The Lateral Grafting of Phenyl Group to Polyurethane Copolymer and its Impact on Molecular Interactions

Abstract

Polyurethane (PU) has been known for shape memory, ease of synthesis, and versatile applications, 1-3 and the crosslinking improved the tensile and shape memory properties. 4-6The grafting of functional groups to polymers has been extensively researched for the development of new functional polymers without harming their basic structure. 7Actually, the pendant naphthalene kept PU flexible at low temperature by hindering the molecular interactions between PU chains.8 PU was made water-compatible by incorporating a pendant carboxylate-ammonium salt group.9 The previous pendant groups were all attached to the PU through a diol-containing pendant group. Here, allophanate bonding was employed as a method for grafting a functional group to PU due to the advantages of having more available linking sites, higher reactivity, and simple reaction conditions.10 The pendant group can change the molecular interaction between PU chains, and the impact of this on the tensile and shape memory properties is very intriguing. The rigid aromatic ring as a pendant group may reduce molecular interactions and contribute to low temperature flexibility. In this investigation, the impact of the grafted phenyl group on the tensile properties and low temperature flexibility is examined. Results and DiscussionThe PU was synthesized using previously reported methods.4-6 The phenyl group was grafted to the PU chain via an allophanate bonding, and the repulsion between the grafted phenyls was intended to disrupt the molecular interactions between the PU chains and allow the PU to be flexible enough to move at low temperature. The PU grafting with diisocyanate has already been well researched and can be carried out under very mild reaction conditions.10 In this experiment, the usage of a catalyst such as dibutyl tin laurate (DBTL) or triethylamine was not necessary because the grafting reaction was fast enough to finish without catalysis. The structure of the phenyl-grafted PU is shown in Figure 1(a). Phenylisocyanate was selected as the pendant group because the phenyl was rigid enough to exert the molecular repulsion and the monoisocyanate structure could obviate the possibility of crosslinking observed in diisocyanate grafting (Figure 1(b)). The content of phenylisocyanate was gradually raised while the hard and soft segment contents were fixed in order to investigate the effect of phenyl grafting (Table 1).The IR spectra of the selected ones (P-0, P-3, and P-5) are compared in Figure 2(a). Intermolecular attractions, such as hydrogen bonding and dipole-dipole interactions, between the hard segments of the PU can be analyzed from the IR spectra. The bonded C=O stretching vibration appears from 1699 to 1706 cm ).11,12 As the phenyl content increased, the bonded carbonyl peak slightly decreased relative to the free carbonyl, suggesting that the molecular interactions were interrupted by the grafted phenyl. The thermal behavior was investigated by DSC from −50 to 250°C (Figures 2(b)). The glass transition temperature (T g ) of the soft segment i...