Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene

Abstract

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Effect of Thermal History and Shear on the Viscoelastic Response of iPP Containing an Oxalamide-Based Organic Compound

“…Additionally, it may be concluded that at 220 °C the complex viscosity decreases slightly with increasing OXA3,6 concentration throughout the evaluated frequency range. In line with previous observations, 24 these findings confirm that OXA3,6 facilitates a viscosity suppression in i PP.…”

“…With respect to the viscosity suppression, earlier, with the help of molecular modeling, we concluded that the OXA3,6 concentration is far too low to facilitate the observed enhancement in molecular relaxation, thus excluding the possibility that OXA3,6 acts as a solvent for i PP. 24 This implies that there should be some sort of interaction between the OXA3,6 molecules and the i PP chains, where the average effect is the speed-up of the chain dynamics, especially the large scale reorientation of the chains (i.e., in the terminal regime). According to the data provided in Figure 3, this interaction should not cause any significant disruption of the entanglement network.…”

“…30 Because of the effective contribution of the high molar mass component in the structure development, i PP was found to be less sensitive to shear in slit flow (i.e., lowered Weissenberg number 31 ) in the presence of OXA3,6 , resulting in the formation of significantly thinner shear layers compared to the pure i PP sample. 24 However, despite the suppression in apparent viscosity, systems containing OXA3,6 exhibited a decrease in the crystallization time and a more homogeneous crystallization morphology. Though these findings are interesting, a clear understanding on the origin and correlation between the viscosity suppression and shear is not available.…”

“…Because the self-assembly of OXA3,6 proceeds via a nucleation and growth mechanism, the resultant particle morphology is found to be strongly dependent on parameters such as the application of shear, concentration, and cooling rates (Figure 1C). 10,24…”

“…Because of the poor visibility of the NA particles, yellow dotted lines are placed over borders of the NA aggregates formed during cooling at a rate of 1 °C/min. Images B and C are adapted with permission from refs (10) and (24), respectively.…”