Structural effects of ionomers on the morphology, isothermal crystallization kinetics and melting behaviors of PET/ionomers
Graphical abstract
Introduction
Poly(ethylene terephthalate) (PET) has been widely used as industrial films and engineering plastics because of its excellent physical properties. However, PET exhibits a rather slow crystallization rate, which limits its application in engineering plastics. Inorganic substances [1], [2], [3] and organic salts especially the alkali metal salts of aromatic carboxylic acid [4], [5], [6] have been used as nucleating agent to enhance the crystallization rate of PET.
Ionomers are polymeric materials having a small number of ionic side groups in the hydrophobic backbone chains, which cause slight molecular weight reduction of PET and have been commercially recognized as the most effective nucleating agents for PET [7]. Ethylene-based ionomers [8], [9], [10], [11], [12], [13], ionic-ended polyesters [14] or styrene-based ionomers [15] have been reported as nucleating agent in PET. In PET/PE blends, the addition of ionomer can improve both the compatibility and crystallization of blends [16], [17], [18], [19], [20], [21]. Yu et al. [8] reported that the ionomer ethylene–sodium acrylate (trade name AClyn) was more effective than ethylene–sodium methacrylate (trade name Surlyn) to enhance the crystallization rate of PET because of their different nucleation mechanism. Their further investigations indicated that the nucleation efficiency of ionomers based on ethylene–acrylic acid depends on the nature of cation [9]. From our previous work, it was found that the styrene–sodium acrylate ionomers initiated heterogeneous nucleation and their chemical structure determined the aggregation behaviors of ionomers, which affected the crystallization behaviors of PET/ionomer.
To further disclose the relationship between the structure of ionomer and the crystallization properties of PET/ionomer, ethylene–sodium acrylate ionomer (AClyn) and styrene–sodium acrylate ionomer (SAA–Na) with the similar molecular weight and mole fraction of sodium acrylate, were mixed with PET. The morphology, isothermal crystallization kinetics and melting behaviors of PET/ionomers were investigated.
Section snippets
Materials
Poly(ethylene terephthalate) with intrinsic viscosity of 0.68 dL/g in phenol/tetrachloroethane mixture (1:1 by weight) was kindly supplied by Sinopec Yizheng Chemical Fibre Company Limited (YCF), China. The ionomer styrene–sodium acrylate (SAA–Na) was obtained by 100% neutralization of the styrene–acrylic acid copolymer, which was prepared by copolymerization in benzene solution with azodiisobutyronitrile (AIBN) as initiator [22] and had molecular weight (Mw) of 2.3 × 103 and 9.2 mol% of acrylic
Morphology of PET/ionomers
The fractured surfaces of PET modified by the styrene–sodium acrylate ionomer (SAA–Na) and ethylene–sodium acrylate ionomer (AClyn) are shown in Fig. 2. In PET/SAA–Na, only white specks existed on the PET matrix surface (Fig. 2(c)), and this feature was more obvious with increasing the content of SAA–Na ((d)). Comparing with the surface of neat PET (Fig. 2(a)), it can be seen that the white specks should be the ionomer SAA–Na, which during the fracture elongated more than PET but was not pulled
Conclusions
Low molecular weight ionomers, ethylene–sodium acrylate (AClyn) and styrene–sodium acrylate (SAA–Na), were mixed with PET. SEM micrographs showed that voids with smooth surfaces and particles of ionomer existed in PET/AClyn, however, in PET/SAA–Na, only particles strongly bonded to the PET matrix because of the presence of benzene group in SAA–Na.
During crystallization, SAA–Na and AClyn acted as heterogeneous nucleating agents at both high and low temperature regions. At the same content beyond
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