A novel epitaxy of isotactic polypropylene (α phase) on PTFE and organic substrates

doi:10.1016/S0032-3861(99)00310-9

Polymer

Volume 41, Issue 7, March 2000, Pages 2613-2625

https://doi.org/10.1016/S0032-3861(99)00310-9 Get rights and content

Abstract

Isotactic polypropylene in its α modification (αiPP) crystallises epitaxially on polytetrafluoroethylene (PTFE) and several hemiacids or salts of substituted benzoic acids via a novel contact plane, namely (110): so far, the only known contact plane involved in αiPP homo- and hetero-epitaxies was (010). In spite of its complicated architecture (alternation of antichiral helices with different azimuthal settings), the (110)_αiPP contact plane displays well defined, if not prominent, rows of methyl side chains parallel to the crystallographic 〈112〉 direction (at 57° to the c-axis) and ≈5.5 Å apart. The matching contact planes of the substrates display linear gratings made of rows of e.g. chlorine atoms or PTFE chains with similar ≈5.5 Å inter-row or interchain distances. Various morphologies are observed in iPP thin films crystallised at different cooling rates in the presence of PTFE; they can be analysed in terms of a succession and interplay of successive epitaxies: initial αiPP/PTFE heteroepitaxy, followed by αiPP/αiPP and γiPP/αiPP homoepitaxies.

Introduction

Epitaxial crystallisation is a widespread phenomenon in isotactic polypropylene (iPP) and is much investigated both for its scientific aspects and economic implications. Epitaxial crystallisation is documented for all three polymorphs of iPP: α, β and γ modifications, and is manifested either when iPP interacts with itself (homoepitaxy) or with “foreign” substrates (heteroepitaxy).

The best known, spontaneous homoepitaxy affects only the α form of iPP and results in a most spectacular and characteristic lamellar branching specific to this polymer and crystal modification [1], [2]. This homoepitaxy has been analysed at a crystallographic and sub-molecular level [3], [4]. It rests on an epitaxy between two (010) contact planes made of isochiral helices. The underlying rotation twin generates two chain orientations at 100° to each other. A similar structural relationship exists between bilayers within the unit-cell of the γ phase of iPP (γiPP), leading to a structure with two populations of chains with non-parallel axes [5], [6]. These two chain orientations are embedded in a single lamella, and do not therefore show up morphologically in the form of lamellar branching. Finally, an epitaxy of the γ phase on the α phase has been recognised early on [7], and rests on the same pattern of crystallographic interactions.

Heteroepitaxy of iPP on various crystalline substrates and nucleating agents is used as a means to enhance the nucleation rate of the α phase [8] or to generate the more elusive β phase [9], [10]. Interestingly, so far only the (010)_αiPP (or the equivalent (001)_γiPP) contact planes involved in the homoepitaxies of the α and γ phases have been found to be also involved in the heteroepitaxies of these phases [11]. This plane is the contact plane when a substrate periodicity of ∼5 Å matches its 〈101〉 interrow distance, as for example in the iPP/PE or iPP/aliphatic polyamides epitaxies [12], [13], [14]. Finally, the β phase is induced by additives which display periodicities near 6.5 Å. Contrary to the previous cases, the epitaxy rests mainly on a matching with the helix axis repeat distance (≈6.5 Å) of the three-fold helices of iPP [10].

In the present article, we describe and analyse a novel type of epitaxy of iPP, which applies exclusively for the α phase (and not the γ phase). Contrary to all known epitaxies of αiPP, the epitaxial relationship involves the (110)_α plane. The epitaxies are observed for a variety of substrates, both low MW ones (hemiacids or salts of substituted benzoic acid) and a polymer, namely poly(tetrafluoroethylene) (PTFE). The present investigation provides a structural explanation for the reported nucleation activity of PTFE towards iPP [15], [16], [17]. It also describes a range of complicated morphologies which are due to an interplay and succession of αiPP/PTFE heteroepitaxy and αiPP/αiPP and γiPP/αiPP homoepitaxies.

Section snippets

Experimental

Samples. Two samples of isotactic polypropylene have been used in the parallel investigations performed in the different laboratories: a sample provided by Elf-Atochem, already used in a study on enhanced nucleation [9] with MW=315,000 and polydispersity ≈5.5, and a sample of trade name Novolene produced by BASF AG Ludwigshafen, Germany with very similar characteristics. Actually, the molecular characteristics are not of major importance in the present study, since crystallographic interactions

Results and analyses

Since some morphologies are quite involved, this section presents first the results obtained with organic salts, and the structural analysis of the epitaxy is described in detail: indeed it is mostly on these substrates that a clear-cut heteroepitaxy is observed, “unspoiled” by the αiPP/αiPP homoepitaxy. This “basic” model once established is used to analyse more complex morphologies created as a result of the iPP/PTFE epitaxy, when further growth takes place in the melt and therefore involves

Conclusion

A novel mode of epitaxial crystallisation of iPP has been observed and analysed. The novel epitaxial relationship is generated by substrates which display in their exposed faces a linear grating of periodicity 5.5–5.6 Å. Representative examples of such gratings are the rows of chlorine atoms in the (100) plane of salts or hemiacids of para substituted benzoic acid, or the interchain distance of PTFE helices. This linear grating is matched in the (110)_αiPP contact plane by rows of methyl groups

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A novel epitaxy of isotactic polypropylene (α phase) on PTFE and organic substrates

Abstract

Introduction

Section snippets

Experimental

Results and analyses

Conclusion

Polymer

Polymer

Prog Polym Sci

Polymer

Prog Polym Sci

Polymer

Res Natl Bur Stand Sect A

J Appl Phys

J Polym Sci B: Polym Phys

Macromolecules

Nature

Macromolecules

J Appl Phys

J Polym Sci B: Polym Phys

Makromol Chem