Travels Between Micro and Macro: Bridging the gap between molecular level descriptions and bulk material behavior

Jane E.G. Lipson, Dartmouth College, DMR 0804593

The Approach to Glassiness: Some liquids crystallize, and one can imagine the melt-to-crystal transition much as a light going from ‘on’ (motion ranging from molecular rotation to sample flow) –to ‘off’ (an immobile, ordered solid). However, many important materials do not crystallize, but turn into glasses; window glass is an obvious example. The transition from melt to glass is not an on/off process, but more like a light on a dimmer switch. Mobility gradually shuts down as the glass transition temperature is approached, however, even very close to the transition there are still regions of localized motion. There is significant interest in understanding more about near-glassymaterials for industrial applications, and we have developed a simple new model which exhibits interesting behavior. In our “Limited Mobility” (LM) model pockets of “free” volume allow segments to move locally (upper left) by allowing for an assisted “hop”. On the other hand, this free volume may sometimes be dormant (upper right); in this case it requires a local event to reactivate. As the glass transition is approached we find that the free volume appears to organize itself as a mobile “skin” around glassy “bubbles” (lower left figure). This heterogeneous local mobility is in sharp contrast to what the system would experience were this same amount of free volume to be randomly dispersed (lower right). We have only begun to explore the implications of our model.

In collaboration with Prof. S.T.Milner, Penn State University

Travels Between Micro and Macro: Bridging the gap between molecular level descriptions and bulk material behavior

Jane E.G. Lipson, Dartmouth College, DMR 0804593

Manipulating blend miscibility through thermodynamic control: It is expensive to develop and manufacture a new polymer. An alternate route to a new material is that of blending polymers that are already being produced. Two polymer melts combining into a homogeneous mixture are ‘miscible’; the alternative, like oil and water, is that they phase separate and are thus ‘immiscible’. However, whether a blend falls into one or the other category also depends on temperature and on composition, as illustrated in the cartoon at the upper left, below. Experiments to map out this behavior are not trivial; thermodynamics provides a guide. Just as a raised ball will drop to the ground, achieving its lowest gravitational potential energy, a mixture seeks its lowest chemical potential energy, here expressed in the form of the ‘free energy of mixing’ (∆G). The free energy actually represents a fine balance between the energetics of mixing (∆H), and the entropy of mixing (∆S), the latter reflecting the increased ability of the mixture to sample a large number of microscopic arrangements, relative to the pure components.

Thermodynamics alone does not yield a route to designing miscible mixtures. For that we have developed a theory that connects thermodynamics to the microscopic nature of a material. Key microscopic parameters include the energies that characterizes the interactions between different (12) and the same (11, 22) types of segments. Collaborating with Dame Professor Julia Higgins (Imperial College, London) we have demonstrated that for an important subset of blends the contribution to ∆G from ∆S correlates with the difference |11-22|, while that from ∆H correlates with the the strength of of 12. The figure at left illustrates how the two contributions coordinate to maintain a balance in ∆G; the points are our results from modeling sixteen experimental blends—all for which we could find complete data. The correlation is very marked; what is also worth mentioning is that a long-held and widely-used relationship between the ’s and ∆H is not the one that works. With these results in hand we are poised to provide practical guidance for predicting miscibility in polymer blends.