Study of supramolecular side‐chain and cross‐linking polymers by complexation of various H‐donor acids with H‐acceptor copolymers containing pendent carbazole and fluorescent pyridyl units

Two H‐bonded acceptor (H‐acceptor) homopolymers 14 and 17 were successfully prepared by polymerization of fluorescent pyridyl monomers PBT and PBOT ( 12 and 13 ), which were synthesized via Sonogashira coupling and Wittig‐Horner reactions. To increase the glass transition temperatures as well as reduce the π‐π stacking of the photoluminescent (PL) H‐acceptor copolymers and their H‐bonded polymer complexes, fluorescent monomers 12 and 13 were copolymerized with N‐vinylcarbazole monomer CAZ (23) to produce H‐acceptor copolymers 15–16 and 18–19 . Supramolecular side‐chain and crosslinking polymers (i.e., H‐bonded polymer complexes) obtained by complexation of light‐emitting H‐acceptor polymers 14–19 with various proton donor (H‐donor) acids 20–22 were further characterized by DSC, POM, FTIR, XRD, and PL measurements. The mesomorphic properties can be tuned from the nematic phase in H‐acceptor homopolymers ( 14 and 17 ) to the tilted smectic C phase in their H‐bonded polymer complexes ( 14/20–21 and 17/20–22 ) by the introduction of H‐donor acids (20–22). Moreover, the PL properties of light‐emitting H‐acceptor polymers can be adjusted not only by the central structures of the conjugated pyridyl cores but also by their surrounding nonfluorescent H‐donor acids. In general, redder shifts of PL emissions in H‐bonded polymer complexes occurred when the light‐emitting H‐acceptor polymers were complexed with H‐donors having smaller pKa values. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2734–2753, 2009     

Temperature induced structural evolution of a fluorene‐thiophene copolymer on rubbed surfaces

The thermal alignment of the liquid crystalline fluorene‐thiophene copolymer (F8T2) on rubbed polyimide surfaces is investigated by ex‐situ and in‐situ X‐ray scattering experiments. The ex‐situ characterization allows an assignment of the observed diffraction peaks to distances between polymer backbones (1.6 nm), distances between the flexible side groups of the polymer chains (0.43 nm), and intramolecular distances of adjacent ring units (0.5 nm). The in‐situ characterization allows a temperature dependent observation of the polymer chain alignment. A gradual alignment process of the polymer backbones is observed for temperatures up to 563 K. Decreasing temperature after the polymer chain alignment is accompanied by a glass transition of the side chains at 380 K. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:1599–1604, 2009     

Novel thermotropic esters‐based polymers with broad nematic processing windows

In this study, a new series of semiflexible liquid crystalline (LC) polyesters and poly(ester‐amide)s were synthesized and characterized. Polymers based on 4‐hydroxybenzoic acid (4‐HBA), 6‐hydroxy‐2‐naphthoic acid (HNA), suberic acid (SUA), and sebacic acid (SEA) were modified with hydroquinone (HQ) and different concentrations of 4‐acetamidophenol (AP) to obtain a polyester and two poly(ester‐amide)s, respectively. All polymers were successfully prepared using conventional melt‐condensation techniques. The polymers were characterized by inherent viscosity measurements, SEC, hot‐stage polarizing microscopy, DSC, and TGA. The mechanical behavior was investigated using DMTA and tensile testing. All linear polymers have T_gs in the range of 50–80 °C and melt between 120 and 150 °C. Our polymers display good thermooxidative stabilities (5% wt loss at ～ 400 °C) and exhibit homogeneous nematic melt behavior over a wide temperature range (ΔN ～ 250 °C). The liquid crystal phase was lost when high concentrations of nonlinear monomers such as 3‐HBA (>27 mol %) and resorcinol (RC) (>23 mol %) were incorporated. The LC polyester based on 4‐HBA/HNA/HQ/SUA/SEA could easily be processed into good quality films and fibers. The films display good mechanical properties (E′ ～ 4 GPa) and high toughness, that is, ～ 15% elongation at break, at room temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6565–6574, 2008     

Interfacial slip at the thermotropic liquid‐crystalline polymer/poly (ethylene naphthalate) interface

The unique rheological properties of a thermotropic liquid‐crystalline polymer (TLCP) were first studied. The thermal and shear history of the TLCP was found to play a critical role in its rheological properties. Crystallites were observed in the TLCP melt even above the melting temperature detected by differential scanning calorimetry. Because interfacial slip had long been suggested as an important reason for viscosity reduction in TLCP/thermoplastic blends, for the first time, interfacial slip at the TLCP/poly(ethylene naphthalate) (PEN) interface was investigated with an energy model. The model quantified the degree of interfacial slip at the TLCP/PEN interface by an energy factor. The calculated energy factors revealed a high degree of interfacial slip at the TLCP/PEN interface. It was proposed that the high rigidity of rodlike TLCP chains and their alignment parallel to the interface prevented mutual entanglements at the TLCP/PEN interface. The lack of mutual entanglements promoted the interfacial slip. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 302–315, 2004     

Development of the morphology and crystalline state due to hybridization of reinforced toughened nylon containing a liquid‐crystalline polymer

A hybrid composite consisting of rubber‐toughened nylon‐6,6, short glass fibers, and a thermotropic liquid‐crystalline polymers (LCP) was investigated by the LCP content being varied. The thermal behavior, morphology, and crystallization behavior due to hybridization were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and wide‐angle X‐ray scattering (WAXS). DSC results indicated that the crystallinity of the glass‐fiber‐reinforced toughened nylon‐6,6 was reduced by LCP addition, particularly 5–10 wt % LCP. DMA data showed that the miscibility between the blended components was maximum at the 5 wt % LCP composition, and the miscibility decreased with increasing LCP content. SEM photomicrographs revealed information consistent with the thermal behavior on miscibility. It was also observed that the 10 wt % LCP composition showed predominantly an amorphous character with FTIR and WAXS. WAXS results indicated that LCP hybridization increased the interplanar spacing of the hydrogen‐bonded sheets of the nylon crystals rather than the spacing between the hydrogen‐bonded chains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 549–559, 2003     

Phase Separation and Organisation of Colloidal Spheres Suspended in Sheared Lyotropic Liquid‐Crystalline Polymers

Summary: Organisation behaviours of spherical particles suspended in sheared, lyotropic, liquid‐crystalline polymer solutions have been investigated using polarizing optical microscopy. We find that in a nematic phase the particles phase separate and adopt anisotropic chain‐like structures along the director. An earring defect is observed around a single particle whereas a cross or strings defect between neighbouring particles is found to serve as a repulsive barrier to prevent the particles from contacting each other. A theoretical analysis is presented to explain this new phenomenon.

An optical micrograph of 0.01 wt.‐% glass spheres suspended in a nematic solution of 40 wt.‐% ethyl cellulose in chloroform under an external shear force.     

Effect of Molecular Weight on Photoinduced Birefringence in a Chiral Liquid Crystalline Azodye Polymer

Summary: The evolution of the photoinduced birefringence in thin films of narrow polymer fractions is studied and compared with the behavior of the non‐fractionated polymer. The Δn_ind value decreases by increasing the degree of polymerization ( ) within the oligomeric range but becomes independent of molecular weight starting from a of ≈70. Thermal pretreatment of the films results in higher photoinduced birefringence. The films show good stability of the photorecording.

Birefringence induced after 10 min, Δn_ind(600) and its growth rate at the same moment versus molecular weight.     

Monodomain Liquid Crystal Main Chain Elastomers by Photocrosslinking

A new synthetic strategy for the preparation of macroscopically oriented smectic main chain liquid crystal elastomers (MC‐LCE) by the photocrosslinking of laterally functionalized polyesters is presented. X‐ray measurements proved the formation of the monodomain and allowed a quantitative determination of the order parameter. The thermo‐actuation of our material at the phase transition from the liquid‐crystalline to the isotropic phase was demonstrated by temperature dependent measurements of the sample length, showing a fully reversible shape change of ≈40%. Mechanical measurements showed that the sample can be stretched by up to 60% along the smectic layer normal without any reorientation of the structure, in contrast to earlier experiments and theoretical predictions. The results are discussed in the context of limited smectic layer correlation in different types of smectic materials.

     

Influence of strain rate and temperature on necking transition in a polydomain smectic main chain elastomer

Smectic main‐chain liquid crystalline elastomers (MCLCE) with polydomain morphology are rare examples of elastomers that can form a neck and undergo cold drawing under tension. However, not all previous studies of the mechanical behavior of smectic MCLCE reported neck formation. The mechanical response of a polydomain smectic MCLCE has therefore been characterized by elongation at varying strain rates and temperatures to identify factors favoring mechanical instability. Yielding and neck formation are increasingly favored as the strain rate increases at constant temperature, or as the temperature decreases toward T_g. As cold drawing proceeds, significant creep occurs continuously within the neck, in contrast to the behavior of certain linear polymers that exhibit a “natural” draw ratio. Thermal imaging during elongation indicates that viscous heating is not a prerequisite for neck formation. Rather, inherent softening of the material during yielding due to morphological changes leads to an enhanced rate of deformation and contraction at the neck. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Isotropic–nematic phase separation and demixing in mixtures of spherical nanoparticles with length‐polydisperse nanorods

An extension of Onsager theory is developed to simulate isotropic–nematic phase separation in a mixture of spheres with length‐polydisperse system of rods. This work is motivated by recent experimental data on nanorod liquid crystals. Prior theoretical investigations indicate that both polydispersity and the presence of spheres should increase the biphasic–nematic phase transition, that is, the nematic cloud point. Results indicate that the phase diagrams undergo drastic changes depending upon both particle geometry and rod length polydispersity. The key geometric factor is the ratio between the sphere diameter and the rod diameter. In general, length fractionation is enhanced by the addition of spheres, which may be experimentally advantageous for separating short nanorods from a polydisperse population. Simulation results also indicate that the nematic cloud and shadow curves may cross one another because of the scarcity of spheres in the shadow phase. In general, these results do indicate that the nematic cloud point increases as a function of sphere loading; however, in certain areas of phase space, this relationship is nonmonotonic such that the nematic cloud point may actually decrease with the addition of spheres. This work has application to a wide range of nanoparticle systems, including mixtures of spherical nanoparticles with nanorods or nanotubes. Additionally, a number of nonspherical particles and structures may behave as spheres, including crumpled graphene and tightly coiled polymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012