Phase transitions of the rapid‐compression‐induced mesomorphic isotactic polypropylene under high‐pressure annealing

Self Cite

Isotactic polypropylene filled with 1 wt.% multi-walled carbon nanotubes (iPP/MWCNTs) were prepared, and their crystallization behavior induced by pressurizing to 2.0 GPa with adjustable rates from 2.5 to 1.3 × 104 MPa/s was studied. The obtained samples were characterized by combining wide angle X-ray diffraction, small angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atomic force microscopy techniques. It was found that pressurization is a simple way to prepare iPP/MWCNTs composites in mesophase, γ-phase, or their blends. Two threshold pressurization rates marked as R1 and R2 were identified, while R1 corresponds to the onset of mesomorphic iPP formation. When the pressurization rate is lower than R1 only γ-phase generates, with its increasing mesophase begins to generate and coexist with γ-phase, and if it exceeds R2 only mesophase can generate. When iPP/MWCNTs crystallized in γ-phase, compared with the neat iPP, the existence of MWCNTs can promote the nucleation of γ-phase, leading to the formation of γ-crystal with thicker lamellae. If iPP/MWCNTs solidified in mesophase, MWCNTs can decrease the growth rate of the nodular structure, leading to the formation of mesophase with smaller nodular domains (about 9.4 nm). Mechanical tests reveal that, γ-iPP/MWCNTs composites prepared by slow pressurization display high Young’s modulus, high yield strength and high elongation at break, and meso-iPP/MWCNTs samples have excellent deformability because of the existence of nodular morphology. In this sense, the pressurization method is proved to be an efficient approach to regulate the crystalline structure and the properties of iPP/MWCNTs composites.

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Structure and Mechanical Properties of Multi-Walled Carbon Nanotubes-Filled Isotactic Polypropylene Composites Treated by Pressurization at Different Rates

Jia

Dong

et al. 2019

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“…The high-pressure treatment was performed on a homemade pressure device, and the sample assembly is shown in Figure 1a, whose detailed information has been described in previous work [28,34]. The experimental pressure and temperature profiles are shown in Figure 1b.…”

Section: Preparation Of Samplesmentioning

confidence: 99%

“…The specimen was placed between two aluminum plates and pressed by two opposing pistons under a desired pressure within a certain time, and the required temperature was supplied by the outer heating jacket. The experimental procedure was as follows: after loading the specimen, a relatively low pressure (10 MPa) was applied, and the sample was heated to 200 • C and annealed at this temperature for 10 min to erase processing histories; in order to ensure that the sample has solidified at the end of pressurization, the desired pressure was set at 1.7 GPa so as to obtain a sufficiently high undercooling [28,34], then the relaxed melt at 200 • C were pressurized to 1.7 GPa within the controllable durations of 2,22,34,45,113,226,453,895,1308, 2125, and 2833 s, respectively, which correspond to the wide pressurization rate ranging from 0.6-750 MPa/s; subsequently, the pressurized samples were immediately cooled down to 40 • C at an average cooling rate of about 10 • C/min under 1.7 GPa; at last, the pressure was completely released to obtain the solidified samples. For comparison, another group of samples were prepared by heated to 200 • C, annealed for 10 min, and then cooled down to 40 • C under atmospheric pressure.…”

Section: Preparation Of Samplesmentioning

confidence: 99%

Synergistic Effect of Pressurization Rate and β-Form Nucleating Agent on the Multi-Phase Crystallization of iPP

Jia

Zhuo

et al. 2021

Self Cite

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and β-form nucleating agent (β-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and β-NA enables the preparation of a unique multi-phase crystallization of iPP, including β-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6–1.9 MPa/s, a significant competitive formation between β- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both β- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, β- and γ- iPP coexist with each other. Moreover, with different β-NA contents, the best pressurization rate for β-iPP growth is the same as 1.9 MPa/s, while more β-NA just promotes the content of β-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of β-iPP, it shows that β-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

“…This process is known as relaxation and significantly determines the degree of orientation generated in the component during the manufacturing process and, thus, local anisotropies with respect to mechanical [ 13 , 14 ], optical [ 10 , 14 ], and shrinkage-related [ 15 ] properties. The speed of the intramolecular conformational ordering process depends on the mobility of the molecular chains and is thus determined by the free volume between the polymer chains of the amorphous melt [ 16 ]. As the free volume between the polymer chains increases with increasing temperature [ 17 ] and decreases with the orientation of the molecular chains [ 11 ], this affects the relaxation times.…”

Section: Introductionmentioning

confidence: 99%

Pressure Equilibrium Time of a Cyclic-Olefin Copolymer

Roth

Drummer

2021

Integrative simulation techniques for predicting component properties, based on the conditions during processing, are becoming increasingly important. The calculation of orientations in injection molding, which, in addition to mechanical and optical properties, also affect the thermal shrinkage behavior, are modeled on the basis of measurements that cannot take into account the pressure driven flow processes, which cause the orientations during the holding pressure phase. Previous investigations with a high-pressure capillary rheometer (HPC) and closed counter pressure chamber (CPC) showed the significant effect of a dynamically applied pressure on the flow behavior, depending on the temperature and the underlying compression rate. At a constant compression rate, an effective pressure difference between the measuring chamber and the CPC was observed, which resulted in a stop of flow through the capillary referred to as dynamic compression induced solidification. In order to extend the material understanding to the moment after dynamic solidification, an equilibrium time, which is needed until the pressure signals equalize, was evaluated and investigated in terms of a pressure, temperature and a possible compression rate dependency in this study. The findings show an exponential increase of the determined equilibrium time as a function of the holding pressure level and a decrease of the equilibrium time with increasing temperature. In case of supercritical compression in the area of a dynamic solidification, a compression rate dependency of the determined equilibrium times is also found. The measurement results show a temperature-invariant behavior, which allows the derivation of a master curve, according to the superposition principle, to calculate the pressure equilibrium time as a function of the holding pressure and the temperature.