Macromolecular Nanotechnology Effect of poly(tetrafluoroethylene) nanofibers on foaming behavior of linear and branched polypropylenes Kinga Jurczuk ⇑ , Andrzej Galeski, Jerzy Morawiec Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland article info Article history: Received 3 October 2016 Received in revised form 9 January 2017 Accepted 19 January 2017 Available online 23 January 2017 Keywords: Nanocomposites Poly(tetrafluoroethylene) nanofibers Strain hardening Foaming Cellular structure abstract The influence of poly(tetrafluoroethylene) (PTFE) nanofibers, generated in situ during shear extrusion, on foaming behavior of linear (L-PP) and long-chain branched (LCB-PP) polypropylenes has been investigated. PTFE nanofibers significantly improved both ther- mal and rheological properties of PPs studied. PTFE nanofibers formed an entangled net- work and induced the strain hardening of PP/PTFE nanocomposites. Foams of neat PP and PP/PTFE nanocomposites were produced using continuous extrusion foaming and autoclave-based bead foaming. The presence of PTFE nanofibers resulted in much smaller foam cells and higher cell concentration as compared to neat PPs. In addition, the autoclave-based bead foaming showed that PTFE nanofibers nucleated cells formation in LCB-PP and participated in controlling the cells growth. The entanglements between PTFE nanofibers prevented excessive expansion of cells. As the result, expanded beads of LCB-PP/PTFE nanocomposite possessed ten times smaller cells, narrower cell size distribu- tion and much higher cell concentration as compared to neat LCB-PP. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Polymeric foams consist of at least two phases: solid polymer matrix and a gaseous phase that contributes to the forma- tion of cells. In general, foamed polymer products are much lighter than their solid counterparts and possess various unique characteristics such as higher specific tensile strength, higher toughness, superior thermal and sound insulation properties [1–3]. Polypropylene (PP) is a semi-crystalline polymer having some unique advantages over polystyrene (PS) and polyethylene (PE) that makes it attractive for foaming applications. PP has high stiffness, excellent chemical resistance, good impact resis- tance, and the most important, can be used at higher service temperatures over PS and PE [4,5]. Unfortunately, low melt strength of linear PP (L-PP) near processing temperatures and lack of strain hardening, also known as extensional thickening during cell growth, cause difficulties in production of closed-cell L-PP foams with a high expansion ratio and uniform cell distribution [6,7]. Several ways to improve the foamability of L-PP are known: (1) modification of chain structure either by long-chain branching or chain grafting using electron-beam irradiation or chemical crosslinking methods [8–10], (2) modification of the molecular weight distribution by adding e.g. di-2-ethylhexyl peroxy carbonate [11], and (3) melt blend- ing L-PP with long-chain branched polypropylenes (LCB-PP) [12,13]. Despite the fact that above methods are effective strate- http://dx.doi.org/10.1016/j.eurpolymj.2017.01.024 0014-3057/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: kinsow@cbmm.lodz.pl (K. Jurczuk). European Polymer Journal 88 (2017) 171–182 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj