Shear degradation of polymer solutions

Shear degradation of polyisobutylene solutions was studied in laminar flow through capillaries. A special apparatus was designed with a view to a controlled shear history. The various effects of initial molecular weight, concentration, temperature, and shear stress at the wall on the final degradation product are combined in a single parameter representing the minimum force required for degrading a polymer sample to its final state. This work is based on the M.Sc. Thesis of A. Kadim, submitted at the Department of Chemical Engineering, Technion—Israel Institute of Technolog, Haifa, Israel.     

Optical anisotropy of polyisobutene

The distribution of chemical bonds in polyisobutene results in a theoretically zero optical and electrical anisotropy of the polymer according to bond polarizability additivity principles as conventionally applied, irrespective of polymer chain conformation. Non-zero values of the segmental anisotropy (～4.4 × 10^?40 C²m²/J) derived from strain-optical birefringence on the polymer in the rubbery state have been recently reported by Liberman et al. who suggest qualitatively that severe crowding of the chemical groups in the polymer affects the anisotropies of the group polarizabilities and gives the observed non-zero segmental anisotropy. We have measured optical, electrical and magnetic anisotropies for the pure liquid polymer using techniques of flow birefringence, depolarized light scattering, electro-optic birefringence and magneto-optic birefringence. Segmental optical anisotropies so derived agree with those measured by Liberman et al. and results of the above measurements taken as a whole lead to an origin for the observed anisotropy which requires locality dependent values of optical and electrical bond polarizability anisotropies to be used in the calculation of the polyisobutene polarizability anisotropy. Our explanation of the polarizability locality dependence differs from the steric overlap suggestion of Liberman et al. and depends upon the electric field interaction between dipoles induced in polarizable bonds.     

Polymer degradation. V. Changes in molecular weight distributions during sonic irradiation of polyisobutene

Gel permeation chromatography was used to follow changes in the molecular weight distribution of a polyisobutene, initial M _v = 466,000, during degradation induced by sonic irradiation. Seven samples taken at times of 200–163,500 sec. were studied. In general, a steady decrease in molecular weight was observed, although a bimodal distribution was present after 20,000 sec. irradiation. Various measures of inhomogeneity, M _w/M _n, U, etc., were examined. The function, σ_n/M _w, where σ_n is the standard deviation of the number distribution, is considered as a measure of the relative distribution. It is shown to be almost constant throughout the degradation.     

Shear degradation of vinylidene chloride copolymers

Thermally induced dehydrochlorination is a well-established and prominent degradation mode for vinylidene chloride copolymers. During extrusion, other processes may represent significant degradation pathways. Under shear in air, both oxidative chain-scission and cross-linking are prominent processes for both vinylidene chloride/vinyl chloride and vinylidene chloride/methyl acrylate copolymers. Both processes are dependent upon shear rate and temperature. The shear-stress dependency can be modeled by a kinetic expression that incorporates shear stress into the Arrhenius preexponential factor. Vinylidene chloride/methyl acrylate copolymers appear to be somewhat more susceptible to oxidative chain-scission than are comparable vinylidene chloride/vinyl chloride copolymers, presumably because of a rapid oxidative attack at exposed methyl acrylate units. Shear-induced degradation of these materials in air is characterized by early predominant chain-scission with cross-linking assuming a greater importance as a function of time. Degradation under shear in a nonoxidative environment is a much simpler process—oxidative chain-scission is suppressed and cross-linking is very similar to that observed in air. © 1994 John Wiley & Sons, Inc.     

The hydrosilylation cure of polyisobutene

Summary A liquid polyisobutene oligomer with unsaturated chain ends undergoes hydrosilylation with HMe₂SiOMe₂SiOMe₂SiH or Si(OMe₂SiH)₄ to give higher molecular weight polymers or elastomers. A major side reaction consumes SiH to give redistributed siloxane in the resulting polymers and gaseous silanes and siloxanes as by-products. A second side reaction results in loss of reactivity in the oligomer due to a shift of the terminal double bond to an internal position. If the side reactions are taken into account, it is possible to forecast quantitatively molecular weight, gel point and modulus from the conversions of >SiH, >C=CH_f2 and the chain entanglement concentration reported for polyisobutene in the literature.     

端羟基遥爪聚异丁烯的制备与表征

使用5-叔丁基-1,3-二（2-氯代异丙基）苯/四氯化钛引发体系,通过可控活性正离子聚合反应制备了含双活性链末端的聚异丁烯,再使用1,3-丁二烯封端制备具有烯丙基氯末端结构的聚异丁烯,并结合四丁基氢氧化铵存在条件下水解的方法制备了双端羟基遥爪聚异丁烯,研究了封端时间对封端反应的影响,探讨了所得聚合物的热稳定性和低温性能。结果表明,在四丁基氢氧化铵存在条件下,由含氯链末端亲核取代方式制备所得聚合物的端羟基转化率高,反应时间短,原料便宜易得且制备工艺简便,所得产物的热稳定性及低温性能优良。     

Immobilization of polyisobutene in semi-interpenetrating polymer network architecture

The entrapment of linear polyisobutene (PIB) in semi-IPN architecture is shown to be as efficient as it is in cross-linkable telechelic PIB based full IPN architectures as far as the suppression of cold flow is concerned. Indeed, homogeneous linear PIB/cross-linked polycyclohexylmethacrylate (PCHMA) semi-IPNs containing from 20 to 70 wt% PIB and synthesized without solvent show no cold flow and higher mechanical properties than those of linear PIB or 50 wt% PIB containing blend. In addition, the particular barrier properties toward gas and water are preserved. Those properties arise from the phase co-continuity morphology of the semi-IPN materials which moreover compares with that of corresponding IPNs. A systematic study of the synthesis conditions (nature of the initiator, temperature, cross-linking density) showed that the reacting mixture viscosity is an important parameter that controls the phase separation degree in the final material.     

聚异丁烯研究进展及其工艺优化

主要介绍了近年来国内外聚异丁烯的生产现状及科研情况；对国内外主要厂家生产聚异丁烯的工业生产技术进行了详细的分析。并针对以上几种生产技术的特点进行了对比；另外，还较全面地论述了BF3引发异丁烯聚合反应原理。阐述了异丁烯聚合过程中主要工艺参数对聚合结果的影响,并提出了异丁烯聚合的较佳的工艺参数；最后，针对我国聚异丁烯生产的具体情况提出了作者的几点建议。     

Alder ene functionalization of polyisobutene oligomer and styrene-butadiene-styrene triblock copolymer

The Alder ene functionalization reaction of double bonds containing macromolecules such as polyisobutene oligomer (PIB) and a styrene-butadiene-styrene triblock copolymer (SBS) samples with maleic anhydride (MAH) or diethyl maleate (DEM) as enophiles is described. The analysis of the products by means of different techniques assesses the addition of the polar molecules to the reactive vinylidene units of the polymer with functional degrees (FD) depending on the type of enophile and polymer reactivity. The role of the reaction conditions and the use as Lewis acids as catalysts are discussed in terms of their influence on the addition reaction extent and on the polymer molecular weight.     

Preparation and properties of polyisobutene/organic montmorillonite hot melt pressure-sensitive adhesive (HMPSA)

To enhance the interfacial adhesion between Na-montmorillonite (Na-MMT) and polyisobutene (PIB) matrix, it is necessary to modify Na-MMT in organic way. Organic montmorillonite (OMMT) was successfully modified by Na-MMT with cetyltrimethyl ammonium bromide (CTAB) intercalation reagent. The X-ray diffraction (XRD) result showed that the d-spacing of Na-MMT was increased from 1.424 to 2.480 nm after organic modification. PIB/OMMT hot melt pressure-sensitive adhesive (HMPSA) samples were prepared by melt-intercalation process. The amount of OMMT was optimized according to the system stability and adhesion performance. The effects of OMMT content on rheological, adhesion and thermal properties of PIB HMPSA were investigated. Adhesion performance and system stability of PIB HMPSA were greatly improved by adding moderate amounts of OMMT. In addition, the sample containing 1 wt% OMMT exhibited optimal adhesion property and excellent stability, meanwhile, its 180° peel strength was 1.19 times greater than pristine sample.     

Shear degradation of poly(vinyl acetate) in toluene solutions by high-speed stirring

A poly(vinyl acetate) (PVAc) of M?_w 750,000 and M?_w/M?_n 5.10 in toluene solution was sheared in a Virtis-60 homogenizer. The polymer concentration was 3.0 to 12.0 g/100ml, and test temperature was 10 ± 0.5°C. The extent of degradation was measured by gel permeation chromatography (GPC). It was concluded that on shearing (i) the molecular weight decreases rapidly at the beginning of shearing and thereafter decreases ever more slowly toward a limiting value, (ii) the molecular weight distribution is narrowed, (iii) no degradation occurs up to 5000 rpm and thereafter increases with stirring speed, (iv) degradation is more at lower concentrations but concentration is not a sensitive variable, and (v) the chain scission occurs randomly. The Mark-Houwing relationship for PVAc in THF at 25°C was derived as [η] = 2.47 × 10^?4 × M?_v^0.644.     

圆盘剪水平剪刃适应多钢种剪切的改进措施

介绍安钢炉卷机组圆盘剪水平剪刃系统的合理改造,增加自控程序功能,确定圆盘剪水平剪刃间隙调节的系数。改造完成后,提高了工作效率,以及钢板切边质量和精准度。     

Synthesis of functional polyisobutene I. Copolymerization of isobutene with 4-vinylbenzyl N,N-diethyldithiocarbamate

Summary A special monomer 4-vinylbenzyl N,N-diethyldithiocarbamate (VBDC) was synthesized firstly, and then the cationic copolymerization of VBDC with isobutene was investigated. The results of ¹H-NMR, EA and GPC (with UV detector) indicated that VBDC could copolymerize with isobutene and form the copolymer, and the units of VBDC incorporated into copolymer chains increased with the increasing of the feed ratio of VBDC. However, there has apparent discrepancy between the VBDC in the monomer feed and the VBDC incorporated into the copolymer chain, which is probably due to the relatively lower reactivity of VBDC. In the presence of VBDC, the MWD is narrower than that of in absence of VBDC under the similar experimental conditions. For cumyl methyl ether/TiCl₄ initiating system, the M_w/M_n could be slightly narrowed from 1.55 (no VBDC) to 1.33 (with VBDC) in the mixed solvents of n-hexane and CH₃Cl (15/10,V/V), while cumyl chloride /TiCl₄ initiating system, the M_w/M_n is narrowed from about 5.0 (no VBDC) to about 1.5 (with VBDC) with n-hexane and CH₂Cl₂ (10/10,V/V) as the mixed solvents. When benzyl N, N-diethyldithiocarbamate (BDC) was used as the model compounds instead of the VBDC, the similar results of M_w/M_n were obtained. These results demonstrated that the VBDC functions as the monomer electron donor (ED) in this polymerization system. Received: 23 August 2001/Revised version: 29 October 2001/ Accepted: 2 November 2001     

Water‐soluble polymers. LXXXII. Shear degradation effects on drag reduction behavior of dilute polymer solutions

Drag reduction measurements were conducted on extensively characterized poly(ethylene oxide) and poly(acrylamide) utilizing a fully automated rotating disk rheometer equipped with an optical tachometer, torque transducer, and software allowing real‐time data acquisition. The instrument sensitivity allowed the study of concentrations as low as 0.1 ppm. In addition, previously immeasurable concentration‐ and time‐dependent shear degradation effects were readily observed. A power law equation was shown to adequately correlate the percentage of drag reduction and the volume fraction for each polymer. Furthermore, an empirical shift factor was utilized to superimpose data from all the systems that were studied. By conducting measurements in the proper concentration and time domains, it was possible to extract the minimal concentration for the maximum drag reduction efficiency in the absence of shear degradation. The resulting values were significantly higher than those previously reported by our laboratories for poly(ethylene oxide) and poly(acrylamide). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1211–1221, 2001     

Factors effecting a loss of flocculation activity of polyacrylamide solutions: Shear degradation,cation complexation,and solution aging

Polyacrylamide and poly(acrylamide-co-acrylic acid) flocculant solutions were subjected to shear degradation in a rotating cone instrument. At constant shear rate, shear degradation was minimized (highest limiting intrinsic viscosity) by the use of a lower solution concentration and high ionic strength of the polymer solution. Resistance to shear degradation also increased with increasing anionic character of the polyacrylamide. Sheared polymers showed reduced performance as flocculants of coal preparation plant tailings, their major commerical application in the U.K. Partially anionic polyacrylamide solutions were completely inactivated in the presence of certain multivalent cations. Aluminium, iron_III, lead, copper, and zinc ions formed complexes with the carboxyl groups on the polymer, resulting in zero flocculation activity. Nonionic polyacrylamides were unaffected. Viscosity–aging of polyacrylamide solutions was observed over a period of several months, but was insufficient to affect the flocculation activity. In the presence of 3% ethanol or methanol, no aging was observed in solutions stored for over a year.     

Shear modification of polyethylene

Polymers like LDPE can be modified by applying a shearing process for an extended period. As a result the melt elasticity is reduced, as evidenced by the decrease in extrudate (die) swell at the exit of a capillary and onset of melt fracture. Melt viscosity was also slightly reduced; melt flow index (measured in short capillary) is highly increased. At the same time, melt density rises. All these parameters vary mostly for LDPE grades that exhibit the highest degree of long-chain branching. It was confirmed that the intrinsic viscosisty of the polymer samples was essentially unaffected, so that chain scission may be ruled out. Moreover, the process itself is shown to be reversible as the initial parameters were recovered by special treatment, due to heating or solvation. The mechanism of shear modification is believed to consist of disentanglement of temporary couplings betwen long branches. The practical utilization of this process will be pursued, as well as the scientific understanding of structuring entangled branched chains.     

Shear modification of polymers

Time dependence of recovery from shear history effects in polymer liquids can be used advantageously to modify the properties of macromolecules. The net result of shearing is a reduction in elasticity and possibly in viscosity of the polymer fluid. The material may be processed subsequently in a transient, less elastic, and less viscous state if the diffusion-related rate of recovery of a more fully entangled equilibrium state is not too rapid. Shear-induced changes in entanglement density are manifested in variations in die swell, die entrance pressure losses, normal stresses, melt strength, flow defects such as melt fracture and sharkskin surfaces, haze of extruded films, application properties of coatings liquids and, other properties. Evidence for shear-modification effects is reviewed in this article. The polymer species that are most amenable to this treatment are identified in general terms; shear modification processes that have been used are summarized. Methods for augmenting the efficiency of mechanical shear modification include addition of plasticizers, fillers, and incompletely miscible polymers. The expedient mentioned last can be particularly effective. Assessments of the degree of shear modification are reviewed and applications to coatings mixtures are also summarized. The time-dependent behavior of the type reviewed in this article limits the success of theoretical approaches, as in the application of constitutive equations to describe polymer-liquid behavior. Attempts to relate Newtonian viscosities and other rheological parameters to polymer characteristics are similarly hindered when shear history effects are not taken into account.