Acoustomechanics of semicrystalline polymers

We develop an acoustomechanical theory for semicrystalline polymers and demonstrate that acoustic radiation force is capable of causing giant deformation in these materials. When a polymer layer is subjected to combined tensile mechanical force in plane and acoustic force （sound pressure） through thickness, it becomes initially homogeneously thin but soon inhomogeneous when the two forces reach critical conditions. Critical conditions for such acoustomechanical instability are theoretically determined based on the J2-deformation theory. We demonstrate that pull-in instability can he acoustically triggered even if the in-plane mechanical force is fixed. Bifurcation in the critical condition for acoustomechanical instability occurs when the polymer exhibits sufficiently large hardening. The findings of this study enable reliability design of novel acoustic actuated devices.

APPLICATION OF ELECTRON DIFFRACTION IN THE STRUCTURE CHARACTERIZATION OF POLYMER CRYSTALS

The dependence of properties on the structure and morphology of semicrystalline polymers offers an effective way to tailor the properties of these materials through structure control. To this end, establishing the structure and property relationship is of great importance. For a right characterization of the crystal structure, several techniques can be used. Among these techniques, electron diffraction has its advantage for determining the crystal structure related to specific formation condition since it can combine with bright and dark fields observation of the sample. This feature article describes the application of electron diffraction in determining the crystal structure of semicrystalline polymers with elaborately selected examples. We focus on how the electron diffraction can be used to disclose the crystal structure, mutual orientation of different crystals, as well as the disorders included in the polymer crystals.

Changes of the Molecular Mobility of Poly(ε-caprolactone) upon Drawing,Studied by Dielectric Relaxation Spectroscopy

Dielectric relaxation spectroscopy（DRS） of poly（ε-caprolactone） with different draw ratios showed that the mobility of polymer chains in the amorphous part decreases as the draw ratio increases. The activation energy of the α process, which corresponds to the dynamic glass transition, increases upon drawing. The enlarged gap between the activation energies of the α process and the β process results in a change of continuity at the crossover between the high temperature a process and the α and β processes. At low drawing ratios the a process connects with the β process, while at the highest drawing ratio in our measurements, the a process is continuous with the αprocess. This is consistent with X-ray diffraction results that indicate that upon drawing the polymer chains in the amorphous part align and densify upon drawing. As the draw ratio increases, the α relaxation broadens and decreases its intensity, indicating an increasing heterogeneity. We observed slope changes in the α traces, when the temperature decreases below that at which τα≈ 1 s. This may indicate the glass transition from the ‘rubbery＇ state to the non-equilibrium glassy state.