Microcellular polylactic acid (PLA) foams with various cell size and cell morphologies were prepared using supercritical carbon dioxide (sc-CO2) solid-state foaming to investigate the relationship between the cell structure and mechanical properties. Constrained foaming was used and a wide range of cell structures with a constant porosity of ∼75% by tuning saturation pressure (8–24 MPa) was developed. Experiments varying the saturation pressure while holding other variables’ constant show that the mean cell size and the mean cell wall thickness decreased, while the cell density and the open porosity increased with increase of pressure. Tensile modulus of PLA foams decreased with increasing the saturation pressure, but the specific tensile modulus of PLA foams was still 15–80% higher than that of solid PLA. Tensile strength and elongation at break first increased with increasing saturation pressure up to 16 MPa and then decreased with further increasing saturation pressure (20 MPa and 24 MPa) at which opened-cell structure produced. Compressive modulus, compressive strength, and compressive yield stress also followed the same variation trend. The results indicated that not only cell size plays an important role in properties of PLA foams but also cell morphology can influence these properties significantly.
The influence of 1-decene as the second monomer on the melt-grafting behavior of maleic anhydride (MAH) onto polypropylene (PP) was studied with differential scanning calorimetry and Fourier transform infrared spectroscopy. We found that the value of the grafting degree increased from 0.68% for pure MAH-g-PP to 1.43% for the system with a 1-decene/MAH molar ratio of 0.3, whereas the maximum value with styrene (St) as the second monomer was 0.98% under an St/MAH molar ratio of 1.0. Compared with the contribution of St/MAHg-PP to the peeling strength between the PP and polyamide (PA) layer for a PP/PA laminated film, the introduction of 1-decene/MAH-g-PP increased the peeling strength from 180 g/15 mm to 250 g/15 mm. 1-Decene inhibited the chain scission behavior of PP. 1-Decene reacted with MAH to form a 1-decene/MAH copolymer or the Alderene reaction product before the two monomers grafted onto PP. The grafting of the reactive product onto PP greatly improved the grafting degree of MAH. What is more, because of the similar chemical structures of 1-decene and PP, the affinity of 1-decene with PP was higher than that of St. Compared with St, the introduction of less 1-decene led to a higher grafting degree and higher peeling strength. Therefore, we concluded that 1-decene was more effective for improving the grafting degree of MAH onto PP.
Although polypropylene (PP) grafted by maleic anhydride (MAH) has been successfully commercialized, the grafting mechanism under the existence of a second monomer is not very clear. To simulate the grafting process of MAH onto PP with 1-decene as the second monomer, the MAH/1-decene copolymer and Alder ene reaction product were prepared and grafted onto PP with dicumyl peroxide as the initiator. We found that the grafting of the copolymer and the reactive product greatly improved the grafting degree of MAH. Particularly, the grafting degree with a 3% content of the Alder ene reactive product was 1.89%, which was 178% higher than that of pure MAH-g-PP, the maximum value of which was 0.68%. The molecular weight of the synthesized product affected the grafting efficiency and crystallization behavior of the grafted system. The system grafted with the Alder ene reaction product showed a higher crystallization temperature and crystalline degree. When the molecular weight of the copolymer was higher than 1700, the improvement of the grafting degree could be neglected. For MAH-g-PP with 1-decene as the second monomer, the possible grafting mechanisms were as follows: first, the formation of MAH/1-decene copolymer or the Alder ene reaction product, and second, the grafting of the previously formed product onto PP, in which, the effect of the Alder ene reaction product was more pronounced.
The phase behavior and phase‐separation dynamics of polystyrene/polyvinyl methyl ether (PS/PVME) blend with a critical composition of 70 vol % PVME were examined with a light scattering technique under a shear‐rate range of 0.1–40 s−1. If the shear rates were less than 8 s−1 and the starting temperatures of the measurement were 343 and 383 K, respectively, two cloud points were observed, whereas after the shear rate was higher than 8 s−1, only one cloud point existed, 20 K higher than that of the static state of the blend. Investigation of the phase‐separation dynamics at 443 K suggested that in the vorticity direction the phase‐separation behavior at the early stage and the later stage can be explained by Cahn–Hilliard linearized theory and the exponent growth law, respectively. Phase separation occurs after a shearing time, which was called a delay time τd. The delayed time τd, the apparent diffusion coefficient, and the exponent term of the blend show strong dependence on shear rates. A theoretical prediction of the phase behavior of PS/PVME under a shear flow field by introducing an elastic energy term into Flory's equation‐of‐state theory was made, and the prediction was consistent with the experimental results. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 661–669, 2003
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