Guidelines
When Using the PCAGuidelines
When Using the PCA
When planning and running experiments using the PCA/DSC system, please review the following additional experimental considerations:
Obtaining a specific intensity at the sample is a trial-and-error process. The following guidelines should help in making those adjustments.
The PCA unit Run Registers can be set up to yield an intensity at the end of the light guide between 500 mW/cm2 and the maximum intensity available from the lamp (up to 19,000 mW/cm2 depending on the age of the lamp).
The intensity at the DSC cell platform is roughly 1/10th that measured at the end of the light guide. (This intensity reduction is due to diffusion through the 20 mm air gap between the light guide and the platform.) This can be further reduced by using the 10 percent or 1 percent transmission neutral density filters provided.
Intensities at the DSC cell platform in the visible region are roughly 1/2 (using the 390 nm filter) and 1/4 (using the 490 nm filter) those obtained under comparable conditions for the standard 320 to 500 nm broadband filter configuration. (These relative intensities are the result of the number of high intensity wavelengths which occur in the spectral region used. See the plot below for the high pressure mercury lamp.)
Baseline offset occurs during most experiments when the cell is exposed to light. This offset is associated with the difference in heat capacity between the sample pan and the empty reference pan. This offset is minimal relative to the exothermic events being measured. Furthermore, its contribution during peak integration can readily be eliminated by back extrapolating the baseline after the cure is complete.
A rise in temperature occurs when the cell is exposed to light. This rise is most noticeable at high intensities (> 100 mW/cm2 ) and is influenced to some extent by the cooling accessory being used. However, due to the speed of most curing reactions and the modest size of the temperature change (< 1°C at 100 mW/cm2 intensity), the kinetics of reaction should not be affected.
Several variables can affect PCA results, including wavelength range, light intensity, template and exposure time. Exposure time is controlled by the length of the isothermal segment places in the experimental method between the point at which light initially enters the cell (i.e., the EVENT is turned “ON”) and the point when light is removed (i.e., the EVENT is turned “OFF”). The shortest exposure time that can be set is 0.6 seconds.
The Q Series™ DSC’s are designed to run with “active” cooling in place throughout their whole temperature range. Several items need to be considered when selecting the cooling accessory that will be used for PCA experiments.
Most PCA experiments are run isothermally at temperatures between ambient and 80°C. The lifetime of the extended range liquid-filled light guide is several years at temperatures up to 35°C, but is reduced to several days at 50°C. During experiments at 80°C with the FACS cooler, the ends of the light guide arms in the holder only reach 35°C after 30 minutes. Since PCA experiments typically last less than five minutes, temperature effects on light guide longevity should be insignificant. Nevertheless, allowing the DSC to sit isothermally at 80°C for prolonged periods with the light guide mounted over the cell is not recommended.
The Finned Air Cooling System (FACS) is not designed for Tzero calibration within 5°C of ambient temperature. It should be used primarily for PCA experiments between 30 and 80°C.
The RCS cooling accessory allows PCA measurements to be made from –50 to 80°C. Since portions of the DSC cell will achieve temperature below ambient during RCS operation, good experimental procedure must be followed to ensure that moisture condensation (from the atmosphere) on the outer surfaces of the system does not occur. The DSC has an auxiliary purge that is automatically activated at the end of the PCA experiment to help minimize any potential for condensation. (This purge automatically shuts off when the next experiment begins.) Loading and unloading samples should always be done when the cell has reached ambient temperature or above.
NOTE: It is recommended that the Unload Temperature Range parameter (selected on the Post Test Parameters window when you set up your experimental procedure) be set to 35 to 40°C. After prolonged PCA experiments at –50°C, the outside of the quartz window on top of the DSC reaches about 17°C. However, at the end of an experiment, in the few minutes it takes for the cell to achieve the recommended 35 to 40°C unload temperature, the outer quartz window temperature has increased back to the 24 to 26°C range.
There may be situations where it is desirable to run an isothermal PCA experiment followed immediately by a DSC heating ramp experiment either to complete the curing process for materials where curing is photoinitiated but completed by heating or to evaluate the glass transition or other thermal properties once partial or full cure has occurred. The TA Instruments PCA has been designed to facilitate rapid conversion between PCA and standard DSC experiments. Simply follow the steps below:
Remove the dual light guide adapter with the light guides still in place. [There is no need to remove the light guide adapter base. In fact, as mentioned previously, once the AutoLid and Autosampler (if present) have been disabled, those capabilities have to be deliberately re-enabled when switching back to standard DSC.] The reason for this arrangement is so that the types of experiments described in this section can be performed without removing the light guide adapter base and potentially changing the alignment of the light guide with the cell platforms.
Replace the inner and outer PCA silver lids and quartz window with the standard DSC inner and outer silver lids and the manual cell cover.
Deselect PCA Operation on the Tools/Instrument Preferences/DSC Page in the instrument control menu.
Set up the desired DSC method and start the experiment.
Another unique feature of the Q Series™ DSC/PCA system is the ability to run quasi-isothermal Modulated DSC® measurements on the material before and after photocuring. Modulated DSC measurements should not be made while the sample is exposed to light because the temperature change generated by the light as well as the exothermic curing process disrupts modulation. However, it is possible to create an experimental method such as the one shown below where a quasi-isothermal MDSC® segment is run before and after the PCA exposure segment. Most materials undergo a decrease in heat capacity as cure progresses because the internal structure becomes more rigid and less molecular motion can occur. See also: Setting Up an MDSC Quasi-Isothermal Procedure.
NOTE: Because the heat capacity signal during PCA light exposure is not valid information, It may be desirable when plotting results to only show the heat capacity before and after exposure as shown in the figure below. This can be accomplished by using the "exclude data" function in Universal Analysis.
The MDSC reversing heat flow signal is directly related to heat capacity. Therefore, measuring the change that occurs in MDSC reversing heat flow as the result of specific PCA exposure conditions, is another option for differentiating similar materials.
Equilibrate at 35°C
Data Storage: ON
Data Collection: 0.1 seconds/data point
Isothermal for 0.5 minute
Modulate + 0.2°C every 40 seconds
Isothermal for 1 minute
Modulate + 0°C every 40 seconds
Isothermal for 0.5 minute
EVENT: ON
Isothermal for 2 minutes
EVENT: OFF
Isothermal for 1 minute
Modulate + 0.2°C every 40 seconds
Isothermal for 5 minutes
Modulated DSC® and MDSC® are registered trademarks of TA Instruments—Waters LLC.
Modulated DSC® and MDSC® are terms which describe proprietary technology invented by Dr. Mike Reading of ICI Paints (Slough UK) and patented by TA Instruments—Waters LLC (U.S. Patent Nos. B1 5,224,775; 5,248,199; 5,335,993; 5,346,306).
