Glass Transition Of Nano-Confined Polymers

The glass transition of nano-confined polymer has attracted more and more attention these years. Despite nearly two decades study of the effect of nano-confinement on the glass transition temperature (Tg) of amorphous materials, the quest to probe the deviation of Tgs in non-equilibrium chains has remained unfulfilled. Here we employed several kinds of methods, including the dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), positron annihilation lifetime spectra (PALS) and broadband dielectric spectroscopy (BDS), to study the glass transition behavior of freeze-dried polymers. We compared the freeze-dried polymers and spin-coated thin polymer films, and find out the intrinsic connections between these two systems.By DMA and DSC measurements, we find that the glass transition temperature of nano-confined polymer is the same as the bulk value. However, by TMA, PALS and BDS tests, the apparent Tg decreases by35K, comparing to the bulk. BDS confirmed that the decreased transition is glass transition. The phenomenon that the apparent Tg depends on the test method has also been found in thin polymer films. PALS was used to detect the microstructure of freeze-dried polymers, and found that the mean free volume size in freeze-dried polymer is bigger than the bulk, and the free volume size distribution is much broader than that in bulk sample. There are two characteristic lengths in freeze-dried polymers, which means the heterogeneity is higher than bulk. Heterogeneity is extremely important for glassy dynamics. When the heterogeneity and the free volume size are changed towards bulk, Tg will shift to bulk value too. Surprisingly, these enlarged free volume and high heterogeneity have also been found in thin polymer films. Therefore, we believe that, for both thin polymer films and the freeze-dried polymers, they are in nano-confined state, and their special structure leads to the different glass transition manner from bulk. Whether sensitive to the volume/density change is the key factor to determine the deviated glass transition temperature in nano-confined systems.The chain packing density is crucial to the glassy dynamics in polymers, and the traditional free volume cannot explain it very well. So we introduced the new and very important theory in condensed matter physics, the jamming theory. This is the first time jamming is systematically introduced into polymer systems. TMA and non-linear mechanical experiments were taken to determine the jamming phase diagram for amorphous polymers. This phase diagram is different from the one for attractive particles, granular spheres and other well studied systems. There is a critical packing density, above which the jamming transition temperature is not affected by packing density or applied stress. Besides, there are two key parameters in jamming theory, temperature and stress, which are of vital importance to material processing. Within the jamming phase diagram, we could process polymers at low temperature.For nano-confined polymers, such as the freeze-dried polymers and thin polymer films, the initial chain entanglement density is lower than the bulk value. It is necessary to study the relaxation towards the bulk entanglement state. However, there is not any theory which could depict the re-entanglement kinetics well. Here we creatively treated the re-entanglement process as the complementary process of stress relaxation within Doi-Edwards model, and concluded a equation to describe the buildup of plateau modulus, so that we can monitor re-entanglement process. Furthermore, this equation was successfully applied on the entanglement recovery of non-linear shear modified polymer solution. Meanwhile, the time scale for recovery of the entanglements is much longer (by2～4orders of magnitude) than the linear relaxation time. The correlation between recovery time and the concentration of precursor solution is in good agreement with previous results from molecular dynamics (MD) simulations. This work will help us understand better about the process of entanglement recovery and the nature of nano-confined systems.