Enthalpic interactions, importance

In polymer solutions or blends, one of the most important thennodynamic parameters that can be calculated from the (neutron) scattering data is the enthalpic interaction parameter x between the components. Based on the Flory-Huggins theory [4T, 42], the scattering intensity from a polymer in a solution can be expressed as... 

For these reasons, it is very important to effectively suppress the enthalpic interactions in SEC so that Benoit s plot is valid for both calibration standards and characterized polymers. This is done by choosing non (inter) active column packings, adjusting the temperature of the experiment, and applying interaction (mainly adsorption) suppressing liquids (single or mixed) as eluents. 

If the backbone as well as the side chains consist of flexible units, the molecular conformation arises out of the competition of the entropic elasticity of the confined side chains and the backbone [ 153 -155]. In this case, coiling of the side chains can occur only at the expense of the stretching of the backbone. In addition to the excluded volume effects, short range enthalpic interactions may become important. This is particularly the case for densely substituted monoden-dron jacketed polymers, where the molecular conformation can be controlled by the optimum assembly of the dendrons [22-26,156]. If the brush contains io-nizable groups, the conformation and flexibility may be additionally affected by Coulomb forces depending on the ionic strength of the solvent [79,80]. 

The concept of entropy-enthalpy compensation resulting in the critical conditions of enthalpic interactions and the molar mass independent sample retention turned out useful also for the understanding several other coupled methods of polymer HPLC. It is accepted [195,196] that the polymer species tend to elute at the critical conditions also when either eluent strength or quality change within the HPLC system in the course of the HPLC experiment that is in the continuous and local gradient methods (Sections 16.5.3, 16.5.4, and 16.5.6). Irrespective of the problems and limitations of LC CC, its concept belongs to the important breakthroughs in polymer HPLC. 

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]. 

The product [r[].M is designated hydrodynamic volume of macromolecules, and forms a basis for important Benoit s universal calibration in SEC (see seetion 11.7.3.1, Columns for Size Exclusion Chromatography and Their Calibration). Coiled, chemically different macromolecules in different solvents that exhibit equal V s elute from the given SEC column within the same retention volume -provided the pore stracture of the column packing remains unaffected by the eluent change and the enthalpic interactions with the column packing are negligible. 

Due to the attractive interaction of monomer with the column packing, also its retention volume exceeds V. An important case of the coupled exclusion - interaction processes is in Figure 3 represented by curve d. It mirrors the full mutual compensation of exclusion and interaction, which leads to the independence of retention volume of polymer molar mass. This situation is important for characterization of complex polymers. It allows elimination or at least suppression of the molar mass effect of macromolecules so that their unbiased separation according to another molecular characteristic can be performed. The experimental conditions leading to this specific elution behavior are called critical conditions of enthalpic interactions. They will be more in detail discussed in section 11.8.3. 

In this case, the enthalpic interactions within the HPLC system exceed the exclusion effects (see Figure 3(e)). The retention volumes of polymer species as a rale exponentially increase with their molar masses. The important limitation of the resulting procedures was presented in section 11.5.2.3. The retention of (high) polymers is usually so intense that the latter do not elute from the column any more. Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to ohgomers - up to molar mass of few thousands g.mol. Still, the reduced sample recovery may affect results of separation even in case of oligomers. The selectivity of enthalpy driven HPLC separation is much higher than in the case of SEC but, naturally, the sequence of molar masses eluted from the column is reversed. If the effect of enthalpy is reduced, problems with sample recovery are mitigated - but at the same time the separation selectivity is reduced. 

The important issue of 2D-LC represents the abovementioned transfer of column effluent between the Id and the 2d columns, which can be done either off-line or online. In the off-hne approach, the fractions from the Id column are collected and successively re-injected into the 2d column. In this case, the unit TF is just a fraction collector. The macromolecules within particular fractions from the Id column are immixed so that resulting overall separation selectivity may be challenged. Moreover, entire procedure is laborious and slow. Various approaches were elaborated for the online transfer of fractions from the Id column into the 2d column. Often, the fractions from the Id column are cut into small parts that are one-by-one gradually transported into the 2d SEC column for independent characterization. This is the method of choice if the first-dimension separation produces broader peaks, such as it does liquid chromatography under critical conditions of enthalpic interactions, LC CC (see section 11.8.3). The operation principle of such chop-and-reinject method is evident from Figure 22. In this case, the TF unit from Figure 21 is a switching valve. 

The third type of enthalpic interactions is simply called nonelectrostatic. One example is solvent cavity formation. If strong solvent-solvent bonds exist in a system (e.g. in water, where hydrogen bonding is prevalent among the water molecules), the solvent may be considered structured. For solvation to occur, a cavity must be formed in the liquid to accommodate the ion and some solvent-solvent bonds must be broken. This effect is expected to be minimal for polyether solvents, which are unstructured and have low cohesive energy densities. For polymer solvents, a more important nonelectrostatic interaction may be the energy associated with the formation of strained conformations required for optimal ion coordination. 

Better theoretical models need to be developed that explicitly incorporate the conformational features of the polymers in a mixture. Such models need to deal with behaviors that range from the limits of rigid rods to random coils in a systematic way. In particular, semi-rigid polymers, that are common to high temperature polymers, need to be studied. In addition to the conformation of the two polymers, an explicit way of incorporating specific enthalpic interactions needs to be defined. The development of the proposed model will allow for a theoretical examination of the relative importance of entropic and enthalpic effects to the phase behavior and miscibility. 

The study reveals an important lesson. Our instinct is to design receptors incorporating more favorable enthalpic interactions, and this can indeed be quite a successful... 

We have seen so far that enthalpic interaction parameters are often crucial in controlling the position of NPs in block copolymer assemblies [69-72,74-78]. Nevertheless, it has been shown impressively that the contribution of entropy to the free energy can become important in controlling the position of NPs in vesicular structures [79]. The co-assembly of NPs decorated with polystyrene-b/ocA -poly (ethylene oxide) (PS-b-PEO) copolymers and free (not surface-bound) block copolymers of the same type leads to vesicles with the NPs being incorporated in the (solvophobic) PS domain. Interestingly, depending on the number of monomeric... 

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