On Perpendicular and Tilted Chains in Lamellar Crystals

1991

Works on strictly uniform ultra-long n-alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (T,), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing T, where chain folding takes over. This remarkable rate inversion, observed for c 2 4 6 H494 and c198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a "self poisoning" phenomenon, where chain depositions, which occur transiently in the "wrong" conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.

Polymer, 1998

A small-angle X-ray scattering (SAXS) study was conducted on melt-crystallized long-chain n-alkanes C 162 H 326 , C 194 H 390 and C 246 H 494 . The main objective was to clarify the structure of lamellar crystals with the layer period a non-integer fraction (NIF) of the period of extended-chain crystals; the NIF form is the product of primary chainfolded melt crystallization Keller, A., Polymer, 1986, 27, 1835) and it transforms subsequently to one of the integer folded forms. The extended-chain form crystallized from melt was also studied. Electron density profiles along the layer normal were reconstructed by inverse Fourier transformation using discrete diffraction intensities. The profiles were compared with the data computed from models. The best fitting models consist of alternating high-and low-density layers with relatively sharp boundaries. The high-density layers, representing the crystalline regions, have thicknesses which closely match either the calculated thickness of extended-chain layers with 35Њ chain tilt (for the extended-chain form) or half that value (for the NIF form). In the extendedchain form a low density intercrystalline layer is observed with thickness between 1 and 3 nm, depending on chain length and temperature. In contrast, the non-crystalline layer in the NIF form is 6 to 8 nm thick. It has constant density and is truly amorphous, as indicated by the close match between SAXS and DSC crystallinities, which are less than two thirds for the as-formed NIF structure. While less than half the chains are folded in the latter structure, those that are have a fold in the middle ('integer folding'). Non-folded chains traverse the crystal layer only once, while their uncrystallized ends (cilia) comprise the amorphous layer. Within the crystalline layer of the NIF form chains are tilted at 35Њ. The fraction of folded, fully crystallized chains increases with decreasing temperature at the expense of the amorphous layer, resulting in a reduction in amorphous layer thickness and overall lamellar periodicity (lamellar 'thinning'). ᭧

Macromolecules, 1998

The crystal structure of two semiflexible dicarboxylic acids forming chains is reported; these compounds are models for the crystal packing of polyesters based on 1,5-or 2,7-dihydroxynaphthalene and aliphatic dicarboxilic acids and, more generally, for crystal packing of semiflexible chains containing conformationally blocked aromatic cores along the chain. In both crystal structures, chain axes are not parallel to each other, this unusual feature being connected, seemingly, with the lateral packing of aromatic cores present in the molecules.

1971

The structure and thermodynamic properties of atactic and isotactic acrylic and methacrylic polymers containing 16-18 carbon atoms in the n-aliphatic side chains, and of ccpolymers of hexadecyl acrylate with isopropyl acrylate were studied by means of x-ray and differential thermal analysis. The crystallization of branched acrylic and methacrylic polymers and of acrylic copolymers proceeds in the form of a hexagonal crystal, regardless of the configuration of the backbone chain. Methods of ordering branched macromolecules are proposed, and the melting points, heats and entropies of fusion determined. The role of flexibility of the backbone chains in ordering and the crystallization processes was determined. I n the case of poly(n-alkyl acrylates) the backbone chain is involved in the crystalline lattice; this is not the case in methacrylates and copolymers of hexadecyl acrylate with isopropyl acrylate. Some similarity war assumed between the structure of biopolymers and synt.hetic branched polymers.

Journal of Polymer Science Part C: Polymer Symposia, 1967

An x-ray, electroii micrusc.ope, and mecliauical properties study was carried out o i l polyethylerie, isotactic polypropylene, and trans-1,4-polyisopretie (gutla-perch) with differelit degrees of chemical irregularity made by halogeriat.iori reactioii of these regular polymers. Thc gradual distortion of chemical regularity causes a change in crystalline laltice parameters, degeneration of crystalline spherulitic structure, and simplification of siipernioleciilar st,ructures of t,he polymers investigated. The introduction of chemical irregrilarity is analogoiis to supercooliiig of a crystalliiie polymer wilh suitable cliaiige of mechanical propert>ies aiid morphological forms of structures. In the case of guttapercha, ehloritiation i i i films also favors the ~1 to 8 transition within the crystalhe state. I t has been found that stretching of vulcanized samples of chlorogutta-percha at temperatures above its melting point proceeds with an essential change iii internal energy and increase of entropy. A suppositioii has been made about the structure peculiarities of chlorogutta-percha which permit the appearance of such an effect. A total piclure of chaiige of different parameters and properties is giveii from the point of view of gradual transformatioii of t,he stereoregular polymer to an irregular amorphous one.

Polymer, 1987

Crystallization rate experiments performed on the uniform alkanes C246H494. and C19sH39s, by both differential scanning calorimetry and in situ X-ray diffraction (using a synchrotron source), have revealed that these rates, including both primary nucleation and growth, pass through a minimum with increasing supercooling. The first (expected) increase and subsequent (unsuspected) decrease correspond to extendedchain (E) crystallization, the renewed increase beyond the minimum corresponding to chain-folded crystallization with the fold period I being smaller than L but larger than L/2, where L is the extended chain length. The anomalously retarded crystallization with increasing supercooling, new even qualitatively, appears to arise through competition between extended-and folded-chain deposition. The attachment of folded chains evidently involves a much lower free energy barrier than does the attachment of extended chains. Even if the former process cannot lead to growth of stable chain-folded crystals above their melting temperature, it seriously hampers the only productive process, chain-extended crystallization. The observed effects, which have come to light owing to the availability of ultra-long and uniform n-alkanes, help to provide new insight into the primary stages of chain-folded crystallization, with many potential consequences, some of which are discussed.

Macromolecules, 1986

A mean-field lattice theory is developed to describe the configurations of long-chain molecules at the crystal/amorphous interface in semicrystalline polymers. Chains are assumed to satisfy continuity and space-filing requirements. The theory permits systematic levels of approximation for correlations among neighboring bonds along the chains subject to the interfacial constraints. We consider the two lowest levels of approximation here: (i) single bonds (two segments) or (ii) bond pairs (three segments). Both models predict that approximately 73% of the chains which emerge from the crystal reenter at sites which are immediately adjacent and that the interfacial region should therefore be small, provided the chains are freely flexible. The models predict that the ratio of chain loop8 to ties in the amorphous region is smaller, and the mean lengths are greater, than predicted by random walk models.

Journal of Polymer Science Part B: Polymer Physics, 2015

The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed highmolecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiation-step propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the ortho-rhombic form. An increase of the unit cell parameter a 0 is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements.

Journal of Polymer Science Part B: Polymer Physics, 1990

Lamellar thickening in folded-chain crystals of the n-alkane C198H398 in solution has been shown to occur a t the crystallization temperature. The transformation is from crystals with their chains folded in half to extended-chain crystals, with no observable intermediate forms. This conforms with the extensive existing evidence that integer-folded forms are thermodynamically preferable. Measurements of the kinetics of lamellar thickening have revealed an unexpected maximum in the thickening rate as a function of temperature: above 75"C, thickening slows down with increasing temperature. This unprecedented result is an indirect consequence of another, equally puzzling, anomaly: the rate of extended-chain nucleation a t the side faces of folded-chain crystals decreases with increasing solution concentration. This unique observation is attributed to "selfpoisoning" of the lateral surface of the growing nucleus by frequent folded-chain depositions which inhibit the process of molecular extension. The same effect has been invoked previously to explain the anomalous crystallization rate minima in long alkanes and its present manifestation under different circumstances is regarded as an important independent validation of the concept of "selfpoisoning."

2013

We study crystallization in a model system for eicosane (C20) by means of molecular dynamics simulation and we identify the microscopic mechanisms of homogeneous crystal nucleation and growth. For the nucleation process, we observe that chains first align and then straighten. Then the local density increases and finally the monomer units become ordered positionally. The subsequent crystal growth process is characterized by a sliding-in motion of the chains. Chains preferably attach to the crystalline cluster with one end and then move along the stems of already crystallized chains towards their final position. This process is cooperative, i.e. neighboring chains tend to get attached in clusters rather than independently.