Elsevier

Polymer

Volume 90, 4 May 2016, Pages 18-25
Polymer

Interfacial crystallization and mechanical property of isotactic polypropylene based single-polymer composites

https://doi.org/10.1016/j.polymer.2016.02.052Get rights and content

Highlights

  • Single polymer composites (SPCs) were prepared with isotactic polypropylene (iPP) fibers and matrix.

  • Fiber introduction temperature could enhance interfacial crystallization.

  • Tensile strength strongly depended on the transcrystallinity, orientation degree of iPP fibers and fiber-matrix adhesion.

Abstract

Isotactic polypropylene (iPP) based single-polymer composites (SPCs) were prepared by introducing iPP fibers into the molten or supercooled homogeneous iPP matrix. The influences of fiber introduction temperature (Ti) on the resultant morphology of transcrystallinity (TC) and mechanical properties of SPCs were investigated via a polarized optical microscopy (POM) and a universal tensile test machine. The effects of interfacial crystallization on mechanical properties were also studied. The tensile strength of SPCs was observed to increase firstly and to reach a maximum value at Ti = 160 °C, and then to decrease with further increasing the Ti. Wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM) and POM were employed to understand the mechanical enhancement mechanism. It is found that the enhanced tensile strength of SPCs was strongly dependent on the synergistic effects of TC, high orientation degree of iPP fibers and good adhesion between the iPP fiber and the matrix.

Introduction

In the past few decades, isotactic polypropylene (iPP) has been the major polymeric construction materials in the light of its impressive consumption. One outstanding advantage is its excellent comprehensive properties, including easy processing, low manufacturing cost and so on [1]. Unfortunately, the intrinsic low mechanical strength of iPP limits its further applications. Hence, considerable efforts have been made in order to further improve its mechanical properties. One of the most common methods is by embedding various fibers (carbon, clays, glass, etc.) in the iPP matrix to produce composites [2], [3], [4], [5]. However, two main problems must be avoided if the heterogeneous fibers are added in the thermoplastic matrix. The first is the interfacial residual stress due to different thermal expansion between heterogeneous fibers and polymer matrix [4]. The second is the weak interfacial adhesion owing to the incompatibility among the heterogeneous components [6]. The interfacial interaction between fiber and matrix is a crucial prerequisite for determining the mechanical property [7]. Among the various methods (including the increased specific surface area of fibers, improved chemical activity of fiber surfaces, and matched compatibility) to enhance the interfacial interaction in the iPP/fibers composites [8], [9], [10], [11], [12], [13], [14], interfacial crystallization such as transcrystallinity (TC) [15] is regarded as an efficient and economical approach [16]. Moreover, TC around the fiber, possessing better load transfer ability than amorphous layers, is believed to be of crucial significance to improve the interfacial interaction between matrix and fibers [17], [18].

On the other hand, in light of recyclability, the presence of heterogeneous additives or inclusions, such as glass fibers, clays and magnetic nanoparticles, is an inevitable obstacle for polymer based composites. Hence, the composite systems with the matrix and the fiber being from the same polymer are preferable candidates. In other words, these systems mean mono-component composites or single-polymer composites (SPCs). The concept of SPCs is not new, which was proposed for the first time by Capiati and Porter four decades ago [19]. Such self-reinforced systems have specific economic and ecological advantages. This can be understood as follows: 1) desired mechanical property can be achieved as a result of the occurrence of TC and good interfacial adhesion for the semicrystalline polymer matrix; 2) SPCs show undoubtedly advantages in terms of recyclability. Hence, up to now, many preparation methods of iPP based SPCs have been proposed [20], [21], [22] and summarized in the recent literature [23], [24], [25]. However, although the influences of crystallization temperature, fiber introduction temperature, fiber molecular weight, and matrix molecular mass on the interfacial morphology of iPP based SPCs have been investigated [26], [27], [28], [29], the effect of interfacial features on the mechanical properties of SPCs has been rarely reported up to date.

In this paper, iPP fiber was introduced into iPP film to prepare iPP based SPCs. The interfacial features of SPCs and their tensile strength as a function of introduction temperature were investigated. The underlying origin for the improved tensile strength of SPCs is discussed based on the results of polarized optical microscopy (POM), scanning electron microscope (SEM), wide-angle X-ray diffraction (WAXD).

Section snippets

Materials

The iPP (T30S) employed in this work was a commercial grade iPP, provided by Dushanzi Petroleum Chemical Co. Ltd, China. Its melt flow index (MFI) and weight-average molecular weight (Mw) were 3.0 g/10 min (230 °C, 21.6 N) and 39.93 × 104 g/mol, respectively.

Melt spinning

The iPP fibers used in this work were melt spun by using a mini co-rotating twin-screw extruder (SJSZ-10A, Wuhan Ruiming plastic and mechanical Co. Ltd) with a length-to-diameter ratio (L/D) of 16 and a die diameter of 3.0 mm. The

Influence of introduction temperature on molecular orientation of iPP fibers

The absorbance intensity difference between the parallel- and perpendicular-polarized FTIR spectra can be employed to evaluate the molecular orientation level [31], [32]. Here, to qualitatively evaluate the effect of different thermal history on the orientation level of iPP fibers, polarized FTIR test was carried out. Fig. 2 shows the polarized FTIR spectra of F-145, F-160, F-165, F-168, F-172 and F-175, respectively. It is clear to find notable difference between the absorbance intensities of

Conclusions

The iPP based SPCs have been successfully prepared by introducing iPP fibers into the molten or supercooled homogeneous matrix. The induced TC by homogeneous fiber as a function of Ti was observed by means of POM. Then the relationship between interfacial features and mechanical property of SPCs was studied. The results show that the tensile strength of SPCs firstly increases and reaches a maximum value at Ti = 160 °C, then decreases with the increase of the Ti. Based on the detailed analysis,

Acknowledgments

We express our great thanks to the National Natural Science Foundation of China (51173171, 11172271), the Major State Basic Research Projects (2012CB025904), Plan for Scientific Innovation Talent of Henan Province and Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University). Z. Guo appreciates the start-up funds from University of Tennessee.

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