Current projects
Semi-cristalline polymers (polyolefins, polyesters, nylons, etc.) represent one of the most
important classes of materials. In addition to the usual polymer characteristics, such as the chain
length and conformation, their properties depend upon their crystallinity, which is dictated by kinetic
and thermodynamic factors. We are particularly interested in their crystallization in thin and
ultrathin films (less than 100 nm in thickness) because nanotechnologies require this for new
applications (microelectronics, coatings, antireflective surfaces, electrochemical devices, photoresists,
sensors, etc.). New experimental techniques, such as AFM microscopy, give access to this information.
In ultrathin films, crystallization occurs in 2-D instead of 3-D because the film is of the order of the
radius of gyration of the polymer chain.
Polylactides exist in two different isotactic forms : poly(L-lactide) (PLLA) and
poly(D-lactide) (PDLA) – two optically active polymers. During the crystallization of
ultrathin films, polylactides give edge-on lamellae that crystallize with a curvature
having an “S” shape with PLLA and a “Z” shape with PDLA. When polylactide
crystallizes in the bulk (with a thickness of several microns), the curvature of edge-on
lamellae in thin films is expressed in the twisting of crystalline lamellae in spherulites.
Therefore, the different polymer organizational levels can be related by the concept of
chirality transfer : two molecules having different configurations give rise in solution
to different conformations (right-handed and left-handed helices), which in turn lead to edge-on lamellae that curve in opposite directions
(lamellar level) and, finally, to lamellae with right hand-side or left hand-side rotation (spherulitic level).
PLLA and PDLA can also form stereocomplexes, i.e. a new
crystalline structure that melts at about 240oC as compared to 180oC for
pure PLLA (or pure PDLA). In ultrathin films, equimolar
stereocomplexes give regular and hexagonal single crystals whereas,
below 30 nm, they become dendritic. However, in non-equimolar PLLA/PDLA blends, single crystals become triangular; the dendritic
branches of these crystals bend clockwise if PLLA is the major component and anti-clockwise if PDLA is the major component. Current
studies aim at understanding this phenomenon which depends on the film thickness, blend composition and crystallization temperature.
More generally, we study the crystallization and morphology of polymer blends in
thick and thin films. For miscible polycaprolactone/polyvinyl chloride (PCL/PVC) thick films,
two rates of crystallization and two different morphologies are observed, phenomena that do
not show up in the crystallization of pure PCL. The first has a classical spherulitic morphology;
the second morphology is weakly birefringent and composed of flat-on lamellae. We have
established that the spherulites are formed in the bulk of the material, whereas the second
morphology is located at the film surface. Differences in crystallization kinetics explain, at least
in part, the PCL enrichment at the film surface (as shown by XPS spectroscopy), but the exact
nature of the morphologies remain to be elucidated.
Finally, we are also studying block copolymers (with Prof. C.G. Bazuin), multiphase systems structured at the nanometric level by
self-assembly. With a poly(styrene)/poly(vinyl pyridine) (poly(S-b-VP)) copolymer, well-ordered nodules, often incorporating a removable
small molecules (SM), are obtained. The SM/PVP interactions control the formation of
vertical nodules or horizontal stripes (with dimensions of a few tens of nm). For
potential biological applications, poly(lactide-b-2-dimethylaminoethyl methacrylates)
(poly(LA-DMAE)) were synthesized. The first block of this copolymer is semi-
crystalline, biodegradable and biocompatible; the second one is water soluble, has
tertiary amine moieties and can complex functional molecules by supramolecular
chemistry. The presence of a semi-crystalline block leads to the stabilization of the
morphology due to its tendency to crystallize; on the other hand, the second block,
which is amorphous, imposes constraints that we are now exploring.
Français
English
important classes of materials. In addition to the usual polymer characteristics, such as the chain
length and conformation, their properties depend upon their crystallinity, which is dictated by kinetic
and thermodynamic factors. We are particularly interested in their crystallization in thin and
ultrathin films (less than 100 nm in thickness) because nanotechnologies require this for new
applications (microelectronics, coatings, antireflective surfaces, electrochemical devices, photoresists,
sensors, etc.). New experimental techniques, such as AFM microscopy, give access to this information.
In ultrathin films, crystallization occurs in 2-D instead of 3-D because the film is of the order of the
radius of gyration of the polymer chain.
Polylactides exist in two different isotactic forms : poly(L-lactide) (PLLA) and
poly(D-lactide) (PDLA) – two optically active polymers. During the crystallization of
ultrathin films, polylactides give edge-on lamellae that crystallize with a curvature
having an “S” shape with PLLA and a “Z” shape with PDLA. When polylactide
crystallizes in the bulk (with a thickness of several microns), the curvature of edge-on
lamellae in thin films is expressed in the twisting of crystalline lamellae in spherulites.
Therefore, the different polymer organizational levels can be related by the concept of
chirality transfer : two molecules having different configurations give rise in solution
to different conformations (right-handed and left-handed helices), which in turn lead to edge-on lamellae that curve in opposite directions
(lamellar level) and, finally, to lamellae with right hand-side or left hand-side rotation (spherulitic level).
PLLA and PDLA can also form stereocomplexes, i.e. a new
crystalline structure that melts at about 240oC as compared to 180oC for
pure PLLA (or pure PDLA). In ultrathin films, equimolar
stereocomplexes give regular and hexagonal single crystals whereas,
below 30 nm, they become dendritic. However, in non-equimolar PLLA/PDLA blends, single crystals become triangular; the dendritic
branches of these crystals bend clockwise if PLLA is the major component and anti-clockwise if PDLA is the major component. Current
studies aim at understanding this phenomenon which depends on the film thickness, blend composition and crystallization temperature.
More generally, we study the crystallization and morphology of polymer blends in
thick and thin films. For miscible polycaprolactone/polyvinyl chloride (PCL/PVC) thick films,
two rates of crystallization and two different morphologies are observed, phenomena that do
not show up in the crystallization of pure PCL. The first has a classical spherulitic morphology;
the second morphology is weakly birefringent and composed of flat-on lamellae. We have
established that the spherulites are formed in the bulk of the material, whereas the second
morphology is located at the film surface. Differences in crystallization kinetics explain, at least
in part, the PCL enrichment at the film surface (as shown by XPS spectroscopy), but the exact
nature of the morphologies remain to be elucidated.
Finally, we are also studying block copolymers (with Prof. C.G. Bazuin), multiphase systems structured at the nanometric level by
self-assembly. With a poly(styrene)/poly(vinyl pyridine) (poly(S-b-VP)) copolymer, well-ordered nodules, often incorporating a removable
small molecules (SM), are obtained. The SM/PVP interactions control the formation of
vertical nodules or horizontal stripes (with dimensions of a few tens of nm). For
potential biological applications, poly(lactide-b-2-dimethylaminoethyl methacrylates)
(poly(LA-DMAE)) were synthesized. The first block of this copolymer is semi-
crystalline, biodegradable and biocompatible; the second one is water soluble, has
tertiary amine moieties and can complex functional molecules by supramolecular
chemistry. The presence of a semi-crystalline block leads to the stabilization of the
morphology due to its tendency to crystallize; on the other hand, the second block,
which is amorphous, imposes constraints that we are now exploring.
Français
English