Elsevier

Polymer

Volume 136, 31 January 2018, Pages 215-223
Polymer

Extension induced phase separation and crystallization in semidilute solutions of ultra high molecular weight polyethylene

https://doi.org/10.1016/j.polymer.2017.12.042Get rights and content

Highlights

  • Solutions of Ultra High Molecular Weight Polyethylene show crystallization at temperatures well above Tm when subjected to strong extensional flow.

  • Two phenomena have been observed during controlled extension.

  • Scenario 1: Flow induced phase separation. This is strongly destabilizing.

  • Scenario 2: Flow induced crystallization. This is strongly stabilizing.

Abstract

We investigate the influence of controlled uniaxial extension on various flow induced phenomena in semidilute solutions of ultra high molecular weight polyethylene. Concentrations range from 9w% to 29w% and the choice of solvent is paraffin oil. The start-up extensional behavior is measured at various Hencky strain rates ε˙ and at two different temperatures (150C and 170C) well above the melting point. For Hencky strains ε>0.9 the qualitative behavior of the samples differ significantly depending on the imposed conditions and the concentration of the samples. Overall we propose two flow scenarios: Scenario 1 - flow induced phase separation resulting in an unstable bulky filament and Scenario 2 - flow induced phase separation and crystallization resulting in a stable deformation and a smooth strongly strain hardening filament. Scenario 2 is observed only at 150C at high ε˙ and high concentrations. Scenario 1, observed at both temperatures, is most pronounced at low rates and/or high concentrations.

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 Tm, 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 Tm. 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 Mw=3500000g/mol and a broad molar mass distribution, supplied by DSM and paraffin oil (containing 0.2w% 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 5w%, 10w% and 20w% UHMwPE. The solutions were extruded directly into a mould and moulded into discs of diameter D0=8mm and height h0=6mm.

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 G' and G'', 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 τd. Due to instrument

Conclusion

Solutions of UHMwPE in extensional flow show a rich variety of behavior at temperatures well above Tm. We find that at ε>0.9 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 150C 150Cflow induced crystallization (Scenario 2) was observed for high WiR and high ϕ. The higher the concentration, the lower the WiR 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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