Studies of kinetic glass transition in a triblock copolymer micellar system

Abstract

If a liquid is cooled sufficiently below the melting point, it becomes metastable with respect to the crystalline state. However, if nucleation is suppressed, one can supercools the liquid without resulting in crystallization. The characteristic time for structural relaxation increases rapidly and at a certain point, it becomes comparable to the duration of experiment time scale. At this point, the liquid is being arrested structurally. We define this structurally arrested matter a glass. Dynamically, a glass transition can be viewed as a transition from ergodic to nonergodic state of matters. In a hard-sphere system, mode coupling theory (MCT) predicts occurrence of a glass transition when the volume fraction exceeds a certain value due to the excluded volume effect. However, Recent MCT calculations for a hard-sphere system with a short-range attraction show that one may observe a new type of structurally arrested state originating from clustering effect, called the "attractive glass", as a result of the attractive interaction. This is in addition to the well-known glass-forming mechanism due to the cage effect in the hard sphere system, called the repulsive glass.

(cont.) The calculations also indicate that, if the range of attraction is sufficiently short compared to the diameter of the hard sphere, within a certain interval of the volume fraction and the effective temperature, the two glass-forming mechanisms can compete with each other. For example, by varying the effective temperature at appropriate volume fractions, one may observe respectively, the glass-to-liquid-to-glass re-entrance or the glass-to-glass transitions. Here we present experimental evidence for both transitions, obtained from small-angle neutron scattering (SANS) and photon correlation spectroscopy measurements taken from dense L64 copolymer micellar solutions in heavy water. We show, a sharp transition between the two types of glass can be triggered by varying the temperature in the predicted volume fraction range. In particular, according to MCT, there is an end point (called A3 singularity) of this glass-to-glass transition line, beyond which the long-time dynamics of the two glasses become identical. Our findings confirm this theoretical prediction.

(cont.) Surprisingly, although the Debye-Waller factors, the long-time limit of the coherent intermediate scattering functions, of these two glasses obtained from PCS measurements indeed become identical at the pre- dicted volume fraction, they exhibit distinctly different intermediate time relaxation. Furthermore, our SANS results on the local structure obtained from volume fractions beyond the end point are characterized by the the same features as the repulsive glass obtained before the end point. A complete phase diagram giving the boundaries of the structural arrest transitions for L64 micellar system is given. Furthermore, SANS experiment shows that, in the region of disordered micellar liquid phase, as the hydrostatic pressure increases, the effective micellar interaction potential changes in response to the pressure perturbation, resulting in a significant increase in the low-k part of the SANS intensity distribution, characterizing the formation of fractal clusters. In addition to the formation of fractal clusters, starting from a structurally arrested state, the applied pressure induces the matrix to evolve from an initial attractive glass state through an intermediate state and ends up in a final liquid state.

(cont.) Inspired by the random phase approximation, an additional structure factor term derived from fractal clusters is added to the adhesive hard sphere structure factor to approximate the variation of the effective interaction potential. Based on this idea, a model for the scattering intensity distribution is developed which successfully explains the measured SANS intensity distributions. One of the great challenges in soft matter sciences is to understanding the structurally arrested state of matter. Although it still remains as an open question, it is well-acknowledged that the understanding of the interaction potential is of fundamental importance. Due to the fact that colloidal particles can be produced with well-defined chemical and physical properties such as shape, size and most importantly, the tunable interaction potential. Combining with theoretical predictions, they serves as convenient model systems to provide key information about the relationship between the rich phase behaviors and various interaction potential, such as van der Waals forces, short-range attraction et al.

(cont.) The experimental studies of kinetic glass transition in a micellar system presented in this thesis may provide useful in- formation to further experimental investigations and theoretical calculations on this rapidly expanding field of research.