## 3 Results and Discussion
### Deterioration Mechanism  
In this study, the burst pressure of the AMX membrane with a PVC backing and binder first increased and then decreased as the alkali treatment time progressed. A similar feature was discovered in the tensile testing. Figure 14: Scheme of forming a cross-linking point. Figure 13: Scheme of the dehydrochlorination. Figure 15: Scheme of the PVC main chain cleaving. In the previous section, the effect of alkali treatment on PVC was discussed. As indicated, it caused dehydrochlorination with the formation of polyene and cross-linking structures, accompanied by PVC polymer main chain cleavage. We considered this main chain cleavage as contributing to the change in the mechanical properties of the AMX membrane, and proposed the model explaining the deterioration mechanism of the membrane as illustrated in Figure 16.  
Figure 16: _Cont._  
Figure 16(a1) shows the original structure of the AMX membrane (Case I), which is a composite with a PVC backing and a chloromethylstyrene and divinyl benzene (CMS-DVB) cross-linked structure containing a PVC binder (hereinafter, a cross-linked structure). The AMX membrane was prepared via the paste method [1]. A paste containing CMS, DVB, PVC binder, and an initiator was applied to the PVC backing cloth. The PVC binder was compatible with the monomers in the paste. In the thermal polymerization of CMS and DVB in the presence of the binder, the cross-linked structure blocks were randomly formed. This is indicated as the girder bridge structure in Figure 16(a1). The PVC binder was incorporated into the cross-linked structure, resulting in the formation of an interpenetrating network (IPN) structure to interconnect the cross-linked structure blocks, indicated as intertwined chains between the girder bridges. The PVC binder is illustrated in an orange solid line. Hence, the Case I membrane was modeled as a girder bridge cluster connected by PVC chains. In this study, the PVC backing was tough, whereas the cross-linked structure was hard and brittle. When a tensile stress was applied to the Case I membrane, not only the constrained IPN comprising the cross-linked structure blocks and PVC, but also the PVC backing exhibited a high tensile strength, owing to their hardness and brittleness before being fractured without presenting a large elongation, as shown in the SS curve in Figure 3a. The cross-linked structure was fractured first by the stress concentration, followed by the degradation of the PVC backing as shown in Figure 16(a2). In the Mullen burst test, the fracture of the pristine membrane was accompanied by a loud rupture sound. The interpenetrating PVC was degraded by the alkali treatment. In the initial stage of degradation, the PVC chain between the cross-linked structure blocks was partially cleaved. It is illustrated in an orange broken line, as shown in Figure 16(b1) (Case II); hence, the girder bridges of the Case II membrane can be transformed more easily than those of the Case I membrane. When stress was applied to the Case II membrane, the girder bridges in the cluster shifted to decrease the stress concentration (Figure 16(b2)), resulting in a yield point and elongation, as shown in the curve in  
Figure 16: Deterioration mechanism of the AMX membrane. (**a1**) Pristine AMX: Case I; (**a2**) Case I membrane at breakage; (**b1**) Mildly alkali treated AMX: Case II; (**b2**) Case II membrane under stretch; (**b3**) Case II membrane at breakage; (**c1**) Severely alkali-treated AMX: case III; (**c2**) Case III membrane at breakage. Figure 3b. With further stretching, the resulting partially degraded IPN structure showed multiple yield points until the PVC backing was degraded seriously, as shown in Figure 16(b3). Although the maximum stress at break decreased, the strain to break increased to 189% compared with that of the pristine membrane, and the area below the SS curve increased, resulting in an improvement in the burst pressure.