Most theoretical concepts of polymer crystallization have evolved around monolamellar single crystals as model systems. However, such approaches do not account for an important and unique aspect of crystallization of long flexible molecules: the correlated stacking of lamellar crystals. In our experimental work, we focus on the growth kinetics of such stacks of lamellae in thin films of poly(nonadecane methylphosphonate). Interestingly, concurrent with a decrease in lateral lamellar growth, we observed an increase in vertical growth, that is, an increase in the number of stacked crystalline lamellae. Intriguingly, in contrast to lateral lamellar growth, the rate of such vertical growth increased with decreasing degree of undercooling. Moreover, we show the possibility of forming three-dimensional polymer quasi-single crystals. Some of the formed stacks of lamellar crystals were about 100 times thicker than the initial film; that is, they had a thickness of about 20 times the contour length of the polymer and contained about 800 stacked lamellae. We propose that growth kinetics of stacking of lamellae is governed by (i) the probability of forming self-induced nuclei, (ii) the detachment probability of crystalline stems, and (iii) the influx of molten polymers toward the growth front.
nonequilibrated chain conformations on crystallization kinetics under quiescent conditions has not yet been examined intensively. On the other hand, interestingly, the importance of flow-induced deviations in chain conformations has received considerable attention. [10][11][12][13][14][15] It has been shown that flow enhances the nucleation probability in polymers, [12,16] can induce intermediate states, [10,16] and can change crystal growth rates. [12] In this context, even at temperatures well above the melting temperature of the polymers, it has been shown through dewetting experiments that shear may generate correlated clusters of monomers exhibiting cooperative dynamics, enhancing the lifetime of nonequilibrated structures in the melt. [7] In this report, we study quiescent crystallization kinetics of polymer films of high molecular weight isotactic polystyrene and address the importance of the degree of deviations in chain conformations from equilibrium. To this end, we systematically varied the time provided for equilibration at an annealing temperature, which was higher than the nominal melting temperature of the polymer (T m DSC = 220 °C). In all our experiments, we have fixed the annealing temperature at 250 °C and the crystallization temperature at 200 °C. Accordingly, the only variable in our study was the time provided for equilibration, which was varied between 0 and 90 min. We have observed an exponential decay of the growth rate and the nucleation density of the crystals with increase in t a , a behavior which we attribute to the relaxation of preparation-induced residual stresses. Interestingly, we have observed a systematic variation in the morphology from spherulites to hexagonal crystals of rather uniform height and finally to hexagonal lamellar crystals, with periodic modulations in height. We provide tentative interpretations based on (preparation-induced) deviations in chain conformations affecting polymer diffusion, in particular to the crystal-melt interface. Experimental SectionIsotactic polystyrene (iPS) of molecular weight 947 kg mol −1 was used (synthesized by Dr. M. Vielhauer in the group of Prof. R. Mülhaupt, Freiburg/Germany, with a dispersity index, Đ = 1.5, and more than 90% of isotactic pentads) [17] with a nominal melting temperature T m DSC = 220 °C, while the equilibrium melting temperature was T m E = 242 °C. [16] Due to the low growth Crystallization KineticsIn this paper systematic changes in the isothermal crystallization kinetics (at T c = 200 °C) of nonequilibrated isotactic polystyrene films, generated through spin coating are reported. Kinetics is tuned by annealing films at T a = 250 °C, i.e., above their nominal melting temperature (T m DSC = 220 °C). An exponential decrease in growth rate and a significant decrease of crystal nucleation density by approximately three orders of magnitude are observed by increasing the time (t a ) provided for equilibration at T a . The characteristic exponential decay time is approximately four orders of magnitude longer than the e...
We report novel morphologies with periodic height modulations of isotactic polystyrene (iPS) crystals, resulting from alternating stacks of correlated lamellae. Systematic experiments were performed on iPS films of several thicknesses (h) for varying degrees of undercooling, ΔT = T m ∞ – T C, where T m ∞ and T C are the equilibrium melting temperature and the crystallization temperature, respectively. We demonstrate that the spatial period (λ), i.e., the mean distance between neighboring stacks of lamellae, exhibits a power-law dependence on h and an exponential dependence on 1/ΔT. We propose that self-induced nucleation of stacked layers caused periodic deviation in the growth rate of iPS crystals, yielding periodic height modulations.
Systematic studies based on well-controllable model systems aim at understanding how crystallization from a melt or solution of randomly coiled polymers leads to the formation of mono-lamellar crystals. However, besides mono-lamellar crystals also various other morphologically simple but yet not well understood structures are found. In particular, stacks of correlated lamellar crystals have been observed since the early days of the study of polymer crystallization. Here, we demonstrate that a recently proposed mechanism of self-induced nucleation within lamellar crystals provides a possibility to explain how in such stacks lamellar crystals can be correlated. Examining various polymer systems, we show that the probability for generating self-induced nuclei depends on the morphology of an initiating dendritic basal lamellar crystal. In addition, we provide evidence that this self-induced nucleation mechanism, together with a high rate of transport of molten polymer to the fold surface, may allow the formation of polymer crystals with similar size in all three dimensions, containing a large number of superposed correlated lamellae.
Star-shaped block copolymers are of interest as versatile and highly functional polymeric building blocks and show interesting solution properties or self-assembly. Herein, we report the synthesis of novel, double hydrophilic...
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