Correlating Polymer Crystals via Self-Induced Nucleation

Hui Zhang, Muhuo Yu, Bin Zhang, Renate Reiter, Maximilian Vielhauer, Rolf Mülhaupt, Jun Xu, and Günter Reiter

Phys. Rev. Lett. 112, 237801 – Published 9 June 2014

Abstract

Crystallizable polymers often form multiple stacks of uniquely oriented lamellae, which have good registry despite being separated by amorphous fold surfaces. These correlations require multiple synchronized, yet unidentified, nucleation events. Here, we demonstrate that in thin films of isotactic polystyrene, the probability of generating correlated lamellae is controlled by the branched morphology of a single primary lamella. The nucleation density $n_{s}$ of secondary lamellae is found to be dependent on the width $w$ of the branches of the primary lamella such that $n_{s} \sim w^{- 2}$ . This relation is independent of molecular weight, crystallization temperature, and film thickness. We propose a nucleation mechanism based on the insertion of polymers into a branched primary lamellar crystal.

Received 26 March 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.237801

Authors & Affiliations

Hui Zhang¹, Muhuo Yu², Bin Zhang¹, Renate Reiter^1,3, Maximilian Vielhauer^3,4, Rolf Mülhaupt^3,4, Jun Xu⁵, and Günter Reiter^1,3,*

¹Physikalisches Institut, Albert-Ludwigs-Universität, 79104 Freiburg, Germany
²State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, 201620 Shanghai, China
³Freiburg Materials Research Center (FMF), Albert-Ludwigs-Universität, 79104 Freiburg, Germany
⁴Institute for Macromolecular Chemistry, Albert-Ludwigs-Universität, 79104 Freiburg, Germany
⁵Institute of Polymer Science and Engineering, Department of Chemical Engineering, School of Materials Science and Technology, Tsinghua University, 100084 Beijing, China

^*guenter.reiter@physik.uni-freiburg.de

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Issue

Vol. 112, Iss. 23 — 13 June 2014

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Images

Figure 1
AFM topographic images showing stacks of lamellae of i-PS ( $M_{w} = 13 000 g / mol$ ) single crystals. In (a) the stack is initiated at the center and in (c) off center of the underlying crystal. The crystals were grown from a (a) 20 nm thick film at $195 ° C$ and a (c) 16 nm thick film at $190 ° C$ . (b) and (d) show the corresponding three-dimensional height images cut along the black dotted lines indicated in (a) and (c). The sizes of the images are (a) $16 \times 16 μ m^{2}$ and (c) $14 \times 14 μ m^{2}$ .
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Figure 2
Series of AFM topographic images of i-PS ( $M_{w} = 13 000 g / mol$ ) single crystals indicating the dependence of the probability of forming secondary lamellae on crystallization temperature $T_{c}$ and film thickness $h$ . Top row: $h$ constant (14 nm) and increasing $T_{c}$ from (a) $150 ° C$ , (b) $160 ° C$ , and (c) $170 ° C$ to (d) $180 ° C$ leads to a decreasing number of secondary lamellae. Bottom row: constant $T_{c}$ of $160 ° C$ and increase in $h$ from (e) 12 nm, (f) 14 nm, and (g) 16 nm to (h) 18 nm leads to an increasing number of secondary lamellae. The scale bar represents $10 μ m$ .
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Figure 3
Dependence on crystallization temperature of (a) growth rate $G$ (the velocity at which the main tip of the underlying crystal advanced); (b) tip radius of curvature $ρ$ (solid) and width of the side branches $w$ (open) of the underlying dendritic crystal; (c) the products of $w^{2} G$ (open) and $ρ^{2} G$ (solid). All graphs contain data for different molecular weights [ $7100 g / mol$ (stars), $13 000 g / mol$ (diamonds, circles and squares), and $48 000 g / mol$ (pentagons)] and different film thicknesses [8 nm (stars and diamonds), 9 nm (pentagons), 10 nm (circles), and 12 nm (squares)].
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Figure 4
Density $n_{s}$ of secondary lamellae as a function of (a) crystallization temperature $T_{c}$ and (b) side branch width $w$ , shown for samples of different molecular weights [ $7100 g / mol$ (stars), $13 000 g / mol$ (diamonds, circles, squares), and $48 000 g / mol$ (pentagons)] and film thicknesses [8 nm (stars and diamonds), 9 nm (pentagons), 10 nm (circles), and 12 nm (squares)]. Graph (a) indicates that $n_{s}$ decreased rapidly with increasing crystallization temperature for all samples. While the slope of this decay is similar for all samples, the curves are shifted to higher temperatures for higher $M_{w}$ . In graph (b), all curves approximately superimpose. The solid line is a linear fit of all secondary lamellar densities $n_{s}$ as a function of side branch width $w$ , yielding a slope of $- 2$ with an error bar of $\pm 0.1$ .
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Figure 5
AFM height image of an i-PS ( $M_{w} = 13 kg / mol$ ) single crystal (a) shows the start of secondary lamellae formation at a junction of the branches of the underlying dendrite. The crystal was grown from an 8-nm thick film at $150 ° C$ . The corresponding three-dimensional height image of the area marked by a red rectangle in image (a) indicates that the secondary lamella grew at a junction of the underlying primary lamella (b). The sizes of the images are $32 \times 32 μ m^{2}$ and $2 \times 1.5 μ m^{2}$ for (a) and (b), respectively.
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