Langmuir 2004, 20, 3925-3932
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On the Structure of Poly(N-isopropylacrylamide) Microgel Particles Brian R. Saunders* Manchester Materials Science Centre, UMIST and the University of Manchester, Grosvenor Street, Manchester M1 7HS, U.K. Received December 18, 2003. In Final Form: March 1, 2004 This investigation presents a study of the internal structure of poly(NIPAM/xBA) microgel particles (NIPAM and BA are N-isopropylacrylamide and N,N′-methylene bisacrylamide, respectively). In this study, x is the wt % of BA used during microgel synthesis. Two values of x were used to prepare the microgels, 1 and 10. The microgel dispersions were investigated using photon correlation spectroscopy (PCS) and small-angle neutron scattering (SANS). These measurements were made as a function of temperature in the range 30-50 °C. Scattering maxima were observed for the microgels when the dispersion temperatures were less than their volume phase transition temperatures. The SANS data were fitted using a model which consisted of Porod and Ornstein-Zernike form factors. The analysis showed that the macroscopic hydrodynamic diameter of the microgel particles and the submicroscopic mesh size of the network are linearly related. This is the first study to demonstrate affine swelling for poly(NIPAM/xBA) microgels. Furthermore, the mesh size does not appear to be strongly affected by x. The data suggest that the swollen particles have a mostly homogeneous structure, although evidence for a thin, low segment density shell is presented. The study confirms that poly(NIPAM/xBA) microgel particles have a core-shell structure. The shell has an average thickness of ∼20 nm for poly(NIPAM/1BA) particles which appears to be independent of temperature over the range studied. The analysis suggests that the particles contained ∼50 vol % water at 50 °C. The molar mass of the poly(NIPAM/1BA) microgel particles was estimated as 6 × 109 g mol-1.
Introduction Microgel particles are cross-linked latex particles that are swollen by a good solvent1. In this article, attention will be focused on poly(NIPAM) (NIPAM is N-isopropylacrylamide) microgel particles. Poly(NIPAM) microgel particles were first reported in a seminal article by Pelton and Chibante2. The interest in these particles arises from the fact that they exhibit a temperature-dependent particle size and that the particle size distributions have a low polydispersity. The temperature-dependent nature of the particle size is due to the temperature-dependence of the polymer-solvent interaction parameter for poly(NIPAM) in water. Linear poly(NIPAM) has a lower critical solution temperature (LCST) of ∼32 °C in water3. Cross-linked poly(NIPAM) microgel particles exhibit a substantial decrease in particle diameter when the temperature exceeds 32 °C in water. This critical temperature is referred to as the volume phase transition temperature (VPTT). The primary objective of this study is to determine the structure of poly(NIPAM) microgel particles. It is expected that the accurate determination of the structure of poly(NIPAM) microgel particles will help their exploitation in areas where precise structural manipulation is needed (e.g., drug delivery). There has been a concerted effort in the scientific literature to determine the structural characteristics of poly(NIPAM) microgel particles. One of the most cited papers in this regard is that by Wu and Pelton.4 They reported that, during the formation of poly(NIPAM/BA) (BA is N,N′-methylene bisacrylamide) particles, BA was (1) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (2) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (3) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352. (4) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272, 467.
preferentially incorporated. More recently, Varga et al.5 used static light scattering to investigate the structure of poly(NIPAM/BA) microgel particles. Their results suggested the presence of a core-shell structure for their most highly cross-linked particles. Kratz and Eimer6 investigated the effect of BA concentration on the volume phase transition of poly(NIPAM/BA) microgel particles. They suggested that the BA was not isotropically distributed within the particle interior. Rather it was suggested that a higher connectivity (and hence BA concentration) was present in the center of the particle. Daly and Saunders7 investigated the volume phase transition of poly(NIPAM/BA) microgel particles using electrophoretic mobility and hydrodynamic diameter measurements. That work provided evidence for a nonuniform structure in which a more highly cross-linked core was present. All of these studies have provided evidence for a core-shell structure. However, the author is not aware of any published work which has attempted to quantify the structure of the core and the shell of microgel particles as well as how this changes with temperature. The present study provides new data that begin to address these problems. Small-angle neutron scattering (SANS) utilizes scattering of neutrons by collisions with the nuclei of materials.8,9 Neutrons are deeply penetrating and are scattered by short-range repulsive interactions. Using contrast variation, it is possible to selectively enhance or reduce the scattering from the sample or parts of the sample (for (5) Varga, I.; Gilanyi, T.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. J. Phys. Chem. B 2001, 105, 9071. (6) Kratz, K.; Eimer, W. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 848. (7) Daly, E.; Saunders, B. R. Phys. Chem. Chem. Phys. 2000, 2, 3187. (8) Higgins, J. S.; Benoit, H. C. Polymers and Neutron Scattering; Clarendon Press: Oxford, U.K., 1996. (9) King, S. M. In Experimental Methods in Polymer Characterisation; Pethrick, R. A., Dawkins, J. V., Eds.; Wiley: New York, 1999.
10.1021/la036390v CCC: $27.50 © 2004 American Chemical Society Published on Web 04/17/2004
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multicomponent systems). In a neutron scattering experiment, the scattered intensity (I(q)) is measured as a function of the scattering vector (q). The scattered intensity is given by
I(q) ) φpVp(∆F)2P(q) S(q)
(1)
where φp and Vp are the volume fraction and volume of the scattering species, respectively. The parameter P(q) is the intraparticle form factor, and S(q) is the interparticle structure factor. The latter factor is unity in dilute dispersions. There have been several investigations of poly(NIPAM) microgel dispersions using SANS. Mears et al.10 investigated the scattering profiles of poly(NIPAM/BA) microgels in the presence of anionic surfactant. They found that the mesh size of the particles increased with the extent of particle swelling. Seelenmeyer et al.11 investigated the temperature-induced deswelling of core-shell particles using a combination of SANS and small-angle X-ray scattering. The particles had a polystyrene core and a cross-linked poly(NIPAM) shell. They found a linear correlation between the mesh size and the volume fraction. The data shown in Figure 10 of their paper obey the following equation: ξ = -14.3φPNIPA + 10.9, where ξ is the mesh size (in nanometers) and φPNIPA is the volume fraction of the network in the shell. Although this relationship could be explained using a qualitative argument, it is not indicative of affine swelling. Poly(NIPAM) macrogels have also been investigated using SANS. One of the earliest studies is that reported by Shibayama et al.12 They investigated the changes in the scattering profiles with increasing temperature up to the VPTT. It was reported that the mesh size increased as the macrogel deswelled. This result appears counterintuitive, as one would expect the mesh size to decrease as the macrogel deswells. Poly(NIPAM/BA) microgels and macrogels are prepared by different polymerization methods, and this has implications for their structures. Poly(NIPAM/BA) microgels are prepared by surfactant-free precipitation polymerization (SFPP) which is conducted above the LCST. The particle formation and growth mechanism is widely believed to be analogous to surfactant-free emulsion polymerization of styrene.13 However, in the case of SFPP, there are no emulsion droplets. The particle growth mechanism is likely to involve a coagulation stage. It is also reasonable to expect the central regions of the final particles to contain a relatively high proportion of the monomers consumed at the earliest stages of the polymerization. There is therefore considerable opportunity for the development of an inhomogeneous structure within the interior of poly(NIPAM/BA) microgel particles. This is in contrast to the method usually used to prepare poly(NIPAM/BA) macrogels, which involves polymerization of NIPAM/BA solutions12 below the LCST. In such cases, the monomer that reacts most quickly is distributed over the entire volume of the macrogel slab. Even though a locally inhomogeneous structure on the scale of ∼10 nm results,12 the average segment density for each monomer will be constant across the macrogel slab. Compared to (10) Mears, S. J.; Deng, Y.; Cosgrove, T.; Pelton, R. H. Langmuir 1997, 13, 1901. (11) Seelenmeyer, S.; Deike, I.; Rosenfeldt, S.; Norhausen, C.; Dingenouts, N.; Ballauff, M.; Narayanan, T.; Lindner, P. J. Chem. Phys. 2001, 114, 10471. (12) Shibayama, M.; Tanaka, T.; Han, C. C. J. Chem. Phys. 1992, 97, 6829. (13) Goodwin, J. W.; Hearn, J.; Ho., C. C.; Ottewill, R. H. Br. Polym. J. 1973, 5, 347.
Figure 1. Possible structures for poly(NIPAM/BA) microgels: (a) homogeneous; (b) core-shell with uniform mesh size in the core and the shell; (c) agglomerated; (d) agglomerated with core-shell structure.
macrogels, poly(NIPAM/BA) microgels can be expected to have significant monomer segment density gradients that may extend across the whole particle. A more inhomogeneous structure is expected. The shape of the monomer segment density profiles for microgels will be determined by a number of factors, including the reactivity ratios for the monomers as well as the proportions of the monomers present. What structures are most likely for poly(NIPAM/BA) microgel particles? Several possible structures are depicted in Figure 1. (Of course there are more structural possibilities than those given in Figure 1.) These structures are limiting cases chosen to accentuate differences in a visually clear manner. Structure a is closest to a macrogel structure. It shows a uniform distribution of the crosslinking monomer. Structures c and d take into account coagulation and growth during SFPP. The large particles comprise agglomerates of nanoparticles that are crosslinked to each other. Structure c is similar to structure a except it is locally inhomogeneous. Structure b is a coreshell model in which the core and the shell each have uniform (but different) mesh sizes. Structure d considers a core-shell structure in which the aggregated particles comprise the core and have a higher segment density than those comprising the shell (cf. structure c). It is likely that the boundaries between the core and shell regions shown for structures b and d are more diffuse than those shown here. It is easy to envisage cases whereby the distinction between structures a and c as well as structures b and d would not be clear, for example, if the nanoparticle size approached very small values. In this work, SANS and photon correlation spectroscopy (PCS) data will be used in an effort to determine the most likely structure of poly(NIPAM/xBA) microgel particles at temperatures that range from below the LCST to well above the LCST. (Note that x ) the wt % of BA used during synthesis.) Experimental Section Materials. NIPAM (Acros Organics, 99%), BA (Aldrich, 98%), 4,4′-azobis-4-cyanopentanoic acid (ABCP, Fluka, 98%), and D2O (Aldrich, 99.9%) were used as received. The water used was Milli-Q quality. Microgel Preparation. Poly(NIPAM/xBA) microgel particles were prepared using SFPP in water at 70 °C. The polymerization volume was 1 L. Water (at pH ) 10) containing NIPAM and BA was placed in a three-necked round-bottomed reactor flask.
Poly(N-isopropylacrylamide) Microgel Particles (Water was adjusted to pH ) 10 using NaOH solution.) The contents were degassed with a nitrogen gas purge and heated to 70 °C. ABCP dissolved in water (at pH ) 10) was added to the stirred reaction flask and the reaction allowed to continue overnight. A nitrogen atmosphere was maintained over the contents during the course of SFPP. A typical preparation for poly(NIPAM/1BA) (i.e., 1 wt % BA) contained 8.40 g of NIPAM, 0.085 g of BA, and 0.52 g of ABCP. The microgel dispersions were purified extensively by dialysis as well as centrifugation and redispersion. PCS Measurements. A NOVA 901 instrument (Group Scientific) was used to obtain PCS measurements. The scattering angle was 90°. All PCS measurements were conducted in D2O and were made at a particle concentration of ∼0.03 wt %. The Stokes-Einstein equation was used to calculate the hydrodynamic diameter. The viscosities of D2O at a given temperature were used for these calculations. It is important to note that Routh and Zimmerman14 recently showed that the hydrodynamic diameters calculated for microgel particles using the StokesEinstein equation are accurate. SANS Measurements. SANS measurements were conducted using LOQ at ISIS (Rutherford Appleton Laboratories, Didcot, U.K.). All SANS measurements were performed using poly(NIPAM/xBA) (x ) 1 and 10) dispersed in D2O. Each microgel dispersion studied contained 2 wt % poly(NIPAM/xBA). The SANS measurements were conducted using the LOQ beamline running at 25 Hz. The scattered intensity data were corrected for sample transmission and background scattering using standard procedures and placed on an absolute scale. For this purpose, calibration with a standard partially deuterated polystyrene polymer blend was employed using established procedures. Each set of data was corrected for the incoherent background. The scattering exponent for the high q contribution (e.g., q-n) was determined from an initial investigation of d ln I(q)/d ln q versus q. For the samples measured at temperatures less than 50 °C, the value of n was 2 for the I(q) data for q values greater than a minimum value (q2,min). A plot of I(q) versus q-2 was then constructed for the I(q) and q range which satisfied q > q2,min. The gradient was then used to calculate a value for A2 using I(q) ) A2q-2 + Binc, where Binc is the incoherent background. Subtraction of each calculated value for A2q-2 from I(q) (over a range of q > q2,min) gave values for Binc, which were averaged (Binc(av)). The advantage of this method is that the value for A2 was calculated using linear regression which is inherently biased to the data sets at the high range of I(q) and q-2. These data sets have the least relative error for I(q). The average value for Binc(av) was subtracted from the scattering data, enabling the coherent scattering for the poly(NIPAM/xBA) microgel particles to be determined. The samples measured at 50 °C did not have a discernible scattering contribution from a form factor at q > 0.12 Å-1. For these samples, a Porod contribution (later) decayed rapidly below the baseline at lower q values. The values for Binc(av) for these samples were obtained by averaging the I(q) values (at q > 0.12 Å-1), that is, by assuming that the scattered intensity above this q value was solely due to incoherent scattering.
Results PCS Measurements. Figure 2 shows the hydrodynamic diameter (dh) values measured for poly(NIPAM/ xBA) (x ) 1 and 10) as a function of temperature. The VPTT is taken as the point of inflection of the curves. The data from Figure 2a show that, compared to poly(NIPAM/ 1BA), poly(NIPAM/10BA) exhibits (i) a smaller extent of deswelling at 50 °C and (ii) a higher VPTT. The VPTTs for poly(NIPAM/1BA) and poly(NIPAM/10BA) are estimated from these data as 33.5 and 37.0 °C, respectively. The values for dh are similar to those reported for related microgels elsewhere.7 The trends for dh with respect to x and TVPTT are similar to those reported for a related system by Varga et al.5 (14) Routh, A. F.; Zimmerman, W. B. J. Colloid Interface Sci. 2003, 261, 547.
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Figure 2. Variation of the hydrodynamic diameter for poly(NIPAM/1BA) (0) and poly(NIPAM/10BA) (9) microgels with (a) temperature and (b) reduced temperature (see text). Table 1. Terminology, Reduced Temperatures, and Hydrodynamic Diameters Relevant to the SANS Data temperature term T/°C T′/°C dh/nm
poly(NIPAM/1BA) T
