טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShaked Emil
SubjectReinforcement of PVA (Polyvinyl Alcohol):
Nanofibers vs. Bulk
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Eyal Zussman
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

In this study, we present the mechanical testing (static and dynamic modes) of electrospun polyvinyl alcohol (PVA)-based nanofibers and bulk. The phenomenon of dynamic reinforcement was observed in both the individual fibers and the bulk. An increase in the elastic moduli, resulting from dynamic creep test, occurs when the deformation rate exceeds a critical value, and can be attributed to the non-equilibrium dynamics of supermolecular structures in the amorphous part of the polymer matrix. One of the unique characteristics of these supermolecular structures is the long relaxation time observed during the evolution of these systems. It was noted that the rate of the modulus increase of the nanofibers is nearly double that of the bulk. This difference can be attributed to the influence of confinement on the polymer matrix of the nanofibers. Also, in the dynamic tests it was revealed that the glass transition temperature, Tg, of the amorphous region of the nanofiber polymer matrix is noticeably higher than that of the bulk. These results are in accordance with the conceptual understandings of the unique characteristics of nanometer-scale objects.

In addition, the characteristics of composites made of PVA nanofiber and cellulose whiskers were also examined. We found that the dependence of the effective tensile modulus on the whisker concentration is a non-monotonic one, i.e. an initial increase is followed by a decrease once the whisker concentration exceeds 2 wt %. Such behavior can be explained in the framework of the aggregation model, where first the cluster size reaches the fiber diameter (cluster confinement); then, the whisker distribution becomes inhomogeneous, resulting in a partial weakening of the composite.