Segmental and crosslink point motion in networks

¹³C- and ³¹P-NMR spin lattice relaxation in the rotating frame have been measured on a series of networks prepared from monodisperse and deliberately bimodal poly(propylene glycols) (PPG) crosslinked with tris(4-isocyanatophenyl) thiophosphate. The T_1pC minima correspond to loss maxima in the DMTA (Dynamic Mechanical Testing) measured at 10Hz. The T_1p^P minima fall at higher temperatures than those of T_1p^C for the same network indicating that these crosslinks lag the segments in frequency of motion at a given temperature. The carbon relaxation is biphasic below T_g of the segments indicating two relaxation domains which we assign to bulklike PPG segments and PPG segments proximal to he crosslink. Lineshape analysis by a diffusional model indicates crosslink reorientation is not isotropic until well above T_g. Relaxation and lineshapes for the bimodal networks indicate that junctions are not uniformly plasticized by the segments.     

Glass transition temperature for epoxy-diamine networks

The glass transition temperature of systems based on epoxy resin and a number of diamines has been determined by using a torsion pendulum. An equation relating composition and crosslink density with the glass transition temperature has been established which gives reasonable predictions of the glass transition temperatures for systems based on aliphatic or aromatic amines and methylated amines and for systems containing a monofunctional epoxy diluent. The equation may be used to predict T_g for systems with non-stoichiometric quantities of curing agent and blends of amines. Deviation of the predicted and observed values for T_g is interpreted in terms of differences between definitions of T_g used by other workers and, also the occurrence of competing side reactions during polymerization which lead to additional crosslinks.     

The relationship between crosslink system, network structure and material properties of carbon black reinforced EPDM

The crosslink density and sulfur-ranks of crosslinks formed during vulcanization of a carbon black reinforced ENB–EPDM compound are analyzed as a function of the selected curing system: Conventional, Semi-Efficient, Efficient and Nitrosamine-safe. Each vulcanization system results in a specific crosslink concentration and sulfur-rank distribution: mono-, di- and polysulfidic of nature. Tensile properties, tear strength and compression set of the vulcanized materials turn out to practically only depend on overall crosslink density, as resulting from the particular curing systems and vulcanization times. All trends in properties coincide when plotted as a function of the overall crosslink density. Surprisingly, the crosslink distribution: the ratios of mono- to di- and polysulfidic crosslinks, has only a minor effect on these properties. The differences in sulfur-rank as a function of the chosen vulcanization system turn out to be too small for EPDM to have a significant effect.     

Investigation of molecular motions of crosslinked polyepichlorohydrin by nuclear magnetic resonance spectroscopy

The molecular motion of crosslinked polyepichlorohydrin (PECH) is studied qualitatively by NMR techniques. The results of temperature dependence of ¹H T₂ and T₁ indicate that the crosslinking (crosslink density < 3%) restricts molecular motions of the polymer even far above its T_g. The ¹H T₁ minimum, corresponding to the large-scale chain-motion of crosslinked PECH, shifts to higher temperatures with increasing crosslink density. ¹H T₂ data also show that the crosslinking hinders free chain motions of the polymer above its T_g. The ¹³C T₁ relaxation time is sensitive to such motional changes as well. ¹³C linewidths of crosslinked PECHs vary with the crosslink density in both the swollen state and the solid state. The mechanism of ¹³C linewidth broadening of crosslinked polymers is discussed in detail. In the case of PECH, the linewidth broadening is caused by changing molecular environment due to crosslinking (such as presence of various chemical shift structures and freezing effects in conformational environment as chain mobility decreases), rather than increasing correlation times, which shorten the relaxation time (T₂) of polymer chains. © 1994 John Wiley & Sons, Inc.     

Structure and property relationships in model diglycidyl ether of bisphenol‐a and diglycidyl ether of tetramethyl bisphenol‐a epoxy systems. I. Mechanical property characterizations

Model epoxy networks, with variations in crosslink density and in epoxy monomer rigidity, were prepared to study how the network structure affects modulus, T_g, and toughness/toughenability of epoxy resins. Diglycidyl ether of bisphenol‐A and diglycidyl ether of tetramethyl‐bisphenol‐A, along with the corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties of epoxies. The present study indicates that, as expected, the backbone rigidity of the epoxy network, not the crosslink density alone, will strongly influence modulus and T_g of epoxy resins. Upon rubber toughening, it is found that the rigidity of the epoxy backbone and/or the nature of the crosslinking agent utilized are most critical to the toughenability of the epoxy. That is, the well‐known correlation between toughenability and the average molecular weight between crosslinks (M_c) does not necessarily hold true when the nature of epoxy backbone molecular mobility is altered. The potential significance of the present findings for a better design of toughened thermosets for structural applications is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2137–2149, 1999     

Infrared spectroscopic studies of polymer transitions. III. Effects of sodium ion on glass transitions in styrene-based ionomers

The glass transition in styrene-based ionomers was investigated by means of infrared spectroscopy and differential scanning calorimetry (DSC). Transition temperatures were determined from the temperature dependence of the peak absorbances of the 1700 and 1745 cm^?1 bands. These transition temperatures agreed with glass transition temperatures (T_g) determined by DSC. With increasing degree of ionization, T_g and the enthalpy ΔH of the residual intermolecular hydrogen bonding increased. The values of T_g obtained were analyzed by the theory of Fox and Loshaek for the effect of crosslinks. It is concluded that sodium ions probably from ionic domains and act as crosslinks to reinforce the residual hydrogen bonding and may increase T_g. The absorbance at 1560 cm^?1 (ν_COO^?) did not change at T_g. This suggests that the glass transition observed here is not due to the onset of the mobility in ionic domains, as has been proposed for ethylene-based ionomers on the basis of dielectric measurements.     

The physical properties of bisphenol-a-based epoxy resins during and after curing. II. Creep behavior above and below the glass transition temperature

The creep behavior of a series of fully cured epoxy resins with different crosslink densities was determined from the glassy compliance level to the equilibrium compliance J_e at temperatures above T_g and at the glassy level below T_g during spontaneous densification at four aging temperatures, 4,4-diamino diphenyl sulfone DDS was used to crosslink the epoxy resins. The shear creep compliance curves J(t) obtained with materials at equilibrium densities near and above T_g were compared at their respective T_gs. T_gs from 101 to 205°C were observed for the epoxies which were based on the diglycidyl ether of bisphenol A. Creep rates were found to be the same at short times, and equilibrium compliances J_e were close to the predictions of the kinetic theory of rubberlike elasticity. Time scale shift factors determined during physical aging were reduced to T_g. At compliances below 2 × 10^?10 cm²/dyn, Andrade creep, where J(t) is a linear function of the cube root of creep time, was observed. The time to reach an equilibrium volume at T_g was found to be longer for the epoxy resin with lower crosslink densities. The increase of density during curing is illustrated for the epoxy resin with the highest crosslink density.     

Novel biphasic structured composite prepared by in situ silica filling in natural rubber latex

The sol‐gel reaction of tetraethoxysilane in natural rubber (NR) latex was conducted to produce in situ silica‐filled NR latex, followed by adding sulfur cross‐linking reagents to the latex in a liquid state. The latex was cast and subjected to sulfur curing to result in a unique morphology in the NR composite of a flexible film form. The contents of in situ silica filling were controlled up to 35 parts per one hundred rubber by weight. The silica was locally dispersed around rubber particles to give a filler network. This characteristic morphology brought about the composite of good dynamic mechanical properties. Synchrotron X‐ray absorption near‐edge structure spectroscopy suggested that the sulfidic linkages of the sulfur cross‐linked composites were polysulfidic, S_x (x ≥ 2), and a fraction of shorter polysulfidic linkages became larger with the increase of in situ silica. The present observations will be of use for developing a novel in situ silica‐filled NR composite prepared in NR latex via liquid‐phase soft processing. Copyright © 2011 John Wiley & Sons, Ltd.     

Studies on diphenylguanidine–accelerated sulfur vulcanization of styrene butadiene rubber in the presence and absence of dicumylperoxide

Diphenylguanidine (DPG) raises the rate of decomposition of dicumylperoxide (DCP) and the crosslinking maxima due to DCP is lowered to some extent by DPG. When the molar proportion of DPG–S is approximately 1:1, no additive results of crosslink formation (as reported for NR) are observed. Zinc oxide and stearic acid increase the rate of crosslinking as well as the crosslinking maxima. In the present study it appears most probable that DPG-accelerated sulfuration of SBR is an ionic process. At a constant level of DCP and sulfur crosslink density increases when the amount of DPG is increased; a constant level of DCP and DPG crosslinking increases with rising sulfur concentration. An increase in the concentration of DPG or sulfur leads to greater formation of the complex as well as a change in its composition. The effect of DPG is more pronounced, for by reducing the number of sulfur atoms more sulfur is available for crosslinking. Sulfur absorbs little SO₂ or H₂S; if it is already saturated, there is no perceptible effect. DPG does absorb H₂S or SO₂ and the rate and maxima of crosslinks is increased. The effect of SO₂ is higher because of the higher K_a values of H₂SO₃ and consequently higher concentration of HSO₃^?. Reversion is a free-radical process inhibited by free DPG present in the system. In the presence of zinc oxide and stearic acid the reaction follows a polar mechanism as well as a radical mechanism.     

Determination of charge density of anionic polyacrylamide flocculants by NMR and DSC

New methods are suggested for the determination of the charge density of acrylamide/acrylate copolymers.¹³C nuclear magnetic resonance spectroscopy was used to determine the comonomer ratio by comparing the peak intensities of the methine carbon in acrylamide and acrylate monomers. Results were compared with those obtained by conductometric and potentiometric titration and were found to be in good agreement. Differential scanning calorimetry was employed to determine the glass transition temperatures (T _g) of the copolymers. A master curve was established by plottingT _g versus charge density of the copolymers previously determined by NMR and conductometric titration. Compositions of poly(acrylamide-co-acrylate) samples can thus be determined by measuring theT _g and reading the percent composition directly from the master curve.     

Radiothermoluminescence. I. Radiothermoluminescence for investigation of thermal and radiation-induced crosslinking of rubber mixtures

Phase heterogeneity of vulcanizates based on cis-polybutadiene (cis-Europrene) and SBR (Europrene 1500) blends obtained by radiation, thermal, two-stage radiation-thermal, and two-stage thermal-radiation processes was investigated by the radiothermoluminescence method. Unvulcanized blends of these polymers are found to be heterogeneous, exhibiting two glass transition temperatures T_g, which coincide with the values for the initial components. Following vulcanization, the difference between the two T_g values for a polyblend decreases. The decrease is more distinct for polyblends crosslinked at high temperatures. When the vulcanization temperature exceeds 140°C and the crosslink density (or 1/M_c) exceeds 5 × 10^?5–1 × 10^?4 mole/cm³, the glass temperature ranges of the crosslinked blend are practically superimposed. Such crosslinked mixtures should be considered pseudohomogeneous in phase.     

Influence of the composition on the glass transition temperature of polyurethane and polyurethane acrylate networks

Linear polyurethane, linear segmented polyurethane, polyurethane networks, and polyurethane acrylate networks of various composition were synthesized. The variation of T_g with the type of macrodiol, its length, and the chemical composition of the polymer were studied in relation with the percentage of soft segments, the molar mass between crosslinks, and the concentration of urethane bonds. In this work, the networks were considered as composed of chain segments of various composition between point-like crosslinks. The chemical heterogeneities of the networks were not taken into account. For polyurethanes, it was shown that T_g values are essentially controlled by the amount of urethane bonds. For polyurethane acrylates, the T_g values are dependent on the amount of urethane bonds but also on the presence of crosslinks whose number is varying with the excess of diisocyanate of the first step three times faster for PUA compared with PU. No clear relation was observed between T_g and the molar mass between point-like crosslinks. Another approach considering the network heterogeneities is indispensable and will be used in a following work. © 1996 John Wiley & Sons, Inc.     

Poly(p-xylene disulfide) and poly(p-xylene tetrasulfide): synthesis,cure and investigation of mechanical and thermophysical properties

Abstract

In this work, two polysulfide polymers were synthesized using aromatic organic monomer (α,α′-dichloro-p-xylene) and sodium disulfide (Na₂S₂) and sodium tetrasulfide (Na₂S₄) aqueous monomers. Then, the curing process of the polymers was carried out at 170° C using a rheometer. The structural characteristics of synthesized and cured samples were identified by Raman and Fourier transform infrared (FT-IR) spectroscopies. Also, morphological and thermophysical properties of samples were studied by using the X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. Moreover, the molecular weight of the synthesized samples was determined by proton nuclear magnetic resonance (¹H NMR). Furthermore, the mechanical properties and hardness of the samples were investigated by tensile test and Shore A. The results showed that in the noncured samples during the increase of sulfur in the polymer structure, solubility was increased whereas it decreased the hardness, melting point (T_m ) and glass transition temperature (T_g ) of polymers. But in cured samples, hardness and T_g increase by increasing sulfur and the mechanical properties also improved. This is due to the increase in crosslinks. Also, Tm and solubility are not observed due to the formation of crosslinks.     

Effect of crosslink density on the pressure relaxation response of polycyanurate networks

The effect of crosslink density on the pressure-volume-temperature (PVT) behavior and on the pressure relaxation response for two polycyanurate networks is investigated using a custom-built pressurizable dilatometer. Isobaric cooling measurements were made to obtain the pressure-dependent glass transition temperature (T_g). The pressure relaxation studies were carried out as a function of time after volume jumps at temperatures in the vicinity of the pressure-dependent T_g, and the pressure relaxation curves obtained were shifted to construct master curves by time-temperature superposition. The reduced pressure relaxation curves are found to be identical in shape and placement, independent of crosslink density, when T_g is used as the reference temperature. The horizontal shift factors used to create the master curves are plotted as a function of the temperature departure from T_g (T − T_g), and they agree well with their counterparts obtained from the shear response. Moreover, the retardation spectra are derived from bulk compliance and compared to those from the shear. The results, similar to our previous work on polystyrene, indicate that at short times, the bulk and shear responses have similar underlying molecular mechanisms; however, the long-time mechanisms available to the shear response, which increase with decreasing crosslink density, are unavailable to the bulk response. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2477–2486, 2009     

Strain-induced post-curing of acrylate networks

The mechanical properties and network structure of photocurable polymers are strongly dependent on processing conditions. Here it is reported that highly crosslinked acrylate systems undergo unexpected additional post-curing during DMTA measurements, resulting in an increase in glass-transition temperature (T_g). A detailed study of the conditions under which this increase in T_g takes place unequivocally shows that a small (0.1%) oscillatory strain applied above T_g is responsible for additional cross-linking reactions. The effect of strain-induced post-curing is confirmed by applying post-curing treatments under oscillatory shear strain in rheological tests. Different acrylate systems were characterized and the results show that the strain induced post-curing depends on the network structure of the polymer. In polymer networks with an initial high crosslink density the effect is pronounced while in polymers with an initial lower crosslink density no shift in T_g is observed.     

NMR relaxation study of crosslinked cis-1,4-polybutadiene

Proton relaxation measurements have been used to investigate the effects of crosslinking on the segmental motion in cis-1,4-polybutadiene samples. The temperature dependence of proton spin–lattice relaxation time T₁ and spin–spin relaxation time T₂ at 60 and 24.3 MHz are reported in cis-1,4-polybutadiene (PB) samples with different crosslink density including uncrosslinked PB and samples with 140, 40, and 14 repeat units between crosslinks. In addition, spin-lattice relaxation times in rotating coordinate frame, T_1p, have also been determined. The relaxation data are interpreted in terms of the effects of crosslinks on segmental chain motions. Because of their sensitivity to low-frequency motion, T₂ data are of major interest. At temperatures well above the T₁ minimum the small T₂ temperature dependence resembles solidlike behavior reflecting the nonzero averaging of dipolar interactions due to anisotropic motion of the chain segments between crosslinks. The magnitude of T₂ at 60°C is found to be proportional to the average mass between crosslinks.     

Effect of high-temperature curing on the crosslink structures and dynamic mechanical properties of gum and N330-filled natural rubber vulcanizates

The effects of high-temperature curing and overcuring on the cure characteristics, crosslink structure, physical properties and dynamic mechanical properties (DMPs) of gum and carbon black (N330) filled natural rubber (NR) vulcanizates cured with conventional (CV), semi-efficient (SEV) and efficient (EV) cure systems, which have about the same total crosslink densities under a moderate curing temperature of 150°C, were investigated. The gum NR vulcanizates cured with CV, SEV and EV curing systems have about the same glass transition temperature (T_g) and tan δ values below the temperature of about 0°C, but showed some apparent differences in the tan δ values increasing in the order CVG′ and tan δ values above T_g higher than those of the gum NR vulcanizates.

High-temperature curing and overcuring cause decreases to various extents in the cure plateau torque, Shore A hardness, 300% modulus and tensile strength, and lead to apparent changes in the DMPs. Typically, there is an increase in T_g of all three kinds of gum and N330-filled NR vulcanizates because of changes in the total crosslink densities and crosslink types. The CV vulcanizates show the most significant change in cure characteristics, physical properties and DMPs since the highest content of polysulfidic crosslinks appears in the CV vulcanizate, causing the highest level of reversion and having a dominant effect on the properties.     

Effects of the hydroxyl value of polyol in rigid polyurethane foams

Rigid polyurethane foams (RPUFs) have been fabricated from crude MDI (CMDI) and polypropylene glycols (PPGs) of various hydroxyl values (OHVs) of 300–600 with an environmentally friendly blowing agent (HFC 365mfc). The closed cell content, compression strength, and dimensional stability of the foam increased with increase in the OHV due to the increased crosslink density, which was evidenced from the increased glass transition temperature (T_g). The cream time, gel time, tack‐free time, and density of the foam showed a minimum at 500OHV. The decrease and increase around the minimum were, respectively, interpreted in terms of increased mixture mobility and extensive allophanate crosslinks, which retard gelling as well as foaming reactions. The thermal conductivity also showed a small minimum at 500OHV. Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamic mechanical and thermal characterization of high-performance polybenzoxazines

Two polybenzoxazines are cured in an autoclave from the polyfunctional benzoxazine monomers, 8,8′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) and 6,6′-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl) ketone. The density and tensile properties of these polybenzoxazines are measured at room temperature. Dynamic mechanical tests are performed to determine the T_g, crosslink density, and the activation enthalpy of the glass-transition process for these two polybenzoxazines. The effect of postcure temperature on the T_g of the polymers is investigated and discussed in terms of crosslink density. Fourier transform infrared (FTIR) spectroscopy is also applied for the molecular characterization of the curing systems. Thermal properties of these polybenzoxazines are studied in terms of isothermal aging and decomposition temperature via thermogravimetric analysis. These two polybenzoxazines show mechanical and thermal properties similar to or better than bismaleimides and some polyimides. They also show very high char yield after being carbonized in a nitrogen atmosphere. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3257–3268, 1999     

Viscoelasticity of crosslinked epoxy polymer in the transition region

The dynamic mechanical properties of highly crosslinked epoxyamine polymer networks with nonrandomly distributed crosslinks were investigated. The transition temperatures of these polymers can be correlated with the number of CH₂ groups between crosslink junctions in the aliphatic amine portions of the network. The steepness of the modulus-temperature curve is also a function of crosslink density. This is in contrast with the case of natural rubber crosslinked by sulfur or by electron irradiation, where the modulus-temperature curves have similar shapes although the glass transition temperature increases with the degree of crosslinking. An empirical distribution function, similar to the one used by Tobolsky for stress relaxation distributions, was used to describe the temperature dispersion of the dynamic moduli. Two parameters, h_g and h_r, are used to characterize the steepness of the dispersion curve below and above the transition temperature, respectively. It is tentatively concluded that h_g correlates with the length of the CH₂ sequences in the amine portion of the polymer. The quantity h_r may be related perhaps to the motion involving the trifunctional nitrogen junction.