Synthesis and characterization of copolymers of poly(m-xylylene adipamide) and poly(ethylene terephthalate) oligomers by melt copolycondensation

Abstract

Poly(m-xylylene adipamide)/poly(ethylene terephthalate) (MXD6/PET) copolymers are synthesized by melt copolycondensation with 1–5 wt% low molecular weight PET oligomers into the MXD6 oligomers at 260 °C. FR-IR and ¹H NMR analysis results indicate that the interchange reaction has occurred between MXD6 oligomers and PET oligomers. The thermal behavior of copolymers shows that the melting temperature of MXD6/PET copolymers decreases with the increasing of amount of PET oligomers, while the crystallization temperature accordingly increases. And the equilibrium temperature T_m⁰ is evaluated to be 251.8 °C for the copolymers with 5 wt% PET oligomer adding, which is very close to that of neat MXD6. The tensile and impact strength of MXD6/PET copolymers are significantly improved than that of pure MXD6 by mechanical properties test, and the microfibril structure in the impact fracture sample's surface reveals the feature of ductile fracture.

Introduction

Poly(m-xylylene adipamide) (MXD6) is a kind of semi-aromatic crystalline nylon resin with distinguished properties such as high rigidity and strength, high modulus, good dimensional stability, and prepared by the polycondensation of adipic acid with m-xylylenediamine (MXDA) [1]. The most remarkable peculiarity of MXD6 is concerned by its excellent gas barrier properties against O₂ and CO₂, which is much lower than that of other conventional polyamide resins, ethylene-vinylalcohol copolymers (EVOH) and poly(vinylidene chloride) copolymers (PVDC) etc.[2]. Owing to this excellent gas barrier properties, especially in high humidity conditions, MXD6 can offer important processing benefits by blending with other polymers, such as polyethylene, polypropylene, and poly(ethylene terephthalate) (PET). It can also be appropriately used as engineering structural materials, in particular, when reinforced with glass fiber [3], [4], [5], [6], [7]. Therefore, MXD6 is attracted in the technical, scientific and commercial interest, especially in the field of multilayer food packaging material application such as multilayer and stretch-blown bottles [3], molding compounds and nonofilaments [2]. However MXD6 is easy to be fractured in the processing process, especially in molding process, so it is important to improve the toughness of MXD6.

Polymer blends and copolymerization are the most conventional methods which can improve many properties including elasticity, toughness, melting temperature, glass transition temperature etc. There have been remarkable numbers of research studies of polymer blending that occur between polyamide and polyester, such as poly(butylene terephthalate)/nylon-6 (PBT/PA6) [8], [9], [10], [11], [12], poly(butylene terephthalate)/nylon-66 (PBT/PA66) [13], [14], [15], [16], poly(ethylene terephthalate)/nylon-6 (PET/PA6) [17], [18], [19], PET/nylon-66 (PET/PA66) [18], [20], and PET/Poly(m-xylylene adipamide) (PET/MXD6) [4], [21], [22], [23], [24], [25]. But it is found that polyamide/polyester blends are incompatible and proper compatibilizers are needed, or the copolymers can be got with the help of an appropriate catalyst [4], [9], [10], [11], [12], [22], [23], [24], [26], [27], [28], [29], [30], [31], for example sodium p-toluene sulfonic acid. Chemical interchange reactions may be occurred when polycondensates such as polyamides, polyesters and polymers with reactive functional groups at the chain terminals or in the backbone are mixed in the molten state, leading to the formation of random, block, segmented, or grafted copolymers, and then affecting the properties of the original polycondensates [27]. But most polymer blending or copolymerization are occurred between higher molecular weight polymers in melting state, thermal degradation usually cannot be neglected, which would result in breaking polymer chain linkage as well as some side reactions to reduce the quality of polymer products. Since polymer oligomers, bearing more reactive function groups at the chain ends or in the backbone, such as polyester oligomer and polyamide oligomer, could directly react with each other [32], [33], that might achieve polymer product with quite good quality.

In fact, some features of PET are very close to that of MXD6, including almost the same reaction temperature and vacuum condition in polycondensation process, similar rheological and crystallization behavior and temperature processing window of PET and MXD6 overlapping [2]. Furthermore, PET is easy to form fiber and has good toughness, the MXD6/PET copolymers by copolycondensation with a small amount of PET oligomers should have improved toughness compared to neat MXD6. In this work, PET oligomers are added during MXD6 oligomer polycondensation process to synthesize MXD6/PET copolymers, and then the thermal and mechanical properties of prepared MXD6/PET copolymers are evaluated.