Transport phenomena and polymer morphology

The presence of the crystals in the semicrystalline polymer reduces the mass fraction, determines the space distribution and modifies the transport properties of the amorphous component which is responsible for practically all the sorption, diffusion, and permeability. The conventional determination of the diffusion coefficient from the sorption or diffusion transient is strongly affected by the geometry of the sample. The spin echo method of the nuclear magnetic resonance in a pulsed magnetic field gradient measures the diffusion directly and hence reflects the anisotropy of the orientation of the amorphous component without any distortion by the shape of the sample. The anisotropy of a single amorphous layer of a thickness of less than 10 nm, however, cannot be detected by this method which measures the molecule displacement over about 300 nm. A very rough estimate of the transport properties of the volume element can be based on the fractional free volume of the polymer. This is particularly helpful in the description of the effects in elastically and plastically deformed material. The elastic tensional strain increases while the plastic strain as a rule decreases the free volume. In the latter case the amorphous component, at least at room temperature, permanently deviates from the thermodynamic equilibrium. A substantial part of this deviation can be removed by annealing. If the annealing of the fibrous material is performed with fixed ends which prevents shrinking the sample upon standing at room temperature tends to return, at least partially, to the transport properties before annealing. The sorption is proportional to the mass fraction of the amorphous component, the permeability to the square of it. The diffusion reflects the anisotropy of the morphology and of its influence on the transport properties of the amorphous component. In the fiber structure the amorphous layers parallel and perpendicular to the fiber axis differ substantially. In spite of the close connection between the morphology and the transport properties one must not forget that this connection is one-sided. A model of the structure with the proper transport parameters of the amorphous component has to yield the observed sorption and diffusion data. But these data are not sufficient for constructing unambiguously the structural model.