Molecular structure, symmetry, and shape as design elements in the fabrication of molecular crystals for second harmonic generation and the role of molecules-in-materials

Molecular materials have emerged as a major theme in contemporary science and technology, thanks to the synergetic confluence of the power of synthetic chemistry, the predictive and analytical capability of condensed matter physics, and the versatility of materials science and engineering. The stringent demands in terms of molecular design and supramolecular assembly to be met in the fabrication of nonlinear optical (NLO) materials for applications such as optical second harmonic generation (SHG) make it a unique arena covering fascinating explorations in chemical synthesis, molecular and materials characterization, theoretical modeling, NLO studies, and photonics technology. Our search for conceptually novel and practically simple but effective approaches to the design of molecular building blocks for crystals exhibiting efficient SHG has led us to the avenues described in this Account. The focus has been on the incorporation of structural, symmetry, and shape features in dipolar NLO-phores to realize noncentrosymmetric or polar molecular assemblies suitable to elicit appreciable SHG responses. Attachment of n-alkyl chains of optimal length and remote functional groups were shown to be effective methods to steer the assembly of achiral NLO-phore units into SHG-active noncentrosymmetric lattices. Computational modeling of molecules, molecular clusters, and molecular assemblies in crystals provides valuable insight into the observed structure-function correlations. A systematic exploration of the impact of the placement of stereogenic centers in strongly zwitterionic NLO-phores on the molecular organization led us to the effective exploitation of C2-symmetric units to form helical superstructures capable of efficient SHG. New materials developed are based on organic and metal-organic molecules as well as coordination polymers. The potential utility of molecular shape could be demonstrated through the realization of perfectly polar organization in a family of screw-shaped dipolar molecules. Combination of optical transparency, thermal stability, and feasibility of fabricating thin films with orientationally ordered crystallites are important aspects of these materials. Several examples presented in this Account highlight the significance of molecules-in-materials by illustrating that not only the individual molecules and their organization in the crystal lattice but also the intermolecular interactions exert critical impact on the nonlinear optical response of the materials. The role of cooperative interactions in some of the cases is pointed out. This Account projects a range of design strategies for molecular SHG crystals and avenues for expanding further on the present observations; the need to address and exploit the contribution of intermolecular interactions is specially noted. The different examples presented illustrate not only the fabrication of new families of materials based on interesting models proposed earlier but also the emergence of new models from the novel materials developed.     

自旋扭带强化传热及旋转特性

旋转扭带是在固定式扭带基础上发展而来的, 因其特殊结构所以能在管内流体作用下产生自旋效果。本文对自旋式扭带旋转特性及强化传热特性进行研究。通过理论及实验研究管内自旋扭带旋转特性后得出:扭率越小, 扭带克服阻力起始旋转需要的流体速度越小;扭带转速与管内流体流速呈一次线性关系, 且扭带节距不变时线性比例基本保持不变。通过实验研究后得出:自旋扭带能达到很好的强化传热性能, 扭率越小其强化传热性能越明显, 同时阻力特性也越明显, 在雷诺数为4×10³～4×10⁴、扭率为3～8时, 换热管内摩擦因子增至1.7～3.5倍, 努赛尔数增幅为10%～37%。本文使用评估指标η对扭带进行综合评价, 得出扭率为7的自旋扭带具有最佳的综合性能。并分别拟合出摩擦因子及努赛尔数与雷诺数、扭率之间的关联式, 提出一种工程上自旋扭带选型方法。     

Phase diagrams of ABC linear triblock copolymers under nanopore confinements

The phase diagrams of triblock copolymers in cylindrical nanopores are investigated using the real-space self-consistent field theory in a two-dimensional space. We concentrate on pores with neutral surfaces and invariable diameters, whose rich variety of phases are considered to originate from pure geometric frustration. A series of triangular phase diagrams are constructed by varying the volume fractions for several sets of interaction parameters. These diagrams are sorted into three categories according to their interaction parameters. The confined phase diagrams exhibit several characteristic features that differ from those observed in the bulk. First, a rich variety of geometric frustration phases with strong symmetries, such as cylindrical and square, are observed in the triangular phase space because of the symmetry constraint in the geometric boundary. Second, the frustrated phases present some novel and complex features compared with those in the bulk, demonstrating that the confined morphologies much more sensitively depend on the subtle variation in the binary interaction parameters than those in the bulk. Meanwhile, the entropic energies of the symmetric melts with equal end block volume fractions are investigated to further understand the geometric frustration phase behaviors in the triangular phase diagrams. The reasonable formation mechanisms of the frustration phases are also discussed.     

Genipin-crosslinking polyvinyl alcohol hollow braids degradable tissue engineering scaffolds: Manufacturing techniques and property evaluations

Tissue engineering has been developed with the aim of improving the regeneration and recovery of impaired tissues and organs. Biodegraded scaffolds serve the aforementioned functions and can also be decomposed by means of metabolism. They have no biological toxicity and save patients from injuries by the second surgery, which makes biodegradable scaffolds a new development trend in the tissue engineering. In this study, the textile engineering and chemical crosslinking techniques are employed to produce biodegradable polyvinyl alcohol (PVA) hollow braids, serving as the tissue engineering scaffolds. The process involves two types of products, including the twisted yarns and hollow braids. The twist number of PVA twisted yarns is changed to form different PVA twisted yarns, which are then used to braided into hollow braids via the braiding technology. Therefore, the hollow braids are basically composed three types of PVA twisted yarns. Next, the surface observation, mechanical properties, and degradation of the products are then evaluated. The test results indicate that PVA twisted yarns exhibit the optimal mechanical properties when being twisted with 3 turns/inch. Any higher twist counts result in over twist in the twisted yarns. The optimal hollow braids are composed of PVA twisted yarns with a twist counts being 3 turns/inch. Afterwards, hollow braids are crosslinking with genipin, thereby obtaining greater mechanical strength of 23.6 N and higher decomposition rate of 0.8. The specified hollow braids are suitable for the use as tissue engineering scaffolds.     

Modeling the Anisotropic Deformation Response of β‐HMX Molecular Crystals

In this paper, we develop a computational model for the anisotropic plastic response of β‐HMX molecular crystals based upon experimental data. While phenomenological models of β‐HMX currently exist, they are inadequate as they cannot account for the orientation dependence of the observed response. The proposed model, on the other hand, successfully predicts the experimentally observed highly anisotropic and orientation dependent yield surfaces of β‐HMX molecular crystals having different crystal orientations. The model also correctly predicts the anisotropic plastic yielding of β‐HMX under uniaxial compression at different temperatures along different crystallographic directions. The model presented in this paper may be extended to predict the yielding and elastic–plastic deformation of other energetic molecular crystals.     

换热管内置螺旋形扭带阻力与传热特性的实验研究

为了研究螺旋形扭带阻力与传热特性,选取了不同宽度（6、7和8 mm）的3种扭率（2.0、3.0、4.0）、3种螺距比（1.5、2.0、2.5）的参数组合下共27根螺旋形扭带插入换热管内进行实验.实验结果表明,插入螺旋形扭带后换热管内流动阻力和传热效果都有明显提高.通过对实验数据的多元线性回归分析,建立了相应的阻力系数和努赛尔数关联式.并且由强化传热综合性能评价分析,在实验雷诺数范围内得出强化传热综合性能评价因子φ=1.063～1.587,证明了实验研究的扭带具有强化传热的应用价值.     

Crystalline host-guest assemblies of steroidal and related molecules: diversity, hierarchy, and supramolecular chirality

Steroidal bile acids and over 50 of their derivatives serve as the hosts of inclusion crystals. These hosts each exhibit their own characteristic inclusion behaviors, which have been explored through more than 300 crystallographic data. The molecules with three-axial chirality combine in asymmetric fashion to form diverse assemblies, which have supramolecular properties, such as recognition and dynamics, through cooperative weak interactions. From an overview of these results, an analogy emerged: the steroidal assemblies may have hierarchical structures, such as primary, secondary, tertiary, and host-guest assemblies, similar to proteins. Accordingly, the assemblies with dimensionality bear supramolecular chirality, such as three-axial, tilt, helical, bundle, and complementary chirality. Such a concept can be extended to other organic substances, such as alkaloids and organic salts. These results move in the direction of supramolecular crystal engineering.     

On the formation of S-profiled lamellae in polyethylene and the genesis of banded spherulites

A study of the habits of individual melt-grown polyethylene crystals has shown how banded spherulites form in this polymer and the essential cause to be the relief of fold surface stress. An initially planar lamella develops a characteristic S-profile over time extending out from central regions. This is a consequence of reducing surface stresses by improving fold packing whose spatial requirements are best met by inclined as opposed to the initial perpendicular fold surfaces. The axis of the S-profile is inclined to b, an inclination which introduces the essential asymmetry into the system, and the lamella slightly twisted around the axis of the S. The twist is amplified by the formation of isochiral giant screw dislocations, systematically to one side of the S for a given radius, giving diagonal linkages and each developing lamella a comparatively large increment of twist. The detailed morphology is readily accounted for on this basis as is the restriction of banding to polymers with (eventually) inclined fold surfaces.     

Thermohydraulics of Turbulent Flow Through Heat Exchanger Tubes Fitted with Circular-rings and Twisted Tapes

The influences of circular-ring turbulators (CRT) and twisted tape (TT) swirl generators on the heat transfer enhancement, pressure drop and thermal performance factor characteristics in a round tube are reported. The circular-ring turbulators were individually employed and together with the twisted tape swirl generators in the heated section of the tube. Three different pitch ratios (l/D1.0, 1.5, and 2.0) of the CRT and three different twist ratios (y/W 3, 4, and 5) of the TT were introduced. The experiments were conducted using air as the working fluid under a uniform wall heat flux condition, for the Reynolds number between 6000 and 20000. The experimental results reveal that the heat transfer rate, friction factor and thermal performance factor of the combined CRT and TT are considerably higher than those of CRT alone. For the range examined, the increases of mean Nusselt number, friction factor and thermal performance, in the tube equipped with combined devices, respectively, are 25.8%, 82.8% and 6.3% over those in the tube with the CRT alone. The highest thermal performance factor of 1.42 is found for the combined device consisting of the CRT with l/D 1.0 and TT with y/W 3. The correlations of the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices are also developed in terms of Reynolds number, Prandtl number, twist ratio and pitch ratio.     

INFLUENCE OF NONUNIFORM TWISTED TAPE ON HEAT TRANSFER ENHANCEMENT CHARACTERISTICS

An experimental study of heat transfer and friction factor characteristics in a circular tube fitted by twisted tapes with nonuniform twist ratios is reported. The twisted tapes are used as swirl generators playing roles as heat transfer enhancers. The nonuniform twisted tapes examined in the present work have (1) sequentially increasing twist ratios (SL), (2) sequentially decreasing twist ratios (LS), (3) repeatedly increasing-decreasing twist ratios (SL/SL), (4) repeatedly decreasing-increasing twist ratios (LS/LS), (5) intermittently increasing-decreasing twist ratios (SL→LS), and (6) intermittently decreasing-increasing twist ratios (LS→SL). In addition, three different typical twisted tapes with twist ratios of 3.0, 4.0, and 5.0 (TT) were also evaluated. Apparently, LS yields monotonically increasing swirling intensity and SL decreasing swirling intensity, resulting in lower heat transfer rate and friction factor than the other four nonuniform twisted tapes, which possess extra fluid fluctuation. However, among the tapes tested, the SL/SL offers the maximum thermal performance factor of around 1.03, which corresponds to Nusselt number of around 36% and friction factor of 3.57 times over those of the plain tube.     

几何扭转的无人机机翼复合材料结构设计

具有几何扭转的无人机机翼其机翼外形是复杂的扭曲面,给制造特别是装配协调带来难度。文中介绍了针对几何扭转的无人机机翼的复合材料结构设计,采用巧妙的工艺设计思想,利用复合材料共固化翼面结构将复杂扭曲面的协调装配简化,从而解决了几何扭转的无人机机翼的低成本制造问题。针对某型无人机所设计的几何扭转机翼已经取得成功应用。     

旋流片支撑缩放管管束传热与流动的数值模拟

通过数值模拟,以水为工质,研究了湍流时旋流片支撑缩放管管束管间的传热综合性能,并与无支撑光滑管管束、无支撑缩放管管束及旋流片支撑光滑管管束的传热性能进行了对比,分析了不同结构参数对其传热综合性能的影响;揭示了旋流片支撑缩放管管间流体速度场与温度场之间的协同性。结果表明:旋流片支撑急扩慢缩型缩放管管束相对急缩慢扩型具有更好的传热综合性能,且都比旋流片支撑光滑管管束的传热综合性能好,但二者都没有对应的无支撑缩放管管束综合性能好;具有小角度跟大扭率结构的旋流片更有利于旋流片支撑管束传热综合性能的提高;相对旋流片支撑光滑管管束与无支撑缩放管管束,旋流片支撑缩放管管束强化传热的原因在于传热场协同的增强作用更为明显。考虑管间支撑物支撑管束与抗振的必要性,旋流片支撑缩放管管束是一种高效的壳程强化传热措施,文中条件下通过优化,最大传热综合性能能达到1.057。     

Dissipative Self‐Assembly of Peptides

In molecular self‐assembly, molecules interact with each other by non‐covalent interactions to form larger structures. The process occurs in‐equilibrium, which means that molecules can leave the assembly and reassemble elsewhere, but that occurs on average with equal rates. For self‐assembly, peptides have proven to be a particularly useful building block, in part because of their versatility. Biology also uses self‐assembly to create functional materials. For example, components of the cytoskeleton, like actin filament and microtubules, are self‐assembled proteins. In other words, biology uses the same building blocks and design rules as supramolecular chemists to create functional structures. However, biological assemblies are vastly more complex than their synthetic counterparts. The discrepancy is in part because proteins are more complex than the peptides we use as building blocks. Another contributing factor to the difference in complexity is that most assemblies in living systems exist out of equilibrium. To use peptides for complex functions as biology does, we should understand and be able to create peptide assemblies out of equilibrium. In this work, we review recent advances towards the creation of peptide assemblies that exist out of equilibrium driven by an external energy source.     

Chirality in Self-Assembling Rod-Coil Copolymers: Macroscopic Homochirality Versus Local Chirality

We study chiral mesophases arising as a result of the interplay between microphase separation and orientational ordering in diblock rod-coil copolymers. It is shown that nearly compositionally symmetric copolymers form a columnar structure with twisted rod-rich domains, whereas there is a suppression of the lamellar morphologies with respect to the columnar one. Using high-resolution three-dimensional self-consistent field simulations, we show that chirality in the unit cell of the hexagonal phase develops in two different ways, leading to either homochiral state or heterochiral (locally chiral) state. Thus, chiral polarization, which occurs when the rigid and flexible blocks are segregated, causes a transition to two degenerate chiral states. In a system with many twisted domains, the magnitude of the chirality charge obeys the binomial distribution with random selection of the twist direction for each of the rod-rich domains. We suggest a model of pseudodynamical structural evolution aimed at understanding of how chirality arises from the achiral state and how it evolves. At the initial stage of the evolutionary process, there exists some waiting time for the onset of irreversible changes in chirality; during this time the system flips between the two chirality states.     

The influence of drawing,twisting, heat setting,and untwisting on the structure and mechanical properties of melt-spun high-density polyethylene fiber

The variation of crystalline morphology and mechanical properties of polyethylene fibers was studied as they were sequentially melt spun, drawn, twisted, heat set, and untwisted. Twisting of as-melt spun fibers was also investigated. The morphology was characterized using wideangle x-ray diffraction, small-angle x-ray diffraction, and scanning electron microscopy techniques. Drawing results in high crystalline orientation, fibrillation, and large increases in modulus and tensile strength. Effects due to variation of spinning conditions were noted. Twisting either as-spun or drawn fibers decreased the axial orientation, modulus, tensile strength, and usually also the elongation to break. The changes in these properties increased with twist angle. Twisting also caused transformation of a small fraction of the sample to the monoclinic form of polyethylene. Heat setting caused healing of voids generated during drawing and increased the perfection and periodicity of the stacking of lamellar crystals along the fiber axis. Heat setting also caused the monoclinic polyethylene to transform back to the orthorhombic form, and it increased the modulus and tensile strength. Untwisting returned the orientation in the fiber to essentially that which it would have if it had not been twisted, but untwisting also resulted in the formation of kink bands.     

Control of assembly size of poly (methacrylic acid)-<Emphasis Type="Italic">grafted</Emphasis> fullerenes in aqueous solution

We synthesized poly(methacrylic acid)-grafted fullerenes (PMA-C₆₀) with different PMA molecular weights and investigated the assembly size formed by PMA-C₆₀ in aqueous solution. The molecular weight of PMA strongly affects the assembly size: PMA-C₆₀ with a larger molecular weight forms micelles with 20 nm diameters while PMA-C₆₀ with a smaller molecular weight forms monodisperse assemblies with 200 nm hydrodynamic diameters. We succeeded in converting the large monodisperse assembly into micelles by adding either ionic species or ethanol. This result provides insight into controlling the assembly size of fullerene-containing assemblies.     

Modelling of chain twist boundaries in poly(vinylidene fluoride) as a mechanism for ferroelectric polarization

It is assumed that the process of ferroelectric polarization of the beta phase of poly(vinylidene fluoride) (PVF₂) in response to the action of the external electric field in direction perpendicular to the molecular axis and to the film, involves movement of the chain twist boundaries. These boundaries, at which every chain is twisted by 180 degrees, separate domains of opposite polarization. The energy barriers that are surmounted as the boundary was advanced one repeat unit were calculated and compared with the energy gained by reversing the polarization of an unfavourably oriented repeat unit in an electric field that produces polarization in PVF₂. It is suggested that the movement of chain twist boundaries, in contradistinction to previously postulated models in which only one chain is twisted at a time, provides a model for the poling of PVF₂ that is feasible energetically and kinetically. Theoretical modelling, analogous to that for Bloch wall that separates domains in magnetic materials, suggest that the process of polarization might be described either as a diffusion process or as the propagation of a soliton along the chains.     

扭曲椭圆管管内传热与压降性能的研究

通过建立扭曲椭圆管单管传热与压降性能测试平台,利用光滑圆管,对测试平台测试结果的准确性进行了验证,同时对扭曲椭圆管的传热以及压降性能进行了实验测试,以测试结果为基础,验证了数值计算模型的准确性。对不同几何尺寸的扭曲椭圆管单管传热与压力性能进行了数值计算,分析了换热管几何参数对传热与压降性能的影响,结果显示:扭曲椭圆管传热性能随着扭曲椭圆管长短轴比A/B的增大而增大,随着扭曲椭圆管扭距S的减小而增大。同时以数值计算结果为基础,拟合得到了8×103相似文献   

Nile red probing for sphere-to-rod-to-wormlike micelle transition in aqueous surfactant solution

Nile red (phenoxazone-9) was used as a fluorescence probe to detect the sphere-to-rod-to-wormlike micellar transition in the aqueous mixtures of cetyltrimethylammonium bromide and sodium salicylate or potassium bromide, and also in the aqueous solution of a Gemini surfactant, ethanediyl-α,ω-bis(dimethyl dodecyl ammonium bromide). The results showed the I_a/I_b of Nile red (the fluorescence intensity ratio of the twisted intramolecular charge transfer band to the planar locally excited band) was a more sensitive index than the I_a (the fluorescence intensity of the twisted intramolecular charge transfer band) to represent the micellar transition. This was attributed to the special molecular structure of Nile red that has a large conjugated ring and the can form hydrogen bond with water molecules at the electron acceptor group. This led to sensitive response of the twisted intramolecular charge transfer state of Nile red to the change of microenvironment.