Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S18: Semi-Crystalline Polymers |
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Sponsoring Units: DPOLY Chair: Vahik Krikorian, Massachusetts Institute of Technology Room: Colorado Convention Center 103 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S18.00001: Tuning Surface and Interface Properties Through Crystal Engineering Stephen Z. D. Cheng, Ryan Van Horn, Wenbin Zhang Surfaces and interfaces are critical for polymer adhesion, wettability, and barrier properties. Therefore, understanding and controlling surface and interface properties has wide ranging impact on a number of emerging fields. To more precisely control these properties, their molecular origin, both chemical and geometrical, need to be elucidated, but the correlation of surface properties to molecular characteristics is inherently difficult due to their 2-D nature. Single crystals can create the well defined surfaces needed to illuminate these relationships. The folds on the crystal surface provide a directional grain. Diblock co-polymers allow the chemical composition of surfaces to be changed and the topology of the surface to be precisely varied by changing the tethering density of the chains. Even the organic/inorganic interface properties can be investigated by attaching crystallizable chains to inorganic particles and utilizing the crystallization to force the 2-D assembly of the particles. In all, single crystals are a versatile way to investigate surface and interface properties. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S18.00002: Temperature Effects on Interlamellar Chain Entanglement and Structural Changes in Isotactic Polypropylene during Uniaxial Tensile Deformation Benjamin Hsiao, Feng Zuo, Jongkahk Keum, Xuming Chen, Hongyu Chen, Jing Li \textit{In-situ} small-angle x-ray scattering, wide-angle x-ray diffraction and \textit{ex-situ} atomic force microscopy techniques were carried out to investigate the structural and morphological changes of isotactic polypropylene (iPP) films during uniaxial tensile deformation at varying temperatures (i.e., room temperature, 60$^{\circ}$C and 160$^{\circ}$C). The mass fractions of amorphous, mesomorphic, and crystal phases were determined. Results indicate that at room temperature, the dominant deformation-induced transition occurs between the crystal and mesomorphic phases, while at high temperature ($>$ 60$^{\circ}$C), the dominant transition occurs between amorphous and crystal (i.e., monoclinic $\alpha $-form) phases. This behavior can be explained by the concept of chain mobility, and the relative strength between the interlamellar entangled amorphous network and the surrounding crystal lamellae.~ It appears that at low temperatures, the interlamellar entanglement network is stronger than the adjacent lamellae, resulting in lamellar fragmentation and formation of oriented mesomorphic phase.~ In contrast, an opposite scenario occurs at high temperatures, resulting in disentanglement of amorphous chains and the growth of crystalline lamellae. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S18.00003: Influence of Stereotacticity Defects on the crystallization of Isotactic Polypropylene Xiaofeng Chen, Rahmi Ozisik, Sanat K. Kumar, Phillip Choi, Wayne L. Mattice Monte Carlo simulations of coarse-grained polypropylene on a high coordination lattice were performed to investigate the ``equilibrium'' crystallization behavior of isotactic polypropylene with and without stereotacticity defects at various concentrations and distributions. The formation of the helical structure, which forms the basic crystalline unit for isotactic polypropylene, is the focus of the current study. Results indicate that the effect of stereo defects is local - limited to two nearest neighboring repeat units on each side of the stereo defect. In addition, the influence of many stereo defects on the formation of helices is the sum of the influences of each individual stereo defect. The presence of stereo defects retards the crystallization temperature compared to isotactic polypropylene with no defects. This is expected as higher undercooling is necessary to form helices with imperfections. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S18.00004: Polymer crystallization enabled carbon nanotube functionalization Christopher Li, Lingyu Li, Bing Li, Cristin Yavorsky CNTs were periodically decorated with polymer lamellar crystals using both controlled solution crystallization and physical vapor deposition methods, resulting in nano hybrid shish-kebab (NHSK) structures. The periodicity of the polymer lamellae varies from 20 - 100 nm. The kebabs are approximately 5 nm thick with a lateral size of ~ 20 nm to micrometers, which can be readily controlled by varying the crystallization conditions. Both polyethylene and Nylon 6,6 have been successfully decorated on multi-walled as well as single-walled CNTs. The detailed formation mechanism was attributed to size dependent soft epitaxy. Since the polymer kebabs can be easily removed, these unique NHSKs can serve as templates to fabricate a variety of CNTs-containing hybrid materials with controlled pattering on the CNT surface. This method thus opens a gateway to periodical patterning on CNTs or similar 1D nanowires in an ordered and controlled manner. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S18.00005: A Second Harmonic Generation Study of Polyethylene Crystallization Howard Wang, Narayan Ch Das, Hongtao Bian, Yuan Guo, Hongfei Wang In situ optical second harmonic generation (SHG) measurements have been applied to studying kinetics of isothermal crystallization in polyethylene melts. The degree of crystallinity is revealed through correlating to the SHG intensity. The kinetics data at various degree of undercooling are analyzed using the standard Avrami equation. The detection sensitivity, error sources, advantages and limitations of SHG for studying polymer crystallization are discussed. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S18.00006: Spectroscopically Deciphering the Difference in Stabilizing Interactions of Poly(lactic acid) Polymorphs Kaoru Aou, Xiguo Zeng, Shaw Ling Hsu Poly(lactic acid), or PLA, crystals have been difficult to analyze, owing to uncertainties in the actual atomic placements. Our previous studies indicate that the stereocomplex has a melting enthalpy of 129 J/g in comparison to 99 J/g for the alpha crystal. The enthalpic interaction is thus responsible for the stereocomplex thermal stability. Here we use spectroscopy to complement published atomic coordinate information to better understand the origins of PLA crystal stability. Spectroscopic analysis shows that methyl-related vibrations change dramatically as the alpha crystal unit cell thermally shrink, whereas the stereocomplex vibrations were almost unchanged. Indeed the closest (CH$_{2})$H...H(H$_{2}$C) distances in the alpha crystals are shorter than that in the stereocomplex crystal indicating that methyl-methyl interactions have a larger impact on alpha crystal stability. We also performed energy calculations on the alpha and stereocomplex structures from which we find the relative contribution of van der Waals and electrostatic interactions in the two crystals. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S18.00007: Effects of Confinement on the Crystallization of Perfectly Linear Polyethylene Sasha Myers, Richard Register The crystallization behavior of diblock copolymers of perfectly linear polyethylene (LPE) and polyvinylcyclohexane (PVCH) is evaluated in a variety of morphologies, with LPE forming either the matrix or the discrete domains. Because the glass transition temperature of PVCH is higher than the crystallization temperature of LPE, the mesoscale morphology is frozen upon cooling, confining crystallization within or around glassy microdomains. Previous work on confined polyethylene crystallization has employed hydrogenated polybutadiene (hPBD) as the ``polyethylene'' block, but in that case, the ethyl branch defects in hPBD control the crystal thickness and crystallinity rather than confinement. For LPE confined within spheres and cylinders, crystal thickness is limited by microdomain size and the degree of crystallinity is reduced. Lamellar morphologies impose lesser limitations on LPE crystallization, because the crystals stack orthogonally to the lamellar microdomains. Crystal thicknesses inferred from melting point depression and measured directly by SAXS on flow-aligned lamellar samples are in good agreement. Maximum thickness is influenced by both diblock structure and thermal history. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S18.00008: AFM Study of the Beta to Alpha Transition in Isotactic Polypropylene Jerold Schultz, Huihui Li, Xiaoli Sun, Shouke Yan $\beta -$iPP cylindrites were produced by introducing an iPP fiber into the iPP melt at about 175 \r{ }C, followed by crystallization in the range 100 $\sim $ 140 \r{ }C. Before $\beta $-phase crystallization has completed, the material is upquenched above 140 \r{ }C and crystallization allowed to continue isothermally in the $\alpha $-phase. Following the completion of crystallization, morphological details of the $\beta \to \alpha $ interfacial region have been studied by means of PLM and SEM. Through SEM observation, there are two seemingly different transformation morphologies, i.e., a fan-shaped transition, and a profuse transformation. They in fact arise from a single phenomenon, i.e., the development of multitudinous fingers at the growth face of multilayer $\beta $-lamellae. When viewed from different directions, one observes either $\alpha $-fans or the profuse initiation of $\alpha $ crystals in the transition region. Further, the fingerlike lathes are found to be curled cylindrically. It is suggested that the shrinkage deriving from the change in chain packing is causal for the fingering phenomenon, and a simple mechanism is presented to illustrate the curling phenomenon of the $\alpha $ crystals. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S18.00009: An investigation of the effect of processing conditions on the lamellar and spherulitic morphology of polyhydroxyalkanoates Yuping Xie, Yvonne A. Akpalu Polyhydroxyalkanoates (PHAs) have recently attracted much interest because of their biodegradability and biocompatibility. Since the ultimate properties of polymers can be controlled by processing conditions, particularly cooling rates, the systematic and thorough understanding of the effects of cooling rates on the final morphology and the resulting mechanical properties of PHAs is necessary and important. In this presentation, the lamellar (tens of nanometers), fibrillar (several hundred nanometers) and spherulitic ($\sim \quad \mu $m) morphologies of poly (3-hydroxybutyric acid) (PHB) and the copolymer poly (3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) crystallized under different cooling rates were studied using small angle X-ray scattering, ultra small angle X-ray scattering, and polarized optical microscopy, respectively. The morphology was observed to depend strongly on cooling rate. The influence of cooling rate on the morphology and mechanical properties such as toughness, tensile strength and overall stress-strain behavior will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S18.00010: Confinement effects in polymer crystal nucleation from the bulk to ``few-chain'' systems Kari Dalnoki-Veress, Michael V. Massa, Jessica L. Carvalho We have studied crystallisation in poly(ethylene oxide) (PEO) droplets with volumes ranging over several orders of magnitude. In all samples, homogeneous nucleation was observed, scaling with the volume of the droplet, down to systems with as few as $\sim 10$ polymer chains. Surprisingly, nucleation was unaffected by the high degree of confinement, despite a large surface to volume ratio and the restriction of chains to lengthscales much smaller than the radius of gyration. Nucleation was also found to be independent of chain length for two molecular weights studied, which differed in size by an order of magnitude. Furthermore, crystallisation of much shorter PEO chains, within spherical domains of diblock copolymer samples, gave results consistent with the homopolymer droplets studied. The results suggest that, for these highly supercooled systems, the formation of a nucleus is influenced by its immediate surroundings, and does not depend on the entire length of the constituent chains. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S18.00011: Architecture dependence of crystallizable sequences in semicrystalline polymers Vikram K. Kuppa, Gregory C. Rutledge Monte Carlo molecular simulations are used to examine the backbone conformations of short loops reconnecting the crystal surfaces of isotactic polypropylene(iPP) and polyethylene(PE). Fractions of alternating trans and gauche torsional sequences for iPP and trans states for PE are identified as the appropriate parameters to measure crystallizability. The higher fraction of gauche dihedrals in backbone conformations of iPP make it more flexible than PE. Short loops of both architectures show a high degree of disorder, as manifested by the large number of unfavorable, non-crystallizable sequences. Density profiles for both polymers show a characteristic decay in crystallizable torsions from the crystal phase to the melt, over the interphase. For iPP, the increased flexibility of the chains leads to a larger population of adjacent re-entry loops on the fold surface, which are spatially correlated to the minimum in density profiles of crystallizable sequences. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S18.00012: Tailor-Made Onion-Like Stereocomplex Crystals in Incompatible Enantiomeric Polylactide Containing Block Copolymer Blends Lei Zhu, Lu Sun, Lixia Rong, Benjamin Hsiao Stereocomplexes formed by blending enantiomeric PLA block copolymers have demonstrated great potential for applications in biomedical devices. Here, we successfully synthesized well-defined enantiomeric PLA containing block copolymers by living ring-opening polymerization of L- and D-lactides from hydroxyl-terminated hydrophilic [poly(ethylene oxide) or PEO] and hydrophobic [poly(ethylene-co-1,2-butylene) or PEB] oligomers. Quantitative stereocomplex formation was achieved by equimolar mixing of the incompatible PEO-$b$-PLLA and PEB-$b$-PDLA. Intriguingly, in the blend of PEB-b-PDLA and PEO-b-PLLA with different PEB and PEO molecular weights, onion-like stereocomplex crystals were observed because of unbalanced surface stresses caused by different PEO and PEB molecular weights. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S18.00013: Crystallization of linear polyethylene in nanoporous cylindrical pores Kyusoon Shin, Euntaek Woo, June Huh, Young-Gyu Jeong The linear polyethylene with nearly monodisperse molecular weight distribution is confined in cylindrical nanopores, and the crystallization behavior of the polyethylene is investigated. The crystalline structure and crystallization kinetics of the linear polyethylene in the cylindrical nanopores, examined by x-ray diffraction and calorimetry, shows noticeable deviations from those of bulk polyethylene. We find the imposed confinement induces significant frustration which enables us to control the crystal structure formation such as crystal orientation. The detailed crystallization kinetics of polyethylene in the cylindrical nanopores together with the crystal structure will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S18.00014: Crystallization of Nucleator Nanofibrils in Polypropylene Melt J. Lipp, Y. Cohen, R.L. Khalfin, M. Shuster, A.E. Terry Self-associating molecules act as nucleating agents in polypropylene (PP) in order to increase the crystallization rate and decrease the crystallite size, by forming a fine network of nanofibrils within the polymer melt. The thermodynamic and kinetic basis for formation of this structure is not clear. Current models usually invoke a spinodal decomposition mechanism, as temperature is lowered into an immiscibility gap. This presentation deals with 1,3:2,4-Di(3,4-dimethylbenzylidene)sorbitol [dMdBS] in PP. The kinetics of structure formation was evaluated using small angle x-ray scattering, including synchrotron measurements. The results indicate a crystallization process by means of a nucleation and growth mechanism, which is controlled by the rate of homogeneous nucleation. The thermodynamic temperature of this process, determined for two different dMdBS concentrations from the temperature dependence of the crystallization half-time, agrees with that obtained by group-contribution calculation of the solubility parameters. dMdBS nanofibril formation has a remarkable effect on PP crystallization in melt-spun fibers. Just 0.4{\%} additive at a moderate spin-draw ratio yields a crystalline morphology comprised of parallel chain-folded lamellae, with the lamellar normal highly aligned along the fiber axis. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S18.00015: Effect of OMS on Crystal Phases of PVDF Crystallized From the Melt B.S. Ince-Gunduz, R. Alpern, D. Amare, K. Burke, P. Cebe, J. Crawford, B. Dolan, S. Jones, R. Kobylarz, M. Koplitz, M. Meleski, M. Reveley, A. Sagiv Addition of extremely small amounts of organically modified silicate (OMS) into poly(vinylidene fluoride) (PVDF) causes the polar beta phase to form preferentially in quenched or cold-crystallized samples. Here, we report on further studies of PVDF/OMS nanocomposites crystallized from the melt. Nanocomposite samples were prepared with 0- 4wt.{\%} OMS, and crystallized from melt at 150$^{\circ}$C for an hour or at 165$^{\circ}$C for 16 hrs. In neat PVDF, these treatments favor the growth of non-polar alpha and polar gamma crystal phases, respectively. SAXS and WAXS, FTIR, and DSC were used to establish the crystal phase. Morphology was studied using POM and AFM. For samples crystallized at 150$^{\circ}$C, formation of beta-PVDF can be seen in nanocomposites even at 0.01wt.{\%} of OMS, and the ratio of beta phase to alpha phase increases as the OMS content increases. POM reveals that highly birefringent alpha spherulites occur together with less birefringent spherulites. FTIR microscopy shows these weakly birefringent spherulites are in beta phase. For samples crystallized at 165$^{\circ}$C, alpha and gamma phases occur together, and the gamma fraction increases with the addition of OMS. Research supported by: NSF-DMR Polymers Program, grant DMR-0406127. [Preview Abstract] |
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