Elsevier

Polymer

Volume 45, Issue 10, May 2004, Pages 3441-3455
Polymer

Lamellar morphology of random metallocene propylene copolymers studied by atomic force microscopy

https://doi.org/10.1016/j.polymer.2004.02.071Get rights and content

Abstract

Four sets of propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene and 1-octene, and a total defect content up to ∼9 mol% (including co-unit and other defects), were studied after rapid and isothermal crystallization. Etched film surfaces and ultramicrotomed plaques were imaged so as to enhance contrast and minimize catalyst and co-catalyst residues. While increasing concentration of structural irregularities breaks down spherulitic habits, the formation of the gamma polymorph has a profound effect on the lamellar morphology. Lamellae grown in the radial axis of the spherulite and branches hereon are replaced in γ-rich copolymers with a dense array of short lamellae transverse or tilted to the main structural growth axis. This is the expected orientation for γ iPP branching from α seeds. Spherulites are formed in copolymers with non-crystallizable units (1-hexene and 1-octene) up to ∼3 mol% total defect content and were observed up to ∼6 mol% in those with partially crystallizable comonomers (ethylene and 1-butene). However, lamellae were observed in all the copolymers analyzed, even in the most defective ones, highlighting the important role of the gamma polymorph in propagating lamellar crystallites in poly(propylenes) with a high concentration of defects. Long periods measured from AFM and SAXS are comparatively analyzed.

Introduction

Understanding the effects of chain microstructure on the morphology and crystallization behavior of poly(propylene) and its copolymers is key to applying these materials to an ever diversifying range of applications. A major advance in expanding the properties of poly(propylenes) is the advent of metallocene catalysts, which allow incorporation of large contents of comonomer, copolymerization of cyclic and other comonomer types that are not easily incorporated with classical Ziegler Natta (ZN) catalysts and excellent control of stereoregularity. These catalysts lead to a new class of random polyolefins, which are inaccessible through conventional heterogeneous ZN catalysts [1], [2].

The series of propylene copolymers studied in this work were synthesized with a metallocene catalyst that yields random comonomer distribution, uniform intermolecular distribution of the comonomer content and narrow molecular weight distribution. Hence, these copolymers allow the study of their crystalline structure as a function of increasing comonomer content and, for different types of comonomer at the same comonomer concentration, while other molecular variables such as molar mass and comonomer distribution remain constant. In previous works, the partitioning of the comonomer unit [3] and the conditions for the formation of the alpha and gamma polymorphs [4] were studied in detail in the same copolymers. The effect of the comonomer in enhancing the fractional content of the gamma polymorph was found to be the same as the role of defects in the homo poly(propylene) chain. Thus, independent of the chemical nature of the co-unit, as the comonomer and/or crystallization temperature increase, the fractional content of the gamma polymorph increases at the expense of the alpha phase. However, differences in the partitioning of the ethylene, l-butene, l-hexene and l-octene units between the crystalline and non-crystalline regions leads to contents of the gamma phase that differ among the copolymers [4]. As the gamma phase is favored in iPPs with short crystallizable sequences, higher gamma contents were found in propylene l-hexenes (PHs) and propylene l-octenes (POs) in which the co-units are rejected from the crystal lattice. In the present work, we extend the on-going structural studies of these copolymers to analyze the lamellar morphologies that develop as a function of increasing crystallization temperature in the same copolymers. The crystallization temperatures are chosen in a range such that each copolymer developed up to 100% of the gamma polymorph. In this manner, the associated lamellar structures corresponding to the pure polymorphs and their mixtures could be identified. Simultaneously, the role of comonomer content on the morphology was also studied. The overall spherulitic morphology was characterized by optical microscopy complemented by scanning electron microscopy (SEM), and the details of the lamellar morphology were quantified by atomic force microscopy (AFM). The simplicity of sample preparation is the major advantage of AFM over transmission electron microscopy (TEM) for a detailed study of lamellar structure. Coupled with permanganic etching [5], the AFM is now recognized as a powerful tool for the characterization of polymeric materials [6], [7], [8], [9], [10], [11]. In the present study, permanganic etching not only enhanced contrast, but also helped to minimize catalyst and co-catalyst residues on the surface of these copolymers that interfered with AFM imaging. The AFM data was used to characterize lamellar long periods, which were then compared to those obtained from classical SAXS methods.

Section snippets

Materials

The propylene copolymers and homopolymer studied herein were synthesized with the same metallocene-type catalyst to ensure a very similar molecular mass and similar concentration of stereo and regio-types of defects. Characterization details are listed in Table 1. The copolymer designation is the same as that used in a previous study using the same samples [4]. Listed in separate columns are molar concentrations of the comonomer, head to head misinsertions of the erythro type and concentration

Structural features of the α and γ polymorphs

Before analyzing the microscopic images, the main structural details of the α and γ polymorphs and related superlamellar morphologies are reviewed in this section.

The crystallographic features and superlamellar organization of the α polymorph have been described in different reviews [12], [13], [14], [15]. The most common monoclinic unit cell, with dimensions a=6.65 Å, b=20.96 Å, c=6.50 Å and β=99.8° comprises four 31 helical chains. The symmetry in this cell calls for alternation in the helical

Conclusions

AFM studies, guided by previous thermodynamic and crystallographic investigations [4], [12], [23] have revealed some striking morphological features of the crystalline state of these copolymers. At the supermolecular level a clear distinction in morphologies appears between copolymers with co-units that partially co-crystallize with the propylene unit (PE and PB), and copolymers with l-hexene and l-octene units that are rejected from the propylene crystalline lattice. While spherulites are

Acknowledgements

Funding of this work by the National Science Foundation, grant numbers DMR-0094485 and DMR0076485 (instrumentation grant) is gratefully acknowledged. We acknowledge the help of Kimberly A. Riddle with SEM imaging and are also grateful to S. Magonov for his assistance with AFM in the initial stages of this project. Helpful comments from one reviewer about orientation and thicknesses of epitaxial γ lamellae are also acknowledged. The research at Oak Ridge was sponsored in part by the U.S.

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