Polymer HPLC chemical structure

Whereas SEC is the dominant technique for the characterisation of polymers, various nonexclusion liquid chromatographic (NELC) methods, such as GPEC and LACCC offer equally valid possibilities for deformulation of complex polymer systems. In fact, molecular characterisation of polymers in the precip-itation/adsorption mode (gradient HPLC) enables differences in chemical structure and composition to be... 

In the case of heterogeneous polymers the experimental methods need to be refined. In order to analyze those polymers it is necessary to determine a set of functions / (M), which describe the distribution for each kind of heterogeneity i This could be the mass distributions of the blocks in a diblock copolymer. The standard SEC methods fail here and one needs to refine the method, e.g., by performing liquid chromatography at the critical point of adsorption [59] or combine SEC with methods, which are, for instance, sensitive to the chemical structure, e.g., high-pressure liquid chromatography (HPLC), infrared (IR), or nuclear magnetic resonance spectroscopy (NMR) [57],... 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

In any 2D HPLC, it is important to attain certain degree of both the complementarity and the orthogonality between the two separation dimensions [255-257]. The so far most universal approach to 2D polymer HPLC assumes the partial or possibly full suppression of the molar mass effect in the first dimension of the separation so that the complex polymer is separated mainly or even exclusively according to its chemical structure. Selected coupled methods of polymer HPLC are to be applied to this purpose. In the second dimension of separation—it is usually SEC—the fractions from the first dimension are further discriminated according to their molecular size. Exceptionally, SEC can be used as the first dimension to separate complex polymer system according to the molecular size. This approach is applicable when the size of polymer species does not depend or only little depends on their second molecular characteristic, as it is the case of the stereoregular polymers... 

High performance liquid chromatography (HPLC) of polymers is often thought to be synonymous with Size Exclusion Chromatography for separation by molar size. The present article deals with nonexclusion chromatography of polymers which enables separation by differences in the chemical structure and composition. 

Frequently, when analyzing a complex polymer, the first step must be the determination of the gross composition. Only when the chemical structures of the polymer components are known, can sophisticated separation techniques, such as gradient HPLC or LC-CC, be adapted to a specific analysis. 

On-line HPLC-NMR offers unique opportunities for analyzing complex polymer systems. Using a selective chromatographic technique, the sample can be separated according to the chemical structure. The structural analysis of the polymer can then be conducted by on-line H-NMR. Because of sensitivity enhancements and inq)roved solvent suppression techniques of the NMR, it is possible to work under experimental conditions which are common in liquid chromatography of polymers, i.e. sufficiently high flow rate, moderate sample concentration, protonated solvents, and on-flow detection. 

The chemical structure of the two polymers can be characterized by several techniques Fourier transform infrared (FTIR) nuclear magnetic resonance (NMR) x-ray diffraction (XRD) transmission electron microscopy (TEM) scanning electron microscopy (SEM) mass spectroscopy (MS) ultraviolet spectrometry (UV) and electron scanning for chemical analysis (ESCA). The chemical structure of the two polymers can be analyzed by IR, NIR, or various types of NMR spectroscopy. Determining the structure of chitin and chitosan usually requires the application of combination of various methods. The combination of IR, NIR, and various techniques of NMR give ample information on the chemical structure. IR, NIR, and various types of NMR are less sensitive than that of other quantitative analysis such as UV, HPLC, GC, and MS. 

In many cases functionality does not affect the hydrodynamic volume of the whole polymer chain significantly and SEC can separate them in terms of molecular weight. In order to separate the polymers in terms of functionality, however, IC has to be used. Pasch and Hiller separated a technical oligo(ethylene oxide) by isocratic RPLC with respect to degree of polymerization and functional end groups, and analyzed the chemical structure by on-line H-NMR detection [ 170]. The experiments was conducted under conditions that are common for HPLC... 

Figure 8.10. HPLC trace of poly(3-hexylthiophene) (P3HT). The flow rate of the polymer solution ( 10 M in tetrahydrofuran) was 1.2 ml min". The inset shows the chemical structure of P3HT. Reprinted with permission from Reference 48. Copyright 1987 American Chemical Society.

Coupled HPLC-NMR measurements performed at slow flow rates in fully deuterated solvents and at room temperature have been made in several studies to determine polymer MWD, to analyze the end-groups and the copolymer chemical composition distribution, and to assess the chemical structure and the degree of polymerization of all oligomer species [176-178]. Gradient HPLC-NMR was used in the analysis of the chemical composition distribution of random poly (styrene-co-ethyl acrylate) copolymers [179]. A major drawback in most of these studies is that the measurements could only be conducted at ambient or slightly elevated temperatures, which limits the method applicability, since many polymers, such as polyethylene, polypropylene, and polyolefin copolymers are soluble at high temperatures. 

Nuclear magnetic resonance (NMR) spectrometers offer spectral capabilities to elucidate polymeric structures. This approach can be used to perform experiments to determine comonomer sequence distributions of polymer products. Furthermore, the NMR can be equipped with pulsed-liied gradient technology (PFG-NMR), which not only allows one to determine self-diffusion coefficients of molecules to better understand complexation mechanisms between a chemical and certain polymers, but also can reduce experimental time for acquiring NMR data. Some NMR instruments can be equipped with a microprobe to be able to detect microgram quantities of samples for analysis. This probe has proven quite useful in GPC/NMR studies on polymers. Examples include both comonomer concentration and sequence distribution for copolymers across their respective molecular-weight distributions and chemical compositions. The GPC interface can also be used on an HPLC, permitting LC-NMR analysis to be performed too. Solid-state accessories also make it possible to study cross-linked polymers by NMR. 

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