Effect Of Phase Structure Of Multiphase Polymers On Foaming Behavior Of Supercritical Carbon Dioxide

Polymer foam is a polymer composite with polymers as matrix,gas as dispersed phase(closed-cell structure)or co-continuous phase structure(open-cell structure).The basic driving force behind the invention and development of polymer foam materials is that it can reduce the density of materials by using biomimetic porous structure without affecting the performance,so as to achieve the goal of lightweight.This kind of polymer material with excellent performance is widely used in our life,such as damping materials,sound insulation materials,windmill blades,aircraft filling materials,automobile damping materials and product packaging,etc.In practical applications,different applications have different requirements for the cellular structure of foam materials,because the cellular structure of polymer foam has great influence on their properties.In recent years,with the increasing demand for low energy consumption and green chemistry,the green and pollution-free foaming process in which sc-CO2 as a physical foaming agent has attracted much attention.Therefore,how to prepare foamed materials with controllable pore structure through sc-CO2 foaming process and realize controllable material properties is a hot topic at present.General purpose polymers,such as polyethylene(PE)and polystyrene(PS),are widely used in our daily life and production.It is necessary to explore new ways to prepare such foamed materials and realize high performance.Based on the foaming behavior of multiphase polymers containing PE and PS,the effect of phase structure(microphase and macrophase)on sc-CO2 foaming behavior of polymers was explored.We try to explore the structural design method of foamed materials from the perspective of molecular chain design and phase structure.The content and results of this paper are as follows:(1)Firstly,the comb-like copolymers PS-cb-(PS/PE)with different microphase separation structures were used for the sc-CO2 batch foaming process.The effect of microphase separation structure on the foaming behavior of the polymers under different crystalline states was investigated.It was found that the foaming behavior of the copolymers was closely related to their microphase structure and crystalline state of the foaming samples.When the foaming temperature was low(?70?),due to the blocking of the lamellar crystalline PE phase during the foaming process,the copolymers with lamellar phase structure would limit cell growth and breakage,which is easy to form bimodal nanoporous structure composed of nanofibers;while for the copolymers with columnar phase structure and PS as the continuous phase,a bimodal cellular structure with a larger size was formed,but the opening degree of cells was reduced.At the same time,the nanofibers almost disappeared,and a large number of nanostructured regions were found.When the foaming temperature was higher(?70?),with the increase of foaming temperature,the cell size increased and the cell density decreased.In addition,with the increase of foaming temperature,the cellular structure changed from open to closed cell(especially lamellar phase),and the cell wall became smoother.When the foaming temperature continued to rise(>90?),the melt strength of polymers decreased with the increase of the foaming temperature,which reduced the control force of the polymer matrix on the cell growth,resulting in the poor foaming performance.When the foaming temperature was 90?,the expansion ratio of the foams was the largest,the cell wall was the thinnest and the cell morphology was the most homogeneous.Meanwhile,as the phase structure of the polymers changed from the lamellar microphase separated structure to the columnar microphase separated structure,the cell size of the samples gradually increased.(2)sc-CO2 foaming was carried out using comb-like copolymers PS-cb-(PS-b-PE)with various microphase separation structure to explore the influence of the microphase separation structure of these topological polymers on their foaming behavior.The results showed that the Sc-CO2 foaming behavior of these copolymers was strongly affected by the microphase separation structure and side chain crystalline structure of these copolymers.When the foaming temperature was low(far below the melting peak temperature),a large number of crystalline grains did not melt under this state(such as 60?),and the polymer exhibited confined space foaming.In the process of rapid pressure relief,the copolymers with spherical phase and columnar phase microphase separation structure were limited by the crystalline phase of PE,and nanosized cellular structure was formed in the amorphous region(mainly PS phase);when the microphase separation structure changed from spherical phase structure to lamellar phase structure,the samples showed a restricted banded distribution of cell morphology.When the foaming temperature was high(e.g.90?),the microphase separation structure played a key role in the foaming behavior and cellular structure.When the microphase separation structure of the sample changes from spherical to lamellar,the elasticity of the sample increased.In this case,the cell morphology became more uniform,the cell size decreased and the cell density increased.Comparatively,the copolymers with lamellar microphase separation structure showed a wider foaming temperature window.(3)Polymer alloys with gradually changing macrophase structure were prepared by solution mixing comb-like copolymer PS-cb-PE with graft copolymer PE-g-PS with spherical microphase separation structure.The effect of introducing macrophase separation structure into graft copolymer blends with microphase separation structure on polymer properties was studied.With the increase of PS volume fraction,the macrophase structure of the blends gradually changed from bicontinuous phase to sea-island phase.At the same time,because a small amount of PE-g-PS insertion affected the crystallization behavior of PE part in PS-cb-PE,the melting point of two blends with bicontinuous phase structure was even lower than that of PS-cb-PE,and for similar reason,the melting point of the blend with sea-island phase structure was lower than that of PE-g-PS.Due to the interaction of molecular chains and the introduction of macrophase structure,the rheological properties of the blends were significantly changed.Especially for S1/S2-2 and S1S2-3,the storage modulus was almost equal to the loss modulus in the low frequency region.In sc-CO2 foaming process,when foamed at low temperature(80? or below),the samples mainly showed confined space foaming behavior within macrophase region due to the influence of PE phase crystallization.Although the phase interface promoted the cell nucleation,the presence of PE crystal phase also increased the binding force of the sample for cell growth.At high temperature(100? and above),when the phase structure of the samples changed from bicontinuous phase to sea-island phase,the increase of PE-g-PS component in the blends significantly improved its foaming performance.At the same time,the introduction of macrophase structure could also make the samples S1/S2-2 and S1/S2-3 obtain more uniform cell morphology at higher foaming temperature 110?.(4)Using copolymers PS-cb-(PS-b-PE),PS-cb-(PS/PE)and PS-b-PE as compatibilizers,we studied the compatibilization of these copolymers with different chain topological structure in the LLDPE/PS blends.At the same time,the effect of compatibilization on the foaming behavior of the blends was investigated by sc-CO2 foaming.Compared with linear diblock copolymers,comb-like copolymers showed better compatibilization effect.The blends compatibilized by PS-cb-(PS-b-PE)had the best mechanical properties and phase morphology in the copolymer compatibilization system of three topological structures.In addition,the compatibilized blends by the copolymer compatibilizer with branched structure had excellent supercritical CO2 foaming performance.At the same time,the effects of foaming temperature and phase structure of the compatibilized LLDPE/PS blends by carbon black(CB)nanoparticles on its foaming behavior were also investigated.It was found that the foaming temperature had an obvious effect on the critical CB content which can significantly improve the foaming performance of the blend with bicontinuous phase structure.However,for the blends with sea-island phase structure,the foaming behavior of the blends had no obvious change except for the influence of CB content on the cell size.