## 3 Results and Discussion
### Effects of Water Micro-Reservoir in the Composite Membranes  
It is believed that proton transport occurs in hydrophilic channel consisting of acidic groups (typically -SO3H) and proton-water clusters (such as H3O+, H5O2+, and H9O4+) [34; 35; 36]. Therefore, besides the concentration of sulfonic acid groups, both physical states and content of the water entrapped in hydrophilic channel are influential to the proton transportation. As proposed above, the assimilation of hydrophilic HPS into Nafion was to enhance water uptake. The temperature effect on water uptake at 100% RH showed the sequence as expected (Figure 5a) where N/s-HPS revealed the largest water-absorption capacity of the three membranes under investigation. Furthermore, their water absorption capability was examined by equilibrating the membranes with different RH% at 20 \({}^{\circ}\)C (Figure 5b). The composite membranes revealed stronger moisture capture capability than the pristine cast membrane. Fundamentally, this implies the existence of a denser distribution of hydrophilic sites in the composite membranes. Furthermore, the condensed states of water entrapped in a membrane can be described by \"free water\" freezing at 0 \({}^{\circ}\)C, \"freezable bound water\" or \"freezable water\" freezing below 0 \({}^{\circ}\)C, and \"non freezable bound water\" or \"non freezable water\" due to absence of hydrogen bonding among water molecules, and hence, no phase transition can be detected. Both free water and freezable water can be identified by the low temperature DSC profiles [37; 38] as shown in Figure 6. The four membranes displayed only freezable water existing below 0 \({}^{\circ}\)C. Assuming that the specific fusion heat of freezable water would remain approximately the same as that of free water (4.18 cal/g) regardless of the shift of freezing point, the amount of this type of water could be calculated (Table 1). Subsequently, the non-freezable water would be the difference between water uptake and the amount of the freezable water. In contrast to the freezing point of freezable water in the Nafion membrane, which is identical to the reported value [38; 39], the freezable water in the three composite membranes showed higher freezing point as expected due to the lodging of waterinside the chamber of HPS. Unlike some hydrophilic polymer matrices, such as poly(vinyl alcohol) [40] and sulfonated polystyrene [41], these three composite membranes contain no free water. This observation can be attributed to the substantially small sizes of dispersed water droplets and a high percentage of water molecules associating with the pendant -SO\({}_{3}\)H groups. Hence, the bulk properties of the water droplets significantly faded. By this interfacial effect, the stronger the interfacial interactions between water droplets and hydrophilic groups, the lower the freezing point. Indeed the water freezing points of the composite membranes showed the order: N/s-HPS \(<\) N/c-HPS \(<\) N/n-HPS, which is determined by the hydrophilic affinity of HPS, _i.e._, the stronger affinity led to stronger interfacial interactions following the order of s-HPS \(>\) c-HPS \(>>\) n-HPS in terms of hydrophilicity. This order also reflects the same trend of freezable water% described in Table 1.  
Figure 5: The water uptake of the various membranes: (**a**) impact of temperature under 100% RH; and (**b**) impact of % relative humidity (RH) relation at 20 \({}^{\circ}\)C.  
Of the three composite membranes, the N/n-HPS membrane contained hydrophobic n-HPS particles that associated with perfluorochains as ascribed above. The analysis of water state further suggests that this association reduce the packing density of perfluoro chains because of the occurrence of the interface between perfluorochain and P(St-DVB). Such perturbation occurring in the hydrophobic domains might also affect the clustering of -SO\({}_{3}\)H groups in the hydrophilic channels to a small extent.