Block phosphorus-containing poly(trimethylene terephthalate) copolyester via solid-state polymerization: Reaction kinetics and sequential distribution
Graphical abstract
Introduction
Poly(trimethylene terephthalate) (PTT) is a partly aromatic semicrystalline polyester first synthesized by Whinfield and Dickson in 1941 [1]. Since the commercialization these years, it has been widely used both as engineering plastic and fiber material for its outstanding properties [2], [3], [4], [5], [6], such as good mechanical properties like, high crystallization rate, and especially the excellent elastic recovery as fiber materials [7], [8].
However, PTT is a typical flammable material despite the presence of phenyl groups in the main chain [9], which restricts its use in many fields. It is generally known that flame-retardant material can be obtained via melt blending, finishing and copolymerization [10], [11], [12]. Among them, copolymerization with functional flame-retardant monomers was proved to be a suitable method for preparation of flame-retardant fiber materials [12], [13], [14]. During direct copolycondensation process, transesterification reaction in the melt always results in a copolyester with random constitution, and therefore decreases melting temperature, crystallization behavior, and crystallinity of the obtained copolyesters [15], [16], [17], [18], [19], [20].
During the authors' previous study [9], a novel inherently flame-retardant poly(trimethylene terephthalate) copolyester with phosphorus-containing linking pendent group has been synthesized. 1H NMR spectrum characterization shows that the copolyesters possess random constitution. The melting point of PTT copolyester decreases from 224.3 °C of neat PTT to 199.5 °C of PTTP20 (containing 20 wt% DDP), and the crystallization temperature decreases from 191.3 °C of neat PTT to 120.4 °C of PTTP20 (at ramping rate of 10 °C/min) [21]. Moreover, it has been reported that the good elastic recovery of PTT is caused by its characteristic crystallization structure [22]. The destruction or decrease of the crystallization area may spoil this characteristic property. Therefore, the molecular chain of PTT copolyester incorporated with functional monomers had better be regular, including flame-retardant ones.
A novel method has been developed to synthesize copolyester with block constitution recently is solid-state polymerization (SSP). In polyester industry, SSP is a widely used method to increase molecular weight below its melting temperature but much higher than its glass transition temperature under vacuum, inert gas or some kind of supercritical flows [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Because polymer chain has enough mobility to occur transesterification reaction at SSP temperature, and can avoid thermal degradation, it's very effective in increasing the molecular weight. Furthermore, this suggests another way to prepare copolymers. James et al [39] prepared copolyesters via SSP with oligomer blends of poly(ethylene terephthalate) (PET) and poly(ethylene napthalate) (PEN) as raw material. They figured out that transesterification occurred mainly in the first 2 h of SSP reaction. 1H NMR spectra analysis indicated that the obtained copolyesters were non-random. Jansen et al [40], [41], [42] prepared another two copolyesters, with blend of PBT (Mn = 16 kg/mol) and 2,2-bis[4-(2-hydroxyethoxy) phenyl]propane (Dianol 220), and blend of PBT (Mn = 16 kg/mol) and bis(2-hydroxyethyl) terephthalate (BHET). The reaction kinetics and microstructure was carefully investigated. The results show that the both copolyesters possess block chemical constitution.
In this study, a flame-retardant monomer named (9,10-dihydro-10-[2,3-di(hydroxycarbonyl) propyl]-10-phospha-phenanthrene-10-oxide, DDP) was incorporated in the amorphous phase of PTT via SSP. DDP was first esterified with 1,3-propane diol, then mixtures consisting PTT prepolymer and different amounts of DDP ester were used as raw materials to undergo SSP. The incorporation reaction kinetics at different reaction time was studied with a least-square method. The sequence distribution of obtained copolyester was analyzed with quantitative 1H NMR spectroscopy. The degree of randomness as a function of both reaction time and DDP content were calculated, and the reason of the results was also proposed. The thermal transition data was also obtained to support the 1H NMR analysis results. It is the first time to synthesis such non-random copolyester consisting of flame-retardant monomer. In the subsequent articles, a detailed investigation on crystallization and flammability of this block copolyester prepared by SSP would be presented.
Section snippets
Materials
Dimethyl terephthalate (DMT, CP grade) was provided by Sinopharm Chemical Reagent Co. Ltd (Shanghai China). 1,3-Propane diol (PDO) (fiber grade), tetrabutyl titanate [Ti(OC4H9)4, AR grade] and zinc acetate [Zn(CH3COO)2, AR grade] were purchased from Kelong Chemical Reagent Factory (Chengdu, China). DDP was received from Weili Flame Retardant Chemicals Industry, Co. Ltd (Chengdu, China). 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP 99.5%) was obtained from Yancheng Biological Products Company
Reaction kinetics
The transesterification between DMT and PDO has been carried out. Then the polycondensation makes the prepolymer predominantly hydroxyl-terminated as Scheme 1 shows. The intrinsic viscosity of resulting p-PTT was measured to be 0.45 dL/g, the melting point was 225 °C. The 1H NMR spectrum of p-PTT was shown in Fig. 1, and the repeating unit number of p-PTT was calculated to be 28 according to following equation:where Dp stands for the repeating unit number of p-PTT chain, Ib and Ie are
Conclusions
In summary, phosphorus-containing PTT copolyester was successfully synthesized with PTT prepolymer and DDP ester via SSP at 200 °C. Intrinsic viscosity test shows that intrinsic viscosities of the copolyesters increase with the increase of tssp, but decrease with the decrease of phosphorus content. The investigation results confirmed that intrinsic viscosity matters not only with the molecular weight but also with chain compositions. The incorporation rate constant kf was calculated with a
Acknowledgments
This work was financially supported by the Natural Science Foundation of China (Grant Nos. 50903047, 50933005 and 51121001) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026). The author would also like to thank the Analysis and Testing Center of Sichuan University for the NMR measurements.
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