Copolymer molar mass chemical composition distribution

Figure 7 Experimental (a) and model (b) molar mass chemical composition distribution (MMCCD) of a high-conversion (95 mol.%) STY/MA emulsion copolymer (Is = 0.33, monomer-to-water ratio (MM) = 0.5,1wt.% n-dodecylmercaptan, 50 °C, M =110000gmol ). Reprinted from van Doremaele, G. H. J. Geerts, F. H. J. M. aan de Meulen, L. J. German, A. L. Polymer 99Z, 33,1512-1518. ...

Continuous thermodynamics provides a simple way for the thermodynamic treatment of polydisperse systems. Such systems consist of a very large number of similar species whose composition is described not by the mole fractions of the individual components but by continuous distribution functions. For copolymers, multivariate distribution functions have to be used for describing the dependence of thermodynamic behavior on molar mass, chemical composition, sequence length, branching, etc. 

When deaUng with copolymer systems, one encounters the special problem of copolymer characterization since a copolymer is far from well-defined only by its chemical formula. Copolymers vary by a number of characterization variables. Molar mass, chemical composition, and distribution functions, tacticity, sequence distribution, branching, and end groups determine their thermodynamic behavior in solution. It is far from clear how these parameters influence the thermodynamic properties in detail. Unfortunately, there usually is not much information in the original papers the available ones are added to each system in this book. 

These materials, however, as a rule exhibit rather broad chemical composition distribution. Block copolymers may contain important amounts of parent homopolymer(s) [232,244,269], In any case, it is to be kept in mind that practically all calibration materials contain the end groups that differ in the chemical composition, size, and in the enthalpic interactivity from the mers forming the main chain. In some cases, also the entire physical architecture of the apparently identical calibration materials and analyzed polymers may differ substantially. The typical example is the difference in stereoregularity of poly(methyl and ethyl methacrylate)s while the size of the isotactic macromolecules in solution is similar to their syndiotactic pendants of the same molar mass, their enthalpic interactivity and retention in LC CC may differ remarkably [258,259]. 

Strictly speaking, Balke s system combined SEC in the first dimension with a mixed mode separation in the second dimension. Since SEC separates with respect to hydrodynamic volume and not molar mass, the copolymers under investigation could not be quantified with respect to molar mass distribution (see discussion in Sect. 3). The effect of the SEC separation was simply to obtain fractions with narrower molar mass distribution as compared to the total sample. Considering this fact, it is clear that for chemically heterogeneous copolymers no quantitative data can be obtained from the first dimension. Only the second dimension, separating with respect to chemical composition, can provide quantitative information on the chemical composition distribution. Accordingly, a coupled information on both MMD and CCD was not available for this system. 

The mapping of ethoxylated fatty alcohols and ethylene oxide-propylene oxide block copolymers by 2D chromatography was discussed by Trathnigg et al. [95]. They combined LAC and SEC and were able to determine the chemical composition distribution and the molar mass distribution of the polyethers. 

Fig. 1 Molar mass distribution with overlaid chemical composition distribution of a styrene-MMA block copolymer with poor block formation.

In this section some of the methods used to analyse for the chemical composition (CQ and chemical composition distribution (CCD) of binary and ternary copolymers (henceforth termed copolymers and terpolymers respectively) are discussed. Some practical examples from the literature are given to illustrate the methods. Further, the determination of the chemical composition of copolymers as function of their molar mass as well as the three-dimensicxial combination of the MMD and CCD, namely MMCCD, are treated. Examples of both emulsion polymers and polymers produced by solution and bulk polymerization are given, because with respect to the determination of molar mass and chemical composition emulsion (co)polymers do not require an essentially different approach. 

Synthetic polymers are produced by chain polymerization or step growth polymerization. Due to differences in the lifetime of activated species or the size and reactivity of the oligomers which are coupled in each reaction step, synthetic polymers are heterogeneous in molar mass. Copolymers are produced from more than one monomer species. In general, the different monomer species are differently incorporated in the polymer chain which causes distribution in chemical composition. Distributions in molar mass and chemical composition are also to be expected in polymers derived from homopolymers by incomplete chemical modifications, e.g. in partially hydrolyzed poly(vinyl acetate) [1]. 

Contrary to the usual organic compounds, polymers are far from being homogeneuos maferials (i.e., polymer chains do not possess the same molar mass and chemical structure). As matter of fact, many synthetic polymers are heterogeneous in several respects. Homopolymers may exhibit both molar-mass distribution (MMD) and end-groups (EG) distribution. Copolymers may also show chemical composition distribution (CCD) and functionality distribution (FTD) in addition to the MMD. Therefore, different kinds of heterogeneity need to be investigated in order to proceed to the structural and molecular characterization of polymeric materials. 

Using on-line coupled SEC- H NMR, the chemical composition of eluates was continuously monitored. It was found that the ethyl acrylate-rich copolymers exhibited a broader molar mass and chemical composition distribution than styrene-rich copolymers. The results indicated that the block character of the copolymers with respect to ethyl acrylate units increased with increasing molar mass. 

Due to the statistical character of the polymer forming reactions, macromolecules are not identical. The macromolecules differ in their molar masses, in the sterically arrangement, and in case of copolymers, in their chemical compositions, and so on. Because of the influence of the molecular structure on the properties, all properties of a polymer must show a distribution. The measuring procedures can only determine a mean value of a distribution curve. The mathematical nature of the mean value depends on the physical basics of the measuring procedure. It is necessary to determine not only a mean value of the property, but also the distribution function of the interesting property. The most important distribution for homopolymers is their molar mass distribution function. 

Polymers vary by a number of characterization variables. The molar mass and their distribution function are the most important variables. However, tacticity, sequence distribution, branching, and end groups determine their thermodynamic behavior in solutions too. For copolymers, the chemical distribution and the average chemical composition are also to be given. Unfortunately, much less iirformation is provided with respect to the polymers or copolymers that were applied in most of the thermodynamic investigations in the original hterature. In many cases, the samples are characterized only by one or two molar mass averages and some additional information (e.g., Tg, T, Pb, or how and where they were synthesized). Sometimes even this information is missed. 

Several attempts have been made to solve the calibration dilemma. Some are based on the tmiversal calibration concept, which has been extended for copolymers another approach to copolymer calibration is multiple detection. The advantages of multiple detection lie in its flexibility and the fact that it yields the composition distribution as well as molar masses for the copolymer under investigation [7]. This method requires molar mass calibration and an additional detector response calibration to determine the chemical composition at each point of the elution profile. No other kind of information, parameters or special equipment are necessary to carry out this kind of analysis and to calculate compositional drift, bulk composition, and copolymer molar mass [7a],... 

The major difficulty in the determination of the copolymer molar mass distribution is the fact that the GPC separation is based on the molecular size of the copolymer chain. Its hydrodynamic radius, however, is dependent on the type of comonomers incorporated into the macromolecule and their placement (sequence distribution). Consequently, there can be a coelution of species having different chain lengths and different chemical compositions. The influence on the chain size of different comonomers copolymerized into the macromolecule can be measured by GPC elution of homopolymer standards of this comonomer. Unfortunately, the influence of the comonomer sequence distribution on the hydrodynamic radius cannot be described explicitly by any theory at present However, there are limiting cases that can be discussed to evaluate the influence of the comonomer placement in a macromolecular chain. 

Fig. 9. Simultaneous determination of molar mass and chemical composition distributions of a complex styrene/MMA block copolymer using GPC in combination with Rl and UV detection.

Abstract The synthesis and characterization of polyolefins continues to be one of the most important areas for academic and industrial research. One consequence of the development of new tailor-made polyolefins is the need for new and improved analytical techniques for the analysis of polyolefins with respect to molar mass, molecular topology and chemical composition distribution. This review presents different new and relevant techniques for polyolefin analysis. The analysis of copolymers by combining high-temperature SEC and FTIR spectroscopy yields information on chemical composition and molecular topology as a function of molar mass. Crystallization based fractionation techniques are powerful methods for the analysis of short-chain branching in LLDPE and the analysis of polyolefin blends. These methods include temperature-rising elution fractionation, crystallization analysis fractionation and the recently developed crystaUization-elution fractionation. 

Determination of Chemical Composition Distribution by Dual Detection Based on detection with independent concentration detectors, this approach provides information on the chemical composition of copolymers or polymer blends and the variation with the molar mass [26]. An independent detector signal is required for each comonomer in the macromolecule. Many results have been published for chemically heterogeneous samples using two detectors. Obviously, there is a practical limit (four signals have been reported so far), as there are not many detectors to choose from. In addition, the approach is limited by the contribution of each detector to band broadening. 

Polymer. The polymer determines the properties of the hot melt variations are possible in molar mass distribution and in the chemical composition (copolymers). The polymer is the main component and backbone of hot-melt adhesive blend it gives strength, cohesion and mechanical properties (filmability, flexibility). The most common polymers in the woodworking area are EVA and APAO. 

State-of-the-art polymeric materials possess property distributions in more than one parameter of molecular heterogeneity. Copolymers, for example, are distributed in molar mass and chemical composition, while telechelics and macromonomers are distributed frequently in molar mass and functionality. It is obvious that n independent properties require n-dimensional analytical methods for accurate (independent) characterization of the different structural parameters. 

A useful approach to detection in polymer HPLC presents the on-line hyphenation of different measurement principles. For example, an RI detector combined with a UV photometer produces valuable additional information on the composition of some copolymers. Further progress was brought with the triple detection RI plus LALS plus VISCO [313], which is especially suitable for branched macromolecules and the tetra detection UV plus RI plus LALS plus VISCO, which enables characterization of some complex polymer systems, exhibiting a distribution not only in their molar mass and architecture, but also in their chemical composition such as long chain branched copolymers. 

The term cross-fractionation (CF) refers to analyses of distributions in differing directions by means of separation processes. Cross-fractionation is a significant tool for the evaluation of the complex distribution which copolymers normally have with respect to molar mass (MMD) and chemical composition (CCD). The idea of CF implies separation by one parameter and subsequent analysis of the fractions obtained for the distribution of the other parameter through another separating process. 

The copolymer can be further fractionated by precipitation from acetone solution to n-hexanc at room temperature. In each case, only the first fraction should be used to obtain narrowly distributed high molar mass copolymer chains for LLS measurement, ll NMR can be used to characterize the copolymer composition. The ratio of the peak areas of the methine proton of the isopropyl group in NIPAM and the two protons neighboring the carbonyl group in VP can be used to determine the VP content. The composition of each NIPAM-co-VP copolymer was found to be close to the feeding monomer ratio prior to the copolymerization. The nomenclature used hereafter for these copolymers is NIPAM-co-VP/x/y, where x andy are the copolymerization temperature (°C) and the VP content (mol%), respectively. The solution with a concentration of as low as 3.0 x 10-6 g/mL can be clarified with a 0.45 cm Millipore Millex-LCR filter to remove dust before the LLS measurement. The resistivity of deionized water used should be close to 18 M 2 cm. The chemical structure of poly(NIPAM-co-VP) is as follows (Scheme 2). 

The structural complexity of synthetic polymers can be described using the concept of molecular heterogeneity (see Fig. 1) meaning the different aspects of molar mass distribution (MMD), distribution in chemical composition (CCD), functionality type distribution (FTD) and molecular architecture distribution (MAD). They can be superimposed one on another, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- or bifunctional, copolymers can be block or graft copolymers, etc. In order to characterize complex polymers it is necessary to know the molar mass distribution within each type of heterogeneity. 

For copolymers, in particular random copolymers, instead of discrete functionality fractions a continuous drift in composition is present (see Fig. 3). To determine this chemical composition drift in interrelation with the molar mass distribution, a number of classical methods have been used, including precipitation, partition, and cross-fractionation [2]. The aim of these very laborious techniques is to obtain fractions of narrow composition and/or molar mass distribution which are then analyzed by spectroscopy and SEC. 

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