## 1 Introduction
In the TPU industry, 1,4-BDO is often used as a chain extender; its straight chain structure is beneficial for forming long chain linear block copolymers. However, according to toxicity data in the TOXNET of the National Library of Medicine on the National Institutes of Health campus in the United States, the toxicity of 1,4-BDO is much greater than that of 1,3-BDO [24]. Additionally, inhaling hazards may occur during the polymerization process; [PERSON] et al. reported a poisoning case caused by 1,4-BDO. Usage of 1,4-BDO may not be entirely safe [25]. With regard to research on chain extenders, researchers have used different types of short chain diols in the polymerization reaction [26; 27]; some have used 1,3-BDO in the polymerization to examine its further application as an alternative use to 1,4-BDO and may have an additional benefit for industrial usage, i.e., modulating the glass-transition temperature to room temperature. Additionally, [PERSON]'s study indicated that the short side chain could be used in regulating the glass-transition temperature and bio-based shape memory PU [28]. Although an enormous amount of research has already been conducted on the key factors that affect the hard/soft zone of isocyanates, research into using butanediol as the chain extender is relatively rare. In recent years, understanding of butanediol has deepened, and research on bio-based short chain diols has already been conducted in different fields [29; 30; 31]. Using the fermentation process to catalyze the reaction, biobased butanediol could be created [32]. To understand the mechanisms behind it, the current study attempted to examine how the chain extender could influence the structure of TPU [33; 34; 35; 36; 37; 38; 39; 40; 41]. In the field of TPU research, the microphase separation of hard and soft chains is a topic worth exploring. The morphology of the microphase would affect the final performance of TPU [42; 43; 44]. Some researchers have discussed the influence of the ratio of hard chain segments and soft chain segments on phase separation [45; 46]. [PERSON] and co-workers proposed that a hydrogen bond would crystallize with TPU partially [47]; thus, the soft chain segment could be used to establish the elastic modulus and flexible characteristics by affecting the cross-linking density of polyurethane [22]. [PERSON] discovered that adding different soft chain segments to the PU elastomer would significantly change its material properties [48]. Studies conducted in recent years have chosen to produce TPU by using different structures, and these studies could be applied in adhesive additives [49; 50; 51; 52; 53; 54]. Researchers have conducted studies on diols to distinguish their bonding properties. Using a fixed NCO/OH ratio, [PERSON] used polypropylene glycols with different molecular weights to prepare TPU pressure-sensitive adhesive [55]; this study highlighted the centrality of hard chain segment content and also confirmed that TPU could be used as an adhesive [56]. [PERSON] also stressed the importance of the chain structure to the crystal, and the set speed of hot melt glue could be confirmed [57]. The results of other studies conducted in recent years have indicated that rheological properties could be used to evaluate complex materials similar to TPU. The viscoelastic properties of adhesives are essential with regard to adhesive performance; additionally, the balance between their elasticity (solid state) and viscosity (liquid state) could help researchers in understanding their properties. Additionally, when rheological methods are used to examine the shear thinning and frequency scanning and to examine their viscoelasticity, the results could be used in confirming whether materials could be candidate adhesives (e.g., hot melt glue or pressure-sensitive adhesive). Based on the foregoing literature review, the researchers in the current study replaced 1,4-BDO with 1,3-BDO in the synthesis of TPUs and examined the thermal, morphological, and rheological effects.