Preparation and Characterization of Micrometer-Sized Poly(N-isopropylacrylamide) Hydrogel Particles

Abstract

The present study deals with preparation and swelling properties of micrometer-sized poly(N-isopropylacrylamide) (PNIPAM) hydrogel particles. Aqueous precipitation polymerization of NIPAM with N,N 0 -methylenebisacrylamide (MBAAm) was performed with varying electrolyte concentrations in the polymerization media. Sodium chloride was used as an electrolyte. According to the particle size analysis by transmission electron microscope, the particle size increased in the range of 0.55 to 1.60 mm with an increase in the electrolyte concentration up to 1.6 wt %. The concentration is based on the NIPAM monomer weight. Dynamic light scattering data showed that swelling ratio of PNIPAM hydrogel particles in water varied from 10 to 40 as the electrolyte concentration at particle preparation increased from 0 to 1.6 wt %. Time-course analysis of particle size showed that an increase in particle size would be caused by aggregation of smaller particles. In conclusion, micrometer-sized PNIPAM particles with high swelling capacity can be obtained by the addition of an electrolyte in polymerization media. Thermosensitive hydrogel particles containing poly(N-isopropylacrylamide) (PNIPAM) moieties have attracted much attention because they exhibit sharp and reversible volume phase transition at the lower critical solution temperature (LCST).1,2 PNIPAM particles also show the dramatic change of their colloidal properties such as hydrophobicity, 3 particle size, 4 electrophoretic mobility, 5,6 colloidal stability, 7,8 rheology 9,10 etc. in response to temperature changes across the LCST. These unique features make PNIPAM particles desirable for numerous applications, including controlled drug delivery, 11,12 control of enzymatic activity, 13,14 tunable optics, 15,16 and bioseparation. 17 Colloidally stable PNIPAM particles were first prepared by precipitation polymerization of NIPAM with N,N 0 -methylenebisacryamide (MBAAm) as a cross-linking agent in an aqueous medium at 60 to 70 C. 18 The resulting particles showed a volume phase transition at the LCST around 32 C. In precipitation polymerization, the initial state of the reaction mixture is a homogeneous solution. When a water-soluble initiator is added to the mixture, initiation and propagation take place largely in the homogeneous medium, followed by phase separation of growing PNIPAM chains because PNIPAM is insoluble in the medium at the reaction temperature. This leads to continuous nucleation and coagulation of the resulting nuclei to form larger particles. Since the first report, various methods have been studied so far for preparation of PNIPAM hydrogel particles. Pelton and co-workers reported that addition of surfactant gave smaller and more uniform PNIPAM particles than those obtained without surfactant.19 Yi et al. reported that monodisperse thermosensitive poly(styrene-co-NIPAM) particles with diameters in the range of 100 to 130 nm could be prepared by emulsifier-free emulsion polymerization with microwave irradiation. 20 In addition, many researchers have focuse...

Comparing laser diffraction and optical microscopy for characterizing superabsorbent polymer particle morphology, size, and swelling capacity

“…These small errors from sample to sample could have resulted in differences between the crosslinking density and could have ultimately been responsible for the spread of the Q s . An increase in the crosslinking resulted in reduced swelling values, whereas less crosslinking yielded increased Q s …”

“…Many different methods have been used to characterize SAP particles. For example, their morphology is commonly investigated with scanning electron microscopy (SEM) for dry particles, environmental scanning electron microscopy for hydrated particles, transmission electron microscopy, and optical microscopy (OM) …”

“…The use of sieves to determine a PSD results in a discontinuous size distribution with large fractional size ranges . Although microscopy techniques are accurate and often used to determine particle sizes, they require image processing, which can be time consuming, and the sample size is limited to the number of particles within a micrograph and the number of micrographs taken. To reduce the time required and increase the sample size, particle size analyzers can be used to obtain accurate PSDs.…”

“…DLS and LD are the preferred methods because of their relatively fast analysis time and accuracy. DLS, however, can only be used for particles up to several micrometers, whereas LD can measure particle sizes ranging from 0.04 to 2000 μm. Particle size measurements over this range make LD a common method for determining the size distribution of micrometer‐sized SAPs …”

Thermal and pH Sensitive Composite Membrane for On‐Demand Drug Delivery by Applying an Alternating Magnetic Field

“…[ 15 ] So far, the majority of the obtained PNIPAM hydrogel particles were limited in the range of hundreds of nanometers, only a few were reported to have micrometer sizes. [ 16–18 ] Since micrometer‐sized PNIPAM particles have a higher swelling capacity compared with the conventional nanometer‐sized ones, a larger volume shrinkage can be generated under the same condition, and faster drug release can be expected. All previously published micrometer‐sized PNIPAM particles have a phase transition temperature of around 32 °C.…”