Correlating Microstructural Characteristic Ratios to Mechanical Properties in Isotactic Polypropylene (iPP)

Abstract

When placed under uniaxial tension, a semicrystalline polymer can behave in either a ductile or brittle manner; however, this behavior is impossible to predict until it has been tested. This research aims to relate the mechanical properties of a variety of isotactic polypropylenes (iPP) to their underlying microstructural characteristics to uncover the brittle-to-ductile transition (BDT) of iPP, which can be used to predict ductility. Six iPP samples of fixed molecular weights and crystallinity, were subjected to a variety of processing conditions meant to alter microstructural characteristics to isolate the BDT. The BDT is determined, in part, by the number of tie molecules present in a sample, which can be represented by characteristic ratios that relate average polymer chain length, R_0, to the minimum length needed to form a tie molecule, 2L_c+L_a, or by the Huang & Brown tie molecule probability, P. This research found that the critical tie molecule concentration that encapsulates the BDT of the iPPs is between a R_0/(2L_c+L_a ) value of 1.75 and 2.08. Similarly, the critical P range necessary for ductility was between 11% and 12%. It is theorized that the BDT is dependent on processing conditions, such as cooling history, but results from this research regarding this is inconclusive. Further implementation of small-angle X-ray scattering and differential scanning calorimetry would provide clarity on this uncertainty. Once the BDT was isolated, this research intended to quantify the natural draw ratios (NDR), a measure of ductility, of the iPPs, and observe whether NDR values decreased with increasing molecular weight, a trend seen in other semicrystalline polymers. This was unable to be performed due to time constraints.