Wide-angle neutron scattering studies of mixed crystals of polyethylene and deuteropolyethylene

Wide-angle neutron scattering studies have been made of a series of mixed crystal blends of polyethylene and deuterated polyethylene in the range 0.68 ≤ K = 4πλ^?1 sinθ ≤ 2.02 Å^?1. Recent calculations by Stamm have shown that regularly folded subunits of small numbers (two to ten) of stems should produce an observable modulation of the diffuse scattering pattern of such mixtures in this K range. For both melt- and solution-crystallized blends, no extra signal was observed above the background scattering level. The data are used to place an upper limit (of about four) on the number of stems that could be regularly folded in the (100) and (010) planes for these materials.     

Determination of the chain conformation of polypropylene in the crystalline state by neutron scattering

Summary Wide and small angle neutron scattering from melt-crystallised isotactic polypropylene mixtures of deuterated and protonated molecules were measured in aQ range of 0.003 Å^–1 to 0,8 Å^–1. The scattering curves between 0.1 Å^–1 and 0.8 Å^–1 are very sensitive to the amount of folding of the chains within a lamella. Different annealing conditions which influence the degree of crystallinit between 0.39 and 0.67 and the long spacing between 125 Å and 245 Å had an insignificant influence on the scattering in this range. For various arrangements of crystalline sequences scattering functions were calculated. Best agreement with the experimental scattering curves was obtained for a mixture of single and double crystalline sequences of equal proportion. An adjacent reentry folding model for the molecular conformation is not consistent with the experimental data.The neutron scattering experiments have been performed at the Institute Laue-Langevin in Grenoble/France and at the Kernforschungsanlage Jülich/Germany.     

聚合物片晶中链折叠模型的对称性讨论

 本文从对称的观点讨论了在聚合物片晶中链折叠的问题.提出只有当分子链在折向相反方向后对称性不改变时,近邻规整折叠才是可能的.它是近邻折叠的必要条件.根据折叠时“对称性不变”这一论点,可以预言分子链近邻规整折叠的可能性及可能的折叠方向.     

聚合物片晶中链折叠模型的对称性讨论

本文从对称的观点讨论了在聚合物片晶中链折叠的问题.提出只有当分子链在折向相反方向后对称性不改变时,近邻规整折叠才是可能的.它是近邻折叠的必要条件.根据折叠时“对称性不变”这一论点,可以预言分子链近邻规整折叠的可能性及可能的折叠方向.     

Mixed-crystal infrared studies of chain folding in polyethylene single crystals: Effect of crystallization temperature

Mixed crystals of polyethylene (PEH) and various-molecular-weight perdeuterated polyethylenes (PEDs) have been prepared at 80°C and their infrared spectra compared with those of samples grown at 55°C. Concentrations of 80 PEH/1 PED were required in the former case to eliminate segregation effects whereas 40 PEH/1 PED sufficed in the latter. Resolution of the observed CD₂ bend contour was most reasonably achieved with a crystalline singlet and two crystalline doublets, in addition to a contribution (ca. 15%) from the noncrystalline component. The singlet, comprising about 20% of the crystalline area, contains contributions from both isolated stems and (200) adjacent reentry folding. Random reentry folding is therefore not a predominant mode of chain organization in polyethylene single crystals. The inner of the two doublets arises from adjacent reentry folding along single (110) planes, and is present for all PED molecular weights. For low-molecular-weight fractions this splitting is consistent with the number of stems of one PED molecule allowed in the crystal. The outer doublet arises from multiple (110) plane adjacency, and is present for intermediate and high molecular weights. An analysis of both doublets suggests that at high molecular weights a single molecule can crystallize with noncontiguous regions of adjacent stem domains.     

A neutron scattering analysis: How does the conformation in polyethylene crystals reflect that in the melt?

Neutron scattering data for melt-crystallized polyethylene have been analyzed in order to clarify to what extent the chain folding is randomly reentrant. No attempt has been made to specify the molecular conformation in every detail, and the emphasis is on distinguishing between different classes of conformation. The most random folding corresponded to a model where the folding is imposed solely by the criterion of the chain segments moving the least possible distance during the crystallization process (a “freezing-in” model). This has been shown not to be compatible with published data. For this model analytic calculations are possible based on the projection of a three-dimensional Gaussian distribution onto a plane. A subunit model is then proposed which requires substantial local rearrangement of the chain as it folds during crystallization, but where the distribution of the subunits within the whole molecule is imposed by the preexisting Gaussian chain of the melt. Arguments based on space filling considerations are invoked, with the postulate of a surface structure which is neither crystalline nor truly amorphous. Anything approaching a random switchboard model (e.g., the freezing-in model which we consider) is contrary to both space filling considerations and to the comparison of observed and calculated neutron scattering. The analytical calculation which was performed for the freezing-in model was employed so as to simplify calculations for the subunit model. For scattering intensities over a wide range of scattering angle it is deduced that only the structure within the subunit need be considered. Numerical computer calculations involving only a small number of stems were then carried out for a number of different subunit structures, and some general features are noted which restrict the type of model which can explain the data. As in previously published analyses, a very high proportion of adjacent folds is not compatible with the results. A row model for the stems within a molecule can achieve good agreement, either with straight rows or with a certain amount of “stagger” incorporated. Up to about 40% of the folds could be adjacent. Models based on two-dimensional random walks did not give good agreement.     

聚乙烯片晶中链折迭区的能态

按照EHMO方法,计算了聚乙烯链段取近邻规则折迭构型的能量以及取全反式构型的能量,并进行了比较和讨论。前者与后者相比,能量的升高小于3％,因此作者认为聚乙烯结晶时发生近邻规则折迭是可能的。     

Polymer crystallization from the melt at large undercoolings

Consideration of crystallization kinetics in high-molecular-weight polymers shows that adjacent reentry is unlikely in melt crystallization and that sections of individual chains will crystallize concurrently at several sites. Hence the characteristic crystallization process will be that of a loop of chain with both ends attached to different sites on the crystal surface. Analysis of this process leads to predictions of crystallinity values for various conditions of chain mobility in the crystal and of entanglements in the amorphous regions. Observations on polymers crystallized at high undercoolings where a crystallinity of about 30% is usually observed suggest that the common case is that of a highly entangled amorphous layer and rapid, local annealing of the chains but with no long-range motion in the crystal. This model of loop crystallization is shown to agree with available small-angle neutron scattering data. The overall crystallization kinetics are in accord with surface nucleation controlled growth which also arises out of the Lauritzen-Hoffman adjacent reentry model and has been shown to fit experimental results on growth rates.     

Mixed-crystal infrared studies of chain-folding in polyethylene single crystals: Low-temperature and molecular weight studies

Infrared spectra of 40 PEH/1 PED mixed crystals have been obtained at low temperature as a function of the molecular weight of the PED. The band contour in the CD₂ bending region was resolved into crystalline and noncrystalline components by a least-squares fitting of Gaussian-plus-Lorentzian bands. The splittings of the crystalline component were analyzed in terms of the theory for finite chains of coupled oscillators, with the help of normal coordinate calculations for certain structures. Together with some results on n-paraffin mixed crystals, our studies show that (i) the spectroscopic observations are associated with interactions of crystalline stems of a single molecule; (ii) the observed crystalline splittings are incompatible with an essentially “switchboard” model of chain folding; (iii) adjacent reentry folding predominates, with folding along single (110) and (200) planes occurring; and (iv) for high molecular weights of PED double (110) fold planes prevail. These results are seen to be consistent with the morphologies known to be associated with the conditions of crystallization.     

Crystallization of short aliphatic chains. II. Example of even fold surface with adjacent fold reentry and of a transition to chain extension

As a continuation of the preceding study on the folding behavior of short polymer chains, an iodine-terminated paraffin having 156 Å peak molecular length and a sharp molecular weight distribution was prepared. The paraffin could be crystallized in lamellae of two different thicknesses: (A) thickness close to half the chain length (the most readily obtained); (B) thickness intermediate between half chain length and fully extended chain. Case A corresponds to each chain folding once with equal stems and with ends at the surface. Degradation behavior revealed that the folds must be of closely equal length giving rise to an even fold surface. In case B the situation is more involved: here the chain ends must turn into the lattice. Adjacent reentry is a necessity throughout. In both cases the lamellar thickness could be increased by annealing up to complete chain extension.     

Quantitative determination of mesomorphic regions in oriented polymers from wide-angle X-ray diffraction

A method for quantitative characterization of the noncrystalline region in poly(ethylene terephthalate) (PET) fibers based on x-ray wide-angle diffraction data is elaborated. The procedure consists in a computational resolution of the diffracted intensity into individual peaks for 010, 11 0, and 100 planes and into isotropic and anisotropic components of diffuse scattering. The results show that the content of mesomorphic regions in the noncrystalline part of PET fibers with draw ratio of λ = 3.0 varies from 12% to 32% depending on the temperature of heat treatment.     

Neutron scattering studies of syndiotactic polystyrene crystals

Neutron scattering experiments are reported on solution grown crystals of syndiotactic polystyrene (sPS) at small and large q, in order to investigate the chain conformation within the lamellar crystals. From the results in both the Guinier and the intermediate ranges, we come to the following conclusions: The chain folds in one or several sheet depending on the molecular weight, comparison between measured and calculated intensity plots give strong indication that a is the fold direction. The statistical model recently built for the spatial arrangement of stems in a sheet-like structure is successful again, and demonstrate a strong statistical preference for adjacent re-entry.     

Neutron scattering studies of the structure of amorphous and crystalline polymers and of polymer blends

The technique of neutron scattering for studying both the structure and dynamics of polymer systems is now well established. In the case of amorphous flexible polymers in most cases the bulk chain dimensions agree rather well with those of the unperturbed coil. Some newer results concerning for example side-chain liquid-crystal polymers and segmented polyurethane elastomers are described. Neutron small-angle scattering can also be used for the investigation of the molecular deformation mechanism during the drawing process of polymers. In the case of semicrystalline polymers the way in which a single macromolecule traverses the crystalline and amorphous phases can also be evaluated by neutron scattering. A method has been proposed for the evaluation of the neutron scattering data without introducing detailed structural models. The only assumption made is that the molecular structure can be described as consisting of “clusters” of crystalline stems which belong to the same molecule. It is shown that this cluster model can be verified experimentally for the cases of poly(ethylene oxide), polypropylene and polyethylene. The structure factor S(q) in compatible polymer blends is usually treated by the random phase approximation according to de Gennes. The temperature dependence of S(q) displayed by some systems, however, appears to be anomalous within this approximation. A new method is presented for evaluating the neutron scattering data which takes into account that the Flory-Huggins interaction parameter χ is not a point-function χδ(τ), but can be interpreted in terms of a structure model χ(τ) which consists of spatially separated positive and negative contributions within a 1–10 å range. Finally the application of neutron scattering to diblock copolymers is discussed and it is shown that the results for the case of polystyrene-poly (p-methylstyrene) are in good agreement with the theoretically expected behaviour.     

Nascent crystallization of a growing chain on a catalyst surface: a nonequilibrium molecular dynamics simulation study

The growing chain molecular dynamics (GCMD) simulation method, a new nonequilibrium molecular dynamics code, is proposed to simulate the polymer chain aggregation behavior during polymerization on a catalyst surface. We found that the growing chain crystallizes on the surface in two stages: the nucleation stage and the crystal growth stage. In the first part of the nucleation period, the short polymerizing chain first absorbs on the surface and can be in either an ordered or disordered structure. Still in the nucleation period, when the chain reaches a degree of polymerization, about 100 bonds, the chain folds into a stable nucleus on the substrate with 3-5 stems. In the crystal growth stage where the polymerization also proceeds, we observed a stem elongation process in combination with a chain folding process. In the stem elongation step, the number of stems in the nucleus remains constant, and all the stems expand together to a length of ca. 5-25 ns. In the subsequent chain folding step, the stem length decreases about 20 bonds within a period of ca. 0.1-0.5 ns. During chain growth, the elongation process and the folding process occur in an alternating and repeated fashion. The crystallization mechanism of the polymerizing chain was discussed.     

An infrared and inelastic neutron scattering spectroscopic investigation on the interaction of eta-alumina and methanol

The industrially important interaction of methanol with an eta-alumina catalyst has been investigated by a combination of infrared spectroscopy (diffuse reflectance and transmission) and inelastic neutron scattering (INS) spectroscopy. The infrared and INS spectra together show that chemisorbed methoxy is the only surface species present. Confirmation of the assignments was provided by a periodic DFT calculation of methoxy on eta-alumina (110). The thermal conversion of adsorbed methoxy groups to form dimethylether was also followed by INS, with DFT calculations assisting assignments. An intense feature about 2600 cm(-1) was observed in the diffuse reflectance spectrum. This band is poorly described in the extensive literature on the alumina/methanol adsorption system and its observation raised the possibility of a new surface species existing on this particular catalyst surface. INS measurements established that the 2600 cm(-1) feature could be assigned to a combination band of the methyl rock with the methyl deformation modes. This assignment was reinforced by an analysis of the neutron scattering intensity at a particular energy as a function of momentum transfer, which confirmed this particular adsorbed methoxy feature to arise from a second order transition. Similar behaviour was observed in the model compound Al(OCH3)3. The anomalous infrared intensity of the 2600 cm(-1) peak in the diffuse reflectance spectrum is a consequence of the different absorption coefficients of the C-H stretch and the combination mode. The implications for catalyst studies are discussed.     

点缺陷引起中子辐照MgO(110)单晶的铁磁性

对MgO(110)单晶进行中子辐照,辐照剂量从1.0 × 10¹⁶到1.0 × 10²⁰ cm^-2。基于黄昆漫散射理论,我们计算了MgO晶体中的立方缺陷和偶极力缺陷引起的X射线漫散射强度分布图。通过X射线漫散射及紫外-可见光(UV-Vis)吸收光谱实验表征了晶体的点缺陷组态,并利用超导量子干涉仪(SQUID)测量了样品的磁性。ω–2θ和摇摆曲线说明MgO单晶经中子辐照后产生了晶格畸变,晶体中存在一定浓度的点缺陷。倒易空间图(RSM)显示中子辐照的MgO单晶存在漫散射现象。与计算得到的漫散射分布图对比分析可知,中子辐照的MgO(110)单晶中产生了弗仑克尔缺陷。UV-Vis吸收光谱表明所有辐照晶体中存在阴离子单空位缺陷。辐照剂量较高(1.0 × 10¹⁹和1.0 × 10²⁰ cm^-2)的样品中存在O空位的聚集。磁性测量显示中子辐照后的MgO(110)单晶在室温下依然是抗磁性,但在低温下具有铁磁性,最大饱和磁化强度达到0.058 emu·g^-1。通过中子辐照的方法,可以使MgO(110)单晶产生点缺陷引起的低温铁磁性。利用F色心交换机制可以解释中子辐照MgO晶体中的O空位缺陷与铁磁性之间的关系。     

Morphology of polypropylene crystals. II. Twinning of lamellar crystals

Twinned crystals were obtained from fractionated isotactic polypropylene of M?_w = 600,000 by isothermal crystallization at 130°C. for 20 hr. from dilute α-chloronaphthalene solution (0.005 wt.-%). Electron microscopic observations confirm that the molecular chains of polypropylene lamellar crystals extend along the [100] direction while the folding itself occurs within the (010) planes in the monoclinic crystal form. On this basis it is shown that polypropylene forms twinned crystals in which the composition plane is the (1k0) planes. It can be deduced that the formation of twin nucleus occurs before twinning, and then growth occurs from the neighboring region of the crystal boundary by chain folding along the [100] direction.     

Structural and Dynamical Properties of n-Alkane Molecules in Clathrate Phase of Syndiotactic Polystyrene

Syndiotactic polystyrene (sPS) forms a clathrate phase with a variety of compounds. Not only rigid molecules but also flexible molecules can be stored in the cavities of the clathrate phase. To clarify the adjustment mechanism of a flexible guest molecule to the sPS clathrate system, the host and guest structures were investigated by means of solid-state ¹³C NMR and Raman spectroscopy, and X-ray diffractometry for the sPS clathrates with a series of n-alkanes from n-hexane to n-decane. Although the 010 spacing of the host sPS lattice expanded slightly on going from n-hexane to n-heptane, it decreased markedly at n-octane and then increased gradually with the chain length of guest n-alkane. The conformational change of guest n-alkane molecules was involved in this anomalous change in the 010 spacing. Majority of the n-hexane and n-heptane molecules took extended chain structures in the clathrates, whereas all longer n-alkanes took bent chain structures. The mean-square displacement of hydrogen atoms in the clathrates was estimated by quasielastic neutron scattering experiments. It was confirmed that the host lattice contraction suppressed thermal motion of the clathrate system.     

Anomalous neutron Compton scattering cross sections in ammonium hexachlorometallates

The authors have performed neutron Compton scattering measurements on ammonium hexachloropalladate (NH(4))(2)PdCl(6) and ammonium hexachlorotellurate (NH(4))(2)TeCl(6). Both substances belong to the family of ammonium metallates. The aim of the experiment was to investigate the possible role of electronic environment of a proton on the anomaly of the neutron scattering intensity. The quantity of interest that was subject to experimental test was the reduction factor of the neutron scattering intensities. In both samples, the reduction factor was found to be smaller than unity, thus indicating the anomalous neutron Compton scattering from protons. Interestingly, the anomaly decreases with decreasing scattering angle and disappears at the lowest scattering angle (longest scattering time). The dependence of the amount of the anomaly on the scattering angle (scattering time) is the same in both substances (within experimental error). Also, the measured widths of proton momentum distributions are equal in both metallates. This is consistent with the fact that the attosecond proton dynamics of ammonium cations is fairly well decoupled from the dynamics of the sublattice of the octahedral anions PdCl(6) (2-) and TeCl(6) (2-), respectively. The hypothesis is put forward that proton-electron decoherence processes are responsible for the considered effect. Decoherence processes may have to do rather with the direct electronic environment of ammonium protons and not with the electronic structure of the metal-chlorine bond.     

Hydrothermal synthesis in the system Ni(OH)(2)-NiSO(4): nuclear and magnetic structures and magnetic properties of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2)

We present the synthesis, characterization by DT-TGA and IR, single crystal X-ray nuclear structure at 300 K, nuclear and magnetic structure from neutron powder diffraction on a deuterated sample at 1.4 K, and magnetic properties as a function of temperature and magnetic field of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2). The structure is formed of chains, parallel to the c-axis, of edge-sharing Ni(1)O(6) octahedra, connected by the corners of Ni(2)O(6) octahedra to form corrugated sheets along the bc-plane. The sheets are connected to one another by the sulfate groups to form the 3D network. The magnetic properties measured by ac and dc magnetization, isothermal magnetization at 2 K, and heat capacity are characterized by a transition from a paramagnet (C = 3.954 emu K/mol and theta = -31 K) to a canted antiferromagnet at T(N) = 29 K with an estimated canting angle of 0.2-0.3 degrees. Deduced from powder neutron diffraction data, the magnetic structure is modeled by alternate pairs of Ni(1) within a chain having their moments pointing along [010] and [010], respectively. The moments of Ni(2) atoms are oppositely oriented with respect to their adjacent pairs. The resulting structure is that of a compensated arrangement of moments within one layer, comprising one ferromagnetic and three antiferromagnetic superexchange pathways between the nickel atoms.