Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D4: Polymer Crystallization: 50 years of Chain Folding |
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Sponsoring Units: DPOLY Chair: Buck Crist, Northwestern University Room: Colorado Convention Center Korbel 2B-3B |
Monday, March 5, 2007 2:30PM - 3:06PM |
D4.00001: Fifty (Plus) Years of Polymer Nano-Science (Art) Invited Speaker: At least one dimension of the fundamental structure of all polymers, on the next hierarchical size scale larger than the repeat distance and unit cell, is on the order of 100 {\AA}; hence these days one uses the label ``nano.'' This includes the coil size in solution and melt, and the morphology of both crystalline and block polymers. For study of morphology, the principle techniques, until recently, have been transmission electron microscopy (TEM) and small angle x-ray diffraction, with polymer TEM being the ``art of producing interpretable artifacts.'' Having first been shown chain folded polyethylene single crystals almost exactly 51 years ago, we will summarize and represent some half century of morphology research, and the resulting art, including addressing the areas of nucleation, growth from solution and melt, and deformation of macromolecular materials. Particular emphasis will be placed on early observations with implications on various current crystallizable polymer morphology proposals including chain folding regularity, spherulite nucleation and growth, molecular mobility in the melt and thin film crystallization. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D4.00002: The Morphology of Crystallizable Polymers: Past and Present. Invited Speaker: A perspective will be presented of the evolution of current phenomenological knowledge and views regarding the crystallization habits and morphological characteristics of polymers since the early 1950's. By the mid-1950's the characteristically spherulitic crystallization of polymers from the molten state under quiescent conditions was well established. The origins of the orientation of the chains in the constituent radiating fine texture of spherulites preferentially normal to the radial direction remained obscure then. This matter was resolved as a consequence of the seminal studies reported in 1957 independently by P.H. Till Jr. [J. Polym. Sci., \underline {26}, 301 (1957)]; A. Keller, [Phil. Mag. \underline {2}, 1171 (1957), and E. W. Fischer [Z. Naturforch., \underline {12a}, 753 (1957)] on solution-grown polyethylene lamellar single crystals. Chain-folding, as proposed explicitly by Keller, resulting in typically lamellar polymer crystal habits, and the radiating lamellar texture of spherulites became generally accepted very shortly thereafter. Among the aspects which have received much attention in subsequent years are the details of chain-folding, the bulkiness of chain- folds, the nature of order/disorder at the fold surfaces in lamellae, the diversity in the lateral growth habits and 3-D conformations of lamellar crystals (hollow pyramidal, dished or bowl-shaped, scrolled, twisted),and the processes underlying the characteristic evolution of spherulites from transient precursor axialites/hedrites. Some of these aspects will be focused on in this summary which will also touch briefly on some current discussion regarding the nature of the nucleation of crystallization from the molten state, and the nature of the lateral propagation of lamellar growth from the molten state. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D4.00003: Insights provided by the build-up, structure and morphology of polymer single crystals Invited Speaker: Polymer single crystals have been essential in the discovery and widespread acceptance of the concept of chain folding and have been investigated in their own right for decades. Their impact on polymer science is however much wider, since they are essential tools for the analysis of growth mechanisms, of small scale (crystal structure) and large scale (morphology) levels of organization. Several illustrative examples are given. Single crystals have been used recently to determine the impact of secondary nucleation and extent of lateral spread in polymer crystal growth. They are invaluable in the elucidation of mechanically unstable crystal structures of polymers and biopolymers (frequently in combination with epitaxial crystallization). They help reveal otherwise out of reach details of the molecular arrangement or rearrangement in crystal structures: structural disorder, impact of chain folding on crystal structure symmetry, mechanisms of crystal-crystal transformation, etc. The lamellar shape often reveals the impact of chain folds, which can explain the three-dimensional architecture of spherulites produced in the bulk. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D4.00004: Laws controlling crystallization and melting in bulk polymers Invited Speaker: When the fundamentals of the structure of semi-crystalline polymers - layer-like crystallites with fold surfaces being embedded in an amorphous matrix - were revealed in the Fifties, considerations about the mechanism of formation started immediately. In the Sixties and Seventies, they became a major field of research and a focus of interest. In the years which followed the approach put forward by Hoffman, Lauritzen and their co-workers [1] gained superiority. The picture envisaged by the treatment - a crystalline lamella with an ordered fold surface and smooth lateral faces, growing layer by layer with a secondary nucleation as rate determining step - is easy to grasp and yields simple relationships. Supercooling below the equilibrium melting point $T_{\rm f}^{\infty}$ is the control parameter determining both the thickness $d_{\rm c}$ and the lateral growth rate of the crystallites $G$. Experiments carried out during the last decade provided new insights and are now completely changing the understanding. They showed in particular \\- that $d_{\rm c}$ is inversely proportional to the distance to a temperature $T_{\rm c}^{\infty}$ distinctly above $T_{\rm f}^{\infty}$\\- that the activation energy determining $G$ diverges at a temperature $T_{\rm zero}$ clearly below $T_{\rm f}^{\infty}$.\\ Further simple relationships concern\\ - recrystallization processes: $d_{\rm c}$ is again inversely proportional to the distance to $T_{\rm c}^{\infty}$\\ - the extension of ordered regions within the lamellar crystallites: it is proportional to $d_{\rm c}$. We interpret the observations as indication that the pathway followed in the growth of polymer crystallites includes an intermediate phase of mesomorphic character. A thin layer with mesomorphic inner structure forms between the lateral crystal face and the melt, stabilized by epitaxial forces. The first step in the growth process is an attachment of chain sequences from the melt onto the growth face of the mesomorphic layer. The high mobility of the chains in the layer allows a spontaneous thickening, up to a critical thickness, where the layer solidifies under formation of block-like crystallites. The last step is a perfectioning of the crystallites, leading to a further stabilization. We constructed a thermodynamic scheme dealing with the transitions between melt, mesomorphic layers and lamellar crystallites, assuming for the latter ones that they exist both in an initial \lq native\rq~and a final \lq stabilized\rq~form. $T_{\rm c}^{\infty}$ and $T_{\rm zero}$ are identified with the temperatures $T_{\rm mc}$ and $T_{\rm am}$ of the (hidden) transitions mesomorphic $\rightarrow $ crystalline and amorphous$\rightarrow $ mesomorphic, respectively. Application of the scheme in a quantitative evaluation of small angle X-ray scattering and calorimetric results yields the equilibrium transition temperatures between the various phases, latent heats of transition and surface free energies [2]. [1] J.D Hoffman, G.T Davis, and J.I. Lauritzen. \newblock In {\em Treatise on Solid State Chemistry {\rm Vol.3, N.B.Hannay Ed.}}, page 497. Plenum, 1976. [2] G.~Strobl.\newblock {\em Eur.Phys.J.E}, 18:295, 2005. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D4.00005: Growth kinetics and morphology of polymer crystals Invited Speaker: Originating from the nature of chain folding, polymer single crystals are quite unique in the growth kinetics and morphology. The developments of the understanding in the past 50 years are discussed and the unsolved important issues will be suggested. Polymer single crystals are thin lamellae with the thickness in the order of 10nm determined by the period of chain folding, which keeps a constant value for the isothermal crystallization. The growth of polymer single crystals is modeled by the kinetics of creation and annihilation of growth steps on a rectangular substrate with the pre-determined thickness. The growth face is therefore regarded as a one-dimensional substrate and the kinks and anti-kinks on the substrate correspond to the growth steps propagating in the opposite directions. The kinetic equations of those kinks proposed by Seto and Frank well describe the transition of growth regime as a crossover from single nucleation to multi-nucleation on the basis of the standard model of chain-folded polymer crystallization with surface nucleation proposed by Lauritzen and Hoffman. However, the analysis of the growth kinetics and morphology of single crystals having curved growth front suggests an unusual behavior of the step propagation velocity. The anomaly can be accounted for by a self-poisoning of the growth step interrupted by polymer chains with folding shorter than required. An entropic barrier of pinning proposed by Sadler and Gilmer is a possible candidate of the self-poisoning and is in accordance with recent computer simulation results suggesting the kinetics on a rugged free energy landscape having a resemblance to protein folding. Therefore, the quantitative evaluation of the kinetic barriers of surface nucleation and pinning has been an important issue. In addition, examination of the kinetics of melting will have valuable information because melting of a crystal must be free from nucleation but can still be limited by the entropic barrier. [Preview Abstract] |
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