J. C. Wittmann

Structure and morphology of poly(propylenes): a molecular analysis

Abstract The progress made in the understanding of the crystal polymorphism and morphology of isotactic and syndiotactic poly(propylene) (iPP and sPP) is reviewed. In 1960, the crystal structure of the stable form of iPP, the α phase, was already solved and the general principles of polyolefin chain conformation and crystallography established. The molecular analysis of the lamellar branching of αiPP—a case of homoepitaxy specific to this polymer and this crystal form—was given in 1986. The γ phase of iPP, solved in 1989, is the first and so far unique example of a polymer structure with non-parallel chain stems. The β phase is the first example of a frustrated structure in polymers, and rests on packing of isochiral helices in the unit-cell. The stable, high temperature form of sPP described in 1988 is based on full antichiral packing of helices, whereas the original structure corresponds to another limiting (but not experimentally observed in pure form) case based on packing of isochiral helices. Details of the molecular structure of iPP and sPP, such as the hand of individual helical stems and setting of chains, are accessible by crystallographic considerations which make use of molecular markers provided by the α-α and α-γ epitaxies in iPP or by direct visualization by atomic force microscopy.

Single crystals of γ phase isotactic polypropylene: combined diffraction and morphological support for a structure with non-parallel chains

Abstract Single crystals of the unusual γ phase of isotactic polypropylene have been produced by thin film growth. Their examination by selected area electron diffraction using a tilting stage enables the exploration of nearly the full reciprocal space of this crystal modification. The results fully support and provide an absolute test for a crystal structure proposed recently by Bruckner and Meille characterized by the coexistence in the unit cell of two sets of 31 helices with non-parallel axes. The structural relationship between α and γ phases makes it possible to determine the left- or right-handed chirality of all helices in the single crystals. The surface structure is investigated by the polymer decoration technique, and occasional α growth on γ crystals is described and analysed.

Phase II to phase I crystal transformation in polybutene-1 single crystals: a reinvestigation

The spontaneous phase II to phase I crystal-crystal transformation of polybutene-1 was investigated by electron diffraction and bright- and dark-field imaging of solution and thinfilm grown single crystals. Whole single crystals were observed to transform with a single phase II to phase I orientational relationship and, in the case of multiple orientations, the transformed areas were not dependent on growth sectors. These results do not support the views of Holland and Miller of a “twinned” phase II to I transformation determined by growth sectors but are consistent with a transformation scheme introduced by Fujiwara. Nucleation and growth of the transformation are further discussed.

Epitaxial crystallization and crystalline polymorphism of poly(1-butene): forms III and II

Abstract The three crystalline phases of isotactic poly(1-butene), which differ by their helix conformation and unit cell parameters, are obtained by bulk crystallization via epitaxy on appropriate organic substrates, in particular aromatic acids or salts. In this paper, the epitaxial relationships between forms III and II are established, based on composite electron diffraction patterns where possible. Form III epitaxy rests mainly on the matching of the chain axis repeat distance; form II is an original illustration of an epitaxy of irrational helices in which the interturn distance plays the main role.

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

Isotactic polypropylene in its a modification (alpha iPP) 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 alpha iPP homo- and hetero-epitaxies was (010). In spite of its complicated architecture (alternation of antichiral helices with different azimuthal settings), the (110)(alpha iPP) contact plane displays well defined, if not prominent, rows of methyl side chains parallel to the crystallographic (112) direction (at 57 degrees to the c-axis) and approximate to 5.5 Angstrom apart. The matching contact planes of the substrates display linear gratings made of rows of e.g. chlorine atoms or PTFE chains with similar approximate to 5.5 Angstrom 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 alpha iPP/PTFE heteroepitaxy, followed by alpha iPP/alpha iPP and gamma iPP/alpha iPP homoepitaxies

Low Tc growth transitions in isotactic polypropylene: β to α and α to smectic phases

SummaryTwo growth transitions are observed by optical microscopy in the low Tc range (105–80°C) of isotactic polypropylene (iPP): β to α and α to (probably) smectic phases. These transitions (notably β to α) may be the cause of the complicated annealing and recrystallization behaviour of β iPP, which is known to depend on the initial crystallization and post-crystallization thermal history.

Self-nucleation and enhanced nucleation of polyvinylidene fluoride (α-phase)

Abstract The nucleation phenomena of poly(vinylidene fluoride) in its α-phase (α-PVDF) are investigated. Self-seeding first investigated helps establish a so-called ‘efficiency scale’, which in turn helps rate the nucleation induced by crystalline additives, and notably PTFE. The thermal and nucleation behavior of α-PVDF is significantly dependent on its microstructure. Samples with higher head-to-head inversions display a significant ‘memory effect’: their initial spherulitic morphology is maintained in successive recrystallizations. The nucleation density can be increased by addition of nucleating agents: PTFE or flavanthrone. The nuclei densities reached are, however, nearly two orders of magnitude lower than for isotactic polypropylene. These results serve as a basis for a companion investigation of the nucleation of blends of PVDF and polyacrylic polymers, the structural characteristics and properties of which can be significantly modified by enhancing nucleation.

Impact of nucleating agents of PVDF on the crystallization of PVDF/PMMA blends

Abstract The enhanced nucleation of poly(vinylidene fluoride) in its blends with PMMA, and its impact on the crystallization kinetics and overall crystallinity of the PVDF/PMMA blends are investigated. Since in the blends the growth rates are significantly slowed down and spread over a wider crystallization range, nucleation has a significant impact on the crystallization process. In particular, crystallization peaks are shifted upwards by up to 30°C for nucleated samples. These higher crystallization temperatures make it possible to reach, upon cooling, high degrees of crystallinity for PVDF/PMMA compositions down to 50/50, i.e. for blends which remain amorphous when not nucleated.

Atomic force microscopy on epitaxially crystallized isotactic polypropylene

SummaryContact faces of thin films of α-phase isotactic polypropylene epitaxially crystallized on benzoic acid are examined by atomic force microscopy (AFM). The AFM images reveal the lamellar structure as well as the methyl side group pattern, thus enabling the discrimination between two structurally different contact surfaces in favour of the ‘four’ face type, with one methyl group per helix turn exposed. Furthermore it has been verified, that the observed helices are right-handed.

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

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.