Entanglement networks of 1,2-polybutadiene crosslinked in states of strain. VII. Stress-birefringence relations

Crosslinks are introduced by γ irradiation into 1,2-polybutadiene while strained in uniaxial extension near T_g with stretch ratio λ₀, thereby trapping a proportion of the entanglements originally present. The stress at any subsequent strain λ is accurately given by the sum σ_N + σ_x, where σ_N is the stress contributed by a trapped entanglement network with λ = 1 as reference and a Mooney–Rivlin stress-strain relation, and σ_x is that contributed by a crosslink network with λ = λ₀ as reference and neo-Hookean stress-strain relation. The birefringence is accurately given as δn = ?_Nσ_N + ?_xσ_x, where the ?'s are the respective stress-optical coefficients. From measurements at λ = λ₀ where σ_x = 0, ?_N can be determined separately. For polymer with 88% 1,2 microstructure, ?_N and ?_x are nearly equal and independent of irradiation dose, though strongly dependent on temperature. For polymer with (95–96)% 1,2, ?_N and ?_x are different (even opposite in sign) and dependent on dose. This behavior is associated with a side reaction of cyclization by the γ irradiation, which is inhibited by the 1,4 moiety in the polymer with lesser 1,2 content. It is responsible for residual birefringence in the state of ease (λ = λ_s) where σ_N = –σ_x and the stress is zero.     

Kinetic aspects of the Diels-Alder reaction between poly(styrene-co-furfuryl methacrylate) and bismaleimide

The crosslinking Diels-Alder reaction between styrene-furfuryl methacrylate copolymer samples (poly(ST-co-FM)) and bismaleimide (BM) at 25 °C in chloroform was studied by following the decay in UV absorbance of the maleimide (MI) group at 320 nm. Reaction conditions were changed by using copolymers with different mole fraction of FM, F_FM, and by employing different initial molar ratios of reactants (furan group within FM and MI group within BM). Second order kinetics were obeyed. ¹³C NMR spectra showed that, even when all reactants had been converted to an insoluble crosslinked network, unreacted MI groups remained, presumably in the form of singly reacted pendant BM molecules. The fractions of MI groups remaining unreacted were found to be 0.49, 0.34 and 0.22 for FM:MI mole ratios in the initial mixture of 2, 1 and 0.5 respectively, when using a copolymer of F_FM=0.1354. An attempt was also made to follow the kinetics of network formation by ¹³C NMR spectroscopy, using the peak areas for reacted and unreacted MI and FM groups, but many of the findings were subject to some uncertainty for reasons, which are discussed. However, because the peak areas were considered reliable for unreacted MI groups, the rate constant, k, was evaluated, thereby. Overall using UV and NMR the values of k lay within the interval (0.8-3.6) × 10⁻⁵ dm³ mol⁻¹ s⁻¹.     

Gel growth in free radical crosslinking copolymerization: Effect of inactive gel radicals

A kinetic model is presented for the post‐gelation period of free‐radical crosslinking copolymerization. The model takes into account the trapped radical centers in the gel forming system. It was shown that the weight fraction of sol, W_s, relates to the number of crosslinked units per weight‐average primary molecule, ε, through the equation where n = 2 for Flory's most probable molecular weight distribution, and n = 3 for primary molecules formed by radical combination. Calculation results demonstrate that the existence of trapped radicals significantly affects the growth rate of the gel molecule. It increases the total radical concentration and accelerates the gel growth. The difference in the predictions with and without considering the trapped radicals becomes significant as the crosslinker concentration decreases or, as the vinyl group reactivity on the crosslinker or on the polymer decreases.     

Syntheses and characterizations of bis(trialkoxysilyl)oligoimides. II. Syntheses and characterizations of fluorinated crosslinkable oligoimides with low refractive indices

Novel crosslinkable fluorinated oligoimides were prepared in two steps. The first involved the synthesis of oligoimides terminated with nadic or allylic double bonds, and the second step was materialized either by a radical addition of mercaptotrialkoxysilane derivatives onto nadic double bonds or a hydrosilylation reaction of hydrogenotrialkoxysilane derivative onto allylic double bonds. Three kinds of crosslinking of the trialkoxysilane end groups were studied. The first kind entailed a thermal self‐crosslinking of trialkoxysilane groups. The second process of crosslinking incorporated a bicomponent system—the crosslinked agent was 1,1,1‐tris(4‐hydroxyphenyl)ethane (TRIOH). The trialkoxysilane groups reacted with the hydroxyl–phenol groups of TRIOH to give thermally stable phenoxysilane bonds as well as a crosslinking network. The last method was also a bicomponent system; the oxalic acid was added into an oligoimide solution where by thermal treatment water was created. The water molecules hydrolyzed the trialkoxysilane groups into silanol groups that polycondensed into a crosslinked network following a sol–gel process. The mechanism of the different crosslinking reactions was investigated by Fourier transform infrared spectroscopy and solid‐state ²⁹Si NMR. The self‐crosslinked material prepared from precursor α,ω‐trimethoxysilyl fluorinated oligomer (M_n = 5500 g · mol^?1) exhibited a 10 wt % loss temperature under air higher than 420 °C and a low birefringence (Δn = 0.008) at 1.300 μm. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2602–2619, 2001     

Dynamic mechanical and dielectric properties of ORMOCER® incorporating functionalized poly(styrene) latexes

Organically modified aluminosilicate hybrid materials incorporating polystyrene and poly(styrene‐co‐hydroxypropyl acrylate) latexes, (3‐glycidyloxypropyl) trimethoxysilane, and aluminum sec‐butoxide [Al(O^sBu)₃] were synthesized by a sol–gel process. The bulk materials obtained were macroscopically homogeneous dispersions with good mechanical properties. Dynamic mechanical and dielectric analyses of these new hybrid materials as a function of the Al(O^sBu)₃ concentration and copolymer composition revealed a series of transitions that represented relaxation processes of the incorporated polymer (glass transition), ?Al? O? Si?, the ?Si? O? Si? part of the network, and segmental motion of unreacted ?Si? (CH₂)₃OCH₂CHCH₂O chains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 860–867, 2001     

Formation,structure, and elasticity of loosely crosslinked epoxy-amine networks. II. Mechanical and optical properties

The mechanical and optical behavior in the dry and swollen states of loosely crosslinked epoxy networks prepared from the diglycidyl ether of bisphenol A, phenylglycidyl ether, and 4,4′-diaminodiphenylmethane was investigated, and the weight fraction of sol in the networks was determined. The crosslinking density was controlled by an excess of diamine and by the fraction of monoepoxide. The reaction proceeded to almost full conversion of epoxy groups. With increasing content of monoepoxide or with increasing excess of diamine, the main transition region is shifted to lower temperatures. The dependence of the viscoelastic modulus on temperature and the optical behavior indicate that the networks are homogeneous. In all cases, the sol fraction is adequately described by the theory of branching processes (cf. Part I). The equilibrium modulus related to the dry state is the same irrespective of whether it is obtained by measurements in the dry or swollen state. The mechanical behavior in the rubbery state can be described by the theory of phantom networks with fully suppressed fluctuations of crosslinking (front factor A = 1) or by the theory of phantom networks with fully released fluctuations of crosslinks (front factor) A = f_e?2/f_e] and contribution of trapped entanglements of the Langley-Graessley type (cf. Part I). In the analysis of the equilibrium behavior, it is advantegeous to use the plot of superimposed dependences of G_e on the gel fraction, which considerably reduces the effect of experimental inaccuracy in determination of composition and degree of conversion.     

Synthesis and thermal properties of crosslinkable phosphazene copolymers

Polyphosphazenes of formula [NP(OC₆H₄CN)_x(OCH₂CF₃)_2?x] (x = 0.04–2) were prepared. The copolymers were crosslinked via cyclotrimerization of the nitrilic function, using acid catalysts (chlorosulfonic acid, aluminum chloride) at elevated temperatures. The thermal properties of the crosslinked cyclomatrix polymer were compared to the linear polymer by thermogravimetric analysis and differential scanning calorimetry. When the 4-cyanophenoxy-2,2,2-trifluoroethoxy ratios were less than 0.2, the crosslinked polymers were soluble in polar organic solvents.     

Polymer effect on electron-donating character of polymeric donor. I. Formation of charge-transfer complex between polymers containing N,N-dimethylaniline group and maleic anhydride

The difference in electron-donating character between polymeric and monomeric donors in their charge-transfer-complex formation reactions was studied to clarify the so-called polymer effect in such reactions. Systems containing maleic anhydride and copolymers of N,N-dimethyl-p-aminostyrene (ASt) with styrene, which were prepared by a conventional free-radical polymerization technique, were found to be suitable for this purpose. In correlations between the mean sequence length μ_ASt of the ASt units in the copolymers and the lowest energy of their charge-transfer transition (λ_max) or the association constant of their complex formation K_CT, a bathochromic shift in λ_max and an increase in K_CT with increasing μ_ASt of copolymers are found. Moreover, there is a difference in their modes of μ_ASt dependence. It was concluded from these results that the electron-donating character of the dimethylaniline group increases with increasing number of groups attached on one polymer chain. Also, there is an interaction among neighboring functional groups on one polymer chain. The interaction may be regarded as a kind of polymer effect. In addition, the difference in μ_ASt dependence between λ_max and K_CT and also thermodynamics of the complex formation are discussed.     

Rheological properties of linear and crosslinked polymer blends: Relation between crosslink density and enhancement of elongational viscosity

Strain‐hardening behavior in the elongational viscosity of binary blends composed of a linear polymer and a crosslinked polymer, in which the molecular chains of the linear polymer were incorporated into the network chains of the crosslinked polymer, was studied. Blending the crosslinked polymer characterized as the gel just beyond the sol–gel transition point greatly enhanced the strain‐hardening behavior in the elongational viscosity, even though the amount of the crosslinked polymer was only 0.3 wt %. However, the crosslinked polymer, which was far beyond or below the sol–gel transition point, had little influence on the elongational viscosity as well as the shear viscosity. The stretching of the chain sections between the crosslink points was responsible for the strain‐hardening behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 228–235, 2001     

Diffusion of polymer molecules into polymer networks: Effect of stresses and contraints

Fine threads of cis-1, 4-polyisoprene, diameter ca. 50 μm, were prepared by drawing from solution and drying. They were crosslinked by reaction with H₂S and SO₂ and then swollen with linear cis-polyisoprene liquids of varied molecular weight M_s, from 1,000 to 24,000 g/mol. Diffusion coefficients were determined from the initial rate of uptake, for both unrestrained and stretched threads. They were found to be in good agreement for stretches of up to about 300%. On the other hand, the equilibrium uptake increased markedly (> 100%) over this range of strain, similar to the increase in swelling observed with low-molecular-weight liquids. Values of diffusion coefficient were also obtained from the rate of deswelling upon release of swollen threads from tension, and from the rate of uptake of polymer liquids by a thin coating of crosslinked polymer, bonded onto glass fibers. Satisfactory agreement was obtained in all cases. A number of simple experiments thus give similar values for the diffusion coefficient of polymer liquids in lightly crosslinked polymer networks, in the range 10^?13?10^?16 m²/s, depending upon the molecular weight M_s of the polymer liquid approximately weight as M^?2_s. The amount of liquid absorbed was strongly dependent on its molecular weight, M_s, the degree of crosslinking of the host material, and applied stresses or constraints.     

Postgel properties in the statistical crosslinking of heterochains. I. Systems with N types of chains

The heterochain crosslinking model describes nonrandom crosslinking of polymer chains and is an extension of the classical Flory/Stockmayer gelation theory. We consider the postgelation relationship for the system consisting of N types of polymer chains, in which the probability that a crosslink point on an i‐type chain is connected to a j‐type chain is explicitly given by p_ij. The analytical solutions for the weight fraction of the sol, the number‐average and weight‐average molecular weights within the sol fraction, and the crosslinking density within the sol and gel fractions are derived for the systems, with each type of chain conforming to the Schulz–Zimm distribution. Illustrative calculations are shown for the systems consisting of two and three types of chains, and the obtained results agree with those from the Monte Carlo method. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2333–2341, 2000     

Complexation of poly(vinyl alcohol)–congo red aqueous solutions. 3. Dynamic light scattering study

Mesoscopic structures of poly(vinyl alcohol)(SINGLEBOND)Congo red (PVA(SINGLEBOND) CR) complexes in aqueous solutions were investigated in terms of dynamic light scattering (DLS) technique. The intensity-intensity time correlation function, g⁽²⁾(t), was analyzed with an equation including a single and a stretched exponential function. Two diffusion coefficients, D_f (fast) and D_s (slow) were evaluated. D_f was converted to the apparent correlation length, ξ_app, via the mode-mode coupling hypothesis. The estimated ξ_app was insensitive to the sol(SINGLEBOND)to(SINGLEBOND)gel transition but decreased with CR concentration. This change may be related to the electrostatic screening effect. On the other hand, D_s oscillates with increasing CR concentration at a specific PVA concentration range. This explains well the reentrant sol(SINGLEBOND)gel(SINGLEBOND)sol(SINGLEBOND)gel transition behavior observed in the PVA(SINGLEBOND)CR systems. D_s seems to represent the mobility of the PVA(SINGLEBOND)CR complexes, which annihilates at the gel point. © 1996 John Wiley & Sons, Inc.     

Effect of solvents on properties of polymer gel-electrolyte based on polyester diacrylate

New polymer gel electrolytes based on polyester diacrylates and LiClO₄ salt solutions in organic solvents are developed for lithium ion and lithium polymer batteries with a high ionic conductivity up to 2.7 × 10^?3 Ohm^?1cm^?1 at the room temperature. To choose the optimum liquid electrolyte composition, the dependence is studied of physico-chemical parameters of new gel electrolytes on the composition of the mixture of aprotic organic solvents: ethylene carbonate, propylene carbonate, and λ-butyrolacton. The bulk conductivity of gel electrolytes and exchange currents at the gel electrolyte/Li interface are studied using the electrochemical impedance method in symmetrical cells with two Li electrodes. The glass transition temperature and gel homogeneity are determined using the method of differential scanning calorimetry. It is found that the optimum mixture is that of propylene carbonate and λ-butyrolacton, in which a homogeneous polymer gel is formed in a wide temperature range of ?150 to +50°C.     

Polymers as reagents and catalysts. part x. Halogenations of acetophenone and 1,3-diketones with polymer-supported reagents

A crosslinked copolymer of styrene and 4-vinylpyridine (40-43% of monomer units) was reacted with hydrogen iodide to give a polymer containing pyridinium iodide residues. Reaction of this with chlorine in chloroform at 0° gave a polymer containing pyridinium tetrachloroiodate residues. In a similar manner but using methyl iodide in place of hydrogen iodide, crosslinked polymers containing N-methylpyridinium iodide and N-methylpyridinium tetrachloroiodate residues were prepared. The latter contained up to three chlorine molecules per iodine atom. Both reagents reacted with acetophenone, thus forming iodomethyl-phenyl ketone ($\text{4}$) and chloromethyl-phenyl ketone ($\text{5}$), the ratios depending on the reagent used and the reaction time. Chlorinations of 5,5-dimethyl cyclohexane-l,3-dione and indane-1,3-dione with polymer-supported reagent ($\text{2}$) resulted in the formation of geminal dichlorides in high yields.     

Radiation-induced crosslinking of polyvinyltrimethylsilane in the presence of ethylene glycol dimethacrylate and allyl methacrylate

Radiation-induced crosslinking of homogeneous glassy polyvinyltrimethylsilane was carried out either by the γ-irradiation of the polymeric films containing 3–20 wt% of ethylene glycol dimethacrylate or by the radiation-induced grafting of allyl methacrylate from vapour phase onto films made of pure polymer. The dependence of grafting value on the absorbed γ-irradiation dose and film thickness was investigated. The modified films were analyzed for the sol/gel content and the dependence of gel fraction yield of crosslinked polymer on absorbed dose, concentration of the crosslinking agents and film thickness.The radiation-chemical yields of crosslinking and degradation as well as gelation doses were calculated. The permeability of oxygen and nitrogen through the crosslinked films was determined.     

Effect of gaseous products of overall electrode process on local solution conductivity and efficiency of operation of flow-through porous electrode

A one-dimensional porous electrode (PE) model and additional consideration of the dependence of the local solution conductivity on its gas saturation was used to study the effect of simultaneous hydrogen evolution on distribution of the potential in PE and the overall rate of the target redox reaction. It was found that this effect depends on the ratio of conductivities of the solid κ_s and liquid κ_l phases and direction of solution supply and can be both negative (rear supply at any κ_s and κ_l, front supply at κ_s ≫ κ_l), and positive (front supply at κ_s ≤ κ_l). However, variation of the target reaction rate in all cases for PE with a high specific surface area is low (10–40%). It is shown that in the terms of the model of a homogeneous gas-liquid mixture, a weak effect of gaseous hydrogen is related to the specific form of profiles κ_l(x) far from the earlier considered ideal (or inverse) liquid-phase conductivity profiles.     

Chromatographic properties ofs-triazines in the presence of soluble β-cyclodextrin polymer

The effect of a water-soluble β-cyclodextrin polymer on the lipophilicity and adsorption strength of 17 substituteds-triazine derivatives was studied by thin-layer chromatography. Beta-cyclodextrin polymer dissolved in the mobile phase modifies the chromatographic behaviour ofs-triazine derivatives and, consequently, higherR _f and lowerR _M values were observed. LiCl exerts an opposite influence, it decreases theR _f and increases theR _M values. The β-cyclodextrin polymer enhances the mobility of thes-triazine derivatives on silica gel and reduces their lipophilicity, thus promoting their penetration through the hydrophilic membranes of the target organism. The presence of LiCl decreases the stability of inclusion complexes. The first and second substituents on thes-triazine ring result in an increase of the inclusion complex stability but — due to steric hindrances — the third substituent decreases it.     

Polymer coupling and theory of on-random crosslinking: an analytical solution

Gel formation is an important feature in free-radical polymer coupling. Due to the different possible combination reactivities of each polymer backbone radical, polymer chains are crosslinked in a non-random manner. Equations of the moments have been derived to predict the pregel molecular weight development and the crosslink density at gel point. This work provides an analytical solution for the differential equations. The model agrees with the Flory-Stockmayer gelation theory under the condition of random crosslinking. The magnitude of deviations from the classical theory for non-random crosslinking depends on the product of the radical termination reactivity ratios (r₁r₂), the ratio of the rate constants of backbone radical generation (k), the ratio of the weight-average chain lengths of primary polymers (y), and the polymer weight fractions (w₂).     

Branching by reactive end groups. III. Synthesis,branching, and analysis of m‐ethynylphenol/p‐t‐butylphenol‐coterminated bisphenol a polycarbonates

The use of m‐ethynylphenol (m‐EP) and p‐t‐butylphenol (PTBP) as coterminators for bisphenol A polycarbonates (BA PCs) provided long‐chain‐branched PCs, partially crosslinked PCs, or both after the thermal reaction of the terminal m‐EP groups, depending on the molar ratio of the chain terminators. Linear m‐EP/PTBP PCs were prepared by solution phosgenation of BA and the two coterminators. Differential scanning calorimetry showed the onset of the m‐EP‐end‐group reaction at about 250 °C by the appearance of a reaction exotherm. The enthalpy (ΔH) of this reaction was roughly proportional to the amount of m‐EP in the PC and to an extent could be used to monitor the progress of the reaction and estimate its kinetics. A complete m‐EP‐end‐group reaction was evident from gel permeation chromatography analysis upon heating under N₂ to 380 °C for 10 min or 360 °C for 60 min. The amount, if any, of gel formed after the m‐EP‐end‐group reaction depended on X_EP; those PCs with a X_EP value less than or equal to 0.33 had little or no gel. The maximum X_EP that precluded the formation of gels after branching was estimated to be about 0.45–0.48. The molecular weight of m‐EP/PTBP PCs increased after branching, as evidenced by gel permeation chromatography analysis. Assuming that the terminal m‐EP groups had a statistical distribution on the polymer chain ends and that they underwent only homopolymerization, the average reacted m‐EP‐group functionality according to estimated gel‐point composition was about 2.8–3.0. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2352–2358, 2000     

Investigations on structural,electrical and magnetic properties of nickel substituted La-ferrites

Nanocrystalline nickel substituted La-ferrites, LaNi_xFe_1−xO₃ (where x = 0, 0.2, 0.4, 0.6 and 1.0) were synthesized by sol–gel autocombustion method. X-ray diffraction (XRD) technique was used to confirm the phase formation. The lattice parameter (a) decreases with increase of Ni content. The surface morphology and elemental analysis of the samples were studied by using scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS). The electrical properties of the samples were measured by two-probe method. The hysteresis parameters viz. saturation magnetization (M_s), coercivity (H_C) and remanence (M_r) are reported as a function of nickel content. The substitution of nickel plays an important role in changing the structural, electrical and magnetic properties of La-ferrites.