Dynamic mechanical behavior of a side-chain liquid crystalline polyacrylate

The phenomenology of the α- and β-relaxation processes in an amorphous and a crystallized specimen of a side-chain LC polyacrylate, based on the azobenzene mesogenic unit, was investigated by thermal and dynamic mechanical methods. The activation energy of the β relaxation of the crystallized sample is equal to that of the amorphous sample. The α-transition process of the amorphous sample was described by the WLF equation. In contrast, the α-relaxation behavior of the amorphous part of the semicrystalline sample was described by a double Arrhenius law broken at T = T_g. This peculiar behavior has been tentatively related to the decrease of the motion characteristic length of the macromolecules confined in the multidomain structure of the semicrystalline state. © 1996 John Wiley & Sons, Inc.     

Sub-TG relaxations in LiClO4-poly (propylene glycol)-2000 solutions

The dielectric permittivity and loss of LiClO₄ solutions in poly (propylene glycol (PPG)), molecular weight 2000, have been measured over a concentration range up to a ratio of Li⁺ to oxygen atoms in PPG of 33.3:100, between 77 and 350 K. The data have been analyzed in both the permittivity and electrical modulus formalisms. Addition of LiClO₄ to poly (propylene glycol) first increases the height of the β-relaxation peak, and ultimately a second sub-T_g relaxation peak at a higher temperature emerges. This is in addition to the β-relaxation peak due to the reorientation of PPG dipoles, whose strength decreases from that in pure PPG-2000. For a fixed temperature, the dc conductivity initially decreases with increasing Li⁺ concentration up to 20 Li⁺ per 100 O atoms and thereafter increases. This concentration corresponds to that at which the T_g of the solution reaches its limiting value of ca. 310 K. It is concluded that the formation of ion pairs causes a second and slower sub-T_g relaxation process and that the increase in the efficiency of chain packing reduces the strength of the β-relaxation of the polymer.     

Pressure dependence of the dielectric properties and phase transitions of polyvinylidene fluoride (PVDF) and a copolymer with trifluoroethylene

The frequency, temperature, and hydrostatic pressure dependences of the dielectric properties, molecular relaxations, and phase transitions in PVDF and a copolymer with a 30 mol% trifluoroethylene were investigated. The β-relaxation peak temperature T_β and the melting temperature T_m of both polymers, and the ferroelectric transition temperature T_c of the copolymer, are strong increasing functions of pressure. The magnitudes of the pressure derivatives of T_β, T_c, and T_m increase in the order of the “transition” temperatures, i.e., T_β(P) < T_c(P) < T_m(P). These results can be qualitatively understood in terms of the nature of the molecular motion and/or reorientation processes involved. The results on the copolymer suggest that pressure should induce a ferroelectric-paraelectric transition in PVDF below T_m, but such a transition was not observed over the limited pressure range of the present experiments. The relaxational dynamic (not static) nature of the melting process in these materials is indicated by the observed dependence of T_m on probe frequency. The frequency (or rate) and strong pressure dependences of T_m of PVDF provide a rational explanation for why it is possible to use this polymer as a piezoelectric shock-wave gauge to relatively high shock pressures and the accompanying high temperatures.     

Emissions of biogenic volatile organic compounds from Fraxinus excelsior and Quercus robur under ambient conditions in Flanders (Belgium)

A dynamic branch enclosure system was used to measure emission rates of biogenic volatile organic compounds (BVOCs) from two common European tree species: Fraxinus excelsior and Quercus robur under ambient conditions in Flanders (Belgium). Both tree species were studied for seasonal variability of BVOC emission rates under natural biotic stress (infestations). Emissions were normalized at standard conditions of temperature and photosynthetic active radiation (PAR) (30°C and 1000?µmol?m^?2?s^?1, respectively). Emission rates from Fraxinus excelsior were highest in May (9.56?µg?g_DW ^?1?h^?1) and lowest in October (1.17?µg?g_DW ^?1?h^?1). This tree species emitted (Z)-β-ocimene, (E)-β-ocimene and α-farnesene during the entire measurement period and additionally isoprene only in May. Quercus robur showed isoprene emission variations according to the seasonal cycle with rates of 30, 106 and 29?µg?g_DW ^?1?h^?1 in May, August and October, respectively. Apart from isoprene, (E)-β-ocimene and β-caryophyllene were emitted through the entire experimental period.     

Molecular dynamics and conduction mechanism of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) terpolymer containing ionic liquid

Dielectric spectroscopy (DS) measurements were performed to probe the segmental dynamics and ion mobility of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) terpolymer dopped with different amounts of tetrabutylammonium tetrafluoroborate ([TBA] [BF₄]) ionic liquid (IL). Differential scanning calorimetry (DSC) was also employed to trace the change in the glass transition temperature (T_g) at different loads of IL. The DSC measurements revealed a remarkable reduction in the PVVH T_g from 344 to 310 K just by adding 20 wt% of IL. The DS measurements revealed three relaxation processes named α, β₁, and β₂. The α-process is related to the segmental motion of PVVH while the β₁ and β₂ are due to the restricted local dynamics of side chains. The segmental relaxation times (α-relaxation) speed up with increasing the concentration of IL due to the plasticization effect of IL on polymer chains. The temperature dependence of α-relaxation follows the Vogel-Fulcher-Tammann (VFT) relation with dynamic glass transition between 323 and 294 K in agreement with the DSC measurements. The β₁ and β₂-relaxations have an Arrhenius temperature dependence. The temperature dependence of ionic conductivity obeys the VFT behavior indicating the coupling between the segmental motion of PVVH chains and ion transport. Polaronic tunneling is the predominant conduction mechanism in PVVH and its composites. The specific capacitance increases with increasing both the temperature and IL concentration.     

Nucleotides. Part LIV. Synthesis of condensed N1-(2′-deoxy-β-D-ribofuranosyl)lumazines,New fluorescent building blocks in oligonucleotide synthesis

Various condensed areno[g]lumazine derivatives 2 , 3 , and 5 – 7 were synthesized as new fluorescent aglycones for glycosylation reactions with 2-deoxy-3, 5-di-O-(p-toluoyl)-α/β-D -erythro-pentofuranosyl chloride ( 10 ) to form, in a Hilbert-Johnson-Birkofer reaction, the corresponding N¹-(2′-deoxyribonucleosides) 15 – 21 . The β-D -anomers 15 , 17 , 19 , and 21 were deblocked to 24 – 27 and, together with N¹-(2′-deoxy-β-D -ribofuranosyl)lumazine ( 22 ) and its 6, 7-diphenyl derivative 23 , dimethoxytritylated in 5′-position to 28–33. These intermediates were then converted into the 3′-(2-cyanoethyI diisopropylphosphoramidites) 34 – 39 which function as monomeric building block in oligonucleotide syntheses as well as into the 3′-(hydrogen succinates) 40 – 45 which can be used for coupling with the solid-support material. A series of lumazine-modified oligonucleotides were synthesized and the influence of the new nucleobases on the stability of duplex formation studied by measuring the T_m values in comparison to model sequences. A substantial increase in the T_m is observed on introduction of areno[g]lumazine moieties in the oligonucleotide chain stabilizing obviously the helical structures by improved stacking effects. Stabilization is strongly dependent on the site of the modified nucleobase in the chain.     

Synthesis of stereoregular poly[(2S,3S)-benzyl β-3-methylmalate]

The asymmetric lactone (3 S, 4 R)-3-methyl-4-benzyloxycarbonyl-2-oxetanone ( 6 ) was anionically polymerized to give an insoluble, crystalline, highly isotactic polymer with (2 S, 3 S)-benzyl β-3-methylmalate repeating units. Solubility was achieved by copolymerization of 6 with the recemic (R, S)-butyl malolactonate ( 7 ). The semicrystalline copolymer was characterized (M̄_n = 107 000, T_g = 29,6°C, T_m = 161°C, [α] = 1,5 deg · dm⁻¹ · g⁻¹ · cm³) and its stereosequence investigated by ¹³C NMR.     

Influence of the mesophase structure on the β-relaxation in liquid crystalline poly(meth)acrylates

The dependence of the rotation of the mesogenic unit around its long axis (β-relaxation) on the actual mesophase in liqid crystalline polymethacrylates and polyacrylates was studied by dielectric spectroscopy in the frequency range from 10⁻² Hz to 10⁶ Hz and in a temperature range from 170 K to 430 K. As mesogenic units derivatives of (p-alkoxy-phenyl)-benzoate were used where different mesophases were achieved by small variation of the mesogenic structure, the spacer length and the tail group of the mesogenic unit. For all samples the temperature dependence of the relaxation rate of the β-relaxation can be described by an Arrhenius equation where both the pre-exponential factor and the activation energy increase significantly with the order of the mesophase. To characterize the structure X-ray measurements were also carried out. The mean lateral mesogenic distance was correlated directly with relaxational quantities.     

Polyesters containing ring units in the main chain. II. Effects of size and mobility of rings on molecular relaxations

Molecular relaxations of polyesters containing C₅–C₁₅ rings in the main chain have been studied by DSC, dielectric dispersion, and NMR. Results are discussed in relation to the size and mobility of the rings. The T_g or α-relaxation peak moves to higher temperature with an increase in the ring size from C₅ to C₁₂, but the effect is accompanied by an even–odd alternation with ring size. The β relaxations in dielectric dispersion reflect local-mode motion of ester groups and are affected by steric interactions with the rings. Motions of the ring methylenes of C₁₂ and C₁₅ ring units are detected below T_g by broad-line NMR.     

Properties of polyethylene modified with phosphonate side groups. II. Dynamic mechanical behavior

The viscoelastic behavior of phosphonate derivatives of phosphonylated low-density polyethylene (LDPE) was studied by dynamic mechanical techniques. The polymers investigated contained from 0.2 to 9.1 phosphonate groups per 100 carbon atoms and included the dimethyl phosphonate derivative and two derivatives for which the phosphonate ester group was an oligomer of poly(ethylene oxide) (PEO). The temperature dependences of the storage and loss moduli of the dimethyl phosphonate derivatives were qualitatively similar to those of LDPE. At low phosphonate concentrations, the α, β, and γ dispersion regions characteristic of PE were observed, while at concentrations greater than 0.5 pendent groups per 100 carbons atoms, only the β and α relaxations could be discerned. At low degrees of substitution, the temperature of the β relaxation T_β decreased from that of PE, but above a degree of substitution of 0.1, T_β increased. This behavior was attributed to the competing influences of steric effects which tend to decrease T_β and dipolar interactions between the phosphonate groups which increase T_β. For the phosphonate containing PEO, a new dispersion region designated as the β′ relaxation was observed as a low-temperature shoulder of the β relaxation. The temperature of the β′ loss was consistent with T_g(U) of the PEO oligomers as determined by differential scanning calorimetry, and it is suggested that the β′-loss process results from the relaxation of PEO domains which constitute a discrete phase within the PE matrix.     

Segmental and secondary dynamics in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) blends

Broadband dielectric spectroscopy was used to study the segmental (α) and secondary (β) relaxations in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) (PVPh/PMMA) blends with PVPh concentrations of 20–80% and at temperatures from ?30 to approximately glass‐transition temperature (T_g) + 80 °C. Miscible blends were obtained by solution casting from methyl ethyl ketone solution, as confirmed by single differential scanning calorimetry T_g and single segmental relaxation process for each blend. The β relaxation of PMMA maintains similar characteristics in blends with PVPh, compared with neat PMMA. Its relaxation time and activation energy are nearly the same in all blends. Furthermore, the dielectric relaxation strength of PMMA β process in the blends is proportional to the concentration of PMMA, suggesting that blending and intermolecular hydrogen bonding do not modify the local intramolecular motion. The α process, however, represents the segmental motions of both components and becomes slower with increasing PVPh concentration because of the higher T_g. This leads to well‐defined α and β relaxations in the blends above the corresponding T_g, which cannot be reliably resolved in neat PMMA without ambiguous curve deconvolution. The PMMA β process still follows an Arrhenius temperature dependence above T_g, but with an activation energy larger than that observed below T_g because of increased relaxation amplitude. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3405–3415, 2004     

Effect of end group chemistry on surface molecular motion of monodisperse polystyrene films

The surface molecular motion of monodisperse polystyrene (PS) with various chain end groups was investigated on the basis of temperature‐dependent scanning viscoelasticity microscope (TDSVM). The surface glass transition temperatures, T_g^ss for the proton‐terminated PS (PS‐H) films with number‐average molecular weight, M_n of 4.9k–1,450k measured by TDSVM measurement were smaller than those for the bulk one, with corresponding M_ns, and the T_g^ss for M_n smaller than ca. 50k were lower than room temperature (293 K). In the case of M_n = ca. 50k, the T_g^ss for the α,ω‐diamino‐terminated PS (α,ω‐PS(NH₂)₂) and α,ω‐dicarboxy‐terminated PS (α,ω‐PS(COOH)₂) films were higher than that of the PS‐H film. On the other hand, the T_g^s for the α,ω‐perfluoroalkylsilyl‐terminated PS (α,ω‐PS(SiC₂C^F₆)₂) film with the same M_n was much lower than those for the PS films with all other chain ends. The change of T_g^s for the PS film with various chain end groups can be explained in terms of the depth distribution of chain end groups at the surface region.     

Effect of finite extensibility on the viscoelastic properties of a styrene–butadiene rubber vulcanizate in simple tensile deformations up to rupture

Stress–strain and rupture data were determined on an unfilled styrene–butadiene vulcanizate at temperatures from ?45 to 35°C and at extension rates from 0.0096 to 9.6 min^?1. The data were represented by four functions: (1) the well-known temperature function (shift factor) a_T; (2) the constant strain rate modulus, F(t,T), reduced to temperature T₀ and time t/a_T, i.e., T₀F(t/a_T)/T; (3) the time-dependent maximum extensibility, λ_m(t/a_T); and (4) a function Ω(χ) where χ = (λ ? 1)λ_m⁰/λ_m, in which λ is the extension ratio and λ_m⁰ is the maximum extensibility under equilibrium conditions. The constant strain rate modulus characterizes the stress–time response to a constant extension rate at small strains, within the range of linear response; λ_m is a material parameter needed to represent the response at large λ; and Ω(χ) represents the stress–strain curve of the material in a reference state of unit modulus and λ_m = λ_m. The shift factor a_T was found to be sensibly independent of extension. At all values of t/a_T for which the maximum extensibility is time-independent, the relaxation rate was also found to be independent of λ. These observations indicate that the monomeric friction coefficient is strain-independent over the ranges of T and λ covered in the present study. It was found that λ_m⁰ = 8.6 and that the largest extension ratio at break, (λ_b)_max, is 7.3. Thus, rupture always occurs before the network is fully extended.     

Polyazomethine with m-tolylazo side groups: thermal,dielectric and conductive behaviour

Using solution polycondensation, a new polyazomethine with m-tolylazo side groups (PAz) exhibiting thermotropic liquid crystalline phase was synthesised and its chemical structure was characterised with generally accepted methods. Its phase transition temperatures were detected with both polarising optical microscopy and differential scanning calorimetry. Using dielectric spectroscopy method, both real and imaginary parts of the permittivity were investigated in wide regions of temperature (from ?100°C to 170°C) and frequency (from 1 Hz to 1 MHz). Analysis of frequency dependent permittivity allowed finding three relaxations (α, β₁ and β₂) in PAz. β-relaxations were described with the Arrhenius equation, whereas α-relaxation was described with the Vogel–Fulcher–Tammann equation. The alternating current conductivity (ACC) of PAz was studied in the same regions of temperature and frequency. The frequency dependent ACC was described with an exponent power equation. Presentation of ACC as a function of inverse temperature allowed us to describe ACC with the Arrhenius equation.     

Pressure and volume dependence of thermal conductivity and isothermal bulk modulus up to 1 GPa for poly(isobutylene)

The thermal conductivity λ and heat capacity per unit volume ρc_p of poly(isobutylene)s, one 2.8 in weight average molecular weight and one 85 kg mol⁻¹ in viscosity average molecular weight (PIB-2800 and PIB-85000), have been measured in the temperature range 170–450 K at pressures up to 2 GPa using the transient hot-wire method. At 297 K and atmospheric pressure, λ = 0.115 W m⁻¹ K⁻¹ for PIB-2800 and λ = 0.120 W m⁻¹ K⁻¹ for PIB-85000. The bulk modulus B_T has been measured in the temperature range 170–297 K up to 1 GPa. At atmospheric pressure, the room temperature bulk moduli B_T are 2.0 GPa for PIB-2800 and 2.5 GPa for PIB-85000 with dB_T/dp = 10 for both. These data were used to calculate the volume dependence of λ, At room temperature and atmospheric pressure (liquid phase) we find g = 3.4 for PIB-2800 and g = 3.9 for PIB-85000, but g depends strongly on temperature for both molecular weights. The difference in g between the glassy state and liquid phase is small and just outside the inaccuracy of g of about 8%. The best predictions for g are given by the theoretical model of Horrocks and McLaughlin. We have found that PIB exhibits two relaxations, where one is associated with the glass transition. The value for dT_g/dp at atmospheric pressure (for the main glass transition) is about 0.21 K MPa⁻¹ for both molecular weights. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1781–1792, 1998     

A re‐examination of the sub‐Tm exotherm in polyamide 6: The roles of thermal history,water and clay

The sub‐T_m exotherms in polyamide 6 (PA6) have been carefully re‐examined by differential scanning calorimetry and X‐ray diffraction, considering the effects of processing and thermal history, addition of water and clay. The results obtained cast doubt on Khanna's proposal that sub‐T_m exotherm in PA6 comes from the release of strain energy absorbed during processing, and suggested that the origin of sub‐T_m exotherm is the γ?α transformation at the premelting temperature, namely, the less thermodynamically stable γ‐form (γ^ns) transforming into the more thermodynamically stable α‐form (α^s). The presence of water or clay in PA6 samples facilitated the formation of γ^ns at corresponding cooling rates, and enhanced the development of sub‐T_m exotherms. During the heating scan of PA6/clay composites, the initial γ^ns can be transformed into more stable (γ^s)^t and α^s at the same time, which can be thought as the origin of their sub‐T_m events. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2385–2393, 2009     

New chiral liquid crystal monomers and cholesteric polyacrylates: synthesis and characterisation

The synthesis of new chiral monomers (M₁ ?M₃ ) based on menthol and the corresponding polyacrylates (P₁ ?P₃ ) is described. The chemical structures, formula and phase behaviour of the obtained monomers and polymers were characterised with FT-IR, ¹H-NMR, elemental analyses, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). The effect of the mesogenic core rigidity, spacer length and menthyl steric effect on the phase behaviour of M₁ ?M₃ and P₁ ?P₃ is discussed. The expected mesophase of the compounds based on menthol can be obtained by inserting a flexible spacer between the mesogenic core and the terminal groups. For the chiral monomers and polyacrylates, their corresponding melting temperature (T _m), glass transition temperature (T _g) and clearing temperature (T _i) increased with an increase of the mesogenic core rigidity; while the T _m, T _g and T _i decreased with increasing the spacer length. M₁ and P₁ showed no mesophase, while M₂ and M₃ all revealed a SmC* and cholesteric phases. P₂ and P₃ only showed a cholesteric phase.     

Relaxation Processes and Glass Transition in Polymer Filled with Nanoparticles

We report the results of the investigations of the influence of filling of polymer with Aerosil nanosize particles on the glass transition and dynamics of the α- and the β-relaxation processes in poly(n-octyl methacrylate) by dielectric spectroscopy and differential scanning calorimetry (DSC). The polymer was filled with hydrophilic and hydrophobic Aerosil particles of 12 nm diameter. In filled polymers the characteristic frequency of the alpha-process was shifted to higher frequencies in comparison with pure bulk polymer at the same temperature. This suggests that the filling of the polymer with nanoparticles has resulted in the shift of its glass transition temperature T_g. This change in T_g was mainly due to the existence of a developed solid particle-polymer interface and the difference in the dynamic behavior of the polymer in the surface layers at this interface compared to the bulk behavior. This result was in agreement with DSC experiments.     

Evidence of dynamic heterogeneity as obtained from dielectric and brillouin light-scattering studies of poly(n-hexylmethacrylate)

We report dielectric relaxation and Rayleigh-Brillouin spectroscopic measurements on the side chain polymer poly(n-hexylmethacrylate), PHMA (T_g = 268 K), exhibiting a broad glass transition region. The dielectric loss curves can be represented by single Havriliak-Negami functions in the temperature range of 260–450 K. The width of the distribution relaxation function is a decreasing function of temperature up to T = 333 K ≊ 1.24 × T_g and remains virtually constant above that temperature. This is interpreted as marking the merging of the α-process with a slow β-relaxation in agreement with the value of the cooperativity length associated with the α-mode. Hence above that temperature, the relaxation times confirm well to an Arrhenius temperature dependence. The hypersonic dispersion deduced from the Brillouin spectra (210–550 K) surprisingly peaks at temperatures near T_g which bears no relation to the main α-relaxation. This structural relaxation is rather associated with the side hexyl group motion showing striking resemblance with the hypersonic dispersion in molecular liquids. It is conceivable that the observed damping in PHMA is dynamically related to the internal plasticization effect of the hexyl group. © 1996 John Wiley & Sons, Inc.     

Nucleosides and nucleotides. Part 7. Four dithymidine monophosphates with different anomeric configurations, their synthesis and behaviour towards phosphodiesterases

The four isomeric dithymidine monophosphates βT_d-βT_d ( 1 ), αT_d-βT_d (2), βT_d-αT_d ( 3 ) and αT_d-αT_d ( 4 ), differing only by the anomeric configurations of the nucleoside units, were prepared from the suitably protected nucleosides and nucleotides. The four dinucleoside monophosphates were tested as substrates of snake venom phosphodiesterase and spleen phosphodiesterase. Both enzymes accept all four compounds as substrates, however, the rate of the hydrolysis is considerably smaller if the enzymic attack takes place at the α-nucleoside moiety of the dinucleoside monophosphate.