Effect of crystallization conditions on the shape of polymer single crystals: Experimental and theoretical approaches (original) (raw)
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A Generic Model for Growth and Morphogenesis of Polymer Crystals in Two Dimensions
Lecture Notes in Physics, 2003
We discuss a generic algorithm to simulate the growth of chain crystals from adsorbed monolayers. Chains are considered as elementary statistical units which can exist in states of different internal order. Non-equilibrium growth is combined with internal reorganization processes with a tendency to improve the crystalline order towards the fully stretched chain state. The thickness of the grown crystals is selforganized as a result of the interplay between a barrier to increase local chain order and the gain of enthalpy by accessing higher degrees of order (longer stems). When the reservoir of liquid chains is exhausted relaxation processes dominate. Since chains located at the crystal rims have a higher mobility they are prone to spontaneous reorganization into higher ordered state. This results in striking and very stable morphological phases such as overgrown rims or hole-rim patterns. Increasing the temperature yields to further morphogenesis. In particular droplet-like patterns can be obtained which show liquid-like features on large scales but are made up of highly ordered crystalline chains. We argue that final melting of polymer crystals is not related to the structure which is crystallized but only to the descendants in the morphogenesis of the chain crystal.
Journal of Polymer Science Part B: Polymer Physics, 2015
The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed highmolecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiation-step propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the ortho-rhombic form. An increase of the unit cell parameter a 0 is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements.
Morphologies of Polymer Crystals in Thin Films
Lecture Notes in Physics, 2007
We present microscopy investigations on the morphology of crystals of poly-2-vinylpyridine-polyethyleneoxid diblock copolymers (P2V P − P EO) in thin films with thicknesses below and above the thickness of a single lamella. For crystallisation temperatures above 45 • C, nucleation is highly unlikely. Thus, the resulting morphologies represent essentially single crystals, allowing us to relate morphology to the kinetics of crystal growth. In several examples we demonstrate the influence of thermal history and film thickness on molecular orientation and pattern formation during crystal growth. We discuss the analogies and differences between crystallisation of small molecules and polymers.
Role of orientation in kinetics of nucleation and growth of crystals in polymers
Journal of Polymer Science Part B: Polymer Physics, 1989
A theoretical framework is provided for generalizing the inferences drawn from the results of earlier experimental studies of kinetics of crystallization in oriented poly(ethy1ene terephthalate). The framework is obtained by combining extensions of classical nucleation theories in polymers '. ' and a theory of crystal growth with anisotropic incorporation of segments into growing crystals.'.-' I t is shown that, while a very strong dependence of rate of primary nucleation on orientation does exist, there is a only a much weaker dependence of rate of crystal growth on the orientation of the crystallizing polymer. The theoretical formulation provided here would allow qualitative estimates for comparison with experiments.
1991
Works on strictly uniform ultra-long n-alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (T,), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing T, where chain folding takes over. This remarkable rate inversion, observed for c 2 4 6 H494 and c198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a "self poisoning" phenomenon, where chain depositions, which occur transiently in the "wrong" conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.
New habits in branched polyethylene single crystals
European Polymer Journal, 2016
Faceted polymer single crystals obtained from solution have been examined for decades. Most of the studies have been performed with linear polyethylene. It is well known that solution-grown linear polyethylene single crystals may exhibit different and often complex morphologies and habits. However, the effect of molecular architecture (i.e., short chain branching) in the morphology of the single crystals remains practically unexplored. At the highest crystallization temperature investigated, the shape of the crystals is lozenge-like, but with curved and slightly truncated {110} faces. As the crystallization temperature decreases, strong changes in the width-to-length ratio and, consequently in the characteristic angles of the single crystal occur. Interestingly, the single crystals obtained at the lowest crystallization temperature explored exhibit a nearly square shape. This phenomenon has not yet been observed in linear high molecular weight polyethylene, for which the characteristic lozenge habit with straight {110} faces is retained as crystallization temperature decreases. The application of the Shcherbina and Ungar approach to fit the unusual shapes of the branched PE single crystals obtained in this work requires a strong decrease in the ratio of the rates of propagation to the right and to the left of the growth edge. This result is probably linked to different steric conditions of chain attachments into non-equivalent niches induced by the presence of branches.
Crystallization in ultra-thin polymer films
Thermochimica Acta, 2005
We present a computer model for polymer crystallization in ultra-thin films where chains are considered as dynamical units. In our model chains can change their internal state of order by cooperative motions to improve thermodynamic stability. The interplay between reorganization, enthalpic interactions and the morphology of crystals enables us to explain many properties of growth, morphogenesis and melting of polymer lamellae. We emphasize the relation between the thermodynamic stability of non-equilibrium crystals and morphological features which are beyond the average thickness of the lamellae. In particular, we show that melting of polymers is preceded by reorganization processes and the stability of polymer crystals is not necessarily related to the structure formed at the crystallization point. The simulations allow for the determination of some non-equilibrium properties such as the internal energy and the non-equilibrium heat capacity. We show that multiple-peak melting endotherms result from morphological transformations. The results of our computer simulations are compared with AFM observations in ultra-thin polyethyleneoxide films.
Kinetics of Crystallisation of Polymers - A Review
Progress in Rubber Plastics and Recycling Technology, 2002
A review covering nucleation modes and models of polymer crystallization kinetics. The classical models assume the rate of crystallization to be related to temperature only. For materials exhibiting low molecular mobility, e.g., polymers, time effects appear justified. Ziabicki's model (51-53) allows the rate to be related to time. In thermal nucleation, this relation stems from the delay of the steady-state condition to become established under specific external conditions. The athermal mechanism of nucleation produces another time effect. It involves no potential barriers to be overcome by a cluster to become a nucleus and proceeds only on account of the change in the criterion for the nucleus stability (critical size) as external conditions are modified. Experiments showed the (iso and non-isothermal) crystallization rate to be directly related to time. The underlying phenomenon involves the athermal nucleation occurring on crystal residues left in the melt and the relaxation effect upon subsequent thermal nucleation. The applicability of Ziabicki's model is demonstrated. MECHANISMS OF NUCLEATION IN THE CRYSTALLIZATION OF POLYMERS The crystallisation of polymers is usually regarded as a transformation comprising two stages: (primary) nucleation, and growth of crystals. Taking into account that the stage of crystal growth can be analysed in categories of (secondary) nucleation on the surface of already existing crystals, crystallisation can be regarded as processes of nucleation occurring in the bulk and on the surface of a crystal. The classical theory of nucleation (1-3) examines the one-dimensional process of formation and growth of aggregates as a result of the This paper was originally published in 'Polimery', Volume 46, No.11-12, p768 (in Polish).
The Controlled Evolution of a Polymer Single Crystal
Science, 2005
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