Toughening of poly(propylene carbonate) using rubbery non-isocyanate polyurethane: Transition from brittle to marginally tough
Graphical abstract
Introduction
Poly(propylene carbonate) (PPC) is an alternating copolymer of carbon dioxide and propylene oxide, it has received intensive attention not only for it comes from cheap, abundant CO2, but also it is biodegradable polymer [1], [2], [3], [4]. Possible application of PPC has been reported, such as tissue scaffolding [5], [6], polymer electrolyte [7], adhesive agents, etc. In addition, considering its remarkable barrier property of oxygen and water, PPC can also be used as packaging material [8], [9], [10]. However, PPC is a brittle polymer, since its elongation at break was below 10% at 20 °C, which severely limits its practical application.
To modify the mechanical properties of PPC, especially the brittleness, plasticizer such as diallyl phthalate [11] or low molecular weight urethane compound [12], etc., has been employed through blending, and the elongation at break of PPC has been improved significantly. However, the tensile strength and the glass transition temperature decreased meanwhile because the low molecular weight plasticizer acted as lubricant leading to more prone to move for PPC chain.
Rubbery particles, which have different elastic property with the matrix, have been proved to be effective in toughening brittle polymers. Generally, toughness of a polyblend is related to its morphological structure, while the toughening mechanism is quite complicate. A commonly accepted view is that the rubbery particles alter the stress state in the matrix around the particles, and special structure such as multiple crazes [13], [14], [15], microvoids [16], [17], shear bands [18], [19], appears during the impact process. Multiple crazes have been observed during the impact process of high impact polystyrene (HIPS) [13], the size of rubbery particles, along with the adhesion between rubbery particles and the matrix, is of vital importance because the rubbery particles must act as craze initiators and craze stoppers. While in nylon/rubber blend, matrix yielding is believed to be the main factor contributing to its toughness [18]. Interparticle distance is the critical parameter determining the brittle–ductile transition, and the adhesion between the matrix and rubber is necessary. Enhanced matrix shear yielding has been observed due to the formation of microvoids in ABS [17] and polycarbonate [16]. Although only a small part of energy dissipated through cavitation, it played an important role in initiating further plastic deformation [20], [21]. Because the stress state around the particles is important in the toughening mechanism, the morphology of the polyblend becomes the definitive factor. As far as we have known, there are few articles discussing the effect of rubbery particles on the toughness of PPC.
Non-isocyanate polyurethane (NIPU), which can be prepared by reaction of bis(cyclic carbonate)s and diamines, is non-toxic and CO2-based with utilization of CO2 in the synthesize of bis(cyclic carbonate)s [22], [23]. Because of the randomly existing units containing primary and secondary hydroxyl groups, it is amorphous with low glass transition temperature, making it to be rubbery particles at 20 °C. Moreover, as shown in Scheme 1, it is rich in hydrogen bonding moiety like amine or hydroxyl group, capable of forming hydrogen bonding with carbonyl or oxygen unit in PPC. Another bonus lies in its water solubility, it can be easily removed from the polyblend by water etching, which makes it suitable for prompt morphology study, thereby understanding influence of morphology change on the toughness of polyblend. Therefore, NIPU might be a kind of ideal rubbery particle to toughen PPC and illustrate the toughening mechanism. In this work, three kinds of NIPU were synthesized and used to toughen PPC (Scheme 1). A transition from brittle to marginally tough was observed accompanying with the debonding of NIPU from the matrix. The effect of the morphology and the intermolecular hydrogen bonding between PPC and NIPU on the toughness has also been discussed.
Section snippets
Materials
Chemicals like 1, 3-propanediamine, 1, 8-diaminooctane and 1, 6-hexanediamine were purchased from Aldrich and used without further purification. Ethylene glycol diglycidyl ether was purchased from Ruipu Material Co. (China) and distilled under vacuum before use. Bis(cyclic carbonate) based on the coupling reaction of CO2 and ethylene glycol diglycidyl ether was prepared by iron complex catalysts.
PPC was supplied by Zhejiang Bangfeng Plastic Co. (China), whose technique was licensed under our
Tensile performance and toughening effect of PPC/NIPU blend
The stress–strain curves of PPC/NIPUs blends were plotted in Fig. 1, and the corresponding parameters were listed in Table 1. Neat PPC displayed brittle fracture with elongation at break of around 10%, it increased to over 30% when NIPU1 loading was over 8 wt%, and yielding points appeared clearly in the stress–strain curves. Similar toughening effect can be observed in PPC/NIPU2 and PPC/NIPU3 blends. When NIPU2 loading was 10 wt%, the elongation at break was above 30%, while the elongation at
Conclusion
PPC/NIPU blends with different NIPU loadings were prepared through melt blending. When the NIPU loading was below 10 wt%, the NIPU dispersed uniformly in PPC matrix, and a transition from brittle to marginally tough occurred when L/d reached a critical value, 1.74, where L and d were center-to-center distance and the diameters of the particles, respectively. When the NIPU loading increased to 10 wt%, the impact strength increased 3 times compared with neat PPC. The debonding of NIPU accounted
Acknowledgments
The work was financially supported by the National Natural Science Foundation of China (Grant No. 51321062 and No. 21134002).
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