Elsevier

Polymer

Volume 42, Issue 16, July 2001, Pages 7033-7047
Polymer

Epitaxy of isotactic poly(1-butene): new substrates, impact and attempt at recognition of helix orientation in form I′ by AFM

https://doi.org/10.1016/S0032-3861(01)00185-9Get rights and content

Abstract

Epitaxial crystallization of isotactic poly(1-butene) (iPBu) is performed on different substrates, which help expand the range of interactive substrates used in a previous study. All three crystal forms of iPBu1, which rest on different helix and unit-cell geometries and symmetries, have been produced by epitaxial crystallization. The different epitaxial interactions are discussed. Epitaxial crystallization of form I′ yields an exposed (110) contact plane. The films have true single crystal structure; they display electron diffraction patterns with sharp peaks, but also characteristic streaks due to the co-existence of up and down helices in the crystal structure (anticline helices). The streaks are modelled with a ‘Diffraction faulted’ program. Further, this same (110) contact face of Form I′ provides the potential to observe in direct space (i.e. by AFM) the up or down orientation of helices. Such a possibility would require differentiation by AFM of a methyl group from an ethyl group of the side chains. The AFM resolution reached in our investigation falls short of doing so, but the problematics could be adapted to other, more suitable polymers.

Introduction

Isotactic poly(1-Butene) (iPBu1) can exist in three different crystal phases which differ by the chain conformation and, as a result, the unit-cell geometry and symmetry. Form II is produced spontaneously on bulk crystallization; it has an 113 helix geometry and a tetragonal unit-cell. Form I is isostructural with Form I′; they have a 31 helix geometry and a trigonal unit-cell; Form I is obtained by spontaneous crystal–crystal transformation of Form II on ageing, whereas Form I′ is produced by direct crystallization. Form III, produced only from dilute solution, has a 41 helix conformation and an orthorhombic cell geometry [1].

Crystallization induced by epitaxy on specific substrates is a very powerful means to induce the various polymorphic forms of polymers [2]. When applied to iPBu1, epitaxial crystallization has made it possible to induce all three forms (II, I′ and III) from the melt [3], [4]. A major advantage of epitaxial crystallization is that it yields highly oriented or even single crystal orientations. This holds for crystal modifications which are unstable to mechanical shear, and cannot be oriented by other means. As an illustration, the crystal structure of Form III of iPBu1 could be reanalyzed on the basis of extensive electron diffraction data gathered from single crystals and epitaxially crystallized films [5].

In the present paper, we explore further various aspects of the epitaxial crystallization of iPBu1. We analyze the epitaxial relationships of yet different substrates and their versatility towards the crystal phases of iPBu1. We amend and complete the epitaxy rules of helical polymers on crystalline substrates, illustrated so far mainly with isotactic polypropylene (iPP) [6], [7]. iPBu1 provides indeed a second polymer for which the whole range of possible interactions are involved: chain axis repeat distance, interchain distance, distance between successive helical turns.

We also exploit the epitaxially crystallized films to analyze details of iPBu1 structure, and more specifically the relative helix sense in Form I or I′. As noted very early on by Natta and Corradini [8], up- and down-pointing helices of iPBu1 Form I are nearly isosteric, which makes it possible to substitute an up- by a down-pointing helix at any chain site (the two helices are defined as anticline rather than antiparallel, since conformational rather than chemical differences are at play; parallel helices are isocline or syncline). A structure based on isocline helices only has space group R3c; statistical half occupancy at each chain site of up- and down-pointing helices corresponds to space group symmetry R-3c. In these very thin films (≈10 nm), up-down chain orientation corresponds to a structural disorder, which generates streaks in the diffraction pattern [4]. These are analyzed with a modelization program.

Oriented films of Form I′ with their exposed (110) contact face provide also a material potentially suited to observe by atomic force microscopy (AFM) the relative orientation of individual helical stems in direct space. Such a visualization would be a further step in the ‘direct’ analysis of polymer crystal structures by AFM, following the observation of the pattern of methyl groups in the contact face of epitaxially crystallized iPP [9], [10], the visualization of the frustrated crystal structure of the β phase of iPP [11] and the observation in direct space of the helical hands (right and left) in syndiotactic polypropylene (sPP) [12]. As will be shown, discrimination between up- and down-pointing iPBu1 helices would require being able to distinguish by AFM a methyl group from an ethyl group in the contact face. Although our images fall slightly short of doing so, it is of interest to illustrate the concepts, and describe the experimental procedures and technical challenges encountered in this endeavour, which may be transposed to other, similar but better adapted systems.

Section snippets

Materials and experimental procedures

Isotactic poly(1-butene) is a low molecular weight material purchased from Aldrich. The nucleating agents used during this work are 4-chlorobenzoic acid (4ClBzAc), 4-bromobenzoic acid (4BrBzAc), 3-fluorobenzoic acid (3FlBzAc) and a nucleating agent patented for the β phase of isotactic polypropylene by New Japan Chemical [13], dicyclohexylterephthalamide (DCHT) of formula:

The experimental procedures are as described in several previous works and reviews [2], [3], [4]. It rests on the production

Results and discussion

The present section is organized in two parts: first, the results of epitaxial crystallization of iPBu1 on different crystalline substrates is presented. This work extends and complements an earlier investigation on the same theme [3], [4]. Next, the issue of helical sense in epitaxially crystallized films of form I′ is addressed, using two different approaches: analysis of the diffraction pattern, which provides a global approach of the disorder, and the more local approach made possible by

Conclusion

Epitaxial crystallization of isotactic poly(1-butene) and generation of its different crystal modifications has been further explored by investigating the impact of various low molecular weight nucleating agents which have been found to be efficient for other polymers, and notably for polyolefins. The new results confirm that all three crystal structures can be induced by appropriate nucleation additives. In particular, we have observed that one substrate—3-fluorobenzoic acid—is able to induce

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