Flow-induced crystallization studies can be preformed using a custom-built mini-extrusion device whose design is inspired by the past work of Janeschitz-Kriegl and Kornfield. A schematic of the device is shown in below. The experimental novelty of this ˇ°short-term shearingˇ± apparatus is its ability to fully control the thermal and flow history of the material, and to separate out the effect of shearing on primary nucleation from crystal growth.
The classical experimental protocol, as established by Kornfield et al, is as follows: To begin an experiment, the molten sample is slowly injected from a thermally isolated reservoir (185ˇăC for polypropylene) into the slit die (0.5mm), which is held at a temperature well above the melt (210ˇăC for polypropylene). Loading is performed at low wall shear stresses to minimize chain alignment. The sample is then held for a short relaxation period (5 min) to erase any orientation or crystal nucleation induced by the loading step. After this anneal period, the polymer flow cell is rapidly cooled to below the melt temperature (Tc). A Tc of 150ˇăC (for polypropylene) was chosen for this work. Cooling occurs quickly, to prevent any premature crystallization prior to shear. Once thermally stabilized, a sharp, short pulse (0.5-10 sec) is applied via the actuator to flow the sample. The actuator and Labview software is used to control the wall shear stress in the die (¦Ów) and pulse time. The ¦Ów is determined from the thin-slit dimensions and a flush-mounted, Dynisco® pressure transducer in the gap. Subsequent crystallization, after and during the pulse, is studied by in situ monitoring of birefringence and turbidity using a HeNe laser and a polarizer / analyzer setup.
The device consists of three polarizers, one positioned before the flow cell, and two placed afterwards. A nonpolarizing beam splitter is used to send the light to each of the two polarizers (one crossed and one parallel, relative to the first polarizer). The device is designed with the capability to remove the flow cartridge and further analyze ex situ the sheared material via microscopy (TEM and OM).
1. Kumaraswamy, G., R.K. Verma, and J.A. Kornfield.Review of Scientific Instruments, 1999. 70(4): p. 2097-2104.
2. Kumaraswamy, G., et al..Abstracts of Papers, 220th ACS National Meeting, Washington, DC, United States, August 20-24, 2000, 2000: p. PMSE-283.