AmericanChemicalSociety.com

Introduction

There is considerable interest in the relationships between colloidal and molecular glass-formers.  The present work addresses these via a combination of light scattering from colloids and mechanical measurements on colloids and polymer glasses. The past year has focused primarily on the area of the light scattering measurements from a thermoresponsive particle dispersion in water. 

Background

For molecular glasses, Kovacs [1] catalogued three aging signatures: (a) intrinsic isotherms (b) asymmetry of approach, and (c) memory.  Intrinsic isotherms involve temperature jumps from T_g to a number of temperatures below T_g.  The set of curves for a thermodynamic quantity as a function of aging time t_w is called the intrinsic isotherms.  Asymmetry of approach involves comparison of up- and down- temperature jumps: (a) first equilibrating the sample at T_f  +  and jumping to T_f; (b) repeating (a) with T_f  - .  It is found that the profile of V vs. t_w is asymmetric between two T-jumps +  and -  although it would be symmetric were the response linear. For the memory experiment, the sample is partially aged at a low temperature T_i after which T is increased to T=T_f<T_g and the volume response is measured. Under the right conditions the response is a non-monotonic volume recovery, i.e., the volume grows through a maximum as “aging” time increases and eventually merges with the V vs. t_w profile obtained upon jumping directly from  T_g to T_f. 

Concentrated colloidal systems exhibit behaviors similar to those of molecular glasses [2].  Aging of colloids has generally been studied by following the evolution of a dynamic response after a shear “melting” perturbation using techniques such as rheology [3] and diffusive wave spectroscopy [4].  There are no prior works on colloidal systems to investigate the aging behavior relevant to the signatures of structural recovery catalogued by Kovacs [1].

Experimental

Particles of poly-n-isopropyl acrylamide, PNIPAAM, dispersed in water swell and de-swell rapidly upon changing temperature [5].  We can change the volume fraction of a PNIPAAM colloidal system by changing  temperature.  The volume fraction is where is the mass fraction and d the diameter. d_c is the diameter of a completely de-swelled (dry) particle. The dynamics of the PNIPAAM system was probed using multi-speckle diffusive wave spectroscopy [6]. 

Results

Figure 1a shows the intensity auto-correlation functions after a down-jump from 31.2^oC to 30^oC.  We see that  increases with t_w.  Figure 1b shows log vs. log t_w for jumps from 31.2^oC to different Ts.  For each set of data, there is an initial aging period where t increases with t_w according to a power law:  . At larger t_w>1000,  levels off.   at the largest t_w is taken as the equilibrium value.

Figure 2a shows that the equilibrium  is greater for lower values of T. The low value of  at the lowest temperature is due the sample not achieving equilibrium. Such behavior is seen in studies of physical aging behavior in molecular glasses when equilibrium is not attained [7] . With  we can convert T  to  at each temperature. The equilibrium  is plotted vs.  in Figure 2b.  varies faster than exponentially in .

Results for asymmetry-of-approach are shown in Figure 4a.  The upper curves correspond to down-jump experiments and the lower ones to up-jump experiments.  We see that for smaller initial  the asymmetry is weak and is stronger for the larger   

For the memory experiments we partially aged the sample at 29^oC for 10³s and then jumped to 30.2^oC.  The response for the up-jump step is plotted in Figure 4b as a solid curve.  The dashed curve is the direct jump from 31.2^oC to 30.2^oC.  The magnitude of the memory effect is larger or at least equal to that of Kovacs' data.

Summary

We have shown for the first time, using thermoresponsive particle colloids, that colloidal glasses exhibit similar aging signatures to those of molecular glasses.  We have demonstrated intrinsic isotherms, asymmetry of approach and memory effects when the colloidal glass is subjected to concentration jumps (induced by temperature change histories) that traverse the colloidal glass concentration.
References

1.   A. J. Kovacs, Forschr. Hochpolym.-Forch. 3, 394 (1963).

2.   P. N. Pusey and W. van Megen, Nature 320, 340 (1986).

3.   G. B. McKenna, T. Narita, and F. Lequeux, J. Rheo. 53, 489 (2009).

4.   S. Kaloun et al. Phys. Rev. E. 72, 11401 (2005).

5.   M. Balauf and Y. Lu, Polymer 48, 1815 (2007).

6.   V. Viasnoff, F. Lequeux, and D. J. Pine, Rev. Sci. Inst. 73, 2336 (2002).

7.    A. Lee and G. B. McKenna, Polymer 29, 1819 (1988).