Effect of transesterification reactions on the crystallization behaviour and morphology of poly(butylene/diethylene succinate) block copolymers

The melting behaviour, the crystallization kinetics and the morphology of block poly(butylene/diethylene succinate) copolymers (PBSPDGS) were investigated by means of differential scanning calorimetry and hot stage optical microscopy. Multiple endotherms were evidenced in the PBSPDGS samples, due to melting and recrystallization processes, similarly to PBS. By applying the Hoffman-Weeks’ method, the of both the homopolymer and the copolymers was derived. The isothermal crystallization kinetics was analyzed according to the Avrami’s treatment. The copolymers with long PBS blocks are characterized by a very similar behaviour with respect to pure poly(butilene succinate), indicating that PBS macromolecular folding is not affected by the presence of non-crystallizable diethylene succinate blocks. On the contrary, the copolymers characterized by very short PBS block length were found to crystallize at a slower rate than the homopolymer. As a matter of fact, a higher value of the work of chain folding was also derived on the basis of Hoffman-Lauritzen nucleation theory. Anyway, in all cases the crystallization mechanism (heterogeneous nucleation and three-dimensional growth) was found to be the same.     

Chain dynamics and thermal stability of boron-tris-containing copolymers

Poly(methyl methacrylate-co-glycidyl methacrylate-tris(hydroxymethyl)aminomethane) (PMGT) copolymers were obtained by copolymerization of methyl methacrylate (MMA) and a chelating monomer, glycidyl methacrylate-tris(hydroxymethyl)aminomethane (GMA-Tris), with potassium persulfate as an initiator. The glass transition temperature (T_g) and the proton spin-lattice relaxation time in the rotating frame () substantiated the formation of random copolymers. Borate-loaded PMGT (BPMGT) complexes were prepared by mixing PMGT and boric acid solution. The formation of coordination bond between PMGT and borate was studied using differential scanning calorimetry, infrared and ¹³C solid-state nuclear magnetic resonance spectroscopy. A single composition dependent T_g was obtained for the PMGT copolymers. The T_g value of BPMGT complex was much higher than that of PMGT copolymer with the same composition. The of the main chains in the PMGT copolymers and BPMGT complexes had one value, and that in the complexes was higher than that in the copolymers. The apparent activation energy (E_a) of the thermo-oxidative degradation of Tris units in complexes was larger than that in copolymers, whereas the E_a value of the MMA-GMA matrix was reversed.     

Investigations on mechanical properties of PI-PS multigraft copolymers

The stress-strain behaviour of multigraft copolymers consisting of a polyisoprene (PI) backbone and grafted polystyrene (PS) arms have been characterized by applying models of rubber elasticity such as Mooney-Rivlin, slip-tube and the extended non-affine tube model. Additionally, the range of low deformation has been investigated by relaxation tests for determining the stress relaxation. Multigraft copolymers show high strain at break and low residual strain caused by the large number of physical cross links resulting from several grafted PS side chains. From the model fits the material parameters and of the slip-tube model, representing the influence of chemical cross links and entanglements effects, respectively, and the n_e/T_e-value (n_e - number of statistical segments between two successive entanglements, T_e - Langley trapping factor) of the extended non-affine tube model, are used to describe the tensile behaviour of these thermoplastic elastomers. The PS content was considered as filler phase taking into account the effect of hydrodynamic amplification. The influence of functionality and the number of branch points per molecule on the strain at break and the tensile strength is explained by the model parameters describing the stress-strain curve at low to medium (?400%, slip-tube, Mooney-Rivlin) and low to high (?900%, extended non-affine tube) elongations. It was observed that for the material with a spherical morphology is increasing with the number of branch points β (each branch point consists of a PI backbone segment, depending on the functionality one, two or four grafted PS arms). For cylindrical and lamellar morphologies the was decreasing with increasing β, which could be reconfirmed by applying the extended non-affine tube model where the n_e/T_e-parameter is increasing with β.     

Heat capacities, order-disorder transitions, and thermodynamic properties of rare-earth orthoferrites and rare-earth iron garnets

Rare-earth orthoferrites, RFeO₃, and rare-earth iron garnets (RIGs) R₃Fe₅O₁₂ (R=rare-earth elements) were prepared by citrate-nitrate gel combustion method and characterized by X-ray diffraction method. Isobaric molar heat capacities of these oxides were determined by using differential scanning calorimetry from 130 to 860 K. Order-disorder transition temperatures were determined from the heat capacity measurements. The Néel temperatures (T_N) due to antiferromagentic to paramagnetic transitions in orthoferrites and the Curie temperatures (T_C) due to ferrimagnetic to paramagnetic transitions in garnets were determined from the heat capacity data. Both T_N and T_C systematically decrease with increasing atomic number of R across the series. Lattice, electronic and magnetic contributions to the total heat capacity were calculated. Debye temperatures as a function of absolute temperature were calculated for these compounds. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

Formation and morphology of “shish-like” fibril crystals of aliphatic polyesters from the sheared melt

We found the formation of “shish-like” fibril crystals of aliphatic polyesters such as poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), poly(12-hydroxydodecanoic acid) (PHDA) and poly(16-hydroxyhexadecanoic acid) (PHHA) from the sheared melt with shear rate  = 5 s⁻¹ observed by polarizing optical microscope (POM). The melting temperature T_ms of obtained fibril crystals of PLLA and PCL were higher than those of spherulites and were close to the equilibrium melting temperature . The small angle X-ray scattering (SAXS) patterns from the bulk sample including fibril crystals, small amount of unoriented small crystals and amorphous showed no peaks arose from the existence of long periods in fibril crystals. These are the evidence that the observed fibril crystals consist of assemblies of a lot of extended chain crystals (ECCs). We observed the morphology of moderately extracted single strand of fibril crystals at the magnification of POM by means of scanning electron microscope. We found that macroscopic fibril crystals of PLLA with diameter d = 10 μm consist of the bundle structure of microscopic fibril crystals with d = 2 μm. From POM observation of the formation of fibril crystals of PLLA and PCL, we showed phase diagrams of molecular weight M and crystallization temperature T_c for the formation of fibril crystals. From these phase diagrams, we evaluated a critical M and T_c for the formation of fibril crystals. Moreover, from the sequential melting and crystallization experiments, it was implied that the entanglement and transesterification play an important role on the formation of fibril crystals of aliphatic polyesters.     

Heat capacity and standard molar enthalpy of formation of crystalline 2,6-dicarboxypyridine (C7H5NO4)

Low-temperature heat capacity C_p,m of 2,6-dicarboxypyridine (C₇H₅NO₄; CAS 499-83-2) was precisely measured in the temperature range from (80 to 378) K with a high precision automated adiabatic calorimeter. No phase transition or thermal anomaly was observed in this range. The thermodynamic functions [H_T − H_298.15] and [S_T − S_298.15] were calculated in the range from (80 to 378) K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, and , by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The thermodynamic properties of the compound were further investigated through differential scanning calorimeter (DSC) and the thermogravimetric (TG) analysis.     

Diffusion paths formation for Cu ions in superionic Cu6PS5I single crystals studied in terms of structural phase transition

In order to understand the structural transformations leading to high ionic conductivity of Cu⁺ ions in Cu₆PS₅I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at Cu₆PS₅I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above T_c delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43c (, z=32). Finally, above increasing disordering of the Cu⁺ ions heightens the symmetry to F-43m (, z=4). In this work, the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions (R₁=0.0397 for F-43c phase, and 0.0245 for F-43m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism.     

Magnetic and transport properties of RCr0.3Ge2 (R=Tb, Dy, Ho and Er) compounds

The magnetic and transport properties of ternary rare-earth chromium germanides RCr_0.3Ge₂ (R=Y and Tb-Er) have been determined. X-ray and neutron diffraction studies indicate that these compounds have the CeNiSi₂-type structure (space group Cmcm) [1]. Magnetic measurements reveal the antiferromagnetic ordering below T_N equal to 18.5 K (R=Tb), 11.8 K (Dy), 5.8 K (Ho) and 3.4 K (Er). From the neutron diffraction data the magnetic structures have been determined. For TbCr_0.3Ge₂ and DyCr_0.3Ge₂ at low temperatures the magnetic ordering can be described by two vectors k₁=(,0,0) and k₂=(,0,), and k₁′=(,0,0) and k₂′=(,0,), respectively. In HoCr_0.3Ge₂ and ErCr_0.3Ge₂ the ordering can be described by one propagation vector equal to (,,0) and (0,0,0.4187(2)), respectively. In DyCr_0.3Ge₂ some change in the magnetic ordering is observed at T_t=5.1 K. In temperature range from T_t to T_N the magnetic ordering is given by one propagation vector k=(,0,0). YCr_0.3Ge₂ is a Pauli paramagnet down to 1.72 K which suggests that in the entire RCr_0.3Ge₂ series the Cr atoms do not carry magnetic moments. All compounds studied exhibit metallic character of the electrical conductivity. The temperature dependencies of the lattice parameters reveal strong magnetostriction effect at the respective Nèel temperatures.     

Glass transition temperature modification of acrylic and dienic type copolymers of 4-chloromethyl styrene with incorporation of (Me3Si)3C- groups

The new functional styrenic monomer, 4-trisylmethyl styrene (TsiMS) [Tsi=trisyl=tris(trimethylsilyl)methyl], was synthesized by reacting 4-chloromethyl styrene (CMS) with trisyllithium (TsiLi) in tetrahydrofuran (THF) solvent in the presence of copper chloride (CuCl). Attempt for the free radical polymerization of TsiMS by α,α^′-azobis(isobutyronitrile) (AIBN) as an initiator at 70 ± 1 °C failed for several periods of times. This result showed that the trisyl group is a highly sterically hindered substituent and, subsequently, TsiMS becomes resistant for polymerization. Therefore, for preparation of new methacrylic, acrylic and dienic copolymers of TsiMS, we firstly synthesized the copolymers of CMS with different monomers such as methyl methacrylate (MMA), ethyl methacrylate (EMA), methyl acrylate (MA), ethyl acrylate (EA), n-butyl acrylate (BA) and isoprene (IP) by free radical polymerization method in toluene solution at 70 ± 1 °C using AIBN initiator to give the copolymers I-VI in good yields. The copolymer compositions were obtained using related ¹H NMR spectra and the polydispersity indices of the copolymers determined using gel permeation chromatography (GPC). The trisyl groups were then covalently attached to the obtained copolymers as side chains by reaction between excess of TsiLi and benzyl chloride bonds of CMS units, to give the copolymers - in 80-92% yields. All the resulted polymers were characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopic techniques. The solubility of all the copolymers was examined in various polar and non-polar solvents. The glass transition temperature (T_g) of all copolymers was determined by differential scanning calorimetry (DSC) apparatus. The T_g value of copolymers containing bulky trisyl groups was found to increase with incorporation of trisyl groups in polymer structures. The presence of trisyl groups in polymer side chains, create new macromolecules with novel modified properties.     

Poly(dithiotriethylene adipate): Melting behavior, crystallization kinetics and morphology

The melting behavior and the crystallization kinetics of poly(dithiotriethylene adipate) (PSSTEA) were investigated by differential scanning calorimetry and hot-stage optical microscopy. The observed multiple endotherms, commonly displayed by polyesters, were influenced by the crystallization temperature (T_c) and ascribed to melting and recrystallization processes. Linear and nonlinear theoretical treatments were applied to estimate the equilibrium melting temperature for PSSTEA, using the corrected values of the melting temperature; the nonlinear estimation yielded a higher value by about 15 °C. Isothermal crystallization kinetics were analyzed according to the Avrami’s theory. Values of the Avrami’s exponent n close to 3 were obtained, independently of T_c, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three-dimensional spherulitic growth. As a matter of fact, space-filling spherulites were observed by optical microscopy at all T_c’s. The rate of crystallization became lower as T_c increased, as usual at low undercooling, the crystallization process being controlled by nucleation.     

Effects of branching characteristics and copolymer composition distribution on non-isothermal crystallization kinetics of metallocene LLDPEs

The effects of branch content (BC) and copolymer composition distribution (CCD) on the non-isothermal crystallization kinetics of metallocene m-LLDPEs were studied using modified Avrami analysis, modulated differential scanning calorimetry (MDSC), and Crystaf. Several m-LLDPEs and an m-HDPE - all having comparable M_w and PDI - were experimented. In addition, a ZN-LLDPE was used for comparison purposes. The branch content, unlike the used cooling rates (2-6 °C/min), significantly affected the crystallization behavior. Crystallization peak temperature, , decreased linearly with increasing BC. All the m-LLDPEs showed primary and secondary crystallizations. The secondary crystallization showed to be more pronounced at high BC. The primary crystallization Avrami parameter n for m-HDPE ranged between 3.72 and 4.50, indicating spherulitic crystal growth whereas that for the m-LLDPEs, varied from 2.02 to 5.70. The ZN-LLDPE (having broader composition distribution) offered higher values of and than the m-LLDPEs with similar BC, M_w, and PDI. It, unlike the m-LLDPEs and m-HDPE, fairly agreed with the crystallization kinetic model proposed by Liu et al. The lamella thickness of the m-LLDPEs, L, calculated as per Gibbs-Thomson equation, showed to be in the range 2-16 nm, depending on BC and it decreased approximately following the relationship: L (nm) = 15.0 e^{(−0.0498BC)}.     

Synthesis, structural determination and magnetic properties of layered hybrid organic-inorganic, iron (II) propylphosphonate, Fe[(CH3(CH2)2PO3)(H2O)], and iron (II) octadecylphosphonate, Fe[(CH3(CH2)17PO3)(H2O)]

Fe[(CH₃(CH₂)₂PO₃)(H₂O)] (1) and Fe[(CH₃(CH₂)₁₇PO₃)(H₂O)] (2) were synthesized by reaction of FeCl₂·6H₂O and the relevant phosphonic acid in water in presence of urea and under inert atmosphere. The compounds were characterized by elemental and thermogravimetric analyses, UV-visible and IR spectroscopy. The crystal structure of (1) was determined from X-ray single crystal diffraction studies at room temperature: monoclinic symmetry, space group P2₁, , , , and β=98.62(3)°. The compound is lamellar and the structure is hybrid, made of alternating inorganic and organic layers along the c direction. The inorganic layers consist of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from the water molecule, separated by bi-layers of propyl groups. A preliminary structure characterization of compound (2) suggests a similar layered structure, but with an interlayer spacing of 40.3 Å. The magnetic properties of the compounds were both studied by a dc and ac SQUID magnetometer. Fe[(CH₃(CH₂)₂PO₃)(H₂O)] (1) obeys the Curie-Weiss law at temperatures above 50 K (, ), indicating a Fe +II oxidation state, a high-spin d⁶ (S=2) electronic configuration and an antiferromagnetic exchange couplings between the near-neighbouring Fe(II) ions. Below , Fe[(CH₃(CH₂)₂PO₃)(H₂O)] exhibits a weak ferromagnetism. The critical temperature of has been determined by ac magnetic susceptibility measurements. Compound (2) shows the same paramagnetic behaviour of the iron (II) propyl derivative. The values of C and θ were found to be and −44 K, respectively, thus suggesting the presence of Fe +II ion in the S=2 spin state and antiferromagnetic interactions between Fe(II) ions at low temperatures. Zero-field and field cooled magnetic susceptibility vs. T plots do not overlap below , suggesting the presence of an ordered magnetic state. The critical temperature, T_N, has been located by the peaks at from the ac susceptibility (χ′and χ″) vs. T plots. Below T_N hysteresis loops recorded in the temperature region show an S-shape, while below 15 K assume an ellipsoid form. They reveal that compound (2) is a weak ferromagnet. The critical temperature T_N in these layered Fe(II) alkylphosphonates is independent of the distance between the inorganic layers.     

Use of bis(trifluoromethyl)peroxy dicarbonate as initiator in the radical homopolymerisation of vinylidene fluoride (VDF) and copolymerisation of VDF with hexafluoropropylene

The radical homopolymerisation in acetonitrile of vinylidene fluoride (or 1,1-difluoroethylene, VDF) and the copolymerisation of VDF with hexafluoropropylene (HFP) initiated by bis(trifluoromethyl)peroxy dicarbonate are presented. Different reactions and different reactants were chosen to monitor the polymerisation in terms of initiating radicals generated from this initiator. Homopolymers and copolymers thus obtained were characterised by and NMR spectroscopy. From the assignments of the characteristic signals, an overall reaction mechanism is proposed that explains each step of the polymerisation. Particularly, an interpretation of the polymer microstructures and the presence of end-groups arising from the radical initiator as well as from eventual transfers is suggested. Among some of the microstructures, the trifluoromethoxy end-group was noted to be present in both PVDF and poly(VDF-co-HFP) (co)polymers, as generated from the decomposition of the initiator. This trifluoromethoxy end-group enabled the assessment of the molecular weights of PVDF and poly(VDF-co-HFP) (co)polymers. Thermal properties of the copolymers were also determined, showing that original fluoroelastomers possessing CF₃ end-groups are obtained endowed with low T_g and good thermal stability.     

Synthesis of poly(methyl methacrylate-co-styrene)-block-polysulfide-block-poly(methyl methacrylate-co-styrene) copolymers by free radical polymerization combined with oxidative coupling

Poly(methyl methacrylate-co-styrene)-block-polysulfide-block-poly(methyl methacrylate-co-styrene) triblock copolymers were synthesized for the first time by the free radical copolymerization of methyl methacrylate (MMA) and styrene (St) in the presence of a thiocol oligomer as a chain transfer agent, followed by chemical oxidation of the remaining SH-end groups. The apparent chain transfer constant of the thiocol SH groups in the copolymerization reaction was estimated from the rate of consumption of the thiol groups versus the overall rate of consumption of the monomers (C_T = 1.28). Based on this value, the chain transfer constant of the thiocol SH groups in St polymerization was calculated . The triblock copolymers synthesized were characterized by SEC and ¹H NMR measurements.     

On permittivity and density of the systems (tetraglyme + dimethyl or diethyl carbonate) and the formulation of Δε in terms of volume or mole fraction

Relative permittivity and density on mixing from T = 288.15 K to 328.15 K and atmospheric pressure have been measured over the whole composition range for CH₃O(CH₂CH₂O)_mCH₃ polyoxyethyleneglycol dimethyl ether with m = 4 (also called 2,5,8,11,14-pentaoxapentadecane or tetraglyme) + (dimethyl or diethyl carbonate). For these systems the deviation of permittivity, Δε, changes sign depending on whether it is defined on the basis of mole fractions of volume fractions. Arguments are put forward that support the choice of the definition in terms of mole fraction. The Redlich-Kister equation has been used to estimate the binary fitting parameters and standard deviations from the regression lines were calculated. The density and excess molar volumes were fitted as a function of temperature and mole fraction to a polynomial equation. The temperature dependence of derived magnitudes, such as the isobaric thermal expansion coefficient, α, , and were computed, due to its importance in the study of specific molecular interactions. Different traditional mixing rules have been applied to predict the permittivity of these mixtures.     

Isomorphy of structural phase transitions in LiTaOSiO4, LiTaOGeO4 and titanite, CaTiOSiO4

Structural phase transitions in LiTaOGeO₄ (LTGO) and LiTaOSiO₄ (LTSO) have been observed using differential scanning calorimetry, X-ray diffraction and MAS NMR spectroscopy. LTGO transforms from P2₁/c to C2/c space group symmetry at , while the isomorphic transition occurs at in LTSO. An analogous phase transition is known to occur in the structurally related mineral titanite, CaTiOSiO₄. Spontaneous strain accompanying this phase transition in LTSO is significantly stronger than in titanite. As in titanite non-vanishing strain components are observable for T_c<T<T_i, with a similar ratio T_i/T_c. MAS NMR spectroscopy in combination with computation of the electric field gradient by first principle methods confirms that the tetrahedral Li coordination environment is retained during the phase transitions in LTGO and in LTSO. In LTSO substantial motional narrowing is observed, indicating increased mobility of the Li cation above . The narrowing of the spinning sidebands is significantly modified immediately above and below the critical temperature.