Bernard Lotz

Phase structural formation and oscillation in polystyrene-block-polydimethylsiloxane thin films

The solvent-induced spherical structure in a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer was obtained and stabilized by preparing both the bulk and thin films from propylene glycol methyl ether acetate (PGMEA) solutions. The diblock copolymer possessed a total molecular weight of 42 kDa with a PS volume fraction of 72.2%, and it formed a cylindrical phase structure in the equilibrium bulk state. During thermal annealing, only changes in the sphere size and packing rearrangement were found. In contrast, a unique structure evolution route was observed during solvent treatments. Under a controlled vapour of a PS selective solvent, an oscillation of the structural transition between spheres and cylinders was observed in the thin films. The kinetics of this oscillation of structural transition was found to be closely related to the solvent vapour concentration and film thickness. This experiment revealed a unique ordering pathway towards the equilibrium structure in the thin film for this strongly segregated PS-b-PDMS diblock copolymer.

Crystal Polymorphism and Morphology of Polylactides

The crystal structures of poly(l-lactic acid) (PLLA), its stereocomplexes, and some features of its crystallization process are reviewed. PLLA exists in different crystal modifications that are based on two different left-handed helix geometries having three units in one turn (31) or ten units in three turns (103). The stable α-phase with a 103 helix exists in two variants: an ordered α-phase produced at high crystallization temperature and a less ordered α′-phase produced at low crystallization temperature. Two structures are based on the 31 helical conformation: an elusive γ-phase, obtained so far only by epitaxial crystallization, and a frustrated trigonal β-phase. The β-phase, first obtained in stretched fibers, could be an intermediate or precursor phase of the low crystallization temperature α′-PLLA variant, which would explain a number of its unusual crystallization features (increased growth rate, thicker lamellae, structural disorder). Stereocomplexes of PLLA and poly(d-lactic acid) (PDLA) are formed through intimate association of left- and right-handed threefold helices. They have remarkably higher melting temperatures than the homopolymers, which is explained by the presence of a dense network of weak CH…OC hydrogen bonds. Single crystals of PLLA are obtained both from solution and thin film growth. Spherulites of chiral polylactides are frequently made of twisted lamellae with a large pitch, with the sense of twist depending on the polylactide chirality.

Supramolecular Crystals and Crystallization with Nanosized Motifs of Giant Molecules

Supramolecular crystals and crystallization are general concepts used to describe broader aspects of ordered structures and their formation in the three-dimensional (3D) bulk and solution and in 2D thin film states at length scales ranging from sub-nanometers to sub-micrometers. Although the fundamental crystallographic principles are still held in these structures, starting from their basic repeating units (motifs), it is not necessary that each atomic position within their motifs possesses translational symmetry in long range order, but could have quasi-long range or short range order. As a result, the motif becomes the smallest unit for constructing 3D or 2D ordered structures that maintain the long range translational order. The formation of these supramolecular ordered structures essentially follows the physical principle of phase transformations, involving either nucleation and growth or spinodal decomposition mechanisms. However, larger ordered structures require stronger and more cooperative interactions to sustain their structures in equilibrium or stable states. We propose utilization of collective secondary interactions, similar to those found in biological and living systems, to generate sufficient interactions and stabilize these structures. Furthermore, when the basic unit of the structure becomes increasingly larger and heavier, thermal (density) fluctuations during the phase transitions may not be sufficiently large to overcome transition barriers of the basic unit. In these cases, external fields might be required to stimulate the magnitude of thermal (density) fluctuation and/or redistribute (thus, decrease) a single transition barrier into several stepwise transition sequences with lower barriers for each transition, and thus increase the speed of phase transformations.

UNUSUAL CRYSTALLIZATION AND MELTING PROCESSES OF AN OPTICALLY ACTIVE POLYOLEFIN: ISOTACTIC POLY((S)-4-METHYL-HEXENE-1)

Polyolefins that bear a chiral side chain (typically an isobutyl group) experience a so-called macromolecular amplification of chirality: the chiral side-chain induces a slight preference for either tg or tg- main chain conformation. This slight conformational bias is amplified cooperatively along the chain, and results in preferred chirality of the main chain helical conformations. As a result, these polymers display a liquid-crystal (LC) phase both in solution and, in the melt as a transient phase on the way to crystallization. The existence of two processes (melt-LC and LC-crystal transitions) results in unconventional behaviors that were first analyzed by Pino and collaborators back in 1975. These polymers also offer a means to test the structural consequences of recently introduced crystallization schemes. These schemes postulate the formation of a transient liquid-crystal phase as a general scheme for polymer crystallization.

Oriented Overgrowths of Poly(l-Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies

The crystallization behavior of an amorphous poly(l-lactide) (PLLA) layer deposited on uniaxially oriented isotactic polypropylene (iPP) substrate is been studied by atomic force microscopy (AFM) and electron microscopy combined with electron diffraction. A patterned PLLA structure with two fixed lamella and chain orientations is observed. Electron diffraction demonstrates that the major lamellar set is oriented with molecular chains perpendicular to the chain direction of the iPP. The minor lamellar set is inclined at ≈64° to both the iPP chain axis direction and the lamellae of the major set as judged from both the bright field electron micrograph and the AFM image. The orientation of the main set is explained in terms of soft epitaxy or graphoepitaxy, in which PLLA chains oriented parallel to the ditches of the iPP substrate caused by alternatively arranged crystalline and amorphous regions. The minor set is due to a homoepitaxy of PLLA with parallelism of the helical paths. The orientation of this minor set of lamellae therefore depends on and can help determine the chirality-l or d-of the PLA investigated.