Retention diagrams

Eariy studies on polymer stationary phares revealed that retention diagrams exhibited certain singularities such as pictured in Fig. 1. They were found to correspond... 

In the temperature region corresponding to segment AB of Fig. 1, the polymer is below its glass transition temperature ( g) and penetration of the solute molecules into the bulk of the polymer phase is precluded. Retention proceeds exclusively by rface adsorption and the corresponding retention diagram is linear. Information on the surface properties of the polymer can be obtained in this temperature range. 

At temperatures below the melting point of the polymer, in region CD, retention proceeds by bulk sorption but the polymer-solute interaction is restricted to the amorphous domains of the stationary phase. Upon meltir, in r on DF, the fraction of amorphous material increases, leading to an incre in retention volume. At temperatures above the melting point, segment FG, a linear retention diagram, corresponding to bulk sorption into the completely amorphous polymer, is obtained. By extrapolation of this line to lower temperatures (dashed line FE), the crystalline content of the stationary phase can be determined by comparison of the experimental retention volume with the extrapolated value. 

Fig. 2. Retention diagrams for (o, A, ) n-decane and (o) n-pen> tane on poly(vinyl chloride) plasticized with dioctyl phthalate...

Fig. 3. ( ) Overall and (o) derivative slopes of ( ) retention diagram for n-h cadecane on pc slytene...

The variation in the of the retention diagram with polymer film thickness... 

The extent of the nonequilibrium region, segment BC of Fig. 1, is dependent on both the polymer-solute system (34) and experimental parameters (13, 33). As the thickness of the polymer film or the flow rate of carrier gas increases, the maximum of the retention diagram shifts towards higher temperatures (13). Equilibrium retention data can, however, be obtained by extrapolation of retention vdumes to zero flow rate. Braun and Guillet (35) have developed a model of the chromatc rq>hic behavior of polymers near Tg which reproduces most features observed experimentally. The effects of the magnitude and temperature depandence of the difihision... 

The GC route is particularly attractive for it requires no a priori information on the polymer. With the exception of X-ray measurements, most methods of measurement involve a comparison of some property of the polymer, such as density, with that of the totally amorphous or crystalline material. Furthermore neither the mass of polymer in the column nor the flow rate of carrier gas need to be measured since a ratio of retention volumes is computed in Eq. (21). It should be added, however, ttiat for the successful application of the method it is essential that the measured retention volumes correspond effectively to equilibrium bulk sorption, both above and below. Low molecular weight compounds are known to exhibit apparently similar discontinuities in retention diagrams at their melting points but this is to be ascribed to a change in retention mechanism, from surface adsorption for the solid to bulk sorption for the liquid stationary phase. For a detailed discussion of retention characteristics of low molecular weight substances near their transition temperatures the reader is referred to a recent review by McCrea (8J). 

The success of the method prompted the design of an Automatic Molecular Probe apparatus for the collection of retention data 82). At a preset cycle time a mixture of solute and noninteracting marker was iiqected into the carrier gas and the output of the GC detector was fed to an electronic peak detector. The temperature in the oven was programmed linearly and recorded with a thermocouple. Upon conversion into digital form, a printout of net retention time and temperature was obtained. Retention diagrams have been obtained with this apparatus for high and low density polyethylene 82, 83). 

Fig. 9. Mdting curve and retention diagram for R-hq>tane on pdly-(ethylene M)...

Wallace et al. (20) correlated GC retention volumes of several poly(vinyl chloride) powders with their uptake of plasticizer. Since the diffusion of plasticizers into polymer powders is controlled by the external surface area, the diffusion coefficient of the plasticizer and some shape factor, a correlation with GC measurements could be expected. It was found that plasticizer absorption ( drying ) took place only when the polymer was heated to a temperature immediately above the glass transition temperature as defined by the minimum of the experimental retention diagram. 

Because of the ease of temperature control in a gas chromatograph, one may explore the polymer using the probe above and below various possible phase transitions, with a linear response being revealed by the retention diagram when operating under equilibrium conditions. 

Information from a molecular probe experiment is usually presented in the form of a retention diagram, that is, a plot of log Vg against 1/T(K). A sample curve for a semi-crystalline polymer is shown in Figure 2. The slope reversals are indicative of phase transitions. Such transitions had been noted (28) as early as 1965 for polyethylene (PE) and polypropylene (PP), but the first comprehensive study of polymer structure using IGC was done in 1968 by Smidsrod and Guillet (1) on poly(N-isopropyl acrylamide) (poly-(NIPAM)). 

Figure 2. Retention diagram for a semi-crystalline polymer. (Reprinted with permission from ref. 3. Copyright 1982 Applied Science.)...

The polymer melting point corresponds to the cusp in the retention diagram. 

Figure 4. Retention diagram for decane on high-density polyethylene. Tm is at the cusp. (Reprinted from ref. 19. Copyright 1971 American Chemical Society.)...

Figure 1. Retention diagram In Vg° versus 1/T of ethyl benzene in poly(2,6-dimethyl-l,4-phenylene oxide) (8.52 wt-% polymer loading).

Specific retention volumes and calculated 12 values for the three sets of polymer-probe pairs are given in Tables II to IV. In all cases, specific retention volumes were corrected for support retention. Retention diagrams for polystyrene and PMMPO are shown in Figures 2 to 3. 

The variation of the retention volume with temperature is such that a plot of the logarithm of the retention volume versus reciprocal of the absolute temperature, called a retention diagram, is linear. The slope of this straight line is related to the enthalpy of the process, solution in the stationary phase (GLQ or adsorption on the surface (GSC), with,... 

Fig. 4. Retention diagrams for ( ) 2-methyl butane on low density polyethylene and for (o) n-pentane on medium density polyethylene...

---