Effect of block molecular weight distribution on the structure formation in block copolymer/homopolymer blends

The effect of homopolymer (hP) addition on the structure formation in lamellar amorphous block copolymers (BCP) with narrow‐ and broad‐molecular weight distribution (MWD) was studied using small‐angle X‐ray scattering and transmission electron microscopy. The systems in our study consist of blends of a poly(styrene‐b‐methyl acrylate) copolymer with block‐selective broad MWD of the poly(methyl acrylate) domain as well as polystyrene and poly(methyl acrylate) hPs with molecular weight less than the corresponding block of the copolymer. Homopolymer addition to the broad MWD domain of the BCP is found to induce structural changes similar to narrow MWD BCP/hP blend systems. Conversely, addition of hP to the narrow MWD domain is found to induce a more pronounced expansion of lamellar domains due to the segregation of the hP to the center region within the host copolymer domain. With increasing hP concentration, the formation of a stable two‐phase regime with coexisting lamellar/gyroid microphases is observed that is bounded by uniform lamellar phase regimes that differ in the distribution of hP within the corresponding narrow MWD block domain. The segregation of low‐molecular weight hP to the center region of the narrowdisperse domains of a broad MWD BCP is rationalized as a consequence of the more stretched chain conformations within the narrowdisperse block that are implied by the presence of a disperse adjacent copolymer domain. The increase of chain stretching reduces the capacity of the narrowdisperse block to solubilize hP additives and thus provides a driving force for the segregation of hP chains to the center of the host copolymer domain. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 106–116, 2012     

SIMULTANEOUS CALIBRATION OF MOLECULAR WEIGHT SEPARATION AND COLUMN DISPERSION OF SEC WITH CHARACTERIZED POLYMER STANDARDS

With the aid of the theoretical relationship between the calibration relation of a SEC column for the monodisperse polymer species under ideal working condition and the effective relations between the molecular weight and the elution volume for characterized polymer samples, a computational procedure for simultaneous calibration of molecular weight separation and column dispersion is proposed. From the experimental chromatograms of narrow MWD polystyrene standards and broad MWD 1,2-polybutadiene fractions the spreading factors of a SEC column was deduced by the proposed method. The variation of the spreading factor with the elution volume is independent upon the polymer sample used.     

H_2O/TiCl_4/Py体系引发苯乙烯正离子聚合

研究了在少量吡啶(Py)存在下由水(H2O)四氯化钛(TiCl4)体系引发苯乙烯于二氯甲烷正己烷中进行碳正离子聚合,分别考察[Py]、[H2O]和[TiCl4]对聚合速率、产物分子量与分子量分布的影响.实验结果表明,少量亲核试剂吡啶(Py)对聚合反应起着重要作用,可有效地降低聚合速率和使分子量分布变窄;随着[H2O]和[Py]降低或[TiCl4]增加,聚合产物的分子量增加,而分子量分布指数(Mw Mn)基本维持在1.8左右;随着[Py]增加,聚合速率降低;随着[H2O]和[TiCl4]增加,聚合速率提高.聚合速率对单体浓度呈一级动力学关系,对Py、H2O和TiCl4的反应级数分别为-0.72、0.72和1.86.聚合速率对TiCl4浓度呈接近二级动力学关系,这可能与体系中TiCl4主要以二聚体形式存在有关.聚合转化率和产物分子量均随着反应时间延长而逐渐增大,PS的数均分子量与转化率呈线性增加关系.     

凝胶色谱中宽、窄分子量分布的峰加宽改正问题

本文用宽分布(M_w/M_n=6.3)及窄分布(M_w/M_n=1.22)两种模型函数,改变一系列加宽因子,“合成”了具有不同加宽程度的GPC谱图,然后计算平均分子量并与原模型分子量进行比较,考察分子量分布对峰加宽改正程度的影响.结果表明:分子量分布宽、窄在峰形改正上有着明显差异,窄分布要比宽分布改正得多.但对于分子量平均值来说,宽分布与窄分布的峰加宽改正程度是相同的.因此,通常认为宽分布试样在平均分子量计算时可以忽略峰加宽改正的说法是一种误解。     

用多分散高聚物标样作凝胶色谱的分子量分离和扩展效应同时校准

根据凝胶色谱柱在理想工作条件下单分散高分子组分的校准关系与多分散试样的实效关系之间的理论联系,建议了一种简单的觅数方法,同时作凝胶色谱柱的分子量分离和扩展因子的校准。用本法从窄分布的聚苯乙烯和宽分布的1,2-聚丁二烯级分的实验谱图得到的所用凝胶色谱柱的扩展因子与淋出体积间的关系相互重合,与试样种类无关。     

气相渗透法中的溶质吸附效应

在气相渗透仪上，用四个低分子量有机化合物和五个已知平均分子量的窄分布聚苯乙烯低聚体，对ＶＰＯ方法中的溶质吸附效应作了研究，提出了一种对吸附效应作改正的方法．所得结果表明，在扣除溶质吸附效应后，仪器常数Ｋ无分子量依赖性，提高了测定低分子量聚合物数均分子量的准确性．     

吡啶类试剂调节H_2O/TiCl_4引发苯乙烯正离子聚合研究

1988年,Kennedy等[1~3]首先实现了由BCl3共引发的苯乙烯(St)及其衍生物活性正离子聚合,但所得到聚合产物的分子量分布宽(Mw/Mn=5~6).苯乙烯活性正离子聚合的共引发剂由BCl3扩展到SnCl4、TiCl4[4~7]和TiCl3(OiPr).对于TiCl4共引发的St活性正离子聚合,所用引发剂为2,4,4-三甲基-2     

气相渗透法测定数均分子量的仪器常数问题研究

讨论了蒸气压渗透计仪器常数的分子量依赖性问题．通过对某些已知分子量化合物的温差信号随时间变化的研究，提出了测量四个低分子一种改进的ＶＰＯ信号的测量手续和数据处理方式，可以得到更为准确的数均分子量．     

Living anionic polymerization of lauryl methacrylate and synthesis of block copolymers with methyl methacrylate

Anionic polymerization of lauryl methacrylate (LMA) with 1,1‐diphenylhexyl lithium in tetrahydrofuran (THF) at ?40 °C resulted in a multimodal and broad molecular weight distribution (MWD) with poor initiator efficiency. In the presence of additives such as dilithium salt of triethylene glycol (G₃Li₂), LiCl, and LiClO₄, the polymerization resulted in polymers with a narrow MWD (≤ 1.10). Diblock copolymers of methyl methacrylate (MMA) and LMA were synthesized by anionic polymerization using DPHLi as initiator in THF at ?40 °C with the sequential addition of monomers. The molecular weight distribution of the polymers was narrow and without homopolymer contamination when LMA was added to living PMMA chain ends. Diblock copolymers with broad/bimodal MWD were obtained with a reverse‐sequence monomer addition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 875–882, 2004     

Characterization of functionalized styrene-butadiene rubber by flow field-flow fractionation/light scattering in organic solvent

Flow field-flow fractionation (FlFFF) using an organic solvent as mobile phase has been effectively utilized for the separation and characterization of functionalized styrene-butadiene rubbers (SBR) that are polymerized and followed by coupling reaction in solution. Separation of broad molecular weight SBR was accomplished by an asymmetrical FlFFF channel in THF under field programming and the molecular weight distribution (MWD) of the SBR sample was determined by on-line measurement of light scattering. In this study, FlFFF has been utilized to characterize high-MW functionalized SBR from the low-MW non-functionalized molecules which were used for coupling reaction to produce high-MW functionalized SBRs, and to determine the coupling number of the functionalized SBRs depending on the type of the coupling reagents. The resulting MWD of the SBR samples prepared by the different coupling reagents (SnCl(4) and a polydimethylsiloxane compound) were compared.     

A selection method for high performance stationary phases in glass capillary GC. Part 1: The molecular weight distribution of polyglycol phases

The width of the molecular weight distribution (MWD) of polymer stationary phases influences their polarity and other gas chromatographic characteristics. The MWD of the most frequently used polyglycol-type stationary phases such as Carbowax, Ucon, Emulphor and Pluronic has been determined and compared. The MWD determined by gel chromatography allows for the characterization of the actual batch of a stationary phase, provides a check on its consistency and helps to screen out the apparently less promising phases.     

Characterization of Nonionic Polyacrylamides by Aqueous Size Exclusion Chromatography Using a DRI/LALLSP Detector System

Abstract

Herein is reported an experimental investigation of the molecular weight characterization of nonionic polyacrylamides by aqueous SEC with a DRI/LALLSP detector system. Methodology for the use of the DRI/LALLSP detector responses to determine the molecular weight calibration curve and the peak broadening parameter, [sgrave]² (variance of a Gaussian instrumental spreading function) over a wide molecular weight range has been developed. The method is based on the use of a broad MWD standard made by blending Polysciences broad MWD standards and a generalized analytical solution of Tung's integral equation for the detector response corrected for peak broadening. Molecular weight averages measured by SEC/DRI/LALLSP are in excellent agreement with those measured offline by LALLSP.     

Molecular weight characterization of soluble high performance polyimides. 2. Validity of universal SEC calibration and absolute molecular weight calculation

Six different soluble high-performance aromatic polyimides, each prepared by solution imidization to three controlled average molecular weights, were analyzed by size exclusion chromatography (SEC) using on-line parallel coupled refractometric and viscometric detectors. N-methylpyrrolidone (NMP) with 0.06 M LiBr and NMP stirred over P₂O₅ were used as mobile phase for four of the polyimides; NMP with 0.06 M LiBr and NMP stirred over P₂O₅ were used as mobile phases for four of the polyimides; NMP with 0.06 M LiBr tetrahydrofuran (THF) and chloroform served as mobile phases for the other two polyimides. For all the samples the stationary phase in the SEC columns was cross-linked polystyrene beads. Molecular weight averages of the polyimides were calculated using universal SEC calibration with polystyrene standards in each solvent. The agreement of the calculated molecular weight averages in the different solvents confirms that the universal SEC calibrations are valid for these semiflexible polymers. There was good agreement with weightaverage molecular weights obtained by low-angle laser light scattering (LALLS) performed in pure NMP. Intrinsic viscosity and molecular weight data for a series of nine samples of one polyimide covering a M_w = 20,000–70,000 g mol^–1 interval were treated to obtain Mark-Houwink-Sakurada constants. Unperturbed chain dimensions of this polyimide were obtained by application of the Stockmayer-Fixman extrapolation procedure to these data. ©1995 John Wiley & Sons, Inc.     

Round‐robin experiment in high‐temperature gel permeation chromatography

The results of an interlaboratory or round‐robin experiment in high‐temperature gel permeation chromatography (HT‐GPC) analysis are presented. The intention was to determine and raise awareness of interlaboratory reproducibility of HT‐GPC techniques. Fifteen laboratories performed analyses of five polyethylene samples and standards SRM 1475 and 1476. Reproducibility, as measured by the interlaboratory standard deviation (s_LAB), was greatly influenced by the breadth of the molecular weight distribution (MWD) and branching. The s_LAB values for the weight‐average molecular weight (M_w) of linear polyethylenes of narrow and broad MWDs were 4 and 14%, respectively. For branched polymers, GPC viscometry methods are shown to measure significantly higher molecular weights than the noncoupled GPC method, with higher variance. For branched polyethylenes measured with GPC viscometry, the reproducibility of M_w was characterized by s_LAB = 18%. Reproducibility of the SRM 1475 standard was better than for unknowns. The results for branched standard SRM 1476 emphasize the important role of the detection method in GPC but call into question the use of this material as a molecular weight standard. For single‐site polyethylene, only a handful of labs measured an MWD that closely matched the Flory distribution. Qualitatively, the responses indicate that many variations in instrument and analytical methods exist among laboratories; this is partly a reflection of the development and refinements that this technique must yet undergo before a truly standard method is widely accepted and practiced. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 905–921, 2002     

Organic nanoparticles as fragmentable support for Ziegler–Natta catalysts

A fragmentable support material for Ziegler–Natta catalysts is presented based on micrometer‐sized aggregates of polystyrene nanoparticles. Hydroxyl anchoring groups are introduced by copolymerization of hydroxymethylstyrene in emulsion process to immobilize the catalysts. The catalytic activity in ethylene slurry polymerizations is found to be directly correlated to the hydroxyl group content of the supports. Furthermore, the fragmentation behavior of dye‐labeled support aggregates into the initial nanoparticles is demonstrated using laser scanning confocal fluorescence microscopy as a nondestructive method. These supported catalysts fulfill two important design criteria, high fragmentability and high catalyst loading, and produce high‐density polyethylene with medium molecular weight distributions (MWDs = 3–4). These values lie between those obtained using single‐site metallocene‐based (narrow MWD < 3) or inorganic supported multi‐site Ziegler–Natta‐based (broad MWD = 4–12) polymerizations without the need of blending. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 15–22     

Laser desorption/ionization on porous silicon mass spectrometry for accurately determining the molecular weight distribution of polymers evaluated using a certified polystyrene standard.

Desorption/ionization on porous silicon-mass spectrometry (DIOS-MS) is a novel soft ionization MS technique that does not require any matrix reagent, ideally resulting in fewer obstructive peaks in the lower mass region. In this study, the etching conditions of porous silicon spots as an ionization platform of DIOS-MS were investigated for determining the molecular weight distribution (MWD) of polymers. To evaluate the accuracy of DIOS mass spectra observed using porous silicon spots prepared under various etching conditions, a certified polystyrene (PS) standard sample with an average molecular weight of ca. 2400 was used as a model sample. By optimizing the etching conditions, the MWD of the PS sample could be accurately observed by DIOS-MS using both p-type and n-type porous silicon spots. Especially, in the case of a suitable n-type spot, an accurate peak distribution with very fewer obstructive background peaks could be observed using the minimum laser power, comparable to the conventional matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS).     

Effect of multiple extrusion on molecular structure of polypropylene impact copolymer

Neat and multiple processed polypropylene impact-copolymer (ICPP) were fractionated using series of hydrocarbon solvents with increasing solvent power. The analyses of the fractions obtained in successive extractions showed significant decrease in weight-average molecular weight (Mw) and narrowing the molecular weight distribution (MWD) of investigated samples after extrusions. Although the changes due to thermooxidation were observed in all phases of the system, the most intensive degradation was found in the prevailing PP homopolymer phase.     

Influence of n‐dodecyl mercaptan as chain transfer agent in dispersion polymerization of styrene

Dispersion polymerization of styrene with n‐dodecyl mercaptans (DDM) as the chain transfer agent was investigated. PS particles with various molecular weight, molecular weight distribution (MWD), and particle diameter were prepared by varying the concentration of DDM and also the addition time of DDM before and after the particle nucleation. The average particle diameter was increased, whereas polymerization rate, molecular weight, and MWD were decreased with increasing DDM concentrations from 0 to 10 wt %. The effect of addition of DDM before and after particle nucleation was studied at 0.4, 0.8, and 1.0 wt % DDM. The addition of DDM before particle nucleation produced PS particles of relatively large particle diameter and low molecular weight when compared with the addition of DDM after particle nucleation. This study shows that particle nucleation occurs in about 5–6 min, which corresponds to the 15–16% conversion, 372–378 nm in D_n , and provides a facile way to control the particle size and interesting information about the particle formation using the delayed addition of DDM. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6612–6620, 2008     

Calculation of Molecular Weight Distributions in Non‐Linear Free‐Radical Polymerization Using the Numerical Fractionation Technique

Mathematical Modeling of non‐linear polymerization systems subject to gel formation is a challenging endeavor. At the gel point, the second and higher molecular weight moments diverge to infinity making it impossible to obtain the molecular weight distribution (MWD). The numerical fractionation (NF) technique utilizes a refinement of the method of moments to model non‐linear polymerization systems that form gel. Since the method of moments yields results in terms of average quantities, some information is lost when reconstructing the MWD using NF. As a consequence, a broad shoulder appears at the high chain length end of the MWD tail. This study demonstrates that the validity of the gamma distribution deteriorates for the broader branched polymer generations and evaluates the performance of various alternative model distributions. Proper selection of the model distribution enhances the NF‐reconstructed MWD.     

Molecular weight distributions of linear polymers: Detailed analysis from GPC data

The GPC method is used widely to measure molecular weights of linear polymers. High-quality GPC data contains detailed information on many aspects of the polymer's molecular weight distribution (MWD). This information can be extracted from the data using computer analysis. Equations have been derived for the two simplest MWD functions in the GPC coordinates: the Flory function (one growing polymer chain produces one polymer molecule), and for the case when two polymer radicals combine into one polymer molecule. The equations were used to analyze MWD of two classes of polymers. The first class includes polymers with narrow MWD: polyethylene, ethylene-propylene and ethylene-hexene copolymers, syndiotactic polystyrene, and radical polystyrene. The second class includes polymers with broad MWD: ethylene copolymers and polypropylene produced with heterogeneous, Ti-based catalysts. The examples demonstrate that the resolution of complex GPC curves into their constitutents serve as an important source of information about kinetics of polymerization reactions. © 1995 John Wiley & Sons, Inc.