Elsevier

Polymer

Volume 99, 2 September 2016, Pages 480-487
Polymer

Microstructure evolution of poly[tetrafluoroethylene-co-(perfluoropropylvinylether)] films under uniaxial deformation

https://doi.org/10.1016/j.polymer.2016.07.044Get rights and content

Highlights

  • •

    Uniaxial drawing of fluoropolymer films imparted significant changes to microstructure and thermophysical properties.

  • •

    Achieved higher crystallinity and very high crystal orientation (fc > 0.9) through drawing.

  • •

    Segmental relaxations were greatly impeded by orientation.

Abstract

This paper presents an investigation of semi-crystalline poly[tetrafluoroethylene -co-(perfluoropropylvinylether)] (PFA) films uniaxially drawn at two temperatures. The extent of crystal unit cell orientation increased significantly with drawing at both draw temperatures as expected, but crystallinity was found to increase with draw ratio only at the lower draw temperature, remaining unchanged at the higher temperature. Considering the evolution of small-angle X-ray scattering patterns as a function of deformation, a model for the changes in PFA lamellar microstructure on drawing was developed. The observed mechanical α and γ relaxations of PFA were assigned to relaxation of rigid and mobile amorphous segments, respectively. Drawing at 200 °C to a draw ratio of 7 leads to a ∼40 °C increase in the α transition temperature compared to the unoriented film, and the findings support the key role of molecular orientation in suppressing the cooperative motions of non-crystalline segments in drawn polymers.

Introduction

Poly[tetrafluoroethylene-co-(perfluoropropylvinylether)] (PFA), a random copolymer consisting primarily of tetrafluoroethylene units, is well known to exhibit a variety of desirable properties (e.g. excellent chemical resistance and low coefficient of friction) [1], [2], [3] and as a result it has been used in a variety of applications including in the area of optical materials [4], [5]. PFA also has an advantage over the widely-used polytetrafluoroethylene (PTFE) homopolymer as it is much easier to process, and can be manufactured into high quality thin films, extending its applications. It exhibits excellent, relatively temperature insensitive, dielectric properties (e.g. high volume and surface resistivity) [6], [7], [8] and is therefore suitable for applications such as insulation of communication cables or components for electronic devices where low electrical energy dissipation is necessary at high frequencies [9], [10].

Although PFA has some advantages over PTFE, there have only been limited systematic microstructural studies on semi-crystalline PFA and related TFE (tetrafluoroethylene) – fluoroether copolymers, and very few on structural changes on mechanical deformation [1], [11], [12]. Fujimori and Hayasaka [1] have propose that poly[tetrafluoroethylene -co-(perfluoroethylvinylether)] (EFA) drawn fibers form oriented lamellar crystal stacks at lower strains, which transform into a tilted lamellar arrangement (herringbone-like) upon further drawing.

The aim of the present work is to investigate in detail the effect of uniaxial deformation on crystalline microstructure and dynamics of PFA films. Both wide- and small-angle X-ray scattering measurements were employed to characterize the microstructural evolution of PFA films on drawing, augmented by differential scanning calorimetry to determine degrees of crystallinity. Chain dynamics were studied using dynamic mechanical analysis at a fixed frequency over a broad temperature range.

Section snippets

Materials

Poly[tetrafluoroethylene-co-(perfluoropropylvinylether)] films (Chemours TeflonTM PFA) were purchased from American Durafilm. PFA is a random copolymer obtained from the polymerization of TFE (-CF2-CF2–) and a perfluoropropylvinylether comonomer (-CF2-CF(OCF2CF2CF3)–). The comonomer content in PFA has been reported to be approximately 7 wt% [1]. The thickness of the as-received PFA film was ∼250 μm.

Drawing of PFA films

PFA films were drawn uniaxially using an Instron Model 5866 in an air oven at 140 and 200 °C.

Stress-strain behavior

A representative stress-strain curve for the initially undrawn PFA film at 140 °C is shown in Fig. 1. It displays an initial linear region (primarily elastic deformation), followed by yielding, a longer plateau region, and an upturn in engineering stress beginning at an extension of ∼250%. These are identified as Regions I, II, III and IV, respectively. The seven points marked on the curve represent the strains at which the microstructural, DSC and DMA measurements were carried out: i.e., draw

Summary

The microstructural evolution induced by uniaxial deformation of semi-crystalline PFA films was investigated using X-ray scattering, augmented by DSC measurements of the degree of crystallinity. Unit cell/lamellar orientation increases rather significantly on extension as expected: the WAXD Hermans' orientation factor reached 0.86 at the maximum draw ratio achievable on drawing at 140 °C. Drawing at 200 °C resulted in a maximum draw ratio of 7.0 and an orientation factor of 0.93. The degree of

Acknowledgments

We gratefully acknowledge the support of the Office of Naval Research through Contract Number N00014-14-C-0205. We would also like to thank Dr. David Shelleman and Nichole Wonderling for their support of this research.

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