Studying the miscibility and thermal behavior of polybenzoxazine/poly(ε-caprolactone) blends using DSC, DMA, and solid state C NMR spectroscopy

Polymer blends of polybenzoxazine (PBZ) and poly(ε-caprolactone) (PCL) were prepared by solution blending of PCL and benzoxazine monomer (B-m), followed by thermal curing of B-m. The miscibility and thermal behavior of these PBZ/PCL blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The FTIR spectra indicated that hydrogen bonding interactions occur between the carbonyl groups of PCL and the hydroxyl groups of PBZ upon curing. The DSC results revealed that this PBZ/PCL blend system has a single glass transition temperature over the entire range of compositions that we investigated. The DMA results indicated that the values of T_g of the PBZ/PCL blends were higher than those of the pure polymers. In addition, at higher PCL concentrations we observed two glass transitions for the PBZ/PCL blends: One, for the PCL component, occurred in the low-temperature region and the other, for the PBZ component, in the high-temperature region; this finding indicates that PCL and PBZ are partially miscible in the amorphous phase. The most pronounced effect of the addition of PCL was to broaden the glass transition region, judging from the E″ peaks and the decrease in the value of the loss tangent (tan δ) in the transition region upon increasing the PCL content. We have also studied the ¹H spin-lattice relaxation times in the laboratory frame, , and in the rotating frame, , as a function of the blend composition. The results are in good agreement with those from the DSC analysis; i.e. the blends are completely homogeneous on the scale of 40-70 nm. The values of indicate that the PCL present in the blends exists in both crystalline and amorphous phases; the mobility difference between these PCL phases is clearly visible from the data. In addition, the amorphous phase of PCL is not miscible with PBZ; i.e. it is larger than 2-4 nm.     

Comparison of poly(ethylene oxide) crystal orientations and crystallization behaviors in nano-confined cylinders constructed by a poly(ethylene oxide)-b-polystyrene diblock copolymer and a blend of poly(ethylene oxide)-b-polystyrene and polystyrene

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer with a number average molecular weight of PEO blocks, =8.8 kg/mol, and a number average molecular weight of PS blocks, =24.5 kg/mol, (volume fraction of the PEO blocks, f_PEO, was 0.26) exhibited a hexagonal cylinder (HC) phase structure. Small angle X-ray scattering results showed that the PEO cylinder diameter was 13.3 nm, and the hexagonal lattice was a=25.1 nm. The cylinder diameter of this HC phase structure was virtually the same as that in the blend system constructed by a PEO-b-PS diblock copolymer (=8.7 kg/mol and =9.2 kg/mol) and a PS homo-polymer (=4.6 kg/mol) in which the f_PEO was 0.32. The cylinder diameter in this blend sample was 13.7 nm and the hexagonal lattice was a=23.1 nm. Comparing crystal orientation and crystallization behaviors of this PEO-b-PS copolymer with the blend, it was found that the crystal orientation change with respect to crystallization temperature was almost identical. This is attributed to the fact that in both cases the PEO block tethering densities and confinement sizes are very similar. This indicates that when the of PS homo-polymer is lower than the PS blocks, the PS homo-polymer is located inside of the PS matrix rather than at the interface between the PEO and PS in the HC phase structure. On the other hand, a substantial difference of crystallization behaviors was observed between these two samples. The PEO-b-PS copolymer exhibited much more retarded crystallization kinetics than that of the blend. Based on the small angle X-ray scattering results, it was found that in the blend sample, the HC phase structure was not as regularly ordered as that in the PEO-b-PS copolymer, and thus, the HC phase structure contained more defects in the blend. This led to a suggestion that the primary nucleation process in the confined crystallization is a defect-controlled process. The mean crystallite sizes were estimated by the Scherer equation, and the PEO crystal sizes are on the scale of the confined size.     

Thermal and spectroscopic properties of zinc perchlorate/poly(vinylpyrrolidone) blends and a comparison with related hydrogen bonding systems

We have investigated the thermal and spectroscopic properties of blends of poly(vinylpyrrolidone) (PVP) with zinc perchlorate. Analyses by differential scanning calorimetry indicates that blending with zinc perchlorate increases the values of T_g of PVP. We calculated the interaction strength of the zinc salt/PVP blends based on an extended configuration entropy model. The presence of ion-dipole interactions between PVP and the zinc salt was confirmed based on Fourier transform infrared (FTIR) and solid-state NMR spectroscopies, which suggest that the zinc cations coordinate with the carbonyl groups of PVP. The single value of measured by solid-state NMR spectroscopy observed for all the zinc salt/PVP blends is smaller than that of pure PVP, which is a finding that indicates that the domain size of this blend system decreases upon increasing the zinc salt content. Based on FTIR and solid-state NMR spectroscopic analyses, we conclude that the ion-dipole interactions in the zinc salt/PVP blend are stronger than the hydrogen bonds in systems such as the poly(vinylphenol) (PVPh)/PVP blend and the PVPh-co-PVP copolymer.     

Kinetics of absorption of carbon dioxide into aqueous blends of 2-(1-piperazinyl)-ethylamine and N-methyldiethanolamine

The kinetics absorption of CO₂ into aqueous blends of 2-(1-piperazinyl)-ethylamine (PZEA) and N-methyldiethanolamine (MDEA) were studied at 303, 313, and 323 K using a wetted wall column absorber. The PZEA concentrations in the blends with MDEA varied from 0 to to see the effect of PZEA as an activator in the blends with two different total amine concentrations (1.0 and ). Based on the pseudo-first-order condition for the CO₂ absorption, the overall second-order reaction rate constants were determined from the kinetic measurements. The kinetic rate parameters were calculated and presented at each experimental condition.     

Copolymer crystallization: Approaching equilibrium

Melt crystallization of random copolymers leads to solids with crystalline fraction w_c and final melting temperature that are substantially below the predictions of Flory's equilibrium crystallization theory. Model ethylene/butene random copolymers, when crystallized as multilayer films by rapid solvent evaporation, exhibit increased w_c (50% relative) and (4 K) compared to melt crystallized values. For a copolymer with 0.92 mol fraction ethylene, the density-derived crystallinity w_c=0.6 is the same as that from Flory's theory, although the maximum observable crystal thickness from remains about 25% of the theory value. These effects are seen because crystallization from solution occurs without many of the constraints to segment dynamics that limit crystalline fraction during melt crystallization. Crystal thickness is dominated by secondary nucleation barriers in both melt and solution. Chain or sequence folding is much more regular in the solution crystallized material, and amorphous layer thickness is reduced from about 8 nm to 3 nm.     

Exothermic oxidations in supercritical CO2: effects of pressure-tunable heat capacity on adiabatic temperature rise and parametric sensitivity

Many oxidation reactions, including H₂ combustion with O₂, have been shown to admit the phenomenon of parametric sensitivity. Given its inertness to oxidation and non-flammable nature, supercritical CO₂ (scCO₂) is a desirable solvent for performing oxidations. Further, for oxidations that employ H₂O₂ as an oxidant, the use of scCO₂ as a solvent has been suggested for producing H₂O₂ in situ by reacting H₂ and O₂. Another significant, and as yet not fully understood, advantage of using scCO₂ is the ability to exploit its liquid-like heat capacity, which exhibits a maximum in the near-critical region (1.01-1.2T_c and 0.9-2.0P_c). It is shown in this modeling study that by performing an oxidation reaction in scCO₂, the temperature rise accompanying the highly exothermic reaction can be effectively controlled. To demonstrate this concept, we simulated the maximum temperature rise (ΔT_ad) for H₂ combustion with O₂ in CO₂ in a constant-pressure adiabatic reactor, at feed temperatures ranging from 300 to and reactor pressures from 1 to . At a feed temperature of , a five-fold reduction in ΔT_ad value (from 209 to ) is predicted by tuning the operating pressure from 1 to . In contrast, the ΔT_ad in N₂ medium is relatively insensitive in the 1- pressure range and is six times greater (roughly ) compared to the value predicted with CO₂ medium at . Further, the values of β (the dimensionless temperature rise parameter) may also be sensitively tuned with pressure in the near-critical region such that parametric sensitivity is minimized. These results indicate that the liquid-like heat capacities of scCO₂ may be exploited to control the adiabatic temperature rise and to ameliorate parametric sensitivity during exothermic reactions, a problem of fundamental and practical significance.     

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, , in the unbaffled tank that can be utilised without severe air incorporation is compared to with baffles. However, at this lower , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε_T gave values of at every feed position where measurement was undertaken. Since the spatially averaged value of φ=1, this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε_T at the feed point may not be valid for such swirling flows.     

Dynamic rheological behavior and morphology near phase-separated region for a LCST-type of binary polymer blends

The dynamic rheological properties and morphology in the vicinity of phase-separated region for poly(methyl methacrylate) (PMMA)/poly(styrene-co-acrylonitrile) (SAN) blends with lower critical solution temperature (LCST) behavior were investigated. When temperature was above the phase separation temperature, i.e. cloud point (T_c) for some PMMA/SAN blends, the slope of plotting versus decreased at low frequencies (terminal region), indicating the appearance of phase-separation and existence of heterogeneous structure. We employed a model dealing with complex modulus of the two phases mixture proposed by Kopnistos et al. for describing the dynamic rheological behaviors of PMMA/SAN blends, according to the assumption that the interfacial tension between the matrix and the dispersed phase was independent of local shear and variation of interfacial area, and that the dispersed spherical droplets were nearly monodispersed. It is found that the predicted results were in qualitative agreement with the experimental data of this study. The ratio of interfacial tension α to the size of dispersed phase R, α/R, was obtained for 80/20 and 60/40 PMMA/SAN blends, and the two different morphology were also observed.     

Synthesis, characterization and thermodynamic properties of poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone)

Poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone) P(MHPMA-co-VP) was synthesized in 1, 4-dioxane solution using benzoyl peroxide (BPO) as initiator at 60 °C. The copolymer was characterized by ¹H ¹³C NMR, FT-IR, DSC, TGA, size exclusion chromatography analysis (SEC) and elemental analysis techniques. According to SEC, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of PMHPMA-co-VP were found to be 58,000, 481,000 g/mol and 8.26, respectively. According to TGA, carbonaceous residue value of PMHPMA-co-VP was found to be 6% at 500 °C. Also, some thermodynamic properties of PMHPMA-co-VP such as the adsorption enthalpy, ΔH_a, molar evaporation enthalpy, ΔH_v, the sorption enthalpy, , sorption free energy, , sorption entropy, , the partial molar free energy, , the partial molar heat of mixing, , at infinite dilution was determined for the interactions of PMHPMA-co-VP with selected alcohols and alkanes by inverse gas chromatography (IGC) method in the temperature range of 323-463 K. According to the specific retention volumes, , the weight fraction activity coefficients of solute probes at infinite dilution, , and Flory-Huggins interaction parameters, between PMHPMA-co-VP-solvents were determined in 413-453 K. According to and , selected alcohols and alkanes were found to be non-solvent for PMHPMA-co-VP at 413-453 K. The glass transition temperature, T_g, of the PMHPMA-co-VP found to be 370 and 363 K, respectively, by IGC and DSC techniques, respectively.