Short communicationIn situ shrinking fibers enhance strain hardening and foamability of linear polymers
Graphical abstract
Introduction
The strain hardening behavior of polymeric melts has important roles in polymer processing. This behavior is indicated by an upturn in the tensile stress growth coefficient, η+E(t) [1], which is often called the extensional viscosity even though this is not a proper term, above the linear curve, which is invariant with rate during extensional deformation. This nonlinear behavior can promote a self-healing effect [2], leading to more homogeneous deformation and less flow instability in the elongational flow. Thus, we can expect that a polymeric material exhibiting strain hardening can be used in processes involving elongational flows, such as foaming, film blowing, and fiber spinning. It is well known that a higher degree of strain hardening under extension can increase the foamability by reducing the cell coalescence [[3], [4], [5], [6]]. The result is foam structures with low bulk density, improved thermal and acoustic insulation properties, and high impact strength. Introducing long-chain branching on linear polymeric chains has been a standard method to increase strain hardening, and its rheology and processing have been studied extensively in the last three decades [[7], [8], [9]]. However, to produce polymers with a controlled branching structure for desired strain hardening is still quite challenging, and once a long-chain branched polymer has been synthesized, the behavior is relatively fixed. We have developed technology to control the strain hardening of linear polymers by compounding them with active-shrinking fibers to tailor the behavior by changing an operating condition, such as temperature. Extensional rheometry and foamed morphology will be demonstrated to support our hypothesis.
Various methods have been studied to improve the processability and properties of PP, including long-chain branching and fibrillated polymer blending [3,[10], [11], [12]]. However, no study has been performed to show fiber materials with shrinking characteristics in polymers to improve strain hardening and thus foamability. We adopted our previous concept that shrinking fibers in mortar can improve the mechanical properties of concrete [13]. While the shrinkage of fibers outside of the matrix occurs upon activation due to a change in temperature, the shrinkage of fibers in the matrix is suppressed by the properties of the matrix, such that the fibers generate compressive stress in the matrix (Fig. 1). We hypothesize that, by compounding a linear polymer matrix with fibers that shrink or contract upon specific stimuli, the composite would show enhanced strain hardening during extensional deformation. Such an enhancement can be utilized in polymer processing, for example foaming, since it helps prevent cell coalescence.
Section snippets
Materials
We have chosen polypropylene (PP) as the matrix because PP is one of the most popular polymers due to its unique combination of desirable chemical and physical properties. For example, PP has a higher temperature resistance and stiffness, and great chemical resistance and impact strength [14]. The conventional linear PP has a low melt strength and strain hardening in extensional flow, which limit the processing window considerably [15,16]. The matrix is isotactic, linear PP, received from
Tensile test properties of HAS, HP fibers
We measured the tensile stress of the fibers alone because it is believed that this stress fundamentally contributes to the strain hardening of the matrix. Single fiber tensile tests were carried out with a tensile test fixture in the rheometer. Once the fibers were loaded below a heat activation temperature (175 °C), without applying any tension, the tensile force reading was electronically zeroed, and the temperature was increased to 180 °C. The fibers were extended at a constant speed of
Conclusion
We hypothesized that shrinking fibers compounded in linear polymer matrices enhance strain hardening in extension and thus foamability. The tensile stress growth coefficient (extensional viscosity) of the composite, tensile stress of a single fiber, and foamed morphology of the material support our hypothesis. Both shrinking HAS and non-shrinking HP fibers induce strain hardening, but the HAS fibers enhance the hardening further due to the higher initial tensile stress resulted from shrinkage.
Acknowledgements
The authors would like to thank the University of Vermont Spark VT program (S1610) for supporting this project.
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