## 2 Thermal and Electrochemical Parameters of Interest
### Glass Transition Temperature  
The glass transition temperature (T\({}_{\text{g}}\)) is the range of temperatures where an amorphous polymer or the amorphous regions in semi-crystalline polymers transition from a more rigid and brittle material to a softer and more flexible material [43; 44]. The T\({}_{\text{g}}\) is dependent on the type of polymer, the degree of crosslinking and the chain length. As the degree of crosslinking and the chain length are subject to change within a given polymer, this quantity is actually a range of temperatures over which structural changes occur but it is generally reported as a single number [45]. Increased crosslinking tends to increase T\({}_{\text{g}}\) because T\({}_{\text{g}}\) is higher when there are fewer atoms between crosslinked sites. In this case, higher T\({}_{\text{g}}\) does not necessarily indicate increased crystallinity as crosslinking can disrupt crystallinity as a result of preventing polymer motion to self-organize into crystallites. It is generally considered desirable for polymer electrolytes to have lower T\({}_{\text{g}}\) values as ionic conductivity usually occurs in amorphous polymer phases. Therefore, materials that are amorphous at lower temperatures tend to have better ionic conductivities over a larger temperature range [46; 47]. The T\({}_{\text{g}}\) of PEO-LiTFSI has been reported to be around \(-35\)\({}^{\circ}\)C depending on the salt concentration [48]. Decreased T\({}_{\text{g}}\) has been linked to improved polymer mobility and therefore improved ionic conductivity. The epoxide-based polymer electrolytes prepared from epoxide-only and silicone-containing reagents that are presented in Table 1 tend to have T\({}_{\text{g}}\) values below 0 \({}^{\circ}\)C which is generally considered to be necessary to produce competitive polymer electrolytes. Some of the epoxide-based polymer electrolytes that were prepared with Jeffamines or plasticizers have T\({}_{\text{g}}\) values above this threshold.  
In the examples that are presented in Table 1, the glass transition temperatures were determined experimentally using differential scanning calorimetry (DSC). This technique involves heating a small quantity of sample while recording the energy that is required to do so. Heat flow in the sample is compared to heat flow in a standard and deviations in the heat flow, that are caused by changes in the sample, are indicated by a deviation from the baseline heat flow [49]. When the T\({}_{\text{g}}\) range is reached, the properties of the polymer change as a result of a re-orientation of the polymer chains [45]. This causes a change in the energy that is required to heat the sample and results in an endothermic inflection point on a plot of heat flow as a function of sample temperature. Factors affecting the T\({}_{\text{g}}\) that may be relevant in the polymer electrolytes that are discussed here are lithium salt content, the possibility of sample hydration or plasticizer addition and the level of crosslinking in the system. Samples with more crosslinking tend to be more rigid and have higher T\({}_{\text{g}}\) values. The extent of crosslinking in the samples presented here is most likely to be amplified by increased epoxide content and increased lithium salt content as lithium salts can bind to polymer chains and cause crosslinking via hydrogen bonding interactions [50]. Absorbed water or solvent, which can occur during synthesis or by exposure to ambient (or even dry) conditions tends to cause increased segmental mobility and increased free volume between polymer chains, both of which decrease T\({}_{\text{g}}\)[51]. Similar changes in polymer electrolytes are expected when plasticizer is added. The impact of plasticizer addition on the T\({}_{\text{g}}\) of epoxide-based polymer electrolytes will be discussed in Section 3.4 of this document.