Polymer crystallization

Outline

1. General background
2. Computational method
3. Some recent results
4. Current research projects
5. Collaboration
6. Related papers

Many polymers, whose properties favor the formation of locally ordered structures, crystallize at low temperatures. However, the connectivity of the long chain molecules usually hinders the formation of perfect crystals and leads to semicrystalline materials. For these heterogeneous materials one would like to better understand the pathways leading to a particular composition of crystalline and amorphous regions as well as the influence of external control parameters on the resulting structures.

We try to tackle this problem by direct molecular dynamics simulations of a mesoscopic coarse-grained model for poly-vinylalcohol (CG-PVA). Our model goes one step further than usual united-atom models and resumes all atoms of a monomer into one sphere, and it contains a specific angular bending potential. This approach appeares to be extremely efficient.^[1,2]

3. Some recent results

Figure: Formation of a crystal lamella as observed with molecular dynamics. The simulation box contains 500 monodisperse chains of length N=400. Right: Visualization of 13 chains of the first crystalline domaine. The crystallization domain is observed to extend over several chain diameters.

With these approximations, the crystallization from the homogeneous nuclueation in the melt up to the formation of chain-folded crystal lamellae can be observed. Furthermore, it yields insight into the nucleation process, the growth of the crystal and into the single chain conformations in the crystal. See the figure.

We also discuss the influence of different simulation parameters, and in particular of the angular potential. We characterized the melt at a reference temperature and looked for correlations with the crystallization temperature determined during continuous cooling. For most quantities as persistence length, radius of gyration or relaxation times, no trivial correlation can be found, except for the fraction of stretched tt conformations in the melt: the higher this fraction, the easier is the crystallization and thus the higher the temperature where ordering starts.

In near future we plan to study the following topics:

• Further improvement of coarse-grained models for chemical polymers,
• Study of single chain properties (collapse with decreasing temperature, etc.)
• Influence of chain length, cooling rate, etc. on the crystallization process and the resulting structures
• Structure formation of crystallizable polymer melts close to solid, structured substrates. Due to the large loss of conformational entropy the crystallization process, especially at weak undercooling, is often triggered by container walls, dispersed foreign particles and other heterogeneities. Our numerical studies (H. Meyer) are accompanied by theoretical developments (A. Johner).

H. Meyer (principal investigator), J.-P. Ryckaert (Bruxelles), J.-U. Sommer (Dresden)

1. H. Meyer et F. Müller-Plathe, J. Chem. Phys. 115, 7807 (2001).

2. H. Meyer et F. Müller-Plathe, Macromolecules 35, 1241 (2002).

3. H. Meyer
   Structure Formation and Chain-Folding in Su­percooled Polymer Melts.
   Some Ideas from MD Simula­tions with a Coarse-Grained Model
   In: Polymer Crystallization: Observations, Concepts and Interpreta­tion, edited by J.-U. Sommer et G. Reiter, Lecture Notes in Physics, Vol. 606 (Springer, Berlin-Heidelberg, 2003).

4. D. Reith, H. Meyer et F. Müller-Plathe, Macromolecules 34, 2335 (2001).

5. D. Reith, H. Meyer et F. Müller-Plathe, Comp. Phys. Comm. 148, 299 (2002).

6. G. Strobl, Eur. Phys. J. E 3, 165 (2000).

7. T. Vettorel, H. Meyer
   Coarse-graining of short polyethylene chains for studying polymer crystallization
   J.Chem. Theo. Comput. 2, 616 (2006)

8. T. Vettorel, H. Meyer, J. Baschnagel, M. Fuchs
   Structural Properties of Crystallizable Polymer Melts: Intrachain and Interchain Correlation Functions
   Phys. Rev. E 75, 041801 (2007)