.

Figure 3 (C): IGC was used as photoinitiator. In this case, the largest latex particles were found (diameter: 180 - 220 nm). The particles possess globular shapes and are interconnected by a (PNIPAM) polymer network with embedded latex nano-particles dia-meter (0.5 - 1.0 nm). We consider this finding as an experimental indication for the formation of latex particles by the association of very small "seeds".

The characterization of the P(MMA/DPB) latex particles after recrystallization revealed remarkable differences in their size distributions and their morphology. In Fig. 4, typical structures obtained from the photoinitiated polymerisation in the system MMA/DPB/PNIPAM/H2O/DMF are shown.

FIGURE 4: (Click on the picture to enlarge.) TEM images of the formed P(MMA/DPB) latex particles after recrystallization. The size of the images is 450 nm x 450 nm.

Figure 9 (A): BN was used as photoinitiator.

Figure 9 (B): BME was used as photoinitiator.

Figure 9 (C): IGC was used as photoinitiator.

All the particles analyzed by TEM showed "hairy features" at the boundaries of the globular polymer particles. We attribute these amenities to the presence of PNIPAM, which becomes incorporated into the latex particles during photopolymerisation. These boundary features could not be removed by a second or a third recrystallization. Therefore, it is likely that crosslinking between P(MMA/DPB) and PNIPAM occurs during polymer synthesis to a certain extent. The presence of PNIPAM as hydrophobic, water soluble polymer in the boundary region of P(MMA/DPB) latex particles enhances the water solubility of these otherwise insoluble materials.

Computer-Assisted Analysis of the P(MMA/DPB)-Morphology and Size-Distribution

The analysis of all available TEM images (20-30 per experiment) was performed by the program IMAGE. The size-distribution of the formed (nano)structured P(MMA/DPB) latex particles is presented in Figure 5. The relative statistical maximum (Frel.) of the latex particle diameter was determined to d = 46 ± 2 nm if BN was used as photoinitiator. A bimodal statistical distribution of particle diameters was recognized if BME was employed (d1 = 8.2 ± 0.8 nm; d2 = 21.0 ± 0.5 nm). Under those conditions, "real nanoparticles" possessing a diameter smaller than 10 nm were formed (approximately 55 rel. percent) during photoinitiated synthesis. In the case of IGC serving as photoinitiator, the statistical maximum was found to be d = 22.4 ± 1.2 nm.

FIGURE 5: (Click on the picture to enlarge.) Size distributions of the formed P(MMA/DPB) latex particles using BN, BME and IGC as photoinitiators after recrystallization on carbon surfaces, calculated using IMAGE.

CONCLUSIONS

This work consists of a first step towards the industrial production of defined P(MMA/DPB) latex (nano)particles using PNIPAM as template polymer. The synthetic procedure takes advantage of the constitution of defined hydrophobic nanodomains formed by collapsed PNIPAM chains in an H2O/DMF environment. The presence of a water mixable, but hydrophobic solvent leads to the formation of swollen PNIPAM nanodomains above its lower critical solution temperature (LCST). Those swollen nanodomains are able to absorb the strongly hydrophobic reagents MMA, DPB and the employed photoinitiators BN, BME or IGC.

After the process of recrystallization in THF/H2O regular latex particles were found employing TEM as characterization method. The smallest diameters of the globular latex particles (d1 = 8.2 ± 0.8 nm; d2 = 21.0 ± 0.5 nm) and the lowest content of incorporated PNIPAM (4.1 %) were detected using BME as photoinitiator. A schematic representation of the process investigated is given in Scheme 1.

SCHEME 1: (Click on the picture to enlarge.)

Mechanistic scheme of the process of photopolymerisation occurring in the system PNIPAM/H2O/DMF.

ACKNOWLEDGEMENTS

The authors would like to thank Prof. Dr. D. Gerthsen and Mr. M. Fotouhi Ardakani for the recording of the TEM-spectra. The authors thank Prof. Dr. André M. Braun for the use of his facilities at the Institute of Environmental Analysis Technology (Lehrstuhl für Umweltmesstechnik) at the Engler-Bunte-Institute, University of Karlsruhe, as well as his valuable advice. Financial support from the Deutsche Forschungsgemeinschaft (DFG, BO 1060/3-1) and the Fond der Chemischen Industrie (FCI) are gratefully acknowledged.

REFERENCES

Pokhrel, M. R.; Janik, K.; Bossmann, S. H. Photoinitiated Synthesis and Characterization of P(MMA/DPB) Polymer Nanoparticles Using Poly(N-isopropylacrylamide) in Aqueous Solutions as a Template, Macromolecules 2000, 33, 3577-3584.

Pokhrel, M. R.; Bossmann, S. H. Photochemical Synthesis of Polymer Nanoparticles in Aqueous Solutions Using Poly-N-Isopropyl-Acrylamide as a Template., J. Inf. Rec. 2000, in print.

and references quoted therein.