| Thermoplastic polyether ester elastomer(TPEE)is a new aromatic block copolymer with superior chemical resistance and excellent mechanical properties at low temperature.The hard segment is crystalline polyester with high glass transition temperature and the soft segment is amorphous polyether.Foaming process can expand its potential applications and has high cost performance of TPEE.Whereas,foaming TPEE by supercritical CO2 is difficult due to both the complex micro-phase structure in solid state and the low melt strength in molten state of TPEE.This thesis focuses on the multi-scale structural controlling of TPEE to improve its foaming ability.The effect of hard segment content on the solubility and diffusion behavior of CO2 as well as the solid foaming behavior of TPEE is investgated.TPEE with different chain extended,chain branched and long chain branched structures are successfully prepared by melt modification,and the melt foaming behavior of these modified TPEEs is related to their rheological properties and cryatallization behavior.TPEE/PTFE nano-fibril enhanced nanocomposites are obtained by twin-screw extrusion process,and the extrusion foaming ability of TPEE is greatly improved.These results can provide theoretical basis and new methods for the designing of chain structure of TPEEs and the controlling strategies of cell morphology of TPEE microcellular foams blown by supercritical CO2.Firstly,solid-state foaming process of TPEE composed of poly(tetra methylene glycol)(PTMG)"soft" blocks and poly(butylene terephthalate)(PBT)"hard" blocks using supercritical CO2(scCO2)as the blowing agent was investigated.The effects of hard segment content on the cryastallization,the solubility and diffusion behavior of CO2 and the foaming behavior were studied.It was found that the hard segment content has a significant influence on the crystallization behavior of TPEE.When the hard segment content rises from 29%to 65%,the melting point rises from 161.2? to 201.9? with a crystallinity rising from 20%to 40%.The results showed that the solubility and diffusivity of CO2 in soft segments(PTMG)is significantly higher than in hard segments(PBT).With higher hard segment content,the foaming window is broadened and shifts to higher temperature range.When the content of hard segment is 29%,the foaming window is from 50? to 160? with cell diameter ranging from 4.6 ?m to 16.5 ?m,cell density between 8.1×107 cells/cm3 and 7.5×108 cells/cm3 and expansion ratio ranging from 1.1 folds to 5.8 folds.When the content of hard segment is 65%,the foaming window is from 165? to 195? with cell diameter ranging from 1.8 ?m to 6.8?m,cell density between 2.0×108 cells/cm3 and 1.1 ×1011 cells/cm3 and expansion ratio ranging from 1.1 folds to 4.2 folds.Secondly,TPEE has been reacted with different multi-functional group modifiers,such as 2,2'-bis(2-oxazoline)(2,2'-BOZ)and triglycidyl isocyanurate(TGIC),to generate long chain TPEEs and highly branched TPEEs respectively by reactive extrusion process.Both chain extension and chain branching processing could improve the complex viscosity and modulus of modified TPEE,but modified TPEEs with branched structure showed distinguished strain-hardening behavior.The branched structure can also accelerate crystallization processes.In the batch molten foaming process,TPEEs with branched chains have a higher expansion ratio,higher cell density and wider foaming window compared with those TPEEs with long chains.The foamed TPEE with long chains have cell diameter ranging from 4.2 ?m to 12.5 ?m,cell density between 5.5×108 cells/cm3 and 2.6×109 cells/cm3,and the expansion ratio ranging from 1.4 folds to 14.2 folds.The branched structure can prevent cells from collapsing to obtain foamed TPEE with high expansion ratio ranging from 1.5 folds to 16 folds,cell diameter ranging from 3 ?m to 13 ?m and cell density between 3.6×108 cells/cm3 and 7.1 ×109 cells/cm3.Long-chain branched(LCB)TPEE could be controllably prepared by the two-step reaction of the linear TPEE with TGIC and 2,2'-BOZ in different sequences to improve their rheological properties and foaming ability.All of the modified TPEEs reacted with TGIC first and then with 2,2'-BOZ have more branching points and longer chains.These samples showed higher molecular weight and broader molecular weight distribution.These samples also had higher viscosity and modulus,longer relaxation time,and stronger strain-hardening effect.Moreover,the greater number of branching points accelerated the non-isothermal crystallization process of LCB TPEE.The molten foaming ability of LCB TPEEs was evaluated via a batch process.The LCB structure improved the melt viscoelasticity and the foaming ability remarkably.The LCB TPEEs have the foaming window ranging from 165?to 200? with cell density between 2.0×109 cells/cm3 and 1.0×1011 cells/cm3.The cell density has been improved in two orders of magnitude comparing to those TPEEs with only long chains or only branched structures.In-situ fibrillated polytetrafluoroethylene(PTFE)enhanced nanocomposites were successfully prepared by mixing TPEE and PTFE using a twin-screw extruder.Well-dispersed,long aspect ratio PTFE nano-fibrils with a diameter of less than 200 nm were generated and interweaved into networks.Differential scanning calorimetry and in-situ polarized optical microscopy showed that the existing PTFE nano-fibrils can greatly accelerate and promote the crystallization of TPEE matrix,and the crystallization temperature can be increased by 6?.Both shearing and elongational viscosities results confirmed that the introduction of PTFE nano-fibrils into TPEE matrix can significantly improve the rheological properties.The remarkable changes in the strain-hardening effect and the melt viscoelastic response,as well as the promoted crystallization behavior,led to substantially improved foaming behavior of PTFE/TPEE nanocomposites in the continuous extrusion process using supercritical CO2 as the blowing agent.The existing PTFE nano-fibrils dramatically decreased the cell diameter and increased cell density,together with a higher expansion ratio and more uniform cell structure.The sample with 5%PTFE fibrils showed the best extrusion foaming ability,with an average diameter of 10.4-14.7 ?m,an expansion ratio of 9.5-12.3 and a cell density of 6.6 ×107-8.6 × 107 cells/cm3. |
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