Polymer crystal growth mechanisms

In the classical Lauritzen-Hoffman theory for the mechanism of polymer crystal growth [106], it is assumed that the observed lamellar thickness corresponds to those crystallites that happen to have the largest growth velocity. However, this picture is hard to reconcile with the experimental observation that the thickness of polyethylene single crystals can be modulated by varying the temperature at which they are grown [117,118]. In fact, simulations by Doye et al. [119,120] suggest that the observed lamellar thickness does... 

Understanding the crystal growth mechanisms of these solution-processable polymers is required to control the self-assembled structure during film formation. Most studies on solution-deposited semiconducting polymer thin films for OTFTs pertain to PATs [6,10,15,23,24,27,30,35,36,44,46-48,51-81]. The key factors affecting self-assembly of these polymers during solution processing are as follows ... 

Classification and Nomenclature of Supramolecular Compounds, p. 267 Clathrate Hydrates, p. 274 Crystal Growth Mechanisms, p. 364 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly in Biochemistry , p. 1257 Supramolecular Polymers, p. 1443... 

The crystallization process can be studied isothermally or non-isothermally. The isothermal study can be used to elucidate the crystallizability of polymers, the nucleation, and the crystal growth mechanism. However, most polymers are processed under non-isothermal conditions. It would be more informative to study the crystallization behavior under both isothermal and non-isothermal conditions. 

Abstract. We review how the nucleation mechanism of polymer crystallization could be assigned to intramolecular processes and what are the preliminary benefits for understanding some fundamental crystallization behaviors. The speculative concept of molecular nucleation and the theoretical model of intramolecular nucleation have been elucidated in a broad context of classical nucleation theory. The focus is on explaining the phenomenon of molecular segregation caused by polymer crystal growth. 

In a parallel study, a biocompatible composite made of sulphonated PSU and HAp was prepared and characterized. The incorporation of HAp was achieved by spontaneous precipitation of HAp at controlled conditions of pH and temperature. The time of the precipitation process was used to regulate the content of inorganic component in the composite. The composite material prepared in this experiment contained 9 wt% of HAp and 91 wt% of polymer. Analysis of the kinetic results of the crystallization process showed that the growth of HAp on the film-shaped composite material followed a polynuclear (nuclei above nuclei) crystal growth mechanism [103],... 

The importance of twinned crystals in demonstrating that nucleation is the relevant growth mechanism has been realized since 1949 [64, 99]6. They were first investigated extensively in polymer crystals by Blundell and Keller [82] and they have recently received increased attention as a means of establishing, or otherwise, the nucleation postulate for lamellar growth [90, 91, 95,100-102]. The diversity of opinion in the literature shows that it is very difficult to draw definite conclusions from the experimental evidence, and the calculations are often founded upon implicit assumptions which may or may not be justified. We therefore restrict our discussion to an introduction to the problem, the complicating features which make any a priori assumptions difficult, and the remaining information which may be fairly confidently deduced. 

After the discovery of the photopolymerization of 2,5-DSP crystals, several types of photoproducts were found, not only the linear polymers, but some other derivatives, e.g. the V-shaped dimer or cyclophane (Hasegawa and Hashimoto, 1992). The photopolymerization occurs in a step-growth mechanism by cyclobutane formation between the excited olefin and the olefin in the ground state. 

We note here that all the information presently available on high molecular weight polymer crystal structures is compatible with the bundle model. While very nearly all crystalline polymer polymorphs involve all-parallel chain arrangements, even the only known exception, namely y-iPP [104,105], where chains oriented at 80° to each other coexist, is characterized by bilayers of parallel chains with opposite orientation. This structure is thus easily compatible with crystallization mechanisms involving deposition of bundles of 5-10 antiparallel stems on the growing crystal surface. Also the preferred growth... 

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