Isothermal growth of low molecular weight polyethylene single crystals from solution. 3. Kinetic studies

Abstract

Single crystals of four polyethylene fractions ranging in molecular weight from 3100 to 11 600 were grown isothermally from xylene solution by using the self-seeding technique. Growth rates of the crystals were measured as a function of solution concentration, molecular weight, and temperature of crystallization. The most interesting resulb concern the temperature dependence of the growth rates. For the higher molecular weight fractions the growth rate decreases monotonically when the crystallization temperature increases. In contrast for fractions in the molecular weight range of 3OOC-4OOO the growth curves show a notched appearance in which two branches can be distinguished. These branches intersect at a transition temperature where the temperature coefficient of the growth rates changes discontinuously. This behavior is interpreted to mean that the transition temperatures correspond to changes in the conformation of the chains deposited on the growth faces. Thus, as the crystallization temperature decreases, there is a transition from extended-to once-folded-chain growth, for a fraction with a molecular weight of 3100, and from once-folded to twice-folded for crystals with a molecular weight of 4050. This interpretation of the growth transitions is rationalized with the observed continuous temperature variation of the lamellar thickness by invoking a model of the growth process in which the chain ends are preferentially rejected to the exterior of the lamellae as cilia. With decreasing crystallization temperature the pendant cilia become longer and form a fold when their length becomes equal to the lamella thickness. In accordance with the kinetic theory of polymer crystal growth based on coherent surface nucleation, the growth rate data for the four fractions were analyzed by plotting log G vs. l/TAT, where G is the growth rate, Tis the crystallization temperature, and AT is the supercooling. Values of the basal surface free energies ue derived from the analysis are compared with those obtained by two independent methods and discussed in relation to the molecular structure of the crystals. It is shown that theory and experiment are consistent for crystals in which n (the average number of folds per molecule) > 1 and acceptable values of u, are obtained. Values of ue for once-folded-chain crystals are too low, possibly reflecting inadequacies in the theory. On the other hand, for extended-chain crystals the growth kinetics follow a completely different law in which G a AT. These results pose a challenge to derive the correct theory for the case of extended and once-folded chains and account for the transition between them.

Structure of Polymer Single Crystals

“…The technique of self-decoration was used by Labaig (1978) in a systematic study of crystal growth rates of sharp fractions of polyethylene similar to those used later for solution crystallization (Leung et al, 1985). This study established the lenticular shape of crystals grown in the bulk.…”

Studies of low molecular weight polyethylene single crystals by differential scanning calorimetry

“…2) of crystals of PE3100 grown at temperatures in the range of 60 to 75 ~ are quite different from those of the higher molecular weight fractions. At heating rates lower than 10 ~ rain, only one peak is observed whereas in the higher molecular weight fractions two peaks can still be observed even at heating rates as low as 0.6 ~ Lamellae thickness data of polyethylene single crystal determined by low angle x-ray diffraction [10] show that in the crystallization temperature range studied in this work, crystals of PE6750 and PE11600 are at least twice folded-chain crystals while the crystals of PE3100 are either extended or once folded-chain crystals. These results suggest that in the low molecular weight region, crystals of shorter chain length and fewer number of foldings per chain can reorganize more rapidly upon heating and are thus more susceptible to heat annealing than higher molecular weight crystals with a higher number of foldings per chain.…”

“…From the calculated value of the extended chain length. (which is 259 A) and the lameUar thickness data measfired by low angle x-ray scattering [10], the number of stems per molecule in the crystal can be calculated and are listed in table 2. According to these resuks, the chain conformation for the crystals grown at 60.1 ~ and 68 ~ is Considering the "fractional stem incorporation" model, the ends of neighboring chains may juxtapose themselves to simulate "continuity" between the chains in each strip, or else voids will be created in the crystals (see fig.…”

“…This suggests that these crystals do not crystallize according to the "fractional stem incorporation" model. Recent studies [10] on the morphology of single crystals of PE3100 fraction grown at 74 ~ show that there is no secterization in the single crystals. This implies the extended-chain nature of the crystals.…”

A Review on Polymer Crystallization Theories

“…Generally speaking, polymer crystallization can be classified into two categories: one is valid for melt-grown crystals and solution-grown crystals with relative high molecular weight, in which the crystal growth rate G is nucleation-controlled and exponentially depends on 1/T∆T; the other is valid for solution-grown crystals with relative low molecular weight (Mw), in which the growth rate G is diffusion-controlled and linearly proportional to supercooling ∆T [150,151].…”