By adding 3-(trimethoxysilyl)propyl acrylate to the silicone/urethane elastomer network, the mechanical properties of the silicone/urethane elastomer can be improved by creating additional covalent bonds at elevated temperature. The mechanical properties of the silicone-containing PUA elastomer can be improved even after complete photopolymerization, and local modulus increase is also possible by heating only the desired region of the elastomer. General uses and properties of polyurethane. a) Stress-strain curves of pure polyurethane (PU) and PU/cellulose nanocrystal (CNC) nanocomposites and storage modulus of PU and PU/CNC nanocomposites as a function of temperature (Aihua P., et al., 2011).
Various methods to improve the mechanical properties of polymeric elastomers. a) Schematic representation of the fabrication process flow of polypropylene (PP)/graphene nanocomposites and representative stress-strain behavior of PP/graphene nanocomposites with different graphene loadings (Pingan, S., et al., 2011). The elastomers were annealed at 80℃ for an hour or two to improve their mechanical properties. The deformation of each patch decreased as the modulus of the film increased.
Introduction
- Polyurethane Acrylate (PUA)
- Mechanical Properties Enhancement of Polymeric Elastomer
- Research Motivation
- Research Objective
Wang, R. et al, 2015; Pervin, R. et al., 2015) which can improve the overall mechanical properties and can also be used as sacrificial bonds. Although there are several methods to improve the mechanical properties of elastomers, the inclusion of other materials such as nanocomposites, fillers and additives to the polymeric matrix may not be the best way to improve mechanical properties. In order to improve the mechanical properties by using nanocomposites, fillers or additives, their surfaces must be treated in a way to ensure high affinity for the polymeric matrix (Ning, N. et al., 2012; Khoshkava, V. et al., 2013 ).
Otherwise, incorporated materials will easily separate from the matrix, crack and ultimately lead to failure of the elastomer rather than improving the material's mechanical properties (Naeim A. Even if it is possible to explain the high affinity of the fillers to the polymeric matrix, there are some additional steps necessary to prepare their surfaces in a way that they can be well incorporated into the matrix. In the case of creating additional bonds or networks in a polymer matrix, is it difficult to adjust the mechanical properties while not changing the composition.of prepolymer solution.
It is thus important to develop a method which is simple, yet provides sufficient mechanical property enhancement of elastomers and the desired degree of improvement. We aimed to develop a new method to improve the mechanical properties of polyurethane elastomers in a selective range while adjusting the extent of improvement even after the complete photocuring. Improving the mechanical properties of elastomers in a selective region was accomplished by forming additional Si-O covalent bonding simply by baking or heating the desired region of the PUA elastomer.
Selectively improved mechanical strength of PUA elastomers was verified by observing the deformation of a spot drawn on a surface of the elastomer and the resistance change of the Pt layer on the surface of the elastomer. a) The composition of the polyurethane prepolymer solution and the photocuring conditions.
Experimental Section
- Materials
- PUA Resin Synthesis
- PUA Elastomer Preparation
- Characterization
To adjust the mechanical properties of the elastomer, the desired area of the elastomer was heated at 80 ℃ for one or two hours. All the samples were prepared by cutting edges in a dumbbell shape by hand with a blade. The edge of PUA elastomers produced with molds had minor cracks, and the edge of PUA. elastomer produced using photomask was softer than the body of the elastomer due to undercross-linking at the edge.
Unlike other elastomers, PUA650-1-60 elastomer showed the greatest difference between heated and unheated area of the elastomer. Since there was a clear difference between local modulus-enhanced area and the rest of the area of PUA elastomers, the selectively modulus-enhanced PUA elastomers were tested again, but in a smaller area with improvement. According to the result of the tensile test (Figures 11 and 12), PUA650-1-50 was selected as the most suitable PUA resin among all the resins for modulus increase test.
As the modulus of the film was improved, the amount of distortion of each spot was reduced. Since it has been verified that the modulus of PUA elastomer can be improved in a selective region, the improvement of the selective modulus can be used to ensure the conductivity in a desired part of the elastomer (Figure 16). To improve the mechanical properties of the elastomer, 3-(trimethoxysilyl)propyl acrylate molecules were photo-cured together with the polyurethane resin to produce a uniform polymeric elastomer.
The method to improve the modulus of the PUA elastomers was very simple: heat the elastomer above 80℃ only in a desired area for specified improvement. Selected modulus improvement was investigated by observing deformation of a dot drawn on a surface of the elastomer and resistance change of Pt layer on the surface of the elastomer.
Result and Discussion
PUA Synthesis
HEA has higher reactivity than hydroxyethyl methacrylate (HEMA) (Lee, T. Y. et al., 2004), so chain termination with HEA rather than HEMA enables faster photopolymerization under UV light of the same intensity in the presence of an appropriate photoinitiator. 2-Methyl-1-propanol was also used to terminate the polymer chain, and 2-methyl-1-propanol-terminated chain end lacks reactivity under UV light in the presence of a photoinitiator. The methyl-terminated chain acts as a type of plasticizer that prevents the polymer chains from being tightly packed or entangled with each other and lowers the viscosity of PUA resin.
Three variables were used to synthesize different types of PUA resin: the molecular weight of PTH, the ratio of PTH to PDMS, and the synthesis temperature (Table 1). Synthesized PUA resins by varying PTH molecular weight, PTH to PDMS ratio, and synthesis temperature. The molecular weight of the synthesized resin was measured by gel permeation chromatography (GPC) (Figure 6 and Figure 7).
While there was almost no difference between the molecular weight of PUA650-X-50 and PUA650-X-60, the molecular weight of PUA2000-X-60 showed a large difference from the molecular weight of PUA2000-X-50. It means that PTH650 can move more freely than PTH2000 in a PUA reaction mixture even at a lower temperature. Active motion enables higher conversion at relatively low temperature and can reduce the molecular weight difference of PUA650-X-50 and PUA650-X-60.
Since PTH2000 is free to move at elevated temperature, the reaction conversion is reduced while PUA is synthesized from PTH2000 at relatively low temperature. The viscosity of honey at room temperature is 10,000 cP, so pure PUA resins are too thick to handle.
Tensile Test of PUA Elastomer
Therefore, we decided to produce PUA elastomers in the form of a simple strip for further tensile testing. a) Broken polyurethane elastomers after tensile test. The samples in the top row were made from PUA synthesized using only PTH2000 as diol monomers, and the samples in the bottom row were made from PUA synthesized using a mixture of PTH2000 and PDMS in a ratio of 3. The samples in the first column were made from a mixt. of prepolymers consists of 40% PUA, 5% Darocur 1173 (photoinitiator, PI), 5% 3-(trimethoxysilyl)propyl acrylate (Silane) and 50% ethanol.
It was expected that PUA elastomers prepared with PUA650 would have higher modulus and lower elongation at break than PUA elastomers prepared with PUA2000. Therefore, the mechanical strength of PUA elastomer can be improved by the formation of new Si-O bonds by simply baking the elastomer. In the presence of PDMS units in the PUA backbone, the PUA elastomers showed higher mechanical property improvement (Table 2 and Figure 13).
The result can be explained by a formation of Si-O bonds not only between 3-(trimethoxysilyl)propylacrylate molecules themselves, but between 3-(trimethoxysilyl)propylacrylate molecules and PDMS parts in a backbone. When PDMS moieties are in the main chain, there are more chances for Si-O bonds to form than in the case of absence of PDMS. While both PUA650 and PUA2000 have PDMS parts in the main chain, the mechanical properties of PUA650 were more affected by elevated temperature baking than those of PUA2000.
Since the molecular weight of PUA650 is lower than that of PUA2000, PUA elastomer made from PUA650 has more PDMS units in its polymer matrix compared to PUA elastomer made from PUA2000 with the same volume. Therefore, the result can be explained as a higher density of Si-O bonds in PUA elastomer made from PUA650 than PUA2000 at a given volume.
Selective Modulus Enhancement of PUA Elastomers
PUA elastomers were prepared from PUA 650-1-50 resin and the elastomers were selectively heated only in the center (Figure 15). After selective modulus enhancement, each film was stretched to 200% of its original length to control the amount of deformation of the spot in the center of the elastomer. The size of the spot in the center of the sample heated for 120 seconds was almost the same even after the sample was elongated to 200% of its original size, while the spot in the first sample was largely deformed under load.
Selectively modulus enhanced samples and the difference between a spot size before and after modulus enhancement. a) Selectively heated PUA650-1-50 elastomer for modulus increase.
Selective Conductivity Securing
Conclusions
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