Session L25: Semi-crystalline Polymers

Polymer Single Crystals as 2D Templates

Polymer single crystals grown from dilute solution provide a novel approach to studying polymer physics. Because these crystals are lamellar in shape, their fold surfaces provide a platform from which to study two-dimensionally ordered structures. By coupling other polymers (forming multiblock copolymers) or inorganic particles (forming organic/inorganic hybrids) to the crystallizable block, it is possible to analyze the inherent physics of confining this assembly to 2D space and to provide an opportunity to use the crystal as a template. These non-crystallizable components are excluded from the single crystal lattice, tethering them to both surfaces. This control over the 2D assembly of these materials allows us to study materials for various biological, optical, and electrical applications.   

Interfacial and confinement effects to the structure of nylon 6 /clay nanocomposites made by chaotic flow.

The structure of polymers within nanocomposites is strongly affected by the confinement of the polymer to the interface with the nanoparticles and the method of blending. In nanocomposites of Nylon 6 and clay particles are made by chaotic blending, the strength of the chaotic flow affects the internal structure on multiple length scales, where the local structure is determined by the interfacial effects between the polymer and the nano-particle. The structural evolution as a results in alternating layers of nylon and nylon/clay regions forming two distinct interfaces, that with the clay and that of the pure nylon and the composite. The structure has been studied by X-ray, AFM and TEM at different chaotic blending strengths. At the interface with the clay particles, the polymer chains orient perpendicular to the interface. While the alpha crystalline form dominated the structure of the polymer in melts, the confinement to the layers results in an enhanced gamma form where hydrogen bonds form in between parallel nylon 6 chains dominate.   

WAXS investigations on Polyethylene -- Carbon Nanofibers Composites

Nanocomposites have been obtained by high-shear mixing of isotactic polyethylene with various amounts of purified nanofiller (vapor grown carbon nanofibers type PR-24AG from Pyrograf Products, Inc) by utilizing a HAAKE Rheomix at 65 rpm and 180 $^{\circ}$C for 9 min followed by an additional mixing at 90 rpm for 5 min. Composites loaded with various amounts of vapor grown carbon nanofibers have been prepared. Various spectroscopic techniques have been used to assess the interactions between the polymeric matrix and carbon nanofibers. Wide angle X - Ray scattering investigations focused on the effect of carbon nanofibers on the crystalline phases of polypropylene and on the overall crystallinity degree of the polymeric matrix. This research aims at a better understanding of the nature and structure of the polymer -- carbon nanofibers interface.   

Crystallization of Propylene-Hexene Random Copolymer

Time resolved small- and wide-angle x-ray scattering (SAXS/WAXS) were used to study the crystallization behavior of propylene-hexene random copolymer containing non-crystallizable hexene segments and crystallizable propylene segments. It was found that the copolymer would follow two crystallization paths depending on temperature, resulting in two distinct crystalline structures. At high crystallization temperatures (e.g. $100^{\circ}\mathrm{C}$), the combined effects of phase separation and high chain mobility greatly enhance the formation of lamellar structure consisting of the alpha-form of isotactic polypropylene (iPP) crystal. At low temperature (e.g. $40^{\circ}\mathrm{C}$), the lack of phase separation and the low chain mobility mainly result in the formation of fringe-micelle structure also with the alpha-form of iPP.   

Probing the crystallisation of polyethylene confined to a system of droplets

We present results on the crystallisation of polyethylene (PE) confined to a system of dewetted droplets. With the droplet system, we have access to a large ensemble of small, isolated volumes of crystallisable material, allowing for a direct measurement of nucleation rates. In our previous work with dewetted droplets of poly(ethylene oxide), we were able to demonstrate that long chains showed the same nucleation behaviour as chains roughly an order of magnitude shorter[1]. In contrast to this, it has been shown that for systems of n-alkanes molecular weight plays a role[2]. By investigating a wide range in molecular weight, the dependence of nucleation on molecular weight will be addressed in PE. [1] M.V. Massa et al., Phys. Rev. Lett. \textbf{97}, 247802 (2006). [2] H. Kraack et al., Macromolecules, \textbf{33}, 6174 (2000).   

Crystalline Morphology of Propylene 1-Octene Random Copolymers

The morphology of isotactic propylene 1-octene random copolymers has been studied by AFM, DSC, WAXS, and FTIR in an octene range of 10-20 mol {\%}. Different morphologies were observed below and above 15 mol {\%}. The morphological components in the higher counit copolymers are not of the lamellae-type, thicker than lamellae observed below 15 mol {\%}, connected and isotropic in their orientation. Their global morphology is developed via nucleation and growth (NG) of spherulitic aggregates. The evolution of heat of fusion with time is also sigmoidal shape, typical of NG-type crystallization mechanism. WAXS diffractograms for the higher counit copolymers are devoid of crystalline reflections, except for small and broad peaks suggesting mesomorphic-like structures, which by FTIR show small contents of the 840 cm$^{-1}$, 12 and higher units regularity bands, and hence formed of short helical sequences. The PO morphology is additionally compared with copolymers with ethylene, 1-butene and 1-hexene counits at matched contents.   

Tailoring the Properties of Poly(ethylene terephthalate) without Addition of Fillers via Solid-State Shear Pulverization

We demonstrate the ability to very strongly tune the physical and mechanical properties of poly(ethylene terephthalate) (PET) by changing the processing conditions of neat PET during solid-state shear pulverization without addition of any fillers or nucleating agents. Using differential scanning calorimetry, we observe a roughly factor of 3 increase in crystallinity of PET that has been pulverized and subsequently melted relative to the unprocessed PET. We also observe a dramatic increase in the rate of crystallization of the pulverized samples. Rheological characterization has demonstrated an increase in viscosity of the pulverized material, which can be ascribed to chain branching in the pulverized product. We also observe significant reductions in the oxygen permeability of the PET with pulverization as well as enhancements in mechanical properties that are commensurate with the modified crystallization properties of the pulverized PET.   

Crystallization of Bromine Substituted Polyethylenes with Precise Placement or Random Distribution

The crystalline properties of a series of bromine containing polyethylenes (PEs) with either a random or an exact placement of the Br atom on each and every 21$^{st}$, 19$^{th}$, 15$^{th}$ and 9$^{th}$ backbone carbon have been studied by DSC, NMR, Raman spectroscopy, WAXS and SAXS. Taking into account a larger strain due to the size of the Br atom, the crystallization behavior is analogous to Cl substituted PEs. While all precision Br-PEs crystallize as homopolymers as demonstrated by, 1. DDMAS solid-state $^{13}$C NMR spectra; 2. development of relatively large crystal thickness, and 3. sharp crystallization and melting peaks, the development of the crystalline state in random analogues is led by selection of most crystalline sequences indicated by broad thermal transitions, and lower crystallinities. WAXS patterns are unique for each type of substitution. For precision Br-PEs WAXS contain reflections corresponding to planes containing the Br atoms which are tilted 35$^{\circ}$ in reference to the chain axis. Due to the accommodation of the large Br atom, the crystals of all precisely substituted Br-PEs studied are conformationally disordered as observed by their Raman and NMR spectra. In contrast, crystals from random analogues display negligible conformational disorder.   

Molecular and Crystalline Microstructure of Ferroelectric Poly(vinylidene fluoride-\textit{co}-trifluoroethylene) Ultrathin Films on Bare and Self-Assembled Monolayer-Modified Au Substrates

There has been much interest in polymorphic crystal structures and ferroelectric properties in polymer materials, as one way of an application for organic memory device. We investigate the molecular and microdomain structure of Poly vinylidene fluroride-\textit{co}-trifluoroethylene (P(VDF-TrFE)) thin films spin-coated on bare and self-assembled monolayers (SAMs)-modified Au substrates. The two types of films display similar crystal morphologies with edge-on needlelike crystalline microdomains. They have, however, a different structure depending on the substrate. When the films are deposited on a bare Au surface, the films preferentially have a (110) contact plane with the substrate but a (100) contact plane when deposited on the Au surface modified by SAMs. The polar $b-$axis, along which the ferroelectric polarization is oriented, is therefore tilted to the film (and substrate) surface normal at 30 and 90\r{ }, respectively. In particular, the orientation of the polar $b$-axis tilted at some 90\r{ } to the normal of the polymer films on a CH$_{3}$ terminated SAM modified Au surface explains the smaller remanent polarization at low initial electrical bias.   

Chirality Information Transfer in Polylactides: From Main-Chain Chirality to Lamella Curvature

The behaviour of ultrathin polymer films is very different from that in the bulk phase. In this work, the crystallization of poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA) was followed using in situ atomic force microscopy over a broad range of temperatures and thicknesses. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which can be related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behaviour was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions without any external nucleation. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behaviour. Moreover, the rotating direction of those lamellae can be directly linked with the sense of twisting of the poly(lactide)s lamellae in banded spherulites. Those observations can lead to a model where the curved crystals in ultrathin films can be considered as half-lamellae, which, when associated together, give twisted complete lamellae.   

Effects of Vitamin E on the Oxidative Reaction of Free Radicals in Ultra-High Molecular Weight Polyethylene

Free radicals in gamma- or x-irradiated ultra-high molecular weight polyethylene (UHMWPE) are investigated as a function of vitamin E (alpha-tocopherol ($\alpha $-T)). $\alpha $-T is mixed with UHMWPE (GUR 1020) powder (e-PE) before (premix) or after (post-mix) irradiation. Pre-mix powder is also compression-molded (CM) to solid pucks (1'' thick and 2.5'' dia.) at 200$^{o}$C under constant force of 20-40 kN. Free radicals are detected using an X-band electron spin resonance (ESR) spectrometer, and oxidation index (OI) (1720 cm$^{-1})$ by FTIR technique. As expected, no measurable OI is detected by FTIR and thus e-PE suffers no loss in its mechanical properties. ESR data, however, suggest that $\alpha $-T quenches polyethylene radicals during and/or immediately after irradiation, but it does not have any effect on the long-term oxidative reaction. The difference between the pre- and post-mix powder is apparent only at the initial stage, and the terminal oxygen-induced radicals (OIR) are produced in all irradiated samples. Both pre- and post-mix powders are found to have equal amount of residual $\alpha $-T radical (tocopheroxyl).   

Self-Assembly and Chain-Folding in Hybrid Coil-Coil-Cube Triblock Oligomers of Polyethylene-b-Poly(ethylene oxide)-b-Polyhedral Oligomeric Silsesquioxane

Self-assembly and chain-folding in well-defined oligomeric polyethylene-block-poly(ethylene oxide)-block-polyhedral oligomeric silsesquioxane (PE-b-PEO-b-POSS) triblock molecules were studied. The triblock oligomers were synthesized by attaching two kinds of functional POSS molecules to a hydroxyl-terminated PE-b-PEO diblock oligomer. In the crystalline state, PE chains tilted 32 degrees from the lamellar normal, and both Ib-POSS and Cp-POSS molecules stacked into four-layer (ABCA) lamellar crystals, having the same trigonal symmetry as in pure POSS crystals. Because the cross-sectional area for a PE chain in the PE crystals (0.216 nm2/chain) at the interface was much smaller than that for a POSS molecule in POSS crystals (1.136 nm2/molecule), the self-assembly and PE chain-folding were substantially affected by the sequence of PE and POSS crystallization when crystallizing from the melt. This study indicated that confinement effect plays an important role on chain-folding of crystalline block oligomers.