Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N4: Polymer Crystallization |
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Sponsoring Units: DPOLY DMP Chair: Freddy Khoury, National Institute of Standards and Technology Room: Baltimore Convention Center 308 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N4.00001: A new approach to study of the onsets of tethered chain overcrowding and highly stretched brush regime utilizing crystalline-amorphous diblock copolymers Invited Speaker: Two series of diblock copolymers, PEO-$b$-PS and PLLA-$b$-PS, were used as templates to generate tethered PS blocks on the single crystal surfaces. Controlled and tunable reduced tethering density, $\tilde {\sigma }$, defined by \textit{$\sigma \quad \pi $} $R_{g}^{2}$ (where \textit{$\sigma $ }is the tethered chain density), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature ($T_{x})$ of different PEO-$b$-PS and PLLA-$b$-PS solutions. The $\tilde {\sigma }$ of the tethered PS chains on the crystal surface increased with increasing $T_{x}$ because at a fixed MW of the PEO or PLLA block, an increase in the lamellar thickness ($d_{CRYST})$ was evidence of a decrease in the number of folds. When we plotted the relationships between 1/$d_{CRYST}$ and $T_{x}$ for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at $\tilde {\sigma }$ = 3.7 ($\tilde {\sigma }$*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (\textit{$\Gamma $}$^{PS})$ was used as a parameter to characterize the PS tethered chain interactions. The relationship between \textit{$\Gamma $}$^{PS}$ and $\tilde {\sigma }$ showed a discontinuous transition at $\tilde {\sigma }$*. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. Based on the scaling laws, the onset of highly stretched brush regime could be identified at $\tilde {\sigma }$ = 14.3 ($\tilde {\sigma }$**). In the \textit{$\Gamma $}$^{PS}$ versus $\tilde {\sigma }$ plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between $\tilde {\sigma }*$ = 3.7 and $\tilde {\sigma }$*$*$ = 14.3. It is defined as the regime where the interaction of the tethered PS chains undergo changes from being non-interacting towards penetration to, finally, chain stretching normal to the surface. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 9:12AM |
N4.00002: Flow-Induced Crystallization Precursor Structure in Entangled Polymer Melt. Invited Speaker: Flow-induced crystallization has long been an important subject in polymer processing. Varying processing conditions can produce different morphologies, which lead to different properties. Recent studies indicated that the final morphology is in fact dictated by the initial formation of crystallization precursor structures (i.e., shish kebabs) under flow. In this talk, factors that affect the shish-kebab formation in entangled polymer melts are systematically reviewed, including the concept of coil-stretch transition, chain dynamics, critical orientation molecular weight, phase transition during shish and kebab formations. In particular, recent experimental results from in-situ rheo-X-ray studies and ex-situ microscopic examinations have been presented to illustrate several new findings of flow-induced shish-kebab structures in polymer melts. (1) The shish entity consists of stretched chains (or chain segments) that can be in the amorphous, mesomorphic or crystalline state. (2) The kebab entity mainly arises from the crystallization of coiled chains (or chain segments), which seems to follow a diffusion-control growth process. (3) A shish-kebab structure with multiple shish was seen in the ultra-high molecular weight polyethylene (UHMWPE) precursor. Based on the above results and recent simulation work from other laboratories, a modified molecular mechanism for the shish-kebab formation in entangled melt is presented. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:48AM |
N4.00003: Curved faces in polymer crystals with asymmetrically spreading growth patches Invited Speaker: Polymer crystals often have curved faces. Understanding such morphology is of major interest since it allows distinction between fundamentally different theories of polymer crystallization. E.g. Sadler's ``roughness-pinning'' theory assumes that the curvature is a result of roughening transition on lateral faces. It has since been shown by Mansfield that the curvature can be explained quantitatively, essentially within the Lauritzen-Hoffman nucleation theory. However, the step propagation rates $v$ implied in their treatment are substantially lower than predicted by the LH theory. The retardation appears to be due to the ``self-poisoning'' or ``pinning'' effect of incorrect chain attachment, effectively demonstrated by the extreme cases of growth rate minima in long-chain monodisperse n-alkanes. Recently crystals of poly(vinylidene fluoride) and alkanes C$_{162}$H$_{326}$ and C$_{198}$H$_{398}$ have been found with habits that can be best described as bounded by curved {\{}110{\}} faces. The interesting feature is the asymmetry of the curvature: while the faces are curved at one end, they are straight at the other. We carried out mathematical analysis of the curvature, generalizing the Mansfield model. We suggest that such asymmetric curvature arises from the propagation rates to the left, $v_{l}$, and to the right, $v_{r}$, being different because of the lack of mirror bisecting planes such as (110). By solving appropriate equations with moving boundaries, we obtained the shape of the growth front $y(x,t)$. Calculated crystal habits gave excellent fits to the observed growth shapes of $a$-axis lenticular crystals of long alkanes and PVDF, as well as of single crystals of PEO. This explains some hitherto poorly understood morphologies and, in principle, allows independent measurements of step initiation and propagation rates in all polymers. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:24AM |
N4.00004: New Paradigms for Polymer Crystallization Invited Speaker: The ordering process of topologically connected chains into a crystalline state is distinctly different from that of low molar mass substances. One of the key differences arises fundamentally from entropic barriers due to substantial reduction in configurational entropy of the system during the ordering process. We have derived a new theoretical model with the following essential features: (1) For a single lamella, the free energy landscape exhibits many metastable states (separated by free energy barriers), and a globally stable state. Among the metastable states, even the first viable state with its free energy just below that of the melt is long-lived, due to the barrier for thickening. The thickness of this long-lived metastable state increases with temperature. However, if enough time is granted for this metastable state to evolve, then the equilibrium thickness would be reached for each temperature. The equilibrium thickness decreases with temperature, until the approach of the equilibrium melting temperature. The equilibrium melting temperature does not correspond to that of extended chain dimensions. (2) The lateral growth faces a free energy barrier, due to temporal crowding of entangled chains at the growth front. A general formula is derived for the growth kinetics of the growth front, providing a crossover description for crystallization of low and very high molar mass polymer chains. The predictions are compared with available experimental data. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 11:00AM |
N4.00005: Growth and form of spherulites: A phase field study. Invited Speaker: Polycrystalline patterns termed spherulites are present in a broad variety of systems including metal alloys, polymers, minerals, and have biological relevance as well (see e.g. semi-crystalline amyloid spherulites and spherultic kidney stones). The fact that similar polycrystalline patterns are observed in systems of very different nature suggests that a minimal model based on coarse-grained fields, which neglects the details of molecular interactions, might be appropriate. Although such a field-theoretic approach disregards most of the molecular scale details of formation, some features such as crystal symmetries can be incorporated via the anisotropies of the model parameters. The rationale for developing such coarse-grained models is the current inability of fully molecular models to address the formation of large scale morphologies. A phase field theory of polycrystalline growth, we developed recently, is applied for describing spherulitic solidification in two and three dimensions. Our model consists of several mechanisms for nucleating new grains at the perimeter of the crystallites, including homogeneous (trapping of orientational disorder and branching in certain crystallographic directions) and heterogeneous (particle-induced nucleation) processes. It will be shown that the diversity of spherulitic growth morphologies arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front. This randomization in the crystal orientation accounts for the isotropy of spherulitic growth at large length-scales and long times. We find the entire range of observed spherulite morphologies can be reproduced by this generalized phase field model of polycrystalline growth. [Preview Abstract] |
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