Synthesis and Characterization of Multiresponsive Core-Shell Microgels

ABSTRACT: We report the synthesis and characterization of temperature and pH responsive hydrogel

particles (microgels) with core-shell morphologies. Core particles composed of cross-linked poly(N-isopropylacrylamide)

(p-NIPAm) or poly(NIPAm-co-acrylic acid) (p-NIPAm-AAc) were synthesized via precipitation polymerization and then used as

 nuclei for subsequent  polymerization of p-NIPAm-AAc and p-NIPAm, respectively. The presence of a core-shell morphology was

 confirmed by transmission electron microscopy (TEM). Thermally initiated volume phase transitions were interrogated via

temperature-programmed photon correlation spectroscopy (TP-PCS) as a function of solution pH. The p-NIPAm-AAc core

 hydrogel displays both a strong temperature and pH dependence on swelling. However, both p-NIPAm-AAc (core)/p-NIPAm

 (shell) and p-NIPAm (core)/p-NIPAm-AAc (shell) particles display a more complex pH dependence than the homogeneous

 particles. Specifically, a multistep volume phase transition appears when the AAc component becomes highly charged at a high

 pH. It is apparent from the measured deswelling curves that a portion of the particle swelling behavior is dominated by p-NIPAm,

regardless of its location in the particle. However, deswelling behavior that is due to a mixture of p-NIPAm-AAc and p-NIPAm is

 evident, as well as a regime that is largely attributed to p-NIPAm-AAc alone. Small differences in the effect of pH on the two

 core-shell particles indicate that the influence of p-NIPAm is somewhat greater when it is localized in the shell.

Microgels composed of environmentally responsive polymers continue to attract attention due to their potential applications in

 numerous fields,1 including drug delivery,2-5 chemical separations,6-8 sensors,9-12 and catalysis.13,14 Perhaps the most widely

 studied class of responsive polymers are temperature responsive poly-(alkylacrylamides), specifically

poly(N-isopropylacryl-amide). 1,15-17 Part of the appeal of these materials stems from the varied physical properties of p-NIPAm

 latices that are modulated by the phase transition, including hydrophobicity,18 particle size,19,20 porosity, refractive index,

 colloidal stability,21 scattering cross section,22,23 electrophoretic mobility,24-26 and rheology.27,28 Because of the versatility of

 these materials, there has also been a growing interest into responsive latices that possess a more advanced architecture and

 hence are multifunc-tional.  Numerous responsive core-shell or core-corona particles have been synthesized either to introduce

spatially localized chemical functionalities to the particle,29-31 to impart thermoresponsivity to nonre-sponsive particles,32 or to

 modify a specific physical property of the latex.27,33,34 Many of the core-shell particles described above contain both a

 responsive and nonresponsive compo-nent.  However, there are many potential applications of multiresponsive core-shell particles

 that are com-posed of two or more environmentally sensitive poly-mers. Such materials present a potentially diverse

 chemo-mechanical system for study, as these particles would be expected to display very complex phase transi-tion

behavior. This is especially true in core-shell systems where one polymer has a chemical or mechan-ical influence over the swelling

 of the other polymeric component. In this paper, we present the synthesis and characterization of a multiresponsive (pH and

 temper-ature) core-shell hydrogel system that displays complex phase transition behavior. The effects of particle archi-tecture

and pH on the temperature-dependent swelling behavior are presented, as well as microscopic investi-gations into the resultant

 particle morphologies.

Materials. All reagents were purchased from Sigma-Aldrich.

N-Isopropylacrylamide monomer (NIPAm) was re-crystallized from hexanes (J.T. Baker) and dried in vacuo prior to use. Acrylic acid

 (AAc), N,N¢-methylenebis(acrylamide) (BIS), sodium dodecyl sulfate (SDS), and ammonium persulfate (APS) were all used as

 received. Water for all reactions, solution preparation, and polymer purification was distilled, purified to a resistance of 18 M¿

 (Barnstead E-Pure system), and filtered through a 0.2 ím filter to remove particulate matter.

Preparation of Core-Shell Hydrogel Nanoparticles.

Hydrogel nanoparticles were prepared via aqueous free-radical, precipitation polymerization under a nitrogen atmo-sphere.

A detailed description of this polymerization method may be found elsewhere.1,35 All particles were purified via dialysis

 (Spectra/Por 7 dialysis membrane, MWCO 10 000, VWR) against frequent changes of stirring H2O for 2 weeks at 5 °C.

Poly(NIPAm) Core. NIPAm (1.4 g) and BIS (0.10 g) were dissolved in 150 mL of H2O and then filtered twice through a

membrane filter (Pall Gelman Metricel). Dissolved gas was removed under vacuum for at least 15 min. SDS (0.057 g) was

then dissolved in the monomer solution, which was filtered once again. H2O (40 mL) was used to transfer and wash

throughout filtration. The solution was placed into a 250 mL  three-neck, round-bottom flask and heated over period of 1 h

with a heating mantle under a constant nitrogen purge and maximum stir rate (Corning magnetic stirrer). After stabilizing

for 15 min at 70 °C, polymerization was initiated via addition of APS (0.069 g) dissolved 10 mL of degassed H2O. The reaction

was allowed to proceed at a temperature of 70 °C for 6 h.

* To whom correspondence should be addressed: e-mail

lyon@chemistry.gatech.edu.

8301 Macromolecules 2000, 33, 8301-8306

10.1021/ma001398m CCC: $19.00 © 2000 American Chemical Society

Published on Web 10/12/2000