Cooperativity during Melting and Molecular Exchange in Micelles with Crystalline Cores

Abstract

Molecular exchange processes are important equilibration and transport mechanisms in both synthetic and biological self-assembled systems such as micelles, vesicles, and membranes. Still, these processes are not entirely understood, in particular the effect of crystallinity and the interplay between cooperative melting processes and chain exchange. Here we focus on a set of simple polymer micelles formed by binary mixtures of poly(ethylene oxide)-mono-n-alkyl-ethers (C n-PEO5) which allows the melting point to be tuned over a wide range. We show that the melting transition is cooperative in the confined 4-5 nm micellar core, whereas the exchange process is widely decoupled and unimeric in nature. As confirmed by differential scanning calorimetry, the total activation energy for ejecting a molecule out of the micellar core below the melting point is the sum of the enthalpy of fusion and the corresponding activation energy in the melt state. This suggests that a "local, single-chain melting process" preludes the molecular diffusion out of the micelle during chain exchange.

Time-resolved small-angle neutron scattering studies of the thermally-induced exchange of copolymer chains between spherical diblock copolymer nanoparticles prepared via polymerization-induced self-assembly

“…stericallystabilized nanoparticles) in both aqueous and non-aqueous media. 39,40,[49][50][51][52][53][54][41][42][43][44][45][46][47][48] Two principal mechanisms have been suggested for this process in the literature: (i) the chain expulsion/insertion mechanism and (ii) the micelle fusion/fission mechanism. 40,[55][56][57][58][59] It appears to be generally accepted that the latter mechanism is energetically demanding, and hence most likely not applicable for diblock copolymer micelles under normal experimental conditions.…”

Spherical Micelles with Nonspherical Cores: Effect of Chain Packing on the Micellar Shape

“… 48 In contrast, C 28 -dhPEO5 exhibits active chain exchange, and even at the lowest experimental temperature, 40 °C, molecular exchange takes place within minutes. 23 Interestingly, the aggregation number still remains unchanged above the melting temperature, and only the core shape is altered.…”

“…Previous SAXS experiments in combination with density measurements and differential scanning calorimetry (DSC) have shown the n -alkyl cores in C n -PEO x micelles crystallize. 19 , 21 , 23 , 29 Increasing the temperature above the melting point T m , melting of the n -alkyl chains can be observed in the SAXS curves by a significant increase in the core scattering contribution caused by the lowered n -alkyl density and therefore increased core contrast (see Figure 2 a for an example data set of C 28 -PEO5). In addition, DSC melting traces revealed a clear endothermic phase transition at T m (see Figure 2 b).…”

“… 19 , 28 Furthermore, our observed transition temperatures are quite close to the bulk melting temperatures of corresponding n -alkanes, 32 and the difference can be excellently described by a simple Gibbs–Thomson behavior. 23 , 29 Nonetheless, one cannot describe the state of the n -alkane blocks within the core as crystalline in a classical sense because the maximum domain size is very small as it is constrained by the micellar core radius R c . To obtain direct evidence of the n -alkyl crystallization and information about their conformation inside the crystalline domains, we performed wide-angle X-ray scattering (WAXS) experiments on selected C n -PEO x micelles.…”

“…In all our previous works on the C n -PEO x system, we used a spherical core–shell model for starlike micelles which yielded very good agreement with experimental small-angle scattering data (see for instance Figure 2 a). 15 , 16 , 19 , 20 , 23 , 29 However, it is not obvious how (partially) crystalline n -alkyl chains could be arranged in a spherical core. Vilgis and Halperin predicted that diblock copolymers with crystallizing solvophobic blocks would form disklike cores, where the core chains align along their axis and the solvophilic chains stick out from the basal planes.…”