Comparison of molecular weight distribution models for polymers characterized by g.p.c.

A semi-empirical model which employs polynomials based on general free radical polymerization kinetics, is developed to describe the molecular weight distribution data as well as to evaluate the average molecular weights of a variety of commercial thermoplastics including polystyrene, poly(methyl methacrylate) and low density polyethylene. These novel expressions are equally applicable to a natural rubber sample with a bimodal distribution. Least-squares methods for the classical Schulz and Flory distribution functions are introduced to handle the g.p.c. data of the above polymers. Comparison of the results collected from various analyses indicates clearly that the polynomial model is the most versatile one in the sense that it can be utilized to smooth out satisfactorily the molecular weight distribution data of many polymers. In general, the Wesslau distribution function is particularly good for the highly branched polyolefin and the Schulz model is fairly effective for the addition polymers of moderately sharp molecular weight distribution, presumably with $\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}\text{=3.0}$. However, the Flory and Tung distributions are found to be rather inferior in the present studies. On the basis of the current findings, a new procedure is suggested to facilitate the computations of the true average molecular weights from g.p.c. data directly.     

Data handling in g.p.c. for routine operations

The main points of importance in g.p.c. for routine operation are discussed. Concerning reproductibility, the major problem is the elution volume control, several methods are compared; the influence of number and precision of detector signal measurements are presented. For calibration, working with THF as the eluant, the use of narrow fraction polystyrene standards is a necessary primary step. It furnishes, by classical summations, number and weight average molecular weights in equivalent polystyrenes; these values are already useful in sample characterization. In estimating true molecular weights, the simplest method is use of the Benoit factor and some practical applications are given. A simple method for taking axial dispersion into account is proposed where the spreading function is not Gaussian.     

Molecular weight determinations of polyamides by N-trifluoroacetylation

By N-trifluoroacetylation (TFA) polyamides become soluble in ordinary organic solvents such as acetone, butanone, methylene chloride, chloroform and tetrahydrofuran. In these solvents light scattering and osmotic measurements (membrane and vapour-phase osmometry) were carried out to determine the weight and number average degrees of polymerization. Comparison with the results on non-TFA-polyamides in various solvents clearly shows that no polymer degradation takes place. The [η]-P relationships of Trogamid^R-TFA and nylon-6-TFA in tetrahydrofuran at 25°C were determined and corrected for polydispersity.     

Prediction of molecular weight distribution of cellulose by using the rheological method

In this article, a rheological method that can predict the molecular weight distribution (MWD) of polymer was introduced. Using this method, the MWDs of four cellulose samples were compared from rheological data of the cellulose / N‐methyl morpholine N‐oxide (NMMO) / H₂O solutions. The MWDs of cellulose also were determined by gel permeation chromatography (GPC) calibrated with narrow distribution polystyrene standards, using 0.5% lithium chloride (LiCl) in N,N‐dimethylacetamide (DMAc) as the eluent. Comparison of the results from rheology and GPC showed that the MW and MWD of cellulose could be roughly inferred from their rheological data. Although the differential MWD obtained from the rheological method was bell shaped and can not reflect the fine characteristics of cellulose as GPC, it may be feasible to compare the MWDs of cellulose by using the rheological method. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 598–603, 2004     

Comparison of g.p.c. elution characteristics and diffusion coefficients of asphaltenes

A theoretical analysis of configurational effects on exclusion chromatography (g.p.c.) and diffusion rates of macromolecules is used to interpret data for the diffusion coefficients of petroleum-derived asphaltenes in tetrahydrofuran at 25 °C. If the asphaltene constituents are considered to be rigid, disc-like molecules, the theory and experiments are consistent in that both show the asphaltene diffusion coefficients to be smaller than those of g.p.c.-equivalent polystyrenes. Two conclusions result from this work: (1) polystyrene is not a good molecular model for predicting the diffusional characteristics of asphaltenes; and (2) there is a broad variation (by a factor of ten) of diffusion coefficients among the constituents of one asphaltene extract.     

Molecular weight distribution of phenol-formaldehyde resols by high performance gel permeation chromatography

Resol resins are particularly difficult to characterize with respect to molecular weight because of the large amounts of caustic soda (10–20% w/w) which they contain. A method is described for solubilizing these materials using trichloroacetic acid, such that molecular weight distribution can be measured using a normal gel permeation chromatography system with μ-Styragel columns and tetrahydrofuran as solvent.     

Melt rheology of segmented polyamides: Effect of block molecular weight

Segmented polyamides, also known as polyether‐ester‐amides, are composed of polyether and polyamide structural units. The rheological behavior of segmented polyamides with respect to the variations in the molecular weight of hard and soft blocks has been studied using a Monsanto Processability Tester. These systems exhibit pseudoplastic flow behavior. The shear viscosity of the segmented polyamides decreases with a decrease in hard block molecular weight up to 1500. However, at low shear rates, the shear viscosity shows marginal change with an increase in soft segment molecular weight. The equilibrium die swell increases with an increase in shear rate, but decreases with increasing temperature. The stress relaxation study of the segmented polyamides reveals that the stress developed during extrusion relaxes exponentially for all the systems. The equilibrium die swell at a fixed temperature and shear rate, the time required to relax a fixed amount of stress and the stress developed after a certain time interval decrease with a decrease in hard block molecular weight up to 1500, but increase with an increase in soft segment molecular weight. The activation energy of the melt flow process increases with the rate of shear in most of the cases. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1739–1747, 1999     

Block copolymer molecular weight determination via gel permeation chromatography: Choosing a combining rule

We present a method for accurately determining the true molecular weights of narrow‐distribution block copolymers, using only a basic gel permeation chromatograph (GPC) equipped with a refractive index detector and calibrated with polystyrene standards. Our approach is based on the well‐known observation that GPC calibration curves for different homopolymers in good solvents are essentially parallel, allowing the curves for different polymers to be described by simple hydrodynamic equivalence ratios r_B versus polystyrene. We present values of r_B, in both toluene and tetrahydrofuran, for various polydiene and hydrogenated polydiene homopolymers commonly incorporated into commercial styrenic block copolymers. These values of r_B must be combined to yield the hydrodynamic equivalence ratio of the block copolymer, from which the block copolymer's true molecular weight can be determined. Three combining rules proposed in the literature are tested against a series of symmetric polystyrene–polybutadiene diblock copolymers of varying molecular weight. A simple linear combining rule accurately represents the results. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2056–2069, 2001     

凝胶渗透色谱法测定瓶级PET切片相对分子质量的制样方法优化

样品制备是凝胶渗透色谱(GPC)测定瓶级PET切片分子质量的一个关键因素,样品剪切方法的差异、烘箱温度的误差、加热时间的变化、溶剂和溶解方法选择的不同等都有可能影响到测试的结果。通过优化以上这些因素可以使得测试结果更为准确。     

Determination of molecular weight and molecular sizes of polymers by high temperature gel permeation chromatography with a static and dynamic laser light scattering detector

Flow-mode static and dynamic laser light scattering (SLS/DLS) studies of polymers, including polystyrene, polyethylene, polypropylene and poly(dimethylsiloxane) (PDMS), in 1,2,4-trichlorobenzene (TCB) at 150 °C were performed on a high temperature gel permeation chromatography (GPC) coupled with a SLS/DLS detector. Both absolute molecular weight (M) and molecular sizes (radius of gyration, R_g and hydrodynamic radius, R_h) of polymers eluting from the GPC columns were obtained simultaneously. The conformation of different polymers in TCB at 150 °C were discussed according to the scaling relationships between R_g, R_h and M and the ρ-ratio (ρ=R_g/R_h). Flow-mode DLS results of PDMS were verified by batch-mode DLS study of the same sample. The presented technique was proved to be a convenient and quick method to study the shape and conformation of polymers in solution at high temperature. However, the flow-mode DLS was only applicable for high molecular weight polymers with a higher refractive index increment such as PDMS.     

Experimental study on the effect of molecular weight and chemical composition distribution on the mechanical response of high-density polyethylene

This article aims to appraise the effect of microstructure comprising molecular weight distribution and chemical composition distribution on the mechanical properties of high-density polyethylene (HDPE). HDPE resins were synthesized using several titanium–magnesium-supported Ziegler–Natta catalysts in the industrial gas phase reactor under the same polymerization condition. Gel permeation chromatography and crystallization elution fractionation (CEF) were conducted on the resins to characterize the molecular weight and comonomer distribution. Crystallization, thermal and rheological behavior were evaluated following differential scanning calorimetry, polarization light microscopy, and rheometric mechanical spectrometry. The resins with higher soluble fraction in trichlorobenzene below 80°C (highly branched low molecular weight chains) exhibited longer crystallization time based on the crystallization kinetic obtained from the Avrami model. Rheological determination of the molecular weight between entanglements (M_e) and the average lamella thickness based on the Gibbs–Thomson equation revealed that the entanglement density and impact strength decreased, and the average lamella thickness increased with an increase in the ratio of CEF eluted fraction below 80°C to the crystallizable fraction in the range of 80–90°C.     

Molecular weight distribution of polyacrylonitrile produced in supercritical CO2

An effectively linear molecular weight calibration curve of polyacrylonitrile (PAN) was obtained using a copolymer standard with a single broad molecular weight distribution. The molecular weights and molecular weight distributions of PAN obtained from precipitation polymerization of acrylonitrile in supercritical CO₂ were quantitated by the calibration curve. The effects of monomer concentration, initiator concentration, the CO₂ pressure, and the total reaction time on the molecular weight and molecular weight distribution were studied in detail. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2338–2341, 2002     

Synthesis and characterization of novel aromatic polyamides derived from 2,2′‐bis(p‐phenoxyphenyl)‐4, 4′‐diaminodiphenyl ether

BACKGROUND: Wholly aromatic polyamides (aramids) are high‐performance polymeric materials with outstanding heat resistance and excellent chemical stabilities due to chain stiffness and intermolecular hydrogen bonding of amide groups. Synthesis of structurally well‐designed monomers is an effective strategy to prepare modified forms of these aramids to overcome lack of organo‐solubility and processability limitations. RESULTS: A novel class of wholly aromatic polyamides was prepared from a new diamine, namely 2,2′‐bis(p‐phenoxyphenyl)‐4,4′‐diaminodiphenyl ether (PPAPE), and two simple aromatic dicarboxylic acids. Two reference polyamides were also prepared by reacting 4,4′‐diaminodiphenyl ether with the same comonomers under similar conditions. M?_w and M?_n of the resultant polymers were 8.0 × 10⁴ and 5.5 × 10⁴ g mol^?1, respectively. Polymers resulting from PPAPE exhibited a nearly amorphous nature. These polyamides exhibited excellent organo‐solubility in a variety of polar solvents and possessed glass transition temperatures up to 200 °C. The 10% weight loss temperatures of these polymers were found to be up to 500 °C under a nitrogen atmosphere. The polymers obtained from PPAPE could be cast into transparent and flexible films from N,N‐dimethylacetamide solution. CONCLUSION: The results obtained show that the new PPAPE diamine can be considered as a good monomer to enhance the processability of its resultant aromatic polyamides while maintaining their high thermal stability. The observed characteristics of the polyamides obtained make them promising high‐performance polymeric materials. Copyright © 2009 Society of Chemical Industry     

VAc/St乳液共聚物的相对分子质量及组成分布

采用3种加料方式[间歇法、滴加苯乙烯（5f）法及滴加预乳化液法]合成乙酸乙烯酯（VAc）／St乳液共聚物。考察了共聚合方式对VAc／St乳液共聚物相对分子质量及其组成分布的影响,间歇法所得乳液共聚物相对分子质量较大,分布较宽;而滴加预乳化液法较小,分布也较窄;滴加St法居中。m（VAc）／m（St）为7：3的乳液共聚物在30℃的四氢呋喃溶剂中Mark-Houwink常数为1．24&#215;10^-3mL／g．扩张因子为0．86。     

凝胶色谱法测试PVC树脂平均分子质量及分子质量分布

叙述了凝胶色谱法的测试原理,介绍了用凝胶色谱测试PVC树脂平均分子质量及分子质量分布的方法。     

凝胶色谱法测定木素分子量及其分布

综述了凝胶色谱法测定本素分子量及其分布的方法和取得的进展。新型高效凝胶位的出现,使凝胶色谱法可以测定多种本素分子量。用凝胶色谱法测定木素的分子量,需要根据样品的性质选择色谱技、流动相和标样,并通过实验确定合适的色谱条件。本素样品可能需要经过适当的预处理以适应色谱柱和流动相的要求。     

Change in the molecular weight distribution of poly(ethylene oxide) caused by the complexation with poly(acrylic acid)

The complexation between poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) was made by using double the molar quantity of either polymer component at pH 2 where the resulting complex completely precipitates. After the removal of the precipitate, PEO or PAA remaining in the supernatant was subjected to gel permeation chromatography to investigate the change in the molecular weight distribution (MWD) caused by the complexation. No remarkable difference is observed in the MWD curves for PEO[1] (M_w=1.37 × 10⁴) before and after the complexation with PAA[1] (M_w=1.10 × 10³) and PAA[2] (M_w=4.16 × 10⁵). However, the MWD curves of PEO[2] (M_w=1.26 × 10⁵) and PAA[2] become shortened and shift to the low molecular weight side after the complexation with PAA[1] or [2] and PEO[2], respectively. This tendency is enhanced by increasing the complexation temperature. From these results, it is indicated that the complexation between PEO and PAA deals with an equilibrium reaction, and the equilibrium constant is dependent on the chain length of both polymer components and also on the complexation temperature.     

凝胶渗透色谱－液相色谱－串联质谱法测定水产品中性激素

建立水产品中性激素的凝胶渗透色谱净化－超高效液相色谱－串联质谱分析方法。采用乙酸乙酯提取样品,经GPC净化后, LC－MS／MS测定,外标法定量。流动相为乙腈和0．1％甲酸水溶液,梯度洗脱,电喷雾正离子多反应模式监测。3种性激素在0．5～20．0μg／L线性范围内,相关系数大于0．998,定量限为0．3μg／kg,回收率为88．6％～96．4％,相对标准偏差为1．84％～6．24％。本方法灵敏度高、重现性好,适用于水产品中性激素的测定。     

GPC法测定PAN的平均相对分子质量及其分布

用凝胶渗透色谱 (GPC)测定PAN的平均相对分子质量及其分布。采用宽分布的PAN标准试样 ,在溶剂二甲基甲酰胺中加入 0 .0 4～ 0 .0 8mol/L的NaNO3替代LiBr ,提高了色谱柱的分离效果。用计算机模拟出PAN的GPC校正曲线方程 ,对宽分布的标准试样进行重复测定 ,结果相对标准偏差均小于 3 .5 %。     

GPC-GC法测定辣椒粉中乙草胺农药残留

建立了GPC-GC测定辣椒粉中乙草胺农药残留的方法。样品中的乙草胺经乙腈提取,凝胶透析色谱(GPC)净化后,在优化的色谱条件下,用中等极性毛细管柱(DB-1701)分离、电子捕获检测器(ECD)测定。结果表明乙草胺的标准曲线为Y=26394x+282.78,相关系数0.9907,试样中3个添加水平回收率为78.5%～97.6%,方法检出限为0.0005mg/kg。