Session P48: Focus Session: Crystallization in Single-, Multicomponent, and Hybrid Systems II

Shear-induced homogeneous crystallisation in confined columns of polymer

In previous work, we have studied the crystallisation of a system of dewetted poly(ethylene oxide) (PEO) droplets. We have shown that this defect-free system nucleates homogeneously within the droplet volume. By capping this droplet system, a parallel plate geometry is obtained with the droplets forming isolated PEO columns (a capillary bridge) between the substrate and the cap. With this geometry we are able to investigate how the application of an oscillatory shear mediates the crystal nucleation step in small, defect-free volumes of material. These studies are typically carried out in bulk systems which are dominated by heterogeneous defects, here we will present our first results on the effect of shear on homogeneous nucleation.   

Chain Folding in Polymer Crystals Detected by Solid-state NMR

Polymer crystallization induces transitions from random coils in the melt states to bilayer structures consisting of chain-folded crystals and disordered amorphous regions. Although the concept of chain folding is well recognized, there have been continuous debates about adjacent re-entry fractions of polymer chains crystallized at different physical conditions. To understand chain-folding structures in complex systems, spatial selectivity to access short-range polymer-polymer interactions is necessary. In this talk, we will propose a novel approach using solid-state NMR and selective isotope labeling for characterizing chain-folding of polymer chains. Spatial selectivity in double quantum NMR reveals adjacent re-entry fractions in the bulk crystals at different conditions.   

Forming hierarchically ordered hybrid materials using polymer crystallization

Hierarchically ordered hybrid materials are of great interest for next generation advanced materials research. In this presentation, we will present our recent results on employing polymer single crystals (PSC) to template nanoparticle assembly, leading to hierarchically ordered hybrid materials. Tailor-made, free-standing NP frames and wires containing single or multiple types of NPs have been obtained by using an in-situ polymer crystallization method. End functionalized poly(ethylene oxide) and polycaprolactone single crystals were used as the templates. Gold and magnetite NPs were successfully patterned as evidenced by transmission electron microscopy experiments. Interaction between NPs, free polymer chain as well as PSC has been systematically monitored using Surface Plasmon Resonance. It has been found that NP-free polymer chain interaction holds the key to forming ordered NP patterns in solution.   

Understanding Molecular Epitaxial Mechanism of the $\gamma $-form Crystal and Chain Tilt in the $\alpha $-form Single Crystal of Isotactic Polypropylene

We attempt to investigate how the epitaxial domination of the crystal morphologies takes place in the $\gamma $-form of the chain-folded crystals using high molecular weight isotactic polypropylene (i-PP) samples with a controlled number of stereodefects. Two different morphologies were identified \textit{via} transmission electron and atomic force microscopies (TEM and AFM). One is needle-like and the other is ``flat''. Based on the tilted selected area electron diffraction (SAED) results from TEM, the microscopic formation mechanism of the ``needle'' and ``flat'' morphology was discussed and it revealed that in the ``flat'' $\gamma $-form crystal, the initial $\alpha $-form single crystal had to have a stem orientation tilted away from the thin film normal within the ac-plane around the b-axis. Elongated $\alpha _{2}$-form lath-like single crystals were grown from thin film melt at $T_{x}$ = 145 $^{\circ}$C -155 $^{\circ}$C using commercial sample. SAED experimental results show that the stems in these lath-like single crystals were tilted at an unusual 17$^{\circ}$ angle around the $b$-axis. This 17$^{\circ}$-stem tilt in the $\alpha _{2}$-form single crystals favors the ($10\bar {2})$ fold surface and appears to depend upon both conformational and chain folding constraints.   

Kinetic Control of the Halogen Distribution in Crystals of Precisely Substituted Polyethylenes

Isothermally crystallized polyethylenes with precise chlorine substitution on each and every 21, 19, 15 or 9 backbone carbon display a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a change in WAXD patterns, a sharp increase in melting temperature, an increase in TG conformers and a decrease in SAXS periodicity. These changes correlate with a different distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the Cl distribution in the crystallites is disordered, while slower crystallization rates favor intermolecular staggering of chlorines and a herringbone structure. The drastic change in morphology is enabled by the precise halogen placement in the chain and driven by the selection of the nucleus stem in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows strategies for generating nanostructures at the lamellar and sub-lamellar level not feasible in classical branched polyethylenes.   

On the Width of the Crystallization Process

Differential Scanning Calorimetry (DSC) is the most frequently used experimental techniques for the study of crystallization process in polymers and polymer-based nanocomposites. The experimental data are discussed within various theoretical approaches. Isothermal crystallization studies are typically discussed within the Avrami theory of phase change. However, the as recorded experimental data represent the time dependence of the derivative of the crystallinity degree (versus time) rather than the crystallinity degree. From the experimental point of view, the DSC parameters that are considered are the DSC peak position, the area under the DSC curve, and an empirical not very well defined onset temperature. However, all experimental data points are involved in the Avrami analysis after integration (in the Avrami analysis of the dependence representing the degree of crystallinity versus time). However, no effort has been done to understand the significance of the width of the as recorded DSC spectrum. A detailed analysis of the significance of the width of the DSC spectrum is presented. The connection between the width of the DSC spectrum and the exponent of the Avrami equation is analyzed in detail for the case in which nucleation is negligible. Tentatively, the as obtained results are extended to non-isothermal crystallization kinetics, recorded for various heating rates and to the Ozawa treatment of crystallization of polymers in non isothermal conditions.   

Crystallization and Ordering of Giant Molecules based on Nano-atoms

To create new functional materials for advanced technologies, control over their hierarchical structures and orders is vital for obtaining the desired properties. We utilized and functionalized fullerene (C60) and polyhedral oligomeric silsesquioxane (POSS), and assembled these particles with polymers to form those hierarchical structures. The structures of these assemblies along with the resulting ordered structures were analyzed to determine their structure-property relationships. One of the most illustrating examples is a series of novel giant surfactants and lipids possessing a well-defined amphiphilic head and polymeric tails. Various architectures of this class of materials have been constructed and their self-assembly processes in solution, in the condensed bulk and thin films have been investigated. Another set of examples are ``nano-atoms.'' These classes of molecules are designed to possess features of molecular Janus particles with various symmetry breakings. When specific interactions are introduced, these ``nano-atoms'': are functioned as building blocks to construct different amplified molecules and further to self-assemble into hierarchical ordered structures. Their thermodynamic phase diagrams and kinetic pathways are explored to understand this new class of materials and their potential applications in modern technologies.   

Morphologies in Semi-Crystalline Precise Acid- and Ion-Containing Polymers

For over a decade we have endeavored to quantify the morphologies in ion-containing polymers using a combination of X-ray scattering and electron microscopy methods. We are motivated by the promise that when we know the polymer physics connecting chemical structure, nanoscale morphology, and physical properties, new strategies for improving polymer properties will be evident. This is particularly challenging in semi-crystalline polymers. Recently, we have been exploring the morphologies in precise acid copolymers and in the ionomers made neutralizing these precise copolymers. The precise copolymers are synthesized by Prof. K. Wagener's group at the University of Florida using acyclic diene metathesis and have functional groups attached to every 9th, 15th or 21st carbon along a linear polyethylene molecule. The precision of these molecules gives rise to well-defined hierarchical structures including acid-decorated polyethylene lamellae and acid or ionic aggregates arranged on cubic lattices. The temperature dependence of these polyethylene-based copolymers is dominated by the crystallization of the polymer. Stretching these materials can induce anisotropy that has facilitated our determination of these hierarchical morphologies and plans are underway to initiate in situ tensile deformation and X-ray scattering to probe the dynamic morphology.   

Nanohybrid shish kebab paper: Crystal growth and film properties

Polyethylene single crystals were uniformly grown heterogeneously from carbon nanotubes (CNTs) in solution, forming the nanohybrid shish kebab (NHSK) structure. We demonstrate that highly uniform, free-standing nanohybrid buckypaper with high CNT contents (13-70\%) could be produced from vacuum-filtrated polymer single crystal-decorated CNTs. In this way, polymer crystals served as unique spacers for CNTs so that uniform hybrid buckypaper films could be obtained without CNT agglomeration. Wetting techniques, thermal analysis, and scanning electron microscopy were used to elucidate the effect of polymer single crystals on the resultant structure. Surface roughness of NHSK paper could be controlled by tuning the polymer single crystal size (CNT separation distance). Superhydrophobic NHSK papers were obtained with high surface adhesion, which mimics the rose petal effect. Conductivity of the NHSK papers also varied with polymer crystal size. Great enhancement of important properties could be achieved through the formation of ternary hybrids. To that end, initiated- and oxidative chemical vapor deposition methods extend NHSK buckypaper applicability by providing functional polymer surfaces. NHSK papers may find applications in sensors, electrochemical devices and coatings.   

Role of lamellar thickness in the kinetics of polymer crystal growth

The lamellar thickness of polymer crystals reflects their thermodynamic metastability and meanwhile decides the linear crystal growth rates on their lateral sides. The traditional theories about the growth kinetics of polymer crystals, like Lauritzen-Hoffman theory and Sadler-Gilmer theory, attributed the effect of lamellar thickness to the free energy barrier at the lateral growth front, in order to explain the slower growth of thicker crystals observed at higher temperatures. We studied the linear growth rates of flat-on-oriented polymer crystals in ultra-thin films, by means of dynamic Monte Carlo simulations of lattice polymers. We found that at the same temperatures, the thicker crystals are actually growing faster. The effect of lamellar thickness has no relation with the free energy barrier; rather, it is only related with the driving force for crystal growth. On the basis of the intramolecular crystal nucleation model, we discussed a reasonable microscopic image on the growth kinetics of polymer crystals.   

Isothermal crystallization kinetics of Poly (lactic acid) studied by ultrafast chip calorimeter

Poly (lactic acid) (PLA) is a biocompatible, biodegradable polymer which has attracted much attention. The crystallization ability, as one of the most factors influencing the physical properties of the biomaterials such as thermal, mechanical, and biodegradable properties, has been widely studied mainly by differential scanning calorimeters. However, although the crystallization of PLA is relatively slow, it's difficult to avoid the crystallization from the nuclei or the structure reorganization of the metastable crystalline formed during the annealing process when we use the normal DSC with the heating rate on the level of tens of K/min. With the chip calorimeter whose scanning rate can go up to 1000 K/s, we can avoid the structure reorganization of metastable crystalline during the heating. In this case we annealed the PLA sample in the 80-120$^{o}$C temperature range and found the relationship between the onset the melting temperature T$_{m}$ and crystallization temperature T$_{c}$ is T$_{m}$= 0.53T$_{c}$+ 213.5 and the equilibrium melting temperature is T$_{m,f }$=179.6$^{o}$C.