## 3 Results and Discussion  
FTIR analysis was carried out to investigate the chemical structure of WPUs and the spectra are displayed in Figure 1. The characteristic peaks of the free-NCO groups in the wavenumber range of 2250-2270 cm\({}^{-1}\) were not found in WPUs spectra, which confirmed the completion of WPU synthesis reaction. Further, a new peak was observed at approximately 3350 cm\({}^{-1}\), which corresponded to the N-H stretching vibration. In addition, a combined peak was detected at 1532 cm\({}^{-1}\), which indicated both N-H out-of-plane bending and C-N stretching. Besides, 2 peaks were observed at 2867 and 2950 cm\({}^{-1}\), which were assigned to the C-H stretching vibrations of urethane bonds. There were strong absorption peaks at 3350 cm\({}^{-1}\) in the NH stretching region, which were attributed to the free NH stretching vibration as well as the hydrogen bonding between carbonyl groups and NH. With an increase in CO content, the NH stretching peaks of CO\({}_{9\%}\)-WPU, CO\({}_{2.50\%}\)-WPU, CO\({}_{5.04\%}\)-WPU, CO\({}_{7.40\%}\)-WPU, and CO\({}_{9.66\%}\)-WPU shifted from 3366 cm\({}^{-1}\) to 3361, 3353, 3351 and 3349 cm\({}^{-1}\) respectively. Castor oil molecules containing carbonyl groups tended to form hydrogen bonds with hydrogen atoms. The shift to lower frequencies indicated the formation of more hydrogen bonds. Figure 2 showed the particle size distributions of the WPUs with different content of CO. As can be seen from Figure 2, CO\({}_{9\%}\)-WPU sample exhibited unimodal PSD and the particle size was around 200 to 500 nm. The polycaprolactone chain segments easily aggregate because of the good regularity so that most of the hydrophilic groups were embedded, resulting in the formation of larger particles during the water emulsification of the polymer. It can be found that CO\({}_{2.50\%}\)-WPU sample exhibited smaller particle size and narrower PSD compared with CO\({}_{9\%}\)-WPU although N-MDEA content (3.30 wt%) that formed ionic centers by quaternization in polyurethane chains decreased. It was interesting to find that CO\({}_{5.04\%}\)-WPU and CO\({}_{7.40\%}\)-WPU exhibited bimodal particle size distribution while particle size became smaller. Unlike CO\({}_{5.04\%}\)-WPU and CO\({}_{7.40\%}\)-WPU, CO\({}_{9.66\%}\)-WPU exhibited unimodal PSD and the particle sizes were around 20 to 50 nm. It can be shown that the particle size of WPU decreased with increasing CO content although the amount of hydrophilic moiety decreased. This can be explained by the fact that the  
Figure 1: FT-IR spectra of WPUs. (**a**) in the wavenumber range of 3100–3600 cm\({}^{-1}\); (**b**) in the wavenumber range of 500–4000 cm\({}^{-1}\)). addition of castor oil disrupted the aggregation of polycaprolactone segment, leaving the hydrophilic group exposed, which helped the polymer to form smaller emulsion ions in the process of the formation of latex particles. The micrographs of the representative regions of all samples were recorded at same magnification and demonstrated in Figure 3. The SEM micrographs of WPU films provided the data on flexibility and rigidity, surface morphology, and phase interaction. From the SEM images of the WPU films, a sophisticated surface morphology was found. What most attracts attention was that there were a number of free-standing small nanorods on the surface of CO\({}_{0\%}\)-WPU film. In CO\({}_{2.50\%}\)-WPU, the width of the trilateral and quadrangular cylinders ranged from 100 nm to 200 nm. Many smaller spots can be seen on the films surface of CO\({}_{5.04\%}\)-WPU and CO\({}_{7.40\%}\)-WPU. The diameters of the well-dispersed dots on the surfaces of the two films were generally \(<\)100 nm. CO\({}_{9.66\%}\)-WPU containing higher content of CO demonstrated a remarkable enhancement in homogeneity among particles, and it was more uniform. Figure 4 showed the TEM photographs of CO\({}_{0\%}\)-WPU, CO\({}_{5.04\%}\)-WPU, and CO\({}_{9.66\%}\)-WPU.