Co-reporter:Junsong Li, Xia Liao, Qi Yang, and Guangxian Li
Industrial & Engineering Chemistry Research October 4, 2017 Volume 56(Issue 39) pp:11111-11111
Publication Date(Web):September 7, 2017
DOI:10.1021/acs.iecr.7b02348
The CO2-induced crystals were successfully utilized to regulate the structure of the foamed poly(L-lactic acid) (PLLA) spherulites with large numbers of cells ranging from nanoscale to microscale in three temperature regions. Region I (60–80 °C) was characterized by the double melting peaks and the nanocells with the dramatically improved cell density. By increasing the temperature to Region II (90–110 °C), more highly perfect crystals and the unique spherulitic foam morphology were developed. In Region III (115–120 °C) double melting endotherms emerged again, and microcells were formed. The double melting endotherms in Regions I and III originated from the imperfect crystals created during the saturation and cooling stage, respectively. A bimodal structure with microcells surrounded by nanocells occurred at 115 °C and 20 MPa. It was shown that cells could nucleate and grow in between the neighboring lamellar stacks and that the cells interacted with the spherulites.
Co-reporter:Xian-Wu Cao;Ting-Ting Zheng;Guang-Jian He;Chang-Qian Chen;Ding-Meng Ke
Industrial & Engineering Chemistry Research January 27, 2016 Volume 55(Issue 3) pp:597-605
Publication Date(Web):2017-2-22
DOI:10.1021/acs.iecr.5b04094
Free-radical reactions of polymers, including biodegradable polymers via reactive extrusion, are normally induced by peroxide chemicals, which are known to lead to the formation of secondary products and impart some performances to the resin. Here, we report an ultraviolet (UV)-induced reactive extrusion, without employing a peroxide initiator, to control chain scission and branching reactions of polylactide (PLA). Through this technique, chain scission reaction of molten PLA induced by UV irradiation during extrusion was promoted to high-level efficiency. Degraded PLA samples had lower complex viscosity and storage modulus, because of random main chain scissions. Long-chain branched (LCB) structure of PLA was obtained when a multifunctional chemical agent, trimethylolpropane triacrylate (TMPTA), was added into the PLA matrix during extrusion. Various rheological plots including viscosity, storage modulus, loss tangent, and Cole–Cole plots were used to distinguish the LCB structures of PLA samples. Thermal and crystallization properties of degraded and branched PLA samples were also investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). For the LCB PLA samples, a distinct crystallization exothermic peak appeared and accompanied by the disappearance of the cold crystallization temperature, demonstrating significantly enhanced crystallization rates. This UV-induced reactive extrusion has nonresidues of peroxide, is highly efficient and easily adjustable, and opens new avenues in potential applications for PLA modification, such as grafting and polymerization.
Co-reporter:Weiqiang Han;Qi Yang;Guangxian Li;Bin He;Wenli Zhu;Zengheng Hao
RSC Advances (2011-Present) 2017 vol. 7(Issue 36) pp:22515-22523
Publication Date(Web):2017/04/19
DOI:10.1039/C7RA03496B
The crystallization behavior, banded spherulite and morphological transition of poly(L-lactide) (PLLA) phases within the block copolymers were investigated. All experimental results showed that the structure and thermal properties of PLLA–PCL copolymers could be adjusted by varying the ratios of the chain length of the two blocks. Morphological results indicated that the banded spherulites of PLLA formed when PLLA-rich copolymers crystallized. PCL segments introduced unbalanced stresses around PLLA lamellar crystals, which resulted in a bending moment responsible for twisting of PLLA lamellar crystals. As the block length ratio of PCL to PLLA increased, an over accumulation of PCL segments influenced the twisting of PLLA lamellae. In addition, it was interesting to find that the banded spherulite morphology changed with increasing the crystallization temperature. The crystallization temperature has an effect on the relationship between the sense of lamellar twisting and the morphological transition of PLLA, which is reflected in the fact that the band spacing of banded spherulites showed strong temperature dependence when the crystallization temperature exceeds 115 °C, while it exhibited weak temperature dependence below 115 °C. In particular, above 125 °C the band spacing disappeared and nonbanded spherulites formed.
Co-reporter:Jian Xiong, Xia Liao, Jingjun Zhu, Zhu An, Qi Yang, Yajiang Huang, Guangxian Li
Polymer Degradation and Stability 2017 Volume 146(Volume 146) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.polymdegradstab.2017.10.012
•Isotatic polypropylene is investigated by exposing to different outdoor climates for four years.•PALS is used to investigate the mechanism of chemi-crystallization and free volume change.•The interaction between functional groups decreases the size of free volume holes.In order to explore the natural weathering mechanism of isotatic polypropylene (iPP) served in different outdoor climates, iPP samples were exposed to four typical climate conditions of China for four years. The microstructural changes of iPP were investigated by positron annihilation lifetime spectroscopy (PALS), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC) and wide-angle X-ray diffraction (WAXD). As the degradation processed, the molecular defects such as carbonyl and hydroxyl groups increased. Because of the formation of the new imperfect crystal caused by chemi-crystallization, the crystallinity increased initially and then reached a plateau. Due to the increase of crystallinity, the amounts of defects in crystalline-amorphous interfacial phase increased and the concentration of free volume holes decreased with exposure time. In addition, the interaction between functional groups constrained the mobility of molecular segments, which persistently decreased the size of free volume holes. The results showed that the freed molecule segments released by chain scission formed new crystals in the amorphous region during the degradation process. In addition, the similar microstructural changes of iPP weathered in different stations showed the similar degradation mechanism. Furthermore, the influence of the temperature on the degradation rate of iPP was more significant than other environmental factors.
Co-reporter:Jianwei Bai, Xia Liao, Erbo Huang, Yong Luo, Qi Yang, Guangxian Li
Materials & Design 2017 Volume 133(Volume 133) pp:
Publication Date(Web):5 November 2017
DOI:10.1016/j.matdes.2017.07.064
•The heterogeneous nucleation effect of nanogaphite improved cell morphology of silicone rubber foams.•The silicone rubber foams with the highest cell density and the smallest cell sizes were obtained.•The content of silica was reduced attributed to the introduction of nanographite.•Incorporation of nanographite significantly improved mechanical properties and thermal decomposition temperature of silicone rubber foams.Microcellular silicone rubber/nanographite foam with the smallest cell diameter and the largest cell density compared to the previous studies was prepared using supercritical carbon dioxide as an environmentally benign foaming agent in this study. Both the cell microstructure and properties of porous composite were investigated in detail. In silicone rubber/nanographite system, the incorporation of nanographite into silicone rubber significantly induced heterogeneous bubble nucleation and enhanced the matrix strength, which resulted in the decreased average cell diameter from 4.97 to 1.12 μm and the increased cell density from 8.87 × 108 cells/cm3 to 1.25 × 1010 cells/cm3. The microcellular rubber foam with decreased cell size, increased cell density and narrow cell distribution was corresponded to the excellent compressive strength and higher hardness, which reduced the content of silica. Furthermore, the thermal decomposition temperature of the obtained foam increased from 380 °C to 413 °C due to the introduction of nanographite. The silicone rubber/nanographite foam with improved performance possessed potential in wide applications.The incorporation of small amount nanographite not only enhanced the matrix strength of silicone rubber, but also acted as heterogeneous nucleation agent during foaming, which decreased the cell size while increased the cell density. Thus, the content of silica was reduced and the mechanical property of silicone rubber foam was increased.Download high-res image (201KB)Download full-size image
Co-reporter:Erbo Huang, Xia Liao, Yunchuan He, Bin He, Qi Yang, Guangxian Li
Polymer 2017 Volume 118(Volume 118) pp:
Publication Date(Web):2 June 2017
DOI:10.1016/j.polymer.2017.04.065
•Three-dimensional porous PLA scaffolds of almost fully interconnected pores were successfully prepared.•PCL/PLA blend underwent a significant coarsening process in scCO2 under proper condition.•The porous scaffold showed good biocompatibility and high potential being used in tissue engineering application.Three-dimensional porous polylactide (PLA) scaffolds of almost fully interconnected pores were successfully prepared by extracting the poly(ε-caprolactone) (PCL) phase from co-continuous PCL/PLA blends. Quiescent annealing of the blends in supercritical CO2 (scCO2) prior to the PCL extraction, reported for the first time in this paper, allowed control over the average pore size from 50 to 150 μm of the PLA scaffolds. It was found that the PCL/PLA blend underwent a significant coarsening process in scCO2 under proper condition, which indicated that the CO2 pressure adjustment could be used as an additional effective tool for morphology control and the scaffold structure could be tailored by careful control of annealing time, temperature and CO2 pressure. Moreover, the structure of the blend could be changed at a relative low temperature in the presence of scCO2, this fact added the advantage over the conventional melt-processed treatment, as minimizing the degradation of PLA to maintain the mechanical properties of the scaffolds. A porous PLA scaffold was used to study the NIH 3T3 fibroblast cell culture and the cells successfully proliferated in the scaffold, which suggested biocompatibility and high potential of this scaffold being used in tissue engineering application.A porous scaffold could be obtained from polymer blending followed by quiescent annealing in supercritical CO2 and extraction of the porogen phase, and the scaffold structure could be easily tailored with the assistance of supercritical CO2.Download high-res image (352KB)Download full-size image
Co-reporter:Shaojie Li;Guangjian He;Chul B. Park;Qi Yang;Guangxian Li
RSC Advances (2011-Present) 2017 vol. 7(Issue 11) pp:6266-6277
Publication Date(Web):2017/01/18
DOI:10.1039/C6RA26457C
In this paper, long-chain branched polylactide (LCB-PLA) prepared by UV-induced reaction extrusion with trimethylolpropane triacrylate (TMPTA) was foamed by supercritical carbon dioxide (scCO2), and the effect of the long-chain branching structure on the cell morphologies of PLA foams was investigated. The LCB-PLA displayed higher complex viscosity, melting point and crystal nucleation potential under scCO2, and these factors could influence the foaming behavior of PLA which was proved by the different cell morphologies of samples foamed after various saturation times. The advantage of LCB-PLA on foaming was remarkable at high temperature and high pressure. LCB-PLA with more than 0.5% TMPTA showed nano-cells while the other samples showed micro-cells at 142 °C under 12 MPa, and the samples displayed elliptic cells with horizontal semimajor axis in linear PLA and circular cells or oval cells with vertical semimajor axis in LCB-PLA with increasing temperature. The improved cell morphology with reduced coalescence, no collapse and uniform cell distribution was also shown in LCB-PLA under higher pressure. All these results were due to the increasing matrix strength and higher crystal nucleation potential of LCB-PLA. The findings indicate that LCB-PLA possesses better foaming behavior at high temperature and high pressure. The wide foaming processing window of LCB-PLA would benefit the high temperature and high pressure foaming of PLA such as bead foaming and continuous extrusion foaming, thus broadening its application.
Co-reporter:Junsong Li, Xia Liao, Weiqiang Han, Wei Xiao, Jiangang Ye, Qi Yang, Guangxian Li, Qianping Ran
The Journal of Supercritical Fluids 2017 Volume 130(Volume 130) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.supflu.2017.08.003
•Millable polyurethane foam was prepared by supercritical CO2.•Viscoelasticity of polyurethane under CO2 was analyzed by high-pressure rheometer.•The increase of crosslinking density depressed the CO2 diffusion.•The effect of viscoelasticity on the cell structure depended on cell diameter.•The tensile strength decreased linearly with cell diameter on a log-log scale.The cell structure of microcellular millable polyurethane/nano-silica composites prepared by combining the solid-state supercritical CO2 foaming with the two-step curing were investigated in detail according to the high-pressure viscoelasticity of pre-cured polyurethane. The complex viscosity and storage modulus of pre-cured polyurethane saturated under supercritical CO2 increased with improving the crosslinking network induced by either pre-curing or nano-silica. Moreover, the raised crosslinking density could depress the CO2 diffusion and thus the reduction of the viscosity and modulus. It was shown that the matrix viscoelasticity impacted the cell morphology in different ways as the cell diameter was varied significantly. The compressive hysteresis results showed that the energy absorption of millable polyurethane elastomer was enhanced after the introduction of microcells. The tensile strength of millable polyurethane nanocomposite foam decreased with increasing the cell diameter, and displayed a linear relationship with cell diameter on a double logarithmic scale.Download high-res image (203KB)Download full-size image
Co-reporter:Erbo Huang, Xia Liao, Chongxiang Zhao, Chul B. Park, Qi Yang, and Guangxian Li
ACS Sustainable Chemistry & Engineering 2016 Volume 4(Issue 3) pp:1810
Publication Date(Web):February 23, 2016
DOI:10.1021/acssuschemeng.6b00008
We used a high-pressure differential scanning calorimeter (HP-DSC) to study polymer plasticization by compressed gases at pressures of up to 30 MPa for polylactide (PLA), polycarbonate (PC), isotactic polypropylene (iPP), and polystyrene (PS). The pressure reached values twice as high as the previously published data. We found that the polymer/carbon dioxide (CO2) system’s heating curves have an unidentified endothermic peak above 5 MPa, which turns out to be from CO2’s phase transition. The HP-DSC could accurately determine the depression of the glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) of various polymers at low pressures by simply starting at a higher temperature to avoid CO2’s phase transition; however, the increased plasticization effect of the dissolved CO2 lowered the Tg to the level of overlapping with CO2’s phase transition phenomena at elevated pressures, and therefore, the depressed Tg could not be measured above 6 MPa for PLA, PC, or PS. On the other hand, the Tc of iPP decreased with an increase in pressure, whereas Tm values of PLA and iPP decreased slightly with an increase in pressure and then remained almost unchanged above a certain pressure, which may indicate an increased hydrostatic pressure effect at elevated pressures.Keywords: Glass transition temperature; High-pressure differential scanning calorimeter (HP-DSC); Melting temperature; Phase transition; Supercritical carbon dioxide;
Co-reporter:Chenfei Jia, Xia Liao, Jingjun Zhu, Zhu An, Qiongwen Zhang, Qi Yang and Guangxian Li
RSC Advances 2016 vol. 6(Issue 37) pp:30986-30997
Publication Date(Web):16 Mar 2016
DOI:10.1039/C5RA26478B
β-Phase isotactic polypropylene (β-iPP) specimens with different contents of β-phase nucleating agent were employed to investigate the deformation-induced microstructure evolution during creep behavior. Morphological investigations by SEM showed that the crystalline morphologies of β-iPP were controlled by the content of the β-phase nucleating agent, namely, well-developed β-spherulites induced by low content of β-phase nucleating agent, bundle-like morphology with imperfect spherulites induced by medium content of β-phase nucleating agent and needle-like morphology induced by high content of β-phase nucleating agent. It was interesting to observe that all samples with different contents of β-phase nucleating agent showed a similar β/α transformation process. However, well-developed β-spherulites, which have integrated crystalline structure, showed poor creep resistance compared with the crystalline morphology nucleated by higher contents of β-phase nucleating agent. For bundle-like morphology, the crystalline phase was imperfect and obtained larger long spacing, resulting in better creep resistance. With respect to needle-like morphology, the crystalline phase was disordered and displayed largest long spacing, resulting in best creep resistance. The results of this work revealed that the creep resistance would be different with different crystalline morphologies. On the other hand, this work provided the evolution of microstructure during deformation to further explain the molecular mechanism of fatigue failure for creep.
Co-reporter:Yuwei Wang;Shaojie Li;Yong Luo;Qi Yang;Guangxian Li
Polymer International 2016 Volume 65( Issue 10) pp:1195-1203
Publication Date(Web):
DOI:10.1002/pi.5175
The effects of oxygen functional groups and alkyl chains at the surface of graphene oxide (GO) on the thermal stability, mechanical properties and foaming behavior of poly(methyl methacrylate) (PMMA) nanocomposites were investigated. Alkyl-functionalized GO (GO-ODA) was prepared by grafting octadecylamine (ODA) on the surface of GO. PMMA/GO and PMMA/GO-ODA nanocomposite were obtained by solution blending and were foamed using supercritical carbon dioxide (scCO2). GO-ODA, with the presence of alkyl chains, showed a better dispersion capability in PMMA matrix than GO with a large amount of oxygen functional groups. In addition, the good dispersion capability increased thermal stability and mechanical strength. In comparison with PMMA/GO samples foamed at 70 °C, PMMA/GO-ODA nanocomposite foams displayed improved cell structures with higher cell density, smaller cell size and more homogeneous cell size distribution, which results from the strong heterogeneous nucleation due to alkyl chains on the GO surface. The foaming behaviors became more complicated at 80 °C as the GO might be intercalated and exfoliated with the aid of scCO2, thus further enhancing the heterogeneous nucleation during the foaming process. The results indicated that the surface chemistry of GO was closely related to the properties and foaming behavior of the nanocomposites. © 2016 Society of Chemical Industry
Co-reporter:Junsong Li, Guangjian He, Xia Liao, Hao Xu, Qi Yang and Guangxian Li
RSC Advances 2015 vol. 5(Issue 46) pp:36320-36324
Publication Date(Web):14 Apr 2015
DOI:10.1039/C5RA03682H
Poly(L-lactic acid) (PLLA) foams with unique nanocellular and needle-like morphology were successfully prepared by combining spherulite templating and supercritical carbon dioxide (CO2) foaming. The corresponding crystalline morphology formed under supercritical CO2 in PLLA was illustrated to investigate the foaming behavior of spherulites in detail. It was found that not only the degree of crystallinity but also the crystalline morphology played a vital role in cell nucleation and growth, thus the foam morphology. Nanocellular structure was primarily generated for the PLLA foamed at 100 °C and 12–24 MPa. Moreover, the morphological transition from approximate circular cells to needle-like cells occurred around 16 MPa at 100 °C because of the constraint of lamellae, and the two different structures coexisted at 100 °C and at pressures ranging from 16 to 24 MPa. The results indicated that the expansion ratio of spherulite was bigger than that of PLLA foam.
Co-reporter:Yuwei Wang, Xia Liao, Yong Luo, Qi Yang, Guangxian Li
Journal of Materials Science & Technology 2015 Volume 31(Issue 5) pp:463-466
Publication Date(Web):May 2015
DOI:10.1016/j.jmst.2015.01.010
The surface chemistry of filler is closely related to the structure and morphology of nanocomposite foams. Changing the property of filler is widely used to control the cell structures and functionalize the composite foams. Surface-functionalized graphene oxide (GO-ODA) was prepared by grafting octadecylamine (ODA) on the surface of graphene oxide (GO) to make the filler disperse better in the nanocomposites and have a strong interfacial interaction with polymer matrix. Poly(methyl methacrylate) (PMMA)/GO-ODA nanocomposite foams were obtained by solution blending and foamed using supercritical carbon dioxide (scCO2). Compared to neat PMMA and PMMA/GO samples, the PMMA/GO-ODA nanocomposite foams showed improved cell structures with smaller size, higher cell density and more homogeneous distribution, which should be attributed to the heterogeneous nucleation caused by well-dispersed GO-ODA nanosheets. This work not only improved the compatibility and interfacial interaction of GO with polymer matrix but also indicated that the modified GO sheets can act as ideal filler to control the cell density, size and size distribution efficiently.
Co-reporter:Lingyun Wu;Jingjun Zhu;Kai Ni;Qiongwen Zhang;Zhu An;Qi Yang;Guangxian Li
Polymer International 2015 Volume 64( Issue 7) pp:892-899
Publication Date(Web):
DOI:10.1002/pi.4862
Abstract
The effect of confinement on glass dynamics combined with the corresponding free volume changes of amorphous polystyrene (PS) in blends with semi-crystalline high-density polyethylene (HDPE) have been investigated using thermal analyses and positron annihilation lifetime spectroscopy (PALS). Two different glass transition temperatures (Tg) were observed in a PS/HDPE blend due to the dissimilarity in the chemical structure, consistent with an immiscible blend. However, Tg of PS in the incompatible PS/HDPE blend showed an upward trend with increasing PS content resulting from the confinement effect, while Tg of the semi-crystalline HDPE component became lower than that of neat HDPE. Moreover, the elevation of Tg of PS was enhanced with a decrease of free volume radius by comparing annealed and unannealed PS/HDPE blends. Positron results showed that the free volume radius clearly decreased with annealing for all compositions, although the free volume hole size agreed well with linear additivity, indicating that there was only a weak interaction between the two components. Combining PALS with thermal analysis results, the confinement effect on the glass dynamics and free volume of PS phase in PS/HDPE blends could be attributed to the shrinkage of HDPE during crystallization when HDPE acted as the continuous phase. © 2015 Society of Chemical Industry
Co-reporter:Shaojie Li;Ting He;Qi Yang ;Guangxian Li
Polymer International 2015 Volume 64( Issue 12) pp:1762-1769
Publication Date(Web):
DOI:10.1002/pi.4977
Abstract
The structural changes and crystallization kinetics of polylactide (PLA) during cold crystallization under CO2 at 80 °C were studied using in situ high-pressure Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra show that PLA can crystallize under air and CO2, and some differences are observed. In the second-derivative spectra, the 1220 cm−1 band is only found for PLA crystallized under CO2, and the tt conformer of PLA crystallized under CO2 is located at 1749 cm−1, while that of PLA crystallized under air is located at 1751 cm−1. From wide-angle X-ray diffraction, only the α′-crystal is observed when PLA is crystallized under air, whereas the α-crystal appears when crystallized under CO2. The crystalline-sensitive bands at 921 and 1458 cm−1 were used to analyze the crystallization kinetics of PLA. When PLA crystallizes under air, the 1458 cm−1 band changes faster than the 921 cm−1 one; when it crystallizes under CO2, the result reverses. This suggests that CO2 hinders interchain interactions while promoting the helix conformation. © 2015 Society of Chemical Industry
Co-reporter:Jing Ji;Jianwei Bai;Xianglin Luo;Qi Yang
Colloid and Polymer Science 2015 Volume 293( Issue 8) pp:2311-2319
Publication Date(Web):2015 August
DOI:10.1007/s00396-015-3621-9
The lamellae structure and the ring-banded features of the concentric ring-banded spherulites grown from six-arm star-shaped poly(ε-caprolactone) (PCL) with different arm length under CO2 condition were investigated by polarized optical microscope, atomic force microscopy, and scanning electronic microscopy in this paper. Experimental results indicated that the concentric ring-banded spherulites with alternative periodically ridge and valley bands with continuous edge-on lamellae formed under the condition of CO2. The band space got narrower, and the number of the ring bands increased in PCL with the increasing pressure due to the plasticization effect of CO2. The band space of the six-arm star-shaped PCL with longer arm length was wider than that of the six-arm star-shaped PCL with shorter arm length whereas the number of ring bands was fewer. Because of the longer arm length which resulted in the higher entanglement, the chain mobility of the star-shaped PCL with longer arm length decreased markedly. The different entanglement resulting from the different arm length structure was considered to be the primary factors influencing the characteristic crystallization of the six-arm star-shaped PCL.
Co-reporter:Chenfei Jia, Qiongwen Zhang, Xia Liao, Jingjun Zhu, Lingyun Wu, Kai Ni, Qi Yang, Zhu An, Guangxian Li
Polymer 2015 Volume 67() pp:92-100
Publication Date(Web):12 June 2015
DOI:10.1016/j.polymer.2015.04.057
•The molecular deformation mechanism of polypropylene during creep including three stages is proposed.•The mechanism explains the transition point during creep deformation.•Positron annihilation lifetime spectroscopy is a powerful microanalytical technique to explore the microstructure changes.•The disaggregation–recrystallization process would lead the free volume to increase in number while decrease in size.The hierarchical microstructure evolution of polypropylene during creep was explored via various methods, such as differential scanning calorimetry (DSC), scanning electron microscope (SEM), two-dimensional small-angle X-ray scattering (2D-SAXS), two-dimensional wide angle X-ray diffraction (2D-WAXD) and positron annihilation lifetime spectroscopy (PALS). The results revealed a correlation among the changes of micron-scale spherulites, nano-scale lamellae, crystalline blocks, atomic scale free volume and the deformation of polypropylene during creep. The elongation of micron-scale spherulites along the creep direction, accompanying with the increase of nano-scale lamellar long spacing, as well as the enlargement and amalgamation of atomic scale free volume were observed at ε below 17%; the imperfect fibrillar crystallites with polymer chains preferentially oriented along the creep direction, formed in the stress-induced crystalline block disaggregation–recrystallization process, were proved by SEM and 2D-SAXS results when ε was between 17% and 55%; the further orientation of polypropylene chains led to a higher degree of orientation and crystallinity. The molecular deformation mechanism of polypropylene during creep included three stages: the intralamellar slipping of crystalline blocks, accompanying with the enlargement and amalgamation of free volume, was activated at small strain (ε ≤ 17%); whereas the stress-induced crystalline block disaggregation–recrystallization process as well as the rearrangement and orientation of chains were proceeded at medium strain (17% < ε ≤ 55%); at last, orientation-induced crystallization occurred at larger strain (ε > 55%).
Co-reporter:Kai Ni;Jingjun Zhu;Yadong Lv;Lingyun Wu
Journal of Polymer Research 2015 Volume 22( Issue 6) pp:
Publication Date(Web):2015 June
DOI:10.1007/s10965-015-0753-z
The microstructure evolution of isotactic polypropylene (PP) exposed to subtropical humid climate of Guangzhou of China, were investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, differential scanning calorimeter, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. Positron data showed that the free volume of PP matrix decreased with involving a shrinking of the free volume hole sizes as the extent of weathering degradation of PP aggravated. The shrinkage of free volume hole sizes may be traced to the loss of mobility of molecules of PP matrix. The increase of the glass transition temperature substantiated undoubtedly the decrease of molecular mobility of PP chains. The increase in crystallinity might increase the amount of rigid amorphous fraction of PP matrix, which induced the loss of molecular mobility. Furthermore, the decrease of ortho-positronium formation ought to be correlated to the increase in crystallinity and the increasing amount of scavenchers which were in this work represented by the carbonyl groups.
Co-reporter:Yifan Zhang, Xia Liao, Xianglin Luo, Suilin Liu, Qi Yang and Guangxian Li
RSC Advances 2014 vol. 4(Issue 20) pp:10144-10150
Publication Date(Web):31 Jan 2014
DOI:10.1039/C3RA47503D
Birefringent Maltese-cross concentric ring-banded spherulites with radial periodic variation of thicknesses grown from six-arm star-shaped poly(ε-caprolactone) (PCL) under the condition of subcritical CO2 were observed in this study. The structure of unique unclassical ring-banded spherulites was found to be different from that of traditional ring-banded spherulites which formed by periodic twisting of lamellar crystals along the spherulite radial direction. Laser scanning confocal microscopy, atomic force microscopy and scanning electronic microscopy images revealed that the ring-banded structure consisted of alternating periodically ridge and valley bands with continuous edge-on lamellae. A two-steps growth mechanism of ring-banded spherulites has been proposed to explain the development of central crystals and periodic ring-bands via CO2.
Co-reporter:Xia Liao, Haichen Zhang, Yuwei Wang, Lingyun Wu and Guangxian Li
RSC Advances 2014 vol. 4(Issue 85) pp:45109-45117
Publication Date(Web):23 Sep 2014
DOI:10.1039/C4RA07592G
In this study, a typical immiscible system, a poly(lactic acid) (PLA)/polystyrene (PS) bioblend is used to investigate the effect of interface and phase structure on bubble nucleation and porous morphologies using supercritical carbon dioxide as a physical foaming agent. A unique microcellular skin-core structure with a porous core and surface, and a nonporous skin layer which is embedded in a solid PLA phase are observed. The involved possible mechanism of the interfacial nucleation and confined foaming behavior in multi-phase systems has been discussed. Because of the higher gas concentration and lower activation energy barrier, bubble nucleation preferentially occurs at the interface of the PS and PLA phases. Due to the constrained effect of the crystalline PLA phase, smaller space could be provided for the expansion of the PS phase during the foaming process, hence gas bubbles in the interior are restrained from further growth. The porous structures of the PS phases are similar when the blends have comparable phase morphology. The results indicated that the confinement effect on the foam behavior is not only related to the crystalline PLA phase but also the phase structure of the blend.
Co-reporter:Daofei Bao;Ting He;Qi Yang;Guangxian Li
Journal of Polymer Research 2013 Volume 20( Issue 11) pp:
Publication Date(Web):2013 November
DOI:10.1007/s10965-013-0290-6
Polymer foams with nanoscale cell structure were prepared from polycarbonate (PC)/poly (lactic acid) (PLA) blends. The immiscible blends with PLA as the dispersed phase were foamed through using batch foaming process and CO2 as the physical foaming agent. Nanoscale cells emerged in both PLA domain and PC matrix of blend with 25 wt.% PLA content under an appropriate foaming condition. In the PLA domain, the formation of nanoscale pores was related to the confined foaming, blend and the crystallinity of PLA component. A transition from closed nanoscale cells to open nanoscale cells emerging in the PLA domain was in connection with the foaming condition. In the PC matrix, the cell property of the nanoscale cells was affected by the PLA phase. Moreover, the average cell sizes of nanocellular foams of both phases could be controlled within 40–130 nm by manipulating the size of PLA domain and saturation pressure.
Co-reporter:Ting He, Xia Liao, Yunchuan He, Guangxian Li
Progress in Natural Science: Materials International 2013 Volume 23(Issue 4) pp:395-401
Publication Date(Web):August 2013
DOI:10.1016/j.pnsc.2013.06.006
In this paper, novel electric conductive polylactide/carbon nanotubes (PLA/CNTs) foams were fabricated by a pressure-quench process using supercritical CO2 as a blowing agent. The morphology of PLA/CNTs nanocomposites prepared by solution blending was characterized using SEM and the results indicate that CNTs well dispersed in PLA matrix. The introduction of CNTs improved the thermal stability of PLA. The morphology and electrical properties of PLA/CNTs foams were characterized and discussed. Depending on the process parameters, such as saturation temperature and pressure, nanocellular or microcellular structure of PLA/CNTs nanocomposites were obtained. The volume resistivity of PLA/CNTs foams was from 0.53×103 Ω cm to 15.13×103 Ω cm, which was affected by cell structure and crystallization of foams oppositely. Foaming reduced the electrical conductivity due to the decrease of CNTs volume content and the break of conductive pathways. However, crystallization increased the electrical conductivity possibly because of the CNTs structural change in which the CNTs were less curled and more connected.
Co-reporter:Xia Liao and Arghavan V. Nawaby
Industrial & Engineering Chemistry Research 2012 Volume 51(Issue 19) pp:6722-6730
Publication Date(Web):April 18, 2012
DOI:10.1021/ie3000997
Foams generated via carbon dioxide (CO2) processing typically exhibit a solid skin layer on the exterior surface and a closed-pore structure with limited interconnectivity in the core section thus limiting its application for biomedical intent. By controlling the properties of poly(l-lactic acid)/poly(d,l-lactic acid) (PLLA/PDLLA) blends and using CO2 with specific processing parameters, skinless foams with interconnected porous structure were prepared in this work using only CO2 as a physical foaming agent, which overcome the necessity to use organic solvents and solid porogens. The crystallization behaviors and sorption kinetics of PLLA and its blends were studied. Addition of PDLLA reduces the crystallinity of PLLA/PDLLA blends while treated with CO2 as compared to neat PLLA. The solubility and diffusion coefficients of CO2 in PLLA and its blends were found to be similar. Furthermore, the effect of PLLA/PDLLA blend ratio and CO2 treatment conditions on the foam morphologies was investigated. Through fine parameter control, well interconnected pore structures with a porous surface were generated. Results indicated that by controlling the physical properties of samples combined with optimizing CO2 foaming process, it is indeed possible to create biodegradable interconnected porous structures for potential biomedical applications.
Co-reporter:Xia Liao;Arghavan V. Nawaby
Journal of Polymer Research 2012 Volume 19( Issue 3) pp:
Publication Date(Web):2012 March
DOI:10.1007/s10965-012-9827-3
The sorption behavior, physical properties, and foam morphologies in poly(L-lactic acid) (PLLA)-CO2 system were studied in this paper. The solubility and diffusion coefficient of CO2 in PLLA in the range of 0 °C to 45 °C and pressure up to 5.5 MPa were investigated. The diffusion coefficients were analyzed to determine the plasticization glass transition temperature (Tg) of the PLLA-CO2 systems. The data of Tgs of PLLA at various CO2 pressures demonstrated that PLLA-CO2 system exhibited a retrograde vitrification behavior, which has never been reported on semi-crystalline polymer-gas system by systematic measurement of solubility data. The sorption curves of PLLA at certain temperatures and pressures exhibited a characteristic keen which indicated the rejection of CO2 from the polymer matrix due to CO2-induced crystallization. The fundamental understanding of PLLA-CO2 interactions was utilized to control the CO2 solubility and crystallinity in PLLA thus the physical properties, in order to develop various unique foam structures.
Co-reporter:Xia Liao;Arghavan Victoria Nawaby;Pamela S Whitfield
Polymer International 2010 Volume 59( Issue 12) pp:
Publication Date(Web):
DOI:10.1002/pi.2910
Abstract
The effect of carbon dioxide (CO2) on the physical properties of poly(L-lactic acid) (PLLA) and on the formation of crystalline domains was investigated. The presence of CO2 in the matrix was found to induce crystallization in PLLA, with the crystallinity increasing with increasing CO2 pressure. The combination of saturation conditions and formation of crystalline domains was studied for its effect on the formation of porous morphologies in PLLA. Moreover, the effect of CO2 on PLLA properties and formation of porous structures was further exploited by first creating crystalline domains in samples using CO2 at various pressures at 25 °C and then re-saturating the same samples with CO2 at a constant pressure of 2.8 MPa and 0 °C. This paper reports on the solubility of CO2 at 25 and 0 °C in PLLA, crystallization and subsequent effect on foam morphologies when processed using different saturation cycles. Unique and intriguing morphologies were obtained by specifically controlling the properties of PLLA. Copyright © 2010 Crown in the right of Canada. Published by John Wiley & Sons, Ltd
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