(164b) Measurement of Glass Transition Point of Polymers Under Carbon Dioxide Using Transmitted Light Intensity

The glass transition and the free volume of polymers are closely related. When low molecular weight substances such as residual solvents and plasticizers are mixed, low molecular weight molecules enter between the polymer chains, weaken the intermolecular interaction between the polymer chains, and increase the mobility of the polymer chains. In other words, the free volume of polymers increases with the mixing of low molecular weight substances. Consequently, the glass transition temperature (T_g) becomes lower than that without the low molecular weight substances. This phenomenon is known as the glass transition point (GTP) depression. When the carbon dioxide (CO₂) is dissolved to polymers by pressurization, a similar phenomenon has also been known to occur. By applying this phenomenon, the forming process of polymers can be performed under the lower temperature and mild condition.

The most typical measurement method of the GTP is the use of differential scanning calorimetry (DSC). Especially, for the measurement under the high pressure gases, the high pressure differential scanning calorimetry (HP-DSC) is adopted. Actually, Huang et al.[1] measured the T_g depression of polymers such as poly(lactide) (PLA), poly(carbonate) (PC) and poly(styrene) (PS) by CO₂ with a HP-DSC. Unfortunately, the HP-DSC is quite expensive. Therefore, to accumulate the experimental data, more simple and acceptable measurement methods are anticipated.

In this work, we proposed a visual method to determine the T_g under high pressure. We visually observed the change in appearance of PS powder (Mw = 35,000, 40 mesh) placed between two sapphire glasses at various temperatures under atmospheric condition. As the powdery PS particles scatter the light, initial photo image at 298 K, temperature lower than the T_g, is dark. With the increase in temperature, at around 373 K, which is the T_g of the PS at ambient condition, the light begun to transmit due to the change in the phase.

We propose here a simple and easy method for the observation of the T_g under high pressure which uses the intensity change of the transmitted light. A new apparatus which has a high pressure thin layer viewing cell equipped with two light intensity sensors (one for source and the other for transmitted light) was constructed. The reliability and applicability of the experimental apparatus was verified with the PS and PMMA.

A powdery polymer sample was placed between the two sapphire windows (0.5 mm in distance). The transmitted light intensity from a halogen light source installed above the cell was measured in real time using a photo sensor placed under the cell. At the same time, for the correction of the fluctuation of the light source, another photo sensor (reference) was placed just next to the light source. In this study, the transmitted light intensity was normalized by dividing the output voltage of the transmitted right with that of the reference light. The output voltage data from the two photo sensors and pressure transducer/indicator, and a Pt resistance thermometer were recorded by using a data logger.

First, under atmospheric condition, the transmitted light intensity of the PS (Mw = 35,000) heated from the room temperature to a predetermined temperature at 0.5 K/min was measured. At around 373 K, the sharp change in the transmitted light intensity was observed. In this work, the GTP was obtained as an intersection point of the leading edge and the base line of the intensity of light transmitted through powdery sample. The obtained results coincided fairy well with the literature and DSC data.

Next, under high pressure CO₂, the similar measurements were conducted. The GTP measurements under high pressure CO₂ was carried out with two different procedures; at constant pressure with changing temperature, and at constant temperature with changing pressure. In either method, it was possible to observe the sharp change in transmitted light intensity at a certain temperature or pressure. The GTPs were determined in the same method as described above. The obtained results were quite well coincided with the data reported previously.

The different molecular weights and types polymers were also measured in the same manner.

In the case of the PS (Mw = 250,000), the change in the transmitted light was small compared with the PS (Mw = 35,000). It is presumably due to the increase in the viscosity of PS. If the viscosity was large, the transmitted light intensity did not change rapidly after the glass transition. Therefore, it is evident that to measure the GTP with the new apparatus, depending on the molecular weight, fine tuning of the temperature scanning rate and pressure changing rate must be considered.

On the other hand, in the case of the PMMA (Mw = 35,000), the transmitted light intensity was changed though, it was much smaller than that of the PS (Mw = 35,000). This fact suggested that the light transmittance differs depending on the type of polymers. The change in the intensity of the transmitted light was small, but it was possible to determine the GTPs. The obtained results also agreed fairy well with the literature and DSC data.

It could be concluded that there do exists a limitation, but the newly constructed apparatus might be the powerful tool for the measurements of the GTP.

[1] E. Huang, X. Liao, C. Zhao, C.B. Park, Q. Yang, G. Li, Effect of Unexpected CO₂’s Phase Transition on the High-Pressure Differential Scanning Calorimetry Performance of Various Polymers, ACS Sustainable Chemistry & Engineering, 4 (2016) 1810-1818.