Instrumentation and Procedure:
1. GC INSTRUMENTAL
Any brand name GC with either FID or TC detectors and with modern temperature and flow rate controls will be sufficient to be used for IGC experiments. The control of the temperature is a critical factor in IGC experiments over eight hours of operation.
Markers are to be used in IGC experiments to account for the dead volume in the plumbing, their retention time should be subtracted from the retention time of the solvent to yield the absolute value of the solvent's retention time. In case of using thermal conductivity detector, air is widely used, but for flame ionization detector, methane is the least non-interactive solute that can be used. Although methane may interact with some non-polar polymers, a correction factor can be established to correct for the contribution of methane to the interaction parameters.
2. COLUMN PREPARATION
Each sample is weighed carefully and dissolved in appropriate solvents (about 100 ml) to form a solution. The polymer solution can then be used to coat the solid support using a known method named as the “soaking method” described in section 3. The loading of the polymer sample can be calculated relative to the weight of the solid support (%wt/wt). Chromosorb W with 60/80 mesh is often used treated with DMCS to deactivate the hydroxyl groups on the surface of the support. A treated support supposed to be an “inert”, however, we would like to caution the reader to generate retention volume data with only support in the chromatographic column (0% polymer). These data can then be subtracted from those measured with coated support. This procedure will eliminate the effect of the active sites left un-deactivated on the solid support’s surface.
The typical loading in a chromatographic column varies between 3 - 12% (wt/wt). Loading higher than 12% may cause a decrease in the porosity of the column. Five feet, 0.25 inch od (or smaller) copper tubing may be used to pack the loaded support. Columns have to be washed in methanol and annealed in an oven at 80 oC for 24 hours. The coated support is then vacuum packed into the copper tubing in small increments until 99% of the tube is filled. The copper tubing ends are then to be plugged with salinized glass wool. Solvent residue may be removed by drying the coated column overnight in a vacuum oven at 100 oC. Prior to any retention measurements, each new column may be conditioned in the chromatography at 120 oC for 8 hours, during which the carrier gas sweeps the system until a low and steady base-line signal is obtained.
For IGC experiments aimed at studying blends or a mixture of two polymers, a minimum of three columns should be prepared for the blend studies; the first contains pure polymer (designated as 2), the second contains pure polymer 9designated as 3), and the third contains the blend. The three columns should be studied under identical conditions. This will allow the measurement of the interaction parameters χ12, χ13, and χ23. If the effect of composition of the blend on the interaction parameter needs to be studied, then actual blend columns of different weight fractions of each polymer and counter polymer needs to be prepared, ranging from 0% to 100% by weight. (Notice that 1 is designated to the solvent).
A number of low molecular weight solutes of different chemical natures are usually selected and injected into the packed column. Their retention times are then to be analyzed to yield various interaction parameters and thermodynamic quantities. Experiments show that the equilibrium (diffusion across the layer) is established reasonably fast when the polymer is kept at a temperature significantly higher (by about 50-70K) than the glass transition temperature of the polymer. However, polymers below their glass transition temperature are not penetrated by solute molecules under GC conditions and the kinetic effect will be dominant.
3. COATING THE POLYMER ONTO THE SUPPORT
Routinely, the coating of polymer (or polymer mixture) onto the inert support is usually done by dissolving the polymer in a suitable solvent, adding the support, slowly evaporating the solvent (usually in a rotary evaporator), drying the coated support in the oven and, sometimes, resieving it. During this lengthy procedure a significant part of the polymer may be lost on the walls of the evaporating vessel. Further losses may occur during resieving. Consequently, most researchers analyze the coated support for its polymer content by either extraction or calcination. However, both methods were shown to be subject to significant errors. To achieve a better control of the amount of polymer, we designed a new method (soaking method) for depositing the whole sample of polymer on support and into the column [see references]. In this method the polymer is dissolved in a solvent as usual. The support is piled on a watch glass or a similar dish and a small amount of the solution is applied on the top of the support pile in ahomogeneous way to cover all support surface. Care is taken to wet the pile as much as possible without letting the solution touch the surface of the dish, either around or under the pile. The solvent is allowed to evaporate and the pile is thoroughly mixed. Then the next portion of the solution is applied and the whole procedure is repeated until all the solution (including several rinsings of the solution flask) is used up. It takes typically 10‑20 applications and requires only a few hours. In our experience no polymer was left on the surface of the dish. Then the support was dried in the oven and quantitatively transferred into the column with standard precautions of quantitative analytical chemistry. The method is fast, the amount of polymer is precisely known and the analysis of the column material is avoided. We have compared a column prepared in the above described manner with a column prepared by traditional coating in an evaporator. (The amount left on the walls of the evaporator vessel represented about 10% of the total polymer.) The chromatographic behavior of both columns was fully comparable. The small systematic differences in Vg values on both columns could be accounted for by the uncertainty in the mass of polymer in the traditional column. In our opinion, these results fully justify the use of the new coating technique.
4. Volume of Solvents Injected
Vanishingly small amounts (0.20 μL) of a series of the selected solutes should be injected into the chromatographic column. This small volume has been tested to yield an absolute value of solutes’ retention volumes. Injection of more the 0.20 μL will yield longer retention time due to peak widening, this will affect the accuracy in the thermodynamic quantities. The thermodynamics only work if there is an equilibrium between the mobile and stationary phase. This can only achieved if the volume of the solvent injected is at vanishing level.
