# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.2 Modification on Polymer Host  
The partial crystallinity nature of polymer host is usually the main culprit behind the low conductivity of SPEs. This issue can be resolved by making some alteration on the polymer itself. For example, biopolymers, i.e., cellulose and chitin, are highly crystalline with rigid polymer chain backbone, even though they can still serve as host in polymer electrolyte. Cellulose can be chemically modified into methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC) and sodium carboxymethyl cellulose (NaCMC), to name a few. Similarly, chitosan structure can be altered into carboxymethyl chitosan, hexanoyl chitosan (HCh), N-methylene phosphonic chitosan, chitosan-N-propyl sulfonic acid, etc. Besides reducing crystallinity, the modified polymer will have greater solubility to dissolve in various solvents, as compared to its predecessor. N-phthaloyl chitosan (PhCh) is the product obtained after the chemical adjustment of chitosan in a solution mixture of phthalic anhydride and DMF under a nitrogen gas environment. Chitosan has been successfully transformed into PhCh with the presence of peaks belonging to the phthalic functional group at wavenumbers 1713 and 1733 cm\({}^{-1}\), as proven by FTIR analysis [58]. Dilute acids of formic, acetic, butyric, lactic, maleic and malic are some solvating agents for chitosan [59, 60], but the water content may deteriorate the performance of some electrochemical devices, such as Li-ion batteries and DSSC. PhCh can dissolve in pyridine, DMAc, DMF, DMSO and \(m\)-cresol [61, 62]. [PERSON] et al. [58] reported that PhCh is more amorphous and thermally stable than the antecedent, chitosan. The former requires a higher decomposition temperature (356 \({}^{\circ}\)C) than the latter (290 \({}^{\circ}\)C). PhCh-based SPE consisting of NH\({}_{4}\)SCN prepared via solution-casting method displayed conductivity of 2.42 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) at 25 \({}^{\circ}\)C and \(l_{ion}\) = 0.91 [58]. The PhCh film had the ambient conductivity of -1.93 \(\times\) 10\({}^{-10}\) S cm\({}^{-1}\)[58]. O-nitrochosan is another derivative of chitosan after reacting with sodium hydroxide and funnel nitric acid, as synthesized by [PERSON] et al. [63]. The appearance of new bands at wavenumbers 1355 and 1646 cm\({}^{-1}\) were attributed to nitro (-O-N=O) functional groups, as evidenced by the FTIR spectrum of o-nitrochitosan [63]. The ambient conductivity was greatly enhanced for the modified chitosan film (5.22 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\)) at room temperature, while the unmodified chitosan film exhibited low conductivity of 8.88 \(\times\) 10\({}^{-10}\) S cm\({}^{-1}\)[63]. The large increment was attributed to the high electronegativity of nitro group [63]. Although no dopant salt was incorporated, the authors believed that o-nitrochitosan can be a promising polymer electrolyte membrane in PEMFC [63]. PEO is excellent to solvate Li\({}^{+}\) ion, which is especially useful for application in Li-ion cells, even though its crystalline portion hampers the conductivity enhancement. Usually, the conductivity of PEO electrolyte containing lithium salts is no greater than 10\({}^{-4}\) S cm\({}^{-1}\) under ambient condition with a low value of lithium-ion transference number (\(t_{Li+}\) < 0.40) [64]. Luckily, its chemical structure can be modified and incorporated with various types of functional groups or moieties. Another strategy to improve the conduction of PEO-based electrolyte is to modify its linear chain into hyper-branched structure. [PERSON] et al. [64] have prepared LiTFSI-containing SPE based on PEO having a hyper-branched arrangement and grafting with another PEO of linear chains to improve the ionic conductivity without sacrificing its mechanical strength. An increment by one order of magnitude was observed when compared to SPE having PEO linear chain, which was attributed to reduction in crystallinity [64].