Heterocyclic Polymer Networks Research Articles

Highly stable CO2/N2 and CO2/CH4 selectivity in hyper-cross-linked heterocyclic porous polymers.

The largest obstacles for landfill/flue gas separation using microporous materials are small adsorption values and low selectivity ratios. This study demonstrates that these adsorption and selectivity challenges can be overcome by utilizing a series of hyper-cross-linked heterocyclic polymer networks. These microporous organic polymers (MOPs) were synthesized in a single step by inexpensive Friedel-Crafts-catalyzed reactions using dimethoxymethane as an external linker. The amorphous networks show moderate Brunauer-Emmett-Teller surface areas up to 1022 m(2) g(-1), a narrow pore size distribution in the range from 6 to 8 Å, and high physicochemical stability. Owing to the presence of the heteroatomic pore surfaces in the networks, they exhibit maximum storage capacities for CO2 of 11.4 wt % at 273 K and 1 atm. Additionally, remarkable selectivity ratios for CO2 adsorption over N2 (100) and CH4 (15) at 273 K were obtained. More importantly, as compared with any other porous materials, much higher selectivity for CO2/N2 (80) and CO2/CH4 (15) was observed at 298 K, showing that these selectivity ratios remain high at elevated temperature. The very high CO2/N2 selectivity values are ascribed to the binding affinity of abundantly available electron-rich basic heteroatoms, high CO2 isoteric heats of adsorption (49-38 kJ mol(-1)), and the predominantly microporous nature of the MOPs. Binding energies calculated using the high level of ab initio theory showed that the selectivity is indeed attributed to the heteroatom-CO2 interactions. By employing an easy and economical synthesis procedure these MOPs with high thermochemical stability are believed to be a promising candidate for selective CO2 capture.

Subglass Cooperative Mechanical Relaxations and Activation Entropy in Heterocyclic Polymer Networks

Measurements of differential scanning calorimetry, dynamic mechanical analysis, and dilatometry have been performed in heterocyclic polymer networks (HPNs), whose effective network density has been gradually varied keeping the overall chemical structure essentially unchanged. Evidence of a growing intermolecular cooperativity for a local relaxation motion is offered by the investigation of the subglass mechanical β-relaxation, whose frequency factor and apparent activation energy strongly increase with increasing cross-linking density from about 1015 s−1 and 44.2 kJ/mol to about 1019 s−1 and 69.8 kJ/mol. The analysis of the characteristics of the mechanical β-relaxation suggests the existence of cooperative transitions of consecutive relaxing units, mainly driven by the crank-shaft motion of the network junctions between the isocyanurate heterocycles building up the structure. Comparison with previous results concerning the dielectric β-relaxation evidence a less cooperative character and a different microscopic origin for the conformational transitions driving the dielectric process. Moreover, a decrease in the thermodynamic fragility with increasing network density has been revealed, which contrasts with the increasing dynamic fragility, as measured by dielectric and mechanical probes.

Secondary relaxations in heterocyclic polymer networks: A study of mechanical spectroscopy

Experiments of differential scanning calorimetry and dynamic mechanical spectroscopy on linear and cross-linked heterocyclic polymer networks (HPNs) have been performed over the temperature range between 120 and 400 K. The temperature dependences of the storage modulus E′ and the internal friction Q −1 of HPNs, investigated by mechanical waves of frequencies ranging between 0.3 and 30 Hz, reveal anelastic behaviors which are mainly governed by the secondary β- and primary α-relaxations. The addition of cross-links in an apparently linear polymer network was accompanied by the shift of the glass transition to higher temperatures. Differently from the α-relaxation whose strength decreases with cross-linking, the β-relaxation strength increases. This observation implies the lack of any dynamical hierarchy between the local and cooperative segmental dynamics in these systems. The β-relaxation follows the Arrhenius law with values of the characteristic frequency ω 0 and the activation energy E act ranging between 10 15 and 10 16 s −1 and 43.7 and 48.7 kJ/mole, respectively. In both the HPNs the β-loss peaks have been quantitatively analyzed by the symmetric double-well potential (SDWP) model.

Low temperature specific heat of heterocyclic polymer networks: Effect of network density

Measurements of low frequency Raman scattering and low temperature specific heat between 2 and 20 K have been performed in two glassy, heterocyclic polymer networks. The effective network densities of heterocyclic polymer networks are varied by changing the ratio of bi- and mono-functional isocyanate monomers, leaving the overall chemical structure essentially unchanged. A boson peak at ∼ 20 cm−1 characterizes the Raman spectra at room temperature of both samples. Below 10 K, the specific heats deviate from a cubic temperature dependence, as predicted by the Debye theory, and reveal an excess specific heat, having the shape of a well-defined peak in a C p/T 3 plot with a maximum at ∼ 5 K. The increase in effective network density is accompanied by a slight decrease in excess specific heat. These observations have been explained in terms of additional low-energy vibrations associated with the monomers building the polymer networks.

Structure‐Property Relationships for Model Heterocyclic Polymer Networks: Effect of Monomer Functionality

The effect of initial monomer functionality on the final properties of densely cross‐linked heterocyclic polymer networks (HPN) was evaluated from comparative studies of the viscoelastic behavior and enthalpy relaxation in the glass transition range of polyisocyanurates prepared from a bifunctional monomer D (1,6‐hexamethylene diisocyanate, HDI) and a trifunctional monomer T (the isocyanurate of HDI). Quantitative analysis of these data suggested that an HPN prepared from the monomer T was chemically more heterogeneous and possessed a lower effective network density (presumably, as a result of a lower mobility of the initial monomer T compared to monomer D).

Structure‐Property Relationships for Model Heterocyclic Polymer Networks: Effect of Network Density

Chain motion dynamics around and through the glass transition region of a series of heterocyclic polymer networks (HPN) were characterized by dynamic mechanical spectroscopy and differential scanning calorimetry. The effective network densities of those HPN prepared by simultaneous trimerization of a bifunctional (1,6‐hexamethylene diisocyanate, HDI) and a monofunctional (hexyl isocyanate, HI) monomers were systematically varied by changing the HDI/HI ratio in the reaction mixture, whereas their overall chemical structure remained essentially unchanged. The increasing difference between the experimental and theoretical values of the mean molar masses of chain strands between network junctions, the lower the HDI/HI ratio, was assumed to be the result of deviations from the exact chemical stoichiometry due to subsidiary reactions between isocyanate groups of the same monomer. The increase of the effective network density was accompanied by narrowing of the glass transition interval and by its systematic shi...

Relationships between conductivity and local topology in heterocyclic polymer networks.

The low frequency complex dielectric relaxation above the glass transition temperature T(g) for a series of well-characterized heterocyclic polymer networks has been analyzed in terms of the electric moduli formalism. It was established that the contribution of ionic conductivity to the electric modulus can be quantitatively separated from the alpha relaxation by using a combination of two Havriliak-Negami (HN) functions. A strong correlation between the mechanisms of both conductivity and segmental mobility was inferred from the similarity of the shape of the HN function for conductivity relaxation to those for the main relaxation. This correlation is further supported by the similarity of the temperature dependencies of the relevant relaxation times corresponding to both processes. The overwhelming contribution of the preexponents D0 in the Arrhenius behavior of the apparent diffusion coefficients can be explained by considering a model implying decreased mean free paths of the diffusing elements and lower activation entropies of diffusion for polymer networks with higher apparent network densities.

Probing the subglass relaxation behavior in model heterocyclic polymer networks by dielectric spectroscopy.

The subglass relaxation (beta) in model heterocyclic polymer networks (HPNs) with a controlled ratio of trimerized mono- and diisocyanates was characterized by dielectric spectroscopy in the frequency domain. The beta relaxation in the investigated HPNs follows the Arrhenius law with unusually low values of the preexponential factor (10(-17)<tau(beta0)<10(-15) s). However, little influence of the local environment, as characterized by the network density, on the apparent activation energies DeltaE(beta) is observed. This fact, combined with their fairly low absolute values (50.4-58.3 kJ/mol), were considered as typical of a noncooperative relaxation in loosely packed regions of a glassy quasilattice. Both the intensity and dielectric strength of the beta relaxation in HPNs increase with increasing apparent network density (i.e., with lower ratios of linear and network structures in the system, L/N). This effect was explained by a model assuming that the total, composition-invariant, free volume available was distributed between densely packed domains comprising linear, two-arm isocyanurate heterocycles (ISHs) and loosely packed, three-arm ISHs, which form continuous, three-dimensional network structures. The experimental data for HPNs confirm Ngai's correlation between the logarithm of the secondary beta-relaxation time and the Kohlrausch-Williams-Watts stretching exponent for the primary alpha relaxation. It is suggested that the absence of conjugated bonds within isocyanurate heterocycles makes them sufficiently flexible to allow for specific conformational transitions, like the "chair-boat-chair" transition in the structurally similar cyclohexyl ring.

Influence of cross-linking on the segmental dynamics in model polymer networks

In an attempt to study the specific influence of cross-linking on the α relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10−1 Hz&amp;lt;F&amp;lt;105 Hz frequency range and in the 123 K&amp;lt;T&amp;lt;493 K temperature interval. The α relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the α-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme clearly shows that fragility increases as polymer network develops.

Anharmonicity and fragility in semi-interpenetrating polymer networks

Two series of semi-interpenetrating polymer networks (semi-IPN) based on the same linear polyurethane and two different heterocyclic polymer networks were characterized in terms of the complex dynamic modulus and the mechanical loss tangent, measured between 150 and 500 K at a frequency varying in the range between 0.3 and 30 Hz. The fragility and the deviations from exponentiality of the dynamics of the segmental motion of semi-IPNs in the glass transition interval have been obtained by modelling the mechanical a-relaxation. It was observed that the incorporation of linear polyurethane softens the heterocyclic polymer networks, lowers their resistance to the temperature-induced structural degradation, and increases the complexity of the long-range segmental dynamics. There has also been found a correlation between the fragility and the anharmonicity of the system, the most fragile polymer being characterized by the largest degree of anharmonicity. By extending the analysis to linear polymers, it was established that a growing anharmonicity is predictive of an increasing fragility.

Structure-Microhardness Relationship in Semi-interpenetrating Polymer Networks

Two series of single-phase, semi-interpenetrating polymer networks (semi-IPNs) based on the same linear polyurethane (LPU) and two different heterocyclic polymer networks (HPNs), respectively, were characterized by the room temperature microhardness H measurement. It is shown that the H value linearly increases with both the mass content of the stiffer component (HPN) and the glass transition temperature of a semi-IPN. The latter dependence can be justified on the assumption that the excess enthalpy of the glass with respect to a hypothetical melt state is a measure of the yield strength of the glassy quasi lattice.

Structure–property relationships for cyanurate-containing, full interpenetrating polymer networks

Full sequential interpenetrating polymer networks (seq-IPN) of cross-linked polyurethane (CPU) and heterocyclic polymer networks (HPN) based on thermally cured dicyanic ether of Bisphenol A (DCE) were characterized by small-angle X-ray diffraction, dynamic mechanical analysis, stretching calorimetry and microhardness measurements. Neat CPU was shown to be a microphase-separated system characterized by a regular, three-dimensional macrolattice of network junctions, embedded in uniform-size microdomains of stiff chain fragments which spanned the continuous matrix of soft chain fragments. In contrast, no large-scale structural heterogeneities were detected in the HPN. The X-ray long spacing (L), the degree of microphase segregation (DMS), the α-relaxation temperature and the mechanical properties (elastic modulus and microhardness) were studied as a function of HPN content. Results are explained in the light of a model that discusses the maximum degree of CPV swelling by molten DCE as a function of composition. It is suggested that predominantly chemical interactions between the molten DCE and the stiff chain fragment microdomains, reinforcing primary physical interactions, are responsible for the observed transition at 40% HPN content to a more homogeneous phase morphology of seq-IPNS.

Dielectric relaxation of heterocyclic polymer networks: Effect of the ratio and nature of the heterocyclic rings

Heterocyclic Polymer Networks (HPN) prepared by trimerization of 1,6-hexamethylene diisocyanate (HMDI) in presence of diglycidyl ether of bisphenol A (DGEBA) and triethylene diamine in two different regimes were characterized by dielectric spectroscopy in the 10−1–105 Hz frequency range and in the 123 K–423 K temperature interval. The dielectric experiments reveal the existence of multiple composition dependent relaxation processes. The γ process, appearing at low temperatures only in partially cured systems is related to the presence of free DGEBA. The β process is associated to local motions of the carbonyl groups present in the isocyanurate and oxazolidone cycles. The α relaxation, which has its origin in the segmental motions above the glass transition temperature, tends to appear at higher temperatures with increasing network junction density and with the degree of curing. The fragility and cooperativity of the α relaxation is discussed in terms of recent relaxation approaches.

Thermal characterization of the glass transition of heterocyclic polymer networks

Heterocyclic polymer networks obtained by copolymerization of an epoxy oligomer and hexamethylene diisocyanate (HMDI) in different proportions have been studied by differential scanning calorimetry (DSC) in their glass transition regions. The glass transition temperature and the specific heat capacity increment at the transition correlate well with sample composition, whereas the index of the distribution of relaxation times seems to be unaffected by composition changes. The structural relaxation process is characterized with an equation for the configurational entropy, whose material parameters are determined by a fit to a wide class of thermal histories.

Relaxation in semi-interpenetrating polymers network of linear polyurethane and heterocyclic polymer networks

New semi-interpenetrating polymers (semi-IPN) of linear polyurethane (PU) and heterocyclic polymer networks prepared by trimerized dicyanate of Bisphenol-A (TDCE) have been analyzed by dynamic mechanical spectroscopy (DMS) in the 130–550 K temperature and 0.3–30 Hz frequency ranges. Single mechanical α a-relaxations suggest a large-scale homogeneity ascribed to the affinity between TDCE and PU. Below the glass transition temperature, the interpenetration affects the local molecular motions as a consequence of modifications in the mutual local environments of pure components.

Kinetics of Formation of Heterocyclic Polymer Networks. 3. Catalyst Effect

Non-isothermal trimerization kinetics of 1,6-hexamethylene diisocyanate (HMDI) in the reaction mixture with epoxy resin (ER) at a fixed ratio HMDI/ER = 85/15 (mass) catalyzed by differential concentrations of triethylene amine (TEA) was studied by differential calorimetry at four heating rates. In contrast to the simple single-step models, the isoconversional approach proved to describe quantitatively the reaction kinetics over the entire range of conversions, The apparent activation energies of the curing process and the corresponding pre-exponential factors tended to decrease smoothly, the higher the conversion, whereas a definite excess of the amine component turned out to be necessary to arrive at the maximum reaction rate.

Kinetics of Formation of Heterocyclic Polymer Networks. 4. Storage Time Effect

The effect of the time of preliminary storage at room temperature on the kinetics of HMDI trimerization at the fixed HMDI/ER ratio (85/25), and different concentrations of the catalyst, TEA, and the evolution of glass transition parameters of the end products in function of the curing reaction regime, were studied by calorimetry. It was established that both the activation energy Eα and the pre-exponential factor ln A increased, the longer the storage time, and both tended to decrease with the conversion α1 The time for the onset of gelation tgel decreased, the higher the catalyst concentration and/or the longer the preliminary storage time the former proved to be a major factor controlling tgel|. The glass transition temperatures Tg for post-cured samples of series HT “internally diluted” with stiff oxazolidone heterocycles were systematically higher than those for samples of series LT cured at 393 K whatever the catalyst concentration and/or the storage time.

Kinetics of Formation of Heterocyclic Polymer Networks. 3. Catalyst Effect

Non-isothermal trimerization kinetics of 1,6-hexamethylene diisocyanate (HMDI) in the reaction mixture with epoxy resin (ER) at a fixed ratio HMDI/ER = 85/15 (mass) catalyzed by differential concentrations of triethylene amine (TEA) was studied by differential calorimetry at four heating rates. In contrast to the simple single-step models, the isoconversional approach proved to describe quantitatively the reaction kinetics over the entire range of conversions, The apparent activation energies of the curing process and the corresponding pre-exponential factors tended to decrease smoothly, the higher the conversion, whereas a definite excess of the amine component turned out to be necessary to arrive at the maximum reaction rate.

Kinetics of Formation of Heterocyclic Polymer Networks. 4. Storage Time Effect

The effect of the time of preliminary storage at room temperature on the kinetics of HMDI trimerization at the fixed HMDI/ER ratio (85/25), and different concentrations of the catalyst, TEA, and the evolution of glass transition parameters of the end products in function of the curing reaction regime, were studied by calorimetry. It was established that both the activation energy Eα and the pre-exponential factor ln A increased, the longer the storage time, and both tended to decrease with the conversion α1 The time for the onset of gelation tgel decreased, the higher the catalyst concentration and/or the longer the preliminary storage time the former proved to be a major factor controlling tgel|. The glass transition temperatures Tg for post-cured samples of series HT “internally diluted” with stiff oxazolidone heterocycles were systematically higher than those for samples of series LT cured at 393 K whatever the catalyst concentration and/or the storage time.

Molecular Mobility in Semi-IPNs of Linear Polyurethane and Heterocyclic Polymer Networks

A study of the thermal and mechanical properties of new semi-interpenetrating polymer networks (IPN's) based on linear polyurethane (PU) and crosslinked trimerized dicyanate (TDC) reveals the existence of structures characterized by the absence of chemical interactions. Two distinct glass transitions are observed in the thermograms, as an indication of the fact that the two polymeric components preserve their molecular structure. The interpenetration affects markedly the glass transition temperatures revealed in the pure components in consequence of modifications in the local environments of the relaxing molecular units in the two phases. The primary and secondary relaxations of these systems show features which can be explained by accounting for the free-volume decrease due to the inclusion of PU in the network of TDC. Below the glass transition two molecular relaxations have been observed which have been ascribed to the secondary relaxation motions characterizing each polymeric component. Both relaxations exhibit a marked non-exponentiality which has been well accounted for in terms of a Gaussian distribution of relaxation times. The results of this analysis suggest that the local motions of TDC, ascribed to phenylene groups in the crosslinks between the cyanate units, are severely restricted by the inclusion of PU, while those of PU are slightly influenced by the presence of TDC.