Molecular characterization of block copolymers by means of liquid chromatography

Abstract

The full adsorption–desorption (FAD) procedure was applied to the selected model di- and tri-block copolymers. The dynamic integral desorption isotherms were measured for various homo- and block-copolymers of poly(methyl methacrylate) and poly(glycidyl methacrylate) in a system of non-porous silica–dichloroethane adsorli–tetrahydrofuran desorli. The aim was to evaluate separation selectivity of the FAD approach toward molar mass and chemical composition of macromolecules. It was demonstrated that under optimum conditions the FAD procedure can discriminate parent homopolymers from di-block copolymers, as well as di-block from tri-block copolymers when the adsorptivities of blocks differ sufficiently. The molar mass of both kinds of polymer chains affected the course of their desorption in present system of adsorbent–adsorli/desorli. Consequently the block copolymers studied could not be effectively fractionated according to their composition by a single FAD procedure. A combined method, full adsorption–desorption plus size exclusion chromatography was proposed for the species with selectively adsorbing blocks to provide a two dimensional fractionation of block copolymers.

Introduction

Block copolymers belong to a group of polymeric materials which are very interesting from both scientific and technological points of view. Recently, several original routes for synthesis of block copolymers were proposed and numerous combinations of chemically different chains were prepared. Unfortunately, the exact and unambiguous molecular characteristics of resulting materials are often missing. For example, little may be known about presence of (small amounts) of both parent homopolymers and various parasitic structures such as multiblock and branched macromolecules in many products of block copolymer synthesis. For example, size exclusion chromatography (SEC) which is often used for block copolymer characterization, discriminates parent homopolymers (precursors) from di-block copolymers if both blocks are sufficiently long but may fail to separate di-block from tri-block species. Further, the knowledge of molar mass distribution of at least one block in block copolymers may be limited because SEC directly produces only semi-quantitative, effective values.

Recently, we have briefly discussed general problems of the separation of complex polymers including various kinds of block copolymers by coupled liquid chromatographic (LC) procedures [1]. We have stressed the well known but often overlooked fact that size exclusion chromatography alone can only exceptionally allow a precise characterization of complex polymers such as copolymers, polymer blends or functionalized polymers which usually exhibit more than one kind of distribution of their molecular characteristics. This is because the separation mechanism of SEC is based on differences in size of polymer species in solution. All three main molecular characteristics of complex polymers, namely their molar mass (MM), chemical composition (CC) and architecture, simultaneously influence the sizes of macromolecules. Therefore, various LC methods must be applied to complex polymer systems. These combined LC methods include at least two separation mechanisms either within one single column or within a series of columns. All this applies also to the block copolymers.

Let us consider a di-block copolymer with polydisperse blocks. Even if molecular architecture of both blocks is constant and known and the composition of macromolecules in effluent is continuously monitored, one has to know the functional dependence between hydrodynamic volume of macromolecules on the one hand and their MM and CC on the other hand in order to properly interpret the SEC chromatogram of the sample in terms of average molar masses and molar mass distributions (MMD) of both blocks. The above dependence is, however, usually unknown. The situation becomes even more complicated as the number of blocks increases.

Eluent gradient liquid adsorption chromatography (LAC) was shown to be a promising method for separation of statistical [2], [3], [4] and graft [5] copolymers according to their chemical composition. The LAC fractions obtained were further off-line separated and characterized by means of SEC [6], [7]. However, separation of block copolymers with eluent gradient liquid adsorption chromatography seems to be ineffective [8].

A very elegant approach to the LC characterization of block copolymers offers methods that combine exclusion (entropic) and adsorption (enthalpic) separation mechanisms in such a way that their effects mutually compensate. In the compensation locus which is also called the ‘point of exclusion–adsorption transition’, chains of homopolymers are eluted from the LC column in one single retention volume, independently of their molar mass. The best known such compensation method is liquid chromatography at the critical adsorption point (LC CAP). The existence of critical adsorption point was experimentally disclosed by Belenkii, Gankina and Tennikov [9], [10], [11] and since then discussed in numerous papers (for review see e.g. [12]). The theory of LC CAP was elaborated by Gorbunov and Skvortsov (for review see [13]) and also by Guttman et al. [14]. The former authors also proposed utilization of LC CAP for separation of block copolymers [15]. In this case, one block of a block-copolymer is eluted under CAP conditions, irrespective of its molar mass. At the same time, another, chemically different block may be eluted under conventional SEC conditions and its length can be determined without interference from the first (LC CAP eluted) block which is chromatographically ‘invisible’. This idea was applied to several model block copolymers by Gankina et al. [15] in thin layer chromatography model and by Zimina et al. [16], [17] and later also by Pasch et al. [18] in column LC arrangement, (for review see [12]). Unfortunately, the LC CAP method suffers from several serious drawbacks reviewed in papers [19], [20]. These include high sensitivity of the critical adsorption point position towards eluent composition, temperature, column packing adsorptive properties and possibly also towards pressure. This behavior seriously limits repeatability and reproducibility of the LC CAP measurements. The LC CAP peaks are often broadened [16], [19], [21] and sometimes even split [19]. Moreover, sample recovery is decreased for high molar masses, especially for macromolecules, which are excluded from the pores of at least a part of the LC column packing [21]. Consequently, alternative coupled LC methods for separation of block copolymers are looked for.

One of these novel methods is based on the combination of full adsorption–desorption approach (FAD) with SEC [22], [23], [24], [25], [26]. FAD includes a complete adsorption of polymer sample to be separated from an adsorption promoting liquid (adsorli) onto an appropriate adsorbent which is packed in an especially designed LC-like (micro) column. In the following series of steps, macromolecules are successively displaced from adsorbent by several different eluents with increasing desorbing strength. Alternatively, temperature can be adjusted in a controlled way to displace adsorbed polymer species from the FAD column. Desorption of macromolecules is generally governed by their molecular characteristics, mainly by their MM and their chemical nature, that is by their CC [24], [25], [26], [27], [28]. Therefore, fractionation of polymers according to these parameters can be anticipated in the course of the FAD process. The FAD fractions can be characterized by different analytical methods in both on-line and off-line arrangements. Very advantageous is the coupling of a SEC instrument with the FAD column [22], [23], [24], [25], [26], [27], [28], [29], [30]. For homopolymers, the on-line SEC enables to easily and rapidly monitor amount, molar mass and molar mass distribution of macromolecules leaving FAD column. In this way, constituents of polymer blends can be discriminated and independently characterized [22], [23], [24], [25], [26].

In our recent paper [31], potential and limitations of the FAD method were elucidated for separation of statistical copolymers. We concluded that the single FAD fractionation reflects simultaneously both molar mass and composition of macromolecules and therefore independent determination of above parameters is difficult. On the other hand, the multidimensional combinations such as SEC/FAD/SEC or LAC/FAD/SEC seem to be promising for molecular characterization of statistical copolymers.

In this series of papers, we shall discuss liquid chromatographic separation of block-copolymers. We shall start with application of FAD method to block-copolymers consisting of two (A-block-B) and three blocks (A-block-B-block-A and B-block-A-block-B). Well-defined di- and tri-block copolymers of various average molar masses have been synthesized by sequential anionic polymerization of methyl methacrylate (A) and glycidyl methacrylate (B).The desorption processes for some model block-copolymers have been investigated in order to evaluate feasibility of the full adsorption–desorption approach to their separation.