Decomposition properties of PVA/graphene composites during melting-crystallization

Highlights

• The weight loss of PVA and PVA/graphene composites during melting-crystallization process is firstly quantified.

• The decomposition decreases melting and crystallization enthalpies, but enhances glass transition temperature.

• The thermal decomposition during melting-crystallization is not homogeneous.

• The isothermal decomposition kinetics of PVA and PVA/graphene composites is proposed.

Abstract

The thermal decomposition of PVA and PVA composites during the melting-crystallization process is still unclear due to indistinct changes in chemical compositions. Using graphene as a model, the decomposition properties of PVA and PVA–graphene composites were systematically analyzed under multiple melting-crystallization cycles. And a series of isothermal decomposition experiments around the melting-crystallization temperature were carried out to simulate the corresponding decomposition kinetics. Based on multiple cycle melting-crystallization, the weight loss of PVA and PVA/graphene composites was successfully quantified. Further morphology investigation and chemical structure analysis indicated that the decomposition was non-uniformly distributed, rendering the possibility of crystallization for PVA and PVA/graphene composites after multiple heating–cooling cycles. In addition, isothermal decomposition analysis based on reduced time plot approach and model-free iso-conversional method indicated that Avrami-Eroffev model could well match the decomposition process of the neat PVA and PG-0.3 composite, while the Avrami-Eroffev and first order models could precisely forecast the decomposition of PG-0.9 composite. Both analyses during multiple cycle melting-crystallization and isothermal decomposition demonstrated that graphene served as decomposition accelerator in the whole thermal decomposition process, and particularly the decomposition of neat PVA and PVA/graphene composites was highly related to the band area ratios of C–H and O–H vibrations in Fourier transform infrared (FTIR) spectrum.

Introduction

Polymer crystallization is an important area of interest in polymer science and engineering since crystallinity regulates various properties such as hardness, modulus, tensile strength and toughness [1]. Due to the superior properties of PVA and PVA composites such as nontoxicity and biodegradability as well as their wide applications [2], the crystallization of different PVA composites has been extensively studied in the past decades such as partially hydrolyzed PVA [3], PVA/SiO₂ [4], PVA/carbon nanotubes [5], [6], PVA/polyamide 6 blend [7] and PVA/attapulgite [8]. However, most of the reports only focused on crystallization kinetics and only a very limited number of studies explored the decomposition probabilities during the melting and crystallization process [3], [6], [9], [10].

Peppas et al. [9] first reported that there was no evidence of decomposition for PVA (99% hydrolysis) during an isothermal crystallization process. Later on, another report suggested that real melting equilibrium could be observed without any decomposition of PVA using a high-vacuum differential scanning calorimetry (DSC) [10], raising a concern about the presence of a high vacuum within the DSC cell [3]. On the other hand, Probst et al. [6] reported that there was decomposition involved during the crystallization of PVA/carbon nanotubes based on a failed repeatability of crystallization exotherms at different cooling cycles. However, a subsequent Fourier transform infrared (FTIR) study by Huang et al. [3] again revealed no evidence of PVA (80% hydrolysis) decomposition during the DSC test, yet suggested a possibility to implement non-isothermal crystallization analysis. Due to inconclusive evidences, crystallization mechanisms of PVA and PVA composites have still been a subject of research with a continuing interest in recent years [11], [12], [13], [14]. Our recent work [15] indicated that the crystallization process for PVA and PVA/graphene composites is in fact the combination of crystallization and decomposition. And the graphene in the PVA matrix regulates the nucleation and crystal growth manners of the PVA, yet resulting in retardation of the entire crystallization. However, the decomposition of PVA and PVA/graphene composites during the crystallization process has not been well analyzed.

The decomposition of PVA or PVA composites has been reported for many years on its kinetics and decomposition products, mainly focusing on the pure thermal decomposition [16], [17], [18], [19], but not the decomposition accompanied by the melting-crystallization process. Accordingly, the focus of this work is aimed at investigating the thermal decomposition of PVA and PVA/graphene composites during their phase transition process. Particularly, morphologies, phase compositions, surface chemical structures, decomposition kinetics and decomposition activation energy of PVA or PVA/graphene composites will be systematically analyzed.