A series of polyurethane (PU) and vinyl ester resin (VER) simultaneous and gradient interpenetrating polymer networks (represented as s-IPN and g-IPN, resp.) curing at room temperature were prepared by changing the component ratios of PU or VER in s-IPN, time intervals, and component ratio sequences of s-IPN in g-IPN. The microstructures of s-IPN and g-IPN were detected by atomic force microscope (AFM), dynamic mechanical analyzer (DMA), and surface constitution scanning of nitrogen element of energy dispersive X-ray spectrum (EDX), respectively. The mechanical properties of s-IPN and g-IPN were studied by values in strain-stress curves detected by electronic multipurpose tester. The results indicated that the morphology and mechanical properties are both affected by PU/VER component ratios in s-IPN, gradient time intervals, and gradient component ratio sequences. Furthermore, the morphology detection by EDX and mechanical properties study both proved the formation of gradient structures in transition regions of g-IPN.
Interpenetrating polymer networks (IPNs) are a special class of polymer alloys synthesized with two or more distinct crosslinked polymer networks held together by permanent entanglement [
In our present papers, a series of polyurethane/vinyl ester resin (PU/VER) simultaneous and gradient IPN curing at room temperature are successfully synthesized by changing the component ratios of PU or VER in IPN and casting the mixture with different component ratios in a mold at various times and by different component ratio sequences. The relationships of morphology of IPN and g-IPN with mechanical properties are studied by the results obtained by strain-stress curves and microstructure observation and detections. The gradient structures formed in system are proved by the results of energy dispersive X-ray spectrum and nitrogen element surface scanning.
A weight amount of trimethylol propane (TMP, as crosslinker of PU) was dissolved in dyhydrated ethyl acetate (EAc) first and then 1,4-butylene glycol (1,4-BD, as chain-extender of PU), epoxy acrylate (supplied by Shanghai Xinhua Resin Plant, the main structure is formed by the chain extention of linear acrylate resin with bisphenol A and chloroepoxy propane and then is crosslinked by aromatic diisocyanate), PU prepolymer (toluene diisocyanate precursor, self prepared by using toluene diisocyanate and polyoxypropylene glyco (PPG, WM: about 3000; the hydroxyl value: about 52.0.), and benzoyl peroxide-N,N’-dimentylaniline (BPO-DMA, as redox initiators of epoxy acrylate) were sequentially added into the above system. After the mixture was thoroughly stirred and degassed under vacuum for 5–10 minutes, a transparent sample was prepared by curing in a mold. A series of simultaneous IPN (s-IPN, represented as
Tensile strength (
Generally, s-IPN shows better elongation at break and tensile strength than those of ordinary polymers and other types of multicomponent systems due to the mutual entanglement, forced compatibility, and synergism of the networks in s-IPN. In the s-IPN of PU/VER, VER is the hard segment contents. Thus, both the values of elongation at break and tensile strength are proportional to the VER content in s-IPN. Figure
Strain-stress curves of s-IPN with different component ratios.
Similar to the effects of VER in s-IPN to mechanical properties, higher modulus of VER in g-IPN also contributes better mechanical properties to g-IPN. Different component ratio of PU and VER results in the different modulus contributions of PU and VER networks in system. The different component sequence of PU to VER networks with different component ratio is another factor to affect mechanical properties of g-IPN. Figure
Mechanical properties calculation results of 70/30 s-IPN and g-IPN with different component sequences.
Samples | ||||
---|---|---|---|---|
2 | 0.8 | 0.70 | 126.94 | |
3 | 0.7 | 0.74 | 140.16 | |
3.5 | 0.6 | 0.91 | 193.78 | |
2 | 1.3 | 0.71 | 75.00 | |
3 | 1.2 | 0.83 | 101.78 | |
3.5 | 1.6 | 0.94 | 89.69 | |
70/30 s-IPN | 2.9 | 1.07 | 49.09 |
Strain-stress curves of g-IPN with different component sequences at different time intervals: (a)
Figure
Mechanical properties calculation results of
2 | 0.8 | 0.70 | 126.94 |
3 | 0.7 | 0.74 | |
3.5 | 0.6 | 0.91 | 193.78 |
Strain-stress curves of g-IPN with different time intervals.
The mechanism of curing reactions for PU and VER is different, and thus the two networks could be formed simultaneously. In most cases of the formation of IPN, to improve the thermodynamic compatibility between two different components, researchers often introduce the graft structures between two networks. In this paper, the amino group in VER can react with the isocyanate group in PU to form the graft-IPN. Figure
AFM photographs of s-IPN: (a) 90/10; (b) 80/20; (c) 70/30 s-IPN.
The bright phase is the PU phase with lower modulus and the darker phase is the VER phase with higher modulus. As shown in Figure
The morphology of IPN also can be studied further by DMA detection. From the loss factor (
Loss factor versus temperature curves of IPN with different component ratios.
Figure
Loss factor versus temperature curves of the g-IPN with different component sequences.
In the present study, considering that g-IPNs contain nitrogen, and its content contributes by PU network, the detection of the content changing of N element along the samples in system can illustrate the changing of PU in g-IPN. Figure
The surface line scanning of element N on the different cross section of g-IPN.
The nitrogen element constitution of surface scans of g-IPN at different parts of layer: (a) 220
A series of simultaneous and gradient IPN with different miscibility and mechanical properties are prepared by varying the component ratios of PU to VER in simultaneous IPN, time intervals, and component ratio sequences in gradient IPN. The elastic properties of IPN can be improved evidently by interpenetrating techniques and gradient structures formed in transitional regions. PU/VER component ratios in s-IPN, time intervals, and component ratio sequences in g-IPN all affect the morphology. The formation of gradient constructions in transition regions of g-IPN can be proved by morphology studies through EDX analysis and mechanical properties detections. The content of nitrogen in g-IPN changes with different trend along the direction of sample. And the morphology observation and detection results are both consistent with the results gained by mechanical detections.
The authors gratefully acknowledge the National Natural Science Foundation of China under Grant no. 50675045, the Program for New Century Excellent Talents in University (NCET-08-0165), and Youth Science and Technology Foundation of Heilongjiang Province, China (QC07C01).