Polyethylene single crystals: Melting and morphology

Determination of important thermodynamic and physical properties of polymers has often been frustrated by the tendency of these materials to reorganize when heated. A novel technique has been developed which permits a more careful examination of the relationships existing between melting and morphology of single crystals. This has lead to the observation that high molecular weight polymers possess much narrower melting ranges than had been previously reported. By using this technique it has been possible to identify distinct transition temperatures for {100} and {110} fold sectors. DTA and DSC data have been previously reported for dried down single crystals. The applicability of this data to single crystals as they exist in suspension is questioned.     

Growth and morphology of single crystals of linear aliphatic polyesters

In order to elucidate the relations between morphological habits and chemical structure of polymers, poly(ethylene sebacate), poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) were crystallized from dilute solutions in n-hexanol, isoamyl acetate etc., and were studied with the electron microscopy and x-ray diffraction. The crystal structure of these polyesters are tentatively determined. Morphological “regularity” and “simplicity” of the single crystals are correlated with the chemical structure of the polymers. The crystallization conditions under which “regular” and “simple” single crystals are obtained are relaxed with increase of methylene sequence length in chemical repeat unit. The Bragg extinction bands in the single crystals of poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) suggest nonplanar nature of these crystals. The molecular chains in the poly(ethylene sebacate) single crystals are inclined from the normal of the basal plane; the fold surface corresponds to the (001) plane.     

Surface morphology of silicon single crystals treated with acid polishing etchants

The specific features of formation of micro-and nanorelief on the (001) surface of silicon single crystals upon treatment with widely used etchants HF HNO₃ H₂O and with poorly studied etchants HF KMnO₄ H₂O were studied. The relief of the etched surface was examined by optical, scanning electron, and atomic-force microscopy. The polishing properties of the etchants and the silicon etching rates were studied in relation to the oxidant content. The polishing properties of the etchants were compared by analyzing statistical distribution of such characteristics of the relief of the etched surface as the height and length of micro- and nanowaves.     

Video-tape observation of the crystal growth and morphology of colloidal single crystals

Crystal growth and morphology of colloidal crystals of silica spheres (81.2 nm in diameter) are observed directly on a video-tape camera. Crystal growth from the round-shaped, small single crystals to the angular-shaped ones is clear. It is observable that the single crystals are packed densely and separated from each other with the grain boundaries. The morphology of colloidal crystals is quite similar to that of typical crystals such as metals, proteins, and ice.     

Deformation of porous molecular networks induced by the exchange of guests in single crystals

A strategy for making molecular networks that are porous and deformable is revealed by the behavior of compound 1, in which groups that form hydrogen bonds are attached to a nominally tetrahedral Si core. Compound 1 crystallizes from various carboxylic acids to produce a porous hydrogen-bonded diamondoid network, with up to 65% of the volume available for including guests. Changing the guests expands or contracts the network up to 30% in one direction, and single crystals can accommodate these exchange-induced deformations without loss of crystallinity. This resilience appears to result in part from the incorporation of flexible Si nodes in an otherwise robust network maintained by multiple hydrogen bonds. In certain cases, exchange is faster than deformation of the network, allowing the isolation of metastable structures with a new guest included in an otherwise unchanged network. Such processes can provide new materials that would be difficult or impossible to obtain in other ways.     

邻苯二酚做螯合剂合成不同粒度、形貌的全硅方钠石分子筛大单晶

将邻苯二酚作为螯合剂加入到合成全硅方钠石分子筛的体系 (SiO2-NaOH -EG -R(R :邻苯二酚 ) )中 ,合成了 15 μm× 15 μm× 15 μm～ 6 0 μm×6 0 μm× 6 0 μm粒度不等的高质量的Si-SOD分子筛单晶 .实验结果表明 :当体系中有邻苯二酚存在时得到的产物具有较大尺寸和完美形貌的单晶 ,而不加邻苯二酚时却得到尺寸很小的孪晶 ;同时 ,体系中邻苯二酚的含量对晶体的粒度也有很大影响 .晶化动力学表明 ,邻苯二酚的加入极大地降低了晶化速度 ,这可能与体系中形成了硅 -邻苯二酚螯合物有关     

Single-molecule single crystals

Various approaches to the preparation and verification of single-molecule single crystals are discussed for polyethylene and poly (oxyethylene). Analytic tools are electron microscopy, electron diffraction, and differential scanning calorimetry. The main difficulty in producing a single-molecule single crystal is to keep crystals from joining during growth.     

Single-chain single crystals

Single-chain single crystals of isotactic polystyrene and poly(ethylene oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single-chain single crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single-chain particles on copper grids, followed by isothermal crystallization. A statistical analysis of the sizes of single-chain crystals was found to match with the known molecular weight distribution of original sample, indicating the particles to be composed of single chain. Observation of the morphology and electron diffraction gave evidence of the single crystal nature. Regular-shaped single-chain crystals were obtained after isothermal crystallization for a longer time. By close observation, several types of morphologies were found for single-chain crystals of isotactic polystyrene and poly(ethylene oxide); in addition to the conventional morphologies observed for multi-chain crystals, new morphologies were observed in both cases. The morphologies of poly(ethylene oxide) were explained according to the crystal structure and twin modes. Tent-like single-chain crystals were often observed. Because of the small size of the crystals, they can avoid collapse on the substrate. The crystalline c-axis of single-chain crystals were found to orient preferably in the direction normal to the substrate. The investigation of electron diffraction and high resolution electron microscopy revealed that the structure of the single-chain crystals of isotactic polystyrene is the same as for multi-chain crystals. A reasonable explanation is given for the unusual resistance to electron irradiation and the missing of lower-index reflections. Regular periodic stripes were found on the top surface of single-chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition, a statistical thermodynamics theory was developed for single-chain crystal. It is found that the equilibrium dimensions are related to molecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.     

Rectangular single crystals of polyacrylonitrile

Rectangular single, multilayered and twinned crystals of polyacrylonitrile have been grown from the fractionated samples in propylene carbonate at different temperatures (140–150°) using Film Formation Method of crystal growth.     

Effect of chain defects on the morphology and thermal behavior of solution-grown single crystals of syndiotactic polypropylene

The morphology of solution-grown single crystals of syndiotactic polypropylene with different degree of stereoregularity is compared. A sector formation phenomenon, found in some monolayer single crystals, is discussed in terms of possible crystallographic fold planes, growth planes, and gemination planes. A correlation between thermodynamic and morphological properties such as apparent enthalpy of fusion, critical long spacing, critical annealing temperature, and the number of configurational chain defects along the macromolecule has been found. Two endothermic peaks are observed in the DSC thermograms of single-crystal aggregates of syndiotactic polypropylene. The low-temperature peak is attributed to melting of crystals or parts of crystals with incorporated chain defects. The high-temperature peak corresponds to the melting endotherm of more regular crystalline zones. The peak-area ratio seems to depend on the degree of stereoregularity.     

The influence of short‐chain branching on the morphology and structure of polyethylene single crystals

The influence of short‐chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed high‐molecular‐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 orthorhombic form. An increase of the unit cell parameter a₀ 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. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1751–1762     

Deformation and microstructure of extended-chain polydiacetylene crystals

The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm^?2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 10⁴ Nmm^?2.     

Annealing behavior of the novel morphology of poly(butylene succinate) lath‐shaped single crystals

Lamellar single crystals of poly(butylene succinate) (PBS) with novel morphologies were prepared from a chloroform/methanol solution by self‐seeding methods. Crystal structures and morphologies were investigated by means of atomic force microscopy (AFM). Lath‐shaped crystal and hexagonal‐shaped crystals coexist in one PBS single crystal and this has a lamellar thickness of around 5–6 nm as determined by AFM. The thickening of lamellae from 5–6 to 7–9 nm occurred during heating from 41 to 84 °C. In situ temperature‐controlled AFM observations demonstrated that the lath‐shaped crystal sections melted first and then the hexagonal sections while the edge of the single crystals remained regular during annealing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1492–1496, 2009     

Studies on the morphology and structure of aromatic polyester films

Oriented films of an aromatic polyester containing a triad ester mesogenic group and a decamethylene flexible spacer were studied by polarized light microscopy, small-angle light scattering, and polarized infrared spectroscopy to determine the nature of the alignment of the polymer chains, induced by shearing, which is responsible for the banded or striped morphology observed. The stripes are apparently composed of fibrils arranged in a zigzag manner containing polymer chains packed in a highly ordered, parallel alignment at an angle of ±26° to the shearing direction.     

Statistical thermodynamics of single-chain single crystals

A simple statistical mechanical theory is developed assuming a chain-folded crystal. Taking account of the side-surface energy of the single-chain single crystal, the partition function and the free energy are calculated. Also, the expressions for equilibrium thickness l_e and melting temperature T_{m, e} of the single-chain single crystal are obtained, and they are similar to that given by a thermodynamic analysis of the single-chain single crystal. It is shown that l_e depends on temperature and on the molar mass of the single-chain single crystal, while T_{m, e} depends on the molar mass. Furthermore, it is found that the side-surface and the fold-surface free energies γ and γ_e can be explained on a molecular basis.     

Single-Molecule single crystals of isotactic polystyrene

Single-molecule single crystals were grown from amorphous droplets of fractionated isotactic polystyrene. The crystals were analyzed by electron microscopy and electron diffraction. The molecular mass distribution could be matched with a statistical analysis of single-molecule particles (amorphous and crystals). Proof was brought that single molecules of isotactic polystyrene do not reach equilibrium dimensions on crystallization, rather assume the lamellar morphology with chain-folded macroconformation, also known from crystallization of polymolecular crystals. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America. US Government contract No. DE-AC05-840R-21400.