Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X30: Polymer Crystallization |
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Sponsoring Units: DPOLY Chair: Ben Hsiao, Stonybrook Room: LACC 505 |
Friday, March 25, 2005 8:00AM - 8:12AM |
X30.00001: Reversible Processes Between the Glass and Melting Transition of Poly(oxyethylene) Wunderlich Bernhard, Wulin Qiu Heat capacity, $C_{p}$, of poly(oxyethylene), POE, with molar mass from 1,500 to 900,000, was analyzed by quasi-isothermal, temperature-modulated DSC. The oligomers form extended-chain crystals which melt fully irreversibly and permit direct measurement of the thermodynamic $C_{p}$. The POE with higher mass yield folded-chain crystals and indicate locally reversible melting. The reversing, apparent $C_{p}$ depends on mass, as well as on the amplitude of modulation. Its maximum in the melting range increases with molar mass, but decreases at larger amplitudes of temperature modulation. After separation from the apparent $C_{p}$, the thermodynamic $C_{p}$ depends on the melting temperature and increases beyond the calculated, vibrational $C_{p}$, from the ATHAS Data Bank. Molar masses of 8,000 to 20,000 have almost the same $C_{p}$, but deviate initially at higher temperature. These quantitative observations permit a more detailed discussion of the origin of the thermodynamic $C_{p}$ and the locally reversible melting of the metastable POE. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X30.00002: Theory of Lamellar Growth in Polymer Solutions Arindam Kundagrami, M Muthukumar We investigate theoretically the crystallization of polymers in dilute solutions. Nucleation-controlled crystal growth in a diffusive environment is considered. The single-crystal lamella is modelled by a cylindrical tablet of fixed thickness undergoing radial growth. The entropic barrier, adjacent to the growing surface, is modelled by a suitable entropic force. The temperature (supercooling) dependence is inherent in the diffusion constant and in the barrier term. By solving the boundary value problem including a moving boundary representing the crystal-solution interface, the temperature and concentration dependence of the growth rate are calculated. Our theoretical results are compared with simulation results, experimental data, and the Lauritzen-Hoffman theory. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X30.00003: Reversible melting of extended-chain and folded-chain polymer crystals Rene Androsch, Bernhard Wunderlich, Hans-Joachim Radusch The reversibility of crystallization and melting of crystals of polytetrafluoroethylene (PTFE) and polyethylene (PE) of extended-chain and folded-chain morphology has been investigated by temperature-modulated differential scanning calorimetry (TMDSC). The total and average specific reversibility of folded-chain crystals is considerably larger than in the case of extended-chain crystals. This experimental observation can be attributed to a different number of points of possible decoupling between crystallized and amorphous sequences along individual, partially crystallized molecules within the globally semi-crystalline superstructure in extended-chain and folded-chain crystals. Due to incomplete melting of the macromolecules, the morphology of the folded-chain crystal allows molecular segments to reversibly melt and crystallize as a function of temperature. The extended-chain conformation, in turn, inhibits reversible melting due to the required molecular nucleation after complete melting of a single molecule. The experimental findings support both, the concepts of lateral-surface activity and molecular nucleation presented earlier. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X30.00004: Modeling of Polymer Melting Jianing Zhang, Murugappan Muthukumar We have employed modified Potts model with anisotropic interaction to simulate melting of polymer crystals. Using this coarse-grained model with fixed lamellar thickness, we have successfully reproduced many experimental results. A broad melting transition is found and becomes broader with decreasing crystallization temperature $T_c$, increasing heating rate $\beta$ or crystallization time $t_c$. The melting temperature $T_m$ depends on $\beta, t_c$ and $T_c$ as follows: $T_m \propto \beta^{0.5}$; $T_m\propto\log t_c$ for intermediate $t_c$ values; $T_m$ is constant in low-$T_c$ region and gradually increases with $T_c$ in high-$T_c$ region, in contradiction with Hoffman- Weeks plot but consistent with experimental observations over a wide $T_c$ range. A new relation for $T_m$ is proposed: $T_m=T_m(\beta=0)+const \sqrt{\beta}\log R $, where $R$ is the average lamellar diameter following the law $R\propto t_c^{1/3}$. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X30.00005: Crystallization of ethylene/alpha-olefin copolymers in shear fields R. Shamsundar, P. Sane, V. Premnath, T.P. Mohandas, Guruswamy Kumaraswamy Metallocene ethylene-co-$\alpha $-olefins represent model materials to investigate the effect of ``non-crystallizable defects'' on crystallization of sheared copolymers. We present results on polymers having similar molecular weight and polydispersity (viz. same chain mobility), but varying comonomer percentage (viz. varying topological contraints on crystallization). Shear crystallization experiments are performed in a Linkam shear cell using optical techniques to monitor phase change. The polymer is melted at an elevated temperature, then sheared at a controlled rate for a fixed duration. After shearing, the polymer is cooled to a temperature chosen such that the quiescent crystallization time at that temperature is around 5000 s. Therefore, the crystallization temperature varies with comonomer content and serves only as a ``read-out'' to determine the effect of shearing. Shearing a copolymer (containing just 1 mole percent comonomer) has almost no effect on crystallization kinetics under conditions where shear greatly accelerates homopolymer crystallization. As chain relaxation dynamics are similar for all our polymers, shear enhanced crystallization kinetics in homopolymers is due to the formation of precursors during shear. These precursors are not formed for copolymers. Thus, while precursor formation happens via a rheological pathway, the ``crystallizability'' of the polymer chain determines the chance of precursor formation. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X30.00006: Examination of Flow-Induced Crystallization Precursor Structures in Polyethylene Blend Films by Reversed Melting Method J. Keum, R. Somani, F. Zuo, L. Yang, I. Sics, B. Hsiao, H. Chen, R. Kolb, C.-T. Lue \textit{In-situ.} SAXS (small-angle X-ray scattering) and WAXD (wide-angle X-ray diffraction) techniques were used to investigate melting behavior of the confined blown films that consist of structures formed during film blowing. The PE blend consisted of 95wt {\%} LLDPE ($\bar {M}_{\mbox{w}} \sim $116 Kg/mole) and 5wt {\%} HDPE. The HDPE possessed a bimodal molecular weight distribution with 80 {\%} of low molecular weight fraction (LMW-HDPE, $\bar {M}_{\mbox{w}} $\textit{$\sim $}99 Kg/mole) and 20 {\%} high molecular weight fraction (HMW-HDPE, $\bar {M}_{\mbox{w}} $\textit{$\sim $}1,100 Kg/mole), respectively. Thus, the final blend contained 1 wt {\%} of HMW-HDPE in the range of the overlap concentration, 0.5 wt {\%}. The study was for examining the evolution of flow-induced crystallization precursors and their thermal stability. The results of the blend compared to neat LLDPE showed that the HMW-HDPE species in the blend significantly improved the crystal orientation. We speculate that the HMW-HDPE formed a network of extended-chain crystals due to their long relaxation times, which, subsequently, generated a scaffold of the oriented nuclei that defined the final morphology. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X30.00007: Unexpected Shish-Kebab Structure in Shear-Induced Polyethylene Melt Benjamin Hsiao, Ling Yang, Rajesh Somani, Lei Zhu Unexpected shish-kebab structure with multiple shish in a sheared polymer blend, containing 2 wt{\%} of crystallizing ultra-high molecular weight polyethylene (UHMWPE) and 98 wt{\%} of non-crystallizing polyethylene copolymer matrix, was observed by field-emission scanning electron microscopy (FE-SEM) examinations of the solvent-extracted UHMWPE component. SEM results indicated that the shish-kebab entity contained several independent shish, instead of a single shish as conventionally observed in dilute polymer solutions or thin films. The formation of the shish-kebab structure in UHMWPE could be attributed to the abrupt coil-stretch transition that occurs only in sections of the chain, rather than its entire contour length, and was confirmed by time-resolved synchrotron WAXD and SAXS measurements. X-ray results support the hypothesis that two populations of chain segments (stretched and coiled) in UHMWPE are induced by shear, where the stretched segments form the basis of the multiple shish and the coiled segments crystallize into the kebabs following a diffusion-controlled process. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X30.00008: Shear-Induced Shish-Kebab Morphology in Polymer Melts - Flow Between Two Parallel Plates versus Coaxial Cylinders Rajesh Somani, Igors Sics, Benjamin Hsiao \textit{In-situ} synchrotron SAXS (small-angle X-ray scattering) studies of shear-induced shish-kebab morphology in isotactic polyprolylene melt under two different flow geometries have been carried out. These geometries included (1) shear between two parallel-plates, where the X-ray beam was perpendicular to the flow and neutral directions, and (2) shear between coaxial cylinders or couette flow, where the X-ray beam was perpendicular to the flow and gradient directions. Time-resolved SAXS patterns revealed the formation of typical shish-kebab morphology under both flow geometries; whereby equatorial streaks due to shish were seen to evolve immediately after shear, followed by meridional maxima due to kebabs. SAXS images obtained under the parallel-plate and couette flow geometries exhibited a striking resemblance indicating the cylindrical symmetry of the shish-kebabs and similar spatial distribution along the orthogonal directions in sheared melts. The high temperature stability of iPP shish-kebabs and its molecular origin will be discussed. The financial support of this work was provided by NSF (DMR-0405432). [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X30.00009: Thermal Stability of Shear-Induced Precursors of Shish-Kebab in a Model Polyethylene Blend by in-situ Rheo-SAXS and -WAXD Feng Zuo, JongKahk Keum, Ling Yang, Rajesh Somani, Benjamin Hsiao \textit{In-situ} rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate thermal stability of shear-induced shish-kebab structures in a model polyethylene (PE) blend. The blend consisted of a low molecular weight non-crystallizing PE matrix (LMWPE, M$_{w}$ = 50,000 g/mol, MWD = 2.2) and a small amount of high molecular weight crystallizing PE (HMWPE, M$_{w}$ = 1,500,000 g/mol, MWD = 1.1). After an imposed steady shear condition in a parallel plate shear stage, rate = 125 s$^{-1}$, duration = 20 s, Temperature = 126.5 $^{o}$C, both SAXS and WAXD results showed that a shish-kebab morphology developed in the PE blend. Subsequently, the shish-kebab crystals were subjected to several thermal cycles, consisting of melting and crystallization of the oriented structures. Although shish formed first, followed by kebabs; surprisingly, the shish-kebab structure was seen to melt as an integrated entity. This suggests that the shish-kebabs contained integrated stretched and coiled chains (or chain segments), even for near monodispersed chains. The HMWPE oriented structures were found to withstand a high temperature of 152 $^{0}$C (holding time = 3 min) and acted as nuclei for crystallization of unstretched LMWPE chains at lower temperatures. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X30.00010: Radial distribution of crystallinity in poly(trimethylene terephthalate) fibers characterized by confocal Raman spectroscopy Jing Wu Confocal Raman spectroscopy is used to characterize the radial distribution of crystallinity in poly(trimethylene terephthalate) (PTT) fibers. Raman scattering spectra are taken at different radial positions from fibers prepared at take-up speeds of 1000, 2500 and 4750 m/min. For each fiber, spectra taken at all radial positions do not show substantial difference in the spectral range above 400 cm$^{-1}$. In the spectral region below 300 cm$^{-1}$, however, spectra exhibit systematic changes along the radial direction. For fibers prepared by low take-up speeds, as Raman scattering volumes move from fiber surface to core, a peak centered at 140 cm$^{-1}$ emerges, suggesting an increase in crystallinity. For fibers prepared at high take-up speeds, the reverse trend is observed - the peak centered at 140 cm$^{-1}$ is observed on the fiber surface, not in the fiber core. The competition between orientation and quenching is responsible for the observed trend. The spectral peak at 140 cm$^{-1}$ lies in the Terahertz (THz) regime. Our data suggest the significance of THz dynamics in polymer crystallization. Detailed discussions will be presented. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X30.00011: Crystal Nucleation of Polymers Confined to Droplets: Memory Effects Michael V. Massa, Michelle S.M. Lee, Kari Dalnoki-Veress The crystallisation kinetics of a supercooled polymer melt is often affected by the thermal history of the sample. The crystal nucleation rate, in particular, can be enhanced by a thermal pre-treatment prior to the final crystallisation (self- seeding). Similarly one can study the memory effects of the supercooled melt to elucidate the earliest stages of crystallisation. We investigate the effects of thermal history on the crystallisation in confined polymer melts. By studying an ensemble of isolated and impurity-free droplets, it is possible to make statistically meaningful measurements of the supercooled melt over a large range of temperatures and over long times, due to the absence of heterogeneously nucleated events. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X30.00012: Study of Onsets of Tethered Chain Overcrowding and Highly Stretched Regime of Brushes via Crystalline-Amorphous Diblock Copolymers Joseph X. Zheng, Huiming Xiong, KyungMin Lee, Christopher Y. Li, Lei Zhu, Ping Huang, Ya Guo, Qing Ge, Roderic P. Quirk, Bernard Lotz, Edwin L. Thomas, Stephen Z.D. Cheng Well-controlled tethered polymer chains can be obtained from the solution grown single crystals of crystalline-amorphous diblock copolymers (such as PS-b-PEO and PS-b-PLLA). Based on the thickness-crystallization temperature relation, the interaction between the tethered polymer chains can be measured. The onset of chain was found as a first-order like transition at reduced tethering density near 3.7-3.8, and the onset of highly stretched brush regime was found as a high-order like transition at reduced tethering density near 15. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X30.00013: Molecular Architecture Induced Chain-Folding in Polymeric Amphiphilic Unimolecular Micelles Lei Zhu, Jianjun Miao, Guoqiang Xu, Lu Tian, Kathryn Uhrich, Carlos Avila-Orta, Benjamin Hsiao An amphiphilic star-like macromolecule (ASM) with an aliphatic core and four poly(ethylene oxide) (PEO) arms was synthesized. Its crystallization and self-assembly behaviors at different crystallization temperatures ($T_{c}$s) were characterized by synchrotron small angle X-ray scattering (SAXS). The overall $d$-spacing of alternating amorphous and crystalline lamellae almost doubled from 11.4 nm for samples quenched into liquid nitrogen to 21.0 nm for $T_{c}$ = 42 \r{ }C. Due to an insufficient arm-to-core ratio in the ASM, chain-folding was forced to occur in the arm PEO crystals, while only extended chain crystals were observed for the PEO oligomer having a molecular weight of 2000 g/mol. In the ASM, gradual transitions from almost once-folded to non-integer folded, and finally to extended chain crystals were observed with increasing $T_{c}$ from -20 \r{ }C to 42 \r{ }C. [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X30.00014: Influence of Angular Potentials on the Crystallization of Model Polymer Chains Hendrik Meyer, Thomas Vettorel, J\"org Baschnagel A simplified polymer model appeared to be extremely efficient for the study of polymer crystallization in molecular dynamics simulations [1]. This model goes one step further than usual united-atom models and resumes all atoms of a monomer into one sphere. It still retains information of the possible conformations in an angular potential with 3 local minima. We systematically varied this angular potential which allows to tune crystallization and melting temperatures. The chain rigidity characterized by the fraction of stretched conformations in the melt is an important driving force for crystallization in these models: the higher this fraction, the easier is the crystallization and thus the higher the temperature where ordering starts. But interestingly, no trivial correlation with other examined quantities, and in particular with the persistence length, could be found. \newline \newline [1] H. Meyer and F. M\"uller-Plathe, J. Chem. Phys. 115 (2001) 7807; Macromolecules 35 (2002) 1241. H. Meyer in Lecture Notes in Physics 606, J.U. Sommer, G. Reiter (eds.) Springer 2003. [Preview Abstract] |
Friday, March 25, 2005 10:48AM - 11:00AM |
X30.00015: Long-range Periodic Structure in Porous High Density Polyethylene Crystallized from the Gel State Shujun Chen, Samuel P. Gido, Souvik Nandi, H. Henning Winter Porous polymeric materials have a variety of applications from filtration membranes, low k dielectric materials to artificial organs. Characterization of such porous materials, however, has been limited to surface techniques such as SEM and adsorption isotherms. Little work has been done on the internal structure characterization of porous materials due to the difficulty in obtaining suitable samples without disrupting the structure. In this research, a comprehensive structural characterization was performed on porous HDPE material using SEM, TEM, electron diffraction and AFM. The porous HDPE material was obtained through crystallization from swollen crosslinked gels (CSX process) in supercritical propane. SEM showed micron-size open porous structure in the CSX processed HDPE. TEM revealed long-range periodic structure in its pore walls, much different from structures found in typical HDPE material. Electron diffraction and AFM results confirmed the presence of long-range lamella stacking. [Preview Abstract] |
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