Factors affecting the swelling of poly(N-isopropylacrylamide) microgel particles: fundamental and commercial implications

Colloids and Surfaces A: Physicochemical and Engineering Aspects 149 (1999) 57–64

Factors affecting the swelling of poly(N-isopropylacrylamide) microgel particles: fundamental and commercial implications Brian R. Saunders *, Helen M. Crowther, Gayle E. Morris 1, Sarah J. Mears, Terence Cosgrove, Brian Vincent School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK Received 26 August 1997; accepted 2 November 1997

Abstract A microgel particle is a cross-linked latex particle which is swollen by a good solvent. Particle swelling is intrinsically related to the nature of the interaction between the polymer and continuous phase. Microgel particles based on PNP [PNP=poly(N-isopropylacrylamide)] are particularly interesting since the parent homopolymer undergoes a coil-toglobule transition in water when the temperature increases above 32°C. In this work, PCS (photon correlation spectroscopy) and SANS (small-angle neutron scattering) are employed in a complementary manner to study the environmentally induced de-swelling of PNP particles. Further, we show that particle de-swelling may be induced at room temperature by addition of alcohols or excluded free polymer (i.e. non-adsorbing free polymer) to the continuous phase. (The extents of particle de-swelling observed using these additives are similar to those achieved by heating the pure microgel particles in water above 32°C.) Particle de-swelling in the presence of added alcohol or free polymer arises from ‘‘co-non-solvency’’ and osmotic de-swelling effects, respectively. Copolymerization of N-isopropylacrylamide (NP) with acrylic acid yields microgel particles whose diameters are sensitive to both pH and temperature. These particles adsorb PbII ions from solution in a reversible manner. The latter property has potential application in water purification. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Microgel; Isopropylacrylamide; Small-angle neutron scattering; Co-non-solvency

1. Introduction A microgel particle is a cross-linked latex particle which is swollen by a good solvent. In this work, we examine poly(N-isopropylacrylamide) (PNP) microgel particles. Microgel particles and macrogels are physically and chemically distinct. Microgel particles flow in the dispersed state and * Corresponding author. Present address: Department of Chemistry, University of Adelaide, North Terrace, Adelaide 5005, Australia. 1 Present address: Ian Wark Research Institute, University of South Australia, The Levels 5095, Australia.

have external interfacial areas that are several orders of magnitude larger than those of macrogels. Microgel particles are frequently synthesized using surfactant-free emulsion polymerization and particle formation involves nucleation, aggregation and growth stages. The growth mechanism favours incorporation of monomers with a high reactivity ratio in a nonuniform manner. There is evidence of a non-uniform distribution of cross-linking monomer within PNP microgel particles [1]. Conversely, macrogels are prepared from quiescent solutions under conditions designed to favour a homogeneous network structure. PNP microgel particles exhibit a continuous

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de-swelling transition in water at approximately 32°C [2] (the lower critical solution temperature, LCST ). PNP particles are swollen at room temperature owing to hydrogen bonding between water and the amide groups. Hydrogen bonding within aqueous amide solutions has been found [3,4] to involve the carbonyl oxygen (R–CNO,H–O–H ) and the protic hydrogen (R∞–N–H,OH ). It is 2 reasonable to expect that analogous bonding exists within PNP particles dispersed in water at room temperature. Heating the solution above the LCST disrupts the water–polymer hydrogen bonding, allowing intra- and inter-chain hydrogen bonding and attractive hydrophobic interactions to dominate, inducing chain collapse. Thermally induced de-swelling occurs for both PNP microgel particles and macrogels [5]. The effects of additives on the swelling of microgel particles have been studied in this department and elsewhere [6–13]. De-swelling may be induced by adding free (i.e. non-adsorbing) polymer [8] to the continuous phase, provided that the free polymer is excluded from the microgel particle interior. Particle de-swelling may also be induced by addition of a co-solvent that competes for the water molecules that hydrate the PNP chains [9]. Conversely, the extent of PNP particle swelling may be increased by addition of surfactant to the continuous phase [11]. The latter work involved an extensive PCS (photon correlation spectroscopy) and small-angle neutron scattering (SANS ) study which revealed that small aggregates of SDS formed within the particles at SDS concentrations above the cmc. We have also shown that the LCST of PNP may be increased by incorporation of acrylic acid (AA) [12]. 1.1. Scattering theory SANS experiments involve measuring the scattered intensity as a function of Q, the momentum transfer vector, which is related to the scattering angle (H) and the wavelength of the neutrons, l, by: Q=

A B AB 4p l

sin

H 2

.

(1)

For a dilute dispersion of spherical particles, the scattering amplitude [14] is dependent on the contrast factor (Dr=r −r , where r is the coherp s ent scattering length density and the subscripts p and s refer to the particles and solvent, respectively) and the geometric interference factor P(Q). We have employed PNP particles dispersed in D O in the present study in order to maximize Dr. 2 The scattered intensity, I(Q), of a dispersion of non-interacting particles is given by: I(Q)=V w (Dr)2P(Q), (2) p p where V and w are the particle volume and the p p volume fraction of the particles, respectively. A number of workers have used the following relationship to analyse scattering from macrogels [5]

A

I(Q)=I (0) exp − G

Q2J2 2

B

+

I (0) L , 1+Q2j2

(3)

where J and j are the spatial correlations associated with the bulk and ‘‘solution-like’’ phases, respectively. Following Mears et al. [11], we employ a combination of the Porod and Ornstein–Zernike equations to fit our SANS data: I(Q)=AQ−4+

I (0) L , 1+Q2j2

(4)

where A is a constant. Eq. (4) assumes that the total scattered intensity is the sum of scattering characteristic of the whole particle (Porod scattering, Q−4 dependence) and that from polymer segments in the lightly cross-linked (solution-like) environment (Omstein–Zernike scattering, dependence). The correlation length, j, is related to the size of the lightly cross-linked ‘‘pouches’’ that separate the smaller highly cross-linked regions [15].

2. Experimental The microgel particles were prepared using surfactant-free emulsion polymerization in water at 70°C. Details concerning the reagents and synthetic methods employed can be found elsewhere

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[8,9,12]. N,N∞-methylene-bisacrylamide (BA) was employed as the cross-linking monomer. The systems are designated as PNP–xBA or PNP–yAA–xBA, where x and y indicate the respective wt.% of BA or AA added with respect to the total monomer mass. PNP–xBA (x=2 and 5) and PNP–5AA–2BA were prepared using 4,4∞-azobis(4-cyanopentanoic acid ) as initiator. The initiator employed for the preparation of PNP–9BA was ammonium persulphate. The nature of the initiator residues is not considered to have a significant effect on the swelling properties of PNP microgel particles. All of the microgel systems were cleaned thoroughly using centrifugation with ‘‘Milli-Q’’ quality water. The microgel systems investigated by SANS were subjected to additional centrifugation cycles using D O. 2 TEM data were obtained using a Hitachi HS7S instrument. PCS measurements were performed using a Brookhaven ‘‘Zetaplus’’ instrument and a microgel particle concentration of about 0.05 wt.%. The coefficient of variation for the hydrodynamic diameter is estimated as 5%. The extent of de-swelling is characterized by the de-swelling ratio (a) which is defined as the ratio of the measured particle volume (with a diameter, d ) to that of the fully swollen particle (with a diameter, d ) i.e. a=(d/d )3. The de-swelling 0 0 ratio and the volume fraction of polymer within the microgel particles (w ) are related by 2 a=w−1 (d /d )3, where d is the diameter of the 2 c 0 c particles in the collapsed state. The latter is obtained from PCS measurements at 50°C. All PCS measurements were conducted at 25°C unless stated otherwise. SANS experiments were performed at the ISIS facility (Rutherford Appleton Laboratories, Didcot, UK ) using a spallation neutron source. The microgel particle concentration employed for these measurements was 2 wt.%. The samples were contained in Hellma quartz cells (20 mm×2 mm) ˚ −1. The and the effective Q range was 0.009–0.25 A scattered intensity data were corrected for scattering from the cell and D O as well as transmission 2 using the Collete program at the ISIS facility. The gradient of plots of I(Q)Q6 versus Q6 gave the incoherent background which was subtracted from

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the spectra to yield the absolute coherent scattering (for the PNP particles). A non-linear regression program which took into account the standard deviation of each data point was used for fitting the SANS data to Eq. (4).

3. Results and discussion 3.1. Microgel particle characterization Particle size data for the microgel systems appear in Table 1. The PNP–xBA (x=2, 5 and 9) particles exhibit thermally induced collapse upon heating to 50°C. Comparisons of the swelling behaviour of the particles are best made using the respective values of a. With the exception of PNP–9BA, the microgel particles de-swell to similar extents when heated to 50°C. The larger value of a for PNP–9BA reflects the higher cross-link density within the particles. [It has been shown [16 ] that w and the 2 molar fraction of BA incorporated (x ) are related BA by w 3x3/5 .] Comparison of the de-swelling ratios 2 BA for PNP–2BA and PNP–5AA–2BA (AA=acryllc acid ) suggests that the latter particles are fully collapsed at 50°C (pH=5). However, the hydrodynamic diameter of PNP–5AA–2BA particles measured at 50°C (pH=3.5) is 315 nm (a=0.055) [12], suggesting an ionic contribution to swelling at pH=5. Fig. 1 shows a TEM micrograph of PNP–5BA particles. Extensive two-dimensional order is evident. The image is similar to those published elsewhere [9,17] and demonstrates that the particles have a narrow size distribution. The highly ordered regions (not shown) were only evident in the periphery of the region occupied by the deposited particles on the TEM grid. The particle concentration and extent of disorder were found to increase with decreasing radius from the centre of the deposited particles. SEM data [8,9] have revealed that PNP particles become flattened during water evaporation. Representative values for the collapsed particle diameter, therefore, cannot be obtained using electron microscopy.

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Table 1 Particle size data for PNP microgel particles System

PNP–9BA PNP–5BA PNP–2BA PNP–5AA–2BA

TEM particle diameter (nm)a,b

455±80 570±55 525±40 560±45

PCS particle diameter (nm) d (25°C ) 0

d (50°C ) c

ac

725 715 675d 830d

455 370 355d 445d,e

0.25 0.14 0.15 0.15

aThe ± numbers are the standard deviation. bParticles deposited on TEM grids at room temperature. cDe-swelling ratio of the particles at 50°C. dMeasured in water at pH=5 containing 10−3 M KNO−. 3 eAverage of values measured at 45°C (485 nm) and 55°C (400 nm).

3.2. Thermally induced de-swelling The de-swelling ratio for PNP–5BA particles as a function of temperature is shown in Fig. 2. ( The particles were dispersed in D O to allow direct 2 comparison of PCS and SANS data.) It is evident that the particles undergo thermally induced de-swelling. The data are qualitatively similar to related measurements performed in water [8,9]. However, the LCST has increased to 34±0.5°C, which is about 2°C higher than that normally observed when the particles are dispersed in water. Shibayama et al. [5] reported an LCST of 34°C for PNP macrogel immersed in D O. They attrib2 uted the upward shift of the LCST to a weaker hydrophobic interaction between PNP and D O 2

Fig. 1. Transmission electron micrograph of PNP–5BA particles. The particles were deposited from water at room temperature. The average apparent diameter is 570 nm.

(compared to PNP and H O). Hydration of amides 2 by water (D O) can occur at the CNO and N–H 2 sites via hydrogen (deuterium) bonding. It is suggested that the deuterium bonding of the amide groups of PNP with D O is stronger than the 2 hydrogen bonding that occurs with water. This result is consistent with the work of Kobayashi et al. [18]. They examined the hydration of amides by H O and D O using infrared spectroscopy and 2 2 reported stronger hydration in the case of D O. 2 SANS data for PNP–5BA particles dispersed in D O at 25, 34 and 50°C appear in Fig. 3. The 2 diameters of the microgel particles are much larger than the dimension range accessible by SANS. The power law dependence of the scattering profiles provides information about the internal structure

Fig. 2. A plot of the de-swelling ratio for PNP–5BA particles dispersed in D O as a function of temperature. 2

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Fig. 4. Variation of the de-swelling ratio for PNP–5BA particles as a function of the molecular weight of added PEG (20 wt.%) Fig. 3. Scattering profiles for PNP–5BA microgel particles dispersed in D O. The data were measured at 25 ($), 34 (6) and 2 50°C (%). The fit for the data at 25°C is shown. The error bars represent ± one standard deviation.

of the particles. The scattering is dominated by the contribution from the lightly cross-linked polymer at 25°C. The gradient increases with temperature and has a value of −4.18 at 50°C, indicative of Porod scattering. Thus, the particles have collapsed to hard spheres at 50°C. The changes in the scattering profiles are consistent with the PCS data measured for the system (Fig. 2). The SANS data for PNP–5BA obtained at 25°C exhibit a linear dependence when plotted as 1/I(Q) ˚ −1) [19]. Linear behaviour versus Q2 (at Q>0.02 A ˚ −1) when the data was also apparent (at Q<0.02 A were plotted as I(Q) versus Q−4 The data obtained at 25°C were fitted using Eq. (4) which yielded a ˚ . Mears et al. [11] reported a value of j=28 A ˚ value of j=18 A for their PNP microgel particles. ( The hydrodynamic diameter of the latter microgel particles was 72 nm.) The relationship between j and a is currently under investigation and will be discussed in a forthcoming publication [19]. 3.3. Osmotic de-swelling De-swelling of PNP microgel particles may also be induced by addition of free polymer to the continuous phase. Fig. 4 shows the variation of the de-swelling ratio of PNP–5BA as a function of the molecular weight of added poly(ethyleneglycol ) (PEG). The particles do not exhibit de-swelling in the presence of PEG with a molecu-

lar weight of 220 g mol−1 (PEG220) indicating that PEG220 penetrates the particles interior and that the two polymers (PNP and PEG) are thermodynamically compatible (i.e. x #0, where the 23 subscripts 2 and 3 refer to the particle network and free polymer, respectively). It can be seen from Fig. 4 that de-swelling of the microgel particles occurs when the molecular weight of PEG is ≥1080 g mol−1 and that a reaches a minimum value in the presence of PEG3490. Exclusion of free polymer occurs when the coil size of PEG exceeds that of the pores of the microgel network. Excluded free polymer decreases the chemical potential of water in the continuous phase. The particles, respond by exuding water with a concomitant increase of the network concentration. Particle de-swelling continues until the chemical potentials of the solvent in the particle interior and free polymer solution are equal. A thermodynamic model has been developed which describes the de-swelling of organic-soluble microgel particles in the presence of free polymer [13]. However, the model has not been used to study the present data because it is based on Flory’s theory of network swelling. As has been noted by others [20,21], application of swelling models based on the Flory theory is not valid for PNP networks swollen by water because of the presence of hydrogen bonding. 3.4. Effect of co-non-solvents Addition of alcohol to the continuous phase is another method for inducing de-swelling of PNP

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microgel particles (see Fig. 5). It can be seen from the figure that a initially decreases with increasing volume fraction, w , of methanol (MeOH ) a and 2-propanol (PrOH ). The particles exhibit re-entrant swelling at high values of w . This a phenomenon has been referred to as co-nonsolvency2 [6 ] and is believed to occur because of clathrate structure formation. ( The clathrate structure consists of aggregated alcohol molecules surrounded by locally ordered water molecules.) Competition for water molecules removes hydrated water molecules from the PNP chains allowing inter- and intra-chain hydrogen bonding to dominate. Hence, the microgel particles de-swell. Encapsulation of the alcohol aggregates becomes inefficient when w >0.4, which results in a disruption of the clathrate structure. Additional solvation of the network structure owing to hydrophobic PNP–alcohol interaction is then favoured, which contributes to the re-entrant swelling observed at w >0.4. a If clathrate structure formation were responsible for de-swelling, then the critical volume fraction of alcohol (w* , which corresponds to the minimum a value of a) should be lower for the larger alcohol (PrOH ). Indeed, this is the case as the values of w* for PNP–9BA in MeOH and PrOH are 0.37 a and 0.30, respectively. Co-non-solvency via clat-

Fig. 5. Variation of the de-swelling ratio of PNP–9BA particles dispersed in aqueous-alcohol solutions as a function of the alcohol volume fraction. The alcohols investigated are MeOH (%) and PrOH (n). 2 ‘‘Co-non-solvency’’ refers to the phenomenon where the solubility of a solute in a solvent mixture is worse than that of the solute in each of the pure solvents.

rate structure formation, therefore, qualitatively explains the de-swelling data shown in Fig. 5. However, other factors, such as the nature of the polymer, also play an important role in conon-solvency [10]. Recent work performed within our laboratory has revealed that increased steric crowding of the amide hydrogen atoms strongly reduces hydration and swelling of substituted PNP microgel particles [22]. Coupled with the sensitivity of the swelling properties of PNP to the continuous phase composition, it appears likely that hydrogen bonding of the type R∞N–H,OH is the interaction that 2 most strongly determines the swelling properties of PNP microgel particles. 3.5. De-swelling by pH variation and PbII uptake Incorporation of co-monomer containing acidic or basic functionality into PNP microgel particles yields particles with pH-dependent swelling properties. Fig. 6 shows the variation of the de-swelling ratio of PNP–2BA and PNP–5AA–2BA particles with the dispersion pH. The swelling of PNP–2BA is unaffected by pH implying that the COO− groups incorporated during initiation do not significantly contribute to swelling. Incorporation of acrylic acid within PNP particles results in pH dependent swelling. The COO− groups are fully

Fig. 6. The influence of dispersion pH on the de-swelling ratios of PNP–2BA (6) and PNP–5AA–2BA (%). The concentration of background electrolyte ( KNO ) was 10−3 M. The effect of 3 added PbII(NO−) (3×10−4 M ) is also shown (&). The values 3 2 of d for PNP–2BA (average value over pH range 3–8) and 0 PNP–5AA–2BA (average value over pH range 6–10) were 650 and 920 mm, respectively.

B.R. Saunders et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 57–64

dissociated at high pH resulting in electrostatic repulsion which favours swelling. ( The pK of a acrylic acid is 4.25 [23].) In addition, Donnan equilibrium ensures a higher concentration of mobile (dissociated) ions within the microgel particle network producing an additional osmotic contribution that favours swelling. It is noteworthy that the pH-induced de-swelling transition of PNP–5AA–2BA is more gradual than that observed for an equivalent macrogel [24]. This result is indicative of an inhomogeneous distribution of AA and cross-links within the microgel particle. Work performed within our laboratory has shown that PNP–5AA–2BA particles can be used to extract PbII cations from aqueous solution [12]. The effect of PbII(NO− )2 on the de-swelling ratio 3 of PNP–5AA–2BA is also shown in Fig. 6. The de-swelling ratio of PNP–5AA–2BA decreases significantly in the presence of PbII at pH=5. In fact, the value for a measured in the presence of PbII is similar to that obtained at pH<4 in the absence of PbII(NO−)2. Further, PbII does not effect the 3 extent of swelling at pH=3.5. Moreover, coordination of PbII by the COO− groups is sufficiently robust that PbII is not expelled when the microgel is heated to 50°C [12] (i.e. the coordination site survives the large-scale structural changes that occur during the de-swelling transition). PbII may only be removed by decreasing the pH [12]. All of these observations point to a strong chelation of the PbII species by PNP–5AA–2BA at pH=5. The identity of the PbII(–COO−) species that x exists at pH=5 is of particular interest. It is certain that direct coordination of the COO− is involved, however, contributions involving coordination of CNO and N–H groups are also likely. Indeed, the structural features of PNP–5AA–2BA resemble closely those of the complexones [25] (i.e. a high density of COO− and N–H groups bonded to a backbone containing C and N atoms). Ethylenediamine tetraacetate, EDTA (a wellknown complexone), forms strong chelation complexes with PbII ( log K =18.3 in 0.1 M KNO 1 3 [25]). It is, therefore, proposed that adsorption of PbII in the presence of PNP–5AA–2BA occurs as a result of coordination by R–O−, CNO and N–H groups in a complexone-type chelation environ-

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ment. Coordination of transition-metal ions (e.g. FeIII, CrIII etc.) should also be possible within PNP–5AA–2BA microgel particles and the systems would be suited to a study of the chelation environment using Mo¨ssbauer spectroscopy and electron paramagnetic resonance. The latter techniques have enabled the coordination environment within polypyrrole films containing transition-metal EDTA complexes to be discerned [26 ].

4. Conclusions This study has shown that a delicate balance exists between interactions favouring swelling of microgel particles (e.g. solvation of the PNP chains via hydrogen bonding with water) and those responsible for collapse (e.g. inter- and intra-chain hydrogen bonding and the elasticity of the network). Hydrogen bonding between water and the polymer at the N–H site plays an important role in the swelling properties. Increasing the extent of crowding of this site produces particles that are not swollen by water (particles having a very low LCST ), whereas decreasing the extent of steric crowding (cf. PNP) should increase the LCST. Pelton et al. [17] found that poly(NP-co-acrylamide) microgel particles have a significantly higher LCST than that of PNP. Steric crowding of the N–H site appears to be an important factor governing the LCST of poly(acrylamide) gels. The work described above has shown that the size of PNP microgel particles may be controlled using temperature, pH and addition of free polymer or alcohol. It is noteworthy that the extents of particle collapse achieved using osmotic de-swelling and co-non-solvency are comparable to that observed when the particles are heated to 50°C in the absence of added alcohol or free polymer. It has also been shown that incorporation of acrylic acid produces a network capable of strongly adsorbing aqueous PbII species. The coordination site within the microgel particles probably resembles that found in metal-complexone chelates. Optimization of metal uptake may yield microgel systems suitable for application in industrial waste-water treatment.

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The SANS data have provided insight into the structure of PNP microgel particles and the processes occurring during particle de-swelling. The internal structure of the particles is believed to consist of clusters of highly cross-linked regions separated by larger lightly cross-linked pouches. ˚ at The average size of the latter is about 28 A 25°C. Thermally induced de-swelling results in the disappearance of the two-phase structure and yields collapsed, hard sphere particles.

Acknowledgments The authors gratefully acknowledge financial support for this work from the EPSRC ( UK ), Paint Research Association ( UK ), Lubrizol ( UK ) and a grant from ISIS (Didcot, UK ). We also thank Dr S. King (Rutherford, Appleton Laboratories, Didcot, UK ).

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