Low Molecular Weight Liquid Research Articles

Toughening of glassy polymers by prepackaged deformation-activated diluents

The use of low molecular weight liquid rubber diluents in concentrations of ∼ 1%, dispersed finely in the form of submicron-sized pools, had been demonstrated in the past by Gebizlioglu and colleagues (Macromolecules 1990, 23, 3968) to be an effective source of toughening of brittle glassy polystyrene through deformation-accentuated plasticization effects of craze matter during the development of craze plasticity. Herein, we present results of a study extending the range of this plasticization effect through the development of a new “precipitation-molding” process that can routinely incorporate diluent pools of up to 4–5% by volume and can increase the tensile toughness by about a factor of 3 over what was achieved before. The experiments demonstrate that the toughening mechanism operates primarily through a substantial reduction of the craze flow stress that promotes an increase in craze plasticity that renders inactive adventitious inclusions that normally initiate fracture. Experiments with all blends at different temperatures and strain rates indicate that the fracture response can be represented by a universal behavior pattern of a single thermally assisted process of fracture of craze matter where the diluent merely modulates the plastic resistance. However, the experimental results show that the sorption of the diluent that controls the plasticization effect during straining competes against the time-dependent fracture process and that the toughening effect decreases monotonically with increasing strain rate and decreasing temperature that limits the effectiveness of the process to a range considerably below what is required for impact response. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1469–1490, 1999

Light Transmission-Light Scattering Reverse Mode Switching of (Liquid Crystalline Polymer/Liquid Crystal)Composite System

ABSTRACTConventional (polymer/liquid crystal) composite films exhibit the normal mode electro-optical switching, that is, the light scattering-light transmission switching upon electric field OFF-ON states, respectively. The “reverse mode” electro-optical switching, that is, the light transmission-light scattering switching upon electric field OFF-ON states, respectively, has been investigated for a novel type of composite system being composed of a side chain type liquid crystalline polymer(LCP) and low molecular weight liquid crystals(LCs). The polarizing optical microscopic observation and X-ray diffraction study revealed that the two types of smectic-like short range ordering were present in the (LCP/LCs) composite system in a nematic state. It was confirmed from electric capacitance measurements of the homogeneous alignment cell that these phases with different smectic-like short range orderings each exhibited a different value of threshold voltage. Although the homogeneous alignment cell of the (LCP/LCs) composite system was in a light transmitting state in the absence of an electric voltage, the cell turned into a light scattering state upon the application of an electric voltage of 5-15 V for the cell of 14 µm thick (0.4−1.1 MV" m−1). The reversible change from the light scattering state to the transmission state was observed after removing the electric voltage. The light scattering state of the (LCP/LCs) composite system upon the application of electric voltage might be due to the appearance of optically heterogeneous structure induced by the different values of the threshold voltage between the two types of smectic-like phases with different short range orderings. These phases might be in a phase-separated state with an optical dimension (several hundred nm). The reversible “reverse mode” electro-optical switching was realized for the (LCP/LCs) composite system in a nematic state in which the two types of smectic-like short range orderings were separately formed in an optical size level.

Synthesis of liquid crystalline polyesterimides by melt polymerization and its application to polymer‐dispersed liquid crystal

N-(4-Carboxyphenyl)trimellitimide (DAI) was synthesized from trimellitic anhydride and p-aminobenzoic acid by a one-step reaction utilizing m-cresol as a solvent. DAI was reacted with hexanediol and nonanediol to give bishydroxyhexyltrimellitimide (BHHI) and bishydroxynonyltrimellitimide (BHNI), respectively. Thermotropic liquid crystalline polyesterimides with BHHI and BHNI as a mesogenic group and 6 and 9 methylene units as a spacer were synthesized by a direct melt-polymerization method similar to the one used in the preparation of commercial polyester. Polyesterimides were characterized by IR, 1H-NMR, DSC, and a polarized optical microscope. Their application as a matrix (binder) of polymer-dispersed liquid crystal (PDLC) was investigated with 4-heptyloxybenzoic acid as a low molecular weight liquid crystal (LMWLC). From the PDLC experiment, polyesterimides were found to be useful as a matrix for the high-temperature PDLC above 100°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1517–1522, 1998

Synthesis of liquid crystalline polyesterimides by melt polymerization and its application to polymer-dispersed liquid crystal

N-(4-Carboxyphenyl)trimellitimide (DAI) was synthesized from trimellitic anhydride and p-aminobenzoic acid by a one-step reaction utilizing m-cresol as a solvent. DAI was reacted with hexanediol and nonanediol to give bishydroxyhexyltrimellitimide (BHHI) and bishydroxynonyltrimellitimide (BHNI), respectively. Thermotropic liquid crystalline polyesterimides with BHHI and BHNI as a mesogenic group and 6 and 9 methylene units as a spacer were synthesized by a direct melt-polymerization method similar to the one used in the preparation of commercial polyester. Polyesterimides were characterized by IR, 1H-NMR, DSC, and a polarized optical microscope. Their application as a matrix (binder) of polymer-dispersed liquid crystal (PDLC) was investigated with 4-heptyloxybenzoic acid as a low molecular weight liquid crystal (LMWLC). From the PDLC experiment, polyesterimides were found to be useful as a matrix for the high-temperature PDLC above 100°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1517–1522, 1998

Relaxational and vibrational dynamics of poly(propylene glycol)

Photon correlation spectroscopy has been used to study structural relaxation dynamics near the glass transition temperature for glass-forming linear chain molecules differing in the number of repeat units. Complementary investigations of the low-frequency vibrational dynamics were performed by means of Raman spectroscopy. The systems studied range from the low molecular weight liquid propylene glycol (PG, molecular weight (MW)=76) to the polymer, poly(propyleneglycol) (PPG, M W=4000). Differences are observed in terms of the fragility of the systems. The findings are attributed to bonding between the hydroxy endgroups which decrease in density as the MW increases.

Electro-Optical Effect Without Hysteresis for (Polymer/Liquid Crystal) Composite Films

Phase-separation behavior of the composite films composed of polymer of a three dimensional network and low molecular weight liquid crystals (LCs) embedded in the network was studied on the basis of the evolution of optical heterogeneity in a casting solution during a film formation process and the morphology of the polymer networks. The electro-optical properties of the composite films based on electric field controlled light scattering were strongly dependent on the size of LC domains which was controlled by the solvent evaporation rate. When poly(diisopropyl fumarate), which is incompatible with the LC, was used as the matrix polymer, no electro-optical hysteresis was detected, while considerable electro-optical hysteresis was done for the composite film consisting of poly(methyl methacrylate), which is fairly compatible with the LC. However, when poly(methyl methacrylate) with high molecular weight (996k) was used as the matrix polymer, hysteresis decreased due to a decrease of compatibility.

Behaviour of the miscibility of poly(ethylene oxide)/liquid crystal blends

The microscopic behaviour of blends of poly(ethylene oxide) with two different low molecular weight liquid crystals (LC) was studied in order to evaluate miscibility. One of the liquid crystal components had a phase transition temperature lower than the melting temperature of poly(ethylene oxide) (PEO), and the other a higher value. The low molecular weight liquid crystal components were 4-cyano-4′-n-heptylbiphenyl (7CB) and p-cyanophenyl p-pentyloxybenzoate (pCP). Thermal analysis and polarized optical and scanning electron microscopy were employed. The melting temperature (Tm) depression of PEO increased with LC content in the blend, suggesting that the PEO was miscible with both liquid crystals in the isotropic phase. The spherulitic structural morphology of the semicrystalline components is affected by the presence of liquid crystals. © 1998 SCI.

Behaviour of the miscibility of poly(ethylene oxide)/liquid crystal blends

The microscopic behaviour of blends of poly(ethylene oxide) with two different low molecular weight liquid crystals (LC) was studied in order to evaluate miscibility. One of the liquid crystal components had a phase transition temperature lower than the melting temperature of poly(ethylene oxide) (PEO), and the other a higher value. The low molecular weight liquid crystal components were 4-cyano-4′-n-heptylbiphenyl (7CB) and p-cyanophenyl p-pentyloxybenzoate (pCP). Thermal analysis and polarized optical and scanning electron microscopy were employed. The melting temperature (Tm) depression of PEO increased with LC content in the blend, suggesting that the PEO was miscible with both liquid crystals in the isotropic phase. The spherulitic structural morphology of the semicrystalline components is affected by the presence of liquid crystals. © 1998 SCI.

Liquid crystalline epoxy resins in polymer dispersed liquid crystal composites

Liquid crystalline epoxy resins can be used as a matrix to encapsulate low molecular weight liquid crystalline droplets to produce PDLC devices. The presence of the low mass liquid crystal decreases the rate of reaction of the resin, that also hardens in a mesomorphic structure for high concentrations of liquid crystal. The optical characterization of the PDLC indicates an improvement of the angular transmission of visible light. ©1997 SCI

Alkaline cupric oxide and nitrobenzene oxidation of alcell® lignin

AbstractA solvolysis lignin has been oxidatively degraded using nitrobenzene and cupric oxide (CuO) as catalysts. The effects of reaction conditions on lignin conversion to identifiable low molecular weight liquid products were quantified. Optimal yields of vanillin and syringaldehyde of 2.2 wt % and 5.9 wt % respectively were obtained using nitrobenzene at 180°C and 2h, whereas both were less than 1 wt % when CuO was used at 225°C and 1h, indicative of the milder oxidative power of CuO for this lignin. The higher total aldehyde yield from solvolysis lignin (8.2 wt %) than kraft lignin (5.1 wt %) when using nitrobenzene indicates it might be a better candidate for producing chemicals via oxidation.

Kinetics of spinodal decomposition in mixtures of low molecular weight liquid crystals and flexible polymers

AbstractIn this paper, the time‐dependent Ginzburg‐Landau model for mixtures containing nematogens has been applied to mixtures of low molecular weight liquid crystals and flexible polymers. Dynamic equations for the time evolution of concentration and orientation fluctuations and the structure factors for these fluctuations are given. It is shown that the coupling between concentration and orientation fluctuations is absent in the isotropic spinodal region, thus the evolution of the structure factors for the concentration fluctuations falls into the Cahn‐Hilliard classic category and it exhibits no maximum in the structure factors of orientation fluctuations. We should emphasize that, in the anisotropic spinodal region, both concentration and orientation structure factors possess a maximum but not coincide with each other and both are shifting to smaller wave numbers according to the scaling relation, qmax ˜ τ−α, as time increases. The value of a closely correlates to the interfacial free‐energy parameters.

Defects and textures in nematic MCLCPs

The primary aim of this article is to present defects and textures in nematic main chain liquid crystal polymers (MCLCPs). Some of the consequences of constructing a nematic phase from long polymeric chains are presented. We concentrate on understanding the role of properties, such as scarcity of chain ends, chain rigidity, high degree of anisotropy in the elasticity, and viscosity, that clearly distinguish MCLCPs from low molecular weight liquid crystals

The Effect of Dissolved Side-Group Polymers on Pattern Dynamics in Nematic Liquid Crystals in a Rotating Magnetic Field

Patterns formed by inversion walls in nematic layers exposed to a rotating magnetic field were studied. Dilute solutions of a mesogenic side group polymethacrylate in a low molecular weight liquid crystal (5CB) were used in comparison with the pure solvent. As found in a previous work, in this system the intensity of backflow (fluid flow induced by director rotation) can be controlled by the polymer concentration due to a specific increase of shear viscosity coefficients. In the synchronous regime of director rotation no significant effects of backflow on the dynamics of the walls are observed. Dynamic solitons known from the synchronous regime were also found at asynchronous rotation, when soliton lattices are formed by continuous nucleation. Here comparison with theory for given values of the lattice period shows soliton currents significantly reduced by backflow. Two of the three additional pattern forming states exclusively found at asynchronous rotation were completely suppressed in solutions with sufficient polymer concentration. The third of these states is affected by backflow in the growth rate of its patterns. Numerical calculations were performed to explain behavior of the patterns in the asynchronous state. For pure 5CB a quantitative comparison with the experiment was possible.

Electrical Deformation of Thermotropic Liquid-Crystalline Polymer Gels

Liquid-crystalline elastomer carrying cyanobiphenyl side groups was synthesized by radical copolymerization of 6-(4′-cyanobiphenyl-4-yloxy)hexyl acrylate and hexamethylene diacrylate. Liquid-crystalline elastomer was swollen in a low molecular weight liquid crystal. Swollen elastomer in the nematic phase quickly deformed by application of DC voltage. The response time of electrical deformation was determined by image analysis.

On the van Krevelen/Hoftyzer relationship for the high-temperature limiting viscosities of polymer melts

The van Krevelen/Hoftyzer limiting viscosities ( T → ∞) of polymer melts are often lower than the high-temperature viscosity of low-molecular weight liquids or the viscosity of air at 20°C and 1 bar. The origin of this surprising result is discussed.

Statistics of wormlike chains. II. Phase transition of polymer liquid crystals and its mixture with low molecular weight liquid crystals

A general self-consistent field (SCF) for the mixture of polymer and low molecular weight (LMW) molecules has been derived by variation principle. Considering a Maier–Saupe type of interaction, the analytical expressions of the SCF for polymer liquid crystals (PLCs) and the mixture of PLCs and LMW liquid crystals are obtained, from which the phase behaviors of PLCs as well as the mixture are studied. The theoretical results are in agreement with experimental results by adjusting a parameter.

Effect of Spacer Length on the Phase Transition of Polymer/LMWLC Composites

Low molecular weight liquid crystals (LMWLCs), 4-[(4-bromoalkyloxy)-phenylazo]benzonitrile (AZO(n)) series were synthesized. Mesogenic monomers, 4-(4-propenoylalkyloxy)-4′-cyanobiphenyl (MCA) were synthesized and both homopolymer and copolymer with a methyl methacrylate (MMA) unit were prepared by free radical polymerization. Polymer/LMWLC composites (P/LCs) were prepared with LMWLC AZO(6) and AZO(8) and non-mesomorphic SCLCPs, PMCA(6)-3 and PMCA(8)-3, where (6) and (8) represent the number of methylene unit in the spacers of both LMWLC and SCLCP. It was observed that ΔTg (defined as Tg of polymer-Tg of P/LC) was smaller and ΔT NI (defined as T NI of LMWLC-T NI of P/LC) was larger, respectively in the case of P/LC samples with the same (6/6 and 8/8) spacer length.

Steep electro-optical response of nematic liquid crystalline molecules induced by addition of polymeric chains

Abstract The steepness increases with a decrease in the ratio of elastic constants of the bend to the splay mode, K 3/K 1. The elastic constants of nematic directors are affected by the geometrical shape of a liquid crystalline molecule and a short range ordering for the alignment of liquid crystalline molecules. The magnitude of K 3/K 1 and the reorientational steepness of the nematic director were evaluated by an electric capacitance measurement of the (side chain-type liquid crystalline polymer (PS6EC)/low molecular weight liquid crystal (E7)) composite system cell. It became apparent from X-ray diffraction studies that the smectic-like short range ordering increases with increasing the fraction of PS6EC even in the nematic state of the composite system. Furthermore, a remarkably steep electro-optical response was successfully achieved.

Some Peculiarities of the Radical Polymerization of Methyl Methacrylate in the Presence of Low Molecular Weight Liquid Crystals

Abstract Radical polymerization of methyl methacrylate (MMA) in the presence of propylbenzene, benzonitrile, their equimolar mixture, and a mixture of nematic liquid crystals (LC) was studied. Chain transfer reaction and unexpected dependence of the initial polymerization rate on LC concentration were revealed by means of UV spectrophotometry, viscometry, and dilatometry. The chain transfer reaction which occurred at the boundary of LC domains in the polymer matrix caused binding of some of the LC molecules to the macromolecules of PMMA. An increase in the initial rate of polymerization and a decrease of polydispersity were explained with formation of the LC phase because growth of the macroradicals continued not only in the homogeneous polymer phase but simultaneously also at the interfacial boundary between the polymer matrix and the LC phase. It is assumed that the LC molecules anchored to the PMMA matrix can affect the mobility of free LC molecules in the LC domains when an external electric field is applied to LC/polymer composite films.

Chevron patterns in liquid crystal flows

We study plane shear and Poiseuille flows of uniaxially nematic liquid crystals for large values of the Ericksen number, ε. This is a non-dimensional quantity that renders rigorous notions of fast flow regimes. (The former corresponds to large values of ε.) The model employed is that due to Ericksen for liquid crystals with variable degree of orientation (Ericksen, 1991). The constitutive equations derived by Kuzuu and Doi (1984) and Marrucci (1981), play an essential role in the analysis. Variables of the problem include the velocity field, v, the pressure, p, the scalar order parameter, s, and the director, n. Recent experimental work involving, both, low molecular weight liquid crystals as well as polymers, has pointed out that the positivity of the ratio α 2 α 3 of the Leslie coefficients does not necessarily guarantee uniform molecular alignment in shear flow regimes, as previously believed. In particular, for fast regimes, the flow breaks up into stripped configurations, chevrons parallel to the velocity field. Taking such observations into account, here, we consider regimes that fulfill the alignment condition, and show that for large values of ε, solutions exhibiting disclinations in the flow region can be found. Moreover we point out that across singular lines, the director experiences jump discontinuities of approximately ±45° to the direction of flow. We construct solutions that present domain structures in the form of stripes parallel to the flow direction, on the plane of shear. The interface between two such stripes corresponds to a nearly isotropic layer.