Effect of Pulsed Magnetic Fields on Crystallization of Polymers
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Effect of Pulsed Magnetic Fields on Crystallization of Polymers Mark N. Levin Nuclear Physics Department, Voronezh State University, Voronezh 394693, RUSSIA, [email protected] ABSTRACT The effect of pre-crystallization pulsed magnetic field (PMF) treatment of linear flexiblechain polymers on kinetics of their crystallization and structure in the crystalline state has been studied in polydimethylsiloxane (PDMS) and polyethylene oxide (PEO). It was established that the PMF treatment can destruct the initial net of non-chemically bonded "physical nodes" of the polymer and thus turn the melt into a metastable state with the increased sensitivity to regimes of crystallization. The effect revealed can lay the foundation for the development of new effective methods of controlled crystallization of the flexible-chain polymers, including fractionation and crystallization of polymorphic polymers in the chosen structural modification.
INTRODUCTION In recent years the unique ability of weak (< 1 T) pulsed magnetic fields (PMF) to modify the real structure and properties of a wide class of non-magnetic solids has been intensively studied. The PMF-induced changes in the microstructure of diamagnetic crystals were particularly observed in Czochralski-grown silicon (Cz-Si) single crystals [1, 2] and later in AIIIBV semiconductor compounds [3] and in the hydrogen-bonded ferroelectrics [4]. The PMF treatments allow one to activate semiconductor surfaces for thin film deposition and to improve characteristics of thin semiconductor layers [5], as well as to control defects in the crystals for low temperature gettering [6], 3-D islanding and detecting of latent defects [7]. It should be pointed out that the energy of the PMF action associated with Zeeman effect is negligible, as the condition µBB ~ 10-3 kT takes place for the applied magnetic fields (here µB is the Bohr magneton, B is the magnetic induction, k is the Boltzmann constant, and T is the absolute temperature). The interpretation of the effects observed is based on the notion of a magnetic field influencing the kinetics of spin-dependent reactions involving radical pairs. It is believed that weak magnetic fields can remove prohibition from the electron transitions with a spin change. This leads to the weakening of the bonds in defect complexes, while the real crystal structure is modified at the expense of thermal and elastic energy of the crystal lattice. The prominent effect of the PMF treatment on polymers was recently discovered in organosilicon polymeric materials [8] and polyethylene oxides [9]. The effect manifests itself in the irreversible changes of temperatures of crystallization (Tc) and melting (Tm) of the polymers. The effect was purposely explained in [8] by cross-linking the polymer chains under the action of PMF on spin-controlled radical reaction of the chain terminal units. The PMF-induced reduction in Tm was associated with a decrease in the size of the supramolecular structures formed during the crystallization of polymers. The aim of the present
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