Extension induced phase separation and crystallization in semidilute solutions of ultra high molecular weight polyethylene
Graphical abstract
Introduction
Ultra high modulus polyethylene fibers are spun from solutions of ultra high molecular weight polyethylene (UHMwPE) [1]. To a large extent fiber spinning processes comprise uniaxial extensional upon extrusion and drawing of the polymeric liquid. The processability of UHMwPE solutions is thus determined by the rheological characteristics primarily in extension, yet controlled rheological studies of PE solutions have been performed only in shear flow [2].
In shear, UHMwPE solutions exhibit significant nonlinear characteristics [3]. The long chains are easily deformed by flow causing the solutions to be highly shear thinning. Several studies on UHMwPE solutions utilize paraffin oil (PO) as solvent. It is a convenient solvent due to its low volatility and the fact that the chemical composition is the same as that of UHMwPE. Extensive work on UHMwPE/PO solutions in shear have been performed by Murase and co-workers [[4], [5], [6], [7]]. Apart from significant shear thinning they discovered other highly nonlinear phenomena in these systems. They found that multiple states of heterogeneities can be initiated by flow under the right conditions. At high shear rates the UHMwPE/PO solutions experience concentration fluctuations that eventually develop into actual phase separation [4]. In addition, at temperatures close to the melting point , the UHMwPE rich phase crystallizes into highly oriented structures [5,6]. These flow induced phenomena have been found to play a huge role in the structural development of UHMwPE fibers during processing [7] as well as the final fiber strength [8,9]. Unfortunately, the deformation in a spin-line is ill defined and lacks control of the deformation. As a result, the imposed deformation is quantified in terms of take-up speed or other measures related to the instrument rather than local deformation of the material. Due to experimental challenges regarding both control of the deformation and the non-stick nature of the sample, studies on polyethylene solutions in controlled extensional flows have, to our knowledge, never been performed.
The purpose of the present study is to characterize solutions of UHMwPE in controlled uniaxial extension. The solvent is PO and the samples are measured at constant deformation rates at temperatures well above . Extension of these non-sticky samples are performed using a filament stretch rheometer (FSR) with a modified sample plate design to prevent slip off. Conditions under which flow induced phase separation and flow induced crystallization (FIC) occur are identified using simultaneous high-speed imaging. We map the regimes under which the different flow induced phenomena occur with respect to imposed deformation rate, polymer concentration, and temperature.
Section snippets
Materials and method
UHMwPE with and a broad molar mass distribution, supplied by DSM and paraffin oil (containing antioxidant: 2,6-di-tert-butyl-p-cresol) were mixed in an extruder at a temperature well above the melting temperature. Three solutions were prepared containing , and UHMwPE. The solutions were extruded directly into a mould and moulded into discs of diameter and height .
Upon cooling to room temperature, some solvent was expelled from the sample due to
Linear rheology
Linear rheology of the three samples along with respective multimode Maxwell model fits are seen in Fig. 3a. The response is given in terms of storage and loss moduli and , respectively. The first crossover is captured for PE/PO-9 while for PE/PO-17 and PE/PO-29 the first crossover are estimated by extrapolating G′, G″ to lower frequencies. The inverse value of the estimated first crossover frequency gives an indication of the average relaxation time by reptation . Due to instrument
Conclusion
Solutions of UHMwPE in extensional flow show a rich variety of behavior at temperatures well above . We find that at the extensional behavior differs fundamentally with concentration, temperature and imposed deformation rate. Overall the observed behaviors can be divided in two Scenarios 1 - Phase separation and 2 - FIC. At flow induced crystallization (Scenario 2) was observed for high and high ϕ. The higher the concentration, the lower the at which FIC can be
Acknowledgement
The authors thank Aage og Johanne Louis-Hansen fonden for financial support as well as Prof. Dimitris Vlassopoulos for fruitful discussions on the work.
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