КОЛЛОИДНЫЙ ЖУРНАЛ, 2013, том 75, № 1, с. 94-100
УДК 541.18
SALT-TUNED PHASE SEPARATION OF POLY(S-co-NIPAM) CORE-SHELL PARTICLES VIA INTERFACIAL IN SITU POLYMERIZATION
© 2013 г. Liping Xua1, Guanghong HeA, Weijun LiuA
a School of Architecture, Anhui University of Architecture, Hefei, 230022 Anhui, People's Republic of China b School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai, 317000 Zhejiang, People's Republic of China Поступила в редакцию 13.01.2012 г.
In this study, temperature-sensitive amphiphilic core-shell nanoparticles of Ж-isopropylacrylamide (NIPAM) were prepared via interfacial in situ polymerization of styrene (S) and NIPAM. Oil soluble cumene hydroperoxide (CHPO) oxidizer and water soluble reductant iron(II) sulfate (FS), polyurethane and hexadecane were used as interfacial initiation pair, surfactant and co-stabilizer, respectively. Radicals are produced and initiate polymerization only when the CHPO and FS are present at oil/water interface. FT-IR and 1H NMR spectroscopy confirmed the co-polymerization of these monomers. The core-shell structure with a diameter 150 nm was corroborated by TEM and FESEM. DSC analysis showed the existence of two glass transition temperatures of the resulting particles. Salt-tuned phase separation behavior of poly(S-co-NIPAM) core-shell particles has been studied by dynamic light scattering. The lower critical solution temperature of the core-shell particles decreased linearly as a function of NaCl concentration that was attributed to the "salt out effect". The variation of particle diameter showed a sigmoidal plot as a function of temperature regardless of the salt concentration.
DOI: 10.7868/S0023291213010163
INTRODUCTION
Miniemulsions are classically defined as aqueous dispersions of relatively stable oil droplets within a size range of 50—500 nm. They can be prepared by shearing a system containing monomer(s), water, surfactant, and costabilizer, usually an osmotic pressure agent, which is water-insoluble in the continuous phase [1]. Due to the small size of the droplets, the large total surface area can effectively compete for radical capture. In comparison with conventional emulsion polymerization, miniemulsion method developed in recent years provides more applications and advantages. For example, inorganic-organic hybrids can be produced using robust inorganic nanoparticles and flexible organic components, since the polymerization is carried out in very small droplets called nanoreac-tors [2, 3]. The particles nucleation occurs primarily within the submicrometer monomer droplets. Recently, miniemulsion polymerization has been used for obtaining nanocomposites and inorganic nanoparticles [4—6] with various interesting morphologies.
Copolymerization of monomers with quite different water solubility through emulsion technology has long been an active field of research, having the goals of synthesizing the surface-functional polymer particles, increasing the stability and modifying the physical properties of the bulk polymers [7—9]. Thermosen-
1 Corresponding author; E-mail: lwj3600@mail.ustc.edu.cn.
sitive polymers in various physical forms such as gels, particles, micelles and capsules have shown intelligent loading and release capabilities for drugs, proteins, nanoparticles, and DNAs upon variation of temperature, ionic strength, pH, solvent and even light etc. [10-13]. Poly(V-isopropylacrylamide) (PNIPAM), a typical thermosensitive polymer, exhibits a lower critical solution temperature (LCST) in aqueous media, below which the polymer is soluble and above which it is in a collapsed phase or water-insoluble [14].
The phase transition of PNIPAM aqueous solution is understood as a result of dehydration of the polymer chains above the LCST, namely the breakage of hydrogen bonds between amide groups of PNIPAM and water molecules, thus collapsing the coiled polymer chains into a globular conformation and inducing polymer aggregation at sufficiently high concentration [15]. PNIPAM hydrogels that exhibit reversible swelling/deswelling have been produced and well studied since 1984 by Tanaka's group [16] and many others. Moreover, grafting polymerization of NIPAM on hydrophobic cores has yielded hairy nanoparticles consisting of thermosensitive hairs, whose hydrodynamic radius is readily tuned by temperature on a scale of hundreds of nanometers [17].
The goal of this study is the preparation of ther-mosensitive poly(S-co-NIPAM) amphiphilic core-shell particles via interfacial in situ miniemulsion polymerization of vinyl monomers using the waterborne
polyurethane (WPU) as surfactant and hexadecane (HD) as costabilizer, respectively. WPU has been investigated by many researchers for the excellent properties, such as solvent resistance, toughness, film formation, and abrasion resistance [18, 19]. Some attention has been paid on the two-component initiator system comprising both water-soluble and oil-soluble components in the emulsion copolymerization of hydrophobic and hydrophilic monomers. It was proposed that the primary radicals would be produced mainly at the oil/water interface, where the hydrophobic oxidant meets the hydrophilic reductant and both of the hydrophobic and hydrophilic monomers are present. In previous works, several groups have taken advantage of this interfacial-initiated method to prepare various functional polymers or hybrid materials [20, 21]. Just recently, Sun and Zheng reported the preparation of temperature-sensitive microspheres via this strategy [22, 23]. However, there have been no reports about synthesizing amphiphilic core-shell particles via interfacial in situ miniemulsion polymerization using WPU as surfactant. The salt-tuned phase separation of the poly(S-co-NIPAM) copolymers has been reported in the text. The response of copolymer to salt concentration upon temperature cycling was monitored by dynamic light scattering (DLS).
EXPERIMENTAL
Materials
Styrene (S), iron(II) sulfate (FS), isophorone di-isocyanate (IPDI) and triethylamine (TEA) were purchased from Shanghai Chemical Reagents Co., China. A portion of S was sequentially washed with NaOH (10 wt %) aqueous solution and distilled water until the pH value of 7.0 was achieved. After dried with anhydrous MgSO4, S was distilled under vacuum prior to use. NIPAM, Cumene hydroperoxide (CHPO) and
95
HD were purchased from Sigma-Aldrich. NIPAM was purified by re-crystallization twice from the mixture of hexane and toluene (30/70). Polycaprolactone diol (PCL-1000, Solvay Company, U.K.) and dimethylol butanoic acid (DMBA, Nippon Kasei Chemical Co. Ltd., Japan) were dried at 90°C under vacuum. All of the reactants and solvents were of analytical grade and used without further purification. Water was obtained from a Milli-Q® Gradient System from Millipore equipped with a QuantumTM cartridge, having a resistivity of18.2 Mfi cm. The pH of solutions was adjusted with NaOH if necessary.
Synthesis of Waterborne Polyurethane
The WPU was synthesized as shown in Scheme 1. The polyaddition reaction between IPDI and diol was conducted in a 500 mL round bottom, four-necked flask, equipped with a mechanical stirrer, a nitrogen gas inlet, and a condenser. The basic protocol for the WPU synthesis was applied as follows. The methanol was used as blocking agent. PCL-1000 (50 g) and IPDI (30 g) were placed in the reactor and heated to 75°C for one hour. Then the DMBA (10 g) was added to the mixture. When the NCO group content reached a given value that was calculated from the amount of all hydroxyl groups to react, polyurethane was capped by methanol (1.0 g). TEA was added to neutralize the product. To calculate the molecular weight of polyurethane, aniline was used as end-capping agent in the preparation of WPU. The NCO groups at the ends of polyurethane molecules reacted with aniline, and the phenyls were introduced into the ends of polyurethane. Based on the 1H-NMR spectrum of polyurethane, one can calculate the molecular weight of WPU. The detailed description of calculations is given in the section of determination the molecular weight of WPU.
NCO
H-O.
+
O
ok
o ^ R
OCN
NCO H_a
I
NCO
HOOC
CH2OH CH2OH
and ROH
R
R
COOH COOH COOH COOH COOH
Scheme 1. Schematic of synthesis of WPU.
KOnnOH^HBIH XyPHAtf TOM 75 № 1 2013
Interfacial-initiated miniemulsion polymerization of S and NIPAM with CHPO (100 mg) and FS (60 mg) as the initiating agents
Polymer NIPAM, M Conversion, % Pw, % GPC data
S NIPAM Mn Mw/Mn
Sample-1 0.10 91 90 86 32000 1.36
Sample-2 0.20 88 88 75 33000 1.58
Sample-3 0.30 87 89 72 38000 1.35
Sample-4 0.40 83 83 62 38000 1.24
Copolymerization S and NIPAM to Prepare Core-Shell Particles via Interfacial in situ Polymerization
The amphiphilic core-shell particles of poly(S-co-NIPAM) were prepared through one-step interfacial mini-emulsion polymerization using WPU and HD as the surfactant and co-surfactant, respectively. The details of polymerization procedure are given in Table. In a typical experiment, WPU (0.6056 g) was dissolved in water (20 mL) and oil-soluble monomer (S, 2.076 g), HD (0.2 g), and CHPO (100 mg) were added. The resulting mixture was emulsified by mechanical stirring at 6000 rpm for 5 min. Finally, the miniemulsification of the samples was completed by subjecting them to a highly efficient sonifier for 10 min. After miniemulsification, the reaction system was heated to 35°C while being purged with nitrogen, then the miniemulsion was mixed with different volumes of a NIPAM aqueous solution (20.0 wt. %) and FS (60 mg) in the batch, and kept for another 12 h to achieve the maximum conversion of monomers. Poly(S-co-PNIPAM) solid particles were obtained by de-emulsification with ethanol, washing with ethanol and dried under vacuum. Conversion of S was determined gravimetrically after the mixture was further dried under vacuum over P2O5 powder. In order to determine the conversion of NIPAM, the mixture was extra
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