Positronium annihilation in amine-cured epoxy polymers

Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between ?78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm³. Two important transition temperatures were found: one corresponds to the glass transition temperature T_g determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below T_g. The sub-T_g transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.     

Mesomorphic and luminescent properties of side chain nematic liquid-crystalline polymers containing Ir(III)

Iridium-containing liquid-crystalline polymers were obtained by graft copolymerisation using poly(methylhydrogeno)siloxane, 1-methyl-4-(4-(4-vinylcyclohexyl)cyclohexyl)benzene (M₁) and an iridium complexes monomer (Ir-M₂). The series of polymers contained different molecular fractions of Ir-M₂ from 0% to 1.2%. All of these polymers showed mesomorphic behaviours. The introduction of small amount of iridium ions endowed liquid-crystalline polymers with luminescent properties. The chemical structures were characterised by IR and ¹H NMR. The mesomorphic properties and phase behaviour were investigated by differential scanning calorimetry, thermal gravimetric analysis, polarising optical microscopy and X-ray diffraction. With an increase of iridium complexes units in the polymers, the glass transition temperature (T_g) did not change significantly; the isotropic temperature (T_i) decreased. All polymers showed typical nematic marble textures, which was confirmed by X-ray diffraction. The temperatures at which 5% weight loss occurred (T_d) were greater than 300°C for the polymers. The introduction of iridium complexes units did not change the liquid-crystalline state of polymer systems. With Ir³⁺ ion contents ranging between 0.6 and 2.4 mol%, luminescent intensity of polymers gradually increased.     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal properties of poly(methyl methacrylate-co-butyl acrylate-co-acrylic acid) modified with divinyl benzene and vinyltrimethoxysilane

The effect of butyl acrylate (BA), divinyl benzene (DVB) and vinyltrimethoxysilane (TMVS) on the thermal properties of poly(methyl methacrylate-co-butyl acrylate-co-acrylic acid) was investigated. Glass transition temperature (T_g), melting temperature (T_m) and specific heat capacity of the copolymers were investigated using Differential Scanning Calorimetry. Thermal stability of the copolymers which is associated with the degradation temperature (T_d) was studied by Thermogravimetric Analysis. Polyacrylates with T_g ranges between -19°Cand 19°C were obtained. With the incorporation of >7 wt% of DVB, the T_g of the copolymer increases from about ?17°C to ?10°C even though they have not undergone UV irradiation. Gel content results prove that crosslinking has occurred in the copolymers. With increasing amount of TMVS from 0 wt% to 7 wt%, the T_m of the copolymers prepared at acidic pH is about 40-60°C higher than that at the alkaline pH. However, the addition of TMVS gives no significant effect to the T_g and T_d of the copolymer films. The thermal stability of the copolymer has improved with increasing amount of BA and DVB, with DVB being more effective. The highest T_d of 425°C with 8% of DVB has been obtained. Consequently, a polyacrylate copolymer with a T_g of about ?13°C, a T_m of 170 °C and a T_d of about 424°C has been successfully synthesized. Hence, the soft polyacrylate with its relatively high T_m and T_d could serve as a superb material especially to be applied in the areas that require high melting temperature and good thermal stability.     

The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study

Positron lifetime measurements, performed in the temperature range 80–300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ₃ and τ₄, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm³ (PE) and between 0.152 and 0.372 nm³ (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of T_g = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at T_g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 10²⁷ holes/m³ is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513–1528, 1998     

Fully aromatic thermotropic liquid crystalline polyesters of 3-phenyl-4,4′-biphenol with 4,4′-benzophenone dicarboxylic acid

A series of fully aromatic, thermotropic polyesters, derived from 3-phenyl-4,4′-biphenol (MPBP), nonlinear 4,4′-benzophenone dicarboxylic acid (4,4′-BDA), and various other comonomers was prepared by the melt polycondensation method and characterized for their thermotropic liquid crystalline behavior by a variety of experimental techniques. The homopolymer of MPBP with 4,4′-BDA had a fusion temperature (T_f) at 240°C, exhibited a nematic liquid crystalline phase, and had a narrow liquid crystalline range of 60°C. All of the copolyesters of MPBP with 4,4′-BDA and either 30 mol % 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA) or 50 mol % terephthalic acid (TA), 2,6-naphthale-nedicarboxylic acid (2,6-NDA) and low T_f values in the range of 210–230°C, exhibited a nematic phase, and had accessible isotropization transitions (T_i) in the range of 320–420°C, respectively. As expected, each of them had a broader range of liquid crystalline phase than the homopolymer. They had a “frozen” nematic, glassy order as determined with the wide-angle X-ray diffraction (WAXD) studies. The morphology of each of the “as-made” polyesters had a fibrous structure as determined with the scanning electron microscopy (SEM), which arises because of the liquid crystalline domains. Moreover, they had higher glass transition temperatures (T_g) in the range of 167–190°C than those of other liquid crystalline polyesters, and excellent thermal stabilities (T_d) in the range of 500–533°C, respectively. © 1995 John Wiley & Sons, Inc.     

The use of photoinitiated free-radical cyclopolymerization in the preparation of novel side-chain liquid-crystalline polydienes

The technique of photoinitiated free-radical cyclopolymerization has been used to produce a variety of novel side-chain liquid-crystalline (SCLC) polymers in which an alicyclic ring structure is incorporated into a polymer backbone. Most of the SCLC polymers gave glass transition temperatures (T_g) around room temperature and clearing points around 60–70°C. However, the poly(1,5-hexadiene) was the exception to this, exhibiting a T_g value of 14.5°C and a clearing point of 132.5°C. Confirmation of the structure of the alicyclic ring along the polymer backbone is extremely difficult to obtain, but evidence is given in the paper to support the ring structure.     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.     

Synthesis of polystyrene-polysiloxane side-chain liquid crystalline block copolymers

The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a T_g well below 25°C and is designed to exhibit a chiral smectic C^* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high T_g glassy block and a low T_g liquid crystalline block.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

Abstract

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T _m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C[dbnd]C, OH, CO₂C₂H₅ and OCH₃ groups).     

Signatures of glass transition in partially ordered phases

Features of glass transition in liquid crystalline nematic and smectic E, B and I_A phases of selected materials observed by means of the polarising microscopy and Fourier transform infrared spectroscopy (FTIR) are reported. Evolution of cracking in the glass and a coincidence of its disappearance temperature with the glass transition temperature (T_g) on heating is shown and discussed in the context of processes that occur in the glass. The shape of temperature dependencies of absorption bands is shown as the signature of the glass transition.     

Binary Mixtures of Liquid Crystalline Ester Bismaleimides

Abstract

A binary system consisting of a chlorohydroquinone-based ester bismaleimide (3-Cl), T _m = 238°C, and a methylhydroquinone-based ester bismaleimide (3-Me), T _m = 251°C, was investigated for the purpose of improving processability by widening the nematic phase range before polymerization. Calculations based on the Schroeder-van Laar equation predicted a system eutectic composition of 41% 3-Me monomer and a eutectic temperature of 202°C. Experiments found the eutectic composition at 35% 3-Me and the eutectic temperature at 218.5°C. Discrepancies between experimental results and theoretical predictions are likely due to error in measured heats of fusion either due to impurities in the samples or due to the reactive nature of the components being considered. Thermal cycling was also found to have a significant melting point depression effect. While significant depression of the system melting point was achieved, polymerization still occurred immediately after melting in all systems evaluated. All mixtures could be polymerized from the nematic phase to yield a solid which retained the nematic orientation of the starting polymer melt.     

Synthesis and thermal transitions of a soluble,main chain,nematic liquid crystalline polymer exhibiting a kinetically trapped,disordered structure

An aromatic copolyester composed of 25 mol % phenyl hydroquinone, 10 mol % isophthalic acid, 40 mol % chloroterephthalic acid, and 25 mol % t-butyl hydroquinone (PICT) has been synthesized. This amorphous, glassy polymer is soluble in common organic solvents such as methylene chloride. Thin, solution-cast films may be prepared which are in a metastable, vitrified, optically isotropic state. On first heating of an isotropic film at 20°C/min in a calorimeter, one glass transition is observed at low temperature (approximately 49°C) and is ascribed to the glass/rubber transition of the metastable, isotropic polymer. This thermal event is followed by a small exotherm due to the development of order during the scan, which results in a second T_g at approximately 125°C. This T_g is associated with the glass/rubber transition of the ordered polymer. Nematic order can be developed by thermal annealing. The lower T_g increases toward the upper T_g as annealing time is increased. For an initially isotropic film annealed at 90°C, the increase of the lower T_g with annealing time and the increase in birefringence observed by optical microscopy are governed by similar kinetics. Isotropization occurs in the temperature range of 250–300°C. The nematic polymer is slightly more dense than its isotropic analog. No detectable differences between isotropic and nematic samples were observed in rotating frame proton spin lattice relaxation times. © 1996 John Wiley & Sons, Inc.     

Study of phase transitions in 4-n-alkoxybenzilidene-4′-n-alkylanilines by one and two dimensional 13C NMR

Abstract

The phase transition of a series of homologous liquid-crystalline compounds, nO.m (4-n-alkoxybenzilidene-4′-n-alkylanilines), from the nematic phase to the smectic A phase has been studied by ¹³C NMR. The order parameters, determined by a two dimensional technique called separated local field spectroscopy combined with off-magic angle spinning, of different molecular segments of these compounds are related linearly to the ¹³C chemical shifts. Changes in the order parameters of the phenyl rings as well as those of the chains during the S_A–N transition depend on the nature of the phase transition. These changes are quantitatively related to the McMillan ratio, which is defined as the ratio between the S_A–N transition temperature (T _SAN) and the nematic to isotropic transition temperature (T _NI), i.e. M = T _SAN/T _NI. The S_A–N transition is first order for M > M _TCP, and second order for M < M _TCP, where TCP is the tricritical point. The value of M _TCP was found to be 0·958 ± 0·004, in excellent agreement with that obtained from spin probe studies (0·959 ± 0·005) reported by Freed and co-workers [1].     

Supramolecular self-assembly of a novel hydrogen-bonded cholesteric liquid crystal exhibiting macromolecular behaviour

The mesomorphic, thermoptic and glass-forming properties of 4-[6-((cholesteryloxy) carbonyl)oxy hexyloxy] benzoic acid (Ch-BA) have been investigated as a novel supramolecular hydrogen-bonded cholesteric mesogen. Fourier transform infrared and ¹H nuclear magnetic resonance studies have confirmed the chemical structure and the hydrogen-bond formation between the mesogens. According to polarising optical microscope observations, the compound exhibited smectic and chiral nematic phases. Differential scanning calorimetry indicated an unexpected glass transition (T _g) around 32°C and a liquid crystalline region between 32 and 122°C, in which the cholesteric phase appeared at 80°C. As a result of the glass formation, samples of Ch-BA which were rapidly cooled below the T _g were found to preserve the long-range ordering of the liquid crystalline state and retained the iridescent colours of the cholesteric phase. These results led to the conclusion that the formation of identical dimers by intermolecular hydrogen-bonding of the terminal carboxylic acids accompanying the lateral packing of the rigid cores, built a trimeric arrangement and this was responsible for the macromolecular behaviour of Ch-BA, despite its relatively simple structure and low molecular weight.     

Acceleration of the cationic polymerization of an epoxy with hexanediol

Thin films of 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexane carboxylate were UV irradiated (1.1 J cm^-2) under isothermal conditions ranging from 0 to 50°C. Under these conditions the polymerization advanced quickly but only to a conversion level of less than 10% before the reaction rate slowed by more than an order of magnitude. This drop off in rate was not caused by the glass transition temperature, T _g, reaching or exceeding the reaction temperature, T _rxn, since the epoxide's T _g remained at least 40°C below T _rxn. Raising the sample temperature above 60°C caused a sharp increase in the conversion level. At 100°C conversion exceeds 80% and the ultimate T _g approaches 190°C. The addition of 10 mass% 1,6-hexanediol, HD, to the epoxy caused the conversion at room temperature to quintuple over the level obtained without the alcohol present. The heat liberated from this alcohol epoxy blend during cure on a UV conveyor belt system caused the sample's temperature to increase by about 100°C above ambient whereas the epoxy alone under these conditions only experienced a modest temperature rise of about 26°C. If the amount of HD in the blend is increased above 10% the heat of reaction at 23°C decreases due to HD being trapped in a nonreactive crystalline phase. Boosting reaction temperatures above 50°C melts the HD crystals and yields significantly improved conversion ratios. As the level of alcohol blended with the epoxy is raised its ultimate T _g is lowered and when the concentration of alcohol in the blend nears 30 mass%T _g drops below room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wide-line <Superscript>1</Superscript>H NMR study of mesomorphic states of 4′-(octyloxy)-4-biphenylcarbonitrile

A sample of 4′-(octyloxy)-4-cyanobiphenyl (8OCB) was studied in the temperature range −60–80°C by wide-line ¹H NMR. The line shapes, half-widths, and second moments were determined. For the smectic phase of 8OCB, the relaxation times T ₁ and T ₂, the correlation time τ_c, and the degree of order were estimated. The ¹H NMR spectral patterns and characteristics were found to be quite different for the crystalline, smectic, and nematic phases of 8OCB, which makes it possible to reliably identify the corresponding transitions and assess the molecular dynamics and molecular order of a structure. The temperature ranges, stability conditions, and other characteristics of the liquid crystalline phases that form on heating 8OCB were determined.     

Synthesis and characterization of wholly aromatic polyesters derived from 1-phenyl-2,6-naphthalene-dicarboxylic acid and aromatic diols

A series of new wholly aromatic polyesters was synthesized by melt polycondensation of 1-phenyl-2,6-naphthalenedicarboxylic acid (PNDA) and diacetates of various aromatic diols. The aromatic diols studied are hydroquinone (HQ), methylhydroquinone (MHQ), phenylhydroquinone (PHQ), (α-phenylisopropyl)hydroquinone (PIHQ), 2,6-naphthalenediol (2,6-ND), 1,4-naphthalenediol (1,4-ND), and 4,4′-biphenol (BP). These polyesters were characterized for their crystallinity, glass transition temperature (T_g), melting temperature (T_m), liquid crystallinity, and thermal stability. In general, crystallinity of the polyesters are very low and the T_g values of the polyesters range from 150 to 172°C depending on the structure of aromatic diols. All of the polymers formed nematic phases above their T_m or T_g. The polyesters derived from PHQ and PIHQ are soluble in chlorinated hydrocarbon solvents. The initial decomposition temperatures of the polyesters are above 400°C under N₂ atmosphere. © 1996 John Wiley & Sons, Inc.     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4′-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4′-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0–23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.