Session J49: Focus Session: Crystallization in Single-, Multicomponent, and Hybrid Systems I

Orientation of conjugated polymers: epitaxy versus mechanical rubbing

Orientation of conjugated polymers like regioregular poly(3-alkylthiophene)s (P3AT) is of high importance as it can be used to exploit their high intrinsic charge transport anisotropy in the elaboration of devices e.g. OFETs and OLEDs. Orientation has been achieved by two different approaches: i) epitaxy and ii) mechanical rubbing. Herein we describe and compare these two orientation methods. On the one hand side, a large palet of structures, nanomorphologies and orientations can be achieved by controlled epitaxial crystallization of polymers like P3HT, PBTTT or polyfluorenes. Not only can epitaxy afford highly oriented and crystalline films of P3ATs, but also highly oriented fibers with a characteristic shish-kebab morphology. The application of Electron Diffraction analysis (rotation-tilt) on highly oriented polymer layers is an original an powerfull method to unravel the crystal packing of conjugated polymers. On the other hand side, mechanical rubbing of conjugated polymers, especially P3HT can also lead to highly oriented films without the use of an orienting substrate. The mechanism of thin film orientation has been analyzed in detail using Transmission Electron Microscopy, Grazing-Incidence X-ray diffraction and optical spectroscopy. It is demonstrated that the molecular weight Mw of the polymer impacts the maximum orientation achieved by rubbing. The Mw-dependence of orientation is explained in terms of chain folding and entanglements that prevent the reorientation and reorganization of the pi-stacked chains, especially for Mw$\ge $50kDa. Electron diffraction and HR-TEM show that epitaxied and rubbed films differ in terms of \textit{intra-lamellar} order within layers of pi-stacked chains. Whereas the epitaxied P3HT films show a semi-crystalline structure with crystalline domains bearing 3D order, the rubbed P3HT films exhibit rather a 2D nematic-like order.   

Solution Assembly of Hybrid Poly (3-hexyl thiophene) and Cadmium Selenide Nanowires

Optimizing morphology of self-assembled systems containing both electron carrying (n-type) and hole carrying (p-type) materials holds promise for the fabrication of improved devices, such as solar cells. In this talk, two routes to formation of hybrid p-n composite fibrils consisting of~crystalline p-type poly(3-hexyl thiophene) (P3HT) nanowires with n-type cadmium selenide (CdSe) quantum dots and nanorods into well-defined structures will be discussed. The first method involves co-crystallization of freely soluble P3HT and P3HT-functionalized CdSe nanorods to form crystalline hybrid nanowires upon addition of a marginal solvent. Transmission electron microscopy reveals that nanorods preferentially orient parallel to and flank the sides of fibers. In a second route to forming hybrid materials, chain-end functionalized P3HT is crystallized into fibrillar nanowires. Introduction of nanoparticles promotes binding at the fibril edge, forming parallel composite pathways or ``superhighways.'' These assembly approaches represent efficient means to organization of conjugated polymers and semiconducting nanostructures, thus offering new opportunities for optoelectronic device design.   

Correlating crystallization and ionic conductivity of PEO/graphene oxide nanocomposite

Polyethylene oxide (PEO) is one of the best candidates for solid state electrolyte due to its chemical stability and strong ability to form complex with lithium salts. Crystallization behavior of PEO directly affects the lithium ion transport, and in turn the ionic conductivity of the electrolyte. By adding two dimensional graphene oxide nanosheets into PEO matrix, mechanical property of the latter is significantly strengthened, while the crystallization behavior of PEO is also altered by the graphene oxide sheets. The crystallization of PEO/graphene oxide nanocomposites was studied by differential scanning calorimetry (DSC) and the orientations of graphene oxide and PEO crystal were studied by small angle X-ray scattering and wide angle X-ray diffraction. PEO/graphene oxide nanocomposite doped with lithium salt was further fabricated and characterized by electrochemical impedance spectroscopy. Anisotropic ionic conductivity was observed for the nanocomposite electrolyte due to the orientation of graphene oxide and directional growth of PEO crystals.   

Nucleation and Crystallization in nucleated Polymers

Crystallization is commonly considered as nucleation followed by a growth process. Here we apply the recently developed technique, differential fast scanning calorimetry (DFSC), for a unique, new look at the crystal growth of poly(epsilon-caprolactone) (PCL) and PCL carbon nanotube composites from 185 K, below the glass transition temperature, to 330 K, close to the equilibrium melting temperature. The DFSC allows temperature control of the sample and determination of its heat capacity during temperature treatments by employing cooling and heating rates from 50 to 50,000 K/s. First, the crystal nucleation and overall crystallization half times were determined simultaneously in the range of temperatures where crystallization of PCL occurs. After attempting to analyze the experiments with the classical nucleation and growth model a new methodology is described, which addresses the specific problems of crystallization of flexible linear macromolecules. The structures seem to range from having practically unmeasurable latent heats of ordering (nuclei) to being clearly-recognizable, ordered species with rather sharp disordering endotherms at temperatures from the glass transition to equilibrium melting (increasingly perfect and larger crystals). The mechanisms and kinetics of growth (if any) involve a detailed understanding of the interaction with the surrounding rigid amorphous fraction (RAF) in dependence of crystal size and perfection. E. Zhuravlev, J.W.P. Schmelzer, B. Wunderlich and C. Schick, Kinetics of nucleation and crystallization in poly(epsilon-caprolactone) (PCL), Polymer 52 (2011) 1983-1997.   

Spherulite Growth in Polymer-Nanoparticle Blends

Blends of polymers with inorganic nanoparticles (NP) were studied by polarized optical and fluorescence microscopy. Silica nanoparticles with a range of diameters from 7 to 100 nm were used. Neat NPs as well as NPs surface-functionalized with a range of groups from strongly to weakly interacting, were blended with poly(ethylene oxide). A purpose-built T-jump microscopy cell was used allowing rapid temperature equilibration at high supercoolings. Lautitzen-Hoffman type analysis revealed that, although the NPs slow down the standard growth rate $G_0 $in the order PEO - Me-treated SiO2 - untreated SiO2 - COOH-treated SiO2 - NH2-treated SiO2, the surface free energy $\sigma $ decreases in the same order. This suggests that the NPs reduce macromolecular mobility, but at the same time help reduce the secondary nucleation barrier to some extent. Other polymers and NP types, including quantum dots, were also studied. The work also examines the spatial distribution of NPs in the spherulitic polymer nanocomposites.   

Synthesis and morphology of hydroxyapatite/polyethylene oxide nanocomposites with block copolymer compatibilized interfaces

In order to exploit the promise of polymer nanocomposites, special consideration should be given to component interfaces during synthesis and processing. Previous results from this group have shown that nanoparticles clustered into larger structures consistent with their native shape when the polymer matrix crystallinity was high. Therefore in this research, the nanoparticles are disguised from a highly-crystalline polymer matrix by cloaking them with a matrix-compatible block copolymer. Specifically, spherical and needle-shaped hydroxyapatite nanoparticles were synthesized using a block copolymer templating method. The block copolymer used, polyethylene oxide-b-polymethacrylic acid, remained on the nanoparticle surface following synthesis with the polyethylene oxide block exposed. These nanoparticles were subsequently added to a polyethylene oxide matrix using solution processing. Characterization of the nanocomposites indicated that the copolymer coating prevented the nanoparticles from assembling into ordered clusters and that the matrix crystallinity was decreased at a nanoparticle spacing of approximately 100 nm.   

Confined crystallization in compatibilized Polyamide 6/High Density Polyethylene blends

Blending polymers can be considered the easiest way to obtain new materials with tuned properties thanks to the possibility to control blend morphologies. The blend characteristics depend on the properties of each component, on composition and on morphologies developed during polymers processing. In case of semi-crystalline blended polymers, mechanical performances are closely related to the crystalline morphology. Therefore, it is essential that crystallinity is maintained after blending in order to keep or enhance the properties. This may be a challenge when the blends exhibit multiphase morphologies with sub-micrometer domain sizes. In this work, we study the crystallization behavior of compatibilized Polyamide 6/High Density Polyethylene (PA6/PE) blends by means of the Differential Scanning Calorimetry technique. Blends with various morphologies (dispersed, stretched dispersed, fibrillar and co-continuous) are obtained by reactive extrusion and varying blend composition and processing parameters. Blend composition and morphology turn out to greatly affect the bulk crystallization temperatures of both PA6 and PE. When the polymer is confined in domains of a few micrometers the crystallization temperature peak shifts to lower temperatures. Thus, the smaller the domain size the lower the crystallization temperature in case of dispersed morphologies. Moreover, in multi-scale morphologies showing polymer droplets in the nanometer range, fractionated crystallization (multiple crystallization peaks) is observed.   

Crystallization and Microphase Separation in Chiral Block Copolymers

Block copolymers composed of chiral entities, denoted as chiral block copolymers (BCP*s), were designed to fabricate helical architectures from self-assembly. A helical phase (denoted H*) was discovered in the self-assembly of poly(styrene)-$b$-poly(L-lactide) (PS-PLLA) BCPs*. To examine the phase behavior of the PS-PLLA, self-assembled superstructures resulting from the competition between crystallization and microphase separation of the PS-PLLA in solution were examined. A kinetically controlled process by changing non-solvent addition rate was utilized to control the BCP* self-assembly. Single-crystal lozenge lamellae were obtained by the slow self-assembly (i.e., slow non-solvent addition rate) of PS-PLLA whereas amorphous helical ribbon superstructures were obtained from the fast self-assembly (i.e., fast non-solvent addition rate). As a result, the formation of helical architectures from the self-assembly of the PS-PLLA reflects the impact of chirality on microphase separation, but the chiral effect might be overwhelmed by crystallization. Consequently, various crystalline PS-PLLA nanostructures in bulk were obtained by controlling the crystallization temperature of PLLA ($T_{c,PLLA})$ at which crystalline helices and crystalline cylinders occur while $T_{c,PLLA}\over {\smash{\scriptstyle=}\vphantom{_x}}$}} T_{g,PS}$, respectively. Anisotropic arrangement of the PLLA crystallites grown within the microdomains was identified. The formation of this exclusive crystalline growth is attributed to the spatial confinement effect for crystallization. While $T_{c,PLLA}\over {\smash{\scriptstyle=}\vphantom{_x}}$}} T_{g,PS}$, the preferential growth may modulate the curvature of microdomains by shifting the molecular chains to access the fast path for crystalline growth due to the increase in chain mobility. As a result, a spring-like behavior of the helical nanostructure can be driven by crystallization so as to dictate the transformation of helices and to result in crystalline cylinders.   

Chain conformation and crystallization in PEO / layered silicate nanocomposites

The polymer chain conformation under confinement and the polymer morphology are investigated in hydrophilic PEO/Na$^{+}$-montmorilonite nanohybrids synthesized by melt and/or solution intercalation. Intercalated hybrids with mono- and bi-layers of PEO chains are obtained for all compositions covering the complete range from pure polymer to pure clay. For low polymer concentrations, where all the polymer chains are intercalated, PEO is purely amorphous. As PEO concentration increases further, the polymer chains adsorbed on the outer surface of the clay particles remain amorphous as well. It is only when there is large amount of excess polymer outside the completely filled galleries that the bulk polymer crystallinity is abruptly recovered. The conformation of the confined or adsorbed polymer chains, as probed by Raman and FTIR spectroscopies, is found more disordered that the PEO melt even at higher temperatures; this is evident by the dramatic increase of the gauche conformations of the C-C bond along the chain backbone. Sponsored by the Greek GSRT ($\Sigma YNEP \Gamma A \Sigma IA$; $09\Sigma YN-42-580)$ and by the EU (CP-IP 246095-2).   

Lamellar Orientation Inversion under Dynamic Interplay between Simultaneous Crystallization and Phase Separation

Crystallization dynamics and lamellar orientation may be affected under the dynamic interplay between crystallization and phase separation for a two component system. If phase separation is really weak, lamellae grow in the radial direction within spherulites. However, when strong phase separation intervenes, the lamellae growth could be oriented in the tangential direction in a concentric alternating concentration ring pattern. This lamellar orientation inversion is reflected by a birefringence inversion under optical microscopy and will be illustrated by a study of the PEO/PMMA system.   

Phase separation of DMDBS from iPP, and controlled crystalline orientation

We report an unexpected dependence of DMDBS phase separation temperature on the molecular weight of the matrix isotactic polypropylene (iPP). DMDBS crystallizes out at lower temperatures for iPP with decreasing molecular weight (and correspondingly lower tacticity). This molecular weight dependence is unique to iPP, and is not observed for either syndiotactic PP or for random ethylene-PP copolymers. We show that thermodynamic Flory-type arguments are unable to rationalize the observed results. We also results on extrusion film casting of iPP containing DMDBS and show that flow-alignment of DMDBS networks template the orientation of PP crystals. The modulus and yield strength increase on addition of DMDBS, relative to the neat iPP. Tensile modulus and yield stress of drawn films increase with the degree of orientation, and we are able to achieve a substantial increase even at relatively low draw ratios.