Rheol Acta (2014) 53:159–164DOI 10.1007/s00397-013-0751-1

ORIGINAL CONTRIBUTION

Microrheology and electrical conductivity of a dilutePNIPAM suspension

Francisco Javier Guevara-Pantoja ·Flor Gomez-Galvan · Ivan Eleazar Cipriano ·Hilda Mercado-Uribe · Jesus Carlos Ruiz-Suarez

Received: 8 August 2013 / Revised: 5 December 2013 / Accepted: 6 December 2013 / Published online: 9 January 2014© Springer-Verlag Berlin Heidelberg 2014

Abstract We study on a suspension of poly(N-isopropylacrylamide) at a low concentration using particle-trackingmicrorheology in two modalities: at constant temperaturesand during a heating ramp. The dilute suspensions do notpresent a glass state at low temperatures, and at high temper-atures, the polymer particles collapse, giving rise to entropicforces that induce aggregation of the tracking colloids. Theviscoelastic moduli of the system, which drastically changewith temperature, allow us to follow this dynamics. Further-more, the viscosity of the system remains approximatelyconstant with temperature but suddenly increases as itpasses the lower critical solution temperature (LCST). Sucheffect is probably associated to hydration before the col-lapse of the polymer, as verified by electrical conductivitymeasurements.

Keywords PNIPAM · Microrheology · Depletion forces

Introduction

Thermosensitive polymers have been extensively studied forapplications in the biomedical industry, especially becausethey are excellent candidates to act as viscosity adjust-ment agents and excipients in pharmaceutical formulationswith controlled release of active pharmaceutical substances(Durmortier et al. 2006; Escobar-Chavez et al. 2006).

One example is the poly(N-isopropylacrylamide) (PNI-PAM), which shows, in aqueous solution, a reversible

F. J. Guevara-Pantoja · F. Gomez-Galvan · I. E. Cipriano ·H. Mercado-Uribe · J. C. Ruiz-Suarez (�)CINVESTAV-Monterrey, PIIT, Nuevo Leon, 66600, Mexicoe-mail: cruiz@mda.cinvestav.mx

swelling-deswelling phase transition above the lower crit-ical solution temperature (LCST). Below this temperature(around 32 ◦C), the polymer chains are hydrophilic, andthe network is swollen with water,above, the chains becomehydrophobic, the network collapses, and water is expelled(Garner et al. 2009). Although many aspects of the LCSTin PNIPAM have been studied using traditional techniques,like UV-Vis spectrophotometry (Zha et al. 2007; Herreraet al. 2010; Kotsuchibashi et al. 2009), dynamic light scat-tering (Brugger and Richtering 2008; Ngai et al. 2006),Fourier transform infrared spectroscopy (Sun et al. 2008),differential scanning calorimetry (Herrera et al. 2010,Brugger and Richtering 2008), and, less frequently, dielec-tric spectroscopy (Ono and Shikata 2007; Gomez Galvanet al. 2012), there are important properties investigated withother means, for instance, by using conventional rheology.Indeed, the rheological behavior of PNIPAM is crucial tounderstand aging, rigidity, swelling mechanisms, and crys-tallization (Purnomo et al. 2006, 2007, 2008; Howe et al.2009; Munk et al. 2011; Senff and Richtering 1999).

In this regard, particle-tracking microrheology has alsobeen explored by Ende et al. (2010) and Colin et al. (2011).In such technique, Brownian particles are added to the sus-pension under study (where normally a drop is required)and followed with a microscope (Hunter and Weeks 2012).From the images obtained at no more than 30 fps, theaveraged mean squared displacement (MSD) of the track-ing particles is obtained. This is the rheological signatureof the system: if the particles are diffusive, the medium isliquid-like; if it is strongly sub-diffusive, the suspension iselastic.

PNIPAM microgels are colloidal soft particles that canbe both compressed and deformed to a degree that isdetermined by the cross-link density and the polymer sol-vent interactions of their constituent polymer network; see

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Lorenzo and Seiffert (2013). It is also well known that withincreasing temperature, after the phase transition, the radiusof gyration of the microgels decreases from around 75 nmto approximately 55 nm and, with it, the effective volumefraction (Purnomo et al. 2007, 2008). This reduction in den-sity, of course, has a rheological impact: the yield stressdrops and the elastic properties vanish (Senff and Richtering1999).

In this work, we perform microrheology measurementsof PNIPAM at a dilute concentration of 0.01 w/w (nor-mally, a suspension of microgels forms a glass or crystalabove 0.07 w/w). Below the phase transition tempera-ture, the tracer particles are strongly diffusive; above, theyrapidly lose mobility by a restrictive field of depletionforces exerted by the collapsed particles. When the tem-perature varies in a slow ramp, as the polymer crosses itsphase transition, the obtained MSD suffer a sudden change,suggesting an increase of viscosity. We also measure theelectric conductivity of the polymer, before and after thetransition, and find that it correlates well with the swelling-deswelling transition and the advent of the entropicinteraction.

Methods

PNIPAM suspension

N-isopropylacrylamide (NIPAM, 97 %), ammonium per-sulfate (APS, 98 %), N,N,N ′,N ′-tetramethylethylenediamine(TEMED, 98 %), N,N ′-methylenebisacrylamide (MBA,99 %), and acrylamide (98 %) were used in the synthesisof the polymers as purchased from Aldrich Chemical Co.,without additional purification. Deionized water (Milli-Q,18.2 M�) was used to prepare all the PNIPAM samples.They were synthesized in the same way as in a previouswork (Gomez-Galvan et al. 2012) and prepared in aqueoussolution at 0.01 w/w. The cross-linker (MBA) employed inthe synthesis was 1 mM. The microgels were purified byrepetitive washing cycles with deionized water, followed bydialysis with distilled water in order to eliminate residualmonomers. Thereafter, the microgels are dried for 24 h atroom temperature in vacuum.

Electrical conductivity experiments

The electrical conductivity of the PNIPAM suspension asa function of temperature was measured with a Keithleyelectrometer (model 6517B) in a device shown schemati-cally in Fig. 1. This is a Plexiglas prism with dimensionsof 20.5 cm long ×16 cm wide ×18 cm high. The surfaceof the piece was machined to make two shallow channelsof 14.5 cm in size, 0.5 cm in width, and 2.7 cm in depth.

Fig. 1 Device to measure the electrical conductivity of PNIPAM

They are located on the sides of the five holes where thesamples are placed. These channels serve as a guide to slidea Plexiglas cube (3 cm big, 3.4 cm wide, and 3 cm high),which contains a small halogen lamp of 20 W for heatingthe polymer. The space between the lamp and the sample isaround 3 mm. The dimensions of the holes are 0.61 cm long,0.32 cm wide, and 0.2 cm deep. The separation betweenthem is 2.5 cm. The oval holes have two cooper poles oneach side. Each pole makes contact with a cooper sheet thatpartially covers a rectangular Plexiglas bar placed belowthe hole. The bar moves by a gear system, which consistsin an external knob and two aluminum rods that also pro-vide stability to the bar. In this way, one can choose thesample to evaluate. Two BNC connectors are connected tothe cooper sheets, which turns out to be connected to theelectrometer.

Microrheology experiments

The averaged MSD of the tracer particles as a functionof time is obtained from a series of images taken witha Leica microscope equipped with a ×100 objective anda CCD camera. The tracer particles were 1 μm chargedpolystyrene colloids in a very dilute concentration (volumefraction of 0.02), so no further stabilization was required.Around 70 particles were always in the field of view ofthe microscope. Only a small drop of the suspension (nor-mally 2 μl) is required, which is one of the neat featuresof this technique. The liquid is injected in a sealed glasscell glued on a copper thin plate, which in turn is gluedon a flat thermoelectric Peltier circuit. Both the Peltierand the copper plates have a small central hole throughwhich the light of the microscope passes. No evaporationis detected during the measuring time. The Peltier device isconnected to a power supply controlled by a computer with

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LabVIEW. The temperature is controlled with a precisionof 0.05 ◦C.

Results and discussion

Figure 2 shows the temperature dependence, from 29 to37 ◦C, of the electrical conductivity of PNIPAM normalizedto the electrical conductivity of deionized water at 29 ◦C,which is shown as a reference. The error bars are the stan-dard deviations of five experiments. Deionized water hasa low conductivity but steadily increases with temperature(water molecules are continuously forming ion pairs H+and OH−, so when temperature increases, the number ofions increases, too). We can clearly see the different behav-ior of PNIPAM. As the temperature increases, the electricalconductivity of the polymer steadily drops to reach aminimum value at around 32 ◦C. This effect is due to aswelling process: water molecules are sequestered by thenetwork and thus the transport of ions drops. Between 32and 33 ◦C, the conductivity does not vary much, indicatingthat the polymer does not hydrate further. Above 33 ◦C, thebehavior is reversed, the PNIPAM particles shrink, wateris expelled, the concentration of ions increases, and theymove freely. Indeed, a further increment of water moleculesin the bulk, which can dissociate more easily as the tem-perature increases, gives rise to a further increment of theconductivity. Indeed, the pH is sensitive to temperaturechanges, because water molecules dissociate due to thermalagitation. Therefore, the higher the temperature, the largerthe thermal energy is to induce dissociation and, thus, theconcentration of ions augments.

This hydration dynamics occurring in the PNIPAM sus-pension, clearly underlined by the change of electrical

Fig. 2 Electrical conductivity of PNIPAM as a function of tempera-ture normalized to the value of water at 29 ◦C. PNIPAM (squares) anddeionized water (circles)

conductivity, is necessarily associated to the rheology of thesystem. So, as said before, in order to asses such correla-tion, we add to the suspension very few colloidal particleswe can watch and follow, for it is the erratic movement ofsuch particles that gives us the rheological properties of thesuspension. Let us first apply the same ramp heating weapplied before to measure the electrical conductivity: i.e.,we slowly heat the suspension while filming the trajecto-ries of the particles dissolved in the polymeric suspension.From these, we obtain the MSD depicted in Fig. 3. TheMSD increases quite linearly, except between 32 and 33 ◦C,where a plateau forms as the suspension crosses the tran-sition. This plateau indicates that the viscosity of PNIPAMsuddenly suffers an increment, reducing the diffusion ofthe tracking particles. This change in viscosity has beenobserved before using a traditional rheometer, where it hasbeen found that the smaller the shear stress applied to thepolymer, the larger the viscosity increment (Howe et al.2009) is. In our case, the shear impact is obviously smallbecause it is due to thermal fluctuations, and for such rea-son, the effect is enhanced. Following the arguments ofHowe et al. (2009), the increment of viscosity is due to theentanglement of the polymer chains and starts as the size ofthe polymer particles begins to decrease significantly. Afterthe plateau, the particles diffuse more freely, and the MSD isagain linear.

Of course, from the MSD shown in Fig. 3, we can-not obtain the rheological moduli because the temperaturevaries. So we now fix the temperature and run the experi-ments as a function of time.

Fig. 3 The MSD of the tracking particles measured during a contin-uous temperature ramp. The phase transition is observed between 32and 33 ◦C, where a change in viscosity is clearly noticeable

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Fig. 4 The MSD of the tracking particles measured at differenttemperatures: 25, 27, 29, 31, 33, and 35 ◦C

Figure 4 shows the MSD at different temperatures: 25,27, 29, 31, 33, and 35 ◦C. Clearly, the tracking particleshave a pure diffusive behavior at long times, except at35 ◦C where they completely lose their liquid-like behav-ior and seem to fall inside a cage. Moreover, before theyget trapped, an interesting jump occurs at 33 ◦C, surelyproduced by the collapse of the PNIPAM particles and therelease of water in the suspension.

As habitually done in the literature (Ende et al. 2010;Colin et al. 2011), we calculate now the moduli of the

Fig. 5 Elastic (squares) and loss (triangles) modulus as function ofangular frequency

suspension as a function of frequency through the followingtwo equations:

|G∗(ω)| = kbT

πa(⟨�r2

(1ω

)⟩)�[1 + α(ω)]

, (1)

α(ω) ≡ d ln(⟨�r2(t)

⟩)

d ln(t)

∣∣∣∣∣t= 1

ω

. (2)

To numerically calculate the moduli using the above equa-tions, we must run a reliable code (see Crocker and Weeks2012).

The viscoelastic nature of the PNIPAM suspension ismore than evident, and clear signs of it are appreciated as we

Fig. 6 a A photograph of the colloidal system where tracking parti-cles are free to move in a dilute sample of PNIPAM. The temperatureis lower than the critical one 29 ◦C. b The same than before, but thistime the temperature is close to the LCST temperature 32 ◦C. Theagglomerates of the tracking particles (not the microgel colloids) startto nucleate by depletion forces. c Above the transition temperature35 ◦C, such depletion forces induced by the PNIPAM microgels finallyaggregate most of the tracking particles

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change the temperature (see Fig. 5). At short tracking times,the particles have a sub-diffusive behavior, and thereafterthey diffuse quite similarly as Brownian particles do, exceptat 35 ◦C. At this temperature, as noted above, the responseis mostly elastic at low frequencies (large times). The rea-son for this behavior has an entropic cause: as the particlesof PNIPAM shrink, they exert an unbalanced osmotic pres-sure that pushes the polystyrene colloids together. Thiswell-known phenomenon gives rise to colloidal gelation(Hunter and Weeks 2012; Crocker et al. 1999; Yodh et al.2001). And once such tracking particles agglomerate, theymove as if they were inside a cage and therefore in anelastic medium. The images shown in Fig. 6 graphicallydepict this scenario, which is, of course, reversible (stress-ing the entropic nature of the interactions). Indeed, whenthe suspension is cooled, the microgel particles rehydrateand swell, and the depletion forces disappear, allowingthe polystyrene particles to disperse again. This is also anindication that there is no absorption between the micro-gels and the polystyrene particles. One may argue thatthis elastic-like behavior is not a property of the suspen-sion but rather a property of the polystyrene agglomerates.However, the fact that the tracking particles cannot dif-fuse freely in this condition, not because they are trappedin a polymeric network but because they are restrictedby entropic forces, highlights the interesting nature of thethermo-sensitive polymer studied here. We need to remarkthat we do not observe aggregation of the microgel particles,possibly because a small remaining charge is left after thesynthesis.

Conclusions

We studied on a dilute suspension of a thermo-sensitivepolymer (PNIPAM) as a function of temperature before,during, and above the phase transition. For such purpose,we implemented a novel cell with a Peltier circuit to con-trol the temperature of the sample or to create a temperatureramp. For the first time, the swelling-deswelling dynam-ics is studied by measuring the electrical conductivity andthen by particle-tracking microrheology. The techniquescomplement each other because they associate the rheo-logical properties of PNIPAM with its hydration dynamics.We also observe that the tracking colloids aggregate drivenby depletion forces created by the much smaller PNIPAMmicrogels.

Acknowledgments This work has been supported by CONACyT,Mexico, under grants 101384 and 154202; F. J. G. P., F. G. G. , andI. E. C. acknowledge fellowships from CONACyT. We acknowledgethe useful discussions with J. L. Arauz-Lara.

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