Modulation On The Physicochemical Properties Of Ldhs And Study On The Crystallization Behavior Of Ldhs/Semi-crystalline Polymer Composites

Semi-crystalline polymer is a type of polymer with incomplete crystallization,in which the proportion of ordered segments(crystal regions)is less than 70%.Semi-crystalline polymers have the disadvantages of slow crystallization rate,difficulty in molding,long molding duration,and low heat resistance,which as a result limit the application and development of such polymer materials.In order to overcome the above drawbacks,a certain amount of nucleating agent is generally added to the semi-crystalline polymers to improve their crystallization behavior,enhance the mechanical properties,and thus expand its application range.As two types of typical semi-crystalline polymer,polyethylene terephthalate(PET)and polyvinylidene fluoride(PVDF)are widely used in functional plastics,product packaging,and petrochemical industries.Layered double hydroxides(also known as hydrotalcite,LDHs)is an important type of inorganic layered materials,with the characteristics of composition tunability,structure diversity and adjustable physicochemical properties,which has been added into many polymers to improve the mechanical properties,thermal stability,and gas barrier properties.In this dissertation,LDHs nanoparticles with different metal elements and interlayer anions were prepared and used as nucleating agents to synthesize PET/LDHs and PVDF/LDHs composites.The effects of physicochemical properties of LDHs on the crystallization behavior of PET and PVDF were investigated.The mechanism of nucleation and crystallization of semi-crystalline polymers was revealed,by using XRD,TEM,POM,DSC technologies and theoretical equations.The obtained semi-crystalline polymer/LDH composite materials with controlled crystallinity are expected to be applied in product packaging,functional separators and other fields.The main research contents and results of this thesis are as follows:(1)Preparation and crystallization behavior of PET/LDHs compositesMgAl-and CaAl-LDHs with different metal elements were prepared by urea-assistant hydrothermal and co-precipitation method,respectively.PET/LDHs composites were further obtained by solution-complex method,in which MgAl-and CaAl-LDHs as two inorganic nucleating agents were dispersed in PET matrix with a certain ratio(1%,3%,5%,7%).Characterizations of TEM,EDS,mapping,etc.prove that LDHs are uniformly distributed in the PET matrix.The effect of different metal compositions of LDHs on the crystallization behavior of PET was further investigated.For both MgAl-and CaAl-LDHs,the crystallization temperature of PET increases with the increase of LDHs loading,and the addition of LDHs greatly shortens the crystallization time of the composite material.When LDHs content reaches 1%,3%,5%,7%,the crystallization temperature of PET/MgAl-LDHx(x indicates LDH addition)increases by 5 ?,18 ?,20 ?,and 28 ?,respectively;while PET/CaAl-LDHx increases by 1 ?,9 ?,19 ? and 27 ? respectivey.Moreover,with the same content of hydrotalcite,the half-crystallization time of PET/MgAl-LDHx composites is also shorter than that of PET/CaAl-LDBHx,indicating a superior nucleating effect of MgAl-LDHs than CaAl-LDHs.The nucleation mechanism of PET induced by LDHs was further explored by means of theoretical formulas.The results show that the addition of LDHs reduces the surface free energy and the nucleation free energy of the polymer chains,thereby reducing the energy barrier for nucleation.In order to further solve the problem of poor compatibility of the inorganic filler with polymers,stearic anion intercalated hydro talc ite(MgAl-LDH-SA)was prepared and incorporated with PET as a nucleating agent.The crystallization temperature of PET/LDH-SA increases by about 5 ? compared with PET/LDH composite,and shows a faster crystallization rate.In addition,the addition of LDHs significantly improves the thermal stability,mechanical properties,and gas barrier properties of PET/LDHs composites.(2)Preparation and crystallization behavior of PVDF/LDHs composite filmsPVDF/LDHs composite films were prepared by phase inversion method using PVDF as solute material,N,N-dimethylacetamide(DMAC)as solvent,LDHs as additives,and deionized water as nonsolvent coagulation bath.The structure and crystal morphology of PVDF were characterized by SEM,EDS mapping analysis,which prove that LDHs were uniformly dispersed in PVDF films.Infrared absorption spectra and XRD characterization demonstrate that the PVDF exists in both a-and P-forms,which is caused by the interaction of solvent and PVDF chains.When the LDH loadings of composite films is increased from 3%to 5%,the relative content of ?-PVDF increased from 1.37%to 3.2%,respectively.It is noteworthy that the content of a-PVDF was also increase upon incorporating with LDHs,indicating that LDHs in composite membranes leads to the promotion of crystalline structure of PVDF.The non-isothermal DSC curve confirms that LDHs changes the crystallization behavior of PVDF and increases the non-isothermal crystallization temperature of pure PVDF by 5 ?.The isothermal crystallization curve confirms that the LDHs as nucleating agent shorten the PVDF crystallization time.In addition,LDHs additives resulted in faster diffusion rate of the solvent and the coagulation bath,and consequently larger pore size of PVDF membranes and higher water flux.The membrane hydrophilic ity was greatly improved due to the addition of LDHs.When the LDHs loadings is 1%,3%,and 5%,the contact angles of the PVDF/LDHs composite membranes decreases from 100° of pure PVDF membrane to 85 °,79 °,and 77 °,respectively.The improvement of hydrophilic ity reduces the attraction and adhesion of hydrophobic impurity and thereby makes the PVDF/LDHs composite a potential material for waste water treatment.