ABSTRACT:The kinetics of fractionated polyethylenes was investigated by the method of dilatometry. A discontinuous change in crystallization rates was observed for molecular weights 4300 and 17300. This discontinuity is shown to be an important factor in clarifying the mechanism of the formation of extended-chain crystals. Extended-chain crystals were obtained within the region where folded-chain crystals were prohibited. Extended-chain crystallization had a mode of three-dimensional growth. It is suggested that the formation of extended-chain crystals at atmospheric pressure is distinct from that under high pressure. A theory of polymer crystallization is proposed in order to explain the transition in crystalJization rates.KEY WORDS Polyethylene I Crystallization Rates I Extended-Chain Crystals I Avrami Exponent I Stochastic Process I Surface Nucleation Rate I Extended-chain crystals can be produced at atmospheric pressure, depending on the crystallization conditions. 1 It was shown, however, that it became harder to give rise to extendedchain crystals as the molecular weight of polymers was increased. In extended-chain crystallization under high pressure, the situation is completely reversed, that is, extended-chain crystals are easily produced with increasing molecular weight. 2 -4 As regards this inconsistency, we consider that it is necessary to clear up whether the mechanism of the formation of extended-chain crystals at atmospheric pressure and at high pressures is the same or not. Under high pressure, extended and folded-chain crystallization are considered to be rival processes from the kinetical point of view. Meanwhile, little is known about the mechanism of the formation of extended-chain crystals at atmospheric pressure, because the kinetics of extended-chain crystals has not been fully investigated.In this study the crystallization rates of three kinds of fractionated linear polyethylenes with different molecular weights were measured in order to get information about the mechanism of the formation of extended-chain crystals at atmoPolymer J., Vol. 11, No.3, 1979 spheric pressure, and the growth mode of extendedchain crystallization was compared with that at high pressures.We also present a theory of the growth rates of m-times folded-chain crystals with finite molecular weight for the purpose of explaining the transition in crystallization rates observed in this experiment.
EXPERIMENTALThe materials used in this work were the viscosity-average molecular weights M. =4300, 17300. and 150000 of linear polyethylene fractions (Mw! M, was in the range 1.2-1.3) that were obtained from Sholex 6000C by the column fractionation method.We studied crystallization kinetics by using dilatometric techniques. The materials which were originally in the form of powder were melted and cooled to room temperature under vacuum. About 300 mg of the materials were put into the dilatometer, whose diameter was 0.8 mm; after being filled with fresh mercury, the dilatometer was evacuated overnight. The dilatometer was p...