Growth Kinetics of Stacks of Lamellar Polymer Crystals

Abstract

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.

A nucleation mechanism leading to stacking of lamellar crystals in polymer thin films

“…(a). Simultaneously, as T C increases, the crystal growth rate decreases, accompanied by an increase in detachment probability of polymers at the crystal growth front and a small increase of the diffusion coefficient of the detached and molten polymers . In our experiments, we observed stacks containing a large number of correlated crystalline lamellae only for T C near the melting temperature.…”

“…). Based on our AFM measurements, the thickness of each lamella within such stacks was found to be around 3 nm, equivalent to the length of a 19 CH 2 sequence between two methyl phosphonate groups, indicating that the methyl phosphonate groups were not included in the lamellar crystal . In the top part of the stack shown in Fig.…”

“…As an attempt to find out if there is a limit to the number of superposed secondary lamellae in such stacks, we performed experiments on thin films of a highly flexible and more rapidly diffusing polymer, namely PE19Me, which thus forms more rapidly and frequently secondary lamellae . This precision polymer has methyl phosphonate groups after every 19 CH 2 sequence along the polymer chain.…”

“…(a) Optical micrograph (size 97 × 92 μm 2 ) showing stacks of superposed lamellar crystals of PE19Me formed at T C = 68 °C in ca 15 nm thin film after crystallization for 14 h. The nucleation density of the crystals was ca 10 3 mm −2 , which was controlled by a self‐seeding temperature protocol . (b) Three‐dimensional representation of AFM image of the stack of lamellar crystals marked by blue square in (a), revealing that ca 40 lamellae were superposed in this stack.…”

“…Thus, to follow the crystal growth in real time, the samples were crystallized at temperatures very near to T M . To compensate for the missing possibility of homogeneous nucleation at T C very near to T M , we adopted a self‐seeding mechanism to control the nucleation density of PE19Me crystals …”