Capillary dilatometer

Dilatometry is a convenient method for measuring polymerization rate. The method is based on a decreasing volume of the examined system along with conversion of monomer to polymer. For simple polymerization, usually carried out in solution in capillary dilatometer, the decrease in volume, Av, is calculated from measurements of the decrease in the level of reacting mixture ho - h = Ah in capillary with radius, r. Using equation ... 

The specific volume vs. temperature data were obtained by using capillary dilatometers. Specific volumes at 25°C., determined in a pycnometer, were used as reference. 

Where the sub-index 0 indicates initial conditions and T is the induction period. kdim in equation 1 has been measured with precision by Kothe and Fischer as fcdim = 2.51x10 exp (-93,500/(/f2)) L moP -s with R in J moP °K and T, the temperature, in °K. In order to obtain initial estimates of the value of fedsma we performed reactions for the system S-MA in presence of OH-TEMPO [N]) in a capillary dilatometer in order to measure the induction period and the conversion - time curve after induction. Different compositions of the pair S-MA and of the nitroxide mixture increased its volume by thermal expansion until thermal equilibrium was established. At that point zero time was marked and the volume contraction of the reaction mixture with time was correlated with conversion via standard calculations that use the density of the monomer mixture and the polymer. ° Table 1 contains a summary of the results and Figure 3... 

Dilatometric Analysis. The density of the films at room temperature and as a function of temperature was determined using a capillary dilatometer with mercury as the filling liquid 16). Dilatometric runs were performed in the temperature range from 20 to 90°C (heating rate 0.25 K/min) to avoid degradation of the polymer. Degradation has been reported in samples kept at 100°C for a long time (7). More details related to these experiments can be found in our previous paper (7). 

I = complex impedance, B = conductivity bridge, C = capillary viscometer, P = pycnometer or dilatometer, V = volumetric glassware, I = instrument, U = method unknown... 

Fig.2.3. Dilatometer in a thermostat. Conversion-time curves are obtained through volume contraction in the capillary...

The dilatometers are emptied as follows.The dilatometer, cooled in ice to below 10 °C, is inclined carefully over a small beaker, the capillary is lifted from the dilatometer, the polymer solution poured into a beaker and the capillary and dilatometer bulb washed... 

A dilatometer consists essentially of a glass tube (the body ), containing the reaction mixture, to which a straight capillary tube is attached (usually, but not always, vertical). The movement of the meniscus of the reaction mixture in the capillary amplifies changes in the volume of the reaction mixture and is measured, as a function of time, with a cathetometer or by some automatic device. Dilatometry has been used for a very long time because it is simple, cheap, convenient and can be made very accurate. 

If the dilatometer body has one access only, namely through the capillary, the filling process is laborious and slow under atmospheric pressure because of air-locks in the capillary, but it can be swift and easy when done under vacuum. Emptying the body can prove difficult after a polymerisation reaction since the reaction mixture becomes very viscous. Therefore it is common practice to cut open the body after a reaction and to repair it for the next. The best way to break open the body of a dilatometer is to score... 

Since the change in the height of the liquid level in the capillary of the dilatometer is proportional to the change of volume of the reaction mixture, which, in turn, is proportional to the change in its density. 

For any polymerisations which are not internally of first order, the apparatus constant A must be determined. This can be done as follows Let the volume of solution in the dilatometer be Vj + nr h (where is the volume of the body of the dilatometer, r the i.d. of the capillary and h the height of the meniscus in the capillary), and let the dependence of the volume... 

Fig. 3.14. Tap dilatometer with loop, L, for stopping distillation into the capillary, C, of the dilatometer, B A mixing chamber, M phial breaker, P phial, D trap to retain phial fragments, T PTFE tap.

I.6.2. A variety of models If the most rigorous technique is not required, for example, if a monomer is to be polymerised at, say, 80 °C by a radical initiator, then solvent and monomer are run into the mixing chamber, the catalyst is added and left to dissolve, the assembly is then attached to a vacuum Une to allow the reaction mixture to be degassed by the conventional freeze-pump-thaw process and to facilitate the filling of the body and the capillary. When this has been done, the dilatometer is thermostatted and the height of the meniscus in the capillary is monitored by means of a catheto-meter. The simple dilatometer adequate for this can be modified for more... 

If one is using a solvent of relatively high vapour pressure the solvent will tend to distil from the residual reaction mixture in the mixing chamber into the capillary because of the difference in vapour pressure between surfaces of different curvature. This can be prevented by incorporating a dry-joint (see p. 47) between the top of the capillary as shown in Fig. 3.12, or by incorporating a small length of capillary with the same i.d. as that used for the dilatometer, as shown in Fig. 3.14. 

Dilatometer. Reliable kinetic data on gamma-induced emulsion polymerization can be obtained only when the polymerization rate is measured continuously (7). The recording dilatometer used in our previous work had some disadvantages. A mercury meniscus traveled down a precision capillary, releasing a thin platinum wire within the capillary. The electrical resistance of this assembly was used as a measure for the... 

Fig. 5.29 Dilatometer capillary height during isothermal crystallization at the temperatures indicated of an a,a>-octadecyloxypoly(oxyethylene) triblock (A/w(PEO) = 11.4kg mol-1) following quenching from the melt (Thierry and Skoulios 1977).

The reaction can be followed in a dilatometer, by watching the decrease in the height of the solution in the capillary tube. 

In the following experiment, a 0.100 mol dm 3 solution of ethylene oxide was studied in the presence of a constant concentration of acid. The following data lists the heights in the dilatometer capillary as a function of time. 

Volume measurements needed to calculate the SFI are made by observing and recording the position of the dye solution meniscus in the graduated capillary tube at one temperature (typically 60°C) or two temperatures at which the sample is fully molten, and at desired measurement temperatures (which will be lower than the clear point of the sample). The sample is brought to the first measurement temperature from 0°C after a standardized tempering procedure. This and subsequent measurement temperatures must be approached from below to avoid supercooling effects. Temperature is controlled by immersing the whole dilatometer in constant temperature water baths or an ice-water bath. 

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