Flow-induced Crystallization in Isotactic Polypropylene

Open Access
Author:
Hamad, Fawzi Ghassan
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 05, 2015
Committee Members:
  • Scott Thomas Milner, Dissertation Advisor
  • Ralph H Colby, Dissertation Advisor
  • Darrell Velegol, Committee Member
  • Enrique Daniel Gomez, Committee Member
  • John B Asbury, Committee Member
Keywords:
  • flow-induced crystallization
  • quiescent crystallization
  • polypropylene
  • rheology
  • microscopy
  • differential scanning calorimetry
Abstract:
Brief intervals of strong flow stretch chains in a semicrystalline polymer melt, which results in an increase in the nuclei number density and a transformation of the crystal structure. This flow-induced crystallization (FIC) phenomenon is explored in this study using highly isotactic polypropylene (iPP) samples. Using one synthesized and five commercial linear isotactic polypropylene samples, we investigate the FIC behavior by imposing shear onto these samples in a rotational rheometer. Equipped with a good temperature control and flexible shear protocol, we apply different temperature and flow conditions. The magnitude of the FIC effect varies with basic processing parameters (shear rate, specific work, crystallization temperature, and shearing temperature) and material properties (tacticity, molecular weight distribution, and particle concentration in the polymer). The scope of this study is to systematically investigate the influences of these parameters on FIC. The FIC effects that are investigated in this dissertation are: crystallization kinetics, persistence time of flow-induced nuclei, and crystal morphology. The crystallization time was measured in the rheometer by monitoring the onset of crystallization after quenching samples sheared above Tm. These samples were subsequently used to study their flow-induced nuclei persistence time and crystal morphology. The lifetime of flow-induced nuclei was determined by measuring the time required to return from FIC back to quiescent crystallization using a differential scanning calorimeter. The crystal morphology was imaged using polarized optical microscopy and atomic force microscopy. We investigated the influence of specific work on the three FIC characteristics, and found three regimes that are separated by the critical work (Wc) and the saturation work (Wsat) thresholds. Below the critical work threshold, the morphology is composed of mostly spherulite crystals, which keep a constant volume, and a small fraction of rice grain (anisotropic) crystals. The number of rice grain crystals increases with specific work, speeding up the crystallization time of the semicrystalline polymer. At critical work, spherulite formation stops, and the morphology consists only of rice grain structures. This morphology allows the sample to crystallize at higher temperatures when cooling at 5 C/min, with the sheared sample crystallizing at 129C compared to the unsheared sample at 113C. In addition, these rice grain structures can withstand significant annealing at elevated temperatures: for example, 2 days of annealing at 210C are required to fully erase these metastable nuclei. For Wc < W < Wsat, the rice grain number density increases with specific work, further strengthening the FIC effect. At the saturation work threshold, the rice grain size, crystallization time, crystallization temperature when cooling at 5 C/min, and nuclei persistence time reach saturation and remain constant for all W > Wsat. Similar to specific work, we find that the crystallization rate (i.e. nuclei number density) increases with shear rate up to a saturation shear rate of about 1 /s for most samples. At this shear rate, only a very small minority of the longest chains (above 10^4 kg/mol) have Rouse times long enough to be stretched by the flow. Further increases in shear rate above saturation shear rate have no effect on the crystallization rate. We varied the crystallization temperature Tc after shearing, and found that the crystallization temperature increases the nuclei number density in the sample in the same way as it does during quiescent crystallization. Remarkably, the minimum crystallization time is essentially independent of undercooling, in the range of Tc we studied. This strongly suggests that the maximum nuclei number density is driven by specific work, as flow likely causes a larger reduction in the nucleation barrier compared to undercooling. The nuclei number density in the sample increases from ~10^16 m^(-3) to 10^18 m^(-3). Shearing isotactic polypropylene at higher temperatures reduced the FIC effect after subsequent quenching. Generally speaking, shearing at higher temperatures results in slower crystallization, but surprisingly, the influence of temperature is rather weak. Flow-induced crystallization persists even when shear is applied well above the equilibrium melting temperature (187C), finally weakening above the Hoffman-Weeks temperature (210C). This is likely due to the long lifetime of flow-induced precursors (crystallize to form rice grains), which remain stable at temperatures below 210C and only start to disappear slowly in prolonged annealing at temperatures above 210C (diminishing the FIC effect). Tacticity was found to govern the maximum nuclei number density in sheared samples; samples with lower isotactic content show a stronger FIC effect. Similarly, it was found that the concentration of particulates (mainly catalyst residue) are crucially important to FIC, samples with lower amounts of particles lowering the FIC nuclei number density. Data shows that the rate at which the crystallization time changes correlates with the prominence of the high molecular weight tail. A sample with a higher molecular weight tail in its distribution exhibits a faster change in crystallization time as a function of specific work. Similarly, increasing the molecular weight of the added component in a blend induces a larger change in the FIC behavior.