Study of the molecular conformation of an aromatic polyamide–hydrazide by intrinsic viscosity

The persistence length of poly(terephthaloyl p-amino benzhydrazide) (X-500) in dimethyl sulfoxide (DMSO) has been determined from intrinsic viscosities measured for several whole-polymer samples. An approximate correction is applied for excluded volume effects, and these corrected values are fitted using the viscosity treatments of Yamakawa and Fujii and of Eizner and Ptitsyn. Both give for the persistence length of X-500 values in the range 30–35 Å. Revised treatment of our previous light scattering data for the same polymer yields 49 Å for the persistence length. Thus the chain extension of X-500 falls between those of para-linked polyamides and those of typical freely coiling macromolecules. Phase diagram studies indicate the solubility of X-500 in DMSO decreases with rising temperature, and can be increased by the addition of LiCl. No anisotropic phase is found in DMSO at the highest volume fraction of polymer, v₂ = 0.19, which can be obtained with 4 g/dl LiCl. The critical concentration of X-500 needed for the appearance of the anisotropic phase depends upon solvent since Morgan reported an anisotropic phase for this polymer in 100% H₂SO₄ when v₂ ≈ 0.13.     

Phase Behavior of Microemulsions Formulated with Sodium Alkyl Polypropylene Oxide Sulfate and a Cationic Hydrotrope

The partial ternary phase diagram of anionic extended surfactant of alkyl polypropylene oxide sulfate C₁₂(PO)₄SO₄ alone and combined with the cationic hydrotrope, tetrabutyl ammonium bromide with water and decane were determined under ambient conditions. Middle phase microemulsion was formulated using salinity scans in the dilute region of surfactant/brine/decane. Visual inspection as well as cross polarizer and optical microscopy were used to detect anisotropy. Spinning drop tensiometer was used to measure interfacial tension (IFT). The first ternary phase diagram using the extended surfactant alone showed three one phase regions, the anisotropic lamellar liquid crystalline phase, L _α and the isotropic L₁ micellar liquid and L₃ sponge phase. In the second ternary phase diagram using the extended surfactant combined with tetra butyl ammonium bromide, an isotropic micellar region, L ₁, appeared in the diluted area of the phase diagram. Meanwhile the L _α phase disappeared completely and the three phase region has a bluish transparent middle phase. Interfacial tension measurements between middle phase and brine, and between decane and brine yielded ultra low values. Calculated IFT values using the characteristic length obtained using De Gennes approximation gave almost half the measured values. The interfacial rigidity was also calculated and compared to values obtained from the literature.     

Concentrated solutions of polydisperse rodlike polymers in the isotropic and lyotropic liquid-crystalline phases. I. The effects of molecular length distribution on phase behavior

The earlier calculations of Moscicki and Williams of the phase behavior of rodlike macromolecules in solution incorporating a Gaussian distribution of rod lengths have been extended to include, in addition to a “basic” Gaussian distribution, a small amount of high-molecular-weight species which are characterized by a δ-function or “box-function” distribution. It is shown that C^*_p, the concentration of polymer at which the biphasic material (isotropic plus anisotropic phase) first appears may be substantially lowered in comparison with that for the system comprising only the basic distribution. The volume fraction Φ_A of anisotropic phase, the compositions of the two phases, and the distribution of species between the phases are calculated for a range of polymer concentration which spans the isotropic, biphasic, and anisotropic phase and, among several features obtained, it is shown that the high-molecular-weight species are essentially responsible for the initial formation of anisotropic phase in the biphasic system. The results of the calculations have important implications for the interpretation of published dielectric relaxation data for polyalkylisocyanates in solution at high concentration and for published shear-flow viscosity data for polyalkylisocyanates and poly(p-benzamide) in solution at high concentration and these are discussed in some detail.     

Phase behaviour and physicochemical properties of microemulsions with a non-ionic surfactant (IGEPAL)

The non-ionic surfactant pentaethylenglycol-4-octylphenyl ether (igepal CA-520) represents a good industrial alternative to the long-tail members of the C_iE_j family. In this paper, the phase behaviour of the microemulsion system igepal CA-520/n-decane/brine is studied in detail. An isotropic phase was found, as well as liquid crystalline and cream-like structures, depending on composition and temperature. Such structures can either form single-phase homogeneous mixtures, or coexist with other structures when phase separation takes place. Below surfactant concentration of about 20%, more complicated phase equilibria develop as temperature changes. The presence of different additives shifts the temperature ranges where the different phases exist, while keeping the general shape of the phase diagram, which agrees with the general rules for non-ionic surfactants. Complementary rheology experiments reveal a change from non-Newtonian to Newtonian behaviour during the phase transition from a lamellar phase to the isotropic microemulsion. A structure of water droplets associated in clusters can be proposed from SANS and electrical conductivity.     

Rheological properties of anisotropic poly(para-benzamide) solutions

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C^*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C^*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C^* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM?)^* ≈ 1.3 × 10⁵ at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C^* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.     

Nematogenic block copolymers via the yamazaki reaction

A novel extension of the Yamazaki reaction is used to prepare block copolymers having rigid blocks of poly-(p-benzamide) (PBA) and semiflexible blocks of polyamide-hydrazide. A PBA prepolymer having M ? 10,000 was synthesized by the usual Yamazaki reaction using triphenylphosphite. As previously reported, higher-molecular-weight PBA could be obtained using 4-N-(4′-aminobenzamido)benzoic acid containing a preformed amide linkage. Addition of p-aminobenzhydrazide and terephthalic acid then led to formation of the polyamide-hydrazide blocks using as the active reactant the diphenylphosphite formed as a by-product in the first polymerization. Evidence that a block copolymer is produced includes an increase in inherent viscosity during the second step, differences in the solubility of the copolymer compared to the homopolymers, and comparison of the phase diagram of the block copolymer in N-methylpyrrolidone having 4% added LiCl with those of a random copolymer, and of mixtures of the two homopolymers. The critical concentration required to form a nematic phase in solutions of the block copolymers is correlated with the length (or axial ratio) of the rigid block, and with its proportion in the copolymer.     

Preparation of polyamides via the phosphorylation reaction. X. A study of higashi reaction conditions

We report a study of the conditions of the phosphorylation reaction for the preparation of aromatic polyamides using the Higashi reaction medium. For poly(p-phenylene terephthalamide) (PPD-T), the optimum conditions are: reaction temperature, 115°C; monomer concentration, C = 0.083 mol/L; and ratio of triphenyl phosphite (TPP) to monomer, 2.0. These optimum conditions produce PPD-T having η_inh = 6.2 dL/g. At temperatures of 120°C and above PPD-T precipitates from the reaction mixture, leading to lower molecular weights. At lower temperatures the reaction mixture gels, and the gel time decreases with increasing reaction temperature. However, polycondensation continues in the gel state. Monomer concentrations C = 0.10 mol/L and above produce precipitation and yield polyamides of lower molecular weight. For the preparation of poly(p-benzamide) (PBA), the optimum ratio of TPP to monomer is 0.6 for either p- aminobenzoic acid or N-4-(4′-aminobenzamido)benzoic acid. In the former case the inherent viscosity of polymer prepared at 115°C showed little dependence upon the concentration of the monomer. The highest value, η_inh = 1.8 dL/g, was obtained with C = 0.40 mol/L and a TPP/monomer ratio of 0.6. However, for the same TPP/monomer ratio, the monomer containing a preformed amide linkage, N-4-(4′-aminobenzamido)benzoic acid, gave PBA with η_inh = 4.6 dL/g when the monomer concentration is 0.33 mol/L. This is the highest value reported for PBA using the phosphorylation reaction. In A?A + B?B polycondensation, examples in which one of the monomers contained one or two preformed amide linkages produced polyamides having η_inh = 7.8 and 8.9 dL/g, respectively.     

X-ray investigations on some liquid crystalline methacrylic polymers with ω-hexyloxysalicylaldimine side groups carrying p -alkyl or p -alkyloxy terminal substituents

X-ray diffraction data for four different liquid crystalline side group methacrylic polymers based on the ω-hexyloxysalicylaldimine moiety with p-alkyl or p-alkyloxy terminal substituents are presented. For the decyloxy, dodecyloxy and octyl derivatives, a smectic C₂ phase occurs over a broad temperature range, while for the tetradecyl derivative a complex behaviour was observed on cooling from the isotropic state. In this last case a smectic A_d phase arises, changing to a smectic C₂ phase and a further smectic C_d phase. A possible explanation for the observed phenomena is presented.     

PHASE EQUILIBRIA IN AQUEOUS LOW MOLECULAR CATANIONIC SURFACTANT SYSTEMS

Phase equilibria in systems alkanoic acid-alkylamine-water have been determined at 298.2 K. In the system hexanoic acid (A) - hexylamine (B) only one isotropic solution phase is formed. This solution phase is in eqilibrium with almost pure water. The strong A₁B₁ complex shows catanionic surfactant properties as concluded from phase equilibria studies in the system A₁B₁-heptane-water. This system behaves like ordinary microemulsions. The dominant feature of the phase diagram with ethanoic acid -dodecylamine is the large D phase. In addition there exist a large solution phase, a hexagonal phase and a small cubic I phase.     

Compatibility and mesophase separation in blends of α-helical polyglutamates

Blends composed of the α-helical polymers, poly-L-glutamates [(? NHC^αHRC′O? )_n, R = ? CH₂CH₂COO? (CH₂)_m(C₆H₅] (Lm) and the corresponding D enantiomers (Dm), have been studied by x-ray diffraction and viscoelastic measurements. Binary blends of L2, D2, L3, and D3 are compatible and form isomorphous mixed crystals at all compositions, whereas other pairs, with the exception of L1/D1, are incompatible. The demixing process is described for a ternary system consisting of L1, D3, and a diluent chloroform at 40°C. The phase diagram comprises four regions, I, IA, A, and AA, with increasing polymer concentration; I: isotropic, A: anisotropic, IA: I–A biphasic, and AA: A–A biphasic. The IA biphasic gap is greater in the ternary system than in the binary ones. The high-molecular-weight component (D3) is partitioned into the A phase in the IA region. The AA separation originates from incompatibility of the polymers. The phase behavior is discussed on the basis of the Abe-Flory theory by incorporating the polymer-polymer interaction parameter.     

Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Phase diagrams of multi‐wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C₆₀‐I _h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic–nematic phase transition temperature (T _NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney‐type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ～0.1–0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C₆₀‐I _h)/E7 blend showed no discernible change of T _NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T _NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.     

Synthesis of comb‐type polycarbosilanes via nucleophilic substitution reactions on the main‐chain silicon atoms

A series of comb‐type polycarbosilanes of the type [Si(CH₃)(OR)CH₂]_n {where R = (CH₂)_mR′, R′ = ? O‐p‐biphenyl? X [X = H (m = 3, 6, 8, or 11) or CN (m = 11)], and R′ = (CF₂)₇CF₃ (m = 4)} were prepared from poly(chloromethylsilylenemethylene) by reactions with the respective hydroxy‐terminated side chains in the presence of triethylamine. The product side‐chain polymers were typically greater than 90% substituted and, for R′ = ? O‐p‐biphenyl? X derivatives, they exhibited phase transitions between 27 and 150 °C involving both crystalline and liquid‐crystalline phases. The introduction of the polar p‐CN substituent to the biphenyl mesogen resulted in a substantial increase in both the isotropization temperature and the liquid‐crystalline phase range with respect to the corresponding unsubstituted biphenyl derivative. For R = (CH₂)₁₁? O‐biphenyl side chains, an analogous side‐chain liquid‐crystalline (SCLC) polysiloxane derivative of the type [Si(CH₃)(O(CH₂)₁₁? O‐biphenyl)O]_n was prepared by means of a catalytic dehydrogenation reaction. In contrast to the polycarbosilane bearing the same side chain, this polymer did not exhibit any liquid‐crystalline phases but melted directly from a crystalline phase to an isotropic liquid at 94 °C. Similar behavior was observed for the polycarbosilane with a fluorocarbon chain, for which a single transition from a crystalline phase to an isotropic liquid was observed at ?0.7 °C. The molecular structures of these polymers were characterized by means of gel permeation chromatography and high‐resolution NMR studies, and the crystalline and liquid‐crystalline phases of the SCLC polymers were identified by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 984–997, 2003     

Effect of n-alkanes and peptides on the phase equilibria in phosphatidylcholine—water systems

Abstract

The phase equilibria in phosphatidylcholine (PC)-n-alkane-²H₂O systems have been studied to elucidate the driving forces for the transition between a lamellar liquid-crystalline (L _α) phase and a reversed hexagonal (H _II) phase. A tentative phase diagram for the system dioleoyl-PC (DOPC)-n-dodecane-²H₂O was determined. DOPC forms an L _α phase up to at least 90°C in excess water. However, an H _II phase was formed at room temperature at both low and high water concentrations in DOPC-n-dodecane-²H₂O mixtures. The phase equilibria were also studied in PC-n-dodecane-²H₂O systems containing PC with different degrees of acyl chain unsaturation. The water and dodecane concentrations required to induce the formation of an H _II (or isotropic) phase increase in the order dilinoleoyl-PC ～ DOPC < 1-palmitoyl-2-oleoyl-PC < dipalmitoyl-PC. The effect of n-alkanes with different chain lengths (C₈–C₂₀) on the phase equilibria in DOPC-n-alkane-²H₂O mixtures was studied. Although the number of alkane carbon atoms added per DOPC molecule was kept constant, the ability of the alkanes to promote the formation of an H _II phase was strongly chain length dependent; the ability decreased when going from octane to eicosane. Finally, some PC-peptide-²H₂O systems were investigated. Gramicidin (hydrophobic) had a similar influence on the phase equilibria as the alkanes. Melittin (amphiphilic) induced the formation of an isotropic phase, while insulin and duramycin (water soluble) had no, or a very limited, ability to induce a non-lamellar phase, respectively. Our results are discussed in the light of simple physical models dealing with the self-assembly of amphiphiles.     

Alternative Diesel Fuel: Microemulsion Phase Behavior and Combustion Properties

The phase behavior of water, diesel, limonene, and ethanol was investigated at ambient temperature using single nonionic alkyl polyglycol ethers (C₁₄E₃). Visual inspection as well as crosspolarizers was used to detect transparency and anisotropy. Ternary phase diagrams were determined. Combustion experiments using a four-cylinder diesel engine were carried out. Isotropic water in diesel microemulsion region (L₂) and anisotropic liquid crystalline region (LC) were found with all combinations. Increasing the ratio of limonene to diesel reduced the microemulsion region while the presence of ethanol increased it on the expense of the LC region. Combustion tests performed on a selected formulation from the ternary phase diagram of water, diesel, ethanol, and C₁₄E₃ revealed substantial reduction of soot, NO_x, and CO₂ emissions compared to neat diesel.     

Synthesis and characterization of polyaryloxyphosphazenes

The synthesis, dilute solution characterization, and thermal analysis of seven polyaryloxyphosphazenes are described. Synthesis is accomplished by the ring-opening polymerization of hexachlorocyclotriphosphazene at 245°C, followed by reaction of polydichlorophosphazene with sodium aryloxide salts in solution at 115°C. Polymers prepared and characterized have the general structure [(ArO)₂PN]_n, with Ar = C₆H₅, m- and p-CH₃C₆H₄, m- and p-ClC₆H₄, p-C₂H₅C₆H₄, or p-CH₃OC₆H₄. Elemental and infrared analyses show these polymers are essentially free of reactive chlorine sites. All the polymers displayed high intrinsic viscosities [η] > 1 dl/g, in tetrahydrofuran or chloroform. Closer examination of the dilute solution properties of two polyaryloxyphosphazenes revealed high molecular weights (M?_w> 6 × 10⁵) and broad molecular weight distributions (M?_w/M?_n > 4.7). The experimental values for the Z-average radii of gyration, 〈S²〉_z^1/2, characterized at near theta conditions, are larger than the calculated values for a freely rotating chain, which suggests that these polymers are relatively linear and not highly branched. Thermal analysis revealed second-order glass transitions between ?37 and +13°C and first-order endothermic transitions between 43 and 160°C for the different polymers. Although crystalline structure can persist above this first-order transition, this temperature can be regarded as a melting temperature or softening temperature at which films can be molded. Decomposition temperatures, measured in argon and oxygen, ranged from 250°C to 400°C.     

Experimental Investigation on Binary Phase Diagram of the Thermotropic Phase Transitions Compounds (n‐CnH2n+1R3)2ZnCl4

The thermotropic phase transitions compounds (n‐C_nH_2n+1R₃)₂ZnCl₄ as well as a series of their binary mixtures were prepared by a solution reflux method from their ethanol solutions. The experimental subsolidus binary phase diagram of [n‐C₁₈H₃₇N(CH₃)₃]₂ZnCl₄‐[n‐(C₁₈H₃₇)₂N(CH₃)₂]₂ZnCl₄ is constructed over the entire composition range by differential scanning calorimetry(DSC) and X‐ray. Experi‐ mental results indicate one stable intermediate phase [n‐C₁₈H₃₇N(CH₃)₃] [n‐(C₁₈H₃₇)₂N(CH₃)₂]ZnCl₄ at W_C18C3Zn %=59.75 %, and two invariant three phase equilibria, which shows two eutectoid temperatures: Te₁ at 310±1 K for eutectoid point W_C18C3Zn %=36.24 %, Te₂ at 313±1 K for eutectoid point W_C18C3Zn %=80.17 %. These three noticeable solid‐solution ranges are α‐phase at the left, ?‐phase at the right, and ψ‐phase in the middle of the phase diagram. It is (n‐C_nH_2n+1R₃)₂ZnCl₄ systems as phase change materials that are characterized the phase transition temperatures T in the range of 310 to 340 K, the transition enthalpies ΔH in the range of 38.40 and 168.72 J/g between two polymorphic forms.     

Review of: “Physico-Chemical Properties of Selected Anionic Cationic and Nonionic Surfactants”. N.M van Os,J.R. Haak and L.M. Rupert (Eds.). Elsevier,Amsterdam, 1993, pp. viii + 608 pages. $245.75.(ISBN 0-444-89691-0)

The phase behavior in water of pentaglycerol monostearate (C₁₈G₅) and pentaglycerol monooleate (C_18:1G₅) surfactants has been studied as a function of temperature and surfactant weight fraction, W _s . The equilibrium phases present at each composition and temperature studied were characterized by means of visual observation under normal and polarized light, differential scanning calorimetry (DSC), and X‐ray scattering, both at small (SAXS) and at wide angle (WAXS). In the temperature range 0–46°C, C₁₈G₅ presents a thermotropic α‐gel structure. However, at higher temperatures, the α‐gel phase melts and a lamellar liquid crystalline (L_α) phase is formed. The amount of water that can be solubilized by α‐gel and L_α was determined by plotting the interlayer distance, d, as a function of the reciprocal of W _s . Water is soluble in the α‐gel phase up to 21 w/w% water concentration and in the L_α phase up to 30 w/w% water concentration. At higher water concentrations, excess water appears and a dispersion of α‐gel (α‐gel+W) and lamellar liquid crystal (L_α+W) in water is formed, respectively. In contrast, C_18:1G₅ is liquid in the whole range of temperatures studied (0–100°C). While at low temperatures, C_18:1G₅ presents a L_α structure, at about 63°C L_α melts and an isotropic liquid reverse micellar solution (O_m) phase is formed. The amount of water that can be solubilized by both O_m and L_α increases with temperature.     

Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships,Variable‐Temperature Powder X‐ray Diffraction,and Small‐Angle X‐ray Scattering Studies

A class of extended 2,5‐disubstituted‐1,3,4‐oxadiazoles R¹‐C₆H₄‐{OC₂N₂}‐C₆H₄‐R² (R¹=R²=C₁₀H₂₁O 1 a , p‐C₁₀H₂₁O‐C₆H₄‐C?C 3 a , p‐CH₃O‐C₆H₄‐C?C 3 b ; R¹=C₁₀H₂₁O, R²=CH₃O 1 b , (CH₃)₂N 1 c ; F 1 d ; R¹=C₁₀H₂₁O‐C₆H₄‐C?C, R²=C₁₀H₂₁O 2 a , CH₃O 2 b , (CH₃)₂N 2 c , F 2 d ) were prepared, and their liquid‐crystalline properties were examined. In CH₂Cl₂ solution, these compounds displayed a room‐temperature emission with λ_max at 340–471 nm and quantum yields of 0.73–0.97. Compounds 1 d , 2 a – 2 d , and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized‐light optical microscopy and differential scanning calorimetry. Structure determination by a direct‐space approach using simulated annealing or parallel tempering of the powder X‐ray diffraction data revealed distinctive crystal‐packing arrangements for mesogenic molecules 2 b and 3 a , leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200–210 °C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable‐temperature X‐ray diffractometry. Nondestructive phase transitions (crystal‐to‐crystal, crystal‐to‐mesophase, mesophase‐to‐liquid) were observed in the diffractograms of 1 b, 1 d , 2 b, 2 d , and 3 a measured at 25–200 °C. Powder X‐ray diffraction and small‐angle X‐ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure–property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.     

Stereospecific polymerization of isobutyl vinyl ether. III. Polymerization with VCl3 • LiCl

The polymerization of isobutyl vinyl ether by vanadium trichloride in n-heptane was studied. VCl₃ ? LiCl was prepared by the reduction of VCl₄ with stoichiometric amounts of BuLi. This type of catalyst induces stereospecific polymerization of isobutyl vinyl ether without the action of trialkyl aluminum to an isotactic polymer when a rise in temperature during the polymerization was depressed by cooling. It is suggested that the cause of the stereospecific polymerization might be due to the catalyst structure in which LiCl coexists with VCl₃, namely, VCl₃ ? LiCl or VCl₂ ? 2LiCl as a solid solution in the crystalline lattice, since VCl₃ prepared by thermal decomposition of VCl₄ and a commercial VCl₃ did not produce the crystalline polymer and soluble catalysts such as VCl₄ in heptane and VCl₃ ? LiCl in ether solution did not yield the stereospecific polymer. It was found that some additives, such as tetrahydrofuran or ethylene glycol diphenyl ether, to the catalyst increased the stereospecific polymerization activity of the catalysts. Influence of the polymerization conditions such as temperature, time, monomer and catalyst concentrations, and the kind of solvent on the formed polymer was also examined.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.