Study On The Relationship Between Structure And Properties Of Polyvinyl Chloride And Poly(vinyl Chloride-co-vinyl Acetate)

As one of the major polymers in the global plastics market,polyvinyl chloride（PVC）and poly（vinyl chloride-co-vinyl acetate）（PVCA）has been widely used in industrial and civil applications duo to its good physical and mechanical properties,excellent corrosion resistance and reasonable cost.However,the rapid development of science and technology has put higher demands on the performance of PVC and PVCA.In this paper,the relationship between the structure and thermal stability,processing properties and solubility of PVC and PVCA has been established through combination of experiments and molecular dynamics simulations to better guide the practice.In this paper,the thermal stability of three representative PVC resins at home and abroad were analyzed by oven aging test,Congo red test and TGA.The evolution of molecular chain structure of PVC during thermal degradation was investigated by UV/Vis,¹H-NMR and ¹³C-NMR,which established the relationship between defect structure and thermal stability of PVC before and after degradation.All the thermal property tests had given consistent results that the PVC samples generated fewer conjugated triene,total double bond and syndiotactic structure during degradation exhibited better thermal stability.The products of PVC degradation process were qualitatively and quantitatively analyzed by TG-IR and Py-GCMS,and it was found that PVC degradation products were dominated by hydrogen chloride（HCl）and low molecular weight hydrocarbons（CH₂）.In addition,the thermal degradation of PVC was divided into two stages:the first stage was the removal of HCl from PVC to form short-chain olefins,haloalkanes and monocyclic aromatic structures（benzene and toluene）;the second stage was the further degradation and cyclisation of PVC molecular chain to form bicyclic and polycyclic aromatic structures.The thermal degradation models of PVC containing different isomers and defect structure were established by molecular dynamics（MD）simulations,the results showed that the syndiotactic structure and the 1,2-dichloride terminal were the most unstable structures.In addition,the specific distribution and formation paths of thermal degradation products during the degradation of PVC were analyzed,and it was further confirmed that HCl and low molecular weight hydrocarbons（CH₂）were the main degradation products of PVC,and defect structure,oxygen atmosphere and high temperatures all contributed to the thermal degradation,which led to a deeper theoretical study of the thermal degradation mechanism of PVC.By copolymerizing with the second monomer vinyl acetate（VAc）to improve the processing properties and solubility of PVC,the experimental results showed that the thermal stability of PVCA with high VAc content was slightly reduced,and the thermal degradation products were dominated by HCl,carboxylic acid and low molecular weight hydrocarbons（CH₂）.PVCA with higher VAc content had a larger loss tangent（tanδ）,lower viscosity（η^*）and lower glass transition temperature（T_g）,thus improving the processing properties of PVCA.In addition,PVCA with higher VAc content had a higher UV transmittance and lower polydispersity coefficient（PDI）in ethyl acetate and butanone,which effectively improved the solubility properties of PVCA.A series of PVCA systems with different VAc contents were modeled to investigate the mechanism of the effect of VAc content on the thermal stability,processing properties and solubility of PVCA,and the results showed that the C-Cl bonds in VC and C-O bonds in VAc had similar thermal stability.In addition,PVCA with higher VAc content exhibited larger molecular chain mobility and free volume fraction（FFV）,smaller intermolecular interactions and molecular chain characteristic ratio（<Ree²>/<r²>）,demonstrating that PVCA with higher VAc content had greater molecular chain flexibility,which was conducive to improve processing properties.Meanwhile,the results of the solubility parameter（δ）and energy of secondary interactions showed that the increase in VAc content reduced the solubility difference between the polymer and the solvent and enhanced the interaction,thus improving the solubility of PVCA.