epitaxial growth to address crystallization issues for P3HT. In particular, it presents
recent results relative to (1) the structure determination of P3HT by electron
diffraction analysis and (2) the impact of molecular weight (M w ) on the semicrystalline nanomorphology and structure of epitaxied P3HT thin films.
2 Epitaxy: Principle and Methods
2.1 Epitaxy of Semicrystalline Polymers: The Case
of Polyolefins
Epitaxy can be defined as the directed growth of a crystalline or semicrystalline
material on the surface of a crystalline substrate. As opposed to inorganic epitaxy,
which implies covalent bonding between the overlayer and the substrate, the
epitaxy of organic materials and polymers is governed by a subtle balance of
weak noncovalent interactions that is often difficult to predict [26, 27]. As a general
rule, the mutual orientation of the overlayer and the substrate can be described by a
2 Â 2 matrix relating the two-dimensional (2D) lattices of both overlayer and
substrate [26, 27]. One-dimensional (1D) epitaxy mechanisms, which imply lattice
matching along a unique preferred direction of the substrate lattice, are also often
encountered in the case of polymers, e.g., for polyethylene lamellae grown epitaxially on single-wall carbon nanotubes [28–30].
Polymer epitaxy on inorganic substrates was first reported in 1950 by Willems
and by Fischer [31, 32] for polyolefins. Wittmann and colleagues performed
systematic studies of polymer epitaxy in the early 1980s [33–35] using transmission
electron microscopy (TEM). Epitaxy was recognized as a major driving force that
could explain the efficiency of certain, albeit not all, aromatic crystals used as
nucleating agents for polyolefins [33–35]. Moreover, TEM proved to be a unique
tool for observing the characteristic structural and morphological features in
epitaxied polymer films and uncovering the underlying crystallization and epitaxial
growth mechanisms. The versatility of TEM lies in the possibility of operating the
microscope in different and highly complementary modes, e.g., conventional
bright field, electron diffraction, dark field, and high resolution (HR-TEM).
Accordingly, complementary informations in real and reciprocal spaces can be
combined for a precise understanding of structural issues at multiple length scales
(molecular ! mesoscopic), e.g., polymorphism, preferred orientation, and contact
planes of polymer crystals and the dimensions of crystalline and amorphous
domains. However, SCPs like most semicrystalline polymers are extremely beam
sensitive, which implies that one must work with low electron doses so as to
preserve the native structure of the polymer as much as possible, especially in
HR-TEM [36, 37].
An instructive example of polyolefin epitaxy concerns the growth of polyethylene (PE) lamellae on aromatic crystals of p-terphenyl and anthracene. On these two
Understanding the Structure and Crystallization of Regioregular. . .
85
recent results relative to (1) the structure determination of P3HT by electron
diffraction analysis and (2) the impact of molecular weight (M w ) on the semicrystalline nanomorphology and structure of epitaxied P3HT thin films.
2 Epitaxy: Principle and Methods
2.1 Epitaxy of Semicrystalline Polymers: The Case
of Polyolefins
Epitaxy can be defined as the directed growth of a crystalline or semicrystalline
material on the surface of a crystalline substrate. As opposed to inorganic epitaxy,
which implies covalent bonding between the overlayer and the substrate, the
epitaxy of organic materials and polymers is governed by a subtle balance of
weak noncovalent interactions that is often difficult to predict [26, 27]. As a general
rule, the mutual orientation of the overlayer and the substrate can be described by a
2 Â 2 matrix relating the two-dimensional (2D) lattices of both overlayer and
substrate [26, 27]. One-dimensional (1D) epitaxy mechanisms, which imply lattice
matching along a unique preferred direction of the substrate lattice, are also often
encountered in the case of polymers, e.g., for polyethylene lamellae grown epitaxially on single-wall carbon nanotubes [28–30].
Polymer epitaxy on inorganic substrates was first reported in 1950 by Willems
and by Fischer [31, 32] for polyolefins. Wittmann and colleagues performed
systematic studies of polymer epitaxy in the early 1980s [33–35] using transmission
electron microscopy (TEM). Epitaxy was recognized as a major driving force that
could explain the efficiency of certain, albeit not all, aromatic crystals used as
nucleating agents for polyolefins [33–35]. Moreover, TEM proved to be a unique
tool for observing the characteristic structural and morphological features in
epitaxied polymer films and uncovering the underlying crystallization and epitaxial
growth mechanisms. The versatility of TEM lies in the possibility of operating the
microscope in different and highly complementary modes, e.g., conventional
bright field, electron diffraction, dark field, and high resolution (HR-TEM).
Accordingly, complementary informations in real and reciprocal spaces can be
combined for a precise understanding of structural issues at multiple length scales
(molecular ! mesoscopic), e.g., polymorphism, preferred orientation, and contact
planes of polymer crystals and the dimensions of crystalline and amorphous
domains. However, SCPs like most semicrystalline polymers are extremely beam
sensitive, which implies that one must work with low electron doses so as to
preserve the native structure of the polymer as much as possible, especially in
HR-TEM [36, 37].
An instructive example of polyolefin epitaxy concerns the growth of polyethylene (PE) lamellae on aromatic crystals of p-terphenyl and anthracene. On these two
Understanding the Structure and Crystallization of Regioregular. . .
85