Semiflexible Liquid Crystalline Polymer Research Articles

Transient shear banding in the nematic dumbbell model of liquid crystalline polymers.

In the shear flow of liquid crystalline polymers (LCPs) the nematic director orientation can align with the flow direction for some materials but continuously tumble in others. The nematic dumbbell (ND) model was originally developed to describe the rheology of flow-aligning semiflexible LCPs, and flow-aligning LCPs are the focus in this paper. In the shear flow of monodomain LCPs, it is usually assumed that the spatial distribution of the velocity is uniform. This is in contrast to polymer solutions, where highly nonuniform spatial velocity profiles have been observed in experiments. We analyze the ND model, with an additional gradient term in the constitutive model, using a linear stability analysis. We investigate the separate cases of constant applied shear stress and constant applied shear rate. We find that the ND model has a transient flow instability to the formation of a spatially inhomogeneous flow velocity for certain starting orientations of the director. We calculate the spatially resolved flow profile in both constant applied stress and constant applied shear rate in start up from rest, using a model with one spatial dimension to illustrate the flow behavior of the fluid. For low shear rates flow reversal can be seen as the director realigns with the flow direction, whereas for high shear rates the director reorientation occurs simultaneously across the gap. Experimentally, this inhomogeneous flow is predicted to be observed in flow reversal experiments in LCPs.

연결기가 반 유연성 액정중합체의 물성에 미치는 영향

Semi-flexible liquid crystalline polymers containing a mesogenic group and an octamethylene flexible spacer in the main chain were synthesized by solution polycondensation. The mesogenic group in the polymer consists of four aromatic rings connected by ester and ketone, ether, sulfide, methylene, sulfone, or isopropylidene linkage groups. This paper discusses effects of the central linker of the mesogenic group on polymer properties. The structures and properties of synthesized polymers were investigated by 1 H-NMR, FT-IR, differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA), X-ray diffractometer (XRD), and polarizing optical microscope (POM). Polymers having bent linkage groups exhibited low thermal transition temperatures, narrow mesophase temperature ranges, low liquid crystallinity, and good solubilities in organic solvents, while those having bulky linkage groups were amorphous and exhibited high glass transition temperatures.

Entropically driven self-assembling of softened multi-walled carbon nanotubes into a reversibly water dispersible coherent solid

A bulk multi-walled carbon nanotube pellet has been produced by self-arrangement after drying the aqueous suspension of the carbon nanotubes (CNTs) oxidized by boiling CNTs in concentrated acid. The solid can be re-dispersed in water and self-assembled again. The centimeter order bulk CNT pellet unexpectedly showed compact domains of highly aligned parts of long CNTs which size is much larger than the domain size. Viscosity measurements confirmed that the system in question is completely different from a rigid-rod nematic. We reveal the mechanism of the entropy-driven self-alignment of treated worm-like CNTs in analogy to semiflexible liquid crystalline polymer. The polymer-like behavior of treated CNTs brings the CNTs into a genus of robust polymers.

STRUCTURAL PROPERTIES OF SEMI-FLEXIBLE MAIN-CHAIN LIQUID CRYSTALLINE POLYMERS

The structural properties of semi-flexible main chain liquid crystalline polymers(LCPs) are investigated by the method of molecular dynamics(MD) simulations. In semi-flexible main chain LCPs, molecular chains are described by the newly developed model called Solo-LJ-SP-GB model. It has been proven that the computational time the model takes is less than 10％of that the traditional GB/LJ model takes during the MD simulations of LCPs, which greatly improves the computational efficiency. During simulating the liquid crystal formation of semi-flexible main chain LCPs, the effects of the number of flexible spacers in LCPs molecules on the thermodynamic properties are studied. It is found that the thermodynamic properties depending on the number of flexible spacers show odd-even effects, which agrees with the existing experimental data. The local orientational correlation time function and translational mobility of semi-flexible main chain LCPs are calculated and compared with the simulation results of the traditional GB/LJ model, which suggests that these two models have similar anisotropy and mobility.

Synthesis and Mesomorphic Properties of Low Molar Mass Phosphorus Derivatives

New liquid crystalline phosphorus derivatives were synthesized as a core model compound and/or a model compound of the component for semiflexible liquid crystalline polymers. Alkoxy, oligo ethylene oxide, or chiral moieties were introduced into the terminal group of rigid moiety in order to investigate the effect of the terminal group on the thermal properties of the phosphorus derivatives. A nematic phase appears for the phosphorus derivative with alkoxy moieties or oligo ethylene oxide chain, while a mesophase is observed for the phosphorus derivative with a chiral moiety. Some phosphorus derivatives were thermally stable until more than 300°C.

Microhardness of compatibilized blends of polypropylene with a semiflexible liquid-crystalline polymer

The Vickers microhardness of blends of isotactic polypropylene and a semiflexible liquid-crystalline polymer (iPP/LCP 90/10 and 80/20 w/w), compatibilized with 2.5, 5 or 10 wt% PP-g-LCP copolymers with different composition has been studied. It has been shown that the microhardness values of uncompatibilized blends are close to the additive ones, while for compatibilized blends a strong positive deviation from additivity has been established. This result is interpreted by the increase in the degree of crystallinity of PP, by the decrease in the surface free energy of PP crystals and by the decrease in the surface free energy of the LC domains when the PP-g-LCP compatibilizer is present. The effect of the composition and concentration of the compatibilizer on the experimental hardness values has also been studied. The values of the microhardness/modulus of elasticity of some of the materials have been obtained. It is demonstrated that according to these values the compatibilized blends take a position closer to the elastic material in the elastic–plastic spectrum than the uncompatibilized blends. The results are interpreted by the compatibilizing efficiency of PP-g-LCP copolymers towards iPP/LCP blends.

On the molecular orientation and viscoelastic behaviour of liquid crystalline polymers: the influence of macromolecular architecture

The influence of macromolecular architecture on the flow‐induced orientation of main‐chain thermotropic liquid crystalline polymers (LCPs) is investigated using in situ wide‐angle X‐ray scattering. In order to get more insight into the interrelationship between microscopic and rheological behaviour, the viscoelastic properties of the nematic melts were also studied. The LCPs studied are wholly aromatics , composed only of mesogenic units, and semiflexibles , which consist of mesogenic units separated by alkyl spacers. Rheo‐X‐ray scattering evidenced the detrimental influence of the flexible spacers on the orientation process. The wholly aromatic LCPs readily orient along the flow direction, even under modest shear flows ( γ = 0.1 s‐1). The semiflexible LCPs, however, display lower levels of orientation than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterization shows that, like conventional flexible‐chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measurements, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behaviour is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associated with a molecular network formed by the less‐than‐rigid molecular chains. Furthermore, the steady shear viscosity is always smaller than the dynamic viscosity, i.e. the Cox‐Merz rule does not hold. This is due to the fact that steady shear induces molecular orientation, whereas oscillatory shear does not. The relaxation of orientation after steady shear showed that the molecular orientation relaxes after hundreds of seconds. The shear stress, however, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.

Crystallization and morphology of fibers prepared from compatibilized blends of polyethylene with a liquid crystalline polymer

Fibers prepared by melt spinning of compatibilized blends of high-density polyethylene (HDPE) with a semiflexible liquid crystalline polymer (LCP) SBH 1 : 1 : 2 of Eniricerche, Italy, have been investigated by differential scanning calorimetry (DSC) and optical microscopy. Two grades of HDPE with different molar mass have been used to prepare the blends. The compatibilizers are an acrylic acid functionalized polyethylene (Escor 5 000 by Exxon) and an ad hoc synthesized graft copolymer of the liquid crystalline polymer grafted to Escor 5 000 (PE-g-SBH). The studies cover the non-isothermal and isothermal crystallization, and the morphology of the fibers. The influence of the matrix molar mass and the maximal draw ratio of the fibers upon PE crystallization rate have been established. It has been shown that the overall PE crystallization rate under non-isothermal and isothermal conditions increases for fibers prepared from blends of lower molar mass HDPE, compatibilized by PE-g-SBH copolymer. The dispersed LCP phase in the latter fibers due to the enhanced interfacial adhesion in the presence of the copolymer compatibilizer appears as fibrils with high aspect ratio. The results have been interpreted by an increase in interfacial area between the SBH fibrils and the matrix phase, which enlarges the number of heterogeneous nuclei and increases the PE nucleation rate. These effects are less pronounced for the fibers compatibilized with Escor, due to the lower draw ratio of the fibers and to the lower aspect ratio of the SBH fibrils.

Liquid crystalline polyoxetanes with two mesogens in the side chain separated by a flexible spacer

The synthesis and thermal behaviour of a new geometry of side chain liquid crystalline polymers consisting on two mesogenic units connected by a flexible spacer as pendent groups of a polyoxetane chain is reported. Changes in the length and parity of the spacer lead to different mesophase properties. This feature is analysed in comparison to similar effects found in dimers and in semiflexible main chain liquid crystalline polymers with analogous chemical structure, where SC* and SCA symmetries have been obtained. Odd-even effects in transition temperatures and enthalpy changes are observed in both monomers and polymers. The symmetry of the mesophases is analysed by means of X-ray diffraction. Evidence for coexistence of different molecular orderings depending on the thermal treatment was found.

Characterization of a PE-g-LCP compatibilizer, prepared by reactive blending of acrylic acid grafted polyethylene (PE) and a semiflexible liquid crystalline polymer (LCP)

Novel graft copolymers (PE-g-LCP) consisting of polyethylene (PE) backbones and liquid crystalline polymer (LCP) branches, synthesized via reactive blending of acrylic acid functionalized PE (Escor 5000 and Escor 5001 by Exxon) with a semiflexible LCP (SBH by Eniricerche S.p.A.; mole ratio sebacic acid (S): 4,4'-dihydroxybiphenyl (B): 4-hydroxybenzoic acid (H) = 1:1:2) have been characterized by differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction. The isothermal crystallization, the morphology, and the crystalline structure of the crude reactive blending products (COPLE and COPHE) and of their fractions soluble in boiling toluene and xylene and of the residue have been investigated. It has been shown that the isothermal crystallization rate of PE segments of the copolymers decreases from toluene fractions to xylene insoluble fractions, however, these segments preserve their crystallizability even in SBH-rich copolymers. LCP does not lose its mesophase behaviour when bonded to the PE backbone and the morphology of the copolymers is not homogeneous. It is characterized by two types of structure typical of both copolymer segments. There are no interactions in the crystal phase or co-crystallization between the copolymer segments, forming separate crystalline phases. The results have been interpreted by a sufficient length of crystallizable segments of PE and SBH grafts and by a random distribution of SBH grafts along the PE backbone. The latter effect is more pronounced for PE-g-SBH copolymers based on higher molar mass functionalized PE (Escor 5001). It has been assumed that the compatibilization activity of these copolymers towards polyethylene/SBH blends is due not only to the identical chemical structure of the copolymer segments and the corresponding blend components but also to the similarity of their crystalline structure, crystallization behaviour, and morphology.

Morphology and rheology of HDPE/LCP blends compatibilized by a novel PE‐g‐LCP copolymer

A novel graft copolymer (PE-g-LCP) consisting of polyethylene (PE) backbones and liquid crystalline polymer (LCP) branches was synthesized via reactive blending of an acrylic acid-functionalized PE (Escor 5000 by Exxon) with a semiflexible LCP (SBH 1 : 1 : 2 by Eniricerche S.p.A.). The crude reactive blending product (COP) was shown by investigation of the fractions soluble in boiling toluene and xylene and of the residue to contain unreacted Escor and SBH, together with the graft copolymer forming the interphase. The compatibilizing activity of COP for PE/SBH blends, compared to that of pure Escor, was investigated using two PE grades. The COP addition into 80/20 PE/SBH blends caused a much stronger reduction of the SBH droplet dimensions and morphology stabilization than did that of pure Escor. The rheological behavior of the samples showed that COP leads to a slight increase of interfacial adhesion in the melt as well and that the effect is more pronounced when lower molar mass PE grade is used as the blend matrix. Melt-spinning tests demonstrated that deformation of the SBH droplets into highly oriented fibrils can be obtained for the blends of lower molar mass PE, compatibilized with small amounts of the novel PE-g-SBH copolymer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2069–2077, 1999

Morphology and rheology of HDPE/LCP blends compatibilized by a novel PE-g-LCP copolymer

A novel graft copolymer (PE-g-LCP) consisting of polyethylene (PE) backbones and liquid crystalline polymer (LCP) branches was synthesized via reactive blending of an acrylic acid-functionalized PE (Escor 5000 by Exxon) with a semiflexible LCP (SBH 1 : 1 : 2 by Eniricerche S.p.A.). The crude reactive blending product (COP) was shown by investigation of the fractions soluble in boiling toluene and xylene and of the residue to contain unreacted Escor and SBH, together with the graft copolymer forming the interphase. The compatibilizing activity of COP for PE/SBH blends, compared to that of pure Escor, was investigated using two PE grades. The COP addition into 80/20 PE/SBH blends caused a much stronger reduction of the SBH droplet dimensions and morphology stabilization than did that of pure Escor. The rheological behavior of the samples showed that COP leads to a slight increase of interfacial adhesion in the melt as well and that the effect is more pronounced when lower molar mass PE grade is used as the blend matrix. Melt-spinning tests demonstrated that deformation of the SBH droplets into highly oriented fibrils can be obtained for the blends of lower molar mass PE, compatibilized with small amounts of the novel PE-g-SBH copolymer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2069–2077, 1999

Isothermal crystallization kinetics of compatibilized blends of polyolefines with a semiflexible liquid crystalline polymer

A study of the isothermal crystallization behaviour and an analysis of the kinetic parameters of compatibilized blends of polyolefines (isotactic poly(propylene) (iPP) and high density polyethylene (HDPE)) with a semiflexible liquid crystalline polymer (LCP) SBH 1:1:2 of Eniricerche, Italy are presented in this work. The compatibilizers are ad hoc synthesized graft copolymers of the liquid crystalline polymer grafted to functionalized poly(propylene) (PPAA) or polyethylene (PEox) - PP-g-SBH and PE-g-SBH copolymers, respectively. It has been established that the LCP dispersed phase in the blends plays a role in nucleation for the polyolefine matrix crystallization. This effect is more pronounced in the poly(propylene) matrix than in the polyethylene matrix, due to the lower crystallization rate of the former. The addition of PP-g-SBH copolymers (2.5-10 wt.-%) to 90/10 and 80/20 iPP/SBH (w/w) blends provokes a drastic increase of the overall crystallization rate of the iPP matrix and of the degree of crystallinity. Contrarily, the addition of PE-g-SBH copolymers (2.5-8 wt.-%) to 80/20 HDPE/SBH (w/w) blends almost does not change or only slightly decreases the PE overall crystallization rate. Moreover, the interpretation of the Avrami exponents indicates that the iPP crystallization mechanism in the compatibilized iPP/SBH blends remains unchanged only until a degree of conversion of 0.7 and changes above this value; the PE crystallization mechanism and the degree of crystallinity in the compatibilized HDPE/SBH blends do not alter. The results were interpreted in terms of some differences in the compatibilization mechanism and efficiency of both types of graft copolymers (PP-g-SBH and PE-g-SBH). The relationships between blend morphology and nucleation phenomena are discussed.

Effect of PP-g-LCP compatibilizer on the structure and crystallization of blends of isotactic polypropylene (PP) with a semiflexible liquid crystalline polymer (LCP)

Blends of i-polypropylene and a semiflexible liquid crystalline polymer (iPP/LCP 90/10 and 80/20 w/w) were compatibilized with 2.5, 5 or 10 wt.-% PP-g-LCP copolymers. The crystal structure, crystallization behaviour and morphology of the compatibilized blends have been investigated by differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and optical microscopy. It is shown that the nucleation rate and crystallization rate of iPP strongly increased in the compatibilized blends in comparison to the uncompatibilized ones. In compatibilized blends, while iPP spherulites get smaller and crystallinity degree increases, the crystal growth mechanism of iPP remains unchanged. An assumption has been made about the mechanism of the compatibilization, using graft copolymers constructed of segments identical to the blended polymers: the PP segments of PP-g-LCP copolymers, if long enough, cocrystallize with bulk iPP, compatibilizing partially the two components; the LCP grafts of the copolymers cocrystallize with the bulk LCP or enter the amorphous phase of the blends. The miscibility of each part of the copolymer compatibilizers with the corresponding bulk component of the blend leads to a reduction of the interfacial tension and to a strong reduction of the dimensions of dispersed LCP phase. The compatibilized iPP/LCP blends display improved crystallization kinetics, enhanced degree of crystallinity and improved interphase adhesion. Consequently, an improvement of the mechanical properties and rheological characteristics should be expected for these blends.

Effect of PP-g-LCP compatibilizer on the structure and crystallization of blends of isotactic polypropylene (PP) with a semiflexible liquid crystalline polymer (LCP)

Blends of i-polypropylene and a semiflexible liquid crystalline polymer (iPP/LCP 90/10 and 80/20 w/w) were compatibilized with 2.5, 5 or 10 wt.-% PP-g-LCP copolymers. The crystal structure, crystallization behaviour and morphology of the compatibilized blends have been investigated by differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and optical microscopy. It is shown that the nucleation rate and crystallization rate of iPP strongly increased in the compatibilized blends in comparison to the uncompatibilized ones. In compatibilized blends, while iPP spherulites get smaller and crystallinity degree increases, the crystal growth mechanism of iPP remains unchanged. An assumption has been made about the mechanism of the compatibilization, using graft copolymers constructed of segments identical to the blended polymers: the PP segments of PP-g-LCP copolymers, if long enough, cocrystallize with bulk iPP, compatibilizing partially the two components; the LCP grafts of the copolymers cocrystallize with the bulk LCP or enter the amorphous phase of the blends. The miscibility of each part of the copolymer compatibilizers with the corresponding bulk component of the blend leads to a reduction of the interfacial tension and to a strong reduction of the dimensions of dispersed LCP phase. The compatibilized iPP/LCP blends display improved crystallization kinetics, enhanced degree of crystallinity and improved interphase adhesion. Consequently, an improvement of the mechanical properties and rheological characteristics should be expected for these blends.

Effect of PP-g-LCP compatibilizer on the structure and crystallization of blends of isotactic polypropylene (PP) with a semiflexible liquid crystalline polymer (LCP)

Effect of PP-g-LCP compatibilizer on the structure and crystallization of blends of isotactic polypropylene (PP) with a semiflexible liquid crystalline polymer (LCP)

Crystallization behavior and morphology of PE-g-LCP copolymers

This study presents DSC and optical microscopy investigations on copolymers of semiflexible liquid crystalline polymer SBH 112 grafted to functionalized low molecular mass polyethylene (PEox) obtained by melt polycondensation or reactive blending procedures. The crystallization behavior of the PE-g-SBH copolymers has been studied under non-isothermal measurement conditions carried out at different cooling rates. The crystallization temperature (Tcr) of the PE component of the copolymers decreases steadily upon increasing the concentration of the SBH grafts. It was found that the copolymers prepared by reactive blending crystallize at slightly higher Tcr than those prepared by polycondensation and with a higher rate, confirmed by the determination of the crystallization rate coefficients (CRC). The results have been interpreted by the fact that the PE crystallizable segments and SBH grafts of the copolymers obtained by reactive blending are longer than those of the copolymers prepared by polycondensation. The overall nonisothermal crystallization kinetics has been studied by the Harnisch and Muschik equation. The results show that the mechanism of the crystallization of the PE phase changes only when the SBH content overruns ca.50%, due to the decrease of both nucleation and crystal growth rates. The morphology of the copolymers crystallized nonisothermally from melt has been examined by polarization microscopy. Fairly homogeneous morphology with tiny PE spherulites is observed for PE-g-SBH copolymers prepared by polycondensation with SBH as the minor phase. No sign of the dispersed LCP domains can be recognized. On the contrary, the morphology of the copolymers prepared by reactive blending is distinctly biphasic. The allegedly longer PE segments crystallize into tiny spherulites too, but the LC domains formed by the long SBH branches present in this type of copolymers appear clearly in the micrographs at room temperature. It is concluded that the copolymers prepared by reactive blending would be more effective as compatibilizers for PE/SBH blends than those prepared by polycondensation.

Structure investigations of PE-g-LCP copolymers

Transesterification products-copolymers of semiflexible liquid crystalline polymer SBH 112 grafted to functionalized low molecular mass polyethylene (PEox) obtained by melt polycondensation or reactive blending procedures have been investigated by wide-angle x-ray scattering (WAXS) and scanning electron microscopy (SEM).

Alternative synthetic strategies for Schiff base derived liquid crystal polymers: A comparative study

Semiflexible liquid crystalline polymers based on Schiff bases as mesogenic cores have been synthesized through different synthetic strategies. The mesogenic cores are derived from 1,5-naphthalenediamine linked to a polymethylenic spacer by means of ether or ester groups. The synthetic strategies differ in the disconnection of the molecules to design the synthesis of the repeating units. Polymers were synthesized either as polyazomethines, where imine bonds are formed in the polymer synthesis, or as polyethers or polyesters, where the ether or ester groups are formed at the polymer-forming stage. The influence of the synthetic method and the linking groups between mesogenic core and flexible spacer on the thermal and mesogenic properties has been analyzed by DSC, thermogravimetry, and optical microscopy. The possibility of obtaining hydroxy-functionalized polymers by different synthetic strategies has been studied as well as the influence of these groups on the thermal and mesogenic properties. © 1996 John Wiley & Sons, Inc.

Morphology and crystallization of blends of linear low density polyethylene with a semiflexible liquid crystalline polymer

The morphology and the crystallization behavior of blends of linear low density polyethylene (LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 1∶1∶2 by Eniricerche, Italy) have been studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and scanning electron microscopy (SEM). The blends possess a two-phase morphology, due to immiscibility of the two components. SEM observations show that dispersion of the minor SBH phase is favored at low (<ca. 15%) SBH concentration. At higher concentrations, the dimensions of the dispersed particles increase strongly. Non-isothermal and isothermal DSC measurements have shown that the LLDPE crystallization rate is strongly enhanced by the addition of SBH. The maximum effect has been found for an SBH concentration of ca. 10% w/w. POM observation shows that the LLDPE spherulite size decreases by ca 50% in the presence of SBH. These results have been interpreted on the basis of a heterogeneous nucleation played by the crystallized SBH particles on the molten LLDPE matrix. The interpretation of the non-isothermal and isothermal data in terms of the Avrami exponents indicates that the nucleation mechanism is not altered by the presence of a dispersed SBH phase. The relationship between blends morphology and nucleation phenomena have been discussed.