Epitaxy of isotactic poly(1-butene): new substrates, impact and attempt at recognition of helix orientation in form I′ by AFM
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
References (27)
- et al.
Prog Polym Sci
(1988) - et al.
Prog Polym Sci
(1990) - et al.
Polymer
(1994) - et al.
Polymer
(1994) - et al.
Polymer
(2000) - et al.
Polymer
(2000) - et al.
Polymer
(1998) - et al.
Polymer
(2000) - et al.
Acta Cryst
(1994) - et al.
Nuovo Cimento Suppl
(1960)
Macromolecules
Macromolecules
Cited by (47)
Effect of crystalline transformation on supercritical CO<inf>2</inf> foaming and cell morphology of isotactic polybutene-1
2023, Journal of CO2 UtilizationMechanism of form I′ formation in polybutene-1/polypropylene blends
2022, PolymerCitation Excerpt :The spontaneous and irreversible phase transformation of form II to form I needs several days or even weeks to complete [21–23], and the transition is accompanied by adverse effects such as shrinkage and deformation of the objects due to change in the density, which has considerably hindered commercial application of the PB [24–27]. Over past decades, extensive efforts have been devoted to seeking methods to either accelerate the transition of form II to form I [28–32], or bypass the solid-state transition by producing form I′ directly [33–39]. It has been found that direct formation of form I′ crystals from the melt can be realized under specific conditions, such as crystallization in ultrathin films [34], on suitable substrates [33], in copolymers with other 1-alkenes [40–45], or in blends with isotactic polypropylene (iPP) [35–39,46,47].
Effects of molecular weight on polybutene-1 cold crystallization from polybutene-1/polypropylene blend
2019, PolymerCitation Excerpt :In order to overcome this practical problem, a lot of research efforts have been devoted to producing form I/I′ directly instead of form II, so that the undesirable solid-state transformation is avoided. It has been reported that stable form I′ modification can be produced directly by crystallization on suitable substrates [28], in ultrathin films [29], in copolymers with other 1-alkenes [30–36], and in blends with iPP [37–40]. Among these approaches, blending with iPP is particularly attractive because it is simple and of low cost.
The crystallization behavior of biodegradable polymer in thin film
2018, European Polymer JournalCitation Excerpt :The existence of the unique interaction usually results in the crystallization of a polymer on the substrate in an unexpected manner and therefore leads to the formation of unique crystalline structure and morphology of the crystallizing polymer, a phenomenon known as epitaxy. It has been well documented that polymer epitaxy could be realized on inorganic [30,123–127], organic [77,128–136], and polymeric substrates [38,39,51,137–160] from solution, melt, and vapor phases. Here we only focus on the epitaxial crystallization of biodegradable polymer.