Three dimensional molecular orientation of isotactic polypropylene films under biaxial deformation at higher temperatures
Graphical abstract
Introduction
Biaxially oriented isotactic polypropylene (iPP) films have been widely used for packaging, overwrap, labels, and laminate films, owing to their advantages of strength at high temperatures, gas barrier properties, and resistance to abrasion and oil [1]. There are typically two types of stretching methods used to create such films: inflation molding, in which films are blown in a tubular shape, and tenter molding, in which films are elongated in the machine direction and/or the transverse direction by tenter clips [2], [3]. Thus, oriented iPP films can be produced by a variety of stretching techniques based on sequential as well as simultaneous drawing processes using various balanced draw ratios at high temperatures near the melting point and at extremely high elongation speeds [4]. Elucidation of the deformation mechanism under biaxial stretching on the molecular scale would undoubtedly be indispensable for designing and controlling various physical properties of biaxially oriented iPP films. However, very few investigations have been performed on transient deformation behavior during biaxial stretching on the basis of molecular orientation.
It is well known that the conformational orientation of atomic groups and molecular chains can be readily estimated from the polarized infrared (IR) dichroic ratios of certain absorption bands when the direction of their transition moment is known [5], [6], [7]. Importantly, the orientation of chain groups in both crystalline and amorphous regions can be analyzed separately and transiently using IR dichroic techniques. Thus, the IR dichroic technique is a suitable method for in-situ measurement of the chain orientation behavior of semicrystalline polymeric films [8], [9].
Our previous studies on in-situ orientation measurements by the IR technique addressed uniaxial as well as biaxial stretching modes of thin iPP films [8], [9], [10], [11]. In this study, we designed a tailor-made biaxial tensile tester with an optical IR device attached to the stretching frame in such a way that IR dichroic data could be obtained in-situ during biaxial deformation. We applied this new rheo-optical technique to investigate the three-dimensional orientation behavior of iPP films under sequential and simultaneous biaxial stretching. The aim of this work was to provide structural insights into the deformation mechanism of iPP under biaxial extension. In many industrial processes, films are considered to include the mesomorphic phase before stretching because they are quenched rapidly by cooled air or a chill roll immediately on discharge from the die in the molten state. Therefore, we annealed a quenched iPP film containing the mesomorphic phase and its stretching behavior was investigated at a high temperature near the melting point.
Section snippets
Material
A film grade commercial iPP with high tacticity (93%), a weight-average molecular weight (Mw) of 400 kg/mol, and a polydispersity index of 4.6 was used in this study. The pellets were compression molded in a laboratory hot press at 230 °C for 5 min between two aluminum sheets under 40 MPa to prepare films with a thickness of about 100 μm. After removal from the hot press, the samples were immediately plunged into an ice water bath maintained at 0 °C to produce transparent mesomorphic iPP films
Equip-biaxial extension
The in-situ measurements of the orientation functions in the MD, TD, and ND were performed in the equi-biaxial mode at an elongation speed of 30 mm/min in both directions from 150 to 165 °C. The variations in the orientation functions of the crystalline and amorphous chains of the film with applied strain are shown in Fig. 5, Fig. 6, respectively. The stress–strain curves in MD (X-direction) and TD (Y-direction) are also presented in these figures. Both stress–strain curves were virtually
Conclusions
The biaxial deformation behavior of iPP film was investigated at elevated temperature near the melting point. We performed in-situ measurement of the variations in the orientation of molecular chains of the crystalline and amorphous phases of the films, as well the degree of crystallinity, under simultaneous and sequential biaxial stretching. A rheo-optical system comprising an FT-IR instrument mounted on a biaxial tensile machine was used to carry out the measurements. We also developed an
Acknowledgments
The authors deeply appreciate financial support by the Kobe Fundamental Research Laboratory of Sumitomo Bakelite Co. Ltd.
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2021, PolymerCitation Excerpt :The helical conformation bands were widely utilized to probe the structural variation of iPP. For example, shear-induced conformational ordering in iPP melt [49], the molecular orientation behaviors of crystalline and amorphous phase of iPP during the deformation process, can be monitored by helical conformation bands [54,55]. In the present work, we focus on the absorption band around 841 cm−1, which is one of the 31 regular helical conformation bands.