Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D18: Focus Session: Crystallization in Confined Geometry II |
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Sponsoring Units: DPOLY Chair: Christopher Li, Drexel University Room: B117 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D18.00001: Crystallization of Hybrid Polymer Systems Stephen Z. D. Cheng, Ryan M. Van Horn, Wenbin Zhang, Xinfei Yu, Chien-Lung Wang, Hao-Jan Sun Our goal is to understand how to incorporate nanoparticles into materials for significant technological advancements by relating physical properties to the ordered microscopic structure. In our group, we have developed an efficient way to synthesize polymer conjugates using C$_{60}$-fullerenes and POSS. By designing new molecular building blocks and using polymer self-assembly (crystallization, microphase separation, specific hydrogen bonding, and $\pi -\pi $ and other interactions), ordered structures of modified nanoparticles were assembled in the bulk and at the interface. In this talk, an example of crystallizable polymers that have been capped with these particles will be discussed as it pertains to forming 2D sheets on the basal surfaces of single crystals. These materials are capable of crystallizing in solution, enabling the formation of highly conducting or insulating sheets on the basal surfaces of the crystals. The effect of these incompressible particles on the crystallization of the polymer block was studied. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D18.00002: Shear Crystallization Studies in Polymer/Carbon Nanotube Dispersions Marilyn Minus, Satish Kumar Shear crystallization studies have been carried out in polymer/carbon nanotube (CNT) dispersions. In these studies shear-flow forces in polymer solutions as well as the unique nucleation ability of CNT (i.e. long, rod-like, nanoscale structure) are utilized to promote extended-chain crystallization and orientation of the polymer on the surface of the CNT. Polymer and CNT concentration, dissolution temperature, solution crystallization temperature, and speed of shearing were varied to determine the best conditions to promote extended-chain polymer crystallization on the CNT surface. Single-wall, few-wall, and multi-wall carbon nanotubes were used in this study to investigate whether the size/diameter of the nanotube (i.e. diameter range: 1-30 nm), play a role in crystal growth for the polymer. These results are presented in this work in an effort to understand the influence of CNT on polymer crystal growth. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D18.00003: Retarded Crystallization in Polyamide/Layered Silicates Nanocomposites caused by an Immobilized Interphase Andreas Wurm, Mohamed Ismail, Christoph Schick, Bernd Kretzschmar, Doris Pospiech For semicrystalline polymers the concept of rigid amorphous fraction (RAF) at the interface between crystalline and amorphous material is well established. On the other hand, it was found that polymer composites based on inorganic nano-sized fillers distributed in an amorphous polymer matrix show a rigid amorphous fraction, too In this contribution we will discuss rigid amorphous fractions in semicrystalline polymer nanocomposites. The rigid amorphous material in this class of polymers originates simultaneously from two different interaction mechanisms: (i) rigid amorphous due to interaction of the polymer with the crystals and (ii) rigid amorphous due to interaction of the polymer with the nanofillers. We present heat capacity data which allow determining fractions of different mobility for different filler contents quantitatively by analyzing the step at glass transition as well as the melting peak. A retarded crystallization behavior due to immobilized polymer segments at the surface of the nanofiller was observed. The remaining mobile amorphous fraction in polyamide-6.6/Nanofil 9 nanocomposites was independent on the filler content as well as the total rigid fraction. Only the distribution of crystalline and RAF changes as a function of filler content. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D18.00004: Nascent Crystallization and Nascent Morphology Xiaozhen Yang It was found that recognition of nascent morphology enables us to control the ultimate property of the polymerized product. The nascent morphology is at mesoscale, its formation regarding to detailed mechanism is at molecular level. Molecular dynamics simulation of growing chain organization on catalyst surface was performed by modifying the codes for normal NVT ensemble into the giant canonical ensemble. We found that a growing chain as simulated from 10 bond length to 1010 bonds undergoes novel behaviors through stages of the nucleation and the crystal growth on the surface. 64 chains growing in a 8$\times $8 matrix was also examined. Variation of the catalyst density distribution was found correlative to the different nascent morphologies. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D18.00005: Reconsideration of Over-All Crystallization Behavior based on Avrami Equation. Norimasa Okui, Kenichiro Usuda, Susumu Umemoto It has been widely known that Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation can express the time dependence of over-all crystallization. In general, KJMA Eq. has been induced on the assumption of sporadic nucleation with the constant nucleation rate or instantaneous nucleation with the constant nuclear density. However, actual nucleation behavior is clearly different from these assumptions, since the number of nuclei increases with time following the s-shaped curve with the limited number of nuclei. In general, the Avrami exponent has been observed to be non-integer number, since it should be integer number in theoretical sence. In this study, over-all crystallization behavior with two-dimensional growth for poly(ethylene succinate) was measured. The over-all crystallization behavior calculated with the actual experimental nucleation rate was completely coincided with the experimental over-all crystallization curve. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D18.00006: Molecular dynamics simulations of polymer crystallization via self-seeding Chuanfu Luo, Jens-Uwe Sommer We use large scale molecular dynamics (MD) to simulate the processes of polymer crystallization with a coarse-grained model. In total we are able to simulate 1000 polymer chains made of 1000 monomers each, a system large enough to compare to experimental relevant, entangled melts. It is found that some micro crystalline domains (MCDs) can survive slightly above the apparent melting temperature after a consistent cooling and reheating cycle. We chose the stablest MCD as a baby seed and let it grow at a constant quenched temperature. A single lamella can be formed via this self-seeding process. We observe the growth pathway and analyze the chain dynamics especially at the growth front.\\[4pt] [1] C. Luo and J. Sommer, Comp Phys. Comm. 180, 1382 (2009)\\[0pt] [2] C. Luo and J. Sommer, Phys. Rev. Lett. 102, 147801 (2009)\\[0pt] [3] J-J. Xu, Y. Ma, W.B. Hu, M. Rehahn and G. Reiter, Nature Materials 8, 348 (2009) [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D18.00007: Rotator phases and nucleation in polyethylene Scott Milner Experimental evidence has accumulated that polymer crystals often nucleate via a metastable, partially ordered ``rotator'' phase. To investigate this idea theoretically, we must calculate the bulk and interfacial free energies of the critical nucleus. We use our recent theory of the crystal-melt interface, which represents the amorphous region as a grafted brush of loops in a self-consistent pressure field, combined with estimates of bulk free energy differences based on experimental data, to calculate nucleation barriers and rates via rotator versus crystal nuclei for polyethylene. We find rotator-phase nucleation is indeed favored throughout the temperature range where nucleation is observed. Our methods can be extended to other polymers. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D18.00008: Simulation of Twist Solitons in Semicrystalline Polyethylene Nathaniel Wentzel, Scott Milner Semicrystalline polyethylene (PE) consists of ordered lamellae separated by amorphous regions. NMR studies have examined the motion of chains in semicrystalline PE and found that the chains diffuse through the lamellae by propagation of ``twist solitons''---localized 180 degree twists of the PE chain. Twist solitons should also be prevalent in PE rotator phases; thermal populations of solitons may play an important role in the free energy of rotator phases, which are stable in oligomeric PE just below melting, and in high molecular weight PE at elevated pressures. We have performed all-atom MD simulations to study the conformations, mobility, and energy of twist solitons in ordered phases of PE, by covalently bonding chains to their periodic image across the periodic boundaries in the z direction. Single twist solitons can be studied in the crystal phase at low temperatures by incorporating a twist in a periodically connected chain, while thermally activated pairs of twists and antitwists are observed at higher temperatures. We speculate that the prevalence of twist solitons in longer alkanes and high molecular weight PE may be responsible for the observed change in chain length dependence of crystal-rotator and crystal-melt transitions above about n=50. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D18.00009: A critical orientation order of aligned 1D-arrays for templating the growth of oriented lamellae Howard Wang, Kaikun Yang, Huaiping Rong, Muhuo Yu Aligned one-dimensional arrays of nanotube and nanowires are interesting because of their anisotropic mechanical, thermal and electrical properties. Forming composites with polymers is one way to stabilize 1D array for applications. If the matrix is a crystalline polymer, the aligned 1D array can affect the orientation order of polymer crystallization. Studies have shown that isolated individual nanotubes could template growth of lamellar perpendicular to the tube axis. The same is true for highly orientated tube bundles. On the other hand, polymers in random tube composites grow random lamellar stacks. So the question is whether there exists a critical alignment order of 1d-arrays, at which orientation of lamellae becomes globally correlated. But the nematic order parameter alone is not sufficient for defining the alignment order of 1D-arrays. If the local orientation is correlated, it could be either longitudinal or transversal or mixed depending on the nature of particular materials system. Given a global nematic order parameter, how does 1D-arrys' local orientation structure affect the lamellar orientation? [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D18.00010: Molecular simulation of crystal nucleation of n-alkane melts Peng Yi, Gregory Rutledge One of the most important phenomena in molecular systems is nucleation of the crystal phase from a homogeneous melt. This phenomenon is particularly interesting for chain molecules due to their strong anisotropy and their conformational flexibility. In this work we report the results of molecular simulations of crystal nucleation of n-alkane from the melt. A realistic united atom force field was employed. The crystal phase and melting behavior were first determined by ramping temperature in a set of molecular dynamics simulations. The adiabatic nucleation trajectory was then sampled using the molecular dynamics simulations and the Monte Carlo umbrella sampling method. A mean-first-passage-time technique was used to determine the critical nucleus and the nucleation rate. The cylindrical nucleus model was found to provide a better quantitative description of the critical nucleus than the spherical nucleus model. We were also able to calculate the interface free energy for the end and side surface of a cylinder nucleus model from the Monte Carlo simulation data without making further assumption. This method can be extended to study longer n-alkane molecules and the change of nucleus conformation as n increases. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D18.00011: Fractality of perimeter of dense polymer chains in 2D Hendrik Meyer, T. Kreer, A. Johner, J. Baschnagel, J.P. Wittmer Self-avoiding polymers in two-dimensional melts are known to adopt compact and segregated configurations. Compactness does obviously not imply Gaussian chain statistics nor does segregation of chains impose disk-like shapes minimizing the average perimeter length of the chains. Using scaling arguments and molecular dynamics simulations with chain length up to 2048 we show that the chain perimeters are highly irregular and characterized by a fractal line dimension 5/4. This result may be verified experimentally from the power-law scaling of the intrachain form factor in the intermediate wavevector regime in agreement with a generalized Porod law for a compact object of fractal border. [H. Meyer et al Phys. Rev. E 79 050802(R) (2009)] [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 5:18PM |
D18.00012: Forty years of confinement! When will RAF be released? Invited Speaker: Crystallization of polymers from the melt occurs from a highly entangled state. The crystals and amorphous chains remain in intimate contact throughout the crystallization process, leading to confinement and formation of the rigid amorphous fraction (RAF). Forty years ago, H. Zachmann's group studied mobility of crystalline and non-crystalline regions, and identified rigid non-crystalline fraction using NMR. B. Wunderlich and co-workers recognized the signature of reduced mobility, i.e., confinement, via a heat capacity deficit at the glass transition relaxation. Using modern thermo-analytical methods, such as quasi-isothermal temperature modulated calorimetry, we investigate the vitrification and devitrification of RAF, its relationship to the crystalline fraction, and present methods to distinguish limiting cases of two different lamellar structural models incorporating RAF. [Preview Abstract] |
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