Peroxide crosslinking of poly(n-alkyl methacrylates)

The action of dicumyl peroxide on poly(n-butyl methacrylate) and poly(n-nonyl methacrylate) produces degradation and crosslinking reactions in both polymers. Crosslinking and degradation of poly(n-alkyl methacrylates) are influenced also by the initial molecular weight of the polymer as well as by the type of alkyl group. The ratio of degradation to crosslinking p/q determined on the basis of the equation of Charlesby and Pinner, S + S^0.5 = (p/q) + (1/qP_n) is for poly(n-butyl methacrylate) of viscosity molecular weight 0.923 × 10⁶ and 2.16 × 10⁶ of 0.78 and 0.60, respectively; for poly(n-nonyl methacrylate) of weight average molecular weight 3.83 × 10⁵, p/q is 0.16. Crosslinking efficiencies (moles of crosslinks per mole of decomposed dicumyl peroxide) of the above polymers are relatively very low: 0.014, 0.005, and 0.039, respectively. The critical concentration of dicumyl peroxide necessary for the formation of gel, provided it undergoes complete decomposition, is for the above polymers 1.82, 1.65, and 0.98 wt.-%, respectively. Under the critical concentration of dicumyl peroxide the limiting viscosity number of poly(n-butyl methacrylate) increases with increasing concentration of dicumyl peroxide. An initial decrease of the value of the limiting viscosity number, which is characteristic for polymers undergoing simultaneous degradation and crosslinking, was not observed.     

Crystallinity and the effect of ionizing radiation in polyethylene. II. Crosslinking in chain-folded single crystals

In order to ascertain whether the crystal core of irradiated polyethylene single crystals is free from crosslinks—as would follow from the results of Part I—the fold surface of irradiated single crystals was shaved off with ozone and the resulting product examined by GPC for molecular weight. It was found that on ozone degradation the entire material, which had been insolubilized by radiation-induced crosslinking, has become soluble hence became available for the GPC analysis. The chromatograms displayed the same peaked development with degradation as the unirradiated crystals leading eventually to single traverse dicarboxylic acids. This proves the absence of crosslinks within the crystal interior of the material as examined by GPC. The appearance of some additional low molecular weight material, is attributed to scission at the radiation-induced double bonds due to ozone which eliminates the possibility of the existence of crosslinks within the lattice such as might provide scission sites for ozone. The conclusion could therefore be reached that, to the extent assessable by our GPC test, there are no radiation-induced crosslinks within the crystal lattice, hence the crosslinks produced must be entirely confined to the fold surface region of the crystals.     

Direct determination of crosslinking and chain scission in polymers

A direct method has been developed for determining G(crosslinks) for irradiated polymers using the analytical ultracentrifuge. The sedimentation velocity technique is employed to follow changes in the molecular weight of a narrow distribution polystyrene sample irradiated in vacuum with ⁶⁰Co γ-rays. It is shown that G(crosslinks) can be determined at low doses before significant structural changes have occurred in the polymer. At about one-fifth of the gel dose G(crosslinks) was found to be 0.019 compared to a value of 0.040 obtained from gel-sol fraction measurements. It is concluded that G(crosslinks) may increase with dose due to processes such as the addition of radicals to double bonds formed during the irradiation.     

Modification of poly(vinyl chloride). XIII. Crosslinking of poly(vinyl chloride) with dithiols and properties of the crosslinked products

PVC was crosslinked by immersing PVC–dithiol blends in ethylenediamine at 30°C. Properties of the products depended on the chain length and chemical structure of the crosslinkage and on the molecular weight of the polymer chain between crosslinks M_c. Crosslinking by the agent of soft structure and long molecular chain resulted in high tensile strength at break and impact strength and low brittle temperature. The use of the crosslinking agent of short molecular chain gave high yield strength, Young's modulus, and heat distortion temperature. The relation of M_c and the chemical structure of the crosslinks to the properties of the crosslinked rigid polymer was discussed in regard to the crosslinking effect and plasticizing effect.     

Crosslinking Index,Molecular Weight Distribution and Rubber Equilibrium Shear Modulus During Polyfunctional Crosslinking of Existing Polymer, 6. Primary Polymer with a Schulz–Zimm Distribution of Molecular Weights

A network model for the crosslinking of already existing polymer molecules with a so‐called Schulz–Zimm distribution of their molecular weights is presented. It is an extension of previously developed statistical network models applied to the crosslinking of primary polymers with several other molecular weight distributions and with crosslinks of any functionality. The model results in the possibility to obtain more insight into the structure of polymers, especially those with narrow distributions of the molecular weight. In more detail, the model can give a perspective on structural network parameters such as the weight fractions of ideal network, of dangling polymer ends, and of those molecules not connected to the network, i. e., the sol fraction, the number of crosslinks in which a polymer molecule is bound, the functionality of the crosslinks, or the average molar mass of the polymer molecules in between the crosslinks M̄_c. Results of calculations are shown for a hypothetical crosslinking process of polymers with various molecular weight distributions. Moreover, the dependency of the network parameters on the polydispersity index and the type of molecular weight distribution is shown. Finally the increase of the functionality of the crosslinks during the ageing process of a 9.9% poly(vinyl chloride) gel as a function of the polydispersity index of the molecular weight distribution is presented.     

Stress-optical properties of poly(oxypropylene) networks

Strain birefringence of poly(oxypropylene) was studied using several poly(oxypropylene) model networks of different crosslink densities. Most of the measurements were carried out in elongation, in both the unswollen and the swollen states, over the temperature range 10–70°C. The optical configuration parameter was found to be Δa = (4.33 ± 0.09) × 10^?24 cm³ at 25°C with a very small temperature coefficient. Theoretical calculations based on rotational isomeric state theory were employed to interpret the experimental data. The results indicate the intermolecular correlations to be low for this polymer, and, contrary to other systems, the stress-optical coefficient C decreased with increasing average molecular weight between crosslinks. This fact was attributed to the end-group effect introduced by the crosslinking agent.     

Friedel–crafts crosslinking methods for polystyrene modification. IV. Macromolecular structure of crosslinked particles

Crosslinked polystyrene particles were prepared by Friedel–Crafts suspension crosslinking of polystyrene using 2,4-dichloromethyl-2,5-dimethyl benzene as crosslinking agent. The polymer was dissolved in nitrobenzene and reaction occurred in a 70 wt % aqueous solution of ZnCl₂ with poly(vinyl alcohol) as a suspending agent. The spherical particles produced were swollen in toluene, chloroform, and tetrahydrofuran to determine their equilbrium polystyrene volume fraction. Analysis of the crosslinked macromolecular structure gave values of number-average molecular weight between crosslinks of M?_c = 900–5900 increasing as the nominal crosslinking ratio X decreased from 0.75 to 0.0625 mol of crosslinking agent per mole of polystyrene repeating unit. Porosimetric analysis contributed to the understanding of the importance of the pore structure for swelling behavior.     

Radiation crosslinking of elastomers. I. Polybutadienes

The chemical and physical crosslink densities of irradiated polybutadiene were determined using a precise and consistent method. This method included the use of a statistical theory of crosslinking modified to include chain reactions and the use of Langley's theory for calculating the number of trapped chain entanglements. The number of crosslinks formed per unit radiation dose, G(X), was measured for a series of polybutadienes containing various isomeric forms of unsaturation. The physical crosslink density and the molecular weight between entanglements were determined from measurements of the compression modulus of swollen samples.     

Features of Preparing the Butyl Acrylate–Acrylonitrile Copolymer by Radiation-Induced Emulsion Polymerization

Some specific features of the radiation-induced emulsion copolymerization of butyl acrylate and acrylonitrile and radiation-induced crosslinking of the resulting copolymer in the latex form were studied. The effect of polymerization conditions on the polymerization rate and the molecular mass of the polymer was determined. The copolymer was shown to undergo efficient crosslinking in the latex form by the action of -radiation. The radiation-chemical yield of crosslinking sites (G) and the average molecular mass (M _c) of chain segments between two adjacent network junctions were determined.     

Molecular weight changes induced in an anionic polydimethylsiloxane by gamma irradiation in vacuum

An anionic almost monodisperse linear polydimethylsiloxane (PDMS) was subjected to gamma irradiation under vacuum at room temperature. The molecular weight changes induced by the radiation process have been investigated using size exclusion chromatography (SEC) with refraction index (RI) and multi angle laser light scattering (MALLS) detectors, to obtain the number and weight average molecular weights of the irradiated samples.The analysis of the data indicates that crosslinking reactions predominated over scission reactions. The results obtained by an SEC-RI have confirmed the presence of small, but measurable amounts of scission.A previously developed mathematical model of the irradiation process that accounts for simultaneous scission and crosslinking and allows for both H- and Y-crosslinks, fitted well the measured molecular weight data. This prediction is in accordance with the experimental data obtained by ²⁹Si-Nuclear Magnetic Resonance spectroscopy (NMR) and previously reported data for commercial linear PDMS (Satti et al., 2008).     

Molecular weight distribution in radiation-induced polymerization. I. γ-radiation-induced free-radical polymerization of liquid styrene

The kinetics of γ-radiation-induced free-radical polymerization of styrene were studied over the temperature range 0–50°C at radiation intensities of 9.5 × 10⁴, 3.1 × 10⁵, 4.0 × 10⁵, and 1.0 × 10⁶ rad/hr. The overall rate of polymerization was found to be proportional to the 0.44–0.49 power of radiation intensity, and the overall activation energy for the radiation-induced free-radical polymerization of styrene was 6.0–6.3 kcal/mole. Values of the kinetic constants, k_p²/k_t and k_trm/k_p, were calculated from the overall polymerization rates and the number-average molecular weights. Gelpermeation chromatography was used to determine the number-average molecular weight M?_n, the weight-average molecular weight M?_w, and the polydispersity ratio M?_w/M?_n, of the product polystyrene. The polydispersity ratios of the radiation-polymerized polystyrene were found to lie between 1.80 and 2.00. Significant differences were observed in the polydispersity ratios of chemically initiated and radiation-induced polystyrenes. The radiation chemical yield, G(styrene), was calculated to be 0.5–0.8.     

Swelling of crosslinked poly-4-vinylpyridine

Polyvinylpyridine of molecular weight 0.77 × 10⁶ was crosslinked by 2.5, 5, and 10% BrCH₂C₆H₄CO(CH₂)₈COC₆H₄CH₂Br and by 10% ClCH₂C₆H₄CH₂Cl; quaternization of the gel was completed with n-butyl bromide. Swelling ratios in aqueous solutions of lithium, sodium, and 4-isopropyl-N-n-butylpyridinium bromide and in methanolic lithium bromide were determined. Selectivity increased in the sequence Li⁺ <Na⁺ <PrN⁺?C₅H₅. The Flory parameter χ₁, which measures interaction between solvent and polymer, decreased from 1.5 kT to nearly zero with increasing density of crosslinks in the aqueous solutions, and from 3 kT to 0.5 kT in the methanol solutions. The inverse proportionality of q^2/3 (q = swelling ratio) to the crosslinking density was approximately verified for swelling of the resins in water, methanol, and dimethylformamide.     

Thermal analysis of polystyrenes crosslinked by p-di(chloromethyl)benzene

Series of progressively crosslinked polystyrene have been prepared by the reaction off linear polystyrene with p-di(chloromethyl)benzene in dichloroethylene solution with stannic chloride as catalyst. By using a DTA technique it has been shown that the glass transition temperature T_g increases with crosslinking very much faster than is accounted for purely by increase in molecular weight. This type of crosslink is only about half as effective in increasing T_g as are crosslinks introduced by copolymerization of styrene with divinylbenzene. Thermal analysis by use of the thermal volatilization analysis (TVA) technique shows that stability, as measured by the evolution of volatile products, decreases as crosslinking progresses, and thermal gravimetric analysis (TGA) confirms this.     

A solid-state 13C-NMR study of crosslinking in polybutadiene by γ radiation: Effect of microstructure and dose

Solid-state ¹³C NMR spectroscopy has been used to determine the decrease in C?C bonds, formation of crosslinks and cis to trans isomerization during the γ irradiation of (a) > 99% cis, 1,4-polybutadiene, (b) 54% trans, 41% cis, 1,4-polybutadiene, and (c) 86% 1,2-polybutadiene. G(-cis C?C) and G(-trans C?C), were similar and decreased with dose from ≈ 40 for 0-1 MGy to 5 for 5-10 MGy. G(-double bonds) and G(crosslink) were comparable, indicating that crosslinking occurred through the double bonds. G(crosslink) was much higher than values derived from physical properties, confirming that NMR measures the total of inter- and intramolecular crosslinking (cyclization). The 1,2 polybutadiene was much more sensitive to crosslinking, and a value of G(-C?C) = 240 was obtained at low doses. Crosslinking evidently proceeds by a kinetic chain reaction in all three types of polybutadiene.     

Effect of interfacial bonding on the strength of adhesion of elastomers. I. Self-adhesion

Two partially gelled (crosslinked) layers of elastomer were pressed into intimate contact and the gelation reaction was then taken to completion. By varying the extent of initial gelation, the degree of chemical interlinking was varied from zero, when two fully reacted sheets were pressed together, up to a level characteristic of the final density of molecular linking within each layer, when they were brought together before any reaction had occurred. The strength of adhesion between the layers was measured under threshold conditions, i.e., at low rates of peel, at high temperatures, and, in some instances, with the layers swollen with a compatible liquid. Linear relations were obtained between the threshold work of detachment per unit of interfacial area and the amount of chemical interlinking, deduced from the kinetics of molecular linking within each layer. At any degree of interlinking, ranging from zero to the fully interlinked state, the work of detachment was lower for networks composed of shorter molecular chains, in accordance with the Lake–Thomas theory for the threshold strength of elastomer networks. By extrapolation to the fully interlinked state, the strength of adhesion corresponding to cohesive rupture was inferred. These values agreed with measured tear strength for polybutadiene gelled by a free-radical process. For a sulfur crosslinking system, and for both free-radical and sulfur crosslinking of poly(ethylene-co-propylene), the threshold tear strength of the elastomer was found to be much higher than the extrapolated value from adhesion measurements. This discrepancy is ascribed to roughness of the tear plane in relatively strong elastomers, in contrast to the smooth separation of flat adhering layers. Adhesion of fully crosslinked sheets was generally low, 1–2 J/m². Higher values, 5–25 J/m², were found with sulfur crosslinking systems, especially those yielding a high proportion of polysulfidic crosslinks. Interlinking via polysulfide crosslink interchange reactions is suggested in these cases.     

Role of chain transfer in heterogeneous polymerization of ethylene

The role of chain transfer was studied for the radiation-induced polymerization of ethylene in precipitating media, namely n-butyl alcohol, tert-butyl alcohol and their mixtures. The affinities of those solvents for polyethylene are similar, but the chain-transfer coefficient of n-butyl alcohol is larger than that of tert-butyl alcohol. The polymerizations were carried out in a reactor of 100 ml under a pressure of 300 kg/cm², at 60°C, dose rate of 3.07 × 10⁴–1.75 × 10⁵ rad/hr in the presence of 50 ml of solvents. The polymerization in tert-butyl alcohol shows the kinetic behavior characteristic of a heterogeneous polymerization, such as rate acceleration, high dose rate dependence of polymerization rate, and low dose rate dependence of polymer molecular weight, whereas the polymerization in n-butyl alcohol does not exhibit such behavior and gives polymer having a molecular weight much lower than that of polymer obtained in tert-butyl alcohol. The polymer formed in tert-butyl alcohol exhibits a bimodal molecular weight distribution measured by gel permeation chromatography. In mixed tert-butyl alcohol and n-butyl alcohol solvent, with increasing fraction of n-butyl alcohol, the two peaks not only shift to lower molecular weight but the higher molecular weight peak becomes relatively small. Eventually, the polymer formed in n-butyl alcohol exhibits a unimodal distribution. Those results are well explained on the basis of the proposed scheme for heterogeneous polymerization.     

Novel polyisobutylene/poly(dimethylsiloxane) bicomponent networks. I. Synthesis and characterization

The synthesis of novel polyisobutylene (PIB)/poly(dimethylsiloxane) (PDMS) bicomponent networks is described. The synthesis strategy (see Figure 1) was to prepare well-defined and -characterized allyl-tritelechelic polyisobutylenes [ϕ(PIB—C—C=C)₃] and SiH-ditelechelic poly(dimethylsiloxanes) (HSi–PDMS–SiH) and then crosslink these moieties by hydrosilation. The ϕ(PIB—C—C=C)₃ was prepared by living isobutylene polymerization followed by end-quenching with allyltrimethylsilane, whereas the HSi–PDMS–SiH was obtained by equilibrium polymerization of octamethylcyclotetrasiloxane and tetramethyldisiloxane. The detailed structures of the starting polymers were characterized by GPC and ¹H-NMR spectroscopy. A series of PIB/PDMS bicomponent networks of varying compositions and average molecular weights between crosslinks (M _c) of ∼ 20,000 g/mol were assembled. Optimum crosslinking conditions were defined in terms of H₂PtCl₆ catalyst concentration, nature of solvent, time, temperature, and stoichiometry of ∼ CH₂CH=CH₂/∼SiH groups, allowing for the convenient synthesis of well-defined model bicomponent networks. Swelling studies and elemental analysis confirm the correctness of the synthetic strategy. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1891–1899, 1998     

The use of model networks with reptating chains to study extraction efficiencies

It is possible to prepare “model” elastomeric networks having known values of the molecular weight M_c between crosslinks by endlinking functionally terminated polymer chains having number-average molecular weights M_n equal to the desired values of M_c. If chains having chemically inert groups at both ends are intentionally included during the preparation of such a system, they will remain unattached, merely reptating through the subsequently formed network structure. This technique was used to prepare a series of tetrafunctional polydimethylsiloxane (PDMS) networks having essentially the same degree of crosslinking (10^?3M_c = 11.3 g mol^?1) and constant amount of diluent in the form of unattached PDMS chains having molecular weights of 10^?3M_d = 26.4, 18.6, 15.8, 9.8, 6.7, 1.2, and 0.70 g mol^?1. Because of the very high mobility of PDMS, it was also possible to introduce essentially the same amount of the same diluents into already formed PDMS networks having the same M_c. Extractions carried out using tetrahydrofuran at room temperature showed that the diluent (“sol fraction”) introduced by swelling the network is more easily removed than that present during the endlinking, possibly because of less convoluted arrangements within the network structure. Chains with the largest values of M_d which were present during the endlinking were found to be very difficult to remove entirely. It is therefore extremely important to carry out exhaustive extractions to obtain reliable values of network sol fractions, particularly when such data are to be used to estimate extents of reaction in the preparation of end-linked elastomers.     

Synthetic thermally reversible gel systems. VII

Polymethacrylylglycinamides (PMG), like polyacrylylglycinamides (PAG), form thermally reversible aqueous gels, but higher molecular weights and/or concentrations are required and the melting points of the gels are lower. The heats of crosslinking for aqueous PMG gels fall in the range of ?5 to ?10 kcal/mole of crosslinks, the same as for aqueous PAG gels, implying that the crosslinks are chemically similar. PMG and PAG are incompatible with each other but both are individually compatible with some types of gelatin. The solubilities of PMG and PAG are similar. Various reagents, however, affect PMG and PAG gels in quite different manners. Aqueous PMG solutions, just outside conditions required for gelation, are rheopectic. Intrinsic viscosities [η] of PMG in 2M NaCNS are about 2.5 times those in water. The Huggins' k′ value for PMG in 2M NaCNS has a value of 0.39–0.40, and both it and [η] are essentially temperature-independent over the range 25–45°C. In water at 25°C for PMG, k′ has an average value of about 1.4. With increasing temperature, for H₂O, there is a considerable increase in [η] which is accompanied by a decrease in the value of k′. Osmotic molecular weight measurements on unfractionated PMG in H₂O at 40°C yield π/c versus c plots having essentially zero slope, implying a value of close to zero for the second virial coefficient, a value of about 0.5 for the polymer–solvent interaction parameter, and a condition close to a θ condition. An approximate viscosity–M _n relationship for polydisperse PMG is [η]_{2M NaCNS, 25deg;C} = 1.7 × 10^?8 M _n^1.5. The low value of K and high value of the exponent do not result from large differences in polydispersity but rather from a stiff, rodlike configuration in solution. This steric hindrance to rotation also manifests itself in the extreme brittleness of PMG films and in a ΔH_p for homopolymerization of only ?6 kcal/-mole. The infrared spectra of MG monomer and PMG are recorded as well as the density and refractive index for PMG. PMG has a glass transition at 226°C by DTA and by TGA, thermal decomposition sets in at about 300°C. From copolymerization with acrylic acid, values of 1.66 and +0.06, respectively, were obtained for the resonance factor Q and the electrical factor e for MG monomer.     

Inter‐ and Intramolecular Crosslinking Kinetics: Partitioning According to Number of Crosslinks

Summary: Intramolecular crosslinking is important in many processes, including nanoparticle fabrication, but quantitative models of the kinetics have not been extensively investigated. We propose a distribution kinetics analysis for simultaneous intramolecular and intermolecular crosslinking. For realistic cases, moment equations can be solved and crosslink density computed according to the number of crosslinks. An analytical solution for the ratio of intramolecular to intermolecular crosslinking rates varies inversely with the initial polymer concentration and initial molecular weight, in agreement with published experimental data.

Time dependence of crosslink density p^cd in a batch reactor for ω₁ = ω₂ = 1 with κ₂ = 1 and κ₁ = 0, 0.2, 0.5, and 1 (lines 1, 2, 3, and 4), and κ₂ = 0 and κ₁ = 1 (line 5). p^cd increases linearly with time for all cases.