Crystallization of a polymer on a surface

Abstract

We have studied the structure and free energy landscape of a semiflexible lattice polymer in the presence of the surface of a polymer crystal. At low temperatures coexistence of two-dimensional integer-folded crystals is observed. As the temperature is increased there is a transition from these crystalline configurations to a disordered coil adsorbed onto the surface. The polymer then gradually develops a three-dimensional character at higher temperatures. We compute the free energy as a function of increasing crystallinity and compare with the free energy profiles assumed by the Lauritizen-Hoffman surface nucleation theory of polymer crystallization. Our free energy profiles exhibit a ''sawtooth'' structure associated with the successive formation of chain folds. However, in the early stages of crystallization our profiles significantly deviate from those assumed by surface nucleation theory because the initial nucleus is not a single stem but two incomplete stems connected by a fold. This finding has significant implications for the theoretical description of polymer crystallization.

Kinetic Monte Carlo simulations of the growth of polymer crystals

“…The possibility of a two-stem nucleus was first suggested by Point 24 and recently strong evidence for this nucleus has been obtained in a study of the free energy profiles along specific crystallization pathways. 37 This finding about the structure of the initial nucleus puts a significant dent into LH theory because its assumption about the nature of the initial nucleus, and the resulting free energy barrier, is crucial to the theory. However, this finding is rendered somewhat irrelevant because the above results also undermine a more fundamental tenet of the LH theory, namely, the assumption that the thickness of a layer is determined by the initial nucleus.…”

“…Such a nucleus is energetically favoured because of the interactions between the two stems. As it avoids the large free energy barrier associated with a single-stem nucleus 37 there is now no advantage in the initial stems being shorter than the average in the layer ͓Fig. 4͑a͔͒.…”

“…Simulations have shown that upon melting of the crystalline polymer a two-dimensional disordered state was formed for all values of ⑀ g , and only at higher temperature does the polymer become more three dimensional. 37 This suggests that Eq. ͑1͒ is an appropriate form by which to describe the coil.…”

“…33 This interaction scheme has recently been used to investigate the phase behavior of isolated homopolymers [34][35][36] and a homopolymer in the presence of a surface. 37 The low temperature behavior is of particular interest since the polymers were found to adopt cuboidal 36 ͑rectangular when on a surface 37 ͒ ''crystalline'' configurations involving chain folding. However, these structures were observed for thermodynamic reasons-they are lowest in energy-whereas chain folding occurs in polymer crystallization for kinetic reasons.…”

High-speed atomic force microscopy for materials science

“…This application generated data that was used to confirm the validity of particular growth models. [124][125][126] This is the first study where high growth rates can be observed with high spatial and temporal resolution (normally fast growth is measured with optical microscopy, which is inherently limited to low spatial resolution by the diffraction limit of light).…”