Performance and properties of materials
may strongly depend on
processing conditions. This is particularly so for polymers, which
often have relaxation times much longer than the processing times
and therefore may adopt preparation dependent nonequilibrated molecular
conformations that potentially cause novel properties. However, so
far it was not possible to predictably and quantitatively relate processing
steps and resulting properties of polymer films. Here, we demonstrate
that the behavior of polymer films, probed through dewetting, can
be tuned by controlling preparation pathways, defined through a dimensionless
parameter
, which is
the appropriate preparation time
normalized with the characteristic relaxation time of the polymer.
We revealed scaling relations between
and
the amount of preparation-induced residual
stresses, the corresponding relaxation time, and the probability of
film rupture. Intriguingly, films of the same thickness exhibited
hole nucleation densities and subsequent dewetting kinetics differing
by up to an order of magnitude, indicating possibilities to adjust
the desired properties of polymer films by preparing them in appropriate
ways.
Using dewetting as a characterization tool, we demonstrate that physical properties of thin polymer films can be regulated and tuned by employing variable processing conditions. For different molecular weights, the variable behavior of polystyrene films of identical thickness, prepared along systematically altered pathways, became predictable through a single parameter P, defined as the ratio of time required over time available for the equilibration of polymers. In particular, preparation-induced residual stresses, the corresponding relaxation times as well as the rupture probability of such films (of identical thickness) varied by orders of magnitude following scaling relations with P. Our experimental findings suggest that we can predictably enhance properties and hence maximize the performance of thin polymer films via appropriately chosen processing conditions.
We use homogenization theory to establish a new macroscopic model for the complex transverse water proton magnetization in a voxel due to diffusion-encoding magnetic field gradient pulses in the case of biological tissue with impermeable membranes. In this model, new higher-order diffusion tensors emerge and offer more information about the structure of the biological tissues. We explicitly solve the macroscopic model to obtain an ordinary differential equation for the diffusion MRI signal that has similar structure as diffusional kurtosis imaging models. We finally present some validating numerical results on synthetic examples showing the accuracy of the model with respect to signals obtained by solving the Bloch-Torrey equation.
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