On the Uniaxial–Biaxial Nematic Phase Transition in Liquid‐Crystalline Polymers and Elastomers

We study how the uniaxial–biaxial nematic phase transition changes its nature when going from a low‐molecular‐weight liquid crystal to a liquid‐crystalline elastomer or polymer (the latter above the Maxwell frequency) and find a qualitative change due to the presence of a coupling to the strain field in these materials. While this phase transition can be of second‐order in low‐molecular‐weight materials, as is also experimentally observed, we show here that the order of this phase transition is changed generically to no phase transition at all or to a first‐order phase transition in mean‐field approximation. We analyze the influence of an external mechanical stress field above the uniaxial–biaxial nematic phase transition and find that either biaxial nematic order is induced, which is linear or quadratic in the stress intensity, or no response to an external stress results at all, depending on the relative orientation of the applied shear with respect to the director of the uniaxial nematic phase.     

A New Catalytic System (2-PhInd)2ZrMe2/Al(iso-Bu)3 for the Synthesis of Stereoblock Polypropylene

A new efficient two-component homogeneous system consisting of (2-PhInd)₂ZrMe₂ and Al(iso-Bu)₃ cocatalyst was developed for the synthesis of elastomeric stereoblock polypropylene. The catalytic behavior of this system was studied in toluene and liquid propylene. It was shown that a change in the Al/Zr molar ratio enables one to vary the system activity, the kinetic characteristics of the polymerization process, and the properties of polypropylene over wide ranges. For example, an increase in the Al/Zr ratio from 50 to 300 mol/mol is accompanied by an increase in the activity by a factor of 4, an increase in the molecular weight from 80000 to 220000, and a decrease in isotactic pentad content from 39 to 16%. It was found that the polymerization rate is proportional to the concentration of Al(iso-Bu)₃ and to the square of the monomer concentration. Some assumptions about the nature of active centers and about reactions leading to the formation of these centers were made. A model for the formation of sequences of isotactic and atactic blocks was proposed. This model takes into account the multiple inversion of the active center (fast dynamic equilibrium) from the iso-specific to the nonspecific state during chain growth. This inversion is due to the coordination of an additional Al(iso-Bu)₃ molecule. This model satisfactorily describes the pentad composition of polypropylene samples as a function of the Al/Zr ratio.     

Polyethylene and elastomeric polypropylene using alumina-supported bis(arene) titanium,zirconium, and hafnium catalysts

Bis(arene) complexes of zerovalent titanium, zirconium, and hafnium supported on partially dehydroxylated, fumed alumina are effective catalysts for the polymerization of olefins. The zerovalent complexes react with surface hydroxyls with loss of one equivalent of arene to yield the active species. The polyethylenes derived from these catalysts are very high molecular weight. Polymerization of propylene yields elastomeric stereoblock polymers which are composed of isotactic and stereoirregular sequences. The polymers are stiffer than polypropylenes obtained with similar catalysts derived from tetra(neophyl)zirconium. The chain microstructures of the various components of the whole polymers have been characterized by ¹³C-NMR and solvent extraction studies. The ether soluble component of these polymers is of a relatively high molecular weight and the microstructure of the backbone is largely stereoirregular. It is the cocrystallization of this fraction of the polymer with the crystalline, isotactic fractions which is critical to the observed elastomeric properties.     

A New Method for Continuous Solid-State Biaxial Orientation of Polymeric Films

A new laboratory-scale method for continuous solid-state biaxial deformation of polymer materials is introduced. The tabletop biaxial drawing tool utilizes a heated metal disc-like triangular mandrel, which allows for continuous solid-state biaxial deformation of a precursor polymer tube with a controlled drawing speed. The new biaxial drawing tool features two independent heating zones. The main mandrel sets the biaxial drawing temperature. In addition, directly after the mandrel, an annealing unit with a separate heating zone is added for better control of important process parameters, such as post-relaxation and strain-induced crystallization. As such, the new biaxial drawing tool is a tabletop analogue of the industrial triple-bubble process. Finally, a detailed theoretical model describing the deformation field and the forces needed to perform biaxial drawing as a function of the biaxial drawing ratio is presented and compared to experimental values, using isotactic polypropylene as model material. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 352-362     

Synthesis and Characterization of Novel Segmented Polyionenes Based on Polydimethylsiloxane Soft Segments

Novel polydimethylsiloxane (PDMS) based polyionenes were synthesized by a modified Menschutkin reaction involving reaction between bromo-terminated PDMS oligomers and various ditertiary amino compounds. In this study, the nature of the hard segment was varied by using various ditertiary amino compounds and in some cases by incorporating chain extenders, while the soft segment content was varied by changing the molecular weight of the PDMS oligomers. The mechanical properties of these materials were found to be dependent on both the nature and amount of the hard segments. These materials also showed distinct evidence of a microphase-separated morphology where under normal conditions, the hard segments formed in what are believed to be cylindrical ion-rich microdomains dispersed randomly in the soft PDMS matrix. When subjected to uniaxial deformation, the ionic cylinders were found to orient along their long axes in the stretch direction.     

Strain-tunable electronic properties and optical properties of Hf2CO2 MXene

Two-dimensional materials have been extensively applied because of their unusual electronic, mechanical, and optical properties. In this paper, the electronic structure and optical properties of Hf₂CO₂ MXene under biaxial and uniaxial strains are investigated by the Heys-Scuseria-Ernzerhof (HSE06) method. Monolayer Hf₂CO₂ can sustain stress up to 6.453 N/M for biaxial strain and 3.072 N/M for uniaxial strain. Monolayer Hf₂CO₂ undergoes the transition from semiconductor to metal under −12% strain whether it is under biaxial or uniaxial strain. With the increasing biaxial compressive strain, the blue shift of Hf-d, O-p, and C-p orbitals in valence band maximum results in the metallization of monolayer Hf₂CO₂, while the red shift of Hf-d and O-p orbitals in conduction band minimum results in the metallization of monolayer Hf₂CO₂ with increasing uniaxial compressive strain. The analysis of optical properties indicates that uniaxial strain weakens the reflectivity and refractive index of monolayer Hf₂CO₂ in the visible-light range. In addition, the effective mass and the charge distribution under biaxial and uniaxial strains are also explored.     

Experimental analysis of the effect of carbon nanoparticles with different geometry on the appearance of anisotropy of mechanical properties in elastomeric composites

The paper presents studies of the properties of elastomeric materials based on the solution-polymerized styrene-butadiene rubber (S-SBR) filled with nanoparticles of different geometry – grainy (nanodiamonds), layered (graphene nanoplates) and fibrous (nanotubes). Despite the introduction of a small amount of fillers, the significant strengthening of the matrix is exhibited. Of particular interest is the behavior of the nanodiamond-filled elastomer composite, for which a double increase in rupture stress with an increase in rupture strain is observed at the filler volume fraction approximately equal to 2%. The structure of composites is investigated by optical and atomic force microscopy. In addition to standard mechanical testing, experiments of successive cyclic loading along each of two mutually perpendicular axes of plane elastomeric samples were conducted with use of a Zwick 4-vector test stand (biaxial testing machine). The phenomenon of induced anisotropy in materials is analyzed. A hypothesis to explain its manifestation is suggested.     

Biaxial tensile behavior and strength of architectural fabric membranes

The utilization of composite fabric membrane materials for large-span membrane structures has attracted considerable attention in recent decades due to enhanced material properties. Biaxial mechanical properties with respect to real engineering applications are essential and indispensable in comparison with uniaxial ones. This study focuses on true biaxial characteristics of a typical polyvinylidene fluoride (PVDF)-coated polyester membrane material in terms of stress-strain characteristics and breaking criteria.The true stress-strain curves obtained from an experimental study, i.e. seven loading ratios on the basis of symmetry and typical conditions, are investigated with digital image correlation method. The interpolation of these curves in combination of least square method achieves a three-dimensional strain surface as a function of warp and weft strains, which is useful to assess reasonable structural behavior. A new breaking criteria intended for architectural fabric membrane is proposed in analogy to Tsai-Hill, Yeh-Stratton and Norris failure criteria. The basic constants in the criteria are determined using experimental results. A comparative analysis between available uniaxial and biaxial criteria shows that the new criteria can cover all criteria due to the fact that biaxial mechanical properties are larger than uniaxial ones. Furthermore, a similar but glued specimen is employed to compare welded specimens. It is obtained that observations, values and curve tendency are similar, demonstrating the suitability of using new specimens to identify true biaxial properties.     

十二烷基苯磺酸钡存在下负离子聚合控制聚苯乙烯立构规整性研究

以正丁基锂(n-BuLi)为引发剂,环已烷为溶剂,通过添加十二烷基苯磺酸钡/四氢呋喃(SDBB/THF),对苯乙烯负离子聚合进行了研究.该体系可在60℃聚合得到立构规整性聚苯乙烯,其丁酮不溶物含量可达80%左右.13C-NMR表征结果显示其微观序列组成以等规结构为主,三元组mm和等规五元组mmmm含量可达57%和45%;DSC谱图中不仅有聚苯乙烯玻璃化温度(100℃),而且在211℃有一熔融峰.GPC结果表明,实测分子量与设计分子量差别较大,且分子量分布较宽。     

硅烷类外给电子体的取代基变化对丙烯聚合影响的研究

合成了2种新型硅烷化合物双环己基二甲氧基硅烷(Donor-H)、双哌啶二甲氧基硅烷(Donor-Py),将其与工业化的环己基甲基二甲氧基硅烷(Donor-C)分别作为外给电子体,用于MgCl2负载的Ziegler-Natta催化剂催化丙烯聚合,通过DSC、GPC、SSA和13C-NMR等分析手段研究了3种外给电子体取代基的变化对催化剂的催化活性、氢调敏感性、聚丙烯的等规度、分子量分布、结晶能力、等规序列分布的影响.结果表明,随着外给电子体取代基体积的增大,外给电子体的给电子能力逐渐增强.与Donor-C相比,随着外给电子体取代基体积的增大,合成的具有较大取代基的Donor-H和Donor-Py用于丙烯聚合时都表现优异的催化性能,特别是新型含有N杂原子的氨基硅烷类Donor-Py为外给电子体的催化剂的催化活性和制备的聚丙烯的等规度最高,聚丙烯的熔融指数可调范围最宽,结晶能力更强.氨基硅烷类Donor-Py制备的聚丙烯SSA热分级后的高等规组分含量最多,可高达64.5%,聚丙烯等规序列长度最长,聚丙烯的等规序列分布最窄,而且13C-NMR结果也表明聚丙烯等规序列长度MSL最长,聚丙烯的分子链最规整.     

Influence of isotacticity and molecular weight on the properties of metallocenic isotactic polypropylene

The relationships between structure and properties have been established for isotactic polypropylene, iPP, homopolymers synthesized by metallocene catalyst systems. These iPPs exhibit different isotacticity degrees and molecular weights, and several thermal treatments during their processing have been applied. The most important factor affecting the structure and properties of these polymers is the isotacticity content. The thermal treatment, i.e., the rate of cooling from the melt, is also important and a clear molecular weight effect has been also found for the sample with lowest M_w. These factors affect the thermal properties, the degree of crystallinity and, therefore, the structural parameters and the viscoelastic behavior. A slow cooling from melt favors the formation of the γ phase instead of the α modification. The storage modulus, Young modulus and microhardness values increase as crystallinity does, independently of the origin of this increase: higher isotacticity or application of a slow crystallization from the melt.     

Structure-property correlations in polypropylene from metallocene catalysts: stereodefective, regioregular isotactic polypropylene

A series of MAO-activated C(1)-symmetric indenyl-ansa-dithienocyclopentadienyl-based zirconocenes have been used to produce isotactic polypropylenes of medium to high molecular weights, of different degrees of stereoregularity, and free from regioerrors. The effect of the presence of rr defects on the polymorphic behavior and mechanical properties of polypropylene is analyzed. The presence of rr defects induces crystallization of gamma form and of disordered modifications intermediate between alpha and gamma forms. A linear relationship between the amount of gamma form and the average length of isotactic sequences has been found. Samples with low concentration of rr defects, up to 3-4%, present high melting temperatures, in the range 160-130 degrees C, and behave as stiff-plastic materials; sample with higher rr content, in the range 4-6% and melting temperatures around 115-120 degrees C are highly flexible thermoplastic materials, and, finally, samples with concentration of rr defects in the range 7-11% and melting temperatures in the range 80-110 degrees C are thermoplastic elastomers with high strength. The fine-tuning of the chain microstructure, achieved by a tailored design of new metallocene catalysts, has allowed production of new polypropylenes having desired properties, intermediate between those of stiff plastic and elastomeric materials.     

Crystallization kinetics of isotactic polystyrene from isotactic-atactic polystyrene blends

In an attempt to facilitate a better understanding of the role of noncrystallizable components on the crystallization kinetics, spherulitic growth rates as well as the morphology and melting behavior of isotactic polystyrene in blends with various molecular weight atactic polystyrenes (900 to 1,800,000) over a wide range of concentrations have been studied. The growth rates are uniformly depressed with increasing amounts of atactic diluent. In addition, they are dependent on the molecular weight of the added polystyrene, generally decreasing in the molecular weight ranges between 4800 and 19,800 and between 51,000 and 1,800,000. However, between these two ranges, anomalous growth rates showing a sudden increase are observed, which may be explained by an increase in the entrapment of the noncrystallizable diluent. An explanation based on morphological observations, which showed an increase in coarseness of the spherulites with increasing molecular weight of the added atactic polystyrene, is offered.     

Enhanced electrical and mechanical properties of multiwall carbon nanotube rubber composites

Multiwall carbon nanotube‐filled elastomers are prepared by solution blending using a sonication process. It is shown that the processing conditions have a strong effect on the composite properties especially on electrical properties, which are very sensitive to nanotube dispersion within the elastomeric matrix. The percolation threshold is seen to be shifted to a lower nanotube content than that previously reported. With regard to the unfilled elastomer, large increases in the elastic and tensile moduli are obtained with the nanotube loading, thus highlighting the potential of this type of particles as reinforcing fillers for elastomeric matrices. Raman spectroscopy under strain has been used to evaluate the strength of the polymer–filler interface. Weak interfacial interactions are deduced, but the debundling of the nanotubes and the orientational effects of the polymeric chains are observed when the composite is submitted to a uniaxial deformation. Copyright © 2012 John Wiley & Sons, Ltd.     

Bifurcational analysis of the isotropic-discotic nematic phase transition in the presence of extensional flow

A bifurcational analysis is performed on a version of Doi's equation of nematodynamics that describes the non-equilibrium isotropic-discotic nematic phase transition in the presence of steady uniaxial extensional flow. The disc-like molecular geometry and the degenerate extensional flow-induced orientation are shown to be the source of a complex bifurcation and multistability behaviour involving two physically equivalent biaxial nematic phases, one uniaxial nematic phase and one uniaxial paranematic phase. Depending on the temperature and the extension rate, the isotropic-discotic nematic transition, involving the two biaxial nematic phases and the uniaxial paranematic phase, may be continuous (2nd order), discontinuous (1st order), or it may exhibit a tricritical non-equilibrium phase transition point. A validation procedure on the validity of the predictions is implemented. The predictions presented here find practical applications in the industrial spinning of mesophase carbon fibres, and also provide new results that increase the present fundamental understanding of the rheology of discotic nematic liquid crystals.     

Phase diagram of the uniaxial and biaxial soft-core Gay-Berne model

Classical molecular dynamics simulations have been used to explore the phase diagrams for a family of attractive-repulsive soft-core Gay-Berne models [R. Berardi, C. Zannoni, J. S. Lintuvuori, and M. R. Wilson, J. Chem. Phys. 131, 174107 (2009)] and determine the effect of particle softness, i.e., of a moderately repulsive short-range interaction, on the order parameters and phase behaviour of model systems of uniaxial and biaxial ellipsoidal particles. We have found that isotropic, uniaxial, and biaxial nematic and smectic phases are obtained for the model. Extensive calculations of the nematic region of the phase diagram show that endowing mesogenic particles with such soft repulsive interactions affect the stability range of the nematic phases, and in the case of phase biaxiality it also shifts it to lower temperatures. For colloidal particles, stabilised by surface functionalisation, (e.g., with polymer chains), we suggest that it should be possible to tune liquid crystal behaviour to increase the range of stability of uniaxial and biaxial phases (by varying solvent quality). We calculate second virial coefficients and show that they are a useful means of characterising the change in effective softness for such systems. For thermotropic liquid crystals, the introduction of softness in the interactions between mesogens with overall biaxial shape (e.g., through appropriate conformational flexibility) could provide a pathway for the actual chemical synthesis of stable room-temperature biaxial nematics.     

Biaxial smectic A liquid crystal in a pure compound

The synthesis and characterisation is reported of a low molecular weight organic compound to exhibit the biaxial smectic A (SmAb) phase, which shows a transition from the partial bilayer uniaxial SmAd phase to the SmAdb phase as the temperature is lowered.     

Strain energy function of swollen polybutadiene elastomers studied by general biaxial strain testing

The stress–strain behavior of polybutadiene elastomers with various degrees of swelling is investigated for general biaxial strain. Although a number of previous observations have suggested that the classical neoHookean (NH) model describes the uniaxial data of an elastomer in a highly swollen state, the NH model evidently fails to reproduce the biaxial data. This result indicates that the successful fit of the NH model for the uniaxial data of highly swollen elastomers, which has long been recognized, is superficial. We show that the biaxial data in all of the various swollen states including the neat (unswollen) state are satisfactorily described by an Ogden‐type strain energy function with a single set of parameters. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 721–728, 2010     

Observation of pentads in deuterated polyacrylonitrile by NMR spectroscopy

The deuterium-decoupled methine proton spectra of polyacrylonitrile-β,β-d₂ were measured at 156°C in dimethylsulfoxide-d₆ solution. Splittings of triad peaks caused by pentad sequences were observed. The isotactic triad resonance was resolved into three peaks and the heterotactic resonance into two peaks, while the syndiotactic resonance was unresolved or resolved into three peaks. The splittings were assigned by comparing the probability of each pentad sequence with observed intensities. The pentad signal shifted to the higher magnetic field with increasing number of meso configurations as neighbors of the central triad. It was observed that the pentad of atactic polyacrylonitrile-β,β-d₂ obeyed Bernoullian statistics but that of isotactic polymer obtained by γ-ray irradiation of the canal complex seemingly obeyed first-order Markov statistics.     

X‐ray diffraction from the uniaxial and biaxial nematic phases of bent‐core mesogens

X‐ray diffraction patterns for the uniaxial and biaxial nematic phases exhibited by rigid bent‐core mesogens were calculated using a simple model for the molecular form factor and a modified Lorentzian structure factor. The X‐ray diffraction patterns depend strongly on the extent of the alignment of the molecular axes as well as the orientation of molecular planes. The X‐ray diffraction can be unequivocally used to identify the biaxial nematic phase, study the uniaxial–biaxial phase transition, and estimate the order parameters of the nematic phase.