Colloid-Polymer Interactions - ACS Publications - American Chemical

Chapter 13

Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 20, 2017 | http://pubs.acs.org Publication Date: July 14, 1993 | doi: 10.1021/bk-1993-0532.ch013

Flocculation of Poly(N-isopropylacrylamide) Latices in the Presence of Nonadsorbing Polymer M . J. Snowden and B. Vincent School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom

Crosslinked latex particles were prepared by the free radical polymerisation of N-isopropyl acrylamide (NIPAM) using N/N methylene bisacrylamide as a crosslinking agent in water at 70ºC. At 25ºC the latex had a diameter of 460nm, whilst on heating to 40ºC the particles shrunk reversibly to become hard spheres having a diameter approximately 230nm. In the presence of electrolytes at 40°C aggregation of the latex occurred as a result of screening the charges on the particle surface. The same latex at 40ºC in the presence of non adsorbing poly(styrene sulphonate) and sodium carboxymethylcellulose flocculated as a consequence of depletion forces operative in the system. The latex redispersed on cooling to 25ºC. A preliminary report (Snowden, M.J. and Vincent, B. J.C.S.Chem Comm, in press) describing the flocculation of poly(N-isopropylacrylamide) latices by the addition of simple electrolyte and non adsorbing poly(styrene sulphonate) illustrated the reversibility of theflocculationof microgel particles under the influence of temperature. It is the objective of this paper to expand on these initial results and to demonstrate the effect of different electrolytes and polymers on particle stability and also to indicate the effect of the polydispersity of the polymer samples on the dispersions. The critical aggregation temperature of particles of poly(NIPAM) copolymerised with acrylamide was initially studied by Pelton and Chibante (1) using CaCl2. Measurements were made as a function of the fraction of acrylamide incorporated in the polymerisation process and it was found that the temperature at which aggregation was first observed incresed linearly with increasing acrylamide concentration. The critical aggregation temperature of a homopolymer sample of poly(NIPAM) as a function of electrolyte concentration is discussed in this paper along with the valency of the metal cation and 0097-6156/93/0532-0153$06.00/0 © 1993 American Chemical Society

Dubin and Tong; Colloid-Polymer Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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the reversibility of the process with respect to temperature is illustrated. We further report the stability behaviour of poly(NIPAM) in the presence of non adsorbing polyelectrolytes in an attempt to illustrate the similarities and differences in the stability of microgel latices compared to the more well known hard sphere colloids and thus illustrate some unique characteristics of poly (NIPAM) latex. Further the effect of polydispersity on the depletion force exerted by non adsorbing polymers is considered, for in a previous publication, Snowden et al (2) studing the effect of non adsorbing poly(styrene sulphonate) and carboxymethyl cellulose on a silica sol noted that for polydisperse samples the crtical flocculation concentration of the polymer became independent of molecular mass and linear charge density. Under such conditions the theoretical models (3,4) developed to describe depletion flocculation were not surprisingly found to be invalid. Experimental Poly(N-isopropyl acrylamide) (NIPAM) particles were prepared by the free radical polymerisation of NIPAM in water at 70°C, in the presence of N^N methylene bisacrylamide, according to the procedure described by Pelton and Chibante (1). Following dialysis against distiled water, transmision electron micrographas showed the particles to be monodisperse spheres having a mean diameter of 460nm ± 12nm. Poly(styrene sulphonate) Mw 46 000 (Mw/Mn 1.05) was purchased from Polymer Laboratories Ltd, Shropshire and a polydisperse sample of poly(styrene sulphonate) having a nominal molecular mass of 50 000 was kindly donated by Unilever. Carboxymethyl cellulose Mw 250 000 and polyethylene oxide Mw 3 000 000, both samples having unquoted polydispersities, were purchased from BDH chemicals Ltd. The temperature dependency of the particle diameter of poly(NIPAM) latex was determined by photon correlation spectroscopy using a Malvern instruments type 7027 dual LOGLIN digital correlator equipped with a krypton-ion laser (λ = 530.9nm). The intensity of scattered light was measured at 90°. Particle size was measured from 25°C to 50°C in increments of 5°C both on heating and cooling using an external water bath. Stability experiments were carried out using dispersions containing 0.1% microgel particles and varying concentrations of sodium chloride, magnesium chloride and lanthinum chloride made up to a total volume of 5 cm^. The samples were placed in a water bath at 25°C and allowed to stand overnight. The extent of any aggregation in the dispersion was estimated from the wavelength dependence of the turbidity of the


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dispersions (5), where η = -[d log turbidity/d log wavelength] The same experiment was repeated at 40°C. The stability of similar dispersions of poly(NIPAM) were studied at both temperatures using two different samples of poly(styrene sulphonate), carboxymethyl cellulose and polyethylene oxide as a function of polymer concentration

Results Figure 1 illustrates the decrease in particle diameter of the poly(NIPAM) latex particles on heating. The initial diameter of 460nm at 25°C decreased to 230nm at 50°C. On cooling however, the particles expanded and adopted their original particle size. This procedure was found to be fully reversible over a number of heating and cooling cycles with no hysterises taking place between the heating and cooling curves. The particles remained dispersed over the temperature range, 25-50°C in water. In the presence of 0.1 mol dm"3 NaCl at 25°C the dispersion also remained stable, however on repeating the same experiment at 40°C, aggregation of the poly(NIPAM) latex takes place. The sample flocculated at 40°C at NaCl concentrations in excess of 0.06 mol dm' , redispersed on cooling to 25°C, having a value for η of -2.6 which is consistent with the particles becoming fully redispersed. This process of heating the samples, inducing flocculation, cooling and gently inverting the dispersion to allow the particles to redisperse can be repeated many times with the process remaining fully reversible on each occasion. Figure 2 illustrates the change in absorbance of the dispersion at 592nm with increasing temperature at several different NaCl concentrations. It can clearly be seen that the onset of aggregation takes place at lower temperatures on increasing the electrolyte concentration in the dispersion. On cooling to 25°C the particles once again slowly redispersed. The critical aggregation concentration of electrolyte at which a 0.1% dispersion of poly (NIPAM) at 40°C destabilised also decreased with an increase in the valency of the cation present. The onset of aggregation took place in 0.06 mol dm" in NaCl, 0.045 mol dm" in MgCh and 0.02 mol dm" in LaCl3. The stability of the poly(NIPAM) particles in the presence of the monodisperse sample of poly(styrene sulphonate) is illustrated in Figure 3. At 25°C the particles remained dispersed up to the highest polymer concentration studied at 0.8%. On repeating the same experiment with the dispersions at 40°C,flocculationwas found to take place at polymer concentrations of 0.6% and above. The samples remained flocculated while the temperature was maintained at 40°C. On cooling, the dispersions to 25°C and gently inverting the container several times, the flocculated samples redispersed fully with their "n" values simultaneously decreasingfrom-1.3 to -2.7. This process was also found to be reversible over a number of heating and cooling cycles. Figure 3 also illustrates the 3

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Figure 2. A plot of turbidity at 592 nm against temperature for 0.1% dipersion of poly(NIPAM) in different solutions of NaCl.


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-3

same experiment carried out in a background electrolyte of 0.01 mol d m NaCl, an insufficient electrolyte concentration to destabilise the particles, the critical flocculation concentration (CFC) of the sodium poly(styrene sulphonate) sample is reduced to 0.35%. The effect of a polydisperse sample of poly(styrene sulphonate) on the stability of a poly(NIPAM) dispersion in water is shown in figure 4. It can clearly be seen that the polydisperse sample induces flocculation at a polymer concentration of.0.75%, a value slightly higher than for the monodisperse sample. The addition of carboxymethyl cellulose to a poly(NIPAM) dispersion at 40°C results inflocculationof the particles taking place at a polymer concentration of 0.55% in water at pH7 (figure 5). In 0.01 mol dm" NaCl the CFC is reduced to 0.30% which is consistent with the result obtained for sodium poly(styrene sulphonate) under the same conditions. The dispersions containing carboxymethyl cellulose also redispersed on cooling to 25°C. 3

The addition of polyethylene oxide (PEO) to the poly(NIPAM) dispersion in a background electrolyte of 0.01 mol dm" NaCl at 40°C did not result in déstabilisation of the latex dispersion up to a polymer concentrations of 3.0%. 3

Discusssion The decrease in diameter.of the poly(NIPAM) latices on heatingfrom25°C to 40°C is a consequence of the increase in the χ parameter for N-isopropylacrylamide/water system. This facilitates more polymer-polymer contacts, hence the particles contract and shrink substantially. At 25°C the particles can be envisaged as spongy microgels, swollen with solvent and having very low Hamaker constants. It is not surprising therefore, that no aggregation of the dispersion takes place under the influence of added electrolyte as the van der Waals attraction energy would be very small. At 40°C the particles shrink forcing out any solvent from the interstitial spaces and become very similar to hard spheres in their nature. Aggregation of the particles now takes place as the Hamaker constant increases, and screening the charges at the particle surface allows close enough approach for coagulation to occur. In the presence of poly(styrene sulphonate) at 40°C the particles flocculate as a consequence of depletion forces operative in the system. It is believed that the polymer does not adsorb onto the particles from water at neutral pH as both species have negative charges. Other hard sphere colloids have been reported to undergo similar depletion flocculation processes with poly(styrene sulphonate) (2,6) under similar conditions but at a fixed temperature. Theflocculatedlatex on cooling down to 25°C redisperses, as a consequence of the particles swelling with solvent and then undergoing conformational rearrangments. The soft nature of the particles at 25°C Dubin and Tong; Colloid-Polymer Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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Figure 3. A plot of d log absorbance/d log wavelength for a 0.1% dispersion of poly (NIPAM) at increasing concentrations of poly(styrene sulphonate) at 25 and 40°C in water at pH7 and in O.Olmol dm-3 NaCl.

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Figure 4. A plot of d log absorbance/d log wavelength for a 0.1% dispersion of poly (NIPAM) at increasing concentrations of monodisperse and polydisperse poly(styrene sulphonate).


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SNOWDEN AND VINCENT

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Figure 5. A plot of d log absorbance/d log wavelength for a 0.1% dispersion of poly (NIP AM) at increasing concentrations of carboxymethyl cellulose at 25 and 40°C in water at pH7 and in O.Olmol dm-3 NaCl. would require very high polymer concentrations to cause déstabilisation. Clarke and Vincent (7) investigating the effect of non-adsorbing polystyrene on polystyrene microgels reported the criticalflocculationconcentration (CFC) of polymer increased significantly as the microgel became swollen and softer. Further in a recent publication Milling et al (8) studying the effect of grafted chain coverage on the depletion flocculation of sterically stabilised silica, reported that the CFC of the free polymer increased with increasing coverage of the grafted polymer layer i.e. "softness" of the particle. In the presence of 0.01 mol dm~3 NaCl the electrostatic repulsive potential is greatly reduced as a result of charge screening and therefore flocculation into a secondary minimum resulting from depletion forces takes place more readily. This behaviour may be predicted quantitatively by current theories of colloid stability describing interacting spherical particles (4,9,10). The criticalflocculationconcentration of the polydisperse polystyrene sulphonate is sligtly higher than a comparable molecular mass monodisperse sample which may be as a result of any shorter polymer chains been able to occupy the interstitial region where polymer chains are normally excluded thus reducing the depletion layer thickness. As carboxymethyl cellulose is also unlikely to adsorb onto the particle surface due to its negative charge,flocculationis also believed to be as a result of depletion forces. Other negatively charged particles have been shown to undergo depletionflocculationin water with carboxymethyl cellulose at similar concentrations (2). The decrease in the CFC of carboxymethyl cellulose in the electrolyte solution is consistent with that observed with poly(styrene sulphonate) and was not unexpected. Dubin and Tong; Colloid-Polymer Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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The poly(NIPAM) dispersions containing polyethylene oxide solutions showed no instability at any polymer concentration. Despite carrying out the experiments in 0.01 mol dm-3 NaCl where the electrical double layer surrounding the particles, l/κ is less than lOnm, thus allowing adsorbed polymer chains to bridge, no instability was observed. Further at higher polymer concentrations there was no evidence of any depletion flocculation operative in the system. Acknowledgements M.J.Snowden would like to acknowledge BP Research, Sunbury for providing a research fellowship and to Drs J.Morgan and S.E.Taylor for useful discussions. Literature Cited 1. Pelton, R.H. and Chibante, P. Colloids and Surfaces, 1986, 20, 247. 2. Snowden, M.J.; Clegg, S.M.; Williams, P.A.; and Robb, I.D. J. Chem Soc. Faraday Trans, 1991, 87, 2201. 3. Fleer, G.J.; Scheutjens, J.H.M.H. and Vincent, B. ACS Symp. Ser, 1984, 240, 245. 4. Vincent, B.; Edwards, J.; Emmett, S. and Croot, R. Colloids and Surfaces, 1986, 18, 261. 5. Long, J.A.; Osmond, D.W.J. and Vincent, B. J.Colloid Interface Sci, 1973, 42, 545. 6. Rawson, S.; Ryan, K. and Vincent, B. Colloids and Surfaces, 1989, 34, 89. 7. Clarke, J. and Vincent, B. J.Chem Soc., Faraday Trans.1. 1981, 77, 1831. 8. Milling, Α.; Vincent, B.; Emmett S. and Jones Α., Colloids and Surfaces, 1991, 51, 185. 9. Derjaguin, B.V. and Landau, L. Acta Physiocochim. U.R.S.S., 1943, 14, 633. 10. Verwey, E.J.W. and Overbeek, J.Th.G. Theory of the stability of Lyophobic Colloids, Elsiever, Amsterdam, 1948. RECEIVED February 23, 1993


Colloid-Polymer Interactions - ACS Publications - American Chemical

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