|Ph.D Student||Lipp Jonathan|
|Subject||Nanofiber Networks in Polymers|
|Department||Department of Chemical Engineering||Supervisor||Professor Yachin Cohen|
|Full Thesis text|
Dibenzylidenesorbitol (DBS) and its derivatives are well known nucleators and "clarifiers" for various semi-crystalline polymers and are commercially used in polymer manufacture processes. The molecule 1,3:2,4-Di(3,4-dimethylbenzylidene)sorbitol (DMDBS), the most common one in this group used widely with polypropylene (PP), was studied in this research, Previous studies have shown the ability of such molecules to self-associate in organic solvents and polymer melts to yield a nano-scale fibrillar network. One of the open issues is the kinetic mechanism of network formation.
The kinetics of DMDBS network formation in PP melt were investigated as a function of temperature using small-angle x-ray scattering (SAXS) including synchrotron radiation. The time-evolution of the fibrillar structure exhibited characteristic time-lag behavior with a strong inverse temperature dependence, indicating a homogenous nucleation and growth mechanism which is controlled by the nucleation rate. The kinetic measurements allowed evaluation of the thermodynamic transition temperature at 0.4 and 1% (w/w) DMDBS in PP as about 180oC and 190oC, respectively, that define the degree of undercooling for a given temperature. This transition is associated with the crystallization of DMDBS fibrils within the PP melt, which is supported by its independent calculation of the melting point depression of DMDBS crystals in PP based on evaluation of thermodynamic interaction parameters. Imaging by electron microscopy showed a hierarchical fibrillar structure, displaying thin fibrils of less than 100nm in diameter composed of thinner fibrils, measuring less then 10nm. Similar examination of DMDBS in polystyrene showed the same hierarchy only on larger dimensions, where the thinner fibrils were above 100nm and the larger ones were more then 1m in diameter. Thus SAXS could not be used for kinetic measurements in this system.
The effect of DMDBS on oriented crystallization and mechanical properties of melt-spun PP fibers was examined. Enhanced Young's moduli were obtained with 0.4% DMDBS in a temperature protocol that allowed DMDBS fibrils to remain intact during spinning process. It is suggested the nanofibrils, oriented by the shear and elongational flow prior to PP crystallization, serve as nucleation centers for its subsequent crystallization in a highly oriented morphology. The reason for the better performance of DMDBS at about 0.4% content in unclear. These observations may stem from the nanofibrillar network structure, whereby at low DMDBS concentration the network is broken to long thin nanofibrils, whereas at higher content more branched or other elaborate structures may ensue.