Synthesis And Properties Of Heat-resistant Modified Film-use Polyester

For the preparation of heat-resisting polyester film, a series of copolyesters were synthesized by introducing liquid crystalline (LC) mesogens into the polycondenstation of terephthalic acid (TPA) and ethylene giycol (EG) in expectation of improving the heat-resistant of polyesters through enhancing the molecular chain rigidities as well as bettering the film processing performance via reducing their crystallization abilities.Concretely, two LC mesogen monomers, p-hydroxybenzoic acid (PHB) and 4,4'- bibenzoic acid (BPDA) were polymerized with TPA and EG through conventional direct esterification technique. The molecular structure and sequence distribution of the resultant copolyesters were investigated by nuclear magnetic resonance (NMR), and their heat-resistance properties were characterized by means of differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA).There ways were adopted to introduce PHB into PET molecular chains, i.e., direct use without modification, transformed into p-acetoxybenzoic acid (PABA) by acetylation or transformed into terephthaloyl bis(4-oxybenzoic) acid (TOBA) by pre-reaction with terephthalic chloride, which resulted in three types of copolyesters, named PHB-PET, PABA-PET and TOBA-PET respectively. It has been found from NMR results that in PHB-PETs the expected rigid PHB self-aligned segments had seldom formed even under the high PHB loading, and the calculated PHB contents were lower than its rate of charge which should be attributed to the low activity of PHB. On the other hand, PABA showed a high activity, especially, a high self-polymerized ability, and formed some relatively long rigid segments in PABA-PETs. However, it was rather difficult to obtain the PABA-PET with enough high molecular weight. While in TOBA-PET, the rigid PHB self-aligned segments could finally formed through the transesterification. DSC analysis indicated that in general the introduction of PHB segments would reducing the crystallization ability of PET, which was demonstrated by the decreased melting point and crystallization temperature, or by the increased crystalline undercooling. Unfortunately, the introducing of PHB could hardly increase the glass transition temperature of PET except for the PABA-PET system, moreover, form TGA it was found PHB modification could just slightly improve the thermal decomposition temperature and residues of PET.In BPDA modified polyesters, named BPDA-PETs, the BPDA contents increased with the increasing BPDA loading, but still did not match each other exactly. DSC analysis results showed with the increasing contents of BPDA, the glass transition temperatures of BPDA-PET copolyester increased steadily which was identified by DMA, however, their melting points decreased accordingly. Furthermore, TGA results showed that with the increasing contents of BPDA, the thermal decomposition temperatures and thermal decomposition residues of the copolyester increased obviously.In this paper , the common DSC, the isothermal crystallization DSC analysis and the non-isothermal Crystallization DSC analysis were adopted to study the crystallization properties of the BPDA-PET copolyesters. The general result was that the introduction of BPDA would destroy the sequence regularity but increase the rigidity of PET macromolecular chains. Specifically, with the increasing contents of BPDA, the crystallization temperature decreased while the crystallization undercooling increased for the copolyesters. Under the isothermal crystallization, the nucleation capability of copolymer was reduced , which lower the crystallization ability of the BPDA-PET copolyester. While under the non-isothermal Crystallization the introduction of BPDA would restrict the movement of copolymer segments and result in a slower crystallization speed.