Crystallization kinetics of long-chain n-alkanes from the melt and from solution

Crystallization of Long-Spaced Precision Polyacetals III: Polymorphism and Crystallization Kinetics of Even Polyacetals Spaced by 6 to 26 Methylenes

In this paper we extend the study of polymorphism and crystallization kinetics of aliphatic polyacetals to include shorter (PA-6) and longer (PA-26) methylene lengths in a series of even long-spaced systems. On a deep quenching to 0 °C, the longest even polyacetals, PA-18 and PA-26, develop mesomorphic-like disordered structures which, on heating, transform progressively to hexagonal, Form I, and Form II crystallites. Shorter polyacetals, such as PA-6 and PA-12 cannot bypass the formation of Form I. In these systems a mixture of this form and disordered structures develops even under fast deep quenching. A prediction from melting points that Form II will not develop in polyacetals with eight or fewer methylene groups between consecutive acetals was further corroborated with data for PA-6. The temperature coefficient of the overall crystallization rate of the two highest temperature polymorphs, Form I and Form II, was analyzed from the differential scanning calorimetry (DSC) peak crystallization times. The crystallization rate of Form II shows a deep inversion at temperatures approaching the polymorphic transition region from above. The new data on PA-26 confirm that at the minimum rate the heat of fusion is so low that crystallization becomes basically extinguished. The rate inversion and dramatic drop in the heat of fusion irrespective of crystallization time are associated with a competition in nucleation between Forms I and II. The latter is due to large differences in nucleation barriers between these two phases. As PA-6 does not develop Form II, the rate data of this polyacetal display a continuous temperature gradient. The data of the extended polyacetal series demonstrate the important role of methylene sequence length on polymorphism and crystallization kinetics.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical star-like micelles i.e. a constant core density profile, ∼r(0), and a diffuse corona density profile, ∼r(-4/3). Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, Nagg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, Nagg ∼ n(2) similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the Cn-PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

A manifestation of the Ostwald step rule: Molecular-dynamics simulations and free-energy landscape of the primary nucleation and melting of single-molecule polyethylene in dilute solution

The paper presents numerical results from extensive molecular-dynamics simulations of the crystallization process of a single polyethylene chain with N=500 monomers. The development of the ordered structure is seen to proceed along different routes involving either the global reorganization of the chain or, alternatively, well-separated connected nuclei. No dependence on the thermal history was observed at the late stages of the crystallization. The folding process involves several intermediate ordered metastable states, in strong analogy with the experiments, and ends up in a well-defined long-lived lamella with ten stems of approximately equal length, arranged into a regular, hexagonal pattern. This behavior may be seen as a microscopic manifestation of the Ostwald step rule. Both the metastable states and the long-lived one are evidenced as the local minima and the global one of the free-energy landscape, respectively. The study of the microscopic organization of the lamella evidenced that the two caps are rather flat, i.e., the loops connecting the stems are short. Interestingly, annealing the chain through the different metastable states leaves the average number of monomers per loop nearly unchanged. It is also seen that the chain ends, the so-called cilia, are localized on the surface of the lamella, in agreement with the experiments, and that structural fluctuations take place on the lamella surface, as noted by recent Monte Carlo simulations. The study of the melting process evidences that the degree of hysteresis is small.

In situ annealing and thickening of single crystals of C294H590 observed by atomic force microscopy

The annealing behavior of twice-folded crystals of the long-chain alkane C294H590 is examined in situ, in real time, by atomic force microscopy (AFM). AFM is capable of following processes in real time provided that the time scale is sufficiently long for several images to be collected during the process. In this paper, we focus on the temperature dependence and the thickened morphology. We are able to investigate where the thickening starts and how this depends on temperature and how melting is influenced by morphology. By following the motion of holes within the crystal, a lower limit for the rate of diffusion of crystalline polyethylene is estimated. We also focus on the substrate effect on the crystal morphology and thickening, using mica, glass, and graphite.

Synthesis, Solid-State Structure, and Surface Properties of End-Capped Poly(l-lactide)

End-capped poly(L-lactide) (PLLA) samples with dodecyl or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester were synthesized by ring-opening polymerization of L-lactide in the presence of zinc dodecanoxide or zinc 2-(2-(2-methoxyethoxy)ethoxy)ethoxide as a catalyst, respectively. On the basis of NMR analysis, it was confirmed that the carboxylic acid chain ends of PLLA molecules were selectively substituted by dodecyl or MEEE ester groups. To evaluate the wettability on the surface of end-capped PLLA films, the advancing contact angle (thetaa) with water was measured. The amorphous PLLA films showed relatively similar thetaa values regardless of the chemical structure of the polymer chain end. In contrast, the thetaa values of semicrystalline films were varied over a wide range, dependent on the chemical structure of the chain end. In addition, the thetaa values of dodecyl ester end-capped PLLA film with low molecular weight increased with an increase in the crystallization temperature. Both the crystallinity and lamellar thickness of dodecyl ester end-capped PLLA films increased with the crystallization temperature. These results suggest that the segregation of the chain ends on the PLLA film surface was strongly affected by the crystallization conditions.

Morphological and kinetic analyses of regime transition for poly[(S)-lactide] crystal growth

Regime transitions of poly[(S)-lactide] (PLA) crystal growth from the melt were investigated by studying the morphological changes and carrying out kinetic analysis using microscopic techniques. PLA thin films with an average layer thickness of 100 nm were isothermally crystallized at a given crystallization temperature after melting at 220 degrees C. Following isothermal crystallization at a temperature below 145 degrees C, uniform two-dimensional spherulites having stacked flat-on lamellar texture were developed throughout the PLA thin films. On the basis of electron diffraction analysis for two-dimensional spherulites of PLA, it was found that the average growth direction of an individual lamellar crystal was parallel to the crystallographic b axis. At temperatures above 150 degrees C, hexagonal lamellar crystals were formed from the melt. Electron diffractograms of these lamellae showed that the crystal had orthogonal packing of PLA molecules and a truncated-lozenge-shaped growth behavior. The growth surfaces of the hexagonal crystal were parallel to either the crystallographic (110) or the (100) plane. The PLA crystal growth rate along the b axis direction was evaluated at various crystallization temperatures of the thin films. Kinetic analysis of crystal growth in the PLA thin film demonstrated that the regime transitions of PLA crystal growth, from regime III to regime II and from regime II to regime I, occur at around 120 and 147 degrees C, respectively. The transition from regime II to regime I induced morphological changes in the crystalline aggregates whereby spherulitic aggregates transformed into hexagonal lamellar stacking. As for the transition between regimes II and III, no obvious morphological change in the spherulitic crystal aggregates was observed.