分子结构参数对顺丁胶浓溶液流变性质的影响

本工作用GPC-Automatic Viscometer方法测定了顺丁胶样的分子量、分子量分布和支化因子,用同轴圆筒粘度计及落球法测定了顺丁胶浓溶液的粘度,主要研究了分子量分布和长链支化对顺丁胶浓溶液非牛顿流动的影响。提出了描述不同分子量分布的顺丁胶浓溶液粘度的切变速率依赖性的简单公式,并讨论了长链支化对顺丁胶浓溶液非牛顿流动的影响。     

用GPC-级分特性粘数法表征高顺式-1,4聚丁二烯的长链支化

本文在Ambler方法和kraus方法的基础上,用GPC-级分特性粘数法来测定聚合物的长链支化度,能同时以g_i、λ_i、G_i、m_i、支化重量百分数等支化参数来表征聚合物的支化分布、支化程度和支化含量等;得到了相应的计算gi、λi、[η]_i等有关的计算公式和计算方法;还研制成了以光导纤维为冷光源的高精度光电自动计时毛细管粘度计和60小时内恒温精度优于±5×10~(-4)℃的超级恒温水浴,使计时精度达到≤±4×10~(-3)秒;并以国产和进口的镍系顺丁橡胶为例,讨论了分子量和支化度多分散性之间的某些关系;从而确定了该法比较合理地、全面地相对比较聚合物的长链支化度。     

Molecular weight and long-chain branching distributions of some polybutadienes and styrene–butadiene rubbers. Determination by GPC and dilute solution viscometry

A method described for the determination of molecular weight and long-chain branching distributions of polymers requires no prior knowledge of the functional relation between branching frequency and molecular weight. It is based on preparative fractionation and viscometric and gel-permeation chromatographic measurements on both fractions and whole polymer. The technique is applied to several polybutadienes and butadiene-styrene copolymers differing widely in method of synthesis and pattern of long-chain branching.     

Novel mechanism for the formation of long-chain branching in polyethylene

Polyethylene produced by a vanadium-based polymerization catalyst contains long-chain branching as determined by NMR and rheology, even though the polymer has very low levels of vinyl unsaturation. A new mechanism is proposed for the formation of the long-chain branching, which involves C H bond activation of the polyethylene backbone through a σ-bond metathesis reaction, followed by ethylene insertion at the new V C bond. Consistent with the proposed C H bond activation mechanism, the polymerization catalyst was also found to insert ethylene into the C H bonds of alkanes such as heptane. A bridged metallocene catalyst was also found to activate C H bonds of alkanes suggesting this new mechanism may explain the formation of long-chain branching in some metallocene-produced polyethylene. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2889–2898, 1998     

Dilute solution properties and phase separation of branched low-density polyethylene

Viscosity, light scattering, and precipitation temperature measurements on dilute solutions of high-density and low-density polyethylene fractions have been carried out and a theory by Flory for phase equilibrium of linear polymers has been extended to branched polymer. From the results, it is shown that the entropy parameter ψ, depends on branching; a method for the determination of long-chain branching in polymer fractions is proposed combining precipitation temperature and molecular weight measurements. The method has been applied to the evaluation of long-chain branching in low-density polyethylene.     

Another Look at Long-Chain Branching

Long-chain branching can occur during radical polymerization and is especially important for polyethylene. An improved method of calculating the effect of long-chain branching on molecular weight distribution is presented. This method uses a probability treatment. The results are more consistent with both kinetic theory and experimental data than the results of previous long-chain branching calculations. In contrast to previous calculations, the present work shows that generation cannot occur from long-chain branching alone.     

Long-Chain Branching under Conditions of Nonuniform Branching Probability

Abstract

Long-chain branching can occur during free radical polymerization and is especially important for polyethylene. An improved method of calculating the effect of long-chain branching on molecular weight distribution was presented in an earlier paper in which the assumption was made that the probability of branching at each monomer unit was constant throughout the polymerization. A method of including a nonuniform probability of branching in the calculations is presented. Calculation results show that the predictions of the two mathematical models are similar and both models fit published data on polyethylene equally well.     

Determination of branching distribution by concurrent GPC and sedimentation velocity experiments

A method of determining the distribution of branching in polydisperse polymer samples is proposed. This method uses data from concurrent gel permeation chromatography and sedimentation-velocity experiments. Tedious fractionation, which must precede other methods of determining long-chain branching, is eliminated. An example of use of the method on the data of a sample of styrene–divinylbenzene copolymer is given.     

Determination of long-chain branching in model polystyrene samples by the GPC-sedimentation method

The GPC–sedimentation velocity method proposed earlier for the determination of long-chain branches was examined quantitatively by using model star-shaped and comb-shaped polystyrene samples of known degree of branching. The results showed that separation by sedimentation velocity was more sensitive to branching variations than separation by GPC. The results showed also that at low and moderate levels of branching the GPC–sedimentation velocity method reflected accurately the amounts of branching in the model samples. At high levels of branching, the method underestimated the amount of branching. The discrepancy, however, appears to have been caused by the inadequacy of the theories used in the interpretation of the raw data and not by any inherent problem in the method itself. The relative sensitivities of the GPC-sedimentation, GPC-viscosity, and GPC–light scattering methods are discussed.     

用凝胶渗透色谱测定聚合物中的支化度

本文在Ambler报导的测定聚合物中支化度的工作基础上,提出了支化度的计算方法和电子计算机计算程序,可用ALGOL-60语言在国产DJS-6电子计算机上计算。讨论了支化模型、短链支化和长链支化分布函数等对计算结果的影响,确定了该方法可以相对比较镍系顺式聚丁二烯中的支化度。     

Processability improvement of polyolefins through radiation-induced branching

Radiation-induced long-chain branching for the purpose of improving melt strength and hence the processability of polypropylene (PP) and polyethylene (PE) is reviewed. Long-chain branching without significant gel content can be created by low dose irradiation of PP or PE under different atmospheres, with or without multifunctional branching promoters. The creation of long-chain branching generally leads to improvement of melt strength, which in turn may be translated into processability improvement for specific applications in which melt strength plays an important role. In this paper, the changes of the melt flow rate and the melt strength of the irradiated polymer and the relationship between long-chain branching and melt strength are reviewed. The effects of the atmosphere and the branching promoter on long-chain branching vs. degradation are discussed. The benefits of improved melt strength on the processability, e.g., sag resistance and strain hardening, are illustrated. The implications on practical polymer processing applications such as foams and films are also discussed.     

Polymerization of vinyl chloride at reduced monomer accessibility. II. Polymerization at subsaturation pressure with emulsion PVC as seed

Vinyl chloride was polymerized at 59–92% of saturation pressure in a water-suspended system at 45–65°C with an emulsion poly(vinyl chloride) (PVC) latex as a seed. A water-soluble initiator was used in various concentrations. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included determination of molecular-weight distribution and degree of long-chain branching by gel permeation chromatography (GPC) and viscometry, thermal dehydrochlorination, and microscopy. The polymerization rate decreases with decreasing pressure but is reasonable even at the lowest pressure. The molecular weight decreases with decreasing pressure and increasing initiator concentration and also with increasing polymerization temperature, if the initiator concentrations are chosen to give a constant initiator radical concentration. The degree of long-chain branching increases with increasing initiator concentration and decreasing monomer pressure but is unaffected by the polymerization temperature, if the initiator radical concentration is kept constant. The thermal stability decreases with decreasing M _n, while the degree of long-chain branching has only a minor influence. The most important factor in the system influencing the molecular parameter is the monomer accessibility.     

Characterization of linear low-density polyethylene: Cross-fractionation according to copolymer composition and molecular weight

The structure of ethylene copolymers modified by α-olefins has become an area of intense investigation since the successful commercialization of so-called linear low-density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene-1. The resins were fractionated according to short-chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x-ray diffraction, ¹³C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short-chain branching, long-chain branching, and the molecular-weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short-chain branching distribution. No significant long-chain branching was found in the LLDPE resins. The short-chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid-state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α-olefin in these commercial LLDPE resins.     

The application of the size exclusion chromatography/ viscometry technique to the determination of molar-mass and long-chain branching in polychloroprene

The coupling of a low-pressure size exclusion chromatography (SEC) with a modified Ubbelohde capillary viscometer is described. This SEC/viscometry system measures the flow time of aliquot fractions (5ml) of the SEC effluent along with the refractive index change. This dual detection leads to the determination of the intrinsic viscosity as a function of the elution volume, thus allowing a precise use of Benoit's universal calibration. Moreover information of the branching factors (degree of long-chain branching, long-chain branching frequency) can be calculated under certain assumptions. As an example the changes in molar-mass distribution and branching factors during mechanical shear degradation (mastication) of special polychloroprene samples were investigated. It is shown that the SEC/viscometry system is especially suitable for the characterization of polymers with broad molar-mass distribution and extremely high molecular tails. The data provided by this method are useful for the investigation of the viscoelastic behaviour of concentated polychloroprene solutions and for quality control of polymers in the rubber and adhesives industries.     

A simulation model for long-chain branching in vinyl acetate polymerization: 1. Batch polymerization

A new simulation model for the kinetics of long-chain branching formed via chain transfer to polymer and terminal double-bond polymerization is proposed. This model is based on the branching density distribution of the primary polymer molecules. The theory of branching density distribution is that each primary polymer molecule experiences a different history of branching and provides information on how each primary polymer molecule is connected with other chains that are formed at different conversions, therefore making possible a detailed analysis on the kinetics of the branched structure formation. This model is solved by applying the Monte Carlo method and a computer-generated simulated algorithm is proposed. The present model is applied to a batch polymerization of vinyl acetate, and various interesting structural changes occurring during polymerization (i.e., molecular weight distribution, distribution of branch points, and branching density of the largest polymer molecule) are calculated. The present method gives a direct solution for the Bethe lattice formed under nonequilibrium conditions; therefore, it can be used to examine earlier theories of the branched structure formation. It was found that the method of moments that has been applied successfully to predict various average properties would be considered a good approximation at least for the calculation of not greater than the second-order moment in a batch polymerization. © 1994 John Wiley & Sons, Inc.     

DETERMINATION OF BRANCHING DISTRIBUTION IN HIGH cis-1,4-POLYBUTADIENE BY GPC AND AUTOMATIC VISCOMETRY

Based on the methods reported by Ambler and Kraus, a method has been developed for the determination of long-chain branching distribution in polymers by the combined use of GPC and intrinsic viscosity data of polymer fractions. In this method, g_i, λ_i, G_i, m_i, the weight percentage of polymer that is branched, etc. can be used simultaneously to characterize the distribution, degree and content of branching in polymers. Some relations between molecular weight polydispersity and branching polydispersity in Nickel-based high cis-1,4-polybutadiene samples are discussed. It was found that the number of long branches λ. per unit molecular weight is a function of molecular weight and all of the samples are highly branched at a molecular weight of about 10~6.     

Rate Constant for Long-Chain Branch Formation in Free-Radical Polymerization of Ethylene

Quantitative measurements on the rate constant for long-chain branching in radical polymerization of ethylene have not yet been cited. This paper presents a convenient method of calculating the ratio of the rate constant for long-chain branching to that for propagation, by means of polymerization parameters, average degrees of polymerization (X_n and X_w), and amount of total unsaturation. The results of calculation based on our experimental data show that the activation energy E_trp -E_p, and the activation volume ΔV ^trp _?-ΔV ?_p were 3.6 kcal/mole and 24 cc/mole, respectively, in radical polymerization of ethylene. The validity of these values is discussed.     

1H NMR Analysis of Long-Chain-Branched Strong Polyelectrolytes Obtained by Vinyl/Divinyl Monomer Copolymerization in Aqueous Medium

Statistical long-chain branching occurring during the free-radical polymerization of the system sodium 4-vinylbenzenesulfonate (SSNa) and N, N′-methylene-bis(acrylamide) (MBAA) in aqueous medium was studied by ¹H NMR spectroscopy and SEC. The former method yielded the degree of conversion and the content of the three monomer units in the chain, i.e., SSNa, MBAA with pendant vinyl groups, and MBAA as branching points. It was found that the number of individual monomer units per a total of 1000 monomer units does not perceivably change with the degree of conversion and only slightly increases with increasing ratio MBAA/SSNa in the feed.     

Gel permeation chromatography of branched polyethylene

The effect of long-chain branching must be considered in gel permeation chromatography to evaluate the molecular weight polydispersity of branched polyethylenes. Osmotic molecular weights of fractions of branched polyethylene were correlated with elution volumes; weight-average and number-average molecular weights of a branched polyethylene were determined. Molecular weight changes on crosslinking polyethylene by ionizing radiation are accompanied by branching and cannot be simply interpreted by gel permeation chromatography.     

Molecular weight distribution and molecular structure of polyethylene produced by radiation-induced polymerization

The molecular weight distribution of polyethylene produced by radiation was calculated according to a kinetic scheme. The calculated molecular weight distribution was compared with the results deduced from gel-permeation chromatography. The observed distribution curve from GPC was broader and showed a lower degree of polymerization than the calculated one. Discrepancies between observed and calculated curves can be explained if the polymer contains nonsteady-state products and if the reaction mechanism includes chain transfer to dead polymer. By this reaction long-chain branching would occur. Several long-chain branches per polymer molecule were indeed found, as inferred from solution properties.